Alveolar epithelium measurement

The diffusing capacity of the lung for carbon monoxide (CO) is a measure of the ability of the alveolar capillary membrane to transfer or conduct gases from the alveoli to the blood. This transport process is entirely a passive one brought about by diffusion. As described previously in Section 2.2, the barriers for diffusion consist of surfactant, alveolar epithelium, interstitital fluid, capillary endothelium, plasma, and the red blood cell membrane. 

We used in vitro models of lung epithelial cell lines or primary cells to determine E25 permeability. Two different cell types were used to mimic the airway and alveolar epithelium of the lung to study transport. Calu-3, a human cell line derived from an airway carcinoma, when grown at an air/liquid interface, differentiate to form a secretory airway epithelium (17). Rat primary epithelial cells isolated as described by Cheek et al. (18) form a tight barrier similar in structure and function to the alveolar surface. Both cell types when grown to confluence form tight junctions and differentiate and polarize so that the apical or air surface has different characteristics than the basolateral or blood side. The typical transepithelial resistance observed was 350 or >1000 ohms-cm for Calu-3 cells or primary rat alveolar cells, respectively. Once an acceptable resistance was achieved, E25 (2 mg/mL) was placed in either the apical or the basolateral chamber. Cell monolayers were incubated at 37°C for up to 3 hours and ELISA measured the amount of E25 that translocated the epithelial layer and appeared in the receiver well. The apparent permeability (Papp) of the epithelium for E25 was calculated as ... 

Alternatively the membrane passage of human airway epithehal ceh lines can be studied in vitro. A number of bronchial epithehal ceh hnes is available, such as the 16HBE14o- and Calu-3 cell hnes. These ceh hnes can be installed in diffusion chambers to measure transport rates [34]. A major disadvantage of the currently used cell hnes is that they provide information about bronchial epithehal transport only. Since bronchial epithelium is very different from alveolar epithehum, the information from these in vitro studies is of limited value for the prediction of the bioavahabihty of pulmonary administered proteins. 

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