Binding of Steroids to Mineralocorticoid Receptors: Implications for in Vivo Occupancy by Glucocorticoids*

The binding of a number of steroids to mineralocorticoidreceptors in rat kidney was studied by measuring theirability to inhibit competitively the binding of [3H]aldosterone.The calculated activities relative to cortisol (taken as 1) andthe equilibrium dissociation constants (K4 values), respectively,were: aldosterone, 51 and 0.86 nM; 11-deoxycorticosterone(DOC), 40 and 1.07 nM; 9α-iluorocortisol, 40.5 and1.03 nM; progesterone, 5.4 and 8.1 nM; β-methasone (9α-fluoro-11β,17α,21-trihydroxy-16β-methyl-pregna-l,4-diene,3,20-dione),4.5 and 9.8 nM; corticosterone, 3.8 and 11.5 nM; prednisolone(llβ,17α,21-trmydroxypregna-l,4-diene-3,20-dione), 1.8 and24.8 nM; dexamethasone (9α-fluoro-llβ,17α,21-trihydroxy-16amethyl-pregna-l,4-diene-3,20-dione), 1.8 and 23.9 nM; and cortisol,1 and 43.8 nM. For comparison, the Kd values for the bindingof some of these steroids to the renal glucocorticoid receptors(obtained by measuring their ability to inhibit [3H]dexamethasonebinding) were: aldosterone, 92 nM; DOC, 77 nM; cortiso...