Inhibition of platelet aggregation by primary amines. Evidence for a possible role of membrane-associated calcium.

The aggregation of human blood platelets by thrombin, adenosine diphosphate, wheat germ agglutinin or ristocetin was inhibited by primary amines. In general, thrombin-induced platelet aggregation was strongly affected by the amines while the effect was weak on cell aggregation by ristocetin. Usually, the diamines were stronger inhibitors of aggregation than the monoamines with cadaverine as the strongest and ethylamine as the weakest inhibitor. At concentration where platelet aggregation was inhibited, the amines neither displaced serotonin from serotonin-loaded platelets nor caused lysis of human red cells. The lectin activity of wheat germ agglutinin on human red cells was not affected by the amines indicating that the amines probably acted on platelets and not on the agglutinin. The clotting activity of thrombin on fibrinogen was partially inhibited by the amines while its esterolytic activity remained unaltered. The inhibitory action of the amines on platelet aggregation could be overcome with small amounts of calcium while other divalent cations tested had little effect. It is suggested that the amines affect platelet aggregation by interfering with the actions of membrane-associated calcium.

Platelet fibrinogen. Identity and initial observations on the mode of its degradation by plasmin.

Fibrinogen has been purified from human platelets. Platelet fibrinogen exhibits a characteristic pattern in agar gel immunoelectrophoresis different from that of plasma fibrinogen. Stepwise plasmin degradation has been used in further elucidation of the molecular properties of the platelet protein. Examination of comparative digests by immunologic and gel electrophoretic methods has revealed that (1) the platelet protein is more resistant to plasmin degradation, (2) the plasmin-produced fragments of platelet fibrinogen differ consistently from those of its plasma counterpart, and (3) platelet fibrinogen is different from fragment X of plasma fibrinogen. It is suggested that platelet fibrinogen may contribute to the stability of the thrombus.

Defective binding of thrombin to platelets in myeloid leukaemia.

Platelets from patients with myeloid leukaemia showed reduced aggregation with collagen or thrombin. These platelets also had a lower capacity to bind thrombin. This lower thrombin binding is due to a decrease in the total quantity of receptors available and not because of a change in the affinity. In the presence of the patients' plasma, the aggregation behaviour of normal platelets induced by thrombin as well as the clotting time of fibrinogen remained unchanged. The results suggest that the platelet dysfunction in myeloid leukaemia is partially due to a membrane defect involving the thrombin receptors which leads to an impaired induction of the initial stimulation.