HORMONAL CONTROL OF GIT SECRETIONS
The GIT produces a large number of hormones many of whose functions are not well understood, although some of them, together with neuronal activi-ties, are concerned with coordinating the secretions of various digestive juices. Endocrine cells are scattered throughout the entire GIT in clusters forming a diffuse portion of the endocrine system . Over 25 peptides have been extracted and characterized from the GIT. No deficiency states are known for any of these peptides although hormone-secreting tumors have been described.
The G cells in the antral and pyloric regions of the stomach produce gastrin. Gastrin occurs in a number of molecular forms, for example gastrin17 and gas-trin34 are composed of 17 and 34 amino acid residues respectively. Gastrin17 is the most active and has a half-life of about 8 min. Its precursor, gastrin34, has a half-life of approximately 40 min. The release of gastrin is stimulated by food entering the stomach from the esophagus and its function is, in turn, to stim-ulate release of gastric juice. Gastric inhibitory peptide (GIP) is a peptide of 43 amino acid residues secreted by the duodenum and upper jejunum. It stimu-lates insulin release , reduces the secretions of gastrin and pepsin and inhibits gastric movements. The hormone, vasoactive intestinal peptide (VIP) is comprised of 28 amino acid residues. It is released in response to dis-tension of the GIT by food. It stimulates the contraction of smooth muscle tissues of the GIT wall and pancreatic exocrine secretions and it also inhibits gastrin and gastric acid release. Pancreatic polypeptide is formed, as its name implies, by the pancreas and inhibits pancreatic hydrogen carbonate and pro-tein secretions. Secretin is also produced in the duodenum and jejunum. It is a 27-amino acid residue peptide with a half-life of 17 min. Its release is trig-gered by acid from the stomach and it functions to stimulate the release of pancreatic juice, the hydrogen carbonate of which helps neutralize the acid in chyme.
Glucose-dependent insulinotrophic polypeptide (gastric inhibitory polypeptide) is also released in the duodenum and jejunum. It inhibits the secretion of gastric acid and stimulates insulin secretion. Mucosal cells in the upper region of the small intestine secrete cholecystokinin (CCK). Two molec-ular forms are produced consisting of 33 and 39 amino acid residues respec-tively. The release of CCK is stimulated by peptides and fatty acids in the food and, in turn, stimulates the release of pancreatic juice and contractions of the gall bladder. Motilin is a 22 amino acid residue peptide that is structurally unrelated to any other GIT hormone produced in the upper small intestine. It controls GIT movements during fasting. The ileum and colon produce peptide YY and neurotensin. The former decreases pancreatic and gastric secretions, while the latter may regulate peristalsis of the ileum. The hormone called sub-stance P is produced along the entire GIT. Its functions include stimulating the secretion of saliva and it is also involved in the vomit reflex.