High-affinity Cortisol receptor activity in the liver of the brook trout, Salvelinus fontinalis (Mitchill)

In vitro binding of [3H]cortisol to brook trout liver cytosol and salt-soluble nuclear extracts demonstrated a high affinity (cytosol association constant, Ka = (0.18 ± 0.03) × 109M−1, n = 4; nuclear extract association constant, Ka = (0.033 ± 0.01) × 109M−1, n = 4) and low maximum binding capacity (cytosol, Nmax = 167 ± 18 fmol∙mg−1 protein, n = 4; nuclear extract, Nmax = 858 ± 81 fmol∙mg−1 protein, n = 4). Kinetics of binding of [3H]cortisol to cytosol at 2 °C revealed association and dissociation rate constants of 6.95 × 104∙M−1∙s−1 and 2.38 × 10−4 s−1, respectively. Gel filtration chromatography using Sephacryl S-300 as the matrix showed a molecular mass for the cytosol binding activity of 319 000 ± 4486 Da and Stokes' radius of 6.23 ± 0.03 nm (n = 3). Competition studies with different steroids indicated considerable specificity for the cytosol binding. The competitive hierarchy was as follows: dexamethasone > triamcinolone acetonide > Cortisol > corticosterone > cortisone > testosterone > estrone > 17β-estradiol > 11-deoxycortisol > progesterone > 11β, 17α, 21-trihydroxy-4-pregnen-3,20-dione-21-phosphate > 17α,20β-dihydroxy-4-pregnen-3-one > pregnenolone. These results support the concept of cytosolic and nuclear forms of glucocorticoid receptors in the liver of brook trout.