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(446) EFFECT OF FSH IN HEIFERS IMMUNIZED AGAINST GnRH. Jaiswal, Rajesh1, Singh, Jaswant1, Mapletoft, Reuben2, Adams, Gregg1, 1 Department of Veterinary Biomedical Sciences, Saskatoon, Saskatchewan2 Department of Large Animal Clinical Sciences, Saskatoon, Saskatchewan ABSTRACT- To clarify the role of FSH in the development of small follicles (<4 mm), a study was designed to determine the effect of low basal FSH on ovarian follicular dynamics and the response of FSH deprived follicles to exogenous FSH. Hereford cross heifers were given 3 doses of an anti-GnRH vaccine (Metamorphix Canada Inc.), or a sham vaccine (controls) at intervals of 28 days (n = 16 per group). Follicular development was monitored by ultrasonography daily for 111 days after the 1st vaccination (i.e., expected maximal GnRH antibody titer). Heifers in both groups were then given a luteolytic dose of prostaglandin and transvaginal ultrasound-guided follicular ablation was done to synchronize follicular wave emergence. Half of the heifers in each group (n = 8) were given a superstimulatory dose of FSH (200 mg Folltropin im divided bid over 3 days). The daily profiles of follicle numbers and diameters were compared between immunized and control groups by analysis of variance for repeated measures using the day of ablation as a reference point (Day −2 to Day 3; ablation = Day 0). Mean plasma FSH concentration was lower in the immunized than in the control group (P = 0.05) primarily as a result of the absence of a pre-wave FSH surge in immunized heifers. There was no difference (P > 0.25) in the number of 1, 3 and 4 mm follicles between groups, but immunized heifers tended to have more 2 mm follicles (P = 0.08), and had no follicles in the ≥5 mm category (P < 0.01) compared to control heifers. Following FSH treatment, the difference in the number of follicles ≥5 mm persisted between groups (P < 0.01). The diameter profile (Days 0 to 3) of each of the 3 largest follicles in immunized heifers was smaller (P< 0.01) than their counterparts in the control group. The diameter profiles of the 3 largest follicles differed from one another in the control group (P < 0.01), but tended to differ (P = 0.09) only between the 1st and the 3rd largest follicles in the immunized group. It was concluded that follicles can grow to 4 mm under low basal circulating concentrations of FSH but not beyond, and follicles that develop under low basal circulating FSH are less responsive or slower to respond to exogenous FSH. Supported by the Natural Sciences and Engineering Research Council of Canada and Metamorphix Canada Inc. KEY WORDS: follicle dynamics, bovine, anti-GnRH, fsh |
