Gonadotrophin responsiveness, aromatase activity and insulin-like growth factor binding protein content of bovine ovarian follicles during the first follicular wave

The aim of this study was to examine the function of granulosa cells and ho rmone concentrations in follicular fluid in bovine ovarian follicles during selection of the first dominant follicle. Ovaries were obtained from beef heifers on days 1-5 after ovulation: follicles > 4 mm in diameter were diss ected and follicular fluid and granulosa cells were collected from individu al follicles. Oestradiol production by granulosa cells after culture with t estosterone was used to determine aromatase activity and responsiveness to gonadotrophins was determined by cAMP production after culture with FSH or LH. Concentrations of oestradiol, progesterone and insulin-like growth fact or binding proteins (IGFBPs)-4 and -5 were measured in follicular fluid. Fo llicles were classified as largest or smaller (days 1 and 2), or dominant o r subordinate (days 3-5). Aromatase activity was greater in granulosa cells from the largest follicle than in granulosa cells from smaller follicles o n days 1, 3, 4 and 5 (P < 0.05). Responsiveness to LH was not detected in g ranulosa cells on day 1, but from day 2 to day 5 cells from the largest fol licle were significantly more responsive than cells from smaller follicles (P < 0.05). Responsiveness to FSH was detected in granulosa cells from all follicles from day I onwards and did not differ between cells from the larg est follicle or smaller follicles on any day. Follicular fluid concentratio ns of oestradiol and the ratio of oestradiol:progesterone were greater and concentrations of IGFBP-4 and -5 were lower in the largest follicle than in smaller follicles from day 2 to day 5 (P < 0.05). In conclusion, selection of the dominant follicle is associated with increased granulosa cell aroma tase activity followed by increased cAMP response to LH and follicular flui d oestradiol concentrations, and decreased follicular fluid concentrations of IGFBP-4 and -5 within 2 days after ovulation.