REGULAR ARTICLE Thrombin formation in vitro in response to shear-induced activation of platelets Anna M. Fallon a , Ulla M. Marzec b , Stephen R. Hanson b , Ajit P. Yoganathan c, ⁎ a School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States b Department of Biomedical Engineering, Oregon Health & Science University, Portland, Oregon, United States c Wallace H. Coulter School of Biomedical Engineering, Georgia Institute of Technology & Emory University, Room 2119 U.A. Whitaker Building, 313 Ferst Dr., Atlanta, GA 30332-0535, United States Received 1 December 2006; received in revised form 23 February 2007; accepted 11 April 2007 Available online 29 May 2007 Abstract Introduction: Thromboembolic events caused by implanted vascular devices present serious medical challenges. In particular bileaflet mechanical heart valves (MHVs) are prone to thrombus formation in the hinge region due to a combination of high shear stress and stagnation regions. Most studies of shear-induced platelet activation and aggregation have been performed using viscometers, parallel plate flow, and other non-physiologic in vitro configurations. The present study investigated these events in a physiogically relevant environment in which thrombin formation in response to shear stress activation of platelets plays a more predominant role. Materials and methods: Anticoagulated (citrated) human blood was placed in a steady flow loop containing a 400 μm round orifice or various MHVs in the leakage position. Simultaneous blood recalcification enhanced the thrombus forming potential of the blood. Aggrastat and AN51 were used to block binding to the platelet GPIIb/IIIa and GPIb receptors, respectively, and aspirin was used to block thromboxane production. Thrombin generation was measured indirectly by the thrombin-antithrombin III assay. Results and conclusions: Aggrastat, AN51, and aspirin all suppressed thrombin formation. Furthermore, histological results suggested important roles for vWF and fibrinogen in a two-step model of thrombus formation. Thus, thrombin is reproducibly formed in this in vitro system, a process that can be suppressed by blocking platelet activation. This system has the potential to investigate mechanisms and interventions for medical devices that contact with blood under varying shear stress conditions. © 2007 Elsevier Ltd. All rights reserved. KEYWORDS GPIb; GPIIb/IIIa; Prostaglandins; Shear-induced platelet activation (SIPAct) Abbreviations: MHV , mechanical heart valve; LDV , laser Doppler velocimetry; vWF , von Willebrand factor; Ca 2+ , calcium; TAT , thrombin-antithrombin III; SIPA, shear-induced platelet aggregation; SIPAct, shear-induced platelet activation. ⁎ Corresponding author. Tel.: +1 404 894 2849; fax: +1 404 894 4243. E-mail address: ajit.yoganathan@bme.gatech.edu (A.P.Yoganathan). intl.elsevierhealth.com/journals/thre 0049-3848/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.thromres.2007.04.006 Thrombosis Research (2007) 121, 397–406