Ask1 regulates granular secretion. (A) Representative tracings of intracellular Ca²⁺ of Fluo-4 loaded washed platelets from WT and Ask1^−/− mice stimulated with ADP (10 μM) (i), convulxin (10 ng/mL) (ii), AYPGKF (100 μM) (iii), and U46619 (5 μM) (iv) in the presence of 1 mM of extracellular Ca²⁺ (left panels). Quantitation of data from more than 3 experiments (right panels). (B) Quantitation of intracellular Ca²⁺ from WT and Ask1^−/− platelets treated with Ca²⁺ ionophore (A23187, 10 μM) (i) and various concentrations of thrombin (ii). (C) Quantitation of ¹⁴C-serotonin release from platelets (2.5 × 10⁸/mL) from WT and Ask1^−/− mice stimulated with various concentrations of thrombin and measured using a liquid scintillation counter. (D) Quantitation of P-selectin surface expression of WT and Ask1^−/− platelets. (E) Representative TEM images of WT and Ask1^−/− null platelets. ns, not significant. Scale bar, 1 μm. *P < .05; **P < .01.