Actions and interactions of ADP, 5-HT, histamine and PAF on equine platelets

Platelets are thought to play a role in equine diseases such as acute lamin itis and verminous arteritis and may be involved in allergic disease. Media tors implicated in the pathophysiology of these conditions activate platele ts and responses may be enhanced by interactions between mediators. The pre sent study compared platelet aggregation, thromboxane production and releas e of radiolabel;ed [H-3]5-HT in response to 5-HT, histamine, ADP and PAF al one and in combination in vitro. PAF caused concentration-related aggregation, [H-3]5-HT release and thrombo xane production. In contrast, ADP caused aggregation and 5-HT induced the r elease of [H-3]5-HT With little effect on other platelet functions. Histami ne had little or no effect on equine platelets. Addition of 5-HT(10 mu M) p rior to ADP significantly displaced the aggregation response curve to the l eft. The profile of responses to PAF, ADP and 5-HT suggests differential activat ion of intracellular signalling pathways regulating these events. The enhan ced response to ADP in the presence of 5-HT may have implications in thromb oembolic disease in the horse. (C) 2000 Harcourt Publishers Ltd.