Ca2+-induced Ca2+ Release via Inositol 1,4,5-trisphosphate Receptors Is Amplified by Protein Kinase A and Triggers Exocytosis in Pancreatic β-Cells

Oleg Dyachok,Erik Gylfe

doi:10.1074/jbc.m407673200

Abstract

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.

Highlights

Glucose is the most important physiological stimulator of insulin secretion from pancreatic ␤-cells
Ca2؉-induced Ca2؉ release (CICR) Is Promoted by Glucagon, glucagonlike peptide-1 amide fragment 7–36 (GLP-1), and other cAMP Agonists—In accordance with earlier data [37, 38], increase of the glucose concentration from 3 to 20 mM induced initial lowering of [Ca2ϩ]i in ␤-cells followed by slow, large amplitude oscillations of [Ca2ϩ]i (Fig. 1A)
The hormones GLP-1 and glucagon potently amplify nutrient-stimulated insulin secretion by raising cAMP, which interacts with a plethora of signal transduction processes, including ion channel activity, intracellular Ca2ϩ handling, and exocytosis of the insulin-containing granules [47]

Summary

Introduction

Glucose is the most important physiological stimulator of insulin secretion from pancreatic ␤-cells. Our study showed that CICR in primary ␤-cells from mice, rats, and human subjects is caused by activation of IP3Rs rather than RyRs. Agents raising cAMP have been found to promote intracellular Ca2ϩ mobilization in insulin-releasing cell lines and pancreatic ␤-cells, and this action was proposed to be mediated by sensitization of either IP3Rs (24 –26) or RyRs (9, 10, 20 –22, 26, 27).

Results

Conclusion