Author index

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⁺.