Effects of 25-Hydroxyvitamin D, on Rat Duodenum, Jejunum, and Ileum

Jejunum and ileum from vitamin D-deficient rats actively secreted calcium when studied in vitro, while duodenum from these animals actively absorbed calcium. Acute repletion with a single physiologic dose of 25-hydroxyvitamin D3 (25-OH D3) (15 i.u.) increased calcium absorption in duodenum and ileum but had no effect on jejunum. Studies of the distribution of 25-OH D3 and its metabolites in these tissues demonstrated that levels of 1,25-(OH)* Da per cell were highest in duodenum, much lower in ileum and lower still in jejunum. The increased calcium absorption in duodenum and ileum was proportional to the levels of 1,25(OH)% Ds in these tissues. This was not the case for jejunum however, because while 1,25-(OH)* D3 was present, calcium transport rates were unchanged. Another finding was a much higher percentage of water-soluble metabolites of 25-OH Da in jejunum and ileum than in duodenum. This observation may relate to an enterohepatic circulation of these compounds. These experiments suggest that previous reports of decreased calcium absorptive responses by lower small bowel (relative to proximal duodenum) to either treatment with vitamin D or prolonged dietary calcium deprivation, can be explained by the differential cellular localization of 1,25(OH):! DB (duodenum > ileum > jejunum).