Platelet‐leukocyte interaction: activation of rabbit platelets by FMLP‐stimulated neutrophils

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Abstract
1 The effect of the chemotactic peptide, N-fonnyl-l-methionyl-l-leucyl-l-phenylalanine (FMLP) was studied on cells in whole rabbit blood or on a mixture of purified rabbit platelets and neutrophils. 2 In blood, FMLP triggered cell aggregation (measured by electrical impedance) which was dependent upon the concentration of FMLP (9.9 ± 0.7 and 5.2 ± 1.2 ohms at 1 and 0.01 μm FMLP respectively). This aggregation was accompanied by a strong decrease in platelet counts (54.6 ± 6.0 and 45.6 ± 3.8% for 1 and 0.01 μm FMLP respectively) and by a smaller decrease in neutrophil counts (25.0 ± 1.9 and 12.9 ± 1.7% at 1 and 0.01 μm FMLP respectively). 3 When purified platelets and neutrophils were co-incubated, the addition of 0.1 μm induced a marked aggregation (50.0 ±1.6 vs. 19.5 ±1.6% of light transmission, n = 8, P < 0.001), ATP secretion (8.4 ± 1.0 vs. 0.1 ± 0.1 nmol ml−1, n = 6, P < 0.001) and a decrease in platelet counts. FMLP induced aggregation of purified neutrophils and release of lysozyme but lacked direct platelet-stimulating effects. The release of lactate dehydrogenase, a cytoplasmic marker and lysozyme were unchanged under the interaction conditions. 4 Platelet activation was reduced by about 30% with 100 μm aspirin or indomethacin and by about 70% with 100 μm BW 755C. Two Paf-acether antagonists, BN 52021 (100 μm) and WEB 2086 (1 μm) suppressed platelet activation by 70–80%. 5 The supernatant of FMLP-stimulated neutrophils induced platelet activation only when bovine serum albumin was present. Rabbit neutrophils stimulated in the presence of serum albumin by 1 μm FMLP formed 2 nm Paf-acether of which half was released to the extracellular medium. 6 Our results indicate that the stimulation of neutrophils by FMLP induces platelet activation in whole blood and on isolated cells and that both arachidonic acid-metabolites and Paf-acether participate in platelet activation.