A rheological technique was employed to study the platelet-fibrin interaction, measuring the clot rigidity (dynamic shear elastic moduli (G')) and the platelet generated contractile force. The roles of plasma fibronectin, actin and platelet membrane glycoproteins in the platelet-fibrin interaction were investigated.
Plasma fibronectin, in the physiological range of concentration, has only minimal effect on the clot rigidity of either PFP or PRP samples. There is essentially no difference in the maximum G'(G'(,max)) of PRP and fibronectin-free PRP. Therefore, fibronectin does not enhance the mechanical properties of plasma clots and probably does not play a role in the fibrin binding to the platelet membranes.
Externally exposed actin on platelet membranes is unlikely to be important in the platelet-fibrin binding. Addition of DNase I has only minimal effect on the G'(,max) of either PFP or PRP. DNase I forms a stable 1:1 complex with actin.
Membrane glycoproteins appear to be important in the platelet-fibrin interaction. Tetravalent ConA, a carbohydrate-binding protein, lowers the G'(,max) and contractile force generation of PRP clots by a small extent. Plasma glycoproteins such as coagulation factors compete with the platelet surface glycoproteins for the ConA binding in PRP. Tetravalent ConA as well as divalent ConA lower the G'(,max) and contractile force generation of washed platelet-fibrin clots dramatically, almost down to those values found for fibrin clots. Inhibition studies with (alpha)-methyl-D-mannoside indicate the ConA effects are specific to binding induced alterations. The tetravalent ConA effects are reversible, while the divalent ConA effects are completely reversible. ConA effects are more likely due to its binding capacity to platelet membranes than to post-binding glycoprotein rearrangement. Thus ConA binding site appears to be important in the fibrin binding to platelet surface. The glycoprotein IIb and III complex is likely to be associated with the fibrin receptor.