97 THE REGULATION OF UTERINE CONTRACTION BY ENDOCRINE-DISRUPTING CHEMICALS IN RATS

Abstract

Environmental oestrogens, class of endocrine-disrupting chemicals, are defined as compounds that bind the oestrogen receptors and elicit or modulate an ER-mediated response. Examples of suspected environmental oestrogenic chemicals include plastic components such as bisphenol A (BPA) and some components of detergents and their biodegradation products, 4-octylphenol (OP). The uterus frequently contracts throughout the entire menstrual cycle, and these contractions have been termed endometrial waves or contractile waves. The balanced contractile waves play an important role in regulating oestrus or menstrual cycles, implantation, maintaining pregnancy, and parturition. The uterine contraction has been known to be regulated by contraction-associated proteins such as oxytocin, oxytocin receptor (OTR), connexin 43, prostaglandin F2 alpha receptor (FP), and 15-hydroxy prostaglandin dehydrogenase (PDGH). Endogenous steroid hormones including oestrogen and progesterone have been shown to regulate contraction-associated proteins and, thereby, modify uterine contraction. Forty female immature rats were divided into 8 groups, each group composed of 5 rats. They were injected subcutaneously daily for 3 days with BPA and OP at doses of 20, 100, and 500 mg kg^–1 of BW per day; 17β-estradiol (E2, 40 µg kg^–1 per day) was a positive control. After 24 h of the final treatment, the rats were euthanized and the uteri were collected for further experiments. Total RNA and protein were extracted from uteri. The mRNA expressions for CaBP-9k, oxytocin, OTR, FP, and PDGH were determined by real-time PCR with specific primers, and the protein levels were examined by Western blot assay. The data were analyzed using a one-way ANOVA. The P-values <0.05 were considered statistically significant. The treatment of oestrogenic compounds significantly regulated the contraction-associated proteins. First, the mRNA levels of CaBP-9k that is a biomarker for evaluating oestrogenicity in the rats were significantly induced by positive control, E2. Chemicals OP and BPA at doses of 100 and 500 mg kg^–1 also induced the gene expression of CaBP-9k. Oxytocin and OTR were highly augmented by OP (40 fold) and BPA (6-fold) at a dose of 500 mg kg^–1 compared with control, whereas PDGH was moderately upregulated by the chemicals. Interestingly, FP was decreased by E2 and high doses of OP and BPA. These results showed that exposure of immature rats to BPA and OP regulated expression of oxytocin, OTR, PDGH, and FP genes in the rat uterus. Taken together, uterine contraction and physiological conformational change of the uterus were affected by exposure to oestrogenic endocrine-disrupting chemicals. Because balanced contractile activity of the uterus maintains regular menstrual cycles, pregnancy, and labor, alteration of the contractility raised by oestrogenic compounds may cause severe adverse effects on the female reproductive system.