Gastrins are a family of sequence-related carboxyamidated peptides produced by endocrine G Cells of the antrum mucosa in response to a number of stimuli associated with digestion. Antral distension, partially digested proteins, amino acids, and vagal stimulation resulting from smelling, tasting, chewing or swallowing food all contribute to gastrin release from G Cell storage. In addition, caffeine, alcohol, hypoglycemia, antacids and elevated calcium levels will also stimulate gastrin release. Increased serum gastrin levels are associated with duodenal ulcers, Helicobacter pylori infections, colorectal carcinomas, and other tumors and cancerous lesions. Gastrin is the most potent stimulator of gastric acid secretion.

Gastrin-I (G17): H2N-Glu-Gly-Pro-Trp-Leu-Glu-Glu-Glu-Glu-Glu-Ala-Tyr-Gly-Trp-Met-Asp-Phe-NH2

Gastrin is synthesized as a 101 residue pre-pro-peptide on the rough endoplasmic reticulum, then post-translationally modified by cleavage and alpha-amidation to result in the active forms G34, G17 and G13/14; Big, Little and Mini-Gastrins respectively. Other forms also exist, but are not considered biologically significant. There are two types of G17 and G34, type II is sulfated at the tyrosine12 residue, while type I is not. Both G34 and G17 circulate and contribute to the stimulation of gastric acid secretion, but have different clearance rates. In man, G17 has a circulating half-life of about 9 minutes while G34 has a half-life of about 35 minutes.

G34 is the major circulating gastrin in fasting serum, but with G17, increases two to three-fold after feeding until both are present in approximately equal amounts. Fasting gastrin concentration is usually less than 30 pmol/l (equivalent to 62.9 pg/ml) while patients with Zollinger-Ellison Syndrome and gastrinomas have levels exceeding 1,000 pg/ml.

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