|Budget Amount *help
¥1,600,000 (Direct Cost : ¥1,600,000)
Fiscal Year 1996 : ¥1,600,000 (Direct Cost : ¥1,600,000)
The concept of classical endocrine control of ovarian function has now been extended to a more complex regulator system, including paracrine and autocrine modulating mechanisms. Among many factors, locally produced intraovarian insulin-like growth factors (IGFs) and the binding proteins (IGFBPs) and renin-angiotensin system (RAS) have been shown to play an important role in the control of folliculogenesis and ovulation. Growth hormone (GH) amplified gonadotropin actions in the process of follicular development and ovulation, at least in part, stimulating ovarian IGF-I production. IGF-I as well as IGFBPs were produced by ovarian granulosa cells. IGF-I acted synergistically with gonadotropins in the stimulation of a variety of granulosa cell functions, including estradiol (E2) and progesterone production and plasminogen activator (PA) activity. Furthermore, rabbit ovarian cells and rat granulosa cells possessed specific IGF type I receptors. The biological effects of IGF-I,including intr
afollicular PA activities and ovarian steroidogenesis, were modulated by a family of IGFBPs in a complex manner. In the ovary IGFBP-3 appeared to neutralize the actions of gonadotropin and IGF-I,probably via its ability to sequester IGF-I,in the process of follicular growth, oocyte maturation, and ovulation.
A functional local RAS is also known to exist in the ovary. Angiotensin II (Ang II) at 2-h intervals induced oocyte maturation, ovulation, and the production of E2 and prostaglandins (PGs) in the in vitro perfused rabbit ovaries in the absence of gonadotropin. In addition, the intrafollicular Ang II content and renin-like activity were enhanced during the ovulatory process by exposure to hCG,and the concomitant addition of saralasin inhibited hCG-induced ovulation in a dose-dependent manner. Captopril, an inhibitor of angiotensin converting enzyme, significantrly inhibited the resumption of meiosis in the ovulated ova and follicular oocytes stimulated by hCG.Autoradiographic study revealed that AT_2 receptors were predominantly located in granulosa cells, whereas AT_1 receptors were more concentrated in the stroma and the thecal layrs. Ang II-stimulated production of E_2 and PGs and ovulation were significantly blocked by PD123319, a selective nonpeptide antagonist for AT2 receptors.
The increase in ovarian IGF-I synthesis by exposure to hCG or GH induced the stimulation of intrafollicular PA activities. IGFBP-3 blocked the stimulatory effects of gonadotropin in the ovulatory process by neutralizing endogenously produced IGF-I,resulting in reduced intrafollicular PA activities. The increase in intrafollicular PA activites significantly stimulated the generation of Ang II in the preovulatory follicles by an activation of prorenin to renin and/or by the direct cleavage of angiotensinogen. The elevation of intrafollicular Ang II stimulated ovarian PG production, leading to follicular growth and ovulation. In conclusion, IGF-I enhances ovarian Ang II production and follicular growth via the stimulation of intrafollicular PA activities. Thus, IGF-I and Ang II may interact with one another in the mechanisms controlling follicular development and ovulation. Less