Impact of preovulatory estradiol concentrations on subsequent luteal function in beef cattle

Authors: MCLEAN, MEGAN - University Of Missouri; Geary, Thomas; Zezeski, Abby; SMITH, MICHAEL - University Of Missouri; SPENCER, THOMAS - University Of Missouri; POHLER, KY - Texas A&M University; REESE, SYDNEY - Texas A&M University; PERRY, GEORGE - South Dakota State University

Submitted to: Systems Biology in Reproductive Medicine
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2022
Publication Date: 4/8/2022
Citation: McLean, M.K., Geary, T.W., Zezeski, A.L., Smith, M.F., Spencer, T.E., Pohler, K.G., Reese, S.T., Perry, G.A. 2022. Impact of preovulatory estradiol concentrations on subsequent luteal function in beef cattle. Systems Biology in Reproductive Medicine. 68(4):286-297. https://doi.org/10.1080/19396368.2022.2038717.
DOI: https://doi.org/10.1080/19396368.2022.2038717

Interpretive Summary: An increase in the hormone, estradiol is what causes animals to be in estrus. An increase in the hormone, progesterone is needed to establish pregnancy in animals. Both hormones are produced by the ovary. Estradiol comes from a growing follicle. After an egg is released (ovulation), the follicle tissue becomes luteal tissue and produces progesterone. Two studies were conducted to examine the effect of estrus or blood estradiol level on progesterone secretion. In study 1, ovulation was synchronized among cows. Cows that were in estrus had greater estradiol than cows not in estrus. Size of the ovarian follicle was not different between these cows. Levels of progesterone in the blood did not differ between cows during luteal tissue growth. Cows that were not in estrus had lower progesterone 17 days later. Cows that were not in estrus were grouped into high or low estradiol group. Subsequent progesterone was not different between these two groups. In study 2, ovulation was synchronized, and cows were allocated to three groups. Control cows received no further treatment. Vehicle cows received an intrafollicular injection of media. Treated cows received an intrafollicular injection of estradiol blocker (Fulvestrant; ICI 182,780). Blood levels of progesterone were not affected by treatment. These results indicate that preovulatory estradiol was not required to prepare luteal tissue to produce progesterone in beef cows.

Technical Abstract: Two experiments were conducted to examine the hypothesis that estrus expression or increased circulating estradiol, during the preovulatory period, can increase subsequent progesterone secretion by the corpus luteum in cattle. In experiment 1, ovulation was synchronized in nonlactating beef cows (n=53). Cows that exhibited estrus prior to GnRH-induced ovulation (d 0) had greater (P<0.01) peak concentrations of estradiol compared to cows that did not express estrus (11.5±0.8 vs. 6.2±0.6 pg/mL), respectively, but there was no difference in ovulatory follicle size (P=0.80) or interval to ovulation (P=0.23). Circulating concentrations of progesterone during luteal formation (d 3 to 7; P=0.70 and P=0.77) or mid-luteal phase (d 8 to 14; P=0.39 and P=0.12) were not affected by estrus expression or estrus by day of sample collection, respectively. During the luteolysis phase (d 15 to 21), estrus (P=0.15) did not affect progesterone, but there tended (P=0.05) to be an estrus by day interaction with cows that did not exhibit estrus having decreased progesterone on d 17. Cows that did not exhibit estrus were grouped into high (>5.5 pg/mL) or low (<4.5 pg/mL) peak concentrations of estradiol and there was no effect of estradiol group (P=0.51) or estradiol by day (P=0.43) on subsequent circulating concentrations of progesterone. In experiment 2, ovulation (d 0) was synchronized following injection of GnRH in nonlactating beef cows (n=35) and cows were allocated to three groups (control, vehicle injection, or an estradiol antagonist (Fulvestrant; ICI 182,780) and balanced for follicle size on d -2 (PG-induced luteolysis). Intrafollicular injection of vehicle (100 µl) or an estradiol antagonist (25 µg Fulvestrant in 100 µl) into the largest follicle occurred on d -2. Concentrations of estradiol increased (P<0.0001) from d -2 to 0 but did not differ among groups (P>0.50). Furthermore, plasma concentrations of progesterone on d 0 through 20 were not affected by treatment (P=0.86). These results indicate that preovulatory estradiol was not required to prepare granulosa cells for luteinization or subsequent luteal progesterone secretion in beef cattle.