Invited review: pulmonary capillary stress failure.

The pulmonary blood-gas barrier is an extraordinary bioengineering structure because of its vast area but extreme thinness. Despite this, almost no attention has been given to its mechanical properties. The remarkable area and thinness come about because gas exchange occurs by passive diffusion. However, the barrier also needs to be immensely strong to withstand the very high stresses in the capillary wall when capillary pressure rises during exercise. The strength of the thin region of the barrier comes from type IV collagen in the basement membranes. When the stresses in the capillary walls rise to high levels, ultrastructural changes occur in the barrier, a condition known as stress failure. Physiological conditions that alter the properties of the barrier include severe exercise in elite human athletes. Animals that have been selectively bred for high aerobic activity, such as Thoroughbred racehorses, consistently break their pulmonary capillaries during galloping. Pathophysiological conditions causing stress failure include high-altitude pulmonary edema and overinflation of the lung, which frequently occurs with mechanical ventilation. Remodeling of the capillary wall occurs in response to increased wall stress in diseases such as mitral stenosis. The barrier is able to maintain its extreme thickness with sufficient strength as a result of continual regulation of its wall structure. How it does this is a central problem in lung biology.     

Thoughts on the pulmonary blood-gas barrier

The pulmonary blood-gas barrier is an extraordinary structure because of its extreme thinness, immense strength, and enormous area. The essential components of the barrier were determined early in evolution and have been highly conserved. For example, the barriers of the African, Australian, and South American lungfish that date from as much as 400 million years ago have essentially the same structure as in the modern mammal or bird. In the evolution of vertebrates from bony fishes through amphibia, reptiles, and ultimately mammals and birds, changes in the pulmonary circulation occurred to limit the stresses in the blood-gas barrier. Only in mammals and birds is there a complete separation of the pulmonary and systemic circulations, which is essential to protect the extremely thin barrier from the necessary high-vascular pressures. To provide the blood-gas barrier with its required strength, evolution has exploited the high ultimate tensile strength of type IV collagen in basement membrane. Nevertheless, stress failure of the barrier occurs under physiological conditions in galloping Thoroughbred racehorses and also apparently in elite human athletes at maximal exercise. The human blood-gas barrier maintains its integrity during all but the most extreme physiological conditions. However, many pathological conditions cause stress failure. The structure of the blood-gas barrier is apparently continually regulated in response to wall stress, and this regulation is essential to maintain the extreme thinness but adequate strength. The mechanisms of this regulation remain to be elucidated and constitute one of the fundamental problems in lung biology.     

Sustained submaximal exercise does not alter the integrity of the lung blood-gas barrier in elite athletes

The extreme thinness of the pulmonary blood-gasbarrier results in high mechanical stresses in the capillary wall whenthe capillary pressure rises during exercise. We have previously shown that, in elite cyclists, 6-8 min of maximal exercise increase blood-gas barrier permeability and result in higher concentrations ofred blood cells, total protein, and leukotrieneB₄ in bronchoalveolar lavage (BAL)fluid compared with results in sedentary controls. To test thehypothesis that stress failure of the barrier only occurs at thehighest level of exercise, we performed BAL in six healthy athletesafter 1 h of exercise at 77% of maximalO₂ consumption. Controls wereeight normal nonathletes who did not exercise before BAL. In contrastwith our previous study, we did not find higher concentrations of redblood cells, total protein, and leukotriene B₄ in the exercising athletescompared with control subjects. However, higher concentrations ofsurfactant apoprotein A and a higher surfactant apoproteinA-to-phospholipid ratio were observed in the athletes performingprolonged exercise, compared with both the controls and the athletesfrom our previous study. These results suggest that, in elite athletes,the integrity of the blood-gas barrier is altered only at extremelevels of exercise.

     

Stress failure in pulmonary capillaries

In the mammalian lung, alveolar gas and blood are separated by an extremely thin membrane, despite the fact that mechanical failure could be catastrophic for gas exchange. We raised the pulmonary capillary pressure in anesthetized rabbits until stress failure occurred. At capillary transmural pressures greater than or equal to 40 mmHg, disruption of the capillary endothelium and alveolar epithelium was seen in some locations. The three principal forces acting on the capillary wall were analyzed. 1) Circumferential wall tension caused by the transmural pressure. This is approximately 25 dyn/cm (25 mN/m) at failure where the radius of curvature of the capillary is 5 microns. This tension is small, being comparable with the tension in the alveolar wall associated with lung elastic recoil. 2) Surface tension of the alveolar lining layer. This contributes support to the capillaries that bulge into the alveolar spaces at these high pressures. When protein leakage into the alveolar spaces occurs because of stress failure, the increase in surface tension caused by surfactant inhibition could be a powerful force preventing further failure. 3) Tension of the tissue elements in the alveolar wall associated with lung inflation. This may be negligible at normal lung volumes but considerable at high volumes. Whereas circumferential wall tension is low, capillary wall stress at failure is very high at approximately 8 x 10(5) dyn/cm2 (8 x 10(4) N/m2) where the thickness is only 0.3 microns. This is approximately the same as the wall stress of the normal aorta, which is predominantly composed of collagen and elastin. The strength of the thin part of the capillary wall is probably attributable to the collagen IV of the basement membranes. The safety factor is apparently small when the capillary pressure is raised during heavy exercise. Stress failure causes increased permeability with protein leakage, or frank hemorrhage, and probably has a role in several types of lung disease.     

Effects of altitude and exercise on pulmonary capillary integrity: evidence for subclinical high-altitude pulmonary edema.

Strenuous exercise may be a significant contributing factor for development of high-altitude pulmonary edema, particularly at low or moderate altitudes. Thus we investigated the effects of heavy cycle ergometer exercise (90% maximal effort) under hypoxic conditions in which the combined effects of a marked increase in pulmonary blood flow and nonuniform hypoxic pulmonary vasoconstriction could add significantly to augment the mechanical stress on the pulmonary microcirculation. We postulated that intense exercise at altitude would result in an augmented permeability edema. We recruited eight endurance athletes and examined their bronchoalveolar lavage fluid (BALF) for red blood cells (RBCs), protein, inflammatory cells, and soluble mediators at 2 and 26 h after intense exercise under normoxic and hypoxic conditions. After heavy exercise, under all conditions, the athletes developed a permeability edema with high BALF RBC and protein concentrations in the absence of inflammation. We found that exercise at altitude (3,810 m) caused significantly greater leakage of RBCs [9.2 (SD 3.1)x10(4) cells/ml] into the alveolar space than that seen with normoxic exercise [5.4 (SD 1.2)x10(4) cells/ml]. At altitude, the 26-h postexercise BALF revealed significantly higher RBC and protein concentrations, suggesting an ongoing capillary leak. Interestingly, the BALF profiles following exercise at altitude are similar to that of early high-altitude pulmonary edema. These findings suggest that pulmonary capillary disruption occurs with intense exercise in healthy humans and that hypoxia augments the mechanical stresses on the pulmonary microcirculation.     

Assessment of inhibited alveolar‐capillary membrane structural development and function in bronchopulmonary dysplasia

Bronchopulmonary dysplasia (BPD) is a chronic lung disease of extreme prematurity and is defined clinically by dependence on supplemental oxygen due to impaired gas exchange. Optimal gas exchange is dependent on the development of a sufficient surface area for diffusion. In the mammalian lung, rapid acquisition of distal lung surface area is accomplished in neonatal and early adult life by means of vascularization and secondary septation of distal lung airspaces. Extreme preterm birth interrupts secondary septation and pulmonary capillary development and ultimately reduces the efficiency of the alveolar‐capillary membrane. Although pulmonary health in BPD infants rapidly improves over the first few years, persistent alveolar‐capillary membrane dysfunction continues into adolescence and adulthood. Preventative therapies have been largely ineffective, and therapies aimed at promoting normal development of the air‐blood barrier in infants with established BPD remain largely unexplored. The purpose of this review will be: (1) to summarize the histological evidence of aberrant alveolar‐capillary membrane development associated with extreme preterm birth and BPD, (2) to review the clinical evidence assessing the long‐term impact of BPD on alveolar‐capillary membrane function, and (3) to discuss the need to develop and incorporate direct measurements of functional gas exchange into clinically relevant animal models of inhibited alveolar development. Birth Defects Research (Part A) 100:168–179, 2014. © 2014 Wiley Periodicals, Inc.     

Pulmonary edema: ischemia reperfusion endothelial injury and its reversal by c-AMP.

A review of the factors that oppose pulmonary edema formation (alveolar flooding) when capillary pressure is elevated are presented for a normal capillary endothelial barrier and for damaged endothelium associated with ischemia/reperfusion in rabbit, rat, and dog lungs. Normally, tissue pressure, the plasma protein osmotic pressure gradient acting across the capillary wall and lymph flow (Edema Safety Factors) increase to prevent the build-up of fluid in the lung's interstitium when capillary pressure increases. No measureable alveolar edema fluid accumulates until capillary pressure exceeds 30 mmHg. When the capillary wall has been damaged, interstitial edema develops at lower capillary pressures because the plasma protein osmotic pressure will not change greatly to oppose capillary filtration, but lymph flow increases to very high levels to remove the increased filtrate and the result is that capillary pressures can increase to 20-25 mmHg before alveolar flooding results. In addition, the mechanisms responsible for producing pulmonary endothelial damage with ischemia/reperfusion are reviewed and the effects of O2 radical scavengers, neutrophil depletion or altering their adherence to the endothelium, and increasing cAMP on reversing the damage to the pulmonary endothelium is presented.     

In vivo porcine left atrial wall stress: Effect of ventricular tachypacing on spatial and temporal stress distribution

Animal models of ventricular tachypacing (VTP) have been successfully used to reproduce the relevant features observed in patients with atrial fibrillation, such as increased atrial pressure and volume, ion-channel alterations and fibrosis. After performing VTP on a healthy Yorkshire pig, we measured an increase in volume of 60%, a two-fold rise in pressure, and a complex pattern of local mechanical, histological and biochemical changes, including a generalized stiffening of the wall. A protocol recently developed was employed to generate computational models of the porcine left atrium mechanics in healthy conditions and after VTP. Comparison of the stress distribution in the healthy vs. VTP case provided a map of how pressure overload affects and modifies left atrium mechanics. Overall, a positive increase in stress was computed after the VTP treatment. Regions of large increase in the stresses post-VTP were the appendage boundaries, the area around the lower pulmonary vein and the area in the front of the atrium towards the appendage. Due to the elevated stress, the back of the atrium mainly modified its mechanical response, while the appendage remodeled both its shape and its mechanical properties. Large changes in the shape of the mitral valve annulus could be observed as a consequence of the remodeling in the front of the atrium. The relation between local mechanical stress and remodeling that emerges from the results is in agreement with our hypothesis that the structural changes in the atrium are a consequence of a stress-mediated mechanism.     

Physiological aspects of high-altitude pulmonary edema.

High-altitude pulmonary edema (HAPE) develops in rapidly ascending nonacclimatized healthy individuals at altitudes above 3,000 m. An excessive rise in pulmonary artery pressure (PAP) preceding edema formation is the crucial pathophysiological factor because drugs that lower PAP prevent HAPE. Measurements of nitric oxide (NO) in exhaled air, of nitrites and nitrates in bronchoalveolar lavage (BAL) fluid, and forearm NO-dependent endothelial function all point to a reduced NO availability in hypoxia as a major cause of the excessive hypoxic PAP rise in HAPE-susceptible individuals. Studies using right heart catheterization or BAL in incipient HAPE have demonstrated that edema is caused by an increased microvascular hydrostatic pressure in the presence of normal left atrial pressure, resulting in leakage of large-molecular-weight proteins and erythrocytes across the alveolarcapillary barrier in the absence of any evidence of inflammation. These studies confirm in humans that high capillary pressure induces a high-permeability-type lung edema in the absence of inflammation, a concept first introduced under the term "stress failure." Recent studies using microspheres in swine and magnetic resonance imaging in humans strongly support the concept and primacy of nonuniform hypoxic arteriolar vasoconstriction to explain how hypoxic pulmonary vasoconstriction occurring predominantly at the arteriolar level can cause leakage. This compelling but as yet unproven mechanism predicts that edema occurs in areas of high blood flow due to lesser vasoconstriction. The combination of high flow at higher pressure results in pressures, which exceed the structural and dynamic capacity of the alveolar capillary barrier to maintain normal alveolar fluid balance.     

Ventilator-induced lung injury: in vivo and in vitro mechanisms

A lung-protective ventilator strategy significantly reduces mortality in patients with acute lung injury. Substantial progress has been made in understanding how mechanical stress can injure the lung, both in terms of alterations in barrier properties of the pulmonary endothelium and epithelium as well as in stimulating proinflammatory responses of macrophages and neutrophils.     

A morphometric study of the lungs of different sized bats: correlations between structure and function of the chiropteran lung.

1. The lungs of four species of bats, Phyllostomus hastatus (PH, mean body mass, 98 g), Pteropus lylei (PL, 456 g), Pteropus alecto (PA, 667 g), and Pteropus poliocephalus (PP, 928 g) were analysed by morphometric methods. These data increase fivefold the range of body masses for which bat lung data are available, and allow more representative allometric equations to be formulated for bats. 2. Lung volume ranged from 4.9 cm3 for PH to 39 cm3 for PP. The volume density of the lung parenchyma (i.e. the volume proportion of the parenchyma in the lung) ranged from 94% in PP to 89% in PH. Of the components of the parenchyma, the alveoli composed 89% and the blood capillaries about 5%. 3. The surface area of the alveoli exceeded that of the blood-gas (tissue) barrier and that of the capillary endothelium whereas the surface area of the red blood cells as well as that of the capillary endothelium was greater than that of the tissue barrier. PH had the thinnest tissue barrier (0.1204 microns) and PP had the thickest (0.3033 microns). 4. The body mass specific volume of the lung, that of the volume of pulmonary capillary blood, the surface area of the blood-gas (tissue) barrier, the diffusing capacity of the tissue barrier, and the total morphometric pulmonary diffusing capacity in PH all substantially exceeded the corresponding values of the pteropid species (i.e. PL, PA and PP). This conforms with the smaller body mass and hence higher unit mass oxygen consumption of PH, a feature reflected in the functionally superior gas exchange performance of its lungs. 5. Morphometrically, the lungs of different species of bats exhibit remarkable differences which cannot always be correlated with body mass, mode of flight and phylogeny. Conclusive explanations of these pulmonary structural disparities in different species of bats must await additional physiological and flight biomechanical studies. 6. While the slope, the scaling factor (b), of the allometric equation fitted to bat lung volume data (b = 0.82) exceeds the value for flight VO2max (b = 0.70), those for the surface area of the blood-gas (tissue) barrier (b = 0.74), the pulmonary capillary blood volume (b = 0.74), and the total morphometric lung diffusing capacity for oxygen (b = 0.69) all correspond closely to the VO2max value. 7. Allometric comparisons of the morphometric pulmonary parameters of bats, birds and non-flying mammals reveal that superiority of the bat lung over that of the non-flying mammal.(ABSTRACT TRUNCATED AT 400 WORDS)     

The PDE inhibitor zaprinast enhances NO-mediated protection against vascular leakage in reperfused lungs

Disruption of endothelial barrier properties with development of noncardiogenic pulmonary edema is a major threat in lung ischemia-reperfusion (I/R) injury that occurs under conditions of lung transplantation. Inhaled nitric oxide (NO) reduced vascular leakage in lung I/R models, but the efficacy of this agent may be limited. We coadministered NO and zaprinast, a cGMP-specific phosphodiesterase inhibitor, to further augment the NO-cGMP axis. Isolated, buffer-perfused rabbit lungs were exposed to 4.5 h of warm ischemia. Reperfusion provoked a transient elevation in pulmonary arterial pressure and a negligible rise in microvascular pressure followed by a massive increase in the capillary filtration coefficient and severe lung edema formation. Inhalation of 10 parts/million of NO or intravascular application of 100 microM zaprinast on reperfusion both reduced pressor response and moderately attenuated vascular leakage. Combined administration of both agents induced no additional vasodilation at constant microvascular pressures, but additively protected against capillary leakage paralleled by a severalfold increase in perfusate cGMP levels. In conclusion, combining low-dose NO inhalation and phosphodiesterase inhibition may be suitable for the maintenance of graft function in lung transplantation by amplifying the beneficial effect of the NO-cGMP axis and avoiding toxic effects of high NO doses.     

High lung volume increases stress failure in pulmonary capillaries.

We previously showed that when pulmonary capillaries in anesthetized rabbits are exposed to a transmural pressure (Ptm) of approximately 40 mmHg, stress failure of the walls occurs with disruption of the capillary endothelium, alveolar epithelium, or sometimes all layers. The present study was designed to test whether stress failure occurred more frequently at high than at low lung volumes for the same Ptm. Lungs of anesthetized rabbits were inflated to a transpulmonary pressure of 20 cmH2O, perfused with autologous blood at 32.5 or 2.5 cmH2O Ptm, and fixed by intravascular perfusion. Samples were examined by both transmission and scanning electron microscopy. The results were compared with those of a previous study in which the lung was inflated to a transpulmonary pressure of 5 cmH2O. There was a large increase in the frequency of stress failure of the capillary walls at the higher lung volume. For example, at 32.5 cmH2O Ptm, the number of endothelial breaks per millimeter cell lining was 7.1 +/- 2.2 at the high lung volume compared with 0.7 +/- 0.4 at the low lung volume. The corresponding values for epithelium were 8.5 +/- 1.6 and 0.9 +/- 0.6. Both differences were significant (P less than 0.05). At 52.5 cmH2O Ptm, the results for endothelium were 20.7 +/- 7.6 (high volume) and 7.1 +/- 2.1 (low volume), and the corresponding results for epithelium were 32.8 +/- 11.9 and 11.4 +/- 3.7. At 32.5 cmH2O Ptm, the thickness of the blood-gas barrier was greater at the higher lung volume, consistent with the development of more interstitial edema. Ballooning of the epithelium caused by accumulation of edema fluid between the epithelial cell and its basement membrane was seen at 32.5 and 52.5 cmH2O Ptm. At high lung volume, the breaks tended to be narrower and fewer were oriented perpendicular to the axis of the pulmonary capillaries than at low lung volumes. Transmission and scanning electron microscopy measurements agreed well. Our findings provide a physiological mechanism for other studies showing increased capillary permeability at high states of lung inflation.     

More than a leak sealant. The mechanical properties of callose in pollen tubes

While callose is a well-known permeability barrier and leak sealant in plant cells, it is largely unknown whether this cell wall polymer can also serve as a load-bearing structure. Since callose occurs in exceptionally large amounts in pollen, we assessed its role for resisting tension and compression stress in this cell. The effect of callose digestion in Solanum chacoense and Lilium orientalis pollen grains demonstrated that, depending on the species, this cell wall polymer represents a major stress-bearing structure at the aperture area of germinating grains. In the pollen tube, it is involved in cell wall resistance to circumferential tension stress, and despite its absence at the growing apex, callose is indirectly involved in the establishment of tension stress resistance in this area. To investigate whether or not callose is able to provide mechanical resistance against compression stress, we subjected pollen tubes to local deformation by microindentation. The data revealed that lowering the amount of callose resulted in reduced cellular stiffness and increased viscoelasticity, thus indicating clearly that callose is able to resist compression stress. Whether this function is relevant for pollen tube mechanics, however, is unclear, as stiffened growth medium caused a decrease in callose deposition. Together, our data provide clear evidence for the capacity of cell wall callose to resist tension and compression stress, thus demonstrating that this amorphous cell wall substance can have a mechanical role in growing plant cells.     

Cytoskeletal regulation of pulmonary vascular permeability.

The endothelial cell (EC) lining of the pulmonary vasculature forms a semipermeable barrier between the blood and the interstitium of the lung. Disruption of this barrier occurs during inflammatory disease states such as acute lung injury and acute respiratory distress syndrome and results in the movement of fluid and macromolecules into the interstitium and pulmonary air spaces. These processes significantly contribute to the high morbidity and mortality of patients afflicted with acute lung injury. The critical importance of pulmonary vascular barrier function is shown by the balance between competing EC contractile forces, which generate centripetal tension, and adhesive cell-cell and cell-matrix tethering forces, which regulate cell shape. Both competing forces in this model are intimately linked through the endothelial cytoskeleton, a complex network of actin microfilaments, microtubules, and intermediate filaments, which combine to regulate shape change and transduce signals within and between EC. A key EC contractile event in several models of agonist-induced barrier dysfunction is the phosphorylation of regulatory myosin light chains catalyzed by Ca(2+)/calmodulin-dependent myosin light chain kinase and/or through the activity of the Rho/Rho kinase pathway. Intercellular contacts along the endothelial monolayer consist primarily of two types of complexes (adherens junctions and tight junctions), which link to the actin cytoskeleton to provide both mechanical stability and transduction of extracellular signals into the cell. Focal adhesions provide additional adhesive forces in barrier regulation by forming a critical bridge for bidirectional signal transduction between the actin cytoskeleton and the cell-matrix interface. Increasingly, the effects of mechanical forces such as shear stress and ventilator-induced stretch on EC barrier function are being recognized. The critical role of the endothelial cytoskeleton in integrating these multiple aspects of pulmonary vascular permeability provides a fertile area for the development of clinically important barrier-modulating therapies.     

An allometric study of pulmonary morphometric parameters in birds, with mammalian comparisons

Comprehensive pulmonary morphometric data from 42 species of birds representing ten orders were compared with those of other vertebrates, especially mammals, relating the comparisons to the varying biological needs of these avian taxa. The total lung volume was strongly correlated with body mass. The volume density of the exchange tissue was lowest in the charadriiform and anseriform species and highest in the piciform, cuculiform and passeriform species. The surface area of the blood-gas (tissue) barrier, the volume of the pulmonary capillary blood and the total morphometric pulmonary diffusing capacity were all strongly correlated with body mass. The harmonic mean thickness of both the blood-gas (tissue) barrier and the plasma layer were weakly correlated with body mass. The mass-specific surface area of the blood-gas (tissue) barrier (surface area per gram body mass) and the surface density of the blood-gas (tissue) barrier (i.e. its surface area per unit volume of exchange tissue) were inversely correlated (though weakly) with body mass. The passeriform species exhibited outstanding pulmonary morphometric adaptations leading to a high specific total diffusing capacity per gram body mass, consistent with the comparatively small size and energetic mode of life which typify passeriform birds. The relatively inactive, ground-dwelling domestic fowl (Gallus gallus) had the lowest pulmonary diffusing capacity per gram body mass. The specific total lung volume is about 27% smaller in birds than in mammals but the specific surface area of the blood-gas (tissue) barrier is about 15% greater in birds. The ratio of the surface area of the tissue barrier to the volume of the exchange tissue was also much greater in the birds (170-305%). The harmonic mean thickness of the tissue barrier was 56-67% less in the birds, but that of the plasma layer was about 66% greater in the birds. The pulmonary capillary blood volume was also greater (22%) in the birds. Except for the thickness of the plasma layer, these morphometric parameters all favour the gas exchange capacity of birds. Consequently, the total specific mean morphometric pulmonary diffusing capacity for oxygen was estimated to be about 22% greater in birds than in mammals of similar body mass. This estimate was obtained by employing oxygen permeation constants for mammalian tissue, plasma and erythrocytes, as avian constants were not then available.(ABSTRACT TRUNCATED AT 400 WORDS)     

Lung heparan sulfates modulate K(fc) during increased vascular pressure: evidence for glycocalyx-mediated mechanotransduction

Lung endothelial cells respond to changes in vascular pressure through mechanotransduction pathways that alter barrier function via non-Starling mechanism(s). Components of the endothelial glycocalyx have been shown to participate in mechanotransduction in vitro and in systemic vessels, but the glycocalyx's role in mechanosensing and pulmonary barrier function has not been characterized. Mechanotransduction pathways may represent novel targets for therapeutic intervention during states of elevated pulmonary pressure such as acute heart failure, fluid overload, and mechanical ventilation. Our objective was to assess the effects of increasing vascular pressure on whole lung filtration coefficient (K(fc)) and characterize the role of endothelial heparan sulfates in mediating mechanotransduction and associated increases in K(fc). Isolated perfused rat lung preparation was used to measure K(fc) in response to changes in vascular pressure in combination with superimposed changes in airway pressure. The roles of heparan sulfates, nitric oxide, and reactive oxygen species were investigated. Increases in capillary pressure altered K(fc) in a nonlinear relationship, suggesting non-Starling mechanism(s). nitro-l-arginine methyl ester and heparanase III attenuated the effects of increased capillary pressure on K(fc), demonstrating active mechanotransduction leading to barrier dysfunction. The nitric oxide (NO) donor S-nitrosoglutathione exacerbated pressure-mediated increase in K(fc). Ventilation strategies altered lung NO concentration and the K(fc) response to increases in vascular pressure. This is the first study to demonstrate a role for the glycocalyx in whole lung mechanotransduction and has important implications in understanding the regulation of vascular permeability in the context of vascular pressure, fluid status, and ventilation strategies.     

Mitogen Activated Protein Kinase Activated Protein Kinase 2 Regulates Actin Polymerization and Vascular Leak in Ventilator Associated Lung Injury

Mechanical ventilation, a fundamental therapy for acute lung injury, worsens pulmonary vascular permeability by exacting mechanical stress on various components of the respiratory system causing ventilator associated lung injury. We postulated that MK2 activation via p38 MAP kinase induced HSP25 phosphorylation, in response to mechanical stress, leading to actin stress fiber formation and endothelial barrier dysfunction. We sought to determine the role of p38 MAP kinase and its downstream effector MK2 on HSP25 phosphorylation and actin stress fiber formation in ventilator associated lung injury. Wild type and MK2^−/− mice received mechanical ventilation with high (20 ml/kg) or low (7 ml/kg) tidal volumes up to 4 hrs, after which lungs were harvested for immunohistochemistry, immunoblotting and lung permeability assays. High tidal volume mechanical ventilation resulted in significant phosphorylation of p38 MAP kinase, MK2, HSP25, actin polymerization, and an increase in pulmonary vascular permeability in wild type mice as compared to spontaneous breathing or low tidal volume mechanical ventilation. However, pretreatment of wild type mice with specific p38 MAP kinase or MK2 inhibitors abrogated HSP25 phosphorylation and actin polymerization, and protected against increased lung permeability. Finally, MK2^−/− mice were unable to phosphorylate HSP25 or increase actin polymerization from baseline, and were resistant to increases in lung permeability in response to HV_T MV. Our results suggest that p38 MAP kinase and its downstream effector MK2 mediate lung permeability in ventilator associated lung injury by regulating HSP25 phosphorylation and actin cytoskeletal remodeling.     

Physiological and structural changes of the lung tissue in male albino rat exposed to immobilization stress

In case of a life-threatening, stressful event, the body prepares for an emergency. Indeed, the lung is unique in which alveolar cells are constantly exposed to physical and chemical stresses. This study aimed to study the impact of immobilization stress on the blood–air barrier and how it initiate and maintain an inflammatory response, plus determining the resolution of lung inflammation and repair. There was a significant increase in the plasma levels of stress markers “corticosterone and catecholamines” with a decrease in surfactant protein A (a lung-injury marker). Chronic stress produced a significant increase in the pulmonary oxidative and inflammatory markers malondialdehyde, tumor necrosis factor α, and induced nitric oxide synthase when compared with that of acute stress. Both stresses provoked marked pulmonary morphological and ultrastructural changes with a significant increase in caspase-3 immunoexpression. There was increasing evidence of lung’s capacity for repair. This process involved edema resolution, cell proliferation, and tissue remodeling in improving the lung-injury, oxidative, and inflammatory markers.     

Topography of pleural epithelial structure enabled by en face isolation and machine learning

Pleural epithelial adaptations to mechanical stress are relevant to both normal lung function and parenchymal lung diseases. Assessing regional differences in mechanical stress, however, has been complicated by the nonlinear stress–strain properties of the lung and the large displacements with ventilation. Moreover, there is no reliable method of isolating pleural epithelium for structural studies. To define the topographic variation in pleural structure, we developed a method of en face harvest of murine pleural epithelium. Silver-stain was used to highlight cell borders and facilitate imaging with light microscopy. Machine learning and watershed segmentation were used to define the cell area and cell perimeter of the isolated pleural epithelial cells. In the deflated lung at residual volume, the pleural epithelial cells were significantly larger in the apex (624 ± 247 μm²) than in basilar regions of the lung (471 ± 119 μm²) (p < 0.001). The distortion of apical epithelial cells was consistent with a vertical gradient of pleural pressures. To assess epithelial changes with inflation, the pleura was studied at total lung capacity. The average epithelial cell area increased 57% and the average perimeter increased 27% between residual volume and total lung capacity. The increase in lung volume was less than half the percent change predicted by uniform or isotropic expansion of the lung. We conclude that the structured analysis of pleural epithelial cells complements studies of pulmonary microstructure and provides useful insights into the regional distribution of mechanical stresses in the lung.