Dual mechanism involved in the hydrolysis of polyphosphoinositides in rat pancreatic islets

We investigated insulin secretion and inositol phosphate formation in intact and permeabilized rat pancreatic islets, the objective being to elucidate mechanisms of activation of phospholipase-C in pancreatic islets. The intact islets prelabeled with myo-[3H]inositol were incubated in Krebs-Ringer bicarbonate buffer containing 10 mM LiCl and 1 mM myoinositol. Glucose, alpha-ketoisocaproate (KIC), and sulfated cholecystokinin (CCK8S) increased insulin secretion and formation of [3H]inositol phosphate, [3H]inositol bisphosphate, and [3H] inositol trisphosphate. Mannoheptulose, a glucokinase inhibitor, inhibited glucose-induced insulin secretion and [3H]inositol phosphate formation; however, it did not inhibit KIC- and CCK8S-induced secretion and formation. Both glucose- and KIC-induced insulin secretion and [3H]inositol phosphate formation were blocked by 2,4-dinitrophenol, an uncoupler of oxidative phosphorylation in the mitochondria. The islets prelabeled with myo-[3H]inositol were permeabilized by digitonin and then incubated in intracellular mimicking medium containing 1 microM Ca2+ and 2.5 mM ATP. Glucose had no effect on [3H]inositol phosphate formation in the permeabilized islets, and CCK8S increased the formation of [3H]inositol phosphates. Thus, phospholipase-C in pancreatic islets is activated not only via ligand-receptor interaction in the plasma membrane in the case of hormone stimulation, but also by metabolic product(s) in the case of fuel stimulation.