Increased platelet activation and calcium mobilization downstream of P2Y₁-dependent signaling in GRK2^−/− platelets. (A) Platelet activation. Platelets from GRK2^−/− and littermate control mice (WT) were stained with fluorophore-conjugated antibodies to either activated integrin α_IIbβ₃ (Jon/A antibody) or P-selectin and measured by flow cytometry. Platelets were stimulated with MRS2365 (MRS). n = 4. (B) Calcium mobilization. Mouse platelets were loaded with fura-2 AM (10 μM) and stimulated with ADP or MRS at the concentrations indicated in the absence of extracellular Ca²⁺. Representative measurements are shown. The results of 6 experiments, ADP (upper), and the results of 4 experiments, MRS (lower), are summarized (mean ± standard error of the mean). MFI, mean fluorescence intensity; ns, not significant.