Immobilization of sodium channel gating charge in crayfish giant axons by the insecticide fenvalerate.

The type II pyrethroid fenvalerate is known to depolarize nerve membranes by keeping sodium channels in a very stable modified open state. We have performed experiments on crayfish giant axons to determine whether the asymmetric charge movement was affected in parallel with changes in sodium current. When all sodium channels were modified by repetitive stimulation in the presence of fenvalerate, on gating charge movement was reduced by 78%. When on gating currents were fractionated into three exponentially decaying components, it was found that fenvalerate selectively depressed the intermediate and slow components, leaving the fast component unchanged. Off gating currents could be fractionated into two exponentially decaying components. The fast component of off charge movement (tau = 50 microseconds at -160 mV) was abolished by fenvalerate, whereas the slow component was suppressed by 50%. These results are consistent with previous conclusions that a large fraction of the intermediate and slow on and slow off components and essentially all of the fast off components are related to sodium channel gating. We conclude that fenvalerate traps the gating charges of sodium channels in the open state.