Effects of suppressing plasma FSH on ovarian follicular dominance in cattle

Abstract

The importance of FSH during development of the dominant follicle of the first wave of follicular growth in cattle was assessed by injecting heifers twice daily with 20 ml saline (n = 6) or 20 ml charcoal-extracted bovine follicular fluid (bFF; n = 10), containing inhibin, on days 6 and 7 of the oestrous cycle. All animals received a luteolytic dose of PGF(2α) on day 8. The interval to oestrus after PGF(2α) in controls was 2.6 ± 0.2 days. Five of ten bFF-treated heifers exhibited oestrus at the same time as controls (2.7 ± 0.1 days), whereas time to oestrus in the remaining five bFF-treated heifers was significantly longer (6.8 ± 0.6 days; P < 0.05). Treatment with bFF suppressed plasma FSH during the 48 h injection period (P < 0.05). Concentrations of FSH were not different in bFF-treated animals that did not display delayed oestrus compared with those in animals that exhibited delayed oestrus. In controls, the dominant follicle of the first wave continued growing during treatment and ovulated after injection of PGF(2α). In all ten bFF-treated animals, growth of the dominant follicle of the first wave was arrested during the treatment period. In bFF-treated animals that did not exhibit delayed oestrus, the dominant follicle resumed growth after luteolysis and ovulated. In bFF-treated animals that displayed delayed oestrus, the dominant follicle regressed after luteolysis and the ovulatory follicle was selected from a newly recruited (second) wave of follicular growth. Maximal increases in concentrations of oestradiol in plasma reflected preovulatory follicular development and occurred at the time of oestrus, 2 or 3 days after PGF(2α) injection in saline- and bFF-treated heifers that did not display delayed oestrus, and 5-8 days after PGF(2α) in animals in which oestrus was delayed. No differences in concentrations of plasma LH or progesterone were detected between groups. Arrested growth of the dominant follicle of the first wave, associated with suppressed concentrations of basal FSH, suggests an important role for FSH during ovarian follicular dominance in cattle.