PROLACTIN-BINDING SITES IN TILAPIA (SAROTHERODON-MOSSAMBICUS) KIDNEY

Highly purified 125I-labeled tilapia (S. mossambicus) prolactin (PRL) binds to microsomal membrane fractions from tilapia kidney but not liver or gill. Specificity studies revealed that up to 35% of the labeled tilapia PRL bound to the kidney membrane could be displaced by 100 ng of unlabeled hormone. Ovine PRL displaced 125I-labeled tilapia PRL from the kidney PRL-binding site. 125I-labeled ovine PRL demonstrated negligible specific binding to microsomal membrane fractions of tilapia kidney, liver, gill, or muscle. Tilapia PRL did not inhibit the binding of 125I-labeled ovine PRL to lactating rat liver membranes. The specific binding of 125I-labeled tilapia PRL to tilapia kidney membranes was greater with freshwater-acclimated than with seawater-acclimated fish suggesting that the tilapia kidney PRL-binding site is subject to physiological regulation.