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

High-affinity, low-capacity estradiol-17 beta (E-2) binding is present in r ainbow trout scale. The K-d and B-max of the scale E-2 binding are similar to those of the liver E-2 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 x mg protein(-1), for scale and liver, respectively), but different from those of the high-affinity, lo w-capacity E-2 binding in plasma (K-d is 4.0 +/- 0.4 nM and B-max is 625.4 +/- 63.1 fmol x mg protein(-1)). The E-2 binding in scale was displaced by testosterone, but not by diethylstilbestrol. Hence, the ligand binding spec ificity is different from that of the previously characterized liver E-2 re ceptor, where E-2 is displaced by diethylstilbestrol, but not by testostero ne. The putative scale E-2 receptor thus appears to bind both E-2 and testo sterone, and it is proposed that the increased scale resorption observed du ring sexual maturation in both sexes of several salmonid species may be med iated by this receptor. No high-affinity, low-capacity E-2 binding could be detected in rainbow trout gill or skin. (C) 2000 Academic Press.