Intestinal absorption of calcium and oxalate in magnesium-deficient rats

To examine the contribution of exogenous calcium and oxalate in magnesium deficiency, intestinal absorption of both calcium and oxalate was studied by preparing brush border membrane vesicles (BBMV). Purity of the BBMV was ascertained biochemically by enrichment of the marker enzyme alkaline phosphatase by 14-fold with a concomitant 90 per cent decrease in the basolateral marker enzyme Na+/K(+)-ATPase in the purified membrane preparation as compared to the respective homogenate in both the groups. Uptake studies were carried out by a rapid filtration technique. The kinetics were studied by measuring the rate of influx as a function of concentration (0.1-1.0 mM). BBMV from both the groups showed a linear positive relationship between the uptake rate and the concentration for both calcium and oxalate, thereby demonstrating that calcium and oxalate are transported through intestinal microvillus membrane by a simple passive diffusion process. However, the rate of uptake of calcium and oxalate was significantly higher in the magnesium-deficient group as compared to the pair-fed control group, as shown by the increase in the slope line for both calcium and oxalate (for calcium, control = 3.88, deficient = 5.86; for oxalate, control = 4.41, deficient = 7.20). Analysis of the lipid composition of the BBM revealed a significant decrease in the cholesterol content (P < 0.01) with a concomitant increase in the triglycerides (P < 0.01) and total fatty acid content (P < 0.001) in the magnesium-deficient group. Thus the results indicate that, although the mechanism of translocation of calcium and oxalate in the intestine is similar in the two groups, magnesium deficiency leads to hyperabsorption of both the ligands through alterations in the lipid composition of the membrane, thereby increasing the risk of stone formation.