LOCALIZATION AND REGULATION BY STEROIDS OF THE ALPHA-SUBUNITS, BETA-SUBUNITS AND GAMMA-SUBUNITS OF THE AMILORIDE-SENSITIVE NA+ CHANNEL IN COLON, LUNG AND KIDNEY

Polyclonal antibodies have been raised against the alpha, beta and gam ma subunits of the amiloride-sensitive Na+ channel. The three subunits were detected by immunohistochemistry at the apical membrane of epith elial cells from the distal colon, the lung and the distal segments of the kidney tubules. No significant labelling was detected in lung alv eoli, suggesting that it is not a major site of expression of the Nachannel. Effects of a low Na+ diet or of dexamethasone treatment were measured at the mRNA level and at the protein level by immunohistochem istry. In the colon, steroids controlled Na+ channel activity via the stimulation of the transcription of beta and gamma subunits. The ii mR NA was constitutively expressed. However, while neither alpha, beta no r gamma proteins were detected in the colon of control animals, they w ere all detected in the colon of steroid-treated animals. In the lung, Nat channel expression was regulated by glucocorticoids the circulati ng level of which was sufficiently high to induce a maximal expression of the three subunits, even in control animals. Adrenalectomy drastic ally reduced expression of the three subunits. A surprising finding wa s the apparent absence of steroid effects on alpha, beta and gamma sub unit expression in the kidney. Neither the expression of the mRNAs nor the expression of the proteins were significantly altered by aldoster one or by dexamethasone. These results could be due to mixed gluco- an d mineralocorticoid regulations in different segments of the kidney tu bule, but their interpretation also requires regulations that are appa rently not found in the lung or colon.