Correlation between fibrinogen binding to human platelets and platelet

Fibrinogen is essential for aggregating platelets with adenosine diphosphate (ADP) and was recently shown to bind to platelets stimulated with ADP. The present work confirms the specific and saturable nature of the platelet-fibrinogen interaction. Binding of 12iodine-labeIed fibrinogen to human gel-filtered platelets was maximal at 1 mm. and the receptors were saturated when the fibrinogen concentration in the suspending medium approached 0.8 mg/mI. Assuming that one fibrinogen molecule interacts with a single receptor. experiments with 9 normal donors revealed the presence of 1 2.896 ± 2456 receptors per platelet. Much of the bound material dissociated from platelets after incubation with apyrase or EDTA. Binding was markedly inhibited at pH 6.5. in the presence of EDTA. and with platelets from 3 thrombasthenic patients but not with those from a patient with the Bernard-Soulier syndrome. Fibrinogen binding was also virtually absent with platelets that had been incubated with EDTA for 8 mm at 37’C and pH 7.8. These platelets could not aggregate when mixed with ADP and adequate CaCl2 and fibrinogen. although they could still change their shape. Thus. ADP-induced binding of fibrinogen correlates with platelet aggregability. T HE IMPORTANCE of fibrinogen as a cofactor in platelet aggregation has been recognized for some time.’ Its mechanism of action, however, has remained largely unknown. An association betwee