Progesterone-induced atresia of the proestrous dominant follicle in the bovine ovary: changes in diameter, insulin-like growth factor system, aromatase activity, steroid hormones, and apoptotic index.

The objective was to determine whether there were decreases in insulin-like growth factors (IGF)-I and -II and increases in low-molecular-mass IGF-binding proteins (IGFBPs) in association with an inhibition of aromatase activity (AA) and follicular fluid estradiol (E2) production during progesterone (P4)-induced dominant follicle atresia in cattle. Twelve cycling cows received a norgestomet ear implant at proestrus for 9 days and were assigned to control (n = 3) or P4-treated (n = 9) groups. Injections of P4 (150 mg, i.m.) were given on Days 3 and 4; Days 3, 4, and 5; or Days 3, 4, 5, and 6 of the implant period. Controls received injections of corn oil on Days 3, 4, 5, and 6. Ultrasonography of the dominant follicle and blood sampling were done daily. Unilateral ovariectomy was done one day after the last injection. The experiment was repeated with the remaining ovary (6 follicles/treatment group). Granulosa cells were cultured with radiolabeled testosterone to measure AA. Steroid hormones, IGF-I, and IGF-II were measured in follicular fluid by RIA. The follicular fluid IGFBP profile was quantified by Western ligand blotting. P4-treated cows showed a drastic reduction in AA in the dominant follicles, and follicular fluid E2 was several times lower than in controls. Moreover, in P4-treated groups, concentrations of follicular fluid IGF-I and IGF-II were lower than in controls. The quantity of low-molecular-mass follicular fluid IGFBPs increased in P4-treated groups. Accumulation of low-molecular-mass IGFBPs with a reduction in IGFs may be a mechanism of dominant follicle atresia during the bovine estrous cycle.