Figure 3. Flecainide inhibition of a non-inactivating mutant of Nav1.5A.

The non-inactivating (QQQ) mutant channels (see text) were expressed in oocytes and current elicited by depolarizing to −10 mV for 400 ms from a holding potential of −100 mV. Flecainide (1–25 μm) induced a time-dependent decay in the current in the otherwise slowly inactivating current. B, the flecainide-induced decay was fitted with either one (control) or the sum of two (flecainide) exponentials and the effective blocking rate (1/τ_f) plotted versus the flecainide concentration. The straight lines are consistent with a bimolecular interaction with slope (k_on) and y-intercept (k_off). Also plotted is the blocking rate measured after reducing the external concentration of Na⁺ by 50%. The association and dissociation rate constants and the blocking affinity (K_D = k_off/k_on) are listed in Table 2. C, the steady-state current measured near the end of the depolarizing pulses was normalized to the current of drug-free controls and plotted versus flecainide concentration. The continuous curves are fits to a single-site binding model [(I/I_o = (1 + [flecainide]/K_D)⁻¹] with equilibrium constants (K_D) of 11.2 ± 1.3 μm (n = 7) and 4.1 ± 0.7 μm (n = 6) for 100 and 50% external Na⁺, respectively.