Protein-containing lipids isolated from the bronchoalveolar lavage (BAL) of porcine lungs induce channel-mediated transport of cations across artificial lipid bilayers. We hypothesize that BAL-derived channels which insert into model membranes resemble apical membrane channels of polarized type II pneumocytes. To compare apical channels with those derived from BAL, apical membrane vesicles were prepared and fused with lipid bilayers.Methods: Type II pneumocytes were harvested from the same juvenile porcine lungs as those providing BAL. Plasma membrane vesicles(enriched 11-fold in apical membrane marker, alkaline phosphatase, and less than 2-fold in other markers) were prepared. Phosphatidylethanolamine: phosphatidylserine (3:1) planar lipid bilayers were painted between compartments of 250mM and 50mM Na+ gluconate (pH 7.0). Addition of membrane vesicles (150 μg protein) to the hypertonic compartment prompted fusion with the lipid bilayer. Single channel analyses were performed while applying holding potentials across bilayers and monitoring activity.Results: Apical membranes - like BAL - donated cation channels to lipid bilayers with: 1) gating between open and closed states; 2) decreased slope conductance (69 pS versus 165 pS for BAL); 3) reversal potential indicative of Na+ selectivity; and 4) voltage-dependent open state probability. Conclusions: 1) Single channel recordings of planar lipid bilayers provide a new approach to characterizing channel activities of type II pneumocytes. 2) Na+ channels at the apical membrane differ electrically from those isolated by BAL in the bilayer model. We speculate that differences in observed conductances relate to constitutional differences in channel sub-unit recombination within the bilayer. (Funded by Children's Health Foundation)