Original Article Haemorrheology of Equine Platelet–Neutrophil Aggregates D. J. Weiss, O. A. Evanson, D. McClenahan and J. J. Fagliari Department of Veterinary PathoBiology, University of Minnesota, St Paul, MN, USA Abstract. The filterability of platelet–neutrophil aggre- gates through filters containing 5 mm pores was determined to evaluate the potential of platelet– neutrophil aggregates to alter blood flow in the microvasculature. Incubation of platelet–neutrophil mixture with platelet-activating factor (PAF) and with 0.5 mm adenosine diphosphate (ADP) resulted in large platelet–neutrophil aggregates which rapidly plugged filter pores. Incubation of platelet–neutrophil mixtures with lower concentrations of ADP resulted in small platelet–neutrophil aggregates that contained only one neutrophil. Filtration of these samples resulted in a 1.3-, 3.2- and 3.7-fold increase in initial filtration pressure at ADP concentrations of 0.05, 0.15 and 0.25 mm, respectively. We conclude that platelet–neutrophil aggregates have reduced deformability and may alter blood flow in the microvasculature. Keywords: Haemorrheology; Platelet activation; Plate- let–neutrophil aggregates Introduction Platelet–neutrophils aggregates have been described in the blood of Thoroughbreds undergoing treadmill exercise and in ponies during the prodromal stages of experimental laminitis (Weiss and Evanson 1997; Weiss et al. 1998a). Platelet–neutrophil aggregates probably occur secondary to intravascular platelet activation (Hamburger and McEver 1990). Activated platelets express the alpha granule-derived glycoprotein P- selectin on their surface that binds to P-selectin glycoprotein ligand-1 (PSGL-1) which is constituitively expressed on neutrophils and monocytes (Rinder et al. 1991). Platelets can also bind to neutrophils through glycoprotein IIbIIIa. Fibrinogen, bound to glycoprotein IIbIIIa can interact with neutrophil CD11/CD18. (Weber and Springer 1997). We have studied agonist-induced and shear-induced equine platelet–neutrophil aggregate formation in vitro (Weiss and Evanson 1997; Weiss et al. 1998b). ADP and platelet-activating factor- (PAF) activated platelets readily formed platelet–neutrophil aggregates. Platelet– neutrophil aggregate formation was inhibited by EDTA indicating that the interaction was calcium-dependent. Subjecting whole blood to physiological shear stress also induced an increase in platelet–neutrophil aggre- gates. Aggregate formation was inhibited by anti- human P selectin but not by anti-horse CD11/CD18. Therefore, shear-induced platelet–neutrophils aggregate formation is probable mediated by P-selectin-PSGL-1 interactions. The clinical significance of platelet–neutrophil aggre- gates is poorly understood. Studies of experimental equine laminitis indicate that activated platelets and/or platelet–neutrophil aggregates may be involved in the pathogenesis of laminitis (Weiss et al. 1997a, 1998a). Platelet–neutrophil aggregates increased early in the prodromal stages of carbohydrate-induced laminitis (Weiss et al. 1997a). When platelet aggregation was inhibited by administration of a competitive inhibitor of glycoprotein IIbIIIa, the increase in platelet–neutrophil aggregates was attenuated and laminitis did not occur. The clinical significance of platelet–neutrophil aggre- gates in horses undergoing strenuous exercise has not been investigated. However, these aggregates have the potential to lodge in the microvasculature and alter blood flow. Comparative Haematology International (1999) 9:55–59 ß 1999 Springer-Verlag London Limited C OMPARATIVE HAEMATOLOGY I NTERNATIONAL Correspondence and offprint requests to: D. J. Weiss, Department of Veterinary PathoBiology, University of Minnesota, St Paul, MN 55108, USA.