Background Many studies have indicated that hyperpolarizing cardioplegia is responsible for myocardial preservation and researchers have suggested that the adenosine triphosphate-sensitive potassium channels (KATP) were the end effectors of cardio-protection.But whether mitochondrial KATP plays an important role in hyperpolarizing cardioplegia is not apparent.The present study investigated the effect of hyperpolarizing cardioplegia containing pinacidil (a nonselective KATP opener) on ischemia/reperfusion injury in rat hearts,especially the role of mitochondrial KATP in pinacidil hyperpolarizing cardioplegia.Methods Sprague-Dawley rat hearts were Langendorff-perfused for 20 minutes with Krebs-Henseleit buffer at 37℃before equilibration.Cardiac arrest was then induced in different treatments:there was no arrest and ischemia in the normal group,the control group were arrested by clamping the aorta,depolarizing caidioplegia (St.Thomas solution containing 16 mmol/L KCI) and hyperpolarizing cardioplegia groups used St.Thomas solution containing 0.05 mmol/L pinacidil and 5 mmol/L KCI to induce cardiac arrest in group hyperkalemic and group pinacidil,in group hyperkalemic + 5-hydroxydecanote (5HD) and Pinacidil + 5HD,5HD (0.1 mmol/L) was added to the above two solutions to block mitochondria KATP channels.Global ischemia was then administrated for 40 minutes at 37℃,followed by 30 minutes of reperfusion.At the end of equilibration and reperfusion,hemodynamics,ultrastructure,and mitochondrial function were measured.Results In the control group,ischemia/reperfusion decreased the left ventricular developed pressure,heart rate,coronary flow,mitochondrial membrane potential,impaired mitochondrial respiratory function,increased reactive oxygen species and left ventricular end diastolic pressure.Damage to myocardial ultrastructure was also evident.Both depolarized arrest and especially hyperpolarized cardioplegia significantly reduced these lesions.5HD partially blocked the beneficial effects of pinacidil cardioplegia but showing no effects on hyperkalemic arrest.Conclusions Pinacidil cardioplegia provides better cardioprotection with preservation of hemodynamics,ultrastructure,and mitochondrial function than traditional cardioplegia.The mitochondria KATP channels may play an important role in the protection mechanism.