Expression of ENaC and other transport proteins in Xenopus oocytes is modulated by intracellular Na ⁺

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 (I _ami ) and conductance (G _ami ) 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 I _ami and G _ami 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.