Duodenal calcium absorption in vitamin D receptor–knockout mice: Functional and molecular aspects

TLDR

Rickets and hyperparathyroidism caused by a defective vitamin D receptor can be prevented by high calcium intake, indicating that intestinal calcium absorption is essential for 1,25(OH)₂ vitamin D regulation of calcium homeostasis. The study measured serum 45Ca accumulation over 10 minutes after oral gavage in two VDR‑knockout mouse strains and found a three‑fold lower area under the curve compared to wild‑type mice. The authors evaluated expression of intestinal genes involved in transcellular calcium transport to assess the molecular basis of impaired absorption. VDR‑knockout mice showed a three‑fold reduction in serum 45Ca uptake, with duodenal CaT1 and ECaC expression markedly decreased (>90% and >60% respectively) regardless of diet, while calbindin‑D9K was only reduced in the Tokyo strain; in wild‑type mice, high calcium intake suppressed CaT1 expression and 1,25(OH)₂D₃ or low calcium diet induced a six‑fold increase, indicating that vitamin D strongly regulates duodenal calcium channel expression and that calcium influx via CaT1, likely coupled to calbindin‑D9K, is a rate‑limiting step.

Abstract

Rickets and hyperparathyroidism caused by a defective vitamin D receptor (VDR) can be prevented in humans and animals by high calcium intake, suggesting that intestinal calcium absorption is critical for 1,25(OH) 2 vitamin D [1,25(OH) 2 D 3 ] action on calcium homeostasis. We assessed the rate of serum 45 Ca accumulation within 10 min of oral gavage in two strains of VDR-knockout (KO) mice (Leuven and Tokyo KO) and observed a 3-fold lower area under the curve in both KO strains. Moreover, we evaluated the expression of intestinal candidate genes involved in transcellular calcium transport. The calcium transport protein1 (CaT1) was more abundantly expressed at mRNA level than the epithelial calcium channel (ECaC) in duodenum, but both were considerably reduced (CaT1>90%, ECaC>60%) in the two VDR-KO strains on a normal calcium diet. Calbindin-D 9K expression was decreased only in the Tokyo KO, whereas plasma membrane calcium ATPase (PMCA 1b ) expression was normal in both VDR-KOs. In Leuven wild-type mice, a high calcium diet inhibited (>90%) and 1,25(OH) 2 D 3 injection or low calcium diet induced (6-fold) duodenal CaT1 expression and, to a lesser degree, ECaC and calbindin-D 9K expression. In Leuven KO mice, however, high or low calcium intake decreased calbindin-D 9K and PMCA 1b expression, whereas CaT1 and ECaC expression remained consistently low on any diet. These results suggest that the expression of the novel duodenal epithelial calcium channels (in particular CaT1) is strongly vitamin D-dependent, and that calcium influx, probably interacting with calbindin-D 9K , should be considered as a rate-limiting step in the process of vitamin D-dependent active calcium absorption.