Thrombocytopenia in Rabbits Induced by Heparin, Protamine, and Polybrene

Injection into rabbits of heparin or one of the heparin‐neutralizing substances protamine and polybrene induced a rapid, transient fall in the number of platelets, without affecting the fibrinogen levels. The lowest platelet counts in the different animals ranged from 4 % to 52 % of the preinjection values. A few seconds after the injection, platelet aggregates were observed in the blood samples. Later the aggregates disappeared, but smaller aggregates reappeared when the platelet count began to rise. Marked pulmonary symptoms in many animals indicated trapping of platelet aggregates in the pulmonary vascular bed. Some possible reasons for the conflicting reports concerning the effect of heparin on platelets in vivo are discussed.