PKC Inhibits Sec61 Translocon-Mediated Sarcoplasmic Reticulum Ca Leak in Smooth Muscle Cells.

PKC inhibitors stimulate Ca release from internal stores in diverse cell types. Our data indicate that this action cannot be explained by an increased agonist-induced IP production or an overloaded SR Ca pool in smooth muscle cells from guinea pig urinary bladder. The incubation of these cells with three different PKC inhibitors, such as Go6976, Go6983, and BIM 1, resulted in a higher SR Ca leak revealed by inhibition of the SERCA pump with thapsigargin. This SR Ca leakage was sensitive to protein translocation inhibitors such as emetine and anisomycin. Since this increased SR Ca leak did not result in a depleted SR Ca store, we have inferred there was a compensatory increase in SERCA pump activity, resulting in a higher steady-state. This new steady-state increased the frequency of Spontaneous Transient Outward Currents (STOCs), which reflect the activation of high conductance, Ca-sensitive potassium channels in response to RyR-mediated Ca sparks. This increased STOC frequency triggered by PKC inhibition was restored to normal by inhibiting translocon-mediated Ca leak with emetine. These results suggest a critical role of PKC-mediated translocon phosphorylation in regulating SR Ca steady-state, which, in turn, alters SR Ca releasing activity.