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We have further characterized at the single channel level the properties of epithelial sodium channels formed by coexpression of
with either wild-type β or 
subunits and with carboxy-terminal truncated β (βT) or (T) subunits in Xenopus laevis oocytes. β and βT channels (9.6 and 8.7 pS, respectively, with 150 mM Li+) were found to be constitutively open. Only upon inclusion of 1 µM 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 β was 0.54 and for βT was 0.50. In comparison, and T channels exhibited different kinetics: 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 channels was 0.48. T (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 . Inclusion of 0.1 µM amiloride in the pipette solution reduced the mean open time of T to 151 ms without significantly altering the closed time. We also examined the kinetics of amiloride block of β, βT (1 µM amiloride), and T (0.1 µM amiloride) channels. β and βT had similar blocking and unblocking rate constants, whereas the unblocking rate constant for T was 10-fold slower than βT. 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 β subunit, whereas a similar deletion in the subunit affects channel kinetics but not Po.
Key Words: epithelial sodium channel • open probability • amiloride block • Xenopus oocyte
Abbreviations: CH, chimera; ENaC, epithelial sodium channel
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