title: Duodenal and ileal calcium absorption in the rat and effects of vitamin D.
abstract: An in situ ligated loop procedure was applied to dissect transmural calcium transport in the intestine into two components, a saturable and a nonsaturable process. The existence of two such processes was confirmed in the duodenum, but ileal calcium transport was devoid of the saturable component. There was a small saturable component in the upper jejunum. The level of CaBP, the vitamin D-dependent cytosolic calcium-binding protein (Mr, approximately or equal to 9,000), corresponded to the magnitude of the saturable component. No CaBP was detected in the ileum. Vitamin D dependence of the saturable component was established by inducing it in the duodenum of vitamin D-deficient animals following intraperitoneal injection of 1,25-dihydroxyvitamin D3. In these same animals, conversely, the ileum did not respond to exogenous 1,25-dihydroxyvitamin D3. This confirms the absence in the ileum of the saturable component of transmural calcium movement and the fact that the nonsaturable component is not vitamin D dependent. Everted sac experiments also showed that duodenal sacs from vitamin D-replete or -repleted animals transported calcium against a chemical gradient, whereas ileal sacs did not. Vitamin D regulation of intestinal calcium absorption thus occurs only in the proximal intestine, even though calcium is absorbed down its chemical gradient all along the small intestine.
mesh_terms: D000818:Animals; D002117:Calcitriol / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D004386:Duodenum / Q000187:drug effects / Q000378:metabolism*; D004872:Ergocalciferols / Q000494:pharmacology*; D007082:Ileum / Q000187:drug effects / Q000378:metabolism*; D007408:Intestinal Absorption / Q000187:drug effects*; D007583:Jejunum / Q000378:metabolism; D007700:Kinetics; D008297:Male; D009928:Organ Specificity; D051381:Rats; D011919:Rats, Inbred Strains; D064030:S100 Calcium Binding Protein G / Q000378:metabolism; D014808:Vitamin D Deficiency / Q000378:metabolism
chemical_list: D004872:Ergocalciferols; D064030:S100 Calcium Binding Protein G; D002117:Calcitriol; D002118:Calcium