Pharmacologic characterization of an endothelinA (ETA) receptor antagonist in conscious rats

The heart as an extravascular target of endothelin-1 in particulate matter-induced cardiac dysfunction

Exposure to particulate matter air pollution has been causally linked to cardiovascular disease in humans. Several broad and overlapping hypotheses describing the biological mechanisms by which particulate matter exposure leads to cardiovascular disease have been explored, although linkage with specific factors or genes remains limited. These hypotheses may or may not also lead to particulate matter-induced cardiac dysfunction. Evidence pointing to autocrine/paracrine signaling systems as modulators of cardiac dysfunction has increased interest in the emerging role of endothelins as mediators of cardiac function following particulate matter exposure. Endothelin-1, a well-described small peptide expressed in the pulmonary and cardiovascular systems, is best known for its ability to constrict blood vessels, although it can also induce extravascular effects. Research on the role of endothelins in the context of air pollution has largely focused on vascular effects, with limited investigation of responses resulting from the direct effects of endothelins on cardiac tissue. This represents a significant knowledge gap in air pollution health effects research, given the abundance of endothelin receptors found on cardiac tissue and the ability of endothelin-1 to modulate cardiac contractility, heart rate, and rhythm. The plausibility of endothelin-1 as a mediator of particulate matter-induced cardiac dysfunction is further supported by the therapeutic utility of certain endothelin receptor antagonists. The present review examines the possibility that endothelin-1 release caused by exposure to PM directly modulates extravascular effects on the heart, deleteriously altering cardiac function.

Skeletal Muscle Vascular Function: A Counterbalance of Insulin Action

Insulin is a vasoactive hormone that regulates vascular homeostasis by maintaining balance of endothelial-derived NO and ET-1. Although there is general agreement that insulin resistance and the associated hyperinsulinemia disturb this balance, the vascular consequences for hyperinsulinemia in isolation from insulin resistance are still unclear. Presently, there is no simple answer for this question, especially in a background of mixed reports examining the effects of experimental hyperinsulinemia on endothelial-mediated vasodilation. Understanding the mechanisms by which hyperinsulinemia induces vascular dysfunction is essential in advancing treatment and prevention of insulin resistance-related vascular complications. Thus, we review literature addressing the effects of hyperinsulinemia on vascular function. Furthermore, we give special attention to the vasoregulatory effects of hyperinsulinemia on skeletal muscle, the largest insulin-dependent organ in the body. This review also characterizes the differential vascular effects of hyperinsulinemia on large conduit vessels versus small resistance microvessels and the effects of metabolic variables in an effort to unravel potential sources of discrepancies in the literature. At the cellular level, we provide an overview of insulin signaling events governing vascular tone. Finally, we hypothesize a role for hyperinsulinemia and insulin resistance in the development of CVD.

Clinical trials with endothelin receptor antagonists: what went wrong and where can we improve?

In the early 1990s, within three years of cloning of endothelin receptors, orally active endothelin receptor antagonists (ERAs) were tested in humans and the first clinical trial of ERA therapy in humans was published in 1995. ERAs were subsequently tested in clinical trials involving heart failure, pulmonary arterial hypertension, resistant arterial hypertension, stroke/subarachnoid hemorrhage and various forms of cancer. The results of most of these trials - except those for pulmonary arterial hypertension and scleroderma-related digital ulcers - were either negative or neutral. Problems with study design, patient selection, drug toxicity, and drug dosing have been used to explain or excuse failures. Currently, a number of pharmaceutical companies who had developed ERAs as drug candidates have discontinued clinical trials or further drug development. Given the problems with using ERAs in clinical medicine, at the Twelfth International Conference on Endothelin in Cambridge, UK, a panel discussion was held by clinicians actively involved in clinical development of ERA therapy in renal disease, systemic and pulmonary arterial hypertension, heart failure, and cancer. This article provides summaries from the panel discussion as well as personal perspectives of the panelists on how to proceed with further clinical testing of ERAs and guidance for researchers and decision makers in clinical drug development on where future research efforts might best be focused.

Darusentan, a selective endothelin A receptor antagonist, for the oral treatment of resistant hypertension

Resistant hypertension is defined as failure to lower blood pressure to target when a patient adheres to the maximum tolerated doses of three antihypertensive drugs including a diuretic. Notwithstanding the wide availability of several antihypertensive agents and the continued recommendation of dietary and lifestyle modifications, the prevalence of resistant hypertension remains high and is expected to increase thus underscoring the need for potential new treatment modalities in resistant hypertension. Endothelin-1 is a long-lasting potent vasoconstrictor and plays a key role in cardiovascular haemostasis. Endothelin mediates its biological activity in humans through the endothelin A and B receptors. The clinical experience and the evidence for therapy with darusentan in resistant systemic hypertension are reviewed. The leading journals that publish basic science and clinical research in the area of cardiovascular diseases and PubMed were scanned. While results from early clinical studies suggested that darusentan might emerge as new treatment option in patients with resistant hypertension, results from recent studies suggests that darusentan appears unlikely to find its way in the armamentarium for treatment of resistant hypertension.

Endothelin A receptor antagonist modulates lymphocyte and eosinophil infiltration, hyperreactivity and mucus in murine asthma

Levels of endothelins are particularly high in the lung, and there is evidence that these peptides are involved in asthma. Asthma is a chronic inflammatory disease associated with lymphocyte infiltration. In the present study, we used a murine model of asthma to investigate the role of endothelins in lymphocyte and eosinophil infiltration into the airway hyperreactivity and mucus secretion. Sensitized C57Bl/6 mice were treated with endothelin ETA receptor antagonist (BQ123) or endothelin ETB receptor antagonist (BQ788) 30 min before an antigen aerosol challenge. After 24 h, dose response curves to methacholine were performed in isolated lungs, FACS analysis of lymphocytes and eosinophil counts were performed in bronchoalveolar lavage fluid and mucus index was determined by histopathology. In sensitized and antigen-challenged mice there is a marked increase in the T CD4+, T CD8+, B220+, Tgammadelta+ and NK1.1+ lymphocyte subsets. Treatment with BQ123 further increased these cell populations. The number of eosinophils, airway hyperreactivity and mucus were all reduced by BQ123 treatment. The BQ 788 had no significant effect on the parameters analyzed. Treatment with BQ123 reduced the endothelin concentration in lung homogenates, suggesting that endothelins exert a positive feedback on their synthesis. We show here that in murine asthma the ETA receptor antagonist up-regulates lymphocyte infiltration and reduces eosinophils, hyperreactivity and mucus.

The role of endothelin pathway in modulation of airway reactivity to methacholine in C57Bl/6 and BALB/c mice

Endothelin peptides have been shown to increase cholinergic neurotransmission in the airway. Genetic differences in airway responsiveness to methacholine where reported in mice. The present study compared the airway reactivity to methacholine in C57Bl/6 and BALB/c mice, the involvement of endothelin on this reactivity and endothelin levels in lung homogenates. Whole airway reactivity was analyzed by means of an isolated lung preparation where lungs were perfused through the trachea with warm gassed Krebs solution at 5 ml/min, and changes in perfusion pressure triggered by methacholine at increasing bolus doses (0.1-100 microg) were recorded. We found that the maximal airway response to methacholine was much greater in C57Bl/6 than in BALB/c (Emax 34+/-2 vs 12+/-1 cmH(2)O, respectively). Bosentan (mixed endothelin A/B receptor antagonist; 10 mg/kg, i.p., 30 min before sacrifice) reduced lung responsiveness to methacholine in C57Bl/6 (58% at EC50 level) but had no effect in BALB/c mouse strain. This effect seems to be mediated by the endothelin ET(A) receptor since it was significantly reduced by the selective endothelin ET(A) receptor antagonist, BQ 123. Immunoreactive endothelin levels were higher in C57Bl/6 than in BALB/c lungs (43+/-5 vs 19+/-5 pg/g of tissue). In conclusion, airway reactivity to methacholine and lung endothelins content varies markedly between C57Bl/6 and BALB/c strains. Endothelins upregulate lung responsiveness to methacholine only in C57Bl/6, an effect achieved through the endothelin ET(A) receptor.

Neutrophils from acute pancreatitis patients cause more severe in vitro endothelial damage compared with neutrophils from healthy donors and are differently regulated by endothelins

OBJECTIVE

There is evidence that endothelin (ET) 1 affect neutrophil functions and that patients with severe acute pancreatitis have increased plasma levels of ETs. Under appropriate conditions, neutrophils are able to injure the endothelium. In the present study, we compared healthy donors with acute pancreatitis patients for neutrophil degranulation and its ability to injure the endothelium and the contribution of ET-1 to this injury.

METHODS

Injury was evaluated by measuring the detachment of endothelial cells (ECV-304) growing in monolayer in coculture with human neutrophils for 4 hours. Neutrophil degranulation was assessed by myeloperoxidase (MPO) activity in coculture supernatants. In some experiments, neutrophils were pretreated with the antagonist of ET(A) receptor (BQ-123, 10(-6) M), which has high affinity for ET-1.

RESULTS

Neutrophils from both healthy donors and acute pancreatitis patients caused detachment of endothelial cells, and levels of MPO activity were increased in coculture supernatants. Neutrophils from acute pancreatitis patients caused significantly higher levels of detachment and MPO in the supernatants. Pretreatment of neutrophils with BQ-123 inhibited the detachment caused by neutrophils from healthy donors but not by neutrophils from acute pancreatitis patients.

CONCLUSIONS

These results show that neutrophils taken from healthy donors damage the endothelium by a mechanism dependent on ETs acting via ET(A) receptor, whereas neutrophils from acute pancreatitis patients cause more severe damage that is not dependent on ETs in the in vitro system used.

Effects of blockade of the endothelin receptor A and inhibition of nitric oxide synthesis on uteroplacental and renal blood flow in awake pregnant rats

OBJECTIVE

The aim of this study was to evaluate the role of the ETA receptor and nitric oxide (NO) in the regulation of uteroplacental and renal blood flow in pregnant rats.

STUDY DESIGN

Regional blood flows were measured by the microsphere technique in pregnant Sprague-Dawley rats (term 23 days). Blood flow was measured before and after ETA receptor blockade (BQ-123, 1 mg/kg) at gestational day (GD) 19 or 21 (n=8 and n=9, respectively). In 2 additional groups blood flow was measured before and after NO synthase inhibition (L-NAME 2 microg/min) at GD 19 or 21 (n=10 in each group).

RESULTS

BQ-123 significantly increased placental and myometrial blood flows by almost 80% and 35%, respectively, at both gestational ages. BQ-123 did not alter renal blood flow. No effect on placental or myometrial blood flow was observed after L-NAME infusion, neither at GD 19 nor GD 21. Renal blood flow decreased by nearly 35% in both groups after L-NAME.

CONCLUSION

There is an important role for endogenous ET in the regulation of uteroplacental blood flow in the rat. NO contributes to a significant vasodilatation in the renal vasculature; however, NO appears not to be involved in the maintenance of uteroplacental blood flow in the awake pregnant rat.

Endothelin and ET(A) receptors in long-term arterial pressure homeostasis in conscious nonhuman primates

This study was designed to quantify the long-term contribution of endogenous endothelin-1 (ET-1) and ET(A) receptors to the regulation of arterial pressure under normal conditions in nonhuman primates. Therefore, mean arterial pressure (MAP) and heart rate were measured 24 h/day with the use of telemetry techniques in conscious cynomolgus monkeys under control conditions, during administration of an ET(A) selective receptor antagonist (ABT-627; 5 mg/kg, 2 times a day by mouth, 4 days), and a 6-day posttreatment period. Systemic ET(A) blockade reduced MAP (24 h) from 89 +/- 3 to 82 +/- 2 and 79 +/- 2 mmHg on days 1 and 4, respectively. Subsequently, MAP remained suppressed for 3 days posttreatment. Heart rate increased from 111 +/- 5 to 122 +/- 4 and 128 +/- 6 beats/min on days 1 and 4 of ABT-627, respectively, and remained above control for 3 days posttreatment. Plasma ET-1 concentration increased from 1.0 +/- 0.3 to 1.9 +/- 0.4 pg/ml in response to ABT-627 (day 4) but decreased to control values 4 days posttreatment. These data demonstrate a physiologically important role for endogenous ET-1 and ET(A) receptors in the long-term regulation of arterial pressure and plasma ET-1 levels in the conscious nonhuman primate.

Enhanced blood pressure sensitivity to DOCA-salt treatment in endothelin ET(B) receptor-deficient rats

The role of endothelin ET(B) receptor-mediated action in the development and maintenance of deoxycorticosterone acetate (DOCA)-salt-induced hypertension was evaluated using the spotting-lethal (sl) rat which carries a naturally occurring deletion in the ET(B) receptor gene. Homozygous (sl/sl) rats treated with DOCA-salt for 1 week exhibited an earlier onset of hypertension than heterozygous (sl/+) and wild-type (+/+) rats (systolic blood pressure, SBP; 156.7+/-3.4 versus 128.8+/-5.3 and 132.9+/-3.7 mmHg, respectively). Four weeks after the start of DOCA-salt treatment, homozygous rats developed marked hypertension, with a SBP of 206. 0+/-4.5 mmHg, compared with 184.8+/-10.7 mmHg in heterozygous and 164.3+/-4.8 mmHg in wild-type rats. Cardiovascular hypertrophy and renal dysfunction observed after 4-weeks treatment with DOCA-salt were more severe in homozygous rats, compared to wild-type and heterozygous animals. These evidences support strongly the view that ET(B) receptor-mediated actions are a protective factor in the pathogenesis of DOCA-salt-induced hypertension.

Maintenance of blood pressure in normotensive dogs by endothelin

The role of endothelin (ET)-1 in blood pressure homeostasis and the interaction with the renin-angiotensin system (RAS) was investigated in normotensive conscious dogs. ETA receptors were blocked by LU-135252 (1-30 mg/kg); trandolapril (2 mg/kg) or losartan (10 mg/kg) was used to inhibit the RAS. LU-135252 in oral doses of 3-30 mg/kg significantly reduced mean arterial pressure (MAP) by approximately 10 mmHg maximally, whereas trandolapril or losartan were without any effect. MAP reduction was more pronounced when LU-135252 was combined with either losartan (-15.5 +/- 3.2 mmHg; 2 h postadministration; P < 0.05) or trandolapril (-30.9 +/- 3.6 mmHg; P < 0.05). When endogenous nitric oxide (NO) generation was blocked but NO concomitantly infused, this synergistic effect on MAP was prevented. The data show that ET-1 contributes to the maintenance of blood pressure via ETA receptors. Furthermore, ET-1 and ANG II play a prominent role in the control of blood pressure by opposing the effects of NO. The pronounced blood pressure fall after combined blockade of ETA receptors and the RAS may be mediated by an enhanced release of NO.

Endothelin-1 and CYP450 arachidonate metabolites interact to promote tissue injury in DOCA-salt hypertension

Inhibition of cytochrome P-450 (CYP450) enzymes with cobalt chloride (CoCl2) prevented hypertension, organ hypertrophy, and renal injury induced by DOCA and salt (1% NaCl) in uninephrectomized (UNx) rats. Systolic blood pressure (SBP) rose to 193 +/- 6 mmHg by day 21 from control levels of 150 +/- 7 mmHg in response to DOCA-salt treatment, a rise that was prevented by CoCl2 (24 mg. kg-1. 24 h-1). The effects of DOCA-salt treatment, which increased protein excretion to 88.3 +/- 6.9 mg/24 h on day 21 from 9.0 +/- 1.1 mg/24 h on day 3, were prevented by CoCl2. CoCl2 also attenuated the renal and left ventricular hypertrophy and the increase in media-to-lumen ratio in hypertensive rats. DOCA-salt treatment increased excretion of endothelin (ET)-1 from 81 +/- 17 to 277 +/- 104 pg. 100 g body wt-1. 24 h-1 associated with a fourfold increase in 20-hydroxyeicosatetraenoic acid (20-HETE) excretion from 3.0 +/- 1.1 to 12.2 +/- 1.9 ng. 100 g body wt-1. 24 h-1 (days 3 vs. 21). CoCl2 blunted these increases by 58 and 72%, respectively. In aortic rings pulsed with [3H]thymidine, ET-1 increased its incorporation. Dibromododec-11-enoic acid, an inhibitor of 20-HETE synthesis, attenuated ET-1-induced increases in [3H]thymidine incorporation. We distinguished effects of CoCl2 acting via CO generation vs. suppression of CYP450-arachidonic acid metabolism by treating UNx-salt-DOCA rats with 1-aminobenzotriazole (ABT), which suppresses CYP450 enzyme activity, and compared these results to those produced by CoCl2. ABT reduced hypertension, as did CoCl2. Unlike CoCl2, ABT did not prevent organ hypertrophy and proteinuria, suggesting that these effects were partially related to CO formation. Blockade of the ETA receptor with BMS-182874 reduced SBP, organ hypertrophy, and proteinuria, indicating the importance of ET-initiated abnormalities to the progression of lesions in UNx-salt-DOCA.

Reduced endogenous endothelin-1-mediated vascular tone in chronic renal failure

BACKGROUND

Endothelin-1 generated by the vascular endothelium contributes to basal vascular tone and blood pressure in healthy humans. Plasma concentrations of endothelin-1, which are elevated in chronic renal failure (CRF), may contribute to increased vascular tone.

METHODS

We investigated the contribution of endogenous and exogenous endothelin-1 to the maintenance of vascular tone in patients with CRF (creatinine > or = 200 mumol/liter) and in age- and sex-matched healthy subjects. In a series of experiments, we measured forearm vascular responses to intra-arterial norepinephrine (30 to 240 pmol/min), endothelin-1 (5 pmol/min), the selective endothelin A (ETA) receptor antagonist BQ-123 (3 mg/hr), the mixed endothelin-converting enzyme and neutral endopeptidase inhibitor phosphoramidon (30 nmol/min), and the selective neutral endopeptidase inhibitor thiorphan (30 nmol/min).

RESULTS

The maximum reduction in forearm blood flow (FBF) to norepinephrine in CRF (33 +/- 7%) was similar to that in controls (43 +/- 7%, P = 0.53). Endothelin-1 also produced a similar reduction in FBF in CRF (35 +/- 6%) and controls (36 +/- 5%, P = 0.81). BQ-123 increased FBF in CRF (11 +/- 4%) but significantly less than in controls (44 +/- 10%, P = 0.02). Phosphoramidon increased FBF in CRF (68 +/- 20%), again significantly less than in controls (181 +/- 41%, P = 0.001). Thiorphan reduced FBF similarly in CRF (22 +/- 6%) and controls (14 +/- 6%, P = 0.39). Responses to phosphoramidon were substantially greater than to BQ-123.

CONCLUSIONS

These studies show that endogenous generation of endothelin-1 contributes to the maintenance of resting vascular tone in patients with CRF, as well as in healthy subjects. Although the contribution of endogenous endothelin-1 to resting vascular tone appears to be reduced in CRF, ETA receptor antagonism, and particularly endothelin-converting enzyme inhibition, should be explored as means by which to reduce vascular tone and blood pressure in patients with CRF.

Effects of two calcium channel blockers on messenger RNA expression of endothelin-1 and nitric oxide synthase in cardiovascular tissue of hypertensive rats

OBJECTIVE

The aim of this study was to investigate the effects of calcium channel blockers on messenger RNA expression of endothelin-1 and endothelial-type nitric oxide synthase in the cardiovascular tissue of stroke-prone spontaneously hypertensive rats (SHRSP).

MATERIALS AND METHODS

The calcium channel blocker nilvadipine (1.0 or 3.2 mg/kg per day) was subcutaneously administered to two groups of SHRSP, from 4 or 8 weeks of age, for 8 weeks and 4 weeks, respectively. For comparison, nifedipine (3.2 mg/kg per day) was similarly administered to SHRSP from 4 weeks of age for 8 weeks. Kidney, heart, aorta and brain tissue samples were obtained when the rats were 12 weeks old. Messenger RNA expression of endothelin-1 and endothelial-type nitric oxide synthase was determined by reverse transcription-polymerase chain reaction followed by Southern blotting and a ribonuclease protection assay, respectively. Results were compared with those in untreated SHRSP and Wistar-Kyoto rats at 12 weeks of age.

RESULTS

Both nilvadipine and nifedipine significantly decreased blood pressure in SHRSP. Although there were no changes in the weights of the kidney and brain, there was a significant decrease in the weight of the left ventricle of the groups treated with nilvadipine (1.0 mg/kg per day: mean +/- SEM 0.282 +/- 0.003 g; 3.2 mg/kg per day: 0.269 +/- 0.005 g) and nifedipine (1 mg/kg/day: 0.281 +/- 0.012 g) for 8 weeks compared with untreated SHRSP (0.301 +/- 0.004 g). Endothelin-1 messenger RNA expression, which was significantly increased by about twofold in the kidney, heart and brain of SHRSP compared with Wistar-Kyoto rats, was normalized by both calcium blockers. Endothelin-1 messenger RNA expression, which was decreased in the aorta of SHRSP, was further decreased by both calcium blockers. While there was no significant difference in endothelial-type nitric oxide synthase messenger RNA expression in the kidney, heart and aorta between the untreated SHRSP and Wistar-Kyoto rats, expression in the aorta was significantly increased in the group treated with these calcium blockers for 8 weeks from 4 weeks of age.

CONCLUSIONS

These results suggest that, in addition to their potent antihypertensive effects, calcium channel blockers may exhibit cardiovasculoprotective and renoprotective effects by modifying mRNA expression of endothelin-1 and endothelial-type nitric oxide synthase in tissue.

Effects of BQ-123 on systemic and renal hemodynamic responses to endothelin-1 in the rat split hydronephrotic kidney

OBJECTIVE

To assess the site of action of endothelin-1 in vessels of different sizes in the kidney in vivo and investigate the function of endothelin A (ET(A)) receptors in mediating renal and systemic vasoconstriction.

DESIGN

The luminal diameters of different vessels were measured and glomerular blood flow in cortical glomeruli was determined by intravital videomicroscopy in the split hydronephrotic kidney of anesthetized female Wistar rats.

METHODS

The rats were infused with endothelin-1 (40 pmol/kg per min) with or without pretreatment with the selective ET(A)-receptor antagonist BQ-123 (0.5 mg/kg). Aortic clamping was used to control renal blood pressure during the endothelin-1 infusion.

RESULTS

Exogenous endothelin-1 induced a significant rise (30+/-3%) in mean arterial pressure and a marked, long-lasting fall in glomerular blood flow (53+/-3%) related to reduction of the inner diameter of arcuate (-30%), interlobular arteries (-33%) and afferent arterioles (-17%). Aortic clamping to normalize renal blood pressure did not attenuate the vasoconstriction and reduction in glomerular blood flow. Pretreatment with BQ-123 significantly reduced both the endothelin-1-induced rise in mean arterial pressure (12+/-1%) and the fall in glomerular blood flow (-23+/-11%). BQ-123 blunted the response to endothelin-1 in arcuate (-12%), interlobular (-11%) and afferent vessels (-5%). Acetylcholine and nitroprusside completely reversed the vasoconstriction in BQ-123-pretreated animals.

CONCLUSIONS

BQ-123 largely prevented the hemodynamic effects of exogenously administered endothelin-1. Our direct in-vivo techniques showed that ET(A) receptors are, at least in part, involved in endothelin-1 -mediated vasoconstriction in the rat kidney, and support the hypothesis that ET(A) receptors may help to control arterial pressure in anesthetized rats.

Endothelin as a regulator of cardiovascular function in health and disease

The endothelins are a family of endothelium-derived peptides that possess characteristically sustained vasoconstrictor properties. Endothelin-1 appears to be the predominant member of the family generated by vascular endothelial cells. In addition to its direct vascular effects, endothelin-1 has inotropic and mitogenic properties, influences homeostasis of salt and water, alters central and peripheral sympathetic activity and stimulates the renin-angiotensin-aldosterone system. Studies with endothelin receptor antagonists have indicated that endothelin-1 probably has complex opposing vascular effects mediated through vascular smooth muscle and endothelial ET(A) and ET(B)receptors. Endogenous generation of endothelin-1 appears to contribute to maintenance of basal vascular tone and blood pressure through activation of vascular smooth muscle ET(A)receptors. At the same time, endogenous endothelin-1 acts through endothelial ET(B) receptors to stimulate formation of nitric oxide tonically and to oppose vasoconstriction. In view of the multiple cardiovascular actions of endothelin-1, there has been much interest in its contribution to the pathophysiology of hypertension. Results of most studies suggest that generation of, or sensitivity to, endothelin-1 is no greater in hypertensive than it is in normotensive subjects. Nonetheless, the deleterious vascular effects of endogenous endothelin-1 may be accentuated by reduced generation of nitric oxide caused by hypertensive endothelial dysfunction. It also appears likely that endothelin participates in the adverse cardiac and vascular remodelling of hypertension, as well as in hypertensive renal damage. Irrespective of whether vascular endothelin activity is increased in hypertension, anti-endothelin agents do produce vasodilatation and lower blood pressure in hypertensive humans. There is more persuasive evidence for increased endothelin-1 activity in secondary forms of hypertension, including pre-eclampsia and renal hypertension. Endothelin-1 also appears to play an important role in pulmonary hypertension, both primary and secondary to diseases such as chronic heart failure. The hypotensive effects of endothelin converting enzyme inhibitors and endothelin receptor antagonists should be useful in the treatment of hypertension and related diseases. Development of such agents will increase knowledge of the physiological and pathological roles of the endothelins, and should generate drugs with novel benefits.

The role of endothelin in hypertension

The endothelins are 21-amino-acid peptides which may play a role in the pathogenesis of hypertension. There is increasing evidence that the endothelins have a central function in mediating end-organ damage in hypertension, and that important effects of endothelin in the pathogenesis of hypertension may be based on the interactions of the endothelins and the renin-angiotensin and the nitric oxide systems.

Hemodynamic effects of a selective endothelin--a receptor antagonist in deoxycorticosterone acetate-salt hypertensive rats

The time-course changes in blood pressure (BP), cardiac output (CO), and total peripheral conductance (TPC) were investigated during a bolus i.v. infusion of BMS-182874, a selective ETA receptor antagonist, in conscious, unrestrained DOCA-salt hypertensive rats and Sham-operated control rats (SHAM). BP was recorded with radiotelemetry devices and CO with ultrasonic transit-time flow probes. In contrast to vehicle-treated controls, BMS-182874 reduced BP and increased TPC in DOCA-salt hypertensive rats but not in SHAM-rats. A small increase in CO tended to oppose the BP-lowering effect of the antagonist. The results suggest that the role of ETA receptors in the maintenance of the hypertensive state in the DOCA-salt model of hypertension is exerted at the level of the resistance vessels and not on factors that regulate CO.

Association of pre-ischaemic disturbances in energy metabolism with postischaemic dysfunction of the rat isolated working heart

1. Metabolic and functional effects of two protocols of preconditioning were compared in rat isolated hearts subjected to 20 min global ischaemia (37 degrees C) and reperfusion (30 min Langendorff + 15 min working). Prior to the ischaemic period, hearts were perfused according to Langendorff (control group) or were preconditioned by three 5 min cycles or two 10 min cycles of ischaemia and reperfusion (PC-I and PC-II groups, respectively). 2. There was no difference in the contractile function between the two preconditioned groups at the onset of sustained ischaemia, although the PC-II group showed enhanced release of adenosine (Ado), inosine, hypoxanthine and xanthine into the interstitium accompanied by losses of tissue adenine nucleotides (sigmaAN = ATP + ADP + AMP), total creatine (sigmaCr = phosphocreatine + creatine) and activation of glycolysis following the preconditioning period. During reperfusion, the PC-I group showed enhanced functional recovery, higher contents of sigmaAN and sigmaCr, and the smallest lactate dehydrogenase release compared with these indices in the control and PC-II groups. Postischaemic myocardial dysfunction was similar in the control and PC-II groups. 3. Functional recovery of hearts in both preconditioned groups was positively correlated with myocardial contents of ATP, sigmaAN and sigmaCr at the end of reperfusion, but not with pre-ischaemic Ado release into the interstitium. The results suggest that pre-ischaemic disturbances of energy metabolism, rather than activation of Ado receptors or stunning, may contribute to efficacy of multiple preconditioning in the rat isolated heart.

Angiotensin-(1-7) contributes to the antihypertensive effects of blockade of the renin-angiotensin system

Angiotensin-converting enzyme (ACE) inhibition alone or in combination with the angiotensin type-I receptor (AT1) antagonist losartan augments circulating levels of the bioactive peptide angiotensin-(1-7) [Ang-(1-7)]. Hence, we determined whether Ang-(1-7) contributes to the hypotensive effects produced by the combined administration of lisinopril and losartan in spontaneously hypertensive rats by blocking the peptide's synthesis with either of two structurally different neprilysin inhibitors. Intravenous administration of CGS 24592 (30 mg/kg) to rats in which blood pressure was normalized by 9 days of therapy with lisinopril and losartan elicited an elevation of mean arterial pressure that was sustained throughout the infusion period and for 20 minutes thereafter. The hypertensive response was associated with a 62% reduction in circulating levels of Ang-(1-7) and no change in plasma angiotensin II (Ang II). Intravenous infusion of one other neprilysin inhibitor (SCH 39370, 30 mg/kg) produced an increase in mean blood pressure of a magnitude similar to that found with CGS 24592. Pretreatment with the nonselective antagonist [Sar1,Thr8]-Ang II abolished any additional pressor effects of either neprilysin inhibitor in spontaneously hypertensive rats treated with lisinopril or losartan. However, neither the endothelin A antagonist BQ123 nor the kinin B2 antagonist HOE 140 had an effect on basal blood pressure or altered the pressor or heart rate effects of the neprilysin inhibitors. These data suggest that inhibition of Ang-(1-7) formation in rats exposed to the combined blockade of Ang II production and activity is associated with a reversal of the antihypertensive actions produced by these therapies. Thus, endogenous Ang-(1-7) functions as a vasodilator hormone in this form of genetic hypertension.

Effect of chronic ETA-selective endothelin receptor antagonism on blood pressure in experimental and genetic hypertension in rats

1. Chronic treatment with a combined ETA/ETB endothelin receptor antagonist has been shown to reduce blood pressure in experimental rat models of hypertension in which endothelin-1 gene overexpression occurs in the walls of blood vessels, particularly small, resistance-sized arteries, but not in those genetic or experimental models of hypertension in which there is no overexpression of vascular endothelin-1. Failure of some experimental models of hypertension to respond to treatment with the combined ETA/ETB endothelin antagonist may be due in part to blockade of vasorelaxant endothelial ETB receptors which could in theory reduce the efficacy of endothelin antagonism. 2. In this study the orally active ETA-selective endothelin antagonists A-127722.5 and LU 135252 were used in chronic experiments on deoxycorticosterone acetate (DOCA)-salt hypertensive rats (which overexpress vascular endothelin-1 and respond with blood pressure lowering to combined ETA/ETB endothelin receptor antagonism), on spontaneously hypertensive rats (SHR) (which do not overexpress vascular endothelin-1 and do not respond with blood pressure lowering to the combined ETA/ETB receptor antagonist), and in 1-kidney 1 clip Goldblatt (1-K IC) hypertensive rats (which present mild overexpression of vascular endothelin-1 but do not respond with blood pressure lowering to the combined ETA/ETB receptor antagonist). Additionally, it has been suggested that interruption of the renin-angiotensin system may sensitize responses to endothelin antagonism. Accordingly, SHR were treated with an angiotensin converting enzyme inhibitor, cilazapril, in addition to the ETA receptor antagonist. 3. Blood pressure of DOCA-salt hypertensive rats was lowered by a mean of 24 and of 27 mmHg (P < 0.01) by A-127722.5 after 4 weeks of treatment, when given orally at two different doses (10 and 30 mg kg-1 day-1), and by 18 mmHg by LU 135252 50 mg kg-1 day-1. 4. SHR treated with A-127722.5 for 8 weeks starting at 12 weeks of age exhibited the same progressive rise in blood pressure as untreated SHR. Addition of cilazapril resulted in similar reduction of blood pressure in A-127722.5-treated and untreated SHR. 5. Treatment of 1-K IC hypertensive rats with the dose of LU 135252 which lowered blood pressure in DOCA-salt hypertensive rats did not cause any reduction in blood pressure relative to untreated rats. 6. These results demonstrate that treatment with either dose of the selective ETA receptor antagonists A-127722.5 or LU 135252 caused reductions in blood pressure similar to those obtained for a combined ETA/ETB endothelin antagonist. Blood pressure was lowered only in hypertensive rats known to overexpress vascular endothelin-1 (DOCA-salt hypertensive rats) but not in those which do not (SHR) or only have mild vascular overexpression of endothelin-1 gene (1-K 1C hypertensive rats). Reduction in activity of the renin-angiotensin system in SHR did not sensitize blood pressure to potential hypotensive effects of an ETA-selective receptor antagonist.

Endothelin-1 transgenic mice develop glomerulosclerosis, interstitial fibrosis, and renal cysts but not hypertension

The human endothelin-1 (ET-1) gene under the control of its natural promoter was transferred into the germline of mice. The transgene was expressed predominantly in the brain, lung, and kidney. Transgene expression was associated with a pathological phenotype manifested by signs such as age-dependent development of renal cysts, interstitial fibrosis of the kidneys, and glomerulosclerosis leading to a progressive decrease in glomerular filtration rate. This pathology developed in spite of only slightly elevated plasma and tissue ET-1 concentrations. Blood pressure was not affected even after the development of an impaired glomerular filtration rate. Therefore, these transgenic lines provide a new blood pressure-independent animal model of ET-1-induced renal pathology leading to renal fibrosis and fatal kidney disease.

Blood pressure and vascular effects of endothelin blockade in chronic nitric oxide-deficient hypertension

Because nitric oxide inhibits the synthesis and vasoconstrictor effect of endothelin-1, the effect of endothelin-1 may be enhanced under conditions of chronic inhibition of nitric oxide synthesis. We studied the effect of chronic therapy with bosentan, a combined endothelin-A/endothelin-B receptor antagonist, on blood pressure and vascular function and structure of small arteries as well as on the reactivity of the aorta in N(omega)-nitro-L-arginine methyl ester (L-NAME)-induced hypertension. Six-week-old Wistar-Kyoto rats were randomly treated for 6 weeks with placebo (control), L-NAME (70 mg/kg per day), or L-NAME plus bosentan (100 mg/kg per day). The treatments were stopped 2 to 3 days before the in vitro experiments so that only the long-term effects of the drugs could be observed. L-NAME increased systolic blood pressure: bosentan did not prevent this effect although the initial blood pressure rise was delayed (P=NS versus L-NAME group). Bosentan administration did not modify the structural alteration of the resistance vessels induced by L-NAME, nor did it improve endothelium-dependent relaxation of resistance vessels or the aorta. However, bosentan therapy markedly increased endothelium-dependent contraction to acetylcholine, which was slightly enhanced by L-NAME. In contrast, bosentan inhibited aortic endothelium-dependent contractions when applied acutely in vitro. This observation, together with the increased maximal vasoconstriction to the thromboxane A2 receptor agonist U46619 after 2 weeks of bosentan administration, suggests that bosentan also interacts with the receptors mediating endothelium-dependent contractions. In conclusion, our experiments suggest a minor role of endothelin in chronic L-NAME-induced hypertension as well as in the concomitant alterations of vascular structure.

Combined inhibition of endothelin and angiotensin II receptors blocks volume load-induced cardiac hormone release

Volume expansion has been shown to increase plasma atrial natriuretic peptide (ANP) levels, but the precise role of paracrine and autocrine factors in stretch-induced cardiac hormone release is not clear. In the present study, we report the effects of endothelin (ET) and angiotensin receptor (AT receptor) antagonists on baseline and atrial stretch-induced immunoreactive ANP (IR-ANP) and immunoreactive N-terminal ANP (IR-NT-ANP) release in vivo by using BQ-123 (ETA receptor antagonist), bosentan (ETA and ETB receptor antagonist), and losartan (AT1 receptor antagonist). Intravenous administration of BQ-123 had no significant effect on baseline hemodynamics in conscious rats, whereas bosentan (10 mg/kg) and losartan (10 mg/kg) decreased slightly (4 to 7 mm Hg, P < .05 to .001) the mean arterial pressure. Both the ETA receptor antagonist BQ-123 and ETA/ETB receptor antagonist bosentan decreased plasma ANP and NT-ANP responses to volume load (P < .05 to .001), whereas the AT1 receptor antagonist losartan had no significant effect on this response. The relative increase in plasma IR-ANP corresponding to a 3 mm Hg increase in right atrial pressure was 2.7-fold in the vehicle-treated group. BQ-123 (0.3 and 1.0 mg/kg) decreased this response 2.5- and 2.1-fold (P < .05); bosentan (3 and 10 mg/kg), 1.7-fold (P < .001) and 1.9-fold (P < .05); and bosentan (10 mg/kg)+losartan (10 mg/kg), 1.6-fold (P < .001). The responses in plasma IR-NT-ANP decreased simultaneously. These results indicate that combined inhibition of ETA/B and AT1 receptors almost completely blocks ANP response to acute volume load. Therefore, our study shows that endogenous paracrine and/or autocrine factors liberated in response to atrial wall stretch rather than myocyte stretch itself are responsible for the activation of ANP peptide secretion in response to acute volume load. Our results also show that ETA receptors are more important in the regulation of mechanical stretch-induced changes in cardiac hormone secretion than AT1 receptors.

Effect of an endothelin antagonist on hemodynamic responses to angiotensin II

We determined changes in blood pressure, cardiac output, and total peripheral conductance evoked by intravenous infusions of angiotensin II (Ang II) in conscious, unrestrained normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) before and after pretreatment with bosentan, a nonselective endothelin antagonist. Blood pressure was recorded by radiotelemetry and cardiac output by ultrasonic transit-time flow probes. Bosentan per se failed to affect basal blood pressure and evoked only small changes in cardiac output and total peripheral conductance in both strains. The pressor effects of Ang II were exaggerated in SHR compared with WKY. Strikingly, bosentan pretreatment blunted the increases in blood pressure, the fall in cardiac output, and the decreases in conductance evoked by lower doses of Ang II but not higher doses of the peptide. This effect was observed in both rat strains but was more pronounced in SHR. These data suggest that endothelin contributes to the hemodynamic effects of Ang II in both SHR and WKY and that endothelin may contribute to the exaggerated pressor responsiveness of SHR to Ang II.

Exogenous endothelin-1 causes renal vasodilation in the fetal lamb

PURPOSE

Endothelin-1 is known to be a potent vasoconstrictor. We investigated the effects and mechanisms of action of endothelin-1 and its receptors in regulating renal vascular tone in the fetal lamb.

MATERIALS AND METHODS

We observed the in vivo effects of endothelin-1, an endothelin-b receptor agonist (4-alanine-endothelin-1), endothelin-a receptor antagonists (BQ-610 and BQ-123), and the inhibition of prostaglandin and nitric oxide synthesis on the response of the renal circulation to endothelin-1 in a chronic preparation in third trimester fetal lambs.

RESULTS

After injection of 250 ng./kg. endothelin-1 into the descending aorta proximal to the renal arteries in 8 fetal animals, renal blood flow increased (4.4 +/- 0.7 ml. per minute per kg., p < 0.001 versus vehicle), as did mean arterial blood pressure (3.0 +/- 0.3 mm. Hg,p < 0.001 versus vehicle). Calculated renal vascular resistance decreased (-1.1 +/- 0.2 mm. Hg per minute per kg./ml., p < 0.001 versus vehicle). After injection of 1,725 ng./kg. 4-alanine-endothelin-1 in 5 animals renal blood flow increased (3.8 +/- 0.4 ml. per minute per kg., p < 0.05 versus vehicle) and mean arterial blood pressure was unchanged (1.6 +/- 1.7 mm. Hg). Calculated renal vascular resistance decreased (-0.8 +/- 0.2 mm. Hg per minute per kg./ml., p < 0.05 versus vehicle). After injection of 0.5 mg./kg. BQ-610 in 6 animals renal blood flow increased (2.3 +/- 0.7 ml. per minute per kg., p < 0.05) and mean arterial blood pressure decreased (-2.7 +/- 0.3 mm. Hg, p < 0.05 versus vehicle). Calculated renal vascular resistance decreased but this difference was not statistically significant (-0.7 +/- 0.3 mm. Hg per minute per kg./ml., p < 0.07). A dose of 1 mg./kg. BQ-123 in 2 animals decreased renal vascular resistance markedly. Infusion of a prostaglandin synthesis inhibitor (1 mg./kg. per minute meclofenamic acid) did not alter the decrease in renal vascular resistance after endothelin-1 (-0.7 +/- 0.4 mm. Hg per minute per kg./ml). In contrast, during infusion of a nitric oxide synthesis inhibitor (1.5 mg./kg. per minute N-omega-nitro-L-arginine) endothelin-1 increased renal vascular resistance (1.2 +/- 0.2 mm. Hg per minute per kg./ml., p < 0.001).

CONCLUSIONS

Endothelin-1 is a vasodilator in the fetal renal circulation, which acts primarily via endothelin-b receptors. Ongoing activity of endothelin-a receptors contributes to renal vascular tone in fetal lambs. The vasodilatory effects of endothelin-1 in the fetal lamb renal circulation are mediated via the nitric oxide system and not via prostanoids.

Systemic endothelin receptor blockade decreases peripheral vascular resistance and blood pressure in humans

BACKGROUND

Although local inhibition of the generation or actions of endothelin-1 has been shown to cause forearm vasodilatation, the systemic effects of endothelin receptor blockade in healthy humans are unknown. We therefore investigated the cardiovascular effects of a potent peptide endothelin ETA/B receptor antagonist, TAK-044, in healthy men.

METHODS AND RESULTS

Two randomized, placebo-controlled, crossover studies were performed. In nine subjects, TAK-044 (10 to 1000 mg IV over a 15-minute period) caused sustained dose-dependent peripheral vasodilatation and hypotension. Four hours after infusion of the highest dose (1000 mg), there were decreases in mean arterial pressure of 18 mm Hg and total peripheral resistance of 665 AU and increases in heart rate of 8 bpm and cardiac index of 0.9 L x min(-1) x m(-2) compared with placebo. TAK-044 caused a rapid, dose-dependent increase in plasma immunoreactive endothelin (from 3.3 to 35.7 pg/mL within 30 minutes after 1000 mg). In a second study in eight subjects, intravenous administration of TAK-044 at doses of 30, 250, and 750 mg also caused peripheral vasodilatation, and all three doses abolished local forearm vasoconstriction to brachial artery infusion of endothelin-1. Brachial artery infusion of TAK-044 caused local forearm vasodilation.

CONCLUSIONS

The endothelin ETA/B receptor antagonist TAK-044 decreases peripheral vascular resistance and, to a lesser extent, blood pressure; increases circulating endothelin concentrations; and blocks forearm vasoconstriction to exogenous endothelin-1. These results suggest that endogenous generation of endothelin-1 plays a fundamental physiological role in maintenance of peripheral vascular tone and blood pressure. The vasodilator properties of endothelin receptor antagonists may prove valuable therapeutically.

Effects of the endothelin ETA-receptor antagonist FR139317 on development of hypertension and cardiovascular hypertrophy in deoxycorticosterone acetate-salt hypertensive rats

We investigated the role of endothelin-1 (ET-1) in the development of hypertension and cardiovascular hypertrophy in deoxycorticosterone acetate (DOCA)-salt hypertensive rats. Two weeks after the start of DOCA-salt treatment, the rats were divided into two groups and were given FR139317 [(R)2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]- carbonyl]amino-4-methyl-pentanoyl]amino-3-[3-(1-methyl-1H-indolyl)] propionyl]amino-3-(2-pyridyl) propionic acid], a specific ETA-receptor antagonist, or its vehicle for 2 weeks. Uninephrectomized rats without DOCA-salt treatment served as controls. Vehicle-treated DOCA-salt rats developed marked hypertension after 4 weeks. FR139317 significantly suppressed the increase in systolic blood pressure with values averaging 163 +/- 8 mmHg (P < 0.05 vs DOCA-salt rats receiving vehicle, 195 +/- 9 mmHg). Morphological studies in the rats given the vehicle showed vascular medial hypertrophy, with a significant increase in the wall area and wall-to-lumen ratio. A marked decrease in vascular wall hypertrophy was observed in the FR139317-treated DOCA-salt rats. The cardiac hypertrophy in DOCA-salt hypertensive rats was also significantly reduced by FR139317. Therefore, these results suggest that ET-1 plays an important role in the development of DOCA-salt hypertension presumably by stimulating the ETA receptor. In addition, we found that an ETA-receptor antagonist effectively reduced cardiovascular hypertrophy in the rats, so the cardiovascular hypertrophy noted in DOCA-salt hypertensive rats may be related to ET-1.

Protective effect of the specific endothelin-1 antagonist BQ610 on mechanical function and energy metabolism during ischemia/reperfusion injury in isolated perfused rat hearts

Endothelin-1 (ET-1) has been suggested to be involved in the pathophysiology of ischemia/reperfusion injury, but direct proof for this is still sparse. We tested whether protection of high-energy phosphate metabolism contributes to the beneficial effects of ETA receptor antagonists during ischemia/reperfusion. In isolated, buffer-perfused rat hearts, isovolumic function was measured by a left ventricular (LV) balloon, and 31P nuclear magnetic resonance spectra were continuously recorded. Two protocols were performed: (a) 15-min control, 30-min total, global ischemia, and 15-min reperfusion; and (b) 15-min control, 15-min total, global ischemia, and 30-min reperfusion. Treatment with BQ610 (1.75 micrograms/min) or saline was started during control and continued throughout the protocol. BQ610 did not affect function or energy metabolism under control conditions. In BQ610-treated hearts subjected to 30-min ischemia, time to ischemic contracture was significantly delayed (treated 10.6 +/- 0.4 min; untreated 8.1 +/- 0.7 min), and end-diastolic pressure (EDP) remained lower (after 30-min ischemia 26 +/- 2 vs. 35 +/- 2 mm Hg). In addition, recovery of mechanical function in BQ610-treated hearts was accelerated during reperfusion. BQ610 did not affect ATP but significantly accelerated and increased creatine phosphate (51 +/- 7 vs. 37 +/- 3%) recovery on reperfusion after 30-min ischemia. BQ610-treated hearts subjected to 15-min ischemia also showed lower EDP during ischemia and accelerated recovery of mechanical function during reperfusion. However, in this case, there were no differences in high-energy phosphate concentrations between treated and untreated hearts. We conclude that the protective action of BQ610 on mechanical function during ischemia/reperfusion injury can be but is not consistently associated with beneficial effects on cardiac high-energy phosphate metabolism.

Changes in collagen metabolism in response to endothelin-1: evidence for fibroblast heterogeneity

Endothelin-1 (Et-1) is a 21-amino acid peptide primarily synthesized by endothelial cells. It was originally classified as a potent vasoconstrictor but recent evidence suggests that it also possesses a wide variety of non-vascular actions. It stimulates fibroblast and smooth muscle cell proliferation and it has been shown to stimulate fibroblast collagen metabolism. However, studies on its ability to regulate collagen production remain incomplete, and its effect on post-translational processing of procollagen has not been studied. This report details the effect of Et-1 on the rates of procollagen synthesis and degradation in two fibroblast cell lines; human foetal lung (HFL-1) and whole foetal rat fibroblasts (Rat 2). Fibroblast cultures were incubated for 24 hr in the presence or absence of Et-1 before procollagen metabolism was determined by measuring hydroxyproline. Non-collagen metabolism was also determined in these cultures from the uptake of tritiated phenylalanine. Et-1 stimulated procollagen synthesis in HFL-1 fibroblasts and reduced synthesis in Rat 2 cells. The response was dose dependent with the greatest effect at 1.10(-6) M Et-1 for both cell types (155 +/- 6% of control (mean +/- SD, n = 6, P < 0.01) and 61 +/- 4% of control (n = 4, P < 0.01) for HFL-1 and Rat 2 fibroblasts, respectively). Non-collagen protein synthesis was increased to 148 +/- 5% of control (P < 0.05) at 1.10(-6) M Et-1. Non-collagen protein synthesis remained unaffected in the HFL-1 fibroblast cultures. Procollagen degradation, expressed as a proportion of total procollagen synthesis, was decreased in HFL-1 fibroblasts (control, 29 +/- 2%; Et-1, 1.10(-6) M; 21 +/- 2%; P < 0.01), and increased in Rat 2 fibroblasts (control 42 +/- 1%; Et-1, 1.10(-6) M; 49 +/- 1%; P < 0.01). Blocking of the EtA receptor for Et-1, using the receptor antagonist-BQ123, abolished the effect of Et-1 on procollagen metabolism in both cell types. These results suggest that different populations of fibroblasts exhibit heterogeneous responses to Et-1. It is concluded that Et-1 may play an important role in the extent and distribution of fibrosis seen in diseases associated with the overproduction of Et-1.

The endothelium of resistance arteries: physiology and role in hypertension

The endothelium plays a very important role in the regulation of vascular function by way of its barrier role, by interaction with circulating cells such as platelets, which may release vasoactive or growth regulating agents, and through production of substances which modulate vascular tone and smooth muscle cell growth, and which may also exert antithrombotic effects. The endothelium of resistance arteries, vessels critically involved in generating resistance to flow and which play an important role in hypertension, has been studied mainly from the point of view of generation of agents which regulate vascular tone and growth. Endothelium-derived relaxing factors such as nitric oxide, prostacyclin, hyperpolarizing factors (EDHF) and possibly C-type natriuretic peptide (CNP), are counteracted by endothelium-derived contracting factors, which include endothelins and contracting factors (EDCF) which are less well characterized and appear to be cyclooxygenase products. In experimental hypertension in animals, and in human essential hypertension, these mechanisms may be altered. There may be a reduced generation of endothelium-derived nitric oxide and enhanced production of EDCF. Some of the mechanisms involved in the role these agents play in the physiology of resistance arteries and pathologically in hypertension will be reviewed.

The endothelin system and its potential as a therapeutic target in cardiovascular disease

Endothelin (ET)-1, an endothelium-derived peptide, is the most potent vasoconstrictor agent described to date. ET-1 also has positive inotropic and chronotropic effects in the heart and is a co-mitogen in both cardiac and vascular myocytes. The major elements of the system involved in formation of ET-1 and its isopeptides, as well as the receptors mediating their effects, have been cloned and characterised. Antagonists of the ET receptors are now available, and selective inhibitors of the ET-converting enzymes are being developed. Early studies using receptor antagonists support the involvement of ET-1 in the pathophysiology of several cardiovascular diseases. The relative merits of ET-converting enzyme inhibitors and receptor antagonists for the treatment of cardiovascular disease are discussed.

Endothelin subtype B receptor-mediated calcium and contractile responses in small arteries of hypertensive rats

Endothelin-1 elicits vasoconstrictor responses through endothelin subtype A receptors, which are located on vascular smooth muscle cells, and vasodilator responses through endothelin subtype B receptors, which occur predominantly on endothelial cells. Endothelin subtype B receptors also may be present on vascular smooth muscle cells, in which they may mediate vasoconstriction. The aims of this study were to determine the presence of vascular smooth muscle vasoconstrictor endothelin subtype B receptors in mesenteric resistance arteries and to assess whether endothelin subtype B receptor-mediated responses differ between spontaneously hypertensive rats and Wistar-Kyoto rats. Contractile responses to the endothelin subtype B receptor agonist sarafotoxin S6c and endothelin-1 were measured simultaneously with [Ca2+]i in endothelium-denuded mesenteric resistance arteries from adult spontaneously hypertensive rats and Wistar-Kyoto rats. To simulate in vivo conditions matched as closely as possible to in vitro conditions, vessels were mounted in a vessel flow chamber in which intraluminal pressure was maintained at 60 mm Hg. Contraction was determined by video imaging to record lumen diameter, and [Ca2+]i was measured by the fura 2 method. Basal [Ca2+]i was significantly higher (P < .01) in hypertensive (170 +/- 4 nmol/L) compared with normotensive rats (134 +/- nmol/L). The endothelin subtype B receptor agonist sarafotoxin S6c increased [Ca2+]i in a concentration-dependent manner. Sarafotoxin S6c-induced [Ca2+]i and contractile responses were significantly lower in hypertensive compared with normotensive rats. These data demonstrate that endothelin subtype B receptors are present in vascular smooth muscle of small arteries and that endothelin subtype B receptor-mediated vasoconstriction occurs through intracellular calcium signaling pathways.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin: potential role in hypertension and vascular hypertrophy

The endothelins are a family of 21-amino acid peptides that are powerful vasoconstrictors. They may also induce vascular hypertrophy. These peptides may participate through these two mechanisms in the pathogenesis of the elevation of blood pressure and/or in the maintenance of hypertension in both experimental animal models and human essential hypertension. This review presents evidence both in favor and against the involvement of endothelins in hypertension. Plasma levels of endothelin-1 are either normal or slightly elevated in experimental and human essential hypertension. Responses of blood vessels to endothelin-1 may be normal or depressed in many models of experimental hypertension and also in essential hypertension in humans. It has recently been demonstrated that endothelin content and mRNA are increased in blood vessels of deoxycorticosterone acetate-salt hypertensive rats. When endothelin receptor antagonists are administered chronically, elevation of blood pressure and development of vascular hypertrophy are blunted in this experimental model of hypertension. In contrast, spontaneously hypertensive rats do not exhibit any increase in either endothelin-1 mRNA or immunoreactive endothelin in blood vessels and fail to respond with lowering of blood pressure to longterm treatment with endothelin receptor antagonists. Blood pressure development in young spontaneously hypertensive rats is also unaffected by long-term administration of endothelin antagonists. Molecular genetic studies appear to support a genetic role of components of the endothelin system in Dahl salt-sensitive rats. In human essential hypertension, there is some evidence of activation of the endothelin system despite depressed responses of small arteries to endothelin-1 and normal circulating levels of endothelin-1 in plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Antihypertensive effects of a novel endothelin-A receptor antagonist in rats

Endothelin is a potent pressor agent mediated primarily through activation of endothelin-A receptors on vascular smooth muscle. Surprisingly, there is no consensus in the literature regarding the role of endothelin itself or endothelin-A receptors in hypertension. The goal of this study was to compare the effects of the novel, selective endothelin-A receptor antagonist BMS-182874 in various models of hypertension. BMS-182874 specifically inhibited the pressor response to endothelin-1 (0.3 nmol/kg IV) in Sprague-Dawley rats in a dose-dependent manner (ED25 = 8 mumol/kg IV) but had no effect on changes in mean arterial pressure brought about by other vasoactive agents. The antihypertensive effects of BMS-182874 were evaluated in conscious deoxycorticosterone acetate (DOCA)--salt hypertensive rats, spontaneously hypertensive rats (SHR), and sodium-deplete SHR. BMS-182874 reduced blood pressure in DOCA--salt hypertensive rats when administered at a dose of 30, 100, or 300 mumol/kg IV. A maximal decrease of approximately 45 mm Hg was observed after treatment with 100 mumol/kg IV. Three days of oral or intravenous treatment with BMS-182874 (100 mumol/kg) elicited a sustained decrease in blood pressure in the DOCA--salt hypertensive rats. In SHR, BMS-182874 decreased blood pressure by approximately 30 mm Hg, but the antihypertensive effects were similar at doses of 75, 150, and 450 mumol/kg PO. In sodium-deplete SHR, BMS-182874 did not significantly reduce blood pressure. In summary, BMS-182874 is a specific, orally active endothelin-A receptor antagonist that is efficacious in mineralocorticoid hypertension in rats but has less effect in sodium-replete and sodium-deplete SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Modeling hemodynamic profiles by telemetry in the rat. A study with A1 and A2a adenosine agonists

The newly developed radiotelemetry system offers a number of advantages for the measurement of blood pressure and heart rate in laboratory animals. However, no available statistical methods permit valid use of the many data gathered with this continuous recording of hemodynamic parameters. This study describes elaboration and testing of mathematical functions as applied to the measurement of the effects of drugs on blood pressure and heart rate in spontaneously hypertensive rats. We used parametric functions analogous to those for pharmacokinetic studies. Curve fitting is in fact the only approach that provides reasonable estimates of hemodynamic kinetic constants. Nonlinear functions were assessed by analyzing telemetric hemodynamic effects induced by three adenosine receptor agonists with different selectivity for the A1 or A2a receptor. After acute administration in conscious rats, the A1 agonist 2-chloro-N6-cyclopentyladenosine induced dose-related hypotension (eg, 0.03 mg/kg; peak, -70 mm Hg; time to peak, 0.34 hour) and bradycardia (eg, 0.03 mg/kg; peak, -186 beats per minute [bpm]; time to peak, 0.38 hour). The A2a agonist 2-hexynyl-5'-N-ethylcarboxamidoadenosine induced dose-related hypotension (eg, 0.003 mg/kg; peak, -36 mm Hg; time to peak, 0.32 hour) with reflex tachycardia (eg, 0.003 mg/kg; peak, 152 bpm; time to peak, 0.35 hour). The nonselective adenosine agonist 5'-N-ethylcarboxamidoadenosine (0.1 mg/kg) induced hypotension (peak, -75 mm Hg; time to peak, 2.2 hours) and bradycardia followed by tachycardia (first peak, -131 bpm; time to peak, 1.26 hours; second peak, 123 bpm; time to peak, 13.9 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of chronic treatment of adult spontaneously hypertensive rats with an endothelin receptor antagonist

We previously showed that endothelin-1 expression was increased in vascular endothelium of deoxycorticosterone acetate-salt hypertensive rats, whereas in spontaneously hypertensive rats (SHR) it is similar to or less than that in normotensive rats. Treatment with the combined endothelin type A/endothelin type B receptor antagonist bosentan moderately reduced blood pressure rise and nearly completely blunted the development of vascular hypertrophy, particularly in small arteries, in the deoxycorticosterone acetate-salt hypertensive model, suggesting a paracrine role for vascular endothelin-1 in the induction of blood vessel hypertrophy in some forms of experimental hypertension. In the present study we examined the effect of chronic oral treatment for 4 weeks of 12-week-old SHR and Wistar-Kyoto rats (WKY) with 100 mg/kg per day bosentan. Blood pressure rose to hypertensive levels similarly in bosentan-treated and untreated SHR; systolic pressure of WKY was also unaffected. The wet weights of the heart, of aortic segments, and of the mesenteric arterial bed were similar in treated and untreated SHR. When coronary, renal arcuate, mesenteric, and femoral small arteries were evaluated on a wire myograph, the media width and media-to-lumen ratio were greater and the lumen diameter was smaller in vessels from SHR relative to those from WKY, except in small arteries from the renal cortex, in which the lumen was not significantly different in both strains. The media cross-sectional area of small arteries fom the four vascular beds was similar in both strains. Identical morphometric parameters were found in the four vascular beds in bosentan-treated and untreated rats of eh strain.(ABSTRACT TRUNCATED AT 250 WORDS)

ETA and ETB receptors mediate contraction to endothelin-1 in renal artery of aging SHR. Effects of FR139317 and bosentan

We characterized vascular endothelin receptors of the renal artery from adult (12 to 16 weeks of age) and old (72 to 76 weeks) spontaneously hypertensive rats (SHR) and age-matched Wistar-Kyoto rats (WKY). Vessels were suspended in organ chambers (37 degrees C, aerated with 95% O2/5% CO2), and isometric tension was recorded. The endothelin-A (ETA) receptor antagonist FR139317, the combined ETA/ETB receptor antagonist bosentan, and the ETB-selective agonist sarafotoxin S6c were used. In old (and less so in adult) SHR, cumulative concentration-contraction curves to endothelin-1 showed a small contraction resistant to FR139317 (10(-5) mol/L) at 3 x 10(-9) to 10(-8) mol/L endothelin-1, which was completely inhibited by bosentan (10(-5) mol/L). This FR139317-resistant contraction to endothelin-1 was not present in WKY. Furthermore, in the presence of FR139317 (10(-5) mol/L), sarafotoxin S6c induced a stronger contraction in old SHR than in WKY (P < .05). In rings contracted with norepinephrine, sarafotoxin S6c caused endothelium-dependent relaxations in both strains; these relaxations were blocked by N omega-nitro-L-arginine methyl ester, indicating that nitric oxide is the mediator. In WKY but not SHR, release of nitric oxide by sarafotoxin S6c increased with age (P < .05). Thus, both ETA and ETB receptors mediate contraction to endothelin-1 in the renal artery from SHR but not WKY. ETB receptors on vascular smooth muscle seem to be unmasked with age in SHR, whereas those on endothelium (mediating nitric oxide release) exhibit more efficient responses with age in WKY.

Antihypertensive actions of the novel nonpeptide endothelin receptor antagonist SB 209670

Indirect evidence has implicated endothelin-1 in the pathogenesis of hypertension. In the present study we examined such a role directly with SB 209670, a novel nonpeptide endothelin receptor antagonist. The antihypertensive and hemodynamic effects of SB 209670 were examined in conscious, unrestrained spontaneously hypertensive (SHR), normotensive Wistar-Kyoto (WKY), and renin-hypertensive rats. Sustained intravenous infusion of SB 209670 (10 micrograms.kg-1.min-1 for 6 hours) produced a significant, reversible reduction in mean arterial pressure in SHR but not in WKY rats. The antihypertensive response to 10 micrograms.kg-1.min-1 SB 209670 (approximately 25 mm Hg reduction in blood pressure) was associated with bradycardia (16% decrease in heart rate) but only a minimal reduction (3%) in cardiac output, because stroke volume was evaluated (by 15%). Therefore, the antihypertensive effect of SB 209670 resulted from a decrease (13%) in total peripheral resistance. A sustained antihypertensive effect could also be observed after intraduodenal administration of SB 209670 (3 mg/kg) in conscious SHR (reduction of approximately 35 mm Hg 5 hours after administration). SB 209670 (3 mg/kg intravenous bolus) did not alter the pressor response or tachycardia observed in pithed SHR after stimulation of thoracolumbar sympathetic outflow. SB 209670 was also antihypertensive in renin-hypertensive rats, lowering blood pressure to an extent similar to that observed in SHR. Thus, the data presented provide evidence to support a role for endothelin-1 in the pathophysiology of two animal models of hypertension.

Role of endothelin-1 and the ETA receptor in the maintenance of deoxycorticosterone acetate-salt-induced hypertension

1. To search for a possible role for endothelin-1 (ET-1) in deoxycorticosterone acetate (DOCA)-salt-induced hypertension, we examined changes in concentration of ET-1 in vascular and renal tissue in DOCA-salt hypertensive rats and evaluated the antihypertensive effect of the ETA receptor antagonist, FR139317. 2. There was an increase in aortic immunoreactive-ET (IR-ET) concentrations in association with hypertension-induced treatment. There were no significant changes in ET-1 levels in the kidney with DOCA-salt treatment. 3. In DOCA-salt hypertensive rats, a significant correlation (r = 0.83, P < 0.01) was found between aortic IR-ET concentrations and systolic blood pressure. 4. High-performance liquid chromatography analysis of the aortic extract from DOCA-salt rats revealed one major component corresponding to the elution position of synthetic ET-1. 5. The intravenous bolus injection of FR139317 (10 mg kg-1) produced a slight decrease in blood pressure in the control rats and in the DOCA-salt hypertensive rat, FR139317 had a more pronounced hypotensive effect. 6. We propose that ET-1 production in vascular tissues is increased in DOCA-salt hypertensive rats. In addition, our study indicates the pathophysiological importance of increased endogenous ET-1 in the maintenance of DOCA-salt-induced hypertension, through interaction of the peptide with ETA receptors.

Endothelin ETA receptor antagonist, BQ-123, blocks the vasoconstriction induced by sarafotoxin 6b in the heart but not in other vascular beds

1. The cardiovascular effects of SRT6b in control and BQ-123, a specific ETA receptor antagonist, pretreated rats were determined in anesthetized rats using a radioactive microsphere technique. 2. Infusion of SRT6b produced an increase in blood pressure and total peripheral resistance, decrease in cardiac output and stroke volume, and no change in heart rate of control or BQ-123 treated rats. 3. SRT6b induced a decrease in blood flow to the heart which was completely blocked by BQ-123 pretreatment. The decrease in blood flow to other organs by SRT6b was not affected by BQ-123 pretreatment. 4. This study indicates that ET receptors in the coronary blood vessels are of a different type (neither ETA nor ETB) to those in other vascular beds.

Effect of a nonselective endothelin antagonist on vascular remodeling in deoxycorticosterone acetate-salt hypertensive rats. Evidence for a role of endothelin in vascular hypertrophy

We have previously shown that the endothelin content in arteries of deoxycorticosterone acetate (DOCA)-salt hypertensive rats is increased. We designed this study to examine, using the new orally active nonselective endothelin receptor antagonist bosentan, whether this increase in vascular endothelin may contribute to elevated blood pressure and vascular hypertrophy in DOCA-salt hypertensive rats. Rats received bosentan (100 mg/kg body wt per day) for 3 weeks mixed with their food. Systolic blood pressure of DOCA-salt hypertensive rats rose to 197 +/- 5 mm Hg, and that of bosentan-treated DOCA-salt hypertensive rats was 177 +/- 4 mm Hg (P < .01). Mesenteric resistance arteries were studied on a wire myograph. The media width, ratio of media width to lumen diameter, and cross-sectional area of the media of resistance arteries of bosentan-treated DOCA-salt hypertensive rats were significantly smaller than those of untreated DOCA-salt hypertensive rats. The lumen diameter and cross-sectional area of the media of vessels of bosentan-treated rats were not different from those of uninephrectomized control rats. Vasoconstrictor responses, which were altered in DOCA-salt hypertensive rats, approached control in the bosentan-treated rats. We conclude that these results with a nonselective endothelin receptor antagonist may suggest a role for endothelin in the elevation of blood pressure and vascular hypertrophy and remodeling in DOCA-salt hypertensive rats.

Reversal of established responses to endothelin-1 in vivo and in vitro by the endothelin receptor antagonists, BQ-123 and PD 145065

1. Endothelin-1 binds almost irreversibly to its receptors and causes prolonged vasoconstrictions. Here we have studied the reversal of established responses to ET-1 in vivo and in vitro by BQ-123, an ETA receptor-selective antagonist, and/or PD 145065, an ETA/ETB receptor non-selective antagonist. 2. In anaesthetized rats pretreated with hexamethonium, infusion of ET-1 (10(-11) mol kg-1 min-1) increased the mean arterial pressure (MAP) from 93 +/- 1.5 mmHg to 137 +/- 2.4 mmHg after 70 min (n = 29). While the ET-1 infusion was continued an additional infusion of BQ-123 caused a gradual dose-dependent reduction in the pressor effect of ET-1. For instance, after a 60 min infusion of BQ-123 (10(-8) mol kg-1 min-1) the MAP was decreased by 29.3 +/- 4.3 mmHg (n = 4). 3. PD 145065 was a much weaker antagonist of the established pressor effects of ET-1. At 10(-8) mol kg-1 min-1 it had no significant effect and even at 10(-7) mol kg-1 min-1 the elevated blood pressure was only reduced by 11.8 +/- 8.0 mmHg (n = 5) after 60 min. Co-infusion of BQ-123 and PD 145065 caused smaller reductions in the established response to ET-1 than infusion of BQ-123 alone. 4. Sustained contractions of rat aortic rings induced by ET-1 (3 x 10(-9) M) and mediated by ETA receptors were slowly reversed by addition of BQ-123 (10(-5) M) or PD 145065 (10(-5) M). For instance,after 40 min the elevated tone was reduced 85.8 +/- 5.6% (n = 6) by PD 145065, and 77.1 +/- 6.7% (n = 6)by BQ-123. Thus, on the rat aortic rings in vitro both antagonists were equally effective against established responses to ET-1.5. ET-1 increased the perfusion pressure of the rat isolated perfused kidney by 138.1 +/- 7.6 mmHg(n = 14). Subsequent co-infusion of BQ-123 or PD 145065 reversed this increase with PD 145065 being more active. For instance, PD 145065 (10-6 M) reversed the increase in perfusion pressure by 56.9 +/- 8.8% (n = 5) and BQ-123 (10-6 M) reversed it by 22.8 +/- 8.0% (n = 5). This fits well with the vasoconstriction induced by ET-1 in the rat kidney being mediated by ETA and ETB receptors.6. Thus, sustained vasoconstrictions to ET-1 in vitro, mediated by either ETA or ETB receptors, may be reversed slowly by the subsequent application of receptor antagonists. Similarly, endothelin antagonists reverse the pressor effects of ET-1 in vivo although co-antagonism of ETA and ETB receptors or the co-administration of an ETA receptor antagonist, BQ-123, and a mixed antagonist, PD 145065 produces less reversal than the application of an ETA receptor-selective antagonist. This may be because PD 145065 also reduces vasodilatations induced by ET-1 in vivo, or could suggest that because of its peptide structure PD 145065 affects the elimination of ET-1.