Impact of preovulatory estradiol concentrations on conceptus development and uterine gene expression

This experiment was conducted to determine the effect of altering preovulatory estradiol concentrations, through manipulation of length of proestrus, on peripheral progesterone concentrations, conceptus development, interferon tau (IFNT) production and uterine gene expression in cattle. Approximately 6 days after a time-synchronized ovulation, all antral follicles (≥5mm) were ablated from the ovaries in beef heifers. To manipulate preovulatory estradiol concentrations, the length of proestrus prior to the GnRH-induced LH surge was altered between treatments. Heifers were administered PGF_2α either -2.5 days (2.5 days of proestrus; HiE2; n=5) or -1.5 days (1.5 days of proestrus; LoE2; n=5) prior to GnRH (Day 0 of the experiment; 6.75 days after follicle ablation). Follicular dynamics and estradiol concentrations were evaluated during proestrus and progesterone concentrations were analyzed in the subsequent estrous cycle. On Day 7, embryos were transferred into all heifers using standard procedures. On Day 15.5 heifers were slaughtered, the reproductive tract was flushed to collect the conceptus and uterine flush media, and the uterine tissue was processed for subsequent analyses. Peripheral progesterone concentrations, conceptus development and IFNT production were similar between treatments. However, amount of nuclear progesterone receptor in the deep glandular epithelium and mRNA concentrations for estradiol receptor alpha (ESR1) in the uterine endometrium were less in the LoE2 than HiE2 treatment. These changes in uterine characteristics in heifers with lower preovulatory estradiol concentrations were not related to aspects of conceptus development monitored, however, it is speculated that the alterations in mRNA and receptor protein detected may contribute to pregnancy failure subsequent to day 15.5 of gestation.