OVARIAN INTRABURSAL ADMINISTRATION OF INSULIN-LIKE GROWTH FACTOR-BINDING PROTEIN INHIBITS FOLLICLE RUPTURE IN GONADOTROPIN-TREATED IMMATURE FEMALE RATS

The effects of an insulin-like growth factor-binding protein (IGF-BP) on rat follicular function were examined by using the technique of ovarian intrabursal (IB) injection. Immature female rats were injected with 15 IU of eCG followed immediately with IB injections of 4-mu-g IGF-BP3 (right ovary). and vehicle (left ovary). Forty-eight hours later, the same animals were either killed (eCG-treated group) or injected with 1-mu-g of hCG as an ovulatory stimulus. These animals were killed 24 h later (eCG/hCG-treated group). Intrabursal administration of IGF-BP3 inhibited ovulations in the eCG/hCG-treated rats by 55% when compared with the contralateral vehicle-treated ovary (p = 0.01). Examination of the ovaries exposed to IGF-BP3 revealed the presence of unruptured follicles containing a matured oocyte and a disintegrated basement membrane, in addition to normal follicles and corpora lutea. In contrast, IB injection of IGF-BP3 had no effect on ovarian weights or circulating estradiol concentrations in the eCG-treated animals, and the ovaries appeared to be morphologically normal. Ligand blotting experiments using [I-125]-labeled insulin-like growth factor I revealed that granulosa cells obtained from both untreated and eCG-treated rats synthesized and secreted two IGF-BPs of M(r) 35 000 and 30 000. Equine chorionic gonadotropin treatment reduced the amount of the 30 000 M(r) form of IGF-BP. These data suggest that locally produced ovarian IGF-BPs may modulate follicle functions in vivo.