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# Gastrointestinal Physiology ## Stimuli for the release of GIP * GIP is secreted by the duodenum and jejunum. * GIP is the only hormone that is released in response to fat, protein, and carbohydrate. * GIP secretion is stimulated by fatty acids, amino acids, and orally administered glucose. ## C...

# Gastrointestinal Physiology ## Stimuli for the release of GIP * GIP is secreted by the duodenum and jejunum. * GIP is the only hormone that is released in response to fat, protein, and carbohydrate. * GIP secretion is stimulated by fatty acids, amino acids, and orally administered glucose. ## Candidate hormones * **Motilin** increases GI motility and is involved in interdigestive myoelectric complexes. * **Pancreatic polypeptide** inhibits pancreatic secretions. * **Glucagon-like peptide-1 (GLP-1)** binds to pancreatic β cells and stimulates insulin secretion. * Analogues of GLP-1 may be helpful in the treatment of type 2 diabetes mellitus. * **Leptin** decreases appetite. * **Ghrelin** increases appetite. ## Paracrines * Are released from endocrine cells in the GI mucosa. * Diffuse over short distances to act on target cells located in the GI tract. * The GI paracrines are somatostatin and histamine. ### Somatostatin * Is secreted by cells throughout the GI tract in response to H+ in the lumen. Its secretion is inhibited by vagal stimulation. * Inhibits the release of all GI hormones. * Inhibits gastric H+ secretion. ### Histamine * Is secreted by mast cells of the gastric mucosa. * Increases gastric H+ secretion directly and by potentiating the effects of gastrin and vagal stimulation. ## Neurocrines * Are synthesized in neurons of the GI tract, moved by axonal transport down the axons, and released by action potentials in the nerves. * Neurocrines then diffuse across the synaptic cleft to a target cell. * The GI neurocrines are vasoactive intestinal peptide (VIP), neuropeptide Y, nitric oxide (NO), GRP (bombesin), and enkephalins. ### VIP * Contains 28 amino acids and is homologous to secretin. * Is released from neurons in the mucosa and smooth muscle of the GI tract. * Produces relaxation of GI smooth muscle, including the lower esophageal sphincter. * Stimulates pancreatic HCO3- secretion and inhibits gastric H+ secretion. In these actions, it resembles secretin. * Is secreted by pancreatic islet cell tumors and is presumed to mediate pancreatic cholera. ### GRP (bombesin) * Is released from vagus nerves that innervate the G cells. * Stimulates gastrin release from G cells. ### Enkephalins (met-enkephalin and leu-enkephalin) * Are secreted from nerves in the mucosa and smooth muscle of the GI tract. * Stimulate contraction of GI smooth muscle, particularly the lower esophageal, pyloric, ileocecal sphincters. * Inhibit intestinal secretion of fluid and electrolytes. This action forms the basis for the usefulness of opiates in the treatment of diarrhea.