Role of ADP-receptor antagonists in the tyrosine phosphorylation of HS1. Aspirin-treated and washed human platelets were stimulated with 100 ng/mL convulxin in the presence or absence of MRS2179 (10 μM), AR-C69931MX (100 nM), or both at 37°C, and the reaction was stopped by adding 2× cell lysis buffer. The stimulation time for convulxin was 30 seconds. HS1 was immunoprecipitated (IP) as described, and the samples were analyzed for tyrosine phosphorylation by Western blotting (IB) using the monoclonal phosphotyrosine (4G10) antibody. The Western blots shown are representative of experiments performed using platelets from 3 different donors.