Intestinal and renal handling of oxalate in magnesium-deficient rats.: Evaluation of intestinal in vivo 14C-oxalate perfusion

Objectives To clarify in vivo , using isolated small intestinal loops perfused with radioactive C-14-oxalate, whether intestinal hyperabsorption or reduced secretion is important in magnesium deficiency (MgD), as this is a potential cause of calcium oxalate urolithiasis. Materials and methods Twenty-four Sprague-Dawley rats were either fed a standard diet (control, 12 rats) or a magnesium-deficient diet (MgD, 12 rats) for 19 weeks. One hour before the animals were killed, a defined length of a small intestinal loop was isolated and filled with 5 mL of 0.9% NaCl and a defined amount of intravenous C-14-oxalate applied. Using this method it was possible to determine the secretion of unlabelled oxalate into the intestinal lumen, from the specific activity in plasma. Results Plasma oxalate levels doubled under MgD; urinary calcium and phosphorus also increased significantly, while urine oxalate tended to decrease. The secretion of oxalate into the intestinal lumen of MgD animals increased significantly, by five times that of the control. The relative supersaturation for calcium oxalate remained constant. Elementary analysis of renal tissue showed an increase in calcium and phosphorus under MgD, in the sense of nephrocalcinosis, but no concretions were detected (no nephrolithiasis). Conclusion In contrast to earlier studies, there is no evidence that hyperoxaluria is responsible for the possible development of urolithiasis in MgD. This was confirmed by calcium phosphate deposits in renal tissue, even though there was no evidence of oxalate urolithiasis. The increase in plasma oxalate seems to be completely compensated by strongly increased oxalate secretion into the intestinal lumen.