Increase in circulating megakaryocyte growth-promoting activity (Meg-GPA) following sublethal irradiation is not related to decreased platelets.

In this study, we have used the sublethally irradiated rat as a model to examine the relationship between platelet count and the plasma level of megakaryocyte growth-promoting activity (Meg-GPA) assayed in vitro. Meg-GPA of irradiated rats whose platelet counts were maintained by platelet transfusions was compared to that of irradiated controls allowed to develop thrombocytopenia. The irradiated controls were given either saline, packed red cells, or no other treatment. Blood values were determined and plasma was collected from all groups 11 days after irradiation, predetermined to be the nadir of platelet count and the peak of Meg-GPA. The Meg-GPA of plasma was assayed by culturing in vitro at a final concentration of 30% with rat marrow in methylcellulose and 2-mercaptoethanol for 7 days at 37 degrees C. Plasma from platelet-transfused irradiated rats, when collected at ambient temperature, contained markedly decreased levels of Meg-GPA compared to that present in plasma of irradiated controls. Subsequent in vitro studies indicated that elevated Meg-GPA levels of plasma from irradiated thrombocytopenic rats were drastically reduced by incubation with platelets in vitro at 37 degrees C. Incubation of the same plasma with platelets at 4 degrees C resulted in much less reduction in Meg-GPA. This suggested that the diminished Meg-GPA observed with plasma from the platelet-transfused irradiated rats may have been due to activation and release of Meg-GPA inhibitors from platelets during plasma collection. To investigate this possibility, experiments were repeated in which platelets of irradiated rats were maintained by platelet transfusion, but plasma was collected at 4 degrees C. Under these conditions, the level of Meg-GPA in plasma of the platelet-transfused irradiated rats was not markedly different from that in plasma of irradiated controls. We conclude that in this experimental model, circulating Meg-GPA level is not related to platelet count.