Role of enhanced Na+ entry in the control of Na,K-ATPase gene expression by serum.

The role of enhanced Na+ entry in the induction of Na,K-ATPase subunit mRNAs by serum was investigated in a "nontransformed" rat liver cell line, Clone 9. Exposure of cells to 10% calf serum resulted in a 1.5-fold increase in the rate of Na+ entry associated with a transient rise in cell Na+ content (twofold at 15 min) and a sustained 1.15-fold rise in cell K+ content. After 6 hr of exposure to serum mRNA alpha 1 and mRNA beta 1 content increased by 1.8- and 2.6-fold, respectively. In nuclear run-on assays, serum stimulated the transcription of the alpha 1 gene approximately 1.9-fold while the transcription rate of the beta 1 gene remained unchanged. In cells incubated in Na(+)-free medium where NaCl was replaced by choline chloride, the induction of mRNA alpha 1 by serum was fully preserved, whereas the increase in mRNA beta 1 was prevented. An unexpected finding was that incubation of cells in Na(+)-free medium alone for 6 hr increased mRNA alpha 1 but not mRNA beta 1 content. These results indicate that Na,K-ATPase subunit mRNAs are differentially induced by serum, and that the induction of mRNA alpha 1, in contrast to that of mRNA beta 1, is transcriptionally mediated and does not require the presence of Na+ in the extracellular medium.