Estradiol Control of Serum Luteinizing Hormone Concentrations in the Bovine

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Abstract
Yearling Holstein heifers ovariectomized 1.5 months previously were untreated (n=5) or given estradiol-17β (E2) via silastic implants (n=5). Serum luteinizing hormone (LH) concentrations did not change in untreated heifers. However, in heifers given E2, serum LH decreased from ∼6 ng/ml to 2 ng/ml or less between 2 and 6 hr and then increased to greater than 50 ng/ml between 14 and 22 hr post-treatment. Average serum LH concentration was maximal (56 ng/ml) at 18.5 hr after estradiol. In a second experiment, heifers ovariectomized 6 months previously were each given 40 μg of gonadotropin releasing hormone (GnRH) intramuscularly. Two days later each heifer was given E2 and assigned randomly to one of two treatment groups. One group was given 40 μg of GnRH 6 hr after E2 and the other, 40 μg of GnRH 12 hr after estradiol. Serum LH increased to between 40 and 90 ng/ml after each GnRH injection. Magnitude of LH release by GnRH was less (P2 than at 6 hr after estradiol. The release of LH in response to GnRH 2 hr after E2 was greater (P<.01) than the release of LH in response to GnRH 6 hr after estradiol. In all cases, increased release of LH following administration of GnRH after E2 was due to an increased duration of elevated serum concentration rather than an increase in maximum serum LH concentration. In addition, heifers given GnRH 6 hr after E2 had a second increase in serum LH concentration (P<.01) between 17 and 22 hr after estradiol. There was no change in serum LH concentration 17 to 22 hr after E2 in heifers given GnRH 12 hr after estradiol. These results suggest that the positive feedback effects of E2 on LH secretion are at least partially due to a progressive increase in pituitary responsiveness to GnRH. This may be achieved by increasing GnRH receptor numbers, GnRH receptor affinity and/or availability of pituitary LH stores. Gonadotropin releasing hormone eliminates the estrogen-induced LH surge presumably by decreasing GnRH receptor availability and/or releasable LH stores in the pituitary. Copyright © 1977. American Society of Animal Science . Copyright 1977 by American Society of Animal Science.

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