Binding of dexamethasone to rat liver nuclei in vivo and in vitro: Evidence for two distinct binding sites

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 (K_d) of approximately 10^-7 M for [³H]DEX at 4°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 × 10⁴ [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, ~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.