Open probability of the epithelial sodium channel is regulated by intracellular sodium.
The regulation of epithelial Na(+) channel (ENaC) activity by Na(+) was studied in Xenopus oocytes using two-electrode voltage clamp and patch-clamp recording techniques. Here we show that amiloride-sensitive Na(+) current (I(Na)) is downregulated when ENaC-expressing cells are exposed to high extracellular [Na(+)]. The reduction in macroscopic Na(+) current is accompanied by an increase in the concentration of intracellular Na(+) ([Na(+)](i)) and is only slowly reversible. At the single-channel level, incubating oocytes in high-Na(+) solution reduces open probability (P(o)) approximately twofold compared to when [Na(+)] is kept low, by increasing mean channel closed times. However, increasing P(o) by introducing a mutation in the beta-subunit (S518C) which, in the presence of [2-(trimethylammonium) ethyl] methane thiosulfonate (MTSET), locks the channel in an open state, could not alone abolish the downregulation of macroscopic current measured with exposure to high external [Na(+)]. Inhibition of the insertion of new channels into the plasma membrane using Brefeldin A revealed that surface channel lifetime is also markedly reduced under these conditions. In channels harbouring a beta-subunit mutation, R564X, associated with Liddle's syndrome, open probability in both high- and low-Na(+) conditions is significantly higher than in wild-type channels. Increasing the P(o) of these channels with an activating mutation abrogated the difference in macroscopic current observed between groups of oocytes incubated in high- and low-Na(+) conditions. These findings demonstrate that reduction of ENaC P(o) is a physiological mechanism limiting Na(+) entry when [Na(+)](i) is high.
Anantharam
Arun
A
Department of Physiology and Biophysics, Weill Medical College of Cornell University, 1300 York Avenue, New York, NY 10021, USA.
Tian
Yuan
Y
Palmer
Lawrence G
LG
D000818:Animals; D020126:Brefeldin A / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D053503:Epithelial Sodium Channels; D025461:Feedback, Physiological / Q000502:physiology*; D005260:Female; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008698:Mesylates / Q000494:pharmacology; D009154:Mutation; D009865:Oocytes / Q000187:drug effects / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000502:physiology*; D015222:Sodium Channels / Q000187:drug effects / Q000235:genetics* / Q000378:metabolism*; D014982:Xenopus laevis
D053503:Epithelial Sodium Channels; D008698:Mesylates; D011500:Protein Synthesis Inhibitors; D015222:Sodium Channels; C109154:(2-(trimethylammonium)ethyl)methanethiosulfonate; D020126:Brefeldin A; D012964:Sodium