Abstract
Mutations in sodium channels are known to confer knockdown resistance to pyrethroid insecticides, such as permethrin and cypermethrin, in various agricultural pests and disease vectors. Double mutations, D(3i28)V and E(3i32)G, were detected in cypermethrin-resistant Helicoverpa armigera and Heliothis virescens populations. However, the role of the two mutations in pyrethroid resistance remains unclear. In this study, we introduced the mutations into the cockroach sodium channel, BgNa(v)1-1a, and examined their effects on channel gating and pyrethroid sensitivity in Xenopus oocytes. D(3i28)V alone and the double mutation, D(3i28)V/E(3i32)G, shifted the voltage dependence of activation in the depolarizing direction by 15 mV and 20 mV, respectively, whereas E(3i32)G had no significant effect. D(3i28)V reduced the amplitude of tail currents induced by permethrin and NRDC 157 (Type I pyrethroids) and deltamethrin and cypermethrin (Type II pyrethroids), whereas E(3i32)G alone had no effect. Intriguingly, the amplitude of Type II pyrethroid-induced tail current from D(3i28)V/E(3i32)G channels was similar to that of BgNa(v)1-1a channels, but the decay of the tail currents was accelerated. Such effects were not observed for Type I pyrethroid-induced tail currents. Computational analysis based on the model of dual pyrethroid receptors on insect sodium channels predicted D(3i28)V and E(3i32)G exert their effects on channel gating and pyrethroid action via allosteric mechanisms. Our results not only illustrate the distinct effect of the D(3i28)V/E(3i32)G double mutations on Type I vs. Type II pyrethroids, but also reinforce the concept that accelerated decay of tail currents can be an effective mechanism of pyrethroid resistance to Type II pyrethroids.