Role of hemodynamics in pulmonary vascular remodeling: implications for primary pulmonary hypertension

Potential damage in pulmonary arterial hypertension: An experimental study of pressure-induced damage of pulmonary artery

Pulmonary arterial hypertension (PAH) is associated with elevated pulmonary arterial pressure. PAH prognosis remains poor with a 15% mortality rate within 1 year, even with modern clinical management. Previous clinical studies proposed wall shear stress (WSS) to be an important hemodynamic factor affecting cell mechanotransduction, growth and remodeling, and disease progress in PAH. However, WSS in vivo is typically at most 2.5 Pa and a doubt has been cast whether WSS alone can drive disease progress. Furthermore, our current understanding of PAH pathology largely comes from small animals' studies in which caliber enlargement, a hallmark of PAH in humans, is rarely reported. Therefore, a large-animal experiment on pulmonary arteries (PAs) is needed to validate whether increased pressure can induce enlargement of PAs caliber. In this study, we use an inflation testing device to characterize the mechanical behavior, both nonlinear elastic behavior and irreversible damage of porcine arteries. The parameters of elastic behavior are estimated from the inflation test at a low-pressure range before and after over-pressurization. Then, histological images are qualitatively examined for medial and adventitial layers. This study sheds light on the relevance of pressure-induced damage mechanism in human PAH.

Pulmonary Arterial Dilatation: Imaging Evaluation Using Multidetector Computed Tomography

Pulmonary artery dilatation comprises a heterogeneous group of disorders. Early diagnosis is important as the presentation may be incidental, chronic, or acute and life threatening depending upon the etiology. Cross-sectional imaging plays an important role, with CT pulmonary angiography being regarded as the first line investigation in the evaluation of pulmonary artery pathologies. Moreover, effects of pulmonary artery lesions on proximal and distal circulation can also be ascertained with the detection of associated conditions. Special attention should also be given to the left main coronary artery and the trachea-bronchial tree as they may be extrinsically compressed by the dilated pulmonary artery. In context of an appropriate clinical background, CT pulmonary angiography also helps in treatment planning, prognostication, and follow-up of these patients. This review mainly deals with imaging evaluation of the pulmonary arterial dilatations on CT with emphasis on the gamut of etiologies in the adult as well as pediatric populations.

RETROSPECTIVE COMPUTER-ASSISTED IMAGE ANALYSIS OF DOPPLER ECHOCARDIOGRAPHY IN PULMONARY HYPERTENSION PATIENTS

Pulmonary hypertension (PH) is diagnosed invasively by right heart catheterization (RHC), which determines patient’s mean and systolic pulmonary artery pressure (mPAP, sPAP) and pulmonary vascular resistance (PVR). This study sought to identify non-invasive echocardiography parameters useful for screening PH. Patients ([Formula: see text]; 19 normotensive; 17 pre-capillary PH; 48 post-capillary PH) who had undergone transthoracic Doppler echocardiography and RHC within 60 days of each other were identified. Tricuspid regurgitant (TR) jet velocities, velocity spectral densities, average flow rates, and Fourier transforms (FFT) of velocity waveforms were calculated via an in-house MATLAB code. Correlations were found between the FFT magnitude at 0 Hz and sPAP and mPAP for normotensive patients; between the MATLAB-calculated TR jet and sPAP and PVR for all PH patients; and between the sum of FFT magnitudes [Formula: see text][Formula: see text]Hz and mPAP, sPAP, and PVR for post-capillary PH patients. Statistical difference was found between the FFT magnitudes at 2 Hz of pre- and post-capillary PH patients ([Formula: see text]). These results suggest non-invasive parameters with clinical utility for estimating RHC measurements and discriminating between PH types, offering a path forward for less invasive and more accessible PH screening protocol.

The outcome of pulmonary hypertension and its association with pulmonary artery dilatation

Background

Pulmonary artery (PA) dilatation is often seen in pulmonary hypertension (PH) and is considered a long-term consequence of elevated pressure. The PA dilates over time and therefore may reflect disease severity and duration. Survival is related to the stage of the disease at the time of diagnosis and therefore PA diameter might be used to predict prognosis. This study evaluates the outcome of patients with pulmonary arterial hypertension (PAH) and chronic thrombo-embolic pulmonary hypertension (CTEPH) and investigates whether PA diameter at the time of diagnosis is associated with mortality.

Methods

Patients visiting an outpatient clinic of a tertiary centre between 2004 and 2018 with a cardiac catheterisation confirmed diagnosis of PAH or CTEPH and a CT scan available for PA diameter measurement were included. PA diameter and established predictors of survival were collected (New York Heart Association (NYHA) class, N‑terminal pro-brain natriuretic peptide (NT-proBNP) level and 6‑min walking distance (6MWD)).

Results

In total 217 patients were included (69% female, 71% NYHA class ≥III). During a median follow-up of 50 (22–92) months, 54% of the patients died. Overall survival was 87% at 1 year, 70% at 3 years and 58% at 5 years. The mean PA diameter was 34.2 ± 6.2 mm and was not significantly different among all the diagnosis groups. We found a weak correlation between PA diameter and mean PA pressure ( r = 0.23, p < 0.001). Male sex, higher age, shorter 6MWD and higher NT-proBNP level were independently associated with mortality, but PA diameter was not.

Conclusion

The prognosis of PAH and CTEPH is still poor. Known predictors of survival were confirmed, but PA diameter at diagnosis was not associated with survival in PAH or CTEPH patients.

Outcomes of infants and children undergoing surgical repair of ventricular septal defect: a review of the literature and implications for research with an emphasis on pulmonary artery hypertension

BACKGROUND

Pulmonary vascular disease resulting from CHDs may be the most preventable cause of pulmonary artery hypertension worldwide. Many children in developing countries still do not have access to early closure of clinically significant defects, and the long-term outcomes after corrective surgery remain unclear. Focused on long-term results after isolated ventricular septal defect repair, our review sought to determine the most effective medical therapy for the pre-operative management of elevated left-to-right shunts in patients with an isolated ventricular septal defect.

METHODS

We identified articles specific to the surgical repair of isolated ventricular septal defects. Specific parameters included the pathophysiology and pre-operative medical management of pulmonary over-circulation and outcomes.

RESULTS

Studies most commonly focused on histologic changes to the pulmonary vasculature and levels of thromboxanes, prostaglandins, nitric oxide, endothelin, and matrix metalloproteinases. Only 2/44 studies mentioned targeted pharmacologic management to any of these systems related to ventricular septal defect repair; no study offered evidence-based guidelines to manage pulmonary over-circulation with ventricular septal defects. Most studies with long-term data indicated a measurable frequency of pulmonary artery hypertension or diminished exercise capacity late after ventricular septal defect repair.

CONCLUSION

Long-term pulmonary vascular and respiratory changes can occur in children after ventricular septal defect repair. Research should be directed at providing an evidenced-based approach to the medical management of infants and children with ventricular septal defects (and naturally all CHDs) to minimise consequences of pulmonary artery hypertension, particularly as defect repair may occur late in underprivileged societies.

A new method for visualizing pulmonary artery microvasculature using synchrotron radiation pulmonary microangiography: the measurement of pulmonary arterial blood flow velocity in the high pulmonary blood flow rat model

BACKGROUND

Increased pulmonary blood flow (PBF) and shear stress may provoke irreversible vascular remodeling, yet invasive visualization of the microvasculature complicates monitoring. A non-invasive imaging methodology would therefore safely provide mechanistic insights into the progression of high PBF-induced vascular remodeling.

PURPOSE

To establish a novel microvasculature visualization method using synchrotron radiation pulmonary microangiography (SRPA) that can also calculate PBF velocity in vivo.

MATERIAL AND METHODS

A high PBF rat model was established by making a fistula between the abdominal aorta and inferior vena cava. After eight weeks, SRPA was performed and the dynamic density changes in the right lower pulmonary artery (PA) were calculated by software. SRPA was performed with a HARP (High-Gain Avalanche Rushing amorphous Photoconductor) receiver. PBF velocity was calculated by contrast medium transit time within the PA. All data were presented as mean ± standard error (SE). Student's t-test was used for comparison between the two groups.

RESULTS

High dynamic spatial and contrast resolution from SRPA in the PA allowed for clear pulmonary microangiography and accurate detection of higher PBF in the rat model (82.3 ± 8.5 mm/s high-PBF group vs. 46.1 ± 4.3 mm/s control group, P < 0.01).

CONCLUSIONS

These novel results demonstrate that SRPA was useful in both visualizing the dynamic flow distribution within the microvasculature and calculating PBF velocity. This newly developed, non-invasive technology may become a powerful tool in clarifying the mechanism of vascular remodeling associated with high PBF-induced shear stress.

Egg and banana sign of severe pulmonary arterial hypertension

The egg and banana sign can be seen on chest computed tomography (CT) in patients with severe pulmonary arterial hypertension (PAH). It is identified by the presence of the pulmonary artery (PA) lateral to the aortic arch with the aortic arch being described as the banana and the PA as the egg.

Changes in main pulmonary artery diameter during follow-up have prognostic implications in pulmonary arterial hypertension

BACKGROUND AND OBJECTIVE

A dilated pulmonary artery (PA) is a common finding in patients with pulmonary arterial hypertension (PAH). Little is known on the variations in PA size over time and whether these changes track with disease severity and/or predict long-term survival.

METHODS

We included patients with PAH who had at least two computed tomography (CT) scans of the chest done on different visits. Both scans matched the use of i.v. contrast.

RESULTS

Pairs of CT scans were compared in 113 PAH patients. During a median (interquartile range (IQR)) time difference between scans of 8 (IQR: 3.5-20.0) months, we noted an increase in main PA diameter of 0.5 ± 1.8 mm (mean ± SD) (P = 0.008). When CT scans were performed >12 months apart (n = 47), the main PA diameter increased or decreased by >1 mm in 40% and 13% of the patients, respectively. An increase in main PA diameter was associated with lower PA compliance, higher right ventricular (RV) systolic pressure, worse RV function and a decline in 6-min walk distance. During a median (IQR) follow-up of 33 (IQR: 4.5-47) months, 53 (46.9%) patients died. The change in PA diameter was a significant predictor of mortality (hazard ratio (HR) per mm increase: 1.33 (95% CI: 1.11-1.61), P = 0.002) when adjusted for difference in time and slice thickness between CT scans, age, gender, PAH aetiology and pulmonary vascular resistance.

CONCLUSION

In PAH patients, an increase in CT-derived main PA diameter over time is associated with progression in pulmonary pressures, RV dysfunction, a decline in functional capacity and higher mortality.

Delayed Microvascular Shear Adaptation in Pulmonary Arterial Hypertension. Role of Platelet Endothelial Cell Adhesion Molecule-1 Cleavage

RATIONALE

Altered pulmonary hemodynamics and fluid flow-induced high shear stress (HSS) are characteristic hallmarks in the pathogenesis of pulmonary arterial hypertension (PAH). However, the contribution of HSS to cellular and vascular alterations in PAH is unclear.

OBJECTIVES

We hypothesize that failing shear adaptation is an essential part of the endothelial dysfunction in all forms of PAH and tested whether microvascular endothelial cells (MVECs) or pulmonary arterial endothelial cells (PAECs) from lungs of patients with PAH adapt to HSS and if the shear defect partakes in vascular remodeling in vivo.

METHODS

PAH MVEC (n = 7) and PAH PAEC (n = 3) morphology, function, protein, and gene expressions were compared with control MVEC (n = 8) under static culture conditions and after 24, 72, and 120 hours of HSS.

MEASUREMENTS AND MAIN RESULTS

PAH MVEC showed a significantly delayed morphological shear adaptation (P = 0.03) and evidence of cell injury at sites of nonuniform shear profiles that are critical loci for vascular remodeling in PAH. In clear contrast, PAEC isolated from the same PAH lungs showed no impairments. PAH MVEC gene expression and transcriptional shear activation were not altered but showed significant decreased protein levels (P = 0.02) and disturbed interendothelial localization of the shear sensor platelet endothelial cell adhesion molecule-1 (PECAM-1). The decreased PECAM-1 levels were caused by caspase-mediated cytoplasmic cleavage but not increased cell apoptosis. Caspase blockade stabilized PECAM-1 levels, restored endothelial shear responsiveness in vitro, and attenuated occlusive vascular remodeling in chronically hypoxic Sugen5416-treated rats modeling severe PAH.

CONCLUSIONS

Delayed shear adaptation, which promotes shear-induced endothelial injury, is a newly identified dysfunction specific to the microvascular endothelium in PAH. The shear response is normalized on stabilization of PECAM-1, which reverses intimal remodeling in vivo.

Significance of main pulmonary artery dilation on imaging studies

Proper and early identification of patients who harbor serious occult illness is the first step in developing a disease-management strategy. Identification of illnesses through the use of noninvasive techniques provides assurance of patient safety and is ideal. PA dilation is easily measured noninvasively and is due to a variety of conditions, including pulmonary hypertension (PH). The clinician should be able to thoroughly assess the significance of PA dilation in each individual patient. This involves knowledge of the ability of PA dilation to accurately predict PH, understand the wide differential diagnosis of causes of PA dilation, and reverse its life-threatening complications. We found that although PA dilation is suggestive of PH, data remain inconclusive regarding its ability to accurately predict PH. At this point, data are insufficient to place PA dilation into a PH risk-score equation. Here we review the causes and complications of PA dilation, define normal and abnormal PA measurements, and summarize the data linking its association to PH, while suggesting an algorithm designed to assist clinicians in patient work-up after recognizing PA dilation.

Blood Flow Simulations for the Design of Stented Valve Reducer in Enlarged Ventricular Outflow Tracts

Tetralogy of Fallot is a congenital heart disease characterized over time, after the initial repair, by the absence of a functioning pulmonary valve, which causes regurgitation, and by progressive enlargement of the right ventricle outflow tract (RVOT). Due to this pathological anatomy, available transcatheter valves are usually too small to be deployed there. To avoid surgical valve replacement, an alternative consists in implanting a reducer prior to or in combination with the valve. It has been shown in animal experiments to be promising, but with some limitations. The effect of a percutaneous pulmonary valve reducer on hemodynamics in enlarged RVOT is thus studied by computational modeling. To this aim, blood flow in the RVOT is modeled with CFD coupled to a simplified valve model and 0D downstream models. Simulations are performed in an image-based geometry and boundary conditions tuned to reproduce the pathological flow without the device. Different device designs are built and compared with the initial device-free state, or with the reducer alone. Results suggest that pressure loss is higher for the reducer alone than for the full device, and that the latter successfully restores hemodynamics to a healthy state and induces a more symmetric flow in the pulmonary arteries. Moreover, pressure forces on the reducer and on the valve have the same magnitudes. Migration would occur towards the right ventricle rather than the pulmonary arteries. Results support the thesis that the reducer does not introduce clinically significant pressure gradients, as was found in animal experiments. Such study could help transfer to patients.

Electrocardiogram-gated 320-slice multidetector computed tomography for the measurement of pulmonary arterial distensibility in chronic thromboembolic pulmonary hypertension

Background

We aimed to study whether pulmonary arterial distensibility (PAD) correlates with hemodynamic parameters in chronic thromboembolic pulmonary hypertension (CTEPH) using electrocardiogram (ECG)-gated 320-slice multidetector computed tomography (MDCT).

Methods and Findings

ECG-gated 320-slice MDCT and right heart catheterization (RHC) was performed in 53 subjects (60.6±11.4 years old; 37 females) with CTEPH. We retrospectively measured the minimum and maximum values of the cross sectional area (CSA) of the main pulmonary artery (mainPA), right pulmonary artery (rtPA), and left pulmonary artery (ltPA) during one heartbeat. PAD was calculated using the following formula: PAD = [(CSAmaximum−CSAminimum)/CSAmaximum]×100(%). The correlation between hemodynamic parameters and PAD was assessed. Mean pulmonary arterial pressure (mPAP) and pulmonary vascular resistance (PVR) were 40.8±8.7 mmHg and 8.3±3.0 wood units, respectively. PAD values were as follows: mainPA (14.0±5.0%), rtPA (12.8±5.6%), and ltPA (9.7±4.6%). Good correlations existed between mainPAD, with mPAP (r = −0.594, p<0.001) and PVR (r = −0.659, p<0.001). The correlation coefficients between rtPAD and ltPAD with pulmonary hemodynamics were all lower or equal than for mainPAD.

Conclusions

PAD measured using ECG-gated 320-slice MDCT correlates with pulmonary hemodynamics in subjects with CTEPH. The mainPA is suitable for PAD measurement.

Prediction of hemodynamic improvement after pulmonary endarterectomy in chronic thromboembolic pulmonary hypertension using non-invasive imaging

Pulmonary endarterectomy (PEA) is the recommended treatment in chronic thromboembolic pulmonary hypertension (CTEPH). Prediction of outcome after PEA remains challenging. In search for pre-operative predictors we evaluated non-invasive parameters measured by chest CT-scan and echocardiography. Between May 2004 and January 2009, 52 consecutive patients with CTEPH who underwent PEA (59.6 % female, mean age 58.9 ± 13.4 years) were included. Prior to surgery, pulmonary artery (PA) diameter indices were calculated by chest CT scan and different echocardiographic measurements to evaluate pulmonary hypertension were obtained. Hemodynamic improvement after PEA was defined as a pulmonary vascular resistance (PVR) <500 dyn s cm(-5) and a mean pulmonary artery pressure <35 mmHg 3 days after PEA. Mortality was evaluated at day 30. Mean pulmonary artery pressure (PAP) at baseline was 40.1 ± 8.5 mmHg, with a PVR of 971 ± 420 dyn s cm(-5). Persistent pulmonary hypertension was observed in 15 patients (28.8 %). Gender, pre-operative mean PAP, PA diameter indices, and tricuspid annular plane systolic excursion were all predictors for hemodynamic improvement after PEA. The indexed PA diameter on CT was the only independent predictor for hemodynamic improvement: 19.4 ± 2.4 versus 22.9 ± 4.9 mm/m(2) in those without improvement (OR 0.76: 95 % CI 0.58-0.99, p = 0.04). All patients who died within 30 days (9.6 %) had persistent pulmonary hypertension, with a post-operative mean PAP of 51.6 ± 14.1 mmHg and PVR of 692 ± 216 dyn s cm(-5). The pre-operative PA diameter indexed for body surface area is the only independent predictor for hemodynamic improvement after PEA in CTEPH patients. In all patients who died within 30 days after PEA, persistent pulmonary hypertension was present.

Pulmonary arterial remodeling in chronic obstructive pulmonary disease is lobe dependent

Abstract Pulmonary arterial remodeling has been demonstrated in patients with severe chronic obstructive pulmonary disease (COPD), but it is not known whether lobar heterogeneity of remodeling occurs. Furthermore, the relationship between pulmonary hypertension (PH) and pulmonary arterial remodeling in COPD has not been established. Muscular pulmonary arterial remodeling in arteries 0.10-0.25 mm in diameter was assessed in COPD-explanted lungs and autopsy controls. Remodeling was quantified as the percentage wall thickness to vessel diameter (%WT) using digital image analysis. Repeat measures mixed-effects remodeling for %WT was performed according to lobar origin (upper and lower), muscular pulmonary arterial size (small, medium, and large), and echocardiography-based pulmonary arterial pressure (no PH, mild PH, and moderate-to-severe PH). Lobar perfusion and emphysema indices were determined from ventilation-perfusion and computed tomography scans, respectively. Overall, %WT was greater in 42 subjects with COPD than in 5 control subjects ([Formula: see text]). Within the COPD group, %WT was greater in the upper lobes ([Formula: see text]) and in the small muscular pulmonary arteries ([Formula: see text]). Lobar differences were most pronounced in medium and large arteries. Lobar emphysema index was not associated with arterial remodeling. However, there was a significant positive relationship between the lobar perfusion index and pulmonary arterial remodeling ([Formula: see text]). The presence of PH on echocardiography showed only a trend to a small effect on lower lobe remodeling. The pattern of pulmonary arterial remodeling in COPD is complicated and lobe dependent. Differences in regional blood flow partially account for the lobar heterogeneity of pulmonary arterial remodeling in COPD.

Reactive oxygen species in pulmonary vascular remodeling

The pathogenesis of pulmonary hypertension is a complex multifactorial process that involves the remodeling of pulmonary arteries. This remodeling process encompasses concentric medial thickening of small arterioles, neomuscularization of previously nonmuscular capillary-like vessels, and structural wall changes in larger pulmonary arteries. The pulmonary arterial muscularization is characterized by vascular smooth muscle cell hyperplasia and hypertrophy. In addition, in uncontrolled pulmonary hypertension, the clonal expansion of apoptosis-resistant endothelial cells leads to the formation of plexiform lesions. Based upon a large number of studies in animal models, the three major stimuli that drive the vascular remodeling process are inflammation, shear stress, and hypoxia. Although, the precise mechanisms by which these stimuli impair pulmonary vascular function and structure are unknown, reactive oxygen species (ROS)-mediated oxidative damage appears to play an important role. ROS are highly reactive due to their unpaired valence shell electron. Oxidative damage occurs when the production of ROS exceeds the quenching capacity of the antioxidant mechanisms of the cell. ROS can be produced from complexes in the cell membrane (nicotinamide adenine dinucleotide phosphate-oxidase), cellular organelles (peroxisomes and mitochondria), and in the cytoplasm (xanthine oxidase). Furthermore, low levels of tetrahydrobiopterin (BH4) and L-arginine the rate limiting cofactor and substrate for endothelial nitric oxide synthase (eNOS), can cause the uncoupling of eNOS, resulting in decreased NO production and increased ROS production. This review will focus on the ROS generation systems, scavenger antioxidants, and oxidative stress associated alterations in vascular remodeling in pulmonary hypertension.

Wall shear stress measured by phase contrast cardiovascular magnetic resonance in children and adolescents with pulmonary arterial hypertension

BACKGROUND

Pulmonary arterial hypertension (PAH) is a devastating disease with significant morbidity and mortality. At the macroscopic level, disease progression is observed as a complex interplay between mean pulmonary artery pressure, pulmonary vascular resistance, pulmonary vascular stiffness, arterial size, and flow. Wall shear stress (WSS) is known to mediate or be dependent on a number of these factors. Given that WSS is known to promote architectural vessel remodeling, it is imperative that the changes of this factor be quantified in the presence of PAH.

METHODS

In this study, we analyzed phase contrast imaging of the right pulmonary artery derived from cardiovascular magnetic resonance to quantify the local, temporal and circumferentially averaged WSS of a PAH population and a pediatric control population. In addition, information about flow and relative area change were derived.

RESULTS

Although the normotensive and PAH shear waveform exhibited a WSS profile which is uniform in magnitude and direction along the vessel circumference at systole, time-averaged WSS (2.2 ± 1.6 vs. 6.6 ± 3.4 dynes/cm(2), P = 0.018) and systolic WSS (8.2 ± 5.0 v. 20.0 ± 9.1 dynes/cm(2), P = 0.018) was significantly depressed in the PAH population as compared to the controls. BSA-indexed PA diameter was significantly larger in the PAH population (1.5 ± 0.4 vs. 0.7 ± 0.1 cm/m(2), P = 0.003).

CONCLUSIONS

In the presence of preserved flow rates through a large PAH pulmonary artery, WSS is significantly decreased. This may have implications for proximal pulmonary artery remodeling and cellular function in the progression of PAH.

An investigation into beef calf mortality on five high-altitude ranches that selected sires with low pulmonary arterial pressures for over 20 years

Producer reports from ranches over 2,438 meters in southwest Colorado suggest that the mortality of preweaned beef calves may be substantially higher than the national average despite the selection of low pulmonary pressure herd sires for over 20 years. Diagnostic investigations of this death loss problem have been limited due to the extensive mountainous terrain over which these calves are grazed with their dams. The objective of the current study was to determine the causes of calf mortality on 5 high-altitude ranches in Colorado that have been selectively breeding sires with low pulmonary pressure (<45 mmHg) for over 20 years. Calves were followed from branding (6 weeks of age) in the spring to weaning in the fall (7 months of age). Clinical signs were recorded, and blood samples were taken from sick calves. Postmortem examinations were performed, and select tissue samples were submitted for aerobic culture and/or histopathology. On the principal study ranch, 9.6% (59/612) of the calves that were branded in the spring either died or were presumed dead by weaning in the fall. In total, 28 necropsies were performed: 14 calves (50%) had lesions consistent with pulmonary hypertension and right-sided heart failure, and 14 calves (50%) died from bronchopneumonia. Remodeling of the pulmonary arterial system, indicative of pulmonary hypertension, was evident in the former and to varying degrees in the latter. There is a need to better characterize the additional risk factors that complicate pulmonary arterial pressure testing of herd sires as a strategy to control pulmonary hypertension.

Wall shear stress is decreased in the pulmonary arteries of patients with pulmonary arterial hypertension: An image-based, computational fluid dynamics study

Previous clinical studies in pulmonary arterial hypertension (PAH) have concentrated predominantly on distal pulmonary vascular resistance, its contribution to the disease process, and response to therapy. However, it is well known that biomechanical factors such as shear stress have an impact on endothelial health and dysfunction in other parts of the vasculature. This study tested the hypothesis that wall shear stress is reduced in the proximal pulmonary arteries of PAH patients with the belief that reduced shear stress may contribute to pulmonary endothelial cell dysfunction and as a result, PAH progression. A combined MRI and computational fluid dynamics (CFD) approach was used to construct subject-specific pulmonary artery models and quantify flow features and wall shear stress (WSS) in five PAH patients with moderate-to-severe disease and five age- and sex-matched controls. Three-dimensional model reconstruction showed PAH patients have significantly larger main, right, and left pulmonary artery diameters (3.5 ± 0.4 vs. 2.7 ± 0.1 cm, P = 0.01; 2.5 ± 0.4 vs. 1.9 ± 0.2 cm, P = 0.04; and 2.6 ± 0.4 vs. 2.0 ± 0.2 cm, P = 0.01, respectively), and lower cardiac output (3.7 ± 1.2 vs. 5.8 ± 0.6 L/min, P = 0.02.). CFD showed significantly lower time-averaged central pulmonary artery WSS in PAH patients compared to controls (4.3 ± 2.8 vs. 20.5 ± 4.0 dynes/cm(2), P = 0.0004). Distal WSS was not significantly different. A novel method of measuring WSS was utilized to demonstrate for the first time that WSS is altered in some patients with PAH. Using computational modeling in patient-specific models, WSS was found to be significantly lower in the proximal pulmonary arteries of PAH patients compared to controls. Reduced WSS in proximal pulmonary arteries may play a role in the pathogenesis and progression of PAH. This data may serve as a basis for future in vitro studies of, for example, effects of WSS on gene expression.

Pulmonary blood flow and pulmonary hypertension: Is the pulmonary circulation flowophobic or flowophilic?

Increased pulmonary blood flow (PBF) is widely thought to provoke pulmonary vascular obstructive disease (PVO), but the impact of wall shear stress in the lung is actually poorly defined. We examined information from patients having cardiac lesions which impact the pulmonary circulation in distinct ways, as well as experimental studies, asking how altered hemodynamics impact the risk of developing PVO. Our results are as follows: (1) with atrial septal defect (ASD; increased PBF but low PAP), shear stress may be increased but there is little tendency to develop PVO; (2) with normal PBF but increased pulmonary vascular resistance (PVR; mitral valve disease) shear stress may also be increased but risk of PVO still low; (3) with high PVR and PBF (e.g., large ventricular septal defect), wall shear stress is markedly increased and the likelihood of developing PVO is much higher than with high PBF or PAP only; and (4) with ASD, experimental and clinical observations suggest that increased PBF plus another stimulus (e.g., endothelial inflammation) may be required for PVO. We conclude that modestly increased wall shear stress (e.g., ASD) infrequently provokes PVO, and likely requires other factors to be harmful. Likewise, increased PAP seldom causes PVO. Markedly increased wall shear stress may greatly increase the likelihood of PVO, but we cannot discriminate its effect from the combined effects of increased PAP and PBF. Finally, the age of onset of increased PAP may critically impact the risk of PVO. Some implications of these observations for future investigations are discussed.

The impact of pulmonary venous hypertension on the pulmonary circulation in the young

OBJECTIVE AND DESIGN

Pulmonary venous hypertension is a well-characterized cause of pulmonary hypertension in adults, but little is known regarding the relationship between left atrial pressure and pulmonary arteriolar resistance in the young. Also, in adults relief of pulmonary venous hypertension results in a marked fall in pulmonary arteriolar resistance, but this could be different in children because vascular changes are more severe in young patients than adults with mitral stenosis. We inspected records of children at Children's Hospital Boston having mitral balloon valvuloplasty, and patients ≤5 years old having mitral valve replacement, to determine (1) the relationship between left atrial pressure and pulmonary arterial pressure and resistance (n = 94 children, median age 17.8 months) and (2) how pulmonary arteriolar resistance changes after mitral valve replacement.

RESULTS

The average indexed pulmonary arteriolar resistance was 7.8 ± 5.9 units and was unrelated to age but was positively related to left atrial pressure. There was great variability in pulmonary arteriolar resistance for any given left atrial pressure. Pulmonary arterial pressure (n = 16) and pulmonary arterial resistance (n = 9) were measured before and after mitral valve replacement (median = 29.4 months old). Despite preoperative indexed pulmonary arterial resistance of ≥5 units in 11 of 15 patients, postoperative pulmonary arterial pressure was substantially lower in all save three, and two patients with high pulmonary arterial pressure still had high left atrial pressure postoperatively (25 mmHg).

CONCLUSIONS

We conclude that in young children, as in adults, pulmonary arterial resistance generally falls greatly with reduction in left atrial pressure.

Egr-1 expression during neointimal development in flow-associated pulmonary hypertension

In flow-associated pulmonary arterial hypertension (PAH), increased pulmonary blood flow is an essential trigger for neointimal formation. Using microarray analysis, we recently found that the early growth response protein 1 (Egr-1) transcription factor is increased in experimental flow-associated end-stage PAH. Its role in PAH development is unknown. Here, we assessed the spatiotemporal expression of Egr-1 during neointimal development in flow-associated PAH. Flow-associated PAH was produced in rats by combining monocrotaline administration with an aortocaval shunt. Animals were sacrificed 1 day before or 1 day, 1 week, or 4 to 5 weeks after flow addition. Egr-1 expression was spatiotemporally assessed using laser microdissection, quantitative real-time PCR and immunohistochemistry. In addition, Egr-1 expression was assessed in a non-neointimal pulmonary hypertension model and in human PAH associated with congenital shunt. In 4 to 5 weeks, rats subjected to increased flow developed PAH with neointimal lesions. Egr-1 mRNA was increased 1 day after flow addition and in end-stage PAH, whereas monocrotaline only did not result in increased Egr-1 mRNA. Directly after flow addition, Egr-1 was expressed in endothelial cells. During disease development, Egr-1 protein expression increased and migrated throughout the vessel wall. In PAH patients, Egr-1 was expressed in vessels with media hypertrophy and neointimal lesions, including plexiform lesions. Thus, Egr-1 may be an important regulator in the development of pulmonary neointimal lesions induced by increased pulmonary blood flow.

Plexiform lesions in the lungs of domestic fowl selected for susceptibility to pulmonary arterial hypertension: incidence and histology

Plexiform lesions develop in the pulmonary arteries of humans suffering from idiopathic pulmonary arterial hypertension (IPAH). Plexogenic arteriopathy rarely develops in existing animal models of IPAH. In this study, plexiform lesions developed in the lungs of rapidly growing meat-type chickens (broiler chickens) that had been genetically selected for susceptibility to IPAH. Plexiform lesions developed spontaneously in: 42% of females and 40% of males; 35% of right lungs, and 45% of left lungs; and, at 8, 12, 16, 20, 24, and 52 weeks of age the plexiform lesion incidences averaged 52%, 50%, 51%, 40%, 36%, and 22%, respectively. Plexiform lesions formed distal to branch points in muscular interparabronchial pulmonary arteries exhibiting intimal proliferation. Perivascular mononuclear cell infiltrates consistently surrounded the affected arteries. Proliferating intimal cells fully or partially occluded the arterial lumen adjacent to plexiform lesions. Broilers reared in clean stainless steel cages exhibited a 50% lesion incidence that did not differ from the 64% incidence in flock mates grown on dusty floor litter. Microparticles (30 μm diameter) were injected to determine if physical occlusion and focal inflammation within distal pulmonary arteries might initiate plexiform lesion development. Three months postinjection no plexiform lesions were observed in the vicinity of persisting microparticles. Broiler chickens selected for innate susceptibility to IPAH represent a new animal model for investigating the mechanisms responsible for spontaneous plexogenic arteriopathy.

Idiopathic pulmonary arterial hypertension: an avian model for plexogenic arteriopathy and serotonergic vasoconstriction

Idiopathic pulmonary arterial hypertension (IPAH) is a disease of unknown cause that is characterized by elevated pulmonary arterial pressure and pulmonary vascular resistance attributable to vasoconstriction and vascular remodeling of small pulmonary arteries. Vascular remodeling includes hypertrophy and hyperplasia of smooth muscle (medial hypertrophy) accompanied in up to 80% of the cases by the formation of occlusive plexiform lesions (plexogenic arteriopathy). Patients tend to be unresponsive to vasodilator therapy and have a poor prognosis for survival when plexogenic arteriopathy progressively obstructs their pulmonary arteries. Research is needed to understand and treat plexogenic arteriopathy, but advances have been hindered by the absence of spontaneously developing lesions in existing laboratory animal models. Young domestic fowl bred for meat production (broiler chickens, broilers) spontaneously develop IPAH accompanied by semi-occlusive endothelial proliferation that progresses into fully developed plexiform lesions. Plexiform lesions develop in both female and male broilers, and lesion incidences (lung sections with lesions/lung sections examined) averaged approximately 40% in 8 to 52 week old birds. Plexiform lesions formed distal to branch points in muscular interparabronchial pulmonary arteries, and were associated with perivascular mononuclear cell infiltrates. Serotonin (5-hydroxytryptamine, 5-HT) is a potent vasoconstrictor and mitogen known to stimulate vascular endothelial and smooth muscle cell proliferation. Serotonin has been directly linked to the pathogenesis of IPAH in humans, including IPAH linked to serotonergic anorexigens that trigger the formation of plexiform lesions indistinguishable from those observed in primary IPAH triggered by other causes. Serotonin also plays a major role in the susceptibility of broilers to IPAH. This avian model of spontaneous IPAH constitutes a new animal model for biomedical research focused on the pathogenesis of IPAH and plexogenic arteriopathy.

Pulmonary vascular disease related to hemodynamic stress in the pulmonary circulation

Hemodynamic stress in the pulmonary vessel is directly linked to the development of vascular remodeling and dysfunction, ultimately leading to pulmonary hypertension. Recently, some advances have been made in our molecular understanding of the exogenous upstream stimuli that initiate hemodynamic pertubations as well as the downstream vasoactive effectors that control these responses. However, much still remains unknown regarding how these complex signaling pathways connect in order to result in these characteristic pathophysiological changes. This chapter will describe our current understanding of and needed areas of research into the clinical, physiological, and molecular changes associated with pressure/volume overload in the pulmonary circulation.

Effects of a localized high-flow anastomosis between the aorta and left lower lobe pulmonary artery on great vessel flow and pulmonary arterial reactivity in the contralateral lung

OBJECTIVES

We sought to assess the effects of a localized anastomosis between the aorta and left lower lobe pulmonary artery on flows through central vessels and on the vascular reactivity of small pulmonary arteries distal or contralateral to the shunt.

METHODS

Flow rates in major vessels and tensions from small pulmonary arteries from the left and right lower lobes were determined 48 hours after creation of an end-to-side anastomosis of the left lower lobe pulmonary artery to the aorta.

RESULTS

Anastomoses increased flow through the left lower lobe pulmonary artery from 194±6 to 452±18 mL/min immediately after anastomosis to 756±19 mL/min by the time of harvest (n=88, P<.05). Flow rates in main pulmonary arteries from hosts with anastomoses were lower (557±26 vs 1033±244 mL/min), whereas aortic root flows were not different from control values (1370±53 vs 1120±111 mL/min; P=.07). Wet/dry weights of both lungs and aortic flow rates were proportional to shunt flow rates. Pulmonary artery rings harvested from the right (unshunted) lobes of high-flow hosts exhibited increased reactivity to the thromboxane agonist U46619 and phenylephrine relative to those of left pulmonary arteries from the same animal or those of control hosts.

CONCLUSIONS

Our studies are the first to identify enhanced reactivity of pulmonary arteries in a lung contralateral to a localized high-output shunt between an aorta and pulmonary artery. These observations suggest that patients with localized systemic-to-pulmonary shunt could exhibit modified vascular tone in remote pulmonary arteries.

Three-dimensional hemodynamics in the human pulmonary arteries under resting and exercise conditions

The biomechanical forces associated with blood flow have been shown to play a role in pulmonary vascular cell health and disease. Therefore, the quantification of human pulmonary artery hemodynamic conditions under resting and exercise states can be useful in investigating the physiology of disease development and treatment outcomes. In this study, a combined magnetic resonance imaging and computational fluid dynamics approach was used to quantify pulsatile flow fields, wall shear stress (WSS), oscillations in WSS (OSI), and energy efficiency in six subject-specific models of the human pulmonary vasculature with high spatial and temporal resolution. Averaging over all subjects, WSS was found to increase from 19.8±4.0 to 51.8±6.7 dynes/cm2, and OSI was found to decrease from 0.094±0.016 to 0.081±0.015 in the proximal pulmonary arteries between rest and exercise conditions (p<0.05). These findings demonstrate the localized, biomechanical effects of exercise. Furthermore, an average decrease of 10% in energy efficiency was noted between rest and exercise. These data indicate the amount of energy dissipation that typically occurs with exercise and may be useful in future surgical planning applications.

Gene expression profile in flow-associated pulmonary arterial hypertension with neointimal lesions

Pulmonary arterial hypertension (PAH) is a pulmonary angioproliferative disease with high morbidity and mortality, characterized by a typical pattern of pulmonary vascular remodeling including neointimal lesions. In congenital heart disease, increased pulmonary blood flow has appeared to be a key mediator in the development of these characteristic lesions, but the molecular mechanisms underlying the pulmonary vascular lesions are largely unknown. We employed a rat model of flow-associated PAH, which induced specific pulmonary neointimal lesions. We identified gene expression profiles in rats specifically related to the addition of increased pulmonary blood flow to monocrotaline and the associated occurrence of neointimal lesions. Increased pulmonary blood flow induced the expression of the transcription factors activating transcription factor-3 (ATF3) and early growth response factor-1 (EGR-1), for which presence was confirmed in neointimal lesions. Monocrotaline alone induced increased numbers of activated mast cells and their products. We further identified molecular pathways that may be involved in treatment with the prostacyclin analog iloprost, a vasoactive compound with clinically beneficial effects in patients with PAH, which were similar to pathways described in samples from patient studies. These pathways, associated with the development of angioproliferative lesions as well as with the response to therapy in PAH, may provide new therapeutic targets.

Simvastatin restores down-regulated GATA-6 expression in pulmonary hypertensive rats

Vascular smooth muscle cell proliferation has been known to be predominant in vascular remodeling of pulmonary hypertensive. The GATA family proteins, a group of zinc finger transcription factors, play an important role during cell proliferation. The aim of present study was to investigate the expression of GATA-6 gene in experimental pulmonary hypertensive rats and explore the effect of regulation of GATA-6 expression by simvastatin on pulmonary vascular remodeling. The male Sprague-Dawley rats model was established with receiving pneumonectomy and monocrotaline (MCT) administration. Right pulmonary artery remodeling in these animals was compared with untreated rats or rats receiving simvastatin. The level of GATA-6 mRNA and protein expression was detected by reverse transcriptase–polymerase chain reaction (RT-PCR) and Western blotting, respectively. Pneumonectomized, MCT-treated rats had significantly increased mean pulmonary arterial pressure (mPAP), RV/(LV + S) ratio (ratio of the right ventricular to left ventricular and septum weights), vascular occlusion scores (VOSs), and percent media wall thickness on day 35, all the indices were significantly decreased after simvastatin administration in these rats. The level of GATA-6 mRNA and protein were markedly decreased in these pneumonectomy and MCT-treated rats, and they were significantly up-regulated in these rats after receiving simvastatin. These results indicate that the development and progression of pulmonary hypertension is prevented by simvastatin by up-regulating GATA-6 expression in the lung tissue.

Evaluation of pulmonary artery stiffness in pulmonary hypertension with cardiac magnetic resonance

OBJECTIVES

This study sought to evaluate indexes of pulmonary artery (PA) stiffness in patients with pulmonary hypertension (PH) using same-day cardiac magnetic resonance (CMR) and right heart catheterization (RHC).

BACKGROUND

Pulmonary artery stiffness is increased in the presence of PH, although the relationship to PH severity has not been fully characterized.

METHODS

Both CMR and RHC were performed on the same day in 94 patients with known or suspected PH. According to the RHC, patients were classified as having no PH (n = 13), exercise-induced PH (EIPH) only (n = 6), or PH at rest (n = 75). On CMR, phase-contrast images were obtained perpendicular to the pulmonary trunk. From CMR and RHC data, PA areas and indexes of stiffness (pulsatility, compliance, capacitance, distensibility, elastic modulus, and the pressure-independent stiffness index beta) were measured at rest.

RESULTS

All quantified indexes showed increased PA stiffness in patients with PH at rest in comparison with those with EIPH or no PH. Despite the absence of significant differences in baseline pressures, patients with EIPH had lower median compliance and capacitance than patients with no PH: 15 (interquartile range: 9 to 19.8) mm2/mm Hg versus 8.4 (interquartile range: 6 to 10.3) mm2/mm Hg, and 5.2 (interquartile range: 4.4 to 6.3) mm3/mm Hg versus 3.7 (interquartile range: 3.1 to 4.1) mm3/mm Hg, respectively (p < 0.05). The different measurements of PA stiffness, including stiffness index beta, showed significant correlations with PA pressures (r2 = 0.27 to 0.73). Reduced PA pulsatility (<40%) detected the presence of PH at rest with a sensitivity of 93% and a specificity of 63%.

CONCLUSIONS

Pulmonary artery stiffness increases early in the course of PH (even when PH is detectable only with exercise and before overt pressure elevations occur at rest). These observations suggest a potential contributory role of PA stiffness in the development and progression of PH.

Effects of pathological flow on pulmonary artery endothelial production of vasoactive mediators and growth factors

BACKGROUND

Alterations in pulmonary blood flow are often associated with the initiation and progression of pulmonary vascular disease. However, the cellular mechanisms involved in mediating flow effects in the pulmonary circulation remain unclear. Depending on the disease condition, flow may be extremely low or high. We therefore examined effects of pathologically low and high flow on endothelial production of factors capable of affecting pulmonary vascular tone and structure as well as on potential underlying mechanisms.

METHODS

Flow effects on pulmonary endothelial release of NO, PGF(1a), ET-1 and TxB(2), on expression of total and phosphorylated eNOS as well as Akt, and on VEGF were examined. Additionally, in a coculture system, effects of flow-exposed endothelial cells on smooth muscle (SM) proliferation and contractile protein were studied.

RESULTS

Compared to physiological flow, pathologically high and low flow attenuated endothelial release of NO and PGF(1a), and enhanced release of ET-1. Physiological flow activated the Akt/eNOS pathway, while pathological flow depressed it. Pathologically high flow altered VE-cadherin expression. Pathologically high flow on the endothelium upregulated alpha-SM-actin and SM-MHC without affecting SM proliferation.

CONCLUSION

Physiological flow leads to production of mediators which favor vasodilation. Pathological flow alters the balance of mediator production which favors vasoconstriction.

Rho kinase-mediated vasoconstriction in rat models of pulmonary hypertension

There is current controversy regarding whether vasoconstriction plays a significant role in the elevated pressure of severe, advanced stages of pulmonary hypertension. Results of acute vasodilator testing using conventional vasodilators in such patients suggest there is only a minor contribution of vasoconstriction. However, there is a possibility that these results may underestimate the contribution of vasoconstriction because the most effective vasodilators have not yet been tested. This issue has not been addressed even experimentally, due mainly to a lack of appropriate animal models. A few animal models that mimic the pathology of human severe pulmonary hypertension more closely (i.e., development of occlusive neointimal lesions in small pulmonary arteries/arterioles) have been introduced, including rat models of left lung pneumonectomy plus monocrotaline injection and vascular endothelial growth factor inhibition plus exposure to chronic hypoxia. We have observed that Rho kinase inhibitors, a novel class of potent vasodilators, reduce the high pulmonary artery pressure of these models acutely and markedly, suggesting that vasoconstriction can significantly be involved in pulmonary hypertension with severely remodeled (occluded) pulmonary vessels. This chapter describes methods used for evaluation of the involvement of Rho kinase-mediated vasoconstriction in rat models of pulmonary hypertension.

Pathogenic mechanisms of pulmonary arterial hypertension

Pulmonary arterial hypertension (PAH) is a complex disease that causes significant morbidity and mortality and is clinically characterized by an increase in pulmonary vascular resistance. The histopathology is marked by vascular proliferation/fibrosis, remodeling, and vessel obstruction. Development of PAH involves the complex interaction of multiple vascular effectors at all anatomic levels of the arterial wall. Subsequent vasoconstriction, thrombosis, and inflammation ensue, leading to vessel wall remodeling and cellular hyperproliferation as the hallmarks of severe disease. These processes are influenced by genetic predisposition as well as diverse endogenous and exogenous stimuli. Recent studies have provided a glimpse at certain molecular pathways that contribute to pathogenesis; these have led to the identification of attractive targets for therapeutic intervention. We will review our current understanding of the mechanistic underpinnings of the genetic and exogenous/acquired triggers of PAH. The resulting imbalance of vascular effectors provoking pathogenic vascular changes will also be discussed, with an emphasis on common and overarching regulatory pathways that may relate to the primary triggers of disease. The current conceptual framework should allow for future studies to refine our understanding of the molecular pathogenesis of PAH and improve the therapeutic regimen for this disease.

α1-Adrenoceptor-dependent vascular hypertrophy and remodeling in murine hypoxic pulmonary hypertension

Excessive proliferation of vascular wall cells underlies the development of elevated vascular resistance in hypoxic pulmonary hypertension (PH), but the responsible mechanisms remain unclear. Growth-promoting effects of catecholamines may contribute. Hypoxemia causes sympathoexcitation, and prolonged stimulation of α1-adrenoceptors (α1-ARs) induces hypertrophy and hyperplasia of arterial smooth muscle cells and adventitial fibroblasts. Catecholamine trophic actions in arteries are enhanced when other conditions favoring growth or remodeling are present, e.g., injury or altered shear stress, in isolated pulmonary arteries from rats with hypoxic PH. The present study examined the hypothesis that catecholamines contribute to pulmonary vascular remodeling in vivo in hypoxic PH. Mice genetically deficient in norepinephrine and epinephrine production [dopamine β-hydroxylase−/− (DBH−/−)] or α1-ARs were examined for alterations in PH, cardiac hypertrophy, and vascular remodeling after 21 days exposure to normobaric 0.1 inspired oxygen fraction (FiO2). A decrease in the lumen area and an increase in the wall thickness of arteries were strongly inhibited in knockout mice (order of extent of inhibition: DBH−/− = α1D-AR−/− > α1B-AR−/−). Distal muscularization of small arterioles was also reduced (DBH−/− > α1D-AR−/− > α1B-AR−/− mice). Despite these reductions, increases in right ventricular pressure and hypertrophy were not attenuated in DBH−/− and α1B-AR−/− mice. However, hematocrit increased more in these mice, possibly as a consequence of impaired cardiovascular activation that occurs during reduction of FiO2. In contrast, in α1D-AR−/− mice, where hematocrit increased the same as in wild-type mice, right ventricular pressure was reduced. These data suggest that catecholamine stimulation of α1B- and α1D-ARs contributes significantly to vascular remodeling in hypoxic PH.

Enhanced α1-adrenergic trophic activity in pulmonary artery of hypoxic pulmonary hypertensive rats

Mechanisms that induce the excessive proliferation of vascular wall cells in hypoxic pulmonary hypertension (PH) are not fully understood. Alveolar hypoxia causes sympathoexcitation, and norepinephrine can stimulate α1-adrenoceptor (α1-AR)-dependent hypertrophy/hyperplasia of smooth muscle cells and adventitial fibroblasts. Adrenergic trophic activity is augmented in systemic arteries by injury and altered shear stress, which are key pathogenic stimuli in hypoxic PH, and contributes to neointimal formation and flow-mediated hypertrophic remodeling. Here we examined whether norepinephrine stimulates growth of the pulmonary artery (PA) and whether this is augmented in PH. PA from normoxic and hypoxic rats [9 days of 0.1 fraction of inspired O2 (FiO2)] was studied in organ culture, where wall tension, Po2, and Pco2 were maintained at values present in normal and hypoxic PH rats. Norepinephrine treatment for 72 h increased DNA and protein content modestly in normoxic PA (+10%, P < 0.05). In hypoxic PA, these effects were augmented threefold ( P < 0.05), and protein synthesis was increased 34-fold ( P < 0.05). Inferior thoracic vena cava from normoxic or hypoxic rats was unaffected. Norepinephrine-induced growth in hypoxic PA was dose dependent, had efficacy greater than or equal to endothelin-1, required the presence of wall tension, and was inhibited by α1A-AR antagonist. In hypoxic pulmonary vasculature, α1A-AR was downregulated the least among α1-AR subtypes. These data demonstrate that norepinephrine has trophic activity in the PA that is augmented by PH. If evident in vivo in the pulmonary vasculature, adrenergic-induced growth may contribute to the vascular hyperplasia that participates in hypoxic PH.

Development of occlusive neointimal lesions in distal pulmonary arteries of endothelin B receptor-deficient rats: a new model of severe pulmonary arterial hypertension

BACKGROUND

Human pulmonary arterial hypertension (PAH) is characterized by proliferation of vascular smooth muscle and, in its more severe form, by the development of occlusive neointimal lesions. However, few animal models of pulmonary neointimal proliferation exist, thereby limiting a complete understanding of the pathobiology of PAH. Recent studies of the endothelin (ET) system demonstrate that deficiency of the ET(B) receptor predisposes adult rats to acute and chronic hypoxic PAH, yet these animals fail to develop neointimal lesions. Herein, we determined and thereafter showed that exposure of ET(B) receptor-deficient rats to the endothelial toxin monocrotaline (MCT) leads to the development of neointimal lesions that share hallmarks of human PAH.

METHODS AND RESULTS

The pulmonary hemodynamic and morphometric effects of 60 mg/kg MCT in control (MCT(+/+)) and ET(B) receptor-deficient (MCT(sl/sl)) rats at 6 weeks of age were assessed. MCT(sl/sl) rats developed more severe PAH, characterized by elevated pulmonary artery pressure, diminished cardiac output, and right ventricular hypertrophy. In MCT(sl/sl) rats, morphometric evaluation revealed the presence of neointimal lesions within small distal pulmonary arteries, increased medial wall thickness, and decreased arterial-to-alveolar ratio. In keeping with this, barium angiography revealed diminished distal pulmonary vasculature of MCT(sl/sl) rat lungs. Cells within neointimal lesions expressed smooth muscle and endothelial cell markers. Moreover, cells within neointimal lesions exhibited increased levels of proliferation and were located in a tissue microenvironment enriched with vascular endothelial growth factor, tenascin-C, and activated matrix metalloproteinase-9, factors already implicated in human PAH. Finally, assessment of steady state mRNA showed that whereas expression of ET(B) receptors was decreased in MCT(sl/sl) rat lungs, ET(A) receptor expression increased.

CONCLUSIONS

Deficiency of the ET(B) receptor markedly accelerates the progression of PAH in rats treated with MCT and enhances the appearance of cellular and molecular markers associated with the pathobiology of PAH. Collectively, these results suggest an overall antiproliferative effect of the ET(B) receptor in pulmonary vascular homeostasis.

Inhaled iloprost reverses vascular remodeling in chronic experimental pulmonary hypertension

RATIONALE

Inhaled iloprost is an effective therapy for pulmonary arterial hypertension (PAH). However, no study to date has addressed the effects of inhaled iloprost on changes to pulmonary vascular structure that occur in PAH.

OBJECTIVES

The present study was designed to investigate chronic antiremodeling effects of inhaled iloprost in monocrotaline (MCT)-induced PAH in rats.

METHODS

Four weeks after a single injection of MCT, after full establishment of PAH, rats were nebulized with iloprost at a dose of 6 microg . kg(-1) . day(-1), or underwent sham nebulization with saline.

RESULTS

After 2 weeks of inhalation therapy, right ventricular pressure and pulmonary vascular resistance were reversed in rats treated with iloprost, but not in sham-treated control animals. Systemic arterial pressure was unaffected. In addition, right heart hypertrophy, the degree of pulmonary artery muscularization, and the medial wall thickness of intraacinar pulmonary arteries regressed in response to iloprost. Furthermore, the MCT-induced increase in matrix metalloproteinase-2 and -9 activities and tenascin-C expression was suppressed.

CONCLUSIONS

We conclude that the inhalation of iloprost reverses PAH and vascular structural remodeling in MCT-treated rats. This regimen suggests the possibility of an antiremodeling therapy in PAH.

Role of lateral cell-cell border location and extracellular/transmembrane domains in PECAM/CD31 mechanosensation

Phosphorylation of tyrosine residues on platelet-endothelial cell adhesion molecule-1 (PECAM-1), followed by signal transduction events, has been described in endothelial cells following exposure to hyperosmotic and fluid shear stress. However, it is unclear whether PECAM-1 functions as a primary mechanosensor in this process. Utilizing a PECAM-1-null EC-like cell line, we examined the importance of cellular localization and the extracellular and transmembrane domains in PECAM-1 phosphorylation responses to mechanical stress. Tyrosine phosphorylation of PECAM-1 was stimulated in response to mechanical stress in null cells transfected either with full length PECAM-1 or with PECAM-1 mutants that do not localize to the lateral cell-cell adhesion site and that do not support homophilic binding between PECAM-1 molecules. Furthermore, null cells transfected with a construct that contains the intact cytoplasmic domain of PECAM-1 fused to the extracellular and transmembrane domains of the interleukin-2 receptor also underwent mechanical stress-induced PECAM-1 tyrosine phosphorylation. These findings suggest that mechanosensitive PECAM-1 may lie downstream of a primary mechanosensor that activates a tyrosine kinase.

Plexogenic pulmonary arteriopathy in a Pembroke Welsh corgi

A 21-month-old, male Pembroke Welsh corgi was referred for investigation of respiratory distress and progressive lethargy. Cardiac evaluation revealed a grade 4 pansystolic murmur over the left and right heart base. A heart murmur, dyspnoea, cyanosis, prolonged capillary refill time and ascites led to the tentative diagnosis of a cardiac malformation with a right-to-left shunt, with likely additional pulmonary disease. Pulmonary hypertension became evident during echocardiography, when the estimated systolic pulmonary artery pressure was over 70 mmHg. Angiography revealed abnormal pulmonary vascular markings consistent with pulmonary hypertension and a small right-to-left shunting patent ductus arteriosus (PDA). The diagnosis of PDA was confirmed at postmortem examination. Histology of the pulmonary arteries showed lesions of plexogenic pulmonary arteriopathy. The question of whether both conditions were separate or part of the same clinical syndrome is discussed in this report.