bluuebunny/crossref_metadata_2025 huggingface dataset:
Rate-dependent changes in intraventricular conduction produced by procainamide in anesthetized dogs. A quantitative analysis based on the relation between phase 0 inward current and conduction velocity.
Authors: Nattel S , Jing W
Published: June 2012

Antiarrhythmic drug effects on maximal upstroke velocity (Vmax) are frequency dependent, which implies that the effects of these drugs on conduction should also be rate dependent. Previous in vivo studies have been limited by assumptions about unchanging propagation pathway, and by the empirical use of a first-order recovery model. To explore time-dependent antiarrhythmic drug-induced conduction slowing in vivo, we used 56-electrode epicardial mapping in chloralose-anesthetized dogs with formalin-induced atrioventricular block. Interval-dependent changes in conduction time were assessed under control conditions and then after three loading and maintenance infusions of procainamide. Under control conditions, epicardial activation time (86 +/- 26 msec at a basic cycle length of 300 msec) was unchanged (87 +/- 24 msec) by pauses up to 6.6 +/- 2.2 seconds. Procainamide caused conduction slowing that dissipated as a function of recovery interval, with 94 +/- 6% recovery over a maximum pause of 6.7 +/- 1.5 seconds, but did not alter activation pattern. Drug-induced changes in conduction were evaluated by use of a mathematical model assuming phase 0 inward current proportional to conduction velocity squared. Conduction changes were better fitted by this "quadratic model" (least sum of squared deviations 3.9 x 10(-3) by mapping in five dogs, 2.7 x 10(-2) by use of QRS duration in nine dogs) than by a monoexponential model (sum of squared deviations 5.7 x 10(-3) by mapping, 3.4 x 10(-2) with QRS; p less than 0.01 vs. quadratic model for each). As predicted by theoretical analysis, recovery time constants from the quadratic model were similar to time constants for procainamide-induced changes in Vmax in vitro, and significantly longer than values obtained with a monoexponential model. Drug-induced changes in QRS duration were highly correlated with simultaneous changes measured by epicardial mapping (r = 0.95, p less than 0.001), indicating that QRS duration is a valid index of drug effects on ventricular conduction. We concluded that procainamide causes interval-dependent changes in ventricular conduction in vivo that are consistent with a proportional relation between phase 0 inward current and the square of conduction velocity. These observations have important potential implications for the dose-dependent and heart rate-dependent effects of antiarrhythmic drugs.
Differences in load dependence of relaxation between the left and right ventricular myocardium as a function of age in rats.
Authors: Capasso J M , Puntillo E , Olivetti G , Anversa P
Published: June 2012

To determine whether the variation in the magnitude of work load sustained by the left and right ventricles during adulthood and senescence affects the load-dependent aspect of relaxation, posterior papillary muscles from the left and right ventricles of rats at 4, 10, and 20 months of age were studied under variably loaded conditions in vitro. Because of differences between the life spans of Fischer and Sprague-Dawley rats, the functional characteristics of relaxation were investigated to evaluate the possibility of a differential age-associated response in these two strains of animals. The kinetic performance of the diastolic phase of myocardial contraction was measured by assessing the relative time during which load bearing occurred in a series of afterloaded isotonic twitches. This measurement was expressed as the ratio of the duration of afterloaded isotonic shortening and relengthening to the time required for isometric force to decline to the same level during isometric relaxation. A ratio of less than unity identified a load-dependent state whereas a value greater than one reflected a load-independent condition. Results showed that the right myocardium was completely load independent whereas the left myocardium was fully load dependent at all physiological afterloads. Aging reduced the load independence of the right ventricle and the load dependence of the left ventricle in Fischer rats. In contrast, no aging effect on the properties of afterloaded isotonic relaxation was seen in Sprague-Dawley rats. In conclusion, distinct differences exist in the mechanical dynamics of inactivation between the left and right ventricular myocardium. Aging reduced these variations in Fischer rats but had no apparent influence in Sprague-Dawley animals up to 20 months after birth.
Mechanisms of termination of reentrant atrial arrhythmias by class I and class III antiarrhythmic agents.
Authors: Spinelli W , Hoffman B F
Published: June 2012

We studied atrial flutter due to circus movement in chronically instrumented conscious dogs to identify the mechanism by which class I and class III antiarrhythmic drugs terminate reentrant excitation. We used a crossover experimental design administering five class I agents and one class III agent, by intravenous bolus followed by intravenous infusion. The class I agents other than lidocaine were almost uniformly effective in terminating the arrhythmia (disopyramide in six of seven dogs, propafenone in six of six, flecainide in seven of seven, and SC-40230 in seven of seven). Termination was preceded by a marked increase in cycle length (ranging from +78% with propafenone to +55% with disopyramide), but with the exception of disopyramide, class I agents did not significantly shorten the excitable gap. With disopyramide the gap decreased from 49 +/- 3% to 28 +/- 3% of the cycle length. With no class I agent did the wavelength of effective refractoriness increase to approach the cycle length of the arrhythmia. Lidocaine, used as a negative control, terminated the reentry in one dog with modest prolongation of the cycle length. Terminations with class I agents correlated with depression of conduction rather than prolongation of refractoriness. In contrast with class I agents, D-sotalol prolonged the cycle length minimally (+10%) and terminated the arrhythmia in six of seven dogs. It decreased the excitable gap from 42 +/- 4% to 26 +/- 6% of the cycle, but it still did not cause the wavelength of effective refractoriness to equal the cycle length. Terminations by D-sotalol seemed to result from either failure of the lateral boundaries of the circus path or reflection within the path.
Acetylcholine causes coronary vasodilation in dogs and baboons.
Authors: Van Winkle D M , Feigl E O
Published: June 2012

Intracoronary administration of acetylcholine or efferent vagal stimulation causes coronary vasodilation in dogs. However, in baboons it has been reported that intracoronary acetylcholine results in a fall in coronary blood flow and that stimulation of the vagi is without effect. The dose response of intracoronary acetylcholine and the effect of efferent vagal stimulation on the coronary circulation were reinvestigated in closed-chest, anesthetized dogs and baboons. The left main coronary artery was cannulated and perfused at constant pressure. alpha-Adrenergic and beta-adrenergic receptors were pharmacologically blocked with phenoxybenzamine and propranolol. Heart rate was held constant by right ventricular pacing. In dogs, intracoronary infusion of acetylcholine (1-300 micrograms/min) elicited a dose-dependent increase in steady-state coronary blood flow and coronary sinus oxygen tension, without a change in myocardial oxygen consumption. Vagal stimulation caused a coronary vasodilation that was attenuated by a metabolically mediated decrease in flow. In baboons, acetylcholine increased steady-state coronary blood flow in the dose range of 1-10 micrograms/min, caused little change at 30 micrograms/min, and decreased flow at 100-300 micrograms/min. Coronary sinus oxygen tension increased in a dose-dependent manner up to 10 micrograms/min. Myocardial oxygen consumption was unchanged in the dose range of 1-10 micrograms/min and declined between 30 and 300 micrograms/min. Efferent stimulation of the vagi resulted in coronary dilation obscured by a metabolic reduction of flow. It is concluded that 1) low doses of acetylcholine elicit a primary coronary vasodilation in both species, but in baboons high doses of acetylcholine cause a reduction of both myocardial oxygen consumption and coronary blood flow below control values and 2) vagal stimulation causes a competition between coronary vasodilation and metabolic reduction of flow in dogs and baboons.
A plasmatic factor may cause platelet activation in acute ischemic stroke.
Authors: Joseph R , Welch K M , Oster S B , Grunfeld S , D'Andrea G
Published: June 2012

To study the pathogenesis of platelet activation in ischemic stroke, ionized calcium ([Cai2+]) was measured in aequorin-loaded gel-filtered platelets in the basal and stimulated state. Basal [Cai2+] was increased in stroke patients maximally 36-72 hours after onset. The increase in [Cai2+] after stimulation with thrombin, collagen, and platelet-activating factor were also greater in stroke patients, but the profiles of these [Cai2+] changes were parallel to control. Cross incubation of control platelets with plasma from stroke patients resulted in raised basal [Cai2+] and caused the release of serotonin from platelets. These results indicate that the higher platelet basal [Cai2+] in stroke patients represents a lowered threshold for activation and that this may be due to a plasmatic factor rather than a primary platelet defect.
The role of alpha 1- and alpha 2-adrenergic receptors in mediation of coronary vasoconstriction in hypoperfused ischemic myocardium during exercise.
Authors: Laxson D D , Dai X Z , Homans D C , Bache R J
Published: June 2012

This study was carried out to test the hypothesis that adrenergic coronary vasoconstriction limits blood flow to hypoperfused regions of myocardium during exercise. The vasoconstrictor influence of alpha-adrenergic receptor subtypes was assessed by use of selective adrenergic blocking agents. Dogs chronically instrumented with a circumflex coronary artery hydraulic occluder and an intra-arterial catheter underwent treadmill exercise in the presence of a coronary stenosis that decreased distal perfusion pressure to 40 mm Hg. Myocardial blood flow was measured with radioactive microspheres (15 microns) before and during selective alpha 1- or alpha 2-adrenergic receptor blockade produced by intracoronary infusion of prazosin (1 microgram/kg/min x 10 min) or idazoxan (1 microgram/kg/min x 10 min), respectively. Coronary perfusion pressure was held equal before and during receptor blockade with the hydraulic occluder. Compared with control exercise, subendocardial blood flow increased during alpha 1-receptor blockade with prazosin from 0.60 +/- 0.14 to 1.12 +/- 0.17 ml/min/g (p less than 0.05), and mean transmural flow increased from 1.07 +/- 0.19 to 1.60 +/- 0.22 ml/min/g (p less than 0.05). In contrast, subendocardial and mean transmural blood flow were not different from control during selective alpha 2-adrenergic receptor blockade with idazoxan (0.48 +/- 0.10 vs. 0.67 +/- 0.14 ml/min/g, p = 0.33, and 0.82 +/- 0.15 vs. 1.02 +/- 0.20 ml/min/g, p = 0.45, respectively). These data indicate that even in the presence of a coronary stenosis that causes substantial myocardial underperfusion during exercise, residual coronary vasoconstrictor tone is present in ischemic myocardium, and this vasoconstriction is mediated predominantly by the alpha 1-adrenergic receptor.
Morphometric analyses of rabbit thoracic aorta after poststenotic dilatation.
Authors: Kukongviriyapan U , Gow B S
Published: June 2012

The aim of this study was to quantify the morphological changes in the arterial wall resulting from poststenotic dilatation (PSD). PSD was produced by placing a split nylon ring around the thoracic aorta of the rabbits at a level of T6-7 during a sterile thoracotomy done under pentobarbital anesthesia. After period of PSD ranging from 1-51 months these rabbits were anesthetized, as were the control animals, and the descending thoracic aorta from the fourth to the eleventh ribs was removed following perfusion fixation with Karnovsky's solution at a constant pressure of 80 mm Hg. The extent of PSD development was variable even though the stenotic ring was the same size in all rabbits. Ultrastructural findings showed degenerative changes of the wall components in the PSD region and were more prominent in the aortas with greater dilatation. Morphometric measurements showed that the PSD was accompanied by a decrease in volume density of both smooth muscle cells (SMCs) and elastin and an increase in collagen and ground substance. These changes were well correlated with degree of dilatation and ratio of internal radius to wall thickness (hence, mean wall stress) but not with duration of PSD. While the number of SMCs per unit volume in the PSD aortas was significantly less than normal (p less than 0.05), there was no significant change in mean cell volume. Although the reduced muscle mass might be expected to lower the capacity of the vessel to maintain tone, previous results show that this does not occur.
EDRF increases cyclic GMP in platelets during passage through the coronary vascular bed.
Authors: Pohl U , Busse R
Published: June 2012

It was investigated whether endothelium-derived relaxing factor (EDRF) increases cyclic GMP (cGMP) content in platelets passing through the coronary bed. Boluses of washed platelets from healthy human donors were injected into the aortic perfusion line of isolated, saline-perfused rabbit hearts under constant flow conditions (28 +/- 2 ml/min). The coronary effluent was collected over 5 seconds, and the cGMP content of platelets was determined by radioimmunoassay. Platelet cGMP amounted to 0.34 +/- 0.11 pmol/mg protein after passage through the unstimulated coronary bed. During stimulation with acetylcholine (1 microM), it increased to 1.6 +/- 0.5 pmol/mg (p less than 0.01; n = 14). Simultaneously, the platelet recovery (measured over 20 seconds after injection) was enhanced (by 45 +/- 11%; p less than 0.01) during endothelial stimulation with acetylcholine. Treatment with the EDRF inhibitor hemoglobin (6 microM) completely abolished the increase in platelet cGMP (p less than 0.01; n = 11) as well as the enhanced platelet recovery (n = 8). Inhibition of EDRF by hemoglobin reduced also the basal platelet cGMP content to 0.17 +/- 0.11 pmol/mg (p less than 0.01). The data indicate that basally released EDRF is able to increase cGMP in platelets during a single passage through the coronary bed. The enhanced recovery of platelets after EDRF stimulation, which coincides with an increase of platelet cGMP, suggests that EDRF plays an important role as inhibitor of platelet activation in the coronary circulation.
Correlation of ventricular area, perimeter, and conotruncal diameter with ventricular mass and function in the chick embryo from stages 12 to 24.
Authors: Keller B B , Hu N , Clark E B
Published: June 2012

Ventricular form and function are interrelated during cardiovascular development. The study of muscle mechanics requires the real-time measurement of length, area, or volume. Because volume measures are not currently possible in the embryonic heart, we tested the hypothesis that end-diastolic (ED) and end-systolic (ES) ventricular perimeter, area, and conotruncal diameter correlate with ventricular mass and function in the stage 12 to stage 24 white Leghorn chick embryo. Video images of the contracting heart were recorded at 60 Hz on 1/2" videotape and studied with a custom image-analysis workstation. ED and ES video fields were selected by maximum and minimum ventricular area and were planimetered for epicardial ventricular perimeter, area, and conotruncal diameter. Data are reported as (mean +/- SEM, n greater than or equal to 8) and were tested by analysis of variance and regression analysis. Heart rate calculated from cycle length increased from 78 +/- 6 beats/min at stage 12 to 162 +/- 5 beats/min at stage 24. ED and ES area increased geometrically versus stage (y = 0.53 - 0.08x + 0.004x2, r = 0.96, p less than 0.001; and y = 0.60 - 0.09x + 0.004x2, r = 0.98, p less than 0.001, respectively). ED and ES perimeter and conotruncal diameter increased linearly versus stage (r = 0.95, p less than 0.001; r = 0.96, p less than 0.001; and r = 0.93, p less than 0.001; r = 0.93, p less than 0.001, respectively). Shortening fraction for each measurement increased from stage 12 to 16 or 18 then decreased.(ABSTRACT TRUNCATED AT 250 WORDS)
Atrial pacing stimulates secretion of atrial natriuretic polypeptide without elevation of atrial pressure in awake dogs with experimental complete atrioventricular block.
Authors: Nishimura K , Ban T , Saito Y , Nakao K , Imura H
Published: June 2012

To clarify whether or not tachycardia stimulates the secretion of atrial natriuretic polypeptide (ANP) without elevation of atrial pressure, we examined the effects of atrial pacing on ANP secretion in awake dogs with normal sinus rhythm and with complete atrioventricular block (CAVB), which was produced surgically by heat cauterization of His' bundle. In four dogs with normal sinus rhythm, atrial pacing increased the atrial rate from 146 +/- 20 to 260 +/- 10 beats/min, with marked elevation of right atrial pressure (from 0.2 +/- 0.1 to 3.9 +/- 0.8 mm Hg) and left atrial pressure (from 0.2 +/- 0.1 to 8.6 +/- 2.8 mm Hg). Along with the hemodynamic changes, the ANP level in plasma obtained from the coronary sinus was increased from 405 +/- 99 to 849 +/- 199 pg/ml (p less than 0.01). In five dogs with CAVB, the ANP level was also significantly increased from 730 +/- 82 to 1,137 +/- 35 pg/ml (p less than 0.01) by rapid atrial pacing (from 164 +/- 20 to 317 +/- 30 beats/min) in spite of the lack of any appreciable changes in either left or right atrial pressure. Furthermore, in the CAVB group, while the ventricular rate was increased by ventricular pacing from 52 +/- 5 to 146 +/- 16 beats/min, atrial pacing was performed simultaneously without atrioventricular (A-V) sequential form. Even under this condition, the ANP level was increased from 480 +/- 172 to 626 +/- 223 pg/ml (p less than 0.05) without any substantial changes in atrial pressure. Such effects of rapid atrial pacing on ANP secretion were suppressed after infusion of autonomic blocking agents.(ABSTRACT TRUNCATED AT 250 WORDS)
Electrical properties of canine subendocardial Purkinje fibers surviving in 1-day-old experimental myocardial infarction.
Authors: Argentieri T M , Frame L H , Colatsky T J
Published: June 2012

The passive electrical properties of subendocardial Purkinje fibers surviving in infarcted regions of canine ventricle 24 hours after coronary ligation were studied by using microelectrode techniques and cable theory. In normal hearts, cells within the subendocardial Purkinje fiber strands were found to be well coupled to each other but electrically isolated from neighboring myocardium. Voltage response to intracellular current injection was consistent with one-dimensional cable behavior and yielded estimates of passive electrical properties in general agreement with previous work on free-running Purkinje strands (membrane length constant, 1.2 +/- 0.1 mm; membrane time constant, 7.3 +/- 0.8 msec; input resistance, 67.4 +/- 7.4 K omega; membrane resistance, 8.2 +/- 0.7 K omega.cm; axial resistance, 0.52 +/- 0.06 M omega/cm; membrane capacitance, 960 +/- 102 nF/cm) (n = 21). On the day after coronary ligation, subendocardial Purkinje fiber action potentials were prolonged and slightly depolarized. Significant increases were measured in input resistance (+40.5%), membrane resistance (+43.9%), and axial resistance (+47.5%), whereas membrane capacitance was found to be significantly decreased (-24.3%) (n = 19). Conduction velocity, membrane length constant, membrane time constant, and the time constant and capacitance for the foot of the action potential remained unchanged. These results are consistent with electrical uncoupling between adjacent cells, which will increase internal resistivity, accompanied by changes in cellular phospholipid content, which can increase membrane resistance and alter membrane capacitance. Alternatively, the results can be explained by a simple model in which the apparent electrical structure is altered by changes in electrical coupling alone, with specific electrical properties remaining constant. Although the mechanisms underlying the observed changes remain uncertain, the present study indicates that myocardial infarction is associated with alterations in the passive electrical structure of surviving subendocardial Purkinje fibers, which, together with changes in action potential configuration, may provide a substrate for the generation of ventricular arrhythmias 24 hours after coronary ligation.
Morphological development of the rat heart growing in oculo in the absence of hemodynamic work load.
Authors: Bishop S P , Anderson P G , Tucker D C
Published: June 2012

We evaluated cardiac muscle development in the absence of hemodynamic work load but in the presence of host factors including blood vessels, nerves, and circulating neurohumoral agents by transplanting 12-day fetal rat ventricle into the anterior eye chamber of adult host rats. Implants were studied by electron microscopy at intervals from 1 to 14 weeks in oculo. For comparison with myocardium developing in oculo, 12-day fetal tissue and 3-, 8-, and 28-day-old normally growing rats were also studied. At 1 week in oculo, myofibrils were laterally located and more frequent than in the 12-day fetus. Fibrils had clear Z bands and H bands, but no M bands. At 10 days in oculo (comparable to birth in normally growing animals), myocyte mitoses were present and tritiated thymidine autoradiography revealed many labeled myocyte nuclei. By 5 weeks in oculo, cells were filled with mature myofibrils with clear M bands and lateral connections between adjacent Z bands. However, myofibril bundles sometimes coursed at sharp angles to each other within single cells. Except for the relative lack of fibrillar polarization and small cell size, ultrastructure of myocytes developing in oculo for 5 or more weeks appeared very similar to myocytes developing in normally growing rats. By 10 weeks in oculo, when in situ growing hearts are clearly in a hypertrophic phase of growth, no mitoses or tritiated thymidine-labeled nuclei were present in myocytes, although labeled nonmyocyte nuclei were present. Morphometric evaluation revealed no change in myocyte diameter or nuclear-to-cytoplasmic ratio from 1 to 3 weeks in oculo, consistent with continued hyperplastic growth. Binucleated cells were present by 3 weeks in oculo and later, and the cytoplasm per nucleus increased fourfold between 3 and 5 weeks in oculo, suggesting conversion to hypertrophic cell growth. We concluded that cells proliferated and differentiated in the absence of a hemodynamic load, but that polarized alignment of myocytes and myofibrils was incomplete.
Presynaptic modulation of efferent sympathetic and vagal neurotransmission in the canine heart by hypoxia, high K+, low pH, and adenosine. Possible relevance to ischemia-induced denervation.
Authors: Miyazaki T , Zipes D P
Published: June 2012

Ischemia in the dog produces denervation of myocardium apical to the ischemic area. To investigate the mechanism(s) of the denervation, we tested the effects of hypoxia and some components of ischemia including high K+, low pH, and adenosine on efferent cardiac autonomic responses. In anesthetized, open-chest dogs, we occluded a diagonal branch of the left anterior descending coronary artery and perfused it with hypoxic Tyrode's solutions (PO2 less than 50 mm Hg). We found that effective refractory period (ERP) shortening induced by bilateral ansae subclaviae stimulation at myocardium basal and apical to the perfusing area did not change during a 20-25 minute period of perfusion with hypoxic normal Tyrode's solution. During perfusion with hypoxic combined Tyrode's solution containing 12 mM K+, pH 6.8, and 10 microM adenosine, ERP shortening at basal sites induced by bilateral ansae subclaviae stimulation remained unchanged but was attenuated at apical sites (16 +/- 1 to 8 +/- 1 msec, mean +/- SEM, n = 35, p less than 0.001), and seven apical sites exhibited denervation (less than or equal to 2-msec shortening). The maximum extracellular K+ concentration of the perfusing area, measured with a K(+)-sensitive electrode, was 5.1 +/- 0.9 mM (N = 3 dogs) during perfusion with normal Tyrode's solution and was 11.8 +/- 0.1 mM (N = 3 dogs) during perfusion with hypoxic combined solution (p = 0.017). In a separate group of dogs, the effects of high K+, low pH, and adenosine in the absence of ischemia were examined. Oxygenated Tyrode's solutions were instilled into the pericardial cavity to superfuse epicardial nerves. The Tyrode's solutions containing high K+ (12 mM), low pH (6.4), or adenosine (10 microM), individually or combined, reduced ERP shortening induced by bilateral ansae subclaviae stimulation in the ventricular intramyocardium to 46%, 55%, 56%, and 33% of each control value obtained during superfusion with normal Tyrode's solution and reduced the magnitude of ERP lengthening induced by bilateral cervical vagal stimulation to 57%, 71%, 61%, and 39%, respectively. ERP responses of the test sites to infused norepinephrine and methacholine, however, remained unaffected by superfusion with combined Tyrode's solution. Thus, high K+, low pH, and adenosine each inhibit efferent sympathetic and vagal neurotransmission presynaptically in the canine heart and may contribute to the development of denervation during early ischemia.
Triggerlike stimulation of cholesterol accumulation and DNA and extracellular matrix synthesis induced by atherogenic serum or low density lipoprotein in cultured cells.
Authors: Orekhov A N , Tertov V V , Kudryashov S A , Smirnov V N
Published: June 2012

A 72-hour incubation of cultured cells with blood sera or plasma of patients suffering from coronary heart disease (CHD) with angiographically assessed coronary atherosclerosis caused a threefold to fourfold elevation of intracellular cholesterol. An elevated cholesterol level in the cells precultured with patients' sera was retained several days after the removal of the examined serum from culture. The accumulation of intracellular cholesterol was accompanied by enhanced synthesis of DNA, total protein, collagen, sulfated glycosaminoglycans, and hyaluronic acid. Enhanced DNA and total protein synthesis was retained for at least 9 days after the serum had been removed from culture. The obtained results suggest that the sera of CHD patients possess an atherogenic potential that manifests itself at the arterial cell level in the stable stimulation of atherosclerotic cellular processes: proliferation, lipidosis, and fibrosis. The examined sera of healthy donors were devoid of such an atherogenic potential. The low density lipoprotein (LDL) fraction (density, 1.030-1.050 g/cm3) obtained from an atherogenic serum had the same atherogenic potential as a whole serum. Atherosclerotic alterations in cultured intimal cells caused by atherogenic LDL were retained for at least 3 days after the removal of the lipoprotein from culture. Preincubation of intimal cells with LDL obtained from healthy donors had no effect on the intracellular cholesterol level or the synthesis of DNA and extracellular matrix. One may assume that the atherogenic potential of CHD patients' sera is related to the presence of LDLs that are qualitatively different from the LDL of healthy subjects.
Heterogeneous changes in epimyocardial microvascular size during graded coronary stenosis. Evidence of the microvascular site for autoregulation.
Authors: Kanatsuka H , Lamping K G , Eastham C L , Marcus M L
Published: June 2012

The purpose of this study was to determine the coronary microvascular sites of autoregulation. The epimyocardial coronary microcirculation was observed through an intravital microscope by stroboscopic epi-illumination in anesthetized open-chest dogs (n = 20). Aortic pressure and heart rate were held constant by an aortic snare and atrial pacing, respectively. Distal pressure of the left anterior descending coronary artery was controlled by a screw occluder on the proximal left anterior descending coronary artery and monitored with a 24-gauge plastic cannula inserted into the branch or distal portion of the left anterior descending coronary artery. Distal pressure of the left anterior descending coronary artery was stepwisely reduced to 59 +/- 1 mm Hg (mild stenosis, n = 20) and 38 +/- 1 mm Hg (severe stenosis, n = 16). In the left circumflex coronary artery area, myocardial blood flow measured with radioactive microspheres to subepicardium, midmyocardium, and subendocardium did not change with the mild and severe stenosis from control. In the left anterior descending coronary artery area, myocardial blood flow to each layer remained at nearly control level with the mild stenosis but was reduced in midmyocardium and subendocardium with the severe stenosis. With the mild stenosis, diameters of coronary arterial microvessels less than 100 microns in diameter dilated, and those larger than 100 microns in diameter did not change. The magnitude of vasodilation in small arterial microvessels was inversely related to control diameter. With the severe stenosis, small arterial microvessels dilated, and simultaneously, large arterial microvessels constricted. Again, the magnitude of vasodilation in small arterial microvessels was inversely related to control diameter.(ABSTRACT TRUNCATED AT 250 WORDS)
Sympathetic neural modulation of cardiac impulse initiation and repolarization in the newborn rat.
Authors: Malfatto G , Rosen T S , Steinberg S F , Ursell P C , Sun L S , Daniel S , Danilo P , Rosen M R
Published: June 2012

We injected neonatal rats with nerve growth factor, the antiserum to nerve growth factor, or placebo for the first 10 days of life. Our goal was to determine the relation between sympathetic innervation of the developing heart, the electrocardiographic expression of cardiac rhythm, and the response of the heart to alpha-adrenergic stimulation with phenylephrine. We were especially interested in the latter area because of the prior demonstration in isolated cell systems of sympathetic neural modulation of a 41-kDa GTP regulatory protein and alpha-adrenergic responsiveness. Ten- to 11-day-old rats treated with nerve growth factor had more complete sympathetic innervation, faster heart rates, and higher levels of the 41-kDa protein than the placebo group. Electrophysiological studies were performed on isolated ventricular septa superfused with Tyrode's solution at 37.0 degrees-37.5 degrees C. The electrophysiological response of septa to 10(-9) and 10(-8) M phenylephrine from the 10-11-day-old nerve growth factor group was comparable with that of 3-week-old control animals. In contrast, 10-11-day-old antiserum-treated rats had an abnormal innervation pattern, lower levels of the 41-kDa protein, and a more immature electrophysiological response to alpha-adrenergic stimulation than the placebo group. In addition, antiserum-treated rats had an abnormally prolonged electrocardiographic QT interval. Our results demonstrate for the first time in intact animals a direct link between sympathetic innervation and alpha-adrenergic receptor-effector coupling as well as the dependence on innervation of the modulation of impulse initiation by alpha-agonists. This sequence of developmental events may be important not only in the regulation of normal cardiac rhythm but also in the expression of certain pathological entities such as the congenital long QT syndrome and the sudden infant death syndrome.
Electrophysiological properties and responses to simulated ischemia in cat ventricular myocytes of endocardial and epicardial origin.
Authors: Kimura S , Bassett A L , Furukawa T , Cuevas J , Myerburg R J
Published: June 2012

In multicellular preparations, there are differences in action potential configuration between endocardium and epicardium, and electrophysiological alterations induced by ischemia are more drastic in epicardium than in endocardium. The present study was designed to examine electrophysiological properties of single cardiac myocytes enzymatically isolated from the endocardial and epicardial surfaces of the cat left ventricle and to determine whether the differential responses to ischemia of intact tissue occur in single cells. Action potentials recorded from the isolated single cells of epicardial surface had lower action potential amplitude and a prominent notch between phase 1 and phase 2, compared with those of the cells isolated from the endocardial surface; these findings are similar to those in intact endocardial and epicardial preparations. Resting membrane potentials recorded from both endocardial and epicardial single cells were sensitive to the change in extracellular K+ concentration and had properties of a K+ electrode. Action potential duration was frequency dependent in both cell types and was shorter in epicardial cells than in endocardial cells at a stimulation rate of 3 Hz. When the cells were superfused with Tyrode's solution that was altered to mimic an ischemic environment in vivo (PO2, 30-40 mm Hg; pH 6.8; [K+], 10 mM; and glucose free), resting membrane potential, action potential amplitude, and action potential duration were reduced, and the refractory period was shortened in both endocardial and epicardial single cells, but there were no differences in the degree of changes in action potentials and refractory periods induced between the two cell types. Action potential changes induced by L-alpha-lysophosphatidylcholine (5-40 mg/l) were also similar in endocardial and epicardial single cells.(ABSTRACT TRUNCATED AT 250 WORDS)
Reduction of ischemic K+ loss and arrhythmias in rat hearts. Effect of glibenclamide, a sulfonylurea.
Authors: Kantor P F , Coetzee W A , Carmeliet E E , Dennis S C , Opie L H
Published: June 2012

Glibenclamide, one of the antidiabetic sulfonylureas, is known to block ATP-dependent K+ channels. We used this drug to determine to what extent K+ loss from acutely ischemic myocardium is mediated via these channels. We also investigated whether glibenclamide would influence ischemic arrhythmias. Isolated rat hearts rendered globally ischemic showed no correlation between early lactate and K+ efflux rates. Cumulative K+ loss during 11 minutes of global ischemia (0.5 ml min-1 g-1) was reduced, from 3.2 +/- 0.3 to 2.5 +/- 0.1 mueq/g (p less than 0.025) by 1 microM glibenclamide and from 3.3 +/- 0.2 to 1.9 +/- 0.2 mueq/g (p less than 0.005) by 10 microM glibenclamide, while lactate efflux was unaltered by the drug. Glibenclamide also exhibited potent antifibrillatory activity, abolishing irreversible ventricular fibrillation during regional ischemia (0/6 vs. 5/6 controls; p less than 0.02) and during global ischemia (0/7 vs. 9/9 controls; p less than 0.01). Heart rate, coronary flow rate, peak systolic pressure, and myocardial oxygen consumption were unaltered by the drug (1 microM). Similarly, glibenclamide (1 microM) did not alter myocardial ATP, phosphocreatine or lactate content, or glucose utilization. Ventricular fibrillation threshold during normoxia was also unaltered by glibenclamide (1 microM). We conclude that K+ loss during acute myocardial ischemia is mediated partly by ATP-dependent K+ channels, and not by a tightly coupled co-efflux with anionic lactate.
Mast cells in the rat heart during normal growth and in cardiac hypertrophy.
Authors: Rakusan K , Sarkar K , Turek Z , Wicker P
Published: June 2012

Mast cells in rat hearts were studied quantitatively during normal postnatal growth and in two types of cardiac hypertrophy. Normally, cardiac mast cell density in 11-12-day-old animals is very low, but increases markedly in the following 2-3 weeks to its highest values, with a subsequent decline toward adult values. At the peak of mast cell density, the percentage of mast cells in close proximity to capillaries is also highest. In adult animals, mast cell counts are significantly higher in the right ventricle than in the left. This relation is preserved even when the right ventricle is hypertrophic, as in rats born at simulated high altitude. Chronic hypertension and swimming have little effect on the mast cell density in rat hearts. Conspicuous changes in the mast cell density at the time of capillary proliferation seem to indicate a special role played by these cells in the formation of new vessels.
Evidence for the existence of inositol tetrakisphosphate in mammalian heart. Effect of alpha 1-adrenoceptor stimulation.
Authors: Kohl C , Schmitz W , Scholz H , Scholz J
Published: June 2012

The time course of the effects of phenylephrine (10 mumol/l) on force of contraction and on inositol phosphates in electrically driven left auricles from rat hearts labeled with [3H]inositol was studied. All experiments were performed in the presence of propranolol (1 mumol/l) and LiCl (10 mmol/l). Products measured after separation with high-performance liquid chromatography were inositol 1-phosphate (1-IP1), inositol 1,4-bisphosphate (1,4-IP2), inositol 1,3,4-trisphosphate (1,3,4,-IP3), inositol 1,4,5-trisphosphate (1,4,5-IP3), and inositol 1,3,4,5-tetrakisphosphate (1,3,4,5-IP4). All inositol phosphates increased after stimulation with phenylephrine. 1,4,5-IP3 was the first compound to rise maximally within 30 seconds; this rise was followed by an increase in 1,3,4,5-IP4 and 1,4-IP2 beginning within 2 minutes. The increase in 1,3,4-IP3 and 1-IP1 was slower and did not reach steady state within 15 minutes. The positive inotropic effect of phenylephrine was maximal after 5 minutes. It is concluded that the increase in the presumed second messengers 1,4,5-IP3 and 1,3,4,5-IP4 coincides with the positive inotropic effect after alpha 1-adrenoceptor stimulation. Since the increase in 1,4,5-IP3 precedes the increase in force of contraction, 1,4,5-IP3 may initiate the positive inotropic effect of alpha 1-adrenoceptor agonists and 1,3,4,5-IP4 maintains the increase in force of contraction.
Effects of proton buffering and of amiloride derivatives on reperfusion arrhythmias in isolated rat hearts. Possible evidence for an arrhythmogenic role of Na(+)-H+ exchange.
Authors: Dennis S C , Coetzee W A , Cragoe E J , Opie L H
Published: June 2012

We investigated the hypothesis that an accelerated Na+o-H+i exchange on reperfusion may lead to a displacement of the 3[Na+] [Ca2+]i/o equilibrium in favor of an arrhythmogenic rise in cytosolic [Ca2+]. Supporting evidence was obtained by subjection of isolated rat hearts to 15 minutes of low-flow (5% of control) ischemia and 2 minutes of reperfusion in the presence of a Krebs-Henseleit HEPES buffer (pH 7.4) containing lactate (10 mM). At first, the [HEPES] was fixed at 5 mM; then, 2 minutes before reflow, either the [HEPES] was varied from 50 to 1 mM to slow H+o washout, or increasing concentrations of 5-(N,N-dimethyl)-amiloride (Ki 7 microM) or 5-(N,N-hexamethylene)-amiloride (Ki 0.2 microM) were added for inhibition of Na(+)-H+ exchange. In each case, reperfusion ventricular arrhythmias were reduced by 69-73% (p less than 0.001).
Ischemic preconditioning slows energy metabolism and delays ultrastructural damage during a sustained ischemic episode.
Authors: Murry C E , Richard V J , Reimer K A , Jennings R B
Published: June 2012

We have shown previously that preconditioning myocardium with four 5-minute episodes of ischemia and reperfusion dramatically limited the size of infarcts caused by a subsequent 40-minute episode of sustained ischemia. The current study was undertaken to assess whether the same preconditioning protocol slowed the loss of high energy phosphates, limited catabolite accumulation, and/or delayed ultrastructural damage during a sustained ischemic episode. Myocardial metabolites and ultrastructure in the severely ischemic subendocardial regions were compared between control and preconditioned canine hearts. Hearts (four to 10 per group) were excised after 0, 5, 10, 20, or 40 minutes of sustained ischemia. All groups had comparable collateral blood flow. Preconditioned hearts developed ultrastructural injury more slowly than controls; evidence of irreversible injury was observed after 20 minutes in controls but not until 40 minutes in preconditioned hearts. Furthermore, after 40 minutes of ischemia, irreversible injury was homogeneous in controls but only focal in preconditioned myocardium. Preconditioning reduced starting levels of ATP by 29%. Nevertheless, it also slowed the rate of ATP depletion during the episode of sustained ischemia, so that after 10 minutes of ischemia, preconditioned hearts had more ATP than controls. However, after 40 minutes, ATP contents were not significantly different between groups. Preservation of ATP resulted from reduced ATP utilization and was not due to increased ATP production. Accumulation of purine nucleosides and bases (products of adenine nucleotide degradation) was limited in preconditioned myocardium. Accumulation of glucose-1-phosphate, glucose-6-phosphate, and lactate also was reduced markedly by preconditioning, due to reduced rates of glycogen breakdown and and anaerobic glycolysis. We propose that preconditioning reduces myocardial energy demand during ischemia, which results in a reduced rate of high energy phosphate utilization and a reduced rate of anaerobic glycolysis. Either preservation of ATP or reduction of the cellular load of catabolites may be responsible for delaying ischemic cell death.
Molecular cloning of gene sequences from rat heart rapidly responsive to pressure overload.
Authors: Komuro I , Shibazaki Y , Kurabayashi M , Takaku F , Yazaki Y
Published: June 2012

Although pressure overload induces cardiac hypertrophy and reexpression of contractile protein isogenes, the molecular mechanism of both is not well understood. We demonstrated the early change in the translational activity of specific cardiac messenger RNA by two-dimensional gel electrophoresis of in vitro translational products. A total of over 400 translational products were detected by fluorography, and the relative predominance of eight species was increased by pressure overload whereas that of two translational products was decreased. We cloned four pressure-overload-responsive complementary DNA clones, pPO-1, -4, -5, and -6, by differential dot blot hybridization. Two clones were increased from the early period after the imposition of pressure overload; the other two were decreased. The expression pattern of each complementary DNA clone in the pressure-overloaded hearts was similar to that in fetal hearts. Pressure overload also induced the additional messenger RNA, which hybridized with pPO-4. This mRNA was also recognized in fetal, neonatal, and adult spontaneously hypertensive rat hearts. The induction of this transcript by pressure overload was not suppressed but, rather, stimulated by cycloheximide. These results suggest that there are some genes that rapidly respond to pressure overload and that may play some role in the development of cardiac hypertrophy.
Changes in myofibrillar activation and troponin C Ca2+ binding associated with troponin T isoform switching in developing rabbit heart.
Authors: McAuliffe J J , Gao L Z , Solaro R J
Published: June 2012

Postnatal development of the mammalian heart is associated with changes in the population of isoforms of the thin filament proteins. We correlated the change in thin filament proteins, which occur in rabbit hearts between 5 days and 22 days of age, with changes in Ca2+ dependence of myofibrillar ATPase activity, force generation, and troponin C Ca2+ binding. The preparations derived from the 5-day-old animals exhibited a high molecular weight isoform of troponin T not found in the hearts of the 22-day-old animals. Other troponin T isoforms were also found to be present in different relative amounts. No other major differences in thin filament protein composition could be identified. Compared with the 5-day-old rabbit heart preparations, the ATPase activity of myofibrils from 22-day-old rabbit hearts exhibited a reduced Ca2+ sensitivity. The pCa50 (negative log of the half-maximal-activity free Ca2+) of the MgATPase activity was shifted by 0.15 pCa units with maturation. Maturation of the myofibrils was also associated with an increased effect of Mg2+ on pCa50. On increasing the Mg2+ from 2 to 10 mM at constant MgATP2-, the pCa50 of 5-day myofibrils was increased (shifted to the right) by 0.39 pCa units for 5-day-old rabbit hearts and 0.45 pCa units for 22-day-old rabbit hearts. Although similar changes in pCa50 of force developed by myofibrils were marginally significant, fibers from hearts of 5-day-old rabbits exhibited a greater Hill coefficient than hearts from 22-day-old rabbits (3.0 vs. 2.1). Despite the increased sensitivity of 5-day-old rabbit hearts to Ca2+, these hearts exhibited significantly less Ca2+ bound to myofibrillar troponin C than did the 22-day-old rabbit hearts. Moreover, the models that best described the Ca2+ binding data are different for the two age groups. Our data indicate that the Ca2+ activation and Ca2+ binding properties of myofibrillar troponin C are altered in developing cardiac myofibrils and that the changes in these properties may be influenced by changes in the troponin T isoforms present in the myofibril.
Quantification of [Ca2+]i in perfused hearts. Critical evaluation of the 5F-BAPTA and nuclear magnetic resonance method as applied to the study of ischemia and reperfusion.
Authors: Marban E , Kitakaze M , Koretsune Y , Yue D T , Chacko V P , Pike M M
Published: June 2012

Calcium has been implicated as a mediator of cell injury in ischemia and reperfusion, but direct measurements of Ca2+ are required to refine this idea. We used nuclear magnetic resonance spectroscopy and the Ca2+ indicator 5F-BAPTA to measure [Ca2+]i in perfused ferret hearts. Several lines of evidence are presented to show that loading with the acetoxymethyl ester of 5F-BAPTA is not significantly complicated by accumulation of partially de-esterified metabolites, compartmentalization into mitochondria, or disproportionate uptake into endothelial cells. During 20 minutes of total global ischemia at 30 degrees C, time-averaged [Ca2+]i increased significantly, reaching peak values roughly three times control at 15-20 minutes. Reperfusion resulted in a persistent elevation of [Ca2+]i during the first 5 minutes, but not afterward. Although the nonlinear response of 5F-BAPTA to [Ca2+] leads to underestimation of the true time-averaged [Ca2+]i, the measured alterations of intracellular Ca2+ homeostasis during ischemia are large compared with the likely errors in quantification. Phosphorus nuclear magnetic resonance spectroscopy of 5F-BAPTA-loaded hearts reveals changes during ischemia similar to those recorded previously in hearts not containing a Ca2+ indicator. Developed pressure recovers to only 50% of control values during reflow, indicating that the presence of 5F-BAPTA in the cytosol does not protect against stunning, at least when the extracellular calcium concentration has been raised to 8 mM. We conclude that 5F-BAPTA provides useful measurements that reveal that time-averaged [Ca2+]i rises during ischemia and returns to control levels soon after reperfusion.