a, voltage dependence of activation for wild-type Nav1.2, E779Q, E837Q, and L840C channels and for wild type with 200 nm Css IV. Conductance-voltage relationships were determined as described under “Experimental Procedures.” b– d, conductance-voltage relationships measured in control (closed symbols) and in the presence of 200 nm Css IV with a +50 mV, 1-ms prepulse preceding the test pulse by 60 ms for E779Q (b), E837Q (c), and L840C (d ) channels. Analogous data in the presence of 1 mm Css IV are shown for E779Q (b) and L840C (d ) as inverted triangles.