Figure 1. Flecainide preferentially inhibits activated states of Nav1.5.

A, use-dependent inhibition was induced by applying a train of 100 depolarizing pulses (−10 mV, 20 ms) at a frequency of 10 Hz. The currents were measured before (Control) and 5 min after application of flecainide (2.5–100 μm). B, the steady-state current measured after 100 pulses (I₁₀₀/I₁) in the presence of flecainide was normalized to the drug-free control and plotted versus the flecainide concentration (Use-dependent). Also plotted is the resting block determined from single depolarizing pulses (−10 mV, 20 ms) applied at 60 s intervals from a holding potential of −120 mV (Resting Block). The continuous curves are a fit to the Hill equation [(I/I_o = (1 + ([Flec]/IC₅₀)⁽ⁿ)⁻¹] where I and I_o are the control and drug-modified current amplitudes and n is the Hill coefficient. The IC₅₀ and n values are 7.4 ± 0.6 μm and 1.5 ± 0.2 for the use-dependent block (n = 5) and 345 ± 15 μm and 1.1 ± 0.06 for the resting block (n = 6). C, the voltage dependence of the flecainide inhibition (25 μm) was determined by varying the voltage of the pulses applied during repetitive stimulation (100 pulses, 10 Hz). The normalized steady-state inhibition after 100 pulses was determined and the fractional inhibition (1 − I_Flec/I_Cont) plotted versus the test voltage. Also plotted is the normalized conductance versus voltage (G–V) relationship determined by briefly depolarizing (20 ms) to voltages between −70 and −5 mV. The conductance at each voltage (G_V) was calculated (G_V = I_Na/(V − V_r), where V_r = reversal potential), normalized to the conductance measured at −10 mV (G_o) and plotted versus the test voltage (V). The continuous curves are fits to the Boltzmann equation (G/G_o = Max/(1 + exp(V_0.5 − V)/k))) with a midpoint (V_0.5) of −42 ± 1.0 mV and maximal inhibition (Max) of 75 ± 2% for the flecainide block (n = 5) and V_0.5 for the current–voltage relationship measured in the presence of flecainide (25 μm) of −36 ± 0.9 mV (n = 4).