Recent advances in our understanding of 1,25-dihydroxyvitamin D3 regulation of intestinal calcium absorption

Calcium is required for many cellular processes including muscle contraction, nerve pulse transmission, stimulus secretion coupling and bone formation. The principal source of new calcium to meet these essential functions is from the diet. Intestinal absorption of calcium occurs by an active transcellular path and by a non-saturable paracellular path. The major factor influencing intestinal calcium absorption is vitamin D and more specifically the hormonally active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). This article emphasizes studies that have provided new insight related to the mechanisms involved in the intestinal actions of 1,25(OH)2D3. The following are discussed: recent studies, including those using knock out mice, that suggest that 1,25(OH)2D3 mediated calcium absorption is more complex than the traditional transcellular model; evidence for 1,25(OH)2D3 mediated active transport of calcium by distal as well as proximal segments of the intestine; 1,25(OH)2D3 regulation of paracellular calcium transport and the role of 1,25(OH)2D3 in protection against mucosal injury.

► A major function of 1,25(OH)2D3 is intestinal calcium transport.
► The proximal and distal intestine contribute to the calcium absorptive process.
► TRPV6 and calbindin-D9k may act together in the calcium absorptive process.
► Active calcium absorption independent of TRPV6 or calbindin-D9k can occur.
► Evidence suggests that 1,25(OH)2D3 can also regulate paracellular calcium transport.