217 LUTEAL REGRESSION AND FOLLICLE DEVELOPMENT FOLLOWING PROSTAGLANDIN-F_2α TREATMENT 3 DAYS AFTER OVULATION IN MARES

Abstract

The corpus luteum (CL) is responsive to exogenous prostaglandin-F_2α (PGF) 1 to 2 days after ovulation in the mare (Troedsson et al. 2001 Theriogenology 55, 1891–1899); however, complete and sustained demise of the CL beginning less than 5 days after ovulation in response to PGF treatment has not been documented. The present study was designed to compare the morphological and physiological response of the primary CL to PGF given at early diestrus with a more conventional treatment given at about mid-cycle. In addition, follicle status pre- and post-treatment were examined and compared between the treatment groups. On the day of pretreatment ovulation (Day 0), riding-type horse mares were randomly assigned to receive a single dose of PGF (Lutalyse, Upjohn, Kalamazoo, MI, USA;; 10 mg/mare, i.m.) on Day 3 (n = 17) or Day 10 (n = 17). Beginning on either Days 3 or 10, transrectal ultrasonography was used to determine follicle and CL diameters, determine luteal tissue gray-scale scores (echogenicity), and to detect ovulation. Follicular and luteal measurements and jugular blood samples were collected daily until the post-treatment ovulation. Structural and functional regression of the CL was indicated by: 1) a progressive decrease (day effect;; P < 0.0001) in mean diameter of the CL beginning 24 h after PGF treatment in the Day 3 and Day 10 groups;; 2) a precipitous decrease (P < 0.009) in mean plasma progesterone concentrations within 24 h in both groups followed by a more gradual decline to basal concentrations by the second day in the Day 10 group or after the fourth day in the Day 3 group;; and 3) an increase (P < 0.02) in mean luteal tissue echogenicity in both groups after the second day following PGF treatment. The mean intervals from PGF treatment to ovulation were not different (P > 0.2) between groups (combined, 9.9 days) but the mean (±SEM) interovulatory interval was shorter (P < 0.0001) in the Day 3 group (13.2 ± 0.9 days;; range, 7 to 20 days) than in the Day 10 group (19.2 ± 0.7 days;; range, 14 to 26 days). The greater the diameter of the largest follicle at the time of PGF treatment, the shorter the interval to post-treatment ovulation in the Day 3 (r = −0.57, P < 0.02) and Day 10 (r = −0.74, P < 0.001) groups. Growth rates of the preovulatory follicles were similar (P > 0.59) between groups (combined, 3.6 mm/day) but the maximum diameter was smaller (P < 0.05) in the Day 3 group (40.5 ± 1.2 mm) compared to the Day 10 group (43.4 ± 0.8 mm). Unexpectedly, more (P < 0.03) double ovulations occurred in the Day 3 group (6/17, 35%) than in the Day 10 group (1/17, 6%). In conclusion, an immature CL at early diestrus responded to PGF treatment in a manner comparable to a mature CL at mid-cycle. The Day 3 group ovulated an average of 6 days earlier than the Day 10 group as a result of the difference in timing of the PGF treatment between groups. Thus, these results warrant a reassessment of the prevailing concept that the equine CL is resistant to PGF-induced regression before 5 days after ovulation, especially when considering the potential benefits of a shortened interovulatory interval and an increased double ovulation rate.