Intrinsic and non-intrinsic regulation of the epithelial Na channel by membrane voltage and intracellular Na

The regulation of the cloned rat epithelial Na channel (rENaC) by membrane voltage and intracellular Na concentration was investigated in the Xenopus oocyte expression system utilizing dual electrode voltage clamp. Large changes of membrane holding voltage from 0 mV to -100 mV (intracellular with respect to ground) caused a transient increase of inward slope conductance at -100 mV. This increase peaked at 119.5 ± 2.7 % (n=7) of control within 1 min, and declined steadily thereafter to 33.7 ± 4.6 % of control within 30 min. A similar pattern of changes of conductance was observed in oocytes subjected to a holding voltage change from 0 mV to -60 mV. The long term (30 min) voltage-induced changes of conductance were not reversed by a 10 fold decrease of external Na, indicating that these secondary changes with membrane hyperpolarization may not be directly attributed to intracellular Na changes. Increasing intracellular Na concentration by direct injection of fixed volumes of 0.25 M Na₂SO₄, caused inhibition of ENaC conductance. This response was also biphasic with a near immediate decrease of inward conductance to 0.788 ± 0.043 (n=8) of control within the first 2 min, and a secondary decrease to 0.593 ± 0.053 (n=6) of control at 30 min. Thus, within the time frame necessary for diffusion of injected Na to the oocyte plasma membrane (assessed from the changes of reversal potential) there was an ∼ 20 % decrease of ENaC conductance. In conclusion, the regulation of the Na channel by membrane voltage and intracellular Na concentration is likely to be complex and may contain an intrinsic component that accounts for the near immediate changes, and a non-intrinsic component that may require the involvement of additional cellular factors to account for the slow secondary response.