An Immortalized Myocyte Cell Line, HL-1, Expresses a Functional δ -Opioid Receptor

The present study characterizes opioid receptors in an immortalized myocyte cell line, HL-1. Displacement of [3H]bremazocine by selective ligands for the mu (μ), delta (δ), and kappa (κ) receptors revealed that only the δ -selective ligands could fully displace specific [3H]bremazocine binding, indicating the presence of only the δ -receptor in these cells. Saturation binding studies with the δ -antagonist naltrindole afforded a Bmaxof 32 fmols/mg protein and a KDvalue for [3H]naltrindole of 0.46 n . The binding affinities of variousδ ligands for the receptor in HL-1 cell membranes obtained from competition binding assays were similar to those obtained using membranes from a neuroblastoma×glioma cell line, NG108-15. Finally, various δ -agonists were found to stimulate the binding of [35S]GTP γ S, confirming coupling of the cardiac δ -receptor to G-protein. DADLE (D-Ala-D-Leu-enkephalin) was found to be the most efficacious in this assay, stimulating the binding of [35S]GTP γ S to 27% above basal level. The above results indicate that the HL-1 cell line contains a functionally coupled δ -opioid receptor and therefore provides an in vitro model by which to study the direct effects of opioids on cardiac opioid receptors.