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Pituitary Responsiveness to Purified Luteinizing Hormone-Releasing Hormone (LH-RH) at Various Stages of the Estrous Cycle in Sheep¹

Endocrine and Polypeptide Laboratories, Veterans Administration Hospital and Tulane Medical School, New Orleans, Louisiana 70140

Abstract

Three mature, whitefaced ewes were used to study the responses to purified porcine LH-RH at various stages of the estrous cycle. The estrus activity of the ewes was checked twice daily with a vasectomized ram. Each ewe received 3 µg LH-RH dissolved in 5 ml saline intracarotidly. Venous blood samples were collected prior to and after administration of LH-RH. Serum LH levels in the samples were quantitated by a double antibody radioimmunoassay. The present experiments show that after administration of LH-RH an elevation of serum LH occurs at all stages of the estrous cycle. The same dose of LH-RH induced a greater change in serum LH during an 8± 4-hr. period on day 1 than at any other stage of the estrous cycle. Thus, present evidence indicates that pituitary responsiveness to LH-RH at estrus is considerably greater than at other stages of the estrous cycle, and may be an important factor in the preovulatory surge of LH.

¹ Supported in part by NIH Fellowship AM-43308-01 and USPHS grant AM-07467 and AM-09094.

² Holder of NIH Postdoctoral Fellowship.

³ The authors wish to thank Dr. D. N. Ward for purified ovine LH, Dr. G. D. Niswender for anti-ovine LH and the Endocrine Study Section of the National Institutes of Health for providing the NIH-LH-S-14 standard used in this study.

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