Localization and regulation by steroids of the ?, ? and ? subunits of the amiloride-sensitive Na+ channel in colon, lung and kidney

Abstract

Polyclonal antibodies have been raised against the alpha, beta and gamma subunits of the amiloride-sensitive Na+ channel. The three subunits were detected by immunohistochemistry at the apical membrane of epithelial cells from the distal colon, the lung and the distal segments of the kidney tubules. No significant labelling was detected in lung alveoli, suggesting that it is not a major site of expression of the Na+ channel. Effects of a low Na+ diet or of dexamethasone treatment were measured at the mRNA level and at the protein level by immunohistochemistry. In the colon, steroids controlled Na+ channel activity via the stimulation of the transcription of beta and gamma subunits. The alpha mRNA was constitutively expressed. However, while neither alpha, beta nor gamma proteins were detected in the colon of control animals, they were all detected in the colon of steroid-treated animals. In the lung, Na+ channel expression was regulated by glucocorticoids the circulating level of which was sufficiently high to induce a maximal expression of the three subunits, even in control animals. Adrenalectomy drastically reduced expression of the three subunits. A surprising finding was the apparent absence of steroid effects on alpha, beta and gamma subunit expression in the kidney. Neither the expression of the mRNAs nor the expression of the proteins were significantly altered by aldosterone or by dexamethasone. These results could be due to mixed gluco- and mineralocorticoid regulations in different segments of the kidney tubule, but their interpretation also requires regulations that are apparently not found in the lung or colon.

Corticosteroid regulation of amiloride-sensitive sodium-channel subunit mRNA expression in mouse kidney

“…Previous studies have found no clear corticosteroid regulation of ENaC or in rat kidney (Renard et al 1995, Asher et al 1996, Escoubet et al 1997, Stokes & Sigmund 1998. These effects on ENaC , and especially , mRNA expression were relatively small and limited to specific renal regions, and thus would probably not have been seen in analyses of the whole kidney.…”

“…Previous work has examined kidney expression of ENaC in the rat (Renard et al 1995, Asher et al 1996, Escoubet et al 1997, Stokes & Sigmund 1998, with modest upregulation of ENaC being reported. However, mineralocorticoid doses used in those rat studies were largely in the pharmacological range (5-to 15-fold greater) (Renard et al 1995, Asher et al 1996, Stokes & Sigmund 1998. In the present study, an upregulation of the level of ENaC mRNA of equal degree was seen across physiological changes in circulating aldosterone.…”

“…Past research has also looked at the effects of dexamethasone on renal ENaC in rats, and most , Asher et al 1996, but not all (Renard et al 1995), studies have shown upregulation of renal ENaC mRNA. This effect appears related to high glucocorticoid receptor occupancy, rather than downregulation of physiological glucocorticoid levels with the HPA axis, and is mimicked by the pure glucocorticoid receptor agonist, RU28362, at high dosage (Stokes & Sigmund 1998).…”

Non-coordinate Regulation of Endogenous Epithelial Sodium Channel (ENaC) Subunit Expression at the Apical Membrane of A6 Cells in Response to Various Transporting Conditions

“…In rat kidney, message for all three subunits is expressed constitutively and changes little with salt restriction (42). Other investigators described modest changes primarily in ␣ subunit mRNA in rat kidney in response to steroid stimulation or adrenalectomy (42,43).…”

Chronic Exposure to Vasopressin Upregulates ENaC and Sodium Transport in the Rat Renal Collecting Duct and Lung

“…8 -10 In contrast, in the kidney, aldosterone affects only the expression of ␣-subunit transcript or protein, 9,11,12 although some investigators found no effect of this hormone on subunit abundance. 4,8,13 Moreover, transgenic mice lacking the mineralocorticoid receptor exhibit a normal abundance of mRNAs encoding ENaC subunits in the kidney. 14 These observations suggest that factors other than aldosterone are involved in the regulation of ENaC in the kidney.…”