Published online before print November 24, 2008, doi: 10.1161/CIRCULATIONAHA.108.784868
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(Circulation. 2008;118:2523-2532.)
© 2008 American Heart Association, Inc.
Coronary Heart Disease |
Angiotensin II Type 2 Receptor Stimulation
A Novel Option of Therapeutic Interference With the Renin-Angiotensin System in Myocardial Infarction?
From the Center for Cardiovascular Research, Institute of Pharmacology, Charité-Universitätsmedizin Berlin, Berlin, Germany (E.K., A.G., J.L., A.F.-L., M.T., F.R., M.S., U.R.K., C.C., P.N., U.K., T.U., U.M.S.); Department of Cardiology and Pneumology, Charité-Universitätsmedizin Berlin, Campus Benjamin Franklin, Berlin, Germany (C.T.); Department of Medicinal Chemistry, Organic Pharmaceutical Chemistry, Uppsala University, Uppsala, Sweden (A.H., M.A.); Deutsches Herzzentrum Berlin, Klinik für Innere Medizin—Kardiologie, Berlin, Germany (T.H., I.P., T.D., K.G.); Philips Clinical Science, Hamburg, Germany (B.S.); and Sahlgrenska University Hospital/Östra, Göteborg, Sweden (B.D.).
Correspondence to Dr Thomas Unger, Center for Cardiovascular Research, Institute of Pharmacology, Charité-Universitätsmedizin Berlin, Hessische Strasse 3–4, 10115 Berlin, Germany. E-mail thomas.unger{at}charite.de
Received April 9, 2008; accepted September 12, 2008.
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Abstract |
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Background— This study is the first to examine the effect of direct angiotensin II type 2 (AT2) receptor stimulation on postinfarct cardiac function with the use of the novel nonpeptide AT2 receptor agonist compound 21 (C21).
Methods and Results— Myocardial infarction (MI) was induced in Wistar rats by permanent ligation of the left coronary artery. Treatment with C21 (0.01, 0.03, 0.3 mg/kg per day IP) was started 24 hours after MI and was continued until euthanasia (7 days after MI). Infarct size was assessed by magnetic resonance imaging, and hemodynamic measurements were performed via transthoracic Doppler echocardiography and intracardiac Millar catheter. Cardiac tissues were analyzed for inflammation and apoptosis markers with immunoblotting and real-time reverse transcription polymerase chain reaction. C21 significantly improved systolic and diastolic ventricular function. Scar size was smallest in the C21-treated rats. In regard to underlying mechanisms, C21 diminished MI-induced Fas-ligand and caspase-3 expression in the peri-infarct zone, indicating an antiapoptotic effect. Phosphorylation of the p44/42 and p38 mitogen-activated protein kinases, both involved in the regulation of cell survival, was strongly reduced after MI but almost completely rescued by C21 treatment. Furthermore, C21 decreased MI-induced serum monocyte chemoattractant protein-1 and myeloperoxidase as well as cardiac interleukin-6, interleukin-1β, and interleukin-2 expression, suggesting an antiinflammatory effect.
Conclusions— Direct AT2 receptor stimulation may be a novel therapeutic approach to improve post-MI systolic and diastolic function by antiapoptotic and antiinflammatory mechanisms.
Key Words: angiotensin • angiotensin AT2 receptor • myocardial infarction • pharmacology
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Introduction |
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Controversy has always existed about the angiotensin II type 2 receptor (AT2R). Initially, after the first description of the AT2R, it was assumed that the AT2R may be without any physiological function. It was not until 5 years after its identification that a first physiological function, antiproliferation, could be ascribed to the AT2R.1,2 Today, it is widely accepted that, in many instances, the AT2R counteracts angiotensin II type 1 receptor (AT1R)–mediated actions, which in most cases is beneficial for the organism.3
Clinical Perspective p 2532
A number of animal studies and the Valsartan in Acute Myocardial Infarction (VALIANT) trial have shown that AT1R blockers improve survival and cardiac function after myocardial infarction (MI).4 It has been speculated that the AT2R contributes to this effect because AT1R blockade leads to an increased synthesis of angiotensin II, which in turn stimulates the unopposed AT2Rs. However, results of animal studies directly addressing the role of the AT2R in MI are again controversial: Although most studies claim that the AT2R improves postinfarct cardiac function, others report either no effect on the outcome or even deterioration.5
The inconsistency of data about AT2R actions may be due in part to the fact that detection and assignment of AT2R-mediated effects have always been difficult. AT2R-mediated effects had to be examined either by treatment of cells or animals with angiotensin II under concomitant AT1R blockade, which resulted in complex experimental protocols, or they were examined in genetically altered animals either overexpressing or lacking the AT2R. The only currently available pharmacological AT2R antagonist, PD123319, lacks selectivity when applied at higher doses and has thus given rise to contradictory findings, especially when applied in vivo.6 An experimental tool to directly stimulate the AT2R under in vivo conditions was lacking until now.
In 2004, the synthesis of the first nonpeptide AT2R agonist (compound 21 [C21]) was published, a compound that allows investigators to specifically and selectively stimulate the AT2R in in vitro and in vivo settings.7 With a Ki of 0.4 nmol/L for the AT2R and a Ki >10 µmol/L for the AT1R, this substance possesses high selectivity for the AT2R.
The present study is the first to examine the role of the AT2R in post-MI cardiac function by direct AT2R stimulation in vivo with the use of the nonpeptide AT2R agonist C21. MI was induced in normotensive Wistar rats by permanent ligation of the anterior descending artery. After a 1-week treatment period, scar volume was assessed by magnetic resonance imaging, and hemodynamic function was assessed by echocardiography and Millar catheter. In a first attempt to address the underlying mechanisms of C21 actions, markers of inflammation and apoptosis were determined in cardiac tissue and plasma. To obtain a first qualitative estimate of the effectiveness of C21, we included a group of animals treated with the AT1R antagonist candesartan as a reference drug.
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Methods |
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Animals
Male normotensive Wistar rats (weight, 200 to 220 g; Harlan Winkelmann, Borchen, Germany) were kept in a specific pathogen–free barrier under standardized conditions with respect to temperature and humidity and were housed on a 12-hour light/12-hour dark cycle in groups of 5 with food and water ad libitum. Animal housing, care, and applications of experimental procedures complied with the Guide for the Care and Use of Laboratory Animals of the State Government of Berlin, Germany.
Myocardial Infarction
Rats were anesthetized with ketamine/xylazine (Sigma Aldrich Chemie, Steinheim, Germany) 80 mg/10 mg/kg IP, intubated, and ventilated with a small-animal ventilator (Starling Ideal Ventilator, Harvard Apparatus) with room air at a rate of 75 cycles per minutes and a tidal volume of 300 µL. A left lateral thoracotomy was performed, and a suture was tightened around the proximal left anterior descending coronary artery. Sham-operated rats underwent the same surgical procedure with the exception of coronary ligature.
Experimental Protocol
Transthoracic Doppler echocardiography was performed 24 hours after MI to assess post-MI baseline cardiac function. For better comparability of outcome between groups, rats were assigned to 2 different groups for later data analysis according to ejection fraction (EF): (1) rats with EF >35% and (2) rats with EF 
35%. All data presented in this study apply to animals with EF >35% unless indicated otherwise. After being allocated to a certain category with respect to EF, animals were further randomly assigned to the following treatments: (1) vehicle; (2) C21 (0.01; 0.03; 0.3 mg/kg per day; C21 was kindly provided by A. Hallberg, University of Uppsala, Uppsala, Sweden; C21 is now available from Vicore Pharma, Göteborg, Sweden); (3) C21+PD123319 (0.03 mg/kg C21; 3 mg/kg PD123319); (4) candesartan (0.1 mg/kg per day); or (5) C21+candesartan (0.03 mg/kg C21; 0.1 mg/kg per day candesartan). A low dose of candesartan (0.1 mg/kg per day) was chosen because in prior experiments of our group, this dose had shown beneficial functional and cellular effects after stroke without affecting blood pressure. Drugs were applied intraperitoneally once daily starting 24 hours after MI to allow for prior determination of EF under untreated conditions. After 7 days of treatment, animals were euthanized, and plasma and hearts were collected. Sham-operated animals served as controls. The number of animals per group was 12 at the time of euthanasia. To achieve this group size, a total of 298 rats underwent successful coronary occlusion.
Transthoracic Doppler Echocardiography
Transthoracic Doppler echocardiography (M-mode and Doppler measurements) was performed on days 1 and 7 after MI in anesthetized rats with the use of the high-resolution imaging system Vevo 770 (VisualSonics Inc, Toronto, Canada).8 For details, see the online-only Data Supplement.
Determination of Left Ventricular and Scar Volumes by Magnetic Resonance Imaging
Cardiac-triggered cine and scar magnetic resonance imaging was performed on a conventional clinical 3.0-T scanner (Philips Achieva CV 3.0 T, Best, Netherlands) equipped with a QuasarDual gradient system (80 mT/m; 200 mT/m per millisecond slew rate) and specifically designed software (release 2.5.1 with implementation of a small-animal software patch developed by GyroTools Ltd, Zurich, Switzerland). For details, see the online-only Data Supplement.
Analysis of Hemodynamic Parameters
On day 7 after MI, before euthanasia, rats were anesthetized with ketamine/xylazine (80 mg/10 mg/kg IP; Sigma). An SPR-407 Mikro-Tip pressure catheter transducer (Millar Instruments, Houston, Tex) was passed through the right carotid artery and inserted into the aorta for the recording of heart rate and arterial pressure under constant pressure monitoring. The catheter was then advanced into the left ventricle for measurement of cardiac parameters. The catheter was connected via PowerLab/4 U (ADInstruments, Spechbach, Germany) to a computer running MacLab (Chart 5 software). Data were recorded over 10 minutes, stored, and later analyzed with Chart software (Blood Pressure Module).
Enzyme-Linked Immunosorbent Assay, Quantitative Real-time Reverse Transcription Polymerase Chain Reaction, and Western Blot Analysis
Enzyme-linked immunosorbent assays for the determination of plasma monocyte-chemoattractant protein-1 and myeloperoxidase, real-time reverse transcription polymerase chain reactions for the measurement of interleukin (IL)-1β, IL-2, and IL-6 in cardiac tissue as well as H9C2 cells, and immunoblotting for the quantification of cardiac caspase-3, Fas ligand, phospho-p38 and phospho-p44/42 mitogen-activated protein kinases (MAPKs) were performed according to standard protocols. Details, primers, and antibodies can be found in the Methods section of the online-only Data Supplement.
Cell Culture Experiments
Neonatal rat cardiomyocytes (H9C2 cell line) were incubated for 10 hours in high-glucose Dulbecco’s modified Eagle’s medium plus 10% fetal bovine serum and treated with 10 ng/mL recombinant tumor necrosis factor-
(TNF-) to induce cytokine expression and mimic the post-MI inflammatory response in the heart. Incubation was performed with or without coincubation with 10–6 mol/L C21 or C21 plus 10–5 mol/L PD123319.
Statistical Analysis
Results were expressed as mean±SEM. Multiple comparisons were analyzed with 1-way ANOVA followed by the Bonferroni post hoc test. Two-group comparisons were analyzed by the 2-tailed Student unpaired t test for independent samples. Differences were considered statistically significant at the value of P<0.05.
The authors had full access to and take full responsibility for the integrity of the data. All authors have read and agree to the manuscript as written.
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Results |
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Basal Parameters
Postinfarct mortality was 31%. Animal losses all occurred within the first 24 hours after MI before first echocardiography and before animals were assigned to certain treatment groups. No animals died on days 2 to 7 after MI. Mean EF was equal in all treatment groups at treatment start (Figure 1). Body and heart weight did not differ between treatment groups or between animals with or without MI (data not shown).
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Hemodynamic Measurements
Hemodynamic parameters obtained on day 7 after MI/sham by Millar catheter in animals with EF >35% after infarct are presented in Table 1 and Figure 2; those obtained in animals with severe impairment of cardiac function (EF 35%) are presented in Table I of the online-only Data Supplement.
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Heart rate, ventricular systolic pressure, and systolic and diastolic blood pressure were not significantly altered by any treatment regimen or surgical procedure. Infarcted animals presented with a significant decrease in contractility as well as in maximal (dP/dtmax) and minimal (dP/dtmin) peak rate of left ventricular pressure. Moreover, left ventricular end-diastolic pressure (LVEDP) was significantly increased as a result of MI. In vivo treatment of rats with the AT2R agonist C12 (0.03 or 0.3 mg/kg body wt) or with the AT1R antagonist candesartan (0.1 mg/kg body wt) significantly enhanced contractility, dP/dtmax, and dP/dtmin, whereas LVEDP was reduced. The AT2R antagonist PD123319 attenuated the effect of C21 on contractility and in part, but not significantly, on dP/dtmin and LVEDP but not on dP/dtmax. The combination of C21 with candesartan was not superior to C21 monotherapy.
In animals with severe postinfarct impairment of cardiac function (EF 35%), C21 (0.03 mg/kg body wt) still led to a significant improvement of LVEDP, contractility, dP/dtmax, and dP/dtmint (Table I of the online-only Data Supplement).
Transthoracic Doppler Echocardiography
Measurements by transthoracic Doppler echocardiography confirmed that no changes in heart rate could be found between groups (Table 2; Table II in the online-only Data Supplement).
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MI caused a significant increase in systolic (LVIDs) and diastolic left ventricular inner diameter (LVIDd) and peak velocity of early filling wave (E), whereas fractional shortening, EF, peak velocity of late filling wave (A), and E-wave deceleration time (EDT) were significantly reduced (Table 2; Figure 3). Treatment with C21 (0.03 or 0.3 mg/kg body wt; only 0.03 mg/kg body wt in rats with EF 35%) or with candesartan (0.1 mg/kg body wt) led to a significant improvement of all of these parameters with the only exceptions being LVIDd and A in rats with EF 35%. The pronounced effect of treatment with C21 on left ventricular wall kinesis and peak velocity of late filling wave is also clearly recognizable on the representative echocardiographic images shown in Figure 4A and 4B.
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The AT2R antagonist PD123319 completely or partly abolished the effects of C21 on LVIDs and LVIDd, fractional shortening, EF, E, A, E/A, and EDT (Table 2; Figure 3). Again, the combination of C21 and candesartan did not elicit any synergistic or additional effects. Whereas Figure 3 illustrates differences in outcome between treatment groups, the impact of different treatment regimens (vehicle, C21, candesartan) on EF in individual animals is shown in Figure 1 (and Figure I in the online-only Data Supplement for animals with EF 35%).
The effect of C21 on postinfarct cardiac function was dose dependent. Although 0.01 mg/kg body wt was widely ineffective (data not shown), cardiac function significantly improved with 0.03 mg/kg body wt; further improvement was only slight, and did not occur in all cases, in animals treated with 0.3 mg/kg body wt (Tables 1 and 2
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Magnetic Resonance Imaging
Magnetic resonance imaging at day 7 after MI/sham revealed that in the C21- but not in the candesartan-treated animals, scar size was significantly smaller than in vehicle-treated rats (Table 3, Figure 4C). Furthermore, C21, the combination of C21 plus candesartan, and, to a lesser extent, candesartan alone significantly improved EF (Table 3).
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Inflammation Markers
Inflammation markers were measured (1) in plasma, (2) in cardiac tissue ex vivo, and (3) in fetal cardiomyocytes in vitro, as follows.
- Plasma monocyte chemoattractant protein-1 (MCP-1) and myeloperoxidase were significantly increased in rats after MI (Figure 5). In rats treated with C21, a significant reduction in MCP-1 and myeloperoxidase levels could be noticed compared with vehicle-treated animals.
- The proinflammatory cytokines IL-1β, IL-2, and IL-6 were determined by real-time reverse transcription polymerase chain reaction in the peri-infarct zone. Expression of all 3 cytokines was increased in hearts of infarcted, vehicle-treated animals (Figure 6). C21 treatment significantly reduced expression levels of IL-1β, IL-2, and IL-6 back to baseline levels or even below. In this experimental setting, C21 proved to be as effective as the AT1R antagonist candesartan. The effect of C21 could be inhibited by cotreatment of animals with the AT2R antagonist PD123319.
- To show that the antiinflammatory effects of C21 were not secondary to any hemodynamic changes or to reduced infarct size, the antiinflammatory properties of C21 were corroborated in the fetal cardiomyocyte cell line H9C2. Cells were incubated for 10 hours with TNF- (10 ng/mL) to mimic the inflammatory response after MI. TNF- treatment led to a significant increase in IL-6 expression in these cells (Figure 6D). Cotreatment of cells with C21 (10–6 mol/LM) significantly reduced TNF-–induced IL-6 expression. This antiinflammatory effect of C21 was inhibited by the AT2R antagonist PD123319 (10–5 mol/L).
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P<0.01 vs C21; MCP-1: #P<0.05 vs vehicle, 
P<0.01 vs C21; n=6 per group).
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P<0.05,
Apoptosis Marker
The apoptosis markers Fas ligand and caspase-3 were measured in cardiac tissue (peri-infarct zone) by immunoblotting (Figure 7). MI led to a significant increase in the expression of both markers. C21 and candesartan reduced this increased expression to a comparable extent. The effect of C21 was blocked by the AT2R antagonist PD123319.
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p44/42 MAPK, p38 MAPK
One week after MI, the cardiac expression of phosphorylated p44/42 MAPK and p38 MAPK was reduced to undetectable levels in vehicle-treated rats (Figure 8). Whereas candesartan treatment led to a slight increase in the expression of both MAPKs, C21 restored phospho-p44/42 and phospho-p38 MAPK expression back to levels seen in sham-operated, noninfarcted animals. This effect of C21 was almost completely blocked by cotreatment with the AT2R antagonist PD123319.
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Discussion |
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Although it is unanimously accepted that stimulation of AT1Rs contributes to tissue damage, inflammation, and unfavorable tissue remodeling after MI, data about the role of the AT2R in this context are controversial.5 This is certainly owed to the fact that, until now, practicable and reliable tools to study AT2R actions by direct AT2R stimulation have not been available. Treatment with the natural ligand angiotensin II primarily elicits effects mediated by the dominating AT1R; the only available AT2R antagonist, PD123319, is nonselective at higher doses6; and the only available AT2 agonist, CGP42112, is a peptide and furthermore has agonistic and antagonistic properties, which renders the interpretation of in vivo data difficult.
We present the first in vivo study using a specific and selective nonpeptide AT2R agonist, C21.7 MI was induced in normotensive Wistar rats by permanent ligation of the left anterior descending coronary artery. Subsequently, animals were treated with C21 for 1 week. Direct AT2R stimulation by C21 improved systolic and diastolic cardiac function after MI, coinciding with a smaller infarct scar in C21-treated animals compared with vehicle treatment. Whereas MI led to an impairment of all parameters measured by echocardiography (LVIDs, EF, fractional shortening for systolic function; LVIDd, E, A, E/A, EDT for diastolic function) or Millar catheter (contractility, dP/dtmax for systolic function; LVEDP, dP/dtmin for diastolic function), respectively, treatment with C21 improved all of these parameters significantly without exception. For most parameters, the effect of C21 could be inhibited by cotreatment with the established AT2R antagonist PD123319, thus providing good evidence that the effects seen by C21 treatment are indeed a result of specific AT2R stimulation.
Previous studies addressing the role of the AT2R in cardiac function and remodeling after MI were performed primarily in genetically altered mice either overexpressing or lacking AT2Rs.5 The vast majority of these studies report a beneficial effect of the presence of AT2Rs on cardiac function after MI. For instance, Bove et al and Yang et al observed improved postinfarct cardiac function in AT2R-overexpressing mice, with NO apparently being of pivotal importance.9–11 Several other studies used male AT2R–/Y mice and reported a deteriorated outcome after MI compared with wild-type mice.12–15 Xu et al16 did not detect any differences between AT2R–/Y and AT2R+/Y mice 4 weeks after MI but detected an impaired response to AT1R blockers in AT2R–/Y. Ichihara et al17 used male AT2R–/Y mice as well, but, in contrast to the other studies using this model, postinfarct scar area was reduced in knockout mice. Nevertheless, mortality in AT2R–/Y mice was higher than in wild-type mice as a result of an increased rate of cardiac ruptures within the first week after ligation. Between weeks 2 and 6 after MI, mortality rate was identical in AT2R–/Y and wild-type mice.
Our study differs from those discussed above because these studies examined effects related to AT2R expression and not to specific receptor stimulation. Admittedly, there may be some endogenous postinfarct AT2R stimulation by elevated cardiac angiotensin II levels. Furthermore, the AT2R has been reported to possess constitutive intrinsic activity,18 and cardiac AT2R expression is increased after MI.19,20 Still, the intensity of such an endogenous stimulation is very unlikely to reach the intensity achieved by stimulation with a specific agonist at an appropriate, pharmacological dosage.
Apart from rats treated with different doses of C21, our study also included a group of animals treated with the AT1R blocker candesartan. We used an AT1R blocker as a reference drug (1) because of its proven efficacy in the improvement of postinfarct cardiac function as shown in animals and clinically4 and (2) because it may interfere with the renin-angiotensin system at least in part by indirect AT2R stimulation. We did not intend to compare the efficacy but rather the quality of actions of both drugs because, at present, with only very limited data and experimental experience with C21 available, it is hard to define what an equally effective dose of C21 or candesartan would be. Nevertheless, in mild to moderate infarctions, both drugs restored a number of cardiac parameters back to levels in noninfarcted animals (ie, with regard to these parameters, both drugs elicited an equal maximal response) (E, A, E/A, EDT). In regard to those parameters, in which a maximal response was not achieved, thus allowing a "comparison" of C21 and candesartan actions (LVIDs, LVIDd, EDP, contractility, dP/dtmin), the improvement by C21 was more effective than with candesartan. This outcome applied to systolic and diastolic function and may point to a superiority of C21.
Outcome of animals treated with the combination of C21 plus candesartan was not superior to that of animals treated with C21 alone, as may have been expected. A likely reason for this finding could be that C21 monotherapy already elicited a maximal functional improvement, which did not admit any further improvement.
To examine the molecular mechanisms underlying the beneficial effect of C21 on post-MI cardiac function, we looked for a putative effect of this drug on the apoptotic and inflammatory response in the peri-infarct zone.
MI led to a significant increase in Fas-ligand and caspase-3 expression in the infarct border zone. C21 significantly reduced the elevated, MI-related Fas-ligand and caspase-3 expression, and this effect could be blocked by the specific AT2R antagonist PD123319. Because apoptosis is a major contributor in postinfarct cardiac remodeling, including the development of infarct expansion,21 prevention of cardiomyocyte apoptosis by C21 may be a major molecular mechanism by which AT2R stimulation preserves cardiac function after MI.
The MAPKs are involved in numerous cellular processes such as apoptosis, inflammation, proliferation, or hypertrophy.
In our study, we found that the activity of p38 MAPK and p44/42 MAPK was reduced to undetectable levels 7 days after MI in vehicle-treated animals and that treatment with C21 was able to completely rescue p38 MAPK and p44/42 MAPK expression in infarcted hearts. This effect was blocked by PD123319, indicating again AT2R specificity of the C21 effect. The effect of candesartan was by far weaker than the effect of C21.
Although MAPKs have traditionally been regarded as mediators of such processes as proliferation, apoptosis, and inflammation, recent data provided evidence that MAPK activation can also result in beneficial effects directly opposed to the aforementioned effects, namely, antiproliferation, cell survival, and anti-inflammation.22 In our experimental setting, the rescue of p38 MAPK and p44/42 MAPK expression by C21 7 days after MI coincided with an improvement of cardiac function and a smaller scar size. Although this is an observation of coincidence, a causal relationship between MAPK activation and improved cardiac function is supported by a recent study by Tenhunen et al,23 in which the authors were able to improve postinfarct cardiac function by rescuing p38 MAPK expression through local adenovirus-mediated p38 MAPK gene transfer. Functionally, MAPK rescue reduced fibrosis and apoptosis (the latter shown by terminal deoxynucleotidyl transferase dUTP nick end labeling staining and, as in our study, by a reduction of caspase-3). Thus, the increase in p38 and p44/42 MAPK expression by C21 may be part of its antiapoptotic effect after MI, which is also reflected by a decrease in caspase-3 and Fas ligand.
In the early phase after acute MI, necrosis of cardiac cells sets into motion an inflammatory response that ultimately promotes fibrosis, scar formation, and thereby heart failure. C21 suppressed this postinfarct inflammatory response, as shown by a significant decrease in cytokine (MCP-1, IL-1β, IL-2, IL-6) expression in plasma and the peri-infarct zone. Again, the effect of C21 could be blocked by the AT2R antagonist, indicating the AT2R specificity of this effect. Because it has been shown for other drugs that inhibition of the inflammatory response contributes to the preservation of cardiac function, it can be assumed that anti-inflammation may be another mechanism of action of AT2R stimulation by C21.
It must be pointed out that this study was performed in normotensive rats and that no changes in blood pressure or heart rate occurred over the time of the experiment either by C21 at all doses used or by candesartan, which was given in a non–blood pressure–lowering dose. Therefore, the effects on cardiac function and cytokine and apoptosis markers as well as MAPK expression are unlikely to be caused by treatment-related hemodynamic changes. This notion is further supported by the inhibitory effect of C21 on IL-6 expression in rat fetal cardiomyocytes in vitro (ie, in an experimental setting independent of any hemodynamic influences).
In sum, this is the first study to use a nonpeptide AT2R agonist for direct stimulation of the AT2R after MI in rats. Treatment with C21 over a period of 7 days led to a pronounced improvement of systolic and diastolic cardiac function coinciding with a smaller scar volume. Furthermore, C21 reduced apoptosis and acted in an anti-inflammatory manner, suggesting that these mechanisms are contributing to the beneficial effect of AT2R stimulation. AT2R stimulation may thus represent a potential future therapeutic option for the improvement of postinfarct cardiac function but also for the prevention of end-organ damage in further organ systems.
Limitations
Because in our study infarct size has only been measured at the end but not before treatment, we cannot exclude an unmeasured difference in infarct size between groups at baseline, which may have influenced the outcome of the study. However, this possibility has been minimized by grouping of animals according to EF before the start of treatment to keep differences in mean EF between groups minimal. This procedure is based on the observation by us and others that EF correlated well with infarct size (r2=0.671; P<0.0001).24 Moreover, assignment of animals to the respective treatment groups was performed by an investigator who was blinded to the outcome of EF measurement in individual animals (only knowing whether EF was >35% or 35%).
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Acknowledgments |
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Sources of Funding
Dr Grzesiak and C. Curato received a grant from the EU Marie Curie graduate program CARDIOVASC (MEST-CT-2005-020268).
Disclosures
Drs Dahlöf and Unger have modest ownership interest in Vicore Pharma and received speaker fees. Dr Steckelings received speaker fees from Vicore Pharma. The other authors report no conflicts.
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References |
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30%), it may be a suitable lead compound for the development of a drug for the blood pressure–independent treatment or prevention of end-organ damage in humans. |
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*Drs Kaschina, Grzesiak, Unger, and Steckelings contributed equally to this work.
The online-only Data Supplement is available with this article at http://circ.ahajournals.org/cgi/content/full/CIRCULATIONAHA.108.784868/DC1.
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