Impact of atrazine on chlorpyrifos toxicity in four aquatic vertebrates.

Atrazine has been shown previously to potentiate chlorpyrifos toxicity in selected invertebrates. This study examined interactions of atrazine and chlorpyrifos in four aquatic vertebrates. Organisms were exposed to binary mixtures of atrazine and chlorpyrifos during toxicity bioassays. Inhibition of cholinesterase (ChE) enzyme activity and chlorpyrifos uptake kinetics were also examined with and without atrazine exposure. Atrazine alone did not affect organisms at concentrations up to 5000 microg/L; however, the presence of atrazine at 1000 microg/L did result in a significant increase in the acute toxicity of chlorpyrifos in Xenopus laevis. Mixed results were encountered with Pimephales promelas; some bioassays showed greater than additive toxicity, while others showed an additive response. No effect of atrazine on chlorpyrifos toxicity was observed for Lepomis macrochirus and Rana clamitans. Atrazine did not affect ChE activity or chlorpyrifos uptake rates, indicating that these toxicodynamic and toxicokinetic parameters may not be related to the mechanism of atrazine potentiation of chlorpyrifos toxicity. Based on the results of this study, it does not appear that a mixture toxicity of atrazine and chlorpyrifos at environmentally relevant concentrations presents a risk to the vertebrate organisms examined in this study.