Lung inflammation increases alveolar permeability, inhibits alveolar fluid reabsorption, and causes edema. The signaling pathways mediating inhibition of transport in alveolar epithelial cells are not clear.

We aimed to identify by which mechanisms inflammation- and hypoxia- activated alveolar macrophages, propagate signals to alveolar epithelium and affect ion transport of alveolar epithelial cells. Primary rat alveolar epithelial cells (ATII) were cultured on filters in absence and presence of alveolar macrophages (MA) and were exposed to hypoxia (1.5% O2) for 24h with LPS (1mg/ml). Cells were also treated with NO-donors and inhibitors of NOS, ERK1/2, MAPK and JunK to identify signaling pathways. To test, whether the presence of macrophages is required for transport inhibition, ATII cell monolayers were also exposed to conditioned media from LPS-stimulated macrophages. Transepithelial transport was measured in Ussing chambers. Nitrite in culture media was measured with the Griess reagent. LPS-stimulation of macrophages inhibited ATII cell ion transport. Hypoxia enhanced this effect. Inhibition of NOS and ERK1/2 prevented transport inhibition caused by LPS stimulation of macrophages when the inhibitors were added either to the macrophages or to ATII cells exposed to conditioned media from LPS treated macrophages. Inhibitors of NOS and ERK1/2 but not MAPK and JunK prevented the increase in nitrite in macrophages exposed to normoxia and hypoxia. Stimulation of alveolar macrophages inhibited alveolar epithelial ion transport by NOS and ERK1/2 dependent mechanisms which points to NO and cytokines as possible mediators. Hypoxia aggravates the inhibitory effect by not yet identified mechanisms.