Permeability of guanidinium through the R666H gating pore. (A) Gating pore currents in representative oocytes expressing R666H channels recorded first in external solution containing 120 mM NMDG at pH 7.4 (left, black traces) and then in external solution containing 60 mM NMDG + 60 mM guanidinium at pH 7.4 (middle, red traces). Right panel (blue traces), gating pore current in a cell with a similar level of expression recorded in external solution containing 60 mM NMDG + 60 mM Na⁺, pH 7.4. Test pulses were recorded in 5-mV increments and shown in 20-mV increments for clarity. (B) Averaged voltage dependence of leak-subtracted and normalized gating pore currents in solutions containing 120 mM NMDG (black circles, n = 4), 60 mM NMDG + 60 mM guanidinium (red circles, n = 4), and 60 mM NMDG + 60 mM Na⁺ (blue triangles, n = 8). Error bars represent SEM.