Inward NMDG⁺ currents recorded from HEK cells stably expressing Kv3.1 channels. (A) The pulse protocol used to record the currents shown in B and C. The cells were held at −100 mV and depolarized for 12 ms at 0.5 Hz to potentials between −20 and +40 mV in 20-mV steps. (B and C) Current recordings from a cell stably expressing Kv3.1 channels with 140 mM of symmetrical NMDG⁺ solutions (B) or with 140 mM NMDG⁺ internal solution and 140 mM TEA⁺ external solution (C). (D) Averaged ratio of Q_off/Q_on as a function of the pulse potentials. The amount of charge displaced during depolarizations and repolarizations (Q_on and Q_off), respectively, was obtained as described in Fig. 1 from the data shown in B and C (n = 4). The horizontal dotted line indicates Q_off/Q_on = 1.0. The solid line overlaying NMDG⁺ data is a fitting of normalized Kv3.1 channel tail currents (right ordinate) obtained with 135 mM K⁺_i/5 mM K⁺_o to a single Boltzmann function. The half-activation voltage (V_1/2) was 3.9 ± 2.9 mV, and the slope factor (k) was 9.7 ± 0.9 mV (n = 7).