High concentrations of latrunculin A inhibit α-granule secretion but stimulate dense granule secretion. (A) Platelets were incubated with the indicated concentration of latrunculin A for 20 minutes. Samples were then exposed to either 100 μM SFLLRN for 10 minutes (•) or 0.2 μM PMA for 20 minutes (▪). Platelets were subsequently analyzed for P-selectin expression by flow cytometry. (B) Platelets were incubated in the presence (□) or absence (▪)of200 μM latrunculin A for 20 minutes and divided into 2 aliquots. One aliquot (Before wash) was exposed to either buffer or 100 μM SFLLRN for 10 minutes as indicated. The second aliquot (After wash) was washed to remove latrunculin A and subsequently exposed to buffer or 100 μM SFLLRN for 10 minutes as indicated. Samples were assayed for P-selectin expression by flow cytometry. (C) Platelets were incubated with the indicated concentration of latrunculin A for 20 minutes. Platelets were then exposed to either buffer (▪) or 100 μM SFLLRN (□) for 10 minutes. P-selectin expression was assayed by flow cytometry to monitor α-granule release. ATP release as assayed using a luciferin-luciferase detection system was used to monitor dense granule release. Data are expressed as percentage of granule release compared to samples treated with agonist alone.