Effects of acute Rho kinase inhibition on chronic hypoxia-induced changes in proximal and distal pulmonary arterial structure and function

Crystal ribcage: a platform for probing real-time lung function at cellular resolution

Understanding the dynamic pathogenesis and treatment response in pulmonary diseases requires probing the lung at cellular resolution in real time. Despite advances in intravital imaging, optical imaging of the lung during active respiration and circulation has remained challenging. Here, we introduce the crystal ribcage: a transparent ribcage that allows multiscale optical imaging of the functioning lung from whole-organ to single-cell level. It enables the modulation of lung biophysics and immunity through intravascular, intrapulmonary, intraparenchymal and optogenetic interventions, and it preserves the three-dimensional architecture, air-liquid interface, cellular diversity and respiratory-circulatory functions of the lung. Utilizing these capabilities on murine models of pulmonary pathologies we probed remodeling of respiratory-circulatory functions at the single-alveolus and capillary levels during disease progression. The crystal ribcage and its broad applications presented here will facilitate further studies of nearly any pulmonary disease as well as lead to the identification of new targets for treatment strategies.

A topological data analysis study on murine pulmonary arterial trees with pulmonary hypertension

Pulmonary hypertension (PH), defined by a mean pulmonary arterial blood pressure above 20 mmHg in the main pulmonary artery, is a cardiovascular disease impacting the pulmonary vasculature. PH is accompanied by chronic vascular remodeling, wherein vessels become stiffer, large vessels dilate, and smaller vessels constrict. Some types of PH, including hypoxia-induced PH (HPH), also lead to microvascular rarefaction. This study analyzes the change in pulmonary arterial morphometry in the presence of HPH using novel methods from topological data analysis (TDA). We employ persistent homology to quantify arterial morphometry for control and HPH mice characterizing normalized arterial trees extracted from micro-computed tomography (micro-CT) images. We normalize generated trees using three pruning algorithms before comparing the topology of control and HPH trees. This proof-of-concept study shows that the pruning method affects the spatial tree statistics and complexity. We find that HPH trees are stiffer than control trees but have more branches and a higher depth. Relative directional complexities are lower in HPH animals in the right, ventral, and posterior directions. For the radius pruned trees, this difference is more significant at lower perfusion pressures enabling analysis of remodeling of larger vessels. At higher pressures, the arterial networks include more distal vessels. Results show that the right, ventral, and posterior relative directional complexities increase in HPH trees, indicating the remodeling of distal vessels in these directions. Strahler order pruning enables us to generate trees of comparable size, and results, at all pressure, show that HPH trees have lower complexity than the control trees. Our analysis is based on data from 6 animals (3 control and 3 HPH mice), and even though our analysis is performed in a small dataset, this study provides a framework and proof-of-concept for analyzing properties of biological trees using tools from Topological Data Analysis (TDA). Findings derived from this study bring us a step closer to extracting relevant information for quantifying remodeling in HPH.

Sensitivity analysis and uncertainty quantification of 1-D models of pulmonary hemodynamics in mice under control and hypertensive conditions

Pulmonary hypertension (PH), defined as an elevated mean blood pressure in the main pulmonary artery (MPA) at rest, is associated with vascular remodeling of both large and small arteries. PH has several sub-types that are all linked to high mortality rates. In this study, we use a one-dimensional (1-D) fluid dynamics model driven by in vivo measurements of MPA flow to understand how model parameters and network size influence MPA pressure predictions in the presence of PH. We compare model predictions with in vivo MPA pressure measurements from a control and a hypertensive mouse and analyze results in three networks of increasing complexity, extracted from micro-computed tomography (micro-CT) images. We introduce global scaling factors for boundary condition parameters and perform local and global sensitivity analysis to calculate parameter influence on model predictions of MPA pressure and correlation analysis to determine a subset of identifiable parameters. These are inferred using frequentist optimization and Bayesian inference via the Delayed Rejection Adaptive Metropolis (DRAM) algorithm. Frequentist and Bayesian uncertainty is computed for model parameters and MPA pressure predictions. Results show that MPA pressure predictions are most sensitive to distal vascular resistance and that parameter influence changes with increasing network complexity. Our outcomes suggest that PH leads to increased vascular stiffness and decreased peripheral compliance, congruent with clinical observations.

Intravenous Delivery of Lung-Targeted Nanofibers for Pulmonary Hypertension in Mice

Pulmonary hypertension is a highly morbid disease with no cure. Available treatments are limited by systemic adverse effects due to non-specific biodistribution. Self-assembled peptide amphiphile (PA) nanofibers are biocompatible nanomaterials that can be modified to recognize specific biological markers to provide targeted drug delivery and reduce off-target toxicity. Here, PA nanofibers that target the angiotensin I-converting enzyme and the receptor for advanced glycation end-products (RAGE) are developed, as both proteins are overexpressed in the lung with pulmonary hypertension. It is demonstrated that intravenous delivery of RAGE-targeted nanofibers containing the targeting epitope LVFFAED (LVFF) significantly accumulated within the lung in a chronic hypoxia-induced pulmonary hypertension mouse model. Using 3D light sheet fluorescence microscopy, it is shown that LVFF nanofiber localization is specific to the diseased pulmonary tissue with immunofluorescence analysis demonstrating colocalization of the targeted nanofiber to RAGE in the hypoxic lung. Furthermore, biodistribution studies show that significantly more LVFF nanofibers localized to the lung compared to major off-target organs. Targeted nanofibers are retained within the pulmonary tissue for 24 h after injection. Collectively, these data demonstrate the potential of a RAGE-targeted nanomaterial as a drug delivery platform to treat pulmonary hypertension.

Markov chain Monte Carlo with Gaussian processes for fast parameter estimation and uncertainty quantification in a 1D fluid-dynamics model of the pulmonary circulation

The past few decades have witnessed an explosive synergy between physics and the life sciences. In particular, physical modelling in medicine and physiology is a topical research area. The present work focuses on parameter inference and uncertainty quantification in a 1D fluid-dynamics model for quantitative physiology: the pulmonary blood circulation. The practical challenge is the estimation of the patient-specific biophysical model parameters, which cannot be measured directly. In principle this can be achieved based on a comparison between measured and predicted data. However, predicting data requires solving a system of partial differential equations (PDEs), which usually have no closed-form solution, and repeated numerical integrations as part of an adaptive estimation procedure are computationally expensive. In the present article, we demonstrate how fast parameter estimation combined with sound uncertainty quantification can be achieved by a combination of statistical emulation and Markov chain Monte Carlo (MCMC) sampling. We compare a range of state-of-the-art MCMC algorithms and emulation strategies, and assess their performance in terms of their accuracy and computational efficiency. The long-term goal is to develop a method for reliable disease prognostication in real time, and our work is an important step towards an automatic clinical decision support system.

Assessing model mismatch and model selection in a Bayesian uncertainty quantification analysis of a fluid-dynamics model of pulmonary blood circulation

This study uses Bayesian inference to quantify the uncertainty of model parameters and haemodynamic predictions in a one-dimensional pulmonary circulation model based on an integration of mouse haemodynamic and micro-computed tomography imaging data. We emphasize an often neglected, though important source of uncertainty: in the mathematical model form due to the discrepancy between the model and the reality, and in the measurements due to the wrong noise model (jointly called 'model mismatch'). We demonstrate that minimizing the mean squared error between the measured and the predicted data (the conventional method) in the presence of model mismatch leads to biased and overly confident parameter estimates and haemodynamic predictions. We show that our proposed method allowing for model mismatch, which we represent with Gaussian processes, corrects the bias. Additionally, we compare a linear and a nonlinear wall model, as well as models with different vessel stiffness relations. We use formal model selection analysis based on the Watanabe Akaike information criterion to select the model that best predicts the pulmonary haemodynamics. Results show that the nonlinear pressure-area relationship with stiffness dependent on the unstressed radius predicts best the data measured in a control mouse.

Structural and hemodynamic properties of murine pulmonary arterial networks under hypoxia-induced pulmonary hypertension

Detection and monitoring of patients with pulmonary hypertension, defined as a mean blood pressure in the main pulmonary artery above 25 mmHg, requires a combination of imaging and hemodynamic measurements. This study demonstrates how to combine imaging data from microcomputed tomography images with hemodynamic pressure and flow waveforms from control and hypertensive mice. Specific attention is devoted to developing a tool that processes computed tomography images, generating subject-specific arterial networks in which one-dimensional fluid dynamics modeling is used to predict blood pressure and flow. Each arterial network is modeled as a directed graph representing vessels along the principal pathway to ensure perfusion of all lobes. The one-dimensional model couples these networks with structured tree boundary conditions representing the small arteries and arterioles. Fluid dynamics equations are solved in this network and compared to measurements of pressure in the main pulmonary artery. Analysis of microcomputed tomography images reveals that the branching ratio is the same in the control and hypertensive animals, but that the vessel length-to-radius ratio is significantly lower in the hypertensive animals. Fluid dynamics predictions show that in addition to changed network geometry, vessel stiffness is higher in the hypertensive animal models than in the control models.

Influence of image segmentation on one-dimensional fluid dynamics predictions in the mouse pulmonary arteries

Computational fluid dynamics (CFD) models are emerging tools for assisting in diagnostic assessment of cardiovascular disease. Recent advances in image segmentation have made subject-specific modelling of the cardiovascular system a feasible task, which is particularly important in the case of pulmonary hypertension, requiring a combination of invasive and non-invasive procedures for diagnosis. Uncertainty in image segmentation propagates to CFD model predictions, making the quantification of segmentation-induced uncertainty crucial for subject-specific models. This study quantifies the variability of one-dimensional CFD predictions by propagating the uncertainty of network geometry and connectivity to blood pressure and flow predictions. We analyse multiple segmentations of a single, excised mouse lung using different pre-segmentation parameters. A custom algorithm extracts vessel length, vessel radii and network connectivity for each segmented pulmonary network. Probability density functions are computed for vessel radius and length and then sampled to propagate uncertainties to haemodynamic predictions in a fixed network. In addition, we compute the uncertainty of model predictions to changes in network size and connectivity. Results show that variation in network connectivity is a larger contributor to haemodynamic uncertainty than vessel radius and length.

RhoA-Rho associated kinase signaling leads to renin-angiotensin system imbalance and angiotensin converting enzyme 2 has a protective role in acute pulmonary embolism

INTRODUCTION

Acute pulmonary embolism (APE) is a cardiovascular disease with high morbidity and mortality. Although the anatomical obstruction of the pulmonary vascular bed initiates APE, recent studies have suggested that vasoconstrictors in the renin-angiotensin system (RAS) play a role in the severity of APE.

MATERIALS AND METHODS

We performed a 5-year retrospective clinical study to analyze the key RAS components in APE patients, including angiotensin converting enzyme (ACE), ACE2, angiotensin II (Ang II) and angiotensin 1-7(Ang(1-7)). The role of RhoA-Rho associated kinase (ROCK) signaling in regulating RAS vasoconstrictors was detected in rat pulmonary artery endothelial cells and in an APE rat model.

RESULTS

In clinical study, we found that the levels of RAS vasoconstrictors were correlated with the clinical classification of APE patients, ACE and Ang II were unregulated, whereas ACE2 and Ang(1-7) were downregulated in the high-risk group compared to the healthy volunteers. In animal study, we found that activated RhoA-ROCK signaling was responsible for the imbalance in RAS vasoconstrictors both in vitro and in vivo, and further evidence indicated that ROCK inhibitors (Y27632 or HA1077) and an ACE2 activator (Resorcinol naphthalein) restored the dysregulated RAS vasoconstrictors significantly and had a protective role in an APE rat model.

CONCLUSIONS

Our study revealed that RhoA-ROCK signaling leads to RAS imbalance in APE patients, and ACE2 activation might be a novel therapeutic target in APE treatment.

Effects of fasudil on pulmonary hypertension in clinical practice

Pulmonary hypertension (PH) is a pathophysiologic disorder that may involve multiple clinical conditions and can complicate the majority of cardiovascular and respiratory diseases. The presence of PH is associated with worse outcomes, but the efficacy of current therapy is still unsatisfactory. Because Rho-kinase (ROCK) plays an important role in the pathogenesis of PH, the ROCK inhibitor fasudil is expected to contribute to PH treatment. In animal models of PH, fasudil reduced pulmonary artery pressure (PAP) and improved survival. Furthermore, the short-term efficacy and safety of fasudil in the treatment of PH are demonstrated in clinical trials. Both PAP and pulmonary vascular resistance in patients with PH are significantly decreased by intravenous or inhaled fasudil without apparent side effect. However, no clinical trial has assessed the long-term efficacy of fasudil in the treatment of PH. Limited data suggest that the mid-term use of fasudil could improve exercise capacity and reduce in-hospital mortality. We also discuss the combined use of fasudil and other drugs for PH treatment. However, these combinations have not yet been evaluated in a clinical trial. According to animal studies, the combination of fasudil with beraprost or sildenafil shows synergistic effects, whereas the combination of fasudil with bosentan has no additional ameliorating effects on PH development.

Contribution of reactive oxygen species to the pathogenesis of pulmonary arterial hypertension

Pulmonary arterial hypertension is associated with a decreased antioxidant capacity. However, neither the contribution of reactive oxygen species to pulmonary vasoconstrictor sensitivity, nor the therapeutic efficacy of antioxidant strategies in this setting are known. We hypothesized that reactive oxygen species play a central role in mediating both vasoconstrictor and arterial remodeling components of severe pulmonary arterial hypertension. We examined the effect of the chemical antioxidant, TEMPOL, on right ventricular systolic pressure, vascular remodeling, and enhanced vasoconstrictor reactivity in both chronic hypoxia and hypoxia/SU5416 rat models of pulmonary hypertension. SU5416 is a vascular endothelial growth factor receptor antagonist and the combination of chronic hypoxia/SU5416 produces a model of severe pulmonary arterial hypertension with vascular plexiform lesions/fibrosis that is not present with chronic hypoxia alone. The major findings from this study are: 1) compared to hypoxia alone, hypoxia/SU5416 exposure caused more severe pulmonary hypertension, right ventricular hypertrophy, adventitial lesion formation, and greater vasoconstrictor sensitivity through a superoxide and Rho kinase-dependent Ca2+ sensitization mechanism. 2) Chronic hypoxia increased medial muscularization and superoxide levels, however there was no effect of SU5416 to augment these responses. 3) Treatment with TEMPOL decreased right ventricular systolic pressure in both hypoxia and hypoxia/SU5416 groups. 4) This effect of TEMPOL was associated with normalization of vasoconstrictor responses, but not arterial remodeling. Rather, medial hypertrophy and adventitial fibrotic lesion formation were more pronounced following chronic TEMPOL treatment in hypoxia/SU5416 rats. Our findings support a major role for reactive oxygen species in mediating enhanced vasoconstrictor reactivity and pulmonary hypertension in both chronic hypoxia and hypoxia/SU5416 rat models, despite a paradoxical effect of antioxidant therapy to exacerbate arterial remodeling in animals with severe pulmonary arterial hypertension in the hypoxia/SU5416 model.

Isolation, culture and identification of pulmonary arterial smooth muscle cells from rat distal pulmonary arteries

The culture of pulmonary arterial smooth muscle cells (PASMCs) is one of the most powerful tools for exploring the mechanisms of pulmonary hypertension (PH). Both pulmonary vasoconstriction and remodeling occur predominantly in distal pulmonary arteries (PA). In this study, we provide our detailed and standardized protocol for easy isolation and culture of PASMCs from rat distal PA to supply every investigator with a simple, economical and useful method in studying PH. The protocol can be divided into four stages: isolation of distal PA, isolation of cells, growth in culture and passage of cells. Rat distal PASMCs were characterized by morphological activity and by immunostaining for smooth muscle α-actin and smooth muscle myosin heavy chain, but not for CD90/Thy-1 or von Willebrand factor. Furthermore, functional assessments were performed, confirming the presence of voltage-dependent Ca²⁺ channels and physiological characteristic of response to hypoxia. In conclusion, we have developed a detailed and simple protocol for obtaining rat distal PASMCs. These PASMCs exhibit features consistent with vascular smooth muscle cells, and they could subsequently be used to further explore the pathophysiological mechanisms of PH.

Pulmonary vascular collagen content, not cross-linking, contributes to right ventricular pulsatile afterload and overload in early pulmonary hypertension

Hypoxic pulmonary hypertension (HPH) is associated with pulmonary artery (PA) remodeling and right ventricular (RV) overload. We have previously uncovered collagen-mediated mechanisms of proximal PA stiffening in early HPH by manipulating collagen degradation and cross-linking using a transgenic mouse strain and a potent collagen cross-link inhibitor, β-aminopropionitrile (BAPN). However, the roles of collagen in distal PA remodeling, overall RV afterload, and RV hypertrophy in HPH remain unknown. Here, we used the same experimental strategy to investigate the effect of pulmonary vascular collagen content and cross-linking on steady and pulsatile RV afterload and on RV hypertrophy in early HPH. Collagenase-resistant mice (Col1a1^R/R) and their littermate controls (Col1a1^+/+) were exposed to normobaric hypoxia for 10 days with or without BAPN treatment. In vivo pulmonary vascular impedance, a comprehensive measure of RV afterload, was measured via simultaneous RV catheterization and echocardiography. Morphology and collagen accumulation were examined using histological techniques and ELISA in lungs and RVs. In both mouse strains, BAPN did not limit increases in pulmonary arterial pressure or pulmonary vascular resistance, indicating a negligible effect of either collagen content or cross-linking on steady RV afterload. However, BAPN prevented the increase in pulse pressure and RV hypertrophy in Col1a1^+/+ mice and these effects were absent in Col1a1^R/R mice, suggesting a role for PA collagen content, not cross-linking, in the pulsatile RV afterload. Moreover, we found a significant correlation between pulse pressure and RV hypertrophy, indicating an important role for pulsatile RV afterload in RV overload in early HPH.

NEW & NOTEWORTHY

The present study found an important role for collagen content, but not collagen cross-linking, in the pulsatile right ventricular (RV) afterload, which is correlated with RV hypertrophy. These results uncover a new collagen-mediated mechanical mechanism of RV dysfunction in early pulmonary hypertension progression. Furthermore, our results suggest that measures and metrics of pulsatile hemodynamics such as pulse pressure and pulse wave velocity are potentially important to cardiovascular mortality in patients with pulmonary hypertension.

Heterogeneous mechanics of the mouse pulmonary arterial network

Individualized modeling and simulation of blood flow mechanics find applications in both animal research and patient care. Individual animal or patient models for blood vessel mechanics are based on combining measured vascular geometry with a fluid structure model coupling formulations describing dynamics of the fluid and mechanics of the wall. For example, one-dimensional fluid flow modeling requires a constitutive law relating vessel cross-sectional deformation to pressure in the lumen. To investigate means of identifying appropriate constitutive relationships, an automated segmentation algorithm was applied to micro-computerized tomography images from a mouse lung obtained at four different static pressures to identify the static pressure-radius relationship for four generations of vessels in the pulmonary arterial network. A shape-fitting function was parameterized for each vessel in the network to characterize the nonlinear and heterogeneous nature of vessel distensibility in the pulmonary arteries. These data on morphometric and mechanical properties were used to simulate pressure and flow velocity propagation in the network using one-dimensional representations of fluid and vessel wall mechanics. Moreover, wave intensity analysis was used to study effects of wall mechanics on generation and propagation of pressure wave reflections. Simulations were conducted to investigate the role of linear versus nonlinear formulations of wall elasticity and homogeneous versus heterogeneous treatments of vessel wall properties. Accounting for heterogeneity, by parameterizing the pressure/distention equation of state individually for each vessel segment, was found to have little effect on the predicted pressure profiles and wave propagation compared to a homogeneous parameterization based on average behavior. However, substantially different results were obtained using a linear elastic thin-shell model than were obtained using a nonlinear model that has a more physiologically realistic pressure versus radius relationship.

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.

Hypoxic pulmonary hypertension in chronic lung diseases: novel vasoconstrictor pathways

Pulmonary hypertension is a well recognised complication of chronic hypoxic lung diseases, which are among the most common causes of death and disability worldwide. Development of pulmonary hypertension independently predicts reduced life expectancy. In chronic obstructive pulmonary disease, long-term oxygen therapy ameliorates pulmonary hypertension and greatly improves survival, although the correction of alveolar hypoxia and pulmonary hypertension is only partial. Advances in understanding of the regulation of vascular smooth muscle tone show that chronic vasoconstriction plays a more important part in the pathogenesis of hypoxic pulmonary hypertension than previously thought, and that structural vascular changes contribute less. Trials of existing vasodilators show that pulmonary hypertension can be ameliorated and systemic oxygen delivery improved in carefully selected patients, although systemic hypotensive effects limit the doses used. Vasoconstrictor pathways that are selective for the pulmonary circulation can be blocked to reduce hypoxic pulmonary hypertension without causing systemic hypotension, and thus provide potential targets for novel therapeutic strategies.

A non-invasive assessment of cardiopulmonary hemodynamics with MRI in pulmonary hypertension

PURPOSE

We propose a method for non-invasive quantification of hemodynamic changes in the pulmonary arteries resulting from pulmonary hypertension (PH).

METHODS

Using a two-element Windkessel model, and input parameters derived from standard MRI evaluation of flow, cardiac function and valvular motion, we derive: pulmonary artery compliance (C), mean pulmonary artery pressure (mPAP), pulmonary vascular resistance (PVR), pulmonary capillary wedge pressure (PCWP), time-averaged intra-pulmonary pressure waveforms and pulmonary artery pressures (systolic (sPAP) and diastolic (dPAP)). MRI results were compared directly to reference standard values from right heart catheterization (RHC) obtained in a series of patients with suspected pulmonary hypertension (PH).

RESULTS

In 7 patients with suspected PH undergoing RHC, MRI and echocardiography, there was no statistically significant difference (p<0.05) between parameters measured by MRI and RHC. Using standard clinical cutoffs to define PH (mPAP>25mmHg), MRI was able to correctly identify all patients as having pulmonary hypertension, and to correctly distinguish between pulmonary arterial (mPAP>25mmHg, PCWP<15mmHg) and venous hypertension (mPAP>25mmHg, PCWP>15mmHg) in 5 of 7 cases.

CONCLUSIONS

We have developed a mathematical model capable of quantifying physiological parameters that reflect the severity of PH.

A novel in vivo approach to assess radial and axial distensibility of large and intermediate pulmonary artery branches

Pulmonary arteries (PAs) distend to accommodate increases in cardiac output. PA distensibility protects the right ventricle (RV) from excessive increases in pressure. Loss of PA distensibility plays a critical role in the fatal progression of pulmonary arterial hypertension (PAH) toward RV failure. However, it is unclear how PA distensibility is distributed across the generations of PA branches, mainly because of the lack of appropriate in vivo methods to measure distensibility of vessels other than the large, conduit PAs. In this study, we propose a novel approach to assess the distensibility of individual PA branches. The metric of PA distensibility we used is the slope of the stretch ratio-pressure relationship. To measure distensibility, we combined invasive measurements of mean PA pressure with angiographic imaging of the PA network of six healthy female dogs. Stacks of 2D images of the PAs, obtained from either contrast enhanced magnetic resonance angiography (CE-MRA) or computed tomography digital subtraction angiography (CT-DSA), were used to reconstruct 3D surface models of the PA network, from the first bifurcation down to the sixth generation of branches. For each branch of the PA, we calculated radial and longitudinal stretch between baseline and a pressurized state obtained via acute embolization of the pulmonary vasculature. Our results indicated that large and intermediate PA branches have a radial distensibility consistently close to 2%/mmHg. Our axial distensibility data, albeit affected by larger variability, suggested that the PAs distal to the first generation may not significantly elongate in vivo, presumably due to spatial constraints. Results from both angiographic techniques were comparable to data from established phase-contrast (PC) magnetic resonance imaging (MRI) and ex vivo mechanical tests, which can only be used in the first branch generation. Our novel method can be used to characterize PA distensibility in PAH patients undergoing clinical right heart catheterization (RHC) in combination with MRI.

Impact of increased hematocrit on right ventricular afterload in response to chronic hypoxia

Chronic hypoxia causes chronic mountain sickness through hypoxia-induced pulmonary hypertension (HPH) and increased hematocrit. Here, we investigated the impact of increased hematocrit and HPH on right ventricular (RV) afterload via pulmonary vascular impedance. Mice were exposed to chronic normobaric hypoxia (10% oxygen) for 10 (10H) or 21 days (21H). After baseline hemodynamic measurements, ∼500 μl of blood were extracted and replaced with an equal volume of hydroxyethylstarch to normalize hematocrit and all hemodynamic measurements were repeated. In addition, ∼500 μl of blood were extracted and replaced in control mice with an equal volume of 90% hematocrit blood. Chronic hypoxia increased input resistance (Z0 increased 82% in 10H and 138% in 21H vs. CTL; P < 0.05) and characteristic impedance (ZC increased 76% in 10H and 109% in 21H vs. CTL; P < 0.05). Hematocrit normalization did not decrease mean pulmonary artery pressure but did increase cardiac output such that both Z0 and ZC decreased toward control levels. Increased hematocrit in control mice did not increase pressure but did decrease cardiac output such that Z0 increased. The paradoxical decrease in ZC with an acute drop in hematocrit and no change in pressure are likely due to inertial effects secondary to the increase in cardiac output. A novel finding of this study is that an increase in hematocrit affects the pulsatile RV afterload in addition to the steady RV afterload (Z0). Furthermore, our results highlight that the conventional interpretation of ZC as a measure of proximal artery stiffness is not valid in all physiological and pathological states.

Patchy deletion of Bmpr1a potentiates proximal pulmonary artery remodeling in mice exposed to chronic hypoxia

Reduced vascular expression of bone morphogenetic protein type IA receptor (Bmpr1a) has been found in patients with pulmonary arterial hypertension. Our previous studies in mice with patchy deletion of Bmpr1a in vascular smooth muscle cells and cardiac myocytes showed decreased distal vascular remodeling despite a similar severity of hypoxic pulmonary hypertension (HPH). We speculate increased stiffness from ectopic deposition of collagen in proximal pulmonary arteries might account for HPH. Pulsatile pressure-flow relationships were measured in isolated, ventilated, perfused lungs of SM22α;TRE-Cre;R26R;Bmpr1a(flox/flox) (KO) mice and wild-type littermates, following 21 days (hypoxia) and 0 days (control) of chronic hypoxia. Pulmonary vascular impedance, which yields insight into proximal and distal arterial remodeling, was calculated. Reduced Bmpr1a expression had no effect on input impedance Z(0) (P = 0.52) or characteristic impedance Z(C) (P = 0.18) under control conditions; it also had no effect on the decrease in Z(0) via acute rho kinase inhibition. However, following chronic hypoxia, reduced Bmpr1a expression increased Z(C) (P < 0.001) without affecting Z(0) (P = 0.72). These results demonstrate that Bmpr1a deficiency does not significantly alter the hemodynamic function of the distal vasculature or its response to chronic hypoxia but larger, more proximal arteries are affected. In particular, reduced Bmpr1a expression likely decreased dilatation and increased stiffening in response to hypoxia, probably by collagen accumulation. Increased PA stiffness can have a significant impact on right ventricular function. This study illustrates for the first time how proximal pulmonary artery changes in the absence of distal pulmonary artery changes contribute to pulmonary arterial hypertension.

Pulmonary vascular mechanics: important contributors to the increased right ventricular afterload of pulmonary hypertension

Chronic hypoxia causes pulmonary vasoconstriction and vascular remodelling, which lead to hypoxic pulmonary hypertension (HPH). Hypoxic pulmonary hypertension is associated with living at high altitudes and is a complication of many lung diseases, including chronic obstructive pulmonary disease, cystic fibrosis and obstructive sleep apnoea. Pulmonary vascular changes that occur with HPH include stiffening and narrowing of the pulmonary arteries that appear to involve all vascular cell types and sublayers of the arterial wall. Right ventricular (RV) changes that occur with HPH include RV hypertrophy and RV fibrosis, often with preserved systolic and diastolic function and ventricular-vascular coupling efficiency. Both vascular stiffening and vascular narrowing are important contributors to RV afterload via increases in oscillatory and steady ventricular work, respectively. The increased blood viscosity that occurs in HPH can be dramatic and is another important contributor to RV afterload. However, the viscosity, vascular mechanics and ventricular changes that occur with HPH are all reversible. Furthermore, even with continued hypoxia the vascular remodelling does not progress to the obliterative, plexiform lesions that are seen clinically in severe pulmonary hypertension. In animal models, the RV changes appear adaptive, not maladaptive. In summary, HPH-induced vascular mechanical changes affect ventricular function, but both are adaptive and reversible, which differentiates HPH from severe pulmonary hypertension. The mechanisms of adaptation and reversibility may provide useful insight into therapeutic targets for the clinical disease state.

Mitigation of radiation induced pulmonary vascular injury by delayed treatment with captopril

BACKGROUND AND OBJECTIVE

A single dose of 10 Gy radiation to the thorax of rats results in decreased total lung angiotensin-converting enzyme (ACE) activity, pulmonary artery distensibility and distal vascular density while increasing pulmonary vascular resistance (PVR) at 2 months post-exposure. In this study, we evaluate the potential of a renin-angiotensin system (RAS) modulator, the ACE inhibitor captopril, to mitigate this pulmonary vascular damage.

METHODS

Rats exposed to 10 Gy thorax only irradiation and age-matched controls were studied 2 months after exposure, during the development of radiation pneumonitis. Rats were treated, either immediately or 2 weeks after radiation exposure, with two doses of the ACE inhibitor, captopril, dissolved in their drinking water. To determine pulmonary vascular responses, we measured pulmonary haemodynamics, lung ACE activity, pulmonary arterial distensibility and peripheral vessel density.

RESULTS

Captopril, given at a vasoactive, but not a lower dose, mitigated radiation-induced pulmonary vascular injury. More importantly, these beneficial effects were observed even if drug therapy was delayed for up to 2 weeks after exposure.

CONCLUSIONS

Captopril resulted in a reduction in pulmonary vascular injury that supports its use as a radiomitigator after an unexpected radiological event such as a nuclear accident.

Vascular remodeling in pulmonary hypertension

Pulmonary hypertension is a complex, progressive condition arising from a variety of genetic and pathogenic causes. Patients present with a spectrum of histologic and pathophysiological features, likely reflecting the diversity in underlying pathogenesis. It is widely recognized that structural alterations in the vascular wall contribute to all forms of pulmonary hypertension. Features characteristic of the remodeled vasculature in patients with pulmonary hypertension include increased stiffening of the elastic proximal pulmonary arteries, thickening of the intimal and/or medial layer of muscular arteries, development of vaso-occlusive lesions, and the appearance of cells expressing smooth muscle-specific markers in normally non-muscular small diameter vessels, resulting from proliferation and migration of pulmonary arterial smooth muscle cells and cellular transdifferentiation. The development of several animal models of pulmonary hypertension has provided the means to explore the mechanistic underpinnings of pulmonary vascular remodeling, although none of the experimental models currently used entirely replicates the pulmonary arterial hypertension observed in patients. Herein, we provide an overview of the histological abnormalities observed in humans with pulmonary hypertension and in preclinical models and discuss insights gained regarding several key signaling pathways contributing to the remodeling process. In particular, we will focus on the roles of ion homeostasis, endothelin-1, serotonin, bone morphogenetic proteins, Rho kinase, and hypoxia-inducible factor 1 in pulmonary arterial smooth muscle and endothelial cells, highlighting areas of cross-talk between these pathways and potentials for therapeutic targeting.

Chronic hypoxia decreases arterial and venous compliance in isolated perfused rat lungs: an effect that is reversed by exogenous L-arginine

Chronic hypoxia (CH)-induced pulmonary hypertension is characterized by vasoconstriction and vascular remodeling, leading to right ventricular dysfunction. Given the role of arterial compliance (C(a)) in right ventricular work, a decrease in C(a) would add to right ventricular work. Nitric oxide (NO) is a potent vasodilator made by NO synthases from L-arginine (L-Arg). However, little is known of the effect of L-Arg on vascular compliance (C(v)) in the lung. We hypothesized that exposure to CH would decrease C(a) and that this effect would be reversed by exogenous L-Arg. Sprague-Dawley rats were exposed to either normoxia or CH for 14 days; the lungs were then isolated and perfused. Vascular occlusions were performed and modeled using a three-compliance, two-resistor model. Pressure-flow curves were generated, and a distensible vessel model was used to estimate distensibility and a vascular resistance parameter (R(0)). Hypoxia resulted in the expected increase in arterial resistance (R(a)) as well as a decrease in both C(a) and C(v). L-Arg had little effect on R(a), C(a), or C(v) in isolated lungs from normoxic animals. L-Arg decreased R(a) in lungs from CH rats and redistributed compliance to approximately that found in normoxic lungs. CH increased R(0), and L-Arg reversed this increase in R(0). L-Arg increased exhaled NO, and inhibition of L-Arg uptake attenuated the L-Arg-induced increase in exhaled NO. These data demonstrate that the CH-induced decrease in C(a) was reversed by L-Arg, suggesting that L-Arg may improve CH-induced right ventricular dysfunction.

Abnormal pulmonary artery stiffness in pulmonary arterial hypertension: in vivo study with intravascular ultrasound

Background

There is increasing recognition that pulmonary artery stiffness is an important determinant of right ventricular (RV) afterload in pulmonary arterial hypertension (PAH). We used intravascular ultrasound (IVUS) to evaluate the mechanical properties of the elastic pulmonary arteries (PA) in subjects with PAH, and assessed the effects of PAH-specific therapy on indices of arterial stiffness.

Method

Using IVUS and simultaneous right heart catheterisation, 20 pulmonary segments in 8 PAH subjects and 12 pulmonary segments in 8 controls were studied to determine their compliance, distensibility, elastic modulus and stiffness index β. PAH subjects underwent repeat IVUS examinations after 6-months of bosentan therapy.

Results

At baseline, PAH subjects demonstrated greater stiffness in all measured indices compared to controls: compliance (1.50±0.11×10^–2 mm^2/mmHg vs 4.49±0.43×10^–2 mm^2/mmHg, p<0.0001), distensibility (0.32±0.03%/mmHg vs 1.18±0.13%/mmHg, p<0.0001), elastic modulus (720±64 mmHg vs 198±19 mmHg, p<0.0001), and stiffness index β (15.0±1.4 vs 11.0±0.7, p = 0.046). Strong inverse exponential associations existed between mean pulmonary artery pressure and compliance (r² = 0.82, p<0.0001), and also between mean PAP and distensibility (r² = 0.79, p = 0.002). Bosentan therapy, for 6-months, was not associated with any significant changes in all indices of PA stiffness.

Conclusion

Increased stiffness occurs in the proximal elastic PA in patients with PAH and contributes to the pathogenesis RV failure. Bosentan therapy may not be effective at improving PA stiffness.

A brief overview of mouse models of pulmonary arterial hypertension: problems and prospects

Many chronic pulmonary diseases are associated with pulmonary hypertension (PH) and pulmonary vascular remodeling, which is a term that continues to be used to describe a wide spectrum of vascular abnormalities. Pulmonary vascular structural changes frequently increase pulmonary vascular resistance, causing PH and right heart failure. Although rat models had been standard models of PH research, in more recent years the availability of genetically engineered mice has made this species attractive for many investigators. Here we review a large amount of data derived from experimental PH reports published since 1996. These studies using wild-type and genetically designed mice illustrate the challenges and opportunities provided by these models. Hemodynamic measurements are difficult to obtain in mice, and right heart failure has not been investigated in mice. Anatomical, cellular, and genetic differences distinguish mice and rats, and pharmacogenomics may explain the degree of PH and the particular mode of pulmonary vascular adaptation and also the response of the right ventricle.

Persistent vascular collagen accumulation alters hemodynamic recovery from chronic hypoxia

Pulmonary arterial hypertension (PAH) is caused by narrowing and stiffening of the pulmonary arteries that increase pulmonary vascular impedance (PVZ). In particular, small arteries narrow and large arteries stiffen. Large pulmonary artery (PA) stiffness is the best current predictor of mortality from PAH. We have previously shown that collagen accumulation leads to extralobar PA stiffening at high strain (Ooi et al. 2010). We hypothesized that collagen accumulation would increase PVZ, including total pulmonary vascular resistance (Z(0)), characteristic impedance (Z(C)), pulse wave velocity (PWV) and index of global wave reflections (P(b)/P(f)), which contribute to increased right ventricular afterload. We tested this hypothesis by exposing mice unable to degrade type I collagen (Col1a1(R/R)) to 21 days of hypoxia (hypoxia), some of which were allowed to recover for 42 days (recovery). Littermate wild-type mice (Col1a1(+/+)) were used as controls. In response to hypoxia, mean PA pressure (mPAP) increased in both mouse genotypes with no changes in cardiac output (CO) or PA inner diameter (ID); as a consequence, Z(0) (mPAP/CO) increased by ~100% in both genotypes (p<0.05). Contrary to our expectations, Z(C), PWV and P(b)/P(f) did not change. However, with recovery, Z(C) and PWV decreased in the Col1a1(+/+) mice and remained unchanged in the Col1a1(R/R) mice. Z(0) decreased with recovery in both genotypes. Microcomputed tomography measurements of large PAs did not show evidence of stiffness changes as a function of hypoxia exposure or genotype. We conclude that hypoxia-induced PA collagen accumulation does not affect the pulsatile components of pulmonary hemodynamics but that excessive collagen accumulation does prevent normal hemodynamic recovery, which may have important consequences for right ventricular function.

Pulmonary vascular wall stiffness: An important contributor to the increased right ventricular afterload with pulmonary hypertension

Pulmonary hypertension (PH) is associated with structural and mechanical changes in the pulmonary vascular bed that increase right ventricular (RV) afterload. These changes, characterized by narrowing and stiffening, occur in both proximal and distal pulmonary arteries (PAs). An important consequence of arterial narrowing is increased pulmonary vascular resistance (PVR). Arterial stiffening, which can occur in both the proximal and distal pulmonary arteries, is an important index of disease progression and is a significant contributor to increased RV afterload in PH. In particular, arterial narrowing and stiffening increase the RV afterload by increasing steady and oscillatory RV work, respectively. Here we review the current state of knowledge of the causes and consequences of pulmonary arterial stiffening in PH and its impact on RV function. We review direct and indirect techniques for measuring proximal and distal pulmonary arterial stiffness, measures of arterial stiffness including elastic modulus, incremental elastic modulus, stiffness coefficient β and others, the changes in cellular function and the extracellular matrix proteins that contribute to pulmonary arterial stiffening, the consequences of PA stiffening for RV function and the clinical implications of pulmonary vascular stiffening for PH progression. Future investigation of the relationship between PA stiffening and RV dysfunction may facilitate new therapies aimed at improving RV function and thus ultimately reducing mortality in PH.

Characterization of the isolated, ventilated, and instrumented mouse lung perfused with pulsatile flow

The isolated, ventilated and instrumented mouse lung preparation allows steady and pulsatile pulmonary vascular pressure-flow relationships to be measured with independent control over pulmonary arterial flow rate, flow rate waveform, airway pressure and left atrial pressure. Pulmonary vascular resistance is calculated based on multi-point, steady pressure-flow curves; pulmonary vascular impedance is calculated from pulsatile pressure-flow curves obtained at a range of frequencies. As now recognized clinically, impedance is a superior measure of right ventricular afterload than resistance because it includes the effects of vascular compliance, which are not negligible, especially in the pulmonary circulation. Three important metrics of impedance--the zero hertz impedance Z(0;), the characteristic impedance Z(C;), and the index of wave reflection R(W;)--provide insight into distal arterial cross-sectional area available for flow, proximal arterial stiffness and the upstream-downstream impedance mismatch, respectively. All results obtained in isolated, ventilated and perfused lungs are independent of sympathetic nervous system tone, volume status and the effects of anesthesia. We have used this technique to quantify the impact of pulmonary emboli and chronic hypoxia on resistance and impedance, and to differentiate between sites of action (i.e., proximal vs. distal) of vasoactive agents and disease using the pressure dependency of Z(C;). Furthermore, when these techniques are used with the lungs of genetically engineered strains of mice, the effects of molecular-level defects on pulmonary vascular structure and function can be determined.