Impaired dense granule release from CalDAG-GEFI–deficient platelets stimulated with low-dose PAR4p. (A) ¹⁴C-serotonin-loaded WT or CalDAG-GEFI–deficient (KO) platelets were activated with various concentrations of PAR4p in the presence or absence of Ro31-8220 (5 μg/mL) under stirring conditions. Activation was terminated 10 minutes after addition of the agonist, and ¹⁴C-serotonin levels in the supernatant (SN) of activated platelets were determined. 0% indicates ¹⁴C-serotonin in SN of resting platelets; 100%, ¹⁴C-serotonin levels in platelet samples after lysis; n = 6. (B) Time course of PAR4p-induced serotonin release. The activation process was terminated at the indicated time points after addition of 1.25 mM of PAR4p; n = 6. (C) WT (black line) or CalDAG-GEFI–deficient (gray lines) platelets were activated with 0.5 mM of PAR4p and/or 5 μM of ADP in the presence or absence of 2-MesAMP (50 μM). Results are representative of 4 experiments.