RESUMO
The role of the Na+/K+-ATPase (NKA) in heart failure associated with myocardial infarction (MI) is poorly understood. The elucidation of its precise function is hampered by the existence of two catalytic NKA isoforms (NKA-α1 and NKA-α2). Our aim was to analyze the effects of an increased NKA-α2 expression on functional deterioration and remodeling during long-term MI treatment in mice and its impact on Ca2+ handling and inotropy of the failing heart. Wild-type (WT) and NKA-α2 transgenic (TG) mice (TG-α2) with a cardiac-specific overexpression of NKA-α2 were subjected to MI injury for 8 wk. As examined by echocardiography, gravimetry, and histology, TG-α2 mice were protected from functional deterioration and adverse cardiac remodeling. Contractility and Ca2+ transients (Fura 2-AM) in cardiomyocytes from MI-treated TG-α2 animals showed reduced Ca2+ amplitudes during pacing or after caffeine application. Ca2+ efflux in cardiomyocytes from TG-α2 mice was accelerated and diastolic Ca2+ levels were decreased. Based on these alterations, sarcomeres exhibited an enhanced sensitization and thus increased contractility. After the acute stimulation with the ß-adrenergic agonist isoproterenol (ISO), cardiomyocytes from MI-treated TG-α2 mice responded with increased sarcomere shortenings and Ca2+ peak amplitudes. This positive inotropic response was absent in cardiomyocytes from WT-MI animals. Cardiomyocytes with NKA-α2 as predominant isoform minimize Ca2+ cycling but respond to ß-adrenergic stimulation more efficiently during chronic cardiac stress. These mechanisms might improve the ß-adrenergic reserve and contribute to functional preservation in heart failure.NEW & NOTEWORTHY Reduced systolic and diastolic calcium levels in cardiomyocytes from NKA-α2 transgenic mice minimize the desensitization of the ß-adrenergic signaling system. These effects result in an improved ß-adrenergic reserve and prevent functional deterioration and cardiac remodeling.