In vivo evidence for the specific binding of human beta-endorphin to the lung and liver of the rat.

Specific binding of human beta-endorphin (beta-EP) was demonstrated in the lung and liver of the rat in vivo by the following lines of evidence, using [125I-Tyr27]beta-EP as a radiolabeled tracer. First, the tissue-to-serum concentration ratios of the intact labeled peptide 15 min after intravenous administration were decreased significantly in the lung and liver by a simultaneous injection of unlabeled beta-EP (48.5 nmol/kg), whereas in the other tissues such a decrease was not observed. Second, serum concentrations of the preadministered labeled peptide were increased rapidly after an additional intravenous injection of unlabeled beta-EP via the femoral vein, but not via the carotid artery into the heart. Third, the immunoreactive labeled beta-EP (125I-beta-EP), which was purified on a Sephadex G-50 column and did not specifically bind to the rat brain membranes, did not accumulate in the lung and liver and was not displaced by unlabeled beta-EP in vivo, in contrast to [125I-Tyr27]beta-EP, the commercially available HPLC-purified labeled peptide. Fourth, an additional injection of dynorphin (1-13) or ethylketocyclazocine (kappa agonist) also increased the serum concentrations of preadministered [125I-Tyr27]beta-EP but injection of Ala2-D-Leu5-enkephalin (delta agonist) or naloxone (mu antagonist) did not, suggesting kappa-type binding sites in the lung.