Inhibition by naloxone of neutrophil superoxide release: a potentially useful antiinflammatory effect

The working hypothesis of many studies of shock has been that naloxone acts by blocking centrally and/or peripherally located opioid receptors. At plasma concentrations used to treat experimental shock (10(-6) M and above), naloxone inhibited the in vitro release of superoxide (O2-) by human neutrophils that were stimulated by the E. coli peptide N-formyl methionyl leucyl phenylalanine (FMLP). Superoxide release stimulated by phorbol 12,13-dibutyrate (PDB) was also inhibited by naloxone. Naloxone had no effect on the FMLP-stimulated release of beta-glucuronidase or lysozyme. Naloxone had no effect on 3H FMLP receptor binding. Studies utilizing 3H naloxone revealed the presence of a ligand-specific naloxone binding site on human neutrophils with a Kd of 1.2 X 10(-5) M, which is close to the ID50 of the inhibitory effect upon O2- release (1.8 X 10(-5). Thyrotropin releasing factor (TRF) had no effect upon 3H naloxone binding or on O2- release. Verapamil, a calcium channel blocker, inhibited 3H naloxone binding, and O2- release while nifedipine, another calcium channel blocker had no effect on either assay except at 10(-4) M, at which concentration 3H naloxone binding as well as the release of O2- were increased. These experiments suggest that the inhibitory effect of naloxone upon O2- release is mediated via a specific binding site.