Staurosporine mobilizes Ca(2+) from secretory granules by inhibiting protein kinase C in rat submandibular acinar cells.

Staurosporine was previously shown to mobilize Ca(2+) from the thapsigargin-insensitive Ca(2+) store in rat submandibular acinar cells. However, the nature of the store is not yet known. Therefore, in the present study, the staurosporine-releasable intracellular Ca(2+) store was characterized. Staurosporine increased the cytosolic Ca(2+) concentration ([Ca(2+)](c)) after the inositol 1,4,5-trisphosphate (IP(3))-sensitive Ca(2+) store was depleted. Ionomycin caused only small increases in [Ca(2+)](c) after the depletion of the IP(3)-sensitive Ca(2+) store, whereas ionomycin+monensin caused large increases. However, ionomycin+monensin did not increase [Ca(2+)](c) when added after [Ca(2+)](c) was increased by staurosporine, indicating that the acidic Ca(2+) store was the main source of Ca(2+). The acidic Ca(2+) store appeared to be associated with secretory granules, since ionomycin+monensin- and staurosporine-induced [Ca(2+)](c) increases were significantly reduced when the acinar cells were degranulated. The effect of staurosporine on [Ca(2+)](c) was mimicked by other protein kinase C inhibitors. Therefore, we conclude that staurosporine mobilizes Ca(2+) from secretory granules, probably through the inhibition of protein kinase C in rat submandibular acinar cells.