Species variability in platelet aggregation response to different agonists

Conflicting data on platelet function in animal species are reported in the literature. In this study, the response of buffalo, horse, pig and sheep platelets to different agonists was assessed. Blood samples were collected from the jugular vein of six healthy subjects of each species and platelet-rich plasma was obtained by centrifugation. Platelet aggregation responses to increasing doses of adenosine 5'-diphosphate (ADP), arachidonic acid, collagen, platelet activating factor (PAF) and ristocetin were measured by a turbidimetric method. Horse platelets were the most responsive to ADP, collagen and PAF, whereas sheep platelets were the most responsive to ristocetin. The response to arachidonic acid varied least between species. PAF was the most effective agonist, inducing a maximum aggregation response at a concentration of 1 micro M for platelets of each species. Conversely, concentrations of ristocetin higher than 1mg/ml induced a maximum aggregation response only with sheep and horse platelets. The different responses of platelets from the four animal species to various agonists may reflect either (1). structural differences (including composition of the platelet membrane and presence of specific agonist receptors), or (2). activation of distinct signalling pathways by the agonist.