Keratinocyte growth factor‐2 intratracheal instillation significantly attenuates ventilator‐induced lung injury in rats

Preservation or restoration of normal alveolar epithelial barrier function is crucial for pulmonary oedema resolution. Keratinocyte growth factor‐2 (KGF‐2), a potent epithelial cell mitogen, may have a role in preventing ventilator‐induced lung injury (VILI), which occurs frequently in mechanically ventilated patients. The aim of the study was to test the role of KGF‐2 in VILI in rats. Forty healthy adult male Sprague‐Dawley rats were randomly allocated into four groups, where rats in Groups HVZP (high‐volume zero positive end‐expiratory pressure) and HVZP+KGF‐2 were given intratracheally equal PBS and 5 mg/kg KGF‐2 72 hrs before 4 hrs HVZP ventilation (20 ml/kg), respectively, while PBS and KGF‐2 were administered in the same manner in Groups Control and KGF‐2, which underwent tracheotomy only with spontaneous breathing. Inflammatory cytokines (tumour necrosis factor‐α, macrophage inflammatory protein 2), neutrophil and total protein levels in bronchoalveolar lavage fluid and surfactant protein mRNA expression in lung tissue were detected; the number of alveolar type II cells, lung water content and lung morphology were also evaluated. The results indicate that pre‐treatment with KGF‐2 showed dramatic improvement in lung oedema and inflammation compared with HVZP alone, together with increased surfactant protein mRNA and alveolar type II cells. Our results suggest that KGF‐2 might be considered a promising prevention for human VILI or other acute lung injury diseases.     

Keratinocyte growth factor attenuates hydrostatic pulmonary edema in an isolated, perfused rat lung model

Hydrostatic pulmonary edema is a common complication of congestive heart failure, resulting in substantial morbidity and mortality. Keratinocyte growth factor (KGF) is a mitogen for type II alveolar epithelial and microvascular cells. We utilized the isolated perfused rat lung model to produce hydrostatic pulmonary edema by varying the left atrial and pulmonary capillary pressure. Pretreatment with KGF attenuated hydrostatic edema formation. This was demonstrated by lower wet-to-dry lung weight ratios, histological evidence of less alveolar edema formation, and reduced alveolar accumulation of intravascularly administered FITC-labeled large-molecular-weight dextran in rats pretreated with KGF. Thus KGF attenuates injury in this ex vivo model of hydrostatic pulmonary edema via mechanisms that prevent increases in alveolar-capillary permeability.     

Novel Peptide for Attenuation of Hyperoxia-induced Disruption of Lung Endothelial Barrier and Pulmonary Edema via Modulating Peroxynitrite Formation

Pulmonary damages of oxygen toxicity include vascular leakage and pulmonary edema. We have previously reported that hyperoxia increases the formation of NO and peroxynitrite in lung endothelial cells via increased interaction of endothelial nitric oxide (eNOS) with β-actin. A peptide (P326TAT) with amino acid sequence corresponding to the actin binding region of eNOS residues 326–333 has been shown to reduce the hyperoxia-induced formation of NO and peroxynitrite in lung endothelial cells. In the present study, we found that exposure of pulmonary artery endothelial cells to hyperoxia (95% oxygen and 5% CO₂) for 48 h resulted in disruption of monolayer barrier integrity in two phases, and apoptosis occurred in the second phase. NOS inhibitor N^G-nitro-l-arginine methyl ester attenuated the endothelial barrier disruption in both phases. Peroxynitrite scavenger uric acid did not affect the first phase but ameliorated the second phase of endothelial barrier disruption and apoptosis. P326TAT inhibited hyperoxia-induced disruption of monolayer barrier integrity in two phases and apoptosis in the second phase. More importantly, injection of P326TAT attenuated vascular leakage, pulmonary edema, and endothelial apoptosis in the lungs of mice exposed to hyperoxia. P326TAT also significantly reduced the increase in eNOS-β-actin association and protein tyrosine nitration. Together, these results indicate that peptide P326TAT ameliorates barrier dysfunction of hyperoxic lung endothelial monolayer and attenuates eNOS-β-actin association, peroxynitrite formation, endothelial apoptosis, and pulmonary edema in lungs of hyperoxic mice. P326TAT can be a novel therapeutic agent to treat or prevent acute lung injury in oxygen toxicity.     

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.     

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.     

Lycium barbarum polysaccharide protects against LPS-induced ARDS by inhibiting apoptosis,oxidative stress,and inflammation in pulmonary endothelial cells

Acute respiratory distress syndrome (ARDS) is a heterogenous syndrome characterised by diffuse alveolar damage, with an increase in lung endothelial and epithelial permeability. Lycium barbarum polysaccharide (LBP), the most biologically active fraction of wolfberry, possesses antiapoptotic and antioxidative effects in distinct situations. In the present study, the protective effects and potential molecular mechanisms of LBP against lipopolysaccharide (LPS)-induced ARDS were investigated in the mice and in the human pulmonary microvascular endothelial cells (HPMECs). The data indicated that pretreatment with LBP significantly attenuated LPS-induced lung inflammation and pulmonary oedema in vivo. LBP significantly reversed LPS-induced decrease in cell viability, increase in apoptosis and oxidative stress via inhibiting caspase-3 activation and intracellular reactive oxygen species (ROS) production in vitro. Moreover, the scratch assay verified that LBP restored the dysfunction of endothelial cells (ECs) migration induced by LPS stimulation. Furthermore, LBP also significantly suppressed LPS-induced NF-κB activation, and subsequently reversed the release of cytochrome c. These results showed the antiapoptosis and antioxidant LBP could partially protect against LPS-induced ARDS through promoting the ECs survival and scavenging ROS via inhibition of NF-κB signalling pathway. Thus, LBP could be potentially used for ARDS against pulmonary inflammation and pulmonary oedema.     

Lung endothelial cells strengthen,but brain endothelial cells weaken barrier properties of a human alveolar epithelium cell culture model

The blood–air barrier in the lung consists of the alveolar epithelium, the underlying capillary endothelium, their basement membranes and the interstitial space between the cell layers. Little is known about the interactions between the alveolar and the blood compartment. The aim of the present study was to gain first insights into the possible interplay between these two neighbored cell layers. We established an in vitro Transwell model of the alveolar epithelium based on human cell line H441 and investigated the influence of conditioned medium obtained from human lung endothelial cell line HPMEC-ST1.6R on the barrier properties of the H441 layers. As control for tissue specificity H441 layers were exposed to conditioned medium from human brain endothelial cell line hCMEC/D3. Addition of dexamethasone was necessary to obtain stable H441 cell layers. Moreover, dexamethasone increased expression of cell type I markers (caveolin-1, RAGE) and cell type II marker SP-B, whereas decreased the transepithelial electrical resistance (TEER) in a concentration dependent manner. Soluble factors obtained from the lung endothelial cell line increased the barrier significantly proven by TEER values and fluorescein permeability on the functional level and by the differential expression of tight junctional proteins on the molecular level. In contrast to this, soluble factors derived from brain endothelial cells weakened the barrier significantly. In conclusion, soluble factors from lung endothelial cells can strengthen the alveolar epithelium barrier in vitro, which suggests communication between endothelial and epithelial cells regulating the integrity of the blood–air barrier.     

Site‐specific and endothelial‐mediated dysfunction of the alveolar‐capillary barrier in response to lipopolysaccharides

Infectious agents such as lipopolysaccharides (LPS) challenge the functional properties of the alveolar‐capillary barrier (ACB) in the lung. In this study, we analyse the site‐specific effects of LPS on the ACB and reveal the effects on the individual cell types and the ACB as a functional unit. Monocultures of H441 epithelial cells and co‐cultures of H441 with endothelial cells cultured on Transwells^® were treated with LPS from the apical or basolateral compartment. Barrier properties were analysed by the transepithelial electrical resistance (TEER), by transport assays, and immunostaining and assessment of tight junctional molecules at protein level. Furthermore, pro‐inflammatory cytokines and immune‐modulatory molecules were evaluated by ELISA and semiquantitative real‐time PCR. Liquid chromatography–mass spectrometry‐based proteomics (LS‐MS) was used to identify proteins and effector molecules secreted by endothelial cells in response to LPS. In co‐cultures treated with LPS from the basolateral compartment, we noticed a significant reduction of TEER, increased permeability and induction of pro‐inflammatory cytokines. Conversely, apical treatment did not affect the barrier. No changes were noticed in H441 monoculture upon LPS treatment. However, LPS resulted in an increased expression of pro‐inflammatory cytokines such as IL‐6 in OEC and in turn induced the reduction of TEER and an increase in SP‐A expression in H441 monoculture, and H441/OEC co‐cultures after LPS treatment from basolateral compartment. LS‐MS‐based proteomics revealed factors associated with LPS‐mediated lung injury such as ICAM‐1, VCAM‐1, Angiopoietin 2, complement factors and cathepsin S, emphasizing the role of epithelial–endothelial crosstalk in the ACB in ALI/ARDS.     

Keratinocyte growth factor protects against Pseudomonas aeruginosa-induced lung injury

We have previously reported that keratinocyte growth factor (KGF) attenuates alpha-naphthylthiourea-induced lung injury by upregulating alveolar fluid transport. The objective of this study was to determine the effect of KGF pretreatment in Pseudomonas aeruginosa pneumonia. A 5% bovine albumin solution with 1 microCi of (125)I-labeled human albumin was instilled into the air spaces 4 or 24 h after intratracheal instillation of P. aeruginosa, and the concentration of unlabeled and labeled proteins in the distal air spaces over 1 h was used as an index of net alveolar fluid clearance. Alveolocapillary barrier permeability was evaluated with an intravascular injection of 1 microCi of (131)I-albumin. In early pneumonia, KGF increased lung liquid clearance (LLC) compared with that in nonpretreated animals. In late pneumonia, LLC was significantly reduced in the absence of KGF but increased above the control value with KGF. KGF pretreatment increased the number of polymorphonuclear cells recovered in the bronchoalveolar lavage fluid and decreased bacterial pulmonary translocation. In conclusion, KGF restores normal alveolar epithelial fluid transport during the acute phase of P. aeruginosa pneumonia and LLC in early and late pneumonia. Host response is also improved as shown by the increase in the alveolar cellular response and the decrease in pulmonary translocation of bacteria.     

Hydraulic conductance of lung endothelial phenotypes and Starling safety factors against edema

Recent permeability studies comparing endothelial cell phenotypes derived from alveolar and extra-alveolar vessels have significant implications for interpreting the mechanisms of fluid homeostasis in the intact lung. These studies indicate that confluent monolayers of rat pulmonary microvascular endothelial cells had a hydraulic conductance (L(p)) that was only 5% and a transendothelial flux rate for 72-kDa dextran only 9% of values determined for rat pulmonary artery endothelial cell monolayers. On the basis of previous studies partitioning the filtration coefficients between alveolar and extra-alveolar vascular segments in rat lungs and previous studies of lymph albumin fluxes and permeability, the contribution of the alveolar capillary segment to total albumin flux in lymph was estimated to be less than 10%. In addition, the Starling safety factors against the edema calculated for the alveolar capillaries are quite different from those estimated for whole lung. Estimates of the edema safety factor due to increased filtration across the alveolar capillary wall based on the low L(p) indicate it is quantitatively the greatest safety factor, although it would be a minor safety factor for extra-alveolar vessels. Also, a markedly higher effective protein osmotic absorptive force for plasma proteins must occur in the capillaries relative to extra-alveolar vessels. The lower L(p) for alveolar capillaries also has implications for the sequence of hydrostatic edema formation, and it also may have a role in preventing exercise-induced alveolar flooding.     

Intact alveolar epithelial permeability and transalveolar fluid absorption after thoracic irradiation in rats.

We have addressed the question of how the alveolar space stays relatively free of fluid when thoracic irradiation injures the pulmonary capillary endothelium and plasma fluid leaks into the interstitium. A single dose of 15 Gy to the thorax of rats significantly increased the pulmonary capillary filtration coefficient and the lung wet/dry weight ratio 2 h after irradiation. However, there was no significant increase in the release of lactose dehydrogenase or leaking of Evans blue dye into the alveolar space, indicating that alveolar epithelial permeability remained intact. We found no significant difference in the basal alveolar fluid clearance between control and irradiated animals. There was also no significant difference in blockage of alveolar fluid clearance by amiloride. This indicates that the function of the alveolar epithelial Na(+) channels is not impaired and that alveolar epithelium absorbs fluid normally. Examination of lung tissue by light microscopy demonstrated accumulation of fluid in the perivascular region but not in the alveolar space. Our data appear to indicate that the alveolar epithelial barrier function is more resistant to radiation than that of the pulmonary capillary endothelium. We conclude that intact alveolar epithelial permeability and normal transalveolar epithelial fluid absorption ability are of critical importance in keeping the alveolar space relatively free of fluid during acute radiation lung injury.     

Mechanisms of lung endothelial barrier disruption induced by cigarette smoke: role of oxidative stress and ceramides

The epithelial and endothelial cells lining the alveolus form a barrier essential for the preservation of the lung respiratory function, which is, however, vulnerable to excessive oxidative, inflammatory, and apoptotic insults. Whereas profound breaches in this barrier function cause pulmonary edema, more subtle changes may contribute to inflammation. The mechanisms by which cigarette smoke (CS) exposure induce lung inflammation are not fully understood, but an early alteration in the epithelial barrier function has been documented. We sought to investigate the occurrence and mechanisms by which soluble components of mainstream CS disrupt the lung endothelial cell barrier function. Using cultured primary rat microvascular cell monolayers, we report that CS induces endothelial cell barrier disruption in a dose- and time-dependent manner of similar magnitude to that of the epithelial cell barrier. CS exposure triggered a mechanism of neutral sphingomyelinase-mediated ceramide upregulation and p38 MAPK and JNK activation that were oxidative stress dependent and that, along with Rho kinase activation, mediated the endothelial barrier dysfunction. The morphological changes in endothelial cell monolayers induced by CS included actin cytoskeletal rearrangement, junctional protein zonula occludens-1 loss, and intercellular gap formation, which were abolished by the glutathione modulator N-acetylcysteine and ameliorated by neutral sphingomyelinase inhibition. The direct application of ceramide recapitulated the effects of CS, by disrupting both endothelial and epithelial cells barrier, by a mechanism that was redox and apoptosis independent and required Rho kinase activation. Furthermore, ceramide induced dose-dependent alterations of alveolar microcirculatory barrier in vivo, measured by two-photon excitation microscopy in the intact rat. In conclusion, soluble components of CS have direct endothelial barrier-disruptive effects that could be ameliorated by glutathione modulators or by inhibitors of neutral sphingomyelinase, p38 MAPK, JNK, and Rho kinase. Amelioration of endothelial permeability may alleviate lung and systemic vascular dysfunction associated with smoking-related chronic obstructive lung diseases.     

Redox regulation of endothelial barrier integrity

Intestinal ischemia-reperfusion is associated with the generation of reactive oxygen metabolites as well as remote, oxidant-mediated lung injury. Oxidants elicit endothelial redox imbalance and loss of vascular integrity by disorganizing several junctional proteins that contribute to the maintenance and regulation of the endothelial barrier. To determine the specific effect of redox imbalance on pulmonary vascular barrier integrity, microvascular permeability was determined in lungs of animals subjected to chemically induced redox imbalance. The effect of redox imbalance on microvascular permeability and endothelial junctional integrity in cultured lung microvascular cells was also determined. Whole lung and cultured pulmonary endothelial cell permeability both increased significantly in response to chemical redox imbalance. Thiol depletion also resulted in decreased endothelial cadherin content and disruption of the endothelial barrier. These deleterious effects of intracellular redox imbalance were blocked by pretreatment with exogenous glutathione. The results of this study suggest that redox imbalance contributes to pulmonary microvascular dysfunction by altering the content and/or spatial distribution of endothelial junctional proteins.     

Genistein‐3′‐sodium sulphonate protects against lipopolysaccharide‐induced lung vascular endothelial cell apoptosis and acute lung injury via BCL‐2 signalling

Under septic conditions, Lipopolysaccharide (LPS)‐induced apoptosis of lung vascular endothelial cells (ECs) triggers and aggravates acute lung injury (ALI), which so far has no effective therapeutic options. Genistein‐3′‐sodium sulphonate (GSS) is a derivative of native soy isoflavone, which has neuro‐protective effects through its anti‐apoptotic property. However, whether GSS protects against sepsis‐induced lung vascular endothelial cell apoptosis and ALI has not been determined. In this study, we found that LPS‐induced Myd88/NF‐κB/BCL‐2 signalling pathway activation and subsequent EC apoptosis were effectively down‐regulated by GSS in vitro. Furthermore, GSS not only reversed the sepsis‐induced BCL‐2 changes in expression in mouse lungs but also blocked sepsis‐associated lung vascular barrier disruption and ALI in vivo. Taken together, our results demonstrated that GSS might be a promising candidate for sepsis‐induced ALI via its regulating effects on Myd88/NF‐κB/BCL‐2 signalling in lung ECs.     

Targeted induction of lung endothelial cell apoptosis causes emphysema-like changes in the mouse

Pulmonary gas exchange relies on a rich capillary network, which, together with alveolar epithelial type I and II cells, form alveolar septa, the functional units in the lung. Alveolar capillary endothelial cells are critical in maintaining alveolar structure, because disruption of endothelial cell integrity underlies several lung diseases. Here we show that targeted ablation of lung capillary endothelial cells recapitulates the cellular events involved in cigarette smoke-induced emphysema, one of the most prevalent nonneoplastic lung diseases. Based on phage library screening on an immortalized lung endothelial cell line, we identified a lung endothelial cell-binding peptide, which preferentially homes to lung blood vessels. This peptide fused to a proapoptotic motif specifically induced programmed cell death of lung endothelial cells in vitro as well as targeted apoptosis of the lung microcirculation in vivo. As early as 4 days following peptide administration, mice developed air space enlargement associated with enhanced oxidative stress, influx of macrophages, and up-regulation of ceramide. Given that these are all critical elements of the corresponding human emphysema caused by cigarette smoke, these data provide evidence for a central role for the alveolar endothelial cells in the maintenance of lung structure and of endothelial cell apoptosis in the pathogenesis of emphysema-like changes. Thus, our data enable the generation of a convenient mouse model of human emphysema. Finally, combinatorial screenings on immortalized cells followed by in vivo targeting establishes an experimental framework for discovery and validation of additional ligand-directed pharmacodelivery systems.     

Dynamics of pulmonary endothelial barrier function in acute inflammation: mechanisms and therapeutic perspectives

The lungs provide a large inner surface to guarantee respiration. In lung alveoli, a delicate membrane formed by endo- and epithelial cells with their fused basal lamina ensures rapid and effective gas exchange between alveolar and vascular compartments while concurrently forming a robust barrier against inhaled particles and microbes. However, upon infectious or sterile inflammatory stimulation, tightly regulated endothelial barrier leakiness is required for leukocyte transmigration. Further, endothelial barrier disruption may result in uncontrolled extravasation of protein-rich fluids. This brief review summarizes some important mechanisms of pulmonary endothelial barrier regulation and disruption, focusing on the role of specific cell populations, coagulation and complement cascades and mediators including angiopoietins, specific sphingolipids, adrenomedullin and reactive oxygen and nitrogen species for the regulation of pulmonary endothelial barrier function. Further, current therapeutic perspectives against development of lung injury are discussed.     

SC5b-9-Induced Pulmonary Microvascular Endothelial Hyperpermeability Participates in Ventilator-Induced Lung Injury

Mechanical ventilation with large tidal volumes can increase lung alveolar permeability and initiate inflammatory responses, termed ventilator-induced lung injury (VILI). VILI is characterized by an influx of inflammatory cells, increased pulmonary permeability, and endothelial and epithelial cell death. But the underlying molecular mechanisms that regulate VILI remain unclear. The purpose of this study was to investigate the mechanisms that regulate pulmonary endothelial barrier in an animal model of VILI. These data suggest that SC5b-9, as the production of the complement activation, causes increase in rat pulmonary microvascular permeability by inducing activation of RhoA and subsequent phosphorylation of myosin light chain and contraction of endothelial cells, resulting in gap formation. In general, the complement-mediated increase in pulmonary microvascular permeability may participate in VILI.     

Staphylococcal phosphatidylinositol‐specific phospholipase C potentiates lung injury via complement sensitisation

Staphylococcus aureus is frequently isolated from patients with community‐acquired pneumonia and acute respiratory distress syndrome (ARDS). ARDS is associated with staphylococcal phosphatidylinositol‐specific phospholipase C (PI‐PLC); however, the role of PI‐PLC in the pathogenesis and progression of ARDS remains unknown. Here, we showed that recombinant staphylococcal PI‐PLC possesses enzyme activity that causes shedding of glycosylphosphatidylinositol‐anchored CD55 and CD59 from human umbilical vein endothelial cell surfaces and triggers cell lysis via complement activity. Intranasal infection with PI‐PLC‐positive S. aureus resulted in greater neutrophil infiltration and increased pulmonary oedema compared with a plc‐isogenic mutant. Although indistinguishable proinflammatory genes were induced, the wild‐type strain activated higher levels of C5a in lung tissue accompanied by elevated albumin instillation and increased lactate dehydrogenase release in bronchoalveolar lavage fluid compared with the plc^? mutant. Following treatment with cobra venom factor to deplete complement, the wild‐type strain with PI‐PLC showed a reduced ability to trigger pulmonary permeability and tissue damage. PI‐PLC‐positive S. aureus induced the formation of membrane attack complex, mainly on type II pneumocytes, and reduced the level of CD55/CD59, indicating the importance of complement regulation in pulmonary injury. In conclusion, S. aureus PI‐PLC sensitised tissue to complement activation leading to more severe tissue damage, increased pulmonary oedema, and ARDS progression.     

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.