Graduate Thesis Or Dissertation
 

Response of the bovine corpus luteum to exogenous gonadotropin-releasing hormone during the estrous cycle

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  • Two experiments were conducted to investigate the effects of gonadotropin-releasing hormone (GnRH), administered twice during the same estrous cycle, on bovine luteal function. Eight mature beef cows, each cow serving as her own control, were injected intravenously (i.v.) with saline on days 2 and 8 of the cycle (day of estrus=day 0 of the cycle), then with 10Oug GnRH on days 2 and 8 of the subsequent cycle. Jugular blood samples were taken immediately prior to an injection and at 15, 30, 45, 60, 120 and 240 min postinjection to characterize changes in serum luteinizing hormone (LH). Blood was also collected on alternate days after an injection until day 16 of the cycle, to characterize serum progesterone levels. Treatment with GnRH did not significantly alter serum progesterone concentrations. Although GnRH caused release of LH on days 2 and 8, the quantity of LH released was significantly greater on day 8 (P<0.0025). Length of the estrous cycle was not altered by treatment (control 20.4 ± .5 vs. treated 20.4 ± .4 days). In Experiment 2, five mature beef cows, each serving as her own control were injected i.v. with 10Oug GnRH on day 2 of the cycle, with saline on days 2 and 8 of the succeeding cycle and then with 10Oug GnRH on days 2 and 8 of the third cycle. Corpora lutea (CL) were obtained per vaginam on day 10 of each cycle. Luteal tissue was sliced and incubated for 2 hr with saline (control) or LH (10ng/m1). Treatment of cows with GnRH on day 2 or days 2 and 8 caused a marked increase in in vitro basal progesterone production (ug/g) by luteal slices (control, 37.45 ± 2.93 vs. GnRH day 2, 43.97 ± 5.03 vs. GnRH days 2 and 8, 53.18 ± 3.03) but suppressed the ability of luteal tissue to respond to LH (P<0.07). Treatment did not affect the initial luteal progesterone concentration as determined by measurement of the steroid in unincubated samples (control, 23.99 ± 1.60 vs. GnRH day 2, 24.08 ± 2.15 vs. GnRH days 2 and 8, 29.80 ± 3.65 ug/g). Results of this study demonstrate that LH, released in response to GnRH, can increase basal progesterone production and yet desensitize luteal cells to further stimulation by the homologous hormone .
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