Relationship between concentrations of immunoreactive insulin-like growth factor-I in follicular fluid and various biochemical markers of differentiation in bovine antral follicles.

Three experiments were conducted to determine the relationship between concentrations of insulin-like growth factor-I (IGF-I) in ovarian follicular fluid and various biochemical markers of follicular differentiation in bovine follicles. In Experiment I, ovaries were removed on Days 7, 14, 28, 42, or 56 after parturition from a total of 21 cows. In Experiment 11, ovaries of 31 cows were removed between Days 20 and 30 postpartum after 48 or 96 h of either saline (0.9% NaCI, 5 ml) or luteinizing hormone-releasing hormone (LHRH, 500 ng/5 ml saline) injections given every 2 h via jugular can nulae. In Experiment III, ovaries of six cows were removed 48-50 h after a 35-mg injection of prostaglandin Fduring the midluteal phase of an estrous cycle. In Experiments I and II, all follicles � 8.0 mm in diameter were removed from each ovary (n = 33 and 46, respectively). In Experiment III, fluid from all follicles > 4 mm in diameter were removed individually (n = 10), and fluid from follicles 1-4 mm in diameter were pooled for each cow. Follicles for each experiment were further categorized as either estrogen-active (E-A, concentration of estradiol > progesterone in follicular fluid) or estrogen-inactive (E-I, concentration of progesterone > estradiol in follicular fluid). Measurements of immunoreactive IGF-I (i-IGF-I) were made after separating IGFs from their binding proteins with an acid- ethanol extraction. Levels of i-IGF-I in follicular fluid (72-149 ng/ml) did not significantly change with time postpartum (Experiment I) nor change in response to LHRH injections (Experiment II). In Experiment III, concentrations of i-IGF-I in fluid of 1-4 mm, 5-12-mm or >12-mm follicles did not differ (p>0. 10). Although E-A follicles contained 15- to 61-fold greater levels of estradiol than E-I follicles across all three experiments, i-IGF-I levels did not differ (p>0. 10) between E-A and E-I follicles. Also, in Experiments I and II, levels of androstenedione in follicular fluid and numbers of granulosa cell human chorionic gonadotropin (hCG) binding sites were not significantly correlated with levels of i-IGF-I in follicular fluid. However, a positive correlation was found between concentrations of i-IGF-I (range 11-248 ng/ml) and progesterone (range 19-1500 ng/ml) in fluid in E-I (but not E-A)follicles of Experiments land II (r = 0.56, p<0.05, andr = 0.82, p<0.01, respectively). Levels of i-IGF-I in follicular fluid were also positively correlated with follicular diameter in E-A follicles of Experiment I (r = 0.62, p< 0.05) and III (r 0.72, p< 0.05). Levels of i-IGF-I were negatively correlated with numbers ofgranu- losa cell follicle-stimulating hormone binding sites (r = -0.54, p<0.Ol), numbers of thecal hCG binding sites (r = -0.33, p< 0.05), and levels of estradiol (r = -0.45, p< 0.01) in E-I follicles of Experiment II. In summary, follicu- lar differentiation in cattle accompanied by dramatic increases in follicular fluid levels of estradiol occurred without changes in levels of i-IGF-I. In contrast, increased concentrations of progesterone in follicular fluid were associated with increased levels of i-IGF-I in individual follicles. Thus, in contrast to results from in vitro studies, it seems unlikely that concentrations of IGF-I in follicular fluid are limiting for ovarian follicular est.radiol production in vivo. However, results of the present study are consistent with the hypothesis that IGF-I levels in follicular fluid may regulate progesterone production in vivo.