Abstract
We have further characterized at the single channel level the properties of epithelial sodium channels formed by coexpression of alpha with either wild-type beta or gamma subunits and alpha with carboxy-terminal truncated beta (betaT) or gamma (gammaT) subunits in Xenopus laevis oocytes. alphabeta and alphabetaT channels (9.6 and 8.7 pS, respectively, with 150 mM Li+) were found to be constitutively open. Only upon inclusion of 1 microM amiloride in the pipette solution could channel activity be resolved; both channel types had short open and closed times. Mean channel open probability (Po) for alphabeta was 0.54 and for alphabetaT was 0.50. In comparison, alphagamma and alphagammaT channels exhibited different kinetics: alphagamma channels (6.7 pS in Li+) had either long open times with short closings, resulting in a high Po (0.78), or short openings with long closed times, resulting in a low Po (0. 16). The mean Po for all alphagamma channels was 0.48. alphagammaT (6.6 pS in Li+) behaved as a single population of channels with distinct kinetics: mean open time of 1.2 s and closed time of 0.4 s, with a mean Po of 0.6, similar to that of alphagamma. Inclusion of 0. 1 microM amiloride in the pipette solution reduced the mean open time of alphagammaT to 151 ms without significantly altering the closed time. We also examined the kinetics of amiloride block of alphabeta, alphabetaT (1 microM amiloride), and alphagammaT (0.1 microM amiloride) channels. alphabeta and alphabetaT had similar blocking and unblocking rate constants, whereas the unblocking rate constant for alphagammaT was 10-fold slower than alphabetaT. Our results indicate that subunit composition of ENaC is a main determinant of Po. In addition, channel kinetics and Po are not altered by carboxy-terminal deletion in the beta subunit, whereas a similar deletion in the gamma subunit affects channel kinetics but not Po.