Tubular reabsorptive capacity for magnesium in the dog kidney

Micropuncture studies were performed on 27 thyroparathyroidectomized dogs to determine the segmental reabsorption of magnesium before and after graded magnesium infusion. Overall kidney reabsorption, as determined by the difference between filtered magnesium and urinary excretion, initially increased with elevation of plasma magnesium up to 3.5 meq/liter. Further elevation of plasma magnesium resulted in the appearance in the urine of all additional increments in filtered magnesium. Thus, renal magnesium reabsorption followed a pattern characteristic of a maximal transport rate (Tm). Evaluation of the proximal tubule by micropuncture demonstrated that net reabsorption of magnesium rose proportional to the increase in filtered magnesium and accounted for a constant fractional reabsorption of 14%. In contrast, magnesium reabsorption in Henle's loop initially increased with low delivery rates but peaked and fell with high plasma magnesium concentrations. Little magnesium reabsorption was observed between the distal collection site and final urine. Accordingly, the overall urinary excretion pattern was a summation of the different effects occurring in the proximal tubule and Henle's loop. Thus, renal magnesium reabsorption is not characterized by a Tm process but is a composite of distinct transport properties of the proximal tubule and the loop of Henle.