Increased erythropoiesis and 2'5'-A polymerase activity in the marrow and spleen of phenylhydrazine-injected rats.

The rate of CFUE production in adult rats varies considerably according to whether the animals are rendered anemic by short or long term treatment with phenylhydrazine. The bone marrow responds during 2 months of phenylhydrazine injections with a progressive increase in CFUE, the spleen shows a rapid and larger rise after 3 daily injections followed by a decline to near zero, control numbers, after prolonged administration of the drug, and the liver never develops erythropoietic activity. Endogenous colonies, that is, colonies that grow in plasma clots without added erythropoietin, are most numerous in bone marrow after phenylhydrazine treatment, are always few in the spleen and are never present in cultures of liver cells. In both marrow and spleen, there is a direct correlation between the rate of erythropoiesis and 2'5'-A polymerase activity. These findings suggest that production of the enzyme may be an early event in erythroid cell differentiation.