['Opening of the mitochondrial permeability transition pore (mPTP) is a crucial event in lethal reperfusion injury. Phosphorylation (inhibition) of glycogen synthase kinase-3beta (GSK3beta) has been involved in cardioprotection. We investigated whether phosphorylated GSK3beta may protect the heart via the inhibition of mPTP opening during postconditioning.', 'Wild-type and transgenic GSK3beta-S9A mice (the cardiac GSK3beta activity of which cannot be inactivated) underwent 60 minutes of ischemia and 24 hours of reperfusion. At reperfusion, wild-type and GSK3beta-S9A mice received no intervention (control), postconditioning (3 cycles of 1 minute ischemia and 1 minute of reperfusion), the mPTP inhibitor cyclosporine A (CsA; 10 mg/kg IV), or the GSK3beta inhibitor SB216763 (SB21; 70 microg/kg IV). Infarct size was assessed by triphenyltetrazolium chloride staining. The resistance of the mPTP to opening after Ca(2+) loading was assessed by spectrofluorometry on mitochondria isolated from the area at risk. In wild-type mice, infarct size was significantly reduced by postconditioning, CsA, and SB21, averaging 39+/-2%, 35+/-5%, and 37+/-4%, respectively, versus 58+/-5% of the area at risk in control mice (P<0.05). In GSK3beta-S9A mice, only CsA, but not postconditioning or SB21, reduced infarct size. Postconditioning, CsA, and SB21 all improved the resistance of the mPTP in wild-type mice, but only CsA did so in GSK3beta-S9A mice.'] Is inhibition of GSK3beta by postconditioning required to prevent opening of the mitochondrial permeability transition pore during reperfusion?