Selection Of Fewer Dominant Follicles In Trio Carriers Given Gnrh Antagonist And Luteinizing Hormone Action Replaced By Nonpulsatile Human Chorionic Gonadotropin

Studying selection of multiple dominant follicles (DFs) in monovulatory species can advance our understanding of mechanisms regulating selection of single or multiple DFs. Carriers of the bovine high fecundity Trio allele select multiple DFs, whereas half-sib noncarriers select a single DF. This study compared follicle selection during endogenous gonadotropin pulses versus during ablation of pulses with Acyline (GnRH antagonist) and luteinizing hormone (LH) action replaced with nonpulsatile human chorionic gonadotropin (hCG) treatment in Trio carriers (n = 28) versus noncarriers (n = 32). On Day 1.5 (Day 0 = ovulation), heifers were randomized: (1) Control, untreated; (2) Acyline, two i.m. doses (Days 1.5 and D3) of 3 mu g/kg; (3) hCG, single i.m. dose of 50 IU hCG on Day 1.5 followed by daily doses of 100 IU; and (4) Acyline + hCG. Treatments with nonpulsatile hCG were designed to replace LH action in heifers treated with Acyline. Acyline treatment resulted in cessation of follicle growth on Day 3 with smaller (P < 0.0001) maximum follicle diameter in Trio carriers (6.6 +/- 0.2 mm) than noncarriers (8.7 +/- 0.4 mm). Replacement of LH action (hCG) reestablished follicle diameter deviation and maximum diameter of DFs in both genotypes (8.9 +/- 0.3 mm and 13.1 +/- 0.5 mm; P < 0.0001). Circulating follicle stimulating hormone (FSH) was greater in Acyline-treated than in controls. Finally, Acyline + hCG decreased (P < 0.0001) the number of DFs from 2.7 +/- 0.2 to 1.3 +/- 0.2 in Trio carriers, with most heifers having only one DF. This demonstrates the necessity for LH in acquisition of dominance in Trio carriers (similar to 6.5 mm) and noncarriers (similar to 8.5 mm) and provides evidence for a role of GnRH-induced FSH/LH pulses in selection of multiple DFs in Trio carriers and possibly other physiologic situations with increased ovulation rate.[GRAPHICS]Summary sentenceNumber of dominant follicles (DFs) was reduced from 2.7 in carriers of the high fecundity Trio allele to mostly a single DF in carriers with gonadotropin pulses ablated by GnRH antagonist and LH action replaced with nonpulsatile human chorionic gonadotropin treatment.