Pansu D, Bellaton C, Roche C, Bronner F
Am J Physiol. 1983 Jun;244(6):G695-700. doi: 10.1152/ajpgi.1983.244.6.G695.
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.
采用原位结扎肠袢法将肠壁钙转运分解为两个部分，即饱和过程和非饱和过程。十二指肠中证实存在这两种过程，但回肠钙转运缺乏饱和成分。空肠上段存在少量饱和成分。钙结合蛋白（CaBP），即维生素D依赖性胞质钙结合蛋白（分子量约为9000）的水平与饱和成分的大小相对应。回肠中未检测到CaBP。通过向维生素D缺乏动物的十二指肠腹腔注射1,25 - 二羟基维生素D3来诱导饱和成分，从而确定了饱和成分对维生素D的依赖性。相反，在这些相同的动物中，回肠对外源性1,25 - 二羟基维生素D3没有反应。这证实了回肠不存在跨壁钙转运的饱和成分，以及非饱和成分不依赖维生素D这一事实。外翻肠囊实验还表明，来自维生素D充足或缺乏动物的十二指肠囊能够逆化学梯度转运钙，而回肠囊则不能。因此，尽管钙沿化学梯度在整个小肠中被吸收，但维生素D对肠道钙吸收的调节仅发生在近端小肠。
