Duodenal and ileal adaptation to dietary calcium restriction: in vivo studies in the rat.

Intestinal adaptation by the growing rat to a low-calcium diet was studied by in situ perfusion of duodenum and ileum in vivo. Rats were fed diets containing either 1.2 or 0.02% Ca for 17-24 days. To study plasma-to-lumen flux and net calcium absorption, rats were loaded parenterally with 45Ca and perfused intraluminally with 3.4 mM calcium. Calcium restriction caused net absorption in ileum to increase fourfold, in duodenum almost twofold. With calcium restriction, plasma-to-lumen flux decreased in duodenum but not in ileum and was small relative to absorption. However, net calcium secretion, when measured in a separate set of animals by intraluminal perfusion of NaCl, decreased in ileum but not in duodenum in response to calcium restriction. The magnitude of adaptation is greater in ileum than duodenum and is largely the result of increased lumen-to-plasma flux. The distal small intestine is probably crucial for calcium homeostasis in dietary calcium deficiency.