Sodium movements in the human red blood cell.

Measurements were made of the sodium outflux rate constant, (o)k(Na), and sodium influx rate constant, (i)k(Na), at varying concentrations of extracellular (Na(o)) and intracellular (Na(c)) sodium. (o)k(Na) increases with increasing [Na(o)] in the presence of extracellular potassium (K(o)) and in solutions containing ouabain. In K-free solutions which do not contain ouabain, (o)k(Na) falls as [Na(o)] rises from 0 to 6 mM; above 6 mM, (o)k(Na) increases with increasing [Na(o)]. 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 [Na(o)] increases from 0 to 6 mM, (i)k(Na) decreases, suggesting that sites involved in the sodium influx are becoming saturated. As [Na(c)] increases, (o)k(Na) at first increases and then decreases; this relation between (o)k(Na) and [Na(c)] 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(c) for Na(o) via the Na-K pump mechanism.