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ENaC gene expression
1 Department of Molecular Cell Physiology, Kyoto Prefectural University of Medicine, Graduate School of Medical Science, Kyoto, Kyoto, Japan
2 Center for Cell and Molecular Signaling and Department of Physiology, Emory University School of Medicine, Atlanta, Georgia, USA
3 Department of Molecular Cell Physiology, Kyoto Prefectural University of Medicine, Graduate School of Medical Science, Kyoto, Kyoto, Japan; Department of Respiratory Molecular Medicine, Kyoto Prefectural University of Medicine, Graduate School of Medical Science, Kyoto, Kyoto, Japan
* To whom correspondence should be addressed. E-mail: marunaka{at}koto.kpu-m.ac.jp.
Hypotonicity stimulates transepithelial Na+ reabsorption in renal A6 cells, but the mechanism for this stimulation is not fully understood. In the present study, we found that hypotonicity stimulated Na+ reabsorption through increases in mRNA expression of the 
subunit of the epithelial Na+ channel (ENaC). Hypotonicity decreases cytosolic Cl- concentration; therefore, we hypothesized that hypotonicity-induced decreases in cytosolic Cl- concentration could act as a signal to regulate Na+ reabsorption through changes in ENaC mRNA expression. Treatment with the flavone, apigenin which activates the Na+/K+/2Cl- cotransporter and increases cytosolic Cl- concentration, markedly suppressed the hypotonicity-induced increase in ENaC mRNA expression. On the other hand, blockade of the Na+/K+/2Cl- cotransporter decreases cytosolic Cl- concentration and increased ENaC mRNA expression and Na+ reabsorption. Blocking Cl- channels with NPPB inhibited the hypotonicity-induced decrease in cytosolic Cl- concentration and suppressed the hypotonicity-induced increase in ENaC mRNA expression. Co-application of NPPB and apigenin synergistically suppressed ENaC mRNA expression. Thus, in every case changes in cytosolic Cl- concentration were associated with changes in ENaC mRNA expression and changes in Na+ reabsorption: decreases in cytosolic Cl- concentration increased ENaC mRNA and increased Na+ reabsorption while increases in cytosolic Cl- concentration decreased ENaC mRNA and decreased Na+ reabsorption. These findings support the hypothesis that changes in cytosolic Cl- concentration are an important and novel signal in hypotonicity-induced regulation of ENaC expression and Na+ reabsorption.
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