Figure 5.8.

The model for state-dependent block of sodium channels by SCBIs predicts that block is voltage dependent only over the range over which inactivation occurs. In order to observe block directly at strong depolarizations, where inactivation is complete, the axon was held at various potentials as in Fig. 5.7, but a 500-ms hyperpolarizing prepulse to − 120 mV was applied just before the 10-ms test pulse during which I_Na was measured (see inset protocol). The prepulse was of sufficient amplitude and duration to remove inactivation completely, without interfering with block. (A) Time course of f_u, the fraction of channels unblocked, following steps from −120 mV to various other potentials, in an axon equilibrated with 1 µM RH-1211. Steady-state block from this and another axon treated with 0.5 µM RH-1211 are plotted in (B). The solid curves in (B) were plotted according to Eq. (5.2), with K_I= 0.14 µM, V_S= 78.2mV, and k=4.44 mV. Reproduced from Salgado (1992) with permission from the American Society for Pharmacology and Experimental Therapeutics.