Subunit Composition and Role of Na+,K+-ATPases in Ventricular Myocytes

Abstract

Na+,K+-ATPases are composed of one alpha and one beta subunit; four alpha and three beta isoforms have been found to date. We elucidated which alpha and beta subunits were present in the ventricular myocytes of rat and guinea-pig and what roles the Na+,K(+)-ATPase isozymes play in cardiac contraction. The presence of the alpha1, alpha2, and alpha3 subunits and the beta1 and beta2 subunits in rat and guinea-pig hearts were confirmed at the protein or mRNA level. Immunocytochemistry showed a patchy presence of alpha1 in the transverse tubules and surface sarcolemma, whereas alpha2 was distributed continuously in the transverse tubules alone. The alpha3 isoform was expressed prominently in the guinea-pig intercalated disc and slightly in the rat. On the other hand, the beta1 isoform was located in the transverse tubules and surface sarcolemma, whereas the beta2 was mainly located in the intercalated disc. The immunocytochemistry and immunoprecipitation findings indicated that the alpha1 and alpha2 form heterodimers with beta1 and the alpha3 with beta2 in ventricular myocytes. The application of low concentrations of ouabain enhanced the amplitudes of twitch without a change in resting tension in rat and guinea-pig ventricular stripts, whereas that of high concentrations resulted in a decrease in twitch with an increase in the resting tension. We thus conclude that the alpha2beta1 and alpha3beta2 isozymes are selectively located in the transverse tubules and intercalated disc of the ventricular myocytes, respectively, and the alpha2beta1 is involved in the regulation of the Ca2+ contents in the SR.