Studies on Vitamin D3 and the Intestinal Absorption of Calcium and Other Ions in the Rachitic Chick

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Abstract
The effect of vitamin D3 on the intestinal absorption of calcium and other ions was systematically studied in the chick. Most of the experiments were performed with ligated duodenum. Under the conditions described, it was observed that a lag period of 8 to 16 hours was necessary before oral vitamin D3 exerted its effect on calcium absorption. When the intralumenal levels of calcium were varied from 0.04 mg of Ca to 25 mg of Ca, a different pattern of response in Ca47 absorption was noted between rachitic and vitamin D3-repleted chicks. The magnitude of Ca47 absorption with time was studied at two levels of calcium, 0.04 and 1.0 mg; here, it was observed that the major difference in absorption due to vitamin D occurred between zero to 10 minutes after isotope administration. The effect of citrate, phosphate, phytate, glycerophosphate and EDTA on Ca47 absorption in the rachitic and vitamin D-treated chick was investigated; acetate was used as reference anion. Citrate had no effect on either group but phosphate and phytate equally depressed Ca47 absorption in the chicks with or without vitamin D supplementation. Glycerophosphate and EDTA had a greater depressing effect on Ca47 absorption in the rachitic than in the vitamin D-treated chicks. With respect to phosphate absorption, vitamin D3 had a slightly favorable effect on phosphate-P32 absorption between zero to 10 minutes after isotope administration but little difference was noted thereafter. When Ca47 and P32–PO4 were simultaneously injected into the duodenal lumen, each ion mutually depressed the absorption of the other; however, vitamin D3 had the usual pronounced effect on Ca47 absorption but there was also an observable effect of the sterol on P32 absorption. It was further observed that vitamin D3 did not alter the intralumenal pH of the duodenum, and that the absorption of Na23 and K43 and distribution of Ca47, Na22 and K43 were not greatly influenced by vitamin D3. Vitamin D3 increased the duodenal absorption of Mg++, Ca++, Sr++, and Ba++, Cs+ and Co++ but absorption of Na+, K+, Cu++, Fe+++ and Zn++ was not significantly enhanced by vitamin D3. Pertinent implication of the above data to the mechanism of vitamin D action were that the vitamin D-mediated response was not readily saturated by increasing levels of intralumenal calcium and that the mechanism was not specific for calcium ion.

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