Creation of an inositol 1,4,5-trisphosphate-sensitive Ca2+ store in secretory granules of insulin-producing cells.

A rapid rise in the cytosolic free Ca2+ concentration due to influx of extracellular Ca2+ or mobilization from intracellular stores is the primary trigger for exocytosis from secretory cells. Our understanding as to the precise role of Ca2+ mobilization has been complicated by the presence of several types of Ca2+ stores in most cells. We now demonstrate that overexpression of the type 3 inositol 1,4,5-trisphosphate (IP3) receptor in insulin-secreting beta TC-3 cells results in the creation of a unique IP3-sensitive Ca2+ pool, restricted to the insulin secretory granules of these cells.