PKC Inhibits Sec61 Translocon-Mediated Sarcoplasmic Reticulum Ca²⁺ Leak in Smooth Muscle Cells

PKC inhibitors stimulate Ca²⁺ release from internal stores in diverse cell types. Our data indicate that this action cannot be explained by an increased agonist-induced IP₃ production or an overloaded SR Ca²⁺ 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 Ca²⁺ leak revealed by inhibition of the SERCA pump with thapsigargin. This SR Ca²⁺ leakage was sensitive to protein translocation inhibitors such as emetine and anisomycin. Since this increased SR Ca²⁺ leak did not result in a depleted SR Ca²⁺ 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, Ca²⁺-sensitive potassium channels in response to RyR-mediated Ca²⁺ sparks. This increased STOC frequency triggered by PKC inhibition was restored to normal by inhibiting translocon-mediated Ca²⁺ leak with emetine. These results suggest a critical role of PKC-mediated translocon phosphorylation in regulating SR Ca²⁺ steady-state, which, in turn, alters SR Ca²⁺ releasing activity.