Effects of insulin-like growth factor-I on follicle growth, oocyte maturation, and ovarian steroidogenesis and plasminogen activator activity in the rabbit

We examined the effects of insulin-like growth factor (IGF)-I on follicular growth, oocyte maturation, and ovarian steroidogenesis and plasminogen activator (PA) activity in vitro, using a perfused rabbit ovary preparation in order to determine whether the follicle-stimulating effects of growth hormone (GH) are mediated by IGF-I. The addition of IGF-I to the perfusate stimulated follicular growth and the resumption of meiosis in follicular oocytes in a dose-dependent manner. There was no significant difference in the production of progesterone by perfused rabbit ovaries between IGF-I-treated and control ovaries, whereas IGF-I increased the production of estradiol (E₂) by perfused rabbit ovaries in a dose-dependent manner. The concomitant addition of a monoclonal antibody recognizing the type I IGF receptor, αIR-3, to the perfusate significantly blocked IGFI- stimulated follicular growth, oocyte maturation, and E₂ production. Intrafollicular PA activity increased significantly 4 h after exposure to 10 or 100 ng/ml of IGF-I and reached maximal levels at 6 h. The percentage increase in follicle diameter at 6 h after exposure to IGF-I was significantly correlated with the intrafollicular PA activity. Treatment with GH resulted in a 2.7fold increase in intrafollicular levels of IGF-I mRNA. The binding of [¹²⁵I]-IGF-1 to rabbit ovarian membrane preparations was inhibited by unlabeled 1GF-I and IGF-II in a concentration-dependent manner. The relative affinity of the IGF-I receptor for IGFI, IGF-II, and insulin was typical of type I binding (IGF-I > IGFII > insulin). Affinity cross-linking of ovarian membranes with [¹²⁵I]-IGF-I revealed a radiolabeled band corresponding to a molecular weight of 135 000, the α subunit of the type I IGF receptor. This band was totally displaced by IGF-I and αIR-3. It was concluded that IGF-I stimulated follicular development, E₂ production, and oocyte maturation by interacting with its specific receptor located in rabbit ovarian membranes.