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

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++ de-creased 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 absorp-tion 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 path-ways. IJ FlUNG the past 53 years, it has become increasingly apparent that there exists an interrelationship between Ca++, Mg and Sr for transport in both the intestine and the renal tubule. The studies of Berglund and Forster (1) indicated that there might be competition between Ca and Mg++ for transport by the renal tubule of a