Physiological determinants of the pulmonary filtration coefficient

An in silico approach to understanding the interaction between cardiovascular and pulmonary lymphatic dysfunction

The lung is extremely sensitive to interstitial fluid balance, yet the role of pulmonary lymphatics in lung fluid homeostasis and its interaction with cardiovascular pressures is poorly understood. In health, there is a fine balance between fluid extravasated from the pulmonary capillaries into the interstitium and the return of fluid to the circulation via the lymphatic vessels. This balance is maintained by an extremely interdependent system governed by pressures in the fluids (air and blood) and tissue (interstitium), lung motion during breathing, and the permeability of the tissues. Chronic elevation in left atrial pressure (LAP) due to left heart disease increases the capillary blood pressure. The consequent fluid accumulation in the delicate lung tissue increases its weight, decreases its compliance, and impairs gas exchange. This interdependent system is difficult, if not impossible, to study experimentally. Computational modeling provides a unique perspective to analyze fluid movement in the cardiopulmonary vasculature in health and disease. We have developed an initial in silico model of pulmonary lymphatic function using an anatomically structured model to represent ventilation and perfusion and underlying biophysical laws governing fluid transfer at the interstitium. This novel model was tested against increased LAP and noncardiogenic effects (increased permeability). The model returned physiologically reasonable values for all applications, predicting pulmonary edema when LAP reached 25 mmHg and with increased permeability.NEW & NOTEWORTHY This model presents a novel approach to understanding the interaction between cardiac dysfunction and pulmonary lymphatic function, using anatomically structured models and biophysical equations to estimate regional variation in fluid transport from blood to interstitial and lymphatic flux. This fluid transport model brings together advanced models of ventilation, perfusion, and lung mechanics to produce a detailed model of fluid transport in health and various altered pathological conditions.

Comparative Study of Capillary Filtration Coefficient (Kfc) Determination by a Manual and Automatic Perfusion System. Step by Step Technique Review

The purpose of calculating the capillary filtration coefficient is to experimentally evaluate edema formation in models of pulmonary ischemia-reperfusion injury. For many years, the obtaining of this coefficient implies a series of manual maneuvers during ex-vivo reperfusion of pulmonary arterial pressure, venous pressure and weight, as well as the calculation of the Kfc formula. Through automation, the calculation of capillary filtration coefficient could be easier and more efficient. To describe an automatic method designed in our laboratory to calculating the capillary filtration coefficient and compare with traditional determination of capillary filtration coefficient as gold standard method. An automatic three valve perfusion system was constructed, commanded by a mastery module connected to a graphical user interface. To test its accuracy, cardiopulmonary blocks of Wistar rats were harvested and distributed in manual (n=8) and automated (n=8) capillary filtration coefficient determination groups. Physiological parameters as pulmonary arterial pressure, pulmonary venous pressure, weight and capillary filtration coefficient were obtained. Results: Capillary filtration coefficient, pulmonary arterial pressure, venous arterial pressure shown no statistical significance difference between the groups. The automated perfusion system for obtaining Kfc was standardized and validated, giving reliable results without biases and making the process more efficient in terms of time and personal staff.

Understanding Vasomotion of Lung Microcirculation by In Vivo Imaging

The balance of lung extravascular water depends upon the control of blood flow in the alveolar distribution vessels that feed downstream two districts placed in parallel, the corner vessels and the alveolar septal network. The occurrence of an edemagenic condition appears critical as an increase in extravascular water endangers the thinness of the air-blood barrier, thus negatively affecting the diffusive capacity of the lung. We exposed anesthetized rabbits to an edemagenic factor (12% hypoxia) for 120 min and followed by in vivo imaging the micro-vascular morphology through a "pleural window" using a stereo microscope at a magnification of 15× (resolution of 7.2 μm). We measured the change in diameter of distribution vessels (50-200 μm) and corner vessels (<50 μm). On average, hypoxia caused a significant decrease in diameter of both smaller distribution vessels (about ~50%) and corner vessels (about ~25%) at 30 min. After 120 min, reperfusion occurred. Regional differences in perivascular interstitial volume were observed and could be correlated with differences in blood flow control. To understand such difference, we modelled imaged alveolar capillary units, obtained by Voronoi method, integrating microvascular pressure parameters with capillary filtration. Results of the analysis suggested that at 120 min, alveolar blood flow was diverted to the corner vessels in larger alveoli, which were found also to undergo a greater filtration indicating greater proneness to develop lung edema.

Permeability and calcium signaling in lung endothelium: unpack the box…

This brief review assesses the role of Ca²⁺ signaling in lung endothelium in regulation of endothelial permeability. The disconnect between experimental and clinical outcomes to date may be due, in part, to the use of tools which yield information about aggregate permeability or Ca²⁺ responses in lung or in endothelial monolayers. The teaching point of this review is to “unpack the box,” i.e. consider the many potential issues which could impact interpretation of outcomes. These include phenotypic heterogeneity and resultant segment-specific permeability responses, methodologic issues related to permeability measures, contributions from Ca²⁺ channels in cells other than endothelium—such as alveolar macrophages or blood leukocytes), Ca²⁺ dynamic patterns, rather than averaged Ca²⁺ responses to channel activation, and the background context, such as changes in endothelial bioenergetics with sepsis. Any or all of these issues might color interpretation of permeability and Ca²⁺ signaling in lung.

Clinical potential of oligonucleotide-based therapeutics in the respiratory system

The discovery of an ever-expanding plethora of coding and non-coding RNAs with nodal and causal roles in the regulation of lung physiology and disease is reinvigorating interest in the clinical utility of the oligonucleotide therapeutic class. This is strongly supported through recent advances in nucleic acids chemistry, synthetic oligonucleotide delivery and viral gene therapy that have succeeded in bringing to market at least three nucleic acid-based drugs. As a consequence, multiple new candidates such as RNA interference modulators, antisense, and splice switching compounds are now progressing through clinical evaluation. Here, manipulation of RNA for the treatment of lung disease is explored, with emphasis on robust pharmacological evidence aligned to the five pillars of drug development: exposure to the appropriate tissue, binding to the desired molecular target, evidence of the expected mode of action, activity in the relevant patient population and commercially viable value proposition.

Endothelial hyperpermeability in severe pulmonary arterial hypertension: role of store-operated calcium entry

Here, we tested the hypothesis that animals with severe pulmonary arterial hypertension (PAH) display increased sensitivity to vascular permeability induced by activation of store-operated calcium entry. To test this hypothesis, wild-type and transient receptor potential channel 4 (TRPC4) knockout Fischer 344 rats were given a single injection of Semaxanib (SU5416; 20 mg/kg) followed by 3 wk of exposure to hypoxia (10% oxygen) and a return to normoxia (21% oxygen) for an additional 2-3 wk. This Semaxanib/hypoxia/normoxia (i.e., SU5416/hypoxia/normoxia) treatment caused PAH, as evidenced by development of right ventricular hypertrophy, pulmonary artery medial hypertrophy, and occlusive lesions within precapillary arterioles. Pulmonary artery pressure was increased fivefold in Semaxanib/hypoxia/normoxia-treated animals compared with untreated, Semaxanib-treated, and hypoxia-treated controls, determined by isolated perfused lung studies. Thapsigargin induced a dose-dependent increase in permeability that was dependent on TRPC4 in the normotensive perfused lung. This increase in permeability was accentuated in PAH lungs but not in Semaxanib- or hypoxia-treated lungs. Fluid accumulated in large perivascular cuffs, and although alveolar fluid accumulation was not seen in histological sections, Evans blue dye conjugated to albumin was present in bronchoalveolar lavage fluid of hypertensive but not normotensive lungs. Thus PAH is accompanied by a TRPC4-dependent increase in the sensitivity to edemagenic agents that activate store-operated calcium entry.

Lung Circulation

The circulation of the lung is unique both in volume and function. For example, it is the only organ with two circulations: the pulmonary circulation, the main function of which is gas exchange, and the bronchial circulation, a systemic vascular supply that provides oxygenated blood to the walls of the conducting airways, pulmonary arteries and veins. The pulmonary circulation accommodates the entire cardiac output, maintaining high blood flow at low intravascular arterial pressure. As compared with the systemic circulation, pulmonary arteries have thinner walls with much less vascular smooth muscle and a relative lack of basal tone. Factors controlling pulmonary blood flow include vascular structure, gravity, mechanical effects of breathing, and the influence of neural and humoral factors. Pulmonary vascular tone is also altered by hypoxia, which causes pulmonary vasoconstriction. If the hypoxic stimulus persists for a prolonged period, contraction is accompanied by remodeling of the vasculature, resulting in pulmonary hypertension. In addition, genetic and environmental factors can also confer susceptibility to development of pulmonary hypertension. Under normal conditions, the endothelium forms a tight barrier, actively regulating interstitial fluid homeostasis. Infection and inflammation compromise normal barrier homeostasis, resulting in increased permeability and edema formation. This article focuses on reviewing the basics of the lung circulation (pulmonary and bronchial), normal development and transition at birth and vasoregulation. Mechanisms contributing to pathological conditions in the pulmonary circulation, in particular when barrier function is disrupted and during development of pulmonary hypertension, will also be discussed.

The vital role of the right ventricle in the pathogenesis of acute pulmonary edema

The development of acute pulmonary edema involves a complex interplay between the capillary hydrostatic, interstitial hydrostatic, and oncotic pressures and the capillary permeability. We review the pathophysiological processes involved and illustrate the concepts in a number of common clinical situations including heart failure with normal and reduced ejection fractions, mitral regurgitation, and arrhythmias. We also describe other rarer causes including exercise, swimming, and diving-induced acute pulmonary edema. We suggest a unifying framework in which the critical abnormality is a mismatch or imbalance between the right and left ventricular stroke volumes. In conclusion, we hypothesize that increased right ventricular contraction is an important contributor to the sudden increase in capillary hydrostatic pressure, and therefore, a central mechanism involved in the development of alveolar edema.

Prevention of ventilator-induced lung edema by inhalation of nanoparticles releasing ruthenium red

The acute respiratory distress syndrome (ARDS), a devastating lung disease that has no cure, is exacerbated by life-supportive mechanical ventilation that worsens lung edema and inflammation through the syndrome of ventilator-induced lung injury. Recently, the membrane ion channel transient receptor potential vanilloid 4 (TRPV4) on alveolar macrophages was shown to mediate murine lung vascular permeability induced by high-pressure mechanical ventilation. The objective of this study was to determine whether inhalation of nanoparticles (NPs) containing the TRPV4 inhibitor ruthenium red (RR) prevents ventilator-induced lung edema in mice. Poly-lactic-co-glycolic acid NPs containing RR were evaluated in vitro for their ability to block TRPV4-mediated calcium signaling in alveolar macrophages and capillary endothelial cells. Lungs from adult C57BL6 mice treated with nebulized NPs were then used in ex vivo ventilation perfusion experiments to assess the ability of the NPs to prevent high-pressure mechanical ventilation-induced lung edema. Poly-lactic-co-glycolic acid NPs (300 nm) released RR for 150 hours in vitro, and blocked TRPV4-mediated calcium signaling in cells up to 7 days after phagocytosis. Inhaled NPs deposited in alveoli of spontaneously breathing mice were rapidly phagocytosed by alveolar macrophages, and blocked increased vascular permeability from high-pressure mechanical ventilation for 72 hours in ex vivo ventilation perfusion experiments. These data offer proof of principle that inhalation of NPs containing a TRPV4 inhibitor prevents ventilator damage for several days, and imply that this novel drug delivery strategy could be used to target alveolar macrophages in patients at risk of ventilator-induced lung injury before initiating mechanical ventilation.

The Pseudomonas aeruginosa exoenzyme Y impairs endothelial cell proliferation and vascular repair following lung injury

Exoenzyme Y (ExoY) is a Pseudomonas aeruginosa toxin that is introduced into host cells through the type 3 secretion system (T3SS). Once inside the host cell cytoplasm, ExoY generates cyclic nucleotides that cause tau phosphorylation and microtubule breakdown. Microtubule breakdown causes interendothelial cell gap formation and tissue edema. Although ExoY transiently induces interendothelial cell gap formation, it remains unclear whether ExoY prevents repair of the endothelial cell barrier. Here, we test the hypothesis that ExoY intoxication impairs recovery of the endothelial cell barrier following gap formation, decreasing migration, proliferation, and lung repair. Pulmonary microvascular endothelial cells (PMVECs) were infected with P. aeruginosa strains for 6 h, including one possessing an active ExoY (PA103 exoUexoT::Tc pUCPexoY; ExoY(+)), one with an inactive ExoY (PA103ΔexoUexoT::Tc pUCPexoY(K81M); ExoY(K81M)), and one that lacks PcrV required for a functional T3SS (ΔPcrV). ExoY(+) induced interendothelial cell gaps, whereas ExoY(K81M) and ΔPcrV did not promote gap formation. Following gap formation, bacteria were removed and endothelial cell repair was examined. PMVECs were unable to repair gaps even 3-5 days after infection. Serum-stimulated growth was greatly diminished following ExoY intoxication. Intratracheal inoculation of ExoY(+) and ExoY(K81M) caused severe pneumonia and acute lung injury. However, whereas the pulmonary endothelial cell barrier was functionally improved 1 wk following ExoY(K81M) infection, pulmonary endothelium was unable to restrict the hyperpermeability response to elevated hydrostatic pressure following ExoY(+) infection. In conclusion, ExoY is an edema factor that chronically impairs endothelial cell barrier integrity following lung injury.

Acute lung injury and pulmonary vascular permeability: use of transgenic models

Acute lung injury is a general term that describes injurious conditions that can range from mild interstitial edema to massive inflammatory tissue destruction. This review will cover theoretical considerations and quantitative and semi-quantitative methods for assessing edema formation and increased vascular permeability during lung injury. Pulmonary edema can be quantitated directly using gravimetric methods, or indirectly by descriptive microscopy, quantitative morphometric microscopy, altered lung mechanics, high-resolution computed tomography, magnetic resonance imaging, positron emission tomography, or x-ray films. Lung vascular permeability to fluid can be evaluated by measuring the filtration coefficient (Kf) and permeability to solutes evaluated from their blood to lung clearances. Albumin clearances can then be used to calculate specific permeability-surface area products (PS) and reflection coefficients (σ). These methods as applied to a wide variety of transgenic mice subjected to acute lung injury by hyperoxic exposure, sepsis, ischemia-reperfusion, acid aspiration, oleic acid infusion, repeated lung lavage, and bleomycin are reviewed. These commonly used animal models simulate features of the acute respiratory distress syndrome, and the preparation of genetically modified mice and their use for defining specific pathways in these disease models are outlined. Although the initiating events differ widely, many of the subsequent inflammatory processes causing lung injury and increased vascular permeability are surprisingly similar for many etiologies.

Regulation and repair of the alveolar-capillary barrier in acute lung injury

Considerable progress has been made in understanding the basic mechanisms that regulate fluid and protein exchange across the endothelial and epithelial barriers of the lung under both normal and pathological conditions. Clinically relevant lung injury occurs most commonly from severe viral and bacterial infections, aspiration syndromes, and severe shock. The mechanisms of lung injury have been identified in both experimental and clinical studies. Recovery from lung injury requires the reestablishment of an intact endothelial barrier and a functional alveolar epithelial barrier capable of secreting surfactant and removing alveolar edema fluid. Repair mechanisms include the participation of endogenous progenitor cells in strategically located niches in the lung. Novel treatment strategies include the possibility of cell-based therapy that may reduce the severity of lung injury and enhance lung repair.

TRPV4 channels augment macrophage activation and ventilator-induced lung injury

We have previously implicated transient receptor potential vanilloid 4 (TRPV4) channels and alveolar macrophages in initiating the permeability increase in response to high peak inflation pressure (PIP) ventilation. Alveolar macrophages were harvested from TRPV4(-/-) and TRPV4(+/+) mice and instilled in the lungs of mice of the opposite genotype. Filtration coefficients (K(f)) measured in isolated perfused lungs after ventilation with successive 30-min periods of 9, 25, and 35 cmH(2)O PIP did not significantly increase in lungs from TRPV4(-/-) mice but increased >2.2-fold in TRPV4(+/+) lungs, TRPV4(+/+) lungs instilled with TRPV4(-/-) macrophages, and TRPV4(-/-) lungs instilled with TRPV4(+/+) macrophages after ventilation with 35 cmH(2)O PIP. Activation of TRPV4 with 4-alpha-phorbol didecanoate (4alphaPDD) significantly increased intracellular calcium, superoxide, and nitric oxide production in TRPV4(+/+) macrophages but not TRPV4(-/-) macrophages. Cross-sectional areas increased nearly 3-fold in TRPV4(+/+) macrophages compared with TRPV4(-/-) macrophages after 4alphaPDD. Immunohistochemistry staining of lung tissue for nitrotyrosine revealed increased amounts in high PIP ventilated TRPV4(+/+) lungs compared with low PIP ventilated TRPV4(+/+) or high PIP ventilated TRPV4(-/-) lungs. Thus TRPV4(+/+) macrophages restored susceptibility of TRPV4(-/-) lungs to mechanical injury. A TRPV4 agonist increased intracellular calcium and reactive oxygen and nitrogen species in harvested TRPV4(+/+) macrophages but not TRPV4(-/-) macrophages. K(f) increases correlated with tissue nitrotyrosine, a marker of peroxynitrite production.