The Effects of 24R, 25‐Dihydroxyvitamin D3 and 24S, 25Dihydroxyvitamin D3 on Phosphate Transport in Vivo

Abstract

The steroid hormone 1, 25‐dihydroxyvitamin D3 [1,25D3] rapidly stimulates the uptake of phosphate in isolated chick intestinal cells, while 24,25‐dihydroxyvitamin D3 [24,25D3] inhibits the rapid stimulation by 1,25D3. in isolated intestinal cells and perfused duodenal loops. Studies were undertaken to determine if 24,25D3 had a similar effect in vivo. 24,25D3 has two isomers which are 24R, 25dihydroxyvitamin D3 [24R,25D3] and 24S, 25dihydroxyvitamin D3 [24S,25D3]. We studied these two isomers separately and tested them over a time course of 1, 5, 10, 15 and 18 h after steroid using chicks on regular diet, but fasted, and chicks on a lower vitamin D diet. All chicks were anesthetized prior to surgical exposure of the duodenal loop and injection of H3 32PO4 in buffer into the lumen. An initial study determined that 3 to 9 min of absorption in vivo was in a linear range, as judged by serum levels of radioactivity. Chicks were then injected IM with either 200 μg 24R,25D3, 20 μg 24S,25D3 or vehicle (1–18 h prior to experimentation) within the same time course studies. The isomers had different effects on phosphate absorption: 24R,25D3 had a hypophosphatemic effect in vivo in which serum levels at 5, 15 and 18 h had decreased to 42%, 39% and 43%, respectively, (P< 0.05) compared with controls. In contrast, 24S, 25(OH)2D3 stimulated intestinal phosphate absorption at the 5h time point by 64%, but had no effect at the other time points tested. Supported by Agriculture and Food Research Initiative Competitive Grant no. 2010‐65206‐20652 from the USDA National Institute of Food and Agriculture.