ERYTHROPOIETIN AND THE POLYCYTHEMIAS *

Erythropoiesis in man is regulated by erythropoietin. This humoral substance provides the means whereby tissue needs for oxygen influence the level of circulating erythrocytes. The usual cause of polycythemia is a decreased oxygen supply to tissues leading to increased erythropoietin output and increased erythropoiesis. However, there is good evidence that disorders in erythropoietin production itself or abnormal marrow activity may also result in elevated red cell levels. In the following discussion, these disorders are examined through the relationship of erythropoietin to erythropoiesis in polycythemic states in man. The method used in these studies is a modification' of the polycythemic mouse assay previously de~cribed,2-~ and features the injection of concentrates of human urine into mice previously made polycythemic by exposure to reduced atmospheric pressure. With the addition of a protein depletion diet, endogenous erythropoiesis, as measured by the incorporation of radioactive iron into newly formed red cells, is reduced to negligible levels (0.02 f 0.01 % utilization). Concentrates of urine are prepared from collections stored at -12' C. The thawed specimens are placed in a dialysis membrane and dialyzed in the cold against a high molecular weight material, polyethylene glycol. Following dialysis, urine sediment is removed by centrifugation, washed with saline, and the supernatants combined and stored frozen for use in the assay. In all patients studied, urine was collected in 24-hour lots or over consecutive six-hour intervals. Estimation of erythropoietin excretion was made by comparing the degree of erythropoiesis induced by the urinary concentrates to dilutions of erythropoietin Standard B. Activity, as measured in this assay, is, by definition, erythropoietin. In the studies to be reported, the regulation of erythropoiesis appears explainable on the basis of a single substance. Control studies undertaken characterized the erythropoietin excretion of normal subjects in both the basal state and in response to limited phlebotomy. Daily erythropoietin output in 19 control subjects averaged 3.8 * 2.3 Standard B units per day and increased as expected in seven individuals subjected to limited bleeding (FIGURE 1 ) . With reduction in hematocrit of only 3 to 7 points, average erythropoietin excretion increased by 53 percent. In the first group of patients studied, polycythemia represented a disorder of oxygen supply to tissue. In six individuals with either cardiac or pulmonary disease, daily erythropoietin excretion varied over a wide range (2.4 to 420.0 units). Again controlled phlebotomy in these patients produced easily demonstrable increases in plasma and urinary erythropoietin levels. In the one patient with hypoxic polycythemia and normal basal erythropoietin excretion, controlled bleeding was attended by a markedly exaggerated increase in erythropoietin output exceeding any of the responses seen in the similarly challenged normals (FIGURE 1).