Vitamin D dependence of in vivo calcium transport and mucosal calcium binding protein in rat large intestine.

Dependence of large intestinal calcium transport on vitamin D has been examined in vitro in colon only. The authors studied calcium fluxes in cecum and colon in vivo by perfusion with 1.6 mM calcium chloride in saline. Tracer 45Ca either was injected parenterally 24 hr before study or was added to the perfusates. For 8--10 wk after weaning, rats had been fed a rachitogenic diet; 48 and 24 hr before study, 50% of the animals were treated with 20,000 IU vitamin E2. In a separate set of animals, mucosal calcium binding protein was analyzed by the Chelex assay method. In comparison with vitamin D-deficient rats, the colon of vitamin D-treated rats showed higher lumen-to-plasma flux and lower plasma-to-lumen flux and net absorption instead of net secretion. In cecum, calcium transport was not significantly altered by vitamin D treatment. Mucosal calcium binding protein was higher in cecum than in colon in both groups and was higher in vitamin-D-treated than in vitamin D-deficient animals in both segments. The current study shows that in the rat colon calcium fluxes both into and out of the lumen as well as net transport are significantly by vitamin D treatment, but that cecal transport rates are not affected. In both cecum and colon, mucosal calcium binding protein increases with vitamin D treatment.