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 (P_i) 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 P_i absorptive process. At a given dose level, the magnitude of the response of intestinal P_i transport was comparable to that of the other two activities. A significant increase in P_i 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 P_i transport tended to reach its maximal level earlier than CaBP or alkaline phosphatase, hinting at a possible separation of P_i 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 P_i 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 P_i absorption. 1,25-Dihydroxyvitamin D₃ is effective in overcoming Sr inhibition of P_i 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 P_i absorption.