Micropuncture study of renal magnesium transport in magnesium-loaded rats.

Mg transport in the deep loop of Henle was studied in 15 young rats (50-60 g) after acute systemic Mg loading (UFMg (ultrafilterable Mg) 4.77 meq/liter). Intratubular Mg was measured with a recently described fluorometric microtechnique. The mean values of the TF/P inulin and TF/UF Mg ratios were 3.32 +/- 0.13 and 4.25 +/- 0.17, respectively. The proportion of filtered Mg recovered in this part of the nephron was therefore 131.2 +/- 5.0%, indicating that an appreciable amount of Mg entered the lumen prior to the sites of puncture. A significant correlation between the TF/P inulin and TF/UF Mg ratios suggests that water reabsorption also contributes to the high concentration of Mg in the loop of Henle during systemic Mg loading. In another series of young rats (90-160 g), similarly loaded with MgCl2 (UFMg 5.81 meq/liter), Mg and inulin were measured in superficial proximal (PT) and distal tubules (DT). Punctures were paired at two sites of the same PT and DT. The Mg concentration increased progressively along the PT in such a way that 90.5% of the filtered load still remained in the late proximal loops. If superficial and deep proximal tubules behave in a similar manner, it may be concluded that the site of entry of Mg is located between the late accessible part of the PT and the bend of the loop of Henle. Only 58.0 +/- 3.0% of the filtered Mg was delivered to the DT, indicating that Mg is extensively reabsorbed in the ascending limb, despite systemic loading. The proportion of filtered Mg did not vary along the DT, indicating no net reabsorption.