RHEOLOGY AND PLATELET-SURFACE ADHESION
Authors: Sakariassen S K , Fressinaud E , Meyer D , Sixma J J , Baumgartner R H 
Published: August 2018

The process of platelet adhesion to sites of vascular injury is pivotal for the arrest of bleeding. The same process may, on the other hand, lead to formation of mural thrombi and may play a role in atherogenesis through the release of platelet-derived growth factor. The events of platelet-surface adhesion may be divided into initial attachment and the subsequent spreading on the surface. These interactions are mediated by a variety of factors, including glycoproteins (GP) in the platelet membrane, von Willebrand factor (vWF) in plasma, and the composition of the surface.However, in most instances their effects on adhesion are dependent on the shear rate.We have investigated the importance of some of these factors in flowing blood at shear rates ranging from those present in large arteries (≃ 200 sec-1) to those present in the microcirculation (≃2600 sec-1). We used annular- and parallel-plate-perfusion chambers with de-endothelial-ized human arteries and collagen-coated surfaces, respectively. A highly reactive surface, such as formed by collagen fibrils, triggers rapid platelet adhesion and thrombus growth on the upstream portion of the surface. This rapid consumption of platelets depletes the boundary layer of the blood flow of platelets, resulting in fewer platelets to adhere further downstream. This effect is most pronounced at shear rates below 650 sec-1, i.e., at conditions with low radial transport of platelets. This phenomenon, apparently physical in nature, we have termed "axial dependence."Deficiency of GPIb results in impaired initial attachment of platelets to subendothelium, a defect which virtually abolishes initial attachment at shear rates above 500 sec-1. However, inhibition of binding of ADP to its putative 100Kd GP receptor by the adenosine analogue 5'-p-fluoro-sulfonyl-adenosine completely prevents initial attachment of platelets. Conversely, deficiency of GPIIb/IIIa results in partially impaired platelet spreading (20-50%), but only at shear rates above 1000 sec-1; deficiency of GPIa results in no platelet spreading, independent of the shear rate. The importance of GPIb and GPIIb/IIIa in adhesion was further demonstrated by experiments using monoclonal antibodies to either GPIb or GPIIb/IIIa.Addition of these antibodies to blood from healthy subjects duplicates the effects found in the natural deficiency states. Furthermore, we used two monoclonal antibodies to vWF, which specifically inhibit either ristocetin-induced binding of vWF to GPIb or thrombin/ADP-induced binding of vWF to GPIIb/IIIa. Each of these antibodies inhibits ≃ 90% of adhesion at 2600 sec-1 shear rate, while no effect is seen at 650 sec-1. Inhibition of binding of vWF, fibronectin, and fibrinogen to GPIIb/IIIa by a dodecapeptide of the γ-carboxy terminus of fibrinogen inhibits ≃ 30% of adhesion in normal blood at 2600 sec-1 shear rate. No effect is present at 650 sec-1 shear rate.These experiments indicate that (1) ADP and/or its putative receptor and GPIb are involved in the early phase of adhesion, whereas (2) GPIa and GPIIb/IIIa are involved in the spreading reaction. It is also apparent that (3) GPIb and GPIIb/IIIa act as receptors for vWF in the process of bridging the platelet to sites of vascular injury, and that (4) vWF is required for both initial attachment and spreading. (5) The shear rate has an unambiguous role in these complex interactions and in the "axial dependence" phenomenon.