Shear-induced platelet activation and platelet-neutrophil aggregate formation by equine platelets

Objectives - To determine whether platelets become activated and form platelet-platelet or platelet-neutrophil aggregates, or both, when subjected to shear. Sample Population - Blood obtained from 3 Thoroughbreds. Procedures - Blood, with PCV adjusted to 32 (low hematocrit) or 60 (high hematocrit)%, was subjected to shear rates of 11.25, 22.5, 45, 90, 225, and 750/s for 3 minutes by use of a cone-plate viscometer. Flow cytometric techniques were used to identify activated platelets, platelet-platelet aggregates, and platelet-neutrophil aggregates. Results - Shear resulted in decreased platelet count, increased mean platelet volume, platelet activation, and formation of platelet-platelet and platelet-neutrophil aggregates. These changes occurred at lower shear rates in blood with high hematocrit. Platelet-neutrophil aggregate formation was inhibited by blocking P-selectin, but not CD11/CD18 receptors. Conclusions - Shear-induced platelet activation and aggregate formation occur at physiologic shear rates. Clinical Relevance - Shear-induced platelet activation may explain the exercise-associated platelet-neutrophil aggregates observed in Thoroughbreds undergoing treadmill exercise.