Calcium absorption and intestinal calcium-binding protein: quantitative relationship.

The concentration of the vitamin D-induced calcium-binding protein (CaBP) and calcium absorption from the duodenum were investigated in chicks with an in vivo ligated-loop technique. The relation between CaBP and calcium absorption was dependent on a) source of vitamin D activity (either vitamin D3 or 1,25-dihydroxycholecalciferol); b) dosage of vitamin D3; c) time after administration of vitamin D3 to rachitic animals. To aid in the interpretation of these results, a phenomenological model was developed in which CaBP was viewed as being linearly related to a portion of calcium absorption. The model, when applied to the data, suggests that there is a "nonfunctional" pool of CaBP the size of which is determined by the vitamin D status of the animal. After correction for this nonfunctional pool, the proportionality between CaBP and calcium absorption is independent of the vitamin D status of the animal.