PKC Inhibits Sec61 Translocon-Mediated Sarcoplasmic Reticulum Ca(2+) Leak in Smooth Muscle Cells

PKC inhibitors stimulate Ca(2+) release from internal stores in diverse cell types. Our data indicate that this action cannot be explained by an increased agonist-induced IP(3) production or an overloaded SR Ca(2+) 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(2+) leak revealed by inhibition of the SERCA pump with thapsigargin. This SR Ca(2+) leakage was sensitive to protein translocation inhibitors such as emetine and anisomycin. Since this increased SR Ca(2+) leak did not result in a depleted SR Ca(2+) 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(2+)-sensitive potassium channels in response to RyR-mediated Ca(2+) sparks. This increased STOC frequency triggered by PKC inhibition was restored to normal by inhibiting translocon-mediated Ca(2+) leak with emetine. These results suggest a critical role of PKC-mediated translocon phosphorylation in regulating SR Ca(2+) steady-state, which, in turn, alters SR Ca(2+) releasing activity.