Cell surface expression and turnover of the alpha-subunit of the epithelial sodium channel

The renal epithelial cell line A6, derived from Xenopus laevis, expresses e pithelial Na+ channels (ENaCs) and serves as a model system to study hormon al regulation and turnover of ENaCs. Our previous studies suggest that the alpha -subunit of Xenopus ENaC (alpha -xENaC) is detectable as 150- and 180 -kDa polypeptides, putative immature and mature alpha -subunit heterodimers . The 150- and 180-kDa alpha -xENaC were present in distinct fractions afte r sedimentation of A6 cell lysate through a sucrose density gradient. Two a nti-alpha -xENaC antibodies directed against distinct domains demonstrated that only 180- kDa alpha -xENaC was expressed at the apical cell surface. T he half-life of cell surface-expressed alpha -xENaC was 24-30 h, suggesting that once ENaC matures and is expressed at the plasma membrane, its turnov er is similar to that reported for mature cystic fibrosis transmembrane con ductance regulator. No significant changes in apical surface expression of alpha -xENaC were observed after treatment of A6 cells with aldosterone for 24 h, despite a 5.3-fold increase in short-circuit current. This lack of c hange in surface expression is consistent with previous observations in A6 cells and suggests that aldosterone regulates ENaC gating and increases cha nnel open probability.