Magnesium transport in the nephron.

Magnesium is filtered at the glomerulus to the extent of 70-80%, of which about 80-95% is reabsorbed by the various nephron segments. Recent evidence from micropuncture and microperfusion experiments has characterized the tubular handling of magnesium. The proximal convoluted tubule reclaims 20-30% of the filtered magnesium, considerably less than the fractional reabsorption of sodium and calcium. The proximal tubule is poorly permeable to magnesium but overall reabsorption is dependent on the filtered load and net water reabsorption. Specific knowledge of magnesium handling in the straight portion of the proximal tubule is lacking. Definitive evidence has been presented to indicate the important role of the thick ascending limb of the loop of Henle. The loop reabsorbs approximately 50-60% of the filtered magnesium or about 80% of that delivered to this segment. In vivo microperfusion studies have shown that elevation of luminal magnesium results in enhanced reabsorption proportional to delivery, whereas elevation of contraluminal magnesium leads to a fall in magnesium transport dependent on the absolute plasma concentration. This suggests a unique magnesium transport interaction on the contraluminal side of the ascending limb membrane that may also involve calcium. Parathyroid hormone increases magnesium reabsorption in the loop; however, its physiological role remains undefined. The distal convoluted tubule reabsorbs only 1-5% of filtered magnesium. Although it is now clear that successive segments of the nephron transport magnesium in different ways, the nature of the cellular mechanisms are unknown. The search for a specific physiological regulator of renal magnesium reabsorption remains unsuccessful. However, recent evidence suggests that overall renal magnesium homeostasis is determined to a large extent by transport in the ascending limb of Henle's loop.