hERG gating currents and voltage dependence of Q-V_on. (A; left) Representative gating currents evoked by 24-ms depolarizing pulses (pulse protocol is shown). At +60 mV, both Ig_on and Ig_off decays were fit with a double exponential (fits are shown in gray). (Right) Expanded Ig_on and Ig_off. For Ig_off, gating current was recorded at −100 mV after depolarization to −80, −30, +20, and +80 mV. Same current scale was used for Ig_on and Ig_off. (B and C) Gating current traces were integrated and charge was plotted as a function of voltage. The Q-V_on plots were best fit with a double Boltzmann function. (B) On-gating current: V_1/2–1 = −35.6 mV, k₁ = 5.6 mV, V_1/2–2 = 28.9 mV, k₂ = 14.5 mV, and r² = 0.99. (C) Off-gating current: V_1/2–1 = −23.8 mV, k₁ = 8.5 mV, V_1/2–2 = 25.8 mV, k₂ = 11.6 mV, and r² = 0.99. (D) Mean data for the two components of Q-V_on: V_1/2–1 = −28.7 ± 2.3 mV, k₁ = 7.7 ± 1.0 mV, V_1/2–2 = 29.4 ± 2.0 mV, and k₂ = 13.2 ± 1.7 mV; n = 3 or 4 at each potential. Between −80 and 0 mV, data were collected in 5-mV steps.