THE ROLE OF THE FREE CYTOSOLIC CALCIUM LEVEL IN BETA-CELL SIGNAL TRANSDUCTION BY GASTRIC-INHIBITORY POLYPEPTIDE AND GLUCAGON-LIKE PEPTIDE I(7-37)

Using the glucose-responsive hamster beta-cell line (hamster insulin tumor cells), we examined the cellular mechanisms by which gastric inhibitory polypeptide (GIP) and glucagon-like peptide 1(7-37) (GLP-1) potentiate glucose-stimulated insulin secretion. Glucose alone increased insulin secretion and increased the free cytosolic calcium levels ([Ca2+]i) without altering cAMP content. When added to glucose-stimulated cells, GIP and GLP-I increased cAMP levels and further increased insulin secretion. At 4 mm but not 0.4 mm glucose, both peptides triggered a dose-dependent rise in [Ca2+]i with ED50s of 0.4 and 0.2 nm for GIP and GLP-1, respectively. The increase in [Ca2+]i was blocked by either chelation of extracellular Ca2+ with EGTA or nimodipine, the voltage-dependent Ca2+ channel blocker. Nimodipine also inhibited the potentiation of glucose-stimulated insulin secretion by GIP and GLP-I without inhibition of the stimulatory effect of these two peptides on cAMP accumulation. Neither peptide altered phosphoinositide metabolism, further underlining that the mobilization of intracellular Ca2+ from endoplasmic reticulum is not involved in the GIP and GLP-I signal transduction pathways. This study establishes that GIP and GLP-1 potentiate glucose-stimulated insulin secretion by increasing extracellular Ca2+ influx through voltage-dependent Ca2+ channels.