Thyroid hormone stimulates Na-K-ATPase activity and its plasma membrane insertion in rat alveolar epithelial cells

Na-K-ATPase protein is critical for maintaining cellular ion gradients and volume and for transepithelial ion transport in kidney and lung. Thyroid hormone, 3,3', 5- triiodo-L-thyronine (T3), given for 2 days to adult rats, increases alveolar fluid resorption by 65%, but the mechanism is undefined. We tested the hypothesis that T3 stimulates Na-K-ATPase in adult rat alveolar epithelial cells (AEC), including primary rat alveolar type II (ATII) cells, and determined mechanisms of the T3 effect on the Na-K-ATPase enzyme using two adult rat AEC cell lines (MP48 and RLE-6TN). T-3 at 10(-8) and 10(-5) M increased significantly hydrolytic activity of Na-K- ATPase in primary ATII cells and both AEC cell lines. The increased activity was dose dependent in the cell lines (10(-9) - 10(-4) M) and was detected within 30 min and peaked at 6 h. Maximal increases in Na-K- ATPase activity were twofold in MP48 and RLE-6TN cells at pharmacological T-3 of 10(-5) and 10(-4) M, respectively, but increases were statistically significant at physiological T3 as low as 10(-9) M. This effect was T-3 specific, because reverse T-3 (3,3', 5'-triiodo-L-thyronine) at 10(-9)-10(-4) M had no effect. The T-3-induced increase in Na-K-ATPase hydrolytic activity was not blocked by actinomycin D. No significant change in mRNA and total cell protein levels of Na-K- ATPase were detected with 10(-9) - 10(-5) M T-3 at 6 h. However, T3 increased cell surface expression of Na-K-ATPase alpha(1)- or beta(1)-subunit proteins by 1.7- and 2-fold, respectively, and increases in Na-K- ATPase activity and cell surface expression were abolished by brefeldin A. These data indicate that T3 specifically stimulates Na-K- ATPase activity in adult rat AEC. The upregulation involves translocation of Na-K- ATPase to plasma membrane, not increased gene transcription. These results suggest a novel nontranscriptional mechanism for regulation of Na-K- ATPase by thyroid hormone.