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  • Title: Sulfonylurea receptors inhibit the epithelial sodium channel (ENaC) by reducing surface expression.
    Author: Konstas AA, Bielfeld-Ackermann A, Korbmacher C.
    Journal: Pflugers Arch; 2001 Aug; 442(5):752-61. PubMed ID: 11512032.
    Abstract:
    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 co-expression studies on Xenopus laevis oocytes, which were assayed for amiloride-sensitive currents (DeltaIami). 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) DeltaIami was reduced by congruent with 53% (or congruent with 45%) compared to DeltaIami measured in matched control oocytes injected with ENaC alone. The inhibitory effect of SUR on DeltaIami was preserved in oocytes expressing ENaC with C-terminally truncated subunits. Co-expression of SURs did not confer sensitivity of DeltaIami 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-HAAAA, a mutant that lacks this signal, still inhibits ENaC currents. Chemiluminescence was reduced by congruent with 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 DeltaIami by reducing surface expression of the channel.
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