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 Cβ activity and receptor-dependent recruitment and trapping of the box domain of RGS4 in GPCRs complexes indicated autonomous functioning of Gq-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 Ca2+ 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 Ca2+ 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.
All Science Journal Classification (ASJC) codes
- Molecular Biology
- Cell Biology