Formation of hydrostatic lung edema in acute left heart failure is traditionally attributed to an imbalance of Starling forces resulting in interstitial and finally, alveolar edema. Recently, mechanosensitive cell responses have been implicated in this scenario. In isolated perfused rat and mouse lungs, we have identified lung endothelial and epithelial responses relevant to its pathogenesis. Hydrostatic stress causes influx of Ca2+ into endothelial cells via mechanosensitive TRPV channels. The endothelial Ca2+ influx increases lung microvascular filtration coefficient via a myosin light chain kinase-dependent mechanism. Simultanesouly, Ca2+ influx stimulates endothelial formation of NO which serves as an intercompartmental signal to adjacent epithelial cells. Endothelial-derived NO impairs alveolar fluid clearance, a protective mechanism driven primarily by epithelial ion transport via amiloride-sensitive Na+ channels and the Na+ ,K+ -ATPase. NO also stimulates alveolar fluid secretion which can be blocked by ouabain or the CFTR-inhibitors glibenclamide or CFTRinh-172, suggesting that the alveolar epithelium actively secretes Cl- via CFTR in hydrostatic stress. These data show that endothelial and epithelial responses contribute importantly to the pathogenesis of hydrostatic lung edema.