Dual roles of mitochondrial K-ATP channels in diazoxide-mediated protection in isolated rabbit hearts

Whether the mitochondrial ATP-dependent potassium (mK(ATP)) channel is the trigger or the mediator of cardioprotection is controversial. We investigat ed the critical time sequences of mKATP channel opening for cardioprotectio n in isolated rabbit hearts. Pretreatment with diazoxide (100 muM), a selec tive mK(ATP) channel opener, for 5 min followed by 10 min washout before th e 30-min ischemia and 2-h reperfusion significantly reduced infarct size (9 +/- 3 vs. 35 +/- 3% in control), indicating a role of mK(ATP) channels as a trigger of protection. The protection was blocked by coadministration of the L-type Ca2+ channel blockers nifedipine (100 nM) or 5-hydroxydecanoic a cid (5-HD; 50 muM) or by the protein kinase C (PKC) inhibitor chelerythrine (5 muM). The protection of diazoxide was not blocked by 50 muM 5-HD but wa s blocked by 200 muM 5-HD or 10 muM glybenclamide administrated 5 min befor e and throughout the 30 min of ischemia, indicating a role of mKATP opening as a mediator of protection. Giving diazoxide throughout the 30 min of isc hemia also protected the heart, and the protection was not blocked by chele rythrine. Nifedipine did not affect the ability of diazoxide to open mKATP channels assessed by mitochondrial redox state. In electrically stimulated rabbit ventricular myocytes, diazoxide significantly increased Ca2+ transie nt but had no effect on L-type Ca2+ currents. Our results suggest that open ing of mKATP channels can trigger cardioprotection. The trigger phase may b e induced by elevation of intracellular Ca2+ and activation of PKC. During the lethal ischemia, mKATP channel opening mediates the protection, indepen dent of PKC, by yet unknown mechanisms.