Sodium dependence of the epithelial sodium conductance expressed in Xenopus laevis oocytes.

The epithelial Na conductance was expressed in Xenopus laevis oocytes by injection of size-fractionated mRNA of bovine tracheal epithelium. Fractionation was achieved by sucrose density gradient centrifugation. Successful expression was analysed by recording current/voltage ( I/V) curves in the presence and absence of amiloride (10 μmol/l). The newly expressed conductance was half-maximally inhibited by 44 nmol/l amiloride and exhibited a selectivity for Na over K of 140∶1. I/V curves obtained at different extracellular Na concentrations ([Na]) were subjected to a Goldman-fit analysis to obtain the relation between Na permeability ( P) and [Na]. The data show that decreasing [Na] from 85 mmol/l to 0.85 mmol/l increased P by more than threefold, which is thought to reflect Na channel inhibition by increasing [Na]. This effect clearly exceeded what can be attributed to concentration saturation of single Na channel conductance (Palmer and Frindt (1986) Proc Natl Acad Sci USA 83:2767). No correlation of inhibition with intracellular Na concentration was observed. Preservation of the [Na]-dependent self-inhibition by the newly expressed Na conductance suggests that it is an intrinsic property of the Na channel protein, probably mediated by an extracellular Na binding site. [ABSTRACT FROM AUTHOR]