| My goal was to examine the influence of insulin-like growth factor-1 (IGF-1), 17-beta estradiol (E2), testosterone (T), and nitrate exposure on various aspects of reproduction in frogs. To accomplish this, I investigated seasonal changes in plasma IGF-1, E2, and T concentrations in a wild population of Rana grylio. I also determined the importance of steroid and growth factor hormones in reproductive physiology by examining ovariectomized Rana catesbeiana for changes in plasma IGF-1, E2, and T concentrations, and changes in oviduct morphology after treatment with known doses of IGF-1, E2, and epidermal growth factor (EGF). Finally, I examined three aquatic frogs species ( Xenopus laevis, R. grylio, and R. catesbeiana ) for the effects of nitrate exposure on changes in plasma IGF-1, E2, and T concentrations, and on oviduct morphology.; I have demonstrated that plasma IGF-1, E2, and T concentrations (and reproductive tissue growth) exhibit a clear seasonal pattern of changes that overlap with changes in environmental variables, such that reproductive condition is optimized to match favorable environmental temperatures. I also demonstrated that E2 is a potent stimulator of oviduct growth, while EGF and IGF-1 do not induce oviductal growth in R. catesbeiana . I also provide the first evidence that exposure to environmentally relevant concentrations of nitrate alters endocrine hormones in Xenopus laevis, R. grylio, and R. catesbeiana. Furthermore, IGF-1 and steroid hormone concentrations are altered with exposure to nitrate at concentrations deemed safe for human drinking water by the US EPA (10 mg/L). In vivo exposure of X. laevis (for 7 continuous days) to nitrate concentrations below 50 mg/L significantly increased plasma IGF-1 concentrations, and inhibited ovarian E2 and T synthesis. In vitro incubation of ovarian tissue (from wild-caught R. grylio) with nitrate concentrations between 0.17 and 33.00 mg/L nitrate (and between 0.20 and 40.60 mg/L nitrite) inhibited E2 and T synthesis after 3 hours of exposure. Lastly, in vivo exposure of R. catesbeiana to nitrate concentrations between 1.65 and 16.50 mg/L increased plasma IGF-1, E2, and T concentrations; and caused oviductal atrophy. These findings demonstrate that exposure to nitrate at extremely low concentrations causes endocrine disruption in frogs. |
