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1 Department of Cell Physiology, Nijmegen Center for Molecular Life Sciences, University Medical Center Nijmegen, Nijmegen, The Netherlands
2 Department of Internal Medicine, Erasmus Medical Center Rotterdam, Rotterdam, The Netherlands
* To whom correspondence should be addressed. E-mail: r.bindels{at}ncmls.kun.nl.
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 17
-estradiol (17-E2), 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) 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 17-E2 increased duodenal gene expression of TRPV5 and TRPV6, but also calbindin-D9K and plasma membrane Ca2+-ATPase (PMCA1b) in ovariectomized rats. 25-Hydroxyvitamin D3-1
-hydroxylase (1-OHase) knockout mice are characterized by hyperparathyroidism, rickets, hypocalcaemia, and undetectable levels of 1,25(OH)2D3 and were used to study the 1,25(OH)2D3-dependency of the stimulatory effects of 17-E2. Treatment with 17-E2 upregulated mRNA levels of duodenal TRPV6 in these 1-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)2D3 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 1-OHase knockout mice, but supplementation with 1,25(OH)2D3 upregulated the expression to significant levels. In conclusion, duodenal TRPV5 and TRPV6 are regulated by 17-E2 and 1,25(OH)2D3, while dietary Ca2+ is positively involved in the regulation of TRPV6 only.
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