Activation of neural cholecystokinin-1 receptors induces relaxation of the isolated rat duodenum which is reduced by nitric oxide synthase inhibitors

Cholecystokinin (CCK) influences gastrointestinal motility, by acting on central and peripheral receptors. The aim of the present study was to determine whether CCK has any effect on isolated duodenum longitudinal muscle activity and to characterize the mechanisms involved. Isolated segments of the rat proximal duodenum were mounted for the recording of isometric contractions of longitudinal muscle in the presence of atropine and guanethidine. CCK-8S (EC50: 39; 95 percent CI: 4.1-152 nM) and cerulein (EC50: 58; 95 percent CI: 18-281 nM) induced a concentration-dependent and tetrodotoxin-sensitive relaxation. Nomeganitro-L-arginine (L-NOARG) reduced CCK-8S- and cerulein-induced relaxation (IC50: 5.2; 95 percent CI: 2.5-18 æM) in a concentration-dependent manner. The magnitude of 300 nM CCK-8S-induced relaxation was reduced by 100 æM L-NOARG from 73 ± 5.1 to 19 ± 3.5 percent in an L-arginine but not D-arginine preventable manner. The CCK-1 receptor antagonists proglumide, lorglumide and devazepide, but not the CCK-2 receptor antagonist L-365,260, antagonized CCK-8S-induced relaxation in a concentration-dependent manner. These findings suggest that CCK-8S and cerulein activate intrinsic nitrergic nerves acting on CCK-1 receptors in order to cause relaxation of the rat duodenum longitudinal muscle.

Role of non-nitric oxide non-prostaglandin endothelium-derived relaxing factor(s) in bradykinin vasodilation

The most conspicuous effect of bradykinin following its administration into the systemic circulation is a transient hypotension due to vasodilation. In the present study most of the available evidence regarding the mechanisms involved in bradykinin-induced arterial vasodilation is reviewed. It has become firmly established that in most species vasodilation in response to bradykinin is mediated by the release of endothelial relaxing factors following the activation of B2-receptors. Although in some cases the action of bradykinin is entirely mediated by the endothelial release of nitric oxide (NO) and/or prostacyclin (PGI2), a large amount of evidence has been accumulated during the last 10 years indicating that a non-NO/PGI2 factor accounts for bradykinin-induced vasodilation in a wide variety of perfused vascular beds and isolated small arteries from several species including humans. Since the effect of the non-NO/PGI2 endothelium-derived relaxing factor is practically abolished by disrupting the K+ electrochemical gradient together with the fact that bradykinin causes endothelium-dependent hyperpolarization of vascular smooth muscle cells, the action of such factor has been attributed to the opening of K+ channels in these cells. The pharmacological characteristics of these channels are not uniform among the different blood vessels in which they have been examined. Although there is some evidence indicating a role for KCa or KV channels, our findings in the mesenteric bed together with other reports indicate that the K+ channels involved do not correspond exactly to any of those already described. In addition, the chemical identity of such hyperpolarizing factor is still a matter of controversy. The postulated main contenders are epoxyeicosatrienoic acids or endocannabinoid agonists for the CB1-receptors. Based on the available reports and on data from our laboratory in the rat mesenteric bed, we conclude that the NO/PGI2-independent endothelium-dependent vasodilation induced by BK is unlikely to involve a cytochrome P450 arachidonic acid metabolite or an endocannabinoid agonist.

Characterization of nerve-induced relaxation of gastrointestinal sphincteric smooth muscle in a South American opossum (Didelphis albiventris)

The presence of inhibitory nonadrenergic noncholinergic (NANC) intrinsic innervation of the circular muscle of the gastrointestinal sphincters of the South American (SA) opossum was investigated in vitro. Isolated circular muscle strips from the esophagogastric and ileocolonic junctions but not from the gastroduodenal (pylorus) region developed spontaneous tension. Tetrodotoxin (TTX, 1 muM) augmented the spontaneous tension only in the ileocolonic junction strips. Electrical field stimulation of esophagogastric and ileocolonic junction strips caused frequency-dependent responses consisting of a relaxation at lower frequencies (<1 Hz) and a biphasic response or contraction at higher frequencies. In the strips from the pyloric region electrical field stimulation abolished the spontaneous activity at lower frequencies and induced contractions at higher frequencies. The responses elicited by electrical field stimulation in the three sphincters were abolished by TTX (1 muM). Electrical field-induced contractions were reduced while relaxations were enhanced by atropine (1 muM). In the presence of atropine (1 muM) and guanethidine (3 muM), electrical field stimulation, nicotine and ATP induced frequency-or concentration-dependent relaxations of the three sphincters that were abolished by TTX (1 muM). Isoproterenol and sodium nitroprusside caused concentration-dependent relaxations which were TTX-resistant. These findings indicate that the sphincteric circular muscle of the SA opossum gastrointestinal tract is relaxed by the activation of intrinsic NANC nerves and therefore can be used as a model for the study of the mechanisms involved in these responses.

ANP as a neuroendocrine modulator of body fluid homeostasis

The role played by the central nervous system (CNS) in the control of body fluid homeostasis has been demonstrated by several authors. The AV3V plays a key role in central control of sodium excretion since its cholinergic, adrenergic, angiotensinergic and osmotic stimulation enhances and its destruction blocks sodium excretion in rats and goats. Cholinergic stimulation of the AV3V induced an increase in plasma ANP as well as a marked elevation in content of the peptide in medial basal hypothalamus, neuro and adenohypophysis. On the other hand, a decline in plasma ANP after AV3V lesions was accompanied by dramatic declines in content of ANP in these same structures. Our previous work has also indicated the essential role of the AV3V region and its ANPergic neurons in the control of ANP release in response to volume expansion (BVE) and indicated that alpha-adrenergic and muscarinic receptors are critical in mediating these responses. Lesions of the AV3V region, or of the median eminence or posterior lobe of pituitary gland blocked the increase in plasma ANP concentration in response to BVE. That this effect is related to blockage of the activity of the brain ANPergic neurons is supported by fyndings in sheep and in rats that the injection of the antiserum directed against ANP into the AV3V region at least partially blocked the BVE-induced release of ANP. We and others have also previously shown that denervation of baroreceptors inhibits ANP release induced by BVE. Activation of the ANP neurons also cause release of ANP from the anterior and neural lobe of pituitary gland. ANP neurons may activate oxytocinergic neurons in the supraoptic and paraventricular, which projects to neural lobe. Oxytocin would circulate to the atria and may directly activate release of ANP from the atrial myocytes, since i.v. or i.p. injection of oxytocin increases sodium excretion as well as elevates plasma ANP. Oxytoxin is present in the neural lobe in large quantity, which could reach the atria myocytes in high concentration and release ANP that circulate to the kidneys and evokes natriuresis to return circulating blood volume to normal.

Effect of chronic estradiol administration on the acute pressor response to aortic coarctation in conscious rats

We investigated the effect of chronic estradiol administration on the pressor response elicited by acute (45 min) partial aortic constriction in conscious Wistar rats and on vascular reactivity to angiotensin II and vasopressin in vitro. Estradiol or vehicle was administered for 7 days to young castrated male and female rats and to female rats that had stopped cycling. In the acute experiment of aortic coarctation in concious rats, carotid pressure was monitored continuously before and for 45 min after partial abdominal aortic coarctation. In ovariectomized females the mean carotid pressure and heart rate before aortic coarctation were significantly lower in estradiol treated animals. Estradiol did not affect the pressor response to aortic coarctation of castrated male rats or ovariectomized female rats but blunted the reflex bradycardia of ovariectomized rats. The onset of the pressor response to aortic coarctation was delayed in aged female rats as compared to the other groups. While estradiol treatment significantly accelerated the onset of hypertension in aged rats, it did not affect the pressor response of castrated animals. Full dose-response curves to angiotensin II and vasopressin were constructed in vitro in the isolated mesenteric arterial bed obtained from similary treated groups. Estradiol did not affectthe vasopressin sensitivity or responsiveness of any group, but caused a significant increase in angiotensin II sensitivity in ovariectomized rats only. In conclusion, these data slow that chronic estradiol administration ot aged female rats accelerate the installation of the pressor response to acute aortic coarctation. In addition, estradiol administration to ovariectomized rats is associated with lower blood pressure and heart rate.

Evidence for the participation of the L-arginine-nitric oxide pathway in neurally induced relaxation of the isolated rat duodenum

1. Electrical field stimulation (EFS) of intrinsic nerves in the rat proximal duodenum induces a frequency-dependent non-adrenergic-non-cholinergic (NANC) relaxation response. 2. The inhibitors of L-arginine-NO synthase L-NG-nitro arginine methyl-ester (L-NAME) and L-NOARG (L-NG-nitro arginine) reduced the NANC relaxations elicited by EFS in a dose- and time-dependent manner; L-NOARG was two times more potent than L-NAME (IC50 = 14.3 vs 25.2 microM) and these effects were partially reverted by the addition of 300-1000 microM L-arginine but not of 300-1000 microMD-arginine. Relaxation caused by vasoactive intestinal peptide (VIP; 0.1 microM) or ATP (20 microM) was not blocked by L-NAME or L-NOARG. 3. The magnitude of the blockade caused by L-NAME and L-NOARG was dependent on the frequency of stimulation. At low frequencies (below 1 Hz) both L-NAME and L-NOARG abolished the relaxations, while at 2 to 8 Hz only partial inhibition was observed (maximal inhibition: 44.6 per cent +/- 5.2 and 63.4 per cent +/- 3.4, respectively) 4. The basal tonus of the duodenum was increased by 10-300 microM L-NAME and 10-300 microM L-NOARG and this effect was blocked by 1 mM L-arginine. 5. Nitric oxide generated from acidified NaNO2 caused a dose-dependent (EC50 = 2.75 microM) relaxation of the duodenum which was not affected by 100 microM L-NAME, 100 microM L-NOARG or 1 microM tetrodotoxin (TTX). 6. NADPH-diaphorase positive neurons and fibers identified by histochemistry were present in the myenteric plexus and along both circular and longitudinal muscle fibers indicating that nitric oxide could be synthetized by these neural structures

Evidence for the involvement of nitric oxide in the electrically induced relaxations of human lower esophageal sphincter and distal pylorus

The aim of the present study was to investigate in vitro the effect of Ng-nitro-L-arginine methyl ester (L-NAME), a nitric oxide (NO) synthase inhibitor, on neurogenic relaxation of human lower esophageal sphincter (LES) and distal pylorus (DP) circular muscle strips induced by electr4ical field stimulation (EFS). Muscle strips obtained from 5 patients who underwent total gastrectomy were suspended in 10-ml organ baths containing Krebs solution for recording isometric tension. L-NAME(30 uM) reduced the amplitude of the EFS-induced relaxation by 85 ñ 9% (N=3) in the LES and by 52 ñ 16% (N=3) in the DP but did not affect sodium nitroprusside-induced relaxation. L-Arginine (300 uM) partially reversed the L-NAME inhibition in the LES and totally in the DP. These findings suggest a role for L-arginine-derived NO in the nerve-mediated NANC relaxation of the human LES and DP