Regulation of a cloned epithelial Na⁺ channel by its β and γ-subunits

Using the Xenopus oocyte expression system, we examined the mechanisms by which the β- and γ-subunits of an epithelial Na⁺ channel (ENaC) regulate α-subunit channel activity and the mechanisms by which β-subunit truncations cause ENaC activation. Expression of α-ENaC alone produced small amiloride-sensitive currents (-43 ± 10 nA, n = 7). These currents increased >30-fold with the coexpression of β- and γ-ENaC to - 1,476 ± 254 nA (n = 20). This increase was accompanied by a 3.1- and 2.7-fold increase of membrane fluorescence intensity in the animal and vegetal poles of the oocyte, respectively, with use of an antibody directed against the α- subunit of ENaC. Truncation of the hast 75 amino acids of the β-subunit COOH terminus, as found in the original pedigree of individuals with Liddle's syndrome, caused a 4.4-fold (n = 17) increase of the amiloride-sensitive currents compared with wild-type αβγ-ENaC. This was accompanied by a 35% increase of animal pole membrane fluorescence intensity. Injection of a 30- amino acid peptide with sequence identity to the COOH terminus of the human β-ENaC significantly reduced the amiloride-sensitive currents by 40-50%. These observations suggest a tonic inhibitory role on the channel's open probability (P(o)) by the COOH terminus of β-ENaC. We conclude that the changes of current observed with coexpression of the β- and γ-subunits or those observed with β-subunit truncation are likely the result of changes of channel density in combination with large changes of P(o).