Ex vivo binding to glucocorticoid receptors in the thymus of the adrenalectomized rat.

The interaction of glucocorticoids with thymic cytosol receptors in the adrenalectomized rat was studied by a method based on their capacity to deplete cytosol receptors when administered in vivo. Three h after a single oral administration of reference steroids at various dose levels, thymus cytosol aliquots were incubated with a saturating concentration of [3H]dexamethasone (30 nM) for 24 h at 0-2 degrees C with and without 2000 nM unlabeled dexamethasone. Bound radioactivity was determined by dextran-coated charcoal adsorption. The depletion of cytosol receptors was dose-dependent for each glucocorticoid, with the following ED50 (mg/kg): fluocinolone acetonide 0.032, dexamethasone 0.09, triamcinolone acetonide 0.12, betamethasone 0.24, deflazacort 0.55, triamcinolone 1.6, prednisolone 4.0, hydrocortisone 17.0. A striking correlation (r 0.991) between ex vivo receptor binding and thymolytic effect was observed. When data from in vitro competition studies were compared with those obtained in ex vivo experiments, the latter were correlated more tightly with the biological response.