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-coup led receptors (GPCR) in polarized exocrine cells to address the questions o f how luminal to basal Ca2+ waves can be generated in a receptor-specific m anner and whether quantal Ca2+ release reflects partial release from a cont inuous pool or an all-or-none release from a compartmentalized pool. Immuno localization revealed that expression of GPCRs in polarized cells is not un iform, with high levels of GPCR expression at or near the tight junctions. Measurement of phospholipase C beta activity and receptor-dependent recruit ment and trapping of the box domain of RGS4 in GPCRs complexes indicated au tonomous functioning of G(q)-coupled receptors in acinar cells. These findi ngs explain the generation of receptor-specific Ca2+ waves and why the wave s 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+ wav e with the same initiation site and pattern was launched by inhibition of s arco/endoplasmic reticulum Ca2+-ATPase pumps of cells continuously stimulat ed with sub-maximal agonist concentration. By contrast, rapid sequential ap plication of sub-maximal and maximal agonist concentrations to the same cel l triggered C2+ waves with different initiation sites. These findings indic ate that signaling specificity in pancreatic acinar cells is aided by polar ized expression and autonomous functioning of GPCRs and that quantal C2+ re lease is not due to a partial Ca2+ release from a continuous pool, but rath er, it is due to an all-or-none Ca2+ release from a compartmentalized Ca2pool.