Summary. Injection of steroid-free bovine follicular fluid (bFF; 2 × 5 ml s.c. 12h apart) into anoestrous ewes lowered plasma FSH concentrations by 70% and after 24 h had significantly (P < 0·01) reduced the number of non-atretic follicles (≥ 1 mm diam.) without influencing the total number of follicles (≥ 1 mm diam.) compared to untreated controls. Hourly injections of FSH (10 μg i.v. NIH-FSH-S12) for 24 h did not influence the number of non-atretic follicles but did negate the inhibitory effects of bFF on follicular viability. Hourly injections of FSH (50 μg i.v., NIH-FSH-S12) + bFF treatment for 24 h significantly increased the total number of non-atretic follicles, and particularly the number of medium to large non-atretic follicles (≥ 3 mm diam.) compared to the untreated controls (both P < 0·01). The 10μg FSH regimen (without bFF) significantly increased aromatase activity in granulosa cells from large ( ≥ 5 mm diam.; P < 0·01) but not medium (3–4·5 mm diam.) or small (1–2·5 mm diam.) follicles compared to controls. The 10 μg FSH + bFF regimen had no effect on granulosa-cell aromatase activity compared to the controls. However, the 50 μg FSH plus bFF regimen increased the aromatase activity of granulosa cells from large, medium and small non-atretic follicles 2·6-, 8·3- and ≥ 11-fold respectively compared to that in the control cells.
Ewes (N = 11) that ovulated 2 follicles had significantly higher plasma FSH concentrations from 48 to 24 h and 24 to 0 h before the onset of a cloprostenol-induced follicular phase (both P < 0·01) than in the ewes (N = 12) that subsequently ovulated one follicle. Hourly FSH treatment (1·6 μg i.v., NIAMDD-FSH-S15) for 24 h but not for any 6 h intervals between 48 and 24 h or 24 and 0 h before a cloprostenol-induced luteolysis also resulted in significant increases (P < 0·05) in the number of ewes with 2 ovulations.
We conclude that (1) the number of non-atretic antral follicles in sheep ovaries is influenced by plasma FSH concentrations; (2) the level of follicular oestradiol biosynthesis can be enhanced by FSH treatment; and (3) sustained elevations of plasma FSH concentrations for 24 h but not 6 h within 48 h of the onset of luteolysis significantly enhances the ovulation rate in Romney ewes.
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