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The possible role of calcium-binding protein induced by 1 alpha,25-dihydroxyvitamin D3 in the intestinal calcium transport mechanism.

The relationship between the appearance of vitamin D-dependent calcium-binding protein (CaBP) and calcium absorption was studied in sequentially isolated duodenal mucosal preparations from vitamin D-deficient chicks and those supplemented with vitamin D3 or 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25-(OH)2D3]. The duodenal calcium absorption activity was significantly increased 6 h after a single iv injection of 0.625 microgram of 1 alpha,25-(OH)2D3, attained a maximum at 12 h, and declined gradually thereafter. On the other hand, only small amounts of CaBP appeared in the crypt and lower villus regions 6 h after 1 alpha,25-(OH)2D3 administration. At 12 h, the CaBP was found in the entire villus, but its content was still much higher in the crypt and lower villus. At 24 h and 48 h, the distribution of CaBP showed the opposite gradient, higher in the villus and lower in the crypt. At 72 h, CaBP was found only in the upper villus. The life time of duodenal mucosal cells was calculated to be 108 h as indicated by the cells labeled with [3H]thymidine. Thus, the movement of CaBP from the crypt to the villus tip was considered to be much faster than the cell migration, suggesting that both crypt and villus cells are capable of producing CaBP. Daily administration of vitamin D3 or 1 alpha,25-(OH)2D3 for 2 weeks resulted in a marked increase in CaBP levels mainly in the mid- and upper villus regions. The higher the intestinal calcium transport activity was, the higher the duodenal CaBP content. These results suggest that CaBP is not necessary in initiating intestinal calcium transport, but it plays an important role in maintaining the enhanced transport mechanism.