The ATP dependence of a ouabain-sensitive sodium efflux activated by external sodium, potassium and lithium in human red cells.

The stimulation of ouabain-sensitive Na+ efflux by external Na+, K+ and Li+ was studied in control and ATP-depleted human red cells. In the presence of 5 mM Na+, with control and depleted cells, Li+ stimulated with a lower apparent affinity than K+, and gave a smaller maximal activation than K+. The ability of Na+, K+ and Li+ to activate Na+ efflux was a function of the ATP content of the cells. Relative to K+ both Na+ and Li+ became more effective activators when the ATP was reduced to about one tenth of the control values. At this low ATP concentration Na+ was absolutely more effective than K+.