Activation of Drosophila sodium channels promotes modification by deltamethrin - Reductions in affinity caused by knock-down resistance mutations

kdr and super-kdr are mutations in houseflies and other insects that confer 30- and 500-fold resistance to the pyrethroid deltamethrin. They correspon d to single (L1014F) and double (L1014F+M918T) mutations in segment IIS6 an d linker II (S4-S5) of Na channels. We expressed Drosophila para Na channel s with and without these mutations and characterized their modification by deltamethrin. All wild-type channels can be modified by <10 nM deltamethrin , but high affinity binding requires channel opening: (a) modification is p romoted more by trains of brief depolarizations than by a single long depol arization, (b) the voltage dependence of modification parallels that of cha nnel opening, and (c) modification is promoted by toxin II from Anemonia su lcata, which slows inactivation. The mutations reduce channel opening by en hancing closed-state inactivation. In addition, these mutations reduce the affinity for open channels by 20- and 100-fold, respectively. Deltamethrin inhibits channel closing and the mutations reduce the time that channels re main open once drug has bound. The super-kdr mutations effectively reduce t he number of deltamethrin binding sites per channel from two to one. Thus, the mutatations reduce both the potency and efficacy of insecticide action.