Paths of hydrophilic solute flow across excised bullfrog lung.

The rate of permeation of radiolabeled glycerol, Cl-, urea, Na+, choline, mannitol, sucrose, raffinose, cyancobalamin, and inulin across the alveolar epithelium of the excised short-circuited bullfrog lung was measured. In addition, the flows of cyancobalamin, angiotensin I, and human calcitonin were estimated by radioimmunoassay. In general, the flow of most solutes was compatible with passive permeation through aqueous channels but estimates of equivalent pore radius were hampered by contamination of tritiated and C-labeled solutes by small radioactive breakdown products. The more rapid permeation of fragments spuriously raised the apparent permeability coefficient of the larger parent compound and, usually, the estimate of the pore radius, Other limitations of the evaluation of equivalent pore radius from relative rates of probe molecule flow across excised epithelia are discussed. The most reliable data can be accounted for by a single population of pores of 1.1 nm radius, a value that lies within the range suggested for the adult mammalian lung.