Relationship between follicular development and the decline in the follicle-stimulating hormone surge in heifers

Experiment 1 was conducted to determine whether progesterone affects the pattern of the FSH surge or follicular development associated with a follicular wave in heifers. On Day 7 (Day 0 = ovulation), heifers were allocated into a group receiving prostaglandin F-2 alpha (PGF(2 alpha); n = 6) or a control group (n = 5). Twenty-four hours later, all detectable follicles (greater than or equal to 2 mm) were ablated (Hour 0). Follicular development was monitored Hours 0, 3, 6, 9, 12, and 16, at 8-h intervals thereafter until Hour 112. To monitor FSH concentrations, blood was sampled at Hours -24, 0, 3, 6, 9, 12, and 16, and at 8-h intervals thereafter until Hour 104. There were no differences (p > 0.05) between the PGF(2 alpha)-treated group and controls in the patterns of the FSH surge or follicular development. Experiment 2 tested the hypothesis that 3-mm follicles do not have FSH-suppressing capacity and that suppression increases as follicles grow beyond 5 mm. Twenty-four hours after an injection of PGF(2 alpha) (Days 6-8), heifers were subjected to either ablation of follicles greater than or equal to 2 mm or ovariectomy. Intact heifers were allocated into four groups (n = 5) in which all follicles of the new wave were ablated upon reaching either 3, 5, or 7 mm or were not ablated (controls). Blood was sampled at 8-h intervals to monitor FSH and estradiol-17 beta. Averaged over Hours 8-120, FSH concentrations (ng/ml) were higher (p < 0.05) in the ovariectomized (2.02 +/- 0.05) and the 3-mm groups (1.91 +/- 0.05) than in the 5-mm (1.52 +/- 0.05), 7-mm (1.35 +/- 0.04), and control groups (1.33 +/- 0.05); and estradiol concentrations (pg/ml) were lower (p < 0.05) in the ovariectomized group (0.19 +/- 0.03) than in the 3-mm (1.48 +/- 0.16), 5-mm (1.56 +/- 0.15), 7mm (2.22 +/- 0.27), and control groups (2.55 +/- 0.49). In conclusion, the presence of endogenous progesterone did not affect FSH patterns or follicular development. Follicles less than or equal to 3 mm had no detectable capacity to suppress FSH. As follicles grew from 3 to 5 mm, they gained the capacity to suppress FSH; however, as follicles grew beyond 5 mm, FSH-suppressing capacity did not increase. The FSH decline was not attributable to an increase in circulating estradiol.