Uptake and binding of glucocorticoids in fish tissues

Cytosols prepared from the liver and various tissues of goldfish (intact or hypophysectomized) and trout (intact) were incubated at 2 degrees C in the presence of tritiated cortisol or dexamethasone (3 x 10(-9) to 3 x 10(-6) mol/l) with or without a 1000-fold excess of unlabelled steroid. In contrast to mammals, the specifically bound component represented a very low fraction of the total bound steroid retained on DEAE cellulose filters and did not show saturation over a large range of concentrations. The subcellular distribution of [3H]dexamethasone was studied in trout liver after intra-vascular injection of the labelled steroid with and without an excess of unlabelled steroid. The amount of protein-bound steroid in the cytosol again corresponded to a small (4%) proportion of the free steroid. The large reduction in the uptake of tritiated dexamethasone, which was induced in both the cytosol and nuclei by competing unlabelled dexamethasone, was interpreted as evidence for mediated entry across cellular and nuclear membranes. These results indicate that high-affinity binding sites are absent, or present only in very small numbers in cytosol from teleost tissues. The entry of glucocorticoids into the nucleus may not require the hormone to be bound to high-affinity cytosolic receptors unless the binding, though quantitatively small, displays a high rate of turnover.