HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Beck, T. W. Articles by Convey, E. M. Search for Related Content PubMed PubMed Citation Articles by Beck, T. W. Articles by Convey, E. M.

Estradiol Control of Serum Luteinizing Hormone Concentrations in the Bovine¹

Michigan State University³, East Lansing 48824

Abstract

Yearling Holstein heifers ovariectomized 1.5 months previously were untreated (n=5) or given estradiol-17ß (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 (P<01) at 48 hr before E₂ 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.

¹ Published with the approval of the Michigan Agricultural Experiment Station as Journal Article No. 7789.

² Present address: Animal Science Department, Oklahoma State University, Stillwater 74074.

³ Animal Reproduction Laboratory, Department of Dairy Science.