THE EFFECT OF ASPIRIN ON THROMBIN-STIMULATED PLATELET-ADHESION RECEPTOR EXPRESSION AND THE ROLE OF NEUTROPHILS

Aims Aspirin has proven clinical efficacy in limiting the thrombotic c omplications of atherosclerotic vascular disease but its mechanism of action remains unclear. Recent evidence suggests the anti-platelet act ion of aspirin may be partly mediated by neutrophil derived nitric oxi de (NO). The aim of the study was to determine the effects of aspirin on thrombin-induced platelet expression of the sc-granule membrane pro tein, P-selectin, and the platelet surface glycoprotein required for a ggregation, GPIIb-IIIa, and to assess whether this was enhanced by the presence of neutrophils. Methods Platelet P-selectin and GPIIb-IIIa r eceptor expression were assessed by flow cytometric analysis of washed platelets stimulated with thrombin (0.025 iu ml(-1), sub aggregatory concentration) alone or after pre-incubation with aspirin (0.05, 0.1, 0.5, 1.0 mg ml(-1)) either in the presence or absence of neutrophils ( 100 platelets per neutrophil). NO release was determined by assay of n itrite in the supernatants from parallel samples. Results In prelimina ry aggregation studies, aspirin at all concentrations inhibited arachi donic acid but not thrombin-induced platelet aggregation Similarly, as pirin at all concentrations failed to inhibit thrombin-induced platele t P-selectin or GPIIb-IIIa expression and this was not influenced by t he presence of neutrophils. A reduction in P-select:in and GPIIb-IIIa receptor density on non-activated platelets co-incubated with unstimul ated neutrophils was associated with NO release from neutrophils, but this was not enhanced by the addition of aspirin. Conclusions These re sults confirm that thrombin-induced platelet cl-granule release, with consequent P-selectin expression, and platelet GPIIb-IIIa expression, are not affected by aspirin inhibition of cyclo-oxygenase and suggest that the anti-thrombotic efficacy of aspirin in vivo may partly depend on other mechanisms. This study did not demonstrate an effect of neut rophils or neutrophil derived NO on aspirin inhibition of platelet adh esion receptor expression.