Endothelin receptor antagonists in a beagle model of pulmonary hypertension: contribution to possible potential therapy?

Efficacy of an endothelin-A receptor antagonist in heart transplantation from asphyxiated canine non-heart-beating donors

OBJECTIVE

Hypoxic perfusion before arrest, an indeterminate period of warm ischemia, and subsequent reperfusion are major causes of cardiac allograft dysfunction in non-heart-beating donors (NHBDs). The present study was undertaken to elucidate the cardioprotective effects of ET(A) receptor antagonist FR139317 for hearts obtained from asphyxiated NHBDs in a canine transplantation model.

METHODS

Hypoxic cardiac arrest was induced in 17 donor dogs. FR139317 (10 mg/kg) was given to 7 of the dogs over a period of 10 min before disconnecting the ventilator. The hearts were preserved with FR 139317-supplemented cardioplegic solution (FR group). The remaining 10 did not receive FR 139317 at any time during the experiment (control group). Orthotopic transplantation was performed after a mean myocardial ischemic time of 4h.

RESULTS

During the agonal period, the highest systolic pulmonary artery pressure in the FR group was lower than that in the control group (47 +/- 14 vs. 58 +/- 27 mmHg). All animals in the FR group were weaned from cardiopulmonary bypass, whereas only five of the controls were weaned, two of which were identified to have dominant right ventricular failure. After transplantation, recovery rates of the left ventricular end-systolic pressure-volume ratio (E(max)) and the maximum first derivative of pressure measured over time (max dP/dt) were not significantly different between the groups, but recovery rates of the cardiac index, left ventricular minimum dP/dt and exponential time constant of LV relaxation (tau) in the FR group were higher than those in the control group.

CONCLUSIONS

The ET(A) receptor antagonist FR 139317 reduced pressure overload on the right ventricle by decreasing the peak pulmonary artery pressure before donor arrest. Cardioprotective effects of this agent for heart transplantation from NHBDs are manifested by preserved diastolic properties of the left ventricle.

Endothelin receptor antagonists

Endothelin receptor antagonists (ERAs) have been developed to block the effects of endothelin-1 (ET-1) in a variety of cardiovascular conditions. ET-1 is a powerful vasoconstrictor with mitogenic or co-mitogenic properties, which acts through the stimulation of 2 subtypes of receptors [endothelin receptor subtype A (ETA) and endothelin receptor subtype B (ETB) receptors]. Endogenous ET-1 is involved in a variety of conditions including systemic and pulmonary hypertension (PH), congestive heart failure (CHF), vascular remodeling (restenosis, atherosclerosis), renal failure, cancer, and cerebrovascular disease. The first dual ETA/ETB receptor blocker, bosentan, has already been approved by the Food and Drug Administration for the treatment of pulmonary arterial hypertension (PAH). Trials of endothelin receptor antagonists in heart failure have been completed with mixed results so far. Studies are ongoing on the effects of selective ETA antagonists or dual ETA/ETB antagonists in lung fibrosis, cancer, and subarachnoid hemorrhage. While non-peptidic ET-1 receptor antagonists suitable for oral intake with excellent bioavailability have become available, proven efficacy is limited to pulmonary hypertension, but it is possible that these agents might find a place in the treatment of several cardiovascular and non-cardiovascular diseases in the coming future.

Cardiopulmonary indications for endothelin receptor antagonists: review of recent efficacy trials

Recent clinical and experimental animal trials indicate that endogenously produced endothelin-1 (ET-1) contributes to the abnormal systemic and pulmonary vascular resistance associated with congestive heart failure (CHF) and pulmonary hypertension (PH). In experimental CHF, the chronic blockade of ET-1 actions by ET receptor antagonists clearly improves haemodynamic status, and improves cardiac structure and survival. The latter is based on limited results. In experimental PH there are consistent reports of prevention and reversal of PH, pulmonary vascular remodelling and right ventricular hypertrophy, independent of the inciting mechanisms. These results in experimental animals illustrate the potential efficacy of the ET receptor antagonists in future clinical trials. With five ET receptor antagonists in clinical development, and more on the way, their potential will soon be realised.

Role of endothelin ETB receptor in the pathogenesis of monocrotaline-induced pulmonary hypertension in rats

We investigated the role of endothelin ETB receptor in the development of monocrotaline-induced pulmonary hypertension, by using the spotting-lethal (sl) rat, which carries a naturally occurring deletion in the endothelin ETB receptor gene. Three weeks after injection of saline or monocrotaline (60 mg/kg, s.c.), hemodynamics, cardiac hypertrophy and endothelin-1 levels in right ventricle were determined. Monocrotaline produced a marked pulmonary hypertension associated with increases in right ventricular pressure and hypertrophy, pulmonary arterial medial thickening and the endothelin-1 levels. The monocrotaline-induced alterations tended to be enhanced in ETB-deficient homozygous rats, compared with cases in wild-type rats. The treatment with selective ETA receptor antagonist ABT-627 [2R-(4-methoxyphenyl)-4S-(1,3-benzodioxol-5-yl)-1-(N,N-di(n-butyl)aminocarbonyl-methyl)-pyrrolidine-3R-carboxylic acid] for 3 weeks (10 mg/kg/day, twice daily) almost completely suppressed the monocrotaline-induced pulmonary hypertension and related organ damage both in ETB-deficient and wild-type animals to the same levels. Thus, we suggest that the antagonism of the ETA receptor is essential for the protection from monocrotaline-induced pulmonary hypertension, irrespective of the presence of the ETB receptors, although a protective role of ETB receptor-mediated action in the pathogenesis of this disease model cannot be ruled out.

Contribution of endothelin to pulmonary vascular tone under normoxic and hypoxic conditions

The contribution of endothelin to resting pulmonary vascular tone and hypoxic pulmonary vasoconstriction in humans is unknown. We studied the hemodynamic effects of BQ-123, an endothelin type A receptor antagonist, on healthy volunteers exposed to normoxia and hypoxia. Hemodynamics were measured at room air and after 15 min of exposure to hypoxia (arterial PO(2) 99.8 +/- 1.8 and 49.4 +/- 0.4 mmHg, respectively). Measurements were then repeated in the presence of BQ-123. BQ-123 decreased pulmonary vascular resistance (PVR) 26% and systemic vascular resistance (SVR) 21%, whereas it increased cardiac output (CO) 22% (all P < 0.05). Hypoxia raised CO 28% and PVR 95%, whereas it reduced SVR 23% (all P < 0.01). During BQ-123 infusion, hypoxia increased CO 29% and PVR 97% and decreased SVR 22% (all P < 0.01). The pulmonary vasoconstrictive response to hypoxia was similar in the absence and presence of BQ-123 [P = not significant (NS)]. In vehicle-treated control subjects, hypoxic pulmonary vasoconstriction did not change with repeated exposure to hypoxia (P = NS). Endothelin contributes to basal pulmonary and systemic vascular tone during normoxia, but does not mediate the additional pulmonary vasoconstriction induced by acute hypoxia.

Endothelin system: the double-edged sword in health and disease

The endothelin system consists of two G-protein-coupled receptors, three peptide ligands, and two activating peptidases. Its pharmacological complexity is reflected by the diverse expression pattern of endothelin system components, which have a variety of physiological and pathophysiological roles. In the vessels, the endothelin system has a basal vasoconstricting role and participates in the development of diseases such as hypertension, atherosclerosis, and vasospasm after subarachnoid hemorrhage. In the heart, the endothelin system affects inotropy and chronotropy, and it mediates cardiac hypertrophy and remodeling in congestive heart failure. In the lungs, the endothelin system regulates the tone of airways and blood vessels, and it is involved in the development of pulmonary hypertension. In the kidney, it controls water and sodium excretion and acid-base balance, and it participates in acute and chronic renal failure. In the brain, the endothelin system modulates cardiorespiratory centers and the release of hormones. More advanced functional analysis of the endothelin system awaits not only additional pharmacological studies using highly specific endothelin antagonists but also the generation of genetically altered rodent models with conditional loss-of-function and gain-of-function manipulations.

Endothelin receptor blockade attenuates air embolization-induced pulmonary hypertension in sheep

We investigated the effects of two types of endothelin receptor antagonists on pulmonary hypertension induced by pulmonary air embolization in awake sheep. We prepared awake sheep with indwelling catheters inserted in blood vessels for continuous monitoring of pulmonary artery pressure, left atrial pressure and systemic arterial pressure. Cardiac output was measured every 30 min. The study consisted of two experiments, one with FR139317 (100 microg/kg/min; (R)2-[(R)-2-[(S)-2-[1-(hexahydro-1H-azepinyl)]-carbonyl]amino-4-++ +methy l-pentanoyl]amino-3-[3-(1-methyl-1H-indolyl)]propionyl)amino-3-(2-pyr idyl)propionic acid), a selective endothelin ET(A) receptor antagonist, and the other with TAK-044 (100 microg/kg/h; cyclo[D-alpha-aspartyl-3-[(4-phenylpiperazin-yl)carbonyl]-L-alanyl -L- alpha- aspartyl-D-2-(2-thienyl) glycyl-L-leucyl-D-tryptophyl] disodium salt), an endothelin ET(A) and ET(B) receptor antagonist. In the paired experiments, air was continuously (4.06 ml/min) infused into the main pulmonary artery for 3 h after the baseline pressures were stabilized. Sheep were treated or not treated with FR139317 or TAK-044. Pulmonary artery pressure was significantly higher than the baseline pressure after the start of air infusion. Both FR139317 and TAK-044 significantly attenuated the increase in pulmonary artery pressure during air embolization. Plasma endothelin -1 levels in both pulmonary and systemic arteries were equally and significantly increased after the start of air infusion. The results indicate that endothelin-1 release is attributable to the development of pulmonary hypertension during the course of air embolization in awake sheep.

Obesity drugs and the heart

Appetite suppressant-related pulmonary hypertension and valvular heart disease are established disorders. Currently, the mechanism of these disorders is not certain. An estimated 6 million Americans and 70 million persons worldwide have been exposed to fenfluramine and dexfenfluramine. The clinical significance and long-term prognosis of cardiovascular effects and, thus, the potential public health effect of these disorders are not known. Longitudinal studies are required to further evaluate these disease processes. In addition, although isolated cases of regression of pulmonary hypertension and valve disease have been reported after the cessation of appetite suppressant therapy, the natural history remains uncertain.

ET-1-induced pulmonary vasoconstriction shifts from ET(A)- to ET(B)-receptor-mediated reaction after preconstriction

Endothelin-1 (ET-1) has been reported to induce pulmonary vasoconstriction via either ET(A) or ET(B) receptors, and vasorelaxation after ET-1 injection has been observed. Our study investigated the effects of ET-1 in isolated rabbit lungs, which were studied at basal tone (part I) and after preconstriction (U-46619; part II). Pulmonary arterial pressure (PAP) and lung weight gain were monitored continuously. In part I, ET-1 (10(-8) M; n = 6; control) was injected after pretreatment with the ET(A)-receptor antagonist BQ-123 (10(-6) M; n = 6) or the ET(B)-receptor antagonist BQ-788 (10(-6) M; n = 6). The same protocol was carried out in part II after elevation of pulmonary vascular tone. ET-1 induced an immediate PAP increase (DeltaPAP 4.3 +/- 0.4 mmHg at 10 min) that was attenuated by pretreatment with BQ-123 (P < 0.05 at 10 min and P < 0.01 thereafter) and that was more pronounced after BQ-788 (P < 0.01 at 10 min and P < 0.001 thereafter). In part II, ET-1 induced an immediate rise in PAP with a maximum after 5 min (DeltaPAP 6.3 +/- 1.4 mmHg), leveling off at DeltaPAP 3.2 +/- 0.2 mmHg after 15 min. Pretreatment with BQ-123 failed to attenuate the increase. BQ-788 significantly reduced the peak pressure at 5 min (0.75 +/- 0.4 mmHg; P < 0.001) as well as the plateau pressure thereafter (P < 0.01). We conclude that ET-1 administration causes pulmonary vasoconstriction independent of basal vascular tone, and, at normal vascular tone, the vasoconstriction seems to be mediated via ET(A) receptors. BQ-788 treatment resulted in even more pronounced vasoconstriction. After pulmonary preconstriction, ET(A) antagonism exerted no effects on PAP, whereas ET(B) antagonism blocked the PAP increase. Therefore, ET-1-induced pulmonary vasoconstriction is shifted from an ET(A)-related to an ET(B)-mediated mechanism after pulmonary vascular preconstriction.

Acute pulmonary hypertension after cardiopulmonary bypass in pig: the role of endogenous endothelin

BACKGROUND

Acute pulmonary hypertension occurring after cardiopulmonary bypass can be a cause of post-operative morbidity and mortality. The purpose of this study was to investigate whether bosentan, a non-peptidic mixed endothelin antagonist affected the pulmonary hypertension induced by experimental cardiopulmonary bypass.

METHODS

Pigs were anesthetized and instrumented to determine hemodynamic measurements. Pigs were randomized to receive either 3 mg/kg bolus + 7 mg/kg per h bosentan (n = 8) or saline (n = 7). All pigs underwent 90 min of cardiopulmonary bypass and were further observed for a 120-min period.

RESULTS

In the control group, cardiopulmonary bypass induced a dramatic pulmonary hypertension (+78 +/- 13%, P < 0.005) and accompanied an increase of pulmonary vascular resistance (+228 +/- 50%, P < 0.005), whereas, in the treated group, bosentan completely prevented these deleterious effects of cardiopulmonary bypass with only a moderate decrease of systemic vascular resistance (-19 +/- 14.6%, P < 0.05).

CONCLUSIONS

The present findings support the hypothesis that endogenous endothelin is a mediator of acute pulmonary hypertension occurring after cardiopulmonary bypass. Bosentan, a mixed endothelin antagonist completely prevented pulmonary hypertension after cardiopulmonary bypass and may, therefore, have therapeutic applications in the management of patients following cardiac surgery.

The ET(A)-Receptor Antagonist LU 135252 Prevents the Progression of Established Pulmonary Hypertension Induced by Monocrotaline in Rats

BACKGROUND: An imbalance between the nitric oxide (NO) and endothelin systems may contribute to the development of pulmonary hypertension (PH). We evaluated the effect of the specific ET(A)-receptor antagonist LU 135252 (LU) in rats with established monocrotaline (MCT)-induced PH and the involvement of NO in the control of pulmonary vascular tone. METHODS AND RESULTS: Two weeks after MCT, rats developed PH with a right ventricular pressure (RVP) of 42.3 +/- 8.5 vs 28.2 +/- 4.1 mmHg for controls (mean +/- SD, P <.05). Daily oral therapy with LU (50 mg/kg) or saline was started 2 weeks post-MCT injection for 20 days. LU increased the survival rate nonsignificantly from 41.7% to 66.7%. The surviving MCT + saline rats showed severe PH (RVP of 82.5 +/- 8.9 mmHg) and RV hypertrophy with a right-to-left ventricle + septum weight ratio of 69.6% +/- 10.2%, which were improved by LU to 53.5 +/- 11.1 mmHg and 53.7% +/- 9.9%, respectively (P <.01). In isolated lungs, pulmonary vascular compliance was reduced by PH and unaffected by LU therapy. After the NO synthase inhibitor N(omega)-nitro-L-arginine (10(-4) mol/L), compliance was further reduced, although much less so, in the LU-treated group (P <.01). CONCLUSIONS: In this model, ET(A) antagonist therapy has a favorable effect on survival and pulmonary hemodynamics and reduces the dependency on NO for the attenuation of reduced vascular compliance.

Discovery and development of an endothelin A receptor-selective antagonist PD 156707

PD 156707 is a highly potent, selective antagonist of the ETA receptor that has demonstrated efficacy in a number of different disease models. The next few years will be exciting in the field of ET research as several compounds progress through clinical development. It is our hope that the efficacy that data demonstrated to date with PD 156707 will some day be translated into real hope for the patients who are waiting beyond the confines of our research laboratories.

Chronic pulmonary hypertension--the monocrotaline model and involvement of the hemostatic system

Monocrotaline (MCT) is a toxic pyrrolizidine alkaloid of plant origin. Administration of small doses of MCT or its active metabolite, monocrotaline pyrrole (MCTP), to rats causes delayed and progressive lung injury characterized by pulmonary vascular remodeling, pulmonary hypertension, and compensatory right heart hypertrophy. The lesions induced by MCT(P) administration in rats are similar to those observed in certain chronic pulmonary vascular diseases of people. This review begins with a synopsis of the hemostatic system, emphasizing the role of endothelium since endothelial cell dysfunction likely underlies the pathogenesis of MCT(P)-induced pneumotoxicity. MCT toxicology is discussed, focusing on morphologic, pulmonary mechanical, hemodynamic, and biochemical and molecular alterations that occur after toxicant exposure. Fibrin and platelet thrombosis of the pulmonary microvasculature occurs after administration of MCT(P) to rats, and several investigators have hypothesized that thrombi contribute to the lung injury and pulmonary hypertension. The evidence for involvement of the various components of the hemostatic system in MCT(P)-induced vascular injury and remodeling is reviewed. Current evidence is consistent with involvement of platelets and an altered fibrinolytic system, yet much remains to be learned about specific events and signals in the vascular pathogenesis.

Reduced pulmonary metabolism of endothelin-1 in canine tachycardia-induced heart failure

OBJECTIVES

Plasma endothelin-1 (ET-1) increases in congestive heart failure (CHF). The pulmonary vascular bed could contribute to this increase through a reduced clearance. We evaluated the effect of tachycardia-induced CHF on pulmonary ET-1 kinetics. To discern between changes due to variations in pulmonary hemodynamics from true alterations of endothelial cell functions, we quantified ET-1 kinetics in isolated rat lungs under variable pressure and flow-rate conditions.

METHODS AND RESULTS

Indicator-dilution studies were performed in anesthetized dogs (n = 14) before and 3 weeks after rapid ventricular pacing and in isolated lungs from healthy rats (n = 4). In isolated lungs, graded increases in perfusion rate from 5-25 ml/min caused gradual reductions in ET-1 extraction from 60 +/- 1.5% to 17 +/- 4.9% (mean +/- S.D.). The capacity to clear ET-1 from the circulation, as computed from the permeability-surface area product (PS), however did not vary over this range of flows. CHF increased plasma ET-1 (11.2 +/- 11.4 vs. 5.2 +/- 1.6 fmol/ml, p < 0.01), did not affect pulmonary ET-1 extraction (29.4 +/- 12.5% vs. 29.9 +/- 12.9%), but decreased the PS (8.3 +/- 5.4 cm3/s vs. 14.4 +/- 9.9 cm3/s, p = 0.038). Contrary to the invariability of the PS in normal isolated rat lungs, CHF was associated with a positive relationship between the PS and pulmonary plasma flow (r = 0.65, p < 0.01). ET-1 binding studies in lung tissues showed no significant variations in ETA and ETB receptors densities but revealed a threefold decrease in binding affinity (p < 0.01) that may explain the reduced clearance.

CONCLUSION

CHF causes a reduction of pulmonary ET-1 clearance that likely contributes to the increased circulating ET-1 levels and reflects pulmonary metabolic dysfunction associated with this condition.

EndothelinA receptor blockade improves nitric oxide-mediated vasodilation in monocrotaline-induced pulmonary hypertension

BACKGROUND

Nitric oxide (NO) and endothelin (ET) have been implicated in the pathogenesis of pulmonary hypertension (PH). Chronic ETA antagonist therapy reduces PH in monocrotaline (MCT)-treated rats. Interactions between the L-arginine-NO pathway and the ET system have been described. We therefore studied the effect of long-term treatment with an oral ETA antagonist (LU 135252) on NO-related vasodilation in isolated lungs from control rats and rats with MCT-induced PH.

METHODS AND RESULTS

Three weeks after MCT injection, PH was associated with an increase in right ventricular pressure (from 27.4 +/- 0.9 to 66.6 +/- 4.1 mm Hg) and a decrease in endothelium-independent vasodilation in response to sodium nitroprusside (10(-10) to 10(-5) mol/L; delta Emax, from 11.1 +/- 0.9 to 2.7 +/- 0.3 mm Hg). Endothelium-dependent vasodilation in response to acetylcholine (10(-9) to 10(-4) mol/L) and the calcium ionophore A23187 (10(-9) to 10(-7) mol/L) remained unaffected. Treatment with LU 135252 did not significantly affect the endothelium-dependent and -independent vasodilations in control rats. However, in MCT-treated rats, LU 135252 therapy significantly reduced right ventricular pressure (39.7 +/- 2.1 mm Hg), potentiated acetylcholine-induced vasodilatation (delta Emax, from 1.6 +/- 0.2 to 3.7 +/- 0.4 mm Hg), and improved the responses to sodium nitroprusside (delta Emax, from 2.7 +/- 0.3 to 5.6 +/- 0.6 mm Hg). LU 135252 did not significantly alter the non-receptor-mediated endothelium-dependent vasodilation to A23187 or pulmonary constitutive NO synthase activity.

CONCLUSIONS

MCT PH is associated with a reduced smooth muscle responsiveness to NO but a maintained endothelium-dependent vasodilatory potency. Long-term ETA antagonist therapy not only restores smooth muscle responsiveness to NO but also increases endothelium-dependent dilation in response to acetylcholine. This mechanism may contribute to the therapeutic benefit of ETA antagonists in PH.

Selective or nonselective endothelin antagonists in porcine hypoxic pulmonary hypertension?

We evaluated the hemodynamic effects of various endothelin (ET) receptor antagonists using selective and non-selective ETA and ETB receptor blockade in hypoxic pigs in vivo. BMS-182874 (10 mg/kg i.v.) TBC-11251z (10 mg/kg i.v.), selective ETA receptor antagonists, attenuated hypoxia (FiO2 0.1)-evoked increase in pulmonary arterial pressure and pulmonary vascular resistance. Bosentan (10 mg/kg i.v.) a non-selective ETA and ETB receptor antagonist, cause essentially similar effects as ETA antagonism alone. No effects were observed after selective ETB blockade using BQ-788 (total dose 1 mg i.v.). The vasoconstrictor effect of exogenous ET was most effectively antagonized by TBC-11251z, followed by BMS-182874 and bosentan. The pulmonary vasodilator effect exerted by exogenous ET during hypoxia was reversed by BQ-788 and bosentan but not by TBC-11251z or BMS-182874. We can therefore conclude that ET contributes to hypoxic pulmonary vasoconstriction primarily through ETA receptor activation.

Prostaglandin E1 antagonizes hypoxic pulmonary vasoconstriction but reduces systemic blood pressure in dogs

OBJECTIVE

To investigate whether prostaglandin E1 (PGE1) directly inhibits hypoxic pulmonary vasoconstriction in dogs.

DESIGN

Prospective, longitudinal study.

SETTING

University research laboratory.

SUBJECTS

Six mongrel dogs in vivo.

INTERVENTIONS

The left thorax of anesthetized and ventilated dogs was opened and the left lower lobe was separately ventilated. The tip of the thermodilution pulmonary artery catheter was introduced into the left lower lobe pulmonary artery. The left lower lobe was ventilated with hyperoxic (95% oxygen and 5% CO2) or hypoxic (95% nitrogen and 5% CO2) gas. By manipulating the occluders placed on both pulmonary arteries, blood flow in the left lower lobe was regulated. Continuous pressure-flow plots for the left lower lobe were then obtained.

MEASUREMENTS AND MAIN RESULTS

Measurements included continuous pressure-flow plot generation, thermodilution cardiac output and blood flow in the left lower lobe, and blood gas analysis. Alveolar hypoxia of the left lower lobe caused blood flow in the left lower lobe to decrease from 371.8 +/- 63.4 to 95.0 +/- 23.4 mL/min and shifted the pressure-flow plot to the right, with a decreased slope and with an increase in the pressure-axis intercept. Subsequently, systemic venous infusion of PGE1 at a rate of 0.3 microg/kg/min had no effect on the pressure-flow plot configuration, blood flow in the left lower lobe, pulmonary vascular resistance, systemic vascular resistance, and PaO2. However, there was a decrease in the pressure-axis intercept of the pressure-flow plot. Infusion of PGE1 at a rate of 3.0 microg/kg/min (high-dose) during hypoxia reduced pulmonary vascular resistance and systemic vascular resistance by 19% and 25%, respectively, and returned the pressure-flow plot toward normal while blood flow in the left lower lobe increased to 122.6 +/- 21.0 mL/min. Consequently, PaO2 decreased from 270 +/- 31 to 144 +/- 32 torr (36.0 +/- 4.1 to 19.2 +/- 4.3 kPa).

CONCLUSION

High-dose PGE1 essentially inhibits hypoxic pulmonary vasoconstriction, at the expense of a deterioration in pulmonary gas exchange and systemic blood pressure in dogs.

Endothelin-1: possible implications in pulmonary vascular disease

The vasoactive properties of endothelin-1 (ET-1) in the animal model very with the tone of the pulmonary vessels, the dose level of ET-1, and the maturation of the vessels. The action of ET-1 is mediated by endothelium-derived nitric oxide, prostaglandins, and electrolytes. Plasma levels of ET-1 are elevated in pulmonary hypertension in both animals and humans. ET-1 antagonists may prove useful in treating pulmonary hypertension in children and adults.