Enhanced dense granule secretion of ADP potentiates integrin α_IIbβ₃activation in PAR4-AP–stimulated EXOC3 KO platelets, which is independent of changes in calcium mobilization, integrin outside-in signaling, and GPIbα surface levels. (A) Washed platelets (1 × 10⁸/mL) from WT and EXOC3 KO platelets were stimulated with increasing concentrations of PAR4-AP (50-300 µM) and monitored for changes in cytosolic calcium levels. The kinetics of mean changes in calcium responses from 4 independent experiments (i) with quantified area under the curve (AUC) analysis (ii) are presented. (B) Washed platelets (2 × 10⁷/mL) were pretreated for 10 minutes with vehicle or 1 µM P₂Y₁₂ receptor antagonist cangrelor (AR-C) before stimulation with 125 µM PAR4-AP and monitored for changes in JON/A binding by using flow cytometry. Washed platelets (2 × 10⁸/mL) from WT and EXOC3 KO platelets were pretreated with vehicle, tirofiban (50 µg/mL) (C) or Nk protease (5 µg/mL) (D), and assessed for changes in PAR4-AP (125 µM)–mediated dense granule secretion responses by lumi-aggregometry. Representative secretion traces (Ci,Di) and peak ATP secretion values (Cii,Dii) are displayed. Data are presented as the mean ± standard error of the mean. n = 4 (A); n = 5 (B); n = 3 (C); n = 4 (D). *P < .05, **P < .01, ****P < .0001 vs indicated sample. RFU, relative fluorescence units.