Platelet aggregation is inhibited by a nitric oxide-like factor released from human neutrophils in vitro

Summary. Thrombin-induced platelet aggregation was inhibited in vitro by washed human neutrophils. Aggregation was inhibited in a neutrophil concentration dependent manner but glutaraldehyde fixed neutrophils had no significant effect on platelet aggregation. The neutrophil-derived inhibitory factor had the pharmacological profile of nitric oxide. Its action was potentiated by both superoxide dismutase and M&B22, 948, a selective cyclic guanosine mono-phosphate (cyclic GMP) phosphodiesterase inhibitor. Haemoglobin lessened this inhibitory action of neutrophils. L-Arginine, the substrate for nitric oxide formation, enhanced inhibition, whereas, manavanine, a structural analogue of L-arginine, prevented it. Nitric oxide release by neutrophils antagonized platelet ATP secretion and thromboxane B₂ release. Inhibition was mediated by nitric oxide activation of guanylate cyclase with a subsequent rise in cyclic GMP. When neutrophils were stimulated with formyl-met-leuphe, there was a further increase in platelet cyclic GMP. This was enhanced by superoxide dismutase, but lessened by haemoglobin. Leukotriene B₄ stimulation of neutrophils promoted inhibition of platelet aggregation. Leukotriene B₄ alone had no direct effect on thrombin-induced aggregation of platelets. Platelets, when incubated with neutrophils and stimulated with calcium ionophore A23187, increased leukotriene B₄ production by neutrophils in a platelet concentration dependent manner. Platelets alone were unable to release leukotriene B₄. The action of platelets in haemostasis is modified as they come into contact with neutrophils. This may be an important physiological mechanism.