Effect of magnesium salt anions on potassium balance in normal and magnesium-depleted rats.

Potassium depletion often accompanies clinically significant magnesium depletion, and magnesium replacement is most frequently undertaken with magnesium sulfate. However, inorganic sulfate acts as a nonreabsorbable anion in the distal nephron, an effect that could potentially interfere with the correction of any associated potassium deficiency. We have investigated the effect of a variety of magnesium salts, including sulfate and nonsulfate forms, on cation balance and skeletal muscle ion composition in normal and magnesium-depleted rats. Even modest amounts of dietary sulfate increased the urinary excretion of potassium in both normal and magnesium-depleted rats. During a 7-day feeding period, diets containing MgSO4 or MgO with an equivalent amount of Na2SO4 resulted in significantly greater urinary excretion of potassium than diets without sulfate. In normal animals, this kaliuresis did not produce hypokalemia or muscle potassium depletion. Magnesium-deficient feeding produced magnesium depletion, kaliuresis, and a decline in both plasma and muscle potassium content. When magnesium was restored to the diet without sulfate, urinary potassium excretion decreased, and both magnesium and potassium deficits were corrected. However, when magnesium refeeding included sulfate salts, magnesium stores returned to normal, whereas the kaliuresis and potassium depletion persisted. When accompanied by a high sodium intake, the kaliuretic effect of sulfate was increased.