Role in platelet adhesion

Several investigators have found that foreign surfaces, when exposed to blood, adsorb plasma proteins (21, 22). Platelet adhesion to this proteinated surface is the first observable event in thrombosis on the foreign surface. If we consider the proteinated surface as an acceptor in a platelet adhesion mechanism (23), the significance of the nature and composition of adsorbed protein is its role in platelet adhesion (24). 

The exposed collagen fiber of the subendothelial layers may form the trap for the circulating platelets. Indeed, when collagen fibers are added to platelet-rich plasma, platelet clumps form at the surface of the fiber. The adhesion requires ADP and is inhibited by EDTA and collagenase. Thus, in vascular injury, the release of an ADP-like substance and the denudation of collagen fiber appear to play a determinant role in platelet adhesion. 

Surface Tension. Interfacial surface tension between fluid and filter media is considered to play a role in the adhesion of blood cells to synthetic fibers. Interfacial tension is a result of the interaction between the surface tension of the fluid and the filter media. Direct experimental evidence has shown that varying this interfacial tension influences the adhesion of blood cells to biomaterials. The viscosity of the blood product is important in the shear forces of the fluid to the attached cells viscosity of a red cell concentrate is at least 500 times that of a platelet concentrate. This has a considerable effect on the shear and flow rates through the filter. The surface stickiness plays a role in the critical shear force for detachment of adhered blood cells. 

Collagen is a major component of connective tissue that becomes exposed at the subendothelium of injured blood vessels. It contributes to platelet adhesion and also plays a role in platelet activation by binding to several receptors on platelets such as integrin a 2(3 1 or glycoprotein VI (GP VI). 

Platelets also express adhesion receptors of the selectin gene family (P-selectin) and of the immunoglobulin gene superfamily (PECAM-1 and ICAM-2) (6). Expression of PECAM-1 and ICAM-2 may play a role in platelet interactions with endothelial cells and leukocytes, respectively. 

Other platelet membrane glycoproteins that are involved in adhesion reactions are GPIV and GPVI, which play a role in platelet interactions with and activation by collagen (6, 7). GPIV also functions as a receptor for the platelet alpha granule protein thrombospondin (2, 7, 8). Table 1 summarizes the functions of platelet membrane receptors. 

H.R. Baumgartner, The role of blood flow in platelet adhesion, fibrin deposition, and formation of mural thrombi, Microvasc. Res. 5(2) (1973) 167-179. 

Radomski, M.W., Palmer, R.M., and Moncada, S., The role of nitric oxide and cGMP in platelet adhesion to vascular endothelium, Biochem. Biophys. Res. Commun., 148, 1482, 1987. 

The intetaction of proteolytically-active a-tfarombin with platelets and other cells of the vasculature, such as endothelial cells and smooth muscle cells, plays a major role in both normal hemostasis and atherosclerosis. Despite extensive studies in numerous laboratories extending back over thirty years, major questions regarding the mechanism of these interactions remain uruesolved. Furthermore, since dirombin can also induce chemotaxis and adhesion of inflammatory cells, and fibroblast mitogenesis, the importance of elucidating the nature of its receptor, or receptors, extends r beyond its role in platelet activation. However, this review will be restricted mainly to considerations of thrombin receptors in human platelets. 

Fibronectin is present in plasma, subendothelium and platelet alpha granules. Despite containing the RGD sequence, the role of fibronectin in adhesive and cohesive properties of platelets seems less important. It has been suggested that plasma fibronectin would not be required for platelets to adhere on vascular extracellular matrices [45]. However, studies with plasma depleted of fibronectin seem to indicate that this adhesive protein could play a role in mediating adhesion of platelet and thrombus formation on collagen rich surfaces [46]. As will be commented on later, the mote recent studies suggest that vascular fibronectin would mediate platelet adhesion under certain experimental conditions [47]. 

To determine if the air nuclei that can be trapped in the roughness of a synthetic material play a major role in cellular adhesion to the material, a series of experiments was conducted in which the air nuclei were removed before the material was exposed to a biological fluid. The biological fluids examined were blood, washed platelet suspensions and fibrinogen solutions. 

Wu Y, Simonovsky FI, Ratner BD, Horbett TA. The role of adsorbed fibrinogen in platelet adhesion to polyurethane surfaces a comparison of surface hydrophobicity, protein adsorption, monoclonal antibody binding, and platelet adhesion. J Biomed Mater Res A 2005 74(4) 722-38. 

Comparable with red blood cells, desialylation of platelets leads to a rapid removal from the circulation (Choi et al. 1972, Greenberg et al. 1979). Despite contradictory results (Nurden and Caen 1976, Gartner et al. 1977, Patschke et al. 1977), there are valid indications that membrane-bound sialic acid does play an important role in the adhesion and aggregation of thrombocytes. (For further details see section IV, Blood Clotting.)... 

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