Effects of calcium buffering on glucose-induced insulin release in mouse pancreatic islets: an approximation to the calcium sensor

1. The properties of the calcium sensor for glucose-induced insulin secreti on have been studied using cell-permeant Ca2+ buffers with distinct kinetic s and affinities. In addition, submembrane cytosolic Ca2+ distribution has been modelled after trains of glucose-induced action potential-like depolar izations. 2. Slow Ca2+ buffers (around 1 mmol l(-1) intracellular concentration) with different affinities (EGTA and Calcium Orange-5N) did not significantly af fect glucose-induced insulin release. Modelling showed no effect on cytosol ic Ca2+ concentrations at the outermost shell(0.05 mu m), their effects bei ng observed in the innermost shells dependent on Ca2+ affinity. 3. In contrast, fast Ca2+ buffers (around 1 mmol l(-1) intracellular concen tration) with different affinities (BAPTA and Calcium Green-5N) caused a 50 % inhibition of early insulin response and completely blocked the late pha se of glucose-induced. insulin response, their simulations showing a decrea se of [Ca2+](i) at both the inner and outermost shells. 4. These data are consistent with the existence in pancreatic beta-cells of a higher affinity Ca2+ sensor than that proposed for neurons. Moreover, th ese data are consistent with the proposed existence of two distinct pools o f granules: (i) 'primed' vesicles, colocalized with Ca2+ channels and respo nsible of the first phase of insulin release; and (ii) 'reserved pool' vesi cles, not colocalized and responsive for the second phase.