Effect of magnesium on rat nephron sodium reabsorption: a segmental analysis

To evaluate the effect of hypermagnesemia on distal nephron sodium reabsorption, renal clearance, micropuncture, microinjection, and electrophysiological studies were performed in the anesthetized rat before and after intravenous administration of MgCl2, MgSO4, and Na2SO4. Along the proximal tubule, MgCl2 caused a 21% decrease and MgSO4 a 17% decrease in fractional sodium reabsorption, while only a 4% decrease was observed with Na2SO4. In the loop segment, the decrease in fractional sodium reabsorption was 7% with MgCl2, 15% with MgSO4, and 12% with Na2SO4. In the distal tubule and collecting duct, MgCl2 and MgSO4 had no effect on fractional sodium reabsorption, whereas Na2SO4 significntaly depressed sodium reabsorption to a greater extent in the collecting duct than in the distal tubule. Microinjection studies, however, showed that both MgSO4 and Na2SO4 depressed lumen-to-plasma sodium movement across the collecting duct. Early and late distal tubular transepithelial potential difference was unaffected by MgCl2, whereas it was increased by both MgSO4 and Na2SO4. The decrease in sodium reabsorption along the distal tubule and collecting duct produced by Na2SO4 and probably MgSO4 may relate to the increased transepithelial electrochemical gradient for sodium transport produced by the poorly reabsorbable sulfate anion.