Approximately 70-80% of plasma magnesium is ultrafilterable. Of this, 7080% is in the ionic form, Mg2+, while the remainder is complexed to phosphate, citrate, and oxalate (Fig. 14).
Approximately 20-30% of filtered Mg2+ is reabsorbed in the proximal tubule. Because this is less than the fraction of filtered water reabsorbed, luminal Mg2+ concentration rises to levels that exceed the ultrafilterable plasma Mg2+ concentration by a factor of about 1.5. Similar to Ca2+, Mg2+ reabsorption in the proximal tubule parallels Na+ and water reabsorption. Thus, Mg2+ reabsorption is enhanced in volume contraction and inhibited in volume expansion. Mg2+ absorption is absent in the descending thin limb of Henle's loop, and in fact there may be some Mg2+ secretion.
The major site of renal Mg2+ absorption is the thick ascending limb of the loop of Henle where 50-60% of filtered Mg2+ is reabsorbed (Fig. 14). Studies of single tubules suggest that the majority of this flux occurs in the cortical thick ascending limb. Similar to Ca2+, there is some question as to the relative magnitudes of passive and active Mg2+ absorption. Studies examining the effect of voltage on Mg2+ absorption suggest that all or most of Mg2+ absorption is passive. However, studies examining the mechanism of hormonal regulation of Mg2+ absorption suggest that a transcellular component also exists in parallel. This cellular mechanism by which Mg2+ is absorbed is poorly defined. Pro-
FIGURE 14. Mg2+ transport along the nephron. Numbers indicate percentage of filtered Mg2+ remaining in the luminal fluid. The majority of Mg2+ absorption occurs in the thick ascending limb.
posed mechanisms have included an apical membrane Mg2+ channel in series with a basolateral membrane Mg2+ pump and a Na/Mg exchanger.
The thick ascending limb is an important site of regulation of Mg2+ transport. Mg2+ absorption here is increased in Mg2+ deficiency and inhibited in hypermagnesemia. Mg2+ absorption is also stimulated by PTH, calcitonin, glucagon, and ADH. Loop diuretics inhibit Mg2+ absorption in this segment by inhibiting the lumen-positive voltage and thus inhibiting passive Mg2+ absorption.
The distal convoluted tubule mediates reabsorption of an additional 2-5% of the filtered Mg2+ load. This flux is also stimulated by PTH, glucagon, vasopressin, and calcitonin. Magnesium absorption in the collecting duct appears to be minimal and is poorly characterized.
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