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XB-ART-34520
J Physiol 2006 Jul 15;574Pt 2:333-47. doi: 10.1113/jphysiol.2006.109173.
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Open probability of the epithelial sodium channel is regulated by intracellular sodium.

Anantharam A , Tian Y , Palmer LG .


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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.

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References [+] :
Abriel, Feedback inhibition of rat amiloride-sensitive epithelial sodium channels expressed in Xenopus laevis oocytes. 1999, Pubmed, Xenbase