Renal and intestinal calcium transport: Roles of vitamin D and vitamin D- dependent calcium binding proteins

A model has been presented here for vitamin D-dependent Ca transport, based on observations of the intestinal Ca absorption process. In this model of vitamin D-dependent Ca transport, processes that occur in different areas of the intestinal epithelial cell combine to result in active transport of Ca²⁺ from the intestinal lumen to the bloodstream. At the brush-border membrane, 1,25(OH)₂D₃ causes a rapid opening of Ca²⁺ channels and transport of Ca²⁺ into the cell in a matter of seconds to minutes by a process that is independent of gene transcription. Inside the cell, 1,25(OH)₂D₃ stimulates transcription of the CaBP-D(9k/28k) gene, with resulting accumulation of CaBP-D(9k/28k) mRNA and protein 1 or more hours after 1,25(OH)₂D₃ treatment. The CaBP-D(9k/28k) has greater affinity for Ca²⁺ than do the brush-border membrane components, so Ca²⁺ movement through the cytosol is facilitated, with Ca²⁺ carried by CaBP-D(9k/28k). At the BLM, 1,25(OH)₂D₃ causes an increase in concentration of the PMCA, and stimulates Ca²⁺-pumping activity. The PMCA has still greater affinity for Ca²⁺ than does the CaBP-D(9k/28k). The combination of these vitamin D- dependent events results in active transport of Ca across the intestinal epithelia. Vitamin D sufficiency is necessary for this response to vitamin D treatment. This model may apply to renal DT cells as well as to intestinal absorptive cells. Vitamin D-regulated factors that are involved in vitamin D- dependent active Ca transport and are present in both renal DT and intestinal epithelial cells include VDR, CaBP-D(9k/28k) and the PMCA. The PMCA is localized to the BLM in both cell types. Both kidney and intestine respond similarly to changes in vitamin D, Ca, or P status. The many similarities between renal DT cells and intestinal epithelia strongly support the application of this model for vitamin D-dependent Ca transport in both tissues.