Mutations in the human gene encoding the nucleotide-binding region in the γ-subunit of AMP-activated protein kinase (AMPK) cause cardiomyopathy with preexcitation syndrome. Mutant AMPK showed reduced binding affinity to nucleotides in vitro raising the possibility that altered regulation of AMPK activity by AMP/ATP could contribute to the disease phenotype. In this study, we determined the sensitivity of AMPK activity to AMP/ATP in the beating hearts using transgenic mice expressing a mutant (N488I, γ2-mutant) or wild-type γ2-subunit (γ2-TG). The [ATP] and [AMP] were unaltered in all hearts but the AMPK activity was increased by 2.5-fold in γ2-mutant hearts freeze-clamped at normal AMP/ATP compared with nontransgenic (WT) or γ2-TG. The increased basal AMPK activity was caused by increased Thr-172 phosphorylation of the α-subunit (p-AMPK, by 4-fold) at normal [ATP] and was not changed by reducing glycogen content by 60% in the γ2-mutant hearts. A reversal of AMP/ATP, caused by ATP degradation, increased p-AMPK by 7-fold in WT but caused no change in γ2-mutant hearts. These results demonstrate that the mutation renders AMPK insensitive to the inhibitory and stimulatory effects of the regulatory nucleotides ATP and AMP, respectively, suggesting that the pathogenesis of the human disease may not be attributable to a simple loss- or gain-of-function.