The regenerating liver produces erythropoietin in response to hypoxia. The amounts of erythropoietin produced in animals subjected to hepatectomy are significantly higher than those observed in sham-operated animals. Hepatic erythropoietin production appears to be dependent upon the stage of regeneration with the highest levels being produced during the period of greatest proliferation and increase in liver mass.
The effect of erythropoietin on the splenic microvascular system of polycythemic CF1 mice was studied using in vivo microscopic methods. Administration of a single dose (3 U) of erythropoietin resulted in an increase in the linear velocity of blood flow through the splenic sinusoids and a reduction in the number of sinusoids storing blood. This response was first seen 4-6 hr after injection; it persisted for 48 hr and was reduced markedly by 72 hr. By 120 hr the spleens were indistinguishable from controls. The response was specific for erythrogenic tissue, since no response was seen in the adjacent nonerythropoietic pancreatic tissue. The results suggest that the splenic microvascular response to erythropoietin may be indirect and may be mediated by the release of a vasoactive metabolite from the erythrogenic tissues surrounding the sinusoids. Erythropoietin-sensitive stem cells are suggested to be the source of such a metabolite.
Sites of erythrogenin production in the neonatal rat were studied. Five-day-old neonates rendered hypoxic displayed no detectable levels of erythrogenin in their kidneys, but highly elevated amounts of this factor were noted in both their livers and spleens. These heightened levels of hepatic and splenic erythrogenin most likely form the basis for the ability of the bilaterally nephrectomized neonatal rat to show little impairment in EP production in response to hypoxia.
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