Effects of 100% O2 breathing on permeability of alveolar epithelium to solute

Modelling lung diffusion-perfusion limitation in mechanically ventilated SARS-CoV-2 patients

This is the first study to describe the daytime evolution of respiratory parameters in mechanically ventilated COVID-19 patients. The data base refers to patients hospitalised in the intensive care unit (ICU) at Arequipa Hospital (Peru, 2335 m) in 2021. In both survivors (S) and non-survivors (NS) patients, a remarkable decrease in respiratory compliance was observed, revealing a proportional decrease in inflatable alveolar units. The S and NS patients were all hyperventilated and their SatO2 was maintained at >90%. However, while S remained normocapnic, NS developed progressive hypercapnia. We compared the efficiency of O2 uptake and CO2 removal in the air blood barrier relying on a model allowing to partition between diffusion and perfusion limitations to gas exchange. The decrease in O2 uptake was interpreted as diffusion limitation, while the impairment in CO2 removal was modelled by progressive perfusion limitation. The latter correlated with the increase in positive end-expiratory pressure (PEEP) and plateau pressure (Pplat), leading to capillary compression, increased blood velocity, and considerable shortening of the air-blood contact time.

Pulmonary Interstitial Matrix and Lung Fluid Balance From Normal to the Acutely Injured Lung

This review analyses the mechanisms by which lung fluid balance is strictly controlled in the air-blood barrier (ABB). Relatively large trans-endothelial and trans-epithelial Starling pressure gradients result in a minimal flow across the ABB thanks to low microvascular permeability aided by the macromolecular structure of the interstitial matrix. These edema safety factors are lost when the integrity of the interstitial matrix is damaged. The result is that small Starling pressure gradients, acting on a progressively expanding alveolar barrier with high permeability, generate a high transvascular flow that causes alveolar flooding in minutes. We modeled the trans-endothelial and trans-epithelial Starling pressure gradients under control conditions, as well as under increasing alveolar pressure (Palv) conditions of up to 25 cmH2O. We referred to the wet-to-dry weight (W/D) ratio, a specific index of lung water balance, to be correlated with the functional state of the interstitial structure. W/D averages ∼5 in control and might increase by up to ∼9 in severe edema, corresponding to ∼70% loss in the integrity of the native matrix. Factors buffering edemagenic conditions include: (i) an interstitial capacity for fluid accumulation located in the thick portion of ABB, (ii) the increase in interstitial pressure due to water binding by hyaluronan (the "safety factor" opposing the filtration gradient), and (iii) increased lymphatic flow. Inflammatory factors causing lung tissue damage include those of bacterial/viral and those of sterile nature. Production of reactive oxygen species (ROS) during hypoxia or hyperoxia, or excessive parenchymal stress/strain [lung overdistension caused by patient self-induced lung injury (P-SILI)] can all cause excessive inflammation. We discuss the heterogeneity of intrapulmonary distribution of W/D ratios. A W/D ∼6.5 has been identified as being critical for the transition to severe edema formation. Increasing Palv for W/D > 6.5, both trans-endothelial and trans-epithelial gradients favor filtration leading to alveolar flooding. Neither CT scan nor ultrasound can identify this initial level of lung fluid balance perturbation. A suggestion is put forward to identify a non-invasive tool to detect the earliest stages of perturbation of lung fluid balance before the condition becomes life-threatening.

Nitrite therapy prevents chlorine gas toxicity in rabbits

Chlorine (Cl₂) gas exposure and toxicity remains a concern in military and industrial sectors. While post-Cl₂ exposure damage to the lungs and other tissues has been documented and major underlying mechanisms elucidated, no targeted therapeutics that are effective when administered post-exposure, and which are amenable to mass-casualty scenarios have been developed. Our recent studies show nitrite administered by intramuscular (IM) injection post-Cl₂ exposure is effective in preventing acute lung injury and improving survival in rodent models. Our goal in this study was to develop a rabbit model of Cl₂ toxicity and test whether nitrite affords protection in a non-rodent model. Exposure of New Zealand White rabbits to Cl₂ gas (600ppm, 45min) caused significant increases in protein and neutrophil accumulation in the airways and ∼35% mortality over 18h. Nitrite administered 30min post Cl₂ exposure by a single IM injection, at 1mg/kg or 10mg/kg, prevented indices of acute lung injury at 6h by up to 50%. Moreover, all rabbits that received nitrite survived over the study period. These data provide further rationale for developing nitrite as post-exposure therapeutic to mitigate against Cl₂ gas exposure injury.

Mitigation of chlorine-induced lung injury by low-molecular-weight antioxidants

Chlorine (Cl(2)) is a highly reactive oxidant gas used extensively in a number of industrial processes. Exposure to high concentrations of Cl(2) results in acute lung injury that may either resolve spontaneously or progress to acute respiratory failure. Presently, the pathophysiological sequelae associated with Cl(2)-induced acute lung injury in conscious animals, as well as the cellular and biochemical mechanisms involved, have not been elucidated. We exposed conscious Sprague-Dawley rats to Cl(2) gas (184 or 400 ppm) for 30 min in environmental chambers and then returned them to room air. At 1 h after exposure, rats showed evidence of arterial hypoxemia, respiratory acidosis, increased levels of albumin, IgG, and IgM in bronchoalveolar lavage fluid (BALF), increased BALF surfactant surface tension, and significant histological injury to airway and alveolar epithelia. These changes were more pronounced in the 400-ppm-exposed rats. Concomitant decreases of ascorbate (AA) and reduced glutathione (GSH) were also detected in both BALF and lung tissues. In contrast, heart tissue AA and GSH content remained unchanged. These abnormalities persisted 24 h after exposure in rats exposed to 400 ppm Cl(2). Rats injected systemically with a mixture of AA, deferoxamine, and N-acetyl-L-cysteine before exposure to 184 ppm Cl(2) had normal levels of AA, lower levels of BALF albumin and normal arterial Po(2) and Pco(2) values. These findings suggest that Cl(2) inhalation damages both airway and alveolar epithelial tissues and that resulting effects were ameliorated by prophylactic administration of low-molecular-weight antioxidants.

Surfactant dysfunction in SP-A-/- and iNOS-/- mice with mycoplasma infection

Surfactant dysfunction was studied in C57BL/6 (B6), B6.SP-A(-/-), and B6.iNOS(-/-) mice with pulmonary mycoplasma infection (10(7) colony-forming units). Cell-free bronchoalveolar lavage (BAL) from uninfected B6.SP-A(-/-) versus B6 mice had a reduced content of very large aggregates (VLA) and an increase in intermediate large aggregates (ILA), with no difference in total large aggregates (LA = VLA + ILA). However, LA from uninfected B6.SP-A(-/-) versus B6 mice contained less protein and were more sensitive to inhibition by serum albumin and lysophosphatidylcholine in pulsating bubble studies in vitro. Infection with Mycoplasma pulmonis caused significant lung injury and surfactant abnormalities in B6.SP-A(-/-), B6.iNOS(-/-), and B6 mice at 24, 48, 72 h after infection compared with uninfected mice of the same strain. Analyses of time-pooled data indicated that mycoplasma-infected B6.SP-A(-/-) and B6.iNOS(-/-) mice had significantly lower levels of LA and higher protein/phospholipid ratios in BAL compared with infected B6 mice. Infected B6.iNOS(-/-) versus B6 mice also had increased minimum surface tensions on the pulsating bubble and decreased levels of surfactant protein (SP)-B in BAL. These results indicate that pulmonary mycoplasma infection in vivo causes lung injury and surfactant abnormalities that are dependent in part on iNOS and SP-A. In addition, SP-A deficiency modifies surfactant aggregate content and lowers the inhibition resistance of LA surfactant in vitro compared with congenic normal mice.

Nucleotide-mediated inhibition of alveolar fluid clearance in BALB/c mice after respiratory syncytial virus infection

Respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract disease in infants and children worldwide. Intranasal infection of BALB/c mice with RSV strain A2, but not ultraviolet-inactivated RSV, for 2 or 4 days reduced basal alveolar fluid clearance (AFC), a seminal function of bronchoalveolar epithelium, and caused loss of AFC sensitivity to amiloride inhibition. Reduced AFC was temporally associated with increased lung water content but was not a consequence of increased epithelial permeability or cell death. Reduced AFC was also not due to decreased transcription of epithelial Na+ channel subunit genes in lung tissue. RSV-mediated inhibition of AFC 2 days after infection was rapidly prevented by addition to the instillate of P2Y receptor antagonists (suramin and XAMR-0721) or enzymes that degrade UTP, but not those that degrade ATP. After UTP degradation, AFC returned to control levels but was no longer sensitive to amiloride. UTP at nanomolar concentrations recapitulated the AFC inhibitory effect of RSV in normal mice and mice infected with RSV for 6 days, indicating that normalization of AFC at this time point is a consequence of cessation of UTP release, rather than P2Y receptor desensitization. We conclude that RSV infection of the bronchoalveolar epithelium results in reduced AFC as a consequence of autocrine feedback inhibition mediated by UTP. These studies are the first to demonstrate AFC inhibition by an important pulmonary viral pathogen. Reduced AFC may result in formation of an increased volume of fluid mucus, airway congestion, and rhinorrhea, all features of severe RSV disease.

Elevated generation of reactive oxygen/nitrogen species in hantavirus cardiopulmonary syndrome

Hantavirus cardiopulmonary syndrome (HCPS) is a life-threatening respiratory disease characterized by profound pulmonary edema and myocardial depression. Most cases of HCPS in North America are caused by Sin Nombre virus (SNV), which is carried asymptomatically by deer mice (Peromyscus maniculatus). The underlying pathophysiology of HCPS is poorly understood. We hypothesized that pathogenic SNV infection results in increased generation of reactive oxygen/nitrogen species (RONS), which contribute to the morbidity and mortality of HCPS. Human disease following infection with SNV or Andes virus was associated with increased nitrotyrosine (NT) adduct formation in the lungs, heart, and plasma and increased expression of inducible nitric oxide synthase (iNOS) in the lungs compared to the results obtained for normal human volunteers. In contrast, NT formation was not increased in the lungs or cardiac tissue from SNV-infected deer mice, even at the time of peak viral antigen expression. In a murine (Mus musculus) model of HCPS (infection of NZB/BLNJ mice with lymphocytic choriomeningitis virus clone 13), HCPS-like disease was associated with elevated expression of iNOS in the lungs and NT formation in plasma, cardiac tissue, and the lungs. In this model, intraperitoneal injection of 1400W, a specific iNOS inhibitor, every 12 h during infection significantly improved survival without affecting intrapulmonary fluid accumulation or viral replication, suggesting that cardiac damage may instead be the cause of mortality. These data indicate that elevated production of RONS is a feature of pathogenic New World hantavirus infection and that pharmacologic blockade of iNOS activity may be of therapeutic benefit in HCPS cases, possibly by ameliorating the myocardial suppressant effects of RONS.

Extracellular glutathione inhibits oxygen-induced permeability changes in alveolar epithelial monolayers

Exposure to high fractional inspired oxygen for 24 h increases permeability of the alveolar epithelium, contributing to the clinical manifestations of oxygen toxicity. Utilizing a model of the alveolar epithelium in which isolated rat type II cells form polarized monolayers on polycarbonate filters [transepithelial resistance (R(t)) > 1 k Omega x cm(2) by day 4], we evaluated the ability of reduced glutathione (GSH) to ameliorate these changes. On day 4, apical fluid was replaced with culture medium containing 1) no additives, 2) GSH (500 microM), or 3) GSH (500 microM) + glutathione reductase (0.5 U/ml) + nicotinamide adenine dinucleotide phosphate (250 microM). Monolayers were exposed (for 24 h) to room air (control) or 95% O(2), each containing 5% CO(2). After 24 h of hyperoxia, R(t) for condition 1 decreased by 45% compared with control (P < 0.001). In conditions 2 and 3, R(t) did not decrease significantly (P = not significant). Hyperoxia-induced decreases in active ion transport were observed for conditions 1 and 2 (P < 0.05), but not for condition 3 (P = not significant). These findings indicate that extracellular GSH may protect the alveolar epithelium against hyperoxia-induced injury. Addition of glutathione reductase and nicotinamide adenine dinucleotide phosphate may further augment these protective effects of GSH.

Surfactant protein-B-deficient mice are susceptible to hyperoxic lung injury

Surfactant protein-B (SP-B) is a small, hydrophobic peptide that plays a critical role in pulmonary function and surfactant homeostasis. To determine whether SP-B protects mice from oxygen-induced injury, heterozygous SP-B(+/-) gene-targeted mice and wild-type SP-B(+/+) littermates were exposed to hyperoxia (95% oxygen for 3 d) or room air. Although specific lung compliance in room air in SP-B(+/-) mice was slightly reduced as compared with that in SP-B(+/+) mice, it was reduced more markedly during hyperoxia (46% versus 25% decrease, respectively). The larger decrease in lung compliance in SP-B(+/-) mice was associated with increased severity of pulmonary edema, hemorrhage and inflammation, lung permeability and protein leakage into the alveolar space. Hyperoxia increased SP-B messenger RNA (mRNA) and total protein concentrations by 2-fold in SP-B(+/+) and SP-B(+/-) mice, but decreased the abundance of SP-B protein in lavage fluid relative to total protein only in SP-B(+/-) mice. Hyperoxia increased SP-B expression, but apparently not enough to maintain SP-B function and lung compliance in the presence of increased protein leakage in SP-B(+/-) mice. Increased alveolar-capillary leakage and relative deficiency of SP-B may therefore contribute to oxygen-induced pulmonary dysfunction in SP-B(+/-) mice. These data support the concept that SP-B plays an important protective role in the lung.

Protein kinase and phosphatase activity in the lungs of normoxic versus hyperoxic rats

Studies were designed to examine aspects of phosphorylation and dephosphorylation in rat lung cells in response to hyperoxic exposure. Protein kinase and phosphatase activities were measured in preparations of lungs from normoxic rats, hyperoxia-exposed rats (95% O2 for 60h), and rats recovering in room air for 1 and 3 days. Protein kinase C (PKC) activity immediately postexposure was significantly lower than in normoxic controls (normoxia 127.1 +/- 13 vs. hyperoxia 101.5 +/- 6 pmol/min mg-1) and continued to decline during the recovery period (85.3 +/- 4 and 78.2 +/- 6 pmol/min mg-1 at 1 and 3 days recovery, respectively). The PKC activity did not translocate from cytoplasm to the membranes. In contrast, PKA activity did not change in response to hyperoxia exposure or recovery. Protein phosphatase activity was decreased significantly by hyperoxia exposure (normoxia 30.7 +/- 3 vs. hyperoxia 21.9 +/- 1 pmol/min microgram-1) but returned to normoxic control levels by 1 and 3 days (24.1 +/- and 31.5 +/- 1 pmol/min microgram -1, respectively). Protein phosphatase activity was inhibited by okadaic acid (Ki = 1 nM) and calyculin A (Ki = 0.61 pM), indicating a type 2A protein phosphatase. Enzyme activities in cultured type II alveolar cells paralleled those observed in whole lung preparations. Decreased enzyme activities in the lung may be located to the development of acute lung injury during hyperoxic exposure.

Effects of acute hyperoxic exposure on solute fluxes across the blood-gas barrier in rat lungs

We investigated effects of acute hyperoxia on solute transport from air space to vascular space in isolated rat lungs. Air spaces were filled with Krebs-Ringer bicarbonate solution containing fluorescein isothiocyanate-labeled dextran (FD-20; mol wt 20,000) and either 22Na+ and [14C]sucrose, or D-[14C]glucose and L-[3H]glucose. Apparent permeability-surface area products for tracers over time (up to 120 min) were calculated for isolated perfused lungs from control rats (room air) and rats exposed to > 95% O2 for 48 or 60 h immediately postexposure. After O2 exposures, mean fluxes for [14C]sucrose and FD-20 were significantly higher than in room-air control lungs. However, amiloride-sensitive Na+ and active D-glucose fluxes were unchanged after hyperoxic exposure. Therefore, it is unlikely that decreases in net solute transport in this lung-injury model contributed to pulmonary edema resulting from O2 toxicity. Increased net solute transport shown to help resolve pulmonary edema after acute hyperoxic exposure must therefore begin during the recovery period. In summary, our data show increases in passive solute fluxes but no changes in active solute fluxes immediately after acute hyperoxic lung injury.

Increased expression and activity of sodium channels in alveolar type II cells of hyperoxic rats

We investigated the cellular and molecular events associated with the increase in sodium transport across the alveolar epithelium of rats exposed to hyperoxia (85% O2 for 7 days followed by 100% O2 for 4 days). Alveolar type II (ATII) cell RNA was isolated and probed with a cDNA for one of the rat colonic epithelial sodium channel subunits (alpha rENaC). The alpha rENaC mRNA (3.7-kb transcript) increased 3-fold in ATII cell RNA isolated from rats exposed to 85% O2 for 7 days and 6-fold after 4 days of subsequent exposure to 100% O2. In situ hybridization revealed increased expression of alpha rENaC mRNA transcripts in both airway and alveolar epithelial cells of hyperoxic rats. When immunostained with a polyclonal antibody to kidney sodium channel protein, ATII cells from hyperoxic rats exhibited a significant increase in the amount of immunogenic protein present in both the plasma membrane and the cytoplasm. When patched in the whole-cell mode, ATII cells from hyperoxic rats exhibited amiloride and 5-(N-ethyl-N-isopropyl)-2',4'-amiloride (EIPA)-sensitive currents that were 100% higher compared with those obtained from air-breathing rats. Single-channel sodium currents (mean conductance of 25 pS) were seen in ATII cells patched in both the inside-out and cell-attached modes. The number and open probability of these channels increased significantly during exposure to hyperoxia. Exposure to sublethal hyperoxia up-regulated both alpha rENaC mRNA and the functional expression of sodium channels in ATII cells.

Hyperoxia-induced alterations of rat alveolar lavage composition and properties

Although lethal exposures of most animal species to oxygen result in a reduced amount of surfactant phospholipids (PL), hyperoxia in rats leads to elevated levels of PL on the alveolar surface. Because of this different response, a study was made of the amount, composition, surface properties, and subfraction distribution (obtained by differential centrifugation) of alveolar lavage materials from rats exposed to > 95% oxygen for 64 h. The exposures lead to severe lung damage, which includes the appearance of pleural effusion, pulmonary edema, and increased protein levels on the alveolar surface. However, the PL levels of lavage fluid are increased two- to threefold, and the PL composition is altered. In O2-exposed rats, only 39(+/- 1)% of the phospholipid is disaturated phosphatidylcholine (DSPC), the major surface active component of surfactant, as compared to 46(+/- 1)% DSPC in lavage from control animals. The distribution of PL and DSPC in subfractions of lavage materials obtained by differential centrifugation is approximately reversed following hyperoxia. In lavage from control animals, 36% of the PL is in the heavier, more dense subfractions and 64% is in the lighter, less dense subfractions, while 72% is heavier and 28% lighter in lavage from O2-exposed animals. Measurements of surface properties with the Wilhelmy balance indicate that the ability of the lavage materials to reduce surface tension is impaired following hyperoxia. Thus, lethal exposures of rats to oxygen lead to increased amounts of surfactant on the alveolar surface, but the surface properties of the surfactant are impaired, probably due to reduced levels of DSPC, increased amounts of protein, and alterations in its physical form.

Oxidative stress causes a protein kinase C-independent increase of paracellular permeability in an in vitro epithelial model

To evaluate the response of an epithelial barrier to a moderate but sustained oxidative stress, we cultured monolayers of Madin Darby canine kidney cells on microporous filters and exposed them to the hypoxanthine-xanthine oxidase (HX-XO) reaction. The transepithelial permeability coefficient for mannitol (Pm) was assessed as a marker of paracellular permeability. When the oxidative stress was limited in intensity and duration (production of 10 nmol/ml/min O2- with generation of 467 +/- 30 nmol/ml H2O2 over 1 h), we observed an increase of Pm with a delay of several hours (324 +/- 65% of baseline by 6 h, P < 0.005). There was complete return to control values by 24 h. The increase of Pm did not appear to be related to a depletion of cellular ATP. Protein kinase C (PKC) activity did not increase, and the rise in Pm was not prevented by CGP 41,251, a specific inhibitor of PKC. By contrast, CGP 41,251 inhibited the Pm increase that was elicited by PDBU, a phorbol ester that activates PKC. In our model, we conclude that a reversible increase of paracellular permeability occurs after oxidative stress independently of ATP depletion or PKC activation. Other, as yet unknown mechanisms have to be involved in this process.

Experimental aspiration trauma: comparison of steroid treatment versus exogenous natural surfactant

The therapeutic efficacy of dexamethasone and a natural pig surfactant preparation was investigated in a rabbit aspiration model. Lung injury was induced by intratracheal administration of 2 mL of a betaine hydrochloride-pepsin mixture per kilogram body weight (BW). Dexamethasone (n = 34) was given intravenously in two doses (D1 = 7.5 mg/kg BW; D2 = 3.75 mg/kg BW; D2 6 h post D1). In different groups D1 was injected at different times (T = -2, 0, +2, and +24 h) before and after aspiration. Natural surfactant (n = 5) was administered 24 h post lung injury in a single dose of 12 mg phospholipids per kilogram body weight. The therapeutic potential was evaluated by measuring static lung compliance and the difference in a lung volume between 0 and 20 mm Hg airway pressure. No therapeutic effect of dexamethasone was seen at any time of application. In contrast, the intratracheal administration of natural surfactant 24 h post injury completely reversed the deterioration of lung mechanic properties.

Hyperoxic lung injury in Fischer-344 and Sprague-Dawley rats in vivo

Supplemental oxygen remains an important therapy for pulmonary insufficiency, despite the potential adverse effects of hyperoxic exposures. Recently, He et al. reported that hyperoxic ventilation more readily damaged isolated perfused lungs from Fischer-344 rats than from Sprague-Dawley rats (Am. J. Physiol. 259:L451), which correlates with the previously reported strain differences in hepatic responses to diquat-induced oxidant stress in vivo (J. Pharmacol. Exp. Ther. 235:172). We therefore examined the differences in hyperoxic lung injury in Fischer-344 and Sprague-Dawley rats in vivo. Adult male rats were exposed to > 95% O2 and were sacrificed after 24, 48, or 60 h. Control animals were maintained in room air. Dramatically greater increases in pleural effusions and bronchoalveolar lavage protein concentrations in response to hyperoxia were observed in the Fischer-344 rats than in the Sprague-Dawley rats (p < .05 at both 48 and 60 h for both measurements). Additionally, the glutathione concentrations in alveolar lining fluid decreased from 800 microM to 115 microM in Fischer-344 rats after 60 h of > 95% O2, but did not change in Sprague-Dawley rats. We conclude that the greater susceptibility of Fischer-344 than of Sprague-Dawley rats to hyperoxic lung injury in vitro reported previously also is observed in vivo and that this strain difference offers unique opportunities to study mechanisms of hyperoxic lung injury.

Immunocytochemical and functional characterization of Na+ conductance in adult alveolar pneumocytes

The purpose of this study was to document the existence, assess the spatial localization, and characterize some of the transport properties of proteins antigenically related to epithelial Na+ channels in freshly isolated rabbit and rat alveolar type II (ATII) cells. ATII cells, isolated by elastase digestion of lung tissue and purified by density-gradient centrifugation, were incubated with polyclonal antibodies raised against Na+ channel protein purified from beef kidney papilla (NaAb), followed by a secondary antibody (goat antirabbit immunoglobulin G conjugated to fluorescein isothiocyanate). Rat ATII cells exhibited specific staining with NaAb at the level of the plasma membrane, which, in most cells, colocalized with that of the lectin Maclura pomiferra agglutinin, an apical surface marker. In Western blots, NaAb specifically recognized a 135 +/- 10-kDa protein in rat ATII membrane vesicles. When patch clamped in the whole cell mode using symmetrical solutions (150 mM Na+ glutamate), ATII cells exhibited outwardly rectified Na+ currents that were diminished by amiloride (10-100 microM) instilled into the bath solution. Ion substitution studies showed that the conductive pathways were three times more permeable to Na+ than K+. Amiloride, benzamil, and 5-(N-ethyl-N-isopropyl)-2',4'-amiloride were equally effective in diminishing 22Na+ flux into rabbit and rat ATII cells (45% inhibition at 100 microM, with IC50 of approximately 1 microM for all inhibitors). Tetraethylammonium chloride (10 mM) or BaCl2 (2 mM), well-known K+ channel blockers, had no effect on 22Na+ uptake. These results indicate that ATII cells express an amiloride-sensitive Na+ conductance, probably a channel, with a lower affinity for amiloride and its structural analogues than the well-established amiloride-sensitive Na+ channels found in bovine renal papila and cultured amphibian A6 kidney cells.

Differential accumulation of surfactant protein A, B, and C mRNAs in two epithelial cell types of hyperoxic lung

The physiologic response of the lung to oxygen toxicity is complex, and similar among all mammals studied. Acute exposure to 100% O2 results in severe decreases in respiratory function and is accompanied by alterations in pulmonary surfactant metabolism, including the regulation of surfactant proteins A, B, and C (SP-A, SP-B, SP-C). Because surfactant proteins and their mRNAs can be expressed in alveolar epithelial type II cells, and nonciliated bronchial epithelial (Clara) cells, we were interested in determining if alterations in the abundance of SP-A, SP-B, and SP-C mRNAs occurred differentially in these two cell types during hyperoxic lung injury. Using quantitative in situ hybridization, we found that hyperoxic lung injury resulted in nearly 20-fold increases in SP-A and SP-B mRNAs in Clara cells, with relatively small (2-fold or less) increases in type II cells. Immunohistochemical analysis suggested a commensurate increase in SP-A protein in Clara cells. SP-C mRNA was only detected in type II cells, and changed little in hyperoxic lung. Because Clara cells are not known to produce surfactant, and appear to lack SP-C mRNA, these observations suggest that increased SP-A and SP-B may serve nonsurfactant functions in hyperoxic lung.

Cell-specific alterations in expression of hyperoxia-induced mRNAs of lung

Hyperoxic lung injury is an unfortunate consequence of ventilatory oxygen therapy that is necessary to sustain life in certain clinical situations. The biochemical events that accompany hyperoxia of the lung, and the molecular mechanisms underlying these events, are incompletely understood. To better understand hyperoxic lung injury, our laboratory has cloned a set of genes corresponding to mRNAs that increase in abundance in the lungs of hyperoxic rabbits. In this report, we focus on three hyperoxia-induced cDNA clones, which encode surfactant apoprotein A (SP-A), the tissue inhibitor of metalloproteinases (TIMP), and metallothionein. In situ hybridizations and RNA dot blots of isolated lung cell populations indicate that the abundance of mRNA encoding all three proteins is increased by hyperoxia in specific cell types. SP-A mRNA increases in type II alveolar epithelial cells and in bronchiolar epithelial cells. TIMP mRNA increases in interstitial fibroblasts, in chondrocytes of the cartilage surrounding airways, and in endothelial cells of a specific subset of vessels, probably venules. Metallothionein transcripts also increase in chondrocytes and pulmonary fibroblasts. A comparison of the increase in these mRNAs during hyperoxic exposure in adults and newborns indicates that adults respond faster and to a greater extent than newborns and suggests that the rate and extent of these increases is correlated with the time course and severity of the injury.

Augmentation of superoxide dismutase and catalase activity in alveolar type II cells

This study tested whether adducts formed by covalent linkage of superoxide dismutase (SOD) or catalase to polyethylene glycol (PEG) could augment SOD and catalase activity in alveolar type II cells and document enhanced resistance to oxidant damage. Alveolar type II cells were isolated from adult, pathogen-free rats. Antioxidant enzymes were added to the medium of cell cultures in various concentrations for periods up to 48 h. Incubation with 500 to 3,000 U of PEG-SOD or 10,000 to 40,000 U of PEG-catalase/10(6) cells produced a dose-response-related increase in intracellular enzyme activity in comparison with controls (untreated or treated with SOD or catalase, inactivated PEG-SOD or PEG-catalase, or PEG alone). Uptake was maximal during the first 4 h. Using fluorescent label (fluorescein isothiocyanate) bound to PEG-catalase, we found intracellular localization of the labeled enzyme. Exposure to H2O2 led to reduced cytotoxicity in cells pretreated with PEG-catalase than in controls. We conclude that supplementation with PEG-SOD or PEG-catalase enhanced the activity of these enzymes in alveolar type II cells and increased their resistance to oxidant stress.

Rat lung alveolar type I epithelial cell injury and response to hyperoxia

Hyperoxia has been shown to cause extensive lung injury, which involves all components of the alveolar septum, although the type I epithelium has generally been reported to be resistant to significant injury. Electron microscopic morphometry was performed to define changes in volumes of subcellular components of alveolar epithelial cells in rats exposed to 85% O2 for 0, 7, and 14 d. Because of their large size, type I cells in control animals actually contain a greater volume of most of the organelles involved in cell metabolism than do type II cells. Hyperoxic exposure causes a dramatic change in the subcellular composition of the average type I cell, suggesting significant injury and/or response. Injury was suggested by the finding that lysosomes plus peroxisomes increased 1,250% after 7 d in hyperoxia and remained elevated by 200% after 14 d of exposure. Volumes of mitochondria, rough endoplasmic reticulum, smooth endoplasmic reticulum, and Golgi apparatus increased by 100%, 51%, 91%, and 500%, respectively, after hyperoxia. Qualitative analysis showed an altered, ruffled air border with focal areas of cytoplasmic translucency (suggesting injury) and focal areas of subcellular hypertrophy. Exposure to hyperoxia was associated with more organelles being found in peripheral or attenuated portions of type I alveolar cells. Since the increase in type I organelles exceeds the volume of these organelles in its progenitor, the type II cell, it is likely that hyperoxia causes hypertrophy of the type I alveolar epithelium itself, independent of simple type II cell differentiation. Because of the large size and wide distribution of the type I cell, dramatic shifts in cell substructure caused by hyperoxia are more difficult to detect and require quantitative analysis to fully ascertain the extent of cell alterations.

Pathophysiology of neonatal lung injury

Respiratory distress in newborn and young infants often develops as a result of acute lung injury, in which disruption of the normal barrier function of the pulmonary endothelium and epithelium causes protein-rich interstitial and alveolar edema. Several conditions may initiate acute lung injury, including aspiration of meconium or gastric contents, bacterial or viral infection, overzealous resuscitation, and birth associated with incomplete lung development that requires ventilatory support with positivepressure mechanical ventilation and high concentrations of inspired oxygen. The latter condition usually occurs after premature birth, but it also may occur as a consequence of impaired fetal lung growth secondary to diaphragmatic hernia or chest compression from lack of liquid in the amniotic cavity. Acute lung injury sometimes progresses to a chronic form of lung disease, which is characterized by edema, fibrosis, airway distortion, and nonuniform inflation of the lungs.

Pulmonary clearance of 99mTc-DTPA during halothane anaesthesia

We studied the integrity of the alveolo-capillary barrier during different forms of anaesthesia by measuring the pulmonary clearance of inhaled 99mTc-DTPA. We studied four groups of rabbits. Groups I and II were anaesthetized with nembuthal only and the fractional concentration of inspired oxygen (F1O2) was 0.30 and 1.00, respectively. Groups III and IV were anaesthetized with 1% halothane and F1O2 was 0.30 and 0.99, respectively. 99mTc-DTPA was administered as a fine aerosol and the clearance of the tracer from the lungs was subsequently measured with a gamma camera. The mean half-life of the tracer in the lungs in Groups I-IV was 60, 58, 59 and 26 min, respectively. The rapid pulmonary clearance of 99mTc-DTPA in Group IV indicates that halothane in combination with high oxygen concentration increases the permeability of the alveolo-capillary barrier. This may be due to effects on the pulmonary surfactant system and/or the alveolar epithelium.

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.

Mitigation of pulmonary oxygen toxicity in premature lambs with intravenous antioxidants

Deficiencies of antioxidants and increased free radical generation may explain the high incidence of bronchopulmonary dysplasia in premature infants. Long-acting antioxidants such as polyethylene glycol (PEG) conjugated superoxide dismutase (SOD), and catalase might modify this process. We delivered 32 premature lambs, 16 pairs of twins, by cesarean section at 125-141 days of gestation (term 146 days) and stabilized them on ventilators in normocapnic hyperoxia for a period of 8 h. One lamb of each twin pair received an intravenous dose of 7,500-50,000 IU/kg of PEG-SOD and of 37,500-1,000,000 IU/kg of PEG-catalase at birth. Their siblings acted as controls. Mean airway pressure, arterial pressure, and heart rate were recorded continuously. Arterial blood gases and pH were obtained every 30 min. After sacrifice, standardized lung biopsies were prepared for quantitative morphometrics and electron microscopy. Administration of PEG antioxidants at birth reduced the influx of neutrophils and macrophages into the lung and damage to arterioles, bronchiolar mucosa, and type II pneumocytes without major changes in alveolar surface area or pulmonary function. These effects were dose-related and detectable even at the lowest doses of PEG antioxidants administered.

Lung management during cardiopulmonary bypass: influence on extravascular lung water

Progressive respiratory insufficiency secondary to cardiopulmonary bypass (CPB) is still a hazard after cardiac surgery. Pathophysiologically, impaired capillary endothelial integrity seems to be the fundamental lesion, followed by increased interstitial fluid accumulation. The reasons for this pulmonary damage are controversial; however, management of the nonperfused lungs during CPB has been widely neglected and may be partly responsible. In this study, 90 patients undergoing coronary artery bypass grafting were randomly divided into six groups (15 patients each) with different management of the lungs during CPB: group 1, lungs collapsed (0/0); group 2, static inflation with +5 cm H2O and F1O2 1.0 (+5/1.0); group 3, static inflation with +5 cm H2O and F1O2 0.21 (+5/0.21); group 4, static inflation with +15 cm H2O and F1O2 1.0 (+15/1.0); group 5, static inflation with +15 cm H2O and F1O2 0.21 (+15/0.21); and group 6, controlled mechanical ventilation as before start of CPB (positive end-expiratory pressure [PEEP] +5 cm H2O; F1O2 1.0) (ventilation). In addition to hemodynamic monitoring, extravascular lung water (EVLW) was measured by means of a double-indicator dilution technique with heat and indocyanine green. Measurements were performed after induction of anesthesia, before onset of CPB, and immediately after weaning from bypass, as well as 60 minutes and 5 hours after termination of CPB. Pulmonary gas exchange (PaO2) and intrapulmonary shunting (Qs/Qt) were also measured. Starting from comparable, normal baseline values, EVLW was increased in all groups after weaning from CPB, with the most pronounced increase in group 4 (maximum, +35%) and group 5 (+40%).(ABSTRACT TRUNCATED AT 250 WORDS)

Serum markers for prediction of pulmonary radiation syndromes. Part I: Surfactant apoprotein

Detection of a biochemical marker indicating radiation lung injury prior to the onset of clinical pathologic events could prove valuable in patient management. An increased level of alveolar surfactant is one of the earliest detectable changes following lung irradiation, starting within hours of irradiation and persisting a maximum of 2-6 weeks. However, because broncho-alveolar lavage is impracticable and endothelial cell damage due to radiation results in changes in permeability of vessel wall with leakage of alveolar proteins into serum, identification of serum markers was sought. A series of experiments in rabbits are described that clearly demonstrate serum surfactant apoprotein is an accurate marker and predictor for later lethal radiation pneumonitis. At 3-7 days after graded single doses to lung, surfactant was found in the serum paralleling the dose response for lethality. Control studies with a physiologic agent such as terbutaline release alveolar surfactant, but no serum surfactant was detected. Monitoring serum surfactant could direct preventive intervention prior to clinicopathologic manifestation of pulmonary radiation syndromes.

Oxidant inhibition of epithelial active sodium transport

The purpose of this study was to quantify the effects of extracellularly generated partially reduced oxygen species on active sodium (Na+) transport across the ventral toad skin, a well-studied epithelium. Sections of skin from decapitated toads were mounted in an Ussing chamber, bathed on both sides with electrolyte solution containing 500 microM xanthine and bubbled continuously with room air. The tissues were short-circuited, and short-circuit current (Isc) and tissue resistance (Rt) were monitored continuously with an automatic voltage clamp apparatus. Fifteen mU/ml of xanthine oxidase (XO), either purchased from Calbiochem or purified from cream, were instilled in either the apical (mucosal) or basolateral (serosal) baths at t = 0 and t = 10 min. Hydrogen peroxide (H2O2) concentrations increased to 200 microM within the first 20 min and then decreased, reaching a value of 40 microM by 60 min. Mean [H2O2] was 90 microM. Instillation of XO in the apical bath resulted in a large decrease in Isc and an increase in Rt, their values being 43% and 160% of their corresponding controls 85 min after the first instillation. Addition of superoxide dismutase and catalase completely prevented these changes. Instillation of XO in the basolateral bath had no effect. Similar physiological responses were obtained using the Calbiochem XO or the purified XO, which contained no measurable protease activity. It was concluded that extracellularly generated partially reduced oxygen species may interfere with active Na+ transport by possibly damaging apical Na+ channel proteins.

Sublethal hyperoxic injury to the alveolar epithelium and the pulmonary surfactant system

We quantified the effects of continuous exposure to 100% O2 on the development of sublethal injury to the pulmonary alveolar epithelium of rabbits. There was a progressive increase in alveolar permeability to solute after 48 h in O2, which coincided with the onset of damage to the pulmonary microvasculature. Rabbits that were exposed to 100% O2 for 64 h and returned to room air for 24 h had, in addition to increased permeability to solute, decreased phospholipid levels, decreased total lung capacity, pulmonary edema, high minimum surface tensions in their bronchoalveolar lavage, and moderate hypoxemia. Intratracheal instillation of calf lung surfactant (CLSE) significantly ameliorated the progression of hyperoxic injury by increasing alveolar phospholipid levels and thus preventing the inhibition of lung surfactant activity by plasma proteins and other high molecular weight components of alveolar edema. We concluded that the alveolar epithelium and the pulmonary microvasculature show similar sensitivity to hyperoxia and that clinical manifestations of hyperoxic lung injury may be due, at least in part, to surfactant dysfunction.

Failure of aerosolised 99mTc DTPA clearance to predict outcome in patients with adult respiratory distress syndrome

The rate of clearance of technetium-99m labelled diethylene triamine pentacetic acid (99mTc DTPA) was measured in 32 patients with adult respiratory distress syndrome to determine if a more rapid clearance rate, possibly reflecting a more severe abnormality of pulmonary function, was associated with a reduced likelihood of recovery from pulmonary failure. Although the mean rate of clearance from lung to blood (T1/2LB) of 99mTc DTPA was more rapid in the patients (T1/2LB = 29 (SEM 3.2) min than in 42 normal subjects (T1/2LB = 59 (1.8)min), there was no difference between the clearance rate in the 18 patients who recovered from respiratory failure (T1/2LB = 31 (5) min) and the 14 who died (T1/2LB = 27 (4) min). Additionally, not all patients studied had abnormally rapid clearance rates. In 12 of the 32 patients the T1/2 fell within the range for normal individuals; this was found more commonly in patients who were predisposed to develop adult respiratory distress syndrome by pancreatitis or massive blood transfusion. These data suggest that a single measurement of 99mTc DTPA clearance in patients with established respiratory failure and adult respiratory distress syndrome is of no value in assessing the likelihood of recovery from this condition.

Effects of 100% oxygen breathing on the capillary filtration coefficient in rabbit lungs

Prolonged exposure to 100% O2 at 1 atm is known to result in a progressive increase of the alveolar epithelial permeability to lipid-insoluble molecules. To investigate whether the damage to the capillary endothelium precedes or follows this event, conscious, unanesthetized rabbits were exposed to 100% O2 from 24 to 66 hr, and (a) the filtration coefficient (Kf) of the pulmonary capillary endothelium in isolated, perfused lungs and (b) the arterial and carbon dioxide gas tensions and right and left heart vascular pressures were measured in intact animals. The mean value of the filtration coefficient (+/- SEM) in air-breathing animals was 0.036 +/- 0.002 ml/(min x Torr x g dry lung). After 48 and 66 hr in 100% O2, it increased by 58 and 114% from its baseline value, respectively. At the later period the lung wet/dry weight of the isolated, but not the intact lungs, increased also from 5.42 +/- .2 to 7.3 +/- .3 (means +/- 1 SEM) due to the combination of a higher capillary conductance and the lack of lymph flow in this preparation. All other variables remained normal throughout the exposure. Thus, in contrast to previous morphological findings, these results indicate that the oxygen damage to the capillary endothelium is progressive and occurs concurrently with the increase of the alveolar permeability to solute but before the appearance of pulmonary edema and the compromise of gas exchange.

Pulmonary oxygen toxicity. Early reversible changes in human alveolar structures induced by hyperoxia

To study the early changes in the lower respiratory tract in persons exposed to periods of hyperoxia usually considered safe, we evaluated 14 normal subjects by bronchoalveolar lavage before and immediately after 16.7 +/- 1.1 hours of breathing more than 95 per cent oxygen. Hyperoxia caused a significant alveolar-capillary "leak" as detected by the presence of increased plasma albumin and transferrin in lavage fluid. These changes were reversible, as shown at repeat lavage in four subjects two weeks after oxygen administration. Hyperoxia for an average of 17 hours did not change the total number or type of lung inflammatory and immune effector cells recovered by lavage (P greater than 0.05, all comparisons). However, alveolar macrophages from subjects exposed to oxygen released increased amounts of fibronectin (P less than 0.05) and alveolar-macrophage--derived growth factor for fibroblasts (P less than 0.01)--mediators thought to modulate fibroblast recruitment and proliferation in the alveolar wall. Thus, although some of the effects of exposure to 17 hours of more than 95 per cent oxygen are reversible, hyperoxia for even this short period lowers the structural or functional barriers that normally prevent alveolar-capillary "leak" and induces processes that can culminate in fibrosis of the alveolar wall.