Characterization of specific corticosterone binding sites in adrenal cortex plasma membrane and their localization by autoradiographic studies

Specific corticosterone binding to calf adrenal cortex plasma membrane was measured using the biologically active radioligand [H-3]corticosterone. Corticosterone binding was found to be time-dependent, saturable and reversible, and was reduced by more than 70% when membranes were pretreated with proteases. The population of corticosterone binding sites in calf adrenal cortex plasma membrane was homogeneous and displayed the following characteristics: equilibrium dissociation constant K-d = 77 +/- 8 nM and maximum specific binding capacity B-max = 70,378 +/- 6,385 fmol/mg protein. The relative affinities of several structural analogues of steroids were deduced from competition assays. From these experiments we can conclude that the plasma membrane binding site characterized is selective for corticosterone and progesterone derivatives, and different from nuclear glucocorticoid, mineralocorticoid, estrogen and progestin receptors. Likewise, this corticosterone binding site is independent of mineralocorticoid and Na+, K+-ATPase digitalis receptors. From autoradiographic studies we suggest these corticosterone binding sites are located in the whole adrenal cortex.