Mitochondrial ATP-dependent potassium channels: Novel effectors of cardioprotection?

Background - Brief interruptions of coronary blood flow paradoxically protect the heart from subsequent prolonged ischemia. The basis of such endogenous cardioprotection, known as 'ischemic preconditioning,' remains uncertain. Pharmacological evidence has implicated ATP-dependent potassium (K(ATP)) channels in the mechanism of preconditioning; however, the effects of sarcolemmal K(ATP) channels on excitability cannot account for the protection. Methods and Results - We simultaneously measured flavoprotein fluorescence, an index of mitochondrial redox state, and sarcolemmal K(ATP) currents in intact rabbit ventricular myocytes. Our results show that diazoxide, a K(ATP) channel opener, selectively activates mitochondrial K(ATP) channels. Diazoxide induced reversible oxidation of flavoproteins with an EC₅₀ of 27 μmol/L but did not activate sarcolemmal K(ATP) channels. The subcellular site of diazoxide action is further localized to mitochondria by confocal imaging of fluorescence arising from flavoproteins and tetramethylrhodamine ethyl ester. In a cellular model of simulated ischemia, inclusion of diazoxide decreased the rate of cell death to about half of that in controls. Both the redox changes and protection are inhibited by the K(ATP) channel blocker 5-hydroxydecanoic acid. Conclusions - Our results demonstrate that diazoxide targets mitochondrial but nor sarcolemmal K(ATP) channels and imply that mitochondrial K(ATP) channels may mediate the protection from K(ATP) channel openers.