(Hypertension. 2001;38:95.)
© 2001 American Heart Association, Inc.
Scientific Contributions |
From the Department of Pharmacology, Erasmus University Rotterdam, The Netherlands.
Correspondence to A.H.J. Danser, PhD, Department of Pharmacology, Room EE1418b, Erasmus University Rotterdam, Dr Molewaterplein 50, 3015 GE Rotterdam, The Netherlands. E-mail danser{at}farma.fgg.eur.nl
| Abstract |
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, and exposed to quinaprilat, captopril, Ang-(1-7), and/or bradykinin. Bradykinin induced complete relaxation (pEC50=8.11±0.07, mean±SEM), whereas quinaprilat, captopril, and Ang-(1-7) alone were without effect. Quinaprilat shifted the bradykinin curve to the left in a biphasic manner: a 5-fold shift at concentrations that specifically block the C-domain (0.1 to 1 nmol/L) and a 10-fold shift at concentrations that block both domains. Captopril and Ang-(1-7) monophasically shifted the bradykinin curve to the left, by a factor of 10 and 5, respectively. A 5-fold shift was also observed when Ang-(1-7) was combined with 0.1 nmol/L quinaprilat. Repeated exposure of porcine coronary arteries to 0.1 µmol/L bradykinin induced B2 receptor desensitization. The addition of 10 µmol/L quinaprilat or Ang-(1-7) to the bath, at a time when bradykinin alone was no longer able to induce relaxation, fully restored the relaxant effects of bradykinin. Angiotensin II type 1 or 2 receptor blockade did not affect any of the observed effects of Ang-(1-7). In conclusion, Ang-(1-7), like quinaprilat and captopril, potentiates bradykinin by acting as an ACE inhibitor. Bradykinin potentiation is maximal when both the ACE C- and N-terminal domains are inhibited. The inhibitory effects of Ang-(1-7) are limited to the ACE C-domain, raising the possibility that Ang-(1-7) synergistically increases the blood pressure-lowering effects of N-domain-specific ACE inhibitors.
Key Words: angiotensin bradykinin angiotensin-converting enzyme inhibitors receptors, bradykinin coronary artery
| Introduction |
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In the present study, we investigated the bradykinin-potentiating effects of Ang-(1-7) in porcine coronary arteries (PCAs), its dependency on ACE, and the possible involvement of AT1 and/or AT2 receptors. The effects of Ang-(1-7) were compared with those of quinaprilat and captopril, 2 ACE inhibitors with preference for the ACE C-and N-domains, respectively.13,14 We also verified the effect of Ang-(1-7) in human coronary arteries (HCAs).
| Methods |
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4 mm in length, suspended on stainless steel hooks in 15-mL organ baths containing Krebs bicarbonate solution, aerated with 95% O2/5% CO2, and maintained at 37°C. Segments containing macroscopically visible atherosclerotic lesions were not used.
PCAs were obtained from 32 pigs age 2 to 3 months (YorkshirexLandrace; weight, 10 to 15 kg). The pigs had been used in in vivo experiments studying the effects of
-adrenoceptor and serotonin receptor agonists and antagonists under pentobarbital (600 mg IV) anesthesia.15 The ethics committee of Erasmus University Rotterdam dealing with the use of animals for scientific experiments approved the protocol for this investigation. Hearts were explanted at the end of the experiment, and the coronary arteries were removed immediately and handled in the same way as HCAs.
Organ Bath Studies
All vessel segments were allowed to equilibrate for at least 30 minutes, and the organ bath fluid was refreshed every 15 minutes during this period. Changes in tissue contractile force were recorded with a Harvard isometric transducer. The vessel segments, stretched to a stable force of
15 mN, were exposed to 30 mmol/L K+ twice. The functional integrity of the endothelium was verified by observing relaxation to 1 nmol/L substance P after preconstriction with 1 µmol/L prostaglandin F2
(PGF2
).16 Subsequently, the tissue was exposed to 100 mmol/L K+ to determine the maximal contractile response to K+. The segments were then allowed to equilibrate in fresh organ bath fluid for 30 minutes. Thereafter, the following experiments were performed.
First, Ang-(1-7) concentration-response curves (CRCs) (0.1 nmol/L to 10 µmol/L) were constructed in HCAs and PCAs, both at baseline and after preconstriction with 10 µmol/L PGF2
, in the absence or presence of the AT1 receptor antagonist irbesartan (1 µmol/L) or the AT2 receptor antagonist PD 123,319 (1 µmol/L).
Second, the effects of Ang-(1-7), quinaprilat, and captopril on bradykinin-induced vasorelaxation were studied in PCAs. Vessels were preincubated for 30 minutes in the absence or presence of Ang-(1-7) (10 pmol/L to 10 µmol/L), quinaprilat (0.1 pmol/L to 0.1 mmol/L), captopril (0.1 pmol/L to 0.1 mmol/L), 10 µmol/L Ang-(1-7)+1 µmol/L irbesartan, 10 µmol/L Ang-(1-7)+1 µmol/L PD 123,319, 10 µmol/L Ang-(1-7)+0.1 nmol/L quinaprilat, or 10 µmol/L Ang-(1-7)+10 µmol/L quinaprilat. Vessels were then preconstricted with 10 µmol/L PGF2
or 1 µmol/L U46619, and CRCs to bradykinin (0.1 nmol/L to 1 µmol/L) were constructed.
Third, the effect of quinaprilat and Ang-(1-7) on desensitized B2 receptors was studied in PCAs. Vessels were preincubated for 30 minutes with or without 1 µmol/L irbesartan or 1 µmol/L PD 123,319. Vessels were then preconstricted with 10 µmol/L PGF2
and exposed 3 times to a concentration of bradykinin (0.1 µmol/L) that is capable of inducing maximal relaxation. Each next exposure was started as soon as the effect of the previous exposure had disappeared, ie, after
15 minutes. After the third exposure, when bradykinin no longer exerted a vasodilatory effect, quinaprilat (10 µmol/L) or Ang-(1-7) (10 µmol/L) was added to the organ bath. Thereafter, when the effects of quinaprilat and Ang-(1-7) had disappeared, a fourth bradykinin dose (0.1 µmol/L) was added to the organ bath.
Statistical Analysis
Data are given as mean±SEM and expressed as a percentage of the contraction in response to PGF2
or U46619. CRCs were analyzed by the logistic function described by de Lean et al17 to obtain pEC50 (-10log EC50) values, EC50 representing the concentration at which 50% of the maximal relaxant effect has been reached. Statistical analysis was by ANOVA, followed by post hoc evaluation (according to Tukey or Dunnett where appropriate). Values of P<0.05 were considered significant.
| Results |
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or in the presence of irbesartan or PD 123,319.
Effects of Ang-(1-7), Quinaprilat, and Captopril on Bradykinin-Induced Relaxation in PCAs
Bradykinin caused complete relaxation of preconstricted PCAs in a concentration dependent-manner (pEC50=8.11± 0.07; n=12; Figure 1, top panel). Quinaprilat (10 µmol/L) and captopril (10 µmol/L), like Ang-(1-7) at this concentration, did not exert any effect on preconstricted PCAs. However, in the presence of 10 µmol/L Ang-(1-7), the bradykinin CRC was shifted
5-fold to the left (pEC50=8.72±0.09; n=12; P<0.05 versus control), whereas in the presence of 10 µmol/L quinaprilat or 10 µmol/L captopril the bradykinin CRC was shifted
10-fold to the left (pEC50=9.03±0.21 and 8.91±0.09, respectively; n=8; P<0.01 versus control) (Figure 1, top panel). The effect of Ang-(1-7) on the bradykinin CRC was not affected by irbesartan or PD 123,319 (Figure 1, bottom panel).
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To study the concentration dependency of the Ang-(1-7)- and ACE-inhibitor-induced leftward shifts, bradykinin CRCs were also constructed in the presence of a wide range of Ang-(1-7), quinaprilat, and captopril concentrations (Figure 2). The leftward shift caused by quinaprilat occurred in a biphasic manner, with a 5-fold shift at concentrations in the subnanomolar range and a 10-fold shift at concentrations >1 nmol/L. This biphasic shift is in agreement with earlier studies13 demonstrating different quinaprilat binding affinities of the ACE C- (Kd=7 pmol/L) and N-domains (Kd=1267 pmol/L) and suggests that the 5- and 10-fold shifts represent C-domain inhibition and complete (ie, C- plus N-domain) ACE inhibition, respectively. The 10-fold leftward shift caused by captopril occurred monophasically (Figure 2), suggesting that captopril is bound with similar affinity by the 2 ACE domains. The 5-fold leftward shift caused by Ang-(1-7) also occurred monophasically (Figure 2), suggesting either that Ang-(1-7) inhibits 1 ACE domain only or that Ang-(1-7) potentiates bradykinin independently of its effects on ACE. To sort out the latter, the effect of 10 µmol/L Ang-(1-7) on top of quinaprilat, either at a concentration that selectively inhibits the ACE C-domain (0.1 nmol/L) or at a concentration that inhibits both ACE domains (10 µmol/L), was studied. Ang-(1-7) affected neither the 5-fold leftward shift of the bradykinin CRC at 0.1 nmol/L quinaprilat (pEC50=8.59±0.39 and 8.68±0.29 with and without Ang-(1-7), respectively; n=13; P=NS) nor the 10-fold shift at 10 µmol/L quinaprilat (pEC50=9.06±0.20 and 9.03±0.21 with and without Ang-(1-7), respectively; n=11; P=NS) (Figure 3). Most likely, therefore, Ang-(1-7), at concentrations up to 10 µmol/L, acts as a selective inhibitor of the ACE C-domain.
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Effect of Ang-(1-7) and Quinaprilat on Desensitized B2 Receptors in PCAs
Repeated exposure of preconstricted vessel segments to 0.1 µmol/L bradykinin produced progressively smaller responses (Figure 4, top panel). The response to the third bradykinin dose was <50% of the response to the first bradykinin dose. Quinaprilat or Ang-(1-7), added to the organ bath after the effect of the third bradykinin dose had disappeared, both at a concentration of 10 µmol/L, completely restored the relaxant effect of bradykinin (90±10% and 83±8% relaxation, respectively; n=8; P=NS). A fourth bradykinin dose, added after the effect of quinaprilat or Ang-(1-7) had disappeared, induced no further effect. The effect of Ang-(1-7) was not different in the presence of irbesartan or PD 123,319 (Figure 4, bottom panel).
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| Discussion |
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The concentrations of Ang-(1-7) required to obtain ACE C-domain inhibition (
1 µmol/L) are in agreement with previously reported values.5,8 These concentrations exceed the in vivo tissue and plasma Ang-(1-7) concentrations in rats and humans by at least 4 orders of magnitude. Ang-(1-7) levels increase during ACE inhibition1821 because ACE inhibitors block the ACE N-domain-mediated degradation of Ang-(1-7).1,8 However, the fact that most ACE inhibitors also block the ACE C-domain will mask any potential C-domain-blocking effect of the high Ang-(1-7) levels during ACE inhibition. Indeed, Ang-(1-7) did not enhance bradykinin-induced vasodilation in the forearm in human subjects treated with an ACE inhibitor.7 Therefore, the effects of Ang-(1-7) described in the present study may only be of physiological importance if sufficiently high concentrations of Ang-(1-7) are reached in the immediate vicinity of ACE (the levels measured per gram tissue do not exclude this possibility) and/or in combination with ACE inhibitors that specifically block the ACE N-domain.
Studies with the N-domain-specific substrate N-acetyl-ser-asp-lys-pro (AcSDKP) have revealed that the ACE inhibitor captopril inhibits AcSDKP hydrolysis 16 times more potently than Ang I hydrolysis.14 Because the ACE N- and C-domains exhibit similar catalytic activities toward Ang I,22 these data suggest that captopril, at low concentrations, preferentially inhibits the ACE N-domain. Using captopril, however, we were unable to make a clear distinction between the effects of ACE N- and C-domain inhibition on the relaxant effects of bradykinin. In contrast, using the ACE inhibitor quinaprilat, which binds to the ACE C-domain with almost 200-fold greater affinity than to the ACE N-domain,13 we could make such a distinction: quinaprilat shifted the bradykinin CRC to the left in a biphasic manner, with an
5-fold shift occurring at concentrations that selectively block the C-domain and an
10-fold shift at concentrations that block both domains. Our inability to find a similar biphasic shift in the presence of increasing concentrations of captopril most likely relates to the limited selectivity of this ACE inhibitor for the ACE N-domain.
Ang-(1-7), like captopril, induced a monophasic leftward shift of the bradykinin CRC. At the highest Ang-(1-7) concentration (10 µmol/L) that was tested, however, the shift was only half of that caused by similar concentrations of quinaprilat or captopril. Adding quinaprilat, at a concentration that selectively blocks the ACE C-domain, to 10 µmol/L Ang-(1-7) did not cause a further leftward shift, thereby demonstrating that Ang-(1-7), at least at this concentration, blocks the ACE C-domain only. Because a concentration of 10 µmol/L is already many orders of magnitude above the measured concentrations of Ang-(1-7) in vivo,1821 we did not evaluate whether even higher concentrations of Ang-(1-7) resulted in ACE N-domain inhibition.
Does ACE inhibition result in bradykinin potentiation by blocking its hydrolysis? Because bradykinin, like Ang I, is cleaved equally well by the 2 ACE domains,22 our finding that blockade of both domains results in a twice as large leftward shift as blockade of 1 domain appears to support this possibility. However, the long half-life of bradykinin in this preparation (30 minutes),3 as well as our previous finding in PCAs that the quinaprilat-induced leftward shift of the ACE-resistant bradykinin analogue [Hyp3-Tyr(Me)8]-bradykinin was not different from that of bradykinin,11 argues against this concept. Moreover, under conditions of B2 receptor desensitization (induced by repeated exposure to a concentration of bradykinin that causes maximal relaxation), when bradykinin itself was no longer active, both quinaprilat and Ang-(1-7) immediately restored the effect of bradykinin. Taken together, therefore, a more likely explanation for the Ang-(1-7)-, quinaprilat-, and captopril-induced bradykinin potentiation is the upregulation (or resensitization) of B2 receptors that has recently been described in Chinese hamster ovary (CHO) cells transfected with the human B2 receptor and human ACE.8,9,23,24 This effect only occurs when ACE and B2 receptors are sterically closely associated, probably forming a heterodimer.24 ACE inhibitors are believed to alter the heterodimer interaction, thereby promoting a conformation in the B2 receptor that affects its sequestration and coupling to second messengers.24 Our findings in intact PCAs show that ACE-B2 receptor crosstalk is not limited to transfected CHO cells and, in addition, suggest that the conformational changes underlying bradykinin potentiation correlate directly with inhibition of the ACE C- and N-domains.
Received December 8, 2000; first decision January 8, 2001; accepted January 12, 2001.
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