Differential regulation of the apical plasma membrane Ca2+-ATPase by protein kinase a in parotid acinar cells

Cross-talk between intracellular calcium ([Ca2+](i)) signaling and cAMP defines the specificity of stimulus-response coupling in a variety of cells. Previous studies showed that protein kinase A (PKA) potentiates and phosphorylates the plasma membrane Ca2+-ATPase (PMCA) in a Ca2+-dependent manner in parotid acinar cells (Bruce, J. I. E., Yule, D. I., and Shuttleworth, T. J. (2002) J. Biol. Chem. 277, 48172 -48181). The aim of this study was to further investigate the spatial regulation of [Ca2+](i) clearance in parotid acinar cells. Par-C10 cells were used to functionally isolate the apical and basolateral PMCA activity by applying La3+ to the opposite side to inhibit the PMCA. Activation of PKA (using forskolin) differentially potentiated apical [Ca2+](i) clearance in mouse parotid acinar cells and apical PMCA activity in Par-C10 cells. Immunofluorescence of parotid tissue slices revealed that PMCA1 was distributed throughout the plasma membrane, PMCA2 was localized to the basolateral membrane, and PMCA4 was localized to the apical membrane of parotid acinar cells. However, in situ phosphorylation assays demonstrated that PMCA1 was the only isoform phosphorylated by PKA following stimulation. Similarly, immunofluorescence of acutely isolated parotid acinar cells showed that the regulatory subunit of PKA (RII beta) translocated to the apical region following stimulation. These data suggest that PKA-mediated phosphorylation of PMCA1 differentially regulates [Ca2+](i) clearance in the apical region of parotid acinar cells because of a dynamic translocation of PKA. Such tight spatial regulation of Ca2+ efflux is likely important for the fine-tuning of Ca2+-dependent effectors close to the apical membrane important for the regulation of fluid secretion and exocytosis.