Changes in equine endometrial oestrogen receptor alpha and progesterone receptor mRNAs during the oestrous cycle, early pregnancy and after treatmentwith exogenous steroids

Two experiments were performed to determine changes in the abundance of oes trogen and progesterone receptor (ER alpha and PR) mRNAs in equine endometr ium during the oestrous cycle and early pregnancy, and under the influence of exogenous steroids. In Expt 1, endometrial biopsies were obtained from n on-mated mares during oestrus and at days 5, 10 and 15 after ovulation, and from pregnant mares at days 10, 15 and 20 after ovulation. There were over all effects of day on the abundance of ER alpha (P = 0.0001) and PR (P = 0. 0014) mRNAs. The amount of ER alpha mRNA decreased at day 10 of pregnancy, and PR mRNA was reduced at day 5 in non-mated mares and at day 15 of pregna ncy, compared with oestrous values. Experiment 2 was conducted to determine the effects of exogenous steroids on endometrial ER alpha and PR mRNAs. En dometrial biopsies were obtained from 19 anoestrous mares that had been tre ated with vehicle, oestradiol, progesterone, or oestradiol followed by prog esterone for either a short or a long duration. The steroid treatment affec ted the abundance of ERa mRNA (P = 0.0420), which was higher (P < 0.05) in the oestradiol group than in the group treated with oestradiol followed by long duration progesterone. The steroid treatment did not affect the abunda nce of PR mRNA. These results demonstrate that the amount of steroid recept or mRNA changes with the fluctuating steroid environment in the uterine end ometrium of cyclic and early pregnant mares, and that the duration of proge sterone dominance may affect ER alpha gene expression. In addition, factors other than steroids may regulate ER alpha and PR gene expression in equine uterine endometrium.