Insulin-like growth factor system in bovine first-wave dominant and subordinate follicles

Estradiol (E(2))-active (Day 5 [D5]), transitional E(2)-active (D8), atretic (D12-I), and E(2)-sustained dominant follicles (DF, D12+I) and associated subordinate follicles (SF) were obtained through use of an experimental model described here. The ovary bearing the DF was surgically removed by colpotomy, and individual follicles were utilized to study changes in concentrations of insulin like growth factor-I (IGF-I) and -II (IGF-II) and changes in amounts and proportions of the different IGF-binding proteins (ICFBP) present in follicular fluid (FF). The ratio of FF E(2) to progesterone (EPR) was utilized to classify follicles into E(2) active (EPR>1) and E(2) inactive (EPR<1). The IGF-I and IGF-II concentrations in FF were similar among experimental groups and between DF and SF. Six different molecular mass bands (49, 43, 35, 30, 28, and 22 kDa) were detected by ligand blot in FF of DF and SF. Immunoprecipitation analysis identified four IGFBPs (-2, -3, -4, and -5) in FF. The 35-kDa band corresponded to IGFBP-2, the 49- and 43-kDa bands to ICFBP3, the 28- and 22-kDa bands to ICFBP-4, and the 30-kDa band to ICFBP-5. No IGFBP-6 was found by immunoprecipitation. Absolute amounts and proportions of low molecular mass IGFBPs (-2, -4, and -5) were increased with atresia of the DF and in SF compared to E(2)-active DF. Conversely, although absolute amounts of IGFBP-3 remained unchanged, their proportion in FF decreased in SF compared to DF. The ratio of IGF-I to ICFBPs decreased with atresia of the DF, possibly leaving less bioavailable ICF-I to increase FSH action at the level of the follicle.