Figure 4.

Selectivity of αS589 mutants to monovalent cations. Macroscopic amiloride-sensitive currents were measured at −100 mV in oocytes superfused with Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺ external solution (see Methods). (A) Amiloride-sensitive currents are normalized to I_Na and presented as mean ± SEM (n = 12–14 per condition). Positive current values correspond to outward currents. I_Na at −100 mV was 29.5 ± 4.0 μA for wt, 49.1 ± 3.4 μA for αS589A, 6.0 ± 0.5 μA for αS589C, and 4.5 ± 0.9 μA for the αS589D mutant. (B) Relationship between the size of the ion and permeability. The normalized amiloride-sensitive currents are plotted as a function of the diameter of the nonhydrated ion. Fits to the equation I_x/I_Na = f × (1-[d/2]/r)², where f is a scaling factor, d is the diameter of the nonhydrated ion, and r is the cylinder radius of the pore, are shown for ENaC wt (solid line), αS589A (long dash), αS589C (short dash), and αS589D (dotted line). The Cs⁺ value for αS589A was omitted in the figure for clarity.