Glucose increases extracellular [Ca2+] in rat insulinoma (INS-1E) pseudoislets as measured with Ca2+-sensitive microelectrodes

Secretory granules of pancreatic beta-cells contain high concentrations of Ca2+ ions that are co-released with insulin in the extracellular milieu upon activation of exocytosis. As a consequence, an increase in the extracellular Ca2+ concentration ([Ca2+](ext)) in the microenvironment immediately surrounding pulls should be expected following the exocytotic event. Using Ca2+-selective microelectrodes we show here that both high glucose and non-nutrient insulinotropic agents elicit a reversible increase of [Ca2+](ext) within rat insulinoma (INS-1E) beta-cells pseudoislets. The glucose-induced increases in [Ca2+](ext) are blocked by pretreatment with different Ca2+ channel blockers. Physiological agonists acting as positive or negative modulators of the insulin secretion and drugs known to intersect the secretory machinery at different levels also induce [Ca2+](ext), changes as predicted on the basis of their described action on insulin secretion. Finally, the glucose-induced [Ca2+](ext) increase is strongly inhibited after disruption of the actin web, indicating that the dynamic [Ca2+](ext) changes recorded in INS-1E pseudoislets by Ca2+-selective microelectrodes occur mainly as a consequence of exocytosis of Ca2+-rich granules. In conclusion, our data directly demonstrate that the extracellular spaces surrounding beta-cells constitute a restricted domain where Ca2+ is co-released during insulin exocytosis, creating the basis for an autocrine/paracrine cell-to-cell communication system via extracellular Ca2+ sensors. 2012 Elsevier Ltd. All rights reserved.