The platelet releasate acts as an agonist and induces P-selectin exposure on platelets tested for recruitment.

Effects of erythrocytes and aspirin ex vivo. PRP (2 × 10⁸platelets/mL), PRP plus erythrocytes (RBCs), or whole blood (WB) were stimulated with collagen (1 μg/mL) and centrifuged to obtain a cell-free releasate within 1 minute. Aliquots of releasate were added as agonist to aspirin-free autologous PRP. P-selectin exposure induced by releasate was measured by flow cytometry of recruited platelets (“Materials and methods”). Releasates were obtained in duplicate for each data point, and duplicate flow cytometric measurements of each sample of releasate were performed. The 4 values so obtained from each donor were averaged; n represents the number of different volunteers studied. Data are mean ± SEM of percentage of platelets that bind PE–CD-62. (A) Effects of releasates from collagen-stimulated PRP, WB, and solvent controls (n = 15; *P < .001, PRP versus WB). (B) Relationship to hematocrit (n = 4; *P <.001, PRP versus PRP plus RBCs). (C) Normal versus glutaraldehyde-fixed RBCs, compared with PRP (hematocrit [Hct] 40%; n = 3; P < .001 normal RBCs versus PRP or fixed RBCs). (D) Effect of aspirin (n = 4; Hct 40%; before versus after ASA ingestion: *P < .01 for PRP, and *P < .0001 for PRP plus RBCs). Significance was determined with Student t test.