Further Studies on the Cytoplasmic 3H-Dexamethasone Receptor in the Liver from Adult and Fetal Rats

In a previous experiment, differences in the binding affinity to dexamethasone (DEX) of the cytoplasmic receptor in the fetal and adult rats were observed. Therefore, the binding characteristics of cytoplasmic receptor, to DEX in livers of adult and fetal rats were further examined to throw insight into the mechanism of agerelated changes in responsiveness to glucocorticoids. Cytoplasmic DEX-receptor complexes in both adult and fetal livers were mainly precipitated with 35% saturation of (NH4) 2SO4 and eluted immediately after a void volume from a Sephadex G-100 column. When the 35% ammonium sulfate fraction was applied to DEAE chromatography, two peaks of the radioactivity bound to protein, eluted with 50mM and 100 mM KCl, were observed in the adult liver. On the other hand, only one peak which was eluted with 50mM KCl was noticed in the fetal liver. Stability of the DEX-receptor complex during incubation at 0 or 23 °C was not significantly different in these two tissues. At least two DEX-binding components which exhibited RBPB 0.28 and RBPB 0.43 in polyacrylamide gel electrophoresis were demonstrated in the cytoplasm from the adult liver, while one clear peak with RBPB 0.25 was observed in thefetal liver. The binding in the adult cytosol to DEX was efficiently inhibited by addition of corticosterone and cortisol and the similar inhibition by these two steroids was observed in the binding of fetal cytosol but in the latter tissue, deoxycorticosterone and progesterone were also competed with DEX binding moderately. Two forms of DEX-receptor complex-one binds only to nuclei and the other binds to DNA and nuclei-were observed in the fetal liver cytosol as well as in the adult one. The binding capacity of these two forms of receptor was almost comparable in cytosols of two tissues examined. The nuclear binding of 3H-DEX in the isolated liver cells was also different in fetal and adult animals.