Characterization of pyrethroid action on ciliary calcium channels in Paramecium tetraurelia

Type-II pyrethroids, including deltamethrin, are highly toxic to Paramecium tetraurelia, an organism that does not possess a voltage-sensitive sodium channel. Previous research has established that deltamethrin is toxic to P. tetraurelia in mortality bioassays at concentrations as low as 10(-10) and 10(-11) M under resting and depolarizing conditions, respectively. Deltame thrin, likewise, stimulated P. tetraurelia backward-swimming behavior, an a voidance behavioral response that is controlled exclusively by Ca2+ uptake via the voltage-sensitive calcium channels associated with the cilia. We ha ve now characterized the action of various calcium channel agonists and ant agonists on the avoidance behavior and Ca2+ influx in P. tetraurelia and ha ve determined that the voltage-sensitive calcium channel associated with th e cilia is blocked by the divalent cation Ni2+ but is insensitive to octano l and amiloride. Radioisotope tracer experiments, using whole cells under r esting conditions, established that the toxic 1R isomer of deltamethrin res ulted in increased Ca2+ influx, while the nontoxic 1S enantiomer produced n o significant effect. Pawn mutants, which lack a functional voltage-sensiti ve calcium channel, were unaffected by deltamethrin. Fluorescent bioassays, under depolarizing conditions, corroborated behavioral and radioisotope ex periments. Specifically, these experiments established that deltamethrin st imulated Ca2+ uptake in a stereospecfic manner and that this uptake was blo cked by the phenethylamine-type calcium channel blocker D595 under physiolo gical conditions. Deltamethrin treatment resulted in a dose-dependent incre ase in Ca2+ uptake and membrane depolarization with concentrations as low a s 10(-11) M. Electrophysiological recordings of whole cells showed that tre atment of 10(-9) M deltamethrin resulted in membrane destabilization, incre ased number of spontaneous action potentials, prolonged repetitive discharg es following stimulation, membrane depolarization, and death by osmotic lys is. Our findings establish that the toxic effect of deltamethrin is structu rally related, dose dependent, and enhanced by depolarization and provide s ubstantial evidence that Type-II pyrethroids, specifically deltamethrin, ac t as potent calcium channel agonists on the ciliary voltage-sensitive calci um channel of P. tetraurelia. (C) 1999 Academic Press.