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 inraluminal perfusion of NaCl, decreased in ileum but not in duodenum in response to clacium restirction. 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.

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Intestinal adaptation by the growing rat to a low-magnesium diet was studied by in situ perfusion of duodenum and ileum in vivo. Rats were fed diets containing either 0.066 or 0.022% Mg for 3 weeks. Magnesium-restricted rats became hypomagnesemic and hypercalcemic. Net magnesium secretion was studied by perfusing an initially magnesium-free saline solution; secretion was higher in duodenum than in ileum, and decreased significantly in the duodenum in response to magnesium restriction. Net magnesium adsorption studied by intraluminal perfusion of 2.5 mM magnesium in saline was greater in duodenum than ileum in rats taking a low-magnesium diet, but duodenal and ileal absorption did not differ in animals taking the normal magnesium diet. Absorption did not adapt significantly to magnesium restriction in either segment. Adaptation of small-intestinal magnesium transport to a low magnesium diet is minimal, consisting mainly of reduced duodenal magnesium secretion.

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