The structural barrier of absorptive mucosae: site difference of the permeability of fluorescein isothiocyanate-labelled dextran in rabbits.

The permeability of fluorescein isothiocyanate-labelled dextran (FD, M.W. 4400-71,200) across nasal, buccal, duodenal, jejunal, ileal, colonic, and rectal mucosae excised from rabbits has been measured to estimate the structural barrier of absorptive mucosae using Ussing-type diffusion chambers. The permeability coefficient of FD in all these mucosae decreased with increasing molecular weight. The rank order of FD permeability did not always correlate with the electrical resistance of the mucosae. Among components of the small intestine (duodenum, jejunum, ileum) and the large intestine (colon, rectum), however, the rank order of FD permeability corresponded to the magnitude of the electrical resistance in each instance; the upper colonic mucosa showed the highest permeability, especially in FD of low molecular weight, and permeability of the rectal mucosa was lowest except for the duodenal mucosa. The nasal mucosa showed the lowest electrical resistance and the highest permeability of those studied, suggesting that it has a leaky structural barrier.