SPECIFIC BINDING-SITES FOR CORTICOSTERONE IN ISOLATED CELLS AND PLASMA-MEMBRANE FROM RAT-LIVER

The specific binding of [H-3]corticosterone to hepatocytes is a nonsaturable, reversible and temperature-dependent process. The binding to liver purified plasma membrane fraction is also specific, reversible and temperature dependent but it is saturable. Two types of independent and equivalent binding sites have been determined from hepatocytes. One of them has high affinity and low binding capacity (K(D) = 8.8 nM and B(max) = 1477 fmol/mg protein) and the other one has low affinity and high binding capacity (K(D) = 91 nM and B(max) = 9015 fmol/mg). In plasma membrane only one type of binding site has been characterized (K(D) = 11.2 nM and B(max) = 1982 fmol/mg). As it can be deduced from displacement data obtained in hepatocytes and plasma membrane the high affinity binding sites are different from the glucocorticoid, progesterone nuclear receptors and the Na+, K+-ATPase digitalis receptor. Probably it is of the same nature that the one determinate for [H-3]cortisol and [H-3]corticosterone in mouse liver plasma membrane. Beta- and alpha-adrenergic antagonists as propranolol and phentolamine did not affect [H-3]corticosterone binding to hepatocytes and plasma membranes; therefore, these binding sites are independent of adrenergic receptors. The binding sites in hepatocytes and plasma membranes are not exclusive for corticosterone but other steroids are also bound with very different affinities.