| [ Summary ] |
Intestinal calcium absorption is an important process involved in the maintenance of Ca2+ homeostasis. The intestinal absorption of calcium involves two pathways : a transcellular route and a paracellular route. Transcellular active calcium transport occurs mainly in the duodenum and gradually decreases in the distal segments. The transcellular component is a three-step process that consists of apical calcium uptake via transient receptor potential vanilloid family calcium channel 6 (TRPV6), cytoplasmic calcium translocation in a calbindin-D9K-bound form, and basolateral calcium extrusion via plasma membrane Ca2+-ATPase (PMCA) isoform 1b. However, solvent drag-induced calcium transport depends on a paracellular sodium gradient created by Na+, K+-ATPase on the lateral membrane. Active transport is upregulated during low calcium intake and downregulated during high calcium intake, when paracellular calcium transport through the tight junctions of the intestine becomes the dominant process. Solvent drag-induced paracellular calcium transport accounts for -70% of the total active duodenal calcium transport, suggesting the physiological significance of solvent drag-induced calcium transport in the duodenum. Based on this data, it is obvious that active paracellular calcium transport in the small intestine plays an indispensable role in calcium homeostasis. |