| Abstract: | The binding of 3H]dexamethasone (DEX) to rat liver nuclei in vitro and in vivo have been compared. In vitro, purified nuclei displayed a single class of specific glucocorticoid binding sites with a dissociation constant (Kd) of approximately 10(-7) M for 3H]DEX at 4 degrees C. The glucocorticoid agonists prednisolone, cortisol, and corticosterone and the antagonists progesterone and cortexolone competed avidly for this site, but the potent glucocorticoid triamcinolone acetonide (TA) competed poorly in vitro. Nuclei isolated from the livers of intact rats contained 1-2 X 10(4) 3H]DEX binding sites/nucleus. Up to 85% of the binding sites were recovered in the nuclear envelope (NE) fraction when NE were prepared either before or after labeling with 3H]DEX in vitro. After adrenalectomy, the specific 3H]DEX binding capacity of both nuclei and NE decreased to 15-20% of control values, indicating sensitivity of the binding sites to hormonal status of the animals. Efforts to restore the binding capacity by administration of exogenous glucocorticoids, however, were unsuccessful. After labeling of rat liver nuclei in vivo by intraperitoneal injection of 3H]DEX or 3H]TA into living animals, the steroid specificity and subnuclear localization of radiolabel were different. Both 3H]TA (which did not bind in vitro) and 3H]DEX became localized to nuclei in a saturable fashion in vivo. With either of these ligands, approximately 20% of the total nuclear radiolabel was recovered in the NE fraction. These results suggest the presence of two separate and distinct binding sites in rat liver nuclei, one which is localized to the NE and binds 3H]DEX (but not 3H]TA) in vitro, and another which is not localized to the NE but binds 3H]DEX and 3H]TA in vivo. |
