Human neutrophils and mononuclear cells inhibit platelet aggregation by releasing a nitric oxide-like factor

Incubation of neutrophils or mononuclear cells with washed platelets (all prepared from human venous blood) resulted in an inhibition of thrombin-induced platelet aggregation that was dependent on the number of nucleated cells added. The inhibition was potentiated by superoxide dismutase and reversed by oxyhemoglobin. In the case of neutrophils the inhibition was associated with an increase in cGMP, whereas with mononuclear cells both cAMP and cGMP were increased. The inhibitory activity of neutrophils or mononuclear cells was prevented by their preincubation with NG-monomethyl-L-arginine methyl ester. L-Arginine reversed the action of NG-monomethyl-L-arginine methyl ester, whereas D-arginine was ineffective. Preincubation of the cells with catalase or mannitol did not prevent their inhibitory action on platelet aggregation. The inhibition of platelet aggregation was not due to platelet damage or to uptake of thrombin by neutrophils or mononuclear cells. It was overcome by increasing the concentration of thrombin and was absent in cell-free supernatants obtained from a suspension of neutrophils or mononuclear cells or from mixtures of platelets with neutrophils or platelets with mononuclear cells. These data provide evidence for the release of a nitric oxide-like factor from human neutrophils and mononuclear cells. In addition, evidence is provided that, as in stimulated murine macrophages and endothelial cells, the precursor of this factor is L-arginine.