Platelet Factor 4 and Adenosine Diphosphate Release during Human Platelet Aggregation

PLATELET FACTOR 4 (PF 4 ) is a low molecular weight, heat stable protein which neutralizes heparin 1,2 . Recent reports indicate PF 4 is released from human platelets during aggregation by adenosine diphosphate (ADP), adrenaline, thrombin and collagen 3,4 . It is known that human platelet aggregation in plasma containing citrate proceeds in two phases in response to adrenaline, ADP and thrombin 5–8 . Reversible platelet aggregation seems to be chiefly the result of changes in the platelet membrane induced by external stimuli. The second phase is accompanied by the selective release of various platelet contents, including adenine nucleotides, 5-hydroxytryptamine and certain lysosomal enzymes 9 . We have examined the release of PF 4 in human platelet-rich plasma with particular reference to the two phases of platelet aggregation. We also compared release of PF 4 with ADP release.