Hypoxia has been shown to cause lung edema and impair lung edema clearance. In the present study, we exposed isolated rat lungs to pO 2 ‫؍‬ 40 mm Hg for 60 min or rats to 8% O 2 for up to 24 h and then measured changes in alveolar fluid reabsorption (AFR) and Na,K-ATPase function. Low levels of oxygen severely impaired AFR in both ex vivo and in vivo models. The decrease in AFR was associated with a decrease in Na,K-ATPase activity and protein abundance in the basolateral membranes from peripheral lung tissue of hypoxic rats. ␤-Adrenergic agonists restored AFR in rats exposed to 8% O 2 (from 0.02 ؎ 0.07 ml/h to 0.59 ؎ 0.03 ml/h), which was associated with parallel increases in Na,K-ATPase protein abundance in the basolateral membrane. Hypoxia is associated with increased production of reactive oxygen species. Therefore, we examined whether overexpression of SOD2, manganese superoxide dismutase, would prevent the hypoxia-mediated decrease in AFR. Spontaneously breathing rats were infected with a replication-deficient human type 5 adenovirus containing cDNA for SOD2. An otherwise identical virus that contained no cDNA was used as a control (Adnull). Hypoxic Adnull rats had decreased rates of AFR (0.12 ؎ 0.1 ml/h) as compared with hypoxic AdSOD2 and normoxic control rats (0.47 ؎ 0.04 ml/h and 0.49 ؎ 0.02 ml/h, respectively), with parallel changes in Na,K-ATPase.Severe hypoxia can occur during ascent to high altitude (1) and in patients with acute respiratory distress syndrome and pulmonary edema. One of the primary defense mechanisms in the lung against alveolar fluid accumulation is the active transport of sodium out of the air spaces, which generates a transepithelial osmotic gradient that leads to alveolar fluid reabsorption (AFR).2 Sodium enters the apical membrane of alveolar epithelial cells through amiloride-sensitive Na ϩ channels (2, 3) and is then transported out across the basolateral membrane by the ouabain-inhibitable Na,K-ATPase (4 -7). Hypoxia has been shown to impair AFR and may contribute to alveolar fluid accumulation (8, 9). However, the mechanisms by which hypoxia impairs AFR and alveolar epithelial sodium transport proteins has not been fully elucidated.A mechanism by which hypoxia might impair AFR is by altering the function of either apical epithelial sodium channels and/or basolateral Na,K-ATPase proteins. Several in vitro studies using cultured alveolar epithelial cells have demonstrated that exposure to hypoxia results in the decrease in epithelial sodium channels and Na,K-ATPase protein abundance (10 -12), which was reversed upon reoxygenation. Other investigators have reported various mechanisms associated with the decrease in alveolar fluid reabsorption in animals exposed to hypoxia in vivo (9,11,13).In the current study, we provide evidence that exposure to hypoxia results in decreased Na,K-ATPase activity and protein abundance at the plasma membrane, which contributes to a decrease in alveolar fluid reabsorption in both in vivo and ex vivo models of hypoxia. These data suggest that (a...

Rationale: Endothelin-1 (ET-1) is increased in patients with highaltitude pulmonary edema and acute respiratory distress syndrome, and these patients have decreased alveolar fluid reabsorption (AFR). Objectives: To determine whether ET-1 impairs AFR via activation of endothelial cells and nitric oxide (NO) generation. Methods: Isolated perfused rat lung, transgenic rats deficient in ET B receptors, coincubation of lung human microvascular endothelial cells (HMVEC-L) with rat alveolar epithelial type II cells or A549 cells, ouabain-sensitive 86 Rb 1 uptake. Measurements and Main Results: The ET-1-induced decrease in AFR was prevented by blocking the endothelin receptor ET B , but not ET A . Endothelial-epithelial cell interaction is required, as direct exposure of alveolar epithelial cells (AECs) to ET-1 did not affect Na,K-ATPase function or protein abundance at the plasma membrane, whereas coincubation of HMVEC-L and AECs with ET-1 decreased Na,K-ATPase activity and protein abundance at the plasma membrane. Exposing transgenic rats deficient in ET B receptors in the pulmonary vasculature (ET-B 2/2 ) to ET-1 did not decrease AFR or Na,K-ATPase protein abundance at the plasma membrane of AECs. Exposing HMVEC-L to ET-1 led to increased NO, and the ET-1-induced down-regulation of Na,K-ATPase was prevented by the NO synthase inhibitor L-NAME, but not by a guanylate cyclase inhibitor. Conclusions: We provide the first evidence that ET-1, via an endothelial-epithelial interaction, leads to decreased AFR by a mechanism involving activation of endothelial ET B receptors and NO generation leading to alveolar epithelial Na,K-ATPase down-regulation in a cGMP-independent manner.

Hypoxia can cause stress and structural changes to the epithelial cytoskeleton. The intermediate filament (IF) network is known to reorganize in response to stress. We examined whether rats exposed to hypoxia had altered keratin IF expression in their alveolar epithelial type II (ATII) cells. There was a significant decrease in keratin protein levels in hypoxic ATII cells compared with those in ATII cells isolated from normoxic rats. To define the mechanisms regulating this process we studied changes to the keratin IF network in A549 cells (an alveolar epithelial cell line) exposed to 1.5% oxygen. We observed a time-dependent disassembly-degradation of keratin 8 and 18 proteins, which was associated with an increase in reactive oxygen species (ROS). Hypoxia-treated A549 cells deficient in mitochondrial DNA or A549 cells treated with a small interfering RNA against the Rieske iron-sulfur protein of mitochondrial complex III did not have increased levels of ROS nor was the keratin IF network disassembled and degraded. The superoxide dismutase (SOD)/catalase mimetic (EUK-134) prevented the hypoxia-mediated keratin IF degradation as did the overexpression of SOD1 but not of SOD2. Accordingly, we provide evidence that hypoxia promotes the disassembly and degradation of the keratin IF network via mitochondrial complex III-generated reactive oxygen species.-Na, N., Chandel, N. S., Litvan, J., Ridge, K. M. Mitochondrial reactive oxygen species are required for hypoxia-induced degradation of keratin intermediate filaments.

Hypoxia can cause stress and structural changes to the epithelial cytoskeleton. The intermediate filament network is known to reorganization in response to stress. We examined whether rats exposed to hypoxia (8% O2; 24 h) had altered keratin IF expression in their alveolar epithelial cells (AEC). There was a significant decrease in keratin protein abundance in hypoxic AEC as compared to AEC isolated from normoxic, control rats. To define the potential mechanisms regulating this process we studied changes to the keratin IF network in human A549 cells exposed to 1.5% oxygen; 24 h. We observed a time dependent disassembly/degradation of keratin 8 and 18 proteins which was associated with an increase in reactive oxygen species (ROS). Hypoxia‐treated A549 cells deficient in mitochondrial DNA or A549 cells treated with a small interfering RNA against the Rieske iron‐sulfur protein of mitochondrial complex III did not have increased levels of ROS, nor was the keratin IF network disassembled/degraded. The catalase/superoxide dismutase (SOD) mimetic (EUK‐134) prevented the hypoxia‐mediated keratin IF degradation as did the overexpression of SOD1, but not SOD2. Accordingly, we provide evidence that hypoxia promotes the disassembly/degradation of the keratin IF network via mitochondrial complex III‐generated reactive oxygen species.

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