Platelet activation

Influence of different anticoagulants on platelet aggregation in whole blood; a comparison between citrate, low molecular mass heparin and hirudin

Anticoagulants used for platelet function studies in vitro may affect platelet responsiveness. In the present study we compared the influence of three different anticoagulants, sodium citrate, low molecular mass heparin, and recombinant hirudin, on platelet aggregation in whole blood in vitro, using impedance aggregometry. ADP and collagen induced aggregation was significantly lower in citrated blood compared to hirudin treated blood, reflecting the importance of extracellular calcium for platelet function. Inhibition of platelet aggregation by aspirin, was more pronounced in citrated blood compared to hirudin treated blood, in agreement with the concept of artifactually enhanced thromboxane generation in media containing low extracellular calcium levels. In blood anticoagulated with low molecular mass heparin, platelet aggregation to collagen tended to be enhanced as compared to hirudin treated blood, whereas platelet responses to ADP at a high concentration were slightly reduced. Of the anticoagulants investigated, the selective thrombin inhibitor hirudin is the most suitable anticoagulant for studies of platelet aggregation in vitro in whole blood.

Tyrosine phosphorylation of the novel protein-tyrosine kinase RAFTK during an early phase of platelet activation by an integrin glycoprotein IIb-IIIa-independent mechanism

A key regulatory event controlling platelet activation is mediated through the phosphorylation of several cellular proteins by protein-tyrosine kinases. The related adhesion focal tyrosine kinase (RAFTK) is a novel cytoplasmic tyrosine kinase and a member of the focal adhesion kinase (FAK) gene family. FAK phosphorylation in platelets is integrin-dependent, occurs in a late stage of platelet activation, and is dependent on platelet aggregation. In this study, we have investigated the involvement of RAFTK phosphorylation during different stages of platelet activation. Treatment of platelets with thrombin induced, in as early as 10 s, a rapid tyrosine phosphorylation of RAFTK in a time- and concentration-dependent manner. Treatment of platelets with thrombin in the absence of stirring or pretreatment of platelets with RGDS peptide prevented platelet aggregation, but not RAFTK phosphorylation. Furthermore, phosphorylation of RAFTK did not require integrin engagement since platelets treated with the 7E3 inhibitory antibodies that block fibrinogen binding to glycoprotein IIb-IIIa did not inhibit RAFTK phosphorylation. Similarly, platelets treated with LIBS6 antibodies, which specifically activate glycoprotein IIb-IIIa, did not induce RAFTK phosphorylation. Stimulation of platelets by several agonists such as collagen, ADP, epinephrine, and calcium ionophore A23187 induced RAFTK phosphorylation. Tyrosine phosphorylation of RAFTK in platelets is regulated by calcium and is mediated through the protein kinase C pathway. Phosphorylation of RAFTK is dependent upon the formation of actin cytoskeleton as disruption of actin polymerization by cytochalasin D significantly inhibited this phosphorylation. The RAFTK protein appears to be proteolytically cleaved by calpain in an aggregation dependent manner upon thrombin stimulation. These results demonstrate that RAFTK is tyrosine-phosphorylated during an early phase of platelet activation by an integrin- independent mechanism and is not dependent on platelet aggregation, suggesting different mechanisms of regulation for FAK and RAFTK phosphorylation during platelet activation.

Characterization of in vitro and in vivo platelet responses to thrombin and thrombin receptor-activating peptides in guinea pigs

Guinea pig platelets are similar to human platelets in their responsiveness to thrombin receptor-activating peptides and other agonists. Therefore, guinea pigs anesthetized with Inactin (90 mg/kg i.p.) were used to assess in vivo activities of thrombin and thrombin receptor-activating peptides (TRAPs) using 111 In-labeled platelets and a microcomputer-based system. The aggregatory responses are expressed as percent change for a 20 min period over basal radioactivity (AUC). Reversible accumulation of platelets occurred in the pulmonary microcirculation in response to stimuli. Human thrombin (50 and 100 U/kg i.v.) caused a dose-related platelet accumulation. Responses of similar magnitude were induced by SFLLRN (TRAP-(1-6)) and Ala-Phe(p-F)-Arg-Cha-HArg-Tyr-NH2 (high-affinity thrombin receptor-activating peptide, 0.03, 0.1 and 0.3 mg/kg i.v.). High-affinity thrombin receptor-activating peptide, a new synthetic oligopeptide agonist, is about 3-fold more potent than TRAP-(1-6), a wild-type sequence. Similarly, high-affinity thrombin receptor-activating peptide is about 4 times more potent than TRAP-(1-6) in the radioligand binding study using platelet membrane. By comparison, high-affinity thrombin receptor-activating peptide manifested an aggregatory activity (EC60 = 1.2 microM) about 15 times more potent than that of TRAP-(1-6)(EC60 = 18.6 microM) in washed guinea pig platelets. The intrapulmonary platelet aggregation in response to thrombin, TRAP-(1-6) and high-affinity thrombin receptor-activating peptide was characterized by long duration (approximately 30 min); a reduction in response (18-54%) tended to occur with repeated challenges, presumably due to desensitization and consumption. The response to thrombin (100 U/kg) was greatly inhibited by (D)-Phe-Pro-Arg-chloromethyl ketone (PPACK), a potent thrombin inhibitor (250 micrograms/kg + 6 micrograms/kg per min i.v. x 30): AUC, 150 +/- 552 vs. 7171 +/- 1052 in the control period (n = 8, P < 0.05). The response to high-affinity thrombin receptor-activating peptide (0.03 mg/kg), which acts on thrombin receptor directly, was not affected by PPACK. It is concluded that guinea pigs are an appropriate preparation for evaluation of in vivo activity of thrombin inhibitors as well as thrombin receptor agonists and antagonists.