The Effect of Vitamin D3 on Lanthanum Absorption: Suggestive Evidence for a Shunt Path
Deciphering the path that calcium takes during the absorptive process will be of significant importance in understanding the calcium absorption mechanism and vitamin D action. The usual model (cf. Calc. Tissue Research 2, 301, 1968) depicts Ca entering the epithelial cell across the brush border, moving through the cytoplasma as the free ion or in association with a complexer, and then being extruded from the cell across the basal membrane by a Ca pump or by a Na-Ca exchange mechanism. Energy is required either directly via ATP input or indirectly via the maintenance of a sodium-gradient. This intracellular path model undoubtedly pertains to that calcium which is actively transported. However, there is sufficient evidence suggesting that Ca is also absorbed by a diffusional process, a process that is vitamin D-dependent, as is the active transport mechanism (cf. WASSERMAN, 1974). The question arises as to whether those calcium ions that are diffusionally transmitted use the same route as those calcium ions being actively transported. Certain kinetic data, such as the enhanced bidirectional calcium permeability of the intestine by vitamin D (WASSERMAN and KALLFELZ, 1962; MARTIN and DELUCA, 1969) and thermodynamic calculations (WASSERMAN and TAYLOR, 1969; WASSERMAN, 1974) suggest a possible non-cytoplasmic (or paracellular) path for calcium translocation.
KeywordsPermeability Gall Lanthanum
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