Vitamin D-dependent intestinal calcium absorption

Abstract

During the past decade, considerable progress has been made in elucidating the mechanism of the vitamin D-dependent stimulation of intestinal calcium transport. A major breakthrough was registered with the discovery of a vitamin D-stimulated CaBP. The intestinal production of this protein has been shown to be inversely regulated by dietary calcium in the vitamin D-replete animal but is nonexistent in animals deficient in the vitamin. Furthermore, the level of calcium transport is directly proportional to the CaBP activity, and histologic localization studies indicate that the protein is present in the intestinal lumen in close association with the absorptive surface. When inhibitors of RNA and protein synthesis are given before the vitamin, the production of CaBP is blocked, which indicates that CaBP may be a functional protein of the hormone 1α,25-(OH)2D. However, examination of microvillar membranes indicates the presence of other proteins in response to 1α,25-(OH)2D administration, and the participation of these proteins in the 1α,25-(OH)2D-mediated increase in calcium absorption is suggested. In addition, changes in the microvillar membrane lipid composition provide support for an additional mechanism(s) by which intestinal cell calcium uptake is augmented after vitamin D administration. A mechanistic summary of these findings is presented in Figure 1.