Transport of water and solutes across bullfrog alveolar epithelium

Water and solute transport properties of the alveolar epithelium of isolated bullfrog lungs were studied. Lungs from Rana catesbeiana were removed and mounted in an Ussing chamber. Unstirred layers on both sides of the tissue were estimated from the time courses of dilution potential development, and the measured transport parameters were corrected for the effect of the unstirred layers. Spontaneous potential difference, short-circuit current, tissue resistance, instantaneous voltage-current relationships, diffusional permeabilities of water and hydrophilic solutes, and hydraulic conductivities were determined. The hydraulic conductivity obtained from hydrostatically driven water flow anomalously decreased with time, and was initially 100 -1,000 times higher than osmotically determined hydraulic conductivity. The equivalent pore radius of the bullfrog alveolar epithelium was estimated to be 0.8-0.9 nm. We conclude that the alveolar epithelium is extremely tight, presenting a major barrier to water and solute flow. This high resistance to water and solute flow may be helpful in maintaining the alveolar lumen relatively free of fluid under normal physiological conditions.