Hepatic estrogen receptors and plasma estrogen-binding activity in the Atlantic Salmon

Livers of male and female immature Atlantic Salmon ( Salmo salar) contain specific high-affinity [ 3H]estradiol binding sites in cytosol ( K d 2–4 n M, concentration about 0.6 pmol/g liver). Low levels of high-affinity binding are detectable in salt extracts of nuclei of untreated fish, but injections of estradiol result in transient depletion of the cytosol binder and in accumulation of high levels of binding sites in nuclear salt extracts ( K d 5–6 n M; concentration about 6 pmol/g liver). Both the cytosol and nuclear binding sites are temperature sensitive and are optimally assayed by incubation at 2°. Both are specific for estradiol and diethylstilbestrol (DES) and no significant competition by dihydrotestosterone (DHT), progesterone, or hydrocortisone is seen. The triphenylethylene nonsteroidal antiestrogen, 4-hydroxytamoxifen, exhibits an affinity comparable to that of estradiol. The nuclear binding activity sediments with a coefficient of 3.6 S in salt-containing sucrose density gradients, and is stable on storage at −20° for several months. The cytosol binder on the other hand is not stable on sucrose density gradients or on prolonged storage. Salmon plasma contains two [ 3H]estradiol binding components, one with a relatively high affinity for [ 3H]estradiol ( k d 13 n M) and the other having a much lower affinity but present in high concentrations. The high-affinity plasma binder exhibits distinctive specificity with no affinity for DES or 4-hydroxytamoxifen but some affinity for DHT and progesterone. These properties serve to distinguish the plasma activity from the intrahepatic estrogen binders. The salmon liver estrogen receptor system has many features in common with typical estradiol receptors from other vertebrates. Immature salmon liver appears to be the richest source of hepatic estrogen receptor so far found for any vitellogenic species.