Evidence for a Splenic Release Factor of Platelets in Chronic Blood Loss Plasma of Rabbits

S ummary . Rabbits subjected to chronic blood loss with iron supplementation raised their platelet count and megathrombocyte number 1.7x and 2.1 x above baseline, respectively. In similarly treated splenectomized animals, no significant response was obtained. When chronic blood loss plasma was injected into recipient animals there was a rise in platelet count and megathrombocyte number of 1.8 x and 3.4 x respectively compared to animals injected with control plasma. In similarly treated splenectomized animals no such rise was obtained; neither was there a rise in platelet count or megathrombocyte number in intact recipient animals when chronic blood loss plasma was obtained from splenectomized animals. It is concluded that the thrombocytosis of chronic blood loss requires the presence of an intact spleen. The data suggest the presence of a release factor which requires the spleen for its elaboration as well as expression.