The mechanism of formamidine synergism of pyrethroids in susceptible house flies and flies with target site resistance (kdr) to pyrethroid insecticides was characterized by measurements of [3H]saxitoxin (STX) binding to nerve membrane sodium channels from untreated and formamidinetreated flies. Two STX binding components, one with high affinity and one with low affinity, were observed in untreated flies of each strain, and the binding constants, Bmax and Kd, were similar for each binding component in each strain. In vivo exposure of flies to the formamidines chlordimeform and BTS 27271 appeared to change the number of STX binding components from two to one and to decrease the concentration at which saturation of STX binding occurred. Also, in vivo exposure to the formamidines changed STX binding cooperativity from negative to positive, based on Hill plot data. This change was greater with BTS 27271 than with chlordimeform and it paralleled the greater synergism of pyrethroid toxicity seen with BTS 27271. With BTS 27271 the binding affinity for STX was significantly greater in SBO than KDR flies, a result that paralleled the greater synergism seen in SBO flies. Our data suggest that formamidines are target site synergists of pyrethroids which act by modifying drug binding cooperativity in target tissue.
All Science Journal Classification (ASJC) codes
- Agronomy and Crop Science
- Health, Toxicology and Mutagenesis