Activation status of platelet aggregates and platelet microparticles shed in sheared whole blood

The role of temperature and shear rate in the activation status of aggregating platelets and platelet microparticles (MPs) was investigated in a modified concentric-cylinder rotational viscometer. Whole blood anticoagulated with citrate was exposed to a range of shear rates typical of cardiopulmonary bypass circuits (0, 1000, 2000 and 4000 s-1) over four temperatures spanning hypothermic to mildly hyperthermic conditions (24, 30, 37 and 42°C) for short durations (100 s). Aliquots of blood were double-stained for CD41 (platelet GPIIb/IIIa) and CD62 (P-selectin). Platelets, platelet aggregates, MPs and red blood cell-platelet and -MP aggregates were identified by flow cytometry by acquiring only CD41-positive particles and differentiating on a plot of CD41 versus forward light scatter. The activation status of each particle was quantified by measuring CD62 expression (α-granule release). A degree of correlation between the shedding of MPs and the formation of platelet–platelet aggregates was observed for the data as a whole (r=0.85 for p<0.01), although this trend was not observed for a shear rate of 4000 s-1. The mean expression of CD62 on both platelets and MPs was maintained at a very low level for all temperature and shear rate combinations. There was, however, a number of very highly activated MPs associated with red blood cells at high shear rates.