Saxitoxin (STX) binding was measured in susceptible (SBO) and pyrethroid-resistant (KDR) female houseflies having only target site insensitivity as a resistance mechanism. In KDR flies, there was a quantitative decrease in STX binding capacity (Bmax) relative to SBO flies coupled with an increase in binding affinity (Kd). Treatment of SBO flies with sublethal doses of cypermethrin resulted in a large decrease in the number of STX binding sites and an increase in STX binding affinity. In KDR flies, identical treatments had the opposite effects. Treatment of both strains with higher doses of cypermethrin resulted in smaller decreases in Bmax values coupled with decreases in binding affinities. The results show that physiological changes in STX binding occur upon exposure to extremely low doses of cypermethrin. The data suggest that the kdr resistant gene may be expressed as changes in STX binding kinetics and that measurements of STX binding in pyrethroid-treated insects may be a useful approach for studying pyrethroid's mode of action and resistance.
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