|Funston, Richard - COLORADO STATE UNIV|
|Seidel, Jr, George - COLORADO STATE UNIV|
Submitted to: Biology of Reproduction
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 7, 1996
Publication Date: N/A
Interpretive Summary: Successful reproduction is dependent on the ability of an animal to release a viable oocyte at the appropriate time of her estrous cycle. Yet little is known about the mechanisms that control the development of the ovarian follicles that support developing oocytes. While several hundred follicles begin to grow during each estrous cycle, only one generally ovulates. Cell lculture studies have demonstrated that the hormone insulin-like growth factor I (IGF-I) enhances the ability of cells from follicles to proliferate and secrete steroids (i.e., estrogens), two aspects associated with follicle maturation. In addition, IGF-I has been shown to be beneficial for in vitro maturation of oocytes. The present study demonstrated that concentrations of IGF-I did not differ between fluid from follicles destined to ovulate and those destined to die. However, the study determined that IGF-binding proteins (IGFBPs) were much less prevalent in follicles destined to ovulate compared to follicles destined to die. As IGFBPs inhibit the action of IGF-I, high levels of IGFBPs in a follicle may prevent IGF-I from promoting the normal maturation processes within a follicle, resulting in the demise of the follicle. This study increases our knowledge of the process occurring during the maturation of ovarian follicles. It is anticipated that a greater understanding of the mechanisms controlling IGFBPs in the ovary may eventually lead to procedures to enhance producers' ability to control or improve reproduction in cattle.
Technical Abstract: Concentrations of IGF-I and profiles of IGF binding proteins (IGFBPs) were determined for serum and fluid from nonovulatory small follicles and the preovulatory follicle collected from cows at the onset of standing estrus or 8.5 and 20.5 h after administration of 100 ug GnRH at the onset of estrus (n=4/time). Concentrations of IGF-I (determined by RIA) did not differ (p>.10) among times, but were less (p<.05) in fluid from small and preovulatory follicles than those in serum. Profiles of IGFBPs (evaluated by ligand blot analysis) differed among serum and fluid from small or preovulatory follicles. Preovulatory follicles contained IGFBP-3 with little or no other IGFBPs detected. Amount of IGFBP-3 and progesterone did not differ with time of sample collection, but estradiol 17B and androstenedione declined markedly (p<.05) after the LH surge. Small follicles contained IGFBP-3 and other IGFBPs determined to be IGFBP-2, -4, and -5 by immunoprecipitation, providing the first identification of IGFBP-4 in bovine follicular fluid. Amount of IGFBP-2 and 28-29 kDa IGFBPs (small form of IGFBP-5 and(or) large form of IGFBP-4) changed in an inverse association with concentrations of androstenedione. The 31 kDa form of IGFBP-5 was positively correlated with concentration of progesterone. Serum contained IGFBP-2, -3, and -4, but not IGFBP-5. Amount of IGFBP-2, but not other IGFBPs, decreased by 50% from estrus to 20.5 h post-GnRH. In summary, amounts of IGF-I or IGFBPs in preovulatory follicular fluid did not change in response to an LH surge, even though concentrations of steroids changed markedly. The absence of IGFBPs other than IGFBP-3 in bovine preovulatory follicles may allow for increased availability of IGF-I, which is proposed to be important for oocyte maturation and ovulation.