Ca2+-induced Ca2+ Release via Inositol 1,4,5-trisphosphate Receptors Is Amplified by Protein Kinase A and Triggers Exocytosis in Pancreatic β-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 Ca2+-induced Ca2+ release (CICR) in pancreatic β-cells from mice and rats and the role of CICR in secretion. CICR was observed as pronounced Ca2+ spikes on top of glucose- or depolarization-dependent rise of the cytoplasmic Ca2+ concentration ([Ca2+]i). cAMP-elevating agents strongly promoted CICR. This effect involved sensitization of the receptors underlying CICR, because many cells exhibited the characteristic Ca2+ spiking at low or even in the absence of depolarization-dependent elevation of [Ca2+]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 Ca2+, failed to mobilize intracellular Ca2+. On the contrary, a high caffeine concentration abolished CICR by interfering with inositol 1,4,5-trisphosphate receptors (IP3Rs). Therefore, the cell-permeable IP3R antagonist 2-aminoethoxydiphenyl borate blocked the cAMP-promoted CICR. Individual CICR events in pancreatic β-cells were followed by [Ca2+]i spikes in neighboring human erythroleukemia cells, used to report secretory events in the β-cells. The results indicate that protein kinase A-mediated promotion of CICR via IP3Rs is part of the mechanism by which cAMP amplifies insulin release.