Gastrin is a 17-residue polypeptide isolated from the antral mucosa. It was isolated originally in two different forms. In one of the forms, the tyrosine residue in position 12 is sul-fated. Both forms are biologically active. Cholinergic response to the presence of food in the gastrointestinal tract provides the stimulus for gastrin secretion. The lowering of pH in the stomach inhibits the secretion of gastrin. The effects of structural modification of gastrin on gastric acid secretion have been reviewed.65 These studies revealed that the four residues at the COOH terminus retain significant biological activity and that the aspartate residue is the most critical for activity. The most important action of gastrin is to stimulate the secretion of gastric acid and pepsin. Other actions of gas-trin include increased secretion of pancreatic enzymes; contraction of smooth muscles; water and electrolyte secretion by the stomach and pancreas; water and electrolyte absorption by the small intestine; and secretion of insulin, glucagon, and somatostatin. A synthetic pentapeptide derivative, pentagastrin, is currently used as a gastric acid secretagogue.

Pentagastrin. Pentagastrin (Peptavlon), a physiological gastric acid secretagogue, is the synthetic pentapeptide derivative: N-t-butyloxycarbonyl-jS-alanyl-l-tryptophyl-l-me-thionyl-l-aspartyl-l-phenylalanyl amide. It contains the COOH-terminal tetrapeptide amide (H • Try • Met • Asp • Phe • NH2), which is considered to be the active center of the natural gastrins. Accordingly, pentagastrin appears to have the physiological and pharmacological properties of the gastrins, including stimulation of gastric secretion, pepsin secretion, gastric motility, pancreatic secretion of water and bicarbonate, pancreatic enzyme secretion, biliary flow and bicarbonate output, intrinsic factor secretion, and contraction of the gallbladder.

Pentagastrin is indicated as a diagnostic agent to evaluate gastric acid secretory function, and it is useful in testing for anacidity in patients with suspected pernicious anemia, at-rophic gastritis or gastric carcinoma, hypersecretion in suspected duodenal ulcer or postoperative stomal ulcers, and Zollinger-Ellison tumor.

Pentagastrin is usually administered subcutaneously; the optimal dose is 6 pg/kg. Gastric acid secretion begins approximately 10 minutes after administration, and peak responses usually occur within 20 to 30 minutes. The usual duration of action is from 60 to 80 minutes. Pentagastrin has a relatively short plasma half-life, perhaps less than 10 minutes. The available data from metabolic studies indicate that pentagastrin is inactivated by the liver, kidney, and tissues of the upper intestine.

Contraindications include hypersensitivity or idiosyncrasy to pentagastrin. It should be used with caution in patients with pancreatic, hepatic, or biliary disease.

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