Role of cholecystokinin in the regulation of gastrointestinal motility.

Cholecystokinin, a hormone released from endocrine cells of the upper small intestine in response to amino acids and fatty acids in chyme, has potent effects on gut smooth muscle contractility. Depending on the region of the gut, the effect of cholecystokinin may be neurally mediated, direct, or both. Neurally-mediated effects may be relaxant or contractile depending on the transmitter released by cholecystokinin. Based on mimicry by infusion of cholecystokinin to postprandial levels and the ability of specific cholecystokinin receptor antagonists to abolish effects of endogenous cholecystokinin released in response to a meal, three physiological effects of cholecystokinin, all of which are neurally-mediated, have been identified: contraction of the gallbladder mediated by cholecystokinin-induced release of acetylcholine, relaxation of the sphincter of Oddi mediated by cholecystokinin-induced release of vasoactive intestinal peptide and inhibition of gastric emptying mediated by cholecystokinin-induced activation of an inhibitory vago-vagal reflex involving vasoactive intestinal peptide-induced relaxation of the gastric fundus. Gallbladder contraction and relaxation of the sphincter of Oddi regulate the flow of bile, and inhibition of gastric emptying regulates the flow of chyme into the duodenum. Combined with the potent stimulation of pancreatic enzyme secretion by cholecystokinin, these effects insure optimal conditions for digestion and absorption of nutrients by the intestine.