bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics of cromakalim‐induced relaxations in the smooth muscle cells of guinea‐pig mesenteric artery and vein
Authors: Nakao K. , Okabe K. , Kitamura K. , Kuriyama H. , Weston A.H.
Published: July 2012

The effects of cromakalim (BRL 34915) on the smooth muscle cells of guinea‐pig mesenteric artery and vein were investigated with microelectrode and tension recording methods.

Cromakalim (> 10 μm) produced membrane hyperpolarization with an increase in ionic conductance. The hyperpolarization occurred to a greater extent and lasted longer in the vein than in the artery.

The hyperpolarization induced by cromakalim in mesenteric vein comprised two components, one of which was Mn sensitive. In mesenteric artery, the hyperpolarization was relatively insensitive to Mn.

From the current‐voltage relationship measured from arterial smooth muscle membranes, the reversal potential of cromakalim was estimated to be −80 mV. The cromakalim‐induced hyperpolarization was not modified in Na‐ or Cl‐deficient solution.

In both mesenteric artery and vein, cromakalim relaxed tissues precontracted with high K with (below 40 mm) or without (above 40 mm) hyperpolarization of the membrane.

In the mesenteric artery, action potentials evoked by electrical stimulation ceased before the generation of hyperpolarization.

Cromakalim produced a cross‐desensitization with nicorandil on the evoked membrane hyperpolarization in mesenteric artery.

It is concluded that the relaxing actions of cromakalim result from the hyperpolarization which follows the opening of Ca‐dependent K channels. The inhibition of a voltage‐dependent Ca current may also be involved in this inhibitory effect.
Control of interval‐force relation in canine ventricular myocardium studied with ryanodine
Authors: Bose D. , Hryshko L.V. , King B.W. , Chau T.
Published: July 2012

The mechanism of post‐extrasystolic, rest and frequency potentiation was studied in canine isolated ventricular muscle.

Ryanodine, which impairs Ca availability from the sarcoplasmic reticulum (SR), reduced the amplitude of the extrasystole less than that of the steady state contraction. Ryanodine also inhibited post‐extrasystolic potentiation and converted rest‐potentiation into rest depression. Rest‐potentiation was blocked preferentially by ryanodine compared to post‐extrasystolic potentiation. An increase in the contribution of extracellular Ca to the extrasystolic contraction could not entirely account for the post‐extrasystolic potentiation.

Prolonged rest, by itself, also caused depression of the first post‐rest contraction. During rest‐potentiation, SR Ca seemed to play a greater role in contraction than transmembrane Ca influx. However, the ability of the ‘release pool’ of Ca in the SR to be reprimed after a contraction was reduced. This was seen as a decrease in post‐extrasystolic potentiation elicited immediately after rest.

A decrease in stimulus interval was associated with a transient decrease in contraction amplitude followed by an increase. An abrupt increase in stimulus interval had the opposite effect. Ryanodine blocked the initial transient changes and accelerated the delayed changes. These results suggest that the transient changes in contraction after sudden changes in drive interval are dependent on the SR.

Transmembrane Ca entry and the rate of recovery of the Ca release process (repriming) in the SR after a contraction seem to be interval‐dependent. The data also indicate that different mechanisms are involved in post‐extrasystolic and rest‐potentiation.

The results are consistent with a model which proposes ‘recirculation’ of activator Ca within the SR after a contraction or of the presence of an appreciable amount of inactivation of the SR Ca release process during normal stimulation. An increased pool of releasable Ca due to longer recirculation time or a time‐dependent decay in the level of inactivation of Ca release from the SR may give rise to rest‐potentiation.
Inhibition by anti‐inflammatory agents of contraction induced by epidermal growth factor‐urogastrone in isolated longitudinal smooth muscle strips from guinea‐pig stomach
Authors: Itoh H. , Muramatsu I. , Patel P. , Lederis K. , Hollenberg M.D.
Published: July 2012

Epidermal growth factor‐urogastrone (EGF‐URO) caused a concentration‐dependent contractile response of longitudinal muscle strips from the gastric body of the guinea‐pig stomach. The contractile response to EGF‐URO was monophasic, with tension returning rapidly to baseline. Desensitization was evident in that further addition of EGF‐URO to the organ bath did not cause a second contraction.

Preincubation with indomethacin, ibuprofen, naproxen and aspirin markedly inhibited the contractions induced by EGF‐URO with an order of potency (indomethacin > naproxen > ibuprofen > aspirin) that reflected the ability of these agents to inhibit cyclo‐oxygenase.

The data indicate that prostanoids mediate the action of EGF‐URO in the longitudinal muscle preparation.

Auranofin (0.5 to 50 μm), a chrysotherapeutic agent with antiproliferative properties used for treating rheumatoid arthritis, also markedly inhibited the EGF‐URO response; however, other gold‐containing compounds (aurothioglucose or gold sodium thiomalate at 30 to 100 μm) failed to cause significant inhibition.

Preincubation of preparations for 2 h with 1 μm hydrocortisone, prednisolone or dexamethasone caused an inhibition of EGF‐URO‐induced contraction of approximately 50%. However, steroids lacking either a 17α‐hydroxyl (corticosterone) or an 1lβ‐hydroxyl (cortisone, deoxycorticosterone, prednisone) substituent did not inhibit the contraction caused by EGF‐URO. For hydrocortisone, the inhibitory effect was half‐maximal at 0.2 μm and was maximal at 1 μm. Cycloheximide (10 μm) blocked the inhibitory action of hydrocortisone and potentiated the contractile action of EGF‐URO.

The ability of a variety of steroidal and non‐steroidal anti‐inflammatory agents to interfere with the action of EGF‐URO in a smooth muscle preparation suggests that these agents may also inhibit the action of EGF‐URO mediated by prostanoids in other target tissues.

The data also point to a potential role for EGF‐URO in regulating gastric motility.
The mechanism of the inotropic action of striatoxin, a novel polypeptide toxin from a marine snail, in isolated cardiac muscle
Authors: Ohizumi Yasushi , Kobayashi Masaki , Muroyama Akiko , Nakamura Hideshi , Kobayashi Jun'ichi
Published: July 2012

Striatoxin (StTX), a novel polypeptide from a marine snail, caused a dose‐dependent increase in contractility in the isolated atria of guinea‐pig and rat in the concentration‐range of 2 × 10−9 to 3 × 10−8m and 3 × 10−8 to 10−6m, respectively.

In guinea‐pig atria, the StTX‐induced inotropic effect was inhibited by tetrodotoxin but not by cimetidine or chlorpheniramine. Practolol, propranolol or reserpine caused only partial block of this inotropic action.

In isolated single cells from rat hearts, StTX caused an increase in the degree and the rate of contraction.

In guinea‐pig atria, StTX provoked action potentials with a plateau phase of long duration without affecting the maximum rate of rise, the amplitude of action potential and the resting membrane potential. This prolongation was also reversed by tetrodotoxin.

In guinea‐pig cardiac myocytes, whole‐cell patch‐clamp experiments showed that StTX slowed Na channel inactivation without affecting the time course of channel activation. The voltage dependence of Na currents was not altered by StTX.

The residual currents, but not peak currents were markedly enhanced by StTX.

These results suggest that StTX causes prolongation of the action potential duration probably due to slowed inactivation of Na inward currents and enhanced residual currents and that this may result in an increase in Ca2+ availability in cardiac muscle cells. This could explain the cardiotonic action of StTX.
The mode of inotropic action of ciguatoxin on guinea‐pig cardiac muscle
Authors: Seino Asami , Kobayashi Masaki , Momose Kazutaka , Yasumoto Takeshi , Ohizumi Yasushi
Published: July 2012

Ciguatoxin (CTX) caused a dose‐dependent increase in the contractile force of the guinea‐pig isolated left atria at concentrations ranging from 0.1 to 10 ng ml−1 with the ED50 value of 0.5 ng ml−1.

In the atria, tetrodotoxin (5 × 10−7 m) inhibited markedly the inotropic action of CTX. The inotropic effect of CTX at low concentrations was abolished by practolol (10−5 m) and reserpine (2 mg kg−1 daily, for 3 days), whereas that of CTX at high concentrations was partially inhibited by both drugs.

In single atrial cells, CTX (3 ng ml−1) produced a marked increase in the amplitude of longitudinal contractions.

CTX (3 ng ml−1) caused marked prolongation in the falling phase of action potentials of atrial strips without affecting the maximum rate of rise of action potentials and membrane resting potentials. The effect of CTX on action potentials was abolished by tetrodotoxin (10−6m).

The whole‐cell patch‐clamp experiments on myocytes revealed that CTX (20 ng ml−1) shifted the current‐voltage curve of Na inward currents by 40 mV in the negative direction. CTX caused a small sustained Na inward current even at resting membrane potentials.

These results suggest that the inotropic action of lower concentrations of CTX is primarily due to an indirect action via noradrenaline release, whereas that of higher concentrations is caused not only by an indirect action but also by a direct action on voltage‐dependent Na channels of cardiac muscle. It is also suggested that CTX activates cardiac muscle Na channels by modifying the voltage‐dependence of channel activation to increase Na inward currents, thus producing cardiotonic actions.
Slow channel inhibitor effects on brain function: tolerance to severe hypoxia in the rat
Authors: Cartheuser Carl F.
Published: July 2012

The protective effects of ten slow channel inhibitor drugs against severe progressive hypoxia were investigated in rats breathing spontaneously during light anaesthesia. Respiration, heart rate, electrocorticogram (ECoG) and/or electroencephalogram (EEG) were recorded.

Tolerance times were monitored from hypoxia onset until cessation of respiration, ECoG, EEG synchronization, and ‘background‐EEG’. Drugs were administered i.v. 5 min before the onset of hypoxia.

Verapamil, gallopamil, and nimodipine resulted in a significant increase of tolerance times; fendiline and bepridil showed a small increase (not significant); bencyclan and prenylamine were ineffective; cinnarizine and diltiazem slightly reduced tolerance times as did flunarizine at low doses.

At protective doses, verapamil, gallopamil, and nimodipine significantly raised the respiration rate but had little or no cardiac depressor effects. Bencyclan showed ventilatory drive but cardiocirculatory depression. A clear‐cut ventilatory drive did not occur with the other ineffective slow channel inhibitors.

It is suggested that the protective actions observed were not due to slow channel inhibition per se, nor to spasmolytic potency or increased cerebral blood flow. Ventilatory drive associated with other cardiopulmonary actions which secondarily raise the brain oxygen supply are likely to be responsible for this effect.
Enhanced oxidative phosphorylation in rat liver mitochondria following prolonged in vivo treatment with imipramine
Authors: Katyare Surendra S. , Rajan Rema R.
Published: July 2012

Effects of prolonged in vivo administration of the tricyclic antidepressant drug imipramine on oxidative energy metabolism in rat liver mitochondria were examined.

Imipramine treatment resulted in an increase in state 3 respiration rates with all the substrates tested as early as one week after treatment; this was sustained through the second week of treatment.

The changes in respiration rates were accompanied by a selective increase in the intramitochondrial cytochrome aa3 and c + c1 contents after both one and two weeks of treatment.

Administration of imipramine did not alter the total liver protein content per g tissue, the mitochondrial protein content per g tissue or the mitochondrial yield.

Kinetic analyses of succinoxidase activity in terms of Arrhenius plots indicated possible alterations in mitochondrial membrane lipid milieu and membrane fluidity after the drug treatment, especially in the second week.
Adrenergic modulation of the release of 5‐hydroxytryptamine from the vascularly perfused ileum of the guinea‐pig
Authors: Racké K. , Schwörer H. , Kilbinger H.
Published: July 2012

Isolated segments of the guinea‐pig ileum were vascularly perfused and the release of 5‐hydroxytryptamine (5‐HT) and 5‐hydroxyindoleacetic acid (5‐HIAA) into the portal venous effluent was determined by h.p.l.c. with electrochemical detection. Test substances were applied via the arterial perfusion medium.

Isoprenaline (0.1 μm) increased the outflow of 5‐HT and 5‐HIAA maximally by about 75% and this was antagonized by propranolol (0.1 μm). Forskolin (1–10 μm) increased the outflow of 5‐HT by approximately 105% and that of 5‐HIAA by approximately 55%. The phosphodiesterase inhibitor AH 21–132 (0.1–1 μm) increased the outflow of 5‐HT and 5‐HIAA by about 70%. Isoprenaline (1 nm) and AH 21–132 (10 nm), which alone had no effect, increased the outflow of 5‐HT and 5‐HIAA by 75%, when applied in combination.

Clonidine (1 μm) reduced the outflow of 5‐HT by 45%, an effect blocked by tolazoline (1 μm), but not by prazosin (0.1μm).

The effects of isoprenaline, forskolin and clonidine were also observed in the presence of tetrodotoxin (1 μm) demonstrating a direct modulation of 5‐HT release from the enterochromaffin cells.

In conclusion, the release of 5‐HT from enterochromaffin cells is facilitated by activation of β‐adrenoceptors and inhibited via α2‐adrenoceptors. Enhancing intracellular cyclic AMP, by direct stimulation of adenylate cyclase with forskolin or by inhibition of phosphodiesterase, also facilitates the release of 5‐HT. The β‐adrenoceptor‐mediated effect on 5‐HT release appears to involve an increase in cyclic AMP, as the effect of isoprenaline was potentiated after inhibition of phosphodiesterase.
Characterization of the binding of [3H]‐CGS 19755: a novel N‐methyl‐d‐aspartate antagonist with nanomolar affinity in rat brain
Authors: Murphy Deborah E. , Hutchison Alan J. , Hurt Steven D. , Williams Michael , Sills Matthew A.
Published: July 2012

CGS 19755 (cis‐4‐phosphonomethyl‐2‐piperidine carboxylic acid), a rigid analogue of 2‐amino‐5‐phosphonopentanoic acid (AP5), is one of the most potent competitive N‐methyl‐d‐aspartate (NMDA) antagonists described. Using Triton‐treated crude synaptic membranes from rat brain, binding studies indicated that [3H]‐CGS 19755 bound with high affinity and selectivity to the NMDA‐type excitatory amino acid receptor.

[3H]‐CGS 19755 binding was saturable, reversible, heat‐labile, pH‐dependent and linear with protein concentration. Specific binding represented 80–85% of the total amount bound.

Using a centrifugation assay, saturation experiments revealed two distinct binding components with Kd values of 9 and 200 nM, and corresponding Bmax values of 0.55 and l.00 pmol mg−1 protein. In contrast, a single binding component with a Kd value of 24 nM and an apparent value of 0.74 pmol mg−1 protein was observed with a filtration assay.

Competition experiments in which both assay techniques were used, showed that [3H]‐CGS 19755 selectively labels the NMDA receptor. The most active inhibitors of [3H]‐CGS 19755 binding were l‐glutamate and CGS 19755 (IC50 values = 100 nm).

In the centrifugation assay, a number of excitatory amino acids were found to generate shallow inhibition curves, and computer analysis indicated the presence of two binding components. The quisqualate receptor ligand AMPA (D,L‐α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionate), kainic acid and the non‐competitive NMDA antagonists, such as phencyclidine, tiletamine and MK‐801, were without activity.

The high affinity binding obtained with [3H]‐CGS 19755 by use of filtration techniques thus permits the more rapid evaluation of compounds as potential NMDA antagonists and agonists. Therefore, this rigid analogue of AP5 is a more suitable radioligand for NMDA receptors than [3H]‐CPP (3‐(±)−(2‐carboxypiperazin‐4‐yl)propyl‐l‐phosphonic acid), the corresponding analogue of 2‐amino‐7‐phosphonoheptanoic acid (AP7).
Effects of the anaesthetic 2,6‐diisopropylphenol on synaptic transmission in the rat olfactory cortex slice
Authors: Collins G.G.S.
Published: July 2012

The effects of the general anaesthetic 2,6‐diisopropylphenol (DIP) on synaptic transmission and the actions of amino acid transmitter candidates have been investigated in rat olfactory cortex slices.

On electrical stimulation of the lateral olfactory tract (LOT), DIP (20 to 200 μm) increased the area of those surface field potentials which reflect γ‐aminobutyric acid (GABA)‐mediated transmission in a concentration‐dependent manner in 6 out of 12 slices. In a series of conditioning experiments, DIP (50 μm) also potentiated GABA‐mediated pre‐ and post‐synaptic inhibition.

Perfusion of slices with DIP (50 μm) potentiated the reduction in the excitability of the terminals of the LOT produced by exogenous GABA in a picrotoxin‐sensitive manner.

DIP (50μm) markedly potentiated the surface depolarizations evoked by GABA, muscimol and 3‐aminopropanesulphonic acid. The effect on the response to 3‐aminopropanesulphonic acid was observed over a concentration range of DIP of 6.25 to 50μm and was not blocked by the benzodiazepine receptor antagonist Ro 15–1788.

In slices in which GABA‐mediated transmission was abolished by picrotoxin (25 μm), DIP (50 μm) had no significant effect on monosynaptically‐evoked excitatory transmission but depressed the areas of those field potentials which reflect di‐/polysynaptic excitations in a concentration‐dependent manner (from between 1.6 and 6.25 to 50 μm).

In a series of conditioning experiments DIP (50 μm) abolished the increase in the excitability of the pyramidal cells evoked on stimulation of deep association fibres.

DIP (50μm) had no significant effect on surface depolarizations evoked by N‐methyl‐d‐aspartate, quisqualate and kainate or by the transmitter candidates l‐glutamate and l‐aspartate.

It is concluded that, at clinically relevant concentrations, DIP potentiates GABA‐mediated transmission probably by an interaction with the GABA receptor complex and inhibits di‐/polysynaptic excitations, possibly by inhibiting the release of excitatory transmitters.
The mechanism by which monoamine oxidase inhibitors give rise to a non‐calcium‐dependent component in the depolarization‐induced release of 5‐HT from rat brain synaptosomes
Authors: Evans Suzanne M. , Collard Keith J.
Published: July 2012

The effects of the monoamine oxidase inhibitors pargyline and nialamide on the Ca2+‐dependency of [3H]‐5‐hydroxytryptamine release from superfused rat brain synaptosomes has been studied in order to evaluate the discrepancies that have occasionally been observed in studying transmitter release by in vivo and in vitro techniques.

The application of K+ pulses of low concentration (12.5–20mm) caused an essentially Ca2+‐dependent release of [3H]‐5‐HT. However, at K+ concentrations above 30 mM, a small non‐Ca2+‐ dependent component appeared.

At high concentrations of K+ (30–55 mm), nialamide (18 μm) or pargyline (1 μm) increased the amount of [3H]‐5‐HT released which could be accounted for by an increase in the non‐Ca2+‐ dependent component of release.

The elevation of the non‐Ca2+‐dependent component of release caused by the monoamine oxidase inhibitors was totally abolished by the inhibitors of the plasma membrane 5‐HT carrier, chlomipramine (500 nm), citalopram (50 nm) and fluoxetine (1 μm).

The results suggest that the non‐Ca2+‐dependent component of release seen with high depolarizing concentrations of K+, particularly in the presence of monoamine oxidase inhibitors, is caused by the efflux of [3H]‐5‐HT through the plasma membrane carrier which seems to be activated during depolarization.

The significance of these findings to the physiological in vivo situation, and to the use of in vitro preparations in the study of transmitter release is discussed.
Contrasting in vitro lymphocyte chemotactic activity of the hydroxyl enantiomers of 12‐hydroxy‐5,8,10,14‐eicosatetraenoic acid
Authors: Bacon K.B. , Camp R.D.R. , Cunningham F.M. , Woollard P.M.
Published: July 2012

The chemotactic activity of 12(R)‐hydroxy‐5,8,10,14‐eicosatetraenoic acid (12(R)‐HETE), 12(S)‐HETE and leukotriene B4 (LTB4) for human mixed peripheral blood lymphocytes has been assessed in a 48‐well microchemotaxis assay. Responses to the standard lymphocyte chemoattractants, zymosan‐activated plasma, casein and N‐formyl‐methionyl‐leucyl‐phenylalanine (fMLP) were also measured.

12(R)‐HETE was shown to be chemotactic for lymphocytes over the range 5 × 10−7 to 5 × 10−5 m. In contrast, negligible chemotactic responses to 12(S)‐HETE were obtained.

LTB4 was 200 times more potent than 12(R)‐HETE as a lymphocyte chemoattractant, although maximal responses to the two agonists were similar.

12(R)‐HETE and LTB4, which are present in extracts of samples from the skin lesions of psoriasis, may be, at least in part, responsible for the lymphocyte infiltrate which is a characteristic feature of this disease.
The interaction of methoctramine and himbacine at atrial, smooth muscle and endothelial muscarinic receptors in vitro
Authors: Eglen R.M. , Montgomery W.W. , Dainty I.A. , Dubuque L.K. , Whiting R.L.
Published: July 2012

The action of methoctramine and himbacine at muscarinic receptors has been studied using guinea‐pig isolated trachea, oesophageal muscularis mucosae, paced left atria, and rat aortic preparations.

Methoctramine (1 × 10−6‐3.2 × 10−4 m), but not himbacine, elicited positive inotropic responses. These responses were enhanced by pretreating the animals with reserpine. The responses in reserpine‐treated animals were not antagonized by phentolamine (1 × 10−6 m) but were antagonized by propranolol (1 × 10−6 m).

Methoctramine, but not himbacine, exhibited allosteric inhibitory effects at cardiac muscarinic receptors, resulting in a curvilinear Schild plot. Deviations from competitive antagonism were also observed in combination dose‐ratio experiments using atropine and methoctramine. At 1 × 10−6 m, the pKB value for methoctramine was 7.88 ±0.15 (mean ± s.e.mean, n = 5). The pA2 value for himbacine at cardiac muscarinic receptors was 8.52 ± 0.06 (n = 3).

At tracheal and oesophageal muscularis mucosal smooth muscle receptors, the Schild plots for both antagonists were linear. The pA2 values for methoctramine at receptors in these two preparations were similar (6.08 ± 0.05 and 6.03 ± 0.09 respectively, n= 4) and were approximately 60 fold less than those values observed at atrial receptors. Himbacine, also exhibited similar values at muscarinic receptors in the trachea and oesophageal muscularis mucosae (7.61 ± 0.05 and 7.57 ± 0.04 respectively, n= 4).

Muscarinic receptors mediating relaxation of the rat aortic endothelium exhibited pA2 values for methoctramine (5.87 ± 0.12, n= 6) which were similar to those observed in the smooth muscle, but not the atria. The pA2 values for himbacine at endothelial muscarinic receptors were approximately 0.5 pA2 units lower than those observed at muscarinic receptors in smooth muscle (6.92 ± 0.80, n= 6). In addition, the Schild slopes for methoctramine and himbacine at these receptors were significantly (P < 0.05) less than unity.

Methoctramine, and to a lesser extent himbacine, are potent and selective antagonists for cardiac muscarinic receptors. However, caution should be used in interpretation of the data with methoctramine in view of the inhibitory allosteric properties and direct inotropic actions of this compound.
Electrophysiological mechanism for the antiarrhythmic action of propafenone: a comparison with mexiletine
Authors: Amerini Sandra , Bernabei Roberto , Carbonin Pierugo , Cerbaiz Elisabetta , Mugelli Alessandro , Pahor Marco
Published: July 2012

The antiarrhythmic potency of propafenone was evaluated in the guinea‐pig isolated heart; arrhythmias were induced with (a) digitalis intoxication and (b) hypoxia followed by reoxygenation.

Propafenone, 0.5 μm, was found to be the minimal but effective antiarrhythmic concentration. The antiarrhythmic activity of propafenone developed slower than that of 10 μm mexiletine, which was the lowest effective concentration under the same experimental conditions.

The electrophysiological effects of propafenone were then studied on sheep cardiac Purkinje fibres (manifesting oscillatory afterpotentials and triggered automaticity induced by barium or strophanthidin) and compared with those of 10 μm mexiletine.

Both 0.5 μm propafenone and 10 μm mexiletine consistently blocked triggered activity in sheep Purkinje fibres. The onset of the effect of propafenone was slower than that of mexiletine.

Unlike mexiletine, propafenone did not reduce the amplitude of oscillatory afterpotentials.

In contrast, propafenone significantly reduced in barium‐ and strophanthidin‐treated preparations.

It is concluded that the antiarrhythmic action of propafenone on digitalis‐ and reoxygenation‐induced arrhythmias is probably due to an electrophysiological mechanism different from that of mexiletine. Mexiletine, by reducing the amplitude of oscillatory afterpotentials, prevents the attainment of the threshold; propafenone, by reducing the excitability of the cell, increases the threshold and consequently an oscillatory afterpotential of the same amplitude will not generate arrhythmias.
Fade and tachyphylaxis of gastric acid secretory response to pentagastrin in rat isolated gastric mucosa
Authors: Hirst Barry H. , Holland Joy , Parsons Michael E. , Price Carolyn A.
Published: July 2012

Gastric acid secretory responses to pentagastrin were characterized in the rat isolated gastric mucosa. In particular, the mechanisms underlying fade, declining response upon continued stimulation, and tachyphylaxis, progressively reduced responses upon repeated stimulation, were investigated.

Pentagastrin, 10−9‐10−7m, resulted in concentration‐related increases in acid secretion, with a mean maximum of 2.65 μmolcm−2h−1 in response to pentagastrin, 10−7m. Higher concentrations of pentagastrin produced sub‐maximal secretory rates; we define this as auto‐inhibition. The responses to all concentrations of pentagastrin demonstrated fade. The rate of fade was correlated with the maximum acid secretory rate, declining at about 36% of the peak over the first 16min.

The Po2, Pco2, [HCO3−], pH, [glucose], [lactate], [Na+] and [K+] did not decline during the fade of the acid secretory response to pentagastrin, 10−7 m. Addition of a second aliquot of pentagastrin was not able to reverse fade, but these tissues were responsive to histamine. Replacement of the serosal solution, before addition of a second aliquot of pentagastrin, increased the acid response from 3% to 24% of the first response.

Serosal solution from donor tissues, allowed to respond to pentagastrin and then the acid secretion to fade, was able to stimulate secretion in fresh recipient tissues, although at lower rates.

Acid secretory responses to a second dose of pentagastrin were not significantly different, whether the tissues were previously unstimulated, or stimulated with pentagastrin washed out after attaining its peak secretory response (after 10–20 min). The second response was significantly reduced if the first response was allowed to fade with the pentagastrin in contact for 100min; i.e. fade significantly influenced the extent of tachyphylaxis.

Proglumide, 10−2m, a gastrin receptor antagonist, and omeprazole, 10−5 m, an inhibitor of the gastric (H+ + K+)‐ATPase, both inhibited pentagastrin‐stimulated acid secretion to similar extents. The second response to pentagastrin after pentagastrin alone, or pentagastrin plus omeprazole were both reduced compared to responses after no stimulation or omeprazole alone, respectively. After pentagastrin plus proglumide, the second response to pentagastrin was not lower than after proglumide alone. Proglumide, but not omeprazole, therefore, prevented pentagastrin tachyphylaxis.

It is concluded that gastrin fade and tachyphylaxis are related phenomena. Part of the fade may be due to release of an inhibitor(s). The major proportion of tachyphylaxis is a result of specific interaction of gastrin with its receptors.
Receptors for ATP in rat sensory neurones: the structure‐function relationship for ligands
Authors: Krishtal Oleg A. , Marchenko Sergey M. , Obukhov Alexander G. , Volkova Tatyana M.
Published: July 2012

The pharmacological properties of the ATP‐activated conductance in isolated sensory neurones of the rat were investigated by use of voltage clamp and concentration clamp techniques.

Adenosine 5′‐triphosphate (ATP), adenosine 5′‐diphosphate (ADP), cytidine 5′‐triphosphate (CTP), cytidine 5′‐diphosphate (CDP) and some derivatives activate these receptors, whereas adenosine 5′‐monophosphate (AMP), cytidine 5′‐monophosphate (CMP) and other naturally‐occurring nucleotides are competitive blockers.

In the sequence of substances, adenosine 5′‐(β,γ‐methylene)‐triphosphonate (APPCP), adenosine 5′‐(β,γ‐difluoromethylene)− triphosphonate (APPCF2P), adenosine 5′‐(β,γ‐dichloromethylene)‐triphosphonate (APPCC12P) and adenosine 5′‐(β,γ‐dibromomethylene)triphosphonate (APPCBr2P), the properties of ligands depend on the radius of the atom linked to the carbon of the diphosphonate group. Thus, APPCP is an agonist, APPCF2P is a partial agonist, while dichloromethylene and dibromomethylene analogues of adenosine 5′‐(β,γ‐methylene)triphosphonate demonstrate features of competitive blockers. APPCC12P is the most effective blocker of ATP‐receptors (inhibition constant Ki = 21 ± 4μm). An adenosyl or adenylyl radical, when connected to the terminal phosphate of ATP, converts the agonist into a partial agonist.

Two especially important parts of the ATP molecule are crucial for the interactions with receptors. They are: (1) the vicinity of C6 of the purine ring and (2) the polyphosphate chain. Some modifications in these regions of the molecule result in the transformation of an agonist into an antagonist.
Detergent and methylene blue affect endothelium‐dependent vasorelaxation and pressure/flow relations in rat blood perfused mesenteric arterial bed
Authors: Randall Michael D. , Hiley C. Robin
Published: July 2012

The autoperfused superior mesenteric arterial bed of the rat was used to study the opposition by carbachol of pressor responses to noradrenaline and the effects of methylene blue, the detergent CHAPS and indomethacin on pressure/flow relations.

Carbachol (1 ng‐3 μg) reduced the pressor response to 1μg noradrenaline in a dose‐dependent manner with an ED50 = 25.5 ± 6.0 ng and a maximum inhibition of 51.8 ± 1.9%. Perfusion of the mesenteric bed with 0.3% CHAPS in saline for 150 s abolished the reduction by carbachol of the noradrenaline pressor response. The effect of carbachol was also abolished by infusion of 1% methylene blue into the mesenteric vacular bed. Indomethacin (5mgkg−1) was without significant effect on the carbachol opposition of noradrenaline pressor responses. None of the 3 treatments had any effect on the response to 1 μg noradrenaline.

Perfusion with CHAPS before determination of pressure/flow relations, or infusion of methylene blue during their determination, steepened the regression of pressure upon flow to the same extent; at all the flow rates used (0.4–3.54 ml min−1) pressure was greater as a result of the treatment than in control animals. Pretreatment with indomethacin had no effect on pressure/flow relations.

It is concluded that carbachol opposition to noradrenaline pressor responses in the blood perfused superior mesenteric arterial bed of the rat shows the characteristics of being mediated by endothelium‐dependent relaxing factor (EDRF). Since vascular resistance increases more rapidly than in controls when the endothelium is functionally inhibited by treatment with CHAPS or perfusion with methylene blue, it appears that EDRF has a role in vivo in the modulation of myogenic vascular tone.
Complex interactions of agonists with α1‐adrenoceptors in intact cells
Authors: Sladeczek Fritz , Schmidt Bernard H. , Cory Robert N. , el Moatassim Chakib , Alonso Richard , Kirk Kenneth L. , Kirk Christopher J. , Rouot Bruno , Bockaert Joël
Published: July 2012

The apparent Ki values of (−)noradrenaline (NA), (+)− and (−)‐adrenaline (Ad), phenylephrine and the mono‐fluorinated NAs (in position 2, 5 or 6) for α1‐adrenoceptors of intact BC3H1 cells labelled with [3H]‐prazosin were greatly dependent on the incubation temperature.

The EC50 values of these compounds for stimulation of the inositol phosphate (IP) accumulation at 37°C were intermediate between their apparent dissociation constants at 2°C (Ki2°) and at 37°C (Ki37°).

The fact that an irreversible blockade of 46% ± 6% (n = 3) of the [3H]‐prazosin binding sites by phenoxybenzamine reduced the maximal IP‐formation induced by NA by 57% ± 5% (n = 3) shows that there is a direct coupling between α1‐adrenoceptors and phospholipase C in BC3H1 cells.

The Ki37°s of all agonists tested were in the same range (0.1 to 1mm) and showed no simple correlation with their EC50 values.

The Ki2° values for all the agonist correlated linearly with their EC50 values but were about 20–100 times lower than the respective EC50 values (except for the partial agonist methoxamine). In order to explain this difference, we propose that the apparent high affinity in the cold could be due to an [3H]‐prazosin‐induced alteration of the active site of the α1‐adrenoceptor, increasing its apparent affinity for catecholamines.
Acetylcholine releases endothelium‐derived hyperpolarizing factor and EDRF from rat blood vessels
Authors: Chen Guifa , Suzuki Hikaru , Weston Arthur H.
Published: July 2012

The effects of haemoglobin and methylene blue on the acetylcholine (ACh)‐induced electrical and mechanical responses of smooth muscle cells were investigated in rat aorta and rat main pulmonary artery.

When the endothelium was intact, ACh induced a transient hyperpolarization and sustained relaxation of tissues precontracted with noradrenaline. Both hyperpolarization and relaxation were absent in preparations without endothelium.

Haemoglobin and methylene blue inhibited the ACh‐induced relaxation, but not the transient hyperpolarization.

In aorta with an intact endothelium, ACh produced an increase in both the rate of 86Rb efflux and tissue cyclic GMP levels. The changes in ion flux were unaffected by either haemoglobin or methylene blue in concentrations which almost abolished the increase in cyclic GMP concentrations.

In arteries with an intact endothelium, indomethacin had no effect on the ACh‐induced electrical and mechanical responses or on the increase in 86Rb efflux and tissue cyclic GMP levels.

It is concluded that in the rat aorta and rat main pulmonary artery, ACh releases two different substances, an endothelium‐derived relaxing factor (EDRF) and a hyperpolarizing factor (EDHF), from the endothelial cells. Neither substance appears to be derived from a pathway dependent on cyclo‐oxygenase. EDHF seems to play a minor role in the relaxation of noradrenaline‐induced contractions.
Effects of neuropeptides and capsaicin on the canine tracheal vasculature in vivo
Authors: Salonen R.O. , Webber S.E. , Widdicombe J.G.
Published: July 2012

The nonadrenergic, noncholinergic nervous system may control the airway vasculature via various neuropeptides. We have perfused the cranial tracheal arteries of the anaesthetized dog and investigated the effects of neuropeptides and capsaicin (which is supposed to release neuropeptides from sensory nerve endings) on the tracheal vasculature by injecting them locally into the perfusion system.

Neurokinin A (NKA, 0.02–20 pmol), calcitonin gene‐related peptide (CGRP, 2–200 pmol) and peptide histidine isoleucine (PHI, 0.02–2 nmol) dose‐dependently decreased tracheal vascular resistance (Rtv). NKA was 10 and 100 times more potent than CGRP and PHI, respectively. The duration of the response to CGRP was greatly prolonged with larger doses. Galanin (0.2–2 nmol) had no appreciable effect on Rtv.

Neuropeptide Y (NPY 0.02–2 nmol) and bombesin (0.02–10 nmol) dose‐dependently increased Rtv. However, the dose‐response curve for bombesin was bell‐shaped suggesting the development of tachyphylaxis with larger doses. In smaller doses, bombesin was twice as potent as NPY. The duration of the response to NPY was prolonged with larger doses.

With the exception of PHI no neuropeptide altered tracheal smooth muscle tone; PHI (1 and 2 nmol) caused small dilatations of the trachea.

The effects of capsaicin (2–100 nmol) were complex. Usually, the vascular response had two dose‐dependent phases: a rapid vasoconstriction followed by a small, longer‐lasting vasodilatation. The tracheal smooth muscle response was usually biphasic, a contraction followed by a relaxation.

According to previous and present data, the order of potency of the neuropeptides on the canine tracheal vasculature is for the vasodilators: NKA > vasoactive intestinal peptide (VIP) > CGRP ≥ substance P > PHI, and for the vasoconstrictors: bombesin > NPY. The longer‐acting neuropeptides (VIP, CGRP and NPY) may be more important than the shorter‐acting neuropeptides (substance P, NKA, PHI and bombesin) as regulators of the airway wall blood flow.
Comparison between effects of caffeine and ryanodine on electromechanical coupling in myocardium of hibernating chipmunks: role of internal Ca stores
Authors: Kondo Noriaki
Published: July 2012

To clarify the cause of uncoupling of Ca influx through Ca channels and the contractility of the myocardium in hibernating chipmunks, the electromechanical effects of two different internal Ca store inhibitors, caffeine and ryanodine, and a cardiotonic agent, isoprenaline, were investigated in papillary muscles of hibernating animals.

Ryanodine (10−6 m), an inhibitor of internal Ca release, abolished the contraction with a marked inhibition of the action potential plateau (APp). In such preparations, an increase in Ca influx induced by isoprenaline (5 × 10−8 m) failed to augment the contraction, indicating uncoupling of Ca influx and contraction.

In ryanodine pretreated preparations, 10mM caffeine produced an early phase of APp, but did not affect the contraction abolished by ryanodine, while a higher concentration of caffeine (25 mm) markedly increased the contraction with an augmentation of the electrical response.

In the absence of ryanodine, caffeine (5mm) almost abolished the contraction with a greater inhibition of APp. In such preparations, isoprenaline greatly increased the contraction with an augmentation of the early phase of APp.

These effects were not significantly affected by additional application of ryanodine, but were inhibited by nifedipine, a Ca channel blocker.

These observations suggest that in cardiac muscles of hibernating animals, lack of the positive inotropic effect of isoprenaline may be attributed to a rapid and effective sequestration of increased cytoplasmic Ca through Ca influx by internal stores, probably by enhancement of their ability to take up Ca.
ZAPA, (Z)‐3‐[(aminoiminomethyl)thio]‐2‐propenoic acid hydrochloride, a potent agonist at GABA‐receptors on the Ascaris muscle cell
Authors: Holden‐Dye L. , Walker R.J.
Published: July 2012

This study is the first report of a compound which is equal in efficacy to γ‐aminobutyric acid (GABA) at the nematode Ascaris muscle GABA‐receptor. The GABA‐receptor at the Ascaris muscle cell which mediates a membrane hyperpolarization and muscle relaxation has eluded classification. The structure‐activity profile of this receptor is not typical of GABAA or GABAB‐receptors. Here we report that the isothiouronium compound ZAPA is as potent as GABA at this receptor. This finding has important implications for the characterization of the Ascaris GABA‐receptor and the design of novel anthelmintics.
The stereospecificity of LY253352 for α1‐adrenoceptor binding sites in the brain and prostate
Authors: Lepor Herbert , Baumann Mary , Shapiro Ellen
Published: July 2012

The stereospecificity of the enantiomers of LY253352, a potent and selective α1‐adrenoceptor antagonist, were studied in the human prostate and canine brain using radioligand receptor binding methods.

The mean equilibrium dissociation constant (KD) in the canine brain and human prostatic adenoma was 84.4 pM and 65.4 pM, respectively.

The α1‐adrenoceptor density in the canine brain was approximately eight fold greater than in the human prostatic adenoma.

The mean Ki values of (−)‐LY253352 and (+)‐LY253352 in the prostate were 0.19 nM and 5.79 nM, respectively.

The mean Ki values of (−)‐LY253352 and (+)‐LY253352 in the brain were 0.29 nM and 34.7 nM, respectively.

This study indicates that the stereochemical specificity of the optical isomers of LY253352 is a manifestation of differential affinities of the enantiomers for α1‐adrenoceptor binding sites.

The differential affinities of (+)‐LY253352 in the brain and prostate are suggestive of subtle unique properties of adrenoceptor binding sites in these tissues.
A comparison of the effects of the calcium entry blockers, verapamil, diltiazem and flunarizine against contractions of the rat isolated aorta and portal vein
Authors: Marriott J.F.
Published: July 2012

The actions of the chemically distinct calcium entry blockers, verapamil (Ver), diltiazem (Dlz) and flunarizine (Flu) have been compared in rat isolated aorta and portal vein.

KCl‐induced contractions of the rat aorta depend exclusively upon extracellular Ca2+, whereas, those induced by noradrenaline (NA) rely upon Ca2+ from intra‐ and extracellular sources. The NA‐induced contraction was pharmacologically dissected under Ca2+‐free conditions revealing a contraction dependent upon intracellular Ca2+ (EGTA‐resistant response) or a low concentration of prazosin which left a contraction which was mediated by extracellular Ca2+ (prazosin‐resistant response).

The portal vein produced spontaneous rhythmic contractions and a sustained contraction to NA and KCl; however, all responses appeared to depend exclusively upon extracellular Ca2+.

In the aorta, contractions which might be expected to depend upon Ca2+‐entry through voltage‐operated channels (KCl‐induced contraction) showed similar sensitivities to Ver, Dlz and Flu whereas, marked differences in the sensitivity to these agents was noted against contractions which appear to depend upon Ca2+‐entry through receptor‐operated channels (prazosin‐resistant response). Only Dlz reduced contractions mediated by intracellular Ca2+ (EGTA‐resistant response).

In the portal vein, Ver and Dlz caused similar pronounced reductions of spontaneous and NA or KCl‐induced contractions. In contrast, these contractions of the portal vein were unaffected by Flu except at a concentration of 10 μM. However, contractions induced by addition of Ca2+ (0–14 mM) to previously depolarized portal veins could be reduced by Flu (100 nμ‐10 μM).

The present study indicates that in the rat aorta, contractions mediated by intracellular Ca2+ and depolarization or receptor‐activated Ca2+ entry can be pharmacologically dissected and that these processes show different sensitivities to calcium entry blockade. Of the agents tested, Ver displays the properties most commonly associated with an ideal calcium entry blocker. Ca2+‐activation mechanisms in the portal vein differ from those in the aorta resulting in a different spectrum of selectivity of the calcium entry blockers studied.
Failure of the inhibition of rat gastric mucosal 5‐lipoxygenase by novel acetohydroxamic acids to prevent ethanol‐induced damage
Authors: Boughton‐Smith N.K. , Whittle B.J.R.
Published: July 2012

The role of leukotriene B4 (LTB4) and LTC4 as mediators of gastric mucosal damage following ethanol challenge in vivo has been investigated using two selective 5‐lipoxygenase inhibitors, BW A4C and BW A137C.

Oral administration of ethanol to rats in vivo, induced macroscopic damage to the gastric mucosa and markedly increased the formation of the 5‐lipoxygenase products, LTB4 and LTC4, from the mucosa ex vivo.

Pretreatment with the acetohydroxamic acids BW A4C and BW A137C (5–50 mg kg−1 p.o.) dose‐dependently reduced ethanol‐stimulated LTB4 and LTC4 formation by the gastric mucosa, with an ID50 of approximately 5 mg kg−1 p.o.

A single oral dose of BW A4C (20 mg kg−1) induced near‐maximal inhibition of mucosal LTB4 formation within 30 min, which was well maintained for 5h, whereas BW A137C (20 mg kg−1 p.o.) induced maximal inhibition between 30 and 60 min after administration, which then diminished over the subsequent 5 h.

The mucosal formation of the cyclo‐oxygenase product, 6‐keto‐prostaglandin F1α, which was unaltered following ethanol challenge, was not inhibited by the acetohydroxamic acids. Likewise, the small increase in mucosal thromboxane B2 formation following challenge was not inhibited by BW A4C.

Neither BW A4C nor BW A137C, at doses that almost completely inhibited the mucosal synthesis of LTB4 or LTC4, reduced the macroscopic gastric mucosal damage induced by ethanol.

Pretreatment with the lipoxygenase inhibitor BW 755C (5–50 mg kg−1 p.o.) did reduce mucosal damage, but there was a dissociation between the degree of protection and the inhibition of leukotriene biosynthesis.

Oral administration of high doses of either BW A4C or BW A137C (300 mg kg−1) did not induce macroscopic gastric damage over a 3 h period.

These findings suggest that the leukotrienes, LTB4 and LTC4 are not the primary mediators of ethanol‐induced acute mucosal damage, but do not exclude their role in more chronic gastric damage and inflammation.
GABAA‐receptor‐mediated inhibition of the delayed increase in intragastric pressure to stimulation of vagal afferent fibres in cats
Authors: Okamoto Toshihiro , Kurahashi Kazuyoshi , Fujiwara Motohatsu , Oikawa Hiroshi
Published: July 2012

The possible involvement of γ‐aminobutyric acidA (GABAA)‐ and GABAB‐receptors in the inhibitory effects of GABA on the delayed increase in intragastric pressure of the stomach to stimulation of vagal afferent fibres in cats was studied.

Cats were anaesthetized with pentobarbitone‐gallamine and pretreated with hexamethonium. GABA inhibited the hexamethonium‐resistant delayed contraction of the stomach in a dose‐dependent manner. Such effects of GABA were antagonized by both bicuculline and picrotoxin.

Muscimol, a GABAA‐receptor agonist, mimicked the inhibitory effects of GABA and the effects of muscimol were antagonized by bicuculline and picrotoxin. The ID50 of muscimol was 10 times less than that of GABA.

In contrast to muscimol, baclofen, a GABAB‐receptor agonist did not mimic the inhibitory effects of GABA.

The present experiments demonstrate that GABAA‐receptors are involved in the inhibitory action of GABA on the delayed contraction of the stomach to vagal afferent stimulation.