Modulation of human neutrophil inflammatory responses by nitric oxide: studies in unprimed and LPS-primed cells

Because nitric oxide (NO) can act both as a regulatory and as a toxic molecule, we have studied N-formyl-methionyl-leucyl-phenylalanine (fMLF)-stimulated responses of human neutrophils (PMNs) during various conditions of NO modulation in unprimed and bacterial lipopolysaccharide (LPS) -primed cells. Effects of various NO modulators were assessed on stimulated superoxide (O2-) generation, granule exocytosis, homotypic aggregation, and rises in intracellular free Ca2+ ([Ca2+]i). Significant differences in the effects of various NO modulators on inflammatory responses of PMNs kept in stirred suspension versus those kept under static and/or adherent conditions, were observed. L-arginine, the physiological substrate for NO synthase (NOS), and NG-nitro-L-arginine methyl ester, an inhibitor of NOS, both caused a 40-50% inhibition of LPS-induced priming of O2- generation in PMNs in stirred suspension, but not in LPS-primed PMNs under static or adherent conditions. The NO donors, sodium nitroprusside and S-nitroso-N-acetylpenicillamine, completely abrogated the LPS-induced priming of O2- generation in PMNs in suspension, while causing only a 40-50% inhibition in PMNs under static or adherent conditions. The Ca2+ ionophore, A23187, prevented the LPS-induced priming of O2- generation without affecting O2- generation in unprimed PMNs. LPS priming of PMNs induced about a twofold increase in fMLF-stimulated homotypic aggregation, exocytosis of secondary granules, and rises in [Ca2+]i. In related studies, we also provide definitive evidence for enzymatic formation of NO in human PMNs and demonstrate a significant decrease in NO levels in LPS-primed PMNs. Taken together, these findings indicate that NO modulates PMN inflammatory responses and plays a protective role in priming and activation processes of inflammatory PMNs.