Detection of thrombopoietic activity in plasma by stimulation of suppressed thrombopoiesis

Rabbits in which thrombocytosis had been produced by five daily transfusions of platelet concentrates had suppressed endogenous thrombopoiesis, as reflected by decreased incorporation of selenomethionine-(75)Se ((75)SeM) into the circulating platelet mass. Rabbits in which endogenous thrombopoiesis had been suppressed by transfusion-induced thrombocytosis were used to detect thrombopoietic activity in rabbit plasma. Thrombopoietic activity was demonstrated in the plasma of both normal and thrombocytopenic donor rabbits. A dose response relationship was observed between the incorporation of (75)SeM into platelets and the dose of plasma administered. Infusion of 20-150 ml of plasma from thrombocytopenic donors increased the incorporation of (75)SeM into platelets from 52 to 107% above control values. A dose response effect also was seen after infusion of normal plasma, but normal plasma produced less effect than comparable doses of plasma from thrombocytopenic donors. Rabbits with transfusion-induced thrombocytosis appear to be more sensitive assay animals for the detection of thrombopoietic activity than animals with normal platelet counts. Changes in the rate of appearance and levels of (75)SeM may primarily indicate changes in platelet protein or platelet size and are apparently more sensitive indicators of the state of thrombopoiesis than are alterations in the numbers of circulating platelets. The results strongly support the concept of a humoral agent, i.e. thrombopoietin, that acts on megakaryocytes to regulate platelet production.

The effect of hypoxia on the platelet count

The occurrence of low arterial oxygen tensions in postoperative patients is well established. The effect of hypoxia on platelets was therefore studied in 36 rabbits. Two groups were exposed to low oxygen mixtures and a third group was exposed to air in an environmental chamber. In all groups the period of exposure was 24 hours. The hypoxic rabbits showed increased platelet counts from the first to nineteenth days after exposure. The difference in counts between both hypoxic groups and the control rabbits was statistically highly significant after exposure. An increase in platelets may well be an important factor in the aetiology of venous thrombosis. It is consequently suggested that a prophylactic régime against thrombo-embolism should include vigorous efforts to prevent hypoxia in the postoperative period.

Thrombocytosis following thrombocytopenia in man

Observations are presented on the changes in the platelet count of patients during the treatment of Addisonian pernicious anaemia with vitamin B₁₂, of thrombocytopenic purpura with prednisone, and of malignant disease with methotrexate.

In each of these clinical situations, thrombocytopenia was succeeded, after a delay of a number of days, by a phase of thrombocytosis.