Platelet Adhesion to Proteins in Microplates : Applications in Experimental and Clinical Research

Abstract: Platelets are crucial for prevention of blood loss after vessel injury. Platelet adhesion to disrupted vessel walls is mediated by receptors such as the GPIb-IX-V complex that binds von Willebrand factor and the collagen-binding integrin α2β1. Also cross-linking of platelets, mediated by αIIbβ3 that binds to fibrinogen, results in platelet aggregation that further contributes to hemostasis. Platelets are also important pathophysiologically because of their role in thrombus formation following atherosclerotic plaque rupture. Pharmacological treatments aimed to prevent such events include use of platelet inhibitors such as acetylsalicylic acid (ASA) and clopidogrel. Despite the presence of several different platelet function assays, no one has so far been considered useful for clinical evaluation of the effect of anti-platelet treatment. The aim of this thesis was to evaluate possible applications in experimental as well as in clinical research for a platelet adhesion assay performed during static conditions. In principle, platelets in plasma are allowed to attach to protein coated microplates. Adhered platelets are then detected by induction of an enzymatic reaction followed by spectrophotometric measurements of the developed product. Our results show that the platelet adhesion assay is able to detect experimentally induced activation as well as inhibition of platelets. The assay also seems useful for investigation of synergistically induced platelet activation, especially when the coated surface consists of albumin. This is exemplified by the combination of lysophosphatidic acid and adrenaline, which induced a synergistically increased platelet adhesion to albumin that was dependent on αIIbβ3-receptors and on the secretion of ADP. Furthermore, secretion of ADP as well as TXA2 seems to contribute to several adhesive reactions investigated with this assay. The dependence on secretion, together with results showing that adhesion to collagen and fibrinogen is dependent on α2β1- and αIIbβ3-receptors respectively, indicate that the adhesive interactions occurring in the assay is in accordance with the general knowledge about platelet function. Regarding clinical applications, we found that platelet adhesion was increased for patients with essential thrombocythemia (ET) compared to controls. This is in line with the in vivo function of ET-platelets since a common complication for ET-patients is thrombosis. Furthermore, the assay was able to detect effects of treatment with clopidogrel in patients with unstable angina. To some extent it also measured the effects of ASA-treatment. In conclusion, our results suggest that the assay is suitable for experimental research and that further studies should be performed aimed at developing the assay into a clinically useful device.

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