Vagal release of IR-VIP and IR-gastrin from the isolated perfused rat stomach

Vagal control of the release of somatostatin, vasoactive intestinal polypeptide, gastrin-releasing peptide, and HCl from porcine non-antral stomach

We studied the secretion of somatostatin and HCl and the release of vasoactive intestinal polypeptide (VIP) and gastrin-releasing peptide (GRP) from isolated, vascularly perfused, porcine non-antral stomach. Electric vagus stimulation increased acid secretion and the release of VIP and GRP and inhibited somatostatin secretion as determined in the venous effluent. Atropine abolished the HCl response and reversed the somatostatin inhibition to a three-fold increase, whereas GRP and VIP responses were unchanged. Both intra-arterial carbachol (10(-6) M) and GRP (10(-8) M) increased acid secretion and inhibited somatostatin secretion. VIP (10(-8) M) increased somatostatin secretion and had no effect on acid secretion. By immunohistochemistry, somatostatin was localized to both open-type and closed-type cells equally spread in the various parts of the gastric glands without particular relation to the parietal cells. Numerous GRP- and VIP-immunoreactive nerve fibers were seen between the glands. It is concluded that the fundic and antral secretion of somatostatin, investigated in a previous study, are differently regulated. The relation of fundic somatostatin release to acid secretion seems to be complex.

Extrinsic control of the release of galanin and VIP from intrinsic nerves of isolated, perfused, porcine ileum

By immunohistochemistry galanin-like immunoreactivity and vasoactive intestinal polypeptide (VIP)-like immunoreactivity were found in nerve cell bodies mostly in the submucous plexus and in nerve fibres in the mucosa, submucosa and muscularis including the myenteric plexus of the porcine ileum and were found to co-exist in most of these structures. Using isolated, perfused porcine ileum we studied the release of galanin and VIP in response to electrical stimulation of the mixed periarterial nerves or to intraarterial infusions of different neuroactive agents. Nerve stimulation (4-10 Hz) inhibited the basal release of galanin and VIP from the ileum (to 69 +/- 6 and 62 +/- 6% of basal release). After infusion of the alpha-adrenergic blocker, phentolamine, (10(-6) M) electrical stimulation increased the release of both galanin and VIP (to 140 +/- 12 and 133 +/- 13% of basal output). This increase was abolished by atropine (10(-6) M) and by hexamethonium (3.10(-5) M). Infusion of norepinephrine (10(-6) M) inhibited, whereas acetylcholine (10(-6) M) stimulated the release of both peptides. The effect of the latter was abolished by atropine. The inhibitory effect of nerve stimulation was not influenced by atropine. Our results suggest that the galanin- and VIP-producing intrinsic neurons receive inhibitory signals by noradrenergic nerve fibers and stimulatory signals mediated by cholinergic nerves, possibly via a cholinergic interneuron.

The effect of glucose and insulin on vagally induced gastrin, bombesin-like immunoreactivity and somatostatin secretion from the perfused rat stomach

Previous studies in the isolated perfused rat stomach have shown that elevated glucose and insulin concentrations modulate BLI and somatostatin release during arterially administered peptidergic stimuli. In the present study the effect of elevated levels of glucose or insulin was examined on vagally induced changes of gastrin, somatostatin and BLI secretion. The lumen of the stomach was perfused with saline pH 7 or pH 2. Vagal stimulation (5 Hz, 1 msec, 10V) increased gastrin and BLI secretion and inhibited somatostatin release. The increase of the perfusate glucose concentration from 100 mg/dl to 150 or 300 mg/dl or the addition of insulin (100 microU/ml) augmented vagally stimulated gastrin release at luminal pH 7 but not pH2. Vagally induced inhibition of somatostatin was attenuated by both concentrations of glucose at either luminal pH while insulin had no effect. BLI secretion was affected neither by elevated glucose nor by insulin. On the other hand, the noncholinergic component of vagally induced BLI secretion in the presence of atropine was augmented by insulin. These data demonstrate that glucose and insulin can modulate vagally activated gastric neuroendocrine functions which could be of relevance during the ingestion of carbohydrate containing meals.

Vagal afferent innervation and regulation of gastric function

In this article, we have presented evidence that vagal capsaicin-sensitive afferent fibers are involved in the regulation of gastric mucosal and motor function. Gastric acid secretion stimulated by gastric distension, histamine and central injection of TRH analog are all partly dependent on vagal capsaicin-sensitive afferent mechanisms. It is possible that as vagal efferent activity releases histamine, the common final pathway is the reduction in the response to histamine. At present, it is unclear as to the mechanism by which capsaicin-sensitive afferents are involved in the secretory response to histamine. With regard to the gastric acid and mucosal blood flow responses to TRH, it is not clear whether the sensory neurons represent a component of the efferent pathway that is activated by TRH or whether their role is to set the sensitivity of, or exert feedback control on this efferent pathway. As perineural capsaicin application decreases peptide content in the peripheral terminal fields of sensory neurons and these peptides may produce local effector functions within the tissue, it is possible that alterations in the gastric responses to TRH result from a decrease in the local effector functions of vagal neurons. From the experiments on electrical stimulation of the vagus nerve, it is evident that antidromic stimulation of vagal afferents can stimulate gastric mucosal blood flow, although under these experimental conditions there was no evidence for a capsaicin-sensitive stimulation of gastric acid secretion. The physiological relevance of this stimulation of gastric mucosal blood flow is at present unclear, but it is possible that physiological stimuli, such as distension or nutrients, may stimulate afferents and signal for an increase in gastric mucosal blood flow. In addition, pathophysiological or noxious stimulation of vagal afferents may also signal for an increase in gastric mucosal blood flow and may play a role in the response of the mucosa to injury.

Identification of vagal efferent fibers and putative target neurons in the enteric nervous system of the rat

The stomach and small intestine receive an efferent innervation from the dorsal motor nucleus of the vagus (DMX). The current experiments were undertaken as a partial test of the hypothesis that the CNS innervates only a small number of command neurons in a restricted number of enteric ganglia. The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into the DMX by iontophoresis, and 10-21 days later PHA-L was visualized in the bowel by immunofluorescence. Varicose vagal efferent fibers, labeled by PHA-L, were found in the myenteric plexus as far distally as the ileo-colic junction. PHA-L-labeled varicose axons were rare in comparison to nonlabeled fibers, entered a minority of myenteric ganglia, and contacted a small proportion of the neurons. Ganglia thus innervated by vagal efferent fibers were more numerous in the stomach than in the small intestine. Within the stomach, these ganglia were common in the antrum than in the corpus and none were found in the wall of the rumen. Innervated ganglia in the small intestine became progressively more sparse distally. No PHA-L-labeled axons were observed in the submucosal plexus, thus raising the possibility that vagal modulation of secretomotor responses involves an intermediate synapse in the myenteric plexus. Nonvaricose bundles of PHA-L-labeled fibers were also observed. These bundles appeared to utilize the connectives of the myenteric plexus as a pathway within which to descend within the bowel. Vagal efferent bundles were found to pass through the pyloric sphincter to enter the small intestine from the stomach; thus vagal fibers can reach the distal intestine by an intraenteric route that is not lesioned by crushing mesenteric nerves. The existence of this pathway affects the interpretation of experiments seeking to utilize such lesions to distinguish intrinsic from extrinsic neurites. Possible target neurons of the vagal efferent innervation were identified by simultaneously demonstrating the immunoreactivities of 5-hydroxytryptamine (5-HT), vasoactive intestinal polypeptide (VIP), enkephalin (ENK), galanin (GAL), and tyrosine hydroxylase (TH) along with that of PHA-L. Vagal terminals in the myenteric plexus appeared selectively to contact 5-HT- and, to a significantly lesser extent, VIP-, but not ENK- or GAL-immunoreactive neurons. Apparent vagal innervation of 5-HT-immunoreactive neurons was significantly more common in the duodenum, where a majority of the 5-HT-immunoreactive cells were encircled by varicose PHA-L-labeled axons, than in the stomach.(ABSTRACT TRUNCATED AT 400 WORDS)

VIP and autonomic neurotransmission

A variety of peptides have been proposed as transmitter candidates in non-cholinergic, non-adrenergic nerves. The nerves containing vasoactive intestinal polypeptide (VIP), which innervate blood vessels, non-vascular smooth muscle, mucosal epithelium and glands comprise a major and wide-spread population of the peptide-containing systems. There is now experimental data supporting the view that VIP is a transmitter in non-adrenergic, non-cholinergic nerves in the digestive tract, respiratory tract and urogenital tract, controlling smooth muscle tone and motility, blood flow and secretion. It is possible that impairment of VIP-containing nerves is involved in a number of autonomic dysfunctions.

Quantitative ultrastructural analysis of enkephalin-, substance P-, and VIP-immunoreactive nerve fibers in the circular muscle of the guinea pig small intestine

The present work was undertaken to determine what proportion of all nerve fibers in the circular muscle of the guinea pig small intestine contain the neuropeptides enkephalin, substance P, and vasoactive intestinal peptide and in which combinations these peptides occur in the fibers. It was envisaged that such an analysis would provide insights into the chemical identity of excitatory and inhibitory nerve fibers that innervate the muscle. Whole-mount preparations from normal and extrinsically denervated gut were labelled with antiserum to the individual peptides or with combinations of antipeptide antisera and processed for electron microscopy. Reactive and nonreactive vesicle-containing nerve fiber profiles were examined and counted in ultrathin sections. Vesicle-containing nerve fiber profiles immunoreactive for enkephalin, substance P, or vasoactive intestinal peptide had similar morphologies in that they all contained variable proportions of small clear and large granular vesicles. In all samples stained for single peptides or combinations of peptides, a small proportion of immunoreactive profiles approached smooth muscle cells to within 15-20 nm with no intervening basal lamina. A total of 14,694 vesiculated nerve fiber profiles from three control and three extrinsically denervated animals were scored for the presence of immunoreactivity to enkephalin, substance P, vasoactive intestinal peptide, or combinations of these peptides. Analysis of variance showed that the number of profiles labelled for substance P was not different from the number of profiles labelled for vasoactive intestinal peptide and that the number labelled with the substance P and vasoactive intestinal peptide antisera simultaneously were not different from the sum of the numbers obtained with each alone. The number of profiles labelled for substance P plus enkephalin was greater than the number labelled for substance P alone and the number labelled with vasoactive intestinal peptide plus enkephalin was greater than that with vasoactive intestinal peptide alone. Simultaneous labelling for substance P and vasoactive intestinal peptide resulted in immunoreactivity in the same number of profiles as did reaction for all three peptides at the same time. In both cases, about 95% of the profiles were labelled. The results from extrinsically denervated muscle were not different from control circular muscle. These results indicate that nearly all the intrinsic nerve fibers supplying the circular muscle of the guinea pig small intestine contain either substance P or vasoactive intestinal peptide but not both.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased levels of vasoactive intestinal peptide (VIP) and oxytocin during suckling in lactating dogs

The aim of the present study was to investigate how suckling influences the plasma levels of oxytocin and vasoactive intestinal peptide in lactating dogs. Blood-samples were drawn from ten lactating beagles during suckling, in week one and week three of the five week long lactation period and the levels of vasoactive intestinal peptide and oxytocin were determined by radioimmunoassay. The immunoreactive levels of vasoactive intestinal peptide and oxytocin (below referred to VIP and oxytocin) increased rapidly in response to the suckling stimulus. The rise of both peptides was significant at both suckling experiments. The origin and role of suckling-released oxytocin is well established. The origin of and function of the VIP released in response to suckling remains unknown.

Effect of vasoactive intestinal peptide, peptide histidine isoleucine and growth hormone-releasing factor-40 on bombesin-like immunoreactivity, somatostatin and gastrin release from the perfused rat stomach

Bombesin-like immunoreactivity (BLI) has been demonstrated in neurons of the gastrointestinal tract and gastric BLI secretion can be demonstrated in response to the classical neurotransmitter acetylcholine. Since structurally related peptides VIP, PHI and GRF have to be considered as peptidergic neurotransmitters it was of interest to determine their effect on gastric BLI secretion. Additionally, somatostatin (SLI) and gastrin secretion was examined. The isolated stomach of overnight fasted rats was perfused with Krebs-Ringer buffer via the celiac artery and the effluent was collected via the portal vein. The gastric lumen was perfused with isotonic saline at pH7 or pH2. All four peptides were tested at a dose of 10(-11) M and 10(-8) M at both pH levels and in addition the effect of VIP and PHI was examined at 10(-14) M and 10(-12) M during luminal pH2. At luminal pH7 VIP and PHI stimulated SLI release at 10(-8) M but had no effect on BLI or gastrin secretion. rGRF and hpGRF were both ineffective on SLI and gastrin release while rGRF inhibited and hpGRF stimulated BLI secretion. This effect was not dose related. At luminal pH2 all four peptides stimulated BLI secretion. Stimulation by PHI was already observed at a dose of 10(-14) M while VIP elicited a stimulatory effect at 10(-12) M. PHI at the two lowest concentrations of 10(-14) and 10(-12) M elicited a stimulation of SLI and gastrin release while the same doses of VIP and the higher doses of all four peptides had no effect on SLI and gastrin secretion at an acidic intraluminal pH.(ABSTRACT TRUNCATED AT 250 WORDS)

Gastrointestinal somatostatin: distribution, secretion and physiological significance

Somatostatin-like immunoreactivity (SLI) has been found throughout the gastrointestinal tract in all species examined. In the stomach it is mainly present in endocrine-type D-cells whereas in the intestine there is also an extensive distribution in enteric neurones. In all regions of the gastrointestinal tract multiple forms of somatostatin exist. A precursor (prosomatostatin) has been partially sequenced, three forms with 20 (SS-20), 25 (SS-25) and 28 (SS-28) amino acids completely sequenced, and somatostatin-14 (SS-14) demonstrated by radioimmunoassay. Both SS-14 and SS-28 exert a wide range of actions on the gastrointestinal tract and there is strong supportive evidence for a role in the regulation of gastric acid and gastrin secretion, gastrointestinal motility and intestinal transport. Both in vivo and in vitro studies on the secretion of gastric SLI into the vasculature have shown that nutrients initiate the process but that subsequent events are regulated by a complex interplay between hormonal and neuronal pathways. GIP is one of the most potent hormonal secretagogues. In the stomach, acetylcholine, opioid peptides and substance P are probably involved in parasympathetic inhibitory pathways and gastrin releasing peptide in stimulatory pathways. The sympathetic nerves are also stimulatory. Regulation of secretion of intestinal SLI has not been so extensively studied. Although SLI is also found in the gastrointestinal lumen the significance is unclear. Despite these advances the exact route of delivery of somatostatin to its target organs is uncertain and paracrine, endocrine and neural pathways may all be involved.

Effect of substance P on somatostatin release from the isolated perfused rat stomach

The effect of substance P (SP) on the release of gastric somatostatinlike immunoreactivity (SLI) was studied. Substance P inhibited both basal SLI release and SLI release stimulated by gastric inhibitory polypeptide or 1-isoproterenol. This inhibitory action of SP was not blocked by atropine or hexamethonium, suggesting that a cholinergic mechanism was not involved. The SP-suppressed SLI release was also not reversed by the administration of naloxone, which indicated that enkephalinergic involvement was absent. However, the SP antagonist [D-Pro2, D-Trp7,9]-SP abolished the inhibition of SLI release by SP. It is concluded that SP might be involved in the control of gastric SLI release and that the inhibitory action of SP was probably mediated directly or indirectly on the gastric D cells through SP receptors.

Pentagastrin-stimulated gastric acid secretion by the isolated perfused rat stomach

The purpose of this present study was to develop a method for stimulation of acid secretion by the isolated perfused rat stomach. Rat stomachs were perfused in situ via the abdominal aorta and celiac axis with Krebs-Ringer bicarbonate buffer in the presence or absence of 10% ovine erythrocytes. The gastric lumen was perfused with distilled water and gastric contents were collected at frequent intervals through a catheter at the pylorus. Sixty minute gastric acid output in response to various concentrations of pentagastrin was determined by titration of gastric contents with 0.01 N NaOH to pH 7.0. During arterial perfusion with Krebs-Ringer bicarbonate buffer in the absence of ovine erythrocytes gastric acid output was 2.50 +/- 0.58 SEM microEq H+/h, which did not increase in response to perfusion with Krebs-Ringer bicarbonate buffer containing pentagastrin. However, inclusion of 10% ovine erythrocytes in the arterial perfusate resulted in substantial stimulation of gastric acid by pentagastrin: maximal acid output, achieved with a pentagastrin dose of 0.6 microgram/kg/h, was 23.5 +/- 3.73 microEq H+/h (p less than 0.01). The results of the present study demonstrate the capacity of the isolated vascularly perfused rat stomach to secrete acid and provide a model for studying interactions of gastrointestinal regulatory peptides and their physiologic roles in the regulation of gastric acid secretion.

Modulation of acetylcholine-induced secretion of gastric bombesin-like immunoreactivity by cholinergic and histamine H2-receptors, somatostatin and intragastric pH

Recently we have shown the release of bombesin-like immunoreactivity (BLI) from the isolated perfused rat stomach. In these experiments we have shown that BLI secretion is stimulated by acetylcholine. Gastric inhibitory peptide (GIP) exerts an inhibitory effect which is dependent on the intraluminal pH. The present study was designed to examine further the exact cholinergic mechanisms and to study the interaction between cholinergic and histaminergic mechanisms as well as the effect of the intraluminal pH. Acetylcholine elicited a dose-dependent increase in BLI and gastrin secretion (10(-6) M and 2 X 10(-6)M), whereas somatostatin release was suppressed at luminal pH 7. Blockade of muscarinic cholinergic receptors by atropine (10(-5)M) and nicotinic cholinergic receptors by hexamethonium (10(-5) M) abolished the effect of acetylcholine on all three peptides. Reduction of the intraluminal pH to 2 also abolished acetylcholine-induced stimulation of BLI and gastrin secretion and the inhibition of somatostatin secretion. Changes of intraluminal pH per se had no effect on the secretion of either peptide. Somatostatin (10(-7) M) reduced both BLI and gastrin secretion during stimulation with acetylcholine. The addition of the H2-receptor antagonist cimetidine (10(-5) M) abolished the effect of both doses of acetylcholine on BLI and somatostatin secretion and also the effect of the lower dose of acetylcholine (10(-6) M) on gastrin secretion during luminal pH 7. At luminal pH 2 cimetidine did not alter BLI and somatostatin secretion in response to acetylcholine, however, gastrin release was augmented in the presence of cimetidine. These data demonstrate that the effect of acetylcholine on BLI, gastrin, and somatostatin secretion is mediated by muscarinic and nicotinic cholinergic receptors and also by histamine H2-receptors. Somatostatin inhibits cholinergically induced BLI secretion. The cholinergic effects on BLI, somatostatin and gastrin secretion are abolished during an acidic intragastric pH. In this isolated perfused rat stomach model the inhibitory effect of intraluminal acid on gastrin secretion is, at least in part, mediated by H2-receptors. This suggests that the secretion of bombesin, a potential peptidergic neurotransmitter is modulated by neural, endocrine and local tissue factors and also by alterations of intragastric pH.

Elevation of plasma gastrin and pancreatic polypeptide by electrical vagal stimulation in sheep: effects of sequential stimulation

Gastrin and pancreatic polypeptide (PP) are released into the circulation by vagal stimulation. The individual effects of the anterior and posterior vagal trunks on the release of these peptides are unknown. Four sheep were anaesthetized and studied acutely: both vagi were dissected at the hiatus and the trunks divided. In two sheep, the distal ends of the anterior trunks were stimulated for 5 min with an 8 V, 1 ms impulse at 10 Hz. After 1 h the posterior trunks were stimulated similarly. In the other two sheep, the posterior trunk was stimulated and after 1 h the anterior vagal trunk was stimulated. The anterior and posterior trunk equally stimulated the release of both gastrin and PP in four animals. The second stimulation in these four animals resulted in an 18-fold greater integrated response of gastrin and 20-fold greater response of PP. This potentiation to the second stimulus was observed in further experiments even when the same trunk, posterior or anterior, was stimulated twice. The similarity of influence of the anterior and posterior trunks for the release of PP suggests the existence of mechanisms for vagally stimulated PP release other than branches direct from the vagal trunks.