Erythropoietic stress, macrocytosis, and hemoglobin switching in HbAA sheep.

Hemoglobin switching and macrocytosis were studied in homozygous hemoglobinAA sheep. An abrupt initiation of erythropoietic stress, accompanied by a pulsed elevation of circulating erythropoietin levels, was induced by phlebotomy. Sequential blood samples were separated according to density on Stractan gradients to isolate cells newly entering the circulation from the marrow each day. Analysis of hemoglobin phenotype and cell volume distribution in these young reticulocytes revealed a distinct temporal separation in the appearance of hemoglobin C and increased cell volume. The appearance of macrocytes within 24 hr of erythropoietin elevation suggests that macrocytosis could be the result of the action of erythropoietin during the late stages of erythroid maturation. The 72-hr delay in the appearance of hemoglobin C indicates that commitment to a particular hemoglobin phenotype occurs at an early stage of differentiation and involves immature erythroid stem cells. The results of this study show that these consequences of erythropoietic stress are initiated at two different developmental stages, resulting in the production of macrocytosis and hemoglobin switching.