Regulation of erythropoiesis during rapid growth.

High levels of plasma erythropoiesis stimulating factor(s) (ESF) have been found in neonatal WLO-mice during the period of rapid growth. If the high ESF activity is due to the concomitant physiological anaemia of infancy alone, it should be possible to block erythropoiesis by hypertransfusion. Mice were hypertransfused starting on day 14, and killed on day 20. Although hypertransfusion reduced the ESF levels by approximately 55% (P less than 0.001), ESF levels were still detectable in the cell culture assay used (P less than 0.001). Moreover, hypertransfused mice showed active erythropoiesis in the bone marrow, and none had a reticulocyte count below 2%. No correlation was found between PCV and ESF in the hypertransfused animals (r = 0.07, P greater than 0.5), nor was there any difference in weight gain between control and hypertransfused mice (P greater than 0.5). These results show that hypertransfusion did not totally supress erythropoiesis in neonatal WLO-mice, which is different from hypertransfused adult mice. The data indicate that the high plasma ESF found in neonatal WLO-mice during the growth period are not due to the anaemia alone. These findings support studies indicating that regulation of erythropoiesis in the neonate differs from the adult. Factors related to growth per se could be responsible for this difference.