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J. Gen. Physiol.,
Volume 112, Number 4, October 1, 1998 423-432
From the Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, Connecticut 06520
We have further characterized at the single channel level the properties of epithelial sodium channels formed by coexpression of 
with either wild-type 
or 
subunits and with carboxy-terminal truncated (T) or (T) subunits in Xenopus laevis oocytes. and T 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 was 0.54 and for T 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 , T (1 µM amiloride), and T (0.1 µM amiloride) channels. and T had similar
blocking and unblocking rate constants, whereas the unblocking rate constant for T was 10-fold slower than
T. 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 subunit, whereas a similar deletion in the subunit affects channel kinetics but not Po.
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