Sulfonylurea receptors inhibit the epithelial sodium channel (ENaC) by reducing surface expression

In the kidney the epithelial Na+ channel (ENaC) is co-expressed with the sulfonylurea receptor (SUR), an ABC protein that shares a high degree of homology with the cystic fibrosis transmembrane conductance regulator (CFTR) and reportedly modifies ENaC in various preparations. To investigate a possible regulatory relationship between SUR and ENaC, we performed coexpression studies on Xenopus laevis oocytes, which were assayed for amiloride-sensitive currents (DeltaI(ami)). Moreover, a chemiluminescence assay was used to investigate the surface expression of extracellular hemagglutinin-tagged SUR1 (SUR1-HA) or HA-tagged ENaC (ENaC-HA). In oocytes co-injected with SUR1/ENaC (or SUR2B/ENaC) DeltaI(ami) was reduced by congruent to 53% (or congruent to 45%) compared to DeltaI(ami) measured in matched control oocytes injected with ENaC alone. The inhibitory effect of SUR on DeltaI(ami) was preserved in oocytes expressing ENaC with C-terminally truncated subunits. Co-expression of SURs did not confer sensitivity of DeltaI(ami) to diazoxide, pinacidil, tolbutamide, or glibenclamide. ENaC does not facilitate the surface expression of SUR1-HA, which is known to be retained in the endoplasmatic reticulum (ER) by an ER-retention/retrieval signal. SUR1-HA(AAA), a mutant that lacks this signal, still inhibits ENaC currents. Chemiluminescence was reduced by congruent to 49% in oocytes co-expressing ENaC-HA/SUR1 compared to that in control oocytes expressing ENaC-HA alone. We conclude that SUR does not interact with ENaC at the level of the plasma membrane but that it inhibits DeltaI(ami) by reducing surface expression of the channel.