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 (> 2000omegacm2) grown on polycarbonate filters, was studied. Model proteins were 14C-cytochrome c, 14C-ovalbumin, granulocyte-colony stimulating factor (G-CSF), 14C-bovine serum albumin (BSA), 125I-transferrin, and 14C-immunoglobulin G. Cytochrome c was extensively metabolized, as indicated by < 10% of the dose being translocated in intact form. This contrasts with 20-80% for the other model proteins studied. The flux of cytochrome c and G-CSF was symmetric in the apical-to-basolateral (ab) and basolateral-to-apical (ba) directions. By contrast, the flux of intact ovalbumin, BSA, transferrin and immunoglobulin G showed asymmetry, with the ab flux being higher by 2-5 times. There was no relationship between ab or ba fluxes and the molecular weights of these four model proteins. Since some of the proteins were translocated at much greater rates than are consistent with restricted diffusion or pinocytosis, receptor-mediated or adsorptive transcytosis may be involved.