The magnesium dependence of sodium‐pump‐mediated sodium—potassium and sodium—sodium exchange in intact human red cells

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Abstract
The Mg content of human red blood cells was controlled by varying the Mg concentration in the medium in the presence of the ionophore A23187. The new Mg levels attained were very stable, which allowed the Mg dependence of the Na pump to be investigated. The effects of Mg occurred at the inner surface of the red cell membrane for the range of Mg concentrations tested (10-7 to 6 .times. 10-3 M). At intracellular Mg2+ concentrations < 0.8 mM the activation of ouabain-sensitive Na-K exchange by internal Mg2+ could be resolved into 2 or 3 components: a small component, about 5% of the maximum flux, apparently independent of Mg2+ concentration < 2 .mu.M; a saturating component with a K1/2 between 30-45 .mu.M, and possibly a component which increases linearly with Mg2+ concentration and which only becomes apparent at concentrations > 0.1 mM. At intracellular Mg2+ concentrations < 0.8 mM, activation of ouabain-sensitive Na-Na exchange by internal Mg2+ could be resolved into 2 components: a small component, about 6% of the maximal flux, which is apparently independent of Mg2+ concentration < 2 .mu.M and a saturating component with a K1/2 of about 9 .mu.M. At Mg2+ concentrations between about 0.2-0.8 mM the rate of Na-Na exchange remained constant at the maximal level. The intracellular concentration of ATP decreased and the ADP concentration increased as the Mg content of the cells was reduced from the normal level. A small increase in ATP and a small decrease in ADP were seen when Mg content was increased above the normal level. The variation in the ATP:ADP ratio from 2.5 at very low Mg levels to about 6 at normal Mg levels can account, at least in part, for the different K1/2 values of Na-K and Na-Na exchange. When the concentration of Mg2+ was increased above about 0.8 mM, both Na-K and Na-Na exchange were inhibited. Na-Na exchange was more strongly inhibited than Na-K exchange.

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