Aggregation and secretion. (A) Aggregation of WT and SG KO platelets: WT and KO platelets were run simultaneously in a dual-channel aggregometer with the concentrations of agents indicated on each panel. Results shown are representative of 2 to 3 independent experiments with very similar results. (B) Serotonin release: PAR4 agonist-induced release of serotonin from WT and SG^−/− platelets was measured after loading platelets with ³H-serotonin. The mean percentage releases (± SD) from 3 separate experiments are shown.