The effect of stress and exogenous cortisol on receptor-like binding of cortisol in the liver of rainbow trout, Oncorhynchus mykiss

Cortisol binding has been identified in cytosols prepared from rainbow trout liver. Binding is of high affinity (kD = 5.1 ± 0.2 nM, n = 23) low capacity (Nmax = 197 ± 12 fmol/mg protein, n = 23), and high specificity, only dexamethasone, cortisol, and Ru38486 being efficient in displacing bound [3H]cortisol. Binding is not due to contamination by blood because plasma displayed no affinity for cortisol under the assay regime employed here and, although whole blood cytosol does specifically bind cortisol, the degree of contamination is demonstrably too low to account for the levels of binding detected in liver cytosol. No specific binding of [3H]cortisol could be detected in liver nuclear extracts, although the simultaneous assay for nuclear estradiol-binding sites was positive. Rainbow trout stressed by confinement displayed a significant reduction in cytosolic [3H]cortisol-binding capacity (with no concomitant detectable appearance of binding within nuclear extracts), 96-hr confinement eliciting a 40% depression in binding capacity relative to unstressed fish. The administration of cortisol via intraperitoneal implants also reduced, significantly, the number of hepatic-binding sites. The results are discussed with reference to anomalies in reported characteristics of teleost glucocorticoid receptors and the phenomenon of downregulation observed in some mammalian systems.