The effect of a GnRH agonist on follicular dynamics and response to FSH stimulation in prepubertal calves

Endocrine control of follicular growth was determined by observing the left ovary of prepubertal calves previously treated with a patent GnRH agonist for 13 days. The ovarian response to hormonal stimulation was determined us ing the right ovaries of the same animals. Three-month-old crossbred calves were assigned to one of the two following treatment groups: 1) saline cont rol for 13 days, with purified porcine FSH for the last 3 days (n = 5); and 2) GnRHa for 13 days, with purified porcine FSH for the final 3 days (n = 5). The left ovaries were removed from all calves after 10 days, and the ri ght ovaries were removed at the end of treatment. Plasma concentrations of FSH, LH and oestradiol-17 beta were followed up during the GnRHa and pFSH t reatments. The maximum macroscopic diameter of the Fl follicle, as determin ed by daily ultrasonography, did not differ between GnRHa-treated calves (f rom 6.6 to 10.4 mm) and the saline control calves (from 6.7 to 10.3 mm). Hi stological analysis of the ovaries showed that the number of follicles > 0. 40 mm in diameter varied greatly for calves of the two groups (from 11 to 2 20 at 10 days). GnRHa significantly increased the mean number of follicles (total and nonatretic) of size class > 5.4 mm as compared to saline control calves (P < 0.05). The FSH treatment significantly increased the mean numb er of follicles 3.00-5.4 and > 5.4 mm in diameter (P < 0.05), with no chang e in the number of follicles smaller than 3.00 mm. The rate of atresia of l arge follicles (3.01-5.40 mm) was significantly reduced by purified porcine FSH treatment in both groups (P < 0.05). In no case did the GnRHa induce o vulation or luteinization of follicles. The LH and FSH concentrations incre ased transiently after GnRHa treatment on the first day, but afterwards, bo th hormones increased to only one sixth of what was observed after the init ial GnRHa injection treatment. This increase in LH and FSH was observed 1 h after GnRHa treatment on each consecutive day of the experiment and were s ignificantly different in the control group (0 h versus 1 h versus 2 h x sa line control versus GnRH agonists groups; P < 0.01) During the superovulato ry treatment, FSH concentrations peaked at around 0.70 ng.mL(-1) in both sa line- and GnRHa-treated groups on the first day but on the last day of suro vulatory treatment, FSH concentrations were higher in GnRHa agonist-treated calves than in the control calves (day 11 versus day 12 versus day 13 x sa line control versus GnRH agonist treatment groups; P < 0.01). LH profiles w ere unchanged by surovulatory treatment. Concentrations of oestradiol-17 be ta increased significantly over the three days (P < 0.001) of the superovul atory treatments in both groups (P < 0.01). These results indicate that GnR H agonist treatment allows recruited antral follicles to pursue their growt h during the early selection process via sustained FSH and LH secretion all owing more than a single large follicle to maintain their growth without go ing to atresia. (C) Inra/Elsevier, Paris.