Kinetics of SFLLRN- and PMA-induced α-granule secretion from platelets. (A) Platelets were stimulated with either 100 μM SFLLRN (•) or 0.2 μM PMA (□). At the indicated times following exposure to agonist, samples were fixed with paraformaldehyde. Samples were then assayed for P-selectin expression by flow cytometry to monitor α-granule secretion. Error bars indicate the standard deviation of 3 to 7 samples. (B) Platelets were stimulated with either 100 μM SFLLRN or 0.2 μM PMA as indicated. Platelets were then pelleted at the indicated times. The supernatants from stimulated platelets were assayed for β-thromboglobulin (β-TG) by immunoblot analysis as an indicator of release of α-granule contents.