Transepithelial Na+ reabsorption across tight epithelia is regulated by aldosterone. The amiloride-sensitive epithelial sodium channel (ENaC) is a major target for the natriferic action of aldosterone. In this study, the effect of aldosterone on ENaC mRNA abundance and the rate of protein synthesis for each of the three ENaC subunits (alpha, beta and gamma) in the A6 kidney cell line were examined. In cells grown on plastic, aldosterone induced a large and rapid increase in epithelial sodium channel (ENaC) beta and gamma subunit mRNA abundance, but this effect is not translated into the synthesis of the corresponding proteins. In cells grown on a porous substrate, amiloride-sensitive electrogenic sodium transport was expressed and was upregulated by aldosterone (300 nM) as early as 1 h after the addition of the hormone. The alpha, beta, and gamma mRNA abundance was not changed by aldosterone during the first 3 h of stimulation, whereas a fourfold increase over control was observed after 24 h. The rate of synthesis of alpha subunit was significantly increased above control already 60 min after aldosterone addition, whereas beta subunit synthesis increased only 6 h after hormone addition, with no significant change for the gamma subunit. The half-lives of each subunit as assessed by 35S methionine pulse-chase experiments were short (between 40 and 50 min) and were not modified by aldosterone. Taking into account the short half-life of ENaC protein and assuming that the synthesis of the alpha subunit is a limiting factor in the assembly and expression of new channels at the cell surface, it is proposed that the aldosterone regulation of sodium transport might be, in part, mediated by de novo synthesis of the channel protein.
Expression and purification of the alpha subunit of the epithelial sodium channel, ENaC
