Acute lung injury leading to acute respiratory distress (ARDS) is a global health concern. ARDS patients have significant pulmonary inflammation leading to flooding of the pulmonary alveoli. This prevents normal gas exchange with consequent hypoxemia and causes mortality. A thin fluid layer in the alveoli is normal. The maintenance of this thin layer results from fluid movement out of the pulmonary capillaries into the alveolar interstitium driven by vascular hydrostatic pressure and then through alveolar tight junctions. This is then balanced by fluid reabsorption from the alveolar space mediated by transepithelial salt and water transport through alveolar cells. Reabsorption is a two-step process: first, sodium enters via sodium-permeable channels in the apical membranes of alveolar type 1 and 2 cells followed by active extrusion of sodium into the interstitium by the basolateral Na+, K+-ATPase. Anions follow the cationic charge gradient and water follows the salt-induced osmotic gradient. The proximate cause of alveolar flooding is the result of a failure to reabsorb sufficient salt and water or a failure of the tight junctions to prevent excessive movement of fluid from the interstitium to alveolar lumen. Cytokine- and chemokine-induced inflammation can have a particularly profound effect on lung sodium transport since they can alter both ion channel and barrier function. Cytokines and chemokines affect alveolar amiloride-sensitive epithelial sodium channels (ENaCs), which play a crucial role in sodium transport and fluid reabsorption in the lung. This review discusses the regulation of ENaC via local and systemic cytokines during inflammatory disease and the effect on lung fluid balance.

A thin fluid layer in alveoli is normal and results from a balance of fluid entry and fluid uptake by transepithelial salt and water reabsorption. Conventional wisdom suggests the reabsorption is via epithelial Na channels (ENaC), but if all Na reabsorption were via ENaC, then amiloride, an ENaC inhibitor, should block alveolar fluid clearance (AFC). However, amiloride blocks only half of AFC. The reason for failure to block is clear from single-channel measurements from alveolar epithelial cells: ENaC channels are observed, but another channel is present at the same frequency that is nonselective for Na over K, has a larger conductance, and has shorter open and closed times. These two channel types are known as highly selective channels (HSC) and nonselective cation channels (NSC). HSC channels are made up of three ENaC subunits since knocking down any of the subunits reduces HSC number. NSC channels contain α-ENaC since knocking down α-ENaC reduces the number of NSC (knocking down β- or γ-ENaC has no effect on NSC, but the molecular composition of NSC channels remains unclear). We show that NSC channels consist of at least one α-ENaC and one or more acid-sensing ion channel 1a (ASIC1a) proteins. Knocking down either α-ENaC or ASIC1a reduces both NSC and HSC number, and no NSC channels are observable in single-channel patches on lung slices from ASIC1a knockout mice. AFC is reduced in knockout mice, and wet wt-to-dry wt ratio is increased, but the percentage increase in wet wt-to-dry wt ratio is larger than expected based on the reduction in AFC.

Background and Purpose We have shown that cholesterol is synthesized in the principal cells of renal cortical collecting ducts (CCD) and stimulates the epithelial sodium channels (ENaC). Here we have determined whether lovastatin, a cholesterol synthesis inhibitor, can antagonize the hypertension induced by activated ENaC, following deletion of the cholesterol transporter (ATP‐binding cassette transporter A1; ABCA1). Experimental Approach We selectively deleted ABCA1 in the principal cells of mouse CCD and used the cell‐attached patch‐clamp technique to record ENaC activity. Western blot and immunofluorescence staining were used to evaluate protein expression levels. Systolic BP was measured with the tail‐cuff method. Key Results Specific deletion of ABCA1 elevated BP and ENaC single‐channel activity in the principal cells of CCD in mice. These effects were antagonized by lovastatin. ABCA1 deletion elevated intracellular cholesterol levels, which was accompanied by elevated ROS, increased expression of serum/glucocorticoid regulated kinase 1 (Sgk1), phosphorylated neural precursor cell‐expressed developmentally down‐regulated protein 4‐2 (Nedd4‐2) and furin, along with shorten the primary cilium, and reduced ATP levels in urine. Conclusions and Implications These data suggest that specific deletion of ABCA1 in principal cells increases BP by stimulating ENaC channels via a cholesterol‐dependent pathway which induces several secondary responses associated with oxidative stress, activated Sgk1/Nedd4‐2, increased furin expression, and reduced cilium‐mediated release of ATP. As ABCA1 can be blocked by cyclosporine A, these results suggest further investigation of the possible use of statins to treat CsA‐induced hypertension.

It has been suggested that voltage-dependent anion channels (VDACs) control the release of superoxide from mitochondria. We have previously shown that reactive oxygen species (ROS) such as superoxide (O) and hydrogen peroxide (HO) stimulate epithelial sodium channels (ENaCs) in sodium-transporting epithelial tissue, including cortical collecting duct (CCD) principal cells. Therefore, we hypothesized that VDACs could regulate ENaC by modulating cytosolic ROS levels. Herein, we find that VDAC3-knockout(KO) mice can maintain normal salt and water balance on low-salt and high-salt diets. However, on a high-salt diet for 2 weeks, VDAC3-KO mice had significantly higher systolic blood pressure than wildtype mice. Consistent with this observation, after a high-salt diet for 2 weeks, ENaC activity in VDAC3-KO mice was significantly higher than wildtype mice. EM analysis disclosed a significant morphological change of mitochondria in the CCD cells of VDAC3-KO mice compared with wildtype mice, which may have been caused by mitochondrial superoxide overload. Of note, compared with wildtype animals, ROS levels in VDAC3-KO animals fed a normal or high-salt diet were consistently and significantly increased in renal tubules. Both the ROS scavenger 1-oxyl-2,2,6,6-tetramethyl-4-hydroxypiperidine (TEMPOL) and the mitochondrial ROS scavenger (2-(2,2,6,6-tetramethylpiperidin-1-oxyl-4-ylamino)-2-oxoethyl)triphenylphosphonium chloride (mito-TEMPO) could reverse the effect of high-salt on ENaC activity and systolic blood pressure in the VDAC3-KO mice. Mito-TEMPO partially correct the morphological changes in VDAC3-KO mice. Our results suggest that knocking out mitochondrial VDAC3 increases ROS, alters renal sodium transport, and leads to hypertension.

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