Epithelial tissue cytosolic transport

In situ models are to evaluate absorption or membrane permeability under the physiologically relevant tissue condition. While the luminal environment can be modulated by the administered solution, the tissue condition is physiologically controlled. The estimated membrane permeability can be, in most cases, assumed to represent the transport across the epithelial cell layer at steady state or quasisteady state. However, one needs to be aware that the involvement of metabolic degradation, which may occur at the cellular surface or within the cytosol, can be a factor leading to biased estimates of membrane permeability and erroneous interpretation of the transport process. Particularly,... 

Induction of Calbindin-D In response to calcitriol administration, there is an increase in mRNA synthesis and then in the synthesis of calbindin-D in intestinal mucosal cells, which is correlated with the later and more sustained increase in calcium absorption. In vitamin D-deficient animals, there is no detectable calbindin in the intestinal mucosa, whereas in animals adequately provided with vitamin D, it may account for 1 % to 3% of soluble protein in the cytosol of the colunmar epithelial ceils. Although the rapid response to calcitriol is an increase in the permeability of the brush border membrane to calcium, the induction of calbindin permits intracellular accumulation and transport of calcium. The rapid increase in net calcium transport in tissue from vitamin D-replete animals is presumably dependent on the calbindin that is already present in deficient animals, there can be no increase in calcium transport until sufficient calbindin has accumulated to permit intracellular accumulation, despite the increased permeability of the brush border. 

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