Magnesium reabsorption in the juxtamedullary loop of Henle: effect of magnesium deprivation.
To determine the contribution of the juxtamedullary loop of Henle to magnesium reabsorption during magnesium deficiency, we performed two-phase micropuncture studies of end-descending limbs in a group of magnesium-deficient rats (n = 7) and in a pair-fed control group (n = 8) given MgCl2 in their drinking water. In the magnesium-deficient rats, daily excretion of magnesium fell to very low values (1.2 +/- 0.2 vs. 52 +/- 12 microM.day-1.100 g body weight-1, p less than 0.05). Plasma magnesium concentration and fractional magnesium excretion during the control phase were nearly 52 and 27%, respectively, of the values observed in pair-fed controls. Fractional magnesium delivery to the end-descending limb did not differ significantly between the two groups. During the acute magnesium repletion phase, fractional magnesium excretion and fractional magnesium delivery to the end-descending limb increased by a similar value in the two groups of rats, despite a lower filtered load of magnesium in the magnesium-deficient group. Absolute magnesium reabsorption upstream to the end-descending limb was lower in the magnesium-deficient rats but was otherwise tightly coupled to the filtered load of magnesium (Y = 0.91 + 0.37 x, r = 0.82, p less than 0.05). Similar observations were made with regards to whole kidney magnesium reabsorption. Our results suggest that, in young magnesium-deficient rats, magnesium reabsorption is tightly coupled to the filtered load of magnesium both in segments upstream to the juxtamedullary end-descending limb and in the whole kidney, and that a reabsorptive defect for magnesium is not evident in this setting.
Division of Nephrology, Royal Victoria Hospital, Montréal, Qué., Canada.
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D000818:Animals; D002118:Calcium / Q000652:urine; D004032:Diet; D005260:Female; D007677:Kidney Function Tests; D007684:Kidney Tubules / Q000378:metabolism*; D008138:Loop of Henle / Q000378:metabolism*; D008274:Magnesium / Q000378:metabolism* / Q000494:pharmacology; D015636:Magnesium Chloride; D008275:Magnesium Deficiency / Q000378:metabolism*; D010710:Phosphates / Q000652:urine; D051381:Rats; D011919:Rats, Inbred Strains
D010710:Phosphates; D015636:Magnesium Chloride; D008274:Magnesium; D002118:Calcium