Lack of effect of opioid peptides, morphine and naloxone on superoxide formation in human neutrophils and HL-60 leukemic cells

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Abstract
There are controversial reports in the literature concerning the effects of opioids on superoxide (O 2 − ) formation in phagocytes, these agents being either inhibitory or stimulatory. We re-examined this issue and compared the effects of the Chemotactic peptide, N-formyl-l,-methionyl-l-leucyl-l-phenylalanine (fMet-Leu-Phe), phorbol myristate acetate (PMA), ATP, platelet activating factor (PAF), cytochalasin B (CB) and prostaglandin E1 (PGE1) with those of various opioids on O 2 − formation in human neutrophils and HL-60 leukemic cells under defined experimental conditions. In the presence of CB, fMet-Leu-Phe and PAF concentration-dependently activated O 2 − formation in neutrophils with EC50 values of 20 nM and 100 nM, respectively. In the absence of CB, fMet-Leu-Phe and PAF were much less effective. PAF synergistically enhanced O 2 − formation induced by fMet-Leu-Phe. ATP at a concentration of 100 μM and the opioids, methionine enkephalin, β-endorphin, dynorphin, [d-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin, [d-Ala2-d-Leu5]-enkephalin and morphine at concentrations between 10 pM to 1 μM did not activate O 2 − formation. ATP but not \-endorphin potentiated fMet-Leu-Phe-induced O 2 − formation. O 2 − formation induced by a maximally stimulatory concentration of PMA (100 ng/ml) was enhanced by fMet-Leu-Phe but was unaffected by methionine enkephalin or PGE1. PMA at a non-stimulatory concentration (2 ng/ml) potentiated the effect of fMet-Leu-Phe but did not induce responsiveness to PAF, ATP or β-endorphin. PGE1 strongly inhibited fMet-Leu-Phe-induced O 2 − formation, whereas morphine, methionine enkephalin and the opioid antagonist, naloxone, were without effect. In HL-60 cells differentiated with dibutyryl cAMP, fMet-Leu-Phe, PAF and ATP but not β-endorphin activated O 2 − formation. Our results show that O 2 − formation is differentially regulated by various classes of intercellular signal molecules and that opioids do not play a role in the regulation of O 2 − formation. The precise definition of the experimental conditions and control experiments with established modulators of O 2 − formation are essential to evaluate the role of opioids in the regulation of this effector system.