| Abstrakt: |
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 μg 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<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).In Experiment 2, 20 beef heifers were assigned randomly into four groups (n=5) of 2 × 2 factorial design to examine the effects of GnRH on LH receptors. Treatment consisted of an i.v. injection of vehicle or 100 μg GnRH on Day 2 of the cycle. Heifers were killed on Day 8 or 14 of the same cycle. At that time the ovaries were removed, and corpora lutea were enucleated, weighed and frozen until assayed for LH receptors. Concentrations of unoccupied LH receptors in luteal plasma membranes were reduced at Days 8 and 14 after treatment (control vs. treated, Day 8, 96.6 vs. 54.7; Day 14, 92.0 vs. 40.7 pmol/μg protein; common SE=3.22, p<0.005). It is suggested that exogenous GnRH acts indirectly in the bovine to suppress luteal function. |
