This investigation aimed to assess whether the mitochondrial ATP-sensitive potassium channel blocker 5-hydroxydecanoate (5-HD) could abolish the protection conferred by fasting and ischemic preconditioning (IPC) and to ascertain whether these effects are associated with glycogen breakdown and glycolytic activity. Langendorff perfused hearts of fed and 24-h fasted rats were exposed to 25 min ischemia plus 30 min reperfusion. IPC was achieved by a 3 min ischemia plus a 5 min reperfusion cycle. 5-HD (100 microM) perfusion begun 5 min before IPC or 13 min before sustained ischemia in the non preconditioned groups. Fasting improved the reperfusion recovery of contraction, decreased the contracture and the lactate production, increased glycogenolysis and did not affect the percentage of viable tissue. 5-HD abolished the effects of fasting on the contractile recovery but did not affect the contracture. 5-HD decreased the lactate production in the fed group, increased the preischemic glycogen content in both nutritional groups and did not affect the ischemic glycogen fall. IPC improved the contractile function but prevented the contracture only in the fed group, reduced lactate accumulation and glycogenolysis and evoked an increase of the viable tissue. 5-HD abolished the effects of IPC on the contractile recovery and did not affect its effect on the contracture, lactate production, glycogenolysis and viable tissue. These data suggest that the mitocondrial ATP-sensitive potassium channel is involved in the effects of fasting and IPC on the contractile function but the other cardioprotective and metabolic effects appear evoked through other mechanisms. Also suggest that besides the inhibition of the mitochondrial potassium channel, other mechanisms mediate the effects of 5-HD.