Regulation of erythropoiesis in suckling rabbits with and without postnatal anemia: partial suppression of production/release of erythropoiesis stimulating factor(s) by iron supplements.

The postnatal anemia in rabbits is accompanied by a marked rise in the plasma erythropoiesis stimulating factor(s) (ESF). The purpose of this study was to establish whether the increase in plasma ESF is only related to the anemia, or whether other mechanisms also are involved. Two matched groups of rabbits were studied from the 15th to the 36th day after birth. One group received iron parenterally and had no postnatal fall in hemoglobin concentration (Hb), the other developed the usual anemia. In both groups plasma ESF was undetectable on the 15th day, and also on the 22nd day, despite a marked fall in Hb in the untreated group and rise in the iron-treated group. Thereafter plasma ESF showed a slight, continuous rise in the nonanemic rabbits, in contrast to a marked, transient rise with maximum on the 29th day in the untreated group. On the 36th day there was no difference between the groups. In the iron-treated group the reticulocyte production rate remained unchanged, while the Hb mass rose continuously. In the untreated animals there was an initial decline in reticulocyte production rate, while Hb mass showed a slight increase. From the 29th day, however, reticulocyte production rate rose to the same level as in the iron-treated group and Hb mass rose markedly. In conclusion, the rise in plasma ESF during the postnatal anemia in rabbits is only in part related to the low Hb. Hypoxia-independent mechanisms, probably related to the growth and maturation per se also are involved. The lack of erythropoietic response to the rise in plasma ESF is due to lack of available iron.