Figure 7.

Lidocaine does not slow the return of site 1304 accessibility after brief depolarizations. In the protocol diagrammed in A, 20-ms exposures to 8 μM MTS-ET were timed to occur 7.5 ms after 20-ms depolarizations to 0 mV. The macroscopic current was assayed between each exposure, and modification rates were measured as shown in Fig. 2. The 7.5 ms reflects the experimentally measured delay between the voltage command to the piezoelectric stack (which occurred exactly at the end of the conditioning pulse) and the commencement of solution exchange (for details, see Vedantham and Cannon, 1998). B shows the average of several modification experiments conducted with or without 1.0 mM lidocaine. The solid lines are exponentials whose time constants are the mean values of the time constants obtained from fits to data from individual experiments. The dashed line reflects the curve that would be expected if the accessibility of site 1304 paralleled the degree of Na⁺ channel availability in 1.0 mM lidocaine during the exposure. The fraction of current recovered in the presence of 1.0 mM lidocaine after 8-, 10-, and 20-ms recovery times was averaged (corresponding to the shaded area in Fig. 6), giving 0.2962. An accessibility of 0.30 predicts a rate of 2.1 s⁻¹ at 8 μM MTS-ET (predicted rate = 0.30 × (R _max − R _min) + R _min), or 0.26 μmol⁻¹ s⁻¹.