Localization of the type 3 inositol 1,4,5-Trisphosphate receptor in the Ca2+ wave trigger zone of pancreatic acinar cells

Agonist-induced cytosolic Ca²⁺ (Ca²⁺;) signals begin as apical-to-basal Ca²⁺, waves in pancreatic acinar cells and in other polarized epithelia. However, the basis of this polarized Ca²⁺, signaling pattern is unknown. Here we use immunocytochemistry to demonstrate that the type 3 inositol trisphosphate receptor is localized to the extreme apex of pancreatic acinar cells, the region which corresponds to the trigger zone from which Ca²⁺; signals originate in this cell type (Kasai, H., Li, Y. X., and Miyashita, Y. (1993) Cell 74, 669-677). We also show that inositol trisphosphate-mediated Ca²⁺ release induces amylase release from permeabilized pancreatic acini. Since Ca²⁺_i signals begin by inositol trisphosphate-mediated Ca²⁺ release, these findings suggest that localization of the type 3 inositol trisphosphate receptor to the trigger zone is responsible for the generation of apical-to-basal Ca²⁺_i waves, and that this organization may be important for regulating apical exocytosis in pancreatic acinar cells.