Regulation of protease-activated receptor 1 (PAR1) on platelets and responsiveness to thrombin receptor activating peptide (TRAP) during systemic inflammation in humans.

Thrombin is a coagulation protease that activates platelets, endothelial cells, leukocytes and mesenchymal cells. Thrombin signaling is mediated at least in part by protease-activated receptors (PARs). As little is known about the in vivo regulation of PAR1, this study aimed to characterize the effects of systemic thrombin formation during human endotoxemia on the regulation of PAR1 and the associated responsiveness of human platelets to thrombin receptor activating peptide (TRAP). Endotoxin (2 ng/kg) was infused into 40 healthy men to study the regulation of PAR1 in systemic human inflammation. The SPAN12 antibody was used to determine the in vivo regulation of PAR1. To measure whether modulation of the PAR1 receptor may be associated with altered platelet reactivity, whole blood was stimulated with TRAP ex vivo. Thrombin generation was determined by prothrombin (F(1+2)) fragment. F(1+2) levels increased almost 9-fold from 0.5+/-0.1 nmol/L to 4.5+/-1.9 nmol/L at 4 h (p<0.001). PAR1 decreased by approximately 8% (p<0.001) within 2 h after endotoxin infusion and stayed at those levels until 6 h. Concomitantly, TRAP induced P-selectin expression maximally decreased by 18% (p<0.001) at 6 h. In conclusion, PAR1 expression is down-regulated on platelets during systemic thrombin formation induced by inflammation in humans which results in decreased responsiveness to subsequent stimulation of the PAR1 receptor.

Recombinant human activated protein C (rhAPC; drotrecogin alfa [activated]) has minimal effect on markers of coagulation, fibrinolysis, and inflammation in acute human endotoxemia.

Inflammatory and procoagulant host responses are closely related in sepsis. The protein C pathway serves as a regulatory pathway with anti-inflammatory and anticoagulant properties. Recently, recombinant human activated protein C (rhAPC) was shown to reduce mortality in severe sepsis. Nevertheless, the effects of rhAPC in humans are still ill defined. The infusion of low endotoxin doses into humans provides a standardized model to study inflammatory and hemostatic mechanisms. Thus, we investigated whether rhAPC acts as an anticoagulant or anti-inflammatory drug in human endotoxemia. There were 24 volunteers randomized to receive either 24 microg/kg per hour rhAPC or placebo intravenously for 8 hours. Lipopolysaccharide (LPS, 2 ng/kg) was administered 2 hours after starting the infusions. rhAPC decreased basal tissue factor (TF)-mRNA expression, and thrombin formation and action. In contrast, rhAPC did not significantly blunt LPS-induced thrombin generation. Consistently, rhAPC did not reduce LPS-induced levels of TF-mRNA or D-dimer and had no effect on fibrinolytic activity or inflammation. Finally, endogenous APC formation was enhanced during endotoxemia and appeared to be associated with inflammation rather than thrombin formation. In conclusion, even low-grade endotoxemia induces significant protein C activation. Infusion of rhAPC decreases "spontaneous" activation of coagulation but does not blunt LPS-induced, TF-mediated coagulation in healthy volunteers, which is in contrast to a number of anticoagulants.