ORAL CALCIUM TRANSIENTLY INCREASES CALBINDIN(9K) GENE-EXPRESSION IN ADULT-RAT DUODENA

In rat intestine, the 9 kilodalton calbindin (CaBP9K) is significantly increased in vivo by 1,25-dihydroxyvitamin D-3 (1,25(OH)(2)D-3) throu gh a vitamin D (D) response element located in the 5'-flanking region of the gene. However, in vitro calcium has also been reported to incre ase CaBP9K gene expression in fetal duodenal culture preparations. The aim of the studies was to investigate whether calcium feeding alone c an influence CaBP9K gene expression in vivo in adult rat duodena by ev aluating the pattern of expression of its mRNA following short- or lon g-term exposure to oral calcium, comparing the data to exposure to the known inducer of the gene, 1,25(OH)(2)D-3. Hypocalcemic D-depleted ra ts were acutely or chronically supplemented with calcium per os, or wi th 1,25(OH)(2)D-3 in the presence or absence of oral calcium. Short-te rm calcium feeding was shown to significantly increase the expression of the CaBP9K gene to a level similar to that observed in 1,25(OH)(2)D -3-treated rats but no additive effect between oral calcium and 1,25(O H)(2)D-3 on the level of its mRNA was observed. Moreover, the calcium effect on CaBP9K gene expression was shown to be independent of the ci rculating ionized calcium concentration and, contrary to the effect of 1,25(OH)(2)D-3, not sustained following long-term exposure. Our data clearly indicate that oral calcium alone has a significant but only tr ansient effect of the expression of the adult rat intestinal CaBP9K ge ne in vivo and that maintenance of its expression requires normalizati on of the D endocrine system.