Decreased intestinal calcium absorption in vivo and normal bush border membrane vesicle calcium uptake in cortisol-treated chickens: Evidence for dissociation of calcium absorption from bush border vesicle uptake

The influence of cortisol on intestinal calcium transport was studied in isolated duodenal loops and brush border membrane (BBM) vesicles of vitamin D-deficient or replete chickens. Four- to five-week-old vitamin D-deficient cockerels were dosed intraperitoneally with 1 μg of 1,25-dihydroxyvitamin D₃ [1,25-(OH)₂D₃] alone 15 hr before sacrifice or in combination with 1, 3, or 5 mg of cortisol 24 and 48 hr before sarcrifice. After a 1-μg dose of 1,25-(OH)₂D₃ the in situ intestinal ligated loop technique revealed a 60% increase in calcium absorption compared to control birds (P ≤ 0.001). However, the administration of cortisol in various doses (3 and 5 mg) to chickens given 1,25-(OH)₂D₃ resulted in significant decreases in intestinal calcium transport in vivo (P ≤ 0.05; P ≤ 0.05). When intestinal BBM vesicles were prepared from birds treated in a manner identical with that described above, there was no observable difference between calcium uptake in BBM vesicles of the 1,25-(OH)₂D₃-treated birds and that of the cortisol plus 1,25-(OH)₂D₃-treated birds. 1,25-(OH)₂D₃-treated and 1,25-(OH)₂D₃ plus cortisol-treated chicks had intestinal BBM vesicle uptakes that were significantly greater than those of vitamin D-deficient controls (P ≤ 0.02; P ≤ 0.025). These data show that in vivo intestinal calcium transport may be markedly reduced in the presence of normal intestinal BBM vesicle calcium uptake. This suggests that factors other than BBM calcium uptake (e.g., protein synthesis or contraluminal membrane events) play an important role in the movement of calcium from the intestinal lumen into the bloodstream and extracellular fluid of the organism.