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(Circulation. 1996;94:2131-2137.)
© 1996 American Heart Association, Inc.

Articles

Vasodilator Effects of Endothelin-Converting Enzyme Inhibition and Endothelin ET_A Receptor Blockade in Chronic Heart Failure Patients Treated With ACE Inhibitors

Michael P. Love, MB, ChB, MRCP; William G. Haynes, MB, ChB, MD, MRCP; Gillian A. Gray, BSc, PhD; David J. Webb, MB, BS, MD, FRCP; John J.V. McMurray, BSc, MB, ChB, MD, MRCP

the Medical Research Council Clinical Research Initiative in Heart Failure, University of Glasgow (M.P.L., J.J.V.M.), and the University of Edinburgh Department of Medicine, Western General Hospital (W.G.H., G.A.G., D.J.W.), Scotland, UK.

	Abstract

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Background The importance of endothelin-1 in chronic heart failure (CHF) is unclear. We therefore investigated the effects of endothelin-converting enzyme (ECE) inhibition and endothelin ET_A receptor blockade in CHF patients treated with ACE inhibitors. We also compared the function of ET_A and ET_B receptors in healthy subjects and patients with CHF.

Methods and Results Locally active doses of study drugs were infused into the nondominant brachial artery while forearm blood flow (FBF) was measured by venous occlusion plethysmography. In CHF patients (n=10), phosphoramidon (a combined ECE and neutral endopeptidase inhibitor) and BQ-123 (an ET_A receptor antagonist) increased FBF by 52±10% (P=.0006) and 31±6% (P=.002), respectively, and thiorphan (a selective neutral endopeptidase inhibitor) reduced FBF by 15±5% (P=.0007). Forearm vasoconstriction to endothelin-1 (an ET_A and ET_B receptor agonist) was significantly blunted in CHF patients compared with control subjects (both n=10; CHF versus control subjects, P<.001), whereas vasoconstriction to sarafotoxin S6c (an ET_B receptor agonist) was significantly enhanced in CHF patients compared with control subjects (both n=10; CHF versus control subjects, P<.05).

Conclusions ECE inhibitors and ET_A receptor antagonists may be useful as vasodilator agents in CHF patients already receiving treatment with an ACE inhibitor. Both ET_A and ET_B receptors can mediate agonist-induced vasoconstriction in healthy subjects and patients with CHF, but further studies are required to clarify the contribution of each receptor subtype in mediating the effects of endogenous endothelin-1.

Key Words: endothelin • heart failure • vasodilatation • vasoconstriction • receptors

	Introduction

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Chronic heart failure is an important and increasingly common healthcare problem. Even with full conventional medical therapy, CHF impairs quality of life more than most other chronic medical illnesses and carries an extremely poor prognosis.¹ The development of novel and more effective therapeutic strategies for CHF is therefore an important priority in cardiovascular medicine.

In terms of symptoms and survival, the greatest impact on the medical management of CHF has been made by the ACE inhibitors² ; angiotensin II receptor antagonists have recently shown similar therapeutic promise.³ This has led to the view that vasodilatation through neuroendocrine modulation may be the most appropriate and effective therapeutic strategy in CHF. Novel therapies capable of achieving a greater reduction in vascular resistance than is possible with current treatment modalities are urgently required.

The endothelins are a family of endogenous peptides with potent vasoconstrictor, antinatriuretic, and mitogenic properties that may be important in the pathophysiology of CHF.⁴ Endothelin-1, the principal endothelin isoform generated in the human vasculature,⁵ has uniquely sustained vasoconstrictor actions and has been shown to contribute to basal vascular tone in humans.⁶ Plasma levels of the peptide are increased twofold to threefold in CHF of all causes in proportion to the symptomatic and hemodynamic severity of the syndrome.^{7 8 9 10 11 12 13 14 15} Big endothelin-1, the biologically inactive precursor of mature endothelin-1, may be preferentially increased in CHF,^{13 15} suggesting enhanced endothelial synthesis or secretion of the peptide. Plasma big endothelin-1 also predicts prognosis, with a higher concentration predicting a greater likelihood of death or need for cardiac transplantation.¹³

We have addressed the contribution of endothelin-1 to peripheral vasoconstriction in CHF by pursuing two possible therapeutic interventions suggested by the cellular processing and actions of endothelin-1.⁴ In study A, we investigated the effects of blocking the generation (via ECE inhibition) and action (via selective endothelin ET_A receptor blockade) of endothelin-1 in a heterogeneous group of patients with conventionally treated CHF. We have also tried to clarify the role of the two principal endothelin receptor subtypes, ET_A and ET_B, in mediating vasoconstriction in vivo.^{16 17} Vascular smooth muscle ET_A receptors were previously believed to be solely responsible for mediating endothelin-1–induced vasoconstriction, whereas endothelial ET_B receptors were believed to mediate vasodilatation through release of prostacyclin and/or nitric oxide. More recently, it has been suggested that vascular smooth muscle ET_B receptors might also mediate vasoconstriction.^{18 19 20 21} Knowledge of the contribution of each receptor subtype to vasoconstriction has obvious implications for the potential therapeutic use of selective or nonselective endothelin receptor antagonists in CHF. In study B, we attempted to clarify the role of constrictor ET_A receptors by comparing the vascular effects of the selective ET_A receptor antagonist BQ-123 in healthy subjects and patients with CHF. In study C, we tried to assess the relative importance of ET_A and ET_B receptors by comparing the vascular effects of endothelin-1 (a nonselective [ET_A and ET_B] receptor agonist) and sarafotoxin S6c (a selective ET_B receptor agonist) in healthy subjects and patients with CHF.

	Methods

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Subjects
Studies were conducted with the approval of the local ethics committee and the written, informed consent of each CHF patient and healthy control subject.

Fourteen patients with chronic (>3 months) stable heart failure participated in study A. No specific cause of CHF was excluded, but no patients had concomitant hypertension, diabetes mellitus, or chronic renal failure. Each was receiving maintenance treatment with a diuretic and a maximal dose of an ACE inhibitor (either captopril 50 mg TID, enalapril 10 mg BID, or lisinopril 10 mg UID). Medication was continued as usual before and during the day of studies because we wished to evaluate the effects of ECE inhibition and ET_A receptor blockade over and above the effects of conventional medical therapy for CHF. Patient characteristics are summarized in Table 1.

View this table: [in this window] [in a new window]   Table 1. Patient Characteristics, Study A (Phosphoramidon, Thiorphan, and BQ-123 Protocols)

Six patients with chronic stable heart failure and 10 healthy control subjects participated in study B. Ischemic heart disease was the underlying cause of heart failure in each case, and no patients had concomitant hypertension, diabetes mellitus, or chronic renal failure. All CHF patients were receiving maintenance treatment with a diuretic, digoxin, and ACE inhibitor, and no control subject was taking any regular medication. The medication of CHF patients was withheld for 24 hours before studies. Patient characteristics are summarized in Table 2.

View this table: [in this window] [in a new window]   Table 2. Patient and Control Subject Characteristics, Study B (BQ-123 Protocol)

Ten patients with chronic stable heart failure and 10 healthy control subjects participated in study C. The underlying cause of heart failure was ischemic heart disease in 9 patients and alcoholic cardiomyopathy in 1; none had concomitant hypertension, diabetes mellitus, or chronic renal failure. All CHF patients were receiving maintenance treatment with a diuretic and ACE inhibitor, and no control subject was taking any regular medication. The medication of CHF patients was withheld for 24 hours before studies. Patient characteristics are summarized in Table 3.

View this table: [in this window] [in a new window]   Table 3. Patient and Control Subject Characteristics, Study C (Endothelin-1 and Sarafotoxin S6c Protocols)

Drug Infusion and FBF Measurement
Studies were performed with subjects resting supine in a quiet clinical laboratory maintained at a constant temperature between 23°C and 25°C. Subjects were asked to abstain from caffeine-containing drinks, alcohol, and cigarette smoking on study days and to fast for at least 2 hours before studies commenced.

With 1% lidocaine used for local anesthesia, an unmounted 27–standard wire gauge steel cannula (Coopers Needle Works) was inserted into the brachial artery of the nondominant arm and connected to a constant-rate infusion pump (Wellmed P1000) via a 16-gauge epidural catheter (Portex Ltd). Physiological saline (0.9%) was infused at 1 mL/min for an equilibration period of 30 minutes before infusion of study drugs. Locally active doses of drugs were dissolved in physiological saline and administered at 1 mL/min.

FBF was measured by venous occlusion plethysmography with indium (gallium)-in-Silastic strain gauges applied to the widest aspect of each forearm.²² The hand circulation was excluded during periods of blood flow measurement by inflation of wrist cuffs to 220 mm Hg. Upper arm cuffs were inflated to 40 mm Hg for 10 in every 15 seconds to prevent venous outflow and obtain plethysmographic recordings. Voltage output from a Vasculab SPG 16 strain-gauge plethysmograph (Medasonics Inc) was transferred to an Apple Macintosh computer (LC III, Apple Computer Inc) for analysis with a MacLab analog-to-digital converter and Chart software (version 3.2.8; both from AD Instruments).

In all studies, FBF was measured in both arms at 10-minute intervals during saline equilibration and at 5-minute intervals during study drug infusion. In study A only, saline was infused for a further 30 minutes after discontinuation of drug infusion with blood flow measured every 10 minutes. The last five measurements of FBF during each 3-minute recording period were averaged, and the mean percentage change from baseline in the ratio of flow between the infused and noninfused arms was calculated. This method of analysis uses the noninfused arm as a contemporaneous control and ensures that the effects of study drugs are distinguished from any other external or environmental factors.²²

Blood pressure and heart rate were recorded in the noninfused arm at 10-minute intervals throughout all studies with a well-validated semiautomated sphygmomanometer (Takeda UA-751, Takeda Medical Inc).

Data Analysis
All results are expressed as mean values with 95% CIs in the text and SEM in figures. The significance of the effect of each study agent on FBF was examined with Statview 4.02 software (Abacus Concepts Inc) for the Apple Macintosh computer to perform repeated-measures ANOVA. An unpaired t test was performed to test the significance of the difference in peak antagonist and agonist effects between healthy subjects and patients with CHF in studies B and C. Differences were considered statistically significant at a value of P<.05.

Detailed Protocols
Study A: ECE Inhibition, NEP Inhibition, and Selective ET_A Receptor Blockade
Three single-blind protocols were performed, each in 10 patients with CHF. Phosphoramidon (Clinalfa AG) was infused at 30 nmol/min for 60 minutes, a dose sufficient to achieve local forearm concentrations equivalent to the IC₅₀ of phosphoramidon for ECE assuming resting FBF of 50 mL/min.²³ Phosphoramidon, however, also inhibits NEP, the enzyme responsible for the degradation of various biologically active peptides, including bradykinin, substance P, natriuretic peptides, angiotensin II, and endothelin-1 itself.^{24 25} We therefore compared its effect with that of thiorphan (Sigma Chemical Co Ltd), a selective inhibitor of NEP.²⁶ Thiorphan was also infused at 30 nmol/min for 60 minutes because it is effectively equipotent with phosphoramidon as an inhibitor of NEP.²⁴ Concentrations of thiorphan >100-fold greater than achieved at this rate of infusion have no inhibitory effect on ECE.²³ The selective ET_A receptor antagonist BQ-123 (American Peptide Co) was infused at 100 nmol/min for 60 minutes, a dose sufficient to achieve local forearm concentrations 10-fold higher than the pA₂ for BQ-123 at the ET_A receptor.²⁷

Study B: Selective ET_A Receptor Blockade
Ten healthy control subjects received brachial artery infusion of the selective ET_A receptor antagonist BQ-123 at 100 nmol/min for 90 minutes, again a dose sufficient to achieve local forearm concentrations 10-fold greater than the pA₂ for BQ-123 at the ET_A receptor.²⁷ We also infused BQ-123 for 90 minutes in 6 more patients with CHF because it was not clear whether a maximal effect had been obtained after 60 minutes of infusion in study A.

Study C: Nonselective (ET_A and ET_B) Receptor Agonism and Selective ET_B Receptor Agonism
On separate days at least 1 week apart and in single-blind fashion, 10 healthy control subjects and 10 patients with CHF received brachial artery infusion in random, balanced order of pharmaceutical grade endothelin-1 (Clinalfa AG) and sarafotoxin S6c (Sigma Chemical Co Ltd) at 5 pmol/min for 60 minutes. This dose of both agonists has previously been shown to cause slow-onset vasoconstriction in human forearm resistance vessels.²¹ Assuming resting FBF of 50 mL/min, infusion of 5 pmol/min of endothelin-1 and sarafotoxin S6c would achieve a local concentration of each peptide in the infused forearm of about 0.1 nmol/L. Endothelin-1 would be expected to act equally on both ET_A and ET_B receptors at this concentration, given that the K_i values for endothelin-1 at ET_A and ET_B receptors are 0.6 and 0.12 nmol/L, respectively.²⁸ Sarafotoxin S6c would be expected to be highly selective for the ET_B receptor (K_i, 0.25 nmol/L) because its local forearm concentration would be at least 70 000-fold lower than its K_i at the ET_A receptor (>7300 nmol/L).²⁸

	Results

Top Abstract Introduction Methods Results Discussion References

 
Heart rate, blood pressure, and FBF in the noninfused arm did not change significantly in any study protocol, confirming that drugs had only local actions on the forearm vasculature of the infused arm and had no systemic hemodynamic effects.

Study A: ECE Inhibition, NEP Inhibition, and Selective ET_A Receptor Blockade
Phosphoramidon caused slow-onset vasodilatation, with a peak increase in FBF of 52±10% at 80 minutes (CI, 32% to 72%; ANOVA versus baseline, P=.0006; Fig 1), ie, 20 minutes after discontinuation of phosphoramidon infusion. Thiorphan caused slow-onset vasoconstriction, with a peak reduction in FBF of 15±5% at 90 minutes (CI, -6% to -24%; ANOVA versus baseline, P=.0007; Fig 1), ie, 30 minutes after discontinuation of thiorphan infusion. BQ-123 caused slow-onset vasodilatation, with a peak increase in FBF of 31±6% at 60 minutes (CI, 20% to 42%; ANOVA versus baseline, P=.002; Fig 1), ie, at the end of BQ-123 infusion.

View larger version (16K): [in this window] [in a new window]   Figure 1. Effect of brachial artery infusion of phosphoramidon 30 nmol/min (), thiorphan 30 nmol/min (), and BQ-123 100 nmol/min (), each for 60 minutes in 10 patients with CHF. Hatched bar represents period of study drug infusion. Phosphoramidon (P=.0006) and BQ-123 (P=.002) caused progressive forearm vasodilatation, whereas thiorphan caused progressive forearm vasoconstriction (P=.0007).

Study B: Selective ET_A Receptor Blockade
BQ-123 caused slow-onset vasodilatation in control subjects, with a peak increase in FBF of 54±10% at 90 minutes (CI, 34% to 74%; ANOVA versus baseline, P<.0001; Fig 2). As in study A, BQ-123 caused slow-onset vasodilatation in CHF patients, with a peak increase in FBF of 33±4% at 90 minutes (CI, 26% to 40%; ANOVA versus baseline, P<.0001; Fig 2). Vasodilatation to BQ-123 tended to be blunted in CHF patients compared with control subjects (CHF versus control subjects, P=.14); the maximal increase in FBF at 90 minutes was not statistically significantly different between CHF patients and control subjects unless the solitary control subject whose FBF was unaffected by BQ-123 infusion was excluded from the analysis (CHF versus control subjects, P=.04).

View larger version (15K): [in this window] [in a new window]   Figure 2. Effect of brachial artery infusion of BQ-123 100 nmol/min for 90 minutes in 6 patients with CHF () and 10 age-matched healthy control subjects (). Hatched bar represents period of BQ-123 infusion. Vasodilatation to BQ-123 tended to be blunted in CHF patients compared with control subjects (P=.14).

Study C: Nonselective (ET_A and ET_B) Receptor Agonism and Selective ET_B Receptor Agonism
Endothelin-1 caused slow-onset vasoconstriction in control subjects and patients with CHF (Fig 3). Endothelin-1 reduced FBF by 41±4% in control subjects (CI, -34% to -49%; ANOVA versus baseline, P<.0001) and by 25±3% in patients with CHF (CI, -19% to -31%; ANOVA versus baseline, P<.0001) after 60 minutes of infusion. Vasoconstriction to endothelin-1 was significantly blunted in CHF patients compared with control subjects (CHF versus control subjects, P<.001). Sarafotoxin S6c also caused slow-onset vasoconstriction in control subjects and patients with CHF (Fig 4). Sarafotoxin S6c reduced FBF by 29±3% in control subjects (CI, -24% to -34%; ANOVA versus baseline, P<.0001) and by 39±4% in patients with CHF (CI, -31% to -47%; ANOVA versus baseline, P<.0001) after 60 minutes of infusion. Vasoconstriction to sarafotoxin S6c was significantly greater in CHF patients than control subjects (CHF versus control subjects, P<.05).

View larger version (15K): [in this window] [in a new window]   Figure 3. Effect of brachial artery infusion of endothelin-1 5 pmol/min for 60 minutes in 10 patients with CHF () and 10 age-matched healthy control subjects (). Hatched bar represents period of peptide infusion. Vasoconstriction to endothelin-1 was significantly blunted in CHF patients compared with control subjects (P<.001).

View larger version (23K): [in this window] [in a new window]   Figure 4. Effect of brachial artery infusion of sarafotoxin S6c 5 pmol/min for 60 minutes in 10 patients with CHF () and 10 age-matched healthy control subjects (). Hatched bar represents period of peptide infusion. Vasoconstriction to sarafotoxin S6c was significantly enhanced in CHF patients compared with control subjects (P<.05).

	Discussion

Top Abstract Introduction Methods Results Discussion References

 
These studies demonstrate that endothelin-1 contributes importantly to peripheral vascular resistance in patients with CHF and suggest that ECE inhibitors and endothelin ET_A receptor antagonists may be useful as vasodilator agents, even in patients who are already receiving treatment with an ACE inhibitor. We have also shown that both ET_A and ET_B receptors can mediate vasoconstriction in the peripheral vasculature of healthy subjects and patients with CHF, a finding that may have important implications for the potential therapeutic use of endothelin receptor antagonists.

Studying the effects of agents that block either the generation or action of endothelin-1 in vivo is the only way to clarify the putative pathophysiological role of the peptide in CHF. A major advance in the field of endothelin research has been the cloning and characterization of ECE,²³ but at present no specific and selective inhibitors of the enzyme exist. Several selective and nonselective endothelin receptor antagonists potentially suitable for human therapeutic use are at various stages of clinical development.²⁹

The best available pharmacological tool at present for studying the effects of blocking the generation of endothelin-1 is the combined ECE and NEP inhibitor phosphoramidon.²³ Phosphoramidon caused a substantial (50%) increase in FBF when a locally active dose was infused into the brachial artery of patients with CHF (Fig 1). Given the broader substrate specificity of NEP than of ECE,^{24 25} this vasodilatation could theoretically have been due to NEP rather than ECE inhibition, causing local accumulation of one of the many dilator substances metabolized by NEP. However, brachial artery infusion of the selective NEP inhibitor thiorphan caused forearm vasoconstriction in patients with CHF (Fig 1), indicating that the vasodilatation observed with phosphoramidon was most likely due to inhibition of ECE, with simultaneous NEP inhibition probably offsetting the magnitude of its overall vasodilator effect. The specific mechanism underlying the effect of thiorphan is likely to be complex, given the many vasoactive factors known to be metabolized by NEP, but the most probable explanation is that of local accumulation of constrictor substances, such as angiotensin II or endothelin-1 itself. Further studies will be necessary to elucidate the mediators underlying the vasoconstrictor response to NEP inhibition.

Further evidence that endothelin-1 contributes to peripheral vascular resistance in CHF is provided by our studies with the selective ET_A receptor antagonist BQ-123. Brachial artery infusion of a locally active dose of BQ-123 for 60 minutes increased FBF by 30% in patients with CHF in study A. Whereas maximal vasodilatation to phosphoramidon was observed 20 minutes after its infusion was discontinued (suggesting persisting inhibition of ECE), the vasodilatory effect of BQ-123 declined immediately after discontinuation of its infusion, in keeping with its action as a competitive antagonist at the ET_A receptor. The maximum response to BQ-123 was probably achieved after 60 minutes of infusion in study A (Fig 1) because the effect of BQ-123 appeared to reach plateau by 60 minutes in study B without any significant incremental vasodilatation after 60 minutes (Fig 2). ECE inhibition appeared to cause greater vasodilatation than selective ET_A blockade in study A, but further studies with varying doses and durations of infusion of both phosphoramidon and BQ-123 would be required to validate any direct comparison of their effects. Our principal aim was to determine whether antiendothelin drugs have therapeutic potential as vasodilator agents in patients with CHF already receiving conventional medical therapy rather than to compare the relative effects of ECE inhibition and ET_A receptor blockade.

Though we investigated only the short-term effects of ECE inhibition and ET_A receptor blockade in a single vascular bed, responses in human forearm resistance vessels are thought to be broadly representative of responses in other resistance beds. Indeed, our observations support and extend the recent findings of Kiowski et al,³⁰ who reported that acute bolus administration of a mixed ET_A and ET_B receptor antagonist, bosentan, significantly reduced systemic and pulmonary vascular resistance in patients with CHF. A crucial difference from our own studies was that these hemodynamic effects were observed in patients whose ACE inhibitors had been discontinued for >4 plasma half-lives. Importantly, our observations suggest that antiendothelin drugs have the potential to offer hemodynamic benefit in CHF patients over and above that already obtained with an ACE inhibitor.

Collectively, our agonist and antagonist studies suggest that both ET_A and ET_B receptors can mediate vasoconstriction in forearm resistance vessels of healthy subjects and patients with CHF. Our agonist studies in healthy control subjects are consistent with similar recently reported studies in healthy volunteers.²¹ The lesser mean constriction to sarafotoxin S6c than to endothelin-1 in control subjects (Figs 3 and 4) implies that both ET_A and ET_B receptor subtypes mediate vasoconstriction, but it is difficult to extrapolate our results to quantify the relative contribution of each receptor subtype in mediating the effects of endogenous endothelin-1; further comparative studies with selective ET_A and ET_B receptor antagonists, when available for use in humans, will clarify this issue.

We found that the forearm vasodilator effect of the selective ET_A receptor antagonist BQ-123 tended to be blunted in CHF patients compared with healthy control subjects (Fig 2). Although this raises the possibility of ET_A receptor downregulation in CHF, any comparison of the effects of BQ-123 between CHF patients and control subjects must be made with extreme caution. The time course of the effect of BQ-123 and the need for brachial artery cannulation do not lend themselves to repeated dose-response studies; consequently, we cannot be certain that we have demonstrated the maximal effect of BQ-123 in the forearm vasculature. Additionally, because we were reluctant to withhold drug treatment from our CHF patients for any longer than 24 hours before studies, it is possible that persisting vascular effects of their medication (in particular of ACE inhibition) may have contributed to the difference observed between patients and control subjects.

Our finding that the vasoconstrictor effect of endothelin-1 was attenuated and that of sarafotoxin S6c was enhanced in the forearm vasculature of CHF patients compared with control subjects (Figs 3 and 4) suggests changes in ET_A and ET_B receptor sensitivity similar to those observed by Cannan et al³¹ in the coronary vasculature of dogs with experimental CHF. However, the functional significance of these apparent alterations in receptor sensitivity is unclear; further studies comparing the hemodynamic effects of selective ET_A and ET_B receptor antagonists are necessary to delineate the relative importance of each receptor subtype in regulating vascular tone. The divergent nature of the agonist responses we observed in patients with CHF, ie, the blunted constrictor effect of endothelin-1 coupled with the enhanced constrictor effect of sarafotoxin S6c, makes it unlikely that the differences observed were simply due to any persisting vascular effect of the patients' medication. Similarly, although the small baseline differences in FBF, mean arterial pressure, and heart rate between patients and control subjects (Table 3) might have influenced vascular responsiveness to each agonist, these differences on their own would not explain the divergent variation in responses in patients with CHF. It is, however, important to acknowledge that our studies cannot definitively exclude the possible existence of a dilator subtype of ET_A receptor sensitive to endothelin-1 or another species of constrictor receptor sensitive to sarafotoxin S6c in patients with CHF.

Our studies with BQ-123 support an important role for ET_A receptors in mediating the constrictor effects of endogenous endothelin-1 in patients with CHF, but the role of ET_B receptors in this regard needs further clarification. Whether selective ET_B receptor blockade would result in a net vasoconstrictor or vasodilator effect in healthy subjects or patients with CHF is not currently known and cannot be determined from our present data. Our observation that forearm vasoconstriction to sarafotoxin S6c was enhanced in patients with CHF might be partly related to endothelial dysfunction,³² resulting in diminished endothelial ET_B receptor–mediated release of dilator substances, but whether this is indicative of a shift in the relative functional importance of endothelial and vascular smooth muscle ET_B receptors in CHF requires further investigation.

In summary, our findings have potentially important therapeutic implications. Our central and most important finding from a clinical perspective was that ECE inhibition and ET_A receptor blockade produced a significant additional reduction in forearm vascular resistance in patients with CHF already treated with conventional medical therapy, including an ACE inhibitor. For a new treatment modality to be of real value in CHF, it will have to offer hemodynamic benefit over and above that obtained with an ACE inhibitor; antiendothelin drugs appear to have this potential. If both ET_A and ET_B receptors do indeed mediate vasoconstriction in patients with CHF, then this suggests that use of a nonselective receptor antagonist or a selective ECE inhibitor would be necessary to achieve optimal inhibition of the constrictor effects of endogenous endothelin-1. The ideal receptor antagonist would probably be one that blocked constrictor ET_A and ET_B receptors but preserved endothelial ET_B receptor–mediated vasodilatation; such antagonists with differential selectivities for constrictor and dilator ET_B receptor subtypes have not yet been developed. Much is still to be learned about the role of the endothelins in human physiology and pathophysiology, but if orally active endothelin receptor antagonists and ECE inhibitors produce sustained systemic hemodynamic changes of the type we have observed in the forearm, then they may represent a further therapeutic advance in the treatment of CHF.

	Selected Abbreviations and Acronyms

 

CHF = chronic heart failure ECE = endothelin-converting enzyme FBF = forearm blood flow NEP = neutral endopeptidase

	Acknowledgments

 
Dr Love was supported by a Bristol-Myers Squibb Cardiovascular Fellowship and Dr Haynes by a Wellcome Trust Advanced Training Fellowship (042145/114). We wish to thank E. Stanley and Dr N. Lannigan of the Pharmacy Department at the Western General Hospital for their assistance with drug preparation.

	Footnotes

 
Reprint requests to Dr MP Love, Bristol-Myers Squibb Cardiovascular Fellow, Clinical Research Initiative in Heart Failure, Wolfson Bldg, University of Glasgow, Glasgow G12 8QQ, Scotland, UK. E-mail m.love@biomed.gla.ac.uk.

Presented in part at the 67th and 68th Scientific Sessions of the American Heart Association, Dallas, Tex, November 14-17, 1994, and Anaheim, Calif, November 13-16, 1995.

Received May 13, 1996; revision received June 5, 1996; accepted June 7, 1996.

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