Distinct specificity for corticosteroid binding sites in amphibian cytosol, neuronal membranes, and plasma

To address mechanisms of corticosteroid action, one needs tools for distinguishing between the major classes of corticosteroid binding sites: neuronal membrane-associated receptors, intracellular ligand-activated transcription factors, and corticosteroid binding globulins (CBG) in plasma. We characterized the binding parameters for three classes of binding sites in an amphibian, Ambystoma tigrinum, and found that each class had a distinct pharmacological specificity. Equilibrium saturation and kinetic experiments indicated that [3H]corticosterone binds to neuronal membranes with high affinity (Kd approximately 0.37 nM). Aldosterone and two synthetic ligands for mammalian intracellular receptors, dexamethasone and RU486, displayed low affinity for brain membrane sites. In cytosol prepared from brain and liver, [3H]corticosterone bound to a single class of receptors with high affinity (Kd approximately 0.75 and 4.69 nM, respectively) and the rank order potencies for steroid inhibition of [3H]corticosterone binding was RU486 > dexamethasone approximately = corticosterone > aldosterone. In kidney and skin cytosol, [3H]corticosterone binding was best fit with a model having a high-affinity and a lower-affinity site; these sites are not consistent with the pharmacology of mammalian Type I (MR) and Type II (GR) receptors. [3H]Corticosterone also bound to presumed CBG in plasma with high affinity (Kd approximately 2.7 nM), but dexamethasone and androgens bound to plasma CBG with equivalently high affinity. These data demonstrate that pharmacological specificity can be a useful tool for distinguishing corticosteroid binding to different classes of binding sites. These data also indicate that there may be marked species differences in the specificity of corticosteroid binding sites.