164 EFFECT OF OVULATION-INDUCING FACTOR (OIF) ON OVARIAN FUNCTION IN CATTLE

Abstract

Systemic administration of ovulation-inducing factor (OIF), discovered recently in seminal plasma of llamas, alpacas (induced ovulators), and cattle (spontaneous ovulators), stimulated ovulation in >90% of female llamas and alpacas. The objective of the present study was to test the hypothesis that purified OIF from llama seminal plasma would induce ovulation in cattle. Peripubertal heifers, weighing 323 ± 27 kg, were used to minimize the confounding effect of spontaneous ovulation. Heifers (n = 11/group) were treated intramuscularly with 1.0 mg/100 kg of purified OIF, 100 µg of GnRH (positive control), or 2.5 mL of phosphate-buffered saline (negative control). Ovarian dynamics were monitored daily by transrectal ultrasonography for 10 days post-treatment. Blood samples were collected at 0.5- to 1-h intervals for 8 h, beginning at the time of treatment. Ovulation occurred in 9/11 (82%) of GnRH-treated heifers and in 1/11 (9%) heifers in each of the OIF- and saline-treated groups (P < 0.05). A surge in plasma LH concentration was detected within 30 min of treatment in the GnRH group (2.2 ± 0.1 ng mL^–1; P < 0.05), but remained at the basal level in the OIF- and saline-treated groups (0.3 ± 0.1 and 0.2 ± 0.1 ng mL^–1, respectively). The onset of regression of the dominant follicle present at the time of treatment was earlier (P < 0.05) in OIF- v. saline-treated heifers (3.1 ± 0.6 days v. 6.0 ± 0.7 days). The interval from treatment to follicular wave emergence was shorter (P < 0.05) in GnRH- and OIF-treated heifers than in those treated with saline (1.1 ± 0.4 days, 1.5 ± 0.3 days, and 3.1 ± 0.3 days, respectively). A similar pattern was observed for emergence of the second follicular wave (5.1 ± 0.7 days, 4.6 ± 0.5 days, and 6.6 ± 0.4 days, respectively). Purified OIF did not induce ovulation in heifers but hastened both the regression of the extant dominant follicle and follicular wave emergence. Results provide a rationale for the hypothesis that OIF from seminal plasma is involved in controlling follicular wave dynamics in spontaneously ovulating species (e.g., Bos taurus) through a suppressive effect on the dominant follicle. The mechanism of action on ovarian follicular wave dynamics, as well as species specificity, remains to be elucidated.