RESUMO
We evaluated the effects of acute hyperoxic exposure on alveolar epithelial cell (AEC) active ion transport and on expression of Na+ pump (Na+-K+-ATPase) and rat epithelial Na+ channel subunits. Rat AEC were cultivated in minimal defined serum-free medium (MDSF) on polycarbonate filters. Beginning on day 5, confluent monolayers were exposed to either 95% air-5% CO2 (normoxia) or 95% O2-5% CO2 (hyperoxia) for 48 h. Transepithelial resistance (Rt) and short-circuit current (Isc) were determined before and after exposure. Na+ channel alpha-, beta-, and gamma-subunit and Na+-K+-ATPase alpha1- and beta1-subunit mRNA levels were quantified by Northern analysis. Na+ pump alpha1- and beta1-subunit protein abundance was quantified by Western blotting. After hyperoxic exposure, Isc across AEC monolayers decreased by approximately 60% at 48 h relative to monolayers maintained under normoxic conditions. Na+ channel beta-subunit mRNA expression was reduced by hyperoxia, whereas alpha- and gamma-subunit mRNA expression was unchanged. Na+ pump alpha1-subunit mRNA was unchanged, whereas beta1-subunit mRNA was decreased approximately 80% by hyperoxia in parallel with a reduction in beta1-subunit protein. Because keratinocyte growth factor (KGF) has recently been shown to upregulate AEC active ion transport and expression of Na+-K+-ATPase under normoxic conditions, we assessed the ability of KGF to prevent hyperoxia-induced changes in active ion transport by supplementing medium with KGF (10 ng/ml) from day 2. The presence of KGF prevented the effects of hyperoxia on ion transport (as measured by Isc) relative to normoxic controls. Levels of beta1 mRNA and protein were relatively preserved in monolayers maintained in MDSF and KGF compared with those cultivated in MDSF alone. These results indicate that AEC net active ion transport is decreased after 48 h of hyperoxia, likely as a result of a decrease in the number of functional Na+ pumps per cell. KGF largely prevents this decrease in active ion transport, at least in part, by preserving Na+ pump expression.