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Department of Medicine, University of Minnesota, Minneapolis, Minnesota 55455
Submitted 4 November 2002 ; accepted in final form 4 April 2003
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-KATPase enzyme using two adult rat AEC cell
lines (MP48 and RLE-6TN). T3 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 T3 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 T3 specific,
because reverse T3
(3,3',5'-triiodo-L-thyronine) at
10-9-10-4 M had no effect. The
T3-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 T3 at 6 h.
However, T3 increased cell surface expression of Na-K-ATPase
1- or 
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.
alveolar cell lines; protein translocation; sodium pump
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