HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:
Guest Access | Sign In via User Name/Password

This Article Abstract Full Text (PDF) Free Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Article Archive Download to citation manager Citing Articles Citing Articles via ISI Web of Science (10) Citing Articles via Google Scholar Google Scholar Articles by Langleben, D. Articles by Giovinazzo, M. Search for Related Content PubMed PubMed Citation Articles by Langleben, D. Articles by Giovinazzo, M.

Etiology-Specific Endothelin-1 Clearance in Human Precapillary Pulmonary Hypertension^*

^* From the Center for Pulmonary Vascular Disease and Divisions of Cardiology (Dr. Langleben), Pulmonology (Dr. Hirsch), and Rheumatology (Dr. Baron), Sir Mortimer B. Davis Jewish General Hospital and Lady Davis Institute for Medical Research (Mr. Langleben and Ms. Giovinazzo), Department of Medicine, McGill University, Montreal, QB; the Research Center of the Montreal Heart Institute (Dr. Dupuis), Montreal, QB; and Division of Rheumatology (Dr. Senécal), CHUM-Hôpital Notre-Dame, Université de Montréal, Montreal, QB, Canada.

Correspondence to: David Langleben, MD, Room E-258, Jewish General Hospital, 3755 Cote Ste Catherine, Montreal, QC, Canada H3T 1E2; e-mail: david.langleben{at}mcgill.ca

Abstract

Study objectives: Endothelin (ET)-1 is a mediator of vascular remodeling seen in human pulmonary hypertension (PH), and it is normally cleared via endothelial ET-B receptors. Increased levels of ET-1 are found in precapillary PH, partly from increased synthesis. We hypothesized that the endothelial dysfunction and vascular remodeling seen in human precapillary PH would also reduce ET-1 clearance.

Design and setting: Case series from a single institutional PH center.

Patients: Thirty-four patients with pulmonary arterial hypertension (PAH; idiopathic [IPAH], n = 19; connective tissue disease [CTD], n = 15) and 11 patients with chronic thromboembolic PH were studied.

Measurements and results: Using indicator dilution methods, the first-pass extraction of radiolabeled ET-1 through the pulmonary circulation, and permeability surface (PS) area, an index of functional microvascular surface available for ET-1 clearance, were determined. Mean extraction for IPAH and thromboembolic PH groups was normal, but it was reduced in PAH from CTD; 69% of all patients studied had normal extraction. The mean PS product was reduced significantly for all three etiologies as compared to normal, but 58% of IPAH patients and 40% of CTD-related PAH patients had normal PS products.

Conclusions: Receptor-mediated ET-1 extraction and functional vascular surface area for clearance vary between etiologies of PAH. However, contrary to our hypothesis, endothelial ET-B receptor-mediated extraction is preserved in many patients. The scientifically significant finding of our study is that high ET-1 levels seen in patients with PAH must be predominantly due to excess synthesis rather than reduced clearance. The finding that endothelial ET-B receptors are still present and functional in PAH may also be of relevance to the choice of selective vs nonselective ET receptor antagonists.

Key Words: endothelin • hypertension, pulmonary • lung

Endothelin (ET)-1 is a potent vasoconstrictor and smooth-muscle mitogen that may in part mediate the pathogenesis of pulmonary hypertension (PH).¹²³ Consequently, there has been intense activity to develop clinically useful ET receptor antagonists.¹⁴⁵⁶ Circulating plasma ET-1 levels are increased in many forms of PH, and the balance of pulmonary circulatory ET-1 production and clearance is shifted from normal net clearance toward net excess production.²⁷⁸⁹¹⁰ This may partly be due to the increased pulmonary ET-1 gene expression and synthesis in PH, particularly at the site of the microvascular remodeling, which principally causes elevated pulmonary vascular resistance.¹¹ ET-1 is taken up from the pulmonary circulation via the endothelial ET-B receptor.¹² Vascular remodeling in patients with PH causes a loss of functional microvascular surface area, which could consequently affect pulmonary ET-1 clearance. ET-1 clearance is reduced in patients with PH due to left-heart disease, but ET-1 clearance in precapillary causes of PH, including pulmonary arterial hypertension (PAH) and chronic thromboembolic PH (CTEPH), has not been previously studied.¹⁰¹³ Thus, we hypothesized that ET-1 clearance would be greatly reduced in precapillary PH. Given the importance of identifying patterns of ET-1 clearance in PH, at routine cardiac catheterization we used a first-pass indicator-dilution method to measure pulmonary ET-1 clearance^810121415 in three groups of patients: two groups with PAH and one group with CTEPH.

Materials and Methods

Study Subjects
Forty-five consecutive patients with precapillary PH (19 patients with idiopathic PAH [IPAH]¹⁶; 15 patients with PAH related to connective tissue disease [CTD]; 11 patients with CTEPH) and meeting the following criteria were studied. During ET-1 sampling, all patients were undergoing routine hemodynamic study as part of their evaluation or prior to initiation of therapy. IPAH was diagnosed when the patients fulfilled the criteria as established by the consensus group of the third World Symposium on Pulmonary Arterial Hypertension.¹⁶ Patients with PAH related to CTD had clinical features and serologic abnormalities that confirmed the underlying disease (systemic sclerosis, n = 11 [limited scleroderma, n = 7; diffuse scleroderma, n = 4]; systemic lupus erythematosus, n = 1; rheumatoid arthritis, n = 1; mixed CTD, n = 1; overlap syndrome, n = 1).¹⁷ Patients with CTEPH had ventilation/perfusion lung scintigraphy and pulmonary angiogram findings consistent with the diagnosis.¹⁸ Intracardiac shunting was excluded with the use of Doppler transthoracic and transesophageal echocardiography with IV injection of agitated saline solution. All patients were receiving supportive therapy for PH including, where appropriate, digoxin, diuretics, calcium blockers, anticoagulants, and oxygen. All patients gave written, informed consent in a protocol approved by the Research Ethics Committee of the Jewish General Hospital and conforming to the Helsinki Declaration of 1975.

Measurement of Pulmonary ET-1 Extraction
Under light sedation and local anesthesia, a thermodilution catheter was inserted via a central vein (internal jugular or femoral) and passed through the right heart into the pulmonary artery using fluoroscopic guidance. An indwelling sheath was inserted into the femoral artery. After a steady baseline state had been obtained (at least 20 min after insertion of the catheters), hemodynamic parameters were measured, including systemic and pulmonary arterial pressures, right atrial pressure, pulmonary arterial wedge pressure, and thermodilution cardiac output. ¹²⁵I-labeled ET-1 in Evans-Blue-albumin solution was injected via the proximal port of the thermodilution catheter, and was rapidly flushed into the right atrium with 10 mL of 0.9% NaCl. With a peristaltic pump (Masterflex; Cole Parmer; Chicago IL), blood was extracted from the femoral arterial sheath, at a rate of 40 mL/min, and collected into serial tubes on a fraction collector (Gilson; Middleton, WI; 1.6 mL per tube) for a total of 62.4 s.

The injection mixture was prepared under sterile conditions. Human albumin (25%) was added to 3 mL of Evans blue dye (5 mg/mL) to obtain a final bolus concentration of 4% albumin. The blue dye binds tightly to albumin and serves as a vascular reference from which tracer ET-1 kinetics are computed; 15 µCi of ¹²⁵I-labeled ET-1 (Perkin-Elmer; Woodbridge, ON; 2,200 Ci/mmol) was then added to the mixture. Saline solution was then added to constitute a final volume of 5 mL. For each injection, 2 mL was taken to constitute the bolus administered to the patient, and the remainder was kept for the preparation of dilution curve standards.

All collected tubes, 1/10 and 1/100-diluted standards, and injection line samples were treated identically. Blood (200 µL) was pipetted from each tube and assayed in a counter to determine ¹²⁵I activity. The remaining samples were centrifuged at 3,000 revolutions per minute for 10 min, and 100 µL of plasma was drawn and added to 1.0 mL of 0.9% NaCl in a spectrophotometer cuvette for measurement of Evans blue dye absorbance (620 nm – 740 nm). The exact quantity of tracers injected was determined by subtracting the activity retained in the injection system from the amount contained in the 2-mL bolus. For each sample, the fractional recoveries of tracer ET-1 and albumin were determined. A plot of the fractional recoveries per milliliter of blood as a function of time was then constructed to obtain the indicator dilution curve for each tracer. The albumin was used as a plasmatic vascular reference tracer. Pulmonary blood flow could then be computed as follows:

where q₀ is the total amount of Evans blue dye bound albumin injected, and the denominator represents the area under the fractional recovery vs time curve for the same tracer. The recirculating tracer was mathematically removed by linear extrapolation of the exponential downslope on a semilogarithmic plot. Mean tracer ET-1 extraction was calculated by the following equation:

where the right term represents the difference in the areas of the fractional recovery vs time curves for tracer albumin and ET-1.

The metabolic capacity of the lungs to clear ET-1 from circulation was then evaluated by computation of the permeability surface (PS) area product for ET-1 removal. The PS product, equivalent to plasmatic clearance, is used to describe the unidirectional uptake of permeating tracers using the indicator dilution technique,¹⁹²⁰ and is an index of functional vascular surface area available for clearance. We have previously validated the use of the PS product to evaluate pulmonary removal of tracer ET-1 and demonstrated its stability over a wide range of pulmonary blood flow, despite the expected variations in mean tracer ET extraction.¹⁵ The PS for ET-1 removal by the pulmonary circulation was computed as follows:

where Fp is the pulmonary plasma flow in milliliters per second, and Ext is mean tracer ET-1 extraction.

Statistical Considerations
Variables studied included clinical and hemodynamic characteristics, calculated pulmonary plasma flow, ET-1 extraction, and PS product. Group summary data are presented as mean ± SD. Comparisons of group data were performed using analysis of variance and the Wilcoxon rank-sum test. Correlations were assessed using least-square regression analysis. Two-tailed p values < 0.05 were considered significant. The ranges of normal for ET-1 extraction and PS product were defined as the mean ± 2 SDs, determined in the control population.⁸

Sample Size Calculation: With a control (normal) group size of 13 patients,⁸ in order to detect an absolute 10% difference between group means for extraction, and to have a two-sided error probability of 0.05 and a probability of correctly rejecting the null hypothesis (power) of 0.95, it was determined that each experimental group would require 15 patients.²¹

Results

Patient Characteristics
There were no significant differences between the disease groups with regard to age; heart rate; mean systemic, right atrial, pulmonary arterial, and wedge pressures; cardiac output; or systemic and pulmonary vascular resistances, with marked abnormalities in the hemodynamic characteristics being seen in all groups (Table 1 ). There was a very good (r = 0.89, p < 0.01) correlation between the thermodilution-derived plasma flow and the indicator dilution-derived plasma flow. However, the regression line was not the line of identity, and there were more points to the right of the line, suggesting that thermodilution cardiac output slightly underestimated plasma flow.

View larger version (9K): [in this window] [in a new window] [Download PPT slide]   Figure 1.. First-pass extraction of ET-1 (as a percentage) in the three groups of patients and in normal patients previously reported.8 Symbols are used to designate data points for individual patients.

View larger version (8K): [in this window] [in a new window] [Download PPT slide]   Figure 2.. PS product (milliliters per second) in the three groups of patients, and in normal patients previously reported.8 Each data point represents a single patient.

Plasma ET-1 Levels, Arterial/Venous Ratio, and Relationship to PS Product
Mixed venous plasma ET-1 levels were elevated in 26 of the 45 patients. There was no relationship between plasma ET-1 levels and disease type, and there was no relationship between plasma ET-1 levels and PS product. Some of the patients with the highest plasma ET-1 levels had PS product levels well within the normal range. Similarly, there was no relationship between arterial/venous plasma ET-1 ratios and PS product.

Discussion

ET-1 may play a detrimental role in human PH.²²² ET receptor blockade improves hemodynamics, functional capacity, and survival in PAH patients.⁴⁵⁶²³However, there is still incomplete understanding of ET homeostasis in the human pulmonary hypertensive lung. While excess pulmonary ET-1 synthesis contributes to the abnormally high levels,¹¹ the importance of altered ET-1 extraction has not been studied. Furthermore, the importance of changes in ET receptor density and availability for binding with ET-1 in a remodeled bed is unknown. This is the first study to examine transpulmonary extraction of ET-1 from blood circulating through the lung of humans with precapillary PH. The first-pass indicator dilution method, using trace amounts of radiolabeled ET-1 and a reference marker to measure pulmonary blood plasma flow, has permitted detailed assessment of pulmonary ET-1 extraction and PS product in a variety of animal models, and in healthy and diseased humans.^810121415 In the absence of ET-B receptor blockers, normal extraction levels cannot be artifactual, and physiologically high levels of ET-1, as were seen in our patients, do not interfere with the assay technique.

Based on the known presence of severe microvascular remodeling and flow restriction in patients with precapillary PH, we hypothesized that extraction and PS product would be markedly reduced in all patients. The study was strongly powered from a statistical standpoint to detect a meaningful (10% absolute) difference between PAH patients and normal subjects. Surprisingly, we found that a substantial proportion (59%) of patients with PAH had preserved ET-1 extraction, as did all patients with CTEPH, and 50% of PAH patients had preserved PS products. Furthermore, mean ET-1 extraction for the IPAH group was not different from control subjects. However, there was heterogeneity in the PAH group, and a majority of patients with CTD had subnormal levels, as compared to most IPAH patients having normal levels of extraction despite similarly severe hemodynamic abnormalities. This is the first suggestion that ET handling can vary between different types of PAH.

Why is ET-1 extraction preserved in many patients, despite a reduced PS product that represents a reduced functional vascular surface area available for extraction? The prolonged pulmonary arterial transit time, owing to the low cardiac output, allows greater time for interaction between ET-1 and the endothelial ET-B receptor, resulting in an apparently normal extraction level. A similar phenomenon has been described for the pulmonary clearance of other circulating peptides.²⁴ Further understanding can be gained from the observation in isolated perfused lungs, with controlled blood flow rates, that extraction normally varies with flow while PS product is normally constant over a wide range of flows.¹⁵

The binding of ET-1 by the ET-A receptor results in vasoconstriction and smooth-muscle proliferation. By contrast, ET-1 binding by the endothelial ET-B receptor results in vasodilation by production of nitric oxide and prostacyclin.⁷²⁵ Moreover, circulating ET-1 appears to be cleared exclusively by the endothelial ET-B receptor.¹² IPAH and PAH related to CTD are associated with marked maladaptive structural remodeling and restriction of the pulmonary microvasculature, in a fashion that may reduce perfused vascular surface area, affect endothelial function, and alter surface receptor expression.²⁶ Our data hint that extraction and PS product may be somewhat worse in CTD as compared to IPAH, offering the possibility that the microvasculopathy may be more severe in CTD and may vary greatly between PAH patients. However, the number of patients we studied was relatively small, and a confirmation of this finding would require larger patient populations. Chronic thromboembolic disease usually results from severe physical plugging by organized clot of proximal lobar and segmental arteries, although it may also affect microvascular function.²⁷

The PS product is an index of functional vascular surface area available for ET-1 extraction. It depends on the amount of perfused microvasculature, the density of ET-B receptors, and their functionality. In a severely remodeled, underperfused microvascular bed such as is seen with precapillary PH, one might expect a severe reduction in PS product. In fact, many of our patients had normal or nearly normal PS product values. One explanation would be that their microvasculature is not as severely restricted as previously imagined. This is unlikely, however, given the known marked histologic abnormalities in these severely pulmonary hypertensive patients. A more plausible explanation might be that the density of ET-B receptors is increased in PH. Indeed, increased ET-B receptor expression and density has been described in distal pulmonary arteries in IPAH, PH associated with congenital heart disease, and CTEPH.²⁸²⁹ The relatively small variance in PS product found in the chronic thromboembolic group may be a chance occurrence. It remains to be determined whether CTEPH causes less heterogeneity of microvascular disease and perfusion between patients than do IPAH- and CTD-induced PAH. Patients with PH from left-heart disease have previously been described to have reduced ET-1 clearance.¹⁰¹³ Given that the pulmonary capillaries are normally the principal site of ET-1 clearance,³⁰ and that pulmonary venous hypertension from left-heart disease should fully distend the pulmonary capillary bed, it is surprising that ET-1 clearance is not increased in these patients. However, the expression of ET-B receptors has not been studied in patients with left-heart disease, and the effects of capillary distention and leak are also unknown. Low receptor expression or capillary injury would result in reduced ET-1 clearance in left-heart disease. These phenomena may in part explain the differences between ET-1 clearance in left-heart disease and in the patients with precapillary PH in the present study. Although both are forms of PH, they represent two very different underlying pathophysiologies.

Approximately 40% of our patients had normal mixed venous plasma ET-1 levels. In a previous study¹¹ of tissue ET-1 levels in PAH, all patients had some degree of increased pulmonary ET-1 expression and synthesis. Thus, the patients in our study with normal ET-1 plasma levels may not have had much spillover of ET-1 into the circulation. Although a weak association between circulating ET-1 levels and pulmonary arterial pressure has been reported, we did not find a relationship between ET-1 extraction and pulmonary hemodynamics. This is the first description of such a lack of relationship. As has previously been described, the balance between production and extraction of ET-1 is complex,⁸ but the findings of our study strongly suggest that the high plasma and tissue ET-1 levels seen in PAH are predominantly due to excess synthesis rather than reduced clearance.²¹¹ Indeed, in our study, some of the patients with the highest plasma ET-1 levels had high extraction levels, suggesting that they must also have markedly increased ET production. However, preserved extraction, as we found, might to some degree moderate levels of ET-1 that would otherwise be even higher. We also found no relationship between the arterial/venous plasma ET-1 ratio and PS product but, as is mentioned above, the former is a crude measure of ET homeostasis in the lung. The number of patients in our study did not permit analysis of the relationship between ET-1 extraction and plasma ET-1 levels by type of precapillary PH.

The novel findings of our study are that ET-1 extraction and functional vascular surface area for extraction vary between etiologies of precapillary PH and that, unexpectedly, extraction is preserved in many patients. Moreover, even in patients in whom extraction is reduced, it is still present to some degree. It is never absent. Thus, the high ET-1 levels seen in PAH are predominantly due to excess synthesis rather than reduced clearance. The finding that endothelial ET-B receptors are still present and functional in PAH may also be of significance for the relative effectiveness of selective vs nonselective ET receptor antagonists, but that question can only be resolved by controlled clinical trials.

Acknowledgements

We are grateful to Alexandre Caron, Alan Moskovic, Nathalie Ruel, and Joseph Khoury for expert technical assistance. Drs. Robert Schlesinger, Leonidas Dragatakis, and Mark J. Eisenberg were of great assistance with cardiac catheterizations. We thank the nurses and technicians of the Jewish General Hospital Cardiac Catheterization Laboratory, and the Coronary Care Unit, for their generous and patient assistance.

Footnotes

Abbreviations: CTD = connective tissue disease; CTEPH = chronic thromboembolic pulmonary hypertension; ET = endothelin; IPAH = idiopathic pulmonary arterial hypertension; PAH = pulmonary arterial hypertension; PH = pulmonary hypertension, PS = permeability surface

This work was funded by operating grants (MOP-42476 and 67145 to DL, MOP-12887 to JD, MOP-68966 to JLS) from the Canadian Institutes of Health Research, and by the Bank of Montreal Center for the Study of Heart Disease in Women at the Jewish General Hospital.

Dr. Langleben is a Chercheur-Boursier Clinicien National (National Clinical Research Scholar), and Dr. Dupuis is a Chercheur- Boursier Clinicien Senior (Senior Clinical Research Scholar), of the Fonds de la Recherche en Sante du Quebec. Dr. Senécal, M. Giovinazzo, and I. Langleben have no conflicts of interest to declare. Drs. D. Langleben, Dupuis, Hirsch, and Baron have served as speakers, investigators and/or consultants for one or more of Actelion Inc., Encysive Corporation, and Myogen Inc.

Received for publication June 15, 2005. Accepted for publication August 27, 2005.

References

1. Yanagisawa, M, Kurihara, H, Kimura, S, et al (1988) A novel potent vasoconstrictor peptide produced by vascular endothelial cells. Nature 332,411-415[CrossRef][Medline]
2. Stewart, DJ, Levy, RD, Cernacek, P, et al Increased plasma endothelin-1 in pulmonary hypertension: marker or mediator of disease? Ann Intern Med 1991;114,464-469[ISI][Medline]
3. Wort, SJ, Woods, M, Warner, TD, et al Endogenously released endothelin-1 from human pulmonary artery smooth muscle promotes cellular proliferation. Am J Respir Cell Mol Biol 2001;25,104-110[Abstract/Free Full Text]
4. Channick, RN, Simonneau, G, Sitbon, O, et al Effects of the dual endothelin-receptor antagonist bosentan in patients with pulmonary hypertension: a randomised placebo-controlled study. Lancet 2001;358,1119-1123[CrossRef][ISI][Medline]
5. Rubin, LJ, Badesch, DB, Barst, RJ, et al Bosentan therapy for pulmonary arterial hypertension. N Engl J Med 2002;346,896-903[Abstract/Free Full Text]
6. Barst, RJ, Langleben, D, Frost, A, et al Sitaxsentan therapy for pulmonary arterial hypertension. Am J Respir Crit Care Med 2004;169,441-447[Abstract/Free Full Text]
7. De Nucci, G, Thomas, R, D’Orleans-Juste, P, et al Pressor effects of circulating endothelin are limited by its removal in the pulmonary circulation and by the release of prostacyclin and endothelium-derived relaxing factor. Proc Natl Acad Sci U S A 1988;85,9797-9800[Abstract/Free Full Text]
8. Dupuis, J, Stewart, DJ, Cernacek, P, et al Human pulmonary circulation is an important site for both clearance and production of endothelin-1. Circulation 1996;94,1578-1584[Abstract/Free Full Text]
9. Yoshibayashi, M, Nishioka, K, Nakao, K, et al Plasma endothelin concentrations in patients with pulmonary hypertension associated with congenital heart defects. Circulation 1991;84,2280-2285[Abstract/Free Full Text]
10. Dupuis, J, Cernacek, P, Tardif, JC, et al Reduced pulmonary clearance of endothelin-1 in pulmonary hypertension. Am Heart J 1998;135,614-620[CrossRef][ISI][Medline]
11. Giaid, A, Yanagisawa, M, Langleben, D, et al Expression of endothelin-1 in the lungs of patients with pulmonary hypertension. N Engl J Med 1993;328,1732-1739[Abstract/Free Full Text]
12. Dupuis, J, Goresky, CA, Fournier, A Pulmonary clearance of circulating endothelin-1 in dogs in vivo: exclusive role of ETB receptors. J Appl Physiol 1996;81,1510-1515[Abstract/Free Full Text]
13. Staniloae, C, Dupuis, J, White, M, et al Reduced pulmonary clearance of endothelin in congestive heart failure: a marker of secondary pulmonary hypertension. J Cardiac Failure 2004;10,427-432[CrossRef][ISI][Medline]
14. Dupuis, J, Goresky, CA, Stewart, DJ Pulmonary removal and production of endothelin in the anesthetized dog. J Appl Physiol 1994;76,694-700[Abstract/Free Full Text]
15. Dupuis, J, Moe, GW, Cernacek, P Reduced pulmonary metabolism of endothelin-1 in canine tachycardia-induced heart failure. Cardiovasc Res 1998;39,609-616[Abstract/Free Full Text]
16. Simonneau, G, Galie, N, Rubin, LJ, et al Clinical classification of pulmonary hypertension. J Am Coll Cardiol 2004;43,5S-12S[Abstract/Free Full Text]
17. Scussel-Lonzetti, L, Joyal, F, Raynauld, JP, et al Predicting mortality in systemic sclerosis: analysis of a cohort of 309 French Canadian patients with emphasis on features at diagnosis as predictive factors for survival. Medicine 2002;81,154-167[CrossRef][Medline]
18. Fedullo, PF, Auger, WR, Rubin, LJ Chronic thromboembolic pulmonary hypertension. N Engl J Med 2001;345,1465-1472[Free Full Text]
19. Crone, C The permeability of capillaries in various organs as determined by use of the "indicator diffusion" method. Acta Physiol Scand 1963;58,292-305[ISI][Medline]
20. Bassingthwaite, JP, Chinard, FP, Crone, C, et al Terminology for mass transport and exchange. Am J Physiol 1986;250,H539-H545
21. Dupont, WD, Plummer, WD Power and sample size calculations. Control Clin Trials 1990;11,116-128[CrossRef][ISI][Medline]
22. Oparil, S, Chen, SJ, Meng, QC, et al Endothelin-A receptor antagonist prevents acute hypoxia-induced pulmonary hypertension in the rat. Am J Physiol 1995;268,L95-L100
23. McLaughlin, VV, Sitbon, O, Badesch, DB, et al Survival with first-line bosentan in patients with primary pulmonary hypertension. Eur Respir J 2005;25,244-249[Abstract/Free Full Text]
24. Orfanos, SE, Langleben, D, Khoury, J, et al Pulmonary capillary endothelium-bound angiotensin-converting enzyme activity in humans. Circulation 1999;99,1593-1599[Abstract/Free Full Text]
25. Eddahibi, S, Springall, D, Mannan, M, et al Dilator effect of endothelins in pulmonary circulation: changes associated with chronic hypoxia. Am J Physiol 1993;265,L571-L580
26. Fishman, AP Clinical classification of pulmonary hypertension. Clin Chest Med 2001;22,385-391[CrossRef][ISI][Medline]
27. Fedullo, PF, Auger, WR, Channick, RN, et al Chronic thromboembolic pulmonary hypertension. Clin Chest Med 2001;22,561-581[CrossRef][ISI][Medline]
28. Davie, N, Haleen, SJ, Upton, PD, et al ETA and ETB receptors modulate the proliferation of human pulmonary artery smooth muscle cells. Am J Respir Crit Care Med 2002;165,398-405[Abstract/Free Full Text]
29. Bauer, M, Wilkens, H, Langer, F, et al Selective upregulation of endothelin B receptor gene expression in severe pulmonary hypertension. Circulation 2002;105,1034-1036[Abstract/Free Full Text]
30. Westcott, JY, Henson, J, McMurtry, IF, et al Uptake and metabolism of endothelin in the isolated perfused rat lung. Exp Lung Res 1990;16,521-532[ISI][Medline]

This Article Abstract Full Text (PDF) Free Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in ISI Web of Science Similar articles in PubMed Alert me to new issues of the journal Add to My Personal Article Archive Download to citation manager Citing Articles Citing Articles via ISI Web of Science (10) Citing Articles via Google Scholar Google Scholar Articles by Langleben, D. Articles by Giovinazzo, M. Search for Related Content PubMed PubMed Citation Articles by Langleben, D. Articles by Giovinazzo, M.