Erythropoietic factors in plasma from neonatal mice. In vivo studies by the exhypoxic polycythaemic mice assay for erythropoietin.

The erythropoiesis stimulating factor(s) (ESF) in plasma from 20-day-old WLO-mice have previously been studied by a cell culture assay, and also by means of gel filtration chromatography and affinity chromatography. It was concluded that the high levels of ESF found in the neonatal mouse plasma probably consisted of erythropoietin (Ep) alone. The objective of the present investigation was to obtain further information of whether this high ESF found in vitro is Ep alone, or Ep in combination with other factors. To accomplish this plasma from 20-day-old WLO mice and a standard Ep were studied in vivo by the exhypoxic polycythaemic mice assay for Ep, with and without preincubation with rabbit anti-Ep serum (AS). Aliquots of some samples were also studied in vitro by the exhypoxic polycythaemic mice assay for Ep, with and without pre- in both assays (P less than 0.001). However, incubation with AS significantly reduced (P less than 0.001) but did not totally block either the in vivo or the in vitro activity of the plasma (P less than 0.005). This also was the case regarding the in vivo activity of the standard Ep (P less than 0.001), while the in vitro activity of this Ep preparation was totally blocked by incubation with AS (P greater than 0.3). These results indicate that a considerable part of the high erythropoietic stimulatory activity found in plasma from 20-day-old mice, with both assays, is Ep. This supports the previous in vitro studies. However, the present results also support the conclusion that part of the activity is due to non-Ep stimulatory factors.