Influence of vitamin D on calcium absorption in the chick

¹ Department of Biochemistry, University of Adelaide, Adelaide, South Australia

The site of absorption of Ca⁴⁵ was studied in rachitic chicks and rachitic chicks given vitamin D₃. Vitamin D₃ markedly increases absorption from the small intestine and, in vivo, similar amounts of calcium are absorbed along the entire small intestine. With everted gut sacs, the distal third of the small intestine transported much more calcium than did the duodenal and middle sections. Thus, interpretations of in vitro results may not always depict the natural in vivo process. Vitamin D₂ had little activity in the chick, but AT-10 series 2 and AT-10 series 3 were almost as active as vitamin D₃ for calcium transport. These results suggest an "active carrier" may be formed by addition of hydrogen or hydroxyl groups to the opened ring B of vitamin D, giving a carrier capable of reversible oxidation-reduction or keto-enol tautomerism. Using metabolic inhibitors, active calcium transport in vitro relied on glycolysis for its energy supply. The transport was independent of the sodium pump.