Clearance Of ET-1 Research Articles

Abstract 2131: Bone Marrow-derived Macrophages May Contribute To Elevation Of Et-1 And Pulmonary Hypertension In Female Bmpr2 +/r899x Mutant Mice

Introduction: Mutation that causes loss of function of bone morphogenetic protein receptor 2 (BMPR2) has been implicated in the pathogenesis of Pulmonary Arterial Hypertension (PAH), a disease that currently has no cure and is characterized by elevated right ventricular systolic pressure and right ventricle hypertrophy. Females experience thrice more incidence of PAH. Elevation in the expression of ET-1, Endothelin-1, has been linked to the development of PAH. It is unclear what is contributing to the elevation of ET-1 observed in female mutants. Hypothesis: Macrophage activation in the microvasculature contributes to ET-1 elevation and PH pathogenesis in female BMPR2 +/R899X mice. Materials/Methods: Lung samples from 12-month-old male and female wild-type and BMPR2 +/R899X mutants were used for analysis of the ET-1 expression via Western Blot (WB). Accumulation of CD45+ET-1+ cells in the lungs was evaluated by immunohistochemistry (IHC). Finally, bone marrow-derived macrophages (BMDM) isolated from male and female mutant mice underwent a 24-hour treatment with 100 ng/mL Lipopolysaccharide (LPS) and vehicle (control) and ET-1 expression was measured in the cell lysate by WB. Results/Discussion: In exploring the source of ET-1, our initial findings indicated an accumulation in inflammatory perivascular cells. A double-blind analysis of the CD45 pan-leukocyte marker revealed a substantial increase in CD45+ cells in female mutants (4.09 +/- 0.68 and 10.46 +/- 1.15 CD45+ cells/microvessel, male and female, respectively; 8 micrographs/animal; 3 mice/group). Colocalization between CD45+ cells and ET-1 suggests these cells may be a source or contribute to ET-1 clearance in the lungs of female mutants. While the identity of CD45+ cells remain unclear, their morphology suggests they are likely macrophages. Indeed, preliminary results indicate a significant rise in ET-1 expression in BMDM from female mutants at baseline and upon LPS exposure. Conclusion: Accumulation of CD45+ cells in female mutant lungs, along with colocalization with ET-1, may contribute to PAH development. Additionally, increased ET-1 expression in macrophages from female mutant mice suggests macrophage activation as a potential factor in ET-1 elevation in female mutant lungs.

Impact of Superoxide Dismutase Mimetic AEOL 10150 on the Endothelin System of Fischer 344 Rats.

Endothelin-1 is a potent vasoconstrictor and mitogenic peptide involved in the regulation of vasomotor tone and maintenance of blood pressure. Oxidative stress activates the endothelin system, and is implicated in pulmonary and cardiovascular diseases including hypertension, congestive heart failure, and atherosclerosis. Superoxide dismutase mimetics designed with the aim of treating diseases that involve reactive oxygen species in their pathophysiology may exert a hypotensive effect, but effects on the endothelin system are unknown. Our objective was to determine the effect of the superoxide dismutase mimetic AEOL 10150 on the basal endothelin system in vivo. Male Fischer-344 rats were injected subcutaneously with 0, 2 or 5 mg/kg body weight of AEOL 10150 in saline. Plasma oxidative stress markers and endothelins (bigET-1, ET-1, ET-2, ET-3) as well as lung and heart endothelin/nitric oxide system gene expressions were measured using HPLC-Coularray, HPLC-Fluorescence and RT-PCR respectively. AEOL 10150 reduced (p<0.05) the circulating levels of isoprostane (-25%) and 3-nitrotyrosine (-50%) measured in plasma 2h and 24h after treatment, confirming delivery of a physiologically-relevant dose and the potent antioxidant activity of the drug. The reduction in markers of oxidative stress coincided with sustained 24h decrease (p<0.05) of plasma levels of ET-1 (-50%) and ET-3 (-10%). Expression of preproET-1 and endothelin converting enzyme-1 mRNA were not altered significantly in the lungs. However preproET-1 (not significant) and ECE-1 mRNA (p<0.05) were increased (10–25%) in the heart. Changes in the lungs included decrease (p<0.05) of mRNA for the ET-1 clearance receptor ETB and the vasoconstriction-signaling ETA receptor (-30%), and an early surge of inducible nitric oxide synthase expression followed by sustained decrease (-40% after 24 hours). The results indicate that interception of the endogenous physiological flux of reactive nitrogen species and reactive oxygen species in rats impacts the endothelin/nitric oxide system, supporting a homeostatic relationship between those systems.

Abstract 16429: Neuro-endocrino-immunological PBMC Signatures Early After Mechanical Support Device Implantation

Background: The neuro-endocrino-immunological (NEI) pathophysiology associated with advanced heart failure (AdHF) progression and arrythmogenesis is not well understood. While it has been shown that mechanical circulatory support device (MCSD) implantation rapidly restores normal hemodynamics, it is unknown as to whether NEI mechanisms also normalize rapidly. We hypothesized that transcriptional activity in PBMC reflecting NEI normalizes shortly after MCSD implantation. Methods: We collected PBMC samples from 25 MCSD patients and 4 healthy controls (CTRL). Samples were obtained at 1 day before, and 1, 3, 5 and 8 days after MCSD. Purified mRNA was subjected to whole-genome NGS analysis. Statistically significant genes, dysregulated between AdHF patients and CTRLs, were subjected to time dependent bioinformatics analysis. Results: We identified 1226 dysregulated gene-transcripts between AdHF patients and CRTLs at baseline. Time dependent analysis provided 344 dysregulated transcripts in AdHF patients across all time-points. An analysis of enriched molecular pathways by dysregulated genes showed 52 pathways with NEI annotations at baseline. The Endothelin pathway, one of the most significant of the 52 pathways, had 17/60 dysregulated transcripts in AdHF patients at baseline and through day 8 after implantation (Fig.1A). Within the Endothelin pathway we found that expression of EDNRB, a gene central to ET1 clearance and ECE-inhibition, is persistently elevated through day 8 after MCSD (Fig.1B). Conclusion: Transcriptional NEI dysregulation is persistent in PBMC 8 days after MCSD implantation. Neuromodulatory strategies targeted at NEI dysregulation and implemented after MCSD implantation may be a mechanism to improve perioperative outcomes. Further studies including validation methods to objectively assess NEI activity are warranted.

No redistribution of lung blood flow by inhaled nitric oxide in endotoxemic piglets pretreated with an endothelin receptor antagonist

Inhaled nitric oxide (INO) improves ventilation-perfusion matching and alleviates pulmonary hypertension in patients with acute respiratory distress syndrome. However, outcome has not yet been shown to improve, and nonresponse is common. A better understanding of the mechanisms by which INO acts may guide in improving treatment with INO in patients with severe respiratory failure. We hypothesized that INO may act not only by vasodilation in ventilated lung regions, but also by causing vasoconstriction via endothelin (ET-1) in atelectatic, nonventilated lung regions. This was studied in 30 anesthetized, mechanically ventilated piglets. The fall in oxygenation and rise in pulmonary artery pressure during a sepsislike condition (infusion of endotoxin) were blunted by INO 40 ppm. Endotoxin infusion increased serum ET-1, and INO almost doubled the ratio between mRNA expression of endothelin receptor A (mediating vasoconstriction) and B (mediating vasodilation and clearance of ET-1) (ET-A/ET-B) in atelectatic lung regions. INO caused a shift in blood flow away from atelectatic lung regions in the endotoxemic piglets, but not during ET receptor antagonism. We conclude that INO in short-term experiments, in addition to causing selective pulmonary vasodilation in ventilated lung regions, increases the ET-A/ET-B mRNA expression ratio in lung tissue. This might augment the vasoconstriction in atelectatic lung regions, enhancing the redistribution of pulmonary blood flow to ventilated lung regions which are reached by INO. Such vasoconstriction may be an important additional factor explaining the effect of INO.

Pathophysiological Roles of Endothelin Receptors in Cardiovascular Diseases

Endothelin (ET)-1 derived from endothelial cells has a much more important role in cardiovascular system regulation than the ET-2 and ET-3 isoforms. Numerous lines of evidence indicate that ET-1 possesses a number of biological activities leading to cardiovascular diseases (CVD) including hypertension and atherosclerosis. Physiological and pathophysiological responses to ET-1 in various tissues are mediated by interactions with ETA- and ETB-receptor subtypes. Both subtypes on vascular smooth muscle cells mediate vasoconstriction, whereas the ETB-receptor subtype on endothelial cells contributes to vasodilatation and ET-1 clearance. Although selective ETA- or nonselective ETA/ETB-receptor antagonisms have been assumed as potential strategies for the treatment of several CVD based on clinical and animal experiments, it remains unclear which antagonisms are suitable for individuals with CVD because upregulation of the nitric oxide system via the ETB receptor is responsible for vasoprotective effects such as vasodilatation and anti-cell proliferation. In this review, we have summarized the current understanding regarding the role of ET receptors, especially the ETB receptor, in CVD.

ETA and ETB receptors are expressed in vascular adventitial fibroblasts

The adventitia has been recognized to play important roles in vascular oxidative stress, remodeling, and contraction. We recently demonstrated that adventitial fibroblasts are able to express endothelin (ET)-1 in response to ANG II. However, it is unclear whether ET-1 receptors are expressed in the adventitia. We therefore investigated the expression and roles of both ET(A) and ET(B) receptors in collagen synthesis and ET-1 clearance in adventitial fibroblasts. Adventitial fibroblasts were isolated and cultured from the mouse thoracic aorta by the explant method. Cultured cells were treated with ANG II (100 nmol/l) or ET-1 (10 pM) in the presence or absence of the ANG II type 1 receptor antagonist losartan (100 μM), the ET-1 receptor antagonists BQ-123 (ET(A) receptor, 1 μM) and BQ-788 (ET(B) receptor, 1 μM), and the ET(B) receptor agonist sarafotoxin 6C (100 nM). ET-1 peptide levels were determined by ELISA, whereas ET(A), ET(B), and collagen levels were determined by Western blot analysis. ANG II increased ET-1 peptide levels in a time-dependent manner. ANG II increased ET(A) and ET(B) receptor protein levels as well as collagen in a similar fashion. ANG II-induced collagen was reduced while in the presence of BQ-123, suggesting a role for the ET(A) receptor in the regulation of the extracellular matrix. ANG II treatment in the presence of BQ-788 significantly increased ET-1 peptide levels. Conversely, the ET(B) receptor agonist sarafotoxin 6C significantly decreased ET-1 peptide levels. These data implicate a role for the ET(B) receptor in the clearance of the ET-1 peptide. In conclusion, both ET(A) and ET(B) receptors are expressed in adventitial fibroblasts, which paves the ground for the biological significance of adventitial ET-1. The ET(A) receptor subtype mediates collagen I expression, whereas the ET(B) receptor subtype may play a protective role through increasing the clearance of the ET-1 peptide.

Diltiazem facilitates endothelin clearance from the blood stream to reduce toxic elevation of plasma endothelin level in rodents.

Pretreatment with diltiazem at a dose of 2 mg kg-1 intravenously protected against sudden death induced by intravenous administration of endothelin-1 (ET-1, 5 nmol kg-1), with an apparent decrease in the plasma immunoreactive-ET-1 (IR-ET-1) in mice. These effects, which were also observed in anaesthetized rats, disappeared in rats with bilateral ligation of the renal arteries. In the latter, the exogenous ET-1-induced elevation of plasma IR-ET-1 tended to be higher than that in the sham-operated controls. Furthermore, in anaesthetized rats, diltiazem inhibited ET-1-induced decreases in renal blood flow and increased renal accumulation of IR-ET-1. These results indicate that part of the clearance of ET-1 from the bloodstream occurs in the kidney, and that diltiazem enhances the elimination of the peptide, presumably by improvement in the renal circulation, this action leading to alleviation of the toxic effects of ET-1.

Proliferative stimulation of the vascular endothelin-1 axis in vitro and ex vivo by infection with Chlamydia pneumoniae

Endothelin-1 (ET-1) is a vasoactive peptide that modifies vascular function via the G-protein coupled transmembrane receptors, Endothelin-A receptor (ETAR) and Endothelin-B receptor (ETBR). Dysregulation of the ET-1 axis plays a role in atherosclerotic development as it triggers cell proliferation, inflammation, and vasoconstriction. The respiratory pathogen Chlamydia pneumoniae (Cp) has been recovered from atherosclerotic lesions, and related to atherogenesis, via activation of vascular small GTPases and leukocyte recruitment. Cp effectively reprograms host cell signalling and is able to enter an intracellular persistent state in vascular cells that is refractory to antibiotics. Upon chlamydial infection, vascular smooth muscle cells, which do not produce significant ET-1 under physiological conditions were switched into a fundamental source of ET-1 mRNA and protein in a p38-MAP-kinase-dependent pathway. Endothelial cells did not overproduce ET-1 but showed upregulation of mitogenic ETAR mRNA and protein while the counterbalancing ETBR, which regulates ET-1 clearance, remained unaffected. This disruption of the ET-1 axis was confirmed in an ex vivo mouse aortic ring model, and resulted in endothelial cell proliferation that could be abrogated by ETAR-siRNA and the selective ETAR-antagonist BQ-123. Chronic chlamydial infection of the vascular wall might represent a permanent noxious stimulus linked to the endothelial cell proliferation characteristic of early atherosclerosis. Suppression of this deleterious paracrine loop by ETAR antagonism opens up a new option of preventing possible vascular sequelae of otherwise untreatable chronic chlamydial infection. In conclusion, this is the first study to demonstrate infection to dysregulate the ET-1 axis towards inducing a proatherogenic proliferative phenotype.

The Endothelin Axis: A Novel Target for Pharmacotherapy of Female Malignancies

The endothelin axis (ET axis), comprising the three peptides endothelin (ET)-1, -2, -3 and their receptors ET(A)R and ET(B)R, is expressed in various cells and tissues. The biologically active ET-1 is formed by endothelin-converting enzyme (ECE) from inactive big-ET-1. ET-1 has emerged as an important peptide in a host of biological functions, including development, cellular proliferation, apoptosis and angiogenesis, thereby playing an important physiological and pathophysiological role. As these effects are mediated by ET(A)R, activation of ET(B)R prevents apoptosis, inhibits ECE expression and mediates the clearance of ET-1. Emerging data indicate that the ET axis is involved in tumourigenesis and tumour progression of various cancers. Expression of the ET axis has been demonstrated in a wide range of human tumours. Since most data have been reported for female malignancies, this review will focus on the role of the ET axis in cancers of the ovary, the cervix and the breast. In ovarian cancer, activation of ET(A)R by ET-1 is a key mechanism in the cellular signalling network promoting cancer growth and progression. Similar effects have been shown for cervical and endometrial cancer. In breast cancer, ET-1 via ET(A)R promotes proliferation and invasion, mediates bone metastases and predicts unfavourable response to chemotherapy. The outstanding role of ET-1 and ET(A)R in carcinogenesis and tumour progression has led to an extensive search for interfering agents, resulting in the development of selective ET(A)R antagonists on the one hand and inhibitors of the endothelin-converting enzyme (ECE) on the other. Targeting the ET axis via ET(A)R or ECE blockade seems to be a promising approach in the treatment of female malignancies.

Dual ETA/ETB vs. selective ETA endothelin receptor antagonism in patients with pulmonary hypertension

Since the identification of endothelin as a key mediator in the pathogenesis of several diseases, including pulmonary arterial hypertension (PAH), the pharmacologic control of the activated endothelin system with endothelin receptor antagonists (ETRA) has been a major therapeutic achievement for the treatment of patients with PAH. To date, dual ET(A)/ET(B) and selective ET(A) receptor antagonists have clinically been evaluated. To answer the question of whether selective or dual ETRA is preferable in patients with PAH, experimental and clinical data with relevance to the pulmonary circulation are reviewed in this article. Whereas experimental and clinical data provide unambiguous evidence that ET(A) receptors mediate the detrimental effects of ET-1, such as vasoconstriction and cell proliferation, the elucidation of the role of ET(B) receptors has been more complex. It has been shown that there is a subpopulation of ET(B) receptors on smooth muscle cells and fibroblasts mediating vasoconstriction and proliferation. On the contrary, there is clear evidence that endothelial ET(B) receptors continue to mediate vasodilation, vasoprotection and ET-1 clearance despite the pathology associated with pulmonary hypertension. More difficult to assess is the net effect of these mechanisms in patients to be treated with ETRA. When considering the available data from controlled clinical trials, nonselectivity does not appear to carry a relevant clinical benefit for the treatment of patients with PAH when compared with selective ET(A) receptor antagonism.

Etiology-Specific Endothelin-1 Clearance in Human Precapillary Pulmonary Hypertension

Endothelin (ET)-1 is a mediator of vascular remodeling seen in human pulmonary hypertension (PH), and it is normally cleared via endothelial ET-B receptors. Increased levels of ET-1 are found in precapillary PH, partly from increased synthesis. We hypothesized that the endothelial dysfunction and vascular remodeling seen in human precapillary PH would also reduce ET-1 clearance. Case series from a single institutional PH center. Thirty-four patients with pulmonary arterial hypertension (PAH; idiopathic [IPAH], n = 19; connective tissue disease [CTD], n = 15) and 11 patients with chronic thromboembolic PH were studied. Using indicator dilution methods, the first-pass extraction of radiolabeled ET-1 through the pulmonary circulation, and permeability surface (PS) area, an index of functional microvascular surface available for ET-1 clearance, were determined. Mean extraction for IPAH and thromboembolic PH groups was normal, but it was reduced in PAH from CTD; 69% of all patients studied had normal extraction. The mean PS product was reduced significantly for all three etiologies as compared to normal, but 58% of IPAH patients and 40% of CTD-related PAH patients had normal PS products. Receptor-mediated ET-1 extraction and functional vascular surface area for clearance vary between etiologies of PAH. However, contrary to our hypothesis, endothelial ET-B receptor-mediated extraction is preserved in many patients. The scientifically significant finding of our study is that high ET-1 levels seen in patients with PAH must be predominantly due to excess synthesis rather than reduced clearance. The finding that endothelial ET-B receptors are still present and functional in PAH may also be of relevance to the choice of selective vs nonselective ET receptor antagonists.

Concomitant antagonism of endothelial and vascular smooth muscle cell ETBreceptors for endothelin induces hypertension in the hamster

In the vascular system, endothelin (ET) type B (ET(B)) receptors for ET-1 are located on endothelial and on venous and arterial smooth muscle cells. In the present study, we investigated the hemodynamic effects of chronic ET(B) receptor blockade at low and high doses in the Syrian Golden hamster. After 16 days of gavage with A-192621 (0.5 or 30 mg.kg(-1).day(-1)), a selective ET(B) receptor antagonist, hamsters were anesthetized with a mixture of ketamine and xylazine (87 and 13 mg/kg im, respectively), and basal mean arterial blood pressure (MAP) and pressor responses to exogenous ET-1 were evaluated. The lower dose of A-192621 (0.5 mg.kg(-1).day(-1)) did not modify basal MAP, whereas the higher dose (30 mg.kg(-1).day(-1)) increased MAP and plasma ET levels. Radio-telemetry recordings confirmed the increase in MAP induced by the higher dose of A-192621 in conscious hamsters. On the other hand, although the lower dose of A-192621 was devoid of intrinsic pressor effects, it markedly reduced the transient hypotensive phase induced by intravenously injected IRL-1620, a selective ET(B) receptor agonist. Finally, A-192621 (0.5 mg.kg(-1).day(-1)) alone or A-192621 (30 mg.kg(-1).day(-1)) + atrasentan (6 mg.kg(-1).day(-1)), a selective ET(A) receptor antagonist, potentiated the pressor response to exogenous ET-1. Our results suggest that, in the hamster, ET(B) receptors on vascular smooth muscle cells are importantly involved in the clearance of endogenous ET-1, whereas the same receptor type on the endothelium is solely involved in the vasodilatory responses to the pressor peptide. Blockade of endothelial and vascular smooth muscle cell ET(B) receptors triggers a marked potentiation of ET(A)-dependent increases in systemic resistance.

NO and NO-independent mechanisms mediate ETBreceptor buffering of ET-1-induced renal vasoconstriction in the rat

Vascular endothelin (ET) type B (ET(B)) receptors exert dilator and constrictor actions in a complex interaction with ET(A) receptors. We aimed to clarify the presence and relative importance of nitric oxide (NO) and other mechanisms underlying the dilator effects of ET(B) receptors in rat kidneys. Complete inhibition of NO production with Nomega-nitro-L-arginine methyl ester (L-NAME, 25 mg/kg iv) enhanced the renal vasoconstriction elicited by ET-1 injected into the renal artery from -15 to -30%. Additional infusion of the NO donor nitroprusside (NP) into the renal artery did not reverse this effect (-29%) but effectively buffered ANG II-mediated vasoconstriction. Similarly, ET-1 responses were enhanced after a smaller intrarenal dose of L-NAME (-22 vs. -15%) and were unaffected by subsequent NP infusion (-21%). These results indicate that the responsiveness to ET-1 is buffered by ET(B) receptor-stimulated phasic release of NO, rather than its static mean level. Infusion of the ET(B) receptor antagonist BQ-788 into the renal artery further enhanced the ET-1 constrictor response to NP+L-NAME (-92 vs. -49%), revealing an NO-independent dilator component. In controls, vasoconstriction to ET-1 was unaffected by vehicle (-27 vs. -20%) and markedly enhanced by BQ-788 (-70%). The same pattern was observed when indomethacin (Indo) was used to inhibit cyclooxygenase (-20% for control, -22% with Indo, and -56% with ET(B) antagonist) or methylsulfonyl-6-(2-propargyloxyphenyl)-hexanamide (MS-PPOH) or miconazole+Indo was used to inhibit epoxygenase alone (-10% for control, -11% with MS-PPOH, and -35% with ET(B) antagonist) or in combination (-14% for control, -20% with Indo + miconazole, and -43% with ET(B) antagonist). We conclude that phasic release of NO, but not its static level, mediates part of the dilator effect of ET(B) receptors and that an NO-independent mechanism, distinct from prostanoids and epoxyeicosatetraenoic acids, perhaps ET(B) receptor clearance of ET-1, plays a major buffering role.

Reduced pulmonary clearance of endothelin in congestive heart failure: a marker of secondary pulmonary hypertension

BackgroundEndothelin-1 (ET-1) levels are elevated in congestive heart failure (CHF) in relation with the severity of pulmonary hypertension. We evaluated whether a reduced pulmonary ET-1 clearance could contribute to this elevation. Methods and resultsWe determined pulmonary ET-1 clearance in 24 patients with CHF in relation with hemodynamics, plasma ET-1, and N-terminal pro-brain natriuretic peptide (NT-proBNP) levels. Pulmonary ET-1 extraction, measured by the single bolus indicator-dilution technique, was reduced to 32±14% in comparison to historic controls (47±7%). Plasma ET-1 clearance by the lungs (924±588 mL/min) was also much lower than in controls (1424±79 mL/min). Clearance correlated inversely with mean pulmonary artery pressure (PAP, r=−.47, P=.017) and pulmonary capillary wedge pressure (r=−.47, P=.017) and positively with the rate of left ventricular (LV) relaxation LV −dP/dt (r=.593, P=.004). After multivariate analysis, only mean PAP and LV −dP/dt were independently correlated with ET-1 clearance (r=−.40, P=.03, and r=.55, P=.005, respectively). Plasma ET-1 levels did not correlate with clearance (r=.038, P=.86), and there was no significant arteriovenous ET-1 gradient. There was a mild nonsignificant correlation between plasma ET-1 and pulmonary artery systolic pressure (r=.38, P=.06), but a strong correlation with right atrial pressure (r=.696, P < .0001) and NT-proBNP levels (r=.51, P=.001), which were maintained after multivariate linear regression (r=.60, P=.001, and r=.32, P=.04, respectively). ConclusionPulmonary ET-1 clearance is reduced in CHF in relation with the severity of pulmonary hypertension. This reduced clearance does not significantly modulate plasma ET-1 levels. Whether this is only a marker of secondary pulmonary hypertension or could modulate pulmonary vascular tone will require further studies.

Endothelin B receptors located on the endothelium provide cardiovascular protection in the hamster.

Endothelins (ETs) act through two receptors, namely ET(A) and ET(B). In the cardiovascular system, the activation of both receptors leads to vasoconstriction. However, ET(B) receptors also mediate endothelium-dependent vasodilatation and clearance of plasma ET-1. With regard to these latter properties, we wanted to assess the contribution of ET(B) receptors and the effects of selective and mixed ET receptor blockade on vascular tone in control Syrian Golden hamsters and in Bio 14.6 cardiomyopathic hamsters after bolus injection of ET-1 and IRL-1620, a selective ET(B) agonist. In 12-week-old anaesthetized control hamsters, ET-1 (0.5 nmol/kg) induced a sustained pressor response which was only partly reduced by the selective ET(A) receptor antagonist BQ-123, suggesting a contribution of ET(B) receptor activation to the vasoconstrictive effects of ET-1. This was confirmed by injection of the selective ET(B) receptor agonist IRL-1620 (1 nmol/kg). However, the pressor response to this agonist was always preceded by a transient vasodilatation, indicating activation of endothelium-located ET(B) receptors. When the selective ET(B) receptor antagonist BQ-788 was administered, the hypotensive phase following IRL-1620 injection was abolished. Interestingly, BQ-788 or a mixture of BQ-788 and BQ-123 significantly potentiated the pressor responses to ET-1. In 12-week-old Bio 14.6 cardiomyopathic hamsters, ET-1 and IRL-1620 induced haemodynamic responses similar to those observed in control hamsters, although the IRL-1620-induced pressor increase was lower. No difference in cardiac prepro ET-1 mRNA expression was observed between the two strains of hamsters. In conclusion, we suggest that endothelium-located ET(B) receptors are involved in the physiological antagonism of ET-dependent protracted pressor effects, and thus may play a protective role in both normal hamsters and those with cardiomyopathy.

Endothelin-1 and endothelin receptor antagonists as potential cardiovascular therapeutic agents.

Endothelin (ET)-1 is an endothelium-derived peptide with potent vasoconstrictor and proliferative properties. The ET system is activated in several cardiovascular disease states associated with functional and structural vascular changes, including hypertension and heart failure. The two ET receptor subtypes are known as ET(A)R and ET(B)R. The former is located mainly on vascular smooth muscle cells and is responsible for mediating vasoconstriction and proliferation. The latter is present predominantly on endothelial cells and mediates vasorelaxation as well as ET-1 clearance. Activation of smooth muscle ET(B)R causes vasoconstriction. Selective ET(A)R antagonists as well as nonselective ET(A)R-ET(B)R antagonists have been developed. Studies with animal models and early-phase clinical trials provided strong evidence that these agents are effective in the treatment of heart failure, essential hypertension, pulmonary hypertension, and atherosclerosis. However, the complexity of biologic effects mediated by two different receptor subtypes complicates therapy with selective versus nonselective ET receptor antagonists. In addition to subtype selectivity and potency, changes in receptor subtype distribution under different pathologic conditions and different patient populations will play a crucial role in the evaluation of these potentially therapeutic drugs.

The therapeutic potential of endothelin receptor antagonists in cardiovascular disease.

Endothelin (ET)-1, a 21-amino acid peptide, is the predominant isoform of the endothelin peptide family. ET-1 is ubiquitously expressed and stimulates vasoconstriction and cell proliferation. Enzymes such as endothelin converting enzymes (ECE), chymases, and non-ECE metalloproteinases contribute to the synthesis of ET-1, which is regulated in an autocrine fashion in vascular and nonvascular cells. Endothelin ET(A) receptors mediate vasoconstriction and cell proliferation, whereas ET(B) receptors are involved in the clearance of ET-1, inhibition of endothelial apoptosis, release of nitric oxide and prostacyclin, and inhibition of ECE-1 expression. Most cardiovascular diseases, such as arterial hypertension, atherosclerosis, restenosis, heart failure, idiopathic cardiomyopathy, pulmonary hypertension, and renal failure are associated with local activation of the endothelin system. Experimental studies and first clinical trials suggest that ET-1 is importantly involved in the functional and structural changes in the cardiovascular system, and that many of the actions of ET-1 are mediated through pressure-independent mechanisms. Endothelin antagonists promise to be successful as a new class of drugs for the treatment of cardiovascular diseases.

Therapeutic potential for endothelin receptor antagonists in cardiovascular disorders.

The endothelins are synthesized in vascular endothelial and smooth muscle cells, as well as in neural, renal, pulmonal, and inflammatory cells. These peptides are converted by endothelin-converting enzymes (ECE-1 and -2) from 'big endothelins' originating from large preproendothelin peptides cleaved by endopeptidases. Endothelin (ET)-1 has major influence on the function and structure of the vasculature as it favors vasoconstriction and cell proliferation through activation of specific ET(A) and ET(B) receptors on vascular smooth muscle cells. In contrast, ET(B )receptors on endothelial cells cause vasodilation via release of nitric oxide (NO) and prostacyclin. Additionally, ET(B) receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Indeed, ET-1 contributes to the pathogenesis of important disorders as arterial hypertension, atherosclerosis, and heart failure. In patients with atherosclerotic vascular disease (as well as in many other disease states), ET-1 levels are elevated and correlate with the number of involved sites. In patients with acute myocardial infarction, they correlate with 1-year prognosis. ET receptor antagonists have been widely studied in experimental models of cardiovascular disease. In arterial hypertension, they prevent vascular and myocardial hypertrophy. Experimentally, ET receptor blockade also prevents endothelial dysfunction and structural vascular changes in atherosclerosis due to hypercholesterolemia. In experimental myocardial ischemia, treatment with an ET receptor antagonist reduced infarct size and prevented left ventricular remodeling after myocardial infarction. Most impressively, treatment with the selective ET(A) receptor antagonist BQ123 significantly improved survival in an experimental model of heart failure. In many clinical conditions, such as congestive heart failure, both mixed ET(A/B )as well as selective ET(A) receptor antagonism ameliorates the clinical status of patients, i.e. symptoms and hemodynamics. A randomized clinical trial showed that a mixed ET(A/B) receptor antagonist effectively lowered arterial blood pressure in patients with arterial hypertension. In patients with primary pulmonary hypertension or pulmonary hypertension related to scleroderma, treatment with a mixed ET(A/B) receptor antagonist resulted in an improvement in exercise capacity. ET receptor blockers thus hold the potential to improve the outcome in patients with various cardiovascular disorders. Randomized clinical trials are under way to evaluate the effects of ET receptor antagonism on morbidity and mortality.

Endothelin A receptor antagonists in congestive heart failure: blocking the beast while leaving the beauty untouched?

Congestive heart failure (CHF) is a disease process characterized by impaired left ventricular function, increased peripheral and pulmonary vascular resistance and reduced exercise tolerance and dyspnea. Thus, mediators involved in the control of myocardial function and vascular tone may be involved in its pathophysiology. The family of endothelins (ET) consists of four closely related peptides, ET-1, ET-2, ET-3, and ET-4, which cause vasoconstriction, cell proliferation, and myocardial effects through activation of ET(A) receptors. In contrast, endothelial ET(B) receptors mediate vasodilation via release of nitric oxide and prostacyclin. In addition, ET(B) receptors in the lung are a major pathway for the clearance of ET-1 from plasma. Thus, infusion of an ET(A) receptor antagonist into the brachial artery in healthy humans leads to vasodilation whereas infusion of an ET(B) receptor antagonist causes vasoconstriction. ET-1 plasma levels are elevated in CHF and correlate both with the hemodynamic severity and with symptoms. Plasma levels of ET-1 and its precursor, big ET-1, are strong independent predictors of death in patients after myocardial infarction and with CHF. ET-1 contributes to increased systemic and pulmonary vascular resistance, vascular dysfunction, myocardial ischemia, and renal impairment in CHF. Selective ET(A) as well as combined ET(A/B) receptor antagonists have been studied in patients with CHF showing impressive hemodynamic improvements (i.e. reduced peripheral vascular and pulmonary resistance as well as increased cardiac output). These results indicate that ET receptor antagonists indeed have a potential to improve hemodynamics, symptoms, and potentially prognosis of CHF which still carries a high mortality.

Acute hemodynamic and neurohumoral effects of selective ETA receptor blockade in patients with congestive heart failure

OBJECTIVESTo investigate the hemodynamic effects of the selective endothelin (ET)A receptor antagonist LU135252 in patients with congestive heart failure (CHF).BACKGROUNDNonselective ETA/B receptor antagonists improve hemodynamics in patients with CHF. Since ETB receptors mediate the release of nitric oxide and the clearance of ET-1, selective ETA antagonists are of special interest.METHODSThe hemodynamic effects of a single oral dose of the selective ETA receptor antagonist LU135252 (1, 10, 30, 100 or 300 mg) were investigated in a multicenter study involving 95 patients with CHF (New York Heart Association II–III) with an ejection fraction ≤35%.RESULTSBaseline ET-1 positively correlated with pulmonary vascular resistance, pulmonary capillary wedge pressure (PCWP), and mean pulmonary artery pressure (MPAP, r = 0.37–0.50, p < 0.0004) but were inversely related to cardiac index (CI; r = −0.36, p = 0.0004). LU135252 dose dependently increased CI and decreased mean arterial pressure and systemic vascular resistance (p < 0.03–0.0002), while heart rate remained constant or decreased slightly. Pulmonary capillary wedge pressure, MPAP, pulmonary vascular resistance and right atrial pressure also decreased significantly (p < 0.035–< 0.0001). Two hours after LU135252, plasma ET-1 did not significantly increase after 1 mg but did so by 23% (p = 0.003), 29% (p = 0.0018), 56% (p < 0.0001) and 101% (p < 0.0001) after 10, 30, 100 and 300 mg, respectively, while plasma catecholamines remained constant.CONCLUSIONSIn patients with CHF, a single oral dose of the selective ETA receptor antagonist LU135252 improves hemodynamics in a dose-dependent manner without activation of other neurohumoral systems and is well tolerated over a wide dose range.