Preconditioning (PreC) and postconditioning (PostC) have been shown to initiate a number of signaling cascades that reduce cell death. However, the mechanisms by which these signals reduce cell death have been elusive. 1 PreC has been shown to phosphorylate and thereby inhibit glycogen synthase kinase (GSK)-3ß, and perfusion with GSK inhibitors has been shown to reduce cell death induced by ischemia/reperfusion, when added before ischemia2 or when added at the start of reperfusion. 3–5 These studies are consistent with data in other tissues showing that inhibition of GSK-3ß reduces apoptosis. Information regarding the mechanism by which inhibition of GSK protects has been provided by Juhaszova et al, 6 who report that inhibition of GSK-3ß delays the opening of the mitochondrial permeability transition pore (MPT)(see the Figure). The MPT is a large-conductance pore in the inner mitochondrial membrane which is opened under conditions associated with ischemia/reperfusion, such as high matrix reactive oxygen species and high matrix calcium. Pharmacological inhibitors of the MPT have been shown to reduce ischemia/reperfusion injury, suggesting that activation of MPT might have a role in ischemia/reperfusion-mediated cell death. However the molecular components of the MPT have not been identified. 7

Juhaszova et al6 showed that myocytes isolated from mice with cardiac specific overexpression of a constitutively active form of GSK-3ß, in which the serine 9 is replaced with alanine, are not protected by PreC or diazoxide. Juhaszova et al also decreased GSK-3ß using interfering RNA and showed that this was protective, whereas decreasing GSK-3α was without effect. These data agree with data from other groups showing that inhibitors of GSK protect and that many types of cardioprotection result in increased phosphorylation of GSK-3ß. 2–6 However, the obligatory role of phosphorylation and/or inhibition of GSK in cardioprotection has been questioned by Nishino et al in this issue of Circulation Research. 8 Nishino et al used GSK-3α/ß knock-in (KI) mice in which the phosphorylation sites on GSK-3α (Ser21) and GSK-3ß (Ser9) are changed to alanine. For controls, they used WT mice that were inbred from the same colony but were not