Sodium Kinetics of Na,K-ATPase α Isoforms in Intact Transfected HeLa Cells

By participating in the regulation of ion and voltage gradients, the Na-K pump (i.e., Na,K-ATPase) influences many aspects of cellular physiology. Of the four α isoforms of the pump, α1 is ubiquitous, α2 is predominant in skeletal muscle, and α3 is found in neurons and the cardiac conduction system. To determine whether the isoforms have different intracellular Na⁺ affinities, we used the Na⁺-sensitive dye sodium-binding benzofuran isophthalate (SBFI) to measure pump-mediated Na⁺ efflux as a function of [Na⁺]_i in human HeLa cells stably transfected with rat Na-K pump isoforms. We Na⁺-loaded the cells, and then monitored the time course of the decrease in [Na⁺]_i after removing external Na⁺. All transfected rat α subunits were highly ouabain resistant: the α1 isoform is naturally resistant, whereas the α2 and α3 isoforms had been mutagenized to render them resistant. Thus, the Na⁺ efflux mediated by endogenous and transfected pumps could be separated by studying the cells at low (1 μM) and high (4 mM) ouabain concentrations. We found that the apparent K_m for Na⁺ efflux attributable to the native human α1 isoform was 12 mM, which was similar to the K_m of rat α1. The α2 and α3 isoforms had apparent K_m's of 22 and 33 mM, respectively. The cells expressing α3 had a high resting [Na⁺]_i. The maximal activity of native α1 in the α3-transfected cells was only ∼56% of native α1 activity in untransfected HeLa cells, suggesting that transfection with α3 led to a compensatory decrease in endogenous α1 pumps. We conclude that the apparent K_m(Na⁺) for rat Na-K pump isoforms increases in the sequence α1 < α2 < α3. The α3 isoform may be suited for handling large Na⁺ loads in electrically active cells.