Determination of epithelial magnesium transport with stable isotopes

Abstract

The inability to adequately determine Mg 2+ flux rates with radiotracer studies has stymied our efforts to understand how magnesium is transported by epithelial cells. To evaluate epithelial Mg 2+ transport, a stable 25Mg isotope was used to measure magnesium uptake into normal and Mg 2+-depleted Madin–Darby canine kidney (MDCK) cells. 25Mg entry rates were significantly increased in Mg 2+-depleted cells relative to those cultured in normal magnesium media, 0.5 mM. 25Mg uptake was inhibited by external La 3+ but not Ca 2+ in both normal and Mg 2+-depleted cells suggesting a specific entry pathway. These results with 25Mg were the same as with microfluorescence determinations using mag-fura-2. We have shown that Mg 2+ entry into epithelial cells reflects transepithelial transport; accordingly, increased Mg 2+ uptake in Mg 2+-depleted cells provides an important intrinsic control of renal magnesium absorption. Furthermore, these studies indicate that cellular Mg 2+ transport may be quantitated with the use of stable isotopes that may be successfully applied to cells other than epithelia.