The transport of dextrans (approximately 4 to approximately 150 kDa) across an in vitro model of the alveolar epithelial barrier was studied to determine the effects of molecular size on pulmonary absorption of macromolecular drugs. Fluorescein isothiocyanate (FITC)-labeled dextrans (FDs) with average molecular weights (all in kDa) of 3.86 (FD4), 9 (FD10), 19.8 (FD20), 40.5 (FD40), 71.6 (FD70), and 156.9 (FD150) were utilized as model macromolecular drugs. Unidirectional fluxes of FDs at 37 and 4 degrees C were measured from the appearance rates of FD in the receiver fluid of open-circuited monolayers (>2000 omega-cm2) of rat alveolar epithelial cells. Apparent permeability coefficients (P(app)) were estimated from the observed flux and the corresponding concentration gradient of FD. Results showed that FD fluxes were the same in both apical-to-basolateral (AB) and opposite (BA) directions at each molecular weight studied. The P(app) was not significantly different at 0.5 and 1.0 mg/mL FD40 donor concentrations. The FD P(app) (x 10(-8)cm/s) decreased gradually from 1.35 for FD4 to 0.32 for FD40, indicating an apparent inverse relationship between P(app) and molecular weight of FD. By contrast, P(app) was about the same at 0.13 for both FD70 and FD150. When experimental temperature was lowered to 4 degrees C, P(app) decreased by approximately 40% for FDs of 4 through 40 kDa, whereas the decrease in P(app) was by approximately 80% for larger FDs of both 70 and 150 kDa. Moreover, these FDs were found to be relatively intact (approximately 90%) in either receiver fluid after 5-h flux experiments without detectable levels of metabolites in the respective donor fluid, suggesting that alveolar epithelial cells allow translocation of FDs intact across the barrier. Equivalent pore analysis, assuming restricted diffusion of FDs of 4-40 kDa via cylindrical, water-filled pores across the cell monolayer revealed a population of large equivalent pores with approximately 5.6 nm radius. These data suggest that smaller macromolecules (radius <5 nm) traverse the alveolar epithelial barrier via paracellular pathways, and that larger (i.e., radius > or = 6 nm) macromolecules likely cross the barrier via other pathways (e.g., pinocytosis).