Abstract
During the first three months after birth lambs produce sequentially three erthryocyte populations of different mean volume as demonstrated by electric sizing methods (Valet, Franz, and Lauf, J. Cell. Physiol. 94 (1978) 215). We separated by centrifugal elutriation the small volume population (type II) red cells of a genotypically low K^+^ (LK) lamb from the population containing the larger volume type I and III cells, an admixture of fetal (I) and adult (III) erythrocytes. The cells were separated at various time intervals after birth and analyzed with respect to their volumes, cation contents, and cation flux properties by means of ^86^Rb uptake. The effect of antiβL on K^+^ pump and leak fluxes was ascertained in unseparated and separated red cells. It was found that the small red cells of population II, transiently present for several weeks, were fully developed LK cells with K^+^ pumps responding characteristically to the stimulatory action of antiβL. In constrast, the larger cells of population I and III were of high K^+^ (HK) nature at early time points, the K^+^ pump activities approximately ten times higher than adult LK cells. These cells constitute an admixture of type I fetal HK cells, and type III reticulocytes which are precursors for the final type III adult LK cells, since antiβL had a small stimulatory effect. At later times, however, only adult type III LK cells predominated. The data directly support our earlier finding that the HKβLK transition in genotypically LK lambs is primarily governed by cellular replacement.