Background—Nitric oxide (NO) has been implicated as a mediator of “second-window” ischemic preconditioning, and mitochondrial ATP-dependent K⁺ (mitoK_ATP) channels are the likely effectors. The links between NO and mitoK_ATP channels are unknown.

Methods and Results—We measured mitochondrial redox potential as an index of mitoK_ATP channel opening in rabbit ventricular myocytes. The NO donor S-nitroso-N-acetyl-dl-penicillamine (SNAP, 0.1 to 1 mmol/L) oxidized the mitochondrial matrix dose-dependently without activating sarcolemmal K_ATP channels. SNAP-induced oxidation was blocked by the selective mitoK_ATP channel blocker 5-hydroxydecanoate and by the NO scavenger 2-(4-carboxyphenyl)-4,4′,5,5′-tetramethylimidazole-1-oxyl-3-oxide. SNAP-induced mitochondrial oxidation was detectable either by photomultiplier tube recordings of flavoprotein fluorescence or by confocal imaging. SNAP also enhanced the oxidative effects of diazoxide when both agents were applied together. Exposure to 1 mmol/L 8Br-cGMP failed to mimic the effects of SNAP.

Conclusions—NO directly activates mitoK_ATP channels and potentiates the ability of diazoxide to open these channels. These results provide novel mechanistic links between NO-induced cardioprotection and mitoK_ATP channels.