Mitochondria can either enhance or suppress cell death. Cytochrome c release from mitochondria and depolarization of the mitochondrial membrane potential (DeltaPsi) are crucial events in triggering apoptosis. In contrast, activation of mitochondrial ATP-sensitive potassium (mitoK(ATP)) channels prevents lethal ischemic injury in vivo, implicating these channels as key players in the process of ischemic preconditioning. We probed the relationship between mitoK(ATP) channels and apoptosis in cultured neonatal rat cardiac ventricular myocytes. Incubation with 200 micromol/L hydrogen peroxide induced TUNEL positivity, cytochrome c translocation, caspase-3 activation, poly(ADP-ribose) polymerase cleavage, and dissipation of DeltaPsi. Pharmacological opening of mitoK(ATP) channels by diazoxide (100 micromol/L) preserved mitochondrial integrity and suppressed all markers of apoptosis. Diazoxide prevented DeltaPsi depolarization in a concentration-dependent manner (EC(50) approximately 40 micromol/L, with saturation by 100 micromol/L), as shown by both flow cytometry and quantitative image analysis of cells stained with fluorescent DeltaPsi indicators. These cytoprotective effects of diazoxide were reproduced by pinacidil, another mitoK(ATP) agonist, and blocked by the mitoK(ATP) channel antagonist 5-hydroxydecanoate (500 micromol/L). Our findings identify a novel mitochondrial pathway that is protective against apoptosis. The results also pinpoint mitoK(ATP) channels as logical therapeutic targets in diseases of enhanced apoptosis and oxidative stress.