Oxidant inhibition of epithelial active sodium transport

Reactive species and pulmonary edema

Pulmonary edema occurs when fluid flux into the lung interstitium exceeds its removal, resulting in hypoxemia and even death. Noncardiogenic pulmonary edema (NPE) generally results when microvascular and alveolar permeability to plasma proteins increase, one possible etiology being oxidant injury. Reactive oxygen and nitrogen species (RONS) can modify or damage ion channels, such as epithelial sodium channels, which alters fluid balance. Experimental systems in which either RONS are increased or protective antioxidant mechanisms are decreased result in alterations of epithelial sodium channel activity and support the hypothesis that RONS are important in NPE. Both basic and clinical studies are needed to critically define the RONS-NPE connection and the capacity of antioxidant therapy (either alone or as a supplement to β-agonists) to improve patient outcome.

Radix paeoniae rubra suppression of sodium current in acutely dissociated rat hippocampal CA1 neurons

The effect of Radix paeoniae rubra (RPR) on voltage-gated sodium channel (VGSC) currents (I(Na)) was examined in freshly isolated rat hippocampal CA1 neurons using whole-cell patch-clamp technique under voltage-clamp conditions. RPR suppressed I(Na) without affecting the current activation, inactivation and deactivation. The amplitude of I(Na) decreased by approximately 18.4% within a few seconds of 0.8 mg/ml RPR exposure. RPR (0.8 mg/ml) shifted the steady-state inactivation curves of I(Na) to negative potentials, with hyperpolarizing direction shift of V(1/2) of 10.0 mV. The time course of I(Na) recovery from inactivation was prolonged significantly by 0.8 mg/ml RPR. RPR (0.8 mg/ml) also enhanced the activity-dependent attenuation of I(Na) and decreased the fraction of activated channels. These results suggested that RPR suppressed hippocampal CA1 I(Na) by shifting the inactivation curve in hyperpolarizing direction, slowing the recovery time course from inactivation, enhancing the activity-dependent attenuation and decreasing the number of activatable channels. RPR suppression on I(Na) might predict the protective effect during brain ischemia in hippocampal CA1 neurons.

Acute hyperoxic lung injury does not impede adenoviral-mediated alveolar gene transfer

The transfer of protective genes to the alveolar epithelium can attenuate lung injury if accomplished before its onset. The pathobiology of acute lung injury (ALI) includes formidable hurdles to gene transfer, including alveoli filled with fluid, inflammatory cells, and cytokines, all of which may impair gene transfer after the onset of injury. We tested the hypothesis that adenovectors could efficiently transduce injured alveoli by exposing adult, male Sprague-Dawley rats to 100% oxygen for 48 or 60 h before endotracheal instillation of either 1 x 10(9) or 4 x 10(9) plaque-forming units of an adenovirus that expresses an Escherichia coli lac Z gene (adbeta-gal) in a surfactant-based vehicle (Survanta). X-gal staining 72 h postinfection revealed transgene expression in all segments of room air control and hyperoxic lungs infected with either dose of adbeta-gal. Net transgene expression in hyperoxic lungs was not different from room air controls despite the presence of pulmonary edema and severe histologic injury. These findings show that adenovectors can efficiently transduce the alveoli of acutely injured, edematous lungs. The data indicate that the pathophysiologic processes of ALI do not impair adenoviral-mediated alveolar gene transfer and provide support for the development of gene therapies for ALI.

Continuous enteral nutrition attenuates pulmonary edema in rats exposed to 100% oxygen

Adult rats exposed to hyperoxia develop anorexia, weight loss, and a lung injury characterized by pulmonary edema and decreased lung liquid clearance. We hypothesized that maintenance of nutrition during hyperoxia could attenuate hyperoxia-induced pulmonary edema. To test this hypothesis, we enterally fed adult male Sprague-Dawley rats via gastrostomy tubes and exposed them to oxygen (inspired O(2) fraction >0.95) for 64 h. In contrast to controls, enterally fed hyperoxic animals did not lose weight and had smaller pleural effusions and wet-to-dry weight ratios (a measure of lung edema) that were not different from room air controls. Enterally fed rats exposed to hyperoxia had increased levels of mRNA for the Na(+)-K(+)-ATPase alpha(1)- and beta(1)-subunits and glutathione peroxidase. These findings suggest that maintenance of nutrition during an oxidative lung injury reduces lung edema, perhaps by allowing for continued expression and function of protective proteins such as the Na(+)-K(+)-ATPase.

Hydrogen peroxide inhibits cAMP-induced Cl- secretion across colonic epithelial cells

We examined the effects of H2O2 on Cl- secretion across human colonic T84 cells grown on permeable supports and mounted in modified Ussing chambers. Forskolin-induced short-circuit current, a measure of Cl- secretion, was inhibited in a concentration-dependent fashion when monolayers were pretreated with H2O2 for 30 min (30-100% inhibition between 500 microM and 5 mM). Moreover, H2O2 inhibited 76% of the Cl- current across monolayers when the basolateral membranes were permeabilized with nystatin (200 micrograms/ml). When the apical membrane was permeabilized with amphotericin B, H2O2 inhibited the Na+ current (a measure of Na+-K+-ATPase activity) by 68% but increased the K+ current more than threefold. In addition to its effects on ion transport pathways, H2O2 also decreased intracellular ATP levels by 43%. We conclude that the principal effect of H2O2 on colonic Cl- secretion is inhibitory. This may be due to a decrease in ATP levels following H2O2 treatment, which subsequently results in an inhibition of the apical membrane Cl- conductance and basolateral membrane Na+-K+-ATPase activity. Alternatively, H2O2 may alter Cl- secretion by direct action on the transporters or alterations in signal transduction pathways.

Inhaled nitric oxide versus conventional therapy: effect on oxygenation in ARDS

A randomized, controlled clinical trial was performed with patients with acute respiratory distress syndrome (ARDS) to compare the effect of conventional therapy or inhaled nitric oxide (iNO) on oxygenation. Patients were randomized to either conventional therapy or conventional therapy plus iNO for 72 h. We tested the following hypotheses: (1) that iNO would improve oxygenation during the 72 h after randomization, as compared with conventional therapy; and (2) that iNO would increase the likelihood that patients would improve to the extent that the FI(O2) could be decreased by > or = 0.15 within 72 h after randomization. There were two major findings. First, That iNO as compared with conventional therapy increased Pa(O2)/FI(O2) at 1 h, 12 h, and possibly 24 h. Beyond 24 h, the two groups had an equivalent improvement in Pa(O2)/FI(O2). Second, that patients treated with iNO therapy were no more likely to improve so that they could be managed with a persistent decrease in FI(O2) > or = 0.15 during the 72 h following randomization (11 of 20 patients with iNO versus 9 of 20 patients with conventional therapy, p = 0.55). In patients with severe ARDS, our results indicate that iNO does not lead to a sustained improvement in oxygenation as compared with conventional therapy.

The use of the frog palate preparation to assess the effects of oxidants on ciliated epithelium

This work was designed to develop a simple method based on the frog palate preparation to study the effects of hydrogen peroxide (H2O2) on ciliated epithelium. For this purpose, five sets (n = 10 per set) of frog palate preparations (Rana catesbeiana) were studied during 35 min after immersion in increasing concentrations of H2O2: 1, 8, 16, 32, and 64 microM. The effects of H2O2 on ciliated epithelium were assessed by measuring transepithelial potential difference (PD) and mucociliary transport (MT). Measurements were performed at 5-min intervals. In addition, the palates submitted to the 64 microM dose were immersed in Ringer's solution and followed by another 30 min to assess the possible recovery after maximal injury. Transepithelial potential difference (PD) was measured by means of agar-filled microelectrodes connected to the high input of a grounded electrometer. Mucociliary transport (MT) was determined by directly monitoring the movement of autologous mucus along the palate surface. Significant decrease in MT was observed in 16 microM and beyond and significant change in PD was observed in 32 microM and 64 microM. Palates submitted to 64 microM of H2O2 returned to their baseline levels of PD and MT within 30 min of recovery in Ringer's solution. In conclusion, the frog palate preparation was shown to be an efficient experimental tool to assess the deleterious effects of H2O2 on the ciliated epithelium.

Inflammatory mediators and otitis media with effusion. An experimental approach using cell culture

Otitis media with effusion is characterized by the presence of an inflammatory cellular infiltrate of the submucosa and a poor ventilation of the middle ear. This result in hypersecretion of mucus and alteration of the mucociliary clearance, which produce accumulation of fluid and cellular debris in the middle ear. The aim of this work was to investigate whether inflammatory mediators such as prostaglandins and oxygen metabolites modulate the absorptive function of the middle ear epithelium. The data we present demonstrated that: (i) among prostanoids, only prostaglandin E2 modulated the rate of sodium transport; (ii) oxidants had a stimulatory effect on ion transport; (iii) the role of reactive oxygen species was mediated by prostaglandin E2. This process might be involved in the impairment of the mucociliary clearance.

Depleted mucosal antioxidant defences in inflammatory bowel disease

Experimental approaches designed to define the role of reactive oxygen and nitrogen species generated by inflammatory cells in the tissue injury seen in inflammatory bowel disease rarely consider the chemical antioxidant defences against such increased oxidant stress in the mucosa. In this investigation, we have analysed components of the aqueous and lipid phase antioxidant mucosal defences by measuring the total peroxyl radical scavenging capacity and the levels of urate, glutathione, alpha-tocopherol, and ubiquinol-10 in paired noninflamed and inflamed mucosal biopsies from inflammatory bowel disease patients. Compared to paired noninflamed mucosa, decreases were observed in inflamed mucosa for total peroxyl radical scavenging capacity (55%, p = 0.0031), urate [Crohn's disease (CD), 62.2%, p = 0.066; ulcerative colitis (UC), 47.3%, p = 0.031], glutathione (UC, 59%, 7/8 patients, ns), total glutathione (UC 65.2%, 6/8 patients, ns), ubiquinol-10 (CD, 75.7%, p = 0.03; UC, 90.5%, p = 0.005). The mean alpha-tocopherol content was unchanged. These observations support our earlier findings of decreased reduced and total ascorbic acid in inflamed IBD mucosa and demonstrate that the loss of chemical antioxidant defences affects almost all the major components. The decreased antioxidant defences may severely compromise the inflamed mucosa, rendering it more susceptible to oxidative tissue damage, hindering recovery of the mucosa and return of epithelial cell layer integrity. The loss of chemical antioxidant components provides a strong rational for developing novel antioxidant therapies for the treatment of inflammatory bowel disease.

Peroxynitrite inhibits sodium uptake in rat colonic membrane vesicles

Peroxynitrite (ONOO-) is a potent oxidizing agent that initiates lipid peroxidation and sulfhydryl oxidation and may be responsible for a portion of the cytotoxicity attributed to superoxide anion (.O2-). We quantified the extent to which ONOO-, xanthine plus xanthine oxidase (XO) and hydrogen peroxide (H2O2), decreased sodium (Na+) uptake into membrane vesicles derived from colonic cells of dexamethasone-treated rats. Carrier-free 22Na+ uptake into vesicles was measured in the presence of an inside-negative membrane potential, produced by the addition of the potassium ionophore valinomycin (10 microM) after removal of all external potassium by cation exchange chromatography. Preincubation of vesicles with either 100 microM or 1 mM ONOO- for 30 s decreased the amiloride-blockable fraction of Na+ uptake by 27 +/- 7% and 65 +/- 2%, respectively (means +/- S.E.; n greater than or equal to 5; P less than 0.05 from control). However, the amiloride-insensitive part of Na+ uptake was not affected, indicating that there was no overt destruction of these vesicles by these ONOO- concentrations. Decomposed ONOO-, hydrogen peroxide (1 microM-10 mM), or xanthine (500 microM) plus XO (10-30 mU/ml), either in the absence or in the presence of 100 microM FeEDTA, did not decrease Na+ uptake. These data suggest that ONOO- is a potent injurious agent that can compromise Na+ uptake across epithelial cells, possibly by damaging Na+ channels.

NO2 decreases paracellular resistance to ion and solute flow in alveolar epithelial monolayers

Primary cultured monolayers of rat alveolar epithelial cells grown on tissue culture-treated Nuclepore filters were exposed to 2.5 ppm nitrogen dioxide (NO2) for 2-20 min. Changes in monolayer bioelectric properties and solute permeabilities were subsequently measured. Exposure to NO2 produced a dose-dependent decrease in monolayer transepithelial electrical resistance (Rt), whereas monolayer short-circuit current was unaffected. Post-exposure monolayer permeability to 14C-sucrose (which primarily crosses alveolar epithelium via the paracellular pathway) increased markedly. That for 3H-glycerol (which permeates through both paracellular and transcellular pathways) increased to a lesser extent. Partial recovery of Rt and solute permeabilities was noted by 48-h post-exposure. The time courses of the decrease in Rt and increase in solute permeabilities were similar. These results suggest that NO2 primarily impairs passive alveolar epithelial barrier functions in vitro, probably by altering intercellular junctions, and does not appear to directly affect cell membrane active ion transport processes. When correlated with results obtained from experimental approaches, studies of in vitro alveolar epithelial monolayers may facilitate investigations of dosimetry, sites, and mechanisms of oxidant injury in the lung.

Characterization of antioxidant activities of pulmonary surfactant mixtures

Instillation of intratracheal surfactant is known to limit the morbidity and mortality of patients and animals with oxidant-induced lung injury. In this study we quantified the antioxidant properties of natural lung surfactant (NLS), consisting of 90% lipid and 10% protein, and of calf lung surfactant extract (CLSE) consisting of 99% lipid and 1% protein. NLS, but not CLSE, contained significant amounts of superoxide dismutase (SOD) and catalase activities (7 U SOD/mumol phospholipid (PL) and 1 U catalase/mumol PL). More than 90% of the SOD activity was abolished by 1 mM KCN, suggesting that this was the CuZn form of the enzyme. In addition, NLS significantly reduced extracellular H2O2 without losing its ability to reach minimum surface tensions below 1 dyn/cm upon dynamic compression. The NLS scavenging of H2O2 could not be accounted for by albumin. The presence of catalase and SOD activities in NLS was also verified by activity stains of proteins separated by native polyacrylamide gel electrophoresis. Intratracheal instillation of 7 ml of NLS (308 mumol PL) into rabbits significantly increased SOD content in type II cells isolated 12 h later. It is concluded that, in addition to promoting alveolar stability, instillation of pulmonary surfactant may offer significant protection to the alveolar epithelium by scavenging extracellularly generated partially reduced oxygen species and by enhancing intracellular antioxidant enzyme content.

Hydrogen peroxide stimulates rat colonic prostaglandin production and alters electrolyte transport

The changes in short circuit current (electrogenic Cl- secretion) of rat colon brought about by xanthine/xanthine oxidase in the Ussing chamber were inhibited by catalase and diethyldithiocarbamate, but not by superoxide dismutase. These results, the reproduction of the response with glucose/glucose oxidase and with exogenous H2O2, and the lack of effect of preincubation with deferoxamine or thiourea implicate H2O2, and not O2- or OH., as the important reactive oxygen metabolite altering intestinal electrolyte transport. 1 mM H2O2 stimulated colonic PGE2 and PGI2 production 8- and 15-fold, respectively, inhibited neutral NaCl absorption, and stimulated biphasic electrogenic Cl secretion with little effect on enterocyte lactic dehydrogenase release, epithelial conductance, or histology. Cl- secretion was reduced by cyclooxygenase inhibition. Also, the Cl- secretion, but not the increase in prostaglandin production, was reduced by enteric nervous system blockade with tetrodotoxin, hexamethonium, or atropine. Thus, H2O2 appears to alter electrolyte transport by releasing prostaglandins that activate the enteric nervous system. The change in short circuit current in response to Iloprost, but not PGE2, was blocked by tetrodotoxin. Therefore, PGI2 may be the mediator of the H2O2 response. H2O2 produced in nontoxic concentrations in the inflamed gut could have significant physiologic effects on intestinal water and electrolyte transport.