Factors that influence absorption and secretion of calcium in the small intestine and colon.

Intestinal epithelium absorbs calcium by an energy-dependent cellular process that is stimulated by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. Calcium entry across the brush border is driven by existing electrochemical gradients; exit across the basolateral membrane against these same gradients is driven by a calcium-activated ATPase, sodium-calcium exchange, or both. The specific cellular sites of 1,25(OH)2D3 action remain to be identified. Calcium transport is independent of phosphate and influenced by sodium. Sodium may alter calcium transport at the brush border through alterations of the transmembrane electrical gradient and at the basolateral membrane by exchange with intracellular calcium. The segmental distribution of calcium active transport is heterogeneous, with maximal flux rates in the proximal portions of small intestine and colon and net secretion in mid- and distal small intestine and mid- and distal colon. 1,25(OH)2D3 converts regions of net secretion in ileum and colon to net absorption. 1,25(OH)2D3-stimulated active phosphate transport is largely confined to areas of low-calcium transport, with maximal phosphate absorption in jejunum, the site of maximal calcium secretion. Calcium secretion, primarily in jejunum and ileum, is nonsaturable, may follow the paracellular pathway, and is stimulated by mucosal sodium and somatostatin.