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INTEGRATIVE |
1 Experimental Cardiology, Thoraxcentre2Internal Medicine, Cardiovascular Research Institute COEUR, Erasmus MC, University Medical Centre Rotterdam, Rotterdam, the Netherlands
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Abstract |
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0.05). MI increased pulmonary arterial pressure and PVR both at rest and during exercise (both P
0.05). The increased pulmonary arterial pressure correlated with the increased plasma ET levels in resting MI swine (r
= 0.71; P
0.01). Furthermore, the pulmonary vasoconstrictor response to ET-1 infusion was enhanced after MI (P
0.05). ETA/ETB blockade decreased PVR in MI swine from 3.6 ± 0.3 to 3.1 ± 0.5 mmHg min l–1 at rest and from 3.4 ± 0.3 to 2.4 ± 0.2 mmHg min l–1 during exercise at 4 km h–1 (both P
0.05). This increased response to mixed ETA/ETB blockade in MI compared to normal swine appeared to be the result of an increased ETA-mediated vasoconstriction, as ETA blockade decreased PVR in MI swine from 3.4 ± 0.4 to 2.8 ± 0.2 mmHg min l–1 at rest and from 3.1 ± 0.3 to 2.6 ± 0.2 mmHg min l–1 at 4 km h–1 (both P
0.05). In conclusion, increased plasma ET levels together with increased pulmonary resistance vessel responsiveness to ET result in an exaggerated pulmonary vasoconstrictor influence of ET in swine with a recent MI. This vasoconstrictor influence is the result of an emergent tonic ETA-mediated vasoconstriction in addition to the exercise-induced ETB-mediated vasoconstriction that is already present in normal swine.
(Received 8 February 2006;
accepted after revision 11 May 2006;
first published online 18 May 2006)
Corresponding author D. J. Duncker: Experimental Cardiology, Thoraxcentre, Erasmus MC, University Medical Centre Rotterdam, PO Box 1738, 3000 DR Rotterdam, the Netherlands. Email: d.duncker{at}erasmusmc.nl
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Introduction |
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In addition to the backwards transmission of left atrial pressure into the pulmonary circulation, an elevated pulmonary vascular resistance (PVR), associated with activation of the endothelin (ET) system, also contributes to pulmonary hypertension secondary to MI (Remetz et al. 1989; Haitsma et al. 2001; Nauser & Stites, 2001). ET-1 is one of the most potent vasoconstrictors known to date and is produced in endothelial cells. Its role in pulmonary hypertension is supported by increased levels of circulating ET, as well as increased local production (Dupuis et al. 1996; Sakai et al. 1996; Moraes et al. 2000). Moreover, there is a correlation between ET plasma levels and the severity of pulmonary hypertension in patients with chronic heart failure (Cody et al. 1992).
Both ETA and ETB receptors have been identified in the pulmonary vasculature (MacLean et al. 1994; Soma et al. 1999). ETA and ETB receptors on vascular smooth muscle induce vasoconstriction, whereas ETB receptors on the endothelium induce vasodilatation through production of NO and prostacyclin. We have previously shown, that endogenous ET-mediated pulmonary vasoconstriction in normal swine occurs predominantly through the ETB receptors (Merkus et al. 2003), indicating that the vasoconstrictor effect of ETB receptors on vascular smooth muscle is larger than the vasodilator effect of ETB receptors on the endothelium, while ETA receptors have virtually no influence on the pulmonary circulation. The endothelial ETB receptors are also responsible for the plasma clearance of ET in the lungs (Fukuroda et al. 1994), which is reduced in relation to the severity of pulmonary hypertension in patients with chronic heart failure (Staniloae et al. 2004). The reduced pulmonary clearance of ET and the increased plasma ET levels after MI suggest that ET can contribute to the aggravation of pulmonary hypertension. Indeed, studies in patients with pulmonary hypertension secondary to MI suggest that ETA as well as ETA/ETB receptor blockade are capable of improving clinical conditions (Motte et al. 2005). However, to date no study has addressed the effect of ET receptor antagonists on PVR after MI.
The aim of the present study was therefore to test the hypothesis that an increased ET-mediated vasoconstrictor influence in the pulmonary circulation contributes to secondary pulmonary hypertension in swine with a recent MI (MI swine). For this purpose, we first investigated in awake resting swine; whether plasma ET levels and pulmonary vascular responsiveness to exogenous ET are increased after MI. Subsequently, we investigated whether the vasoconstrictor influence of endogenous ET was increased in the pulmonary circulation and assessed the role of ETA and ETB receptor subtypes in this process. Because in normal swine a pulmonary ET-mediated vasoconstrictor influence emerges during exercise, but is negligible under resting conditions (Merkus et al. 2003), the vasoconstrictor influence of endogenous ET in MI swine was studied both at rest and during treadmill exercise.
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Methods |
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Studies were performed in accordance with the Council of Europe Convention (ETS123)/Directive (86/609/EEC) for the protection of vertebrate animals used for experimental and other scientific purposes, and with approval of the Animal Care Committee at Erasmus MC, University Medical Centre Rotterdam. A total of 47 Yorkshire x Landrace swine (2–3 months old, 22 ± 1 kg at the time of surgery) of either sex entered the study. After completing all experimental protocols, animals were killed by an intravenous overdose of pentobarbitone sodium.
Surgery
Swine were sedated with ketamine (30 mg kg–1 I.M.), anaesthetized with thiopental (10 mg kg–1 I.V.), intubated and ventilated with a mixture of O2 and N2O (1 : 2) to which 0.2–1% (v/v) isoflurane was added (Stubenitsky et al. 1998; Duncker et al. 2001). Anaesthesia was maintained with midazolam (2 mg kg–1 + 1 mg kg–1 h–1 I.V.) and fentanyl (10 µg kg–1 h–1 I.V.). Under sterile conditions, the chest was opened via the fourth left intercostal space and a fluid-filled polyvinylchloride catheter was inserted into the aortic arch and pulmonary artery for blood pressure measurement (Combitrans pressure transducers, Braun, Melsungen, Germany) and blood sampling. An electromagnetic flow probe (14–15 mm, Skalar) was positioned around the ascending aorta for measurement of cardiac output. A polyvinylchloride catheter was also inserted into the left atrium to measure pressure. In all 47 swine, the proximal part of the left coronary circumflex artery (LCx) was exposed, but the LCx was permanently occluded with a silk suture to produce an MI in only 22 of the animals (Haitsma et al. 2001; van der Velden et al. 2004). Three MI swine died during surgery due to recurrent fibrillation. Catheters were tunnelled to the back and animals were allowed to recover, receiving analgesia (0.3 mg buprenorphine I.M.) for 2 days and antibiotic prophylaxis (25 mg kg–1 amoxicillin and 5 mg kg–1 gentamycin I.V.) for 5 days.
Resting protocols
Experimental design. Resting studies were performed 17 ± 1 days after surgery with animals resting quietly on the treadmill. A total number of 19 normal and 18 MI swine were used in the resting protocols. In the first resting protocol we evaluated the effects of MI on resting haemodynamic variables and plasma ET levels. In the second resting protocol we evaluated the pulmonary resistance vessel responses to ET by intravenously infusing exogenous ET-1.
Pulmonary arterial pressure and plasma levels of ET. Plasma levels of ET were determined in 15 normal and 18 MI swine. As we have recently shown that exercise does not affect plasma levels of ET in either normal or MI swine (Merkus et al. 2005), we elected to determine plasma ET levels only under resting conditions. Arterial blood samples (5 ml) in all swine were collected in tubes containing EDTA. Samples were centrifuged (3000g; 10 min; 4°C) and plasma was stored at –80°C. Plasma levels of ET-like immunoreactivity were determined using a radioimmuno assay from Euro-Diagnostica (Malmö, Sweden), which has a cross reactivity of 100% towards ET-1, 48% towards ET-2 and 109% towards ET-3 (Merkus et al. 2003). Because production of ET-2 and ET-3 appears to be absent in the cardiovascular system of the swine (Kjekshus et al. 2000), the concentrations measured with the radioimmuno assay most probably represent ET-1. Resting haemodynamic measurements consisting of aortic, left atrial and pulmonary arterial blood pressures and cardiac output were obtained.
Exogenous ET-1 infusions. With swine (n = 6 normal and n = 3 MI) resting quietly, resting haemodynamic measurements were obtained. Subsequently, ET-1 was infused i.v. via the pulmonary arterial catheter at rates of 20 and 40 pmol kg–1 min–1 and resting haemodynamic measurements were again obtained at the end of each 10-min infusion period.
Exercise protocols
Experimental design. Studies were performed 11 ± 1 days after surgery with animals exercising on a motor-driven treadmill. The two exercise protocols were performed on different days and in random order. A total number of 14 normal and nine MI swine were used in the exercise protocols, of which eight normal and eight MI swine were also used in the resting protocols. In the first exercise protocol, we evaluated the vasoconstrictor influence of endogenous ET, using a mixed ETA/ETB receptor antagonist. In the second protocol, we assessed the role of the ETA receptor subtype in the vasoconstrictor influence of endogenous ET in MI swine, using a selective ETA receptor antagonist. We refrained from studying the effects of a selective ETB receptor antagonist, because the ETB receptor is responsible for clearance of ET in the lungs (Fukuroda et al. 1994; Merkus et al. 2003). Hence, ETB blockade increases ET levels, which can then act through the ETA receptor to cause vasoconstriction, thereby confounding interpretation of the results. Consequently, the contribution of the ETB receptor must be derived from the difference in response to the ETA and the ETA/ETB receptor antagonists.
Effects of combined ETA and ETB receptor blockade. With swine (n = 13 normal and n = 9 MI) lying quietly on the treadmill, resting haemodynamic measurements consisting of left atrial, aortic and pulmonary arterial blood pressures and cardiac output were obtained and arterial and mixed venous blood samples were collected. Haemodynamic measurements were repeated and rectal temperature measured with animals standing on the treadmill. Subsequently, a four-stage exercise protocol (1, 2, 3 and 4 km h–1) was begun with each stage lasting 2–3 min. Haemodynamic variables were measured and blood samples collected during the last 30 s of each exercise stage, when haemodynamic variables had reached a steady state. At the conclusion of the exercise protocol, animals were allowed to rest for 90 min. Then, animals received the mixed ETA and ETB receptor antagonist tezosentan (a gift from Dr Clozel, Actelion Pharmaceuticals Ltd, Allschwill, Switzerland) intravenously over 10 min at a dose of 3 mg kg–1, followed by a continuous infusion of 6 mg kg–1 h–1 I.V. (Merkus et al. 2003), and the exercise protocol was repeated. Tezosentan has a pA2 of 9.5 for ETA and a pA2 of 7.7 for ETB receptors, indicating only a 63-fold selectivity for ETA compared to ETB receptors (Clozel et al. 1999). Moreover, we have previously shown that the employed dose of tezosentan blocks the pressor response to intravenous ET infusion and results in an increase in plasma ET levels, indicating that both ETA and ETB receptors are blocked (Merkus et al. 2003).
Effects of ETA receptor blockade. Swine (n = 9 normal and n = 8 MI) received the ETA receptor antagonist EMD 122946 (a gift from Professor Schelling, E. Merck Darmstadt, Darmstadt, Germany) at a dose of 3 mg kg–1 intravenously, 90 min after they underwent the exercise protocol under control conditions (Merkus et al. 2003). EMD 122946 has a pA2 of 9.5 for ETA and a pA2 of 6.0 for ETB receptors, indicating a 3200-fold selectivity for ETA compared to ETB receptors (Mederski et al. 1998). EMD 122946 in a dose of 3 mg kg–1 blocks the pressor response to intravenously infused ET. In contrast, this dose of EMD 122946 does not block ETB receptors, as ETB-mediated clearance of ET is not affected by administration of EMD 122946 as demonstrated by unaltered ET plasma levels (Merkus et al. 2003). Resting measurements were obtained and the exercise protocol repeated 5 min after completion of the infusion.
Blood gas measurements
Blood samples obtained in the exercise protocols, were kept in syringes on-ice until the conclusion of each exercise trial. Measurements of PO2 (mmHg), PCO2 (mmHg) and pH were then immediately performed with a blood gas analyser (Acid-Base Laboratory Model 505, Radiometer, Copenhagen, Denmark). Oxygen saturation (%) and haemoglobin level (g dl–1) were measured with a haemoximeter (OSM3, Radiometer). Blood O2 content (µmol ml–1) was computed as (Hb x 0.621 x O2 saturation) + (0.00131 x
PO2). Body O2 consumption 
was calculated as the product of cardiac output and the difference in O2 content between arterial and mixed venous blood (Stubenitsky et al. 1998; Duncker et al. 2001).
Data analysis
Digital recording and off-line analysis of haemodynamic variables have been described in detail elsewhere (Stubenitsky et al. 1998; Duncker et al. 2001). Systemic vascular resistance was computed as mean aortic blood pressure divided by cardiac output. PVR was computed as mean pulmonary arterial pressure minus mean left atrial pressure divided by cardiac output.
Statistical analysis
The effect of MI on pulmonary arterial pressure and plasma ET levels was assessed using an unpaired t test. The relation between endogenous ET and pulmonary arterial pressure was assessed using linear regression, whereas statistical analysis of the effect of exogenous ET on systemic vascular resistance and PVR in normal versus MI swine was performed using analysis of variances (ANOVA). To test for the effects of MI and drug treatment (EMD 122946 or tezosentan) on the relation between 
and systemic vascular resistance or PVR, regression analysis was performed using MI, drug treatment and 
, as well as their interaction, as independent variables and assigning a dummy variable to each animal. Statistical significance was accepted when P
0.05. Data are presented as mean ±
S.E.M.
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Results |
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After MI, mean resting pulmonary arterial pressure increased from 15 ± 1 to 24 ± 3 mmHg (Fig. 1A), which was in part the result of the increased mean left atrial pressure (normal swine, 4 ± 1 mmHg; MI swine, 12 ± 2 mmHg). Resting ET plasma levels were elevated in MI (3.8 ± 0.4 pmol l–1) compared to normal swine (2.7 ± 0.3 pmol l–1, P
0.05) (Fig. 1B). Furthermore, a significant correlation existed between pulmonary arterial pressure and ET levels in MI swine (r
= 0.708; P
0.01) (Fig. 1C), supporting a role for ET in pulmonary hypertension after MI.
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Exercise up to 4 km h–1 in normal swine resulted in a tripling of 
which was met by a doubling of cardiac output and a 
40% increase in O2 extraction (Fig. 3). The increase in cardiac output was principally due to an increase in heart rate, as stroke volume increased by only 7%. Mean aortic blood pressure was minimally affected (Table 1), implying that the increase in cardiac output was balanced by a similar decrease in systemic vascular resistance. Mean pulmonary arterial pressure almost doubled in normal swine during exercise (Fig. 3). However, the transpulmonary pressure gradient (pulmonary arterial pressure minus left atrial pressure) increased slightly less than cardiac output, reflecting a small decrease in PVR during exercise.
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15% increase in PVR (Fig. 3) The role of ET in the regulation of vascular tone
In normal swine, ETA/ETB receptor blockade with tezosentan had no effect on the pulmonary circulation under resting conditions (Table 1 and Fig. 4). During exercise, however, tezosentan reduced pulmonary arterial pressure with no effect on left atrial pressure and caused a small increase in cardiac output (Table 1), implying a decrease in PVR. The tezosentan-induced decrease in PVR increased progressively with incremental exercise intensity (Fig. 4). In contrast, the ETA receptor antagonist EMD 122946 had no effect on pulmonary arterial or left atrial pressure in normal animals either at rest or during exercise (Table 2), so that PVR remained unchanged (Fig. 4). Hence in normal swine, the vasoconstrictor influence of ET on the pulmonary circulation during exercise is mediated by ETB receptors.
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Administration of the ETA/ETB receptor antagonist tezosentan to normal swine resulted in a decrease in systemic vascular resistance, reflecting vasodilatation in the systemic vasculature at rest and during exercise. There were no differences between the response to tezosentan and EMD 122946 in the systemic circulation of normal swine (Tables 1 and 2). In MI swine the ETA/ETB receptor blockade and ETA receptor blockade resulted in similar decreases of systemic vascular resistance compared to in normal swine. These findings indicate that the ET-dependent vasoconstrictor influence in the systemic circulation is principally mediated by ETA and is not increased after MI.
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Discussion |
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In agreement with previous studies from our laboratory (van Kats et al. 2000; Haitsma et al. 2001, 2002), MI swine (2 weeks after an MI) have left ventricular (LV) dysfunction, characterized by a lower stroke volume and increased left atrial pressure. The accompanying increase in heart rate resulted in an essentially maintained cardiac output, indicating that overt congestive heart failure is absent (van Kats et al. 2000). In agreement with clinical observations (Cody et al. 1992), and with previous observations from our own laboratory (van Kats et al. 2000; Haitsma et al. 2001, 2002), MI swine were characterized by increases in pulmonary arterial pressure and PVR. Furthermore, similar to clinical observations (Cody et al. 1992), we also found that the severity of pulmonary hypertension and the increase in plasma levels of ET were highly correlated. Moreover, the range of plasma ET levels after MI, which demonstrate increases of up to 4-fold as compared to normal swine, is similar to that found in patients with pulmonary hypertension secondary to MI (Cody et al. 1992). These observations indicate that our porcine model recapitulates several aspects of secondary pulmonary hypertension observed in patients after MI.
Elevation of plasma ET levels in pulmonary hypertension
In agreement with previous studies from our laboratory (Haitsma et al. 2001; Merkus et al. 2005), circulating levels of ET were increased after MI. These elevated ET levels could have been the result of decreased ET clearance, increased ET production, or both.
ET clearance. The pulmonary circulation, together with the kidneys and the liver, is the predominant site in the body for clearance of ET, with single-pass clearance reaching values of up to 30–50% (Dupuis et al. 1994, 1996). This clearance is mediated through binding of ET to the ETB receptor on the pulmonary endothelium (Fukuroda et al. 1994). In the pulmonary circulation of healthy individuals the ET-production and clearance are balanced, such that there is no net difference between pulmonary arterial and venous ET plasma levels (Dupuis et al. 1994, 1996). In secondary pulmonary hypertension, increased plasma ET levels can be the result of decreased clearance of ET in the lungs. Indeed, in both experimental models and humans with heart failure, clearance of ET was shown to be reduced (Dupuis et al. 1998a,b; Staniloae et al. 2004), although this was not a unanimous finding (von Lueder et al. 2004).
ET production. ET is produced from big-ET by endothelin-converting enzyme (ECE)-1, while big-ET is produced from preproendothelin by furin-like enzymes (Galie et al. 2004). Hence, increases in either ECE activity or in preproendothelin levels could have contributed to an increase in ET production. The rate-limiting step in the production of ET is thought to be the conversion from big-ET to ET by ECE-1. Increased ET production in pigs with pacing-induced heart failure was attributed to an increased ECE-1 activity (von Lueder et al. 2004). The ECE-1 isoform that is involved is still incompletely understood, but is most likely to be the ECE-1a isoform, as ECE-1a mRNA is up-regulated while ECE-1c mRNA appears to remain unaltered (Ergul et al. 2001). However, an up-regulation of ECE-1 mRNA could not be confirmed in rats with either mild LV dysfunction or overt heart failure (Lepailleur-Enouf et al. 2001). Conversely, Lepailleur-Enouf et al. (2001) as well as others (Tonnessen et al. 1998) reported that increased preproendothelin mRNA levels in the lungs contribute to increased ET production in rats with MI-induced heart failure. However, in animals with mild LV dysfunction (resembling the porcine MI model in the present study) preproendothelin mRNA levels were still normal (Lepailleur-Enouf et al. 2001), suggesting that increased preproendothelin expression might be a feature of more advanced stages of heart failure. It should be noted that in all these studies, involving protein and/or mRNA expression, total lung tissue was used. As ECE-1 as well as preproendothelin are also present in bronchial epithelial cells (Saleh et al. 1997; Takizawa et al. 1998; Ahmed et al. 2000; Yap et al. 2000), interpretation of these studies using total lung tissue is difficult. Future studies using only pulmonary resistance vessels, are required to investigate the molecular alterations underlying an increased production of ET in more detail.
The mechanisms underlying the reported increases in ECE activity and/or preproendothelin levels in pulmonary hypertension secondary to LV failure are incompletely understood, but could be caused, at least in part, by endothelial dysfunction. Thus, in patients with secondary pulmonary hypertension due to heart failure, basal pulmonary NO production is reduced (Moraes et al. 2000). NO is capable of limiting ET-production (Lavallee et al. 2001; Alonso & Radomski, 2003; Kelly et al. 2004), and in support of this, we recently found that loss of NO synthesis, resulted in an enhanced pulmonary vasoconstrictor influence of ET in normal swine (Houweling et al. 2005). However, we previously failed to observe a loss of the vasodilator influence of NO in the pulmonary circulation after MI (Haitsma et al. 2002), suggesting that at this stage of LV dysfunction, the increased plasma ET levels are not the result of loss of NO bioavailability. Future studies, investigating the effects of ET receptor antagonists following NO synthase inhibition in swine with pulmonary hypertension after MI, are required to test this hypothesis more rigorously.
ET receptor subtypes involved in pulmonary hypertension
ET-induced constriction of large pulmonary arteries is principally mediated by ETA receptors (MacLean et al. 1994; Perreault & Baribeau, 1995), whereas constriction in the smaller pulmonary resistance vessels is mediated by ETB receptors (MacLean et al. 1994). These findings are supported by observations that the density of ETA receptors in the lung decreases with decreasing vessel size, whereas the density of ETB receptors, both on the endothelium and the smooth muscle, increases (Soma et al. 1999).
In normal swine, blockade of either ETA receptors alone or combined blockade of ETA and ETB receptors had no effect on PVR at rest, indicating that endogenous ET does not contribute to resting tone in the pulmonary resistance vessels (Merkus et al. 2003). During exercise, however, an ET-mediated vasoconstriction became apparent that was ETB receptor-mediated with no contribution of ETA receptors (Merkus et al. 2003; Houweling et al. 2005). This ETB-mediated vasoconstriction that limits the pulmonary vasodilatation in response to exercise (Merkus et al. 2003), is consistent with the localization of the ETB receptors on the pulmonary resistance vessels (Soma et al. 1999).
In swine with secondary pulmonary hypertension, the increased pulmonary vasodilator response to mixed ETA/ETB receptor blockade was increased in MI compared to normal swine. Both plasma ET levels and sensitivity of the pulmonary circulation to exogenous ET (via intravenous infusion) were increased, and probably acted in concert to increase the pulmonary vasoconstrictor influence of ET in MI swine. Although we cannot entirely exclude the possibility that other neurohormones, including noradrenaline and angiotensin II, contributed to the increased pulmonary resistance vessel tone after MI, it is important to note that mixed ETA/ETB receptor blockade abolished the differences in PVR between normal and MI swine (see Fig. 4). This finding is consistent with the concept that an increased ET-mediated vasoconstrictor influence was principally responsible for the increase in pulmonary resistance vessel tone after MI.
Although the ETA receptor does not contribute to the regulation of pulmonary vasomotor tone in normal swine at rest or during exercise, an ETA-mediated vasoconstriction emerged after MI. This ETA-mediated vasoconstriction appeared tonic in nature, so that it did not increase further during exercise. This increased vasoconstrictor influence of the ETA receptor may also have contributed to the increased pulmonary sensitivity to exogenous ET. This is in accordance with a study in pigs with pacing-induced heart failure, which showed that ETA receptor mRNA and ETA binding are increased (Ergul et al. 2001). Similarly, data in dogs showed that, whereas pulmonary vasoconstriction in response to exogenous ET is predominantly mediated through the ETB receptor in normal dogs, there is an ETA-mediated component in dogs with pacing-induced heart failure (Tadano et al. 2004). Conversely, Docherty & MacLean (1998) investigated the effect of ET on isolated pulmonary arterioles in animals with MI using ET receptor agonists and antagonists, and found no contribution of the ETA receptor to pulmonary vasoconstriction in either normal rabbits or rabbits with MI. Similarly, several studies in rats showed no alterations in ETA mRNA and/or protein expression in the lung after MI (Kobayshi et al. 1998; Lepailleur-Enouf et al. 2001). However, in all studies, involving protein and/or mRNA expression, total lung tissue was used. Because ETA as well as ETB receptors are also present in bronchial airway smooth muscle and lung alveoli (Goldie, 1998), interpretation of these studies using total lung tissue is difficult.
Comparison of the vasodilator effect of ETA and ETA/ETB receptor blockade in normal and MI swine suggests that the contribution of the ETB receptor to the regulation of pulmonary vasomotor tone remained unaltered after MI. These data are in apparent contrast to the majority of studies on ETB receptor expression, both at the level of mRNA (Ivy et al. 1998; Lepailleur-Enouf et al. 2001) and the level of protein binding (Kobayshi et al. 1998; Ergul et al. 2001), which indicate that ETB receptor expression as well as ETB-mediated clearance are reduced in lungs of animals with heart failure. However, in these studies, ETB receptors on the endothelial cells and on the vascular smooth muscle cells cannot be distinguished. It is therefore possible that ETB receptors on the endothelium, which are responsible for vasodilatation and clearance, are reduced to the same extent as ETB receptors on vascular smooth muscle, which induce vasoconstriction. The net effect would then be that the ETB-mediated vasoconstriction in response to endogenous ET would remain more or less constant, while clearance is reduced.
Clinical relevance
In the treatment of pulmonary hypertension, ET antagonism is a promising therapy. There have been several clinical trials that have demonstrated improvement in haemodynamic variables and exercise capacity and reduction in the number of clinical events after administration of an ETA receptor antagonist or a mixed ETA/ETB receptor antagonist (Channick et al. 2001; Rubin et al. 2002; Barst et al. 2004; Galie et al. 2005). It has been proposed that ETA receptor blockade would be a good option because it leaves the ETB-mediated clearance of ET as well as the endothelium ETB-mediated pulmonary vasodilatation unaffected. However, comparison of clinical trials using either ETA or combined ETA/ETB receptor antagonists (Motte et al. 2005), suggests that exercise capacity of patients with pulmonary hypertension improves more with combined ETA/ETB than with ETA receptor antagonists. The observations in the present study in MI swine provide further support for the use of combined ETA/ETB receptor antagonism in patients with secondary pulmonary hypertension. This important issue is currently being investigated in the prospective STRIDE-2 clinical trial, comparing treatment with ETA receptor antagonism versus combined ETA/ETB receptor antagonism (Langleben et al. 2004; Cleland et al. 2005).
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), while the data of normal swine are shown as mean ±
S.E.M. (
). Inclusion of both MI and normal swine resulted in a highly similar correlation between pulmonary arterial pressure (PAP) and ET plasma levels (PAP = 4.9 x ET + 3.5, r
= 0.703), as compared to inclusion of MI swine alone (PAP = 5.2 x ET + 3.6; r
= 0.708). *P
under control conditions in 14 normal (
. Normal swine are represented by circles and MI swine are represented by squares; control exercise trials are shown as open symbols, exercise trials in the presence of antagonists are shown as filled symbols. *P
P
P