In vitro effect of opioid agonist and antagonist on superoxide release by granulocytes

The effect of the opioid agonist morphine and of the (-) and (+) stereoisomers of the antagonist naloxone were studied on the O2-generation from human granulocytes. Morphine or naloxone had no effect on basal or phorbol myristate acetate (PMA) stimulated O2-generation, while equimolar (-) naloxone and morphine concentrations (1 x 10-13 - 1 x 10-7 M) inhibited the stimulated O2-generation. The effect of (-) naloxone was stereospecific, suggesting the involvement of opioid receptors. The unmasking of non opioid effects of morphine could be responsible for the inhibition of O2-generation. It is suggested that the opioid control of oxidative metabolism in human granulocytes could involve multiple receptors mediating opposite effect.