Ranitidine potentiates ileum contractions caused by GABA and electrical stimulation

GABA-evoked contractions of the guinea pig ileum were significantly potentiated by the histamine H2-receptor antagonist ranitidine in concentrations above 10 microM. To help define the mechanism of this interaction, the present study compared the effects of ranitidine on contractile responses of the guinea pig ileum to GABA, acetylcholine (A Ch) and electrical stimulation of intrinsic cholinergic neurons. Ranitidine, at concentrations that potentiated responses to GABA, also potentiated contractions induced by transmural electrical stimulation. The ability of ranitidine to amplify these latter responses was antagonized by atropine. Contractile responses to exogenous A Ch, however, were unaffected by ranitidine at any concentration. These results suggest that prejunctional, rather than postjunctional mechanisms, are of primary importance in the interaction between ranitidine and GABA.     

Evidence for A1 and A2 adenosine receptors in guinea pig trachea

The adenosine analogs [5'-N-ethylcarboxamideadenosine (NECA), 2-Chloro-adenosine (2-ClA), R-phenylisopropyladenosine (R-PIA), N6-cyclohexyl adenosine (CHA), and N6-cyclopentyladenosine (CPA)] produced both relaxation and contraction responses in isolated guinea-pig trachea. A concentration-related relaxation response was observed in trachea which were precontracted with either histamine or KC1. This response followed an order of analog potency that was indicative of the A2 receptor subtype (NECA greater than 2-ClA greater than R-PIA greater than CPA greater than CHA). Theophylline, an adenosine-receptor antagonist, blocked this relaxation response. In addition, a concentration-related contractile response was produced with adenosine analogs in those trachea that were not previously contracted. In contrast, the contractile response followed an analog potency indicative of the A1 receptor subtype (R-PIA greater than 2-ClA = CPA = CHA). This contractile response was not mediated by cholinergic, adrenergic or histaminergic receptors. 2-ClA induced a biphasic response, while NECA only relaxed these tissue under basal tone. Unlike the relaxation response, these contractile responses were not attenuated by theophylline, but were blocked by 1,3 dipropyl-8-(2 amino-4-chlorophenyl)xanthine (PACPX). These findings confirm the existence of two subpopulations of adenosine receptors in guinea pig trachealis muscle.     

Is the Adenosine Receptor Modulation of Histamine-Induced Accumulation of Inositol Phosphates in Cerebral Cortical Slices Mediated by Effects on Calcium Ion Fluxes?

In this report, we show that under conditions designed to provide an initially uniform incorporation of [3H]inositol into mouse and guinea pig cerebral cortical slices prior to agonist stimulation, the accumulation of 3H-inositol phosphates (3H-InsPx, x = 1-4) induced by histamine in mouse and guinea pig cerebral cortical slices increased in a quasilinear manner with increasing added calcium. Raising the ambient calcium ion concentration failed to reduce the adenosine receptor-mediated inhibition of the histamine-induced 3H-InsPx response in mouse cerebral cortical slices. Similarly, the potentiation of the histamine response by adenosine receptor activation in guinea pig cerebral cortical slices was unaffected by lowering the added calcium ion concentration. The presence of the calcium ionophore A23187 (33 microM) produced 3H-InsPx responses in both mouse and guinea pig cerebral cortical slices, which were not affected by the presence of the stable adenosine analogue 2-chloroadenosine. A23187 also potentiated the accumulation of 3HInsPx induced by histamine in both species. Both the inhibitory and potentiatory modulations of the histamine response by 2-chloroadenosine in mouse and guinea pig, respectively, were still apparent in the presence of A23187. These results indicate that the histamine-induced 3H-InsPx accumulations in both mouse and guinea pig cerebral cortical slices are sensitive to variations in calcium ion concentrations. However, the adenosine receptor modulations of the histamine responses are relatively insensitive to fluctuations in either extra- or intracellular calcium ion concentrations, and thus cannot be mediated by effects on calcium ion movements.     

Contractile responses to adrenergic nerve stimulation are enhanced with removal of endothelium in rat caudal artery

Removal of the endothelium from isolated perfused rat caudal arteries produced a two fold increase in the contractile response to transmural nerve stimulation. Pretreatment with 6-hydroxydopamine eliminated the contractile response to adrenergic nerve stimulation but failed to uncover any vasodilatory effect of electrical stimulation, either directly on smooth muscle or via non-adrenergic nerves. Endothelial removal also produced two and four fold enhancement of the contractile responses to the selective alpha 1- and alpha 2-adrenoceptor agonists methoxamine and B-HT 920. However, pKB values for prazosin and yohimbine versus both agonists indicate that both methoxamine and B-HT 920 are acting primarily at alpha 1-adrenoceptors in this tissue. These results provide evidence that endothelial factors released either at basal levels or by the stimulation of agonists play a significant physiological role in modifying the contractile responses of blood vessels.     

A comparative and regional study of potassium induced relaxation of vascular smooth muscle

Summary The current study was undertaken to assess species and regional variations in the relaxation of vascular smooth muscle in response to potassium and in the ouabain sensitivity of this relaxation. The effect of species variation was investigated through the use of tail arteries from rats, dogs, cats, monkeys, and pigs; the effect of regional variation was studied in tail, middle cerebral, femoral, and posterior coronary arteries from baboons. Helical strips from all of these vessels were made to contract with norepinephrine or serotonin in a potassium-free solution. The vessels relaxed when potassium was added back to the solution. Strips of tail artery from rats, dogs, and monkeys showed greater relaxation in response to potassium than did strips from pigs and cats. Helical strips from tail, cerebral, and coronary arteries of the baboon relaxed to a greater degree in response to potassium than did strips from the femoral artery. Ouabain produced a concentration-dependent decrease in the magnitude of potassium relaxation in all vessel types. Half-maximal inhibition occurred at approximately 10^–8 to 10^–7 M in all arterial strips except those obtained from rat tail artery (5×10^–5 M). The inhibition of potassium relaxation by ouabain was fully reversed by 30 min exposure to a ouabain-free solution in only the rat tail artery strips. A component of potassium-induced relaxation in tail artery strips from monkeys and baboons was not inhibited by ouabain. The results show that the magnitude of response, potassium and ouabain sensitivity, and recovery from ouabain treatment of potassium relaxation are species related. The regional bed from which the vascular smooth muscle is derived also determines the magnitude and potassium sensitivity of the relaxation. These parameters of potassium-dependent relaxation may reflect corresponding differences in the electrogenic pumping of sodium and potassium among various animal species and various regional vascular beds.Abbreviations ATPase adenosine triphosphatase - PSS physiological salt solution - C contractile magnitude from baseline in milligrams - R relaxation measured as residual force above baseline in milligrams - SEM standard error of the mean These studies were supported by NHLBI grant HL-18575Dr. Webb was a Post-doctoral Research Fellow of the Michigan Heart Association during this investigation     

Neuropeptide Y co-exists and co-operates with noradrenaline in perivascular nerve fibers

Neuropeptide Y (NPY)-immunoreactive nerve fibers were numerous around arteries and few around veins. NPY probably co-exists with noradrenaline in such fibers since chemical or surgical sympathectomy eliminated both NPY and noradrenaline from perivascular nerve fibers and since double staining demonstrated dopamine-beta-hydroxylase, the enzyme that catalyzes the conversion of dopamine to noradrenaline, and NPY in the same perivascular nerve fibers. Studies on isolated blood vessels indicated that NPY is not a particularly potent contractile agent in vitro. NPY greatly enhanced the adrenergically mediate contractile response to electrical stimulation and to application of adrenaline, noradrenaline or histamine, as studied in the isolated rabbit gastro-epiploic and femoral arteries. The potentiating effect of NPY on the response to electrical stimulation is probably not presynaptic since NPY affected neither the spontaneous nor the electrically evoked release of [3H]noradrenaline from perivascular sympathetic nerve fibers.     

Neuropeptide Y and vasoactive intestinal polypeptide in cerebral arteries of the rat: relationships between innervation pattern and mechanical response

Possible relationships between the density of peptide innervation and the contractile response of rat cerebral arteries to exogenously applied neuropeptide Y (NPY) and vasoactive intestinal polypeptide (VIP) were examined. The effects of NPY on membrane potential and reactivity of cerebral arteries to exogenous norepinephrine also were studied. In normally innervated arteries there was no apparent correlation between degree of innervation and response to NPY. Marked, prolonged tachyphylaxis to NPY and VIP was observed following brief exposure to these peptides. Surgical removal of the superior cervical ganglia or the sphenopalatine ganglia greatly reduced and, in some cases, eliminated NPY- or VIP-immunoreactive perivascular nerves from cerebral arteries. However, responses of denervated middle cerebral arteries to exogenous NPY or VIP were not different from responses of innervated arteries. Doses of NPY that induced maximal contraction caused no change in membrane potential of the middle cerebral artery. NPY also did not alter the response of cerebral arteries to exogenous norepinephrine. Finally, electrical stimulation of normal or denervated arteries caused only minor constrictor or dilator responses. These results do not support a substantial role for peptidergic perivascular nerves in regulation of pial arterial contractility in the rat.     

Histamine dependence of pentagastrin-stimulated gastric acid secretion in rats.

Does gastrin stimulate gastric acid secretion by direct action on oxyntic cells, by releasing histamine, or by being potentiated by histamine? Previous studies in the mouse pointed to gastrin-regulated histamine release. Guinea pig and rat are well known to vary in their sensitivity to histamine. Therefore, the effects of histamine and pentagastrin were compared quantitatively on isolated, lumen-perfused, stomach preparations from these species in the absence and presence of histamine H2-receptor blockade. The loss of potency of histamine in the rat was mirrored by a loss of potency of pentagastrin consistent with the idea that pentagastrin acts by releasing histamine. In the rat, a well-defined pentagastrin curve was obtained in the presence of histamine H2-receptor block as though pentagastrin acts both directly on the oxyntic cell and indirectly by releasing histamine. It was not necessary to invoke a potentiating interaction between histamine and pentagastrin at the oxyntic cell; the two effects appeared simply to add. Potentiation was observed, however, between other combinations of stimuli, for example, between vagal nerve and pentagastrin stimulation. The physiological consequences of these results are discussed.     

Cyclic AMP-Generating Systems in Cell-Free Preparations from Guinea Pig Cerebral Cortex: Loss of Adenosine and Amine Responsiveness Due to Low Levels of Endogenous Adenosine

The accumulations of radioactive cyclic AMP elicited by adenosine, norepinephrine, and histamine in adenine-labeled vesicular entities of a particulate fraction from guinea pig cerebral cortex are greatly reduced as a result of prolonged preincubation. The presence of adenosine deaminase during preincubations largely prevents the loss of adenosine, norepinephrine and histamine responses. Adenosine deaminase was inactivated by deoxycoformycin prior to stimulation of cyclic AMP accumulation by adenosine or amines. If adenosine deaminase is not inactivated, responses to norepinephrine are not significant and histamine responses are reduced by 50%. Adenosine deaminase cannot restore responsiveness of the cyclic AMP-generating systems. It is proposed that, in particulate fractions of guinea pig cerebral cortex, low levels of adenosine cause a slow loss of receptors and/or coupling of receptors to cyclic AMP-generating systems.     

The inhibitory actions of acebutolol and propranolol on the contractile response to 5-hydroxytryptamine in various isolated vascular smooth muscles

Pretreatment with acebutolol or propranolol at high concentrations had an inhibitory effect on the contractile response to 5-hydroxytryptamine (5-HT) in most vascular smooth muscles such as rabbit aorta and basilar, mesenteric, renal, femoral arteries and cat coronary artery. The inhibitory actions of both agents were generally greater than on the responses to excess Ca2+ and potassium. In rabbit renal arteries, acebutolol had no effect on the response to 5-HT but inhibited the responses to excess Ca2+ and potassium. Propranolol had a marked inhibitory effect on the response to 5-HT. In all preparations used, the contractions induced by norepinephrine (NE) and histamine showed a much greater resistance to the effect of acebutolol and propranolol than the contractions induced by 5-HT, Ca2+ and potassium. Nifedipine had no inhibitory effect on the response to 5-HT in most of the preparations. Nifedipine inhibited the response to 5-HT only in the basilar arteries. The inhibitory actions of propranolol on the response to 5-HT was greater than that of acebutolol. The inhibitory action of acebutolol and propranolol on the response to 5-HT may be related to mechanisms other than the beta-adrenoceptor blocking action of the drugs. The possible mechanisms of inhibitory action of both beta-adrenoceptor antagonists on 5-HT are discussed.     

Effect of melatonin on neurogenic vasoreactivity: formation and modulation of the vessel response

The effect of melatonin (0.1 microM) on the contractile response to electrical field stimulation (EFS) of the juvenile rat tail artery segment was studied. The spontaneous decrease in reactivity of the segment observed in the course of our experiments was accompanied with a melatonin-evoked increase in the reactivity which was proportional to this decrease and was not connected with a sensitization of the segment to the substance. Perfusion of the segment with an acidic solution leads to a more pronounced inhibition of the response as well as to a greater melatonin-evoked protentiation of the response. Noradrenaline-evoked response was not augmented by melatonin. The results suggest that the potentiating effect of melatonin on the EFS-evoked response of the juvenile rat tail artery depends on degree of the change in the artery reactivity and was not due to change in sensitivity of postjunctional membrane to noradrenaline.     

Epithelial modulation of allergen and drug effects in guinea pig airways.

The effect of egg albumin (EA) challenge on tracheal tube preparations from sensitized guinea pigs was studied with regard to EA permeability, histamine release and penetrability, and the contractile response of the preparation. We used a plethysmographic method that allowed simultaneous measurement of changes in smooth muscle tension and collection of samples for determination of mediators. Our results clearly show that epithelial damage potentiates the contractile response to histamine, potassium ions, and acetylcholine. Epithelial damage did not alter the maximal contractile response in preparations challenged with high antigen concentrations (EA, 1 mg/ml), but histamine release measured in the perfusate increased substantially. The permeability of the preparations to EA was greater when the epithelium was damaged. No increase in the permeability in response to the EA challenge was observed. The present study has demonstrated that guinea pig airway epithelium constitutes a barrier for both antigen and drugs. We also present a method for recording contractile responses from intact whole tracheal preparations, in which the epithelium can still act as a barrier, as is the case in vivo.     

Depolarizing agent-induced cyclic AMP accumulation in isolated rat spinal cord

The stimulation of cyclic adenosine 3',5'-monophosphate (cyclic AMP) accumulation by the depolarizing agents K+, ouabain and veratridine, was studied in rat and guinea pig spinal cord tissue slices. Significantly increased accumulation of cyclic AMP was produced by each of the agents in a concentration-dependent manner. Veratridine and ouabain were equipotent (EC50 = 5 x 10(-5)M) and approximately 500 fold more potent than K+ (EC50 = 10(-2)M). Depolarizing agent-induced cyclic AMP accumulation in slices from guinea pig spinal cord was approximately double the response in rat spinal cord. Maximum stimulation occurred within 2.5 min of incubation with these agents and lasted for at least 30 min. Regional studies demonstrated that the maximal accumulation of cyclic AMP occurred to a greater degree in tissue slices from the dorsal section of spinal cord from both rat and guinea pig. Whereas the ouabain and veratridine stimulatory responses are completely dependent on extracellular Ca++, the K+ response is only partially dependent. Stimulation due to ouabain and veratridine is dependent, and K+ is independent, of release of neurohumoral substances such as norepinephrine or adenosine from spinal neurons. These experiments indicate the possible modulatory role of depolarization-linked events in regulating the spinal cord cyclic AMP system.     

Adenosine A(2a)-receptor activation increases contractility in isolated perfused hearts

Adenosine A(2a)-receptor activation enhances shortening of isolated cardiomyocytes. In the present study the effect of A(2a)-receptor activation on the contractile performance of isolated rat hearts was investigated by recording left ventricular pressure (LVP) and the maximal rate of LVP development (+dP/dt(max)). With constant-pressure perfusion, adenosine caused concentration-dependent increases in LVP and +dP/dt(max), with detectable increases of 4.1 and 4.8% at 10(-6) M and maximal increases of 12.0 and 11.1% at 10(-4) M, respectively. The contractile responses were prevented by the A(2a)-receptor antagonists chlorostyryl-caffeine and aminofuryltriazolotriazinyl-aminoethylphenol (ZM-241385) but were not affected by the beta(1)-adrenergic antagonist atenolol. The adenosine A(1)-receptor antagonist dipropylcyclopentylxanthine and pertussis toxin potentiated the positive inotropic effects of adenosine. The A(2a)-receptor agonists ethylcarboxamidoadenosine and dimethoxyphenyl-methylphenylethyl-adenosine also enhanced contractility. With constant-flow perfusion, 10(-5) M adenosine increased LVP and +dP/dt(max) by 5.5 and 6.0%, respectively. In the presence of the coronary vasodilator hydralazine, adenosine increased LVP and +dP/dt(max) by 7.5 and 7.4%, respectively. Dipropylcyclopentylxanthine potentiated the adenosine contractile responses with constant-flow perfusion in the absence and presence of hydralazine. These increases in contractile performance were also antagonized by chlorostyryl-caffeine and ZM-241385. The results indicate that adenosine increases contractile performance via activation of A(2a) receptors in the intact heart independent of beta(1)-adrenergic receptor activation or changes in coronary flow.     

Pharmacological actions of tentacle extract of the jellyfish, Acromitus rabanchatu, occurring in the Bay of Bengal

Tentacle extract of A.rabanchatu, produced a fall of blood pressure in cat, rat and guinea pig. Hypotension produced in cat remained unantagonized by blockers of acetylcholine, histamine and 5-HT. On isolated guinea pig heart, the extract significantly reduced the rate and amplitude of contraction leading to irreversible cardiac arrest. In cats and rats, the respiratory rate and amplitude was decreased significantly and resulted in temporary apnoea. The extract also produced vasoconstriction in perfused rat hindquarter preparation and increased cutaneous capillary permeability. The extract produced contraction in several isolated smooth muscle preparations. Contraction on guinea pig ileum was partly antagonized by atropine and cyproheptadine. On isolated rat phrenic nerve diaphragm and chick biventer cervicis, the extract produced irreversible blockade of the electrical stimulation-induced twitch responses. Haemolytic and myonecrotic activity was exhibited by the extract. LD50 was found to be 7.7 mg/kg (iv, mice).     

Effect of L-prolyl-L-leucyl-glycinamide (MIF-I) on some neutrotransmitter-receptor interactions

Some neurotransmitter-receptor interactions have been studied in an attempt to determine how L-prolyl-L-leucyl-glycinamide (MIF-I) exerts its antiparkinson effect. MIF-I affected neither the contractile responses of isolated mouse vas deferens and guinea pig ileum to noradrenaline, acetylcholine, substance P and histamine, nor the inhibitory effects of dopamine and GABA on the rat vas deferens and guinea pig ileum. MIF-I, as well as L-leucine and Pro-Leu, antagonized the contractile response of the ileum to 5-hydroxytryptamine (5-HT). Behavioural tests were used to examine the action of MIF-I on CNS transmitter-receptor interactions. MIF-I did not modify the circling produced by either dopamine agonists in nigro-striatal lesioned rats of 5-HT agonists in rats with a lesion of the medial raphe nucleus. MIF-I affected neither 5-hydroxytryptophan-induced head twitches in mice, which is a measure of 5-HT receptor stimulation, nor striatally-evoked head turning in the rat, which is a model for brain GABA function. It is concluded that MIF-I, at the doses used, does not directly modify the function of any of the CNS transmitter examined. Other possibilities to explain its antiparkinson action are discussed.     

Effect of streptozotocin diabetes on motor and inhibitory transmission in rat anococcygeus.

The effect of streptozotocin diabetes of 4-week duration on the adrenergic motor transmission and on the nonadrenergic, noncholinergic, inhibitory transmission in the rat anococcygeus was investigated by recording contractile and relaxant activity of isolated muscle preparations taken from diabetic and age-matched control animals. The neurogenic contractile responses to electrical field stimulation were significantly reduced in the preparations from diabetic rats. The inhibitory transmission remained unaffected in the diabetic rats. Concentration--response curves showed no change in sensitivity of the diabetic anococcygei to noradrenaline. The maximum tension generated was also similar in preparations from diabetic and nondiabetic animals. The contractile responses to electrical field stimulation were significantly greater in preparations from diabetic rats treated for 4 weeks with either sorbinil (20 mg.kg-1.day-1 orally) or myo-inositol (667 mg.kg-1.day-1 orally) when compared with the untreated diabetic controls; the sensitivity to noradrenaline was identical in all three groups. It is concluded that streptozotocin diabetes causes a significant reduction of adrenergic contractile responses of the anococcygeus to electrical field stimulation by a prejunctional mechanism, and the reduction can be prevented by treating the animals with the aldose reductase inhibitor sorbinil or with myo-inositol.     

Effects of serotonin1-like receptor agonists on autonomic neurotransmission.

Serotonin1A receptor agonists, 8-hydroxy-2-(di-n-propylamino)tetralin and 10-methyl-11-hydroxyaporphine, inhibited electrical stimulation-induced contraction of the guinea-pig ileum. These agonists also inhibited the pressor and tachycardiac responses to low frequency (0.25 Hz) but not to high frequency (2.0 Hz) electrical stimulation of the sympathetic nervous system in pithed rats. Serotonin1B receptor agonist RU 24969 inhibited pressor and tachycardiac responses to both low and high frequencies of stimulation in pithed rats. In the cat nictitating membrane, serotonin1A receptor agonists did not alter contractions elicited by electrical stimulation (0.1-3.0 Hz). Serotonin not only contracted the cat nictitating membrane but also facilitated contractile responses to low frequency (0.1-1.0 Hz) stimulation. The contractile effect of serotonin in the cat nictitating membrane was blunted by bretylium, methysergide, and ketanserin, but not by metoclopramide. The facilitatory effect of serotonin was antagonized by methysergide, but not by ketanserin, pindolol, propranolol, or metoclopramide. These results suggest that serotonin1A receptors modulate autonomic neurotransmission in the guinea-pig ileum and pithed rats, but not in the cat nictitating membrane. Serotonin contracts the cat nictitating mebrane via serotonin2 subtypes, while facilitating stimulated contractile responses through the serotonin1-like receptors.     

Chloride channel activity of vascular smooth muscle in the spontaneous hypertensive rats

To characterize the activity of the Ca2+-activated Cl- channels in vascular smooth muscle (VSM) of the spontaneous hypertensive rats (SHR), the isolated mesenteric vascular beds and tail artery strips were preparated from SHR and Wistar rats aged 7-8 weeks. The changes in contractile response to norepinphrine (NE) were taken as an index of vascular mortion. Results showed that the contractile responses of mesenteric arteries and tail arteries to NE in SHR were significantly greater than that in Wistar rats. The inhibition magnitude of the contractile response by Ca2+-activated Cl- channel blocker, niflumic acid in SHR was significantly less than that in Wistar rats. Decreasing the extracellular Cl- concentration increased the contractile response to NE significantly, but the amplitude of enhanced contractile response in SHR was greater than that in Wistar rats. It can be concluded that NE-induced contraction was enhanced in SHR, which is partly due to an increase in Cl- efflux through the Ca2+-activated Cl- channels. The chloride channel activity may be increased in association with the elevation of blood pressure.     

Isolation, partial purification and pharmacodynamics of a slow contractile substance in the venom sac extract of the wasp Vespa cincta Fabr

The venom of V. cincta contains acetylcholine (ACh), histamine and 5-hydroxytryptamine (5-HT). Blockers of these agonists did not block completely the hypotensive and smooth muscle contractile activity of venom. On smooth muscle, there was a residual slow contraction. The active substance which produced this slow contraction was separated by solvent extraction, gel filtration and TLC. The purified material (which has been provisionally designated "Vecikinin") lowered cat, rat and guinea pig blood pressure, increased amplitude of cardiac contraction, and increased capillary permeability. Vecikinin contracted several smooth muscle preparations (rat uterus, rat ascending colon, guinea pig ileum, guinea pig colon and rat ileum), while relaxing rat duodenum. Its contractile activity was not lost on boiling, but acid or alkali-boiling reduced its contractile activity. It was inactivated on incubation with chymotrypsin and carboxypeptidase but not with trypsin, pepsin or leucine aminopeptidase. It is a peptide, appears to be of low molecular weight, and could be distinguished from substance P, angiotensin, bradykinin and hornet or wasp kinin.