Agglutination of rabbit platelets in plasma by the thrombin inhibitor D-phenylalanyl-L-prolyl-L-arginyl chloromethyl ketone.

The specific thrombin inhibitor D-phenylalanyl-L-prolyl-L-arginyl chloromethyl ketone (FPRCH2Cl, PPACK) has been used as an anticoagulant with human blood to permit the preparation of platelet-rich plasma in which the physiological concentration of Ca2+ is maintained. When we attempted to use FPRCH2Cl (40 microM) as an anticoagulant for rabbit blood, clotting was prevented for at least 6 h at room temperature, but nearly all the platelets agglutinated. Thromboxane B2 was not formed and indomethacin and prostaglandin E1 were not inhibitory. The presence of citrate delayed this agglutination, but only EDTA (not EGTA) prevented it, indicating a dependence on Mg2+. Addition of FPRCH2Cl to suspensions of rabbit platelets in Tyrode-albumin solution had no effect on responses to ADP or collagen, but completely blocked all responses to human or rabbit thrombin. Thus plasma is required for the agglutination. Intravenous injection of FPRCH2Cl (5 mg/3 kg rabbit) reduced the platelet count to less than 50% by 15 to 25 min. Thus FPRCH2Cl cannot be used as an anticoagulant for rabbit blood when platelets are to be examined, and has a profound effect on the platelet count in vivo in rabbits. These effects should be considered when FPRCH2Cl is used as an antithrombotic agent for in vivo experiments with rabbits.