The transition from HK to LK phenotype in the red cells of newborn genetically LK lambs

Red cells from newborn lambs were separated into different age populations by centrifugation, and cells with fetal hemoglobin (Hb) were distinguished from those with adult Hb by an acid elution technique. Changes were followed during development in rates of K+ transport (active and passive), numbers of Na+/K+ pump sites per cell, cell volumes, and numbers of Lp and L1 antigen sites per cell. These changes were correlated with the percentage of cells with adult hemoglobin. (The Lp and L1 antigens are associated with K+ transport in that specific alloantibody against Lp, anti-Lp, stimulates active transport, and anti-L1 inhibits passive transport.) Active K+ transport decreased during development because of a decline in number of Na+/K+ pumps (from measurements of ouabain binding) and because of an alteration in the affinity of the pumps for intracellular K+ (from kinetic studies in which the intracellular K+ concentration was varied). Cells with fetal Hb had fewer Lp sites and were larger than cells with adult Hb. As transport properties changed, the number of Lp sites increased and continued to increase after all the cells had adult Hb Cells with fetal Hb had as many L1 sites as lamb cells with adult Hb, but the number of L1 sites was less than those found previously for adult sheep. A population of small cells with intermediate K+ concentration and intermediate numbers of Lp sites appeared soon after birth. The various points of evidence suggested that the developmental process leading to cells with adult transport properties was a gradual one and did not coincide precisely with the switch from fetal to adult Hb.