Potassium fluxes in the rat reticulocyte. Ouabain sensitivity and changes during maturation

K+ turnover is markedly enhanced in the rat reticulocyte, both influx and efflux rates being increased by factors of approximately 3 over the corresponding rates in adult cells. These accelerated fluxes are observed despite the absence of any appreciable change in intracellular K+ concentration during the course of maturation. Qualitative characteristics of the active transport process for K+ influx appear to be identical in reticulocytes and mature erythrocytes with regard both to K+ sensitivity, and to ouabain sensitivity as a function of external K+ concentration. The number of ouabain binding sites per unit volume of cells, however, is increased by a factor of approximately three in the reticulocyte and thus correlates well with the observed degree of enhancement of active K+ influx in these cells. Half-maximal rates of ouabain-sensitive K+ influx are observed at external K+ concentrations well below 1 mM for both reticulocytes and mature erythrocytes. It is concluded that the enhanced rate of K+ accumulation in the reticulocyte can be quantitatively attributed to an increased number of pump units which are qualitatively identical to those in the mature cell, and which function at a near-maximal rate at the ambient K+ concentration present in normal rat plasma.