¹ From the Digestive and Hereditary Diseases Branch, National Institute of Arthritis and Metabolic Diseases, National Institutes of Health, Bethesda, Maryland 20014

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^-10M. 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.

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