Binding of [3H]Dexamethasone by Rat Placenta*

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Abstract
[3H]Dexamethasone binding was measured in the basal zone trophoblast (BZT) and the labyrinthine zone (LZ) of placenta from Wistar rats on days 19–22 of pregnancy. Cytosol from both zones was incubated with [3H]dexamethasone for 4 h at 0–4 C, and high affinity, low capacity sites were measured by Scatchard plot analysis in BZT and LZ, tissues which demonstrated similar concentrations of sites and affinity constants. Specific binding was decreased by omitting mercaptoethanol from the incubation buffer, by adding N-ethylmaleimide, or by increasing the temperature to 16 or 37 C. Competition studies revealed that the rank order of inhibition of [3H]dexamethasone binding by nonradioactive steroids was dexamethasone » progesterone = corticosterone » testosterone = 17β-estradiol. Dexamethasone-binding sites could be distinguished from progestinbinding sites by competition analysis as well as by the absence of 17α,21-dimethyl-19-nor-pregna-4,9-diene-3,20-dione ([3H]-R5020) binding in LZ, whereas the corticoid was bound to both zones. Nuclei from BZT and LZ took up radioactive steroid when exposed to cytosol prelabeled with [3H]dexamethasone, but little radioactivity was present in nuclei incubated with buffer containing [3H]dexamethasone. After in vivo administration of [3H]dexamethasone to pregnant rats, radioactivity was recovered in nuclei and cytosol from both placental zones. Furthermore, whole homogenates or cytosol from BZT or LZ did not metabolize [3H]dexamethasone to any significant extent. These results suggest that rat placenta contains dexamethasonebinding sites which resemble the glucocorticoid receptors described in other tissues. (Endocrinology108: 1697, 1981)