The release mechanism of platelet-activating factor during shear-stress induced platelet aggregation.

We previously reported that 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine (platelet-activating factor, PAF), released from activated platelets stimulated with thrombin plus collagen, is associated with platelet microparticles. In the present study, we found that PAF is concentrated and associated with microparticles released from high shear stress-induced activated platelets. The total amount of PAF released from 3 x 10(8) platelets under high shear stress (108 dyne/cm2) was 3.2 +/- 0.6 x 10(-15)mol (n = 5, mean +/- S.D.). Eighty percent of the PAF released from the platelets was recovered in the microparticle fraction after ultracentrifugation in the presence of albumin. Under high shear stress, PAF was not released from platelets within 3 minutes, although microparticles were released. In conclusion, microparticles released from activated platelets in the rheological condition of high shear stress are major carriers of PAF.