SHEAR-INDUCED PLATELET ACTIVATION AND PLATELET-NEUTROPHIL AGGREGATE FORMATION BY EQUINE PLATELETS

Objectives-To determine whether platelets become activated and form pl atelet-platelet or platelet-neutrophil aggregates, or both, when subje cted 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, 22 5, and 750/s for 3 minutes by use of a cone-plate viscometer. Flow cyt ometric techniques were used to identify activated platelets, platelet -platelet aggregates, and platelet-neutrophil aggregates. Results-Shea r 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 w as inhibited by blocking P-selectin, but not CD11/CD18 receptors. Conc lusions-Shear-induced platelet activation and aggregate formation occu r at physiologic shear rates. Clinical Relevance-Shear-induced platele t activation may explain the exercise-associated platelet-neutrophil a ggregates observed in Thoroughbreds undergoing treadmill exercise.