The presence of high-affinity, low-capacity estradiol-17β binding in rainbow trout scale indicates a possible endocrine route for the regulation of scale resorption

High-affinity, low-capacity estradiol-17β (E₂) binding is present in rainbow trout scale. The K_d and B_max of the scale E₂ binding are similar to those of the liver E₂ receptor (K_d is 1.6 ± 0.1 and 1.4 ± 0.1 nM, and B_max is 9.1 ± 1.2 and 23.1 ± 2.2 fmol × mg protein^-1, for scale and liver, respectively), but different from those of the high-affinity, low-capacity E₂ binding in plasma (K_d is 4.0 ± 0.4 nM and B_max is 625.4 ± 63.1 fmol × mg protein⁻¹). The E₂ binding in scale was displaced by testosterone, but not by diethylstilbestrol. Hence, the ligand binding specificity is different from that of the previously characterized liver E₂ receptor, where E₂ is displaced by diethylstilbestrol, but not by testosterone. The putative scale E₂ receptor thus appears to bind both E₂ and testosterone, and it is proposed that the increased scale resorption observed during sexual maturation in both sexes of several salmonid species may be mediated by this receptor. No high-affinity, low-capacity E₂ binding could be detected in rainbow trout gill or skin.