Alteration of Cholecystokinin-Mediated Phosphatidylinositol Hydrolysis in Pancreatic Acini From Insulin-Deficient Rats: Evidence for Defective G Protein Activation

Abstract

Insulin deficiency leads to a decreased ability of cholecystokinin octapeptide (CCK-8) to raise cytosolic free-calcium levels in the pancreatic acinar cell. To elucidate the mechanisms underlying this defect, we studied the effects of CCK-8 on phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis in pancreatic acini prepared from nondiabetic and streptozocin-induced diabetic rats. Analysis by high-pressure liquid chromatography indicated that, in diabetic rat acini, the CCK-8-mediated increase in [3H]inositol 1,4,5-trisphosphate ([3H]IP3) levels was delayed, and the increase in [3H]inositol 1,3,4,5-tetrakisphosphate ([3H]IP4) levels was markedly attenuated compared with nondiabetic rat acini. The expected increase in the mass levels of IP3, measured in a competitive binding assay, was reduced in the diabetic group after incubation with CCK-8, carbachol, and bombesin. Phospholipase C activity was decreased by 30% in diabetic rat acini, whereas the specific activity of PIP2 and the amount of myo-[3H]inositol in the free and trichloroacetic acid-precipitable pools were similar in both groups. The nonhydrolyzable analogue of GTP guanosine-5'-O-(3'-thiotriphosphate) rapidly enhanced IP3 levels in permeabilized acini, and the percent increase above basal was greater in the diabetic group. When added for 5 s or 2 h, insulin did not alter basal or CCK-8-stimulated [3H]IP3 and [3H]IP4 levels in either nondiabetic or diabetic rat acini. However, after a 4-h incubation, insulin increased basal [3H]IP3 and [3H]IP4 levels in diabetic rat acini and potentiated the actions of CCK-8 on both inositol phosphates. Insulinlike growth factor I did not alter [3H]IP3 and [3H]IP4 levels either acutely or after a 4-h incubation. These findings point to a defect in the signal-transduction pathway that is activated by CCK-8 and other calcium-mobilizing agonists in the diabetic rat pancreas and suggest that insulin, acting via its own receptor, exerts long-term regulatory effects on PIP2 hydrolysis in the pancreatic acinar cell.