Magnesium Transport

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-



Outer Medulla

Inner Medulla

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.

Was this article helpful?

0 0
Supplements For Diabetics

Supplements For Diabetics

All you need is a proper diet of fresh fruits and vegetables and get plenty of exercise and you'll be fine. Ever heard those words from your doctor? If that's all heshe recommends then you're missing out an important ingredient for health that he's not telling you. Fact is that you can adhere to the strictest diet, watch everything you eat and get the exercise of amarathon runner and still come down with diabetic complications. Diet, exercise and standard drug treatments simply aren't enough to help keep your diabetes under control.

Get My Free Ebook

Post a comment