Reversibility of the association of fibrinogen with rabbit platelets exposed to ADP

Fibrinogen enhances the adenosine diphosphate (ADP) induced aggregation of washed rabbit platelets and has previously been shown to associate with platelets upon the addition of ADP. This association has been investigated further by measuring the binding of 126I-flbnnogen to platelets exposed to ADP under physiologic conditions of aggregation and deaggregation. To determine the extent of 126I-fibrinogen binding, the platelets were removed by rapid centrifugation and the radioactivity in the platelet pellet was measured; trivalent chromium-51 was used as a space marker. If fibrinogen is present before ADP is added, the fibrinogen binds maximally to the platelets as they change shape in response to ADP. When ADP is not removed from the platelet suspension, deaggregation is slow, and fibrinogen also binds when added shortly after the ADP, but the amount that binds is less. The ability of the platelets to bind fibrinogen upon exposure to ADP is lost rapidly if an enzyme that removes the ADP is present in the suspension. A large portion of the 125I-fibrinogen bound to platelets during ADP stimulation, can be displaced by unlabeled fibrinogen added shortly after the ADP. Bound fibrinogen dissociates from platelets when they deaggregate upon removal of the ADP stimulus, and characterization of this fibrinogen by SDS-PAGE did not show any change resulting from its reactions with platelets. If the platelets are allowed to recover and then are stimulated a second time with ADP, they aggregate and bind fibrinogen to the same extent as platelets not previously aggregated with ADP. Thus, fibrinogen binding sites become available when platelets are exposed to ADP, become unavailable within a short time, and, if conditions are such that the platelets can recover, fibrinogen binding sites may become available again upon subsequent addition of ADP. It appears that neither the fibrinogen nor the fibrinogen binding site is permanently altered during the processes of aggregation and deaggregation.