Effects of hyperoxia on type II cell Na-K-ATPase function and expression

Alveolar fluid is resorbed using active Na⁺ transport primarily through basolateral sodium-potassium-adenosinetriphosphatase (Na-K-ATPase) and apical Na⁺ channels that are particularly dense on the alveolar type II (ATII) epithelial cells. During lung injury with pulmonary edema, continued or accelerated Na⁺ and fluid resorption is critical for a favorable outcome. However, little is known of how ATII cell Na⁺ transport is affected during injury. These experiments examined the effects of acute lung injury on ATII cell Na-K-ATPase activity and expression using an established model of rats exposed to 100% O₂ for 60 h. Na-K-ATPase activity of ATII cells isolated immediately after exposure was assessed by ouabain-sensitive ⁸⁶Rb⁺ uptake in intact cells and by ouabain-sensitive P(i) production by cell membranes. In the presence of 1 mM ouabain, ouabain-sensitive Rb^- uptake was not different between normoxic and hyperoxic cells, but the apparent Na-K-ATPase maximal velocity (V(max) of hyperoxic cell membranes was 75 ± 8% of normoxic membranes (P < 0.05). On Western blots of ATII cell membranes, α₁-subunit protein significantly decreased with hyperoxia (35 ± 9% of normoxia; P < 0.05), whereas the amounts of the β-subunit were unchanged (P > 0.051). On Northern blots of ATII cell total RNA, steady-state levels of both the α₁- and β₁-subunit mRNA increased after hyperoxia (α₁ = 2.5 ± 1.3-fold; β₁ = 4.6 ± 2.5-fold). Thus despite hyperoxic decreases in Na-K-ATPase V(max) and the amount of α₁-protein, Rb⁺ uptake by Na-K-ATPase in intact cells was unchanged. The mRNA levels, protein amounts, and enzyme activity did not respond in parallel to hyperoxic injury, and the activity in intact cells correlated best with the amounts of the β-subunit, the limiting component in de novo pump assembly in many tissues.