AQP4 and acute respiratory distress syndrome: However, there have also been reports suggesting that pulmonary inflammation and edema were associated with a marked reduction in the AQP1 mRNA expression level in the lung [18, 19] and the AQP1 mRNA expression level was negatively correlated with the level of the wet-to-dry weight ratio (wet/dry weight ratio) [20]. AQP4 mRNA expression is upregulated in alveolar type II (ATII) cells [21] and AQP4 is believed to regulate fluid exchange between the alveolar space and the alveolar epithelium barrier in ALI [22].