Voltage dependence of block by amiloride in oocytes expressing WT and αS583C ENaC. (A) I_Na-V relationships for WT in 20 mM NaCl with no amiloride or 0.1 µM amiloride (amil). (B) Plot of ln(I₀/I_amil – 1) versus voltage where I₀ and I_amil are currents in the absence and presence of amiloride, respectively. The slope of the relationship obtained by linear regression gives K_amil = 64 ± 11 nM, z•δ = 0.03 ± 0.01 (n = 6). (C) I_Na-V relationships for αS583C in 110 mM NaCl with no amiloride or 10⁻⁶ M amiloride. (D) Plot of ln(I₀/I_amil – 1) versus voltage. The slope of the relationship obtained by linear regression gives K_amil = 2.0 ± 0.2 µM, z•δ = 0.12 ± 0.01 (n = 4).