Evidence that sodium deprivation influences vitamin D dependent rat renal calcium binding protein

In order to provide some insight concerning the role of renal calcium binding protein (CaBP) in the functioning of the mammalian kidney, the response of renal CaBP to dietary alterations was examined. Three week old rats were fed diets deficient in calcium, phosphorous or sodium supplemented with vitamin D for a four week period. The specific activity of renal CaBP (as measured by the chelex resin assay; Ca²⁺ bound protein/Ca²⁺ bound resin per mg protein) in the 28,000 M_r region was found to increase four fold in rats fed the low phosphorus diet and two fold rats fed the low calcium diet when compared to rats fed the control diet. Renal CaBP/mg protein from rats fed the low sodium diet decreased 50% from the control values. Changes in renal CaBP were confirmed by polyacrylamide gel analysis of the 28,000 M_r fraction by densitometric tracing using a purified CaBP marker. The greater response to dietary phosphorus restriction suggests that renal CaBP may be regulated by a mechanism different from that of intestinal CaBP. The decrease in renal CaBP in rats fed the low sodium diet suggests for the first time that sodium is required for vitamin D dependent distal tubular calcium transport processes.