For humans and rodents, duodenum is a very important site of calcium absorption since it is
exposed to ionized calcium released from dietary complexes by gastric acid. Calcium
traverses the duodenal epithelium via both transcellular and paracellular pathways in a
vitamin D-dependent manner. After binding to the nuclear vitamin D receptor, 1, 25-
dihydroxyvitamin D 3 [1, 25 (OH) 2 D 3] upregulates the expression of several calcium
transporter genes, eg, TRPV5/6, calbindin-D 9k, plasma membrane Ca 2+-ATPase 1b, and …