Antihypertensive actions of the novel nonpeptide endothelin receptor antagonist SB 209670

Alternative Approach to Access 5-Hydroxy-1H-pyrrol-2-(5H)-ones from Base-Induced Tandem Intramolecular Cyclization of Sulfur Ylide with Ketones and 1,3-Hydroxy Rearrangement

An easily adaptable protocol for the preparation of 5-hydroxy-1H-pyrrol-2(5H)-ones from readily available starting materials has been reported. The reaction of sulfur ylides with carbonyl compounds is a common approach to synthesizing epoxides. Alternatively, we have developed a method with mild reaction conditions wherein sulfur ylide underwent an intramolecular cyclization with a ketonic carbonyl group in a highly efficient way and was followed by 1,3-hydroxy rearrangement to produce 5-hydroxy-1H-pyrrol-2(5H)-ones in excellent yields. The present method offers a straightforward approach to synthesize 5-hydroxy-1H-pyrrol-2(5H)-ones from sulfur ylides without the aid of transition metal in one-pot operation, which involves sequential cyclization and rearrangement reaction. The formation of 5-hydroxy-1H-pyrrol-2(5H)-ones is supported by different spectroscopic techniques, including X-ray crystallographic data and 2D NMR studies (COSY, HSQC, HMBC, and DEPT).

Antitumor Activity of a 5-Hydroxy-1H-Pyrrol-2-(5H)-One-Based Synthetic Small Molecule In Vitro and In Vivo

Alternative chemo-reagents are in great demand because chemotherapy resistance is one of the major challenges in current cancer treatment. 5-hydoxy-1H-pyrrol-2-(5H)-one is an important N-heterocyclic scaffold that is present in natural products and medicinal chemistry. However, its antitumor activity has not been systematically explored. In this study, we screened a panel of 5-hydoxy-1H-pyrrol-2-(5H)-one derivatives and identified compound 1d as possessing strong anti-proliferative activity in multiple cancer cell lines. Cell cycle analysis revealed that 1d can induce S-phase cell cycle arrest and that HCT116 was sensitive to 1d-induced apoptosis. Further analysis indicated that 1d preferentially induced DNA damage and p53 activation in HCT116 cells and that 1d-induced apoptosis is partly dependent on p53. Furthermore, we showed that 1d significantly suppressed tumor growth in xenograft tumor models in vivo. Taken together, our results suggest that 5-hydoxy-1H-pyrrol-2-(5H)-one derivatives bear potential antitumor activity and that 1d is an effective agent for cancer treatment.

Endothelin A antagonist LU-135252 and trandolapril in the treatment of the Cohen-Rosenthal diabetic hypertensive rat

BACKGROUND

Hypertension and non-insulin-dependent diabetes mellitus (NIDDM) often occur simultaneously and the combination requires vigorous control of hypertension. This can generally be achieved by a combination of antihypertensive drugs. The present study examines the antihypertensive and possible hypoglycemic effects of combined therapy with endothelin A (ETA) receptor antagonist LU-135252 and angiotensin-converting enzyme (ACE) inhibitor trandolapril in male Cohen-Rosenthal Diabetic Hypertensive (CRDH) rats.

METHODS

Rats were divided into four groups as follows: group I served as control; group II--LU-135252 30 mg/kg/day; group III--trandolapril 0.1 mg/kg/day and group IV--both LU-135252 30 mg/kg/day and trandolapril 0.1 mg/kg/day. Systolic blood pressure (SBP) and plasma glucose levels were evaluated at the beginning of the experiment and after 2, 4 and 6 weeks.

RESULTS

SBP decreased significantly in all treated groups after 2, 4 and 6 weeks of treatment compared to baseline. Maximum decrease was in group IV (combination) from 174.8+/-3.7 to 136.1+/-2.4 mmHg (22%) (p<0.0001); in group III (trandolapril) from 165.8+/-2.7 to 137.5+/-2.9 mmHg (17%) (p=0.0002); and in group II (LU-135252) and from 169.1+/-3.1 to 147.8+/-2.5 mmHg (12%) (p=0.0004). Glucose levels in plasma decreased significantly after 6 weeks of treatment. Maximum decrease was in group IV: from 501.0+/-42.8 to 178.6+/-7.3 mg/dl (62%) (p<0.0001); in group III from 428.2+/-47.7 to 146.8+/-5.6 mg/dl (63%) (p<0.0001); and in group II from 491.2+/-39.3 to 272.2+/-28.3 mg/dl (42%) (p=0.0002).

CONCLUSION

The SBP decrease was additive when both drugs were given together. Thus, combination of ETA antagonist and ACE inhibitor appears to offer a rational fixed-dose antihypertensive therapy, which is superior to that of either drug alone. The decrease in glucose level, which was the least impressive while on LU-135252 alone, was more prominent during combination after 2 weeks, although without further decrease.

Significance of endothelin on volume homeostasis in obese Zucker rats

1. The present study explored the impact of endothelin (ET) and its interaction with renal sympathetic nerves in the control of volume homeostasis in obesity. 2. Groups of lean and obese Zucker rats with innervated and denervated kidneys were subjected to an acute isotonic saline volume expansion (VE), 10% bodyweight and renal haemodynamics and excretory function were evaluated following administration of SB209670 (a non-selective ET antagonist) or UK 350 926 (a selective ETA receptor antagonist). 3. Volume expansion in untreated obese rats resulted in a blunted cumulative urine sodium excretion (CuUNaV) after 40 min, VE compared with untreated lean rats being 36 and 51% in the denervated and innervated kidneys, respectively (both P < 0.001). 4. In lean rats, both SB209670 and UK 350 926 caused a depressed ability to excrete the saline load and CuUNaV after 40 min, VE compared with untreated being decreased by 51 and 60% in the denervated and innervated kidneys, respectively (both P < 0.001). 5. SB209670 and UK 350 926 given to obese rats reduced (both P < 0.001) CuUNaV after VE by 43% in denervated kidneys compared with untreated obese rats, whereas they were without effect on the magnitude of the excretory response in innervated kidneys. 6. These findings show that the blunted renal excretory responses to VE present in obese rats were not mediated by ET. Conversely, ET, acting through ETA receptors, plays a role as a diuretic and natriuretic factor in maintaining volume homeostasis by, at least in lean rats, renal nerve-independent mechanisms.

Role of endothelin in regulation of resistance, fluid-exchange, and capacitance functions of the systemic circulation

This article reviews data at the in vivo whole animal and human level. The importance of both flow and pressure recordings and of the methods used to record these variables is emphasized. Exogenous administration of endothelin-1 evokes a transient depressor response mediated by endothelial endothelinB receptors, but the predominate effect of endothelin-1 is a sustained increase in blood pressure resulting from increases in total peripheral resistance. Resistance in the superior mesenteric, renal, and hindquarter vascular beds of animals and forearm resistance in humans is increased. Both endothelinA and, to a lesser extent, endothelinB receptors on vascular smooth muscle mediate the increases in resistance. Endothelin-1 evokes decreases in the precapillary/postcapillary resistance ratio, resulting in increased capillary pressure and net transcapillary filtration. Endothelin-1 evokes increases in mean circulatory filling pressure in animals and in constriction of the human dorsal hand vein. This venoconstrictor activity is mediated primarily through endothelinA and to a lesser extent endothelinB receptors. Endogenously generated endothelin contributes to the hemodynamic effects of angiotensin and vasopressin in certain animal models of hypertension. Antagonists of endothelin evoke modest hemodynamic changes in healthy humans and in some healthy animals, and they decrease vascular resistance dramatically in several salt-sensitive rat models of hypertension and also in some hypertensive human subjects. Thus, endogenously generated ET appears to play a modest role in the healthy organism, but it likely plays a major role in many pathophysiological states as described in companion articles in this issue.

Endothelin as a causative factor of blunted volume reflex in diabetic rats

1. The study investigated whether endothelin (ET) contributed to the diabetes-associated alterations in volume reflex and characterised the receptor subtype that might be involved. The influence of renal sympathetic nerves on these aspects of ET was also examined. 2. Groups of nondiabetic and streptozotocin-induced diabetic rats were subjected to an acute isotonic saline volume expansion (VE), 10% body wt in the presence and absence of ET antagonists. 3. Cumulative urine sodium excretion (CuU(Na)V) after VE in diabetic rats reached values of 116+/-10 in the denervated and 74+/-6 micromol min(-1) g kidney wt(-1) in the innervated kidneys, which were both less (both P<0.001) than those achieved in the nondiabetic rats, at 267+/-9 in the denervated and 183+/-10 micromol min(-1) g kidney wt(-1) in the innervated kidney, respectively. 4. Diabetic rats pretreated with a nonselective ET(A)/ET(B) antagonist had an enhanced CuU(Na)V in the denervated kidneys by 37% (P<0.01) compared to that of untreated diabetic rats. At both doses of SB209670 these increments were less than the values obtained previously in nondiabetic rats (both P<0.01). The ET(A)/ET(B) antagonist had no meaningful effect on CuU(Na)V in the innervated kidneys of the diabetic rats, whereas previous studies in nondiabetic rats showed the response to be depressed. The CuU(Na)V responses to VE in diabetic rats given the selective ET(A) antagonist were not different from those observed in untreated diabetic rats, irrespective of whether or not the renal nerves were present. In nondiabetic rats, the ET(A) antagonist had an action similar to the mixed antagonist. 5. These findings demonstrate that activation of ET(B) receptors contributes to the depressed ability to excrete a saline load in diabetes mellitus, but its impact is obscured by the influence of the renal nerves.

Structure-activity relationships of a novel class of endothelin-A receptor antagonists and discovery of potent and selective receptor antagonist, 2-(benzo[1,3]dioxol-5-yl)-6-isopropyloxy-4-(4-methoxyphenyl)-2H-chromene-3-carboxylic acid (S-1255). 1. Study on structure-activity relationships and basic structure crucial for ET(A) antagonism

A novel series of endothelin-A (ET(A)) selective receptor antagonists having a 2H-chromene skeleton are described. A lead compound, 2-(benzo[1,3]dioxol-5-yl)-2H-chromene-3-carboxylic acid (3), was found by modifications of our own angiotensin II antagonist. A structure-activity relationship (SAR) study of 3 reveals that the structural requirements essential for potent and selective ET(A) receptor binding affinity are the m,p-methylenedioxyphenyl, carboxyl, and isopropoxy groups at the 2-, 3-, and 6-positions, respectively, on the (R)-2H-chromene skeleton. The substituent at the 4-position is also important for improving the activity, and various hydrophobic functional groups of 6-9 A such as liner, branched, and cyclic aliphatic groups, unsubstituted and substituted aryl groups, and even halogen atoms were acceptable. These results suggest that (R)-2-(benzo[1,3]dioxol-5-yl)-6-isopropoxy-2H-chromene-3-carboxylic acid, formula 108, is the crucial basic structure to be recognized by the ET(A) receptor. The most potent compound is (R)-48 (S-1255), which binds to the ET(A) receptor with an IC(50) value of 0.19 nM and is 630-fold selective for the ET(A) receptor than for the ET(B) receptor. This compound has 55% oral bioavailability in rats. On the basis of the SAR, the roles of each substituent in the receptor binding are discussed.

Cardiovascular effects of endothelin-1 and endothelin antagonists in conscious, hypertensive ((mRen-2)27) rats

SB 209670 is a potent antagonist of the vasoconstrictor (ET(A)- and ET(B)-receptor-mediated) and vasodilator (ET(B)-receptor-mediated) effects of endothelin, whereas SB 234551 is relatively selective for the constrictor (ET(A)-receptor-mediated) effects. Since we had previously found SB 209670 exerted antihypertensive, vasodilator effects in conscious, heterozygous, transgenic ((mRen-2)27) (abbreviated to TG) rats, here we compared the two antagonists in that model, and assessed their chronic effects on responses to exogenous endothelin-1. We did this to test our global hypothesis, namely, that SB 209670, but not SB 234551, would cause inhibition of the depressor effects of exogenous endothelin-1 in vivo, and that this differential effect would be associated with a more marked antihypertensive action of SB 234551 in TG rats. SB 209670 and SB 234551 (infused for 50 h) exerted similar, sustained, antihypertensive effects in TG rats. The antihypertensive effects of the antagonists occurred at times when the pressor effects of exogenous endothelin-1 were not significantly inhibited. Furthermore, SB 234551 did not exert a greater antihypertensive effect than SB 209670 at a time (i.e., 2 - 4 h) when the depressor effects of endothelin-1 were abolished by the latter, but not by the former (although this differential action was lost after 24 h infusion). The results caused us to reject the hypothesis that selective antagonism of the vasoconstrictor effects of endothelin-1 would result in SB 234551 exerting a greater antihypertensive effect than SB 209670 in TG 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.

Differential haemodynamic effects of endothelin receptor antagonist, SB 209670, in conscious hypertensive and normotensive rats

Endothelin has been implicated in the pathogenesis and/or maintenance of hypertension. Endothelin receptor antagonists lower blood pressure in the spontaneously hypertensive rat (SHR), but the regional haemodynamic effects of such drugs in the SHR remain unknown. The aim of this study was to examine the regional haemodynamic effects of the endothelin receptor antagonist, (+/-)-(1S,2R,3S)-3-(2-carboxymethoxy-4-methoxyphenyl)-1-(3, 4-methylenedioxyphenyl)-5-(prop-1-yloxy)-indane-2-carboxylic acid (SB 209670), in SHR and Wistar-Kyoto (WKY) rats. Rats underwent a two-stage operation for implantation of Doppler flow probes and intravascular catheters. Recordings were made of mean arterial pressure (MAP), heart rate (HR) and renal (Ren), mesenteric (Mes) and hindquarters (HQ) blood flows and conductances (Cond). SHR and WKY received 4.5 h infusions of saline or SB 209670 (5 mg/kg priming dose+1 or 5 mg/kg/h, i.v.). SB 209670 lowered blood pressure in both SHR (-23+/-2 mm Hg) and WKY rats (-13+/-1 mm Hg). In addition, a lower dose infusion of SB 209670 also had an antihypertensive effect in SHR (-15+/-5 mm Hg). In SHRs which received the higher dose of antagonist, Ren, Mes and HQ Cond were significantly increased as was the HQ Cond in a low-dose group. In WKY rats, SB 209670 decreased Ren blood flow whilst increasing Mes and HQ blood flows and Cond. SB 209670 also attenuated the regional vasoconstrictor effects of endothelin-1, except in the Mes circulation in SHR. This study illustrates that SB 209670 causes differential haemodynamic effects in SHR and WKY rats. In SHR, the antihypertensive effect of SB 209670 was accompanied by a generalised vasodilatation in the Ren, Mes and HQ vascular beds. In WKY rats, the hypotensive effect of SB 209670 was accompanied by Mes and HQ vasodilatation, but with Ren vasoconstriction. Thus, endothelin is involved in the maintenance of blood pressure and vascular tone in both SHR and WKY rats, but the haemodynamic profiles of these effects differ between the two strains.

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 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.

Enhanced involvement of endothelin in the haemodynamic sequelae of endotoxaemia in conscious, hypertensive, transgenic ((mRen-2)27) rats

1. Age-matched (3-4 months old) male, heterozygous, hypertensive, transgenic ((mRen-2)27) rats (abbreviated to TG rats) and the normotensive control animals (homozygous, Hannover Sprague-Dawley rats (abbreviated to SD rats), were chronically instrumented for the assessment of regional haemodynamic responses to continuous lipopolysaccharide (LPS) infusion (150 microg kg(-1) h(-1), i.v.) 2. The early (1-2 h) hypotension in SD rats (-11+/-3 mmHg; n=7) was significantly less than that in TG rats (-35+/-3 mmHg; n=8), but by 24 h mean arterial blood pressure (MAP) in both strains of rat was not different from the pre-LPS value (SD rats: baseline, 108+/-3 mmHg; 24 h LPS, 112+/-4 mmHg; TG rats: baseline, 171+/-2 mmHg; 24 h LPS, 169+/-3 mmHg). At this stage in the SD rats there was a renal vasodilatation (delta vascular conductance, 29+/-10 [kHz mmHg(-1)]10(3)) but not in TG rats (delta vascular conductance 2+/-3[kHz mmHg(-1)]10(3)). 3. Co-infusion of LPS and the non-selective endothelin receptor antagonist, SB 209670 (600 microg kg(-1) bolus, 600 microg kg(-1) h(-1)) between 24 and 31 h in SD rats caused a fall in MAP of 16+/-2 mmHg accompanied by hindquarters vasodilatation (delta vascular conductance 11+/-3 (kHz mmHg(-1))10(3)). In TG rats, under the same conditions, the fall in MAP was -60+/-6 mmHg, and there were renal, mesenteric and hindquarters vasodilatations (delta vascular conductance, 23+/-5, 32+/-7, and 14+/-4 (kHz mmHg(-1))10(3), respectively). All effects, except the hindquarters vasodilatation, were greater in TG than in SD rats. 4. In TG rats infused with LPS alone for 31 h, between 24 and 31 h the fall in MAP was -17+/-4 mmHg, and the changes in renal, mesenteric and hindquarters vascular conductances were 5+/-3, -4+/-5, and 12+/-4 (kHz mmHg(-1)10(3), respectively. 5. Administration of the angiotensin (AT1)-receptor antagonist, losartan (10 mg kg(-1), i.v.) following co-infusion of LPS and SB 209670 between 24 and 31 h caused similar falls in MAP in SD and TG rats (-12+/-3 and -14+/-4 mmHg, respectively). 6. These results, together with previous findings, are consistent with a relative enhancement of the contribution of endothelin to the maintenance of cardiovascular status in endotoxaemic TG rats, particularly through a mesenteric vasoconstrictor action.

Nephroprotection of an ET(A)-receptor blocker (LU 135252) in salt-loaded uninephrectomized stroke-prone spontaneously hypertensive rats

The present study was designed to assess whether the orally active and highly specific endothelin A (ET(A)) receptor antagonist LU 135252 affects progressive renal dysfunction in a hypertensive rat model of renal damage, ie, the uninephrectomized (UNX) stroke-prone spontaneously hypertensive rat (SHRsp). The animals were examined on a normal salt (0.25%) diet and, to sensitize the kidney to hypertensive injury, also on a high salt (3%) diet. Stereological methods were used to quantify indices of glomerulosclerosis, vascular damage, and tubulointerstitial damage. Treatment with LU 135252 (100 mg/kg body wt) did not affect systolic blood pressure (BP) in animals on a normal salt diet during the whole period of the experiment (18 weeks) or in salt-loaded animals until week 10; subsequently, BP was slightly but significantly lower in salt-loaded UNX-SHRsp given LU 135252. Between weeks 6 and 12, 40% of the untreated UNX-SHRsp on a high salt diet, but none on a standard salt diet, died; such mortality was completely prevented by LU 135252. Indices of renal damage were more abnormal in salt-loaded UNX-SHRsp compared with UNX-SHRsp on a normal salt diet. Development of glomerulosclerosis and tubulointerstitial and vascular damage in UNX-SHRsp on high salt was completely prevented by LU 135252. The respective indices were no longer significantly different from those of salt-loaded sham-operated SHRsp controls. In the less severely damaged kidneys of UNX-SHRsp on normal salt, treatment with LU 135252 tended to ameliorate the indices, but the difference was not statistically significant. The results document a role of the ET system, specifically of ET(A) receptors, in the development of progressive renal injury in salt-loaded UNX-SHRsp. LU 135252 completely prevented death and renal damage resulting from salt loading.

Renal endothelin in FK506-induced nephrotoxicity in spontaneously hypertensive rats

FK506, a major immunosuppressive agent, often causes nephrotoxicity accompanied by renal vasoconstriction. It is recognized that endothelin (ET) plays a role in the cyclosporin A-induced nephrotoxicity, but the involvement of ET in the FK506-induced renal dysfunction is still poorly understood. We elicited nephrotoxicity by daily administration of FK506 in spontaneously hypertensive rats, and we examined the renal gene expression of ET and its receptors and the effects of an ET receptor antagonist on FK506-induced renal dysfunction. FK506 administration (4 mg/kg/day, i.m.) for 14 days induced nephrotoxicity, including a renal vasoconstriction and a decrease in glomerular filtration rate. The renal dysfunction was accompanied by an increase in ET-1 mRNA levels, while ET(B)-receptor mRNA was unaffected. Continuous administration of an ET(A)/ET(B) antagonist, TAK-044 (3 mg/day, s.c.), which effectively blocked systemic and renal vascular responses to exogenously administered ET-1, partially attenuated the FK506-induced renal vasoconstriction. However the reduced glomerular filtration rate were not affected by TAK-044. Thus, although enhanced gene expression of ET-1 in the kidney is involved in the renal vasoconstriction, ET does not play a major role in the FK506-induced renal dysfunction.

Role of endothelin in mediating the attenuated renal hemodynamics in Dahl salt-sensitive hypertension

The aim of this study was to evaluate the role of endothelin (ET) in the hypertension associated with giving a high sodium diet in Dahl salt-sensitive (DS) rats. To achieve this goal, we examined the effects of intravenous infusion of the nonspecific ET(A)-ET(B) antagonist on arterial pressure and renal function in conscious, chronically instrumented DS and Dahl salt-resistant (DR) rats. After 3 weeks on a high sodium (8%) diet, mean arterial pressure (MAP) in DS rats (166+/-3 mm Hg) was significantly higher than in DR rats (124+/-3 mm Hg). Baseline glomerular filtration rate (GFR) and renal plasma flow (RPF) in DS rats (1.92+/-0.25 mL/min and 7.07+/-0.80 mL/min) were lower than in DR rats (2.52+/-0.21 mL/min and 7.98+/-0.85 mL/min), respectively. Renal vascular resistance was significantly higher in DS rats (32.78+/-5.88 mm Hg x mL(-1) x min(-1)) than in DR rats (24.60+/-5.04 mm Hg x mL(-1) x min(-1)). Intravenous infusion of the ET antagonist SB 209670 at a dose of 30 microg x kg(-1) x min(-1) for 75 minutes caused a significant decrease in MAP in DS rats (from 166+/-3 to 144+/-4 mm Hg). In contrast, the effect of the ET antagonism on MAP in DR rats was not significant. ET-antagonist infusion tended to improve GFR and RPF in DS but not in DR rats. To determine the renal effects of ET antagonism independent of the systemic hemodynamic responses, we examined the effects of the same ET antagonist in rats chronically implanted with a renal interstitial catheter. Arterial pressure in DS rats (181+/-5 mm Hg) was significantly higher than in DR rats (135+/-3 mm Hg). Renal interstitial infusion of SB 209670 at a dose of 200 ng x kg(-1) x min(-1) for 60 minutes caused no change in MAP in DS or DR rats. Intrarenal ET antagonism significantly increased GFR (25%), RPF (30%), urine flow (32%), and urinary sodium excretion (25%) in DS rats, while it had no significant effect in DR rats. Fractional excretion of sodium was not significantly changed by renal interstitial infusion of the ET antagonist in DS rats, indicating that improved renal excretory function in DS rats is most likely due to the associated improvement in renal hemodynamics. We conclude that ET may play a role in the attenuated renal hemodynamics and possibly the development of Dahl salt-sensitive hypertension.

Temporal differences between the involvement of angiotensin II and endothelin in the cardiovascular responses to endotoxaemia in conscious rats

1. Male, Long Evans rats were instrumented chronically with pulsed Doppler probes and intravascular catheters to allow assessment of regional haemodynamic changes during i.v. infusion of lipopolysaccharide (LPS, 150 micrograms kg-1 h-1). 2. In the presence of the AT1-receptor antagonists, losartan (10 mg kg-1 + 10 kg-1 h-1), the initial (1-2 h) hypotensive and renal, mesenteric and hindquarters vasodilator responses to LPS were enhanced significantly. Thereafter these effects waned, but between 8-23 h after the onset of LPS infusion, a further fall in mean atrial blood pressure (MAP) and increases in renal and hindquarters flows and conductances occurred. All these changes were significantly greater than seen with losartan or LPS alone, and exceeded the sum of their effects. 3. In the presence of captopril (2 mg kg-1 + 2 mg kg-1 h-1), the initial hypotensive and renal vasodilator responses to LPS were enhanced, but less so than in the presence of losartan. However, the effects of LPS in the presence of losartan and captopril together were not different from those in the presence of losartan alone. These observations indicate that the ability of captopril to inhibit the degradation of bradykinin had no additional influence, and the differences between the effect of captopril and losartan on the initial effects of LPS were probably due to more effective suppression of the action of angiotensin II by losartan. 4. In the absence of LPS, co-infusion of losartan and the non-selective endothelin antagonist, SB 209670 (600 micrograms kg-1 + 600 micrograms kg-1 h-1), caused a substantial, progressive hypotension (-25 +/- 2 mmHg at 24 h) accompanied by increases in renal, mesenteric and hindquarters vascular conductances (31 +/- 13, 44 +/- 9 and 45 +/- 12%, respectively), indicating an involvement of angiotensin II and endothelin in the maintenance of normal cardiovascular status in conscious, Long Evans rats. 5. In the presence of losartan and SB 209670, the initial, LPS-induced fall in MAP (-42 +/- 2 mmHg) was not different from that in the presence of losartan (-39 +/- 4 mmHg), and the increases in renal, in mesenteric, and in hindquarters vascular conductances were similar in the two conditions. However, there was no recovery in MAP, and there were persistent renal, mesenteric and hindquarter vasodilatations. 6. In all experiments involving LPS, administration of the V1- receptor antagonist, d(CH2)5-O-Me-Tyr-AVP (10 micrograms kg-1), 23 h after the start of LPS infusion caused additional hypotension and mesenteric vasodilatation, particularly. This effect was most marked in animals pretreated with losartan and SB 209670. 7. The results indicate that the initial (1-2 h) depressor and dilator effects of LPS infusion in conscious Long Evans rats are opposed by the actions of angiotensins II, rather than endothelin. However, between 2-8 h after the onset of LPS infusion the involvement of endothelin develops and that of angiotensin II fades. By 24 h after the start of infusion of LPS, the pressor and vasoconstrictor actions of endothelin wane, and a role of vasopressin is apparent. At no stage is there clear evidence for an involvement of bradykinin in the haemodynamic sequelae of endotoxaemia in this model.

Effect of selective blockade of endothelin ETB receptors on the liver dysfunction and injury caused by endotoxaemia in the rat

1. We investigated the effects of the selective endothelin (ET)A receptor antagonist BQ-485 and the selective ETB receptor antagonist BQ-788 on circulatory failure, multiple organ dysfunction syndrome (MODS) and the alterations in acid base balance caused by endotoxaemia in the anaesthetized rat. 2. Male Wistar rats were anaesthetized (thiopentone sodium; 120 mg kg-1, i.p.) and received a continuous infusion of vehicle (saline, 0.6 ml kg-1h-1, i.v.), BQ-485 (10 nmol kg-1 min-1, i.v.) or BQ-788 (10 nmol kg-1 min-1, i.v.). Fifteen min later, animals received a bolus injection of either saline (0.9% NaCl, 1 ml kg-1, i.v.) or E. coli lipopolysaccharide (LPS, 10 mg kg-1, i.v.). 3. Injection of LPS resulted in a fall in blood pressure from 115 +/- 4 mmHg (time 0) to 82 +/- 4 mmHg at 360 min (n = 15) as well as a hyporeactivity to the pressor responses to noradrenaline (NA, 1 microgram kg-1, i.v.). Infusion of BQ-788 attenuated the delayed hypotension (at 360 min: 100 +/- 4 mmHg, n = 7; P < 0.05) and significantly enhanced the pressor responses elicited by NA (at 60 to 240 min). In contrast, treatment of LPS-rats with BQ-485 augmented the hypotension (at 360 min), but did not affect the vascular hyporeactivity elicited by endotoxaemia. 4. Endotoxaemia for 360 min resulted in rises in the serum levels of urea and creatinine (indicators of renal failure), glutamate-oxalate-transferase (GOT) and glutamate-pyruvate-transferase (GPT) (indicators of hepatocellular injury), and bilirubin and gamma-glutamyl transferase (gamma GT) (indicators of liver failure) as well as nitrite (indicator of the induction of nitric oxide synthase; iNOS). Treatment of LPS-rats with BQ-788, but not with BQ-485, attenuated the degree of liver injury and failure, while neither BQ-788 nor BQ-485 affected the acute renal failure or the induction of iNOS caused by endotoxin. 5. Endotoxaemia also caused (within 15 min) an acute metabolic acidosis (falls in pH, HCO3-and base excess) which was compensated by hyperventilation (fall in PaCO2). Treatment of LPS-rats with BQ-788 or BQ-485 did not affect the metabolic acidosis caused by LPS. 6. Thus, the selective ETB receptor antagonist BQ-788 attenuated (i) the delayed hypotension, (ii) the vascular hyporeactivity to NA as well as (iii) the degree of hepatocellular injury and dysfunction caused by endotoxin in the anaesthetized rat. In contrast, the selective ETA receptor antagonist did neither attenuate the circulatory failure nor the liver or renal dysfunction associated with endotoxaemia. We propose that the prevention of the hepatocellular dysfunction and injury caused BQ-788 in endotoxaemia is due to an improvement in oxygen delivery to the liver secondary to (i) inhibition of pre-sinusoidal constriction, (ii) inhibition of sinusoidal constriction, and (iii) improvement in perfusion pressure.

Influence of aminoguanidine and the endothelin antagonist, SB 209670, on the regional haemodynamic effects of endotoxaemia in conscious rats

1. We compared the regional haemodynamic responses to lipopolysaccharide (LPS; 150 micrograms kg-1 h-1, i.v.) in the presence of saline, aminoguanidine (AG; 45 mg kg-1 bolus, 45 mg kg-1 h-1 infusion), or AG and the non-selective endothelin receptor antagonist, SB 209670 (600 micrograms kg-1 h-1), in conscious, chronically instrumented, Long Evans rats (350-450 g; n = 8 in all groups). We used AG because there is evidence that it is a selective inhibitor of inducible nitric oxide synthase (iNOS), although recently it has been claimed AG also inhibits constitutive NOS. 2. Infusion of LPS in the presence of saline caused an early, transient hypotension (1-2 h) and a renal vasodilatation, with a secondary, delayed fall in mean arterial blood pressure (MAP), progressive tachycardia, and renal and hindquarters vasodilatation. 3. AG alone caused a rapid (within 30 s) transient rise in MAP (delta 27 +/- 3 mmHg), accompanied by tachycardia and regional vasoconstrictions, but no reduction in regional flows, indicating the pressor effect of AG was, probably, largely due to an increase in cardiac output. These effects are not consistent with AG inhibiting constitutive NOS. In the presence of AG, LPS still caused an early, transient fall in MAP accompanied by a renal vasodilatation, but thereafter there was a significant rise in MAP (17 +/- 3 mmHg, 3 h after onset of LPS infusion) accompanied by bradycardia and marked mesenteric and hindquarters vasoconstrictions. However, 23 h after the onset of co-infusion of AG and LPS all variables were not different from baseline, except heart rate and renal vascular conductance, which were increased. 4. In the presence of AG and SB 209670, LPS caused progressive hypotension and increases in renal, mesenteric and hindquarters vascular conductances. Hence, SB 209670 prevented the rise in MAP and the regional vasoconstrictions seen with AG and LPS, indicating an involvement of endothelin in these events. 5. In the presence of AG and SB 209670, 23 h after the onset of LPS infusion, the AT 1-receptor antagonist, losartan (10 mg kg-1), and the V 1-receptor antagonist, d(CH2)5-0-Me-Tyr-AVP (10 micrograms kg-1, 10 micrograms kg-1 h-1) caused additional incremental falls in MAP and increases in renal, mesenteric and hindquarters vascular conductances. Under these circumstances, MAP was lower and regional vascular conductances higher than in the other experiments following administration of losartan and d(CH2)5-0-Me-Tyr-AVP. Thus, although the findings are consistent with AG inhibiting iNOS, thereby revealing the pressor and vasoconstrictor actions of endothelin released by LPS, it is clear that LPS activates a very powerful hypotensive/vasodilator mechanism(s) which is resistant to AG, and whose full influence is only unmasked when the actions of endothelin, angiotensin II and vasopressin are inhibited.

The arterial wall: a new pharmacological and therapeutic target

In recent years, two key concepts having numerous interrelationships were advanced for the understanding of various cardiovascular diseases: the "endothelial dysfunction" and the "arterial remodelling". Both endothelial dysfunction and arterial remodelling occur in various pathologies including essential hypertension, heart failure, atherosclerosis, restenosis after angioplasty, and pulmonary hypertension, and have modified the therapeutic approach by offering new pharmacological targets: specific receptors not only at the site of the vascular smooth muscle cells but also on the endothelial cells, growth factors that stimulate proliferation of smooth muscle, and receptors and enzymes of the extra-cellular matrix. Among the various substances under research, the present review will discuss angiotensin II receptor antagonists, endothelin receptor antagonists, nitrates-NO donors, potassium channel activators, and substances interfering with proteoglycans and other components of the extra-cellular matrix.

Effects of the endothelin receptor antagonist, SB 209670, on circulatory failure and organ injury in endotoxic shock in the anaesthetized rat

1. This study investigates the effects of the non-selective ETA/ETB receptor antagonist, SB 209670, on systemic haemodynamics, renal function, liver function, acid-base balance and survival in a rat model of endotoxic shock. 2. Injection of E. coli lipopolysaccharide (LPS, 10 mg kg-1, i.v.) resulted in increases in the serum levels of tumour necrosis factor-alpha (TNF-alpha, maximum 60 min after LPS), endothelin-1, (ET-1; maximum 120 min after LPS), and interferon-gamma (IFN-gamma, maximum 180 min after LPS). 3. Injection of LPS also resulted in a fall in blood pressure from 113 +/- 3 mmHg (time = 0) to 84 +/- 4 mmHg at 360 min (n = 15) as well as a hyporeactivity to the vasoconstrictor responses elicited by noradrenaline (NA, 1 microgram kg-1, i.v.). Pretreatment of rats with a continuous infusion of SB 209670 (3 mg kg-1, i.v. bolus + 100 micrograms kg-1, i.v. infusion commencing 15 min prior to LPS) significantly augmented the hypotension as well as the vascular hyporeactivity to NA caused by endotoxaemia. 4. Pretreatment of LPS-rats with SB 209670 (3 mg kg-1, i.v. bolus given 15 min prior to LPS) or infusion of SB 209670 (bolus dose and infusion as above) resulted in a reduction in 6 h-survival from 71% (control) to 30% and 13%, respectively. 5. Endotoxaemia for 4 h resulted in rises in the serum levels of urea and creatinine (indicators of renal failure), but not in the serum levels of bilirubin, GPT and GOT (indicators of liver dysfunction and/or hepatocellular injury). Pretreatment of LPS-rats with SB 209670 (3 mg kg-1, i.v. bolus 15 min prior to LPS) significantly augmented the serum levels of creatinine, bilirubin, GPT and GOT caused by endotoxin. In addition, endotoxaemia caused, within 15 min, an acute metabolic acidosis (falls in pH, HCO3- and base excess) which was compensated by hyperventilation (fall in PaCO2). Pretreatment of LPS-rats with SB 209670 (3 mg kg-1, i.v. bolus) significantly augmented the metabolic acidosis caused by LPS. 6. Thus, the non-selective ETA/ETB receptor antagonist, SB 209670, augments the degree of (i) hypotension, (ii) vascular hyporeactivity to noradrenaline, (iii) renal dysfunction and (iv) metabolic acidosis caused by endotoxin in the anaesthetized rat. In contrast to rats treated with LPS alone, LPS-rats treated with SB 209670 exhibited liver dysfunction and hepatocellular injury. We propose that the release of endogenous ET-1 serves to maintain blood pressure and subsequently organ perfusion in septic shock.

Effects of dexamethasone and SB 209670 on the regional haemodynamic responses to lipopolysaccharide in conscious rats

1. Male (350-450 g) Long Evans rats were chronically instrumented to permit regional haemodynamics to be monitored in the conscious state. In the first experiment, either saline (0.4 ml h-1) or dexamethasone (3 mg kg-1, 125 micrograms kg-1 h-1) was infused continuously for 24 h, before co-infusion of lipopolysaccharide of (LPS, 150 micrograms kg-1 h-1) for 24 h. Dexamethasone prevented the delayed (5-24 h) fall in mean arterial blood pressure (MAP) and the renal and hindquarters vasodilatation seen with LPS infusion alone, but not the initial (about 2 h) fall in MAP or renal vasodilatation. However, at this dose, dexamethasone itself caused a significant rise in MAP and regional vasoconstrictions. 2. In the second experiment, dexamethasone at a lower dose (12.5 micrograms kg-1 h-1) had only slight pressor and vasoconstrictor effects. However, in its presence, infusion of LPS caused a substantial and progressive rise in MAP (maximum at 8 h, +32 +/- 3 mmHg) together with persistent mesenteric and hindquarters vasoconstriction and a transient renal vasodilatation. 3. In the third experiment, the non-selective endothelin antagonist, SB 209670 (600 micrograms kg-1 h-1), blocked the slight pressor and regional vasoconstrictor effects of the lower dose of dexamethasone. Furthermore, in the presence of dexamethasone and SB 209670, infusion of LPS caused marked, but transient hypotension (nadir at 5 h, -24 +/- 2 mmHg) and renal and mesenteric vasodilatation. 4. At the end of all experimental protocols, sequential administration of the AT1-receptor antagonist, losartan, followed by the V1-receptor antagonist, (+)-(CH2)5-O-Me-Tyr, vasopressin, caused effects indicating a variable involvement of angiotensin and vasopressin in the maintenance of cardiovascular status. 5. Collectively, the results indicate that, in the conscious rat, dexamethasone interacts with vasoconstrictor and vasodilator mechanisms, and hence its influence on the haemodynamic responses to LPS cannot be attributed, simply, to inhibition of the activity of inducible nitric oxide synthase and/or cyclo-oxygenase-2.

Regional haemodynamic effects of antagonists of angiotensin II, endothelin and adrenoceptors in conscious, vasopressin-deficient, genetically hypertensive rats

1. Male, vasopressin-deficient, normotensive (DI/N) and hypertensive (DI/H) rats were chronically instrumented (all surgery under sodium methohexitone anaesthesia) to allow assessment of resting haemodynamic status and responses to antagonism of AT1-receptors (Experiment 1), ET(A-) and ET(B-) receptors (Experiment 2) or adrenoceptors (Experiment 3). 2. Before any treatment, mean arterial blood pressure (MAP) was higher, and hindquarters vascular conductance was consistently lower in all groups of DI/H rats than in DI/N rats. 3. In Experiment 1, losartan (10 mg kg-1 i.v.), an AT1-receptor antagonist, was given 5 h after s.c. injection of saline, (DI/N, n = 8; DI/H, n = 8) or hyperoncotic polyethylene glycol, (DI/N, n = 9; DI/H, n = 9) to induce isosmotic hypovolaemia. In the volume-replete state, losartan caused similar small falls in MAP in the two groups (maximum delta MAP; DI/N, -9 +/- 2; DI/H, -15 +/- 5 mmHg), but the mesenteric and hindquarters vasodilatations were greater in DI/N rats. In the volume-depleted state the effects of losartan were augmented (delta MAP; DI/N, -32 +/- 3; DI/H. -31 +/- 3 mmHg), but its vasodilator effects were still greater in DI/N than in DI/H rats. 4. In Experiment 2, infusion of the ET(A-)ET(B-)receptor antagonist, SB 209670 (600 micrograms kg-1 h-1; DI/N, n = 8; DI/H, n = 9), had haemodynamic effects that were not different from those during saline infusion in DI/N (n = 7) and DI/H rats (n = 8). 5. In Experiment 3, sequential administration of the beta 2-adrenoceptor antagonist, ICI 118551 (0.2 mg kg-1 bolus, 0.1 mg kg-1 h-1 infusion), the alpha 2-adrenoceptor antagonist, idazoxan (0.75 mg kg-1 bolus, 1 mg kg-1 h-1 infusion), and losartan (10 mg kg-1 bolus) had only slight haemodynamic effects in DI/N (n = 8) and DI/H (n = 9) rats. Subsequent administration of the alpha 1-adrenoceptor antagonist, prazosin (0.5 mg kg-1 bolus, 0.8 mg kg-1 h-1 infusion) caused marked hypotension, although MAP was still higher in DI/H (95 +/- 4 mmHg) than in DI/N (75 +/- 4 mmHg) rats. However, in this circumstance there were no significant differences between renal, or mesenteric, or hindquarters vascular conductances in the two groups. 6. The results indicate that the hypertension and hindquarters vasoconstriction in DI/H rats is not dependent on AII or endothelin. Moreover, the relative elevation in MAP in DI/H persists in the presence of antagonism of beta 2, alpha 2- and alpha 1-adrenoceptors, in spite of no significant difference in regional vascular conductances.

Haemodynamic effects of losartan and the endothelin antagonist, SB 209670, in conscious, transgenic ((mRen-2)27), hypertensive rats

1. Hypertensive transgenic (TGR(mRen-2)27) (abbreviated to TG) rats (n = 6) and their normotensive Sprague-Dawley (SD) control strain (n = 7) were chronically instrumented for the measurement of cardiac haemodynamics. The hypertension in TG rats (mean blood pressure 181 +/- 9 mmHg) was entirely attributable to a reduction in total peripheral conductance (TG rats = 169 +/- 7, SD rats = 292 +/- 15 microliters min-1 mmHg-1 100g-1) since cardiac index was not different in the two strains (TG rats = 30.5 +/- 1.2, SD rats = 29.5 +/- 1.6 ml min-1 100g-1). 2. In other animals instrumented for the assessment of regional haemodynamics, the extent of peripheral vasoconstriction was similar in renal, mesenteric and hindquarters vascular beds in the TG rats (reduction in vascular conductance relative to SD rats = 42%, 46% and 49%, respectively). 3. During an 8 h observation period with saline infusion, or following injection of losartan (10 mg kg-1) in SD rats there was no hypotension or regional vasodilation. With infusion of the endothelin antagonist, SB 209670 (10 micrograms kg-1 min-1), there was a slight hypotension, but no significant vasodilation; co-administration of losartan and SB 209670 caused a similar profile of effect, although the hypotension was increased. 4. With the same experimental protocol in TG rats, losartan caused a biphasic, progressive fall in mean arterial blood pressure accompanied by renal, mesenteric and hindquarters vasodilation. Although the response to SB 209670 was not biphasic, its hypotensive and vasodilator effects were not different from those of losartan after 8 h. In the combined presence of losartan and SB 209670, mean arterial blood pressure (116 +/- 5 mmHg) was significantly lower than with SB 209670 (132+/-4 mmHg) or losartan(136 +/- 6 mmHg) alone, and renal, mesenteric and hindquarters vascular conductances (61 +/- 3, 90+/-14 and 52+/-4 [kHz nmHg-1]103, respectively) were higher than the corresponding values following either SB 209670 (49 +/- 4, 52 +/- 4 and 34 +/- 3 [kHz mmHg- 1]103, respectively) or losartan (43 +/- 5, 59 +/- 13 and 35+/-4 [kHz mmHg-1]103, respectively) alone. These results indicate the maintenance of hypertension inTG rats is dependent upon renal, mesenteric and hindquarters vasoconstriction, mediated by angiotensinII (AII) and endothelin (ET). Since we found that plasma ET-1 levels in TG rats (12.06+/-2.87 pmol 1-1)were lower than in SD rats (21.53 +/- 3.94 pmol 1-1), then it is possible that locally-generated, rather than circulating ET-l contributes to the widespread vasoconstriction in TG rats.

Enhancement of the hypotensive and vasodilator effects of endotoxaemia in conscious rats by the endothelin antagonist, SB 209670

In conscious, chronically-instrumented rats, the non-selective endothelin antagonist, SB 209670 (10 micrograms kg-1 min-1), caused marked enhancement of the fall in mean arterial blood pressure during infusion of lipopolysaccharide (LPS) for 24 h (LPS alone = -6 +/- 3 mmHg; LPS+SB 209670 = -30 +/- 2 mmHg). This effect was accompanied by a conversion of the mesenteric vasoconstriction to a substantial mesenteric vasodilatation and an augmentation of the hindquarters vasodilatation, seen with LPS alone. Notably, the marked renal hyperaemic vasodilatation during LPS infusion was not affected significantly by SB 209670. These results indicate that endothelin, directly and/or indirectly, plays a pivotal role in the cardiovascular sequelae of endotoxaemia in conscious rats, and prevents marked hypotension, particularly by opposing mesenteric vasodilatation.

In vivo pharmacological characterization of the non-peptide endothelin receptor antagonist SB 209670

1. The aim of the present study was to assess the ability of SB 209670, a high affinity non-peptide endothelin receptor antagonist (0.4 and 18 nM Kis at human cloned ETA and ETB receptors, respectively), to inhibit the haemodynamic actions of endothelin-1 in vivo. 2. Systemic administration of (+/-)-SB 209670, given either as a bolus i.v. injection or as a continuous i.v. infusion, did not alter basal haemodynamic parameters in the anaesthetized rat. 3. Infusion of (+/-)-SB 209670 (10 micrograms kg-1 min-1) selectively inhibited the depressor and carotid vasodilator response to exogenous endothelin-1: 100 micrograms kg-1 min-1 was required to inhibit significantly the biphasic haemodynamic actions of endothelin-1. The haemodynamic actions of angiotensin II and calcitonin gene-related peptide were unaltered by 100 micrograms kg-1 min-1 (+/-)-SB 209670. 4. Bolus i.v. administration of (+/-)-SB 209670 (1 mg kg-1) selectively inhibited the depressor and carotid vasodilator actions of endothelin-1: 10 mg kg-1 (+/-)-SB 209670 was required to inhibit the secondary vasoconstrictor actions of endothelin-1. 5. (+/-)-SB 209670 (10 mg kg-1) was also effective at antagonizing the pressor actions of endothelin-1 in the conscious rat for up to 3 h after intraduodenal administration thereby demonstrating that the antagonist was bioavailable upon enteric administration. This dose of (+/-)-SB 209670 did not alter basal haemodynamic parameters in the conscious rat. 6. Thus, ( +/- )-SB 209670 is an effective endothelin receptor antagonist in vivo. Using the doses defined in this study, SB 209670 may, therefore, serve as a useful tool for understanding the role of endogenous endothelin-I in the control of cardiovascular function under both physiological and pathophysiological conditions.