366 THE BOVINE MODEL FOR REPRODUCTIVE AGING: SUPEROVULATORY RESPONSE

Abstract

Two experiments were done to test the hypotheses that aging in cattle is associated with a reduction in the number of follicles recruited into an ovarian follicular wave, and a reduction in the ovarian response following gonadotropin treatment for superovulation. Old cows (13 to 16 years old; n = 6 to 9) and their young daughters (3 to 6 years old; n = 7 to 9) were treated to induce an ovarian superstimulatory response twice in each of two consecutive years (i.e. 31 and 33 superstimulatory cycles, respectively). In Experiment 1, follicular wave emergence was synchronized using a single treatment of estradiol (5 mg) combined with progesterone (100 mg) at the time of CIDR-B (Bioniche Animal Health, Belleville, Ontario, Canada) insertion. Superstimulatory treatment consisted of a total dose of 50 mg NIH-FSH-P1 (Folltropin-V; Bioniche Animal Health) per 100 kg body weight divided into twice daily intramuscular injections over 4 days. CIDR devices were removed at the time of the last FSH treatment and 500 ¼g cloprostenol (Estrumate; Schering-Plough Animal Health, Montreal, Quebec, Canada) was administered twice along with last two FSH treatments to induce luteolysis. Ovulation was induced using 25 mg porcine LH (Lutropin-V; Bioniche Animal Health) administered 24 h after the last FSH treatment. Ovulations were detected by ultrasonography at 24, 48, and 72 h after LH treatment. In Experiment 2, follicular wave emergence was synchronized with ultrasound-guided follicle ablation in the presence of a functional corpus luteum. Superstimulatory treatment was given as in Experiment 1. Ultrasound-guided follicle aspirations were performed 48 to 72 h after the last FSH for the purpose of another experiment. In both experiments, the ovarian follicular response was monitored daily by ultrasonography. There were no differences (P > 0.3) in mean body weight and dose of FSH treatment between old cows and their daughters. Fewer 2-5-mm follicles (P < 0.01) were detected at the expected time of wave emergence in old cows than in their daughters. In Experiment 1, the difference in mean number of ovulations in old and young cows (32 ± 4 vs. 40 ± 3) did not reach significance (P = 0.11), but fewer follicles e6 mm (P < 0.01) were detected on the day before ovulation in old cows than in their daughters. The number of follicles e6 mm detected just before ovulation (Experiment 1) or follicular aspiration (Experiment 2) in successive superstimulatory cycles was correlated within animals (r > 0.8; P < 0.0001). Similarly, the number of ovulations in successive superstimulatory cycles (Experiment 1) was correlated within animals (r = 0.6; P = 0.04). In conclusion, aging was associated with fewer 2-1 mm follicles at the time of wave emergence, a lower follicular response, and a tendency for a lower ovulatory response subsequent to gonadotropin treatment. The follicular and ovulatory response to superstimulatory treatment was repeatable within individuals.