Changes in Na+-ATPase and Na,K-pump during maturation of sheep reticulocytes

Changes in the sodium pump and sodium-activated ATP hydrolysis (Na+-ATPase) were followed during maturation of low potassium (LK) and high potassium (HK) sheep reticulocytes released into circulation following massive bleeding. In vitro maturation of LK cell reticulocytes resulted in a progressive decrease in cellular K+, ouabain-sensitive 86Rb+ uptake, and Na+-ATPase activity. Although in vitro maturation (up to 10 days) did not result in changes in kinetic behaviour from that typical of reticulocytes to that typical of mature cells, kinetic changes were clearly evident after longer term maturation in vivo. This was shown for HK cells by comparing the characteristics of Na+-ATPase (response to K+) of the lightest (youngest) and heaviest (oldest) cell fractions of blood obtained at various periods during recovery following massive bleeding. It is concluded that the kinetic difference between immature and mature cells is due to cell maturation and reflects either a selective change and (or) loss of certain type(s) of pump sites.