Gonadotropin-releasing hormone-induced alteration of bovine corpus luteum function

Two experiments were conducted to evaluate effects of gonadotropin-releasing hormone (GnRH) on the function of the bovine corpus luteum during the estrous cycle. In Experiment 1, 10 beef heifers were assigned randomly into two groups; each heifer served as her own control. Heifers in Group I (n = 5) were injected i.v. with vehicle (saline) on Day 2 of the cycle (Day 0 = day of estrus) followed by an i.v. injection of 100 micrograms GnRH on Day 2 of the subsequent estrous cycle. Group II (n = 5) heifers were treated similarly except injections were given on Day 10 of the estrous cycle. All heifers were bled via the jugular vein at 15 min intervals beginning 30 min prior to injection and for 3 h after injection. Blood samples were also taken on alternate days after injection through Day 16 of the cycle. Gonadotropin-releasing hormone caused a significant release of luteinizing hormone (LH) on both treatment days with the peak occurring at 15 to 30 min postinjection. Treatment with GnRH on either Day 2 or 10 caused a reduction in serum progesterone levels on Days 12, 14 and 16 of the cycle (Group I, control 3.99, 3.97; 4.07 vs. treated 2.63, 3.45, 2.87; Group II, control 3.18, 3.82, 4.13 vs. treated 2.50, 2.82, 3.17 ng/ml, respectively; common SE = 0.24 p less than 0.03). Length of the estrous cycle did not differ between groups (Group I, control 20.7 vs. treated 20.9; Group II, control 20.7 vs. treated 21.1 days, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)