Decline in Serum Follicle-Stimulating Hormone Concentrations Alters Key Intrafollicular Growth Factors Involved in Selection of the Dominant Follicle in Heifers1

Declining FSH after a transient rise coincides with selection of a dominant follicle (DF) and atresia of the remaining cohort follicles (subordinates) in cattle. The objectives of this study were to determine 1) whether intrafollicular amounts of inhibins, activin-A, insulin-like growth factor I (IGF-I), and IGF-I-binding proteins (IGFBP) are altered during selection of the first-wave dominant follicle (DF1) and 2) whether these biochemical markers are FSH dependent. Beef heifers received six or eight 6-h injections of saline (controls) or eight 6-h injections of recombinant bovine FSH (1 mg/injection) at 38 to 42 h after estrus (Day 0). Daily ultrasound scanning was used to define selection of DF1. Controls (n = 6 per group) were ovariectomized 1) on Day 3 of the estrous cycle before DF1 selection (preselection follicles) and 2) after DF1 selection on Day 4.8 +/- 0.5. In controls, FSH declined between Days 2 and 3 and selection of DF1 occurred between Days 3 and 5. During this interval, intrafollicular estradiol concentrations increased > 5-fold in DF1, yet declined 4-fold in subordinates (p < 0.05). In DF1, total IGF-I increased 1.3-fold (p < 0.05), whereas the amounts of the 40- to 47-kDa and the 35-kDa IGFBP (ligand hybridization) decreased 2.4- and 2.5-fold, respectively (p < 0.05), compared to values in preselection follicles on Day 3; total dimeric inhibin-A decreased 1.8-fold (p < 0.05). In contrast, amounts of the 30- to 32-kDa IGFBP increased 12.4-fold (p < 0.05) in subordinates on Day 4.8 compared with preselection follicles on Day 3, while the amount of inhibins > 34 kDa decreased 4- to 9-fold (p < 0.05). In FSH-treated heifers, both selection of DF1 and atresia of subordinates were delayed by 2.2 days. Preselection follicles recovered on Day 4.9 +/- 0.1 from FSH-treated heifers were similar (p > 0.05) in almost all biochemical parameters to preselection follicles from control heifers; however, they differed markedly from both DF1 and subordinate follicles recovered from control heifers on Day 4.8 +/- 0.5. In conclusion, the decline in FSH beginning after Day 2 of the heifer estrous cycle causes differential alterations in FSH-dependent growth factors and hormones within the cohort of preselection follicles, simultaneously inducing growth and enhanced estradiol-producing capacity of the DF and atresia of subordinate follicles.