JNK1^−/− platelets display abnormal platelet granule secretion. (A) Dense granule secretion was evaluated by measuring ATP release after the aggregation of WT or JNK1^−/− platelets induced by PAR4-AP (100 and 150μM), thrombin (40 and 100 mU/mL), or collagen (0.8-1.2 μg/mL) with stirring. Results are expressed as the amount of ATP released (picomoles) by 10⁸ platelets. (B) α-Granule secretion was determined by flow cytometry evaluation of P-selectin exposure in WT or JNK1^−/− platelets. Platelets were activated with PAR4-AP (75-150μM) or thrombin (10 and 50 mU/mL) and labeled with FITC-labeled rat anti–mouse P-selectin mAb (Wug.E9). Flow cytometry experiments were performed without stirring to prevent platelet aggregation. The levels of P-selectin exposed at the surface are expressed as mean fluorescence intensities (MFI) plus or minus SEM, in arbitrary units, from at least 3 experiments. *P < .05, **P < .01, ***P < .001 (unpaired Student t test).