Singlet oxygen inhibits agonist-induced P-selectin expression and formation of platelet aggregates

Major mediators of activated polymorphonuclear leukocytes (PMN) are the oxidants HOC1 and chloramine, which are a source for the nonradical photon-emitting oxidant singlet oxygen (O-1(2)). We were interested in a possible platelet modulating activity of O-1(2). As a stable O-1(2) source we chose the mild oxidant chloramine T(R) (CT), which mimics the natural chloramine N-chloro-taurine. Freshly drawn native whole blood from donors (n = 5) was incubated at 0 to 3 mM CT for 1 minute at 37 degreesC. Then saline, 10 muM adenosine diphosphate (ADP), 5 mug/mL collagen, or 6.25 muM thrombin receptor activator peptide (TRAP) were added and the mixtures were allowed to incubate for 3 minutes at 37 degreesC. Aliquots of activated blood were fixed in 1% para-formaldehyde. After removal of the fixative, platelets were labeled with anti-CD61-FITC and anti-CD62P-PE antibodies and analyzed by flow cytometry. An oxidant concentration-dependent decrease in the expression of P-selectin appeared (at 3 mM CT to 39, 23, and 20% of the 100% saline control level for ADP, collagen, and TRAP, respectively). There was also an oxidant concentration-dependent decrease in the formation of platelet aggregates (at 3 mM CT to 8, 12, and 13% of the 100% saline control level for ADP, collagen, and TRAP, respectively; the 50% effective dose was 1.0 to 1.5 mM chloramine). In ADP- and TRAP-stimulated platelets, an oxidant-mediated increase in platelet fragments appeared (at 3 mM CT: three- to fourfold of the initial value). The addition to the blood of 30 mM of the oxyradical scavenger mannitol in contrast to excess methionine did not antagonize these oxidative modulations of platelet activation. The results were confirmed using equimolar concentrations of NaOCI and N-chloro-taurine. This study shows that O-1(2) inhibits platelets, decreasing the expression of CD62P and the formation of platelet aggregates. Activated PMN might modulate hemostasis, shifting it into an antithrombotic stale. The physiologic signal action and the direct anticoagulant action of O-1(2) (released by chloramines such as vancomycin) might be a new principle for pharmacologic intervention in atherothrombosis.