Increased integrin activation, α-granule exocytosis, aggregation, and ATP release in platelets from Grk5^−/− mice. (A) Platelets from Grk5^−/− and littermate control (WT) mice were stained with antibodies to activate ɑ_IIbβ₃ (Jon/A; left panel) or P-selectin (right panel) and measured by flow cytometry. Platelets were stimulated with Par-4–AP (AYPGKF) (n = 3). Platelet reactivity in response to Par-4–AP (n = 5) (B), representative aggregation traces (C), and summarized data (D) for platelets stimulated with thrombin (mean ± SEM; n = 3-6 mice at each tested concentration). (E) ATP release for platelets from Grk5^−/− and littermate control mice (WT) stimulated with Par-4–AP (AYPGKF). The results of 3 experiments (mean ± SEM) are summarized. MFI, mean fluorescence intensity; ns, not significant.