Platelet activation in a circulating flow loop: combined effects of shear stress and exposure time.

Measurement of small changes in platelet activation state (PAS) in circulating stenotic systems in vitro has been problematic because of a paucity of real-time assay methods and circulation systems of low platelet-activating potential. PAS was measured by a modified prothrombinase assay in which activated platelets provide the essential cofactors in the activation of prothrombin by factor Xa. Chemical modification of the prothrombin ensures that the thrombin produced, while assayable, does not activate platelets. Human platelets were circulated in loops in which exposure to shear stress was adjusted by independently varying flow rate, viscosity, and the time of exposure to shear. Although with some differences in platelet response to different conditions of stress, the PAS directly increased with time of circulation, shear stress, and time of exposure to shear. The results show that low-level platelet activation caused by shear stress in a circulation loop can be quantitatively assessed in near-real time in a system of tube geometry. They confirm previous results obtained in non-circulating systems that exposure of platelets to shear conditions on the same order as found in the vasculature causes significant platelet activation, and that this activation is dependent on both shear stress and time of exposure.