Altered ovarian steroids in Xenopus laevis exposed to environmentally relevant concentrations of nitrate

Contamination of aquatic ecosystems with anthropogenic sources of nitrate is of increasing concern. Anurans are especially at risk for overexposure to nitrate because they occupy aquatic environments during embryonic, larval, and adult stages. Whereas there have been numerous studies examining the effect of nitrate on frog eggs and tadpoles, the effects of nitrate on adult frogs have been largely neglected. In this study, we examined whether brief exposure to environmentally relevant concentrations of aquatic nitrate influenced ovarian steroid synthesis and ovarian follicle size in adult African Clawed Frogs (Xenopus laevis). We exposed frogs for seven days to nitrate at concentrations of 24.8 or 49.5 mg/l in order to simulate a pulse of nitrate exposure. Relative to controls, nitrate-exposed frogs exhibited suppressed ex vivo synthesis of testosterone and estrogen. Follicles from nitrate-exposed frogs also exhibited an increased stage-4 diameter at both nitrate concentrations and a decreased stage-5 and -6 diameter at the highest nitrate concentration. These results indicate that anuran ovarian steroidogenesis and follicle size are modified by even a brief exposure to environmentally relevant concentrations of nitrate.