Cytoplasmic and nuclear progestin receptors in human myometrium during the menstrual cycle and in pregnancy at term.

Exchange assays have been developed for the determination of the total progestin in receptor sites (unoccupied and occupied with endogenous hormone) in the cytosol and nuclei of human myometrium using radiolabeled R5020 (17,21-dimethyl-19-nor-4,9-pregnadiene-3,20-dione) as ligand. These assays have been applied to the measurement of cytosol and nuclear progestin receptor sites in the myometrium during the menstrual cycle and in pregnancy at term. Both the cytosol receptor levels as well as the sum of cytosol and nuclear receptor levels were highest during the proliferative phase of the cycle, dropped moderately but significantly in the secretory phase, and decreased markedly in pregnancy at term. Despite the marked drop in cytosol receptor levels, nuclear receptor levels in pregnancy at term were similar to those observed in the proliferative or secretory phase. In addition, the fraction of the total cellular receptor that was associated with the nucleus increased from 5.5% in the proliferative phase to 60% in pregnancy at term. The decrease in cytosol progestin receptor concentration as well as the relative increase in nuclear localization of the receptor in human myometrium in pregnancy at term may be related to the high circulating progesterone levels.