Osmotic permeabilities across corneal endothelium and antidiuretic hormone-stimulated toad urinary bladder structures

Osmotic permeabilities of several epithelial structures have been determined with novel optical procedures based on specular microscopy. The osmotic permeabilities of several tissue layers were determined by continuously monitoring the position of the apical tissue borders while an osmotic flow was imposed across those layers. The values found were (in micrometer/s; mean +/- SE): corneal epithelium, 137 +/- 30 (n = 5): antidiuretic hormone stimulated toad bladder, 429 +/- 64 (n = 6); and corneal endothelium, 711 +/- 34 (n = 7). In addition, the osmotically-induced transient change in thickness of the corneal endothelial cells was determined with the help of a computer, and the apparent osmotic permeability measured for the apical membrane was 1420 +/- 160 micrometer/s (n = 5). It is concluded that the osmotic permeability across the endothelial layer is sizably larger than had been previously detected and that osmotic flows across such layer largely traverse the cellular membranes. With osmotic permeability values (per unit of cell membrane area) as large as presently reported, isotonic fluid transport by epithelia can be explained simply on the basis of local osmotic gradients.