| Vestibular endolymph has an unusually-low concentration of Na +, which is important for the vestibular transduction processes. Abnormality of endolymphatic-ion homeostasis is thought to be one of the causes of Meniere's disease, a syndrome that is often treated with glucocorticoids. We sought to determine whether semicircular canal duct (SCCD) contributes to endolymphatic-ion homeostasis by providing an active absorptive pathway for Na+ under glucocorticoid control and whether glucocorticoids regulate a known cAMP-mediated Cl- secretion by SCCD.; Measurements of short circuit current (Isc) from primary cultures of rat SCCD epithelium demonstrated stimulation (7-24 hrs) by the glucocorticoids hydrocortisone, corticosterone, prednisolone, and dexamethasone over physiologically and therapeutically relevant concentrations and its block by amiloride and benzamil, inhibitors of the epithelial sodium channel (ENaC). By contrast, aldosterone stimulated Isc only at unphysiologically-high concentrations. The action of all steroids was blocked by mifepristone but not by spironolactone. Dexamethasone-stimulated Isc was also partially inhibited by basolateral ouabain and Ba2+(IC50: 210 muM), indicating the participation of Na+,K+-ATPase and inwardly rectifying potassium channel (Kir) channels in Na+ absorption. Dexamethasone not only up-regulated the transcript expression of alpha-ENaC; beta 2 & beta3 Na+,K+-ATPase; Kir2.1, Kir2.2, Kir2.4, Kir3.1, Kir3.3, Kir4.2 and Kir7.1; serum-and glucocorticoid-regulated kinase1 (Sgk1) and neural precursor cell expressed developmentally downregulated 4-2 kinase1 (Sgk1) and neural precursor cell expressed developmentally downregulated 4-2 (Nedd4-2), but also down-regulated the glucocorticoid receptor (GR) and 11beta-hydroxysteroid dehydrogenase type1 (1beta-HSD1). Dexamethasone altered transcript levels (alpha-ENaC & Sgk1) by activation of the GR selectively. Proteins were present for all three subunits of ENaC and Sgk1 and expression of alpha- and gamma-ENaC was up-regulated by dexamethasone. These observations are consistent with the glucocorticoid-regulation of Na + transport by SCCD.; Forskolin, cAMP analogs, 3-isobutyl-1-methylxanthine (IBMX), and RO-20-1724 stimulated Isc in the presence of amiloride. Apical genistein (30 muM) stimulated Isc in the absence and presence of 1 muM forskolin. Forskolin-stimulated Isc was partially inhibited by apical CFTR inh-172, an inhibitor of the cystic fibrosis transmembrane conductance regulator (CFTR). Transcript and protein were present for CFTR. Forskolin stimulation was also partially inhibited by basolateral Ba2+(IC 50: 270 muM) ouabain, and bumetanide, indicating the participation of Kir, Na+,K+-ATPase, and Na+,K +,2Cl- cotransporter in Cl- transport. Forskolin stimulation was greater after incubation (24 hr) with the corticosteroids and steroid effect was blocked by mifepristone but not by spironolactone. These observations are consistent with the glucocorticoid-regulation of Cl - transport by SCCD. |
