Dominant mutations in the gamma 2 regulatory subunit of AMP-activated protein kinase (AMPK), encoded by the gene PRKAG2, cause glycogen storage cardiomyopathy. We sought to elucidate the effect of the Thr400Asn (T400N) human mutation in a transgenic mouse (TG(T400N)) on AMPK activity, and its ability to protect the heart against ischemia-reperfusion injury. TG(T400N) hearts had markedly vacuolated myocytes, excessive accumulation of glycogen, hypertrophy, and preexcitation. Early activation of myocardial AMPK, followed by depression, and then recovery to wild-type levels was observed. AMPK activity correlated inversely with glycogen content. Partial rescue of the phenotype was observed when TG(T400N) mice were crossbred with TG(T400N) mice, which overexpress a dominant negative mutant of the AMPK alpha 2 catalytic subunit. TG(T400N) hearts had greater infarct sizes and apoptosis when subjected to ischemia-reperfusion. Increased AMPK activity is responsible for glycogen storage cardiomyopathy. Despite high glycogen content, the TG(T400N) heart is not protected against ischemia-reperfusion injury. (C) 2007 Elsevier Inc. All rights reserved.