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

The Causal Relationship between Endothelin-1 and Hypertension: Focusing on Endothelial Dysfunction, Arterial Stiffness, Vascular Remodeling, and Blood Pressure Regulation

Hypertension (HTN) is one of the most prevalent diseases worldwide and is among the most important risk factors for cardiovascular and cerebrovascular complications. It is currently thought to be the result of disturbances in a number of neural, renal, hormonal, and vascular mechanisms regulating blood pressure (BP), so crucial importance is given to the imbalance of a number of vasoactive factors produced by the endothelium. Decreased nitric oxide production and increased production of endothelin-1 (ET-1) in the vascular wall may promote oxidative stress and low-grade inflammation, with the development of endothelial dysfunction (ED) and increased vasoconstrictor activity. Increased ET-1 production can contribute to arterial aging and the development of atherosclerotic changes, which are associated with increased arterial stiffness and manifestation of isolated systolic HTN. In addition, ET-1 is involved in the complex regulation of BP through synergistic interactions with angiotensin II, regulates the production of catecholamines and sympathetic activity, affects renal hemodynamics and water–salt balance, and regulates baroreceptor activity and myocardial contractility. This review focuses on the relationship between ET-1 and HTN and in particular on the key role of ET-1 in the pathogenesis of ED, arterial structural changes, and impaired vascular regulation of BP. The information presented includes basic concepts on the role of ET-1 in the pathogenesis of HTN without going into detailed analyses, which allows it to be used by a wide range of specialists. Also, the main pathological processes and mechanisms are richly illustrated for better understanding.

Endothelin B receptor dysfunction mediates elevated myogenic tone in cerebral arteries from aged male Fischer 344 rats

The human brain requires adequate cerebral blood flow to meet the high demand for nutrients and to clear waste products. With age, there is a chronic reduction in cerebral blood flow in small resistance arteries that can eventually limit proper brain function. The endothelin system is a key mediator in the regulation of cerebral blood flow, but the contributions of its constituent receptors in the endothelial and vascular smooth muscle layers of cerebral arteries have not been well defined in the context of aging. We isolated posterior cerebral arteries from young and aged Fischer 344 rats, as well as ET_B receptor knock-out rats and mounted the vessels in plexiglass pressure myograph chambers to measure myogenic tone in response to increasing pressure and targeted pharmacological treatments. We used an ET_A receptor antagonist (BQ-123), an ET_B receptor antagonist (BQ-788), endothelin-1, an endothelin-1 synthesis inhibitor (phosphoramidon), and vessel denudation to dissect the roles of each receptor in aging vasculature. Aged rats exhibited a higher myogenic tone than young rats, and the tone was sensitive to the ET_A antagonist, BQ-123, but insensitive to the ET_B antagonist, BQ-788. By contrast, the tone in the vessels from young rats was raised by BQ-788 but unaffected by BQ-123. When the endothelial layer that is normally enriched with ET_B1 receptors was removed from young vessels, myogenic tone increased. However, denudation of the endothelial layer did not influence vessels from aged animals. This indicated that endothelial ET_B1 receptors were not functional in the vessels from aged rats. There was also an increase in ET_A receptor expression with age, whereas ET_B receptor expression remained constant between young and aged animals. These results demonstrate that in young vessels, ET_B1 receptors maintain a lower myogenic tone, but in aged vessels, a loss of ET_B receptor activity allows ET_A receptors in vascular smooth muscle cells to raise myogenic tone. Our findings have potentially important clinical implications for treatments to improve cerebral perfusion in older adults with diseases characterized by reduced cerebral blood flow.

Effects of equol on deoxycorticosterone acetate salt-induced hypertension and associated vascular dementia in rats

Oxidative stress is the major cause of neuronal cell degeneration observed in neurodegenerative diseases including vascular dementia (VaD), and hypertension has been found to increase the probability of VaD. Here, we investigated the effects of equol in deoxycorticosterone acetate (DOCA)-salt-induced hypertensive rats (DHRs) and the associated VaD. The systolic blood pressure of rats treated with low- (10 mg per kg body weight) and high-dose (20 mg per kg body weight) equol for 4 weeks was lower than that of the control group by 12.18 and 17.48% in a dose-dependent manner, respectively (p < 0.05), which was regulated by inhibiting angiotensin-converting enzyme (ACE) activity and increasing the nitric oxide (NO) production. Equol-treated DHRs showed a significant decrease in both the swimming distance and time required to reach the escape platform (78.20 to 82.56%, p < 0.05). In addition, the probe trial session and working memory test indicated that equol improved the long- and short-term memory of the rats. Moreover, the brain antioxidant activity was increased by elevating the activities of superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) levels, and the malondialdehyde (MDA) content and acetylcholinesterase (AChE) activity were decreased, indicating that equol suppressed oxidative stress. In conclusion, we demonstrated that equol exhibited comparable blood pressure (BP)-lowering and VaD-improving effects with the clinically used drug, lisinopril in DHRs while there was a positive correlation between the doses. Therefore, this bioactive compound may be useful for developing functional foods, thereby extending the application of equol-containing crops.

Endothelin in the female vasculature: a role in aging?

Cardiovascular diseases are the leading cause of morbidity and mortality in the world. Aging is associated with an increased incidence of cardiovascular disease. Premenopausal women are relatively protected from vascular alterations compared with age-matched men, likely due to higher levels of the female sex hormones. However, these vasoprotective effects in women are attenuated after menopause. Thus, the vascular system in aging women is affected by both the aging process as well as loss of hormonal protection, positioning women of this age group at a high risk for cardiovascular diseases such as hypertension, myocardial infarction, and stroke. The endothelin system in general and endothelin-1 (ET-1) in particular plays an important role in the pathogenesis of vascular dysfunction associated with aging. Evidence suggests that the female sex steroids can interfere with the vascular expression and actions of ET-1 via several mechanisms, which may further contribute to pathological processes in the vasculature of aging women. In this review, we have summarized hormone-dependent vascular pathways whereby ET-1 may mediate the deleterious effects of aging in postmenopausal females.

Endothelin receptor antagonists as antihypertensives: the next frontier

The endothelin system is a pivotal player along the continuum of cardiovascular disease. There is convincing evidence that the system not only exerts a potent pressor effect but also promotes end-organ damage independent from blood pressure changes. The role of endothelin receptor antagonists (ERAs) in the treatment of hypertension is rapidly evolving. Recent studies demonstrate a formidable antihypertensive effect. Utility of ERAs is likely to be greatest in patients with resistant hypertension. Beyond blood pressure lowering, ERAs exert several properties that may confer additional protection, including effects on endothelial function, atherosclerosis, arterial stiffening, renal function and proteinuria. The full potential of this class will only be realized when the results of ongoing and future studies in hypertension, heart failure and other forms of cardiovascular disease are completed.

Drug discovery for overcoming chronic kidney disease (CKD): the endothelin ET B receptor/nitric oxide system functions as a protective factor in CKD

Accelerated cardiovascular disease (CVD) is a frequent complication of renal disease. Chronic kidney disease (CKD) develops hypertension and dyslipidemia, which in turn can contribute to the progression of renal failure. There is general agreement that endothelin-1 (ET-1), which acts through the two subtypes of receptor ET(A) and ET(B), plays important physiological roles in the regulation of normal cardiovascular function and that excessive ET-1 production is linked to CVD and CKD. Although selective ET(A) or nonselective ET(A)/ET(B) receptor antagonisms have been recognized as a potential strategy for treatment of several cardiovascular disease, it remains unclear which of the antagonisms is suitable for the individuals with CKD because upregulation of the nitric oxide (NO) system via ET(B) receptor is responsible for renal function such as natriuresis, diuresis, and glomerular hemodynamics. Our findings clearly indicate that the blockade of ET receptors, in particular ET(A)-receptor antagonism, not only produces a potential renoprotective effect in CKD but also reduces the risk of CVD. In contrast, pharmacological blockade or genetic deficiency of ET(B) receptor seems to aggravate CKD and CVD in several experimental models of rats. Moreover, preliminary evidence in patients with CKD also suggests that both selective ET(A)- and nonselective ET(A)/ET(B)-receptor blockade decreases blood pressure but that selective ET(A) blockade has additional desirable effects on renal hemodynamics. Thus, at least in CKD, these findings support the notion that ET(B) receptor-mediated actions produce a renoprotective effect and that nonselective ET(A)/ET(B)-receptors blockade seem to offer no advantage over selective ET(A) antagonism, and if anything may potentially reduce the benefits.

Efficacy and safety of darusentan: a novel endothelin receptor antagonist

OBJECTIVE

To summarize the role of the endothelin system (ETS) in cardiovascular disease (CVD) and evaluate the potential usefulness of darusentan, a selective endothelin type A (ET(A)) receptor antagonist, in the treatment of hypertension and chronic heart failure (CHF).

DATA SOURCES

A literature search was conducted in MEDLINE (1966-February 2008), International Pharmaceutical Abstracts (1970-February 2008), and EMBASE (1990-February 2008) using the search terms endothelin, darusentan, LU 135252, hypertension, and heart failure.

STUDY SELECTION AND DATA EXTRACTION

Studies evaluating the role of the ETS in CVD and the pharmacology, pharmacokinetics, safety, and efficacy of darusentan for the treatment of hypertension and CHF were included.

DATA SYNTHESIS

Darusentan represents a novel treatment strategy for patients with resistant hypertension. Its safety and efficacy have been evaluated in the treatment of hypertension and CHF. Darusentan selectively blocks the ET(A) receptor, promoting vasodilatation and preventing several proliferative and inflammatory processes, while promoting the actions of the ET(B) receptor. Studies in patients with stage 2 or resistant hypertension concluded that darusentan safely and effectively lowers blood pressure. Darusentan's unique mechanism of action, dose-dependent blood pressure-lowering profile, once-daily dosing regimen, and sustained 24-hour blood pressure-lowering effect are valuable features. Darusentan is well tolerated, with only peripheral edema, headache, and nasal symptoms being reported more frequently than with placebo. Endothelin receptor antagonists, including darusentan, have been associated with a decrease in hemoglobin and hematocrit and are teratogens. Darusentan does not appear to cause hepatotoxicity. Additional studies in CHF are warranted to assess the safety and efficacy of darusentan, especially given its association with peripheral edema and decreased red blood cell count.

CONCLUSIONS

Given the important role of the ETS in hypertension and available data with darusentan, selective antagonism of the ET(A) receptor represents a promising approach to managing resistant hypertension. Darusentan's role will be more clearly elucidated by ongoing Phase 3 studies.

The future of endothelin-receptor antagonism as treatment for systemic hypertension

Endothelin (ET) is an endogenous peptide secreted predominantly by endothelial cells that mediates its effects via vasoconstriction and hypertrophy of vascular smooth muscle. Because the role of ET has been described in multiple pathologic processes in cardiovascular disease, including hypertension, there has been a strong interest in the development of therapeutic agents that inhibit ET receptors. ET receptor antagonists have shown much promise in disease states such as pulmonary arterial hypertension, essential hypertension, and various forms of secondary hypertension. This review serves to summarize the current role of ET and ET receptor antagonists in both the pathophysiology and the treatment of hypertension.

The role of endothelin receptor antagonists in cardiovascular pharmacotherapy

Endothelin (ET) is a hormone produced predominantly by endothelial cells which has been recognised to play a significant role in the development of several cardiovascular disease states. In order to combat the deleterious effects of ET, several ET-receptor antagonists (ETRA) are currently in clinical development. The agents developed thus far inhibit the actions of ET through either selective inhibition of the ET(A) receptors or non-selective inhibition of both ET(A) and ET(B) receptors. However, due to the differing proportions of the two receptor subtypes in various tissues, animal models and pathologies, it remains a matter of debate whether receptor selective agents impart significant clinical benefits over non-selective agents. This paper seeks to briefly summarise the important preclinical and clinical effects that have been reported in the literature and will attempt to provide a rationale for the use of both types of ETRAs in the treatment of both systemic and pulmonary hypertension as well as chronic heart failure (CHF).

Arterial Pressure Response to the Antioxidant Tempol and ET B Receptor Blockade in Rats on a High-Salt Diet

We hypothesized that increased superoxide contributes to mean arterial pressure (MAP) regulation in male Sprague-Dawley rats fed a high-salt diet and/or during endothelin (ET B ) receptor blockade. Four groups on either a normal- or a high-salt diet were studied for 1 week: (1) control; (2) tempol, a superoxide dismutase mimetic, in their drinking water (1 mmol/L); (3) A-192621, an ET B antagonist, in their food (10 mg/kg daily); or (4) both tempol and A-192621. Without ET B blockade, tempol had no effect on MAP (telemetry) in rats on the normal-salt diet but significantly reduced MAP in rats on the high-salt diet (100±3 vs 112±2 mm Hg, P <0.05). On the normal-salt diet, A-192621 increased MAP with or without tempol. Under high-salt conditions, tempol attenuated the increase in MAP produced by A-192621, but only during the initial days of treatment. Plasma 8-isoprostanes were increased in all rats on the high-salt diet and were further increased after 3 days of A-192621 but not after 7 days; tempol inhibited the increase produced by A-192621 but had no influence on the increase produced by high salt. H 2 O 2 excretion was significantly higher in rats on a high-salt diet for the 7-day drug treatment compared with those on a normal-salt diet. Tempol further increased H 2 O 2 excretion in rats on a high-salt diet, an effect accelerated in A-192621–treated rats. These data suggest that blood pressure lowering by tempol in rats on a high-salt diet may be unrelated to reductions in superoxide and that renal H 2 O 2 may account for the limited ability of tempol to attenuate hypertension produced by ET B receptor blockade.

Role of endothelin in mediating postmenopausal hypertension in a rat model

Cardiovascular disease is the leading cause of death in women after menopause. Hypertension, a major cardiovascular risk factor, becomes more prevalent after menopause. The mechanisms responsible for the increase in blood pressure (BP) in postmenopausal women are unknown. We have recently characterized the aged, postestrous-cycling (PMR) spontaneously hypertensive rats (SHR) as a model of postmenopausal hypertension. The purpose of the present study was to determine whether endothelin plays a role in the increased BP in PMR. Premenopausal female SHR, aged 4-5 mo (YF), and PMR, aged 16 mo, were studied. Expression of preproendothelin-1 mRNA was not different in either renal cortex or medulla between PMR and YF (n = 7-8/group). In contrast, ET-1 peptide expression was significantly higher in renal cortex of PMR than in renal cortex of YF, but there was no difference in medullary ET-1. Expression of endothelin ET(A) receptor (ET(A)R) mRNA was lower in renal cortex and medulla of PMR than of YF. Additional groups of rats (n = 6-7/group) were treated for 3 wk with the ET(A)R antagonist ABT-627 (5 mg.kg(-1).day(-1)). BP was significantly higher in PMR than in YF. ET(A)R antagonist reduced BP in PMR by 20% to the level found in control YF. ET(A)R antagonist had no effect on BP in YF. These data support the hypothesis that the increase in BP in PMR is mediated in part by endothelin and the ET(A)R.

Novel mechanisms responsible for postmenopausal hypertension

Blood pressure increases in many women after menopause. Hypertension is one of the major risk factors for cardiovascular disease. However, the mechanisms responsible for the postmenopausal increase in blood pressure are yet to be elucidated. Various humoral systems have been proposed to play a role in postmenopausal hypertension, such as changes in estrogen/androgen ratios, increases in endothelin and oxidative stress, and activation of the renin-angiotensin system (RAS). In addition, obesity, type II diabetes, and activation of the sympathetic nervous system are common in postmenopausal women and may also play important roles. However, progress in elucidating the mechanisms responsible for postmenopausal hypertension has been hampered by the lack of a suitable animal model. The aging female spontaneously hypertensive rat (SHR) exhibits many of the characteristics found in postmenopausal women. In this review, some of the possible mechanisms that could play a role in postmenopausal hypertension are discussed, as well as the characteristics of the aged female SHR as a model to study.

Role of endothelins in animal models of hypertension: focus on cardiovascular protection

Investigation of the regulation of vascular function by endothelium-derived factors has been a prominent topic of research in the field of hypertension during the last decade. Of the different endothelial factors, endothelins, which play an important role in vasodilatation-vasoconstriction balance, have been the subject of great interest and an impressive number of publications. This peptide, a very potent vasoconstrictor, triggers as well events involved in growth, proliferation, matrix production and local inflammation. In parallel, its role in hypertension has evolved from a simple vasoconstrictor to a central local regulator of vascular homeostasis contributing not only to the elevation of blood pressure, but also to the complications of hypertension. This review summarizes research on endothelins and its receptor antagonists in experimental hypertension, with special emphasis on vascular remodeling and target-organ protection.

Left but not right cardiac hypertrophy in atrial natriuretic peptide receptor-deficient mice is prevented by angiotensin type 1 receptor antagonist losartan

Mice with a genetic deletion of the atrial natriuretic peptide (ANP) receptor, guanylyl cyclase A (GC-A -/-), have chronic arterial hypertension and cardiac hypertrophy from the first day of life. To characterize the role of the angiotensin II and endothelin systems in the development of this cardiovascular phenotype, the effects of chronic treatment with either the angiotensin type I (AT1) receptor antagonist losartan or the endothelin A receptor antagonist BSF208075 were tested. Losartan almost completely reversed systemic arterial hypertension and left ventricular hypertrophy of GC-A -/- mice. This was accompanied by a marked regression of the left ventricular mRNA expression of cardiac hypertrophy markers such as ANP and brain natriuretic peptide and a significant reduction of left ventricular and pulmonary interstitial collagen accumulation. BSF208075 had no effect on any of these cardiovascular parameters. Intriguingly, GC-A -/- mice also showed a very marked right ventricular hypertrophy, which was not reversed by losartan or BSF208075 treatment. Analyses of components of the renin-angiotensin system (RAS) revealed an inhibition of renal and systemic RAS contrasting with increased local left ventricular angiotensin II levels in GC-A -/- mice. Collectively, the results suggest that RAS plays a more important role than the endothelin system in the pathogenesis of arterial hypertension as well as left ventricular hypertrophy and fibrosis in GC-A gene-disrupted mice.

Endothelin A receptor blockade decreases expression of growth factors and collagen and improves matrix metalloproteinase-2 activity in kidneys from stroke-prone spontaneously hypertensive rats

This study hypothesizes that endothelin-1 induces renal damage by increasing expression of growth/inflammatory factors, important in renal fibrosis. Male stroke-prone spontaneously hypertensive rats (SHRSPs) (8-weeks, n = 24) were randomized into three groups: control group, high-salt group (4% NaCl), and salt plus an endothelin A receptor antagonist, BMS 182874 (40 mg/kg/d). After 20 weeks treatment, rats were killed. Messenger RNA (mRNA) expression of renal preproendothelin-1, endothelin A and B receptors, and procollagen I and III was evaluated by reverse transcription polymerase chain reaction. Expression of transforming growth factor (TGF)-beta1 and basic fibroblast growth factor (bFGF) was determined by immunoblotting. Matrix metalloproteinase-2 (MMP-2) activity was measured by zymography. In salt-loaded SHRSPs, preproendothelin-1 mRNA expression was increased 1.6-fold, and endothelin A receptor mRNA expression was decreased (70% of control). Salt-loaded SHRSPs had increased renal expression of TGF-b1 and procollagens. MMP-2 activity was augmented fivefold. BMS decreased (p < 0.01) expression of TGF-beta1, bFGF, and procollagen I and reduced MMP-2 activity. Thus severe hypertension and renal dysfunction in salt-loaded SHRSPs are associated with increased expression of renal endothelin-1, growth factors, and collagen. BMS treatment alleviated these effects, suggesting that nephroprotection by endothelin A receptor blockade is mediated by normalizing expression of growth factors, reducing extracellular matrix deposition, and decreasing MMP activity.

Function and regulation of endothelin-1 and its receptors in salt sensitive hypertension induced by sensory nerve degeneration

To determine the role of endothelin-1 (ET-1) and its receptors in salt-sensitive hypertension induced by sensory nerve degeneration, selective ET(A) antagonist (ABT-627) and ET(B) antagonist (A-192621) were used. Newborn Wistar rats were given vehicle or 50 mg/kg capsaicin subcutaneously on the first and second days of life. After the weaning period, male rats were divided into eight groups, and subjected to the following treatments for 2 weeks: control + normal salt diet (Con+NS, 0.5%), control + high salt diet (Con+HS, 4%), control + high salt diet + ABT-627 (Con+HS+ABT-627), control + high salt diet + A-192621 (Con+HS+A-192621), capsaicin + normal salt diet (Cap+NS), capsaicin + high salt diet (Cap+HS), capsaicin + high salt diet + ABT-627 (Cap+HS+ABT-627), capsaicin + high salt diet + A-192621 (Cap+HS+A-192621). Both ABT-627 (5 mg/kg/d) and A-192621 (30 mg/kg/d) were given by oral gavage twice a day. Mean arterial pressure (MAP, mm Hg) was higher in Con+HS+A-192621 (141 +/-11) than in Con+NS (94 +/- 10), Con+HS (95 +/- 5), and Con+HS+ABT-627 (97 +/- 6) (P<0.05). MAP was also higher in Cap+HS (152 +/- 6) and Cap+HS+A-192621 (180 +/- 7) than in Cap+NS (99 +/- 3) and Cap+HS+ABT-627 (104 +/- 5) (P<0.05), and it was higher in Cap+HS+A-192621 than in Cap+HS (P<0.05). Enzyme immunometric assay showed that ET-1 plasma concentration (pg/mL) was higher in Con+HS+A-192621 (7.59 +/- 0.78) than in Con+NS (2.68 +/- 0.56), Con+HS (2.50 +/- 0.92), and Con+HS+ABT-627 (3.54 +/- 0.79) (P<0.05). ET-1 plasma concentration was also higher in Cap+HS (8.95 +/-.16), Cap+HS+ABT-627 (9.82 +/- 1.22) and Cap+HS+A-192621 (10.97 +/- 0.57) than in Cap+NS (3.06 +/- 0.73) (P<0.05). We conclude that blockade of the ET(A) receptor prevents the development of salt sensitive hypertension induced by sensory nerve degeneration, indicating that activation of the ET(A) receptor by increased plasma ET-1 level contributes to elevation of blood pressure in this model. In contrast, blockade of the ET(B) receptor leads to an increase in blood pressure in both normal and sensory nerve degenerated rats fed a high salt diet. These results suggest that ET(B) plays an antihypertensive role in response to high salt intake under both normal and sensory nerve degenerated conditions.

Chronic exposure to carbon monoxide and nicotine: endothelin ET(A) receptor antagonism attenuates carbon monoxide-induced myocardial hypertrophy in rat

The aims of the present study were to determine the effects of endothelin ET(A) receptor antagonism on carbon monoxide (CO)-induced cardiac hypertrophy and endothelin-1 (ET-1) expression and to compare myocardial effects of chronic nicotine with CO exposure. Female Sprague-Dawley rats (n = 84) were randomized to three groups exposed 20 h/day to CO (200 ppm), nicotine (500 microg/m3), or air for 14 consecutive days. In each exposure group, animals were randomized to ET(A) receptor antagonist LU 135252 in drinking water (0.5 mg/ml) or placebo. Myocardial ET-1 and atrial natriuretic peptide (ANP) expression was measured by competitive RT-PCR and plasma ET-1 by immunoassay. Carboxyhemoglobin was 22.1 +/- 0.3% in CO-exposed animals and 2.8 +/- 0.3% in controls. Plasma nicotine was 57 +/- 7 ng/ml and plasma cotinine was 590 +/- 23 ng/ml in nicotine-exposed animals and below detection levels in controls. CO exposure induced a 21% increase in right ventricular hypertrophy (p < 0.01), a 7% increase in left ventricular hypertrophy (p < 0.01), a 25% increase in right ventricular ET-1 expression (p < 0.05), and an eightfold increase in ANP expression (p = 0.08). ET(A) receptor antagonism reduced right ventricular hypertrophy by 60% (p < 0.05) with no significant effect on left ventricular hypertrophy or myocardial ET-1 expression. Chronic nicotine exposure did not significantly affect cardiac weights or ANP and ET-1 expression. We conclude that ET(A) receptor antagonism reduces right ventricular hypertrophy induced by chronic CO exposure, whereas CO-induced myocardial ET-1 expression remains unchanged.

An update on the status of endothelin receptor antagonists for hypertension

Endothelin receptor antagonists (ETRA) are actively developed by the pharmaceutical industry for several cardiovascular indications. In the context of hypertension, preclinical studies are increasingly focused on prevention or regression of end-organ damage and drug combination than on control of arterial pressure in monotherapy, as most experimental models have already been studied. In general, the antihypertensive effect of ETRA is limited but the overwhelming efficacy of this class of drugs to prevent several end-organ damages warrants judicious combination. However, the few studies looking at regression of hypertension-induced cardiovascular alterations proved less successful, suggesting that ETRA should be used early in the treatment of hypertension to obtain full benefit. Judging from the progression of ongoing trials and the development of new trials patients suffering from pulmonary hypertension and heart failure may be the first to benefit from this new class of drugs. However, it is expected that once on the market, responsive subsets of hypertensive patients will be identified and will benefit from end-organ protection.

Endothelin receptor antagonists and cardiovascular diseases of aging

Our understanding of the role of the endothelin system in human cardiovascular physiology and pathophysiology has evolved very rapidly since the initial description of its constituent parts in 1988. Endothelin-1 (ET-1) is the predominant endothelin isoform in the human cardiovascular system and has potent vasoconstrictor, mitogenic and antinatriuretic properties which have implicated it in the pathophysiology of a number of cardiovascular diseases. The effects of ET-1 have been shown to be mediated by 2 principal endothelin receptor subtypes: ET(A) and ET(B). The development of a range of peptidic and nonpeptidic endothelin receptor antagonists represents an exciting breakthrough in human cardiovascular therapeutics. Two main classes of endothelin receptor antagonist have been developed for possible human therapeutic use: ET(A)-selective and nonselective antagonists. Extensive laboratory and clinical research with these agents has highlighted their promise in various cardiovascular diseases. Randomised, placebo-controlled clinical trials have yielded very encouraging results in patients with hypertension and chronic heart failure with more preliminary data suggesting a possible role in the treatment and prevention of atherosclerosis and stroke. Much more research is needed, however, before endothelin receptor antagonists can be considered for clinical use.

Use of A-192621 to provide evidence for involvement of endothelin ET(B)-receptors in endothelin-1-mediated cardiomyocyte hypertrophy

Increased plasma levels of endothelin-1 correlate with the severity of left ventricular hypertrophy in vivo. The aim of the study was to determine the relative contribution of stimulation of endothelin ET(A) and endothelin ET(B) receptors, and the associated activation of protein kinase C, to the hypertrophic response initiated by endothelin-1 in adult rat ventricular cardiomyocytes maintained in culture (24 h). Endothelin-1 (10(-7) M) increased the total mass of protein and the incorporation of [14C] phenylalanine into protein to 26% and 25% greater (P<0.05) than respective basal values. The total content of RNA and the incorporation of 2-[14C] uridine into RNA were increased by 23% and 21%, respectively, by endothelin-1 (10(-8) M). Actinomycin D (5x10(-6) M), an inhibitor of transcription, abolished the incorporation of [14C] phenylalanine and the increased protein mass elicited by endothelin-1 (10(-8) M). The selective agonists at the endothelin ET(B) receptor, sarafotoxin 6c (10(-7) M) and endothelin-3 (10(-7) M), increased the incorporation of [14C] phenylalanine to 13% and 13% greater than respective basal values. The incorporation of [14C]phenylalanine in response to endothelin-1 (10(-7) M) was reduced by 50% (P<0.05) by the selective antagonist at endothelin ET(A) receptors, ABT-627 (10(-9) M), while the response to sarafotoxin 6c was not attenuated. The selective antagonist at endothelin ET(B) receptors, A192621 (10(-10) M), abolished the response to sarafotoxin 6c (10(-7) M) and attenuated the response to endothelin-1 (10(-7) M) by 43% (P<0.05). The selective inhibitor of protein kinase C, bisindolylmaleimide (5x10(-6) M) attenuated the response to sarafotoxin 6c (10(-7) M) by 78% and that to endothelin-1 (10(-7) M), elicited in the presence of A192621 (10(-10) M), by 52%. In conclusion, these data implicate endothelin ET(B) receptors, in addition to endothelin ET(A) receptors, in endothelin-1-mediated cardiomyocyte hypertrophy and provide evidence for the involvement of protein kinase C, at least in part, in the hypertrophic signalling pathways associated with activation of each receptor subpopulation.

Endothelin: role in experimental hypertension

The role of endothelins (ET) in blood pressure elevation remains controversial. Data supporting involvement of the ET system in different forms of genetic and experimental hypertension in the rat has appeared in the literature in recent years. Production of endothelin (ET)-1 may be enhanced in several experimental rat models of hypertension. Examples of these exhibiting increased preproendothelin-1 mRNA or peptide in the vasculature include salt-sensitive forms like deoxycorticosterone (DOCA)-salt hypertension, DOCA-salt treated spontaneously hypertensive rat (SHR) and Dahl salt-sensitive rats, and other models like stroke-prone SHR, angiotensin II-infused rats and fructose-fed rats, and possibly 1-kidney 1 clip (1-K 1C) Goldblatt hypertensive rats. SHR, 2-kidney 1 clip (2-K 1C) Goldblatt hypertensive rats and chronic N(omega)-nitro-L-arginine methyl ester (L-NAME)-treated hypertensive rats do not appear to exhibit an ET-1 component. Significant vascular growth, and a hypotensive response and regression of vascular growth after treatment with an ET antagonist demonstrate the endothelin-dependency present in some hypertensive models. Severity of high blood pressure elevation, salt-sensitivity and insulin resistance may be common denominators of involvement of the ET system in hypertension. ET antagonism in hypertension may result in regression of vascular damage, prevention of stroke and renal failure and improvement of heart failure. Whether the same is true in human hypertension remains to be established.

Myocardial fibrosis in DOCA-salt hypertensive rats: effect of endothelin ET(A) receptor antagonism

BACKGROUND

To test the hypothesis that endothelin-1 contributes to cardiac fibrosis, cardiac collagen deposition was studied in deoxycorticosterone acetate-salt (DOCA-salt) hypertensive rats, in which the endothelin system is activated. The effects of the ET(A)-selective endothelin receptor antagonist A-127722 were evaluated.

METHODS AND RESULTS

A-127722 (30 mg/kg per day) was administered for 4 weeks. Myocardial fibrosis was evaluated after Sirius red F3BA staining. Systolic blood pressure was 103+/-1.6 mm Hg in unilaterally nephrectomized rats (Uni-Nx), 202+/-3.2 mm Hg in DOCA-salt rats (P:<0.01 versus Uni-Nx), and 182+/-3.1 mm Hg in ET(A) antagonist-treated DOCA-salt rats (P:<0.01 versus DOCA-salt or Uni-Nx). In DOCA-salt rats, interstitial and perivascular collagen density was increased in the subendocardial and midmyocardial regions of the left ventricle (3- to 4-fold, P:<0.05), whereas in subepicardial myocardium, the increase was predominantly perivascular. The ET(A) antagonist prevented cardiac fibrosis in DOCA-salt rats. Procollagen I and III mRNA, which were increased in hearts of DOCA-salt rats, were normalized by ET(A) antagonist treatment. TGF-beta(1) mRNA and TGF-beta(1) protein increased at 1 week in DOCA-salt rats and were lowered in ET(A) antagonist-treated rats.

CONCLUSIONS

ET(A) receptor-mediated collagen deposition in hearts of DOCA-salt rats results from increased procollagen synthesis associated with an initial increment in expression of TGF-beta(1). These results support the hypothesis of a role for endothelin-1 in cardiac collagen deposition in mineralocorticoid hypertension, which may have pathophysiological and pharmacological implications in hypertensive heart disease.

Exaggerated vascular and renal pathology in endothelin-B receptor-deficient rats with deoxycorticosterone acetate-salt hypertension

BACKGROUND

Endothelin (ET)-1 plays an important role in the pathogenesis of deoxycorticosterone acetate (DOCA)-salt-induced hypertension. We evaluated the pathological role of ET(B) receptors in DOCA-salt-induced hypertension, cardiovascular hypertrophy, and renal damage by using the spotting-lethal (sl) rat, which carries a naturally occurring deletion in the ET(B) receptor gene.

METHODS AND RESULTS

Homozygous (sl/sl) rats exhibit abnormal development of neural crest-derived epidermal melanocytes and the enteric nervous system, and they do not live beyond 1 month because of intestinal aganglionosis and intestinal obstruction. The dopamine ss-hydroxylase (DssH) promoter was used to direct ET(B) transgene expression in sl/sl rats to support normal enteric nervous system development. DssH-ET(B) sl/sl rats live into adulthood and are healthy, expressing ET(B) receptors in adrenal glands and other adrenergic neurons. When homozygous (sl/sl) and wild-type (+/+) rats, all of which were transgenic, were treated with DOCA-salt, homozygous rats exhibited earlier and higher increases in systolic blood pressure than did wild-type rats. Chronic treatment with ABT-627, an ET(A) receptor antagonist, completely suppressed DOCA-salt-induced hypertension in both groups. Renal dysfunction and histological damage were more severe in homozygous than in wild-type rats. Marked vascular hypertrophy was observed in homozygous rats than in wild-type rats. Renal and vascular injuries were significantly improved by ABT-627. In DOCA-salt-treated homozygous rats, there were notable increases in renal, urinary, and aortic ET-1, all of which were normalized by ABT-627.

CONCLUSIONS

ET(B)-mediated actions are protective in the pathogenesis of DOCA-salt-induced hypertension. Enhanced ET-1 production and ET(A)-mediated actions are responsible for the increased susceptibility to DOCA-salt hypertension and tissue injuries in ET(B) receptor-deficient rats.

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.

Renal endothelin in hypertension

Due to the potent vasoconstrictor action of endothelin-1 and its synthesis throughout the vasculature and other tissues, most investigators believe that it is an active participant in the pathogenesis of hypertension. However, the autocrine and paracrine nature of the endothelin system has made its role difficult to define. In recent years, it has become apparent that endothelin-1 contributes to the regulation of renal salt and water excretion and that it is a major contributor to the hypertension associated with salt-dependency. Evidence suggests that endothelin-1 within the renal medulla is activated in conditions of salt loading and inhibits reabsorption of sodium in a nitric oxide-dependent manner. Blockade of endothelin A receptors lowers arterial pressure in animal models of salt-dependent hypertension. Furthermore, circulating levels of endothelin-1 are generally higher in African-Americans compared to white Americans as is the prevalence of salt-dependent hypertension. Therefore, it would appear that use of endothelin A-selective receptor antagonists should be targeted to those individuals at risk for salt-dependent hypertension. Blockade of endothelin B receptors would not be desirable because of their important role in eliminating a salt load.

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.

Endothelin enhances and inhibits adrenal catecholamine release in deoxycorticosterone acetate-salt hypertensive rats

Endothelin (ET) and the sympathoadrenal system contribute to the development and maintenance of deoxycorticosterone acetate (DOCA)-salt hypertension. ET can act directly on the adrenal medulla to enhance the release of catecholamines. In addition, the level of ET peptide is increased in the adrenal glands of DOCA-salt hypertensive rats. Therefore, we tested the hypothesis that ET enhances adrenal medullary catecholamine release during DOCA-salt hypertension. The infusion of exogenous ET-1 into an isolated, perfused adrenal gland preparation resulted in an increase in the basal release of norepinephrine (NE) and epinephrine (EPI) in control and DOCA-salt hypertensive rats. Nerve-stimulated (0.3 Hz) release of NE was significantly inhibited during ET-1 infusion in the DOCA-salt hypertensive rats but not in the control rats. The role of endogenous ET on basal and nerve-stimulated NE and EPI release was also examined. An infusion of either BQ-123 (10(-7) mol/L), an ET(A) receptor antagonist, or BQ-788 (10(-7) mol/L), an ET(B) receptor antagonist, did not alter basal NE or EPI release in either control or DOCA-salt hypertensive rats. BQ-788 did not alter nerve-stimulated release of NE and EPI. In contrast, the nerve-stimulated release of EPI, but not NE, was enhanced during BQ-123 infusion in DOCA-salt hypertensive rats. Nerve-stimulated NE and EPI release was unaffected by BQ-123 in the control rats. These data suggest that ET can stimulate adrenal medullary catecholamine release in normotensive and DOCA-salt hypertensive rats. However, ET also inhibits adrenal medullary catecholamine release in DOCA-salt hypertensive rats.

Endothelin receptor antagonists: novel agents for the treatment of hypertension?

Excitement always greets the development of a new class of therapeutic drug, representing as it does the combined efforts of the pharmaceutical industry, research laboratories and clinicians. Endothelin (ET)-receptor antagonists are being actively developed as new therapeutic agents for cardiovascular diseases, and may also be of use in other pathological conditions. Based on early and indirect evidence, ET has been implicated in the pathophysiology of hypertension; the receptor antagonists have been studied quite extensively in this setting at the preclinical level. We now possess direct evidence that such drugs are effective as antihypertensives in some experimental models of hypertension. Furthermore, the ability of ET-receptor antagonists to prevent hypertension-induced end-organ damage is also well documented. Their capacity to reverse already established target organ alterations remains poorly defined. Based on our current preclinical and clinical knowledge, this review presents the anticipated clinical usefulness of these new drugs, both in terms of blood pressure reduction and the protection of target organs.

Prolonged blockade of endothelin ET(A) receptors decreases vascular reactivity in the aorta of spontaneously hypertensive rats in vitro

We investigated the effect of prolonged endothelin-1 type A (ET(A)) receptors blockade on the constrictor response to phenylephrine and the dilator response to acetylcholine (ACh) in isolated aortic rings from normotensive [Wistar-Kyoto (WKY)] rats and spontaneously hypertensive rats (SHRs). Animals were treated for 2 weeks with the ET(A)-receptor blocker LU135252 (50 mg/kg/day; n = 8). LU135252 treatment did not affect blood pressure in both strains. In isolated aortic segments, dilation to ACh and contractions to phenylephrine were decreased only in SHRs. Nitric oxide (NO) synthesis blockade (L-NAME, 0.1 mM) inhibited 90+/-11% (WKY rats) and 76+/-8% (SHRs) of ACh-induced dilation. Cyclooxygenases blockade (indomethacin, 10 microM) had no effect in both strains. Endothelium-derived hyperpolarizing factor(s) (EDHF) blockade (KCl, 20 mM) suppressed the remaining ACh-induced dilation in both strains. Treatment with LU135252 significantly decreased NO-dependent dilation, as compared with controls [70+/-8% vs. 90+/-11% (WKY rats) and 54+/-6% vs. 76+/-8% (SHRs) of total dilation; p<0.05]. On the other hand, EDHF-dependent dilation was significantly higher in the LU135252 groups [29+/-5% vs. 10+/-3% (WKY rats) and 44+/-7% vs. 19+/-4% (SHRs) of total dilation; p<0.05]. Thus prolonged ET(A)-receptor blockade decreased the responsiveness to phenylephrine and ACh in SHR aortas and changed the proportion of dilator agents in ACh-induced dilation.

Stimulation of the renin-angiotensin system by endothelin subtype A receptor blockade in conscious dogs

Previous studies in dogs have shown additive or even synergistic effects of combined angiotensin-converting enzyme inhibition and either nonselective endothelin subtype A/B (ETA/B) or selective endothelin subtype A (ETA) receptor blockade on renal vascular resistance and mean arterial blood pressure. A possible mechanism underlying this interaction may be a stimulation of the renin-angiotensin system during endothelin (ET) receptor blockade. We therefore investigated whether plasma renin activity and renin release are regulated by the ETA receptor. Experiments were made in conscious, chronically instrumented dogs receiving either saline or the selective ETA receptor antagonist LU 135252 (10 mg/kg IV). Eighty to 100 minutes after the administration of LU 135252 (n=5), heart rate (99+/-7 versus 81+/-6 bpm; P<0.05) and renal blood flow (327+/-40 versus 278+/-36 mL/min; P<0.05) were increased significantly, whereas mean arterial blood pressure tended to be lower (93+/-5 versus 105+/-7 mm Hg). These changes were associated with a 2-fold increase in plasma renin activity (0.74+/-0.12 versus 0.37+/-0.10 ng angiotensin I per milliliter per hour; P<0.05). Measurements of renin release at various renal perfusion pressures (n=5) with the use of a vascular occluder implanted around the left renal artery revealed that ETA receptor blockade did not alter renin release at resting renal perfusion pressure (78+/-25 versus 71+/-39 U/min) but strongly enhanced the sensitivity of pressure-dependent renin release <80 mm Hg approximately 2.2-fold. In conclusion, selective ETA receptor blockade is associated with a stimulation of the circulating renin-angiotensin system, which results from both a sensitization of pressure-dependent renin release and a larger proportion of blood pressure values falling into the low pressure range, where renin release is stimulated. These find-ings strengthen the view that ET and the renin-angiotensin system closely interact to regulate vascular resistance and provide a physiological basis for synergistic hypotensive effects of a combined blockade of both pressor systems.

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.

Different contributions of endothelin-A and endothelin-B receptors in the pathogenesis of deoxycorticosterone acetate-salt-induced hypertension in rats

We investigated the involvement of actions mediated by endothelin-A (ETA) and endothelin-B (ETB) receptors in the pathogenesis of deoxycorticosterone acetate (DOCA)-salt-induced hypertension in rats. Two weeks after the start of DOCA-salt treatment, rats were given ABT-627 (10 [mg/kg]/d), a selective ETA receptor antagonist; A-192621 (30 [mg/kg]/d), a selective ETB receptor antagonist; or their vehicle for 2 weeks. Uninephrectomized rats without DOCA-salt treatment served as controls. Treatment with DOCA and salt for 2 weeks led to a mild but significant hypertension; in vehicle-treated DOCA-salt rats, systolic blood pressure increased markedly after 3 to 4 weeks. Daily administration of ABT-627 for 2 weeks almost abolished any further increases in blood pressure, whereas A-192621 did not affect the development of DOCA-salt-induced hypertension. When the degree of vascular hypertrophy of the aorta was histochemically evaluated at 4 weeks, there were significant increases in wall thickness, wall area, and wall-to-lumen ratio in vehicle-treated DOCA-salt rats compared with uninephrectomized control rats. The development of vascular hypertrophy was markedly suppressed by ABT-627. In contrast, treatment with A-192621 significantly exaggerated these vascular changes. In vehicle-treated DOCA-salt rats, renal blood flow and creatinine clearance decreased, and urinary excretion of protein, blood urea nitrogen, fractional excretion of sodium, and urinary N-acetyl-beta-glucosaminidase activity increased. Such damage was overcome by treatment with ABT-627 but not with A-192621; indeed, the latter agent led to worsening of the renal dysfunction. Histopathologic examination of the kidney in vehicle-treated DOCA-salt rats revealed tubular dilatation and atrophy as well as thickening of small arteries. Such damage was reduced in animals given ABT-627, whereas more severe histopathologic changes were observed in A-192621-treated animals. These results strongly support the view that ETA receptor-mediated action plays an important role in the pathogenesis of DOCA-salt-induced hypertension. On the other hand, it seems likely that the ETB receptor-mediated action protects against vascular and renal injuries in this model of hypertension. A selective ETA receptor antagonist is likely to be useful for treatment of subjects with mineralocorticoid-dependent hypertension, whereas ETB-selective antagonism alone is detrimental to such cases.

Endothelin antagonists

The very potent endogenous vasoconstrictor endothelin was discovered in 1988. We know now that there are three isoforms (1, 2, and 3) and two receptor subtypes (A and B). A whole range of peptide and non-peptide antagonists has been developed, some selective for A or B receptors and others with non-selective A/B antagonistic activity. So far the main application of these agents has been experimental--ie, endothelin blockers are used to throw light on disease mechanisms, most notably cardiovascular and renal. However, the non-selective antagonist bosentan and a few other agents have been studied clinically. Evidence so far from preclinical studies and healthy volunteers and from the limited number of investigations in patients permits a listing of the potential areas of clinical interest. These are mainly cardiovascular (eg, hypertension, cerebrovascular damage, and possibly heart failure) and renal. Clouds on the horizon are the need to show that these new agents are better than existing drugs; the possibility of conflicting actions if mixed A/B antagonists are used; and animal evidence hinting that endothelin blockade during development could be dangerous.

Early-onset but not late-onset endothelin-A-receptor blockade can modulate hypertension, cerebral edema, and proteinuria in stroke-prone hypertensive rats

-The ability of endothelin receptor blockade to prevent and to treat established cerebral and renal injury was explored in salt-loaded stroke-prone spontaneously hypertensive rats (SHRSP) with the endothelin receptor subtype-A antagonist A127722. SHRSP were subjected to 1% NaCl intake. The start of treatment with A127722 (35 and 70 mg. kg-1. d-1, respectively) was either synchronized with salt loading or initiated after the first observation of cerebral edema with T2-weighted magnetic resonance imaging. In untreated control animals median survival was 54 days (range, 32 to 80 days) after the start of salt loading. Early-onset A127722 treatment increased median survival to 233 days (range, 92 to 407 days; P<0.05 versus controls) with 35 mg/kg and to 124 days (range, 97 to 169 days; P<0.05 versus control) with 70 mg/kg. The development of cerebral edema was prevented, and systolic blood pressure and proteinuria were dose-dependently reduced. However, all rats in the 70-mg/kg treatment group developed hemorrhages in the basal ganglia shortly before death. Late-onset A127722 treatment failed to affect survival, systolic blood pressure, or proteinuria. Nevertheless, cerebral edema was reduced but not as well as in early-onset treatment. Development of hypertension, cerebral edema, and proteinuria was prevented in SHRSP when A127722 treatment was initiated at the start of salt-loading. However, A127722 treatment did not prolong survival in SHRSP with cerebral edema. This suggests that in SHRSP the endothelin A receptor participates actively in the development of increased blood pressure and initiation of organ damage but participates minimally in established malignant hypertension and progression of target-organ damage.

Mechanical properties of mesenteric resistance arteries from Dahl salt-resistant and salt-sensitive rats: role of endothelin-1

OBJECTIVE

Hypertension has been associated with altered structure and mechanics of resistance arteries. This study assessed whether the passive mechanics of mesenteric small arteries are altered in hypertensive Dahl salt-sensitive rats (Dahl-SS) vs normotensive Dahl salt-resistant rats (Dahl-SR). The role of endothelin-1 in determining small artery mechanics was also studied.

DESIGN

Male Dahl-SR and Dahl-SS were treated with high sodium (4% NaCl) for 10 weeks. Subgroups of each strain were treated with the ETA-endothelin receptor antagonist, A-127722 (30 mg/kg/d), concomitantly. Third-order branches of mesenteric arteries were mounted in a pressure myograph and exposed to intraluminal pressures ranging up to 140 mmHg. Media thickness and lumen diameter were measured at each pressure, from which wall mechanics were calculated.

RESULTS

Tail-cuff systolic blood pressure was elevated with high sodium in Dahl-SS vs Dahl-SR. At the given dose, A-127722 decreased blood pressure in Dahl-SS slightly but significantly. Lumen diameter was lesser and media-lumen ratios were greater in vessels from Dahl-SS. Under isobaric conditions, vessels from Dahl-SS exhibited decreased wall stress and incremental elastic modulus. However, there were no changes in isobaric incremental distensibility or elastic modulus in relation to wall stress. A-127722 attenuated the changes in media:lumen ratio and isobaric stress observed in Dahl-SS.

CONCLUSIONS

Dahl salt-sensitive hypertension was associated with altered structure of mesenteric resistance arteries, which decreased wall stress. Endothelin-1 may be involved in these changes. These arteries exhibit decreased isobaric stiffness and no difference in geometry-independent stiffness, indicating that the decreased lumen diameter is a result of eutrophic remodeling.

Different contributions of the endothelin ET(A) receptor to hypertension induced by acute or chronic inhibition of nitric oxide synthesis

The effects of FR139317((R)2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]carb onyl] amino-4-methyl-pentanoyl] amino-3-[3-(1-methyl-1H-indoyl)]propionyl]-amino-3-(2-pyridyl)prop ionic acid), an endothelin ET(A) receptor antagonist, on systemic and renal haemodynamic responses and excretory responses to chronic or acute nitric oxide (NO) synthase inhibition with NG-nitro-L-arginine (NOARG) have been examined. An intravenous bolus injection of FR139317 (10 mg kg(-1)) to chronic NO-deficient hypertensive rats (2.74 mM NOARG in drinking water for 4 weeks) elicited only a slight decrease in mean arterial pressure (MAP), to the same extent as seen in normotensive control rats. Injection of this drug induced no alteration of the renal haemodynamics of this chronic hypertensive model. Urine formation in control rats was significantly reduced by administration of FR139317. No significant decrease in urine formation was observed in the chronic NO-deficient rats. Acute intravenous injection of NOARG (5 mg kg(-1)) induced a gradual and significant increase in MAP, with a significant decrease in renal blood flow. A slight but insignificant diuretic effect was observed. In animals pretreated with FR139317 (10 mg kg(-1) i.v.) NOARG induced a significantly less potent increase in MAP, whereas similar renal haemodynamic responses to NOARG were observed. In contrast to the FR139317-untreated group, urine formation tended to decrease after administration of NOARG. These results suggest that endothelin, via the ET(A) receptor, contributes to the systemic pressor response to acute NO synthase inhibition, although renal vasoconstriction and functional changes induced by acute NO synthase inhibition are independent of ET(A) receptor-related effects. These results imply that action of endothelin via the ET(A) receptor is not involved in the maintenance of sustained hypertension induced by chronic NO synthase inhibition.

ETA receptor blockade attenuates the hypertension but not renal dysfunction in DOCA-salt rats

Endothelin (ET)-1 has potent renal and systemic vasoconstrictor properties, and thus we investigated whether ET-1 plays a role in increasing blood pressure and decreasing renal function in DOCA-salt hypertension. After a right nephrectomy, rats had DOCA or placebo pellets implanted subcutaneously and were given saline or tap water to drink, respectively. Additional groups of rats were given the ETA receptor antagonist A-127722 in their water. Rats were maintained in metabolic cages for monitoring excretory function and food and water intake. Three weeks after surgery, mean arterial pressure (MAP) was recorded in the conscious rats via a carotid artery catheter. As expected, DOCA-salt rats had significantly higher MAP compared with uninephrectomized controls (197 +/- 6 vs. 133 +/- 3 mmHg). Creatinine clearance, used as an estimate of glomerular filtration rate, was significantly reduced in DOCA-salt rats (2.9 +/- 0.4 vs. 6. 8 +/- 0.3 dl . day-1 . 100 g-1 body wt in controls). ETA receptor blockade with A-127722 significantly reduced MAP (156 +/- 8 mmHg) but had no effect on creatinine clearance of DOCA-salt-treated rats (2.8 +/- 0.3 dl . day-1 . 100 g-1 body wt). Plasma ET-1 levels were significantly raised after DOCA-salt treatment (1.4 +/- 0.5 pg/ml vs. 0.4 +/- 0.1 pg/ml in controls). A-127722 treatment increased circulating ET-1 levels in both placebo (2.3 +/- 0.5 pg/ml) and DOCA-salt (5.6 +/- 0.7 pg/ml) rats. However, ET-1 mRNA expression in renal cortical and medullary tissue was not affected by either A-127722 or DOCA-salt treatments. Thus ETA receptors appear to play a role in the maintenance and development of DOCA-salt hypertension but not in the accompanying reduction of renal function.

Angiotensin II and endothelin-1 receptor antagonists have cumulative hypotensive effects in canine Page hypertension

OBJECTIVE

To evaluate whether the cumulative hypotensive effect of the endothelin-1 receptor antagonist bosentan, previously demonstrated in the presence of an angiotensin converting enzyme inhibitor, persists under angiotensin II receptor blockade with losartan.

DESIGN

The model of hypertension was canine renovascular hypertension (Page hypertension).

METHODS

Ten conscious dogs, studied on two occasions, were administered losartan (a 0.1 mg/kg bolus plus 90 min infusion at 0.1 mg/kg per min) and then bosentan vehicle (experiment I) or losartan and then two cumulative doses of bosentan (a 0.3 mg/kg bolus plus 30 min infusion at 0.7 mg/kg per min; and a 3 mg/kg bolus plus 30 min infusion at 7 mg/kg per min; experiment II).

RESULTS

At the end of the study, mean aortic pressure in dogs had decreased by 14% in experiment I (from 139 +/- 4.7 to 119 +/- 4.7 mmHg, P<0.05), whereas a 28% reduction occurred in experiment II (from 145 +/- 8.9 to 104 +/- 5.0 mmHg, P<0.005), corresponding to an additional 14% decrease after administration of bosentan (P<0.005 between groups). This cumulative effect of bosentan was related to a decrease in systemic vascular resistance (from 1220 +/- 119 to 847 +/- 189 mmHg/ml per min per kg x 10(3), P<0.05). Plasma angiotensin II level increased similarly in both experiments (in experiment I from 133 +/- 43 to 622 +/- 145 pg/ml, P=0.01; in experiment II from 198 +/- 63 to 771 +/- 134 pg/ml, P<0.005) whereas plasma endothelin-1 level increased only in experiment II (from 3.8 +/- 0.4 to 32.7 +/- 3.2 pg/ml, P<0.001).

CONCLUSION

The cumulative hypotensive effect of bosentan suggests that, besides angiotensin II, endothelin-1 is independently involved in the pathophysiology of hypertension, which presents new therapeutic perspectives.

Coronary vascular reactivity is improved by endothelin A receptor blockade in DOCA-salt hypertensive rats

Endothelin-1 (ET-1) is thought to play an important role in the development of deoxycorticosterone acetate (DOCA)-salt hypertension. Because hypertension is associated with an increased incidence of coronary artery disease, this study was designed to determine if coronary vascular contraction to ET-1 is altered in DOCA-salt hypertensive rats and to determine the effect of chronic treatment of DOCA-salt rats with the selective ETA receptor antagonist A-127722. Male Sprague-Dawley rats were divided into four groups: DOCA, Placebo, DOCA + A-127722, and Placebo + A-127722. A-127722 was administered in drinking water at a concentration of 8 mg/100 ml. After 3 wk, mean arterial pressure (MAP) was significantly enhanced in DOCA-salt compared with Placebo rats. A-127722 significantly inhibited the increase in MAP. Contraction to ET-1 (10(-11) to 3 x 10(-8) M) was measured in isolated coronary and mesenteric small arteries (200-300 micron, intraluminal diameter) maintained at a constant intraluminal pressure of 40 mmHg and was significantly impaired in vessels from DOCA-salt compared with Placebo rats. Dose-dependent contractions to KCl were also inhibited in coronary, but only minimally impaired in mesenteric, arteries of DOCA-salt rats. Inhibition of nitric oxide synthase activity did not restore contraction to ET-1 in coronary small arteries. However contractions to ET-1 were enhanced in mesenteric small arteries. Chronic treatment with A-127722 significantly restored contraction to ET-1 in coronary, but not in mesenteric, arteries of DOCA-salt rats. Because ETA receptor blockade impairs the development of hypertension and improves coronary vascular reactivity, these data indicate that ET-1 plays an important role in coronary vascular dysfunction associated with DOCA-salt hypertension.

Endothelin-A receptor antagonist combined with hydralazine improves survival and renal function in hypertensive rats

To investigate the role of endothelin (ET) in severe hypertension, endothelial dysfunction hypercholesterolemic stroke-prone spontaneously hypertensive rats (SHRSP on a 5% cholesterol diet) were additionally fed with 1% NaCl and 0.023% nitro-L-arginine. Under these conditions, all untreated rats died within 30 days (median 17 days). A significant prolongation of survival (median 33 days) was achieved by combination treatment with hydralazine and the ETA receptor antagonist LU 135252. Monotherapy was less effective (LU 135252 18 days; hydralazine 28 days). Likewise, only treatment with the combination completely prevented the increase in systolic arterial pressure (SAP) seen in the control group during the first 10 days and delayed development of hypertension during the subsequent observation period. The superior efficacy of the combination was also reflected by improved kidney function. After 20 days of treatment, proteinuria had only increased to 1,272 +/- 135 mg/kg/day, a reduction of 45% compared to the untreated control group (2,300 +/- 346 mg/kg/day; p < 0.05). In this animal model of aggravated hypertension and endothelial dysfunction, the combination of LU 135252 with hydralazine was superior compared to either monotherapy. Therefore, the combination of an ETA receptor antagonist with vasodilators may be a potent therapy to improve blood pressure, renal function, and survival in severe hypertension with concomitant metabolic disease.

Vascular endothelin-1 expression and effect of an endothelin ETA antagonist on structure and function of small arteries from stroke-prone spontaneously hypertensive rats

There is some evidence that the endothelin (ET) system may participate in blood pressure elevation and in vascular hypertrophy in stroke-prone spontaneously hypertensive rats (SHR-SP). To further understand the involvement of the ET system in this hypertensive model, we examined preproendothelin-1 (preproET-1) mRNA abundance in blood vessels of 5-week and 18-week SHR-SP in comparison to SHR, and treated 12-week old SHR-SP with the ETA-selective receptor antagonist A-127722.5 (30 mg/kg/day in the drinking water) for 10 weeks. Abundance of preproET-1 mRNA by Northern blot analysis was increased more than twofold in aorta and mesenteric arteries of SHR-SP relative to SHR at 18 weeks but not at 5 weeks of age. SHR-SP treated with A-127722.5 had a tail-cuff systolic blood pressure at 22 weeks of age of 241 +/- 2 mm Hg vs. 251 +/- 3 mm Hg in untreated SHR-SP (p < 0.05). Heart:body weight ratio was no different in both groups, but aortic segment:body weight ratio was slightly but significantly smaller in treated SHR-SP (p < 0.05). Pressurized mesenteric small arteries from treated SHR-SP had a smaller media width (12.6 +/- 0.6 microns vs. 14.9 +/- 0.5 microns; p < 0.05) and media:lumen ratio (5.8 +/- 0.2% vs. 7.3 +/- 0.3%; p < 0.01), whereas media cross-sectional area and lumen diameter tended to decrease and increase, respectively, without achieving statistical significance. Acetylcholine-induced relaxation was improved in treated SHR-SP (99.6 +/- 0.6% vs. 90.0 +/- 3.6%; p < 0.05), whereas relaxation responses to sodium nitroprusside were similar in both groups. These results show increases of preproET-1 expression in blood vessels that appear to be secondary to blood pressure elevation. There is a small ET-dependent component in blood pressure elevation and in conduit and resistance artery changes in adult stroke-prone SHR.

Apoptosis in aorta of deoxycorticosterone acetate-salt hypertensive rats: effect of endothelin receptor antagonism

BACKGROUND

Apoptosis or programmed cell death could be greater than normal in various cardiovascular disorders, particularly in the heart. Apoptosis might contribute to remodeling of blood vessels in hypertension and could participate in regulation of vascular hypertrophy/hyperplasia.

OBJECTIVE

To investigate apoptosis in deoxycorticosterone acetate (DOCA)-salt hypertension and to determine whether endothelin-1, whose expression is enhanced in these rats, plays a role in apoptosis.

METHODS

We administered two orally active endothelin-A (ET[A])-selective receptor antagonists, A-127,722.5 (30 mg/kg per day) and LU 135,252 (50 mg/kg per day), to establish whether antigrowth effects of these ET(A) antagonists are in part mediated through apoptosis. Apoptosis was evaluated by radiolabeling of 3' OH ends of fragmented DNA, extracted from aortas, using terminal deoxynucleotidyl transferase, to show the presence of internucleosomal DNA splicing as 'DNA laddering'. Its presence was confirmed by in-situ end-labeling.

RESULTS

Systolic blood pressure was slightly but significantly lower in treated than it was in untreated DOCA-salt hypertensive rats by a mean of 26 mmHg (P < 0.01) after 4 weeks of treatment with A-127,722.5 and by 19 mmHg (P < 0.01) in rats treated with LU 135,252. Aortic cross-sectional area (CSA) was significantly greater (P < 0.001) in DOCA-salt rats than it was in uninephrectomized controls. This increased CSA was normalized by both ET(A) antagonists. DOCA-salt rats exhibited a greater degree of apoptosis (evaluated by DNA 'laddering') in aorta (353 +/- 14 pixels/microg DNA) than did control rats (232 +/- 10 pixels/microg DNA, P < 0.01). The magnitude of apoptosis was significantly greater (P < 0.01) in aorta of endothelin-antagonist-treated than it was in aorta of untreated DOCA-salt hypertensive rats. In-situ end-labeling confirmed that more apoptosis had occurred in the media of aorta from DOCA-salt hypertensive rats and the further increase found after treatment with the ET(A) antagonists.

CONCLUSION

An increase in apoptosis occurs in aorta of DOCA-salt hypertensive rats, probably as a physiologic counterpart of growth in this hypertensive model. ET(A) antagonists may act in part by accentuating the apoptosis, thereby inducing a blunting of vascular growth, which could also contribute to their antihypertensive effects.