Na Self Inhibition of Human Epithelial Na Channel : Temperature Dependence and Effect of Extracellular Proteases

The regulation of the open probability of the epithelial Na⁺ channel (ENaC) by the extracellular concentration of Na⁺, a phenomenon called “Na⁺ self inhibition,” has been well described in several natural tight epithelia, but its molecular mechanism is not known. We have studied the kinetics of Na⁺ self inhibition on human ENaC expressed in Xenopus oocytes. Rapid removal of amiloride or rapid increase in the extracellular Na⁺ concentration from 1 to 100 mM resulted in a peak inward current followed by a decline to a lower quasi-steady-state current. The rate of current decline and the steady-state level were temperature dependent and the current transient could be well explained by a two-state (active-inactive) model with a weakly temperature-dependent (Q₁₀act = 1.5) activation rate and a strongly temperature-dependant (Q₁₀inact = 8.0) inactivation rate. The steep temperature dependence of the inactivation rate resulted in the paradoxical decrease in the steady-state amiloride-sensitive current at high temperature. Na⁺ self inhibition depended only on the extracellular Na⁺ concentration but not on the amplitude of the inward current, and it was observed as a decrease of the conductance at the reversal potential for Na⁺ as well as a reduction of Na⁺ outward current. Self inhibition could be prevented by exposure to extracellular protease, a treatment known to activate ENaC or by treatment with p-CMB. After protease treatment, the amiloride-sensitive current displayed the expected increase with rising temperature. These results indicate that Na⁺ self inhibition is an intrinsic property of sodium channels resulting from the expression of the α, β, and γ subunits of human ENaC in Xenopus oocyte. The extracellular Na⁺-dependent inactivation has a large energy of activation and can be abolished by treatment with extracellular proteases.