Intestinal triamcinolone acetonide receptors of the eel (Anguilla rostrata)

The binding of [6,7-3H]triamcinolone acetonide (TA) to intestinal mucosa of freshwater-adapted silver eels was studied. The cytoplasmic preparations bound the ligand with an equilibrium dissociation constant (KD) of 2.28 +/- 0.37 nM and the maximal number of binding sites (Nmax) was 960 +/- 55 fmol/mg of protein (+/- SE, n = 13). Scatchard analysis indicated the presence of a single species of binding sites. Binding was abolished following treatment of the cytosol with trypsin, N-ethylmaleimide, or Mersalyl, but DNase or RNase treatment had little effect. The competition hierarchy of radioinert steroids on the formation of the [3H]TA-receptor complex was TA greater than dexamethasone greater than cortisol greater than 11-deoxycortisol. Aldosterone, DOC, corticosterone, 11-dehydrocorticosterone, progesterone, testosterone, or estradiol-17 beta did not compete. Sedimentation of the [3H]TA-receptor complex on a linear sucrose gradient (10-30% + 10% v/v glycerol) yielded single peaks in the absence or presence of 0.4 M KCl in the gradient (6 S or 3.5 S respectively). Following heat activation the receptor-ligand complex was freely translocated to homologous nuclei in vitro, though the activated complex did not bind to DNA-cellulose. It was concluded that the eel intestinal mucosal cytosol contains a high-affinity-low capacity steroid receptor system. This is the first instance that such a system was demonstrated in fish tissue.