Summary. The relation between the active potassium influx in the human red blood cell and the extracellular potassium concentration does not appear to be consistent with the Michaelis-Menten model, but is adequately described by a model in which two potassium ions are required simultaneously at some site or sites in the transport mechanism before transport occurs. The same type of relation appears to exist between that portion of the sodium outflux that requires the presence of extracellular potassium and the extracellular potassium concentration. Rubidium, cesium, and lithium, which are apparently transported by the same system that transports potassium, stimulate the potassium influx when both potassium and the second ion are present at low concentrations, as is predicted by the two-site model. IntroductionThe human red blood cell concentrates potassium and extrudes sodium against electrochemical gradients. The magnitude of both the active potassium influx and of a portion of the sodium outflux is a function of extracellular potassium concentration. The form of this relation has

Red blood cells exposed to ouabain are capable of net Na outflux against an electrochemical gradient; the net outflux is inhibited by the diuretic, furosemide. In ouabain-treated cells, both the unidirectional Na outflux and the unidirectional Na influx are inhibited by furosemide. Furosemide also inhibits the ouabain-sensitive Na-Na exchange accomplished by the Na-K pump in K-free solutions. From the interaction of extracellular K, furosemide, and ouabain with the transport system, it seems possible that the ouabain-insensitive Na outflux is accomplished by the same mechanism that is responsible for the ouabain-sensitive Na-K exchange. The ouabain-insensitive Na outflux is increased by extracellular Na, and the influx increases as the intracellular Na increases. In fresh cells, high extracellular K concentrations decrease the ouabain-insensitive Na outflux and increase the ouabain-insensitive Na influx. When the rate constant for sodium outflux and the rate constant for sodium influx in ouabaintreated cells are plotted against the extracellular K concentration, the curves obtained are mirror images of each other. In starved cells, extracellular K increases the ouabain-insensitive Na outflux as does extracellular Na, and it has little effect on the Na influx.

Measurements were made of the sodium outflux rate constant, OkN., and sodium influx rate constant, ikN,, at varying concentrations of extracellular (Nao) and intracellular (Na,) sodium. kN. increases with increasing [Nao] in the presence of extracellular potassium (Ko) and in solutions containing ouabain. In K-free solutions which do not contain ouabain, kN. falls as [Nao] rises from 0 to 6 mM; above 6 mM, kN,, increases with increasing [Nao]. Part of the Na outflux which occurs in solutions free of Na and K disappears when the cells are starved or when the measurements are made in solutions containing ouabain. As [Nao] increases from 0 to 6 mM, ikN decreases, suggesting that sites involved in the sodium influx are becoming saturated. As [Na] increases, kN at first increases and then decreases; this relation between kN, and [Na] is found when the measurements are made in high Na, high K solutions; high Na, K-free solutions; and in (Na + K)-free solutions. The relation may be the consequence of the requirement that more than one Na ion must react with the transport mechanism at the inner surface of the membrane before transport occurs. Further evidence has been obtained that the ouabain-inhibited Na outflux and Na influx in K-free solutions represent an exchange of Na. for Nao via the Na-K pump mechanism.