Adhesion of platelets to surface-bound fibrinogen under flow

After platelet activation, fibrinogen mediates platelet-platelet interactions leading to platelet aggregation. In addition, fibrinogen can also function as a cell adhesion molecule, providing a substratum for adhesion of platelets and endothelial cells. In this report, we studied the adhesion of platelets to surface-immobilized fibrinogen under flow in different shear rates. Heparinized whole blood containing mepacrine-labeled platelets was perfused for two minutes at various wall shear rates from 250 to 2,000 s^-1 in a parallel plate flow chamber. The number of adherent fluorescent platelets was quantitated every 15 seconds with an epifluorescent videomicroscope and digital image processing system. When compared with platelet adhesion and aggregation seen on glass surfaces coated with type I bovine collagen, a significant increase in platelet adhesion was observed on immobilized fibrinogen up to wall shear rates of 800 s^-1. The adherent platelets formed a single layer on fibrinogen-coated surfaces. Under identical conditions, no significant adhesion was observed on fibronectin- or vitronectin-coated surfaces. Although platelet adhesion to collagen was substantially inhibited by the platelet inhibitors prostaglandin E₁ and theophylline, these inhibitors had no effect on platelet adhesion to fibrinogen. Platelets adhered to recombinant homodimeric wild-type (γ400- 411) fibrinogen, but not to the recombinant homodimeric γ' variant of fibrinogen. Platelet adhesion to recombinant fibrinogen with RGD to RGE mutations at positions α95-97 and α572-574 was similar to that with plasma- derived fibrinogen. These results show that platelets adhere to fibrinogen- coated surfaces under moderate wall shear rates, that the interaction is mediated by the fibrinogen 400-411 sequence at the carboxy-terminus of the γ chain, and that the interaction is independent of platelet activation and the RGD sequences in the α chain.