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Abstract Metabolic alkalosis leads to renal magnesium conservation, whereas metabolic acidosis is associated with urinary magnesium wasting. Micropuncture studies suggest that these actions affect magnesium transport in the distal tubule. The cellular mechanisms of acidbase changes were investigated in an immortalized mouse distal convoluted tubule (MDCT) cell line. Intracellular free Mg2+ concentration ([Mg2+]i) was determined by microfluorescence using the Mg2+- responsive dye, mag-fura 2. Mg2+ transport was assessed as a function of change in [Mg2+]i with time following placement of Mg2+-depleted cells into a buffer containing 1.5 mM magnesium. The uptake rate of Mg2+, d( [Mg2+]i)ldt 9 into Mg 2+-depleted cells determined with a buffer solution of pH 7.4 was 178 t 21 nM/s. Mg2+ uptake at pH 8.0 was markedly increased 278 2 35 r&I/s, whereas transport at pH 6.0 was significantly reduced to 121 ? 15 nM/s. Mg2+ uptake at pH 7.4 was not stimulated with 20 or 40 mM bicarbonate, nor were the differences in Mg2+ uptake with pH associated with changes in membrane voltage. Mg2+ uptake was stimulated with membrane hyperpolarization at pH 6.0 but not at pH 8.0. Chlorothiazide (10m4 M), which stimulates Mg2+ uptake by hyperpolarizing the membrane voltage, increased uptake at pH 6.0,59 t 14%, but decreased it at alkaline pH of 8.0, -55 t 3%. Accordingly, MDCT cells become refractory to the stimulating effects of hyperpolarization at alkaline pH values. These studies show that protons may directly affect Mg2+ transport in MDCT cells.