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 mMLi⁺) were found to be constitutively open. Only upon inclusion of 1 μM amiloride in the pipette solution couldchannel 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: αγchannels (6.7 pS in Li⁺) had either long open times with short closings, resulting in a high P_o (0.78), or shortopenings 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 pSin Li⁺) behaved as a single population of channels with distinct kinetics: mean open time of 1.2 s and closed timeof 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 reducedthe mean open time of αγ_T to 151 ms without significantly altering the closed time. We also examined the kineticsof amiloride block of αβ, αβ_T (1 μM amiloride), and αγ_T (0.1 μM amiloride) channels. αβ and αβ_T had similarblocking and unblocking rate constants, whereas the unblocking rate constant for αγ_T was 10-fold slower thanαβ_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.