Characteristics of aldosterone binding by epidermal cytosol in the tail of the larval bullfrog were investigated. Temperature-dependent binding experiments showed that the specific binding of [3H]aldosterone to the tail cytosol was thermolabile. A 3-hr incubation at 0° was the optimum assay condition required for reaching equilibrium. Separation of bound and free hormone was performed using a hydroxylapatite method. Specific binding of aldosterone was observed in the tail epidermis but not in the tail mesenchyme. Saturation analysis revealed that specific binding of [3H]aldosterone to the epidermal cytosol reached maximum between 20 and 40 nM. Scatchard analysis for the cytosol of the tall epidermis yielded a straight line with a dissociation constant (Kd) of 7.0 nM and the maximum number of binding sites (Bmax) of 56.4 fmol/mg protein. Sucrose density gradient analysis of crude cytosol revealed a specific peak of radioactivity in the 8-9 S area. Steroid-binding specificity revealed a significant displacement of the [3H]aldosterone by radioinert aldosterone and, to a lesser extent, by cortisol, whereas 17β-estradiol and testosterone competed very poorly. However, corticosterone and dexamethasone, glucocorticoids in mammals, were better competitors of the aldosterone binding, whereas ZK91587, a selective synthetic antagonist for mineralocorticoid receptor in mammals, scarcely inhibited the aldosterone binding. These results suggest that the aldosterone-binding site may also be a corticosterone receptor and that there may be no other receptors specific for aldosterone at least in the tail epidermis.
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