Opening of Mitochondrial K ATP Channels Attenuates the Ouabain-Induced Calcium Overload in Mitochondria

We tested whether opening of mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channels depolarizes mitochondrial membrane potential (DeltaPsi(m)) and thereby prevents the mitochondrial Ca(2+) overload. With the use of a Nipkow disk confocal system, the mitochondrial Ca(2+) concentration ([Ca(2+)](m)) and DeltaPsi(m) in rat ventricular myocytes were measured by loading cells with Rhod-2 and JC-1, respectively. Exposure to ouabain (1 mmol/L) for 30 minutes produced mitochondrial Ca(2+) overload, and the intensity of Rhod-2 fluorescence significantly increased to 173+/-16% of baseline (P<0.001). Treatment of myocytes with the mitoK(ATP) channel opener diazoxide (100 micromol/L) blunted the ouabain-induced mitochondrial Ca(2+) overload (131+/-10% of baseline; P<0.001 versus ouabain). Moreover, diazoxide significantly depolarized the DeltaPsi(m) and reduced the intensity of JC-1 fluorescence during application of ouabain to 89+/-2% of baseline (P<0.05). These effects of diazoxide were blocked by the mitoK(ATP) channel blocker 5-hydroxydecanoate (500 micromol/L). These results indicate that opening of mitoK(ATP) channels prevents a mitochondrial Ca(2+) overload in association with DeltaPsi(m) depolarization and thereby protects myocardium against ischemic damage.