Intracellular Ca2+ channels initiate physiological glucose signaling in beta cells examined in situ

Abstract

AbstractObjectiveInsulin release from pancreatic beta cells is driven by cytosolic [Ca2+]c oscillations of several different time scales that are primarily attributed to plasma membrane ion channel activity. However, the majority of past studies have been performed at supraphysiological glucose concentrations above 10 mM using electrophysiologic approaches that solely measure plasma membrane ion fluxes. The role of endoplasmic reticulum (ER) Ca2+ stores in glucose-stimulated Ca2+ signaling remains poorly understood.MethodsIn this study, we hypothesized new, brighter [Ca2+]c sensors coupled with high-resolution functional Ca2+ imaging could be used to test a previously unappreciated role for the ryanodine and IP3 intracellular Ca2+ release channels in [Ca2+]c oscillations stimulated by increases from 6 mM to 8 mM glucose.ResultsUsing mouse pancreas tissue slices exposed to physiological glucose increments, our results show that glucose-dependent activation of IP3 and ryanodine receptors produces two kinetically distinct forms of compound events involving calcium-induced Ca2+ release. Ca2+ release mediated by IP3 and ryanodine receptors was sufficient to generate Ca2+ oscillations and necessary for the response to physiological glucose, which could be initiated in the absence of Ca2+ influx across the plasma membrane through voltage-gated Ca2+ channels.ConclusionsIn aggregate, these data suggest that intracellular Ca2+ receptors play a key role in shaping glucose-dependent [Ca2+]c responses in pancreatic beta cells in situ. In our revised model, the primary role for plasma membrane Ca2+ influx at physiological glucose concentrations is to refill ER Ca2+ stores.