Modulation of Ca2+ mobilization by protein kinase C in rat submandibular acinar cells

The effects of protein kinase C (PKC) activation and inhibition on the inositol 1,4,5-trisphosphate (IP3) and cytosolic Ca2+ ([Ca2+]i) responses of rat submandibular acinar cells were investigated. IP3 formation in response to acetylcholine (ACh) was not affected by the PKC activator phorbol 12-myristate 13-acetate (PMA), nor by the PKC inhibitor calphostin C (CaC). The ACh-elicited initial increase in [Ca2+]i in the absence of extracellular Ca2+ was not changed by short-term (0.5 min) exposure to PMA, but significantly reduced by long-term (30 min) exposure to PMA, and also by pre-exposure to the PKC inhibitors CaC and chelerythrine chloride (ChC). After ACh stimulation, subsequent exposure to ionomycin caused a significantly (258%) larger [Ca2+]i increase in CaC-treated cells than in control cells. However, pre-exposure to CaC for 30 min did not alter the Ca2+ release induced by ionomycin alone. These results suggest that the reduction of the initial [Ca2+]i increase is due to an inhibition of the Ca2+ release mechanism and not to store shrinkage. The thapsigargin (TG)-induced increase in [Ca2+]i was significantly reduced by short-term (0.5 min), but not by long-term (30 min) exposure to PMA, nor by pre-exposure to ChC or CaC. Subsequent exposure to ionomycin after TG resulted in a significantly (70%) larger [Ca2+]i increase in PMA-treated cells than in control cells, suggesting that activation of PKC slows down the Ca2+ efflux or passive leak seen in the presence of TG. Taken together, these results indicate that inhibition of PKC reduces the IP3-induced Ca2+ release and activation of PKC reduces the Ca2+ efflux seen after inhibition of the endoplasmic Ca2+-ATPase in submandibular acinar cells.