AUTORADIOGRAPHIC DETERMINATION OF PERMEATION PATHWAY OF PERMEABILITY PROBES ACROSS INTESTINAL AND TRACHEAL EPITHELIA

Mucosal permeability studies are used to assess intestinal and respira tory barrier functions. Our ability to interpret results of permeabili ty studies are hampered by our lack of understanding of absorptive pat hways of permeability markers. The alm of this study was to visually t race the pathway of permeability probes across the small intestinal an d tracheal epithelia by using electron microscopic autoradiography and cytochemistry. We saw a constant rate of mucosal to serosal permeatio n of intestinal permeability probes polyethylene glycol 400 (PEG-4001 and mannitol across the everted small intestinal sac and of the pulmon ary permeability probe bovine serum albumin (BSA) across the tracheal epithelia. Electron microscopic tracing of the permeation pathways of tritiated PEG-400 and tritiated mannitol revealed that the majority of the probes traversed the intestinal epithelium paracellularly within 1 half distance (1650 angstrom) of the intercellular space. It is inte resting that we also found a small but significant transcellular trans port of permeability probes. Goblet cells also absorbed permeability p robes transcellularly, but in an ''all or none'' fashion. Similar path ways were identified in studies utilizing the commonly used pulmonary permeability probes iodine 125-labeled BSA and horseradish peroxidase to determine the routes of transfer in the airway epithelia. In the no rmal unperturbed trachea, these large permeability probes traversed th e pulmonary epithelic transcellularly via endocytosis. On barrier disr uption by cytochalasin D, the probes permeated predominantly via the p aracellular pathway. This study also demonstrates some of the similari ties and differences in transmucosal pathways shared by intestinal and pulmonary epithelia.