Competition Between Calcium, Strontium, and Magnesium for Absorption in the Isolated Rat Intestine

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Abstract
Rats 1-2 mo. old were placed on a low calcium or a low calcium and low magnesium diet. Absorption in isolated rat intestine was studied in vitro. Duodenal segments which actively took up and actively transported Ca45++ well, actively took up and actively transported Sr85++ well; where Ca45++ active uptake and active transport was poor, that of Sr85++ was low. The uptakes of Ca45++, Sr85++, and Mg28++ by the gut tissue were greater than the amounts transported. Ca++, Sr++, and Mg++ decreased the active uptake of Ca45++ and Sr85++ by the duodenum. Ca++ decreased the rate of active transport to the serosal media of Ca40++ and Sr85++. Ca++ was more effective in decreasing the active uptake of Sr55++ than was Sr++. The active uptake of Ca45++ by the ileum was reduced by Mg++ and Sr++. The active uptake of Mg28++ by the ileum was reduced by Ca++. The rate at which Mg28++ crosses and is taken up by the small intestine increases as one progresses from the proximal duodenum to the distal ileum. Ca++ was actively transported by the colon. Ca++, Sr++, and Mg++ appear to share a common pathway for absorption by the gut, but in the various areas of the intestine the rate of absorption of a particular divalent cation and the degree of competition between the divalent cations varies. Therefore, the absorption of a particular cation may involve also other pathways.