At first, all GI peptides were believed to originate from endocrine cells. Powerful cytochemical techniques for the localization of peptides, however, have shown that many were located in nerves throughout the mucosa and smooth muscle of the gut.
Vasoactive Intestinal Peptide
Currently, three peptides are known to function physiologically in the gut as neurocrines (Table 4).
TABLE 2 Candidate Hormones
Site of release
Pancreatic bicarbonate secretion Pancreatic enzyme secretion Inhibits
Gastric secretion Gastric emptying Stimulates
Insulin release Inhibits
Gastric secretion Gastric emptying
Protein (fat)a (Glucose) Fat
Glucose a Parentheses indicate an item of lesser importance.
Integration of Regulatory Mechanisms TABLE 3 Paracrines
Other peptide hormone release Gastric acid secretion Stimulates
Gastric acid secretion
GI, gastrointestinal; ECL, enterochromaffin-like.
Site of release
GI mucosa Pancreatic islets
Vagus inhibits release
Oxyntic gland mucosa ECL cell
Vasoactive intestinal peptide (VIP) is chemically a member of the secretin-glucagon family (see Fig. 7). It is found only in nerves and appears to mediate the relaxation of GI smooth muscle. VIP may also relax vascular smooth muscle to physiologically mediate vasodilation. Many of the effects of VIP on relaxing smooth muscle may be mediated by nitric oxide, the synthesis of which is stimulated by VIP. When injected into the bloodstream, VIP has many of the effects of its relatives, secretin, GIP, and glucagon. It stimulates pancreatic fluid and bicarbonate secretion and water and electrolyte secretion from the intestinal mucosa. It also inhibits gastric secretion.
VIP appears to be the agent responsible for pancreatic cholera or watery diarrhea syndrome. The symptoms of this frequently lethal disease result from the secretion of a peptide from a pancreatic islet cell tumor. The peptide has been identified as VIP in both tumor tissue and plasma samples from these patients. Because VIP stimulates the secretion of cholera-like fluid from intestinal mucosa, it is probably the causative agent.
A variety of biologically active peptides has been isolated from amphibian skin. Many of these have mammalian counterparts with physiologic functions. Bombesin is one of these. It was first isolated from the skin of a frog, for which it is named. The mammalian counterpart of bombesin is GRP, which is found in nerves of the gastric mucosa. GRP is released by vagal stimulation and appears to mediate vagally stimulated gastrin release.
Two pentapeptides isolated from brain activate opiate receptors and have been found in nerves of the mucosa and smooth muscle of the GI tract. They are identical except for the C-terminal amino acid, which is methionine in one (met-enkephalin) and leucine in the other (leu-enkephalin). The enkephalins contract smooth muscle and appear to mediate the contraction of the lower esophageal, pyloric, and ileocecal sphincters. They may also function physiologically in peristalsis.
Opiates have been used for years to treat diarrhea. Their effectiveness is attributed to the fact that they slow intestinal transit. The enkephalins are also potent inhibitors of intestinal fluid secretion. Thus, the anti-diarrheal property of opiates is probably due to this action as well as the motility effects.
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