Early adaptation of renal magnesium reabsorption in response to magnesium restriction.

Dietary magnesium restriction with hypomagnesia is normally associated with diminished urinary magnesium excretion. Young rats were pair fed control magnesium diets (0.05% MgSO4) and magnesium-restricted diets (less than 0.01% Mg), and tubular magnesium reabsorption was assessed to determine the importance of filtered load (plasma magnesium) and to establish the presence of cellular adaptation of magnesium transport. Urinary magnesium excretion decreased from 17.2 +/- 2.7 to 5.9 +/- 1.2% over 20 h on low-magnesium diets without a change in plasma concentration (0.61 +/- 0.02 mM). This cellular adaptation was rapid (within 5 h), specific (without effect on sodium and calcium), and sensitive (without change in plasma concentration). Micropuncture studies demonstrated that cellular adaptation occurred within the loop of Henle. After 20 h, plasma magnesium fell with an associated further decrease in fractional magnesium excretion. Accordingly, cellular adaptation of magnesium transport occurs with magnesium-deficient diets, and alterations in filtered magnesium and plasma magnesium concentration are not necessary for magnesium conservation.