The stomach and intestine secrete various enzymes and hormones that aid in the digestive process. The hormones secreted include gastrin, cholecystokinin, secretin, and gastric inhibitory peptide (GIP). Of these, only gastrin is currently of diagnostic significance.
Gastrin is secreted by the gastrin cells (G cells) of the gastric antrum, the pylorus, and the proximal duodenum in response to vagal stimulation and the presence of food (especially protein) in the stomach. Gastrin stimulates the secretion of acidic gastric juice and pepsin and the release of pancreatic enzymes. It also stimulates motor activities of the stomach and intestine, increases pyloric relaxation, constricts the gastroesophageal sphincter, and promotes the release of insulin.
Cholecystokinin is secreted by the duodenal mucosa in response to the presence of fats. It opposes the actions of gastrin, stimulates contraction of the gallbladder, relaxes the sphincter of Oddi, and with secretin, controls pancreatic secretions. Secretin is secreted by the duodenal mucosa in response to the presence of peptides and acids in the duodenum. It also opposes the actions of gastrin, and with cholecystokinin, controls pancreatic secretions. GIP inhibits gastric motility and secretion and stimulates secretion of insulin.