Decreased concentrations and affinities of oestrogen and progesterone receptors of intrauterine tissue in human pregnancy.

Cytoplasmic and nuclear receptors for oestrogen and progesterone were measured in non-pregnant myometrium and endometrium and compared to concentrations found in decidua of ectopic pregnancy (6-8 weeks gestation) and therapeutic abortions (8-16 weeks). Amnion, chorion, placenta, decidua and myometrium at full term pregnancy were also assayed for the same receptors. High affinity binding was confirmed in the non-pregnant tissue; in early pregnancy, decreases in concentrations of cytoplasmic receptors were demonstrated, these decreases becoming more marked as pregnancy progressed in the 1st trimester. Nuclear receptor concentrations were not significantly different. Significant decreases in the occurrence of positive receptors with the progression of pregnancy were also demonstrated for cytoplasmic and nuclear oestrogen and nuclear progesterone receptors. Tissue at full term pregnancy had no detectable receptors, irrespective of whether the patients were in labour or not. Increasing the range of the labelled steroids failed to demonstrate any low affinity binding sites and pre-assay removal of endogenous hormones also had no effect on receptor status. When endogenous hormones were removed, displaceable binding was demonstrated in the presence of excess unlabelled ligand. However, this binding did not conform with receptor dynamics on Scatchard analysis. Heating the cytosol prior to assay or failure to remove endogenous steroid hormones eliminated this binding. Cytosolic oestrogen and progesterone levels increased significantly in the decidua of therapeutic abortions, whilst term pregnant tissue had the highest concentration of endogenous hormones.