Regulation by Vitamin D of Intestinal Phosphate Absorption

Abstract
The role of vitamin D3 and its active metabolite, 1,25-dihydroxyvitamin D3, in the regulation of intestinal phosphate (Pi) absorption was studied in chick jejunum under a variety of experimental conditions. In a time-course and dose-response study, comparison was also made to well known vitamin D effects such as the induction of intestinal calcium-binding protein (CaBP) as an indicator of calcium transport, and to the stimulation of alkaline phosphatase activity, to elucidate their relationship, if any, in the Pi absorptive process. At a given dose level, the magnitude of the response of intestinal Pi transport was comparable to that of the other two activities. A significant increase in Pi absorption could be detected as early as 12 h after repletion of vitamin D-deficient chicks with 500 I.U. vitamin D3. Vitamin D-induced Pi transport tended to reach its maximal level earlier than CaBP or alkaline phosphatase, hinting at a possible separation of Pi transport from these vitamin D responses. Phosphate transport was shown to adapt readily to nutritional requirements. Both low dietary phosphorus and low calcium caused an adaptive increase in Pi absorption. A high content of strontium in the diet reduced the capacity of the jejunum to absorb phosphate. These changes are consistent with alterations in the biosynthesis of 1,25-dihydroxyvitamin D3, suggesting an important role of this sterol in the in vivo regulation of Pi absorption. 1,25-Dihydroxyvitamin D3 is effective in overcoming Sr inhibition of Pi transport and also has a direct effect on this system in vitamin D-deficient chicks. This latter response can be prevented by simultaneous administration of cycloheximide, indicating the involvement of a protein synthetic step in the vitamin D-dependent stimulation of intestinal Pi absorption.