Influence of Vitamin D on in Vivo Intestinal Calcium Transport in Normal Rats 1

Abstract

Summary

Small intestinal calcium transport was compared in normal controls and in body weight matched rats injected with vitamin D₂ (either 500 or 10,000 U). The vitamin was injected 48 hr before study. Flux into the lumen of ⁴⁵Ca (injected 24 hr before study) and calcium and net calcium absorption were measured under steady-state conditions in duodenum-proximal jejunum and in terminal ileum. Flux of ⁴⁵Ca and calcium into the lumen was significantly lower in the rats treated with vitamin D₂ (10,000 U) than in controls, in both proximal and distal intestinal segments. However net calcium absorption was not significantly increased in the vitamin-treated groups compared with controls, in contrast to the behavior of vitamin D-deficient animals in which the main effect of this vitamin is to increase absorption. Thus, the effects of vitamin D differ in normal and depleted rats and this may be explained by differences in vitamin D metabolism in normal and deficient rats. The finding that vitamin D decreased flux of calcium into the lumen in normal rats suggests that this effect, if prolonged, may play a role in enhancing or maintaining the hypercalcemia of vitamin D toxicity.