Effects of diet calcium and 1,25-dihydroxyvitamin D3 on colon calcium active transport.

Unidirectional fluxes of calcium were studied in the absence of electrochemical gradients across rat descending colon segments in vitro. Dietary calcium restriction and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]enhanced absorption by increasing the mucosal-to-serosal fluxes, whereas secretory (serosal-to-mucosal) fluxes were unchanged. Low-calcium diet also stimulated calcium uptake by everted gut sac segments of ascending as well as descending colon, whereas transverse colon was unresponsive. These results show that the colon is a target organ for 1,25-(OH)2D3 and demonstrate the participation of colon in the intestinal adaptation to calcium deprivation.