Effect of RhoA inactivation by C3 exoenzyme on platelet aggregation. Washed platelets were incubated for 4 hours at 37°C with vehicle buffer (“No C3”) or 400 μg/mL of C3 (“C3”). Then citrated platelet-poor plasma was added back, and platelet aggregation was measured in stirred samples by aggregometry. In this system, light transmittance is assigned a value of 0% in nonaggregated, platelet-replete samples and 100% in platelet-free samples. Vertical bars indicate a 20% aggregation response. With this technique, primary aggregation is generally < 50% and secondary aggregation > 50%. (A and B) Show tracings from two separate experiments representative of four experiments.