Reduced granule release upon activation in BAK/BAX-deficient platelets. (A-C) ATP, serotonin, and PF4 release after platelet aggregation in response to PAR4-AP (50 µM). Platelets were washed and counts adjusted before agonist stimulation. (A) ATP levels were determined in platelet releasates by cell titer glo assay. Bak^+/+Bax^+/+ (n = 17), Bak^+/+Bax^fl/fl (n = 4), Bak^−/−Bax^fl/fl (n = 17), Bak^+/+Bax^{Pf4∆/Pf4∆} (n = 3), and Bak^−/−Bax^{Pf4∆/Pf4∆} (n = 9) mice. (B) Serotonin and (C) PF4 levels were quantified in platelet releasates by enzyme-linked immunosorbent assay. Serotonin included Bak^+/+Bax^+/+ (n = 10), Bak^+/+Bax^fl/fl (n = 8), Bak^−/−Bax^fl/fl (n = 9), Bak^+/+Bax^{Pf4∆/Pf4∆} (n = 7), and Bak^−/−Bax^{Pf4∆/Pf4∆} (n = 7) mice. PF4 n = 4 to 6 mice per genotype. Data are presented as mean ratio/wild-type (wt) ± SD. One-way ANOVA with Dunnett’s multiple comparison test. **P < .005; ***P < .001; n.s., not significant.