Summary
A variety of data suggest that fibrinogen binding is necessary but not sufficient
for platelet aggregation: post fibrinogen binding events may play an important role.
The present study compared fibrinogen binding and platelet aggregation in response
to dithiothreitol (DTT) and ADP. DTT induced saturable and specific fibrinogen binding
(Kd 0.07 + 0.02 μM, Bmax 15,000 + 3000 molecules/platelet) which supported complete
platelet aggregation as determined by single platelet counting. The aggregates were
small, however, and more readily dissociated by EDTA than their ADP-treated counterparts,
despite quantitatively similar fibrinogen binding. Unlike fibrinogen bound to ADP-stimulated
platelets, fibrinogen bound to DTT-treated platelets remained sensitive to dissociation
by EDTA over a 3 h time course, retained its ability to support aggregation, even
when aggregation was induced 60 min after the initial platelet exposure to fibrinogen,
and remained accessible to polyclonal antibodies and plasmin. Confocal scanning laser
microscopy showed only surface clustering of fibrinogen bound to DTT-treated platelets
over the 3 h time course compared to rapid fibrinogen clearing from the surface of
ADP-stimulated platelets. These data suggest that post fibrinogen binding events involved
in the stabilization of fibrinogen binding and/or the redistribution of bound fibrinogen
may play important roles in regulating platelet aggregation.
