Defective shape change after G₁₃ stimulation and partially reduced aggregation in RhoA^−/− platelets. Washed platelets were stimulated with the indicated agonists, and light transmission was recorded on a Fibrintimer 4-channel aggregometer. ADP measurements were performed in prp. (A-B) Representative aggregation curves of 3 individual experiments and representative scanning electron microscopy pictures of wt and RhoA^−/− platelets after stimulation with 0.01μM U46619 (right) are depicted. Scale bar indicates 2 μm. (C) Mean percentage of maximal aggregation ± SD of each group (**P < .01; ***P < .001). U46 indicates U46619; thr, thrombin; PAR, PAR-4 peptide; and coll, collagen.