Immature uterotrophic assay is more sensitive than ovariectomized uterotrophic assay for the detection of estrogenicity of p-nonylphenol in Sprague-Dawley rats

Many efforts have been made to develop assays for detecting endocrine disrupters (EDs). Among them, uterotrophic assay has been known efficient for detecting EDs, especially estrogenic compounds. This study was performed to compare the immature uterotrophic assay with an ovariectomized assay using p-nonylphenol (NP), a weakly estrogenic compound. NP was given to either immature or ovariectomized rats subcutaneously or orally (only immature) at doses of 10, 100, and 1000 mg/kg for 3 days. After treatment with NP, the rats were examined for parameters such as uterine weight, uterine weight per body weight ratio, luminal epithelial height of uterus and vagina, diameter of uterine ducts, and number of uterine glands. Both systems were shown to increase uterine weight in a dose-dependent manner. In the immature system (subcutaneous injection), uterine weight, diameter of uterine duct and vaginal luminal epithelial height were significantly increased at 100 mg/kg/day, while in the ovariectomized system these parameters were not significant at the same dose (except for vaginal luminal epithelial height). These results suggest that the immature system (subcutaneous injection) might be most sensitive to detecting a weakly estrogenic compound and that the measurement of vaginal epithelium is a good end-point.