Characterization of the block of sodium channels by phenytoin in mouse neuroblastoma cells

The interaction of phenytoin (DPH) with membrane ionic channels of cultured N1E-115 neuroblastoma cells was studied. The single suction pipette technique was used for voltage clamp and intracellular perfusion. When the cells were held at -80 mV for periods of 1 min or more, DPH (20-100 μM) inhibited inward sodium current in a dose-dependent manner (resting block); resting block was relieved by hyperpolarizing cells to -100 mV for 1 min. A hyperpolarizing shift of the slow inactivation curve for the Na current was induced by DPH and can explain the effect of holding potential on the resting block. The fast Na inactivation curve, however, was not affected. During repetitive pulsing the DPH-induced inhibition of Na current was enhanced (conditioned block). Conditioned block was both voltage- and frequency-dependent. Conditioning pulses to potentials which do not appreciably open Na channels also produced conditioned block; prolongation of conditioning pulses even to durations longer than the time for maximal steady-state inactivation of the Na current progressively increased the extent of conditioned block, suggesting that DPH can interact with inactivated and closed Na channels. The time course of recovery from voltage-dependent inactivation of sodium current during conditioned block was both slowed and exhibited voltage dependence. Recovery occurred faster when membrane potential during the recovery period was more negative. We conclude that DPH blocks Na channels both by increasing the fraction of channels in an inactivated state and by delaying the transition from inactivated to closed but available channels. This effect is enhanced by depolarizing membrane potential and increasing the frequency of stimulation. These effects of DPH may have important implications with regard to the anticonvulsant activity of DPH.