1-(2,4-xylyl)guanidine blocks R666G gating pore currents. (A) Sodium gating pore current in a representative cell was first measured in 120 mM Na⁺, 1.8-mM Ca²⁺ external solution (black traces), and after addition of 5 mM 1-(2,4-xylyl)guanidine carbonate to external solution (red traces). The holding potential was 0 mV. Leak-subtracted currents are plotted against the voltage in the bottom panel. (B) Guanidinium gating pore current in a representative cell first measured in solution containing 115 mM NMDG, 10 mM guanidine sulfate, 1.8 mM Ca²⁺ (blue traces), and after addition of 5 mM 1-(2,4-xylyl)guanidine carbonate to external solution (red traces). The holding potential was 0 mV. Leak-subtracted current-voltage relationships are plotted in the bottom panel.