160 Effect of a gonadotrophin-releasing hormone antagonist on the fate of the dominant ovarian follicle and emergence of the next follicular wave in alpacas

A study was designed to evaluate the effects of a gonadotrophin-releasing hormone (GnRH) antagonist (cetrorelix) on ovarian function in alpacas. We tested the hypotheses that (1) cetrorelix will cause regression of growing- and static-phase dominant follicles and (2) a new follicular wave will emerge at a consistent time after treatment. The ovaries of mature nonpregnant alpacas were examined daily by transrectal ultrasonography to detect the day of wave emergence (Day 0). Alpacas were allocated randomly to three groups (n = 7 per group) and given cetrorelix on Day 5 (mid-growing dominant follicle), Day 10 (mid-static phase), or no treatment (Control). Cetrorelix treatment consisted of 1.5 mg cetrorelix acetate (0.5 mg/mL in 5% D-Mannitol) im twice, 24 h apart. Ovarian examinations continued until Day 6 of the next follicular wave. On average, the dominant follicle entered the static phase on Day 7.6 ± 0.55 (data combined from Control and Day 10 groups). The mean day of onset of dominant follicle regression was earlier in the Day 5 group than in the Day 10 and Control groups (Day 8.7 ± 0.89, Day 12.9 ± 0.34, Day 13.6 ± 0.90, respectively; P < 0.001) and the maximum dominant follicle diameter was smaller in the Day 5 group than in the Day 10 and Control groups (7.4 ± 0.5 mm, 10.9 ± 0.70 mm, 10.2 ± 0.93 mm; P = 0.005). The duration of the static phase of the dominant follicle was shortest in the Day 5 group, intermediate in the Day 10 group, and longest in the Control group (3.9 ± 0.63 days, 4.9 ± 0.40 days, 6.3 ± 0.52 days; P = 0.015). The inter-wave interval was shortest in the Day 5 group, intermediate in the Control group, and longest in the Day 10 group (12.7 ± 0.36 days, 14.57 ± 0.81 days, 17.71 ± 0.52 days; P < 0.001). The interval from first treatment to emergence of the next wave was virtually identical in the Day 5 and Day 10 groups (7.7 ± 0.36 days vs 7.7 ± 0.52 days; P > 0.99; range: 7–9 days). In conclusion, our hypotheses were supported. Treatment with two doses of cetrorelix (a GnRH antagonist) caused the regression of the growing and static phase dominant follicle in alpacas leading to synchronous emergence of the next wave eight days later.

This research was supported by NSERC and Saskatchewan ADF and received student support from WCVM Interprovincial fund and NSERC. The authors thank Ms. Ella Macquisten for animal handling and treatment help.