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a polypeptide hormone secreted by the mucous membrane of the pylorus of the stomach; it is secreted when the pylorus is stretched or acted on by chemical stimulants (for example, products of the decomposition of food) or impulses from the vagus nerves. Gastrin increases the secretion of gastric and pancreatic juices, as well as of bile, and alters the tone and motility of the stomach and intestines. An increase in the hydrochloric-acid content of the pylorus (when acidic gastric juice enters it) inhibits the secretion of gastrin. Two kinds of gastrin, which are very similar in structure and physiological properties, have been distinguished. Gastrin has been synthesized chemically.