Effects of cations on ouabain binding by intact human erythrocytes.

Abstract

The present studies demonstrate that extracellular cations alter ouabain binding by intact human erythrocytes and that this alteration reflects a change in the apparent affinity but not the capacity for ouabain binding. Monovalent cations exert their effect at a single site (the “monovalent cation site”) and each monovalent cation can inhibit competitively the effect of other monovalent cations. Sodium, lithium, and cesium increase while potassium and rubidium decrease the apparent affinity with which ouabain is bound. Divalent cations exert their effect at a single site (the “divalent cation site”) which is functionally distinct from the site at which monovalent cations act and show mutual competitive inhibition. Each divalent cation studied increases the apparent affinity with which oubain binds to the erythrocyte membrane. Magnesium and calcium, but not barium, can also alter the effect of monovalent cations on ouabain binding. To interpret our findings we have proposed that the erythrocyte membrane has a “receptor complex” composed of a monovalent cation site, a divalent cation site and a glycoside-binding site. The number and type of cations occupying the cation sites determine the affinity of the glycoside-binding site for ouabain.