Insulin 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 primarily 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. In 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. Using 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. In 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.

Beta cells couple stimulation by glucose with insulin secretion and impairments in this coupling play a central role in diabetes mellitus. To clarify the effect of cAMP and the role of Epac2A in intracellular calcium signals and intercellular coupling, we performed functional multicellular calcium imaging in beta cells in mouse pancreas tissue slices after stimulation with glucose and forskolin in wild-type and Epac2A knock-out mice. Increased cAMP evoked calcium signals in otherwise sub-stimulatory glucose and beta cells from Epac2A knock-out mice displayed a faster activation. During the plateau phase, beta cells from Epac2A knock-out mice displayed a slightly higher active time in response to glucose compared with wild-type littermates, and increased cAMP increased the active time via a large increase in oscillation frequency and small decrease in oscillation duration in both Epac2A knock-out and wild-type mice. Functional network properties during stimulation with glucose did not differ in Epac2A knock-out mice, but the presence of Epac2A was crucial for the protective effect of increased cAMP in preventing a decline in beta cell functional connectivity with time. Finally, increased cAMP prolonged beta cell activity during deactivation in an Epac2A-independent manner.