Overview The adhesion-aggregation of platelets to a site of vessel wall injury is a quintessential blood flow phenomenon. Firstly, platelets are driven to the vicinity of the vessel wall by a form of convective diffusion in which red cells both mechanically augment the effective platelet diffusivity (Turitto et al., Ind. Eng. Chem. Fund. 11:216-223, 1972; Grabowski et al., Ind. Eng. Chem. Fund. 11:224-232, 1972) and enhance the near-wall piatelet concentration (Ti11es and Eckstein, Microvasc Res., In press, 1987). Secondly, red cells subjected to physiologic shear forces are capable of secreting sufficient adenine nucleotides to induce primary platelet aggregation without themselves undergoing frank lysis (Reimers et al, Blood 64:1200-1206, 1984). This "humoral" effect of erythrocytes is likely to contribute to primary hemostasis in a shear stress-dependent manner. Thirdly, endothelial cells are able to modulate platelet aggregation at a site of vessel injury by producing prostacyclin (and perhaps other antithrombotic substances) in a manner which increases with vessel shear rate (Grabowski et al, Blood 62:301a, 1983); production for a large range of arterial shear rates appears to be limited by plasma-borne substrate (arachidonate). This manner of production ensures a concentration of prostacyclin in the near-wall region which remains relatively independent of shear rate.Imaging primary hemostasis. In our work, epi-fluorescence videomicroscopy has allowed real time imaging of platelet adhesion-aggregation to a simulated vessel wall injury. The injury model is an endothelial cell monolayer (ECM) across which, prior to ECM exposure to flowing blood, a 6-0 sterile suture is drawn in a direction transverse to flow. Microinjuries result which measure 70 ± 15μm (Mean ± SD) in width. The fluorescent label is the TAB murine monoclonal antibody (courtesy of Dr. R.P. McEver) directed against human platelet GPIIB, together with a fluorescein-conjugated goat F(ab')2 against murine inmunoglobulin. The injured ECM's, grown to confluence on rectangular cover glasses precoated with microfibrillar collagen, comprise one wall of a flow chamber mounted on a vertical microscope stage. On microinjury sites and at shear rates of 100 to 700 sec-1, computer-enhanced video images show adherence, remodelling and growth of chains of platelet aggregates. Aligned with the flow direction, these chains have a spacing of approximately 30)im, a length similar to the average endothelial cell diameter. One may speculate that such chains provide a scaffold for wound healing insofar as they are likely rich in agents chemotactic for leukocytes and in platelet-derived growth factor.Modulatory role of endothelium. When the ECM's are pre treated with 1.0 mM FC lysine acetyl sal icy late (LA), aggregate length increases (P<0.001) up totwo-fold, outflow levels by RIA of serum thromboxane B2 increase (8 of 8 paired runs), and outflow levels of prostacyclin by RIA for 6-Keto PGFiot decrease (5 of 7 paired runs). The Table gives data for one of four similar experiments at 270 sec-1 and following five minutes of flow. These data imply that products of ECM which are inhibitable by aspirin modulate local adhesion-aggregation; their inhibition, as by vasculitis or drugs, may give rise to thrombotic states.Bleeding disorders. Aggregate length is reduced in von Willebrand's disease (4 patients), Hermansky-Pudlak syndrome (2 patients), and after 300 mg oral aspirin (Tablet 4 donors). The reduction in the first two, however, is greater (P<0.01) than that for oral aspirin. With oral aspirin, further, there is a paradoxic increase in the percent platelet coverage of the injury area.
Summary. Rheology has profound effects on the rate, structure, and modulation of primary hemostasis. Many of these effects can be studied via real-time, epi-fluorescence videomicroscopy of platelet adhesion-aggregation to a site of injury to an endothelial cell monolayer exposed to flowing blood. The model described has application to the study of thrombotic and hemostatic disorders and unstable angina.
Platelet von Willebrand factor: evidence for its involvement in platelet adhesion to collagen
