The effects of sodium and potassium on ouabain binding by human erythrocytes

Ouabain binding by the human erythrocyte membrane is reversible, exhibits a high degree of chemical specificity, and can be detected at ouabain concentrations as low as 1 x 10(-10)M. The relation between ouabain binding and ouabain concentration can be described by a rectangular hyperbola permitting determination of the maximal binding (B(max)) and the ouabain concentration at which ouabain binding is half-maximal (K(B)). Reducing the external sodium concentration increased K(B), while reducing the external potassium concentration decreased K(B). Neither cation altered B(max) The reciprocal of K(B) was a linear function of the sodium concentration at sodium concentrations ranging from 0 to 150 mM. Conversely, the relation between the reciprocal of K(B) and the external potassium concentration was nonlinear, and raising the potassium concentration above 4 mM produced no further increase in K(B). These results are compatible with a model which postulates that the erythrocyte membrane contains a finite number of receptors each composed of a glycoside-binding site and a cation-binding site. When sodium occupies the cation-binding site, the affinity of the glycoside site for ouabain is increased; when potassium occupies the cation-binding site the affinity of the glycoside site for ouabain is decreased.