Enzymic diversification of the sodium pump in sheep red cells

It is well-established that sheep are genetically dimorphic with respect to the K+ content of their mature red cells and that high-K+ (HK) and low-K+ (LK) red cells differ with respect to the activity and kinetic properties of the sodium pump and Na+-ATPase. In contrast, reticulocytes of both types have a high K+ content and are similar with respect to their pump and Na+-ATPase kinetic characteristics. Reticulocytes differ, however, from mature cells of either HK or LK sheep. The first part of this paper describes a comparative study of the side-specificity of Na+ and K+ interactions with Na--ATPase of inside-out membrane vesicles prepared from mature HK and LK red cells. The results indicate that the genetic difference between the two is expressed as differences in apparent affinities for both Na+ and K+ and that these differences are evident at the cytoplasmic as well as at the extracellular membrane surface. The second part of this paper describes a study of the nature of the maturation-associated changes in the sodium pump system of sheep reticulocytes. Maturation was followed both in vitro (long-term incubation at 37 degrees of reticulocytes) and in vivo (changes in density gradient fractionated HK cells followed at intervals during recovery from massive bleeding. The results support the conclusion that the dimorphism in the sodium pump of sheep red cells is a result of specific changes in Na+-ATPase and that these changes proceed differently during maturation of the two types of sheep red cells.