Detection of corticosteroid type I binding sites in heart

Both high affinity (type I) and low affinity (type II) corticosteroid binding sites are detected in cytosolic extracts of atrial and ventricular tissue when [3H]aldosterone is used as the ligand. In the presence of RU-28362, which blocks binding of [3H]hormone to the low affinity type II sites, [3H]aldosterone binds to a single class of high affinity sites. The apparent Kd for binding of the hormone to the type I sites was 1.0 nM in atrial cytosol and 0.75 nM in ventricular cytosol. The concentration of type I sites in atrial (12 fmol/mg protein) and ventricular cytosols (11 fmol/mg protein) is comparable to reported values in renal (17-31 fmol/mg protein) cytosol. Activation of the type I hormone-receptor complexes to the DNA-binding form was examined using chromatography on DEAE-Sephadex anion-exchange resin. The unactivated hormone-receptor complex elutes at a concentration of 400 mM KCl. Following heat treatment (25 degrees C, 30 min) the [3H]aldosterone-receptor complex is transformed to a low salt eluting (200 mM KCl), DNA-binding form. Activation is blocked by inclusion of 10 mM sodium molybdate during heat treatment.