Measurement of solute fluxes in isolated rat lungs.

Most previous studies in isolated perfused lungs have utilized measurements of solute flow from alveolar to vascular space to characterize the barrier and transport properties of the alveolar epithelium. In this study, we measured flux of a series of nonionic hydrophilic solutes and sodium across the alveolar epithelium of the isolated rat lung from perfusate to airspace (P-->A), as well as from airspace to perfusate (A-->P). Apparent permeability-surface area products (PS) were calculated from the rates of isotope appearance downstream in either the airspace or the perfusate. Equivalent pore analysis of data for P-->A solute flow demonstrated a small pore population with radius 0.6 nm occupying 85% of the total pore area and a large pore population with radius 3.8 nm occupying 15% of the total area. Similar analysis of A-->P solute flux demonstrated a small pore population of 0.6 nm occupying 86% of the total pore area and a large pore population with radius 2.9 nm occupying 14% of total pore area. The ratio (R) of PSP-->A divided by PSA-->P was 0.8 for the nonionic hydrophilic solutes, while R for sodium was 0.5. In the presence of amiloride and ouabain, R for sucrose was unchanged while R for sodium increased to 0.8 due to a fall in PSA-->P. The difference between R for sodium and R for the passively transported solutes, and the reduction in this difference in the presence of sodium transport inhibitors, are consistent with active sodium reabsorption by the intact alveolar epithelium. Differences in measured unidirectional passive solute fluxes probably result from unequal effective surface areas for diffusion from vascular space to airspace and vice versa in the anatomically complex mammalian lung.