Intestinal Absorption of Phosphate in the Chick: Effect of Vitamin D3 and Other Parameters

Abstract

The effect of various parameters, including vitamin D₃, on the intestinal absorption of ²²P-phosphate in the chick was investigated. Translocation was determined by the in situ ligated loop technique and measurements were made of radionuclide leaving the intestinal lumen (absorption), accumulation by scraped mucosa (in gut tissue) and that entering the blood (transferred to body). It was observed, initially, that ²²P was rapidly translocated across all segments of the small intestine (duodenum, jejunum, ileum) and vitamin D₂ positively affected the total process in each segment. Because of the ease of showing a responsiveness to vitamin D₃ by the ileum, the subsequently described results were obtained with this segment. In a time course experiment, it was noted that ²²P accumulation by mucosa proceeded at a more rapid rate than the release of radionuclide to blood, indicating that the latter was the limiting step. The absorptive process was saturable as shown by a study in which increasing levels of stable phosphate were present in the absorption fluid. An effect of vitamin D₃ on the transfer of ²²P from mucosa to body was evident, as well as an effect on the uptake phase. Arsenate, EHDP and L-phenylalanine inhibited ²²P translocation, but primarily in the vitamin D₃-repleted chicks. This suggested, as did the study of the effect of various levels of stable phosphate, that ²²P interacts with some intestinal component in the transport path. Since NaEGTA (a Ca-chelator) and thorough washing of the ileal lumen did not alter ²²P absorption and mucosal uptake, it was evident that ²²P₁ was not absorbed as a coion to calcium. A direct relationship between serum Ca levels and the degree of ³²P absorption became evident, although this correlation (r = 0.82) might be fortuitous. Vitamin D₃, when given to rachitic chicks, elicited an enhanced ²⁵P absorption at 16 hours; the appearance of vitamin D-dependent CaBP was evident at 12 hours. Studies on the nature of ³²P in mucosal tissue after absorption indicated that the radionuclide was largely indistinguishable from ortho-phosphate, and that only a small fraction was present as phospholipid-²²P. No phosphate binder in mucosal supernatant fluid, analogous to CaBP, was detected under the specific conditions employed.