Differential expression of Na-K-ATPase isoforms in rat alveolar epithelial cells

Lung Na-K-ATPase has been shown to contribute to vectorial Na⁺ transport and edema clearance. The α₁- and β₁-Na-K-ATPase subunits have been localized to alveolar type II (ATII) cells, and the α₂-Na-K-ATPase has been reported in rat lung homogenates. Expression of Na-K-ATPase α₁-, α₂- , and β₁-subunits was investigated in rat ATII cells cultured for 7 days, a period during which they lose their phenotypic markers and differentiate to an alveolar type I (ATI)-like cell phenotype. Differentiation of ATII cells to an ATI-like phenotype resulted in a decrease of α₁- and an increase of α₂-mRNA and protein abundance without changes in the β₁-subunit. Thus ATI-like cells exhibited a mixture of α₁- and α₂-isoforms. Nuclear run- on analysis suggests that these changes were transcriptionally regulated. The existence of the distinct functional classes of Na-K-ATPase in ATII and ATI- like cells was confirmed by ouabain inhibition of Na-K-ATPase activity. Ouabain inhibition of ATII cells was consistent with expression of the α₁- isozyme [50% inhibitory concentration (IC₅₀) = 4 x 10^-5 M], whereas, in ATI-like cells, it was consistent with the presence of both α₁- and α₂- isozymes (IC₅₀ = 9.0 x 10^-5 and 1.5 x 10^-7 M, respectively); [³H]ouabain binding studies corroborated these findings. Our results indicate that, during ATII cell cytodifferentiation with time in culture, there is a shift in isoform composition that may reflect physiological functions of alveolar epithelial cells.