Unoccupied and Occupied Estrogen Receptors in Myometrial Cytosol and Nuclei from Nonpregnant and Pregnant Women*

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Abstract
Exchange assays were developed and used to measure the total concentration of estrogen receptor sites (unoccupied and occupied with endogenous hormone) in the cytosol and nuclei of human myometrium during the menstrual cycle and in pregnancy at term, before and during labor. A major fraction of the nuclear receptors appear to be unoccupied (not complexed with estrogen). These sites were detected in both crude and highly purified myometrial nuclei, indicating that they are located in the nucleus and do not represent a cytoplasmic or other cellular contaminant. The distribution of estrogen receptors between the cytosol and the nucleus varied in pregnant and nonpregnant patients. In pregnant patients, the receptor was detected only in the nucleus, while in nonpregnant patients most of the receptor was found in the cytosol. The levels of total receptor were considerably lower (5 times) in pregnancy at term than during the menstrual cycle. No significant differences in cytosol receptor levels or in unoccupied nuclear receptor levels were observed between the proliferative and secretory phases of the cycle. The levels of occupied nuclear receptors were significantly higher in the proliferative than in the secretory phase. The levels of total and occupied nuclear receptors in pregnancy at term were similar to those found during the proliferative phase and appeared to decrease somewhat during labor, although the differences were not statistically significant.