Polarized expression of G protein-coupled receptors and an all-or-none discharge of Ca2+ pools at initiation sites of [Ca2+]i waves in polarized exocrine cells

In the present work we examined localization and behavior of G protein-coupled receptors (GPCR) in polarized exocrine cells to address the questions of how luminal to basal Ca2+ waves can be generated in a receptor-specific manner and whether quantal Ca2+ release reflects partial release from a continuous pool or an all-or-none release from a compartmentalized pool. Immunolocalization revealed that expression of GPCRs in polarized cells is not uniform, with high levels of GPCR expression at or near the tight junctions. Measurement of phospholipase C beta activity and receptor-dependent recruitment and trapping of the box domain of RGS4 in GPCRs complexes indicated autonomous functioning of G(q)-coupled receptors in acinar cells. These findings explain the generation of receptor-specific Ca2+ waves and why the waves are always initiated at the apical pole. The initiation site of Ca2+ wave at the apical pole and the pattern of wave propagation were independent of inositol 1,4,5-trisphosphate concentration. Furthermore, a second Ca2+ wave with the same initiation site and pattern was launched by inhibition of sarco/endoplasmic reticulum Ca2+-ATPase pumps of cells continuously stimulated with sub-maximal agonist concentration. By contrast, rapid sequential application of sub-maximal and maximal agonist concentrations to the same cell triggered C2+ waves with different initiation sites. These findings indicate that signaling specificity in pancreatic acinar cells is aided by polarized expression and autonomous functioning of GPCRs and that quantal C2+ release is not due to a partial Ca2+ release from a continuous pool, but rather, it is due to an all-or-none Ca2+ release from a compartmentalized Ca2+ pool.