Glucocorticoids stimulate cation absorption by semicircular canal duct epithelium via epithelial sodium channel

Abstract

The semicircular canal duct (SCCD) epithelium is a vestibular epithelial domain that was recently shown to actively contribute to endolymph homeostasis by Cl(-) secretion under control of beta(2)-adrenergic stimulation. By analogy to other Cl(-) secretory epithelia, we hypothesized that SCCD also provides an active absorptive pathway for Na(+) under corticosteroid control. Measurements of short-circuit current (I(sc)) demonstrated stimulation (7-24 h) by the glucocorticoids hydrocortisone (EC(50) 13 nM), corticosterone (33 nM), prednisolone (70 nM), and dexamethasone (13 nM) over physiologically and therapeutically relevant concentrations and its block by amiloride (IC(50) 470 nM) and benzamil (57 nM), inhibitors of the epithelial sodium channel (ENaC). I(sc) was also partially inhibited by basolateral ouabain and Ba(2+), indicating the participation of Na(+)-K(+)-ATPase and a K(+) channel in Na(+) transport. By contrast, aldosterone stimulated I(sc) only at unphysiologically high concentrations (EC(50) 102 nM). The action of all steroids was blocked by mifepristone (RU-486; K(d) approximately 0.3 nM) but not by spironolactone (K(d) approximately 0.7 microM). Expression of mRNA for the alpha-, beta-, and gamma-subunits of ENaC was demonstrated in the presence and absence of glucocorticoids. These findings are the first to identify SCCD in the vestibular labyrinth as a site of physiologically significant ENaC-mediated Na(+) absorption and osmotically coupled water flux. They further demonstrate regulation of Na(+) transport by natural and therapeutic glucocorticoids. The results provide for the first time an understanding of the therapeutic benefit of glucocorticoids in the treatment of Meniere's disease, a condition that is associated with increased luminal fluid volume.

Distribution of glucocorticoid receptors and 11 beta-hydroxysteroid dehydrogenase isoforms in the rat inner ear

“…However, functional regulation by glucocorticoids in the peripheral vestibular system has already been proven. For instance, corticosteroids are known to have a protective effect also in the vestibular maculae (Pondugula et al, 2004) and to regulate ions such as ENaC in animal experiments of the semicircular canal duct epithelium (Pondugula et al, 2006;Taura et al, 2006). In the present study, immunoreactivity of GRs was observed in the cytoplasm of the VHCs, the DCs, and the VN.…”

Perilymph Pharmacokinetics of Markers and Dexamethasone Applied and Sampled at the Lateral Semi-Circular Canal

“…In cases of endolymphatic hydrops it is possible that modification of fluid transport in one region of the endolymph space may be sufficient to alter the status of all of it (analogous to an entire balloon collapsing when a portion is vented). Pondugula et al (2004) showed that cation uptake into the semicircular canals was upregulated by Dex. Kim and Marcus (2009) found a similar Dex-induced upregulation of sodium transport into the sacculus.…”

Endolymphatic Sodium Homeostasis by Extramacular Epithelium of the Saccule

“…For investigation of the effect of dexamethasone via GR and/or MR, dissected saccules were divided into four groups and incubated for 24 h in DMEM-F12 medium (Invitrogen) supplemented with 100 U/ml penicillin (Sigma) and 100 g/ml streptomycin (Sigma) at 37°C in a 5% CO 2 atmosphere with saturated humidity. Each of two groups was incubated in the presence or absence of 100 nM cyclodextrin-encapsulated dexamethasone (Sigma), a concentration that corresponds to the physiological and therapeutic concentration of dexamethasone (Balis et al, 1987;Braat et al, 1992) and to the concentration that produced a peak response from semicircular canal epithelial cells (Pondugula et al, 2004). The other two groups were treated with 100 nM dexamethasone in the presence and absence of either the GR antagonist mifepristone (100 nM, Sigma) or MR antagonist spironolactone (100 nM, Sigma).…”

“…Recently, glucocorticoids were reported to regulate ion homeostasis of inner ear fluids via genomic pathways in semicircular canal (Pondugula et al, 2004(Pondugula et al, , 2006), Reissner's membrane (Kim et al, 2009), and nongenomic pathways in stria vascularis . In the present report, Na ϩ absorption in saccular extramacular epithelium was found to be stimulated by glucocorticoids via genomic regulation through GR, not through MR, as determined by the long time scale of stimulation and the differential effects of GR and MR antagonists.…”

“…We sought functional evidence of electrogenic transepithelial ion transport by extramacular epithelium of adult mouse saccule and examined whether it is controlled by glucocorticoids via the glucocorticoid receptor (GR) and/or the mineralocorticoid receptor (MR) by electrophysiologic and pharmacologic means. The experimental strategy used in the current study of transepithelial transport by the saccule is based on that used for the study of Na ϩ transport across many epithelia, including others in the inner ear (Pondugula et al, 2004;Pondugula et al, 2006;Kim et al, 2009). …”