Effect of parathyroid hormone on renal calcium and magnesium reabsorption in magnesium deficient rats

Tubular magnesium reabsorption was investigated by recollection micropuncture and in vivo microperfusion techniques in acutely thyropara-thyroidectomized rats made magnesium deficient by dietary deprivation. Henle's loop which normally reclaims the major portion of filtered magnesium was examined by elevation of intraluminal magnesium concentration. The transport capacity in these conditions was significantly lower in magnesium deficient rats (41%) compared to normal animals (71%) at comparable magnesium delivery rates. Acute infusion of MgCl₂ further depressed loop magnesium reabsorption independent of intraluminal magnesium delivery. Parathyroid hormone did not alter magnesium transport capacity in magnesium deficient rats but resulted in enhanced transport in acutely hypermagnesemic deficient rats. Calcium reabsorption followed a similar qualitative pattern as magnesium with respect to loop function and urinary excretion. These results are consistent with a depressed transport capacity for magnesium in the loop of Henle of magnesium deficient rats which is independent of intraluminal magnesium delivery and circulating parathyroid hormone level.