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

Abstract

The primary objective of this study was to examine tile functional role of the Na,K-ATPase α1 isoform in the regulation of cardiac contractility. Previous studies using knock-out mice showed that the hearts of animals lacking one copy of the α1 or α2 isoform gene exhibit opposite phenotypes. Hearts from α2+/- animals are hypercontractile, whereas those of the α1+/- animals are hypocontractile. The cardiac phenotype of the α1+/- 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 α1 isoform and a ouabain-resistant α2 isoform of the Na,K-ATPase, and we analyzed cardiac contractility following selective inhibition of the α1 isoform by ouaboain. Administration of ouabain to these animals and to isolated heart preparations selectively inhibits only the activity of the α-isoform without affecting the activity of the α2 isoform. Low concentrations of ouabain resulted in positive cardiac inotropy in both isolated hearts and intact animals expressing the modified α1 and α2 isoforms. Pretreatment with 10 μM 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 α-isoform and the Na/Ca exchanger in cardiac sarcolemma. The α1 isoform co-immunoprecipitated with the Na/Ca exchanger and vice versa. These results demonstrate that the α1 isoform regulates cardiac contractility, and that both the α1 and α2 isoforms are functionally and physically coupled with the Na/Ca exchanger in heart.