Patent Publication Number: US-4923874-A

Title: Use of 8-azapurin-6-one derivatives for control of hypertension

Description:
FIELD OF THE INVENTION 
     This invention is in the field of cardiovascular therapeutics and relates to a class of fused pyrimidine compounds for use in control of hypertension. Of particular interest is a class of 8-azapurin-6-one derivatives for use in lowering blood pressure in hypertensive subjects. 
     BACKGROUND OF THE INVENTION 
     It has been known for some time that the vasorelaxant effect of the nitrovasodilators, endothelium-derived relaxing factor (EDRF) and atrial natriuretic peptides is caused by stimulation of guanylate cyclase and an elevation in intracellular cGMP levels in vascular smooth muscle cells. In particular, the effects of 8-azapurin-6-one compounds on vascular smooth muscle have been investigated. For example, the effect of four different nitrovasodilators on cGMP levels and relaxation of bovine coronary artery tissue in the presence of 2-o-propoxyphenyl-8-azapurin-6-one has been investigated [W. R. Kukovetz et al, Naunyn-Schmiedeberg&#39;s Arch. Pharmacol., 310, 129-138 (1979)]. In an investigation of the vasorelaxant effects of endothelium derived relaxing factor (ERDF) on rat and rabbit aorta, it was found that 2-o-propoxyphenyl-8-azapurine-6-one potentiated the effects of spontaneously released EDRF [W. Martin et al, J.P.E.T., 237(2), 539-547 (1986)]. The potentiation of atriopeptin-II-induced relaxation of rabbit aorta by 2-o-propoxyphenyl-8-azapurine-6-one has been confirmed [W. Martin et al, Br. J. Pharm., 89, 557-561 (1986)]. In subsequent tests, however, involving an infusion of rat with atriopeptin II, it was found that blood pressure lowering was due to a drop in cardiac output and not due to arterial vasodilation. [R. W. Lappe et al, Circ. Res., 56, 606-612 (1985)]. Thus, in vitro determinations of vasorelaxant activity are not predictive of blood pressure lowering activity in mammals. The inhibitory effects of a class of dibenzoquinazoline diones on cGMP phosphodiesterase from bovine renal artery have been investigated. In this study, it was shown that these compounds lowered blood pressure when injected into spontaneously hypertensive rats. [R. F. G. Booth et al, Biochem. Pharm., 36 (20), 3517-3521 (1987)]. 
     The compound 2-o-propoxyphenyl-8-azapurin-6-one) has been shown to inhibit allergen-stimulated histamine release from mast cells and was initially developed as an orally active prophylactic agent for the treatment of asthma [N. Frossard et al, Br. J. Pharm., 73, 933-938 (1981)]. The mechanism of the mast cell stabilizing effect of this compound is believed to be inhibition of cGMP phosphodiesterase, the enzyme responsible for the metabolism of cGMP, and a subsequent increase in intracellular cGMP levels in mast cells [N. Frossard et al, Br. J. Pharm., 73, 933-938 (1981); R. M. Rudd, Br. J. Dis. Chest., 77, 78-86 (1983)]. U.S. Pat. No. 3,819,631 describes a class of 8-azapurin-6-one compounds, including specifically the compound 2-o-propoxyphenyl-8-azapurin-6-one for use in the treatment of asthma. 
     Other uses for the compound 2-o-propoxyphenyl-8-azapurin-6-one have been investigated. For example, the antiviral activity of this compound was examined along with other fused pyrimidines [J. E. McCormick et al, Proc. R. Ir. Acad., Sect. B, 83, B(1-16), 125-138 (1983)]. 
     DESCRIPTION OF THE INVENTION 
     Treatment of hypertension in a mammal susceptible to or afflicted with hypertension is accomplished by administering a therapeutically-effective amount of an antihypertensive 8-azapurin-6-one compound selected from a family of compounds of Formula I: ##STR1## wherein R represents one or more groups selected from alkyl, alkenyl, alkoxy, alkenyloxy, aralkyl and, aralkoxy, the alkyl portion of any of said groups being in linear or branched configuration; or a pharmaceutically-acceptable salt thereof; with the proviso that at least one of R is an alkoxy group. 
     A preferred class of compounds within Formula I consists of those compounds wherein R represents one or more groups selected from alkyl, alkenyl, alkoxy, alkenyloxy, phenylalkyl and phenylalkoxy, the alkyl portion of any of said groups being in linear or branched configuration and containing one to about twenty carbons, and more preferably one to about ten carbon atoms, or a pharmaceutically-acceptable salt thereof; with the proviso that at least one of R is a linear or branched alkoxy group having one to about twenty carbon atoms, and more preferably one to about ten carbon atoms. 
     A still more preferred class of compounds within Formula I consists of those compounds wherein R is one or more groups selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, iso-butyl, sec-butyl, tert-butyl, n-pentyl, iso-pentyl, neopentyl, n-hexy, n-heptyl, ethenyl, n-propenyl, iso-propenyl, methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, n-pentyloxy, iso-pentyloxy, neopentyloxy, n-hexyloxy, n-heptyloxy, allyloxy, phenmethyl, phenethyl, phenpropyl, phenmethoxy (i.e., benzyloxy), phenethoxy and phenpropoxy; or a pharmaceutically-acceptable salt thereof, with the proviso that at least one of R is an alkoxy group selected from methoxy, ethoxy, n-propoxy, iso-propoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, n-pentyloxy, iso-pentyloxy, neopentyloxy, n-hexyloxy, n-heptyloxy, allyloxy and benzyloxy. 
     A more highly preferred class of compounds of Formula I consists of those compounds wherein R represents one or more groups selected from methyl, ethyl, n-propyl, isopropyl, n-butyl, methoxy, ethoxy, n-propoxy, iso-propoxy, n-butoxy, iso-butoxy, sec-butoxy, tert-butoxy, n-pentyloxy, iso-pentyloxy, neopentyloxy, n-hexyloxy, allyloxy and benzyloxy; or a pharmaceutically-acceptable salt thereof; with the proviso that R includes at least one or two alkoxy groups attached at the 2-,4- or 5- positions of the phenyl ring. 
     An even more highly preferred class of compounds of Formula I consists of those compounds of Formula 1 listed in Table I. 
     
                       TABLE I                                                     
______________________________________                                    
Compound #                                                                
          Formal Name                                                     
______________________________________                                    
 1        8-aza-2-(2-n-propoxyphenyl)purin-6-one                          
 2        8-aza-2-(2-methoxyphenyl)purin-6-one                            
 3        8-aza-2-(2-ethoxyphenyl)purin-6-one                             
 4        8-aza-2-(2-isopropoxyphenyl)purin-6-one                         
 5        8-aza-2-(2-n-butoxyphenyl)purin-6-one                           
 6        8-aza-2-(2-isobutoxyphenyl)purin-6-one                          
 7        8-aza-2-(2-sec-butoxyphenyl)purin-6-one                         
 8        8-aza-2-(2-tert-butoxyphenyl)purin-6-one                        
 9        8-aza-2-(2-n-pentyloxyphenyl)purin-6-one                        
10        8-aza-2-(2-isopentyloxyphenyl)purin-6-one                       
11        8-aza-2-(2-n-hexyloxyphenyl)purin-6-one                         
12        8-aza-2-(2-benzyloxyphenyl)purin-6-one                          
13        8-aza-2-(2,4-dimethoxyphenyl)purin-6-one                        
14        8-aza-2-(2,5-dimethoxyphenyl)purin-6-one                        
15        8-aza-2-(2,5-dibenzyloxyphenyl)purin-6-one                      
16        8-aza-2-(5-benzyloxy-2-methoxyphenyl)-                          
          purin-6-one                                                     
17        8-aza-2-(5-tert-butyl-2-methoxyphenyl)-                         
          purin-6-one                                                     
18        8-aza-2-(2-methoxy-5-methylphenyl)purin-                        
          6-one                                                           
19        8-aza-2-(2-methoxy-3,5-dimethylphenyl)-                         
          purin-6-one                                                     
20        8-aza-2-(2-allyloxyphenyl)purin-6-one                           
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     The term &#34;alkoxy&#34; is intended to embrace monovalent radicals consisting of an alkyl residue of up to about twenty carbon atoms, whether in linear or ramified (i.e. branched) configuration, which residue is attached to through an oxygen atom to the remainder of the molecule. The term &#34;alkoxy&#34; also embraces radicals having more than one alkyl-oxygen atom, fragments. Thus, the term &#34;alkoxy&#34; as used herein will include those radicals having two or more alkoxy portions, i.e., &#34;alkoxyalkoxy&#34; radicals such as ethoxymethoxy and ethoxyethoxy radicals. The term &#34;alkenyloxy&#34; is analogous to &#34;alkoxy&#34; and embraces alkoxy groups which further include a carbon-carbon double bond so as to make, for example, an allyloxy group. The term &#34;aralkyloxy&#34; is analogous to &#34;alkoxy&#34; and embraces alkoxy groups which further include an aryl radical to make, for example, a benzyloxy group. Where the term &#34;alkyl&#34; is used, alone or within other terms such as &#34;alkenyl&#34; or &#34;aralkyl&#34;, the term alkyl embraces linear or branched radicals having one to about twenty carbon atoms, and more preferably one to about ten carbon atoms. &#34;Alkenyl&#34; groups may have one or a plurality of carbon-carbon double bonds, but preferably will contain a single carbon-carbon double bond such as ethenyl and propenyl. &#34;Aryl&#34; groups are exemplified by phenyl and naphthyl, with phenyl preferred. 
     A most preferred class of compounds of Formula I consists of compounds wherein R is a single alkoxy group selected from methoxy, ethoxy, n-propoxy, n-butoxy and n-pentoxy, attached at the 2- position phenyl ring of Formula I, that is, at the ortho position relative to attachment of the phenyl ring to the 8-azapurine-6-one nucleus, examples of which are included within the compounds listed in Table I namely, compounds #1, #2, #3, #5 and #9. 
     A most highly preferred compound of Formula I for use in treatment of hypertension is Compound #1 of Table I having the structure below: ##STR2## 
     Compounds of Formula I including Compound #1, specifically, may be prepared in accordance with procedures described in U.S. Pat. No. 3,819,631. 
     Within this class of 8-azapurin-6-one compounds and derivatives of the invention are the pharmaceutically acceptable salts of the compounds of Formula I, including base addition salts. The term &#34;pharmaceutically-acceptable salts&#34; embraces salts commonly used to form alkali metal salts and to form addition salts of free bases. The nature of the salt is not critical, provided that it is pharmaceutically-acceptable. Suitable pharmaceutically-acceptable base addition salts of compounds of Formula I include alkali metal salts, e.g., salts of sodium and potassium, and ammonium salts and amine salts, e.g., salts made from N,N&#39;-dibenzylethylenediamine, chloroprocaine, choline, diethanolamine, trietholanamine, ethylenediamine, diethylamine, triethylamine, meglumine (N-methylglucamine) and procaine. All of these salts may be prepared by conventional means from the corresponding compound of Formula I by reacting, for example, the appropriate base with the compound of Formula I. 
     Compounds of general Formula I can possess one or more asymmetric carbon atoms and are thus capable of existing in the form of optical isomers as well as in the form of racemic or non-racemic mixtures thereof. The optical isomers can be obtained by resolution of the racemic mixtures according to conventional processes, for example by formation of diastereoisomeric salts by treatment with an optically active acid or base. Examples of appropriate acids are tartaric, diacetyltartaric, dibenzoyltartaric, ditoluoyltartaric and camphorsulfonic acid. Then, the mixture of diastereoisomers may be resolved by crystallization followed by liberation of the optically active bases from these salts. A different process for separation of optical isomers involves the use of a chiral chromatography column optimally chosen to maximize the separation of the enantiomers. Still another available method involves synthesis of covalent diastereoisomeric molecules by reacting compounds of Formula I with an optically pure acid in an activated form or an optically pure isocyanate. The synthesized diastereoisomers can be separated by conventional means such as chromatography, distillation, crystallization or sublimation, and then hydrolyzed to deliver the enantiomericaly pure compound. The optically active compounds of Formula I can likewise be obtained by utilizing optically active starting materials. These isomers may be in the form of a free acid, a free base, an ester or a salt. All of these steroisomers, optical isomers, diastereoisomers, as well as mixtures thereof such as racemic mixtures, are within the scope of the invention. 
     Many therapeutic antihypertensive agents are available for use in reducing blood pressure, such as clonidine, labetalol, metoprolol, minoxidil, prazosin, reserpin and rescinnamine. Also, there are many diuretic agents available which act on the kidney to increase natriuresis and reduce fluid volume, such as amiloride, benzthiazide, bumetanide, chlorothiazide, chlorthalidone, cyclothiazide, ethacrynic acid, furosemide, hydrochlorothiazide, methylclothiazide, metolazone, spironolactone, trichlormethiazide and triamterene. Since all known blood-pressure-lowering agents typically promote sodium retention, because of a drop in pressure at the kidney, a combination of antihypertensive drug and diuretic is required for most patients. In addition a patient suffering from congestive heart failure and edema typically will take a combination of an antihypertensive drug, in order to accomplish blood pressure reduction, and a diuretic agent, in order to reduce fluid volume. An example of one such combination therapy currently available is the blood pressure reducer metoprolol and the diuretic hydrochlorothiazide contained in a single formulation. 
     An advantage provided by compounds of Formula I in treatment of hypertension is the benefit of simultaneous drop in mean arterial pressure and an increase in natriuresis resulting from the action of a single compound. Compound # 1 in particular provides this unexpected combination of properties, that is, a reduction in mean arterial pressure along with an increase in sodium excretion. This combination of properties in one agent eliminates the need for diuretic therapy and may, in particular, provide therapeutic benefit to those hypertensive patients suffering from severe edema. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     FIG. 1 is a graph showing effects of Compound #1 on mean arterial pressure in anesthetized normotensive rats at four infusion rates; 
     FIG. 2 is a graph showing effects of Compound #1 on urinary sodium excretion in anesthetized normotensive rats at four infusion rates; 
     FIG. 3 is a graph showing effect of Compound #1 on levels of plasma cGMP in anesthetized normotensive rats; 
     FIG. 4 is a graph showing effect of Compound #1 on urinary excretion of cGMP in anesthetized normotensive rats; 
     FIG. 5 is a graph showing effect of Compound #1 on levels of plasma cAMP in anesthetized normotensive rats; 
     FIG. 6 is a graph showing effect of Compound #1 on urinary excretion of cAMP in anesthetized normotensive rats; 
     FIG. 7 is a graph showing changes in blood pressure and heart rate in anesthetized normotensive rats infused with Compound #1. 
     FIG. 8 is a graph showing effects of Compound #1 on blood pressure in anesthetized normotensive rats. 
     FIG. 9 is a graph showing effects of Compound #1 on mesenteric blood flow in anesthetized normotensive rats; 
     FIG. 10 is a graph showing effects of Compound #1 on hindquarters blood flow in anesthetized normotensive rats. 
     FIG. 11 is a graph showing effects of Compound #1 on renal blood flow in anesthetized normotensive rats. 
     FIG. 12 is a graph showing effects of Compound #1 on mesenteric vascular resistance in anesthetized normotensive rats. 
     FIG. 13 is a graph showing effects of Compound #1 on hindquarters vascular resistance in anesthetized normotensive rats. 
     FIG. 14 is a graph showing effects of Compound #1 on renal vascular resistance in anesthetized normotensive rats. 
     FIG. 15 is a graph showing effect on mean arterial pressure when Compound #1 is administered intragastrically to conscious spontaneously hypertensive rats; 
     FIG. 16 is a graph showing the effect on heart rate when Compound #1 is administered intragastrically to conscious spontaneously hypertensive rats; and 
     FIG. 17 is a graph showing effect on body weight when Compound #1 is administered intragastrically to conscious spontaneously hypertensive rats. 
    
    
     BIOLOGICAL EVALUATION 
     EXAMPLE I 
     Compound #1 was infused intravenously into normotensive rats to determine depressor and natriuretic effects by measuring blood pressure and sodium excretion. 
     Male Sprague Dawley rats (200-350 g) were obtained from Charles River Laboratories and housed in the laboratory animal facility of the McDonnell Sciences Building in laminar flow cage racks. Animals were allowed access to rat chow and water ad libitum until the morning of the experiment. The rats were anesthetized with Inactin (Byk Gulden), 100 mg/kg I.P. A tracheostomy was performed by inserting PE205 polyethylene tubing (1.5-2.0 cm in length) into the trachea about 2-4 mm below the larynx. The right carotid artery and jugular vein were then catheterized using PE 50, and the catheters were filled with heparinized saline (30 U/ml). A PE 205 urine catheter was inserted into the bladder via a midline incision of the abdomen for urine collection throughout the experiment. 
     Mean arterial pressure was recorded on a Gould system which included Statham transducers (P23ID), universal or pressure processors, pre-amplifiers, and a 2800S 8-channel recorder. The recorder was calibrated each day with a mercury manometer to 50-150 mmHg full scale. Urine was collected in gravimetrically-weighed tubes for 10 minute collection periods. Na +  concentrations were read on an Instrument Laboratories 940 flame photometer and were expressed as uEq/ml. Urine density was assumed to be 1 g/cc. 
     Thirty-three mg of Compound #1 (M and B 22,948; May and Baker, LTD Batch PSS6) was weighed into a tared 12×75 mm Borosilicate test tube on a Mettler AE240 analytical balance. 0.5 ml of 0.25N NaOH was added and the tube was vortexed vigorously for 2-3 minutes or until the compound dissolved. 5 ul aliquots of 0.05N HCl were then added with continuous vortexing until approximately 50 ul of HCl had been added. The pH of the solution was then measured using a pH indicator strip and was approximately 7.5. The volume of the solution was then measured and an equal volume of distilled water was added to achieve a final concentration of 30 mg/ml. 
     The rats were allowed to equilibrate for 45-60 minutes following the surgery. An intravenous infusion of 0.9% NaCl was then started at 3 ml/hr to obtain basal urine flow and blood pressure measurements. Forty minutes after the start of the saline infusion the rats received either a continuation of the saline infusion or Compound #1 at 0.34, 0.68, 1.00 or 2.72 mg/kg/min. Solution of Compound #1 were added to the 0.9% NaCl solution and infused at 3 ml/hr for 30 minutes, followed by an infusion of 0.9% NaCl for an additional 90 minutes. Results are reported in Tables II-XI and in FIGS. 1 and 2. 
     
         __________________________________________________________________________
II. EFFECTS OF 0.9% NaCl (3 ML/HR)                                        
ON MEAN ARTERIAL PRESSURE                                                 
IN ANESTHETIZED RATS                                                      
TIME                                                                      
    MAP (mm Hg)                                                           
(min)                                                                     
    RAT 1                                                                 
        RAT 2                                                             
            RAT 3                                                         
                RAT 4                                                     
                    RAT 5                                                 
                        RAT 6                                             
                            RAT 7                                         
                                MEAN SEM                                  
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10  122 125 121 130 132 130 140 129  2                                    
20  121 127 122 127 128 128 140 128  2                                    
30  120 126 119 127 130 131 139 127  2                                    
40  120 123 119 123 125 126 139 125  2                                    
50  116 123 116 122 125 125 138 124  2                                    
60  116 122 114 118 120 126 137 122  3                                    
70  117 121 113 120 124 123 135 122  2                                    
80  118 119 110 123 126 125 136 122  3                                    
90  118 120 115 114 122 120 135 121  2                                    
100 120 120 110 122 124 122 132 121  2                                    
110 115 116 110 120 122 120 128 119  2                                    
120 120 114 110 122 126 117 132 120  2                                    
130 118 118 108 120 124 120 130 120  2                                    
140 117 115 106 115 124 116 132 118  3                                    
150 118 116 112 110 126 115 124 117  2                                    
160 113 120 114 108 122 116 125 117  2                                    
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         __________________________________________________________________________
 III. EFFECTS OF 0.9% NaCl (3 ML/HR)                                      
ON URINARY SODIUM EXCRETION                                               
IN ANESTHETIZED RATS                                                      
TIME                                                                      
    URINARY SODIUM EXCRETION (uEq/min)                                    
(min)                                                                     
    RAT 1                                                                 
        RAT 2                                                             
            RAT 3                                                         
                RAT 4                                                     
                    RAT 5                                                 
                        RAT 6                                             
                            RAT 7                                         
                                MEAN SEM                                  
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10  0.09                                                                  
        0.09                                                              
            0.12                                                          
                0.29                                                      
                    0.92                                                  
                        0.51                                              
                            0.77                                          
                                0.40 0.11                                 
20  0.07                                                                  
        0.11                                                              
            0.04                                                          
                0.32                                                      
                    0.61                                                  
                        0.40                                              
                            0.39                                          
                                0.28 0.07                                 
30  0.04                                                                  
        0.07                                                              
            0.21                                                          
                0.31                                                      
                    0.24                                                  
                        0.27                                              
                            0.40                                          
                                0.22 0.04                                 
40  0.04                                                                  
        0.02                                                              
            0.17                                                          
                0.29                                                      
                    0.18                                                  
                        0.31                                              
                            0.20                                          
                                0.17 0.04                                 
50  0.05                                                                  
        0.02                                                              
            0.42                                                          
                0.21                                                      
                    0.19                                                  
                        0.26                                              
                            0.15                                          
                                0.19 0.04                                 
60  0.05                                                                  
        0.01                                                              
            0.42                                                          
                0.20                                                      
                    0.24                                                  
                        0.30                                              
                            0.14                                          
                                0.19 0.05                                 
70  0.05                                                                  
        0.01                                                              
            0.24                                                          
                0.25                                                      
                    0.40                                                  
                        0.24                                              
                            0.11                                          
                                0.19 0.05                                 
80  0.07                                                                  
        0.01                                                              
            0.37                                                          
                0.21                                                      
                    0.79                                                  
                        0.35                                              
                            0.10                                          
                                0.27 0.09                                 
90  0.07                                                                  
        0.02                                                              
            0.28                                                          
                0.26                                                      
                    1.19                                                  
                        0.33                                              
                            0.12                                          
                                0.32 0.13                                 
100 0.10                                                                  
        0.05                                                              
            0.18                                                          
                0.20                                                      
                    0.97                                                  
                        0.32                                              
                            0.15                                          
                                0.28 0.10                                 
110 0.16                                                                  
        0.06                                                              
            0.31                                                          
                0.28                                                      
                    2.27                                                  
                        0.53                                              
                            0.12                                          
                                0.53 0.26                                 
120 0.25                                                                  
        0.03                                                              
            0.36                                                          
                0.78                                                      
                    4.09                                                  
                        1.05                                              
                            0.16                                          
                                0.96 0.47                                 
130 0.49                                                                  
        0.02                                                              
            2.10                                                          
                0.72                                                      
                    3.68                                                  
                        0.81                                              
                            0.11                                          
                                1.13 0.43                                 
140 0.82                                                                  
        0.02                                                              
            1.57                                                          
                2.64                                                      
                    3.63                                                  
                        1.49                                              
                            0.17                                          
                                1.48 0.43                                 
150 1.36                                                                  
        0.02                                                              
            1.35                                                          
                1.75                                                      
                    4.19                                                  
                        3.40                                              
                            0.17                                          
                                1.75 0.51                                 
160 1.17                                                                  
        0.03                                                              
            1.42                                                          
                2.36                                                      
                    5.07                                                  
                        6.63                                              
                            0.17                                          
                                2.41 0.83                                 
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         ______________________________________                                    
IV. EFFECTS OF COMPOUND #1 (0.34 MG/KG/MIN)*                              
ON MEAN ARTERIAL PRESSURE                                                 
IN ANESTHETIZED RATS                                                      
TIME  MAP (mm Hg)                                                         
(min) RAT 1   RAT 2    RAT 3  RAT 4 MEAN   SEM                            
______________________________________                                    
10    133     124      120    128   126    2                              
20    130     122      117    129   125    2                              
30    127     122      116    125   123    2                              
40    124     122      115    127   122    2                              
50    103     96       112    110   105    3                              
60    100     92       110    106   102    3                              
70     98     89       106    104    99    3                              
80     97     99       112    110   105    3                              
90    101     99       112    114   107    3                              
100   102     98       113    113   107    3                              
110   100     100      115    114   107    3                              
120   106     99       114    116   109    3                              
130   106     101      113    115   109    3                              
140   105     102      114    118   110    3                              
150   110     100      113    114   109    3                              
160   110     100      115    112   109    3                              
______________________________________                                    
 *The Compound #1 infusion was started at 40 minutes and was terminated at
 70 minutes.                                                              
 
    
     
         ______________________________________                                    
V. EFFECTS OF COMPOUND #1 (0.34                                           
MG/KG/MIN)* ON URINARY SODIUM                                             
EXCRETION IN ANESTHETIZED RATS                                            
TIME  URINARY SODIUM EXCRETION (uEq/min)                                  
(min) RAT 1   RAT 2    RAT 3  RAT 4 MEAN   SEM                            
______________________________________                                    
10    0.01    0.09     0.90   0.70  0.43   0.17                           
20    0.01    0.08     0.88   0.70  0.42   0.17                           
30    0.01    0.04     1.06   0.63  0.44   0.20                           
40    0.01    0.02     0.97   0.60  0.40   0.18                           
50    0.02    0.03     1.49   1.54  0.77   0.33                           
60    0.16    0.07     1.72   2.69  1.16   0.49                           
70    0.15    0.07     1.89   2.99  1.28   0.55                           
80    0.15    0.08     4.48   2.92  1.91   0.84                           
90    0.51    0.10     7.66   3.24  2.88   1.35                           
100   1.36    0.40     9.88   4.72  4.09   1.66                           
110   1.68    1.60     10.80  5.61  4.92   1.68                           
120   2.57    3.68     10.68  6.29  5.81   1.40                           
130   2.12    5.44     9.79   7.07  6.11   1.24                           
140   1.87    7.36     10.00  7.03  6.57   1.32                           
150   2.08    3.38     8.06   5.89  4.85   1.03                           
160   2.29    7.18     7.59   6.50  5.89   0.95                           
______________________________________                                    
 *The Compound #1 infusion was started at 40 minutes and was terminated at
 70 minutes.                                                              
 
    
     
         ______________________________________                                    
VI. EFFECTS OF COMPOUND #1                                                
(0.68 MG/KG/MIN)* ON MEAN                                                 
ARTERIAL PRESSURE IN ANESTHETIZED RATS                                    
TIME  MAP (mm Hg)                                                         
(min) RAT 1   RAT 2    RAT 3  RAT 4 MEAN   SEM                            
______________________________________                                    
10    130     126      126    125   127    1                              
20    133     124      122    125   126    2                              
30    132     126      118    124   125    2                              
40    133     125      120    123   125    2                              
50    110     100      96     96    101    3                              
60    114     97       88     86    96     5                              
70     99     90       84     79    88     3                              
80    102     92       87     86    92     3                              
90    103     92       86     83    91     3                              
100   107     94       88     84    93     4                              
110   110     96       90     82    95     5                              
120   113     94       92     84    96     5                              
130   115     96       91     84    97     5                              
140   118     99       92     82    98     6                              
150   117     99       91     82    97     6                              
160   123     102      96     82    101    7                              
______________________________________                                    
 *The Compound #1 infusion was started at 40 minutes and was terminated at
 70 minutes.                                                              
 
    
     
         ______________________________________                                    
VII. EFFECTS OF COMPOUND #1                                               
(0.68 MG/KG/MIN)* ON URINARY                                              
SODIUM EXCRETION IN ANESTHETIZED RATS                                     
TIME  URINARY SODIUM EXCRETION (uEq/min)                                  
(min) RAT 1   RAT 2    RAT 3  RAT 4 MEAN   SEM                            
______________________________________                                    
10    0.30    0.15     0.73   0.15  0.33   0.11                           
20    0.27    0.10     0.58   0.16  0.28   0.08                           
30    0.54    0.08     0.56   0.12  0.33   0.10                           
40    0.53    0.07     0.90   0.20  0.43   0.14                           
50    0.60    0.10     0.71   0.64  0.51   0.11                           
60    0.85    0.28     0.72   1.58  0.86   0.21                           
70    2.40    0.73     0.89   1.20  1.31   0.29                           
80    5.43    2.92     0.58   0.91  2.46   0.87                           
90    8.30    6.97     0.98   1.38  4.41   1.46                           
100   10.53   7.96     2.32   4.23  6.26   1.43                           
110   11.95   8.74     3.94   7.39  8.01   1.28                           
120   11.56   8.20     3.43   7.52  7.68   1.29                           
130   10.71   7.41     5.82   7.13  7.77   0.81                           
140   8.65    6.65     5.78   5.88  6.74   0.52                           
150   6.86    6.46     7.72   5.33  6.59   0.38                           
160   5.76    6.17     11.34  4.24  6.88   1.20                           
______________________________________                                    
 *The Compound #1 infusion was started at 40 minutes and was terminated at
 70 minutes.                                                              
 
    
     
         ______________________________________                                    
VIII. EFFECTS OF COMPOUND #1                                              
(1.00 MG/KG/MIN)* ON                                                      
MEAN ARTERIAL PRESSURE IN ANESTHETIZED RATS                               
TIME  MAP (mm Hg)                                                         
(min) RAT 1   RAT 2    RAT 3  RAT 4 MEAN   SEM                            
______________________________________                                    
10    120     116      126    108   118    3                              
20    122     116      124    106   117    3                              
30    120     118      123    104   116    3                              
40    118     117      123    102   115    4                              
50    102     86       102    90    95     3                              
60    87      87       92     88    89     1                              
70    80      83       87     81    83     1                              
80    78      84       84     80    82     2                              
90    75      84       87     78    81     2                              
100   75      88       87     79    82     2                              
110   76      86       85     80    82     2                              
120   77      86       88     80    83     3                              
130   81      94       90     80    86     3                              
140   88      95       95     80    90     3                              
150   90      92       96     80    90     3                              
160   92      90       101    78    90     4                              
______________________________________                                    
 *The Compound #1 infusion was started at 40 minutes and was terminated at
 70 minutes.                                                              
 
    
     
         ______________________________________                                    
IX. EFFECTS OF COMPOUND #1                                                
(1.00 MG/KG/MIN)*                                                         
ON URINARY SODIUM                                                         
EXCRETION IN ANESTHETIZED RATS                                            
TIME  URINARY SODIUM EXCRETION (uEq/min)                                  
(min) RAT 1   RAT 2    RAT 3  RAT 4 MEAN   SEM                            
______________________________________                                    
10    0.06    0.21     0.23   0.17  0.17   0.03                           
20    0.10    0.16     0.25   0.16  0.17   0.02                           
30    0.05    0.18     0.25   0.16  0.16   0.03                           
40    0.03    0.15     0.32   0.15  0.16   0.05                           
50    0.27    0.10     0.26   1.74  0.59   0.30                           
60    3.36    0.04     0.68   1.35  1.36   0.56                           
70    5.22    0.04     3.27   1.00  2.38   0.90                           
80    6.03    0.11     9.02   1.23  4.10   1.61                           
90    7.29    0.92     12.76  1.58  5.64   2.15                           
100   8.31    2.86     14.79  5.33  7.82   2.00                           
110   10.46   4.41     13.29  7.70  8.97   1.47                           
120   11.34   3.86     12.16  9.20  9.14   1.45                           
130   17.59   4.03     11.77  9.77  10.79  2.17                           
140   20.06   3.50     9.53   8.87  10.49  2.68                           
150   17.09   2.53     8.47   7.35  8.86   2.35                           
160   14.27   2.50     6.92   6.79  7.62   1.89                           
______________________________________                                    
 *The Compound #1 infusion was started at 40 minutes and was terminated at
 70 minutes.                                                              
 
    
     
         ______________________________________                                    
X. EFFECTS OF COMPOUND #1 (2.72 MG/KG/MIN)*                               
ON MEAN ARTERlAL PRESSURE                                                 
lN ANESTHETlZED RATS                                                      
TIME  MAP (mm Hg)                                                         
(min) RAT 1   RAT 2    RAT 3  RAT 4 MEAN   SEM                            
______________________________________                                    
10    119     108      126    112   116    3                              
20    120     107      123    112   116    3                              
30    119     110      127    108   116    3                              
40    121     108      123    110   116    3                              
50    80      82       84     75    80     2                              
60    78      66       73     63    70     3                              
70    58      57       69     55    60     2                              
80    61      55       67     58    60     2                              
90    62      58       65     60    61     1                              
100   62      58       64     58    61     1                              
110   61      58       64     57    60     1                              
120   61      58       63     56    60     1                              
130   62      60       63     56    60     1                              
140   57      60       62     57    59     1                              
150   54      59       64     56    58     2                              
160   50      58       60     56    56     2                              
______________________________________                                    
 *The Compound #1 infusion was started at 40 minutes and was terminated at
 70 minutes.                                                              
 
    
     
         ______________________________________                                    
XI. EFFECTS OF COMPOUND #1                                                
(2.72 MG/KG/MIN)* ON URINARY                                              
SODIUM EXCRETION IN ANESTHETIZED RATS                                     
TIME  URINARY SODIUM EXCRETION (uEq/min)                                  
(min) RAT 1   RAT 2    RAT 3  RAT 4 MEAN   SEM                            
______________________________________                                    
10    0.10    0.17     0.15   0.07  0.12   0.02                           
20    0.06    0.14     0.19   0.08  0.12   0.02                           
30    0.08    0.12     0.12   0.11  0.11   0.01                           
40    0.06    0.11     0.13   0.14  0.11   0.01                           
50    0.10    0.18     0.06   0.14  0.12   0.02                           
60    --      0.11     0.04   --    0.08   0.02                           
70    --      --       0.02   --    0.02   0.00                           
80    --      --       0.08   --    0.08   0.00                           
90    --      --       0.33   --    0.33   0.00                           
100   0.80    --       2.22   --    1.51   0.41                           
110   2.24    --       4.20   --    3.22   0.57                           
120   3.20    --       5.77   --    4.49   0.74                           
130   3.47    --       5.77   --    4.62   0.66                           
140   2.29    0.30     5.83   0.18  2.15   1.02                           
150   0.45    0.41     4.49   0.21  1.39   0.80                           
160   0.05    0.32     2.26   0.08  0.68   0.41                           
______________________________________                                    
 *The Compound #1 infusion was started at 40 minutes and was terminated at
 70 minutes.                                                              
 
    
     Animals infused with 0.9% NaCl at 3 ml/hr for 160 minutes demonstrated a slight decrease (approximately 10 mmHg) in mean arterial pressure (MAP) (Table II). These animals served as controls for the rats infused with Compound #1. The urinary excretion of Na increased slightly during the 160 minutes of saline infusion in these animals, as shown on Table III. In a second group of rats, Compound #1 was infused during the 40-70 minute period and MAP and urinary sodium excretion (UNaV) were measured and compared to the rats infused with saline. As shown on Table IV, an infusion of Compound #1 at 0.34 mg/kg/min reduced pressure by approximately 20 mmHg, while a 30 minute infusion at 0.68 mg/kg/min resulted in a 30 mmHg fall in mean arterial pressure (Table VI). These infusions of Compound #1 increased the urinary excretion of sodium to peak values of 6.5 and 8.0 uEq/min, respectively (Table V and Table VII). When Compound #1 was infused at 1.00 mg/kg/min, MAP was reduced by approximately 40 mmHg (Table VIII) and a peak value of 11 uEq/min was reached for urinary sodium excretion (Table IX). At the highest infusion rate tested (2.72 mg/kg/min), MAP fell to approximately 60 mm Hg (Table X). At this very low blood pressure, urine flow was so reduced that urine samples could not be obtained from some of the animals for all collection periods. This accounts for the missing values in Table XI. Those animals producing enough urine for sodium measurements demonstrated a peak sodium excretion rate of 4.5 uEq/min (Table XI)--a rate which was still significantly elevated compared to the saline-infused control animals. 
     The blood pressure data from these experiments are summarized in FIG. 1 and the sodium excretion data are graphically illustrated in FIG. 2. A thirty minute infusion of Compound #1 into anesthetized rats resulted in a dose-dependent fall in MAP which was reasonably rapid in onset and very prolonged. Although a slight recovery of pressure was observed at the lower infusion rates, at all infusion rates, MAP in the Compound #1-infused rats was significantly lower than pressure in the saline-infused control animals 90 minutes after the Compound #1 infusions were terminated. Urinary sodium excretion was increased significantly at all rates of Compound #1 infusion compared to the saline-infused control rats. However, the increase in UNaV was blunted somewhat in the rats infused at 2.72 mg/kg/min. This was obviously due to the very low blood pressure in these animals (approximately 60 mmHg). While the peak of the depressor response occured at 70 minutes, when the drug infusions were terminated, the natriuretic responses were significantly delayed. Peak natriuretic responses were typically observed between 120 and 140 minutes. 
     In summary, these data clearly illustrate that Compound #1 infused at 0.34-2.72 mg/kg/min into anesthetized rats results in a significant fall in mean arterial blood pressure. This compound also causes a very prominent simultaneous natriuresis, which reaches its peak value approximately one hour after the fall in pressure reaches its maximum. 
     EXAMPLE II 
     In order to determine the mechanism of action of Compound #1, this compound was infused intravenously into normotensive rats to measure cyclic nucleotides (cAMP and cGMP) levels in plasma and urine. Measurements of cAMP and cGMP levels were made in separate groups of test rats. 
     The test animals were instrumented as described in Example I. A second group of rats (400-500 g) was anesthetized and a PE50 catheter was implanted in the femoral artery of these &#34;donor&#34; rats. 2 ml of blood was drawn from these animals through the femoral artery line and infused into the test rats when blood from the test animals was drawn for measurement of cGMP or cAMP. 
     Compound #1 was prepared for administration and mean arterial pressure was recorded and urine collected from the test rats as described in Example I. For cGMP plasma samples, an equal volume of ice-cold 10% trichloracetic acid (TCA) was added to the sample and this mixture was centrifuged at 2500 g at about 4° C. for 15 minutes. The supernatant was applied to a 0.6×5.0 cm column of Dowex 50W-X8 (H + ), 100-200 mesh, equilibrated with water. The first 4 mls of effluent was collected, lyophilized and then assayed for cGMP using a commercially available radioimmunoassay kit (New England Nuclear). Samples were run in triplicate and extraction efficiencies were consistently greater than 90%. Urine samples were diluted 1/50 and then assayed in triplicate for cGMP. For cAMP, plasma and urine samples were diluted 1/50 and then assayed in triplicate for cAMP using a standard radioimmunoassay kit (New England Nuclear). 
     The rats were allowed to equilibrate for 45-60 minutes following the surgery. An intravenous infusion of 0.9% NaCl was then started at 3 ml/hr to obtain basal urine flow and blood pressure measurements. Forty minutes after the start of the saline infusion, 2 ml of blood was drawn from the rats for cyclic nucleotide measurements and immediately replaced with 2 ml of blood from the donor rats. The blood was collected in chilled tubes containing 0.1% EDTA and was centrifuged at about 4° C. at 2500 g for 10 minutes. The rats then received either a continuation of the saline or Compound #1 at 1.00 mg/kg/min for 30 minutes. At the end of the 30 minute infusion, a second blood sample was taken. Samples were also taken at 60 and 120 minutes after the Compound #1 infusion was terminated. Urine was collected throughout the experiment every 10 minutes for calculation of urine flow rates and for cyclic nucleotide measurements. Results are reported in Tables XII-XIX and in FIGS. 3-6. 
     
         ______________________________________                                    
XII. EFFECTS OF 0.9% NaCl (3 ML/HR) ON PLASMA                             
cGMP IN ANESTHETlZED RATS                                                 
PLASMA cGMP (pmoles/ml)                                                   
TIME  RAT                                                                 
(min) 1      RAT 2   RAT 3 RAT 4 RAT 5 MEAN   SEM                         
______________________________________                                    
 0    3.4    2.8     3.9   4.6   1.8   3.3    0.5                         
 30   3.8    3.9     4.0   4.6   1.8   3.6    0.5                         
 90   3.1    2.5     3.7   3.1   2.5   3.0    0.2                         
150   4.0    3.0     4.4   4.4   2.1   3.6    0.5                         
______________________________________                                    
 
    
     
         ______________________________________                                    
XIII. EFFECTS OF COMPOUND #1 (1.00 MG/KG/MIN)*                            
ON PLASMA cGMP IN ANESTHETIZED RATS                                       
PLASMA cGMP (pmoles/ml)                                                   
TIME  RAT                                                                 
(min) 1      RAT 2   RAT 3 RAT 4 RAT 5 MEAN   SEM                         
______________________________________                                    
 0    2.0    5.0     2.5   7.0   2.4   3.8    0.4                         
 30   19.8   18.6    12.8  10.0  18.0  15.8   1.9                         
 90   20.0   23.9    17.1  8.7   14.7  16.9   2.3                         
150   11.6   14.5    14.2  10.4  11.8  12.5   0.7                         
______________________________________                                    
 *The Compound #1 infusion was started at 0 minutes and was terminated at 
 30 minutes.                                                              
 
    
     
         ______________________________________                                    
XIV. EFFECTS OF 0.9% NaCl (3 ML/HR) ON THE                                
URINARY EXCRETION OF cGMP                                                 
IN ANESTHETIZED RATS                                                      
cGMP EXCRETION (pmoles/min)                                               
TIME  RAT                                                                 
(min) 1      RAT 2   RAT 3 RAT 4 RAT 5 MEAN   SEM                         
______________________________________                                    
 0    18.6   17.1    19.6  34.2  18.9  21.7   3.2                         
 30   18.0   17.1    18.9  22.0  10.7  17.3   1.9                         
 90   15.3   13.4    18.6  28.7  16.5  18.5   2.7                         
150   26.1   13.7    18.5  16.6  12.8  17.5   2.4                         
______________________________________                                    
 
    
     
         ______________________________________                                    
XV. EFFECTS OF COMPOUND #1 (1.0 MG/KG/MIN)*                               
ON THE URINARY EXCRETION OF cGMP                                          
IN ANESTHETIZED RATS                                                      
cGMP EXCRETlON (pmoles/min)                                               
TIME  RAT                                                                 
(min) 1      RAT 2   RAT 3 RAT 4 RAT 5 MEAN   SEM                         
______________________________________                                    
 0    19.5   19.9    18.5  27.4  19.6  21.0   1.6                         
 30   18.0   12.3    37.8  40.4  41.5  30.0   6.2                         
 90   54.1   53.4    68.3  44.1  59.2  58.7   2.7                         
150   23.4   43.5    44.1  43.4  47.1  40.3   4.3                         
______________________________________                                    
 *The Compound #1 infusion was started at 0 minutes and was terminated at 
 30 minutes.                                                              
 
    
     
         ______________________________________                                    
XVI. EFFECTS OF 0.9% NaCl (3 ML/HR) ON PLASMA                             
cAMP IN ANESTHETIZED RATS                                                 
PLASMA cAMP (pmoles/ml)                                                   
TIME  RAT    RAT    RAT  RAT  RAT                                         
(min) 1      2      3    4    5    RAT 6 MEAN   SEM                       
______________________________________                                    
 0    29.7   99.7   17.4 41.2 24.9 48.0  43.5   12.1                      
 30   20.1   45.3   19.1 47.8 21.9 32.7  31.2   5.3                       
 90   28.4   52.1   17.6 35.9 19.1 26.9  30.0   5.2                       
150   31.4   55.2   17.4 26.5 14.8 22.0  27.9   5.9                       
______________________________________                                    
 
    
     
         ______________________________________                                    
XVII. EFFECTS OF COMPOUND #1 (1.00 MG/KG/MIN*)                            
ON PLASMA cAMP IN ANESTHETIZED RATS                                       
PLASMA cAMP (pmoles/ml)                                                   
TIME  RAT    RAT    RAT  RAT  RAT                                         
(min) 1      2      3    4    5    RAT 6 MEAN   SEM                       
______________________________________                                    
 0     40.1   35.1   34.1                                                 
                         3.5  22.0 32.2  27.9   5.5                       
 30   101.6  135.1  118.2                                                 
                         50.1 39.1 37.7  80.3   17.6                      
 90    96.3  106.4  120.3                                                 
                         30.6 20.9 22.6  66.2   18.0                      
150    55.5   78.1   44.0                                                 
                         25.5 16.9 23.9  40.7   9.5                       
______________________________________                                    
 *The Compound #1 infusion was started at 0 minutes and was terminated at 
 30 minutes.                                                              
 
    
     
         ______________________________________                                    
XVIII. EFFECTS OF 0.9% NaCl (3 ML/HR) ON THE                              
URINARY EXCRETION OF cAMP                                                 
cAMP EXCRETION (pmoles/min)                                               
TIME  RAT    RAT    RAT  RAT  RAT                                         
(min) 1      2      3    4    5    RAT 6 MEAN   SEM                       
______________________________________                                    
 0    309.9  173.3  151.2                                                 
                         217.2                                            
                              405.8                                       
                                   175.2 238.8  40.6                      
 30   173.5  219.1  122.8                                                 
                         140.9                                            
                              144.0                                       
                                   244.2 174.1  19.6                      
 90   103.3  238.6   81.2                                                 
                         112.1                                            
                              223.7                                       
                                   288.8 174.6  35.2                      
150   139.8  263.5  110.8                                                 
                         121.0                                            
                              194.6                                       
                                   169.3 166.5  23.2                      
______________________________________                                    
 
    
     
         ______________________________________                                    
XIX. EFFECTS OF COMPOUND #1 (1.00 MG/KG/MIN)*                             
ON THE URINARY EXCRETION OF cAMP                                          
cAMP EXCRETION (pmoles/min)                                               
TIME  RAT    RAT    RAT  RAT  RAT                                         
(min) 1      2      3    4    5    RAT 6 MEAN   SEM                       
______________________________________                                    
 0    314.5  164.7  197.4                                                 
                         291.9                                            
                              183.3                                       
                                   136.0 214.6  29.4                      
 30   537.7  131.7  116.7                                                 
                         351.2                                            
                               88.0                                       
                                   268.8 249.1  71.0                      
 90   375.1  141.6  124.9                                                 
                         129.3                                            
                              118.0                                       
                                    97.7 164.4  42.5                      
150   324.5  346.3  148.7                                                 
                         114.9                                            
                               69.4                                       
                                    88.0 182.0  49.8                      
______________________________________                                    
 *The Compound #1 infusion was started at 0 minutes and was terminated at 
 30 minutes.                                                              
 
    
     This experiment was conducted to determine if an increase in intracellular cGMP concentrations might be involved in mediating the depressor and natriuretic effects of Compound #1 when this compound is infused intravenously into anesthetized rats, as described in Experiment 1. Plasma levels of cGMP remained at a constant level of approximately 3-3.5 pmoles/ml in the rats receiving the saline infusion (Table XII, FIG. 3). In rats receiving Compound #1 at 1 mg/kg/min, plasma levels of cGMP increased significantly from a basal level of around 4 pmoles/ml to 17 pmoles/ml (Table XIII, FIG. 3). This increase in plasma cGMP in the Compound #1 infused animals persisted long after the drug infusion was terminated, just as the blood pressure fall in these rats in response to Compound #1 was persistent (See FIG. 1). The urinary excretion of cGMP was also significantly increased in the rats infused with Compound #1, whereas the excretion of cGMP in the rats receiving saline remained at a stable level of approximately 18-20 pmoles/min throughout the experiment (Table XIV, XV). However, the peak of this increased excretion of cGMP in response to Compound #1 was delayed by approximately one hour compared to the peak increase in plasma cGMP (Table XV, FIG. 4). This is similar to the delay we observed in the natriuretic response to Compound #1 (FIG. 2). In these experiments, the plasma levels of cGMP paralleled the blood pressure lowering response to Compound #1, whereas the urinary excretion of cGMP correlated with the natriuretic action of this drug. 
     It has been demonstrated that in isolated rat aorta, intracellular levels of cAMP as well as cGMP are increased in response to Compound #1 treatment, although the increase in cGMP is twice as large as the increase in cAMP [(P. Schoeffter et al, Biochem. Pharm., 36:3965-3972, (1987)]. 
     Therefore it was of interest in these experiments to determine whether Compound #1 infused at 1 mg/kg/min also increased plasma levels of cAMP and the urinary excretion of cAMP. Plasma levels of cAMP were increased significantly in the rats infused with Compound #1, compared to the saline-infused control rats (Table XVI, Table XVII and FIG. 5). However, unlike the increase in cGMP plasma levels in response to Compound #1 infusion, the increase in plasma cAMP was transient and did not persist after the drug infusion was terminated. The urinary excretion of cAMP was also transiently elevated in the Compound #1 infused rats (Table XVIII, Table XIX and FIG. 6). Therefore Compound #1 is not 100% selective for cGMP phosphodiesterase, but also probably inhibits cAMP phosphodiesterase in the rat. However, the changes in mean arterial blood pressure and urinary sodium excretion observed with Compound #1 infusion in these studies more closely correlated with the changes in cGMP rather than cAMP. This is suggestive evidence that the physiological effects observed with Compound #1 may be mediated by increases in intracellular cGMP levels. 
     EXAMPLE III 
     Compound #1 was infused intravenously into normotensive rats to determine if the blood pressure fall was due to reductions in regional vascular resistance. Male Sprague-Dawley rats (300-425 g), obtained from Harlan Laboratories, were anesthetized with Inactin (Byk Gulden), 100 mg/kg I.P. A tracheostomy was performed by inserting PE205, 1.5-2.0 cm in length into the trachea about 2-4 mm below the larynx. The right femoral artery and the right femoral vein were catheterized using PE50, and the catheters were filled with heparinized saline (30 U/ml). Mean arterial pressure and heart rate were recorded on a Gould system which included Statham transducers (P23ID), universal and pressure processors, pre-amplifiers and a 2800S 8-channel recorder. The recorder was calibrated each day with a mercury manometer such that 0-200 mmHg read full scale. 
     For regional blood flow measurements, minaturized pulsed-Doppler flow probes (Valpey-Fisher) connected to a Doppler flowmeter (University of Iowa Bioengineering) were used. For placement of the probes, the abdominal cavity was exposed through a midline laparotomy. Four-millimeter lengths of the superior mesenteric, right renal artery and the lower abdominal aorta were then isolated carefully to avoid damage to any nearby nerves. The probe on the lower abdominal aorta monitors blood flow to the hindquarters, which is predominately a measure of skeletal muscle blood flow. Appropriately-sized probes were placed around the isolated blood vessels and loosely secured with 6-0 opthalmic silk. The insulated wire leads of the flow probes were anchored to the abdominal wall, thereby maintaining proper orientation of the probe relative to the vessel. Following closure of the abdominal incision, the wire leads from each probe were connected to the Doppler flow meter. 
     The Doppler flowmeter system was used to determine blood velocity in kHz Doppler shift, which has been shown to be directly proportional to absolute blood flow as determined with an electromagnetic system [J. R. Haywood et al., Amer. J. Physiol., 241(10): H-273-H-278, (1981)]. An empirical value of relative resistance can be calculated using the following formula: ##EQU1## 
     The calculated empirical value is proportional to the absolute resistance in each vascular bed. In addition, changes in vascular resistance can be calculated as the percent change from control by the formula listed below: ##EQU2## Expressing resistance changes as percent of control allows the opportunity to make meaningful comparisons between animals, despite differences in baseline resistance. Furthermore, by normalizing the data, resistance changes in different vascular beds can be compared. 
     Compound #1 was prepared for administration as described in Example I. Following instrumentation of the rats, control measurements of each parameter were obtained following sixty minutes of 0.9% NaCl infusion through the femoral vein at 33 ul/min. Compound #1 was then administered in serial doses at 0.34, 1, 2 and 3 mg/kg/min. Each dose was infused iv for fifteen minutes; however, cardiovascular variables were averaged only during the last five minutes of this period. Results are reported in Tables XX-XXVIII and in FIGS. 7-14. 
     
         __________________________________________________________________________
XX. CHANGES IN MEAN ARTERIAL PRESSURE IN RESPONSE                         
TO COMPOUND #1 INFUSION IN ANESTHETIZED RATS                              
DOSE   Δ MAP (mm Hg)                                                
(mg/kg/min)                                                               
       RAT 1                                                              
           RAT 2                                                          
               RAT 3                                                      
                   RAT 4                                                  
                       RAT 5                                              
                           RAT 6                                          
                               RAT 7                                      
                                   RAT 8                                  
                                       RAT 9                              
                                           RAT 10                         
                                                MEAN SEM                  
__________________________________________________________________________
0.34    -1  -7  -5 -14  -3  -6   0  -1 -30 -14   -8  3                    
1.00    -8 -18 -18 -24 -15 -15  -9 -11 -41 -26  -19  3                    
2.00   -24 -33 -42 -43 -30 -41 -33 -30 -56 -48  -38  3                    
3.00   -34 -48 -62 -51 -49 -52 -45 -47 -67 -67  -52  3                    
__________________________________________________________________________
 
    
     
         __________________________________________________________________________
XXI. CHANGES IN HEART RATE IN RESPONSE TO COMPOUND #1                     
INFUSION IN ANESTHETIZED RATS                                             
DOSE   HEART RATE (beats/min)                                             
(mg/kg/min)                                                               
       RAT 1                                                              
           RAT 2                                                          
               RAT 3                                                      
                   RAT 4                                                  
                       RAT 5                                              
                           RAT 6                                          
                               RAT 7                                      
                                   RAT 8                                  
                                       RAT 9                              
                                           RAT 10                         
                                                MEAN SEM                  
__________________________________________________________________________
0.34   14  -8  34  -5   -1 17  11   3  -22 -6    4   5                    
1.00   26  -8  58   7  -13 18  30  14  -10 -4   12   7                    
2.00   28  -8  50  18    4  9  43  18  -11 -2   15   7                    
3.00   30   2  47  43   21 20  46  28   -3  4   24   6                    
__________________________________________________________________________
 
    
     
         __________________________________________________________________________
XXII. EFFECTS OF COMPOUND #1 ON MEAN ARTERIAL PRESSURE                    
IN ANESTHETIZED RATS                                                      
DOSE   MAP (mm Hg)                                                        
(mg/kg/min)                                                               
       RAT 1                                                              
           RAT 2                                                          
               RAT 3                                                      
                   RAT 4                                                  
                       RAT 5                                              
                           RAT 6                                          
                               RAT 7                                      
                                   RAT 8                                  
                                       RAT 9                              
                                           RAT 10                         
                                                MEAN SEM                  
__________________________________________________________________________
Control                                                                   
       92  110 127 121 114 116 111 113 124 130  116  3                    
0.34   111 110 107 122 116 94  91  103 114 112  108  3                    
1.00   99  98  97  109 104 83  84  92  102 102  97   3                    
2.00   84  75  78   85  82 68  68  77  102  83  80   3                    
3.00   65  64  70   65  63 57  58  62   66  65  64   1                    
__________________________________________________________________________
 
    
     
         ______________________________________                                    
XXIII. EFFECTS OF COMPOUND #1 ON MESENTERIC                               
BLOOD FLOW IN ANESTHETIZED RATS                                           
DOSE   MESENTERIC BLOOD FLOW (kHz)                                        
(mg/kg/                                                                   
       RAT    RAT    RAT  RAT  RAT  RAT                                   
min)   1      2      3    4    5    6    MEAN   SEM                       
______________________________________                                    
Control                                                                   
       5.0    4.6    0.82 1.5  4.4  4.7  3.5    0.8                       
0.34   5.9    5.3    0.91 1.8  5.2  6.4  4.3    0.9                       
1.00   6.0    6.3    0.95 1.8  5.9  6.4  4.6    1.0                       
2.00   6.1    6.6    0.82 1.65 6.2  5.9  4.5    1.1                       
3.00   5.4    6.0    0.61 1.55 5.2  6.2  4.2    1.0                       
______________________________________                                    
 
    
     
         __________________________________________________________________________
XXIV. EFFECTS OF COMPOUND #1 ON HINDQUARTERS BLOOD                        
FLOW IN ANESTHETIZED RATS                                                 
DOSE   HINDQUARTERS BLOOD FLOW (kHz)                                      
(mg/kg/min)                                                               
       RAT1                                                               
           RAT2                                                           
               RAT3                                                       
                   RAT4                                                   
                       RAT5                                               
                           RAT6                                           
                               MEAN SEM                                   
__________________________________________________________________________
Control                                                                   
       4.5 2.3 2.4 3.8 2.2 2.1 2.9  0.4                                   
0.34   5.7 2.6 2.6 5.0 2.25                                               
                           2.45                                           
                               3.4  0.6                                   
1.00   5.8 2.6 2.45                                                       
                   4.7 2.1 2.25                                           
                               3.3  0.6                                   
2.00   4.9 2.5 1.9 4.0 2.15                                               
                           2.0 2.9  0.56                                  
3.00   4.7 2.9 1.78                                                       
                   3.1 2.27                                               
                           1.8 2.8  0.4                                   
__________________________________________________________________________
 
    
     
         ______________________________________                                    
XXV. EFFECTS OF COMPOUND #1 ON RENAL BLOOD                                
FLOW IN ANESTHETIZED RATS                                                 
DOSE     RENAL BLOOD FLOW (kHz)                                           
(mg/kg/min)                                                               
         RAT1    RAT2    RAT3  RAT4  MEAN   SEM                           
______________________________________                                    
Control  4.0     3.2     3.9   2.7   3.5    0.3                           
0.34     4.2     3.7     4.0   2.9   3.7    0.3                           
1.00     4.2     3.85    3.9   2.75  3.7    0.3                           
2.00     2.9     3.25    3.4   2.05  2.9    0.3                           
3.00     2.0     2.5     2.6   1.85  2.2    0.2                           
______________________________________                                    
 
    
     
         __________________________________________________________________________
XXVI. EFFECTS OF COMPOUND #1 ON MESENTRIC RESISTANCE                      
IN ANESTHETIZED RATS                                                      
DOSE   % Δ MESENTERIC RESISTANCE                                    
(mg/kg/min)                                                               
       RAT1                                                               
           RAT2                                                           
               RAT3                                                       
                   RAT4                                                   
                       RAT5                                               
                           RAT6                                           
                               MEAN SEM                                   
__________________________________________________________________________
0.34   -15 -18 -13 -27 -19 -31 -21  -3                                    
1.00   -24 -39 -26 -34 -36 -36 -33  -2                                    
2.00   -40 -51 -32 -42 -49 -48 -44  -3                                    
3.00   -41 -57 -30 -45 -52 -57 -47  -4                                    
__________________________________________________________________________
 
    
     
         __________________________________________________________________________
XXVII. EFFECTS OF COMPOUND #1 ON HlNDQUARTERS                             
RESISTANCE IN ANESTHETIZED RATS                                           
DOSE   HINDQUARTERS RESISTANCE                                            
(mg/kg/min)                                                               
       RAT1                                                               
           RAT2                                                           
               RAT3                                                       
                   RAT4                                                   
                       RAT5                                               
                           RAT6                                           
                               MEAN SEM                                   
__________________________________________________________________________
0.34   -22 -16 -10 -33 -4  -20 -18  4                                     
1.00   -28 -25 -14 -35 -8  -19 -22  4                                     
2.00   -33 -35 -12 -38 -23 -31 -29  4                                     
3.00   -39 -54 -29 -28 -44 -36 -38  4                                     
__________________________________________________________________________
 
    
     
         ______________________________________                                    
XXVIII. EFFECTS F COMPOUND #1 ON RENAL                                    
RESISTANCE IN ANESTHETIZED RATS                                           
DOSE     % Δ RENAL RESISTANCE                                       
(mg/kg/min)                                                               
         RAT1    RAT2    RAT3  RAT4  MEAN   SEM                           
______________________________________                                    
0.34     -4      -13     -26   -17   -15    5                             
1.00     -14     -24     -33   -21   -23    4                             
2.00     -4      -28     -36   -18   -22    7                             
3.00     20      -25     -32   -30   -17    12                            
______________________________________                                    
 
    
     The effects of Compound #1 administration on mean arterial pressure (MAP) and heart rate (HR) are illustrated in Tables XX, XX1 and FIG. 7. A dose-dependent fall in MAP was observed, which was associated with a slight tachycardia. The absolute changes in MAP are shown in Table XXII and FIG. 8. Mesenteric blood flow increased at all doses of Compound #1 and hindquarters flow increased at 0.34 and 1.0 mg/kg/min, but this effect was not evident at higher doses (Tables XXIII, XXIV and FIGS. 9,10). Renal blood flow was unchanged at lower doses but increased at doses of Compound #1 which dropped blood pressure substantially (Table XXV, FIG. 11). These changes in regional blood flow were then used to calculate changes in regional vascular resistance. Dose-dependent reductions in resistance were observed in both the mesenteric and hindquarters beds (Tables XXVI, XXVII and FIGS. 12,13). A decrease in renal resistance was observed at the lower doses of Compound #1, but further reductions were not seen at the higher doses (Table XXVIII, FIG. 14). These data indicate that decreases in regional vascular resistance contribute to the fall in pressure produced by Compound #1, with a greater vasodilation in the mesenteric and skeletal muscle beds compared to the renal bed. 
     EXAMPLE IV 
     Compound #1 was administered intragastrically to spontaneously hypertensive rats (SHR) to determine blood pressure lowering activity in a hypertensive model. Spontaneously hypertensive rats (12-14 weeks old) were obtained from Charles River Laboratories. The animals were anesthetized with 35 mg/kg of Brevital Sodium (Lilly) given I.P. Catheters (PE 50) were sterilized, filled with heparinized saline (30 U/ml) and then implanted in the right femoral artery. The arterial line was sewn to the leg muscles, threaded under the skin and then exteriorized behind the scapulae. A wire plug was inserted into the end of the catheter until blood pressure and heart rate recordings were made. 
     Pulse pressure and heart rate were recorded on a Gould system as described in Example I. Mean arterial pressure was calculated according to the following formula: MAP=Diastolic Pressure+1/3 Pulse Pressure. The animals were conscious during blood pressure recordings and were unrestrained. 
     Compound #1 was prepared for administration as described in Example I. The compound was administered intragastrically to the rats using a Perfektum animal feeding needle (16G-2&#34; straight) after the animals were lightly anesthetized with ether. 
     One day following implantation of the catheters, baseline measurements of MAP and HR were measured between 12 and 2 pm for two consecutive days. For these measurements, the rats were removed from their cages and placed in large, cardboard boxes 45 minutes prior to beginning the recordings to allow the animals to acclimate to the new environment. MAP and HR were then recorded for one hour. The values for these parameters were calculated every 10 minutes and then averaged over the one hour sampling period. One-half of the animals were then randomly selected to receive 200 mg/kg/day of Compound #1 intragastrically for five days, followed by a four day recovery period during which the rats were administered an equal volume of distilled water. The other half of the rats received distilled water throughout the study. The animals were dosed each morning and mean arterial pressure and heart rate were measured 4-5 hours later. Results are reported in Tables XXIX-XXXVII and in FIGS. 15-17. 
     
         ______________________________________                                    
XXIX. MEAN ARTERIAL PRESSURE IN SPONTANE-                                 
OUSLY HYPERTENSIVE RATS PRIOR TO VEHICLE/                                 
DRUG TREATMENT                                                            
            MAP (mmHg)                                                    
RAT           DAY 1    DAY 2                                              
______________________________________                                    
1             130      150                                                
2             159      139                                                
3             124      151                                                
4             149      140                                                
5             179      139                                                
6             161      129                                                
7             143      132                                                
8             133      125                                                
9             138      126                                                
10            141      141                                                
11            134      123                                                
12            128      148                                                
13            165      159                                                
14            160      147                                                
              146 ± 4                                                  
                       139 ± 3                                         
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         __________________________________________________________________________
XXX. EFFECTS OF VEHICLE ADMINISTRATION ON                                 
MEAN ARTERIAL PRESSURE IN SPONTANEOUSLY HYPERTENSIVE                      
RATS                                                                      
MAP (mmHg)                                                                
DAY RAT1                                                                  
        RAT2                                                              
            RAT3                                                          
                RAT4                                                      
                    RAT12                                                 
                        RAT13                                             
                            RAT14                                         
                                MEAN SEM                                  
__________________________________________________________________________
3   145 133 162 153 158 176 164 156  5                                    
4   154 157 151 147 162 160 --  155  2                                    
5   145 153 151 143 150 167 --  152  4                                    
6   135 139 161 130 154 156 --  146  5                                    
7   143 153 137 169 131 --  --  147  7                                    
8   140 --  154 151 141 --  --  147  4                                    
9   153 126 151 125 154 --  --  142  7                                    
10  168 138 153 148 121 --  --  146  8                                    
11  147 161 179 --  136 --  --  156  9                                    
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         __________________________________________________________________________
XXXI. EFFECTS OF ORAL ADMINISTRATION OF COMPOUND #1                       
(200 MG/KG/DAY) ON MEAN ARTERIAL PRESSURE IN SPONTANE-                    
OUSLY HYPERTENSIVE RATS                                                   
MAP (mmHg)                                                                
DAY*                                                                      
    RAT5                                                                  
        RAT6                                                              
            RAT7                                                          
                RAT8                                                      
                    RAT9                                                  
                        RAT10                                             
                            RAT11                                         
                                MEAN SEM                                  
__________________________________________________________________________
3   159 151 147 144 143 139 137 146  3                                    
4   108 151 119 149 139 117 140 132  6                                    
5   122 128 131 121 96  100 126 118  5                                    
6   141 124 107 126 114 106 137 122  5                                    
7   124 121 119 125 120 --  137 124  3                                    
8   152 130 142 152 121 --  151 141  5                                    
9   141 121 137 142 133 --  140 136  3                                    
10  120 141 120 132 --  --  135 130  4                                    
11  139 115 167 150 --  --  133 141  9                                    
__________________________________________________________________________
 *Day 3-7  200 mg/kg/day                                                  
 *Day 8-11  Recovery                                                      
 
    
     
         ______________________________________                                    
XXXII. HEART RATE IN SPONTEANOUSLY HYPERTEN-                              
SIVE RATS PRIOR TO VEHICLE/DRUG TREATMENT                                 
          HEART RATE (beats/min)                                          
RAT         DAY 1      DAY 2                                              
______________________________________                                    
1           333        396                                                
2           --         349                                                
3           300        351                                                
4           400        371                                                
5           387        376                                                
6           --         351                                                
7           376        329                                                
8           339        310                                                
9           346        307                                                
10          402        380                                                
11          366        300                                                
12          380        359                                                
13          --         354                                                
14          379        346                                                
            364 ± 10                                                   
                       348 ± 11                                        
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         __________________________________________________________________________
XXXIII. EFFECTS OF VEHICLE ADMINISTRATION ON HEART RATE                   
IN SPONTANEOUSLY HYPERTENSIVE RATS                                        
HEART RATE (beats/min)                                                    
DAY RAT1                                                                  
        RAT2                                                              
            RAT3                                                          
                RAT4                                                      
                    RAT12                                                 
                        RAT13                                             
                            RAT14                                         
                                MEAN SEM                                  
__________________________________________________________________________
3   400 364 400 359 367 341 364 371  8                                    
4   374 380 333 394 354 316 --  359  12                                   
5   341 336 324 367 350 326 --  341  7                                    
6   263 277 298 330 350 342 --  313  14                                   
7   289 321 333 383 347 348 --  337  13                                   
8   309 263 292 349 371 294 --  313  16                                   
9   298 283 --  358 --  --  --  313  23                                   
10  340 259 --  366 262 --  --  307  27                                   
11  279 342 --  --  332 --  --  317  21                                   
__________________________________________________________________________
 
    
     
         __________________________________________________________________________
XXXIV. EFFECTS OF ORAL ADMINISTRATION OF COMPOUND #1                      
(200 MG/KG/DAY) ON HEART RATE ON SPONTANEOUSLY                            
HYPERTENSIVE RATS                                                         
HEART RATE (beats/min)                                                    
DAY*                                                                      
    RAT5                                                                  
        RAT6                                                              
            RAT7                                                          
                RAT8                                                      
                    RAT9                                                  
                        RAT10                                             
                            RAT11                                         
                                MEAN SEM                                  
__________________________________________________________________________
3   351 391 400 325 344 282 351 349  15                                   
4   383 391 362 355 328 --  349 361  9                                    
5   370 386 398 326 273 --  305 343  20                                   
6   343 396 343 299 370 --  331 347  14                                   
7   322 365 --  319 350 --  375 346  11                                   
8   337 350 381 364 321 --  324 346  10                                   
9   325 329 --  343 274 --  277 310  14                                   
10  266 366 289 315 --  --  265 300  19                                   
11  279 296 --  330 --  --  318 306  11                                   
__________________________________________________________________________
 *Day 3-7  200 mg/kg/day                                                  
 *Day 8-11  Recovery                                                      
 
    
     
         ______________________________________                                    
XXXV. BODY WEIGHT IN SPONTANEOUSLY HYPER-                                 
TENSIVE RATS PRIOR TO VEHICLE/DRUG                                        
TREATMENT                                                                 
          BODY WEIGHT (grams)                                             
RAT         DAY 1     DAY 2                                               
______________________________________                                    
1           256       251                                                 
2           266       256                                                 
3           264       266                                                 
4           268       268                                                 
5           241       230                                                 
6           266       264                                                 
7           271       268                                                 
8           263       263                                                 
9           258       259                                                 
10          287       278                                                 
11          258       261                                                 
12          265       261                                                 
13          264       257                                                 
14          261       260                                                 
            263 ± 3                                                    
                      260 ± 3                                          
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         __________________________________________________________________________
XXXVI. EFFECTS OF VEHICLE ADMINISTRATION ON BODY WEIGHT                   
IN SPONTANEOUSLY HYPERTENSIVE RATS                                        
BODY WEIGHT (grams)                                                       
DAY RAT1                                                                  
        RAT2                                                              
            RAT3                                                          
                RAT4                                                      
                    RAT12                                                 
                        RAT13                                             
                            RAT14                                         
                                MEAN SEM                                  
__________________________________________________________________________
3   253 271 265 267 259 257 255 261  3                                    
4   235 266 254 265 259 257 248 255  4                                    
5   231 264 248 259 252 255 243 250  4                                    
6   224 251 250 259 245 246 246 245  4                                    
7   223 252 241 261 246 254 --  246  5                                    
8   225 255 243 267 243 252 --  248  6                                    
9   224 254 239 261 244 240 --  244  5                                    
10  230 261 234 255 231 229 --  240  6                                    
11  234 269 235 241 221 219 --  237  7                                    
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         __________________________________________________________________________
XXXVll. EFFECTS OF ORAL ADMINISTRATION OF COMPOUND #1                     
(200 MG/KG/DAY) ON BODY WEIGHT IN SPONTANEOUSLY                           
HYPERTENSIVE RATS                                                         
BODY WEIGHT (grams)                                                       
Day*                                                                      
    Rat5                                                                  
        Rat6                                                              
           Rat7                                                           
               Rat8                                                       
                  Rat9                                                    
                     Rat10                                                
                         Rat11                                            
                              Mean                                        
                                  SEM                                     
__________________________________________________________________________
3   233 270                                                               
           273 262                                                        
                  259                                                     
                     283 263  263 6                                       
4   215 265                                                               
           261 250                                                        
                  253                                                     
                     256 242  249 6                                       
5   214 259                                                               
           258 241                                                        
                  243                                                     
                     246 241  243 6                                       
6   217 262                                                               
           255 252                                                        
                  229                                                     
                     232 237  241 6                                       
7   224 253                                                               
           242 245                                                        
                  217                                                     
                     229 228  234 5                                       
8   225 261                                                               
           252 254                                                        
                  219                                                     
                     --  226  240 7                                       
9   231 262                                                               
           253 262                                                        
                  225                                                     
                     --  231  244 7                                       
10  226 260                                                               
           249 246                                                        
                  231                                                     
                     --  278  240 6                                       
11  207 254                                                               
           256 229                                                        
                  -- --  210  231 10                                      
__________________________________________________________________________
 Day* 3-7  200 mg/kg/day                                                  
 Day* 8-11  Recovery                                                      
 
    
     Mean arterial pressure in this group of spontaneously hypertensive rats (SHRs) averaged 146 and 139 mmHg, respectively, on the two days pressures were measured prior to beginning the vehicle or drug treatments (Table XXIX). Variations in MAP of this degree are quite common in studies where blood pressure is measured in a conscious, unrestrained spontaneously hypertensive rat. Intragastric administration of the vehicle (distilled water) for nine days did not lower MAP (Table XXX, FIG. 15). In the rats treated with Compound #1 at 200 mg/kg/day, MAP was significantly reduced to a low of 118 mmHg on the third day of drug treatment (Table XXXI, FIG. 15). A mean pressure of 118 mmHg is similar to the mean pressure recorded in Wistar Kyoto rats, the strain of animal which is typically used as the control for the SHR. This reduction in MAP was rapidly and completely reversed, since on the first day of vehicle treatment in these animals, MAP was not significantly different from MAP in the vehicle-treated group (FIG. 15). Heart rate in the vehicle-treated rats was reduced from an average of 348 beats per/min on Day 2 to 317 beats/min on Day 11 (Tables XXXII and XXXIII, FIG. 16). Heart rate in Compound #1 treated rats was reduced to 306 beats/min, a reduction which was similar to the fall in heart rate observed in the vehicle-treated group (Table XXXIV, FIG. 16). Thus, the blood pressure-lowering effects of Compound #1 administered chronically are not associated with tachycardia. Reflex tachycardia can sometimes be a problem with the administration of peripheral vasodilators, particularly when pressure drops rapidly. 
     Body weight in the vehicle and drug-treated rats was monitored throughout the study to detect any potential adverse effects of the drug on the general health of the animals. Body weight in this group of SHRs averaged 263 and 260 grams, respectively, on the two days prior to beginning the vehicle/drug treatment (Table XXXV). At the end of the study, body weight in the vehicle treated rats had fallen to 237 grams, whereas body weight in the Compound #1 treated group averaged 231 grams when the study was terminated (Tables XXXVI and XXXVII, FIG. 17). Therefore administration of the drug did not appear to adversely effect the general health of the treated rats. 
     In summary, oral administration of Compound #1 effectively reduced mean arterial blood pressure in SHRs to normotensive levels. This reduction was rapidly and completely reversed when the drug treatment was terminated and the animals were allowed to recover. Most importantly, this fall in pressure in the SHR was not accompanied by reflex tachycardia. Oral administration of Compound #1 also did not appear to have an adverse effect on the general health of the treated rats. These data indicate that the drug is both effective and well tolerated at a dose of 200 mg/kg/day. 
     Examples I-IV demonstrate that Compound #1 is effective on acute treatment therapies in simultaneously lowering blood pressure and promoting sodium excretion when infused intravenously into anesthetized rats. The mechanism of this depressor and natriuretic action may be mediated by cGMP since an increase in plasma cGMP levels and the urinary excretion of cGMP occur in parallel with these actions. Although an increase in cAMP plasma levels and the urinary excretion of cAMP also occurred in the Compound #1 infused rats, these changes did not correlate well with the depressor and natriuretic actions of this compound. When Compound #1 is administered chronically to spontaneously hypertensive rats, mean arterial blood pressure is reduced to normotensive levels without any sign of reflex tachycardia. The fall in mean arterial pressure observed when Compound #1 is infused into anesthetized rats appears to be due to reductions in regional vascular resistance, with more vasodilation in the mesenteric and hindquarters bed compared to the renal bed. These data indicate that Compound #1 may be an effective antihypertensive agent in man.