QUANTITATIVE APPROACHES TO DELINEATE PARACELLULAR DIFFUSION IN CULTURED EPITHELIAL-CELL MONOLAYERS

When using cultured cell monolayers to determine the mechanism of tran scellular diffusion of molecules, it may be important to identify the fraction that moves through the paracellular route or passively diffus es through tight junctions. We characterized the apparent diameter of the junctional pore in a variety of epithelial cell monolayers (Caco-2 , MDCK, alveolar). Using hydrophilic extracellular permeants varying i n molecular radii and charge (neutral, anionic, cationic, zwitterionic ), rate-determining steps and factors of the paracellular route were q uantitatively delineated by the model for molecular size-restricted di ffusion within a negative electrostatic field of force. Protonated ami nes permeated the pores faster than their neutral images while organic anions were slower. With increasing molecular size the influence of c harge diminished. This approach was used to quantify the relationship between permeant radius and transepithelial electrical resistance and to analyze changes in junctional pore size as a function of pharmacolo gical perturbation, such as in the use of absorption promoters or adju vants.