Erythroid accelerating activity of rat serum in early stage of drug induced hemolysis.

An increase in the number of erythroblasts can be seen to some extent in the bone marrow of rats in the early stage of experimentally induced hemolytic anemia prior to any elevation in the plasma erythropoietin (Epo) level. This observation suggests that there is another erythroid stimulating factor present other than Epo. We studied the enhancing effect of serum, taken sequentially during experimentally induced hemolysis in rats, on erythroid proliferation, differentiation and maturation in vitro. Single intraperitoneal injection of 60 mg/kg of acetylphenylhydrazine (APH) induced self-limited hemolytic anemia in rats, in which the hematocrit dropped rapidly with a nadir at day 4 after APH injection, followed by a gradual increase with return to normal level by day 8. Serum obtained consecutively every day after APH injection from day 1 to day 7 was applied to an in vitro culturing system of erythroid progenitors. Addition of day 1 serum, in which an elevation of Epo level had not occurred, to a conventional methyl-cellulose culture of rat bone marrow mononuclear cells (BM-MNCs) resulted in a significant increase in the number of colonies derived from colony forming unit erythroid, but not in burst forming unit erythroid. This erythropoietic activity of the serum was particularly evident in the presence of Epo. In the liquid culture of BM-MNCs, day 1 serum also showed some enhancing effect on erythroblast formation. We were able to see significant differences in these erythroid enhancing activities induced by serum drawn on day 1 in comparison to the serum drawn on subsequent days. These results suggest that an unknown erythroid enhancing factor besides Epo stimulates erythropoiesis in the early stage of hemolytic anemia or sudden hypoxia before there is a measurable rise in the serum Epo level. We propose that this factor be termed erythroid accelerating factor (EAF).