The role of Pre-H2 domains of alpha- and delta-epithelial Na+ channels in ion permeation, conductance, and amiloride sensitivity.

Epithelial Na(+) channels (ENaC) regulate salt and water re-absorption across the apical membrane of absorptive epithelia such as the kidney, colon, and lung. Structure-function studies have suggested that the second transmembrane domain (M2) and the adjacent pre- and post-M2 regions are involved in channel pore formation, cation selectivity, and amiloride sensitivity. Because Na(+) selectivity, unitary Na(+) conductance (gamma(Na)), and amiloride sensitivity of delta-ENaC are strikingly different from those of alpha-ENaC, the hypothesis that the pre-H2 domain may contribute to these characterizations has been examined by swapping the pre-H2, H2, and both (pre-H2+H2) domains of delta- and alpha-ENaCs. Whole-cell and single channel results showed that the permeation ratio of Li(+) and Na(+) (P(Li)/P(Na)) for the swap alpha chimeras co-expressed with betagamma-ENaC in Xenopus oocytes decreased significantly. In contrast, the ratio of P(Li)/P(Na) for the swap delta constructs was not significantly altered. Single channel studies confirmed that swapping of the H2 and the pre-H2+H2 domains increased the gamma(Na) of alpha-ENaC but decreased the gamma(Na) of delta-ENaC. A significant increment in the apparent inhibitory dissociation constant for amiloride (K(i)(amil)) was observed in the alpha chimeras by swapping the pre-H2, H2, and pre-H2+H2 domains. In contrast, a striking decline of K(i)(amil) was obtained in the chimeric delta constructs with substitution of the H2 and pre-H2+H2 domains. Our results demonstrate that the pre-H2 domain, combined with the H2 domain, contributes to the P(Li)/P(Na) ratio, single channel Na(+) conductance, and amiloride sensitivity of alpha- and delta-ENaCs.