Expression of fibrinogen on the surface of ADP-stimulated platelets: comparison of human and rabbit platelets.

Fibrinogen is a cofactor in the aggregation of human platelets and must be added to suspensions of washed human platelets for extensive aggregation to occur in response to ADP. Although the patterns of fibrinogen binding and dissociation during platelet aggregation and deaggregation are similar for human and rabbit platelets, washed rabbit platelets aggregate extensively when stimulated with ADP even in the absence of added fibrinogen. To determine whether secreted fibrinogen was present on the surface of ADP-stimulated platelets and available to support aggregation, the binding of 125I-F(ab')2 fragments of anti-fibrinogen antibodies to formaldehyde-fixed platelets was measured. Although no fibrinogen was detected on human platelets either before or after treatment with ADP, fibrinogen was expressed on the surface of rabbit platelets after stimulation with ADP. However, this secreted fibrinogen did not dissociate during deaggregation of the platelets. Thus, the aggregation of rabbit platelets in the absence of exogenous fibrinogen may be supported by secreted fibrinogen, but a fibrinogen-independent component has not been completely ruled out.