Plasma albumin is essential for collagen-induced platelet aggregation.

We found that platelets must have albumin on the surface to respond to collagen and aggregate. Albumin, however, was not absolutely necessary for ADP-, platelet activating factor-, serotonin- or thrombin-induced aggregation, while fibrinogen was required for ADP- or serotonin-induced aggregation. Immunofluorescent microscopy revealed that albumin was retained on gel-filtrated platelets but not on washed platelets. Albumin was not required for platelet adhesion to immobilized collagen. Without albumin thromboxane formation upon collagen-stimulation was diminished. These data suggest that albumin is essential in some step(s) that results in production of thromboxane A2.