Regulation of the epithelial Ca2+ channels in small intestine as studied by quantitative mRNA detection

The epithelial Ca2+ channels TRPV5 and TRPV6 are localized to the brush border membrane of intestinal cells and constitute the postulated rate-limiting entry step of active Ca2+ absorption. The aim of the present study was to investigate the hormonal regulation of these channels. To this end, the effect of 17beta-estradiol (17beta-E-2), 1,25-dihydroxyvitamin D-3 [1,25(OH)(2)D-3], and dietary Ca2+ on the expression of the duodenal Ca2+ transport proteins was investigated in vivo and analyzed using real-time quantitative PCR. Supplementation with 17beta-E-2 increased duodenal gene expression of TRPV5 and TRPV6 but also calbindin-D9K and plasma membrane Ca2+-ATPase (PMCA1b) in ovariectomized rats. 25-Hydroxyvitamin D-3-1alpha-hydroxylase (1alpha-OHase) knockout mice are characterized by hyperparathyroidism, rickets, hypocalcemia, and undetectable levels of 1,25(OH)(2)D-3 and were used to study the 1,25(OH)(2)D-3-dependency of the stimulatory effects of 17beta-E-2. Treatment with 17beta-E-2 upregulated mRNA levels of duodenal TRPV6 in these 1alpha-OHase knockout mice, which was accompanied by increased serum Ca2+ concentrations from 1.69 +/- 0.10 to 2.03 +/- 0.12 mM ( P < 0.05). In addition, high dietary Ca2+ intake normalized serum Ca2+ in these mice and upregulated expression of genes encoding the duodenal Ca2+ transport proteins except for PMCA1b. Supplementation with 1,25(OH)(2)D-3 resulted in increased expression of TRPV6, calbindin-D9K, and PMCA1b and normalization of serum Ca2+. Expression levels of duodenal TRPV5 mRNA are below detection limits in these 1α-OHase knockout mice, but supplementation with 1,25(OH)(2)D-3 upregulated the expression to significant levels. In conclusion, TRPV5 and TRPV6 are regulated by 17β-E-2 and 1,25(OH)(2)D-3, whereas dietary Ca2+ is positively involved in the regulation of TRPV6 only.