Lidocaine block of human heart sodium channels expressed in Xenopus oocytes

The tertiary amine lidocaine is used clinically for preventing cardiac arrhythmias, and has been widely studied on mammalian tissue. Xenopus oocytes were used as an expression system to study the effect of lidocaine on a sodium (Na) channel, derived from a full-length human heart (hH1) cDNA clone. The concentration dependence of the lidocaine block of hH1 Na current was consistent with a binding stoichiometry of 1:1. At low frequency stimulation, and at holding potentials ≤-100 mV, the IC₅₀ was 226 μm, comparable to values found in mammalian cardiac cells. Lidocaine also shifted the steady-state inactivation of hH1 Na current to hyperpolarized potentials in a dose-dependent manner. Our experiments suggest that lidocaine block is state dependent, with high affinity for an inactivated state (K_I=11 μm) and low affinity for the resting state (K_r=3.9 mm). The quaternary amine derivative of lidocaine, QX-314, had no effect on Na current at an extracellular concentration of 1 mm.