ABSTRACT
Calcium homeostasis in mammalian species is acutely regulated by the concentration of soluble calcium ions, which reach both intracellular and extracellular body compartments following absorption from the gastrointestinal tract. The intestine represents the principal organ where calcium ions enter the body compartmental pools by two primary intestinal transport mechanisms present in the intestinal lumen. In general, calcium transport at this level regulates calcium bioavailability from different food matrices, as the intestinal mucosal layer is the only biological barrier for calcium to cross in order to reach the portal and, subsequently, systemic circulations. Intestinal calcium transport involves translocation of soluble calcium ions from the intestinal lumen to the lateral space occupied by the lamina propria, where two distinct pathways, namely, cellular and paracelluar mechanisms, exist. Calcium ions are transported via the cytosolic compartment in the cellular pathway, facilitated by vitamin D, and are moved by the tight junctions between adjacent cells by passive absorption in the paracellular pathway. The rate of calcium transport in the cellular mechanism is saturable and attains a constant value at a certain intralumenal calcium concentration. Alternatively, the rate of calcium transport in the nonsaturable, paracellular pathway increases as a linear function of soluble concentration of the ion. Prominent cofactors that may enhance paracellular calcium transport include lactose, some amino acids, and some bioactive peptides, such as caseinphosphopeptides (CPP) from milk proteins.
