Role of protein kinase C and Ca2+ in glucose-induced sensitization/desensitization of insulin secretion

The role of protein kinase C and Ca2+ in glucose-induced sensitization/desensitization of insulin secretion was studied. A 22-24 h exposure of mouse pancreatic islets to glucose (16.7 mmol/l) in TCM 199 culture medium, with 0.26 mmol/l or 1.26 mmol/l Ca2+, reduced total islet protein kinase C activity to approx. 85% and 60% of control values, respectively. At 0.26 mmol/l Ca2+ in TCM 199 medium, exposure to glucose (16.7 mmol/l) led to a potentiation of both phase 1 and phase 2 of glucose-induced insulin secretion, and caused a shift in the dose-response curve with 10 mmol/l and 16.7 mmol/l glucose exhibiting equipotent effects in stimulation of insulin secretion. In glucose-sensitized islets, the phorbol ester 12-O-tetradecanoylphorbol 13-acetate (0.16 mumol/l) did not further potentiate induction of secretion by 10 mmol/l or 16.7 mmol/l glucose. At 3.3 mmol/l glucose, however, phorbol ester-induced secretion was augmented, and was characterized by a faster onset of secretion in glucose-sensitized islets relative to control islets. In contrast, a partial reduction in arachidonic acid (100 mumol/l)-induced insulin release was observed in glucose-sensitized islets in the absence of extracellular Ca2+. Increasing the Ca2+ concentration to 1.26 mmol/l in TCM 199 during the 22-24 h exposure to glucose (16.7 mmol/l) led to inhibition of phase 1 and abolition of phase 2 of glucose (10 mmol/l, 16.7 mmol/l)-induced insulin secretion. In addition, this treatment abolished phorbol ester-induced and arachidonic acid-induced insulin secretion at 3.3 mmol/l glucose. Altogether, these data suggest that sensitization of insulin secretion is caused by a preferential down-regulation of the inhibitory effects of protein kinase C, leading to an increased first phase, and an increased coupling of glucose to the stimulatory effects of protein kinase C during the second phase of glucose-induced insulin secretion. Desensitization of insulin secretion appears to be a consequence of sustained Ca2+ influx, inducing extensive down-regulation of protein kinase C and also causing deleterious effects on islet cell function in protein kinase C-deprived islets.