Cardiac receptors for glucocorticoids in the rat: Factors involved in [ 3H]dexamethasone binding and nuclear translocation of [ 3H]dexamethasone-receptor complexes in heart and liver

Abstract

The conditions of (i) the binding of [ 3H]dexamethasone to cytoplasmic receptors and (ii) the interaction between [ 3H]dexamethasone-receptor complexes and the nucleus have been studied in heart of intact and adrenalectomized rats. This study was performed in parallel on heart and liver, the latter being take as a reference target organ. 1. (i) We hav confirmed the presence of high affinity glucocorticoid receptors in the rat heart. The concentration of cardiac receptors in intact animals (45 ± 12 fmol/mg protein) is much lower than that of liver receptors (600 ± 120 fmol/mg protein). Adrenalectomy induces an increas in receptor concentration in both organs without a change in affinity for [ 3H]dexamethasone. This increase, which is more marked in the heart (5.5-fold) than in liver (1.8-fold), can probably not be ascribed to a progressive liberation of receptors previously occupied by endogenous glucocorticoids nor to a decreased transfer of receptors to the nucleus of adrenalectomized animals. Thus adrenalectomy appears to induce a true increase in the number of receptors. 2. (ii) Heart nuclei bind both heart and liver [ 3H]dexamethasone-receptor complexes with the same efficiency, confirming the lack of organ specificity of nuclear binding for steroid hormone-receptor complexes. The apparent saturation of rat nuclei by glucocorticoid-receptor complexes is dut to a macromolecular inhibitor of nuclear binding present in heart cytosol. This factor interacts directly with activated complexes but not with nuclear receptor sites.