Nerve membrane sodium channels were assayed by measuring [3H]saxitoxin (STX) binding and its displacement by tetrodotoxin (TTX) from house fly head membrane preparations. Activity was measured in a susceptible strain and in strains with single or combinations of genes for target site resistance to pyrethroids and metabolic resistance to insecticides. The affinities (Kd values) for STX binding were similar in all strains, indicating that qualitative differences in sodium channels associated with STX binding were not involved in resistance to pyrethroid insecticides. The amount of STX binding (Bmax) was the same in a susceptible strain and a strain with only target site resistance to pyrethroids, indicating that quantitative differences in sodium channels were not involved in resistance. In cotrast, the amount of STX binding was reduced in strains with metabolic resistance to insecticides. The results explain contradictions reported from several previous studies of STX binding in relation to pyrethroid resistance and suggest that the use of strains combining target site and metabolic resistance was responsible for the previous findings.
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