cAMP-enhancing agents "permit" stimulus-secretion coupling in canine pancreatic islet beta-cells

Isolated canine islets of Langerhans differ from isolated islets of other species (including rodents and man) in that elevated glucose concentrations are unable to stimulate insulin secretion. Here we demonstrate that addition to the perifusate of isobutylmethylxanthine (IBMX), forskolin or 8-CPT-cAMP, all of which enhance cytosolic cAMP, permits insulin secretion in response to glucose, leucine or tolbutamide. These cAMP enhancers increase secretogogue-induced electrical activity in beta-cells and restore depolarization-induced, Ca(2+)-dependent granule exocytosis measured as stepwise increases in membrane capacitance. We propose that the primary permissive action of cAMP is to tightly link Ca2+ entry to insulin granule release, while a secondary action is to tighten the link between glucose metabolism and cell depolarization.