Progesterone concentration, estradiol pretreatment, and dose of gonadotropin-releasing hormone affect gonadotropin-releasing hormone-mediated luteinizing hormone release in beef heifers

Dias, F.C.F.; Colazo, M.G.; Kastelic, J.P.; Mapletoft, R.J.; Adams, G.P.; Singh, J.

doi:10.1016/j.domaniend.2010.04.002

« Previous Next »Domestic Animal Endocrinology
Volume 39, Issue 3 , Pages 155-162, October 2010

Progesterone concentration, estradiol pretreatment, and dose of gonadotropin-releasing hormone affect gonadotropin-releasing hormone-mediated luteinizing hormone release in beef heifers

F.C.F. Dias
- Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
M.G. Colazo
- Dairy Research and Technology Centre, Alberta Agriculture and Rural Development, Edmonton, Alberta, Canada
J.P. Kastelic
- Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, Alberta, Canada
R.J. Mapletoft
- Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
G.P. Adams
- Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
J. Singh
- Department of Veterinary Biomedical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
- Address for correspondence: Department of Veterinary Biomedical Sciences, University of Saskatchewan, 52 Campus Dr, Saskatoon, SK, S7N 5B4, Canada; Phone: (306) 966-7410; Fax: (306) 966-7405

published online 28 June 2010.

Abstract

We examined whether progesterone (P₄)-induced suppression of LH release in cattle can be overcome by an increased dose of exogenous gonadotropin-releasing hormone (GnRH) or pretreatment with estradiol (E₂). In Experiment 1, postpubertal Angus-cross heifers (N = 32) had their 2 largest ovarian follicles ablated 5 d after ovulation. Concurrently, these heifers were all given a once-used, intravaginal P₄-releasing insert (CIDR), and they were randomly assigned to be given either prostaglandin F_2α (Low-P₄) or no treatment (High-P₄) at follicle ablation, and 12 h later. Six days after emergence of a new follicular wave, half of the heifers in each group (n = 8) were given either 100 or 200 μg of GnRH i.m. Plasma luteinizing hormone (LH) concentrations were higher in the Low- vs High-P₄ groups, and in heifers given 200 vs 100 μg of GnRH (mean ± SEM 15.4 ± 2.2 vs 9.1 ± 1.2, and 14.8 ± 2.1 vs 9.8 ± 1.4 ng/mL, respectively; P ≤ 0.01). Ovulation rate was higher (P = 0.002) in the Low-P₄ group (15/16) than in the High-P₄ group (6/16), but it was not affected by GnRH dose (P = 0.4). In Experiment 2, heifers (n = 22) were treated similarly, except that 5.5 d after wave emergence, half of the heifers in each group were further allocated to be given either 0.25 mg estradiol benzoate i.m. or no treatment, and 8 h later, all heifers were given 100 μg GnRH i.m. Both groups treated with E₂ (Low- and High-P₄) and the Low-P₄ group without E₂ had higher peak plasma LH concentrations compared to the group with high P₄ without E₂ (12.6 ± 1.8, 10.4 ± 1.8, 8.7 ± 1.3, and 3.9 ± 1.2 ng/mL, respectively; (P < 0.04)). However, E₂ pretreatment did not increase ovulation rates in response to GnRH (P = 0.6). In summary, the hypotheses that higher doses of GnRH will be more efficacious in inducing LH release and that exogenous E₂ will increase LH release following treatment with GnRH were supported, but neither significantly increased ovulation rate.