Dose-response and time course relationships for vitellogenin induction in male western fence lizards (Sceloporus occidentalis) exposed to ethinylestradiol.

The long-term goal of this research is to develop and validate an in vivo reptile model for endocrine-mediated toxicity using fence lizards (Sceloporus spp.). One of the best defined estrogenic responses in oviparous vertebrates is induction of the yolk precursor protein, vitellogenin (Vtg). In this study, dose-response and time course relationships for Vtg induction were determined in male western fence lizards (Sceloporus occidentalis) given intraperitoneal injections of 17alpha-ethinylestradiol (EE2). Plasma Vtg was quantified directly with an antibody-capture enzyme-linked immunosorbent assay (ELISA) and indirectly using plasma alkaline-labile phosphate (ALP) in order to compare these two methods. Both ELISA and ALP predicted similar median effective dose (ED50 [dose causing a 50% maximal response]) values for plasma Vtg induction (0.167 mg/kg for ELISA and 0.095 mg/kg for ALP). In addition, both ELISA and ALP detected significant Vtg induction at a dose of 0.0003 mg/kg of EE2, which was the lowest dose used in our study. A decrease in body weight at the highest dose (10 mg/kg) and an increase in hepatosomatic index at the four highest doses were observed. Serial dilutions of plasma from an EE2-exposed male revealed a high correlation between plasma Vtg and ALP determinations in this species. In conclusion, our data show that plasma ALP may be a suitable alternative for measuring plasma Vtg compared with developing a Vtg ELISA in fence lizards exposed to estrogenic compounds.