Maintenance of fetal hemoglobin (HbF) elevations in the baboon by prolonged erythropoietic stress

We have previously shown that acute erythropoietic (Ep) stress by hemolysis or hypobaric hypoxia causes elevations of HbF in the baboon. The magnitude of these elevations is genetically determined, ranging from 3% to 60% (low, intermediate, and high responders). These genetic differences in HbF levels among animals are mainly due to differences in the number of HbF-containing cells ("F-cells"). The present study was undertaken to study the influence of prolongation and of the severity of Ep stress on HbF levels and the number of F-cells. The packed cell volume (PCV) of the blood of 4 animals, approximately 3 yr old, was maintained at 20% by daily phlebotomies, and the animals were exposed to varying degrees of hypobaric hypoxia for up to 40 days. In these experiments, the number of F-cells increased rapidly and reached individually constant levels ranging from 60% to 80%, when the PCV reached 20%, and no further increase was observed regardless of the subsequent degree of hypoxia. On the other hand, HbF levels, and with it the values for HbF per F-cell, increased proportionally to the severity of the Ep stress and could be maintained at a constant level dependent on the degree of the hypoxia, e.g., at 19,000 feet HbF levels of one animal remained 20%-25% throughout the duration of the exposure of 14 days. These data are indicative of separate control of F-cell numbers and of the levels of HbF per F-cell. It appears that with the increase of Ep stress, those Ep stem cells that have retained the HbF program are mobilized into maturation. A model, attempting to explain this phenomenon is presented.