Effects of estradiol on PGF_2α synthesis and corpus luteum function during early pregnancy in beef heifers

This study investigated the effects of estradiol benzoate (EB) administered 13 days post-ovulation on PGF2α release and corpus luteum function in pregnant and non-pregnant heifers. In Exp. 1, Nelore (Bos indicus) heifers, either inseminated or non-inseminated, were randomly assigned on Day 13 (D13) to receive 0, 1, or 2 mg of EB. Blood samples were collected at baseline (H0) and hourly from H3 to H12 to assess plasma P4 and PGFM concentrations. In a subgroup of pregnant heifers, blood samples were also collected to determine plasma E2 concentrations. Doppler ultrasonography was performed daily from D13 to D19 for monitoring the luteal function, and pregnancy was determined on D28. Luteolysis was earlier (P < 0.05) in non-inseminated heifers treated with 1 or 2 mg EB than in the controls (16.3 ± 0.2 vs. 17.3 ± 0.6 days). Pregnancy rate was lower (P < 0.05) in the EB-1 (50 %; 8/16) and EB-2 (29.2 %; 7/24) groups than in the EB-0 group (90 %; 9/10). The average PGFM concentrations were greater (P < 0.05) in the EB-1 and EB-2 groups than in the EB-0 group, regardless of gestational status. In Exp. 2, inseminated (n = 39) and non-inseminated (n = 21) Nelore heifers received either 0 or 1 mg of EB on D13. Three hours later, endometrial cytology was performed and samples were evaluated by qPCR. Expression of OXTR and PGR was greater and IL1β was lower in EB-treated heifers (P < 0.05). The ESR2 abundance was lower (P < 0.05) in pregnant heifers, regardless of EB treatment. In conclusion, an elevation of circulating E2 at late diestrus upregulates the OXTR and PGR expression in the endometrium, inducing PGF_2α release and luteolysis, which negatively impact on pregnancy establishment, especially in heifers treated with 2 mg EB.