Background—Opening of the mitochondrial permeability transition pore (mPTP) is a crucial event in lethal reperfusion injury. Phosphorylation (inhibition) of glycogen synthase kinase-3 (GSK3) has been involved in cardioprotection. We investigated whether phosphorylated GSK3 may protect the heart via the inhibition of mPTP opening during postconditioning.

Methods and Results—Wild-type and transgenic GSK3-S9A mice (the cardiac GSK3 activity of which cannot be inactivated) underwent 60 minutes of ischemia and 24 hours of reperfusion. At reperfusion, wild-type and GSK3-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 GSK3 inhibitor SB216763 (SB21; 70 µg/kg IV). Infarct size was assessed by triphenyltetrazolium chloride staining. The resistance of the mPTP to opening after Ca²⁺ 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 GSK3-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 GSK3-S9A mice.

Conclusion—These results suggest that S9-phosphorylation of GSK3 is required for postconditioning and likely acts by inhibiting the opening of the mitochondrial permeability transition pore.