Mitochondrial ATP-Sensitive Potassium Channels Attenuate Matrix Ca Overload During Simulated Ischemia and Reperfusion Possible Mechanism of Cardioprotection

Mitochondrial ATP-sensitive potassium (mitoKATP) channels play a key role in ischemic preconditioning of the heart. However, the mechanism of cardioprotection remains controversial. We measured rhod-2 fluorescence in adult rabbit ventricular cardiomyocytes as an index of mitochondrial matrix Ca concentration ([Ca ]m), using time-lapse confocal microscopy. To simulate ischemia and reperfusion (I/R), cells were exposed to metabolic inhibition (50 minutes) followed by washout with control solution. Rhod-2 fluorescence gradually increased during simulated ischemia and rose even further with reperfusion. The mitoKATP channel opener diazoxide attenuated the accumulation of [Ca 2 ]m during simulated I/R (EC50 18 mol/L). These effects of diazoxide were blocked by the mitoKATP channel antagonist 5-hydroxydecanoate (5HD). In contrast, inhibitors of the mitochondrial permeability transition (MPT), cyclosporin A and bongkrekic acid, did not alter [Ca ]m accumulation during ischemia, but markedly suppressed the surge in rhod-2 fluorescence during reperfusion. Measurements of mitochondrial membrane potential, m, in permeabilized myocytes revealed that diazoxide depolarized m (by 12% at 10 mol/L, P 0.01) in a 5HD-inhibitable manner. Our data support the hypothesis that attenuation of mitochondrial Ca overload, as a consequence of partial mitochondrial membrane depolarization by mitoKATP channels, underlies cardioprotection. Furthermore, mitoKATP channels and the MPT differentially affect mitochondrial calcium homeostasis: mitoKATP channels suppress calcium accumulation during I/R, while the MPT comes into play only upon reperfusion. (Circ Res. 2001;89:891-898.)