TxA₂ released from WT platelets restores aggregation of CalDAG-GEFI^−/− platelets. (A) Aggregation traces of platelets (10⁸ cells) activated with 10 μg/mL collagen. The aggregation response of WT (black line), CalDAG-GEFI^−/− (knockout [KO], light gray line), or CalDAG-GEFI^−/− platelets containing 10% WT platelets (dark gray line) was studied. WT platelets were pretreated with 1 mM aspirin (acetylsalicylic acid) before addition to CalDAG-GEFI^−/− platelets to inhibit TxA₂ release. CalDAG-GEFI^−/− platelets were pretreated with 40 μg/mL blocking antibody to α_IIbβ₃ to demonstrate that the aggregation was integrin-dependent. Traces are representative of 3 individual experiments. (B) Representative images showing aggregates of calcein green–labeled WT and/or calcein red–labeled KO (CalDAG-GEF^−/−) platelets fixed with 3.7% formaldehyde 10 minutes after stimulation and visualized with fluorescence microscopy. Scale bar = 25 μm.