High resolution analysis of the cytosolic Ca²⁺events in beta cell collectives in situ

Abstract

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.

The Role of cAMP in Beta Cell Stimulus-Secretion and Inter-cellular Coupling

“…This seems to be true only in glucose-stimulated conditions, while exendin-4, a high affinity GLP-1 agonist, facilitates CICR [107]. Moreover, latest results suggest that even under glucose stimulation intracellular Ca 2+ stores play a crucial role in shaping beta cell [Ca 2+ ]IC responses [108].…”

“…While opinions differ regarding the type of receptor, it is generally accepted that the mobilization of intracellular stores induced by GLP-1 is cAMP-dependent [95] and acts via receptor sensitization [112]. Recent research shows that both types of receptors are important, at least in glucose-induced Ca 2+ signaling [108].…”

“…While both slow and fast [Ca 2+ ]IC oscillations rely on periodic entry of Ca 2+ , the fast pattern also seems to depend on mobilization of intracellular Ca 2+ stores and agents that increase cAMP promote the appearance of regular fast [Ca 2+ ]c oscillations [108,119] Older studies, where GLP-1 was applied in pulses, ranging from 30 s [96] to a few minutes, typically report an a rapid [Ca 2+ ]IC peak, occasionally followed by a smaller, sustained [Ca 2+ ]IC elevation [89,96]. Holz et al described two main temporal components of the GLP-1-induced [Ca 2+ ]IC dynamics and proposed a mechanism behind them.…”

The Role of cAMP in Beta Cell Stimulus–Secretion and Intercellular Coupling

“…Since thapsigargin failed to reduce insulin secretion, it was previously believed that CICR was of minor physiological importance in GSIS [131]. With utilization of Ca 2+ indicators with enhanced sensitivity and high-resolution Ca 2+ imaging, the latest research suggests that, even under glucose stimulation, intracellular Ca 2+ stores play a crucial role in shaping beta cell Ca 2+ responses [132]. Contrary to that, it is agreed that exendin-4, a high affinity GLP-1 agonist, facilitates CICR and that this mechanism importantly contributes to beta cell Ca 2+ response [133][134][135].…”

“…While opinions differ regarding the type of receptor, IP3 or RYR, it is generally accepted that the mobilization of intracellular stores induced by GLP-1 is cAMP-dependent [143] and acts via receptor sensitization [138]. Recent research shows that both types of receptors are important, at least in glucose-induced Ca 2+ signaling [132].…”