Detection of a P-glycoprotein related pump in Chironomus larvae and its inhibition by verapamil and cyclosporin A.

A membrane associated ATP-dependent efflux pump, similar in function to mammalian P-glycoprotein, was detected in anal papillae of Chironomus riparius larvae. Immunohistochemical analysis of larval tissues, using monoclonal antibodies against P-glycoprotein, was supplemented by functional in vivo and in vitro assays which confirmed the existence of a mechanism for transporting xenobiotic substances. The in vitro ATPase activity of homogenate fractions increased in the presence of typical P-glycoprotein substrates (vinblastine, actinomycin D or ivermectin). This increase was unaffected by inhibitors of other membrane ATPases (sodium azide, EGTA, ouabain), but sensitive to vanadate, cyclosporin A and verapamil which inhibit mammalian P-glycoprotein mediated ATP-consumption. Sublethal concentrations of specific P-glycoprotein-inhibitors such as verapamil or cyclosporin A synergistically enhanced the mortality of C. riparius towards ivermectin. Although cyclosporin A originates from entomopathogenic fungi, its mode of action in insects and its function during infection are not understood. Our results lend some credit to the hypothesis that this compound is possibly released to promote poisoning of the infected host by xenobiotics which are normally removed by a P-glycoprotein related pump. The putative role of insect P-glycoprotein homologues in the context of multiple resistance towards insecticides in discussed.