Diazoxide, a selective opener of the mitochondrial
ATP-sensitive potassium channel, has been shown to elicit tolerance to
ischemia in cardiac myocytes and in perfused heart. However, the mechanism of this cardioprotection is poorly understood. Because
reactive oxygen species (ROS) are recognized as important intracellular signaling molecules and have been implicated in ischemic preconditioning, we examined
diazoxide-induced ROS production in adult cardiomyocytes. Cells treated with 50 micromol/L
diazoxide showed a 173% increase in ROS production relative to baseline.
5-Hydroxydecanoate was found to attenuate the
diazoxide-induced increase in ROS generation. The
diazoxide-induced increase in ROS also was abrogated by the addition of either the
antioxidant N-acetylcysteine (NAC) or N-
mercaptopropionylglycine. We also examined the ability of NAC to block the protective effects of
diazoxide in the perfused rat heart. After 20 minutes of global
ischemia and 20 minutes of reflow, hearts perfused with 100 micromol/L
diazoxide before
ischemia showed significantly improved postischemic contractile function relative to untreated hearts (84% versus 29% of initial left ventricular developed pressure, respectively). Hearts treated with
diazoxide in the presence of 4 mmol/
L NAC recovered 53% of initial left ventricular developed pressure, whereas hearts treated with NAC alone recovered 46% of preischemic function. Using (31)P NMR spectroscopy, we found that, similar to preconditioning,
diazoxide significantly attenuated
ischemia-induced intracellular acidification and enhanced post- ischemic recovery of
phosphocreatine levels, both of which were blocked by cotreatment with NAC. These data suggest that the cardioprotective actions of
diazoxide are mediated by generation of a
pro-oxidant environment.