Vitamin D dependence and aboral gradient of in vivo intestinal calcium transport in the rat.

Rates of small-and large-intestinal calcium transport and their dependence on low doses of vitamin D were compared. Young rats were depleted of vitamin D by feeding a rachito-genic diet for 8--9 weeks; 48 and 24 hr prior to study, half were repleted with 200 IU vitamin D2. Duodenum and ileum, or cecum and colon, were perfused separately in situ with 1.6 mM calcium and tracer 45Ca in saline. There was an aboral gradient for net calcium transport and lumen-to-plasma flux with higher values in proximal than distal segments. Vitamin D repletion increased net calcium flux and lumen-to-plasma flux in duodenum and ileum, but not cecum and colon. Plasma-to-lumen flux (calculated as the difference between lumen-to-plasma flux and net flux) tended to be higher in colon than other segments and was not altered by vitamin D repletion.