The Regulation of Intestinal Calcium Transport by Vitamin D

Abstract

THERE is much evidence that metabolites of vitamin D are fundamentally involved in calcium absorption by the intestine. Cholecalciferol (vitamin D₃) is hydroxylated at C₂₅ in the liver^1,2 and then at C₁ in the kidney^3–5. 1,25-dihydroxycholecalciferol is thought to be the active form, a hormone which promotes Ca transport by the intestine^5–8. Ca translocation across the mucosal epithelial cell involves entry across the brush border and exit across the basal-lateral membrane. The site and manner of action of vitamin D metabolites has not been unequivocally established, but has been suggested to be at the basal-lateral membrane⁹, the brush border^10,11, or both^12,13. Kinetic studies in the normal rat led to the conclusion that entry across the brush border was rate-limiting for Ca translocation across the epithelium¹⁴. This would itself suggest that 1,25-dihydroxycholecalciferol regulated Ca absorption by acting at the brush border.