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Expression of ENaC and other Transport Proteins in Xenopus Oocytes is Modulated by Intracellular Na+Kusche-Vihrog K.1,* · Segal A.2,* · Grygorczyk R.3 · Bangel-Ruland N.1 · Van Driessche W.2 · Weber W.-M.1
1 Institute of Animal Physiology, Westphalian Wilhelms-University Muenster,2Laboratory of Physiology, KU Leuven, Campus Gasthuisberg, Leuven,3Research Centre, Centre hospitalier de l’Université de Montréal - Hôtel-Dieu, 3850 Saint-Urbain, Montréal,*These authors contributed equally Corresponding Author
Institute of Animal Physiology, WWU Muenster
Hindenburgplatz 55, 48143 Muenster (Germany)
Tel. +49 251 83 21782, Fax: +49 251 83 21785
The expression of the epithelial Na+ channel (ENaC) is tissue-specific and dependent on a variety of mediators and interacting proteins. Here we examined the role of intracellular Na+ ([Na+]i) as a modulator of the expression of rat ENaC in Xenopus laevis oocytes. We manipulated [Na+]i of ENaC-expressing oocytes in the range of 0-20 mM by incubating in extracellular solutions of different [Na+]o. Electrophysiological, protein biochemical and fluorescence optical methods were used to determine the effects of different [Na+]i on ENaC expression and membrane abundance. In voltage-clamp experiments we found that amiloride-sensitive ENaC current (Iami) and conductance (Gami) peak at a [Na+]i of ∼10 mM Na+, but were significantly reduced in 5 mM and 20 mM [Na+]i. Fluorescence intensity of EGFP-ENaC-expressing oocytes also followed a bell-shaped curve with a maximum at ∼ 10 mM [Na+]i. In Western blot experiments with specific anti-ENaC antibodies the highest protein expression was found in ENaC-expressing oocytes with [Na+]i of 10-15 mM. Since ENaC is also highly permeable for Li+, we incubated ENaC-expressing oocytes in different Li+ concentrations and found a peak of Iami and Gami with 5 mM Li+. The influence of [Na+]i on the expression is not ENaC-specific, since expression of a Cl- channel (CFTR) and a Na+/glucose cotransporter (SGLT1) showed the same dependence on [Na+]i. We conclude that specific concentrations of Na+ and Li+ influence the expression and abundance of ENaC and other transport proteins in the plasma membrane in Xenopus laevis oocytes. Furthermore, we suggest the existence of a general mechanism dependent on monovalent cations that optimizes the expression of membrane proteins.
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