Glucose stimulates voltage- and calcium-dependent inositol trisphosphate production and intracellular calcium mobilization in insulin-secreting beta TC3 cells

The cellular processes leading to a rise in the intracellular free Ca2+ concentration ([Ca2+](i)) after glucose stimulation and Kt depolarization were investigated in insulin-secreting beta TC3 cells. Stimulation with 11.2 mM glucose causes inositol 1,4,5-trisphosphate production and release of Ca2+ from intracellular stores. A strong correlation was observed between the changes in Ins(1,4,5)P-3 concentration and the rise in [Ca2+](i), consistent with the former compound being responsible for release of Ca2+ from intracellular stores. The increase in Ins(1,4,5)P-3 production was reduced by 68 +/- 4% when [Ca2+](i) was kept low on glucose stimulation by loading cells with the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-NNN'N'-tetra- acetic acid (BAPTA). The Ins(1,4,5)P-3 production was prevented in cells hyperpolarized with diazoxide, an opener of ATP-sensitive K+-channels, consistent with the membrane potential controlling the rate of Ins(1,4,5)P-3 synthesis. Depolarizing K+ concentrations evoked changes in [Ca2+](i) and Ins(1,4,5)P-3 production in both the presence and the absence of extracellular Ca2+, and from the relation between the extracellular K+ concentration and membrane potential we found a half-maximal Ins(1,4,5)P-3 production by a 28 mV depolarization from a resting potential of -56 mV and by a rise in [Ca2+](i) of 390 nM. We conclude that stimulation-induced changes in membrane potential and [Ca2+](i) are important in controlling Ins(1,4,5)P-3 production in beta TC-3 cells and that glucose-stimulated Ca2+ mobilization from intracellular stores is due to voltage-dependent Ins(1,4,5)P-3 production and depends on the concurrent increase in [Ca2+](i).