Changes in ovarian function associated with circulating concentrations of estradiol before a GnRH-induced ovulation in beef cows

Authors: LARIMORE, E - SOUTH DAKOTA STATE UNIVERSITY; AMUNDSON, OLIVIA - SOUTH DAKOTA STATE UNIVERSITY; BRIDGES, G - UNIVERSITY OF MINNESOTA; MCNEEL, ANTHONY; Cushman, Robert - Bob; PERRY, GEORGE - SOUTH DAKOTA STATE UNIVERSITY

Submitted to: Domestic Animal Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2016
Publication Date: 8/25/2016
Publication URL: http://handle.nal.usda.gov/10113/63213
Citation: Larimore, E.L., Amundson, O.L., Bridges, G.A., McNeel, A.K., Cushman, R.A., Perry, G.A. 2016. Changes in ovarian function associated with circulating concentrations of estradiol before a GnRH-induced ovulation in beef cows. Domestic Animal Endocrinology. 57:71-79. doi:10.1016/j.domaniend.2016.06.001.

Interpretive Summary: Cows that do not demonstrate behavioral estrus during a fixed time artificial insemination protocol have decreased serum estradiol concentrations and decreased fertility. The current study was designed to investigate the mechanisms leading to decreased follicular estradiol production and to determine if administration of exogenous gonadotropin releasing hormone could increase luteinizing hormone secretion and improve subsequent follicular estradiol production. Cows were provided with hormonal treatments to synchronize estrus and serum estradiol was measured to classify cows as low or high estradiol based on estradiol concentrations. Genes involved with stimulation of steroid production and with the production of steroids were increased in the cows with high estradiol concentrations. Administration of exogenous gonadotropin hormone tended to increase luteinizing hormone pulse frequency in cows with low estradiol concentrations, and this increased estradiol production, but at a slower rate that cows with high estradiol. Thus, the administration of gonadotropin releasing hormone to cows with low estradiol will not overcome the sub-fertility issues, and interventions that up regulate the entire steroidogenic pathway will be necessary in these cows.

Technical Abstract: These studies were conducted to evaluate causes for differences in circulating concentrations of estradiol prior to a GnRH-induced ovulation and to determine if exogenous GnRH administration could alter LH secretion and subsequent follicular estradiol production. Beef cows (Experiment 1; n = 32, Experiment 2; n = 30) were synchronized by an injection of GnRH (100µg; i.m.) on d -7 and an injection of PGF2a (25mg; i.m.) on d 0. In Experiment 1, blood samples were collected every 3 h from PGF2a on d 0 to h 33 after PGF2a and at slaughter (h 36 to 42; n = 10). Cows were assigned to treatment group based on circulating concentrations of E2: HighE2 vs LowE2. At slaughter, follicular fluid (FF) and granulosa cells were collected from the dominant follicle (DF). In Experiment 2, blood samples were collected every 8 h from PGF2a until the dominant follicle was aspirated via ultrasound-guided follicular aspiration to collect FF and granulosa cells (h 38 to 46). In Experiment 1, there was no difference in abundance of CYP11A1 (P = 0.13) or FSHR (P = 0.36), and there was only a tendency (P = 0.10) for a difference in follicular fluid concentrations of progesterone between treatments. HighE2 had increased abundance of 3ß-HSD (P = 0.01), CYP19A1 (P < 0.01), and LHR (P = 0.02) compared to LowE2. HighE2 also had greater concentrations of estradiol (P < 0.01) and androstenedione (P = 0.02) in the follicular fluid of the DF. In Experiment 2, HighE2 had increased abundance of mRNA for CYP11A1 (P < 0.01), 3'-HSD (P < 0.01), CYP19A1 (P < 0.01), and LHR (P = 0.03) in granulosa cells versus either LowE2 or GnRHLowE2. There was a tendency (P = 0.07) for LH pulse frequency to be increased in both the GnRHLowE2 and HighE2 compared to LowE2, but there was no effect on area under the curve (P = 0.92) or mean concentrations (P = 0.92). HighE2 cows experienced an increase in circulating concentrations of estradiol, and cows in the GnRHLowE2 treatment had elevated estradiol at h 7, 8 and 16 compared to cows in the LowE2 treatment. However, concentrations of estradiol declined from h 16 to ASP. In conclusion, animals with greater concentrations of circulating estradiol prior to fixed-time AI experienced an up-regulation of the steroidogenic pathway during the preovulatory period, and treatment with exogenous GnRH was able to impact circulating concentrations of estradiol, but the response was at a slower rate and reduced magnitude in LowE2 compared to HighE2 animals. Thus management methods to up regulate the entire steroidogenic pathway are necessary.