Similar kinetics for Rap1 activation and aggregation in platelets stimulated with PAR4p. (A) Time course for the aggregation response (top panel) and for Rap1 activation (bottom panel) in WT or CalDAG-GEFI–deficient (CalDAG-GEFI^−/−) platelets stimulated with 1.0 mM of PAR4p. Samples for the determination of Rap1-GTP and total Rap1 levels were withdrawn from the aggregation tubes at t = 15, 45, or 180 minutes after addition of the agonist (indicated by black arrows). Results are representative of 3 independent experiments. (B) Time course for the aggregation response (top panel) and for Rap1 activation (bottom panel) in 2-MesAMP-pretreated CalDAG-GEFI^−/− platelets stimulated with 1.5 mM of PAR4p in the presence or absence of 10 μM of epinephrine. Samples for the determination of Rap1-GTP and total Rap1 levels were withdrawn from the aggregation tubes at the indicated time points. Results are representative of 3 independent experiments.