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Effect of Pretreating Heifers with Human Chorionic Gonadotropin and Estradiol on Subsequent In Vitro Luteal Tissue Progesterone Biosynthesis^{1, 2,}

Cornell University,⁵ Ithaca, New York 14850

Abstract

The use of in vivo treatments of HCG, estradiol or HCG plus estradiol was investigated as a method to provide large amounts of LH-sensitive luteal tissue for more extensive in vitro studies. HCG (total dose 11,000 IU) treatments on days 1 to 7 of the estrous cycle produced tissue that was sensitive to LH added in vitro, but day 11 corpora lutea were only slightly larger than normal. Total doses of 13,000 and 15,000 IU HCG increased the weight of the corpora by factors of two and three, but reduced, or abolished the LH response in vitro. Treatment with 5,000 IU HCG on either day 8 or 9 did not substantially increase the weight of the corpora. Basal day 11 in vitro progesterone synthesis was high, probably due to the recent in vivo luteotrophic treatment. However, the tissue was not capable of further stimulation with LH in vitro. Pretreatment with HCG plus estradiol in two experiments resulted in tissues with poor in vitro LH sensitivity. Pretreatment with 3 mg estradiol at 4, 6.5 or 12.5 hr. before removal of the corpus on day 11 also reduced the sensitivity of the tissue to LH in vitro.

These data support the theory that in vivo luteotrophic treatment may deplete a preformed precursor of progesterone, which reduces or abolishes LH sensitivity in vitro. However, results obtained when estradiol was given at levels and times used in these experiments did not support the idea that estradiol reduces endogenous gonadotropins.

¹ Supported in part by funds provided under the Regional Research Project, NE-41 entitled. "Endocrine Factors Affecting Reproduction in the Bovine Female".

² The authors wish to express thanks for assistance during the research to Raymond Saatman, Carol Hall and Timothy Dennis.

³ Supported by NIH Pre-doctoral Fellowship, 4-F1-GM-25, 746-02.

⁴ Present address: Department of Animal and Range Sciences, Montana State University, Bozeman 59715.

⁵ Department of Animal Science and Division of Biological Sciences.