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 agg regating platelets and platelet microparticles (MPs) was investigated in a modified concentric-cylinder rotational viscometer. Whole blood a nticoagulated with citrate was exposed to a range of shear rates typic al of cardiopulmonary bypass circuits (0, 1000, 2000 and 4000 s(-1)) o ver four temperatures spanning hypothermic to mildly hyperthermic cond itions (24, 30, 37 and 42 degrees C) for short durations (100 s). Aliq uots of blood were double-stained for CD41 (platelet GPIIb/IIIa) and C D62 (P-selectin). Platelets, platelet aggregates, MPs and red blood ce ll-platelet and -MP aggregates were identified by flow cytometry by ac quiring only CD41-positive particles and differentiating on a plot of CD41 versus forward light scatter. The activation status of each parti cle was quantified by measuring CD62 expression (alpha-granule release ). A degree of correlation between the shedding of MPs and the formati on of platelet-platelet aggregates was observed for the data as a whol e (r=0.85 for p < 0.01), although this trend was not observed for a sh ear 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 she ar rate combinations. There was, however, a number of very highly acti vated MPs associated with red blood cells at high shear rates.