An investigation into the mechanism of action of atrazine and its effects on developing Rana clamitans and Xenopus laevis.

摘要

Concern for the amphibian class has increased in the last two decades. A high incidence of deformed frogs among certain populations and a growing awareness of amphibian population declines across the globe have contributed to the fear that amphibians are facing environmental stressors that may be deleterious to their future survival. Pesticide exposure, whether from agricultural runoff or atmospheric deposition, is a suspected environmental stressor thought to be adversely affecting amphibian communities. Atrazine, one of the most widely used herbicides in the United States and Canada, has been reported to cause abnormal reproductive development in amphibian populations via disruption of the endocrine system. This laboratory investigation was undertaken to elucidate the mechanism of action of atrazine in amphibians and to examine the effects of atrazine on developing frogs. Two frog species, the native green frog ( Rana clamitans) and the non-native African-clawed frog ( Xenopus laevis), were chronically exposed to waterborne test solutions, including 0, 0.1, 1.0, 10, or 25 μg/L atrazine, 0.005% ethanol (EtOH), or 0.1 mg/L estradiol or dihydrotestosterone carried in 0.005% EtOH. Test solutions were renewed by 50% every 72 hrs. Survival, metamorphosis, growth, sex ratios, as well as gonad and larynx development were examined as endpoints in this study. In addition, sex steroid concentrations, and P450 aromatase activity were examined in X. laevis frogs. Aromatase, an important enzyme in reproductive development, is responsible for converting androgens into estrogens. Atrazine exposure at environmentally relevant concentrations did not significantly affect frog mortality in either species. Time to metamorphosis as well as length and weight of frogs at metamorphic completion did vary between treatments in some cases, but these parameters were not consistently affected across atrazine concentrations. Sex ratios were not altered in the green frog or the African-clawed frog in response to atrazine exposure. However, when green frog tadpoles were exposed to DHT, over 97% of the metamorphosing frogs were classified as males. Neither laryngeal nor gonadal development appeared to be affected by atrazine in the exposed frogs. As expected DHT-exposed frogs had significantly larger larynx muscle areas compared to frogs in other treatments. Sex steroid levels, including 17β-estradiol and testosterone, were not affected in response to atrazine exposure. Likewise, aromatase activity in the brains and gonads of atrazine-exposed frogs was not different from controls. However, aromatase activity was significantly altered in E2 exposed frogs. In conclusion, chronic atrazine exposure at environmentally relevant concentrations did not affect amphibian survival, growth, or reproductive development. Based on these findings, it appears that low-level atrazine exposure does not disrupt the amphibian endocrine system via interference with the steroidogenic enzyme, aromatase. Future research using a greater range of atrazine concentrations and focusing on the possible interactions of atrazine with the amphibian endocrine system, including the hypothalamus-pituitary axis, is necessary to discount atrazine as an endocrine disruptor in amphibians.