Mitochondrial Ca2+-activated K+ channels in cardiac myocytes -: A mechanism of the cardioprotective effect and modulation by protein kinase A

Background - The large-conductance Ca2+- activated K+ (BKCa) channel in the cardiac inner mitochondrial membrane (mitoK(Ca) channel) has been shown to protect the heart against ischemic injury. However, questions about the cardioprotective mechanism and the kinase-mediated regulation of mitoK(Ca) channels remain to be answered. Methods and Results - Flavoprotein fluorescence in guinea pig ventricular myocytes was measured to assay mitoKCa channel activity. The mitochondrial Ca2+ concentration ([Ca2+](m)) and membrane potential (DeltaPsi(m)) were measured by loading cells with rhod-2 and JC-1, respectively. Cell death was assessed by trypan blue permeability. The BKCa channel opener NS1619 reversibly increased the flavoprotein oxidation in a concentration-dependent manner. NS1619 ( 30 mumol/L) attenuated the ouabain (1 mmol/L)-induced elevation of [Ca2+](m) with accompanying depolarization of DeltaPsi(m). These effects of NS1619 were completely antagonized by the BKCa channel blocker paxilline (2 mumol/L) but not by the mitochondrial ATP-sensitive K+ (mitoK(ATP)) channel blocker 5-hydroxydecanoate (500 mumol/L). Paxilline, however, failed to block the oxidative effect of diazoxide (100 mumol/L), a mitoK(ATP) channel opener. The combined application of submaximally effective concentrations of NS1619 (10 mumol/L) and diazoxide (30 mumol/L) produced additive effects. NS1619 (30 mumol/L) blunted the rate of cell death during exposure to ouabain; this cardioprotective effect was prevented by paxilline. Activation of cAMP-dependent protein kinase by 8-bromoadenosine 3'5'-cyclic monophosphate (0.5 mmol/L) and forskolin (10 mumol/L) potentiated the NS1619-induced flavoprotein oxidation. Conclusions - Opening of mitoK(Ca) channels, which is modulated by cAMP-dependent protein kinase, depolarizes the DeltaPsi(m) and attenuates the mitochondrial Ca2+ overload. Our study further indicates that mitoK(Ca) channel activation confers cardioprotection in a manner similar to but independent of mitoK(ATP) channel activation.