Immunocytochemical localization of the vitamin D-dependent calcium binding protein in chick duodenum.

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Abstract
The vitamin D-dependent Ca-binding protein (CaBP) of chick duodenum was localized by immunocytochemistry and by radioimmunoassay. Light microscopically, CaBP was seen to be present in the absorptive cells of the villi while in other cell types of the villi and the crypts, including goblet cells and endocrine cells, no CaBP was seen. At the EM level, CaBP was localized in the cytosol and the euchromatin of the nucleus, but not in membrane-bounded cytoplasmic compartments. Quantitative evaluation of the immunocytochemical protein A-Au label showed that the terminal web and the cytosol of basal cellular regions were most highly labeled while the brush border was weakly labeled. The radioimmunoassay evaluation of intestinal subcellular fractions indicated that 96% of the homogenate CaBP is in the cytosol high-speed supernatant fraction. Collectively, these results support the hypothesis that the vitamin D-dependent intestinal CaBP may play a role in either regulation of intracellular Ca concentration or movement of Ca across the epithelial cell, rather than directly in the initial step(s) of Ca absorption across the brush border membrane from the gut lumen.

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