No abstract

In order to better understand the role of glucocorticoid receptor in the hormonal action of glucocorticoid, depletion and replenishment of the cytoplasmic receptor and kinetics of induction of tyrosine aminotransferase (TAT) in rat liver were examined after administration of dexamethasone (Dex) and prednisolone. The extent of the receptor depletion and the duration of the depletion period were dose-dependent and were well correlated not only with the biological potency of the steroids administered but also with the kinetics of TAT induction. A linear relationship between the amount of depleted receptor and the kinetics of TAT induction was observed. Pretreatment of animals with a large dose of Dex reduced the response of TAT induction by Dex which was administered 24 h later. This seems to be attributable, at least in part, to the incomplete replenishment of the receptor and also to the increased rate of replenishment of the receptor. The reduced affinity of the partially replenished receptor to glucocorticoid might also participate in lowering the response to the second injection. In conclusion, depletion and replenishment of the cytoplasmic glucocorticoid receptor appeared to be directly responsible for the physiological action of glucocorticoid.

No abstract

Administration of steroid hormones evokes a rapid fall in the amount of cytoplasmic receptors in target tissues. This has been considered to be a consequence of the receptor translocation to nuclei, but the physiological significance of depletion of the cytoplasmic receptors after hormone administration has not been fully elucidated. In the present study, depletion and replenishment of the cytoplasmic glucocorticoid receptor in rat tissues were examined after the administration of glucocorticoids of different biological potencies. Dose dependent depletion was observed in all tissues examined and doses required for complete depletion of the receptor were correlated with biological potencies of steroid administered. The receptor in the heart and the skeletal muscle was relatively sensitive to hormone injection while more than 4 times the amount of steroids was required to induce a similar effect in the thymus and the spleen. The duration of the period of depletion of the receptor in cytosols was also dose dependent and correlated to the biological potency of steroid administered. Replenishment took place earlier in the thymus and the spleen than in the heart and the skeletal muscle. Significantly lower binding affinity was observed in the replenished receptors. The administration of cycloheximide in a dose which inhibits more than 95% of 3H-leucine incorporation did not influence either the depletion or the replenishment of the receptor induced by hormone injection. In conclusion, depletion and replenishment of the cytoplasmic glucocorticoid receptor appeared to be closely correlated to the physiological action of hormones.

3H-Dexamethasone binding sites with a Kd of approximately 0.7 nM and a maximum number of binding sites of approximately 0.3 pmoles/mg protein were demonstrated in the uterine cytosol of adrenalectomized rats only if dithiothreitol was present in the incubation mixture and the simultaneous presence of molybdate further enhanced the binding in the cytosol. The binding sites exhibited a high specificity for glucocorticoids and were depleted in a dose-dependent manner from cytosol after administration of dexamethasone to animals. The depletion was not due to the occupation of the binding sites by the dexamethasone administered and the rate of depletion was correlated with the inhibition of uterine growth induced by estrogen administration. The cytosol labeled with 3H-dexamethasone in the presence of dithiothreitol bound to DNA-cellulose efficiently after heating at 25 degrees C for 30 min and the binding was inhibited by pyridoxal 5'-phosphate added to the reaction mixture. The effect of heating on the DNA-cellulose binding was abolished by molybdate in the incubation mixture. From these observations, it was concluded that 3H-dexamethasone binding sites in the rat uterus were physiologically active glucocorticoid receptors.