Interrelation between renal transport systems of magnesium and calcium

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Abstract
Infusion of magnesium chloride markedly increased the excretion of calcium in dogs. A slight but significant increase in the excretion of magnesium occurred following the infusion of calcium chloride. Utilizing the stop-flow technique it was noted that magnesium and calcium achieved minimal concentration ratios in the distal segments of the nephron. Infusion of magnesium chloride during stop-flow studies increased U/PCa/U/PCr ratios both in the proximal and distal samples. These observations support the postulate that magnesium and calcium compete for a common reabsorptive system in the renal tubules. In several stop-flow experiments Mg28 and Ca45 were injected along with inulin 1 or 2 minutes before releasing the ureteral occlusion. Mg28 and Ca45 appeared simultaneously in the urine prior to the appearance of inulin, suggesting transtubular flux of these cations. Such observations, while indicating bidirectional movement of these ions across the tubular epithelium, do not imply active or net secretion.