(MC). The present study was performed to determine the specific isoform(s) of conventional PKC involved in activating SOC in MC. Fura 2 fluorescence ratiometry showed that the thapsigargin-induced Ca 2ϩ entry (equivalent to SOC) was significantly inhibited by 1 M Gö -6976 (a specific PKC␣ and I inhibitor) and PKC␣ antisense treatment (2.5 nM for 24-48 h). However, LY-379196 (PKC inhibitor) and 2,2Ј,3,3Ј,4,4Ј-hexahydroxy-1,1Ј-biphenyl-6,6Ј-dimethanoldimethyl ether (HBDDE; PKC␣ and ␥ inhibitor) failed to affect thapsigargin-evoked activation of SOC. Single-channel analysis in the cell-attached configuration revealed that Gö -6976 and PKC␣ antisense significantly depressed thapsigargin-induced activation of SOC. However, LY-379196 and HBDDE did not affect the SOC responses. In inside-out patches, application of purified PKC␣ or I, but not II or ␥, significantly rescued SOC from postexcision rundown. Western blot analysis revealed that thapsigargin evoked a decrease in cytosolic expression with a corresponding increase in membrane expression of PKC␣ and ␥. However, the translocation from cytosol to membranes was not detected for PKCI or II. These results suggest that PKC␣ participates in the intracellular signaling pathway for activating SOC upon release of intracellular stores of Ca 2ϩ