The recent observation that knock-out of protease-activated receptor-4 (PAR4) ablates thrombin signaling in mouse platelets and protects against ferric chlorideinduced thrombosis of mouse mesenteric arterioles suggests that thrombin's actions on platelets can play an important role in thrombosis. Complete ablation of thrombin signaling would be difficult to achieve in human beings because human platelets have 2 thrombin receptors that are each capable of mediating transmembrane signaling. However, it is possible that complete ablation of thrombin signaling in platelets is not necessary for an antithrombotic effect. In mouse platelets, PAR3 functions as a cofactor that binds thrombin and promotes productive cleavage of PAR4, and thrombin responses are decreased but not absent in Par3 ؊/؊ platelets. We now report that Par3 ؊/؊ mice were protected against ferric chlorideinduced thrombosis of mesenteric arterioles and against thromboplastin-induced pulmonary embolism. Surprisingly, Par3 ؊/؊ and Par4 ؊/؊ mice showed similar degrees of protection in these models and similar prolongation of tail bleeding times. Thus, even a partial decrease in mouse platelet responsiveness to thrombin protected against thrombosis and impaired hemostasis in some settings. These results demonstrate the importance of PAR3's unusual cofactor function and underscore the relative importance of thrombin's actions on platelets in vivo. They also suggest that PAR inhibition might be explored for the prevention or treatment of thrombosis in human beings. (Blood. 2002;100:3240-3244)

The coagulation protease Factor Xa (Xa) 1 triggers a variety of cellular responses that may be important for inflammatory reactions to tissue injury. Protease-activated receptors (PAR1, PAR2, and PAR4) can mediate Xa signaling in heterologous expression systems. However, other candidate Xa receptors have been described, and the extent to which one or more PARs account for Xa signaling in relevant differentiated cells is unknown. We examined Xa signaling in endothelial cells from wildtype and PAR-deficient mice. Wild-type endothelial cells responded to agonists for PAR1, PAR2, and PAR4. Relative to wild-type, Xa-triggered phosphoinositide hydrolysis was reduced by 60 -75% in Par2 ؊/؊ endothelial cells, by 20 -30% in Par1 ؊/؊ endothelial cells, and by ϳ90% in Par2 ؊/؊ endothelial cells treated with a PAR1 antagonist. Similar results were obtained when ERK1/2 phosphorylation was used to assess Xa signaling. Thus PAR2 is the main endogenous Xa receptor in these endothelial cell preparations and, together, PAR2 and PAR1 appear to account for ϳ90% of endothelial Xa signaling. By contrast, in fibroblasts, PAR1 by itself accounted for virtually all Xa-induced phosphoinositide hydrolysis. This information is critical for the design and interpretation of knockout mouse studies to probe the possible roles of Xa signaling in vivo.The coagulation protease factor Xa (Xa) is generated at sites of vascular injury and inflammation by the actions of the tissue factor/VIIa (TF⅐VIIa) 1 and VIIIa⅐IXa complexes (1, 2). The formation of Xa is localized to the surfaces of cells and membrane vesicles, and in addition to binding Va to form the prothrombinase complex, Xa can directly regulate cellular behavior. For example, Xa can cause endothelial cells to release cytokines (3, 4), display adhesion molecules (4), proliferate (5), and trigger endothelial-dependent vasorelaxation (6). Importantly, studies in animal models of septic shock and the recent clinical trial of activated protein C strongly suggest that coagulation proteases contribute to organ damage and death in this syndrome (7)(8)(9)(10)(11)(12). Such studies have also raised the possibility that coagulation proteases upstream of thrombin might contribute to the pathogenesis of septic shock via activities unrelated to thrombin generation (11). These considerations motivate efforts to identify the receptors that mediate Xa responses in endothelial cells and other cell types.Protease-activated receptors (PARs) are G protein-coupled receptors that mediate signaling to thrombin and other proteases by a mechanism that requires proteolytic cleavage of the receptor (13). Because inhibitors that block the proteolytic activity of Xa also ablate its ability to trigger cellular responses, PARs are candidates for mediating Xa signaling (14,15). Indeed, there is substantial evidence that Xa can signal via PARs. For example, expression of PAR1, PAR2, and PAR4 in Xenopus oocytes confers calcium signaling in response to Xa (16). TF⅐VIIa complex can also activate PAR2 expressed in oocytes and...

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Purpose: To test the potential of magnetic resonance imaging (MRI) in early detection of left ventricular (LV) dysfunction in patients with pulmonary regurgitation and normal LV ejection fraction after repair of tetralogy of Fallot.Materials and Methods: Patients (n ¼ 18) with repaired tetralogy of Fallot and pulmonary regurgitation were prospectively recruited. Healthy volunteers (n ¼ 10) were used as control. Tagging MR images were acquired at the base, mid, and apical LV levels for assessing segmental rotation and circumferential strain. Cine MR images and velocity-encoded MR images were also acquired for assessment of biventricular volumes and biventricular function and pulmonary regurgitant fraction, respectively. Mean values were compared between groups using unpaired Student's t-test.Results: Patients presented with preserved global LV function (LVEF of 59 6 5%). A significant decrease in LV peak circumferential strain was seen in patients compared with normal volunteers at the basilar (À15.6 6 4.5% vs. À17.6 6 4.4%; P < 0.01) and apical (À14.46 6.1% vs. À17.36 5.1%, P < 0.01) slices. LV peak rotation was also delayed in patients compared with volunteers at the basilar (6.1 6 2.6 vs. 4.2 6 0.6 ; P < 0.01) and mid (8.0 6 1.7 vs. 4.9 6 1.0 ; P < 0.01) slices.Conclusion: MRI can detect early regional LV dysfunction in patients with preserved LVEF after repair of tetralogy of Fallot. MR may be a useful technique for guiding clinical decisions in these patients in order to prevent future global LV deterioration.

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