The rates of active transport of calcium in vitro by everted gut-sacs prepared from the proximal small intestine of the rat have been quantified and expressed in absolute units. A maximal rate of transport has been measured. The bulk of the calcium transferred to the serosal surface of the gut-sac is ionized calcium, suggesting that the process is an active cation transport mechanism. The active transfer is relatively specific for Ca++, and no significant accumulation of Mg++, Sr++, Ba++ or K+ in the fluid bathing the serosal surface could be demonstrated. The active transport of calcium in vitro is greater with gut-sacs from growing than from older rats, and it is greater with gut-sacs from pregnant than from nonpregnant rats. The results suggest that the active transport mechanism can increase the intestinal absorption of calcium facultatively to meet the needs of the organism.

Vitamin D is required for the active transport of calcium in vitro. Small doses of vitamins D2 and D3 restore the mechanism in depleted rats, and this provides a sensitive bio-assay for the vitamin, independent of an antirachitic effect. Vitamin D influences calcium transfer in all segments of the small intestine, and maximal increments are observed in the duodenum. The effect of vitamin D requires oxidative metabolism in vitro, is maximal where active transport is maximal, and the sterol increases the maximal rates of active transport of calcium. Consequently, vitamin D influences calcium transport by affecting primarily the active mechanism rather than by simple diffusion. Experiments with various monosaccharides demonstrate that two distinct steps are involved in the active transport mechanism in duodenum. Vitamin D is required for both of the steps.

The small intestine of the rat responds facultatively to a diet low in Ca by increasing the active transport of the cation. The effects of calcium deprivation were studied with everted gut sacs and with duodenal slices in vitro, and the experiments demonstrate that following this stimulus almost the entire small intestine of a young rat can transfer calcium from the mucosa to the serosa against concentration gradients. The active transport is maximal in duodenum, less in ileum, and least in the mid small intestine. Following the low-Ca diet, duodenal gut sacs transport Sr89 against concentration gradients, although strontium is transferred much less readily than is calcium. Vitamin D is required for the adaptive response of the active transport in duodenum and ileum. Younger rats respond to Ca deprivation earlier and more markedly than older animals. Neither thyroparathyroidectomy, hypophysectomy, or adrenalectomy prevent response to the low-Ca diet, although these ablations do affect the active transport mechanism in rats on a given diet.