Inositol (1,4,5)-Trisphosphate Dynamics and Intracellular Calcium Oscillations in Pancreatic β-Cells

Abstract

Glucose-stimulated insulin secretion is associated with transients of intracellular calcium concentration ([Ca²⁺]_i) in the pancreatic β-cell. We tested the hypothesis that inositol (1,4,5)-trisphosphate [Ins(1,4,5)P₃] [Ca²⁺]_i release is incorporated in glucose-induced [Ca²⁺]_i oscillations in mouse islets and MIN6 cells. We found that depletion of intracellular Ca²⁺ stores with thapsigargin increased the oscillation frequency by twofold and inhibited the slow recovery phase of [Ca²⁺]_i oscillations. We employed a pleckstrin homology domain–containing fluorescent biosensor, phospholipase C∂ pleckstrin homology domain–enhanced green fluorescent protein, to visualize Ins(1,4,5)P₃ dynamics in insulin-secreting MIN6 cells and mouse islets in real time using a video-rate confocal system. In both types of cells, stimulation with carbamoylcholine (CCh) and depolarization with KCl results in an increase in Ins(1,4,5)P₃ accumulation in the cytoplasm. When stimulated with glucose, the Ins(1,4,5)P₃ concentration in the cytoplasm oscillates in parallel with oscillations of [Ca²⁺]_i. Maximal accumulation of Ins(1,4,5)P₃ in these oscillations coincides with the peak of [Ca²⁺]_i and tracks changes in frequencies induced by the voltage-gated K⁺ channel blockade. We show that Ins(1,4,5)P₃ release in insulin-secreting cells can be stimulated by depolarization-induced Ca²⁺ flux. We conclude that Ins(1,4,5)P₃ concentration oscillates in parallel with [Ca²⁺]_i in response to glucose stimulation, but it is not the driving force for [Ca²⁺]_i oscillations.