Gonadotropin releasing hormone-induced alteration of corpus luteum function in beef heifers Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/5425kf96s

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  • Two experiments were conducted to evaluate the effects of gonadotropin releasing hormone (GnRH) on the function of the bovine corpus luteum during the estrous cycle. In Experiment 1, 10 heifers were assigned randomly into two groups with each heifer serving as her own control. Heifers in Group I (n=5) were injected intravenously (iv) with vehicle (saline) on day 2 of the cycle (day 0=day of estrus) followed by an iv injection of 100 ug GnRH on day 2 of the subsequent estrous cycle. Group II (n=5) heifers were similarly treated except injections were given on day 10 of the estrous cycle. All heifers were bled via the jugular vein at 15 minute intervals beginning 30 minutes prior to injection and for 3 hours 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 minutes postinjection. Treatment with GnRH on either day 2 or 10 of the cycle 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, P < .03). Length of the estrous cycle did not differ among the 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 heifers were assigned randomly into four groups (n=5) of 2 x 2 factorial design to examine the effects of GnRH on luteal LH receptors. Treatment consisted of an iv injection of vehicle or 100 ug GnRH on day 2 of the cycle. Heifers were sacrificed on day 8 or 14 of the same cycle at which time the ovaries were removed, corpora lutea enucleated, weighed and frozen until assayed for LH receptors. Concentrations of unoccupied LH receptors in the luteal plasma membranes were reduced at day 8 and 14 after treatment (control vs treated, day 8, 96.6 vs 54.7; day 14, 92.0 vs 40.7 pmol/ug protein, P < .05). Data from the present study suggests that exogenous GnRH may act indirectly via the released LH to cause down regulation of luteal LH receptors with a consequent attenuation of progesterone synthesis.
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