Interaction of external alkali metal ions with the Na-K pump of human erythrocytes: a comparison of their effects on activation of the pump and on the rate of ouabain binding.

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Abstract
The effects of external alkali metal ions on the rate of ouabain binding and on the rate of the Na-K pump were examined in human red blood cells. In Na-containing solutions, K, Cs and Li decreased the rate of ouabain binding. For K and Cs, the kinetics of this effect were similar to those for their activation of the pump. In Na-free (choline-substituted) solutions the rate of ouabain binding was decreased by K whereas it was promoted by Cs and Li. External Na increased the rate of ouabain binding whether or not external K was present, and the kinetics of this effect were not the same as those for inhibition of the pump by Na. Not only do the cations affect ouabain binding at the external loading sites on the pump from which ions are translocated inward, but there are apparently additional sites on the external aspect of the pump at which cations can promote ouabain binding and which can be occupied by Li, Na and Cs. These latter sites are apparently those from which Na is discharged after outward translocation by the pump.