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Influence of Gonadotropic Hormones on Estradiol-Induced Corpus Luteum Regression in the Ewe¹

U. S. Department of Agriculture, Beltsville, Maryland 20705²

Abstract

The injection of estradiol into the ewe after the middle of the estrous cycle resulted in early regression of corpora lutea (CL) (Hawk and Bolt, 1970). In contrast, estradiol administered in the early part of the estrous cycle caused extended luteal maintenance (Piper and Foote, 1968). Bolt, Kelley and Hawk (1970) noted that the injection of estradiol on day 3 caused the release of endogenous LH, as indicated by an increase in plasma LH concentrations similar to the LH peak observed at estrus (Niswender et al., 1968).

This study was done to determine whether the luteal regression resulting from an injection of estradiol on day 10 could be prevented by administration of (1) exogenous gonadotropic hormones on day 3, (2) estradiol on day 3 when it will cause endogenous gonadotropin release, or (3) gonadotropic hormones on day 10 at the time the estradiol was administered.

Eighty-four parous Rambouillet ewes of about 8 years of age and weighing approximately 55 kg were checked for estrus twice daily using vasectomized rams.

¹ Appreciation is expressed to Dr. G. D. Niswender for the LH antiserum, Dr. L. E. Reichert, Jr. for the purified LH preparation, Mr. W. B. Welch, Jr. of Ayerst Laboratories for the APL, to the Endocrine Study Section for NIH-LH-S12 reference standard, to Dr. B. Cook for furnishing the computer program used to calculate the assay results and to Dr. L. Miller for making several modifications in the program.

² Animal Science Research Division, A.R.S. This study was supported in part by a reimbursable agreement with the National Institute of Child Health and Human Development. National Institutes of Health, Bethesda, Maryland.