J. Fahrenkrug

Vagal, Cholinergic Regulation of Pancreatic Polypeptide Secretion

THE EFFECT OF EFFERENT, PARASYMPATHETIC STIMULATION UPON PANCREATIC POLYPEPTIDE (PP) SECRETION WAS STUDIED IN THREE WAYS: (a) Plasma PP concentrations increased in response to insulin-induced hypoglycemia in both normal subjects, from 11 pM (9.5-12.5) to 136 pM (118-147), n = 8 (median and interquartile range) and in duodenal ulcer patients, from 33 pM (21-52) to 213 pM (157-233), n = 7. The PP response to hypoglycemia was diminished by atropine in normal subjects (P < 0.005) and completely abolished by vagotomy in the duodenal ulcer patients. (b) Electrical stimulation, 8 Hz, of the vagal nerves in anesthetized pigs induced an increase in portal PP concentrations within 30 s from 32 pM (28-39) to 285 pM (248-294), n = 12. Minimal stimulatory frequency was 0.5 Hz and maximal stimulatory frequency 8-12 Hz. Atropine inhibited the PP response to electrical stimulation. Median inhibition with 0.5 mg of atropine/kg body wt was 74%, range 31-90%, n = 6. The response was eliminated by hexamethonium. Adrenergic alpha and beta blockade did not influence the release of PP in response to vagal stimulation. (c) Acetylcholine stimulated, in a dose-dependent manner, the secretion of PP from the isolated perfused porcine pancreas, half-maximal effective dose being 0.19 muM; maximal PP output in response to 5 min stimulation was 228 pmol, range 140-342 pmol, n = 5. Atropine completely abolished this response.The results of the present study together with the previously demonstrated poor PP response to food in vagotomized patients, indicate that vagal, cholinergic stimulation is a major regulator of PP secretion.

Immunohistochemical localization of a vasodilatory polypeptide (VIP) in cerebrovascular nerves

Brain vessels are richly supplied with sympathetic (adrenergic) and parasympathetic (cholinergic) nerves, which have been visualized by a variety of techiquesS,7,10,13, 14. In the course of our ultrastructural studies 7,13 on the cerebrovascular innervation of rats and cats, we observed a third type of nerves which, by the appearance of the vesicles present in their axon terminals, did not conform with either of the two reported systems of autonomic nerves. These axons were characterized by the predominance of large vesicles, 85-200 nm in diameter, containing a moderately electron-dense core filling the entire vesicle. They rather resembled nerves designated as p-type fibres 2 because of the similarity of the large granular vesicles to those of the polypeptidestoring neurosecretory nerves 1. This led us to search for vasoactive peptides in perivascular nerves of the brain. Vasoactive intestinal polypeptide (VIP), a potent hypotensive and vasodilatory agent 11, has been isolated from the porcine small intestines le,17. In a recent study we were able to demonstrate the occurrence of VIP, not only in endocrine-like cells, but also in a rich system of gastrointestinal nerves, some of which had a perivascular localization 12. It was therefore decided to examine, by immunohistochemistry, whether VIP occurred in the perivascular pial nerves. The tissues were taken from various regions at the base of the brain of adult cats, and included large pial arteries with underlying brain tissue. The material consisted of both unoperated and sympathectomized animals (bilateral removal of the superior cervical ganglia 1 week prior to killing). The preparations were frozen to the temperature of liquid nitrogen in a propane-propylene mixture, freeze-dried, vapour-fixed with diethylpyrocarbonate 15, embedded in paraffin in vacuo, sectioned at 5/~m thickness, and subjected to an indirect immunofluorescence method 3,1z for the demonstration of VIP immunoreactivity.

Nervous release of vasoactive intestinal polypeptide in the gastrointestinal tract of cats: possible physiological implications.

1. The release of vasoactive intestinal polypeptide (VIP) into blood from the gastrointestinal tract was studied when eliciting autonomic nervous effects known to be mediated via non‐adrenergic, non‐cholinergic nerve fibres. All studies were performed on animals given atropine. 2. Electrical stimulation of the low threshold vagal fibres to the stomach did not significantly change gastric volume or VIP concentration in the venous effluent from the stomach. Stimulating the high threshold fibres, on the other hand, produced a gastric relaxation concomitant with a significant increase of venous plasma VIP concentrations. When eliciting a similar vagal relaxation of the stomach by distending a balloon the oesophagus a significant increase of venous plasma VIP concentration was also recorded. 3. Mechanical stimulation of the mucosa of the small bowel increased intestinal blood flow and a significant increase of venous plasma VIP concentration was observed. 4. Stimulation of the pelvic nerves to the colon produced a transient vasodilation and a significant increase of VIP in the venous effluent from the large bowel. A maintained vasodilation in the colon was induced by mechanically stimulating the rectal mucosa. This vascular response was accompanied by a significant raise of venous plasma VIP concentration. 5. The results demonstrate that all the studied nervous effects known to be mediated via non‐adrenergic, non‐cholinergic nerve fibres were accompanied by significant increases of the VIP concentration in the venous effluent. The possible physiological implications of these findings are discussed and it is proposed that VIP may be a neurotransmitter in the gastrointestinal tract.

Influence of the autonomic nervous system on the release of vasoactive intestinal polypeptide from the porcine gastrointestinal tract.

1. The release of vasoactive intestinal polypeptide from the gastrointestinal tract in response to stimulation of the vagus nerves, the splanchnic nerves and to intra‐arterial infusion of acetylcholine (ACh) was examined in pigs. 2. Stimulation of the vagus nerves caused an abrupt increase in the release of vasoactive intestinal polypeptide. The amount of the peptide released depended on the frequency at which the nerves were stimulated. Maximum release was obtained at 8 Hz. 3. Atropine and beta‐adrenergic blocking agents failed to diminish the vagally induced release of vasoactive intestinal polypeptide, while the response was completely blocked by hexamethonium and increased after alpha‐adrenergic blockade and after splanchnicotomy. 4. Intra‐arterial infusion of ACh closely imitated the response to vagal stimulation, but the release of vasoactive intestinal polypeptide induced by ACh was abolished by atropine. 5. Stimulation of the splanchnic nerves caused a decrease in the release of vasoactive intestinal polypeptide, an action which was annulled by alpha‐adrenergic blockade, but still present after the adrenal glands were isolated from the circulation. The inhibitory effect of splanchnic stimulation significantly diminished the vagally induced release of vasoactive intestinal polypeptide. 6. The results demonstrate a dual innervation with opposing effects on the neurones containing vasoactive intestinal polypeptides. The possible physiologic implication of this finding is discussed.

VIP (vasoactive intestinal polypeptide)-containing nerves of intracranial arteries in mammals.

SummaryImmunohistochemical and radioimmunochemical investigations have shown, in various species, the occurrence of numerous nerve fibres containing vasoactive intestinal polypeptide (VIP) in connection with blood vessels of the central nervous system. Pial arteries from pig, cat, and rat have the richest supply of VIP nerve fibres; those of cow, dog, guinea pig and hamster have an intermediary number of nerves, while only few are found in pial arteries from the monkey, rabbit, gerbil, and mouse. The regional variation in VIP-nerve density follows the order: cerebral arteries > cerebellar arteries > basilar > vertebral > spinal cord arteries. Unilateral extirpation of either the pterygopalatine or the superior cervical ganglia does not affect the amount or distribution of VIP fibres in the wall of brain vessels of the ipsilateral side.Measurement of the VIP content by radioimmunoassay shows mean concentrations in the pial arteries varying between 19 and 82 pmol/g tissue wet weight. Regional and species variations in measured VIP levels are similar to the variations in distribution of immunoreactive nerve fibres.

A rich VIP nerve supply is characteristic of sphincters

VASOACTIVE INTESTINAL PEPTIDE (VIP), isolated from extracts of porcine duodenum by Said and Mutt1, was at first thought to be a hormone2. Immunohistochemical studies have since revealed that VIP has a neuronal localisation. VIP-containing nerves occur throughout the body, being particularly frequent in the digestive tract3–7, the genitourinary tract8–10 and the upper respiratory tract11. The nerves storing VIP or other neuropeptides (substance P, somatostatin and enkephalin) apparently represent additional types of autonomie nerves, distinct from the adrenergic and cholinergic ones. A great proportion or the peptidergic nerves seems to originate in nerve cell bodies located close to or within the innervated organ5–9,11. The physiological significance of these new types of nerves is a matter of speculation, but a knowledge of their precise anatomical distribution will assist in defining their targets. We have previously observed that structures believed to exert a sphincter function receive a particularly rich supply of such nerves. We have now examined several sphincters, recognised or anticipated, and have established the presence of VIP nerves in all of them.

Peptide-containing nerve fibres in the gut wall in Crohn's disease.

Neurones containing VIP, substance P, or enkephalin were studied by immunocytochemistry in intestinal specimens from 27 patients with Crohns disease. Also several endocrine cell systems in the gut were examined. The results were compared with those from a control group of 26 patients. The relative frequency of various endocrine cells did not differ overtly from that in controls. Vasoactive intestinal polypeptide and substance P nerve fibres were distributed in all layers of the gut wall, including the submucosal and myenteric plexuses, whereas enkephalin fibres were restricted to the smooth muscle layer and the myenteric plexus. The distribution and frequency of the peptide-containing nerve fibres were the same in Crohns disease patients as in control patients. A proportion of these nerve fibres, however, were notably coarse in the Crohns disease patients. This was particularly apparent in the afflicted parts of the intestine although it was noted also in non-afflicted parts. The concentration of VIP and substance P (expressed as pmol/g wet weight) did not, however, exceed that of the control group.

VIP Innervation of the Gallbladder

By immunohistochemistry, vasoactive intestinal polypeptide (VIP) was localized to nerve fibres and nerve cell bodies in the gallbladder wall of several mammals, including man. There is thus a morphological basis for accepting the powerful actions of VIP on gallbladder motility as physiological. VIP (vasoactive intestinal polypeptide), first thought of as a gut hormone, has recently been localized to a widely distributed system of nerves in the gut wall 1,2. In addition, nerves displaying VIP immunoreactivity are present in the wall of brain vessels and in the hypothalamus 1,3. Among known effects of VIP are relaxation of the gallbladder and inhibition of CCK-induced gallbladder contraction4,5. These observations prompted a search for VIP in the gallbladder wall. The present report deals with the immunohistochemical demonstration of VIP nerves in the gallbladder of several species, including man.

Release of vasoactive intestinal polypeptide from the cat small intestine exposed to cholera toxin.

During a four hour observation period vasoactive intestinal polypeptide (VIP) is released in increasing amounts from the feline small intestine exposed to cholera toxin. As VIP is known to be located almost exclusively in the intestinal nerves, the present findings strongly suggest that cholera toxin activates the enteric nervous system. The findings of this and other studies performed in this laboratory lead to the proposal that the choleraic secretion is, at least in part, secondary to the activation of intramural nervous reflexes in the gut.

Secretion pattern of secretin in man: regulation by gastric acid.

Median concentration of plasma secretin in the fasting state in 11 achlorhydria patients, 17 normal subjects, eight duodenal ulcer patients, and 11 Zollinger-Ellison patients was 0.3, 1.2, 2.5, and 5.9 pmol x 1(-1), respectively. Aspiration of gastric acid normal subjects and duodenal ulcer patients was followed by a significant lowering of the plasma secretin concentration. In normal subjects insulin-induced hypoglycaemia resulted in increased secretin levels when gastric acid was allowed to enter the duodenum, whereas no changes were observed when gastric acid was aspirated. Simultaneous measurements of intraduodenal pH and plasma secretin concentration in the fasting state and in response to a meal showed that rapid falls in intraduodenal pH were followed by short-lived increments in plasma secretin concentration. These changes in pH and in secretin levels were diminished after cimetidine. It is concluded that gastric acid in man does trigger release of secretin and that secretin is secreted intermittently both in the fasting state and in response to a meal when boluses of acid enter the duodenum.