Ann Thorac Surg 2002;73:1185-1188
© 2002 The Society of Thoracic Surgeons
Original article: cardiovascular
Endothelin blockade potentiates endothelial protective effects of ace inhibitors in saphenous veins
Lawrence Ko, BSa,
Andrew Maitland, MDa,
Paul W.M. Fedak, MDa,
Aaron S. Dumont, MDa,
Mitesh Badiwala, BSa,
Fina Lovren, PhDa,
Christopher R. Triggle, PhDa,
Todd J. Anderson, MDa,
Vivek Rao, MD, PhDa,
Subodh Verma, MD, PhD*a
a Division of Cardiac Surgery, The University of Toronto, Toronto General Hospital, Toronto, Ontario, and Division of Cardiac Surgery, The University of Calgary, Calgary, Alberta, Canada
* Address reprint requests to Dr Verma, Division of Cardiac Surgery, The University of Toronto, Toronto General Hospital, 200 Elizabeth St, 14th Floor, Eaton Wing EN 14-217, Toronto, ON M5G 2C4, Canada
e-mail: subodh.verma{at}sympatico.ca
Presented at the Poster Session of the Thirty-seventh Annual Meeting of The Society of Thoracic Surgeons, New Orleans, LA, Jan 29–31, 2001.
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Abstract
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Background. Angiotensin II and endothelin-1 are potent endothelium-derived contracting factors. The effects of acute endothelin antagonism on endothelial function in saphenous vein from patients treated with and without angiotensin-converting enzyme inhibitors were compared.
Methods. Vascular segments of saphenous vein were obtained perioperatively from 14 patients on angiotensin-converting enzyme inhibitors and 29 controls. In vitro endothelium-dependent and -independent responses to acetylcholine and sodium nitroprusside were assessed by constructing isometric dose-response curves in precontracted rings in the presence and absence of bosentan (endothelinA/B receptor antagonist) and BQ-123 (endothelinA antagonist) using isolated organ baths. Percent maximum relaxation and sensitivity were compared between interventions.
Results. Endothelium-dependent relaxation to acetylcholine was augmented in the angiotensin-converting enzyme inhibitor-treated group (p < 0.005). Both specific and mixed endothelin receptor blockade improved acetylcholine-mediated relaxation in the angiotensin-converting enzyme inhibitor-treated and untreated groups (p < 0.02). The effects of these antagonists were endothelium specific as endothelium-independent responses to sodium nitroprusside remain unaltered.
Conclusions. These data demonstrate that (1) chronic angiotensin-converting enzyme inhibition improves endothelial function in saphenous veins, and (2) this effect can be further augmented by acute endothelin blockade. These data suggest that antagonism of both angiotensin II and endothelin may be important in attenuating saphenous vein arteriosclerosis.
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Introduction
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The endothelium assumes a central role in the preservation of vascular homeostasis through the release of a number of endothelium-derived autocrine and paracrine substances [1–3]. Perturbations of endothelial function predispose the vasculature to increased tone, altered reactivity and vasomotion, structural remodeling, enhanced leukocyte adherence, and platelet aggregation [1, 3–5]. Endothelial dysfunction of bypass graft conduits, such as saphenous veins, is an important mediator of graft arteriosclerosis [4, 5].
A burgeoning body of investigation has attempted to elucidate and therapeutically target the putative mechanisms underlying endothelial dysfunction of bypass conduits [6]. Potent vasoconstrictors, such as angiotensin II and endothelin-1, have been implicated as mediators of this vascular dysfunction [7]. Individually, both angiotensin II [7] and endothelin-1 [8] are known to evoke potent contraction of vascular smooth muscle, potentiate the contractile responses of other substances, and mediate smooth muscle proliferation and migration [9]. Recent data have accrued to suggest that pharmacologic blockade of these substances enhances/restores function of the endothelium [7, 8, 10, 11]. The interrelationship of angiotensin II with endothelin-1 is intriguing, and has received considerably less attention. There appear to be commonalities in the pathways of angiotensin II and endothelin-1 receptor activation and interactions in their production/release [9, 12–15]. Limited examination of combined therapy directed toward both angiotensin II and endothelin-1 has been undertaken most recently in experimental models of heart failure and hypertension [9, 15]. Possible bypass graft endothelial protection afforded by combined angiotensin-converting enzyme (ACE) inhibitor (ACEI) and endothelin receptor blocker therapy has not been previously studied.
Toward this end, the present study was conducted to examine the effects of acute endothelin receptor blockade on endothelial function of saphenous vein bypass grafts from patients undergoing elective coronary artery bypass grafting with and without preoperative ACE inhibitor treatment. We herein demonstrate that acute endothelin receptor antagonism augments the endothelial protective actions of ACE inhibitor in human bypass conduits.
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Material and methods
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This study was approved by the Ethics Committee for Scientific Research at the University of Calgary. Forty-three patients undergoing elective coronary bypass grafting were studied. Fourteen patients who were treated with ACE inhibitor for 6 months or more before enrollment were assigned to one group (ACE inhibitor treated), and 29 patients who had never been on ACE inhibitor comprised the control group (untreated). All patients provided written informed consent before enrollment. Saphenous vein segments (2 to 3 cm) from each patient were harvested, placed in oxygenated physiologic salt solution, and transferred promptly to the pharmacology laboratory within 30 minutes. The vessels were cleaned of adherent tissue and sectioned into rings of approximately 5 mm. The rings were suspended in an isolated tissue bath comprised of oxygenated Krebs-Ringer bicarbonate buffer maintained at 37°C and bubbled with 95% O2 and 5% CO2. The buffer had the following composition (in millimoles per liter): NaCl (118), KCl (4.7), CaCl2 (2.5), KH2PO4 (1.2), MgSO4 (1.2), NaHCO3 (25), dextrose (11.1), and disodium calcium edetate (0.026). Isometric dose-response curves were subsequently constructed. A progressive resting tension of 4 g was applied, based on preliminary data demonstrating this tension creating the optimal tension-length relationship. All experiments were performed in the presence of indomethacin (10-5 mol/L) to prevent the synthesis of vascular prostaglandins. The investigators performing the tissue bath experiments were blinded to the presence or absence of preoperative ACE inhibitor therapy. After equilibration for 60 minutes, the dose-response curves were recorded using the following protocol in each segment obtained: (1) cumulative dose-response curves to phenylephrine (10-8 to 10-5 mol/L); (2) cumulative dose-response curves to acetylcholine (10-9 to 10-5 mol/L) in rings precontracted with phenylephrine at the concentration that evoked 75% of maximum contraction (ED75); (3) cumulative dose-response curves to acetylcholine in the presence of bosentan (endothelin receptor types A and B antagonist, 3 µmol/L for 20 minutes) and BQ-123 (endothelin receptor type A antagonist, 1 µmol/L for 20 minutes); and (4) cumulative dose-response curves to sodium nitroprusside (10-10 to 10-5 mol/L) in the presence and absence of bosentan (3 µmol/L for 20 minutes) and BQ-123 (1 µmol/L for 20 minutes). The tissues were allowed to equilibrate between steps for 60 to 90 minutes. Bosentan and BQ-123 at the concentrations implemented are known to be potent endothelin receptor antagonists [16, 17].
Bosentan was kindly provided from Actelion Ltd. (Basel, Switzerland). All other drugs and chemicals were obtained from Sigma (St. Louis, MO).
Results are expressed as mean ± standard error and n represents the number of patients studied in each group. Percent maximum relaxation and agonist sensitivity (the negative log of the concentration that causes 50% of maximal response evoked by the respective agonist) were compared between interventions. The agonist sensitivity provides an estimate of the sensitivity of the tissue to the agonist, (within concentrations of) acetylcholine and sodium nitroprusside. Percent maximum relaxation values were compared between treatments with a paired two-tailed t test. The dose-response curves were compared with repeated measures analysis of variance followed by a Newman Keul’s test for post-hoc comparisons. A two-sided p value less than 0.05 was deemed to be statistically significant.
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Results
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Patient characteristics
Demographic information of the two groups is depicted in Table 1. Both groups were similar with respect to age, systolic blood pressure, total serum cholesterol, and random serum glucose.
Vascular reactivity
The functional values of endothelium-dependent vasorelaxation are summarized in Table 2.
Acetylcholine evoked dose-dependent vasorelaxation in both groups (Fig 1).
This effect was eliminated in endothelium-denuded tissue (data not shown). Saphenous veins from the ACE inhibitor-treated group exhibited improved endothelial function; the percent acetylcholine-induced relaxation was greater in the ACE inhibitor-treated versus untreated groups (p < 0.005). Treatment with both bosentan and BQ-123 significantly improved endothelium-dependent vasorelaxation to acetylcholine in both groups (p < 0.02, Fig 1). Furthermore, enhancement of endothelium-dependent vasomotion was equally improved by both bosentan and BQ-123 (p < 0.05). The beneficial effects of both ACE inhibitor and endothelin antagonists were specific for the endothelium, as responses to the endothelium-independent vasodilator sodium nitroprusside were not altered by treatment with ACE inhibitor or endothelin antagonism treatment (p > 0.05, data not shown).
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Fig 1. Endothelium-dependent vascular relaxation to acetylcholine in saphenous veins from untreated and angiotensin converting enzyme inhibitor-treated groups. Percentage maximum relaxatory responses to acetylcholine (%Emax) in vessel rings precontracted with the concentration that evoked 75% of maximum contraction to phenylephrine are depicted and compared between groups. *p < 0.05, different from all other groups; **p < 0.05, different from all other groups. (ACh = acetylcholine.)
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Comment
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The present study examined the effects of combined ACE inhibitor and endothelin receptor antagonists on endothelial function in human bypass conduits. We demonstrate the ability of acute endothelin receptor antagonists to augment the chronic endothelial protective actions of ACE inhibitor in human saphenous veins.
The regulatory function of the endothelium is disturbed in response to cardiovascular risk factors [3]. Such dysfunction of the endothelium results from a relative imbalance in endothelium-derived constricting and relaxing factors and is thought to produce detrimental functional and ultimately structural changes including arteriosclerosis [3–5]. Angiotensin II and endothelin-1 are two candidate vasoconstrictor and pro-proliferative substances that have been characterized in the development and propagation of endothelial dysfunction [7, 10]. Emerging data have demonstrated that therapy targeted at inhibiting the production/action of angiotensin II and endothelin-1 individually improves endothelial function. However, the possible interactions/synergism of angiotensin II and endothelin-1 in vascular dysfunction are not well established. A preliminary line of investigation has examined the effects of combined therapy targeting both angiotensin II and endothelin-1 in experimental models of heart failure and hypertension [9, 15]. The role of concomitant therapy in improving human bypass graft conduit function has not been previously examined.
Traditional thinking postulated that the vascular effects of angiotensin II result from circulating angiotensin II, whereas the effects of endothelin-1 have been attributed to abluminally secreted (locally produced) endothelin-1 by endothelial and vascular smooth muscle cells. In addition, the notion that these peptides are independently synthesized and secreted has been traditionally held [13]. More recently, data have surfaced that question this assertion. Angiotensin II has proven to be a powerful stimulant of endothelin production and release in vascular endothelial and vascular smoother muscle cells [18, 19]. Angiotensin II also potentiates the ability of endothelin to induce vascular hypertrophy [9]. The ACE inhibitors have been shown to inhibit both basal and insulin-stimulated endothelin-1 secretion in vitro and in vivo [12, 20, 21]. Endothelin antagonism in certain experimental models of angiotensin II dependent hypertension abrogates elevated blood pressure, inflammation, and target end-organ damage [14, 22]. It should also be emphasized that angiotensin II and endothelin-1 receptors are coupled to similar G proteins [13] and the signaling pathways and subsequent intracellular events are quite similar. Experimental data reveal that interactions between endothelin and angiotensin II receptor type 1 (AT1) transduction pathways exist [15, 23, 24]. Recently, Kusaka and coworkers [13] demonstrated that coronary microvascular endothelial cells cosecrete angiotensin II and endothelin-1 by a regulated pathway. Therefore, it appears that the interactions between the renin-angiotensin and endothelin systems are complex and intradependent. It is presently unclear as to whether or not these pathways are best conceptualized as series or parallel systems.
A limited body of investigation has examined the clinically relevant effects of combined ACE inhibitor and endothelin antagonism. Combined AT1 and endothelin receptor blockade has recently been shown in a model of congestive heart failure to improve left ventricular pump function and myocardial contractility [15]. Combined AT1 and endothelin receptor blockade has also demonstrated beneficial synergistic effects in experimental hypertension and specifically reduced blood pressure and end-organ damage [9]. Clearly, further efforts targeted at delineating the beneficial effects of this combined therapy in cardiovascular disease are warranted. Long-term studies examining this therapy would be particularly important.
The study has an important limitation. The degree of ACE inhibition present in the tissues may be variable, depending on the duration and type of ACE inhibitor used. A prospective study evaluating a single ACE inhibitor (at a specified dose) would be required to answer this concern.
In conclusion, results from the present study document the ability of endothelin receptor antagonists to augment the endothelial protective actions of ACE inhibitor in human venous bypass conduits. Endothelial dysfunction plays a key role in the initiation and propagation of graft arteriosclerosis. Results from this study suggest that maximal endothelial protection can be conferred by pharmacologic antagonism of both angiotensin II and endothelin-1.
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Acknowledgments
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Aaron S. Dumont and Paul W. M. Fedak are Fellows of the Heart and Stroke Foundation of Canada. Subodh Verma is a Fellow of the Medical Research Council of Canada (currently named the Canadian Institutes of Health Research, CIHR). Todd J. Anderson is a Scholar of the Alberta Heritage Foundation of Canada. This work was supported by research grants from the Heart and Stroke Foundation of Alberta (Todd J. Anderson) and the Medical Research Council of Canada (Christopher R. Triggle). The ongoing support of the cardiovascular surgeons (Drs Maitland, Kieser, Kidd, and Bayes) is kindly acknowledged. Bosentan was a gift from Acetlion Ltd.
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Top
Abstract
Introduction
