| Abstract: | The use of in vivo treatments of human chorionic gonadotropin (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) treatments to heifers on Days 1 to 7 of the estrous cycle produced tissue that was senitive to LH added in vitro, but Day 11 corpora lutea of the heifers 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 2 and 3, but reduced or abolished the LH response in vitro. Treatment with 5000 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 2 experiments resulted in tissues with poor in vitro LH sensitivity. Pretreatment with 3 mg estradiol at 4, 6.5, or 12.5 hours 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 LG 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. |
