Functional differences between α subunit isoforms of the rat Na,K-ATPase expressed in Xenopus oocytes

The functional properties of the three most widely distributed alpha subunit isoforms of the Na,K-ATPase are not well known, particularly concerning the voltage dependence of their activity and cation binding kinetics. We measured the electrogenic activity generated by Na,K-ATPases resulting from co-expression of the rat alpha1, alpha2* or alpha3* subunits with the rat beta1 subunit in Xenopus oocytes; alpha2* and alpha3* are ouabain-resistant mutants of the alpha2 and alpha3 isoform, which allowed selective inhibition of the endogenous Na+,K+-pump of the oocyte. In oocytes expressing the three isoforms of the alpha subunit, K+ induced robust outward currents that were largely ouabain-sensitive. In addition, ouabain-sensitive inward currents were recorded for all three isoforms in sodium-free and potassium-free acid solutions. The very similar voltage dependence of the Na+,K+-pump activity observed in the absence of extracellular Na+ indicated a similar stoichiometry of the transported cations by the three isoforms. The affinity for extracellular K+ was slightly lower for the alpha2* and alpha3* than for the alpha1 isoform. The alpha2* isoform was, however, more sensitive to voltage-dependent inhibition by extracellular Na+, indicating a higher affinity of the extracellular Na+ site in this isoform. We measured and controlled [Na+](i) using a co-expressed amiloride-sensitive Na+ channel. The intracellular affinity for Na+ was slightly higher in the alpha2* than in the alpha1 or alpha3* isoforms. These results suggest that the alpha2 isoform could have an activity that is strongly dependent upon [Na+](o) and [K+](o). These concentrations could selectively modulate its activity when large variations are present, for instance in the narrow intercellular spaces of brain or muscle tissues.