抄録
Na+,K+-ATPases are composed of one α and one β subunit; four α and three β isoforms have been found to date. We elucidated which α and β 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 α1, α2, and α3 subunits and the β1 and β2 subunits in rat and guinea-pig hearts were confirmed at the protein or mRNA level. Immunocytochemistry showed a patchy presence of α1 in the transverse tubules and surface sarcolemma, whereas α2 was distributed continuously in the transverse tubules alone. The α3 isoform was expressed prominently in the guinea-pig intercalated disc and slightly in the rat. On the other hand, the β1 isoform was located in the transverse tubules and surface sarcolemma, whereas the β2 was mainly located in the intercalated disc. The immunocytochemistry and immunoprecipitation findings indicated that the α1 and α2 form heterodimers with β1 and the α3 with β2 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 α2β1 and α3β2 isozymes are selectively located in the transverse tubules and intercalated disc of the ventricular myocytes, respectively, and the α2β1 is involved in the regulation of the Ca2+ contents in the SR.
