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SUMMARY
Plasma cortisol concentrations, measured by competitive protein-binding, were examined in intact and hypophysectomized goldfish (Carassius auratus L.) adapted to fresh water or to 210 mm-sodium chloride solutions. The mean plasma cortisol concentration of freshwater-adapted fish (6·6 ± 1·8 (s.e.m.) μg/100 ml plasma) increased after stress and intraperitoneal injections of mammalian corticotrophin. Hypophysectomy resulted in a reduction in plasma cortisol concentration to about 2 μg/100 ml plasma.
Transfer of fish to sodium chloride solutions caused rapid, but transitory increases in the plasma cortisol concentrations in intact, but not in hypophysectomized fish. After 3 days in the sodium chloride solution the cortisol levels were similar to those of control fish kept in fresh water. The plasma concentrations of this corticosteroid in goldfish appear to be unrelated to external salinity, although a 'mineralocorticoid' action of the hormone cannot be excluded.
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Cytosols prepared from the liver and various tissues of goldfish (intact or hypophysectomized) and trout (intact) were incubated at 2 °C in the presence of tritiated cortisol or dexamethasone (3 × 10−9 to 3 × 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 intravascular 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.
