Effect of PAR3 IgG on mouse platelet ATP secretion in response to α-thrombin or U46619. Washed platelets were incubated with PAR3 IgG (60 μg/mL) for 5 minutes at 37°C and then stimulated with the indicated concentration of α-thrombin (A) or U46619 (B). The maximum response to each concentration of agonist was quantitated and expressed as a percentage of the response of platelets incubated with preimmune IgG only to 10 nmol/L α-thrombin. (In separate experiments, preimmune IgG had no significant effect on platelet responses.) Each point represents a single aggregometry tracing. The data shown were replicated in two separate experiments.