Non-coordinate regulation of endogenous epithelial sodium channel (ENaC) subunit expression at the apical membrane of A6 cells in response to varioustransporting conditions

In many epithelial tissues in the body (e.g. kidney distal nephron, colon, airways) the rate of Na+ reabsorption is governed by the activity of the ep ithelial Na+ channel (ENaC), ENaC activity in turn is regulated by a number of factors including hormones, physiological conditions, and other ion cha nnels. To begin to understand the mechanisms by which ENaC is regulated, we have examined the trafficking and turnover of ENaC subunits in A6 cells, a polarized, hormonally responsive Xenopus kidney cell line. As previously o bserved by others, the half-life of newly synthesized ENaC subunits was uni versally short (similar to2 h). However, the half-lives of alpha- and gamma -ENaC subunits that reached the apical cell surface were considerably long er (t(1/2) > 24 h), whereas intriguingly, the half-life of cell surface bet a -ENaC was only approximately 6 h. We then examined the effects of various modulators of sodium transport on cell surface levels of individual ENaC s ubunits, Up-regulation of ENaC-mediated sodium conductance by overnight tre atment with aldosterone or by short term incubation with vasopressin dramat ically increased cell surface levels of beta -ENaC without affecting alpha- or gamma -ENaC levels. Conversely, treatment with brefeldin A selectively decreased the amount of beta -ENaC at the apical membrane. Short term treat ment with aldosterone or insulin had no effect on cell surface amounts of a ny subunits, Subcellular fractionation revealed a selective loss of beta -E NaC from early endosomal pools in response to vasopressin, Our data suggest the possibility that trafficking and turnover of individual ENaC subunits at the apical membrane of A6 cells is non-coordinately regulated. The selec tive trafficking of beta -ENaC may provide a mechanism for regulating sodiu m conductance in response to physiological stimuli.