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DEVELOPMENT AND LOCALIZATION OF VITAMIN D-DEPENDENT CALCIUM-BINDING PROTEIN TO THE BRUSH BORDER MEMBRANE IN NEONATAL RAT KIDNEY

In order to investigate the role of vitamin D in renal tubular calcium reabsorption we measured vitamin D-dependent calcium-binding protein (CaBP) in renal homogenates and determined its localization in suckling and weanling rat kidney. One pup was taken from each of five litters at weekly intervals from 7-35 days post partum. CaBP was measured in kidney homogenates by radioimmunoassay and localized in freeze-substituted sections using an immunoperoxidase technique. We found only a 30% increase in CaBP in renal homogenates between 7 and 14 days, and none thereafter. We therefore studied two groups of four animals at 2 and 6 days of age and found a 104% increase between 2 and 6 days. Immunocytochemical studies revealed localization of the CaBP to the brush border of cells in the proximal tubule which was more pronounced with increasing age. The presence of vitamin D-dependent CaBP, a molecular marker of the vitamin D endocrine system in the brush border of the proximal tubule of rat kidney suggests that this is a site of vitamin D-dependent calcium transport analogous to that in the intestine. The developmental appearance and cellular localization noted in the kidney are, however, very different from that seen in the enterocyte, in which the CaBP is present at low levels until weaning, when it undergoes a five-fold increase, and rather than being a brush border protein, is widespread throughout the cytoplasm.