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.
Dyachok
Oleg
O
Department of Medical Cell Biology, Uppsala University, Uppsala, Sweden.
Gylfe
Erik
E
D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000502:physiology*; D002478:Cells, Cultured; D000242:Cyclic AMP / Q000502:physiology; D017868:Cyclic AMP-Dependent Protein Kinases / Q000502:physiology*; D005089:Exocytosis; D005934:Glucagon / Q000494:pharmacology; D052216:Glucagon-Like Peptide 1; D053496:Inositol 1,4,5-Trisphosphate Receptors; D007515:Islets of Langerhans / Q000378:metabolism*; D051379:Mice; D008820:Mice, Obese; D010446:Peptide Fragments / Q000494:pharmacology; D011498:Protein Precursors / Q000494:pharmacology; D051381:Rats; D017208:Rats, Wistar; D018160:Receptors, Cytoplasmic and Nuclear / Q000502:physiology*; D012433:Ryanodine / Q000494:pharmacology
D015220:Calcium Channels; C506926:ITPR1 protein, human; D053496:Inositol 1,4,5-Trisphosphate Receptors; D010446:Peptide Fragments; D011498:Protein Precursors; D018160:Receptors, Cytoplasmic and Nuclear; D012433:Ryanodine; D052216:Glucagon-Like Peptide 1; D005934:Glucagon; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D002118:Calcium