Aldosterone-induced increase in the abundance of Na+ channel subunits

The highly selective, amiloride-blockable Na+ channel is a major target to the natriferic action of the mineralocorticoid aldosterone. This rat epithelial Na+ channel (rENaC) has been recently cloned from colon and is composed of three homologous subunits denoted alpha-, beta-, and gamma-rENaC (C. M. Canessa, L. Schild, G. Buell. B. Thorens, I. Gautschi, J.-D. Horisberger, and B. C. Rossier. Nature Lend. 367: 463-467, 1994). We have tested the effects of corticosteroids on the abundance of mRNA coding for each subunit in kidney cortex and distal colon. Chronic treatment of rats with aldosterone or dexamethasone evoked in kidney cortex a small induction of alpha-rENaC and no change in beta- and gamma-rENaC. In distal colon, however, beta- and gamma-rENaC were strongly induced by either aldosterone or dexamethasone, whereas alpha-rENaC was constitutively expressed. Most of the aldosterone-induced increase in beta- and gamma-rENaC mRNA took place during 3-24 h after plasma aldosterone was elevated. A similar differential induction of rENaC subunits in kidney and colon was also evoked by a Na+-free diet. The effects of salt deprivation were reversed by resalinating rats with a half time of <2 h, suggesting a high turnover rate of at least beta- and gamma-rENaC. The data are consistent with the possibility that induction of channel subunits contributes to the chronic but not the acute response to aldosterone in the colon. Such a mechanism is not likely to play a major role in cortical collecting ducts.