Inhibition of amiloride-sensitive sodium-channel activity in distal lung epithelial cells by nitric oxide

Effect of nitric oxide on epithelial ion transports in noncystic fibrosis and cystic fibrosis human proximal and distal airways

The airways of patients with cystic fibrosis (CF) exhibit decreased nitric oxide (NO) concentrations, which might affect airway function. The aim of this study was to determine the effects of NO on ion transport in human airway epithelia. Primary cultures of non-CF and CF bronchial and bronchiolar epithelial cells were exposed to the NO donor sodium nitroprusside (SNP), and bioelectric variables were measured in Ussing chambers. Amiloride was added to inhibit the Na+channel ENaC, and forskolin and ATP were added successively to stimulate cAMP- and Ca2+-dependent Cl−secretions, respectively. The involvement of cGMP was assessed by measuring the intracellular cGMP concentration in bronchial cells exposed to SNP and the ion transports in cultures exposed to 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one, an inhibitor of the soluble guanylate cyclase (ODQ), or to 8Z, a cocktail of 8-bromo-cGMP and zaprinast (phosphodiesterase 5 inhibitor). SNP decreased the baseline short-circuit current ( Isc) and the changes in Iscinduced by amiloride, forskolin, and ATP in non-CF bronchial and bronchiolar cultures. The mechanism of this inhibition was studied in bronchial cells. SNP increased the intracellular cGMP concentration ([cGMP]i). The inhibitory effect of SNP was abolished by 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, an NO scavenger (PTIO) and ODQ and was partly mimicked by increasing [cGMP]i. In CF cultures, SNP did not significantly modify ion transport; in CF bronchial cells, 8Z had no effect; however, SNP increased the [cGMP]i. In conclusion, exogenous NO may reduce transepithelial Na+absorption and Cl−secretion in human non-CF airway epithelia through a cGMP-dependent pathway. In CF airways, the NO/cGMP pathway appears to exert no effect on transepithelial ion transport.

Gasotransmitters: novel regulators of epithelial na(+) transport?

The vectorial transport of Na(+) across epithelia is crucial for the maintenance of Na(+) and water homeostasis in organs such as the kidneys, lung, or intestine. Dysregulated Na(+) transport processes are associated with various human diseases such as hypertension, the salt-wasting syndrome pseudohypoaldosteronism type 1, pulmonary edema, cystic fibrosis, or intestinal disorders, which indicate that a precise regulation of epithelial Na(+) transport is essential. Novel regulatory signaling molecules are gasotransmitters. There are currently three known gasotransmitters: nitric oxide (NO), carbon monoxide (CO), and hydrogen sulfide (H(2)S). These molecules are endogenously produced in mammalian cells by specific enzymes and have been shown to regulate various physiological processes. There is a growing body of evidence which indicates that gasotransmitters may also regulate Na(+) transport across epithelia. This review will summarize the available data concerning NO, CO, and H(2)S dependent regulation of epithelial Na(+) transport processes and will discuss whether or not these mediators can be considered as true physiological regulators of epithelial Na(+) transport biology.

Ion transport by pulmonary epithelia

The lung surface of air-breathing vertebrates is formed by a continuous epithelium that is covered by a fluid layer. In the airways, this epithelium is largely pseudostratified consisting of diverse cell types such as ciliated cells, goblet cells, and undifferentiated basal cells, whereas the alveolar epithelium consists of alveolar type I and alveolar type II cells. Regulation and maintenance of the volume and viscosity of the fluid layer covering the epithelium is one of the most important functions of the epithelial barrier that forms the outer surface area of the lungs. Therefore, the epithelial cells are equipped with a wide variety of ion transport proteins, among which Na⁺, Cl⁻, and K⁺ channels have been identified to play a role in the regulation of the fluid layer. Malfunctions of pulmonary epithelial ion transport processes and, thus, impairment of the liquid balance in our lungs is associated with severe diseases, such as cystic fibrosis and pulmonary oedema. Due to the important role of pulmonary epithelial ion transport processes for proper lung function, the present paper summarizes the recent findings about composition, function, and ion transport properties of the airway epithelium as well as of the alveolar epithelium.

Lipopolysaccharide modifies amiloride-sensitive Na+ transport processes across human airway cells: role of mitogen-activated protein kinases ERK 1/2 and 5

Bacterial lipopolysaccharides (LPS) are potent inducers of proinflammatory signaling pathways via the activation of nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinase (MAPK), causing changes in the processes that control lung fluid homeostasis and contributing to the pathogenesis of lung disease. In human H441 airway epithelial cells, incubation of cells with 15 µg ml⁻¹ LPS caused a significant reduction in amiloride-sensitive I_sc from 15 ± 2 to 8 ± 2 µA cm⁻² (p = 0.01, n = 13) and a shift in IC₅₀ amiloride of currents from 6.8 × 10⁻⁷ to 6.4 × 10⁻⁶ M. This effect was associated with a decrease in the activity of 5 pS, highly Na⁺ selective, amiloride-sensitive <1 µM channels (HSC) and an increase in the activity of ∼18 pS, nonselective, amiloride-sensitive >10 µM cation channels (NSC) in the apical membrane. LPS decreased αENaC mRNA and protein abundance, inferring that LPS inhibited αENaC gene expression. This correlated with the decrease in HSC activity, indicating that these channels, but not NSCs, were comprised of at least αENaC protein. LPS increased NF-κB DNA binding activity and phosphorylation of extracellular signal-related kinase (ERK)1/2, but decreased phosphorylation of ERK5 in H441 cells. Pretreatment of monolayers with PD98059 (20 µM) inhibited ERK1/2 phosphorylation, promoted phosphorylation of ERK5, increased αENaC protein abundance, and reversed the effect of LPS on I_sc and the shift in amiloride sensitivity. Inhibitors of NF-κB activation were without effect. Taken together, our data indicate that LPS acts via ERK signaling pathways to decrease αENaC transcription, reducing HSC/ENaC channel abundance, activity, and transepithelial Na⁺ transport in H441 airway epithelial cells.

Amiloride-insensitive Na+and fluid absorption in the mammalian distal lung

The ability of the distal lung epithelia to actively transport Na+, with Cl−and water following, from the alveolar spaces inversely correlates with morbidity and mortality of infants, children, and adults with alveolar pulmonary edema. It is now recognized, in contrast to many other Na+transporting epithelia, that at least half of this active transport is not sensitive to amiloride, which inhibits the epithelial Na+channel. This paper reviews amiloride-insensitive Na+and fluid transport in the mammalian distal lung unit under basal conditions and speculates on potential explanations for this amiloride-insensitive transport. It also provides new information, using primary cultures of rat fetal distal lung epithelia and alveolar type II cells grown under submersion and air-liquid interface culture conditions, regarding putative blockers of this transport.

Acute lung injury edema fluid decreases net fluid transport across human alveolar epithelial type II cells

Most patients with acute lung injury (ALI) have reduced alveolar fluid clearance that has been associated with higher mortality. Several mechanisms may contribute to the decrease in alveolar fluid clearance. In this study, we tested the hypothesis that pulmonary edema fluid from patients with ALI might reduce the expression of ion transport genes responsible for vectorial fluid transport in primary cultures of human alveolar epithelial type II cells. Following exposure to ALI pulmonary edema fluid, the gene copy number for the major sodium and chloride transport genes decreased. By Western blot analyses, protein levels of alphaENaC, alpha1Na,K-ATPase, and cystic fibrosis transmembrane conductance regulator decreased as well. In contrast, the gene copy number for several inflammatory cytokines increased markedly. Functional studies demonstrated that net vectorial fluid transport was reduced for human alveolar type II cells exposed to ALI pulmonary edema fluid compared with plasma (0.02 +/- 0.05 versus 1.31 +/- 0.56 microl/cm2/h, p < 0.02). An inhibitor of p38 MAPK phosphorylation (SB202190) partially reversed the effects of the edema fluid on net fluid transport as well as gene and protein expression of the main ion transporters. In summary, alveolar edema fluid from patients with ALI induced a significant reduction in sodium and chloride transport genes and proteins in human alveolar epithelial type II cells, effects that were associated with a decrease in net vectorial fluid transport across human alveolar type II cell monolayers.

Reduction in alveolar macrophages attenuates acute ventilator induced lung injury in rats

OBJECTIVE

Alveolar macrophages are the sentinel cell for activation of the inflammatory cascade when the lung is exposed to noxious stimuli. We investigated the role of macrophages in mechanical lung injury by comparing the effect of high-volume mechanical ventilation with or without prior depletion of macrophages.

DESIGN AND SETTING

Randomized sham-controlled animal study in anesthetized rats.

METHODS

Lung injury was induced by 15 min of mechanical ventilation (intermittent positive pressure ventilation) using high peak pressures and zero end-expiratory pressure. The mean tidal volume was 40+/-0.7 ml/kg. One group of animals was killed immediately after this period of volutrauma (HV), while in a second group normoventilation was continued for 2 h at a tidal volume less than 10 ml/kg (HV-LV). One-half of the animals were depleted of alveolar macrophages by pretreatment with intratracheal liposomal clodronate (CL2MDP).

MEASUREMENTS

Arterial blood gas, blood pressure. After kill: lung static pressure volume curves, bronchoalveolar fluid concentration for protein, macrophage inflammatory protein 2, tumor necrosis factor alpha, and wet/dry lung weight ratio (W/D).

RESULTS

During HV and HV+LV oxygenation, lung compliance, and alveolar stability were better preserved in animals pretreated with CL2MDP. In both groups W/D ratio was significantly greater in ventilated than in nonventilated animals (4.5+/-0.6), but the increase in W/D was significantly less in CL2MDP treated HV and HV-LV groups (6.1+/-0.4, 6.6+/-0.6) than in the similarly ventilated nontreated groups (8.7+/-0.2 and 9.2+/-0.5).

CONCLUSIONS

Alveolar macrophages participate in the early phase of ventilator-induced lung injury.

Functional similarities between pleura and the renal proximal tubule – membrane and cellular considerations

The small amount of liquid that, under physiological conditions, is presented in the pleural cavity has been the focus of extensive research for more than a century. However, there are still unanswered questions and considerable controversies about the nature of the forces governing its movement into and out of the pleural cavity. Early in the 20th century has been proposed that pleural fluid turnover is simple based on the balance between hydraulic and colloid osmotic pressures existing across the pleural membranes. This original hypothesis has not been validated by data accumulating over the last 20 years. Pleural tissues and renal proximal tubules present high water permeability, small transepithelial electrical resistance (22.02 Omega cm2) and the same cation transportation such as Na+ channels, Na+-K+ ATPase channels, and Na+-H+ exchanger. In contrast to previous conflicting theories concerning pleura fluid movement, the same functional characteristics suggest the hypothesis that physiology of pleura is similar to proximal tubules.

Downregulation of ENaC activity and expression by TNF-alpha in alveolar epithelial cells

Sodium absorption by an amiloride-sensitive channel is the main driving force of lung liquid clearance at birth and lung edema clearance in adulthood. In this study, we tested whether tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine involved in several lung pathologies, could modulate sodium absorption in cultured alveolar epithelial cells. We found that TNF-alpha decreased the expression of the alpha-, beta-, and gamma-subunits of epithelial sodium channel (ENaC) mRNA to 36, 43, and 16% of the controls after 24-h treatment and reduced to 50% the amount of alpha-ENaC protein in these cells. There was no impact, however, on alpha(1) and beta(1) Na(+)-K(+)-ATPase mRNA expression. Amiloride-sensitive current and ouabain-sensitive Rb(+) uptake were reduced, respectively, to 28 and 39% of the controls. A strong correlation was found at different TNF-alpha concentrations between the decrease of amiloride-sensitive current and alpha-ENaC mRNA expression. All these data show that TNF-alpha, a proinflammatory cytokine present during lung infection, has a profound influence on the capacity of alveolar epithelial cells to transport sodium.

Histone H1.2 is a substrate for denitrase, an activity that reduces nitrotyrosine immunoreactivity in proteins

Several reports have described an activity that modifies nitrotyrosine-containing proteins and their immunoreactivity to nitrotyrosine Abs. Without knowing the product of the reaction, this new activity has been called a "denitrase." In those studies, some nonspecific proteins, which have multiple tyrosine residues, e.g., albumin, were used as a substrate. Therefore, the studies were based on an unknown mechanism of reaction and potentially a high background. To solve these problems, one of the most important things is to find a more suitable substrate for assay of the enzyme. We developed an assay strategy for determining the substrate for denitrase combining 2D-gel electrophoresis and an on-blot enzyme assay. The resulting substrate from RAW 264.7 cells was Histone H1.2, an isoform protein of linker histone. Histone H1.2 has only one tyrosine residue in the entire molecule, which ensures the exact position of the substrate to be involved. It has been reported that Histones are the most prominent nitrated proteins in cancer tissues. It was also demonstrated that tyrosine nitration of Histone H1 occurs in vivo. These findings lead us to the idea that Histone H1.2 might be an intrinsic substrate for denitrase. We nitrated recombinant and purified Histone H1.2 chemically and subjected it to an on-blot enzyme assay to characterize the activity. Denitrase activity behaved as an enzymatic activity because the reaction was time dependent and was destroyed by heat or trypsin treatment. The activity was shown to be specific for Histone H1.2, to differ from proteasome activity, and to require no additional cofactors.

Measurement of nasal potential difference in mild asthmatics

STUDY OBJECTIVE

To determine whether ion transport or barrier function across the nasal epithelium are altered in asthmatics.

DESIGN

In this pilot study, the nasal potential difference (PD) was measured using the technique established by Knowles and colleagues. A flowing agar bridge made electrical contact with the surface of the nasal epithelium along the floor of the nose. This bridge was referenced to a cutaneous electrode to determine the PD across the nasal epithelium. Changes in nasal PD in response to amiloride, chloride-free medium, and chloride-free medium containing isoproterenol were measured, and responses of asthmatics and healthy control subjects were compared.

PARTICIPANTS

Measurements were made in eight adult nonasthmatic subjects and 6 adult asthmatic subjects. All asthmatics had mild intermittent asthma.

MEASUREMENTS AND RESULTS

Continuous measurements of nasal PD were obtained while the nasal surface was perfused consecutively with saline solution (NaCl-containing solution), saline solution plus 100 micro mol/L amiloride, chloride-free solution plus amiloride, and chloride-free solution with amiloride plus 10 micro mol/L isoproterenol (a beta-adrenergic agonist). No significant differences in baseline PD or change in PD in response to changes in perfusate were found between the two groups.

CONCLUSIONS

Our results suggest that ion transport and barrier function of patients with mild asthma are normal. Therefore, by contrast to cystic fibrosis, changes in salt and water transport across airway epithelium may not contribute to accumulation of mucous secretions in asthma.

High tidal volume ventilation induces NOS2 and impairs cAMP- dependent air space fluid clearance

Tidal volume reduction during mechanical ventilation reduces mortality in patients with acute lung injury and the acute respiratory distress syndrome. To determine the mechanisms underlying the protective effect of low tidal volume ventilation, we studied the time course and reversibility of ventilator-induced changes in permeability and distal air space edema fluid clearance in a rat model of ventilator-induced lung injury. Anesthetized rats were ventilated with a high tidal volume (30 ml/kg) or with a high tidal volume followed by ventilation with a low tidal volume of 6 ml/kg. Endothelial and epithelial protein permeability were significantly increased after high tidal volume ventilation but returned to baseline levels when tidal volume was reduced. The basal distal air space fluid clearance (AFC) rate decreased by 43% (P < 0.05) after 1 h of high tidal volume but returned to the preventilation rate 2 h after tidal volume was reduced. Not all of the effects of high tidal volume ventilation were reversible. The cAMP-dependent AFC rate after 1 h of 30 ml/kg ventilation was significantly reduced and was not restored when tidal volume was reduced. High tidal volume ventilation also increased lung inducible nitric oxide synthase (NOS2) expression and air space total nitrite at 3 h. Inhibition of NOS2 activity preserved cAMP-dependent AFC. Because air space edema fluid inactivates surfactant and reduces ventilated lung volume, the reduction of cAMP-dependent AFC by reactive nitrogen species may be an important mechanism of clinical ventilator-associated lung injury.

Early changes in alveolar fluid clearance by nitric oxide after endotoxin instillation in rats

Alveolar fluid clearance may be inhibited and/or stimulated under pathologic conditions. We examined the early change of alveolar fluid clearance after endotoxin instillation in adult rats. We employed electron paramagnetic resonance nitric oxide (NO) trapping technique with iron complex with N,N-diethyldithiocarbamate as an NO trapping agent. We found that lung NO signals reached the highest magnitude by 6 hours after endotoxin instillation. NO production was accompanied by increases in lung cyclic guanosine monophosphate levels. Alveolar fluid clearance decreased significantly 6 hours after the administration of the endotoxin and increased further at 24 hours. These changes were shown to be related to the function of amiloride-sensitive sodium ion channels. Treatment with gadolinium chloride and aminoguanidine significantly decreased lung NO and cyclic guanosine monophosphate levels and completely ameliorated the decrease in alveolar fluid clearance. In addition, the increase in alveolar fluid clearance at 24 hours returned to normal levels after treatment with gadolinium chloride and aminoguanidine. We found immunoreactive inducible nitric oxide synthase to be abundantly expressed in the cytoplasm of alveolar macrophages. Our results suggest that alveolar endotoxin inhibits alveolar fluid clearance at 6 hours by NO. NO is produced via inducible NO synthase in endotoxin-stimulated alveolar macrophages and was also shown to increase alveolar fluid clearance at 24 hours.

Pulmonary oedema fluid induces non-alpha-ENaC-dependent Na(+) transport and fluid absorption in the distal lung

To determine if pulmonary oedema fluid (EF) alters ion and fluid transport of distal lung epithelium (DLE), EF was collected from rats in acute heart failure. EF, but not plasma, increased amiloride-insensitive short circuit current (I(sc)) and Na(+)-K(+) ATPase protein content and pump activity of DLE grown in primary culture. Inhibitors of Cl(-) transport or cGMP-gated cation channels had a significant (P < 0.05), but limited ability to block the increased I(sc). EF increased amiloride-insensitive, but not amiloride-sensitive, DLE apical membrane Na(+) conductance. The level of mRNA encoding epithelial sodium channel (ENaC) subunits was unchanged (alpha, beta), or decreased (gamma, P < 0.05) in EF-exposed DLE. EF also induced an amiloride-insensitive increase in the potential difference across murine tracheal cysts. Distal lung explants from late gestation wild-type and alpha-ENaC-deficient fetal mice, which normally expand due to liquid secretion, decreased in size due to liquid absorption when exposed to EF. Trypsin digestion or heat treatment of EF abrogated the ability of EF to increase amiloride-insensitive I(sc) in DLE and liquid absorption by distal lung explants. Thus proteins or protein-dependent factors within cardiogenic EF induce an alpha-ENaC-independent and amiloride-insensitive apical membrane Na(+) conductance and liquid absorption in the distal lung.

Rapid effects of 17beta-estradiol and progesterone on sheep visceral and parietal pleurae via a nitric oxide pathway

We investigated the effects of 17beta-estradiol and progesterone on transepithelial electrical resistance (R(TE)) in sheep visceral and parietal pleurae. Specimens of intact pleurae from adult female sheep were used. The samples were transferred to the laboratory within 30 min after death of the animal in a Krebs-Ringer solution at 4 degrees C. The pleura was then mounted as a planar sheet in Ussing-type chambers, and electrical measurements were made. There was an increase in R(TE) in all of the samples examined after addition of 17beta-estradiol and progesterone in visceral and parietal pleurae. This increase was rapid within 1 min, lasted for ~15 min, returned to the basal level within 30-45 min, and was dose dependent. Tamoxifen, an estrogen receptor antagonist, did not significantly eliminate the effect of 17beta-estradiol. Furthermore, no steroid receptors were identified in cytosolic preparations of visceral and parietal pleura with ligand binding assays. The estrogen- and progesterone-induced increase in R(TE) in both visceral and parietal pleurae was affected by addition of an inhibitor of nitric oxide synthase. Indeed, previous administration of N(omega)-nitro-L-arginine methyl ester prevented the increase in R(TE) by 17beta-estradiol and progesterone. These results suggest that 17beta-estradiol and progesterone induce an increase in R(TE) in both visceral and parietal pleura and thus alter the transepithelial permeability. The effect of steroids may be accounted for by rapid release of nitric oxide in pleura.

Lung epithelial fluid transport and the resolution of pulmonary edema

The discovery of mechanisms that regulate salt and water transport by the alveolar and distal airway epithelium of the lung has generated new insights into the regulation of lung fluid balance under both normal and pathological conditions. There is convincing evidence that active sodium and chloride transporters are expressed in the distal lung epithelium and are responsible for the ability of the lung to remove alveolar fluid at the time of birth as well as in the mature lung when pathological conditions lead to the development of pulmonary edema. Currently, the best described molecular transporters are the epithelial sodium channel, the cystic fibrosis transmembrane conductance regulator, Na+-K+-ATPase, and several aquaporin water channels. Both catecholamine-dependent and -independent mechanisms can upregulate isosmolar fluid transport across the distal lung epithelium. Experimental and clinical studies have made it possible to examine the role of these transporters in the resolution of pulmonary edema.

Lack of amiloride-sensitive transport across alveolar and respiratory epithelium of iNOS(-/-) mice in vivo

The extent to which endogenously generated nitric oxide alters Na(+) transport across the mammalian alveolar epithelium in vivo has not been documented. Herein we measured alveolar fluid clearance and nasal potential differences in mice lacking the inducible form of nitric oxide synthase [iNOS; iNOS(-/-)] and their corresponding wild-type controls [iNOS(+/+)]. Alveolar fluid clearance values in iNOS(+/+) and iNOS(-/-) anesthetized mice with normal oxygenation and acid-base balance were ~30% of instilled fluid/30 min. In both groups of mice, fluid absorption was dependent on vectorial Na(+) movement. Amiloride (1.5 mM) decreased alveolar fluid clearance in iNOS(+/+) mice by 61%, whereas forskolin (50 microM) increased alveolar fluid clearance by 55% by stimulating amiloride-insensitive pathways. Neither agent altered alveolar fluid clearance in iNOS(-/-) mice. Hyperoxia upregulated iNOS expression in iNOS(+/+) mice and decreased their amiloride-sensitive component of alveolar fluid clearance but had no effect on the corresponding values in iNOS(-/-) mice. Nasal potential difference measurements were consistent with alveolar fluid clearance in that both groups of mice had similar baseline values, which were amiloride sensitive in the iNOS(+/+) but not in the iNOS(-/-) mice. These data suggest that nitric oxide produced by iNOS under basal conditions plays an important role in regulating amiloride-sensitive Na(+) channels in alveolar and airway epithelia.

Regulation of anion secretion by nitric oxide in human airway epithelial cells

Nitric oxide (NO) is continuously produced and released in human airways, but the biological significance of this process is unknown. In this study, we have used Calu-3 cells to investigate the effects of NO on transepithelial anion secretion. An inhibitor of NO synthase, NG-nitro-L-arginine methyl ester, reduced short- circuit current (I(sc)), whereas an NO donor, S-nitrosoglutathione (GSNO), increased I(sc), with an EC50 approximately 1.2 microM. The NO-activated current was inhibited by diphenylamine-2-carboxylate, clotrimazole, and charybdotoxin. Selective permeabilization of cell membranes indicated that NO activated both apical anion channels and basolateral potassium channels. An inhibitor of soluble guanylate cyclase, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, prevented activation of I(sc) by NO but not by 8-bromo-cGMP, suggesting that NO acts via a cGMP-dependent pathway. Sequential treatment of cells with forskolin and GSNO or 1-ethyl-2-benzimidazolinone and GSNO showed additive effects of these chemicals on I(sc). Interestingly, GSNO elevated intracellular Ca2+ concentration ([Ca2+]i) but had no effect on I(sc) activated by thapsigargin. These results show that NO activates transepithelial anion secretion via a cGMP-dependent pathway that involves cross talk between NO and [Ca2+]i.

Factors directly affecting impulse transmission in inflammatory demyelinating disease: recent advances in our understanding

Demyelination and inflammation both contribute to the neurological deficits characteristic of multiple sclerosis and Guillain-Barré syndrome. Conduction deficits attributable to demyelination are well known, but it is becoming clear that factors such as nitric oxide, endocaine, cytokines, and antiganglioside antibodies also play significant roles. Demyelination directly affects conduction and also causes changes in both the distribution and repertoire of expressed axolemmal ion channels, which in turn affect impulse propagation and can promote hyperexcitability. In conducting axons, sustained trains of impulses can produce intermittent conduction failure, and, in the presence of nitric oxide exposure, can also cause axonal degeneration. Other factors impairing impulse transmission include nodal widening, glutamate toxicity, and disturbances of both the blood-brain barrier and synaptic transmission.

Mechanisms of TNF-alpha stimulation of amiloride-sensitive sodium transport across alveolar epithelium

Because tumor necrosis factor (TNF)-alpha can upregulate alveolar fluid clearance (AFC) in pneumonia or septic peritonitis, the mechanisms responsible for the TNF-alpha-mediated increase in epithelial fluid transport were studied. In rats, 5 microg of TNF-alpha in the alveolar instillate increased AFC by 67%. This increase was inhibited by amiloride but not by propranolol. We also tested a triple-mutant TNF-alpha that is deficient in the lectinlike tip portion of the molecule responsible for its membrane conductance effect; the mutant also has decreased binding affinity to both TNF-alpha receptors. The triple-mutant TNF-alpha did not increase AFC. Perfusion of human A549 cells, patched in the whole cell mode, with TNF-alpha (120 ng/ml) resulted in a sustained increase in Na(+) currents from 82 +/- 9 to 549 +/- 146 pA (P < 0.005; n = 6). The TNF-alpha-elicited Na(+) current was inhibited by amiloride, and there was no change when A549 cells were perfused with the triple-mutant TNF-alpha or after preincubation with blocking antibodies to the two TNF-alpha receptors before perfusion with TNF-alpha. In conclusion, although TNF- alpha can initiate acute inflammation and edema formation in the lung, TNF-alpha can also increase AFC by an amiloride-sensitive, cAMP-independent mechanism that enhances the resolution of alveolar edema in pathological conditions by either binding to its receptors or activating Na(+) channels by means of its lectinlike domain.

Effects of SNP, ouabain, and amiloride on electrical potential profile of isolated sheep pleura

The fluid and solute transport properties of pleural tissue were studied by using specimens of intact visceral and parietal pleura from adult sheep lungs. The samples were transferred to the laboratory in a Krebs-Ringer solution at 4 degrees C within 1 h from the death of the animal. The pleura was then mounted as a planar sheet in a Ussing-type chamber. The results that are presented in this study are the means of six different experiments. The spontaneous potential difference and the inhibitory effects of sodium nitroprusside (SNP), ouabain, and amiloride on transepithelial electrical resistance (R(TE)) were measured. The spontaneous potential difference across parietal pleura was 0.5 +/- 0.1 mV, whereas that across visceral pleura was 0.4 +/- 0.1 mV. R(TE) of both pleura was very low: 22.02 +/- 4.1 Omega. cm2 for visceral pleura and 22.02 +/- 3.5 Omega. cm2 for parietal pleura. There was an increase in the R(TE) when SNP was added to the serosal bathing solution of parietal pleura and to the serosal or mucosal bathing solution in visceral pleura. The same was observed when ouabain was added to the mucosal surface of visceral pleura and to either the mucosal or serosal surface of parietal pleura. Furthermore, there was an increase in R(TE) when amiloride was added to the serosal bathing solution of parietal pleura. Consequently, the sheep pleura appears to play a role in the fluid and solute transport between the pleural capillaries and the pleural space. There results suggest that there is a Na+ and K+ transport across both the visceral and parietal pleura.

DETANONOate, a nitric oxide donor, decreases amiloride-sensitive alveolar fluid clearance in rabbits

Inhaled nitric oxide (NO) has been administered to animals to selectively reduce pulmonary hypertension via NO donors such as the NONOates. However, vectorial Na(+) transport across confluent monolayers of alveolar type II (ATII) pneumocytes has been decreased by NO. We tested the hypothesis that administration of the NO donor, DETANONOate, would decrease alveolar fluid clearance (AFC) in the rabbit in vivo. We instilled a solution of 5% albumin in 0.9% NaCl with 3 mM DETANONOate into anesthetized rabbits. Two hours later, similar AFC values were measured in the presence and absence of 3 mM DETANONOate (38 +/- 12% versus 43 +/- 13%; mean +/- SD). However, animals coadministered 1 mM amiloride with one of three doses of DETANONOate (100 microM, 300 microM, or 3 mM) had significantly (p < 0.05) greater AFC values (23 +/- 8, 20 +/- 14, 28 +/- 12%, respectively) than those administered amiloride alone (10 +/- 7%). When 5% albumin in a Cl(-)-free solution was administered in the presence or absence of 100 microM DETANONOate, neither AFC values nor alveolar Cl(-) concentrations were different. DETANONOate decreases the amiloride-sensitive fraction of AFC but does not decrease total AFC. DETANONOate does not influence total AFC secondary to an increase in the amiloride-insensitive fraction of AFC that is not associated with a decrease in alveolar Cl(-) secretion.

Regulation of an amiloride-sensitive Na+-permeable channel by a beta2-adrenergic agonist, cytosolic Ca2+ and Cl- in fetal rat alveolar epithelium

1. In cell-attached patches formed on the apical membrane of fetal alveolar epithelium, terbutaline (a specific beta2-adrenergic agonist) increased the open probability (Po) of an amiloride-sensitive Na+-permeable non-selective cation (NSC) channel (control, 0.03 +/- 0.04; terbutaline, 0.62 +/- 0.18; n = 8, P < 0. 00001) by increasing the mean open time 100-fold without any significant change in the mean closed time and without any change in the single channel conductance (control, 27.8 +/- 2.3 pS; terbutaline, 28.2 +/- 2.1 pS; n = 8). 2. The Po of the unstimulated channel increased when the apical membrane was depolarized due to a decrease in the closing rate and an increase in the opening rate, while the Po of the terbutaline-stimulated channel did not depend on the membrane potential. 3. Increased cytosolic [Ca2+] also increased the Po of the channel in a manner consistent with one Ca2+-binding site on the cytosolic surface of the channel. Terbutaline increased the sensitivity of the channel to cytosolic Ca2+ by shifting the concentration of cytosolic Ca2+ ([Ca2+]c) required for half-maximal activation to a lower [Ca2+]c value, leading to an increase in Po. 4. An increase in the cytosolic Cl- concentration ([Cl-]c) decreased the Po of the channel consistent with two Cl--binding sites by increasing the closing rate without any significant change in the opening rate. Terbutaline increased Po by reducing the effect of cytosolic Cl- to promote channel closing. 5. Taken together, these observations indicate that terbutaline activates a Ca2+-activated, Cl--inhibitable, amiloride-sensitive, Na+-permeable NSC channel in fetal rat alveolar epithelium in two ways: first, through an increase in Ca2+ sensitivity, and second, through a reduction in the effect of cytosolic Cl- to promote channel closing.