Response of the bovine corpus luteum to increased secretion of luteinizing hormone induced by exogenous gonadotropin releasing hormone

Abstract

Two experiments were conducted to investigate the response of the bovine corpus luteum to surges of luteinizing hormone (LH) induced by natural gonadotropin-releasing hormone (GnRH) administered twice during the same estrous cycle. In experiment 1, eight mature beef cows, each cow serving as her own control, were injected intravenously (iv) with saline on days 2 and 8 of the cycle (day of estrus = day 0 of the cycle), then with 100 μg 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 quantitate changes in serum luteinizing hormone. Blood was also collected on alternate days after an injection until day 16 of the cycle, to characterize changes in serum progesterone concentrations. Although exogenous GnRH caused release of LH on days 2 and 8 of the cycle, the quantity of LH released was greater on day 8 (P<.025). Serum levels of progesterone after treatment with GnRH on day 8 of the cycle did not differ significantly from those observed during the control cycles of the heifers. Because exposure of the bovine corpus luteum to excess LH, induced by GnRH early during the estrous cycle, causes attenuated progesterone secretion during the same cycle, these data suggest that a second surge of endogenous LH may ameliorate the suppressive effect of the initial release of LH on luteal function. Duration 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 iv with saline on days 2 and 8 of the estrous cycle. After an intervening cycle of normal duration, cows were injected iv with 100 μg GnRH on days 2 and 8 of the succeeding cycle. Corpora lutea were obtained per vaginam on day 10 of each cycle. Luteal tissue was sliced and incubated for 2 hr with saline (control) or LH (10 ng/ml). Treatment of cows with GnRH on days 2 and 8 caused a significant increase in in vitro basal progesterone production (μg/g) by luteal slices (control, 37.45 ± 2.93 vs. GnRH days 2 and 8, 53.18 ± 3.03) but suppressed the ability of luteal tissue to respond to LH (P<.07). Results of this latter experiment suggest that a second surge of GnRH-induced LH secretion on day 8 of the cycle promoted an increase in basal progesterone production but desensitized luteal cells to further stimulation by the gonadotropin.