AJP - Renal Physiology, Vol 248, Issue 1 145-F151, Copyright © 1985 by American Physiological Society
Effect of magnesium loading on magnesium delivery to the juxtamedullary end-descending limb
D. R. Roy
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.
