Detection of nonopioid β ‐endorphin receptor in the rat myocardium

Two selective agonists of nonopioid β ‐endorphin receptor, synthetic peptides TPLVTLFK (octarphin) and SLTCLVKGFY (immunorphin), were labeled with tritium to specific activity of 29 and 25 Ci/mmol, respectively. Both labeled peptides were found to bind to high‐affinity naloxone‐insensitive binding sites on the membranes isolated from the rat myocardium (Kd = 2.0 ± 0.2 and 2.5 ± 0.3 nM, respectively). The [ 3 H]octarphin specific binding to the myocardial membranes was inhibited by unlabeled β ‐endorphin (Ki = 1.9 ± 0.2 nM) and immunorphin (Ki = 2.2 ± 0.3 nM). The [ 3 H]immunorphin specific binding with the membranes was inhibited by unlabeled β ‐endorphin (Ki = 2.3 ± 0.3 nM) and octarphin (Ki = 2.4 ± 0.3 nM). The binding specificity study revealed that these binding sites were insensitive not only to naloxone but also to α ‐endorphin, γ ‐endorphin, [Met 5 ]enkephalin and [Leu 5 ]enkephalin. Thus, β ‐endorphin, immunorphin and octarphin bind to the common high‐affinity naloxone‐insensitive receptor of the rat myocardial membranes. Copyright © 2011 European Peptide Society and John Wiley & Sons, Ltd.