Deletion of GRK2 in platelets causes increased platelet accumulation in the shell region, shortens tail bleeding time, and enhances thrombosis. P2Y₁₂ inhibition eliminates the phenotypic difference at the site of injury. (A) Laser-induced thrombus formation in WT and GRK2^−/− mice treated with the P2Y₁₂ antagonist (cangrelor). Cangrelor or vehicle (saline) was administered by IV immediately before each of the laser injuries. (Ai) Confocal intravital fluorescence microscopy was then performed to follow platelet accumulation (CD41) over 3 minutes. (Aii) Platelet accumulation at the site of injury as measured by CD41 of 46 injuries in 3 WT mice treated with saline, 52 injuries in 4 GRK2^−/− mice treated with saline, 44 injuries in 4 WT mice treated with cangrelor, and 39 injuries in 4 GRK2^−/− mice treated with cangrelor. Data sets were compared by using an unbalanced two-way analysis of variance with Tukey’s post hoc test. (B) Hemostatic response was measured by tail bleeding. Tails of anesthetized mice were transected and the time to complete arrest of bleeding (as defined by no bleeding recurring within 2 minutes) was recorded for each mouse. n = 9 mice per genotype. (Ci) Representative images of anti-glycoprotein IX (GPIX)–labeled thrombi in lungs harvested from WT and GRK2^−/− mice treated with ADP in the absence or presence of clopidogrel (P2Y₁₂ antagonist). (Cii and Ciii) Thrombosis scores, representing the mean thrombus area and number of thrombi, of 5 WT mice vs 5 GRK2^−/− mice are summarized in each group. P ≤ .05 was considered statistically significant. (D) The carotid arteries of WT and GRK2^−/− mice were injured by 7.5% FeCl₃. Flow rates through the carotid artery were measured with a Doppler flow probe after vascular injury. (Di) Quantitation of the percentage of mice that formed a stable clot, unstable clot, or no occlusion. A χ² test was conducted, producing a P value of .011. (Dii) The probability of forming unstable occlusion at a respective time interval for WT or GRK2^−/− mice. Experiments were terminated at 30 minutes. Kaplan-Meier curves were analyzed by using the log-rank test. n = 6 mice per genotype. Data are presented as mean ± standard error of the mean. AUC, area under the curve. ns, not significant.