Effect of side of the corpus luteum and pregnancy on estrogen and progesterone receptor expression and localization in the endometrium of mares

The effect of side of corpus luteum on uterine gene expression and protein localization of estrogen receptor α (ERα) and progesterone receptor (PR) in healthy cyclic and pregnant mares 13 days after ovulation (day 0) was investigated. Transcervical biopsies were performed to collect endometrium ipsilateral and contralateral regarding the side of corpus luteum on day 13 post-ovulation in cyclic (n = 6) and pregnant (n = 6) mares. Blood samples were collected daily from day 0 until the day of biopsy for 17β-estradiol (E2) and progesterone (P4) determinations. Receptor expression was determined by immunohistochemistry and transcript expression by real time RT-PCR. Serum E2 and P4 concentrations were not affected by reproductive status. The contralateral horn presented higher percentage of positive cells for ERα than the ipsilateral horn (P < .05), but side did not affect PR. ERα showed low staining and no main effect of pregnancy was found, but pregnant mares had lower protein expression of PR (19.8 vs. 40.4 ± 5.3%, P < .01). The contralateral horn tended to present higher expression of ERα mRNA (1.33 vs. 0.97 ± 0.17, P < .10) and PR mRNA (1.96 vs. 1.57 ± 0.52, P < .09). ERα mRNA relative expression was lower in the pregnant group (0.88 vs. 1.44 ± 0.19, P < .05). The interaction of reproductive status and side of corpus luteum tended to affect PR mRNA expression as pregnant mares had a lower PR mRNA content in the ipsilateral horn than cyclic mares. To our knowledge, this is the first study that describes the behavior of steroid receptors in the endometrium of mares regarding side of corpus luteum.