| Abstract: | Alveolar fluid clearance driven by active epithelial Na+ and secondary Cl− absorption counteracts edema formation in the intact lung. Recently, we showed that impairment of alveolar fluid clearance because of inhibition of epithelial Na+ channels (ENaCs) promotes cardiogenic lung edema. Concomitantly, we observed a reversal of alveolar fluid clearance, suggesting that reversed transepithelial ion transport may promote lung edema by driving active alveolar fluid secretion. We, therefore, hypothesized that alveolar ion and fluid secretion may constitute a pathomechanism in lung edema and aimed to identify underlying molecular pathways. In isolated perfused lungs, alveolar fluid clearance and secretion were determined by a double-indicator dilution technique. Transepithelial Cl− secretion and alveolar Cl− influx were quantified by radionuclide tracing and alveolar Cl− imaging, respectively. Elevated hydrostatic pressure induced ouabain-sensitive alveolar fluid secretion that coincided with transepithelial Cl− secretion and alveolar Cl− influx. Inhibition of either cystic fibrosis transmembrane conductance regulator (CFTR) or Na+-K+-Cl− cotransporters (NKCC) blocked alveolar fluid secretion, and lungs of CFTR−/− mice were protected from hydrostatic edema. Inhibition of ENaC by amiloride reproduced alveolar fluid and Cl− secretion that were again CFTR-, NKCC-, and Na+-K+-ATPase–dependent. Our findings show a reversal of transepithelial Cl− and fluid flux from absorptive to secretory mode at hydrostatic stress. Alveolar Cl− and fluid secretion are triggered by ENaC inhibition and mediated by NKCC and CFTR. Our results characterize an innovative mechanism of cardiogenic edema formation and identify NKCC1 as a unique therapeutic target in cardiogenic lung edema.Traditionally, the formation of cardiogenic pulmonary edema has been attributed to passive fluid filtration across an intact alveolocapillary barrier along an increased hydrostatic pressure gradient. However, recent studies show that cardiogenic edema is critically regulated by active signaling processes. Activation of mechanosensitive endothelial ion channels increases lung vascular permeability (1), whereas alveolar epithelial cells lose their physiological ability to clear the distal airspaces from excess fluid by their capacity to actively transport ions across the epithelial barrier (2–4).In the intact lung, the predominant force driving alveolar fluid clearance is an active transepithelial Na+ transport from the alveolar into the interstitial space. A major portion of the apical Na+ entry is mediated by the amiloride-inhibitable epithelial Na+ channel (ENaC), with basolateral Na+ extrusion through the Na+-K+-ATPase (5). Cl− and water are considered to follow paracellularly for electroneutrality and osmotic balance. In cardiogenic lung edema, the physiological protection against alveolar flooding provided by an intact alveolar fluid clearance is largely attenuated (3, 4). Previously, we have outlined the signaling events at the alveolocapillary barrier that underlie this inhibition of alveolar fluid clearance by showing that hydrostatic stress increases endothelial NO production in lung capillaries (6), which in turn, blocks alveolar Na+ and liquid absorption by a cGMP-dependent inhibition of epithelial ENaC (2).Unexpectedly, however, we observed that increased hydrostatic pressure not only blocks alveolar fluid clearance but reverses transepithelial fluid transport, resulting in effective alveolar fluid secretion that accounts for up to 70% of the total alveolar fluid influx at elevated hydrostatic pressure (2). This effect is not explicable by impaired alveolar fluid clearance and/or passive fluid leakage, and thus, it points to a previously unrecognized and potentially therapeutically exploitable pathomechanism in cardiogenic lung edema, namely alveolar fluid secretion driven by active transepithelial ion transport.Here, we aimed to analyze alveolar fluid secretion and its underlying cellular mechanisms in cardiogenic lung edema. We considered the Cl− channel cystic fibrosis transmembrane conductance regulator (CFTR) as a putative key ion channel in this scenario, because it permits bidirectional permeation of anions under physiologically relevant conditions (7). Hence, the direction of Cl− flux by CFTR may reverse depending on actual electrochemical gradients, thus turning an absorptive into a secretory epithelium or vice versa. This notion is supported by reports describing CFTR as both an absorptive and secretory channel in the regulation of alveolar fluid homeostasis (8, 9). By a combination of indicator dilution, imaging, and radioactive tracer techniques for the measurement of alveolar ion and fluid fluxes in the isolated lung, we show a critical role for CFTR-mediated Cl− secretion in cardiogenic lung edema and identify the Na+-K+-2Cl− cotransporter 1 (NKCC1) as a therapeutic target in this pathology. |
