Submitted to: Society for the Study of Reproduction Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: March 28, 2013
Publication Date: July 1, 2013
Citation: Larimore, E.L., Swanson, O.L., McNeel, A.K., Cushman, R.A., Perry, G.A. 2013. Changes in ovarian function associated with circulating concentrations of estradiol prior to a GnRH-induced ovulation in beef cows [abstract]. Biology of Reproduction Supplement (46th Annual Meeting of the Society for the Study of Reproduction). p. 256 (Abstract #555). Technical Abstract: Previous reports suggest increased circulating concentrations of estradiol prior to GnRH induced ovulation improved conception rates and pregnancy maintenance in beef cattle, and cultured granulosa cells from animals with high antral follicle numbers produced more estradiol and had increased expression of CYP19A1. Therefore, the objective of this study was to evaluate causes for differences in circulating concentrations of estradiol between cows with high and low circulating concentrations of estradiol prior to a GnRH induced ovulation and their relationship to follicle populations. Beef cows (n=32) were synchronized by an injection of GnRH (100µg; i.m.) on d -7 and an injection of PGF2alpha (25mg; i.m.) on d 0. Estrus was monitored every 3 h from d 0 to slaughter. Follicular size and development was determined by transrectal ultrasonography on d -7, -4, and 0, and blood samples were collected every 3 h from PGF2alpha until h 33 and at slaughter (h 36 to 42). Cows were slaughtered (n=9) on d2 and ovaries collected. Follicular fluid was aspirated from the dominant follicle and from all small follicles (1-5 mm; pooled within animal). Granulosa cells were separated from the follicular fluid by centrifugation, and RNA was extracted from dominant follicle granulosa cells. All visible antral follicles were counted and a representative piece of one ovary was fixed for histological evaluation. Circulating and follicular fluid concentrations of estradiol were determined by RIA and animals were classified as either HighE2 (peak estradiol ' 6 pg/mL; n=4) or LowE2 (peak estradiol = 4.5 pg/mL; n=5). There was no difference (P>0.80) in dominant follicle diameter between the LowE2 and HighE2 cows. HighE2 cows tended to have an increased follicular growth rate (P=0.06; 1.25±0.19 mm/day) from d -4 to d 0 compared to LowE2 cows (0.68±0.17 mm/day). HighE2 cows had increased circulating concentrations of estradiol (P<0.01; 7.9±0.94 pg/mL) at time of slaughter compared to LowE2 cows (3.2±0.84 pg/mL). HighE2 cows also had greater concentrations of estradiol (P<0.01; 1,565±196 ng/ml) and androstenedione (P=0.02; 73.9±15.2 ng/ml) in the follicular fluid of the dominant follicle, and higher concentrations of estradiol in follicular fluid of small follicles (P=0.02; 3±0.29 ng/ml) compared to LowE2 cows (398±175, 9.9±13.6, and 1.8±0.26 ng/ml, respectively). Furthermore, expression of CYP19A1 (P<0.01) and LHR (P=0.02) were greater in HighE2 cows (10.1±0.81; 16.6±3.1) compared to LowE2 cows (4.8±0.62; 3.7±2.4). The number of primordial and primary follicles per section did not differ between HighE2 and LowE2 cows (P=0.54). However, HighE2 cows tended to have fewer secondary follicles per section (0.32±0.25 vs. 0.96±0.21; P=0.06) and had a greater number of visible antral follicles (57.0±6.3 vs. 30.8±5.6; P = 0.02) than LowE2 cows, indicating that, although the ovarian reserve is not greater in these cows, the secondary to tertiary follicle transition was enhanced in cows with high E2. Therefore, cows with greater circulating concentrations of estradiol during the preovulatory period had increased ability to produce estradiol and increased numbers of visible antral follicles. Thus, selection for both follicle numbers and estradiol concentrations during the preovulatory period may be a better indicator of fertility than either trait alone.