Previous studies from our laboratory have characterized a naloxone-insensitive β-Endorphin (β-End) receptor on the human pro-monocytic cell line U937. Since monocytes are macrophage precursors, we sought to identify and characterize this site on fully differentiated effector macrophages. Mice (ICR females, 6-8 wk old) were injected (i.p.) with 1 mL of thioglycollate to induce an inflammatory response. Elicited cells were harvested 3 d later by lavage. Macrophages were enriched by adherence and analyzed via radioreceptor assay (with [125I]β-End, 2,000 Ci · mmol-1) as either intact cells or membrane preparations. Scatchard analysis revealed a single saturable binding site for β-End (K(d) = 9.75 ± 2.6 x 10-9 M; 8218 ± 2360 sites/cell). Competition studies showed that other opiate receptor ligands including naloxone, DAMGO, U69593, or 2,5 DPDP-enkephalin were ineffective at displacing [125I]β-End when compared to unlabeled β-End. Analysis of competition studies utilizing fragments and analogs of β-End revealed that β-End (6-31) and β-End (1-5, 16-31) were equipotent, and N-acetylated β-End was less potent, than β-End (1-31) in displacing [125I]β-End binding. In contrast, β-End (1-27) and β-End (28-31) were ineffective. In summary, we have identified a naloxone-resistant β-End binding site on murine peritoneal macrophages that is similar to one we have previously characterized on U937 cells and cultured murine splenocytes.
- β-endorphin receptor
ASJC Scopus subject areas
- Biochemistry, Genetics and Molecular Biology(all)
- Pharmacology, Toxicology and Pharmaceutics(all)