Glucocorticoid regulation of genes in the amiloride-sensitive sodium transport pathway by semicircular canal duct epithelium of neonatal rat

The lumen of the inner ear has an unusually low concentration of endolymphatic Na+, which is important for transduction processes. We have recently shown that glucocorticoid receptors (GR) stimulate absorption of Na+ by semicircular canal duct (SCCD) epithelia. In the present study, we sought to determine the presence of genes involved in the control of the amiloride-sensitive Na+ transport pathway in rat SCCD epithelia and whether their level of expression was regulated by glucocorticoids using quantitative real-time RT-PCR. Transcripts were present for alpha-, beta-, and gamma-subunits of the epithelial sodium channel (ENaC); the alpha(1)-, alpha(3)-, beta(1)-, and beta(3)-isoforms of Na+-K+-ATPase; inwardly rectifying potassium channels [IC50 of short circuit current (I-SC) for Ba2(+): 210 mu M] Kir2.1, Kir2.2, Kir2.3, Kir2.4, Kir3.1, Kir3.3, Kir4.1, Kir4.2, Kir5.1, and Kir7.1; sulfonyl urea receptor 1 (SUR1); GR; mineralocorticoid receptor (MR); 11 beta-hydroxysteroid dehydrogenase (11 beta-HSD) types 1 and 2; serum- and glucocorticoid-regulated kinase 1 (Sgk1); and neural precursor cell-expressed developmentally down-regulated 4-2 (Nedd4-2). On the other hand, transcripts for the alpha(4)-subunit of Na+-K+-ATPase, Kir1.1, Kir3.2, Kir3.4, Kir6.1, Kir6.2, and SUR2 were found to be absent, and I-SC was not inhibited by glibenclamide. Dexamethasone (100 nM for 24 h) not only upregulated the transcript expression of alpha-ENaC (similar to 4-fold), beta(2)-subunit (similar to 2-fold) and beta(3)-subunit (similar to 8-fold) of Na+-K+-A\TPase, Kir2.1 (similar to 5-fold), Kir2.2 (similar to 9-fold), Kir2.4 (similar to 3-fold), Kir3.1 (similar to 3-fold), Kir3.3 (similar to 2-fold), Kir4.2 (similar to 3-fold), Kir7.1 (similar to 2-fold), Sgk1 (similar to 4-fold), and Nedd4-2 (similar to 2-fold) but also downregulated GR (similar to 3-fold) and 11 beta-HSD1 (similar to 2-fold). Expression of GR and 11 beta-HSD1 was higher than MR and 11 beta-HSD2 in the absence of dexamethasone. Dexamethasone altered transcript expression levels (alpha-ENaC and Sgk1) by activation of GR but not MR. Proteins were present for the alpha-, alpha-, and alpha- subunits of ENaC and Sgk1, and expression of alpha- and alpha- ENaC was upregulated by dexamethasone. These findings are consistent with the genomic stimulation by glucocorticoids of Na+ absorption by SCCD and provide an understanding of the therapeutic action of glucocorticoids in the treatment of Meniere's disease.