Gastrin is a 17-residue polypeptide isolated from the antralmucosa. It was isolated originally in two different forms. Inone of the forms, the tyrosine residue in position 12 is sulfated.Both forms are biologically active. Cholinergic responseto the presence of food in the gastrointestinal tract providesthe stimulus for gastrin secretion. The lowering of pHin the stomach inhibits the secretion of gastrin. The effects ofstructural modification of gastrin on gastric acid secretionhave been reviewed. These studies revealed that the fourresidues at the COOH terminus retain significant biologicalactivity and that the aspartate residue is the most critical foractivity. The most important action of gastrin is to stimulatethe secretion of gastric acid and pepsin. Other actions of gastrininclude increased secretion of pancreatic enzymes; contractionof smooth muscles; water and electrolyte secretion bythe stomach and pancreas; water and electrolyte absorption bythe small intestine; and secretion of insulin, glucagon, and somatostatin.A synthetic pentapeptide derivative, pentagastrin,is currently used as a gastric acid secretagogue.
