THE HIGHLY SELECTIVE LOW-CONDUCTANCE EPITHELIAL NA CHANNEL OF XENOPUS-LAEVIS A6 KIDNEY-CELLS

In Na-reabsorbing tight epithelia, the rate-limiting step for Na trans port is the highly selective low-conductance amiloride-sensitive epith elial Na channel (type 1 ENaC). In rat distal colon, type 1 ENaC is ma de of three homologous subunits. The aim of this study was to identify the corresponding genes of the renal channel from the kidney-derived A6 cell line of Xenopus laevis. Three homologous subunits were identif ied and coexpressed in the Xenopus oocyte system. The reconstituted ch annel had all the characteristics of the native type 1 ENaC described in A6 cells: I)high selectivity, 2) low single-channel conductance, 3) slow gating kinetics, and 4) high affinity for amiloride. Transcripts for alpha-,beta-, and gamma-subunits of the Xenopus epithelial Na cha nnel (xENaC) were detected in A6 kidney cells, Xenopus kidney, lung, a nd to a lesser extent in stomach and skin. Each subunit of the xENaC s hares similar to 60% overall identity with the corresponding rat homol ogue (alpha, beta, and gamma rENaC). Our data suggest that the triplic ation of the ENaC subunits occurred before the divergence between mamm alian and amphibian lineages.