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

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Abstract
Polyclonal antibodies have been raised against the α, β and γ 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 β and γ subunits. The α mRNA was constitutively expressed. However, while neither α, β nor γ 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 α, β and γ 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.

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