Journal
JOURNAL OF GENERAL PHYSIOLOGY
Volume 120, Issue 2, Pages 133-145Publisher
ROCKEFELLER UNIV PRESS
DOI: 10.1085/jgp.20028612
Keywords
ENaC; activation energy; trypsin; protease; sodium transport
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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 (Q(10)act = 1.5) activation rate and a strongly temperature-dependant (Q(10)inact = 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 alpha, beta, and gamma 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.
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