Modification of slow inactivation of single sodium channels by phenytoin in neuroblastoma cells.

Modifications of Na+ channels by phenytoin (PT), an anticonvulsant drug, were examined. Previous work using voltage-clamp methods indicated that PT could interact with inactivated states of the channel to reduce excitability. Single-channel analysis was used to test the idea that the fast inactivation process was not required for modification of the channel. The hypothesis that PT could interact with open or slow inactivated states to produce a drug-bound, long duration, nonconducting state was also tested. Currents due to the opening of single Na+ channels were measured in inside-out patches of membrane excised from N1E-115 mouse neuroblastoma cells grown in tissue culture. After the removal of the fast inactivation process enzymatically, the average Na+ current in response to a step depolarization decayed due to the slow inactivation process. The time constant of decay decreased as a function of the concentration of PT. The average current appeared to be caused by extensive reopening of Na+ channels. During maintained depolarization, the reopening of Na+ channels occurred in bursts interrupted by long silent periods, due to the slow inactivated state. PT decreased the burst duration and increased the interval between bursts. The average open time of Na+ channels was reduced in the presence of PT. All of the alterations were enhanced as the concentration of PT was increased. The amplitude of current through the open channel was not effected by PT. PT was able to modify the Na+ channel in the absence of fast inactivation. The results suggest that PT can bind to the Na+ channel and produce a nonconducting state from which the probability of a channel opening is small. These modifications could underly the selective block of action potentials during chronic depolarization of the membrane or during high frequency discharge.