Passive Intercellular Pathway in Amphibian Epithelia

PHYSIOLOGIC studies of transporting epithelia generally indicate that passive shunts (or “leak” pathways for water and ions) exist in parallel with transport systems. Most notably, Ussing^1–3 defines this pathway as an extracellular channel in amphibian skin and has shown that a hypertonic external bath decreases the transepithelial electrical resistance, whereas a hypertonic internal bath has the opposite effect. Similar results have been obtained with toad urinary bladder⁴, but in virtually all of the epithelia studied by electron microscopy, tight junctions⁵ have been found at the luminal end of intercellular spaces. Apparent fusion of adjacent plasma membranes and the inability of electron-dense tracer molecules to pass through such regions^5–8 suggest that they may be tight seals, preventing extracellular transepithelial flow. It is shown that these junctions are reproducibly altered when electrical resistance is changed by hypertonic solutions.