Glucocorticoid-binding proteins of rat heart cytosol and their possible role in the transfer of glucocorticoids into nuclei

Binding of [³H]glucocorticoids by cytosol and nuclei isolated from rat heart has been studied. Two types of glucocorticoid-binding proteins (binder I and binder II) were found in the heart cytosol of adrenalectomized rats. Binder I is similar in its properties to serum transcortin. It does not bind dexamethasone, has fast kinetics of corticosterone binding, is insensitive to SH-blockers, thermostable (not inactivated at short exposure to 50°C), and precipitates in the range of 40 to 60% saturation by ammonium sulphate. Binder II corresponds in its properties to dexamethasone binding proteins of typical target tissues for glucocorticoids. It is characterized by relatively slow kinetics of dexamethasone binding, is sensitive to SH-blockers, highly thermolabile (inactivated completely at short exposure to 37°C when not bound to steroids) and precipitates at 0 to 25% saturation with ammonium sulphate. Corticosterone was found to exert strong competitive inhibitory effect on the binding of [³H]dexamethasone by binder II which demonstrates approximately equal affinity of this protein to both steroids. Nuclei isolated from hearts of normal rats are not able to accumulate [³H]dexamethasone and [³H]corticosterone in the absence of cytosol at 4°C and 20°C. [³H]Corticosterone in the complex with binder I, in the absence of binder II, is not accumulated by nuclei. Nuclei are able to accumulate [³H]dexamethasone, bound to binder II at 4°C. Accumulation is increased by 4-fold after 20 min incubation at 20°C. An increase in temperature of higher than 20°C or prolongation of incubation over 20 min decreases accumulation of protein-bound dexamethasone by nuclei. Our data demonstrate the role of binder II as a heart cytoreceptor which might participate directly in the transfer of glucocorticoids into cell nuclei.