Mechanism of ENaC expression in an aldosterone-sensitive epithelial cell. (a) In a resting state, few ENaCs, which facilitate sodium reabsorption in a rate-limiting fashion, are resident in the apical membrane. Factors known to enhance ENaC surface expression and activity are counterbalanced by retrieval of these channels from the membrane through the ubiquitination pathway mediated by Nedd4-2. (b) Shortly after aldosterone exposure and binding to the MR, transcriptional stimulation of Sgk1 leads to phosphorylation of Nedd4-2, which subsequently disrupts ENaC/Nedd4-2 interactions. In this situation, ubiquitination of ENaCs is reduced, thus favoring ENaC residence in the apical membrane and enhanced sodium reabsorption. Afterward, stimulation by aldosterone requires successive activation of several genes resulting in sustained sodium reabsorption across the epithelium. PI3K, phosphatidylinositol-3′-kinase; ERK, extracellular signal-regulated kinase; PKA, protein kinase A; Na/K-ATPase, sodium/potassium ATPase.