Suppression in the Secretion of Follicle-Stimulating Hormone and Luteinizing Hormone, and Ovarian Follicle Development in Heifers Continuously Infused with a Gonadotropin-Releasing Hormone Agonist1

Abstract

A novel experimental model was developed in cattle to investigate the requirement for FSH and LH during ovarian follicle growth and development. On Day 5 of the estrous cycle, 7 heifers were each implanted with an osmotic minipump containing a GnRH agonist (GnRHa), Buserelin (release rate, 2.5 micrograms/h). Another 7 heifers served as controls. Each minipump was replaced 28 days later with a second pump, which was left in place for a further 20 days. Blood samples were collected daily throughout the experimental period, and frequent samples were also collected on both days of minipump insertion and at 10 days after insertion of the second pump. The ovaries of all heifers were scanned daily by real-time ultrasonography to monitor follicular dynamics. All controls displayed 2 or 3 waves of FSH and follicular development per estrous cycle during the experiment. Insertion of the first minipump produced a large LH and FSH surge and induced ovulation in all 7 animals. Within 8 days of the start of treatment, serum LH concentrations fell to basal levels; they then remained constant at this level throughout the infusion period, only beginning to recover 4-5 days after the termination of infusion. After the initial increase, FSH returned to basal levels before showing a normal wave that was coincident with the emergence, growth, and regression of a dominant follicle. However, despite the peak levels of FSH, dominant follicles from the next wave failed to grow beyond 7-9 mm; they remained at this size for 3 wk until 3-4 days after insertion of the second minipump, when FSH fell precipitously to reach low levels that were maintained throughout the remainder of the infusion. After this fall in FSH concentrations, these follicles regressed rapidly, and no antral follicles > 4 mm were detected until after the termination of treatment. Thereafter, FSH concentrations increased significantly; the increase was accompanied by the emergence of a follicular wave and development of a dominant follicle, with estrus observed 8-11 days later. In conclusion, this study has demonstrated clearly that in cattle the early stages of follicle development (< or = 4 mm) are not dependent on acute support by gonadotropins. However, FSH is required for further growth of follicles up to 9 mm, while LH pulses are indispensable for follicle development beyond 9 mm in diameter. The model developed in this study should be valuable for studying the control of ovarian follicle development and atresia in vivo.

Gonadotropin requirements for dominant follicle selection in GnRH agonist-treated cows

“…This time period coincides with the time when peripheral FSH concentrations reach a nadir (Adams et al 1992), supporting the hypothesis that dominant follicle selection is associated with a shift from FSH to LH dependence. Administration of a gonadotropin releasing hormone (GnRH) agonist that suppressed circulating concentrations of LH, but not FSH, resulted in follicular arrest at 7 to 9 mm in diameter (Gong et al 1996). The preceding data suggest that LH is important for continued follicular development.…”

“…To achieve this goal, we utilized the GnRH agonist (GnRHa) model developed by Gong et al (1996). Previous studies (Gong et al 1996, Garverick et al 2002 have shown that this treatment results in suppression of the gonadotropins, FSH and LH, which subsequently results in suppression of follicular growth to , 5 mm in diameter.…”

“…To achieve this goal, we utilized the GnRH agonist (GnRHa) model developed by Gong et al (1996). Previous studies (Gong et al 1996, Garverick et al 2002 have shown that this treatment results in suppression of the gonadotropins, FSH and LH, which subsequently results in suppression of follicular growth to , 5 mm in diameter. Using this model, our specific aims were: (1) to determine if LH is required for continued follicular growth, (2) to examine the effects of exogenous FSH and LH on steroidogenesis via follicular fluid steroid content and follicular expression of steroidogenic enzymes, and (3) to determine if expression of LHr mRNA by granulosa cells is temporally or hormonally regulated.…”

Efeito do eCG ou benzoato de estradiol associado ao norgestomet na taxa de concepção de vacas de corte submetidas à IATF no pós-parto

“…De fato, alguns autores relataram que os folículos tornam-se responsivos as gonadotrofinas exógenas em um intervalo pós-parto inferior a 30 dias. 3,41 Segundo Crowe et al 42 , o eCG ao suprir a falta de LH, proporciona ao folículo a capacidade de se desenvolver até a fase pré-ovulatória, considerando que tal evento é dependente de LH 43 . Verificou-se que a ativação dos receptores de LH e FSH no folículo promove um aumento na concentração de adenosina monofosfato cíclico (AMPc) intrafolicular, evento que promoveria a ativação de enzimas intracelulares que contribuiriam para incrementar produção de E 2 pelo folículo.…”

Follicular dynamic and ovulation in cattle – a review

“…This takes 3 to 4 days. LH pulses are indispensable for follicle development beyond 9 mm in diameter (GONG et al, 1996;KANITZ et al, 2001). Endocrine pattern of gonadotropins is followed by characteristic changes in follicles.…”