Alveolar epithelium permeability to small solutes: developmental changes

To determine whether alveolar epithelium permeability to small lipid-insoluble solutes changes during development we measured transport across the blood-gas barrier in isolated Ringer-perfused lungs from prenatal, 1-day-old, 4-wk-old, and adult rabbits. Radioactive test molecules, one of which was always sucrose, were dissolved in Ringer solution and instilled into the trachea of degassed lungs. Samples taken from recirculating perfusate were used to calculate permeability-surface area (PS) products. Results were expressed as the ratio (PS)/(PS)sucrose, and as absolute permeability. Lungs from 4-wk-old rabbits were studied most thoroughly; the (PS)/(PS) sucrose ratios obtained are urea 4.0, erythritol 1.3, mannitol 0.98, L-glucose 1.4, and D-glucose 5.6. These and other data imply that the most lipid-insoluble molecules (erythritol, mannitol, L-glucose, and sucrose) are transported by a nonselective bulk process. Urea transport is primarily through lipid membranes; D-glucose seems to involve a special pro...