Abstract

This study examined the actions of phenytoin, carbamazepine, lidocaine, and verapamil on rat brain type IIA Na+ channels functionally expressed in mammalian cells, using the whole-cell voltage-clamp recording technique. The drugs blocked Na+ currents in both a tonic and use-dependent manner. Tonic block was more pronounced at depolarized holding potentials and reduced at hyperpolarized membrane potentials, reflecting an overall negative shift in the relationship between membrane potential and steady state inactivation. Dose-response relationships with phenytoin supported the hypothesis that the voltage dependence of tonic block resulted from the higher affinity of the drugs for inactivated than for resting channels. At -62 mV, approximately 50% of the Na+ channels were blocked by phenytoin at 13 microM, compared with therapeutic brain levels of 4-8 microM. The use-dependent component of block developed progressively during a 2-Hz train of 40-msec-long stimulus pulses from -85 mV to 0 mV. At 2 Hz, verapamil was the most potent use-dependent blocker, lidocaine and phenytoin had intermediate potencies, and carbamazepine was least effective. The use-dependent block resulted from drug binding to open and inactivated channels during the depolarizing pulses and the slow repriming of drug-bound channels during the interpulse intervals. Verapamil, lidocaine, and phenytoin all bound preferentially to open channels, but this open channel block was most striking for verapamil. Use-dependent block was less pronounced at hyperpolarized membrane potentials, due to more rapid repriming of drug-bound channels. The results indicate that type IIA Na+ channels expressed in a mammalian cell line retain the complex pharmacological properties characteristic of native Na+ channels. These channels are likely to be an important site of the anticonvulsant action of phenytoin and carbamazepine. Lidocaine and verapamil, drugs with well characterized effects on peripheral Na+ and Ca2+ channels, are also effective blockers of these brain Na+ channels.