Interaction of Progesterone, GnRH and Estradiol in the Control of LH Release in Castrate Heifers1

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Abstract
Thirteen ovariectomized heifers were randomly divided into two groups for each of three experiments. In the first experiment, 10 mg estradiol-17/β (E) was injected intramuscularly into all heifers. Seven heifers also received intramuscularly (im) 25 mg progesterone (P) at -12, 0, +12 and +24 hr relative to E injection. Estradiol had a biphasic effect on serum luteinizing hormone (LH) measured at various intervals. Serum LH was significantly (P<.01) depressed at 12 hr but preovulatory-like peaks of LH were noted between 20 and 32 hr after E. Neither the inhibitory feedback of E at 12 hr nor the stimulatory feedback at about 24 hr was significantly affected by P treatment. In the second experiment, 40 µg gonado-tropin-releasing hormone (GnRH) was injected im into all 13 heifers. Seven heifers also received 25 mg P at -28, -16 and -4 hr relative to GnRH. Peaks of serum LH were observed 1 to 3 hr post-injection in five of six heifers receiving only GnRH. Only three of seven heifers presented with P exhibited LH peaks after GnRH and their peaks tended to be smaller. Average serum LH for all heifers was decreased for at least 3 hr post-GnRH by progesterone pretreatment. In the third experiment, the conditions of experiment 2 were duplicated except that all heifers received E injections 16 hr before GnRH. In contrast to the results of experiment 2, P no longer decreased the GnRH-induced LH release. In conclusion, exogenous P did not antagonize the inhibitory nor the subsequent stimulatory feedback of injected E on LH release (experiment 1). Exogenous P did inhibit GnRH-induced release of LH in ovariectomized heifers in the absence of E (experiment 2) but not in the presence of exogenous E (experiment 3). Copyright © 1976. American Society of Animal Science . Copyright 1976 by American Society of Animal Science.

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