Correlation of plasma erythropoiesis stimulating factor(s) and immunoreactive erythropoietin levels during rapid growth in the mouse.

During the early neonatal period of rapid growth in the mouse, increased plasma levels of erythropoiesis stimulating factor(s) (ESF) have been found when measured by an in-vitro bioassay technique. It is unclear whether these increased ESF levels represent increased levels of circulating erythropoietin (Ep) alone or Ep in combination with other less-defined erythropoietic stimulatory factors. To examine this issue, plasma from neonatal mice of varying post-natal ages and from normoxic and hypoxic adult mice was studied. We found that plasma Ep levels measured by radioimmunoassay (RIA) correlated significantly with in-vitro bioassayed ESF levels (r = 0.84, P less than 0.0001, n = 21). Although an in-vivo bioassay for plasma Ep proved too insensitive for rigorous correlation with data from the RIA and in-vitro bioassay, the in-vivo data were in qualitative agreement with the other two, more sensitive, assays. In all three assays the highest plasma levels were observed in the 20-day-old mice and in adult mice which had been subjected to hypobaric hypoxia for 8 h. Based on the strong agreement of the results obtained with the RIA and the in-vitro bioassay in both neonatal and adult mouse plasma, we conclude that the high plasma ESF levels of 20-day-old mice measured with the in-vitro bioassay are largely immunochemically identifiable Ep. However, the data also suggest the presence of non-Ep factors in neonatal plasma which stimulate the in-vitro bioassay.