Studies on the platelet surface-associated von Willebrand factor.

The contribution of platelets to the prevention and control of bleeding depends not only on the plasma and vessel wall von Willebrand (vW) factor, but also on the vW factor originating from platelets. Although platelet vW factor is mainly stored in the alpha-granules, small amounts of vW factor have been detected on the platelet surface under conditions without direct stimulation. This communication focuses on the small amount of vW factor associated with the surface of platelets. EDTA-washed platelets in a calcium-free medium containing prostaglandin E1 (PGE1) were exposed to anti-vW factor F(ab')2 fragments. The platelet-associated anti-vW factor antibodies were separated from the free fragments by gel chromatography. Normal platelets were compared to platelets from patients with Bernard Soulier syndrome, Glanzmann's thrombasthenia, and Hermansky-Pudlak syndrome. These experiments showed that, in the absence of extracellular calcium, vW factor can be detected on the surface of washed platelets independent of glycoproteins Ib and IIb-IIIa, and despite markedly reduced intracellular calcium content. This particular platelet surface expression of vW factor is probably the result of disturbing the platelet during the in vitro manipulation. von Willebrand factor is an extremely important element in the multiple molecular interactions required to stop and anchor the platelet in areas of high flow rates. Mechanical perturbation of platelets leading to surface expression of the vW factor helps increase its availability for such critical interactions.