Reducing the ATP sensitivity of the sarcolemmal K_ATP channel is predicted to lead to active channels in normal metabolic conditions, and hence cause shortened ventricular action potentials and reduced myocardial inotropy. We have generated transgenic (TG) mice that express an ATP-insensitive K_ATP channel mutant (Kir6.2[N2-30,K185Q]) under transcriptional control of the -MHC promoter. Strikingly, myocyte contraction amplitude is increased in TG myocytes (15.68±1.15% vs. 10.96±1.49%), even though K_ATP channels in TG myocytes are very insensitive to inhibitory ATP. Under normal metabolic conditions, steady state outward K⁺ currents measured under wholecell voltage clamp are elevated in TG myocytes, consistent with threshold K_ATP activation, but neither the monophasic action potential measured in isolated hearts nor transmembrane action potential measured in right ventricular muscle preparations are shortened at physiological pacing cycles. Taken together, these results suggest that there is a compensatory remodeling of excitation-contraction coupling in TG myocytes. While there are no obvious differences in other K⁺ conductances, peak L-type Ca²⁺ current density (-16.42±2.37 pA/pF) in the transgenic is increased compared with the wild type (-8.43±1.01 pA/pF). Isoproterenol approximately doubled both I_Ca and contraction amplitude in wild type myocytes, but failed to induce a significant increase in TG. Baseline and isoproterenol-stimulated cAMP concentrations are not different in wild type and transgenic hearts suggesting that the enhancement of I_Ca in the latter does not result from elevated cAMP. Collectively, the data demonstrate that a compensatory increase in I_Ca counteracts a mild activation of ATP-insensitive K_ATP to maintain the action potential duration and elevate the inotropic state of TG hearts.