Staurosporine-induced Ca2+ mobilization in rat mandibular salivary acini

The effect of the protein kinase C inhibitor, staurosporine, on intracellular Ca2+ homeostasis was investigated in rat mandibular salivary acinar cells loaded with fura-2. Fura-2 fluorescence was measured at 510 nm while the excitation wavelength was alternated between 340 nm and 380 nm. The ratio of fluorescence intensity (F(340/380)) was used as an index of [Ca2+]i. Stimulation of acinar cells with 10 microM carbachol (CCh) induced a rapid increase in F(340/380) followed by a slow decrease to a sustained elevated level. Addition of 1 microM staurosporine in the presence of CCh caused a further increase in F(340/380). In order to examine whether the staurosporine-induced increase in F(340/380) could be attributed either to the Ca2+ entry pathway or to the mobilization of Ca2+ from intracellular Ca2+ stores, 1 microM staurosporine was added in the presence of CCh after Ca2+ had been omitted from the perfusate. Even in the absence of extracellular Ca2+, F(340/380) still increased slowly from 0.75 +/- 0.05 to 1.57 +/- 0.24 after a delay ranging between 5 min and 10 min. However, the IP3-sensitive intracellular Ca2+ stores did not seem to play a major role in this phenomenon because staurosporine still increased F(340/380) by 0.6 +/- 0.10 after the complete depletion of IP3-sensitive Ca2+ stores by the exposure of cells to 1 microM thapsigargin, a microsomal Ca2+ ATPase inhibitor, in Ca2+-free conditions. These results suggest that staurosporine mobilizes Ca2+ from IP3-insensitive intracellular Ca2+ stores in rat mandibular salivary acinar cells.