In vitro and in vivo indicators of premature platelet activation. (A) Platelet counts in WT, RGS18^−/−, RGS10^−/−, and RGS10^−/−18^−/− mice treated daily for 10 days with 50 mg/kg aspirin (ASA; cyclooxygenase inhibitor) and 1.875 mg/kg prasugrel (pras; P2Y₁₂ inhibitor) by oral gavage, followed by 5 days without treatment. n = 4; mean ± SEM. *P ≤ .05 for WT and RGS10^−/− vs RGS18^−/− vs RGS10^−/−RGS18^−/−. #P ≤ .05 for WT and RGS10^−/− vs RGS18^−/− and RGS10^−/−RGS18^−/−. Data showing the impact of these drugs on platelet activation is included in supplemental Figure 4. (B-C) Flow cytometric analysis of anti-TLT-1 binding (B) and TO staining (C) prior to and 5 days after drug treatment.