CD151^–/– platelets have normal alpha and dense granule secretion. (A-B) Surface expression of P-selectin was determined for washed platelets stimulated by thrombin (A) or PAR-4 agonist peptide (B) at different concentrations and then stained with either a buffer control and FITC–P-selectin mAb for both wild-type and CD151^+/+ platelets. FITC-labeled samples were analyzed on a FACS Calibur analyzer. Results are representative of 3 independent experiments. (C) Control CD151^+/+ (▪) and CD151^–/– (□) murine platelets were loaded with [³H]5-hydroxytryptamine (5-HT) and stimulated with thrombin (1 or 5 U/mL) or PAR-4 agonist peptide (250 or 500 μM) for 5 minutes. Platelets were pelleted by centrifugation at 13 900g (13 000) rpm for 5 minutes, and 5-HT secretion into the medium was determined by scintillation spectrometry. Results are expressed as a percentage of the total tissue content after substraction of release under basal conditions and are representative of 3 experiments. Fluor indicates fluorescence.