Effect of platelet agonists on the binding of platelet-activating factor to human platelets.

The binding of PAF in vitro to its receptors on platelets gradually becomes irreversible. This property has been applied to the measurement of PAF in circulating blood, the concept being that in vivo contact with endogenous PAF diminishes subsequent in vitro binding of 3H-labeled PAF. Here we show that, when platelets are stimulated with thrombin, the subsequent binding of [3H]PAF is greatly reduced, probably due to proteolytic damage of the PAF receptors. In addition, [3H]PAF binding is decreased, when the cell's energy status is low, which is seen during thrombin-induced secretion. Thus, the detection of PAF in circulating blood based on the binding to PAF receptors on platelets is severely hampered when platelets make contact with thrombin.