Effect of synthetic gastric inhibitory polypeptide (GIP) on insulin and glucagon secretion was studied in vivo and in vitro in the rat. Intravenous administration of 1 μg./kg. GIP along with 0.625 gm./kg. glucose caused a more prominent rise of plasma insulin than did 0.625 gm./kg. glucose alone. The suppression of plasma glucagon levels induced by glucose was attenuated partially but not significantly by the concomitant administration of GIP. GIP (1 μg./kg. i.v.) alone raised both plasma insulin and glucagon levels. In in-vi tro experiments with isolated pancreatic islets, GIP significantly augmented insulin release induced by either 8.3 mM or 16.7 mM glucose, whereas the augmentation of glucagon release was observed at 3.3 mM, 8.3 mM, and 16.7 mM glucose concentrations. Three peptides, consisting of 1–28, 22–43, and 15–43 ammo acids of GIP, failed to potentiate insulin and glucagon secretion. These results suggest that synthetic GIP has a stimulating effect on insulin and glucagon secretion.

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