Insulin-like growth factor I and insulin-like growth factor-binding proteins in bovine serum and follicular fluid before and after the preovulatory surge of luteinizing hormone

Abstract

Previous studies have shown that concentrations of insulin-like growth factor (IGF)-binding proteins (BPs) in bovine follicles before the preovulatory surge of LH are inversely associated with estradiol concentrations, such that decreases in IGFBPs occur with advanced development of antral follicles and atresia is associated with increases in IGFBPs. In the present study, we evaluated serum and follicular fluid at different times after the preovulatory surge of LH to determine whether changes in IGF-I and IGFBPs are associated with the marked changes in the steroidogenic capacity that occur in follicles during this period. Serum and fluid from small follicles (≤ 5-mm diameter) and the preovulatory follicle were collected from cows at the onset of standing estrus or 8.5 and 20.5 h after administration of 100 μg GnRH at the onset of estrus (n = 4/time). Concentrations of IGF-I (determined by RIA) did not differ (p > 0.10) among times but were lower (p < 0.05) 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 and 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 a 13- to 15-fold reduction in estradiol-17β and androstenedione occurred between 8.5 and 20.5 h after the LH surge. Small follicles contained IGFBP-2, -3, -4, and -5 (determined by immunoprecipitation). The amount of 28-29-kDa IGFBPs (small form of IGFBP-5 and [or] glycosylated form of IGFBP-4) was inversely associated 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. Circulating IGFBP-2, but not other IGFBPs, decreased by 50% from estrus to 20.5 h after GnRH. In summary, amounts of IGF-I or IGFBPs in preovulatory follicular fluid did not change in response to an LH surge, even though estradiol and androstenedione decreased 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.