*Gastrin is secreted by the “G” cells of the antrum of the stomach in response to stimuli associated with
ingestion of a meal, such as distention of the stomach, the products of proteins, and gastrin releasing peptide, which is released by the nerves of the gastric mucosa during vagal stimulation.
*The primary actions of gastrin are :
(1) stimulation of gastric acid secretion and
(2) stimulation of growth of the gastric mucosa.
*Cholecystokinin is secreted by “I” cells in the mucosa of the duodenum and jejunum mainly in response to digestive products of fat, fatty acids, and monoglycerides in the intestinal contents. This hormone strongly contracts the gallbladder and relaxes the sphincter of Oddi, expelling bile into the small intestine where the bile in turn plays important roles in emulsifying fatty substances, allowing them to be digested and absorbed. Cholecystokinin also inhibits stomach contraction moderately. Therefore, at the same time that this hormone causes emptying of the gallbladder, it also slows the emptying of food from the stomach to give adequate time for digestion of the fats in the upper intestinal tract. It also stimulates release of insulin, stimulates secretion of enzymes by pancreatic acinar cells and increases bowel motility.
*Secretin was the first gastrointestinal hormone discovered and is secreted by the “S” cells in the mucosa of the duodenum in response to acidic gastric juice emptying into the duodenum from the pylorus of the
stomach. Secretin has a mild effect on motility of the gastrointestinal tract and acts to promote pancreatic
secretion of bicarbonate which in turn helps to neutralize the acid in the small intestine. Secretin stimulates bicarbonate and water release from pancreatic ductal cells. It also stimulates the flow of bile and inhibits gastrointestinal mobility. Although secretin releases gastrin from gastrinomas, it inhibits gastrin release from the normal stomach.
*Gastric inhibitory peptide(GIP) is secreted by the mucosa of the upper small intestine, mainly in response to fatty acids and amino acids but to a lesser extent in response to carbohydrate. It has a mild effect in decreasing motor activity of the stomach and therefore slows emptying of gastric contents into the duodenum when the upper small intestine is already overloaded with food products.
*Motilin is secreted by the upper duodenum during fasting, and the only known function of this hormone
is to increase gastrointestinal motility. Motilin is released cyclically and stimulates waves of gastrointestinal
motility called interdigestive myoelectric complexes that move through the stomach and small
intestine every 90 minutes in a fasted person. Motilin secretion is inhibited after ingestion by mechanisms
that are not fully understood.