Association Between Preovulatory Concentrations of Estradiol and Expression of Uterine Milk Protein Precursor, Inhibin Beta A, and Proenkephalin.

Abstract

Eliminating the preovulatory surge of estradiol decreased embryo survival following embryo transfer, uterine weight, uterine protein, RNA to DNA ratio, and the rate of protein synthesis in sheep. Furthermore, cows that did not exhibit standing estrus (decreased preovulatory concentrations of estradiol) around the time of gonadotropin-releasing hormone (GnRH)-induced ovulation had decreased pregnancy success compared to cows that did have elevated preovulatory concentrations of estradiol. Thus, induction of ovulation alone may not be sufficient for maximum pregnancy success. Uterine environment, as influenced by endometrial protein expression, plays a central role in early embryo development and survival. Therefore, the objective of the present experiment was to characterize changes in expression of uterine milk protein precursor, inhibin beta A, and proenkephalin between cows with high (highE2; n = 6) and low (lowE2; n = 5) concentrations of estradiol at time of GnRH induced ovulation (day 0). Beef cows were treated with 100 μg GnRH on day -9, 25 mg PG on day -2, and 100 μg GnRH on day 0. Uterine horn biopsies (collected ipsilateral to the ovulatory follicle) and blood samples were collected on days 0 and 16. Total cellular RNA was extracted from all biopsies and relative mRNA levels for uterine milk protein precursor, inhibin beta A, proenkephalin, and polyubiqutin were determined by real-time RT-PCR and corrected for polyubiqutin. Circulating concentrations of estradiol on day 0 were greater (P < 0.001) in highE2 cows (8.33 ± 1.13 pg/mL) compared to lowE2 cows (2.72 ± 1.24 pg/mL). Relative expression of uterine milk protein precursor was greater (P = 0.04) in highE2 cows compared to lowE2 cows on day 0 (12.51 ± 3.07 vs. 2.10 ± 3.37, respectively). However, expression decreased (P = 0.002) from day 0 to day 16 in highE2 cows, and there was no difference (P = 0.95) in uterine milk protein precursor expression between highE2 and lowE2 cows on day 16 (1.11 ± 0.21 vs. 1.13 ± 0.34, respectively). Relative expression of inhibin beta A was greater (P = 0.03) in highE2 cows compared to lowE2 cows on day 0 (8.81 ± 1.64 vs. 2.82 ± 1.80, respectively), and expression decreased (P = 0.002) in highE2 cows from day 0 to 16. On day 16, there was no difference (P = 0.74) in inhibin beta A expression between highE2 and lowE2 cows (0.28 ± 0.03 vs. 0.29 ± 0.05, respectively). Expression of proenkephalin was not detectable in either highE2 or lowE2 cows on day 0. However, relative expression of proenkephalin on day 16 was greater (P = 0.02) in highE2 cows (10.40 ± 1.95) compared to lowE2 cows (2.70 ± 2.49). In summary, preovulatory concentrations of estradiol regulated uterine expression of uterine milk protein precursor, inhibin beta A, and proenkephalin during the subsequent estrous cycle. Reduced fertility among cows with decreased concentrations of estradiol prior to GnRH-induced ovulation may be associated with effects of changes in uterine protein expression on uterine environment.