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An attempt to evaluate the role of the liver in extrarenal erythropoietin production was made by measuring the content of erythropoietin in homogenates and perfusates from hypoxic rat livers. Extracts from livers from nephric or anephric animals rendered both anemic and hypoxic showed no detectable erythropoietin despite the fact that both plasma and kidney extracts contained large amounts of erythropoietin. This lack of measurable erythropoietin in the liver is not caused by degradation of erythropoietin during the extraction procedure because exogenously added rat erythropoietin was recovered to the same extent from livers or kidney homogenates. More likely, however, it is caused by the fact that extrarenal erythropoietin production accounts for only one-fifth of total erythropoietin production and that the liver mass is about six times that of both kidneys. Consequently, the erythropoietin content of 1 g of liver should be about one-thirtieth of that of 1 g of kidney, an amount that is below the limit of detection of the assay. On the other hand, the 2-h in situ perfusates of livers from similarly stimulated animals contained significant amounts of secreted erythropoietin. It is concluded that the liver participates actively in extrarenal erythropoietin production in the adult rat. However, the small amount expected to be present in tissue homogenates cannot be detected with our current bioassay.

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