Effect of magnesium loading on magnesium delivery to the juxtamedullary end-descending limb.

Previous micropuncture and microperfusion studies in acutely hypermagnesemic rats have yielded conflicting results with respect to magnesium transport in Henle's loop. The following experiments were performed to reexamine, by micropuncture of papillary end-descending limb, whether magnesium undergoes intratubular secretion in magnesium-loaded rats. Group 1 animals served as normal controls; group 2 animals received an acute intravenous magnesium load; group 3 animals were orally magnesium loaded for 3 wk before receiving an acute intravenous magnesium load during micropuncture; group 4 animals were acutely thyroparathyroidectomized and water loaded before receiving an acute magnesium load. Fractional magnesium delivery to the end-descending limb did not differ from the corresponding value observed in the superficial proximal tubule in normal animals (67 +/- 5.3 vs. 76 +/- 7.6%). Acute magnesium loading raised plasma magnesium concentration and fractional magnesium excretion more than twofold but did not change fractional magnesium delivery to the end-descending limb or superficial nephron significantly from control values (75 and 73%). Chronic oral magnesium loading raised daily urinary magnesium excretion threefold (183 vs. 53 mumol X day-1 X 100 g body wt-1, P less than 0.05), but acute magnesium loading in this group did not significantly alter fractional delivery to the end-descending limb (85 +/- 10%, NS). Increasing intratubular flow rate while acutely raising plasma magnesium concentration (group 4) also did not induce intratubular magnesium secretion. The absence of significant changes in fractional magnesium delivery to the end-descending limb during magnesium loading suggests that intratubular magnesium secretion, if at all present, is very small and of questionable significance.