Role of the Endothelin System in Secondary Pulmonary Hypertension Related to Air Embolism: Lessons Learned from Testing Four Classes of Endothelin Blockers in a Rat Model

A rat model of acute pulmonary air embolism (APAE) was developed. These animals had a higher right ventricular systolic pressure (RVSP) (+ 69% at 15-minute peak, and 21-34% at 30-180 minutes), as well as a reduced mean arterial blood pressure (10-20% at 60-180 minutes), heart rate (20-26% at 60-180 minutes) and PaO2 (9-11% at 30-180 minutes) compared with control rats. The role of the endothelin (ET) system, known to be involved in pulmonary hypertension of various etiologies, was investigated by evaluating the effect of the four classes of ET blockers: ET-converting enzyme inhibitor (ECEi) (CGS 35066), selective endothelin-A receptor antagonist (ETA-Ra) (Atrasentan, ABT-627), endothelin-B receptor antagonist (ETB-Ra) (A-192621) or mixed endothelin-A/endothelin-B receptor antagonist (ETA/B-Ra) (A-182086) in this animal model. All four were effective, to various degrees, at reducing the APAE-induced rise in RVSP. The relative efficacy of those compounds in reducing the acute elevation (15 minutes) of RVSP was ECEi >or= ETA/B-Ra >> ETA-Ra = ETB-Ra. The sustained elevation (30-180 minutes) of RVSP was totally abolished by ECEi and attenuated by other ET blockers with a relative efficacy of ETA-Ra > ETA/B-Ra >or= ETB-Ra. ET receptor antagonists did not affect right ventricular basal tone (control rats) whereas ECEi reduced it by up to 12% after 2 hours. The APAE reduction in mean arterial blood pressure was unaffected by ETARa, was completely normalized by ETB-Ra, but was further reduced by either ETA/B-Ra or ECEi. The basal mean arterial blood pressure in control rats was unaffected by ETA-Ra, was elevated by ETB-Ra, but was depressed by ETA/B-Ra and ECEi. All ET blockers maintained normal oxygen saturation in APAE. These results support a role for ETs in rat APAE, since ET blockers can attenuate the cardiopulmonary deterioration and blood gas exchange. However, modulation of the central hemodynamic profile is more complex and may limit the usefulness of some ET blockers.

Interaction between endothelial heme oxygenase-2 and endothelin-1 in altered aortic reactivity after hypoxia in rats

The aim of this study was to determine whether increased expression of heme oxygenase (HO) contributes to impairment of aortic contractile responses after hypoxia through effects on reactivity to endothelin-1 (ET-1). Thoracic aortas from normoxic rats and rats exposed to hypoxia (10% O2) for 16 or 48 h were mounted in organ bath myographs for contractile studies, fixed in paraformaldehyde, or frozen in liquid nitrogen for protein extraction. In rings from normoxic rats, the HO inhibitor tin protoporphyrin IX (SnPP IX, 10 microM) did not alter the response to phenylephrine or ET-1. In rings from rats exposed to 16-h hypoxia, maximum tension generated in response to these agonists was higher in endothelium-intact but not -denuded rings in the presence of SnPP IX. In rings from rats exposed to 48-h hypoxia SnPP IX increased contraction in endothelium-intact but not -denuded rings. In endothelium-intact aortic rings from rats exposed to 16-h hypoxia incubated with endothelin A receptor-specific antagonist BQ-123 (10(-7) M), SnPP IX did not alter phenylephrine-induced contraction. Aortic ET-1 protein levels, measured by radioimmunoassay, were increased in rats exposed to hypoxia for 16 and 48 h. Western blotting showed that HO-1 and HO-2 protein were increased after 16 h of hypoxia and returned to near-control levels after 48 h. Increase in HO-1 protein was detected in endothelium-intact and -denuded rings. Removal of endothelium abolished the increase in HO-2 immunoreactivity. Immunohistochemistry localized expression of HO-1 protein to vascular smooth muscle, whereas HO-2 was only detected in endothelium. HO-2 is expressed by aortic endothelial cells early during hypoxic exposure and impairs ET-1-mediated potentiation of contraction to alpha-adrenoceptor stimulation.

Classification and risk stratification of patients with acute chest pain using a low discriminatory level of cardiac troponin T

BACKGROUND

Cardiac troponins are the biochemical markers of choice for the evaluation of acute coronary syndromes (ACS). Using the first-generation test, most studies related adverse outcome to > 0.20 or 0.10 microg/l cardiac troponin T (cTnT) levels. With the highly sensitive and specific second- and third-generation assays, cTnT is undetectable in most healthy individuals.

HYPOTHESIS

We evaluated whether a lower cTnT level, within 24 h of admission, could indicate an increased risk of future complications.

METHODS

During 1998-1999, clinical data were collected in 260 patients with ACS. Cardiac troponin T was measured at arrival, and 4, 8, and 12-24 h thereafter. The maximum cTnT value was then used to assess, over a 15-month follow-up period, the cumulative risk of death or myocardial infarction (MI), as well as rates of events according to quartiles of cTnT values.

RESULTS

Patients with < or = 0.03 microg/l cTnT levels had the lowest rate of adverse events and the best Kaplan-Meier event-free survival curve. Increasing cTnT levels were associated with stepwise increases in mortality rates and with a constant 10-fold increase in MI rates during follow-up.

CONCLUSIONS

A low threshold cTnT elevation is recommended to assess the risk of ACS. All cTnT elevations > 0.03 microg/l predict a higher risk of MI during follow-up, whereas increasing values predict mortality in relation to the amount of elevation.

Activation of the right ventricular endothelin (ET) system in the monocrotaline model of pulmonary hypertension: response to chronic ETA receptor blockade

Although activation of the endothelin (ET) system contributes to pulmonary hypertension, modifications of the cardiopulmonary ET system and its responses to chronic ET receptor blockade are not well known. To investigate this, rats were injected with monocrotaline (60 mg/kg intraperitoneal) or saline, followed with treatment with the selective ETA receptor antagonist LU135252 (LU; 50 mg.kg(-1).day(-1)) or with saline. After 3 weeks, haemodynamics, cardiac hypertrophy, ET-1 levels and cardiopulmonary ET-receptor-binding profile were evaluated. Monocrotaline (n =7) elicited marked pulmonary hypertension and right ventricular hypertrophy compared with controls (n =8). Both variables were substantially attenuated by LU therapy (n =8; P <0.05 for both). After monocrotaline, right ventricular ET-1 levels were more significantly increased than in the left ventricle (+198% compared with +127%; P <0.05). ETB receptor density was augmented (3-fold) in the right ventricle, whereas that of ETA receptors was not affected. LU treatment also significantly attenuated these alterations (P <0.05). In the lungs, ET-1 levels were not increased after monocrotaline, whereas the balance of ETB to ETA receptors was altered, with a trend toward a lower percentage of ETB than in the control rats. LU treatment did not affect these variables in the lungs. Therefore monocrotaline-induced pulmonary hypertension and right ventricular hypertrophy are associated with the up-regulation of ET-1 and ETB receptors in the right ventricle. These alterations are attenuated with the reduction of pulmonary hypertension and right ventricular hypertrophy after chronic blockade of the ETA receptors, supporting the role of the ET system in right ventricular hypertrophy.

Role of endothelins in septic, cardiogenic, and hemorrhagic shock

Shock is a condition where blood flow is inadequate for tissue needs. In all forms of shock, the concentrations of endothelins (ETs) are elevated, and they are especially high in septic shock. The rise in ETs plasma levels may initially have some positive homeostatic effects, for ETs can help restore normal vascular tone. However, high levels of ETs compromise the appropriate matching of flow to tissue needs and contribute to the pathophysiology of shock. Attempts at regulating the effects of ETs by the use of pharmacological blockers is made complicated by important interactions between the ETA and ETB receptors and potentially different effects on different tissues. We conclude that antagonism of ET receptors is unlikely to be helpful for cardiogenic or hemorrhagic shock. Furthermore, selective blockade is unlikely to be helpful. However, moderate doses of a mixed ET receptor antagonist may be of use for the management of septic patients.

The endothelin system and its role in acute myocardial infarction

Immediately after an acute myocardial infarction (AMI) or in models of ischemia-reperfusion injury, cardiac endothelin (ET) system is markedly activated, and plasma levels of ET are increased. In the heart, expression of the main components of the ET system (ET-1 peptide, both receptor subtypes ETA and ETB, though not endothelin converting enzyme) are increased both at the gene level and protein level, in the viable myocardium, and--even more substantially--in the necrotic area. Despite these conspicuous abnormalities, the role of ET in this setting remains unclear. In the absence of human data, most short-term studies in animals (in terms of hours to up to 8 days post-AMI) and in the reperfused ischemic heart, have found beneficial effects of ET receptor blockade on survival rate, incidence of arrhythmias, cardiac function, and morphology. In contrast, many studies in which a long-term ET inhibition was started immediately post-infarction and the late effects were examined in animals with ensuing chronic heart failure (14-100 days postinfarction), adverse effects were also observed, such as scar thinning, further ventricular dilation, or even a worse survival rate. It appears that the ET system plays a dual role during the early post-AMI period. At present, it is not clear whether the short-term beneficial effects or long-term adverse effects of ET receptor blockade would prevail. Acute use of short-acting ET receptor antagonists in patients with AMI complicated by an acute heart failure is an attractive possibility that also remains to be investigated.

Role of endothelins in congestive heart failure

Despite major advances in conventional medical therapy, patients with heart failure continue to experience significant morbidity and mortality. Endothelin-1 (ET-1) is a potent vasocontrictor and mitogenic peptide that is activated in heart failure. There is increasing experimental and clinical evidence in support of an important role of ET-1 in the pathophysiology of heart failure. Manipulation of the activity of ET-1, especially using endothelin receptor blockers, has allowed for the further elucidation of the role of this neurohormonal system and development of novel therapeutic strategies in heart failure. Published clinical studies of these agents to date have involved relatively small numbers of patients with severe heart failure, followed for a relatively short period of time, and have mainly examined surrogate endpoints. Large-scale trials that address to hard clinical outcomes are ongoing and their results forthcoming. A key question that remains concerns whether selective ETA or dual ETA-ETB receptor blockade will be more effective.

Myocardial contractile responsiveness to endothelin-1 in the post-infarction rat model of heart failure: effects of chronic quinapril

Cardiac endothelin-1 (ET-1) levels and ET receptor expression are increased in congestive heart failure (CHF). In order to determine whether this results in increased responsiveness of ET-A or ET-B receptors to ET-1, we evaluated the contractile effects of ET-1 in isolated papillary muscles isolated from hearts of control rats and from rats 4 weeks post myocardial infarction (MI) having received no therapy or chronic quinapril therapy. The ET-1 dose-response was biphasic in normal muscles. The use of the selective ET-A receptor antagonist BQ123 and the selective ET-B receptor antagonist BQ788 revealed that the initial decrease in tension was the result of ET-B receptor stimulation. Blockade of nitric oxide (NO) production with L-NAME abolished the initial decrease in tension. MI resulted in CHF that was partially reversed by quinapril. In MI, the positive inotropic effects of ET-1 were enhanced due to the loss of the initial ET-B receptor mediated decrease in tension, as well as an increase in the positive inotropic effects of ET-A receptors. This was associated with an increase in ET-A and ET-B receptor mRNA and a decrease in cardiac ecNOS protein. Four weeks of therapy with quinapril attenuated the positive inotropic effects of ET-1 and prevented the increase in ET-A receptor mRNA. Although quinapril did not restore the effects of ET-B receptor stimulation or prevent the increase in ET-B mRNA, it did restore cardiac ecNOS protein expression. Thus, the inotropic response to ET-1 is biphasic due to an overall positive inotropic effect of ET-A receptor stimulation and an ET-B receptor mediated decrease in contractility at low ET-1 concentrations which appears to be mediated by cardiac ecNOS (NO). In post-MI CHF, responsiveness to ET-A receptors increases and the ET-B mediated negative inotropic response is lost despite an increase in both receptor subtypes. Quinapril therapy attenuates these effects and normalises cardiac ecNOS protein.

Long-term effects of nonselective endothelin A and B receptor antagonism in postinfarction rat: importance of timing

BACKGROUND

Some controversy exists as to the effects of endothelin (ET) receptor antagonism on long-term post-myocardial infarction (MI) evolution, particularly as it relates to the timing of the intervention after MI (<24 hours versus 10 days).

METHODS AND RESULTS

Sham rats and rats surviving an acute MI for >20 hours (n=301) were assigned to treatment with saline or the nonselective ET(A) and ET(B) receptor antagonist LU 420627 (LU) started <24 hours (early) or 10 days (late) after MI and continued for 100 days. Long-term LU treatment led to increased mortality of rats with large MI, regardless of the timing of initiation of therapy. Early initiation of LU reduced survival from 61% to 16% (P<0.001 versus untreated), and later initiation reduced survival to 36% (P=0.012 versus untreated and P<0.001 versus early initiation). Early initiation of LU led to scar thinning, further left ventricular (LV) dilatation, LV dysfunction, and an excessive rise in right ventricular systolic pressure. Later initiation of LU did not modify scar formation but resulted in LV dilatation and dysfunction compared with the untreated group. Cardiac fibrosis tended to increase in the LU-treated MI groups. LU in the sham group reduced cardiac endothelial constitutive nitric oxide synthase but did not modify the changes that occurred with a large MI.

CONCLUSIONS

The use of the nonselective ET(A) and ET(B) receptor antagonist LU results in reduced survival, ventricular dilatation, and dysfunction whether started early or late after MI. Early initiation of LU resulted in scar expansion and a particularly unfavorable outcome.

Transient involvement of endothelin in hypertrophic remodeling of small arteries

OBJECTIVE

This study was designed to evaluate the capacity of norepinephrine (NE) to induce hypertrophic remodeling of small arteries in rats, and to determine the involvement of endothelin (ET) to initiate and maintain it.

DESIGN AND RESULTS

Treatment with NE (2.5 microg/kg per min) for 14 or 28 days produced a similar inward hypertrophic remodeling, characterized by a smaller lumen, but increased media thickness and cross-sectional area. Arterial stiffness was reduced. Histological evaluation confirmed the hypertrophic nature of remodeling. Concomitant administration of LU135252 (ET-receptor antagonist) for the first 14 days of NE administration prevented the development of hypertrophy, without altering arterial mechanics. Treatment with the same antagonist from day 14 to day 28 of NE or angiotensin II (Ang II) treatment failed to regress established vascular hypertrophy. In contrast, normalization of arterial structure was observed with prazosin, an alpha-adrenergic blocker. Endothelin content in small mesenteric arteries showed a transient elevation following chronic NE administration.

CONCLUSIONS

Increased circulating NE levels are associated with hypertrophic remodeling of small arteries, in which ET plays an initiating role. However, the maintenance of vascular hypertrophy is ET-independent, either in the presence of augmented circulating levels of NE or Ang II. Thus, early rather than late treatment with ET-receptor antagonists may be a preferable approach to limit small artery-mediated end-organ damage in cardiovascular diseases.

Regional distribution of endothelin-1 and endothelin converting enzyme-1 in porcine endotoxemia

Endothelin-1 (ET-1) levels are markedly increased in sepsis. Since ET-1 is primarily transcriptionally regulated, there should be a corresponding increase in pre-pro-endothelin-1 (ppET-1). Our objective was to determine whether ppET-1 is increased in pigs with a low systemic vascular resistance. We also examined the distribution of ET-1 and the regulation of endothelin-converting enzyme 1 (ECE-1), the rate limiting enzyme in ET-1 production. We anesthetized and ventilated 16 pigs. We measured arterial, pulmonary, and central venous pressures, as well as cardiac output. ET-1 was measured by radioimmunoassay in plasma and in multiple tissues. We infused 20 microg/kg of endotoxin over 2 h and then sacrificed the animals. ppET-1 and ECE-1 mRNA were assessed by Northern analysis. We performed immunohistochemistry for the assessment of tissue ET-1 and ECE-1. The systemic vascular resistance rose at 30 min, but fell by 120 min. Plasma ET-1 more than doubled by 2 h. However, there was no change in the concentration of ET-1 in any tissue except in the pulmonary artery. By immunohistochemistry, there was also no change in ET-1 in aorta, vena cava, heart, lung, liver, and kidney. Distribution of ECE-1 followed that of ET-1 on immunohistochemistry. There was a significant increase in ppET-1 mRNA in liver, kidney papillae, and vena cava, and a tendency for an increase in other tissues. This was paralleled by an increase in ECE-1 mRNA. In conclusion, the amount of ECE-1 mRNA and protein parallel those of ET-1. Endotoxemia is associated with a marked increase in plasma ET-1 and an increase in ppET-1 and ECE-1 mRNA in multiple tissues; however, there was no significant change in tissue ET-1 except in the pulmonary artery. The rise in plasma levels without a change in tissue levels suggests a greater release into the vasculature in sepsis than under normal conditions.

Intravascular ultrasound assessment of pulmonary vascular disease in patients with pulmonary hypertension

BACKGROUND

Measurements of pulmonary pressure and resistance are still considered to be the "gold standard" in the evaluation of pulmonary hypertension (PH), despite their limitations in predicting irreversible disease. Hemodynamic assessment also only provides a global evaluation of the pulmonary vascular bed, whereas PH is an inhomogeneous disease of the vessel wall.

METHODS AND RESULTS

We assessed the value of intravascular ultrasound (IVUS) in 30 patients with suspected PH and correlated the structural changes in distal pulmonary arteries found on IVUS with conventional hemodynamic data. Plasma endothelin (ET)-1 levels and pulmonary ET-1 extraction also were measured as markers of the severity of PH. The anatomic abnormalities revealed by IVUS were more severe in the lower lobes than in the upper lobes, as evidenced by the greater percentage of wall thickness (WT), the smaller lumen diameter/WT and lumen area/total vessel area (p < 0.05 for each). IVUS anatomic indexes correlated directly with hemodynamic data (eg, with pulmonary arterial systolic pressure; r = 0.56; p < 0.001) and ET-1 levels but inversely with pulmonary ET-1 extraction.

CONCLUSION

Patients with PH have greater pulmonary arterial WT that is more severe in the lower lobes than in the upper lobes. The severity of structural abnormalities found on IVUS is directly correlated with hemodynamic findings and ET-1 levels. IVUS may provide useful additional information in the assessment of patients with PH.

Pulmonary metabolism of endothelin 1 during on-pump and beating heart coronary artery bypass operations

BACKGROUND

Coronary artery bypass operations are associated with increased circulating levels of the powerful vasoconstrictor endothelin 1. The pulmonary circulation is an important site for both production and clearance of endothelin 1. Lung endothelial injury resulting from cardiopulmonary bypass could modify pulmonary endothelin 1 metabolism through an increase in production, a reduction in removal, or a combination of both.

METHODS

Pulmonary endothelin 1 kinetics were quantified by using the indicator-dilution technique in patients undergoing coronary artery bypass grafting with (n = 11) or without cardiopulmonary bypass (ie, beating heart; n = 10). Mixed venous endothelin 1 levels were also measured in samples from the pulmonary artery, and systemic levels were obtained from the radial artery.

RESULTS

Pulmonary artery endothelin 1 levels were similar before and after cardiopulmonary bypass, with means of 1.59 +/- 0.37 pg/mL and 1.33 +/- 0.15 pg/mL (P =.45), respectively. Systemic endothelin 1 levels, however, increased after bypass from 1.64 +/- 0.22 pg/mL to 2.07 +/- 0.16 pg/mL (P =.01). In the beating heart group, endothelin 1 levels before and after the operation were similar in the pulmonary artery (1.25 +/- 0.27 pg/mL and 1.45 +/- 0.31 pg/mL, respectively; P =.38), as well as in the radial artery (1.70 +/- 0.26 pg/mL and 1.73 +/- 0.35 pg/mL, respectively; P =.92). The capacity to clear endothelin 1 from the pulmonary circulation, as computed from the permeability-surface area product for endothelin 1, was not affected by cardiopulmonary bypass before and after the operation (25.19 +/- 2.67 mL/s and 23.12 +/- 4.39 mL/s, respectively; P =.49). It was similar and also unaffected in the beating heart group.

CONCLUSION

Coronary artery bypass grafting with cardiopulmonary bypass is associated with an increase in systemic endothelin 1 levels. The mechanism involved is not related to a decreased pulmonary clearance of endothelin 1 from the systemic circulation but rather to an increased endothelin 1 release by the lungs.

Effectiveness of a nonselective ET(A/B) and a selective ET(A) antagonist in rats with monocrotaline-induced pulmonary hypertension

BACKGROUND

Both nonselective ET(A/B) receptor and selective ET(A) receptor antagonists can reduce pulmonary hypertension (PH) and right ventricular hypertrophy (RVH) in various animal models. Depending on their net effects after blockade of endothelial and smooth muscle ET(B) receptors, nonselective ET(A/B) antagonists could be more or less effective than selective ET(A) antagonists.

METHODS AND RESULTS

Two weeks after injection of saline or 60 mg/kg monocrotaline (MCT), rats received 50 mg x kg(-1) x d(-1) of a selective (LU135252) or nonselective (BSF420627) antagonist for 3 weeks. This resulted in 4 groups: control (n=15), MCT (n=60), MCT+ET(A) (n=39), and MCT+ET(A/B) (n=40). Five-week survival was 35% in the MCT group; this was increased to 56% in the MCT+ET(A) group (P:=0.10) and to 67% in the MCT+ET(A/B) group (P:=0.0015). Drug administration was stopped 48 hours before hemodynamic measurements to evaluate the chronic effects of therapy: PH in the MCT group (RV systolic pressure 87+/-1 mm Hg) was improved similarly in both MCT+ET(A) and MCT+ET(A/B) groups (72+/-3 and 70+/-3 mm Hg, respectively, P:<0.05). Severe RVH in the MCT group (RV/left ventricle+septum weight ratio 73+/-1%) was not affected by the selective antagonist (70+/-2%) but was reduced to 54+/-2% in the MCT+ET(A/B) group (P:<0.01). Pulmonary resistive properties, assessed from isolated lung pressure-flow relationships, were improved similarly in survivors from both treated groups.

CONCLUSIONS

Both the nonselective ET(A/B) antagonist BSF420627 and the selective ET(A) antagonist LU135252 are effective in this model of PH. Similar direct comparative studies in other models of PH and with various dosage regimens are warranted to define the optimal pharmacological approach of PH when ET receptor antagonists are used.

Influence of dual ET(A)/ET(B)-receptor blockade on coronary responses to treadmill exercise in dogs

We hypothesized that endothelin (ET) release during exercise may be triggered by alpha-adrenergic-receptor activation and thereby influence coronary hemodynamics and O(2) metabolism in dogs. Exercise resulted in coronary blood flow increases (to 1.88+/-0.26 from 1.10+/- 0.12 ml x min(-1) x g(-1)) and in a fall (P<0.01) in coronary sinus O(2) saturation (17.4+/-1.5 to 9.6+/-0.7 vol%), whereas myocardial O(2) consumption (MVO(2)) increased (109+/-13% from 145+/-16 microl O(2) min(-1) x g(-1)). Tezosentan, a dual ET(A)/ET(B)-receptor blocker, slightly reduced mean arterial pressure (MAP) and increased heart rate throughout exercise. The relationship between coronary sinus O(2) saturation and MVO(2) was shifted upward (P<0.05) after tezosentan administration; i.e., as MVO(2) increased during exercise, coronary sinus O(2) saturation was disproportionately higher after ET-receptor blockade. After propranolol, tezosentan resulted in significant decreases (P<0.05) in left ventricular pressure, the first derivative of left ventricular pressure over time, and MAP during exercise. As MVO(2) increased during exercise, coronary sinus O(2) saturation levels after tezosentan became superimposable over those observed before ET-receptor blockade. Thus dual blockade of ET(A)/ET(B) receptors alters coronary hemodynamics and O(2) metabolism during exercise, but ET activity failed to increase beyond baseline levels.

Blockade of endothelin receptors markedly reduces atherosclerosis in LDL receptor deficient mice: role of endothelin in macrophage foam cell formation

OBJECTIVE

We evaluated the direct effects of long-term blockade of ET(A) and ET(B) receptors using a mixed endothelin (ET) receptor antagonist, LU224332, in the low density lipoprotein receptor (LDL-R) knockout mouse model of atherosclerosis.

METHODS

Four groups of LDL-R deficient mice were studied: control mice fed normal chow (group I); mice fed a high cholesterol (HC, 1.25%) diet alone (group II), HC fed animals treated with LU224332 (group III); and mice fed normal chow treated with the LU compound (group IV). All treatments were continued for 8 weeks at which time the animals were sacrificed and the aortae were removed and stained with oil red O. Atherosclerotic area (AA) was determined by quantitative morphometry and normalized relative to total aortic area (TA).

RESULTS

Cholesterol feeding resulted in a marked increased in total plasma cholesterol ( approximately 15 fold) and widespread aortic atherosclerosis (AA/TA: group I: 0.013+/-0.007; group II: 0.33+/-0. 11; P<0.001). Atherosclerotic lesions were characterized by immunohistochemistry as consisting mainly of macrophages which also showed high levels of ET-1 expression. Treatment with ET antagonist significantly reduced the development of atherosclerosis (AA/TA: group III: 0.19+/-0.07, P<0.01 vs. group II), without altering plasma cholesterol levels and blood pressure. The direct effect of LU224332 on macrophage activation and foam-cell formation was determined in vitro using a human macrophage cell line, THP-1. Treatment of the THP-1 cells with LU224332 significantly reduced cholesterol ester and triacylglycerol accumulation and foam-cell formation on exposure to oxidized LDL (P<0.01 and P<0.05, respectively).

CONCLUSION

We conclude that a nonselective ET receptor antagonist substantially inhibited the development of atherosclerosis in a genetic model of hyperlipidemia, possibly by inhibiting macrophage foam-cell formation, suggesting a role for these agents in the treatment and prevention of atherosclerotic vascular disease.

Importance of local production of endothelin-1 and of the ET(B)Receptor in the regulation of pulmonary vascular tone

Interaction between locally released endothelin-1 (ET-1) and the endothelial ET(B)receptor could modulate pulmonary vascular tone. We evaluated pulmonary ET-1 clearance and ET-1-ET(B)receptor interaction in the modulation of pulmonary vascular tone. Controls and rats with Monocrotaline (MCT)-induced pulmonary hypertension (PH) were studied. Lungs were isolated and perfused under constant pressure. The effect of the selective ET(B)antagonist BQ-788 (10(-12)-10(-8)mole) on perfusion flow rate and(125)I-ET-1 extraction was determined. Baseline(125)I-ET-1 extraction was reduced from 62+/-5% in controls to 49+/-10% in PH (P=0.012). BQ-788 inhibited extraction with a higher half-inhibitory dose in the MCT group (-Log ID(50)= 8.9+/-0.4 vs. 9.5+/-0.1, P=0.03). BQ-788 induced a mild reduction in perfusion flow rate of 0.7+/-0.3 ml/min in controls. In the MCT group, this occurred at a lower dose and was more pronounced with a maximal reduction of 3.3+/-0.7 ml/min (P<0.01 vs. control). ET-1 was undetectable in the effluent at baseline but was present in similar concentrations in both groups after ET(B)blockade. Addition of 2 pg/ml ET-1 to lung perfusate did not modify pulmonary ET-1 clearance or the effect of BQ788 on perfusion flow rate in control lungs. In normal rat lungs, the ET(B)receptor plays a minor regulatory role on vascular tone. In MCT hypertension however, despite a reduction in ET(B)mediated extraction, luminal production of ET-1 attenuates the increase in pulmonary vascular tone.

Role of ET(A) receptors in the regulation of vascular reactivity in rats with congestive heart failure

Endothelium-derived nitric oxide (NO) and endothelin (ET)-1 interact to regulate vascular tone. In congestive heart failure (CHF), the release and/or the activity of both factors is affected. We hypothesized that the increased ET-1 production associated with CHF may result in a reduced smooth muscle sensitivity to NO. The aim of this study was to evaluate the effects of a chronic treatment with the ET(A)-receptor (ET receptor A) antagonist LU-135252 (LU) on cerebrovascular reactivity to sodium nitroprusside (SNP) in the rat infarct model of CHF. Rats were subjected to coronary artery ligation and were treated for 4 wk with placebo (n = 24) or LU (50 mg. kg(-1). day(-1), n = 29). CHF was associated with a decreased (P < 0.05) efficacy of SNP to induce relaxation of isolated middle cerebral arteries. Furthermore, neither NO synthase inhibition with N(omega)-nitro-L-arginine (L-NNA) nor endothelial denudation affected the efficacy of SNP. Thus the endothelium no longer influences smooth muscle sensitivity to SNP. LU treatment, however, normalized (P < 0.05) smooth muscle sensitivity to SNP. Sensitivity of ET-1-induced contraction was increased in CHF only in the presence of L-NNA, whereas contraction induced by ET(B) receptor (receptor B) stimulation was increased (P < 0.05) in endothelium-denuded vessels. LU treatment restored these changes in reactivity and revealed a significant endothelium-dependent ET(B)-mediated relaxation after NO synthase inhibition. In conclusion, CHF decreases and uncouples cerebrovascular smooth muscle sensitivity to SNP from endothelial regulation. The observation that chronic ET(A) blockade restored most of the changes associated with CHF suggests that activation of the ET-1 system importantly contributes to the alteration in vascular reactivity observed in experimental CHF.

Endothelin receptor blockade attenuates lipopolysaccharide-induced pulmonary nitric oxide production

Increased nitric oxide (NO) synthesis by the inducible nitric oxide synthase (iNOS) has been shown to contribute to the development of acute lung injury and delayed hypotension in animals injected with bacterial lipopolysaccharides (LPS). Recent evidence indicates that endothelin-1 (ET-1) is also elevated in septic humans and in animals. To assess the contribution of ETs to LPS-induced pulmonary NO production and iNOS expression, we used P1/fl, a 22 amino acid peptide, to selectively antagonize endothelin-A receptors. Anesthetized, mechanically ventilated rats were injected with either saline or LPS (E. coli endotoxin, 20 mg/kg) and studied for 5 h. Two other groups of rats were pretreated 15 min earlier with P1/fl peptide (20 microg/kg). Unlike saline-treated rats, rats injected with LPS showed a progressive decline in arterial pressure and a significant rise in plasma ET concentration and serum nitrite-nitrate level. In the lungs, LPS injection elicited a several-fold rise in lung iNOS activity and exhaled NO concentration and increased lung wet/dry ratio significantly. Pretreatment with P1/fl peptide eliminated the decline in arterial pressure, the rise in lung wet/dry ratio, lung NOS activity, and iNOS protein expression and significantly attenuated the increase in pulmonary exhaled NO production but had no effect on plasma ET concentration. We conclude that activation of ET-A receptors by rising ET-1 concentration enhances NO production and iNOS expression in the respiratory and vascular systems and contributes to both LPS-induced hypotension and acute lung injury.

Kinetics of endothelin-1 binding in the dog liver microcirculation in vivo

Endothelin-1 (ET-1) is a 21-amino acid peptide produced by vascular endothelial cells that acts as a potent constrictor of hepatic sinusoids. Hepatic binding of tracer (125)I-labeled ET-1 was investigated in anesthetized dogs with the multiple-indicator dilution technique with simultaneous measurements of unlabeled immunoreactive ET-1 plasma levels. Despite 80% binding to albumin, tracer (125)I-ET-1 was avidly extracted by the liver, with only 15 +/- 6% of the peptide surviving passage through the organ. Exchange of ET-1 between plasma and binding sites, probably located on the surface of liver cells, was quantitatively described by a barrier-limited, space-distributed variable transit time model. Reversible and irreversible parallel binding sites were found. Reversible and irreversible plasma clearances of unbound (125)I-ET-1 were 0.084 +/- 0.033 ml. s(-1). g liver(-1) and 0.17 +/- 0.09 ml. s(-1). g liver(-1), respectively, and the dissociation rate constant for reversible binding was 0.24 +/- 0.12 s(-1). The specific ET(A) receptor antagonist BMS-182874 did not modify binding to either site. The nonspecific ET(A)/ET(B) antagonist LU-224332 dose-dependently reduced irreversible binding only. ET-1 levels in the hepatic vein were significantly lower than in the portal vein but were not different from those in the hepatic artery. The ratio between hepatic vein and portal vein levels (0.64 +/- 0.31) was considerably higher than survival fractions, suggesting a substantial simultaneous release of newly synthesized or stored ET-1 by the liver. These results demonstrate both substantial clearance and production of ET-1 by the intact liver. Hepatic ET-1 clearance is mediated by the ET(B) receptor, with the presence of reversible, nonspecific ET-1 binding at the liver surface

Cholesterol reduction rapidly improves endothelial function after acute coronary syndromes. The RECIFE (reduction of cholesterol in ischemia and function of the endothelium) trial

BACKGROUND

Cholesterol lowering reduces coronary events. One mechanism could be improvement of endothelial function. In line with this hypothesis, this study investigates whether cholesterol reduction can result in rapid improvement of endothelial function after acute coronary syndromes.

METHODS AND RESULTS

Patients with acute myocardial infarction or unstable angina and total cholesterol levels at admission >/=5.2 mmol/L or LDL >/=3.4 mmol/L were randomized to placebo (n=30) or pravastatin 40 mg daily (n=30) for 6 weeks. Brachial ultrasound was used to measure endothelium-dependent flow-mediated dilatation (FMD) and response to endothelium-independent nitroglycerin. Changes in the levels of markers of platelet activation, coagulation factors, and plasma endothelin levels were also assessed. Total and LDL cholesterol levels were similar at admission and before randomization in both groups. With pravastatin, but not with placebo, they decreased by 23% (P<0.05) and 33% (P<0.01), respectively. FMD was unchanged with placebo, 5.43+/-0.74% (mean+/-SEM) to 5.84+/-0.81%, but increased with pravastatin, 4.93+/-0.81% to 7.0+/-0.79% (P=0.02), representing a 42% relative increase. Responses to nitroglycerin were similar during the time course of the study in the 2 groups. Markers of platelet activity, coagulation factors, and endothelin levels were not affected by pravastatin.

CONCLUSIONS

Cholesterol reduction with pravastatin initiated early after acute coronary syndromes rapidly improves endothelial function after 6 weeks of therapy.

Endothelin A receptor blockade causes adverse left ventricular remodeling but improves pulmonary artery pressure after infarction in the rat

BACKGROUND

Endothelin A (ETA) receptor antagonists have been shown to improve ventricular remodeling and survival in rats when started 10 days after infarction. Whether starting them earlier would have a more or less beneficial effect is uncertain.

METHODS AND RESULTS

Rats surviving an acute myocardial infarction (MI) for 24 hours (n=403) were assigned to saline or the ETA receptor antagonist LU 127043 or its active enantiomer LU 135252 for 4 weeks. Chronic LU treatment had no effect on survival, with 46% of LU rats and 47% of saline-treated rats with large MI surviving to the end of the study. LU treatment led to scar thinning, further left ventricular (LV) dilatation, an increase in LV end-diastolic pressure, and an increase in wet lung weight (P<0.05). Despite this detrimental effect on LV function, LU led to a significant decrease in RV systolic (50+/-2 to 44+/-2 mm Hg, P<0.05 vs saline) and right atrial pressures. LU treatment also prevented the increase in pulmonary ET-1 found in saline-treated rats with large MI but did not modify the increase in cardiac ET-1 in hearts with large MI.

CONCLUSIONS

The early use of the ETA receptor antagonists LU 127043 or its active enantiomer LU 135252 after infarction in the rat leads to impaired scar healing and LV dilatation and dysfunction. This is accompanied by a decrease in RV systolic and right atrial pressures and a decrease in pulmonary but not cardiac ET-1 levels. It would thus appear that the early use of ETA receptor antagonists after infarction may be detrimental.

Reduced pulmonary clearance of endothelin-1 contributes to the increase of circulating levels in heart failure secondary to myocardial infarction

BACKGROUND

The pulmonary vascular bed is a major site for endothelin-1 (ET-1) clearance. A reduced clearance could contribute to the increase in circulating ET-1 levels found in heart failure (HF). We therefore evaluated the effect of HF on pulmonary ET-1 clearance and on plasma ET-1 concentrations.

METHODS AND RESULTS

Rats with myocardial infarction (n=24) were compared with sham-operated rats (n=22). The lungs were isolated and perfused at a constant flow rate of 10 mL/min. Pulmonary ET-1 clearance was measured by the single-bolus indicator-dilution technique with 125I-labeled ET-1. Infarct rats developed HF with mild pulmonary hypertension. ET-1 extraction was reduced by HF from 63+/-1.5% to 41+/-4.5% (mean+/-SEM, P<0.001). Mixed venous (MV) and aortic ET-1 levels doubled with HF. There was a plasma ET-1 gradient across the lungs of sham rats (MV-aortic levels, 0.21+/-0.12 pg/mL) but not in lungs of HF rats (0.01+/-0.17 pg/mL). Plasma ET-1 levels correlated closely and inversely with ET-1 extraction (P<0.001).

CONCLUSIONS

HF is associated with reduced pulmonary ET-1 clearance that contributes to the increase in circulating levels.

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.

Production of endothelins by the ventilatory muscles in septic shock

Circulating endothelin-1 (ET-1) concentration increases significantly in animal models of sepsis. The main mechanism responsible for this rise in ET-1 levels is believed to be upregulation of ET-1 synthesis in various organs, such as the lungs and heart. In this study we investigated whether ET-1 is synthesized in the ventilatory muscles and whether this synthesis is regulated in septic shock. Conscious rats were injected with Escherichia coli endotoxin (lipopolysaccharide [LPS]) and killed 6, 12, and 24 h later. A fourth group of rats was injected with normal saline and served as a control. The diaphragm was excised at the end of the experiment and quickly frozen. Diaphragmatic ET-1 level was measured with radioimmunoassay, and messenger RNA (mRNA) expression of ET-1 precursor prohormone (preproET-1), preproET-3, and endothelin-converting enzyme was measured with reverse transcription-polymerase chain reaction. LPS injection elicited an early (within 6 h) and prolonged rise in diaphragmatic ET-1 concentration. In addition, mRNA levels of preproET-1 and preproET-3 rose by about 4- and 3-fold within 6 to 12 h of LPS injection, whereas mRNA of endothelin-converting enzyme increased by more than 10-fold and peaked within 24 h of LPS injection. Immunostaining with anti-ET-1 antibody revealed positive ET-1 staining in the endothelium and somatic muscle fibers of septic diaphragms. These results indicate that diaphragmatic muscle fibers synthesize significant amounts of ET-1 in septic shock and that the rise in ET-1 production is due to upregulation of ET precursors and the converting enzyme.

Radioreceptor assay of an endothelin A receptor antagonist in plasma and urine

Orally active nonpeptide antagonists of endothelin (ET) receptors may prove beneficial in the treatment of cardiovascular and renal disease. The pharmacodynamics and pharmacokinetics of these drugs are not sufficiently known, and practical methods for their analysis have not been developed. We describe a simple, sensitive, and reproducible radioreceptor assay (RRA) for LU135252, a selective antagonist of the ETA receptor, using porcine aortic smooth muscle membranes as the acceptor and 125I-endothelin-1 as the ligand. With methanol extraction of plasma and urine samples, recovery of LU135252 ranged from 79% to 91% at 60–1000 nmol/L. The logit-log transformed calibration curves constructed with LU135252 added to plasma or to urine were linear (r = 0.993 ± 0.005, n = 11) in the range from 18.7 to 2400 nmol/L. The detection limit with plasma- and urine-based calibration curves was 19 nmol/L. The interassay coefficient of variation was 12.6% at 70 nmol/L (n = 9) and 6.5% at 590 nmol/L (n = 9). Endothelin-1 did not interfere in the RRA at pathophysiologically and clinically relevant concentrations [up to 15 pmol/L (40 pg/mL)]. When LU135252 was added to plasma, the signal was completely stable after storage for 1 week at 4 °C, although there was a modest loss of the signal after 24 h at room temperature. The practical performance of this RRA was then tested in plasma samples obtained from (a) rats after a single oral administration of LU135252, (b) from coronary-ligated rats chronically treated with LU135252, and (c) in plasma and urine samples obtained from dogs during intrarenal infusion of LU135252.

Radioreceptor assay of an endothelin A receptor antagonist in plasma and urine

Orally active nonpeptide antagonists of endothelin (ET) receptors may prove beneficial in the treatment of cardiovascular and renal disease. The pharmacodynamics and pharmacokinetics of these drugs are not sufficiently known, and practical methods for their analysis have not been developed. We describe a simple, sensitive, and reproducible radioreceptor assay (RRA) for LU135252, a selective antagonist of the ETA receptor, using porcine aortic smooth muscle membranes as the acceptor and 125I-endothelin-1 as the ligand. With methanol extraction of plasma and urine samples, recovery of LU135252 ranged from 79% to 91% at 60-1000 nmol/L. The logit-log transformed calibration curves constructed with LU135252 added to plasma or to urine were linear (r = 0.993 +/- 0.005, n = 11) in the range from 18.7 to 2400 nmol/L. The detection limit with plasma- and urine-based calibration curves was 19 nmol/L. The interassay coefficient of variation was 12.6% at 70 nmol/L (n = 9) and 6.5% at 590 nmol/L (n = 9). Endothelin-1 did not interfere in the RRA at pathophysiologically and clinically relevant concentrations [up to 15 pmol/L (40 pg/ mL)]. When LU135252 was added to plasma, the signal was completely stable after storage for 1 week at 4 degrees C, although there was a modest loss of the signal after 24 h at room temperature. The practical performance of this RRA was then tested in plasma samples obtained from (a) rats after a single oral administration of LU135252, (b) from coronary-ligated rats chronically treated with LU135252, and (c) in plasma and urine samples obtained from dogs during intrarenal infusion of LU135252.

Chronic endothelin-1 blockade preserves myocardial contractility in dilated cardiomyopathy

Endothelin-1 (ET-1) is known to have positive inotropic effects in isolated cardiac muscle strips. ET-1 levels are elevated in congestive heart failure (CHF). We investigated the effects of ET-1 on contractility and cardiac relaxation (lusitropy) of the intact healthy murine heart and myocarditic/cardiomyopathic heart by chronic oral treatment with a mixed ETA/ETB blocker SB217242. Chronic ET-1 blockade of normal hearts resulted in depression of contractility and lusitropy of the normal heart but preservation and enhancement of contractility and lusitropy in myocarditic animals, in which ET-1 cardiac content is elevated. This suggests that ET-1 is important in the basal contractility and relaxation of the normal heart but that its chronic elevation in CHF causes impairment of cardiac systolic and diastolic performance.

Acute renal effects of endothelin-A blockade: interspecies differences

The acute renal effects of LU135252 (LU), a selective endothelin-A (ETA) receptor antagonist, were studied in conscious rats after i.p. administration of 1-10 mg/kg LU, and in clearance studies in anesthetized dogs during left intrarenal infusion of 0.01-0.1 mg/kg/min. In the rat (n = 12), LU (10 mg/kg i.p.) decreased diuresis (-36%), excretion of Na (-55%) and Cl (-38%) but not of K and creatinine, as measured in 8-h collections in metabolic cages. Excretion of oral NaCl load (5% of body weight) during 4 h decreased from 68 +/- 2% (vehicle) to 50.5 +/- 5% (LU; n = 12, p < 0.01). Blood pressure was not affected. In contrast, left intrarenal LU infusion at 0.01, 0.03 and 0.1 mg/kg/min in the dog (n = 4) had no effect on renal hemodynamics or excretory function, whereas it mildly decreased blood pressure. In addition, intrarenal LU (0.03 mg/kg/ min; n = 6) had no effect on the renal response to volume expansion (7% bw) by 0.9% NaCl i.v. These markedly different effects of acute ETA blockade were observed at similar systemic plasma levels of LU in the two species. It is concluded that in the rat, but not in the dog, acute blockade of ETA receptors can impair renal excretory function, most likely at the tubule level. This interspecies difference in the role of endogenous ET in the regulation of renal function is probably due to a different ET receptor profile and distribution in rat and dog kidneys.

Endothelin-1 regulates tone of isolated small arteries in the rat: effect of hyperendothelinemia

Chronic elevation of plasma endothelin-1 (ET-1) levels has been reported in several pathological conditions. To investigate the consequences of increased circulating ET-1 on vascular responsiveness, Sprague-Dawley rats (n=16) were chronically instrumented with a minipump delivering ET-1 at a constant dose for 7 days. Plasma ET-1 levels were more than doubled in treated (0.98+/-0.09 pmol/L; P<.05) versus untreated sham-operated rats (0.43+/-0.04 pmol/L), whereas systolic arterial blood pressure increased (139+/-5 versus 128+/-4 mm Hg in untreated rats; P<.05). After rats were killed, segments of middle cerebral (MCA) and mesenteric (MES) arteries were mounted on an isometric myograph. ET-induced contraction was shifted to the right in ET-1-treated animals and not modified by BQ123 (an ET(A) receptor antagonist); bosentan (ET(A/B) receptor antagonist) prevented ET-1-induced contraction in both groups. After inhibition of nitric oxide synthase with N(omega)-nitro-L-arginine (L-NNA), both phenylephrine and oxymetazoline (an alpha2-adrenoceptor agonist) induced MCA contraction. The sensitivity to phenylephrine was decreased in ET-1-treated compared with control rats (P<.05). Sensitivity to phenylephrine-induced contraction was decreased by BQ123 in control rats only. In contrast, L-NNA revealed greater oxymetazoline-induced contractions in treated compared with control MCA rings (P<.05); this potentiation was blunted by bosentan but unaffected by BQ123. Removal of the endothelium revealed a direct constrictor effect of oxymetazoline that was insensitive to L-NNA alone or combined with bosentan; however, oxymetazoline induced significantly lower constriction in treated rat MCA segments. Responses to oxymetazoline were also blunted in treated compared with untreated denuded MES arteries. In conclusion, chronic elevated plasmatic ET-1 decreases smooth muscle cell sensitivity to contractile agonists both in MCA and MES rings. In cerebral vessels, endothelial alpha2-adrenoceptor-dependent stimulation induced greater contractile responses in treated rats which were sensitive to bosentan, suggesting that oxymetazoline stimulates ET-1 release from the endothelium. This may represent a compensatory mechanism for the loss of smooth muscle sensitivity.

Enhancement of aortic contractility by endothelin following prolonged hypoxia in vivo

The purpose of this study was to determine if endothelin-1 (ET-1) mediates endothelium-dependent enhancement of rat aortic contractility following exposure to hypoxia. Rats breathed room air or 10% oxygen for 12 or 48 h. Thoracic aortas and plasma were analysed for ET-1 content by radioimmunassay. Aortic rings were mounted in organ bath myographs for measurement of isometric tension during activation by phenylephrine (10(-9)-10(-4) M), in the presence and absence of BQ-123. In some rings, the endothelium was removed. Plasma ET-1 levels were 0.79+/-0.09 pg/ml, 2.00+/-0.36 and 1.88+/-0.21 pg/ml, in normoxic rats and rats exposed to hypoxia for 12 or 48 h respectively (P<0.001, 12 or 48 h vs. control). Aortic ET-1 concentrations were 202.3+/-20.8 fg/mg in normoxic rats, compared to 274.9+/-40.6 fg/mg and 292.4+/-24.4 fg/mg in rats exposed to hypoxia for 12 and 48 h, respectively (P<0.01, 12 or 48 h vs. control) and 155.0+/-43.1 fg/mg in de-endothelialized aortas from rats exposed to hypoxia for 48 h (P>0.05 vs. normoxic controls). Maximum tension during phenylephrine-induced contraction was 0.46+/-0.04 mg/g and 0.33+/-0.03 mg/g in endothelialized rings from rats exposed to hypoxia for 48 h in the presence and absence of BQ-123, respectively (P<0.05 for difference), and 0.28+/-0.07 mg/g in rings in which the endothelium had been removed. Local endothelin release is an important mechanism by which the responsiveness of the systemic vasculature to agonists may be preserved during hypoxia.

Reduced pulmonary clearance of endothelin-1 in pulmonary hypertension

OBJECTIVE

Pulmonary hypertension (PHT) is associated with increased endothelin-1 (ET-1) levels that correlate with the severity of the disease. The pulmonary circulation is an important site for ET-1 metabolism and may modulate plasma ET-1 through an increase in production, a reduction in removal, or a combination of both. We measured and compared pulmonary metabolism of circulating ET-1 in controls and in patients with PHT.

METHODS AND RESULTS

The indicator-dilution technique was combined with measurements of ET-1 levels to quantify pulmonary metabolism of ET-1 in controls (n = 13) and in patients with PHT (n = 17). ET-1 levels doubled in PHT (p < 0.05) and, although there was no difference between aortic and pulmonary artery levels in controls (0.68+/-0.09 and 0.61+/-0.08 pg/ml, respectively, p = 0.22), they tended to be higher in PHT (1.23+/-0.26 vs 1.07+/-0.19 pg/ml, p = 0.08). Pulmonary extraction of tracer iodine-125-ET-1 was reduced from 47%+/-2.0% in the controls to 34%+/-3.6% in PHT (p = 0.005) and inversely correlated with the severity of pulmonary hypertension (r = -0.524, p = 0.03). Consequently, circulating ET-1 clearance was reduced by PHT from 1424+/-77 ml/min to 892+/-119 ml/min (p < 0.001). Pulmonary production of circulating ET-1 (in picograms per minute) was not different but the quantity of ET-1 that survives passage through the lungs was increased by PHT (1860+/-359 pg/min vs 992+/-152 pg/min, p = 0.037).

CONCLUSION

PHT is associated with a reduced pulmonary clearance of ET-1 that contributes to the increase in circulating levels.

Impaired endothelin-1 release in tilt-induced syncope

The neurohumoral events associated with neurocardiogenic syncope remain unclear. The simultaneous assessment of changes in endothelium-dependent and independent hormones and in autonomic balance in patients with tilt-induced syncope has been incompletely studied. Forty-six healthy subjects aged between 21 and 83 years (mean +/- SEM 47 +/- 3) underwent a 30-minute head-up tilt test at 60 degrees. Fourteen subjects (10 females and 4 male subjects) exhibited syncope at 16 +/- 2 minutes into the tilt. Hemodynamics were recorded every 5 minutes and blood samples for the measure of catecholamines, endothelin-1 (ET-1), and angiotensin-II (AT-II), were drawn at baseline, and 5, 10, 15, and 30 minutes into the tilt and immediately before syncope. Heart rate variability was analyzed by 5-minute segments during the test. Both catecholamines and ET-1 levels increased consistently in response to head-up tilt in subjects able to tolerate the test. Epinephrine increased to a greater extent before syncope. In contrast, ET-1 failed to increase at any time during the tilt and just before syncope. AT-II increased at 30 minutes into the tilt only in the control group. Finally, power in high-frequency bands decreased less in the group with syncope. Thus, compared with subjects able to tolerate a head-up tilt test, patients with syncope exhibit a greater increase in adrenomedullary activation, no significant increase in ET-1 levels, and a blunting in the decrease of vagal tone before syncope. The lack of increase in ET-1 during tilt may play a role in the inability to support orthostatic stress.

Endothelin reactivity and receptor profile of pulmonary vessels in postobstructive pulmonary vasculopathy

Chronic ligation of one pulmonary artery results in pulmonary vascular remodeling and bronchial angiogenesis, collectively known as postobstructive pulmonary vasculopathy (POPV). To determine whether the reactivity of pulmonary vessels to endothelins (ET) was altered in POPV and to explore potential mechanisms, we ligated the left main pulmonary artery of 18 rats. Four weeks later, using a lung explant technique, we compared POPV lungs with controls for contractile responses of intrapulmonary vessels to ET-1 and ET-3 and for relaxant responses to ET-1 and sodium nitroprusside (SNP) after precontraction with U-46619. Morphometric measurements were made on vessels studied pharmacologically. Competition receptor binding studies with 125I-labeled ET-1 and unlabeled ET-1 and BQ-123 were performed using membrane proteins of pulmonary vessels. We found, in arteries, that contractile responses to ET-1 and ET-3 were significantly increased and that relaxant responses to ET-1 but not to SNP were reduced; in veins, only relaxation to SNP was increased. Morphometry showed that arteries and veins in POPV had reduced diameters without altered muscle thickness. Receptor binding studies showed that the proportion of ETA receptors in arteries was significantly increased in POPV (66%) vs. controls (54%). We conclude that, in POPV, the increase in reactivity to ET-1 and ET-3 is primarily related to an augmented proportion of ETA receptors.

Age- and gender-related changes in endothelin and catecholamine release, and in autonomic balance in response to head-up tilt

1. There is an increase in circulating levels of vasoconstrictive hormones and an alteration in baroreceptor responsiveness with aging. The role of changes in endothelium-dependent and -independent vasoconstrictive hormones in relation to age and gender, with simultaneous assessment of autonomic balance in response to head-up tilt, has been incompletely studied. 2. Sixteen young [25 +/- 3 years (mean +/- SEM)] and 16 older normal volunteers (68 +/- 7 years) underwent a 30 min head-up tilt test at 60 degrees. Haemodynamics were measured every 5 min and blood samples for neurohormone measurement were drawn at baseline, 5, 10, 15 and 30 min into the test. Heart rate variability was analysed in 5 min segments at the baseline, and during the test. The younger subjects exhibited a greater increase in heart rate and diastolic blood pressure, despite lower absolute levels of noradrenaline (norepinephrine) and endothelin-1. Analysis of heart rate variability yielded a decrease in both high- and low-frequency bands in the aged; power at low-frequency decreased only in the young subjects. The age-related differences in blood pressure and noradrenaline levels were markedly attenuated in the female subjects. In addition, endothelin-1 levels and power spectral measurements at low frequency were the lowest in younger females throughout the tilt. 3. Despite attenuated cardiovascular response to tilt, both systemic adrenergic 'drive' and endothelin-1 levels increase in parallel with aging. Thus, endothelium-dependent and -independent vasoconstrictive hormone levels increase with age in the resting state and in response to neurohumoral stimulation in humans.

Endothelin-1 myocardial clearance, production, and effect on capillary permeability in vivo

Myocardial metabolism of endothelin-1 (ET-1) and its effect on coronary microcirculatory exchanges were obtained in anesthetized dogs by combining the indicator-dilution technique with immunoreactive ET-1 measurements. The myocardium extracted 17.7 +/- 4.6% of tracer ET-1 (n = 12). Simultaneously measured ET-1 levels in the aorta (0.97 +/- 0.46 pg/ml) and coronary sinus (0.96 +/- 0.53 pg/ml) were not different, supporting a production of ET-1 by the heart that balances the amount extracted. Intracoronary infusion of ET-1 (5 ng.kg-1.min-1) increased coronary sinus ET-1 levels approximately 50-fold, decreased coronary blood flow per unit of interstitial space by approximately 30% (P = 0.006), and increased myocardial microcirculatory transit times (n = 6). Permeability to albumin was unaffected by ET-1, whereas the permeability-surface area product for sucrose decreased following derecruitment of myocardial capillaries. We conclude that there is a normal myocardial metabolic balance of ET-1 and that the heart marginally contributes to circulating ET-1. Pharmacological doses of ET-1 may adversely affect myocardial metabolism by reducing blood flow and the permeability-surface area product for small circulating substances.

The orally active ET(A) receptor antagonist (+)-(S)-2-(4,6-dimethoxy-pyrimidin-2-yloxy)-3-methoxy-3,3-diphe nyl-propionic acid (LU 135252) prevents the development of pulmonary hypertension and endothelial metabolic dysfunction in monocrotaline-treated rats

Pulmonary hypertension is associated with endothelial dysfunction that may mediate or contribute to the disease process; among those abnormalities is an increase in circulating endothelin-1 levels. We investigated the effect of the orally active endothelin A receptor antagonist LU 135252 (LU) on the development of monocrotaline (MCT)-induced pulmonary hypertension and endothelial metabolic dysfunction. Rats were assigned to four groups by receiving a single dose of MCT or saline, followed by once-daily gavage with LU (50 mg/kg) or saline for 3 weeks. Plasma immunoreactive endothelin-1 levels doubled after MCT and were unaffected by LU therapy. The MCT-induced increase in right ventricular systolic pressure (72.5 +/- 15.9 mmHg) and hypertrophy (right ventricle/[left ventricle plus septum weight]; 0.58 +/- 0.08) were reduced by LU to 42.7 +/- 8.5 mmHg (P < .01) and 0.42 +/- 0.05 (P < .01), respectively. LU, however, did not modify MCT-induced pulmonary artery medial hypertrophy. Pulmonary vascular endothelial metabolic activity was evaluated in isolated lungs by measuring endothelium-bound angiotensin-converting enzyme activity using a synthetic angiotensin-converting enzyme substrate, 3H-benzoyl-phenylalanly-glycyl-proline. MCT reduced fractional 3H-benzoyl-phenylalanly-glycyl-proline hydrolysis (0.488 +/- 0.051, P < .01) which was normalized by LU therapy (0.563 +/- 0.050). LU treatment alone had no significant effect on any of these parameters. We conclude that the endothelin A antagonist LU reduces MCT-induced pulmonary hypertension and right ventricular hypertrophy and restores endothelial metabolic function. These results support the development of endothelin antagonists for the treatment of pulmonary hypertension and associated endothelial metabolic abnormalities.

Renal natriuretic effects of atrial natriuretic peptide in dogs with alloxan-induced acute pulmonary edema

OBJECTIVE

To test the effect of atrial natriuretic peptide (ANF) in dogs with acute pulmonary edema in the absence of urinary sodium retention. ANF is normally a potent natriuretic agent, although this effect is attenuated in generalized edema or acute renal failure.

DESIGN

Animal study.

SUBJECTS

Twenty-nine dogs.

INTERVENTION

Induction of acute pulmonary edema with intravenously administered alloxan and administration of ANF according to five protocols.

OUTCOME MEASURES

Natriuretic effect of ANF before and after the induction of pulmonary edema during the protocols.

RESULTS

In six control animals, induction of pulmonary edema was associated with diuresis (mean 0.38, standard error of the mean [SEM] 0.003 mL/min before alloxan administration v. mean 0.75, SEM 0.11 mL/min after); natriuresis (mean 60, SEM 8 mumol/min before v. mean 103, SEM 12 mumol/min after); and a decline in blood pressure (mean 114, SEM 7 mm Hg before v. mean 93, SEM 9 mm Hg after) and in the glomerular filtration rate (mean 52, SEM 3.3 mL/min before v. mean 36, SEM 3.7 mL/min after). When isoncotic dextran solution was infused in dogs with pulmonary edema, blood pressure was maintained but the glomerular filtration rate still declined by 42% and there was natriuresis. When the renal arteries were clamped for 5 minutes during the infusion of alloxan, diuresis and natriuresis were prevented, but the glomerular filtration rate still declined, although blood pressure was maintained. ANF administered intravenously during pulmonary edema induced a further significant natriuresis in all experimental protocols. Catalase, administered intravenously as a bolus just before the alloxan infusion, prevented pulmonary edema and the associated renal changes.

CONCLUSIONS

Although alloxan appears to be directly nephrotoxic, renal damage caused by this compound does not impair the natriuretic effect of ANF in acute pulmonary edema.

Changes in plasma endothelin-1 and Big endothelin-1 induced by transjugular intrahepatic portosystemic shunts in patients with cirrhosis and refractory ascites

BACKGROUND/AIMS

Endothelin-1 (ET-1) is a potent vasoconstrictor that may be involved in the pathogenesis of splanchnic and renal hemodynamic changes associated with portal hypertension. The aim of this study was to measure the concentration of ET-1 and of its precursor Big endothelin-1 (Big ET-1) in the systemic circulation as well as in the splanchnic and renal venous beds and to evaluate changes after the relief of portal hypertension following transjugular intrahepatic portosystemic shunt placement.

METHODS

Plasma concentrations of ET-1 and of Big ET-1 were measured in the vena cava, renal vein, hepatic vein and portal vein in ten patients with cirrhosis and refractory ascites before and 1-2 months after transjugular intrahepatic portosystemic shunt. The porto-caval gradient, creatinine clearance, plasma aldosterone and renin activity, as well as daily urinary sodium excretion were measured at the same time.

RESULTS

The plasma concentration of ET-1 and Big ET-1, respectively, in peripheral blood of normal volunteers were 0.28 +/- 03 and 3.95 +/- 0.34 pg/ml; the concentrations of both peptides were higher in patients with cirrhosis, both in vena cava (0.61 +/- 0.14 and 10.01 +/- 1.47 pg/ml), hepatic vein (0.62 +/- 0.13 and 13.93 +/- 1.77 pg/ml), portal vein (1.21 +/- 0.12 and 17.84 +/- 1.98 pg/ml) and renal vein (0.76 +/- 0.12 and 14.21 +/- 1.55 pg/ml). Moreover ET-1 and Big ET-1 concentrations were more elevated in the portal vein than in the vena cava (+98% and +70%) and slightly higher in the renal vein as compared to the vena cava (+25% and +42%). After transjugular intrahepatic portosystemic shunt, a rise in creatinine clearance and urinary sodium excretion (+49%; and +53%) was observed together with a marked reduction in plasma aldosterone and renin activity (-59% and -49%). ET-1 and Big ET-1 concentrations remained unchanged in the vena cava whereas a significant reduction of ET-1 and Big ET-1 occurred both in the portal vein (-43% and -44%) and in the renal vein (-53% and -29%). Portal vein and renal vein concentrations of both peptides became similar to vena cava levels.

CONCLUSIONS

Splanchnic and renal hemodynamic changes occurring in patients with cirrhosis and refractory ascites could be related to the production of ET-1 by splanchnic and renal vascular beds. This was abolished by transjugular intrahepatic portosystemic shunt, which could explain the exacerbation of systemic vasodilation and the improvement in renal perfusion observed after the procedure.

Human pulmonary circulation is an important site for both clearance and production of endothelin-1

BACKGROUND

Animal studies suggest a major role of the pulmonary circulation in the clearance of circulating endothelin-1 (ET-1). The contribution of the human pulmonary circulation to plasma ET-1 clearance, however, has never been quantified. The absence of an AV gradient in plasma ET-1 has previously been interpreted as evidence that the lungs do not have a role in modulating circulating ET-1 levels. This study was designed to quantify and discern between pulmonary ET-1 clearance and production in humans.

METHODS AND RESULTS

We studied 13 subjects by combining the multiple indicator-dilution technique with the measurement of immunoreactive ET-1 (irET-1). All patients had normal left ventricular ejection fractions (61 +/- 7%, mean +/- SD) and baseline hemodynamics. Mean pulmonary ET-1 extraction was 47 +/- 7%. The ET-1 extracted does not return to circulation and can be characterized by a sequestration rate constant: Kseq = 0.048 +/- 0.019 s-1. There was no significant difference between irET-1 levels from the pulmonary artery and aorta (0.61 +/- 0.29 and 0.68 +/- 0.33 pg/mL, respectively; P = .22); the normal lung consequently produces an amount of ET-1 that is quantitatively similar to the amount that has been extracted.

CONCLUSIONS

The human lung is an important site for both clearance and production of ET-1. There is a normal physiological balance of ET-1 across the pulmonary circulation, which explains the absence of difference in AV ET-1 levels despite a 47 +/- 7% clearance. Reduced pulmonary clearance or increased production of this peptide may contribute to the increase in circulating levels found in various cardiovascular conditions.

Pharmacokinetics and renal metabolism of atrial natriuretic factor during rat pregnancy

The present studies were done to determine if the attenuation of the natriuretic and diuretic effects of atrial natriuretic factor (ANF) during rat pregnancy was caused by an increase in its metabolism. It was found that the plasma half-life (min), plasma clearance (ml.kg-1.min-1), and volume of distribution (ml/kg) of ANF were, respectively, 2.5 +/- 4, 115 +/- 19, and 371 +/- 44 in pentobarbital-anesthetized virgin rats (n = 6) and not different from the corresponding values of 3.1 +/- 0.5, 124 +/- 26, and 526 +/- 120 in 20-day gravid animals (n = 6). Rates of metabolism of ANF (pmol.min-1.microgram protein-1) by renal cortical membranes from virgin (n = 5) and gravid (n = 5) rats were, respectively, 45 +/- 0.6 and 45 +/- 0.5; likewise, cortical membrane neutral endopeptidase activities in virgin and 20-day gravid rats (n = 7) did not differ. It is concluded that the attenuation of the renal effects of ANF during pregnancy is not caused by changes in its systemic or renal metabolism but might be due to a decrease in guanylate cyclase-linked renal ANF receptors and/or receptor-mediated effects.

Increased endothelin expression in a rat cardiac allograft model of chronic vascular rejection

Endothelins (ET) are potent vasoconstrictors that are directly mitogenic for vascular smooth muscle cells and fibroblasts. It is possible that the vasoconstrictor and mitogenic effects of ET could play a significant role in the vascular remodeling process that occurs in chronic vascular rejection (CVR). We have previously shown that cardiac allografts in the indefinitely surviving major histocompatibility complex identical WF.1L (RT1(1)) to Lewis (LEW) (RT1(1)) inbred rat strain combination provide a highly reproducible model of progressive CVR. The objective of this investigation was to measure endothelin-1 ventricular content of WF.1L-LEW cardiac allografts and to determine the immunohistochemical patterns of ET cellular reactivity at well defined posttransplant time periods. Data were compared with those obtained in similar studies of LEW-LEW syngeneic: heart grafts as well as all recipients' own hearts. The ventricular ET-1 content of the WF.1L cardiac allografts was markedly higher (4.3-, 7.0-, and 4.8-fold at 20, 40, and 60 days, respectively) than in corresponding recipients' hearts. Also, the increase in ventricular ET-1 levels as compared with the recipients' hearts rose significantly only in the allograft group. No comparable differences were observed in the syngeneic heart graft controls. Allografts consistently showed ET staining of intimal myocytes at sites of occlusive and subocclusive intimal proliferation associated with CVR. Allografts also showed ET cellular staining in areas of reparative fibrosis associated with indolent interstitial rejection and ischemic myocardial damage. The results of this study strongly suggest that ET may play a significant role in the pathogenesis of CVR.

Differentiation of embryonal carcinoma cells to a neural or cardiomyocyte lineage is associated with selective expression of endothelin receptors

Endothelins (ETs) were initially characterized as potent vasoactive peptides acting through at least two distinct receptors, ETA and ETB. Subsequently, their significant growth- and hypertrophy-promoting properties in cardiac and other cells were recognized. We investigated the expression of endothelin receptors during differentiation of a pluripotential embryonal carcinoma cell line (P19) to a cardiomyocyte or a neural lineage. These cells resemble those of the inner cell mass of the blastocyst, and their differentiation is believed to closely mimic critical events in early embryogenesis. Differentiation of P19 to a cardiomyocyte lineage, by aggregation and exposure to dimethyl sulfoxide resulted in induction of ETA receptors as demonstrated by radioligand binding studies, Northern blotting, and reporter gene analysis. Moreover, the P19 differentiated to a cardiac lineage responded to ET-1 with a 3-fold increase in the secretion of atrial natriuretic peptide. In contrast, differentiation to a neural lineage, by aggregation and exposure to retinoic acid, was associated with the induction of predominantly ETB. Therefore, selective differentiation of the P19 led to the differential expression of endothelin receptors in a pattern consistent with that observed in normal myocardial and neural tissue. The induction of endothelin receptors in a model system of early embryogenesis provides strong support for the critical role of this peptide/receptor family in differentiation and development. As well, this model system is well suited for the study of the mechanisms controlling endothelin receptor expression during differentiation.

Attenuation of renal effects of atrial natriuretic factor during rat pregnancy

The influence of pregnancy on renal responses to atrial natriuretic factor (ANF) was determined in urethane-anesthetized Sprague-Dawley rats. Infusions of ANF caused a significantly greater increase in urinary excretion of fluid, sodium, and potassium in virgin than in pregnant (13-15 days and 21 days) rats. Guanosine 3',5'-cyclic monophosphate (cGMP) excretion, mean arterial pressure, plasma immunoreactive ANF, and glomerular filtration rate (GFR) following ANF infusions were not different in virgin and gravid rats, although increments in GFR over basal were greater in virgin than in gravid animals. Renal responses to ANF normalized during postpartum and were attenuated by progesterone treatment of virgin rats. Natriuretic effects of infusions of ANF plus ANF-(4-23) (a ligand for clearance receptors) or of ANF plus thiorphan (an endopeptidase inhibitor) in virgin and pregnant rats did not differ; ANF-(4--23) and thiorphan alone caused greater natriuresis in pregnant than in virgin rats. Effects of ANF on cGMP production by collecting duct cells isolated from virgin and pregnant rats did not differ. We concluded that the attenuation in the renal effects of ANF during pregnancy might be mediated by progesterone by an increase in the intrarenal metabolism of ANF and might reflect physiological adjustment to facilitate fluid/electrolyte expansion.

Lack of role for nitric oxide (NO) in the selective destabilization of endothelial NO synthase mRNA by tumor necrosis factor-alpha

The constitutive expression of endothelial nitric oxide (NO) synthase (cNOS) is essential for the physiological regulation of vascular tone and structure. The mechanism of downregulation of steady state cNOS mRNA in human umbilical vein endothelial cells exposed to tumor necrosis factor-alpha (TNF-alpha) was investigated by using Northern blot analysis of total cellular RNA. TNF-alpha produced a dose- and time-dependent decrease in cNOS mRNA expression that was near maximal at 10 U/mL and 6 hours of exposure, respectively. In contrast, steady state expression of endothelin-1 and plasminogen activator inhibitor-1 (PAI-1) mRNA was upregulated by TNF-alpha. The pharmacological generation of NO using sodium nitroprusside (10 mumol/L) and S-nitroso-acetylpenicillamine (100 to 400 mumol/L) had no effect on cNOS mRNA levels, and TNF-alpha-induced downregulation of cNOS was not prevented by coincubation with the inhibitors of NO synthesis N omega-nitro-L-arginine methyl ester (1 mmol/L) and NG-monomethyl L-arginine (10 mmol/L). Under control conditions, cNOS and PAI-1 mRNA were stable after treatment with actinomycin D for periods greater than 24 hours, whereas endothelin-1 message was rapidly degraded (half-life, < 1 hour). Pretreatment with TNF-alpha (30 U/mL) selectively reduced that half-life of cNOS mRNA to less than 12 hours without altering the stability of PAI-1 message. TNF-alpha-induced destabilization of cNOS mRNA could be partially prevented by coincubation with cycloheximide (1 mumol/L) but was not reproduced by addition of sodium nitroprusside.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma endothelin-1 response to acute hypotension induced by vasodilating agents

Cyclic GMP (cGMP) dependent vasodilating agents (natriuretic peptides, nitric oxide) inhibit secretion of endothelin-1 (ET-1) in cultured endothelial cells. However, in circulatory conditions associated with acute hypotension, a marked increase in plasma ET-1 has repeatedly been observed. Therefore, after administration of cGMP-dependent agents in hypotensive dose, the net effect of these opposing influences on ET-1 release may shed light on the mechanisms determining circulating levels of this peptide. We have studied the effect of a hypotensive dose of atrial natriuretic peptide (n = 16), 8-Br-cGMP (n = 5), and papaverin (n = 7) on plasma ET-1 in anesthetized dogs. All agents produced marked increases in the peptide level at the end of infusion (178, 280, and 240% of the last preinfusion level, respectively) and a mean arterial blood pressure (MAP) decrease of 19, 18, and 42 mmHg (1 mmHg = 133.3 Pa), respectively. In all three protocols, plasma ET-1 continued to rise when the hypotensive agent was discontinued and remained elevated for 2-3 h postinfusion, even though MAP was normalized. There was a close positive correlation between the maximal increment in plasma ET-1 and the maximal decrease in MAP (r = 0.67, p < 0.001). These results show that acute hypotension due to directly acting vasodilators is a potent stimulus for systemic release of ET-1, even when due to agents known to inhibit ET-1 production in cultured endothelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of cyclic nucleotides in the regulation of endothelin-1 production by human endothelial cells

The regulation of endothelin-1 (ET-1) production by endothelial cells is likely of crucial physiological importance in the maintenance of vascular homeostasis. The aim of the present study was to explore the possible role of cyclic nucleotides in the control of ET-1 production in human umbilical vein endothelial cells (HUVEC). ET-1 release was determined by measuring levels of immunoreactive ET-1 in HUVEC-conditioned media after 6-h incubations. In the presence of 10% fetal calf serum (FCS) there was a threefold increase in ET-1 release compared with serum-free conditions (1.96 +/- 0.17 vs. 0.56 +/- 0.06 pg/micrograms protein), respectively. Inhibition of protein kinase (PK) C using staurosporine (10 nM) reduced basal ET-1 release by approximately 50% and completely prevented the response to FCS. In contrast, the addition of other PK inhibitors had little effect on basal or serum-stimulated ET-1 release at the concentrations used. N6,2'-O-dibutyryladenosine 3',5'-cyclic monophosphate (DBcAMP) produced significant alterations in ET-1 release depending on the basal level of production. Under serum-free conditions of low basal ET-1 production, DBcAMP increased ET-1 release by 68 +/- 22% but only at the highest concentration studied (1 mM). The dose-response relationship for DBcAMP was potentiated by KT-5720 (0.1 microM), an inhibitor of PKA, with a significant shift to 10-fold lower concentrations, whereas it was blocked by KT-5823 (4 microM), which can inhibit PKG.(ABSTRACT TRUNCATED AT 250 WORDS)

Renal response to ANP in normal dogs during extreme inhibition of distal Na+ transport

Atrial natriuretic peptide (ANP) is thought to exert its major effect within the cortical and inner medullary collecting ducts (CCD and IMCD) by inhibiting Na+ transport along conductive channels and electroneutral pathways. These transport routes are also thought to be inhibited by a combination of amiloride, thiazide and bradykinin. We tested the ability of normal dogs to respond to ANP when various combinations of these Na+ transport inhibitors were present. In 24 dogs ANP raised UNaV from 31 +/- 6 to 223 +/- 41 mu Eq/min (P < 0.05), a delta of 192 mu Eq/min. Bradykinin alone did not depress delta UNaV in response to an ANP infusion. In the presence of extreme natriuresis caused by amiloride and thiazide, the response to ANP was magnified, presumably due to augmented Na+ delivery to the CCD and IMCD. When distal delivery of Na+ to one kidney was controlled by aortic clamping in the presence of amiloride, thiazide and bradykinin. delta UNaV in response to ANP was depressed (48 vs. 168 mu Eq/min). We conclude that in the presence of extreme inhibition of Na+ transport within the collecting ducts, ANP can still cause a further natriuresis, probably in the absence of augmented distal Na+ delivery.