17β-Estradiol Receptor and Progesterone and 20α-Hydroxy-4-Pregnen-3-one Content of the Developing Corpus Luteum of the Rabbit

Abstract

Rabbits were rendered pseudopregnant with a single injection of hCG [human chorionic gonadotropin] (Day 0) and then sacrificed at 18 h through 8 days after injection. The corpora lutea were dissected free from adhering tissue, weighed and homogenized. After an aliquot was taken for radioimmunoassay of progesterone and 20.alpha.-hydroxy-4-pregnen-3-one (20.alpha.-dihydroprogesterone) content, a high speed supernatant (cytosol) was prepared centrifically for quantification of cytoplasmic 17.beta.-estradiol binding. The luteal 17.beta.-estradiol binding proved to be specific to estrogens and was of high affinity and low capacity. Cytosol preparations from endometrium, ovarian interstitium and corpus luteum contained significant estrogen binding activity while other tissues including spleen, kidney, liver and muscle had only low activity. When cytoplasmic estrogen receptor was measured in corpora lutea obtained daily over the first 8 days of pseudopregnancy, activity was found to be low on Days 1-3, increased abruptly between Days 3 and 4, and thereafter remained elevated. Estrogen receptor concentrations averaged approximately 100 fmol [femtomoles] 17.beta.-estradiol bound/mg cytosol protein on Days 4-8. The luteal weight and content of progesterone and 20.alpha.-dihydroprogesterone increased through Day 8 although when expressed as steroid concentration, the progestins in the corpora lutea remained unchanged over the 8 days of tissue collection. Using the weight and the steroid content as a measure of luteal function, it is concluded that the rabbit corpus luteum grows and functions continuously through the first 8 days of pseudopregnancy despite marked alterations in cytoplasmic 17.beta.-estradiol receptors over this interval.