Subunit Composition Determines the Single Channel Kinetics of the Epithelial Sodium Channel

We have further characterized at the single channel level the properties of epithelial sodium channels formed by coexpression of α with either wild-type β or γ subunits and α with carboxy-terminal truncated β (β(T)) or γ (γ(T)) subunits in Xenopus laevis oocytes. αβ and αβ(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 (P (o)) for αβ was 0.54 and for αβ(T) was 0.50. In comparison, αγ and αγ(T) channels exhibited different kinetics: αγ channels (6.7 pS in Li(+)) had either long open times with short closings, resulting in a high P (o) (0.78), or short openings with long closed times, resulting in a low P (o) (0.16). The mean P (o) for all αγ channels was 0.48. αγ(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 P (o) of 0.6, similar to that of αγ. Inclusion of 0.1 μM amiloride in the pipette solution reduced the mean open time of αγ(T) to 151 ms without significantly altering the closed time. We also examined the kinetics of amiloride block of αβ, αβ(T )(1 μM amiloride), and αγ(T) (0.1 μM amiloride) channels. αβ and αβ(T) had similar blocking and unblocking rate constants, whereas the unblocking rate constant for αγ(T) was 10-fold slower than αβ(T). Our results indicate that subunit composition of ENaC is a main determinant of P (o). In addition, channel kinetics and P (o) are not altered by carboxy-terminal deletion in the β subunit, whereas a similar deletion in the γ subunit affects channel kinetics but not P (o).