Ca(2+)-induced Ca(2+) release via inositol 1,4,5-trisphosphate receptors is amplified by protein kinase A and triggers exocytosis in pancreatic beta-cells

Hormones, such as glucagon and glucagon-like peptide-1, potently amplify nutrient stimulated insulin secretion by raising cAMP. We have studied how cAMP affects Ca(2+)-induced Ca(2+) release (CICR) in pancreatic beta-cells from mice and rats and the role of CICR in secretion. CICR was observed as pronounced Ca(2+) spikes on top of glucose- or depolarization-dependent rise of the cytoplasmic Ca(2+) concentration (Ca(2+)](i)). cAMP-elevating agents strongly promoted CICR. This effect involved sensitization of the receptors underlying CICR, because many cells exhibited the characteristic Ca(2+) spiking at low or even in the absence of depolarization-dependent elevation of Ca(2+)](i). The cAMP effect was mimicked by a specific activator of protein kinase A in cells unresponsive to activators of cAMP-regulated guanine nucleotide exchange factor. Ryanodine pretreatment, which abolishes CICR mediated by ryanodine receptors, did not prevent CICR. Moreover, a high concentration of caffeine, known to activate ryanodine receptors independently of Ca(2+), failed to mobilize intracellular Ca(2+). On the contrary, a high caffeine concentration abolished CICR by interfering with inositol 1,4,5-trisphosphate receptors (IP(3)Rs). Therefore, the cell-permeable IP(3)R antagonist 2-aminoethoxydiphenyl borate blocked the cAMP-promoted CICR. Individual CICR events in pancreatic beta-cells were followed by Ca(2+)](i) spikes in neighboring human erythroleukemia cells, used to report secretory events in the beta-cells. The results indicate that protein kinase A-mediated promotion of CICR via IP(3)Rs is part of the mechanism by which cAMP amplifies insulin release.