High Affinity (Type 1) Aldosterone-Binding Sites in Rat Epididymis

The effect of adrenal steroids on epididymal ion fluxes has previously been shown by ablation/replacement studies. In the present study, we have demonstrated that in the presence of excess RU26988 [3H]aldosterone binds to a single class of sites in epididymal cytosol, with high affinity [dissociation constant (Kd)22 C, 1 nM; range 0.6-1.3 nM] and limited capacity (Bmax 12 +/- 3 fmol/mg protein, mean +/- SE). The specificity of these epididymal sites is identical with that of type 1 sites in other tissues: aldosterone equals corticosterone more than dexamethasone more than testosterone more than dihydrotestosterone. Sixteen hours after 6 micrograms estradiol benzoate im to suppress gonadotropin and testosterone production, available type 1 sites in epididymal, but not renal, cytosols were significantly increased. We interpret these studies as showing 1) that adrenal steroids may modulate epididymal ion fluxes via type 1 receptors; 2) that whether corticosterone and/or aldosterone modulates these fluxes is yet to be determined; and 3) that despite its modest affinity for type 1 sites, testosterone may occupy a proportion of such sites in the epididymis, reflecting its very high local tissue concentration.