CALCIUM AND OXALATE UPTAKE BY THE RENAL BRUSH-BORDER MEMBRANE-VESICLES IN MAGNESIUM-DEFICIENT RATS

Calcium and oxalate uptake by renal brush border membrane vesicles (BB MV) was examined in magnesium-deficient and pair-fed control rats. Upt ake studies were carried out by rapid filtration technique and rate of influx of calcium and oxalate as a function of extravesicular concent ration (0.1 mM - 1.0 mM) examined. Calcium uptake by renal BBMV exhibi ted saturable kinetics while oxalate uptake followed a biphasic transp ort mechanism showing saturable kinetics at low oxalate concentrations and passive diffusion at higher concentrations. In magnesium deficien cy the kinetics of calcium and oxalate uptake by renal BBMV remained u naltered but the rate of uptake was significantly enhanced at all the extravesicular concentrations studied. Double reciprocal plot for calc ium uptake showed no change in Vmax but a decrease in Km (2.08 mM) in magnesium deficient rats as compared to pair-fed controls (Km = 5.00 m M). Similar plot for oxalate uptake showed an increase in Vmax (7.69 n moles/8 min/mg protein) in magnesium deficient group as compared to pa ir-fed controls (5.55 nmoles/8 min/mg protein), while Km remained unch anged. The results of the present study indicate high risk of calcium oxalate stone formation in magnesium - deficient rats due to hyperabso rption and retention of calcium and oxalate by the renal tubular brush border membrane.