HIV-1 transgene expression impaired alveolar epithelial barrier function in vivo and increased permeability of alveolar epithelial monolayers in vitro. Panel A shows the relative lung liquid clearance, as reflected by lung tissue wet:dry ratios 30 min following intratracheal challenge with 2 cc of saline (see Methods for details), in wild type (WT) vs. HIV-1 transgenic (HIV-1 Tg) rats. The wet:dry ratios in HIV-1 Tg rats were increased ~2-fold higher above baseline than WT rats (* P < 0.05), reflecting a significantly decreased ability to clear saline from the lung. Note that the baseline wet:dry ratio of the rat lung is 4.5, and therefore the data are plotted to reflect the relative increases from this baseline in each group. Each value represents the mean ± SEM of 6 rats. Panel B shows the relative paracellular permeability of alveolar epithelial monolayers derived from WT vs. HIV-1 TG rats, as reflected by the flux of 3H-sucrose across each monolayer in 2 hrs (see Methods for details). Alveolar epithelial monolayers derived from HIV-1 Tg rats had an ~3-fold increase (* P < 0.05) in paracellular permeability when compared to alveolar epithelial monolayers derived from WT rats. Each value represents the mean ± SEM of alveolar epithelial monolayers from 6 rats.