Dietary magnesium restriction with hypomagnesia is normally associated with diminished urinary magnesium excretion. Young rats were pair fed control magnesium diets (0.05% MgSO4) and magnesium-restricted diets (less than 0.01% Mg), and tubular magnesium reabsorption was assessed to determine the importance of filtered load (plasma magnesium) and to establish the presence of cellular adaptation of magnesium transport. Urinary magnesium excretion decreased from 17.2 +/- 2.7 to 5.9 +/- 1.2% over 20 h on low-magnesium diets without a change in plasma concentration (0.61 +/- 0.02 mM). This cellular adaptation was rapid (within 5 h), specific (without effect on sodium and calcium), and sensitive (without change in plasma concentration). Micropuncture studies demonstrated that cellular adaptation occurred within the loop of Henle. After 20 h, plasma magnesium fell with an associated further decrease in fractional magnesium excretion. Accordingly, cellular adaptation of magnesium transport occurs with magnesium-deficient diets, and alterations in filtered magnesium and plasma magnesium concentration are not necessary for magnesium conservation.

In previous work with the method of multiple indicator dilution (MID), we have established that a spatially distributed model of transcapillary exchange proposed by Goresky, Ziegler, and Bach (GZB) accurately describes, at the in vivo whole-organ level, the handling of extracellular indicators in the canine renal cortex. To date, however, it has not been possible to assess the key hypothesis that GZB corresponds to the actual local mechanism of exchange in vivo and is not just a compact summary of the kidney's average whole-organ behavior. By adapting the MID method to high speed computed tomography (CT), we are now able to report that the GZB mechanism is an accurate description of renal cortical transcapillary exchange down to volumes of cortical tissue comprising no more than a few per cent of the total cortical mass, i.e., containing no more than a few thousand nephrons. A small bolus of iohexol (radiopaque extracellular indicator) or iodipamide ethyl ester microparticles (radiopaque plasma indicator) injected into the renal artery was followed by CT as it passed through the kidney and into the renal vein. Time-attenuation value curves of the two contrast media obtained from the renal vein and from regions of interest in the cortex were then modeled with the GZB mechanism and with a more complex formulation that includes GZB as a limiting case. When applied to the data, the models converged to GZB as the best fit for each region examined. The GZB mechanism is found to provide excellent agreement with the regional data.

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SUMMARYAnaesthesized rats were infused with 100 ,ul min-of 09 % saline or saline containing 2 or 4 mmol 1-1 of magnesium chloride and renal function was studied. Magnesium had no effect on urine flow rate, excretion of sodium or potassium, or glomerular filtration rate (GFR). Infusion of either dose of magnesium increased calcium excretion and calcium concentration in the urine. The increased urinary calcium excretion is due to a decrease in tubular reabsorption of calcium.