The α1 isoform of Na,K-ATPase regulates cardiac contractility and functionally interacts and co-localizes with the Na/Ca exchanger in heart

The primary objective of this study was to examine the functional role of the Na,K-ATPase alpha1 isoform in the regulation of cardiac contractility. Previous studies using knock-out mice showed that the hearts of animals lacking one copy of the alpha1 or alpha2 isoform gene exhibit opposite phenotypes. Hearts from alpha2(+/-) animals are hypercontractile, whereas those of the alpha1(+/-) animals are hypocontractile. The cardiac phenotype of the alpha1(+/-) animals was unexpected as other studies suggest that inhibition of either isoform increases contraction. To help resolve this difference, we have used genetically engineered knock-in mice expressing a ouabain-sensitive alpha1 isoform and a ouabain-resistant alpha2 isoform of the Na,K-ATPase, and we analyzed cardiac contractility following selective inhibition of the alpha1 isoform by ouabain. Administration of ouabain to these animals and to isolated heart preparations selectively inhibits only the activity of the alpha1 isoform without affecting the activity of the alpha2 isoform. Low concentrations of ouabain resulted in positive cardiac inotropy in both isolated hearts and intact animals expressing the modified alpha1 and alpha2 isoforms. Pretreatment with 10 muM KB-R7943, which inhibits the reverse mode of the Na/Ca exchanger, abolished the cardiotonic effects of ouabain in isolated wild type and knock-in hearts. Immunoprecipitation analysis demonstrated co-localization of the alpha1 isoform and the Na/Ca exchanger in cardiac sarcolemma. The alpha1 isoform coimmunoprecipitated with the Na/Ca exchanger and vice versa. These results demonstrate that the alpha1 isoform regulates cardiac contractility, and that both the alpha1 and alpha2 isoforms are functionally and physically coupled with the Na/Ca exchanger in heart.