Fibronectin expression on the platelet surface occurs in concert with secretion.

Thrombin stimulation of human platelets causes increased cellular adhesiveness for other platelets (aggregation) and surfaces and increased surface expression of platelet fibronectin antigen. Aggregation occurs concurrently with secretion. In these studies, the threshold thrombin dose for surface expression of fibronectin, as measured by binding of F(ab')2 antifibronectin, was similar to that for serotonin secretion. Moreover, both processes occurred at similar rates, and inhibition of secretion was associated with inhibition of antifibronectin binding. Thus a hypothesis is proposed in which adhesive proteins within platelet granules become expressed on the platelet surface as a direct consequence of the secretory process. This cluster of adhesive proteins may then contribute to increased cellular adhesiveness.