Rates of protein transport across rat alveolar epithelial cell monolayers

The transport of model proteins, ranging from 12,300 to 150,000 Da, across tight rat alveolar epithelial cell monolayers (>2000 Omega cm(2)) grown on polycarbonate filters, was studied. Model proteins were C-14-cytochrome c, C-14-ovalbumin, granulocyte-colony stimulating factor(G-CSF), C-14-bovine s erum albumin (BSA), I-125-transferrin, and C-14-immunoglobulin G. Cytochrom e c was extensively metabolized, as indicated by < 10% of the dose being tr anslocated in intact form. This contrasts with 20-80% for the other model p roteins studied. The flux of cytochrome c and G-CSF was symmetric in the ap ical-to-basolateral (ab) and basolateral-to-apical (ba) directions. By cont rast, the flux of intact ovalbumin, BSA, transferrin and immunoglobulin G s howed asymmetry, with the nb flux being higher by 2-5 times. There was no r elationship between ab or ba fluxes and the molecular weights of these four model proteins. Since some of the proteins mere translocated at much great er rates than are consistent with restricted diffusion or pinocytosis, rece ptor-mediated or adsorptive transcytosis may be involved.