Characterization of Pyrethroid Action on Ciliary Calcium Channels in Paramecium tetraurelia

Abstract 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. Deltamethrin, likewise, stimulated P. tetraurelia backward-swimming behavior, an avoidance behavioral response that is controlled exclusively by Ca 2+ uptake via the voltage-sensitive calcium channels associated with the cilia. We have now characterized the action of various calcium channel agonists and antagonists on the avoidance behavior and Ca 2+ influx in P. tetraurelia and have determined that the voltage-sensitive calcium channel associated with the cilia is blocked by the divalent cation Ni 2+ but is insensitive to octanol and amiloride. Radioisotope tracer experiments, using whole cells under resting conditions, established that the toxic 1R isomer of deltamethrin resulted in increased Ca 2+ influx, while the nontoxic 1S enantiomer produced no significant effect. Pawn mutants, which lack a functional voltage-sensitive calcium channel, were unaffected by deltamethrin. Fluorescent bioassays, under depolarizing conditions, corroborated behavioral and radioisotope experiments. Specifically, these experiments established that deltamethrin stimulated Ca 2+ uptake in a stereospecfic manner and that this uptake was blocked by the phenethylamine-type calcium channel blocker D595 under physiological conditions. Deltamethrin treatment resulted in a dose-dependent increase in Ca 2+ uptake and membrane depolarization with concentrations as low as 10 −11 M. Electrophysiological recordings of whole cells showed that treatment of 10 −9 M deltamethrin resulted in membrane destabilization, increased number of spontaneous action potentials, prolonged repetitive discharges following stimulation, membrane depolarization, and death by osmotic lysis. Our findings establish that the toxic effect of deltamethrin is structurally related, dose dependent, and enhanced by depolarization and provide substantial evidence that Type-II pyrethroids, specifically deltamethrin, act as potent calcium channel agonists on the ciliary voltage-sensitive calcium channel of P. tetraurelia.