Endothelin receptor antagonism

Safety, pharmacokinetics, and pharmacodynamics in healthy Chinese volunteers treated with SC0062, a highly selective endothelin-A receptor antagonist

This study evaluated the safety, tolerability, pharmacokinetics (PK), pharmacodynamics (PD), and food effects (FE) of SC0062, a highly active endothelin-A (ETA ) receptor antagonist, in healthy subjects. The primary objectives of this first-in-human phase I study, comprised of single-ascending-dose, multiple-ascending-dose, and FE parts, were to characterize the safety and tolerability of SC0062, and FE. The secondary objectives were to determine the PK behavior of SC0062 and its major active metabolite M18, whereas exploratory objectives focused on PD effects, principally effects on endothelin-1 (ET-1) and total bile acids (TBA). Single doses of 10 to 100 mg and multiple daily doses of 20 and 50 mg for 6 days were well tolerated. SC0062 was rapidly absorbed and plasma exposure of SC0062 and M18 increased disproportionately with dose, achieving steady state by day 3, with accumulation ratios of 1.22 and 1.89 on day 6 for SC0062 and M18, respectively. The geometric mean (geometric standard deviation) terminal elimination half-life (t1/2 ) values of SC0062 and M18 were 7.25 (1.70) h and 13.73 (1.32) h, respectively. Plasma ET-1 concentrations were dose-proportional, whereas plasma TBA concentrations behaved erratically. Following a single 50 mg dose of SC0062 after a high-fat meal, Cmax values for SC0062 and M18 increased by 41% and 32%, respectively, and median Tmax values for SC0062 were 3 h longer than fasting values; exposure was unaffected. These favorable safety, PK, and PD results provide a foundation for further studies of SC0062 in pulmonary arterial hypertension, chronic kidney disease, and other relevant indications.

Role of Chrononutrition in the Antihypertensive Effects of Natural Bioactive Compounds

Hypertension (HTN) is one of the main cardiovascular risk factors and is considered a major public health problem. Numerous approaches have been developed to lower blood pressure (BP) in hypertensive patients, most of them involving pharmacological treatments. Within this context, natural bioactive compounds have emerged as a promising alternative to drugs in HTN prevention. This work reviews not only the mechanisms of BP regulation by these antihypertensive compounds, but also their efficacy depending on consumption time. Although a plethora of studies has investigated food-derived compounds, such as phenolic compounds or peptides and their impact on BP, only a few addressed the relevance of time consumption. However, it is known that BP and its main regulatory mechanisms show a 24-h oscillation. Moreover, evidence shows that phenolic compounds can interact with clock genes, which regulate the biological rhythm followed by many physiological processes. Therefore, further research might be carried out to completely elucidate the interactions along the time–nutrition–hypertension axis within the framework of chrononutrition.

Endothelin receptor B2 (EDNRB2) is responsible for the tyrosinase-independent recessive white (mo(w) ) and mottled (mo) plumage phenotypes in the chicken

A mutation that confers white plumage with black eyes was identified in the Minohiki breed of Japanese native chicken (Gallus gallus domesticus). The white plumage, with a few partially pigmented feathers, was not associated with the tyrosinase gene, and displayed an autosomal recessive mode of inheritance against the pigmented phenotype. All F1 offspring derived from crosses with mottled chickens (mo/mo), which show characteristic pigmented feathers with white tips, had plumage with a mottled-like pattern. This result indicates that the white plumage mutation is a novel allele at the mo locus; we propose the gene symbol mo(w) for this mutant allele. Furthermore, the F1 hybrid between the mo(w) /mo(w) chicken and the panda (s/s) mutant of Japanese quail (Coturnix japonica), whose causative gene is the endothelin receptor B2 (EDNRB2) gene, showed a mo(w)/mo(w) chicken-like plumage, suggesting the possibility that the mutations in parental species are alleles of the same gene, EDNRB2. Nucleotide sequencing of the entire coding region of EDNRB2 revealed a non-synonymous G1008T substitution, which causes Cys244Phe amino acid substitution in exon 5 (which is part of the extracellular loop between the putative fourth and fifth transmembrane domains of EDNRB2) in the mutant chicken. This Cys244Phe mutation was also present in individuals of four Japanese breeds with white plumage. We also identified a non-synonymous substitution leading to Arg332His substitution that was responsible for the mottled (mo/mo) plumage phenotype. These results suggest that the EDN3 (endothelin 3)-EDNRB2 signaling is essential for normal pigmentation in birds, and that the mutations of EDNRB2 may cause defective binding of the protein with endothelins, which interferes with melanocyte differentiation, proliferation, and migration.

Antivascular therapy for multidrug-resistant ovarian tumors by macitentan, a dual endothelin receptor antagonist

Endothelin receptors (ETRs) are often overexpressed in ovarian tumors, which can be resistant to conventional therapies. Thus, we investigated whether blockage of the ETR pathways using the dual ETR antagonist macitentan combined with taxol or cisplatinum can produce therapy for orthotopically growing multidrug-resistant (MDR) human ovarian carcinoma. In several studies, nude mice were injected in the peritoneal cavity with HeyA8-MDR human ovarian cancer cells. Ten days later, mice were randomized to receive vehicle (saline), macitentan (oral, daily), taxol (intraperitoneal, weekly), cisplatinum (intraperitoneal, weekly), macitentan plus taxol, or macitentan plus cisplatinum. Moribund mice were killed, and tumors were collected, weighed, and prepared for immunohistochemical analysis. The HeyA8-MDR tumors did not respond to taxol, cisplatinum, or macitentan administered as single agents. In contrast, combination therapy with macitentan and taxol or macitentan and cisplatinum significantly decreased the tumor incidence and weight and significantly increased the survival of mice and their general condition. Multiple immunohistochemical analyses revealed that treatment with macitentan and macitentan plus taxol or cisplatinum inhibited the phosphorylation of ETRs, decreased the levels of pVEGFR2, pAkt, and pMAPK in tumor cells after 2 weeks of treatment and induced a first wave of apoptosis in tumor-associated endothelial cells followed by apoptosis in surrounding tumor cells. Our study shows that ovarian cancer cells, which express the endothelin axis and are multidrug resistant, are exquisitely sensitive to treatment with a dual ET antagonist and can be resensitized to both taxol and cisplatinum. This combined therapy led to a significant reduction in tumor weight.

Macitentan (ACT-064992), a tissue-targeting endothelin receptor antagonist, enhances therapeutic efficacy of paclitaxel by modulating survival pathways in orthotopic models of metastatic human ovarian cancer

Potential treatments for ovarian cancers that have become resistant to standard chemotherapies include modulators of tumor cell survival, such as endothelin receptor (ETR) antagonist. We investigated the therapeutic efficacy of the dual ETR antagonist, macitentan, on human ovarian cancer cells, SKOV3ip1 and IGROV1, growing orthotopically in nude mice. Mice with established disease were treated with vehicle (control), paclitaxel (weekly, intraperitoneal injections), macitentan (daily oral administrations), or a combination of paclitaxel and macitentan. Treatment with paclitaxel decreased tumor weight and volume of ascites. Combination therapy with macitentan and paclitaxel reduced tumor incidence and further reduced tumor weight and volume of ascites when compared with paclitaxel alone. Macitentan alone occasionally reduced tumor weight but alone had no effect on tumor incidence or ascites. Immunohistochemical analyses revealed that treatment with macitentan and macitentan plus paclitaxel inhibited the phosphorylation of ETRs and suppressed the survival pathways of tumor cells by decreasing the levels of pVEGFR2, pAkt, and pMAPK. The dose of macitentan necessary for inhibition of phosphorylation correlated with the dose required to increase antitumor efficacy of paclitaxel. Treatment with macitentan enhanced the cytotoxicity mediated by paclitaxel as measured by the degree of apoptosis in tumor cells and tumor-associated endothelial cells. Collectively, these results show that administration of macitentan in combination with paclitaxel prevents the progression of ovarian cancer in the peritoneal cavity of nude mice in part by inhibiting survival pathways of both tumor cells and tumor-associated endothelial cells.

Mechanisms involved in the long-term modulation of tyrosine hydroxylase by endothelins in the olfactory bulb of normotensive rats

The olfactory bulbs play a relevant role in the interaction between the animal and its environment. The existence of endothelin-1 and -3 in the rat olfactory bulbs suggests their role in the control of diverse functions regulated at this level. Tyrosine hydroxylase, a crucial enzyme in catecholamine biosynthesis, is tightly regulated by short- and long-term mechanisms. We have previously reported that in the olfactory bulbs endothelins participate in the short-term tyrosine hydroxylase regulation involving complex mechanisms. In the present work we studied the effect of long-term stimulation by endothelins on tyrosine hydroxylase in the rat olfactory bulbs. Our findings show that endothelin-1 and -3 modulated catecholaminergic transmission by increasing enzymatic activity. However, these peptides acted through different receptors and intracellular pathways. Endothelin-1 enhanced tyrosine hydroxylase activity through a super high affinity ET(A) receptor and cAMP/PKA and CaMK-II pathways, whereas, endothelin-3 through a super high affinity atypical receptor coupled to cAMP/PKA, PLC/PKC and CaMK-II pathways. Endothelins also increased tyrosine hydroxylase mRNA and the enzyme total level as well as the phosphorylation of Ser 19, 31 and 40 sites. Furthermore, both peptides stimulated dopamine turnover and reduced its endogenous content. These findings support that endothelins are involved in the long-term regulation of tyrosine hydroxylase, leading to an increase in the catecholaminergic activity which might be implicated in the development and/or maintenance of diverse pathologies involving the olfactory bulbs.

Altered endothelin receptor binding in response to nitric oxide synthase inhibition in the pregnant rat

The authors evaluate the expression of endothelin-1 (ET-1) and its receptors in the uterus and placenta during maternal nitric oxide synthase (NOS) inhibition. Timed-pregnant rats received L-NAME (2.5 mg/kg/h) or saline from day 14 to 21 of gestation. Uterine and placental tissues collected on day 21 were assayed for preproET-1, ET( A), and ET(B) mRNA expression; localization and expression of ET-1 and receptor proteins; and receptor activity. NOS inhibition did not affect preproET-1 mRNA expression in the placenta or uterus. ET(A) expression decreased in the uterine free wall, but no other changes in receptor mRNA expression were observed in the uterus or placenta. ET-1 and receptor proteins were unchanged. Placental ET(A) and ET(B) receptor binding decreased. Uterine ET(A) receptor binding decreased in the placental bed. ET-1, a prominent mediator during NOS inhibition, is not of uterine or placental origin. Reduced receptor binding activity is the primary means by which these tissues regulate their response to ET-1 in the setting of NOS inhibition.

Long-term modulation of tyrosine hydroxylase activity and expression by endothelin-1 and -3 in the rat anterior and posterior hypothalamus

Brain catecholamines are involved in the regulation of biological functions, including cardiovascular activity. The hypothalamus presents areas with high density of catecholaminergic neurons and the endothelin system. Two hypothalamic regions intimately related with the cardiovascular control are distinguished: the anterior (AHR) and posterior (PHR) hypothalamus, considered to be sympathoinhibitory and sympathoexcitatory regions, respectively. We previously reported that endothelins (ETs) are involved in the short-term tyrosine hydroxylase (TH) regulation in both the AHR and PHR. TH is crucial for catecholaminergic transmission and is tightly regulated by well-characterized mechanisms. In the present study, we sought to establish the effects and underlying mechanisms of ET-1 and ET-3 on TH long-term modulation. Results showed that in the AHR, ETs decreased TH activity through ETBreceptor activation coupled to the nitric oxide, phosphoinositide, and CaMK-II pathways. They also reduced total TH level and TH phosphorylated forms (Ser 19 and 40). Conversely, in the PHR, ETs increased TH activity through a G protein-coupled receptor, likely an atypical ET receptor or the ETCreceptor, which stimulated the phosphoinositide and adenylyl cyclase pathways, as well as CaMK-II. ETs also increased total TH level and the Ser 19, 31, and 40 phosphorylated sites of the enzyme. These findings support that ETs are involved in the long-term regulation of TH activity, leading to reduced sympathoinhibition in the AHR and increased sympathoexcitation in the PHR. Present and previous studies may partially explain the cardiovascular effects produced by ETs when applied to the brain.

Endothelin-B receptors on suprachoroidal melanocytes mediate an endothelin-1-induced increase in the intracellular calcium concentration of rabbit ocular suprachoroidal tissue

The physiological properties of endothelin-1 in ocular choroidal melanocytes are not well-known, although endothelin-1 increases the intracellular calcium concentration through endothelin-B receptors in skin melanocytes. We studied the effect of endothelin-1 on choroidal melanocytes in rabbit ocular suprachoroidal tissue. Fura-2 microfluorophotometry indicated that endothelin-1 and endothelin-B receptor selective agonists, endothelin-3 and BQ3020, increased the intracellular calcium concentration of the tissue. Endothelin-B receptor selective antagonist BQ788 blunted the response. Immuno-electron microscopy showed that endothelin-B receptor-like immunoreactivity was located only on melanocytes. These results indicate that endothelin-1 increases intracellular calcium concentration in rabbit suprachoroidal tissue via endothelin-B receptors on melanocytes.

Short-term regulation of tyrosine hydroxylase activity and expression by endothelin-1 and endothelin-3 in the rat posterior hypothalamus

Brain catecholamines are involved in several biological functions regulated by the hypothalamus. We have previously reported that endothelin-1 and -3 (ET-1 and ET-3) modulate norepinephrine release in the anterior and posterior hypothalamus. As tyrosine hydroxylase (TH) is the rate-limiting enzyme in catecholamine biosynthesis, the aim of the present work was to investigate the effects of ET-1 and ET-3 on TH activity, total enzyme level and the phosphorylated forms of TH in the rat posterior hypothalamus. Results showed that ET-1 and ET-3 diminished TH activity but the response was abolished by both selective ET(A) and ET(B) antagonists (BQ-610 and BQ-788, respectively). In addition ET(A) and ET(B) selective agonists (sarafotoxin S6b and IRL-1620, respectively) failed to affect TH activity. In order to investigate the intracellular signaling coupled to endothelins (ETs) response, nitric oxide (NO), phosphoinositide, cAMP/PKA and CaMK-II pathways were studied. Results showed that N(omega)-nitro-l-arginine methyl ester and 7-nitroindazole (NO synthase and neuronal NO synthase inhibitors, respectively), 1H-[1,2,4]-oxadiazolo[4,3-alpha]quinozalin-1-one and KT-5823 (soluble guanylyl cyclase, and PKG inhibitors, respectively) inhibited ETs effect on TH activity. Further, sodium nitroprusside and 8-bromoguanosine-3',5'-cyclic monophosphate (NO donor and cGMP analog, respectively) mimicked ETs response. ETs-induced reduction of TH activity was not affected by a PKA inhibitor but it was abolished by PLC, PKC and CaMK-II inhibitors as well as by an IP(3) receptor antagonist. On the other hand, both ETs did not modify TH total level but reduced the phosphorylation of serine residues of the enzyme at positions 19, 31 and 40. Present results suggest that ET-1 and ET-3 diminished TH activity through an atypical ET or ET(C) receptor coupled to the NO/cGMP/PKG, phosphoinositide and CaMK-II pathways. Furthermore, TH diminished activity may result from the reduction of the phosphorylated sites of the enzyme without changes in its total level. Taken jointly present and previous results support that ET-1 and ET-3 may play a relevant role in the modulation of catecholaminergic neurotransmission in the posterior hypothalamus of the rat.

Expression of endothelin 1 and its receptors in the hypoxic pregnant rat

Endothelin 1 (EDN1) plays a primary role in the pathophysiology of hypoxia-induced fetal growth restriction in the rat. In this study we evaluated the effects of chronic maternal hypoxia on the expression of endothelin and its receptors and on receptor binding activity in the uterus and placenta of the rat, in order to elucidate their roles in hypoxia-induced fetal growth restriction. Timed-pregnant Sprague-Dawley rats were maintained in either a normoxic or a normobaric hypoxic (12% O(2)) atmosphere from Gestational Days 18-21. Uterine and placental tissues collected on Gestational Day 21 were assayed for Edn1, Ednra, and Ednrb (endothelin receptors) mRNA expression by real-time quantitative RT-PCR, for localization of EDN1 and its receptors by immunohistochemistry, for EDNRA and EDNRB protein expression by Western blot, and for receptor binding activity by homologous competitive binding assays. EDN1 mRNA expression was significantly increased in the hypoxic placenta, but not in the uterus, compared with normoxic controls. Immunohistochemistry revealed increased EDN1 specifically in the labyrinth of the placenta. Receptor mRNA levels were not significantly affected by hypoxia, but EDNRA protein expression was significantly decreased specifically in the uterine placental beds. Receptor binding decreased significantly in response to hypoxia in all tissues investigated, compared with controls. These results suggest that chronic maternal hypoxia results in increased expression of EDN1 in the placenta but not in the uterus, and that reduced binding activity, rather than regulation of receptor expression, is a mechanism by which these tissues regulate the local hemodynamic response to increased endogenous placental EDN1 in the setting of hypoxia.

Endothelin limits insulin action in obese/insulin-resistant humans

The normal action of insulin to vasodilate and redistribute blood flow in support of skeletal muscle metabolism is impaired in insulin-resistant states. Increased endogenous endothelin contributes to endothelial dysfunction in obesity and diabetes. Here, we test the hypothesis that increased endogenous endothelin action also contributes to skeletal muscle insulin resistance via impairments in insulin-stimulated vasodilation. We studied nine lean and seven obese humans, measuring the metabolic and hemodynamic effects of insulin (300 mU . m(-2) . min(-1)) alone and during femoral artery infusion of BQ123 (an antagonist of type A endothelin receptors, 1 micromol/min). Endothelin antagonism augmented skeletal muscle responses to insulin in obese subjects through changes in both leg blood flow (LBF) and glucose extraction. Insulin-stimulated LBF was significantly increased in obese subjects only. These changes, combined with differential effects on glucose extraction, resulted in augmented insulin-stimulated leg glucose uptake in obese subjects (54.7 +/- 5.7 vs. 107.4 +/- 18.9 mg/min with BQ123), with no change in lean subjects (103.7 +/- 11.4 vs. 88.9 +/- 16.3, P = 0.04 comparing BQ123 across groups). BQ123 allowed augmented leg glucose extraction in obese subjects even in the face of NOS antagonism. These findings suggest that increased endogenous endothelin action contributes to insulin resistance in skeletal muscle of obese humans, likely through both vascular and tissue effects.

Second trimester amniotic fluid endothelin-1 concentrations and subsequent development of intrauterine growth restriction

OBJECTIVE

The purpose of this prospective study was to record endothelin-1 (ET-1) concentrations in the second trimester amniotic fluid and compare these values in women who developed intrauterine growth restriction (IUGR) later in pregnancy with those with uneventful pregnancies.

METHOD

Amniotic fluid was retrieved by amniocentesis from 125 women in the second trimester of pregnancy. The levels of ET1 were measured by a sensitive and specific radioimmunoassay.

RESULTS

From the 125 women included in the study 12 had pregnancies that later developed IUGR and 88 had uneventful pregnancies. The ET1 concentration was significantly higher (P<0.005) in women who later developed IUGR than in normal pregnancy (106 pg/ml versus 64.7 pg/ml).

CONCLUSION

The amniotic fluid concentration of ET1 is elevated by the second trimester in women who later develop IUGR.

Phosphatidylinositol 4,5-bisphosphate reverses endothelin-1-induced insulin resistance via an actin-dependent mechanism

Phosphatidylinositol (PI) 4,5-bisphosphate (PIP(2)) plays a pivotal role in insulin-stimulated glucose transport as an important precursor to PI 3,4,5-trisphosphate (PIP(3)) and a key regulator of actin polymerization. Since endothelin (ET)-1 impairs insulin sensitivity and PIP(2) is a target of ET-1-induced signaling, we tested whether a change in insulin-stimulated PIP(3) generation and signaling, PIP(2)-regulated actin polymerization, or a combination of both accounted for ET-1-induced insulin resistance. Concomitant with a time-dependent loss of insulin sensitivity, ET-1 caused a parallel reduction in plasma membrane PIP(2). Despite decreased insulin-stimulated PI 3-kinase activity and PIP(3) generation, ET-1 did not diminish downstream signaling to Akt-2. Furthermore, addition of exogenous PIP(2), but not PIP(3), restored insulin-regulated GLUT4 translocation and glucose transport impaired by ET-1. Microscopic and biochemical analyses revealed a PIP(2)-dependent loss of cortical filamentous actin (F-actin) in ET-1-treated cells. Restoration of insulin sensitivity by PIP(2) add-back occurred concomitant with a reestablishment of cortical F-actin. The corrective effect of exogenous PIP(2) in ET-1-induced insulin-resistant cells was not present in cells where cortical F-actin remained experimentally depolymerized. These data suggest that ET-1-induced insulin resistance results from reversible changes in PIP(2)-regulated actin polymerization and not PIP(2)-dependent signaling.

Effects of nitric oxide and prostacyclin on hemodynamic response by big endothelin-1 in near term fetal sheep

AIMS

To measure plasma concentrations of endothelin (ET)-1, NO metabolites (nitrate/nitrite; NOx) and 6-keto PGF1 alpha (PGF1 alpha) in maternal and fetal sheep blood, and to evaluate the effects of big ET-1 on hemodynamic response, blood gases and NO and 6-keto PGF1 alpha production in near term fetal sheep.

METHODS

Hemodynamic parameters were measured during infusion of big ET-1 into the carotid vein in chronically catheterized fetal sheep on day 125 of gestation. Fetal arterial blood samples were obtained for ET-1, PGF1 alpha) and nitrate/nitrite (NOx) measurements.

RESULTS

ET-1, NOx and PGF1 alpha plasma concentrations were all significantly higher in fetal compared with the maternal plasma. Big ET-1 significantly decreased fetal systolic and diastolic blood pressure and significantly increased fetal heart rate. Big ET-1 stimulated plasma PGF1 alpha), but not NOx , concentration.

CONCLUSIONS

Circulatory regulating factors in the fetus were up-regulated. The effects of ET-1 on fetal hemodynamic response may be mediated via prostacyclin, but not via the NO pathway.

Endothelin and the regulation of uteroplacental perfusion in nitric oxide synthase inhibition-induced fetal growth restriction

The vasoactive mediators nitric oxide and endothelin are both produced in and active in the uterine and placental vasculature. Inhibition of nitric oxide synthase (NOS) results in fetal growth restriction. Endothelin (ET-1) is upregulated in the setting of NOS inhibition. Our purpose was to determine the impact of ET-1 on uterine and placental perfusion in the pregnant rat treated with a NOS inhibitor. Timed-pregnant Sprague-Dawley rats were treated with L-NAME (2.5 mg/kg/h), with and without A-127722 (10 mg/kg/day), or their respective vehicles, for 1, 4, or 7 days beginning on day 14 of gestation. Blood flow to various organs was determined by microsphere infusion. Maternal and fetal plasma nitrate/nitrite (NOx) was determined by fluorometric assay. Uterine and placental perfusion was significantly decreased by NOS inhibition and was restored to normal by ETA antagonism at 1 and 4 days of infusion but not at 7 days. Maternal plasma NOx, but not fetal plasma NOx, was significantly decreased by NOS inhibition alone. ETA antagonism in combination with NOS inhibition significantly lowered fetal plasma NOx. These results indicate that ET-1 is an important regulator of uterine and placental perfusion in the NOS inhibition model of fetal growth restriction. Our results also suggest that maternal administration of L-NAME does not result in significant transport of L-NAME across the placenta, but that addition of an ETA antagonist results in increased placental perfusion, allowing L-NAME greater access to the fetal compartment.

Mast cells promote homeostasis by limiting endothelin-1-induced toxicity

Endothelin-1 (ET-1) is a 21-amino-acid peptide, derived from vascular endothelial cells, with potent vasoconstrictor activity. ET-1 has been implicated in diverse physiological or pathological processes, including the vascular changes associated with sepsis. However, the factors that regulate ET-1-associated toxicity during bacterial infections, or in other settings, are not fully understood. Both the pathology associated with certain allergic and autoimmune disorders, and optimal host defence against bacterial and parasitic infections are mediated by mast cells. In vitro, mast cells can produce ET-1 (ref. 11), undergo ET-1-dependent and endothelin-A receptor (ET(A))-dependent activation, and release proteases that degrade ET-1 (ref. 14). Although the potential relationships between mast cells and the ET-1 system thus may be complex, the importance of interactions between ET-1 and mast cells in vivo is obscure. Here we show that ET(A)-dependent mast-cell activation can diminish both ET-1 levels and ET-1-induced pathology in vivo, and also can contribute to optimal survival during acute bacterial peritonitis. These findings identify a new biological function for mast cells: promotion of homeostasis by limiting the toxicity associated with an endogenous mediator.

Endothelin receptors as novel targets in tumor therapy

The endotelin (ET) axis, that includes ET-1, ET-2, ET-3, and the ET receptors, ET_A and ET_B, plays an important physiological role, as modulator of vasomotor tone, tissue differentiation and development, cell proliferation, and hormone production. Recently, investigations into the role of the ET axis in mitogenesis, apoptosis inhibition, invasiveness, angiogenesis and bone remodeling have provided evidence of the importance of the ET-1 axis in cancer. Data suggest that ET-1 participates in the growth and progression of a variety of tumors such as prostatic, ovarian, renal, pulmonary, colorectal, cervical, breast carcinoma, Kaposi's sarcoma, brain tumors, melanoma, and bone metastases. ET-1 receptor antagonists beside providing ideal tools for dissecting the ET axis at molecular level have demonstrated their potential in developing novel therapeutic opportunity. The major relevance of ET_A receptor in tumor development has led to an extensive search of highly selective antagonists. Atrasentan, one of such antagonists, is orally bioavailable, has suitable pharmacokinetic and toxicity profiles for clinical use. Preliminary data from clinical trials investigating atrasentan in patients with prostate cancer are encouraging. This large body of evidence demonstrates the antitumor activity of endothelin receptor antagonists and provides a rationale for the clinical evaluation of these molecules alone and in combination with cytotoxic drugs or molecular inhibitors leading to a new generation of anticancer therapies targeting endothelin receptors.

Pharmacokinetics and pharmacodynamics of the ETA-selective endothelin receptor antagonist SPP301 in healthy human subjects

SPP301 is a competitive antagonist of ET-1 with a high selectivity for the ETA receptor. A double-blind, placebo-controlled study was performed to investigate the tolerability, pharmacokinetics, and pharmacodynamics of SPP301 after single oral doses in male healthy subjects; doses of 5, 20, 50, 100, and 200 mg were given to different groups of 4 or 8 subjects each. The effect of food on the pharmacokinetics of SPP301 was assessed for the 50-mg dose according to a sequential design in the same subjects. At regular intervals, blood pressure and pulse rate, plasma levels of ET-1 and of SPP301 and its hydroxymethyl metabolite, and urinary excretion of the parent drug and its metabolite were determined. SPP301 was generally well tolerated. At doses >20 mg, adverse events that are typical for vasodilating agents-namely, headache, nausea and vomiting, dizziness, and postural hypotension-were observed. Maximum plasma levels of SPP301 were reached within 4.5 hours. Cmax and AUC values increased linearly with doses up to 100 mg. The apparent terminal half-life was quite constant over the whole dose range and ranged from 7.5 to 15.2 hours. Urinary excretion of SPP301 was below 0.1% of any dose. Cmax and AUC of the metabolite amounted only to about 5% of the values for SPP301. Concomitant food intake had no effect on the overall exposure but increased average peak plasma concentrations of SPP301 by around 50%. Plasma ET-1 increased nearly twofold at the 5-mg SPP301 dose, with no further relevant increase at higher doses. In conclusion, SPP301 is an active ET-1 antagonist and is well tolerated. The pharmacokinetics of the drug and its metabolite are linear up to 100 mg. Food does not affect overall exposure of SPP301 but increases Cmax. Urinary excretion of SPP301 is below 0.1% of the dose administered.

Modulatory effect of endothelin-1 and -3 on neuronal norepinephrine release in the rat posterior hypothalamus

Based upon the existence of high density of ET-receptors on catecholaminergic neurons of the hypothalamus, we studied the effects of endothelin-1 (ET-1) and endothelin-3 (ET-3) on neuronal norepinephrine (NE) release in the rat posterior hypothalamus. The intracellular pathways and receptors involved were also investigated. Neuronal NE release was enhanced by ET-1 and ET-3 (10 etaM). The selective antagonists of subtype A and B ET receptors (ETA, ETB) (100 etaM BQ-610 and 100 etaM BQ-788, respectively) abolished the increase induced by ET-1 but not by ET-3. The PLC inhibitor, U73122 (10 microM), abolished ET-1 and ET-3 response. GF-109203X (100 etaM) (PKC inhibitor) blocked the increase in NE release produced by ET-3 and partially blocked ET-1 response. The inositol 1,4,5-trisphosphate-induced calcium release inhibitor, 42 microM 2-APB, inhibited the stimulatory effect induced by ET-3 but not by ET-1. The PKA inhibitor, 500 etaM H-89, blocked the increase in neuronal NE release evoked by ET-1 but not by ET-3. Our results showed that ET-1 as well as ET-3 displayed an excitatory neuromodulatory effect on neuronal NE release in the rat posterior hypothalamus. ET-1 through an atypical ETA or ETB receptor activated the PLC/PKC signalling pathway as well as the cAMP pathway, whereas ET-3 through a non-ETA/non-ETB receptor activated the phosphoinositide pathway. Both ETs would enhance the sympathoexcitatory response elicited by the posterior hypothalamus and thus participate in cardiovascular regulation.

Endothelin and the regulation of uterine and placental perfusion in hypoxia-induced fetal growth restriction

OBJECTIVE

Normal placental function is dependent on maintenance of uteroplacental perfusion. Endothelin, a potent vasoconstrictor, is produced in and is active in the uteroplacental vasculature. The purpose of this study was to determine the role of endothelin in the regulation of uteroplacental perfusion under normal conditions, and in hypoxia-induced fetal growth restriction.

METHODS

Timed-pregnant Sprague-Dawley rats, outfitted with arterial catheters, were maintained in either a normoxic or a normobaric hypoxic (12% oxygen) atmosphere from day 18 to 21 of gestation. During this time, the endothelin receptor A antagonist, A-127722, or its vehicle was administered. Regional blood flow was determined on gestational day 21 using 57Co-labeled microspheres. Data were analyzed by analysis of variance with statistical significance accepted at P<.05.

RESULTS

Both placental and uterine placental bed perfusion were significantly decreased by hypoxia and returned to normal values with the endothelin antagonist (P<.01 and P<.05, respectively). Fetal weights were significantly lower in the hypoxic group (P<.001) and restored to control levels by the antagonist.

CONCLUSION

In the rat, endothelin contributes little to the regulation of uteroplacental perfusion under normal conditions. Hypoxia results in a decrease in perfusion of the uteroplacental bed and of the placenta, and perfusion in both of these beds is normalized by endothelin A receptor antagonism. We conclude that endothelin plays a primary role in the pathophysiology of hypoxia-induced fetal growth restriction by reducing uteroplacental perfusion.

Nitric oxide and endothelin in oxygen-dependent regulation of vascular tone of human umbilical vein

We investigated the possible contribution of nitric oxide (NO) and endothelin (ET) to oxygen-dependent regulation of human umbilical vein vascular tone by simultaneous registration of intracellular membrane potential and isometric tension of vessel strips with and without NO synthase inhibition [10-4 M N omega-nitro-L-arginine methyl ester (L-NAME)], ETA receptor blockade (10(-5) M BQ-123), or ETB receptor blockade (10(-7) M BQ-788) at Po2 values in the bath solution between 5 and 104 mmHg. Increasing PO2 above the physiological intrauterine range resulted in depolarization and an increase of isometric tension, whereas lowering PO2 resulted in hyperpolarization and a decrease in isometric tension. Removal of the endothelium reversed these effects. At PO2 values below 39 mmHg, intact preparations treated with either L-NAME, BQ-788, or BQ-123 were more depolarized than controls. In the case of treatment with L-NAME or BQ-123, this was accompanied by an increase in isometric tension. We conclude that it is NO that mediates the hypoxic hyperpolarization and vasodilatation of the human umbilical vein and that ET, via activation of ETB1 receptors on endothelial cells, contributes to this effect.

Fibroblast growth factor mediates hypoxia-induced endothelin-- a receptor expression in lung artery smooth muscle cells

We have previously demonstrated that endothelin (ET)-1 and its subtype A receptor (ET-AR) expression are increased in lung under hypoxic conditions and that activation of ET-AR by ET-1 is a major mediator of hypoxia-induced pulmonary hypertension in the rat. The present study tested the hypothesis that the hypoxia-responsive tyrosine kinase receptor-activating growth factors fibroblast growth factor (FGF)-1, FGF-2, and platelet-derived growth factor (PDGF)-BB stimulate expression of the ET-AR in pulmonary arterial smooth muscle cells (PASMCs). Quiescent rat PASMCs were incubated under hypoxia (1% O2), or with FGF-1, FGF-2, PDGF-BB, vascular endothelial growth factor, ET-1, angiotensin II, or atrial natriuretic peptide under normoxic conditions for 24 h. FGF-1 and -2 and PDGF-BB, but not hypoxia, vascular endothelial growth factor, ET-1, angiotensin II, or atrial natriuretic peptide, significantly increased ET-AR mRNA levels. FGF-1-induced ET-AR expression was inhibited by FGF-receptor inhibitor PD-166866, MEK inhibitor U-0126, transcription inhibitor actinomycin D, and translation inhibitor cycloheximide. In contrast, the stimulatory effect of FGF-1 on ET-AR mRNA expression was not altered by PI3 kinase, PKA, PKC, or adenylate cyclase inhibitors. PASMC ET-AR gene transcription, assessed by nuclear-runoff analysis, was increased by FGF-1. These results provide novel finding that ET-AR in PASMCs in vitro is unresponsive to hypoxia per se but is robustly simulated by tyrosine kinase receptor-associated growth factors (FGF-1, FGF-2, PDGF-BB) that themselves are stimulated by hypoxia in lung. This observation suggests a novel signaling mechanism that may be responsible for overexpression of ET-AR in lung, and may contribute to the hypoxia-induced pulmonary vasoconstriction, hypertension, and vascular remodeling in hypoxia-adapted animal.

ETA and ETB receptor function in pancreatitis-associated microcirculatory failure, inflammation, and parenchymal injury

The role of endothelin (ET)A and ETB receptor function in experimental pancreatitis is still not fully understood. Using a rat model of sodium taurocholate-induced pancreatitis and intravital microscopy, we therefore studied whether selective inhibition of ETA receptor function or combined ETA and ETB receptor blockade affects the development of pancreatitis-associated microcirculatory failure, inflammation, and parenchymal injury. Pretreatment with 10 mg/kg body wt of a combined ETA/B receptor antagonist, which is thought to mediate a simultaneous inhibition of both receptors, did not attenuate the pancreatitis-induced microcirculatory failure, inflammatory response, and parenchymal tissue injury. In contrast, pretreatment with a low concentration of the combined ETA/B receptor antagonist (4 mg/kg body wt), which predominantly inhibits the ETA receptor, revealed an improvement of some microcirculatory disorders and a significant attenuation of leukocyte recruitment and tissue injury. Furthermore, pretreatment with a selective ETA receptor antagonist (1 microg/kg body wt) almost abolished pancreatitis-associated capillary constriction, restored functional capillary density, and, consequently, improved overall nutritive perfusion. Importantly, the maintenance of an appropriate microcirculation by selective ETA receptor inhibition was accompanied by a significant attenuation of the inflammation-associated leukocytic response and by a marked reduction of parenchymal injury. Thus our study indicates that pancreatitis-associated development of microcirculatory failure, inflammation, and parenchymal injury is caused by ETs coupling onto the ETA receptor, which therefore may represent a promising target for novel strategies in the treatment of pancreatitis.

Development of agonists of endothelin-1 exhibiting selectivity towards ETA receptors

(1) Endothelin-1 (ET-1) is a bicyclic 21-amino-acid peptide causing a potent and sustained vasoconstriction, mainly through the ET(A) receptor subtype. So far, no selective ET(A) agonists are described in the literature. (2) A series of truncated and chemically modified ET-1 analogues were obtained through solid-phase peptide synthesis and their biological activity was assessed on rat thoracic aorta rings (ET(A) receptors) and guinea-pig lung parenchyma strips (ET(B) receptors). (3) Structure-activity studies led to the identification of ET-1 fragments exhibiting an ET(A) selective agonistic activity. (4) In particular, [D-Lys(9)]cyclo(11-15) ET-1(9-21) was the most potent peptide. It appeared as a full agonist of ET(A) receptors, being under two orders of magnitude less potent than ET-1 (EC(50): 2.3 x 10(-7) vs 6.8 x 10(-9) M). Interestingly, even a linear formylated analogue, [Ala(11,15), Trp(For)(21)]ET-1(9-21), showed a selective ET(A) activity (EC(50): 3.0 x 10(-6) M). None of the numerous analogues of the series exhibited substantial effects in the guinea-pig lung parenchyma bioassay. (5) Thus, this study describes the first compounds showing a significant bioactivity in an ET(A) pharmacological preparation while being inactive in an ET(B) paradigm. They show that the ET-1 pharmacophores, responsible for the ET(A)-mediated actions, are located within the 9-21 segment of the molecule. Moreover, the bicyclic structure of ET-1 does not appear as essential for the ET(A)-related vasoconstriction. Results also suggest that the positive charge of the Lys(9) side chain participates in an intramolecular ionic bond with the carboxylate function of Asp(18).

The endothelin axis: emerging role in cancer

Collectively, the endothelins and their receptors--referred to as the endothelin (ET) axis--have key physiological functions in normal tissue, acting as modulators of vasomotor tone, tissue differentiation, development, cell proliferation and hormone production. Based on new data, the ET axis also functions in the growth and progression of various tumours. Preliminary results from clinical trials, such as those with atrasentan--an ET(A)-receptor antagonist--in prostate cancer, are encouraging. The place of ET-receptor antagonists in cancer therapy for a range of malignancies merits further investigation.

Determination of atrasentan by high performance liquid chromatography with fluorescence detection in human plasma

Atrasentan is an endothelin antagonist selective for the ET(A) receptor in development at Abbott Laboratories for the treatment of cardiovascular disease and cell proliferation disorders. A simple and sensitive chromatographic method for the determination of atrasentan in human plasma has been developed and validated. The analytical method involves acidification of the plasma samples with 0.3 N HCl prior to extraction with 1:1 (v:v) hexane/tert-butylmethylether. The organic extract was evaporated to dryness, reconstituted with 20:80 (v:v) acetonitrile/0.05 M K(2)HPO(4) and washed with 75:25 (v:v) hexane/tert-butylmethylether. The organic layer was discarded and the aqueous layer was injected into the HPLC. Atrasentan and internal standard (ABT-790) were separated from interference using a 250 x 4.6 mm, 5 microm, 120 A Phenomenex Spherisorb C(8) analytical column with a 50 x 4.6 mm, Alltech Absorbosphere 5 microm CN guard cartridge using a mobile phase consisting of 25:15:5:55 (v:v:v:v) acetonitrile/isopropanol/methanol/0.05 M K(2)HPO(4), pH 7.0, at a flow rate of 1.0 mL/min. Fluorescence detection was achieved using lambda(ex) 278 nm and lambda(em) 322 nm. For a 1.0 mL plasma sample volume, the limit of quantitation was approximately 200 pg/mL. The method was linear from 0.2 to 1300 ng/mL (r(2) = 0.9986). Inter- and intra-day assay RSD (n = 6) were less than 10%. Mean accuracy determinations showed the quality control samples to range between 94 and 99% of the theoretical concentration.

Role for endothelin-1-induced superoxide and peroxynitrite production in rebound pulmonary hypertension associated with inhaled nitric oxide therapy

Our previous studies have demonstrated that inhaled nitric oxide (NO) decreases nitric oxide synthase (NOS) activity in vivo and that this inhibition is associated with rebound pulmonary hypertension upon acute withdrawal of inhaled NO. We have also demonstrated that inhaled NO elevates plasma endothelin-1 (ET-1) levels and that pretreatment with PD156707, an ETA receptor antagonist, blocks the rebound hypertension. The objectives of this study were to further elucidate the role of ET-1 in the rebound pulmonary hypertension upon acute withdrawal of inhaled NO. Inhaled NO (40 ppm) delivered to thirteen 4-week-old lambs decreased NOS activity by 36.2% in control lambs (P<0.05), whereas NOS activity was preserved in PD156707-treated lambs. When primary cultures of pulmonary artery smooth muscle cells were exposed to ET-1, superoxide production increased by 33% (P<0.05). This increase was blocked by a preincubation with PD156707. Furthermore, cotreatment of cells with ET-1 and NO increased peroxynitrite levels by 26% (P<0.05), whereas preincubation of purified human endothelial nitric oxide synthase (eNOS) protein with peroxynitrite generated a nitrated enzyme with 50% activity relative to control (P<0.05). Western blot analysis of peripheral lung extracts obtained after 24 hours of inhaled NO revealed a 90% reduction in 3-nitrotyrosine residues (P<0.05) in PD156707-treated lambs. The nitration of eNOS was also reduced by 40% in PD156707-treated lambs (P<0.05). These data suggest that the reduction of NOS activity associated with inhaled NO therapy may involve ETA receptor-mediated superoxide production. ETA receptor antagonists may prevent rebound pulmonary hypertension by protecting endogenous eNOS activity during inhaled NO therapy.

Endothelin-1 mediates prostaglandin F(2alpha)-induced luteal regression in the ewe

A diversified series of experiments was conducted to determine the potential role of endothelin-1 (ET-1) in ovine luteal function. Endothelin-1 inhibited basal and LH-stimulated progesterone production by dispersed ovine luteal cells during a 2-h incubation. This inhibition was removed when cells were preincubated with cyclo-D-Asp-Pro-D-Val-Leu-D-Trp (BQ123), a highly specific endothelin ET(A) receptor antagonist. Administration of a luteolytic dose of prostaglandin F(2alpha) (PGF(2alpha)) rapidly stimulated gene expression for ET-1 in ovine corpora lutea (CL) collected at midcycle. Intraluteal administration of a single dose of BQ123 to ewes on Day 8 or 9 of the estrous cycle mitigated the luteolytic effect of PGF(2alpha). Intramuscular administration of 100 microg ET-1 to ewes at midcycle reduced plasma progesterone concentrations for the remainder of the estrous cycle. Following pretreatment with a subluteolytic dose of PGF(2alpha), i.m. administration of 100 microg ET-1 caused a rapid decline in plasma progesterone and shortened the length of the estrous cycle. These data complement and extend previously published reports in the bovine CL and are the strongest evidence presented to date in support of a role for ET-1 in PGF(2alpha)-mediated luteal function in domestic ruminants.

Endothelin- and sarafotoxin-induced receptor-mediated calcium mobilization in a clonal murine osteoblast-like cell line, MC3T3-E1/B

Previous studies have demonstrated that, in osteoblast-like MC3T3-E1 cells, various endothelin peptides and their homologous sarafotoxins generate prostaglandin E(2) (PGE(2)) release through an ET(A) receptor subtype. In this study, biphasic Ca(2+) signals elicited with endothelin (ET)-1, ET-2, ET-3, beta-ET, S6a1, and S6b (ET/S6) were measured by microspectrofluorimetric methods in cell suspensions loaded with Fura-2 acetoxymethylester (Fura-2 AM). Phospholipase C (PLC)-dependent calcium activation mechanisms seem to be involved. We found evidence of Ca(2+) release from thapsigargin-sensitive and non-thapsigargin-sensitive intracellular Ca(2+) stores as well as Ca(2+) transmembrane inflow through multiple voltage-independent and Ni(2+)-sensitive cation channels. Using an ET(A) receptor antagonist, BQ-123, we showed that this receptor was coupled to Ca(2+) mobilization. All agonists tested, except S6c (an ET(B)-receptor-specific agonist) induced receptor desensitization. Our results demonstrate that the ET/S6-induced Ca(2+) signaling pathway is mediated via an ET(A)-receptor subtype in MC3T3-E1/B cells.

Functional and biochemical evidence for capsaicin-induced neural endothelin release in isolated working rat heart

In isolated working rat heart, capsaicin elicited a concentration-dependent constriction of coronary arteries accompanied by decline of all cardiac parameters recorded (heart rate, coronary and aortic flow, left ventricular developed pressure, and first derivative of left ventricular developed pressure). The following evidence suggests that capsaicin-induced changes are mediated by endothelin of neural origin: (1) the capsaicin (10 nM)-evoked decrease in coronary flow resulting in deterioration of cardiac functions was mimicked by endothelin (0.1 nM); (2) the selective endothelin ET(A) receptor antagonist, cyclo (D-alpha-aspartyl-L-propyl-D-valyl-L-leucyl-D-tryptophyl) (1 microM), abolished the cardiac effects provoked by capsaicin (10 nM); (3) reduction of extracellular Ca2+ concentration from 2.4 to 1.2 or 0.6 mM inhibited the cardiac effects of capsaicin (10 nM) but not those induced by endothelin (0.1 nM); (4) perfusion of the heart with 0.1% (v/v) Triton X-100 damaged the endothelium and reversed the enhancement of coronary flow evoked by bethanechol (1 microM), decreased the basal flow, but was without effect on capsaicin-induced coronary constriction; (5) in response to capsaicin challenge (10-100 nM), the endothelin concentration measured in coronary effluent by means of radioimmunoassay increased up to sevenfold but remained unchanged in the presence of 0.6 mM Ca2+; (6) no reduction of coronary flow was induced by capsaicin (100 nM) applied to the heart of rats which were desensitised by capsaicin (150 mg/kg). It is concluded that, in the rat heart, capsaicin acting on VR1 capsaicin receptors elicits a release of endothelin from the sensory nerve terminals.

Chronic endothelin-1 treatment leads to heterologous desensitization of insulin signaling in 3T3-L1 adipocytes

We recently reported that insulin and endothelin-1 (ET-1) can stimulate GLUT4 translocation via the heterotrimeric G protein G alpha q/11 and through PI3-kinase--mediated pathways in 3T3-L1 adipocytes. Because both hormones stimulate glucose transport through a common downstream pathway, we determined whether chronic ET-1 pretreatment would desensitize these cells to acute insulin signaling. We found that ET-1 pretreatment substantially inhibited insulin-stimulated 2-deoxyglucose uptake and GLUT4 translocation. Cotreatment with the ETA receptor antagonist BQ 610 prevented these effects, whereas inhibitors of G alpha i or G beta gamma were without effect. Chronic ET-1 treatment inhibited insulin-stimulated tyrosine phosphorylation of G alpha q/11 and IRS-1, as well as their association with PI3-kinase and blocked the activation of PI3-kinase activity and phosphorylation of AKT: In addition, chronic ET-1 treatment caused IRS-1 degradation, which could be blocked by inhibitors of PI3-kinase or p70 S6-kinase. Similarly, expression of a constitutively active G alpha q mutant, but not the wild-type G alpha q, led to IRS-1 degradation and inhibited insulin-stimulated phosphorylation of IRS-1, suggesting that the ET-1-induced decrease in IRS-1 depends on G alpha q/11 and PI3-kinase. Insulin-stimulated tyrosine phosphorylation of SHC was also reduced in ET-1 treated cells, resulting in inhibition of the MAPK pathway. In conclusion, chronic ET-1 treatment of 3T3-L1 adipocytes leads to heterologous desensitization of metabolic and mitogenic actions of insulin, most likely through the decreased tyrosine phosphorylation of the insulin receptor substrates IRS-1, SHC, and G alpha q/11.

Single-dose pharmacokinetics of atrasentan, an endothelin-A receptor antagonist

The pharmacokinetics of 1, 10, 23.25, and 139.5 mg doses of atrasentan was assessed in a placebo-controlled, double-blind, single oral dose study in 24 healthy male subjects. Atrasentan was well tolerated. Atrasentan pharmacokinetics was linear in the 1 to 23.25 mg dose range, with some dose dependency in the highest dose group. Harmonic mean terminal half-life was similar across all dose groups (20-25 h). Apparent oral clearance was low (12 L/h) for the highest dose group compared with the other three dose groups (21-27 L/h). The apparent volume of distribution was large (approximately 6 L/kg), consistent with extensive tissue distribution.

Endothelin and pulmonary hypertension

Biochemical and molecular biological evidence indicates that endothelin (ET)-1 and its receptors are selectively upregulated in the lung during exposure to hypoxia, while functional evidence indicates that ET-1 is a major mediator of hypoxia-induced pulmonary vasoconstriction and vascular remodeling. Hypoxia stimulates ET-1 gene transcription and peptide synthesis in cultured endothelial cells, and plasma ET-1 levels are increased in patients with primary pulmonary hypertension, and in humans exposed to high altitude, while immunoreactive ET-1 and ET-1 mRNA levels are increased in pulmonary artery endothelial cells of patients with primary pulmonary hypertension. Rats exposed to normobaric hypoxia exhibit increased pulmonary artery pressure, increased ET-1 peptide levels in plasma and lung, and selective increases in steady-state ET-1 and ET(A) and ET(B) receptor mRNA levels in lung but not in organs perfused by the systemic vasculature. The observations that both ET-1 and its major vascular smooth-muscle cell receptor are upregulated in response to hypoxia suggest that ET-1 may be a mediator of hypoxia-induced pulmonary hypertension. Moreover, hypoxic pulmonary vasoconstriction and vascular remodeling can be prevented and reversed by administration of either an ET(A)-selective or a combined ET(A) and ET(B) receptor antagonist. These findings support the hypothesis that endogenous ET-1 plays a major role in hypoxic pulmonary vasoconstriction/hypertension, right heart hypertrophy, and pulmonary vascular remodeling and suggest that ET-receptor blockers may be useful in the treatment and prevention of hypoxic pulmonary hypertension in humans.

The acute and chronic stimulatory effects of endothelin-1 on glucose transport are mediated by distinct pathways in 3T3-L1 adipocytes

We have recently shown that pretreatment with endothelin-1 (ET-1) for 20 min stimulates GLUT4 translocation in a PI3-kinase-dependent manner in 3T3-L1 adipocytes (Imamura, T. et al., J Biol Chem 274:33691-33695). This study presents another pathway by which ET-1 potentiates glucose transport in 3T3-L1 adipocytes. ET-1 treatment (10 nM) leads to approximately 2.5-fold stimulation of 2-deoxyglucose (2-DOG) uptake within 20 min, reaching a maximal effect of approximately 4-fold at approximately 6 h, and recovering almost to basal levels after 24 h. Insulin treatment (3 ng/ml) results in an approximately 5-fold increase in 2-DOG uptake at 1 h, and recovering to basal levels after 24 h. The ETA receptor antagonist, BQ 610, inhibited ET-1 induced glucose uptake both at 20 min and 6 h, whereas the ETB receptor antagonist, BQ 788, was without effect. Interestingly, ET-1 stimulated 2-DOG uptake at 6 h, not at 20 min, was almost completely blocked by the protein-synthesis inhibitor, cycloheximide and the RNA-synthesis inhibitor, actinomycin D, suggesting that the short-term (20 min) and long-term (6 h) effects of ET-1 involve distinct mechanisms. GLUT4 translocation assay showed that 20 min, but not 6 h, exposure to ET-1 led to GLUT4 translocation to the plasma membrane. In contrast, 6 h, but not 20 min, exposure to ET-1 increased expression of the GLUT1 protein, without affecting expression of GLUT4 protein. ET-1 induced 2-DOG uptake and GLUT1 expression at 6 h were completely inhibited by the MEK inhibitor, PD 98059, and partially inhibited by the PI3-kinase inhibitor, LY 294002, and the G alpha i inhibitor, pertussis toxin. The PLC inhibitor, U 73122, was without effect. These findings suggest that ET-1 induced GLUT1 protein expression is primarily mediated via MAPK, and partially via PI3K in 3T3-L1 adipocytes.

The endothelin system in septic and endotoxin shock

The view of the endothelium as a passive barrier has gradually changed as a number of endothelium-derived substances have been discovered. Substances like nitric oxide, prostaglandins and endothelins have potent and important properties, involving not only the circulation as such but also the response to stimuli like inflammation and trauma. The endothelin system, discovered in 1988, has not only strong vasoconstrictor properties, but also immunomodulating, endocrinological and neurological effects exerted through at least two types of receptors. Septic shock, a condition with high mortality, is associated with vast cardiovascular changes, organ dysfunction with microcirculatory disturbances and dysoxia. In the experimental setting, endotoxaemia resembles these changes and is, as well as septic shock, accompanied by a pronounced increase in plasma endothelin levels. The pathophysiology in septic and endotoxin shock remains to be fully elucidated, but several studies indicate that endothelial dysfunction is one contributing mechanism. Activation of the endothelin system is associated with several pathological conditions complicating septic shock, such as acute respiratory distress syndrome, cardiac dysfunction, splanchnic hypoperfusion and disseminated intravascular coagulation. Through the development of both selective and nonselective endothelin receptor antagonists, the endothelin system has been the object of a large number of studies during the last decade. This review highlights systematically the findings of previous studies in the area. It provides strong indications that the endothelin system, apart from being a marker of vascular injury, is directly involved in the pathophysiology of septic and endotoxin shock. Interventions with endothelin receptor antagonists during septic and endotoxin shock have so far only been done in animal studies but the results are interesting and promising.

Endothelin-receptor antagonists: current and future perspectives

Despite much effort over recent years to design and develop endothelin-receptor antagonists, these compounds are far from becoming new drug entities. This article will review preclinical data on select endothelin-receptor antagonists as well as clinical data on bosentan, the only molecule currently in Phase III clinical trials. Though efficacious, bosentan is less potent than the angiotensin converting enzyme (ACE) inhibitor, enalapril, in patients with hypertension. We will therefore discuss the possible reason(s) for this low potency, the consequences thereof, and a few therapeutic areas where endothelin-receptor antagonists could find better use.

The contractile effects of endothelins on the smooth muscle of the rat prostate gland

Endothelin-1 elicited tonic contractions of rat prostatic smooth muscle that were unaffected by prazosin (0.5 microM), tetrodotoxin (1 microM) or guanethidine (10 microM). The rank order of potency of the endothelin isopeptides was endothelin-1>endothelin-2> or =endothelin-3. Sarafotoxin S6B was approximately equipotent with endothelin-1 in eliciting tonic contractions, but neither of the selective endothelin ET(B) receptor-agonists, sarafotoxin S6C (0.1 nM-0.3 microM) and BQ3020 (Ac-[Ala (11,15)]endothelin-1(6-21); 0.1 nM-0.3 microM), affected prostatic smooth muscle tone. The selective endothelin ET(A) receptor antagonist, BQ123 (cyclo(D-Asp-L-Pro-D-Val-L-Leu-D-Trp; 1 microM), attenuated responses to endothelin-1, -2 and -3, while the non-selective endothelin receptor antagonist bosentan (1 microM) and the selective endothelin ET(B) receptor antagonist BQ788, (Dmpc-gamma-MeLeu(9)-D-Trp(l-CO(2)CH(3))-D-Nle-OH; 1 microM) attenuated responses to endothelin-3 only. Contractions induced by exogenous administration of noradrenaline were unaffected by preincubation of tissues in BQ123 (1 microM) indicating the selectivity of this antagonist. These data suggest that endothelins mediate contractions of the rat prostate by action at endothelin ET(A) receptors.

Evidence for vasoconstriction mediated by the endothelin-B receptor in domestic swine

Endothelin-1 (ET-1), a potent vasoactive and mitogenic peptide, has been implicated in a number of cardiovascular diseases, including congestive heart failure, neointimal hyperplasia associated with restenosis, and hypertension. The vasoconstriction induced by ET-1 is thought to be mediated mainly by its action on ET(A) receptors on vascular smooth muscle cells. Recent studies have indicated that vasoconstriction also may be mediated by stimulation of an ET(B)-receptor subtype. Increased use of the pig as a cardiovascular model prompted us to examine the receptor profile in this species using ABT-627, a potent, nonpeptide antagonist of the ET(A) receptor. The precursor to ET-1, big ET-1 (0.02 nmol/kg/min), was infused intravenously in domestic swine, resulting in a sustained increase in mean blood pressure of 38 +/- 3 mm Hg. After stabilization of the pressor response, ABT-627 (0.1-10 microg/kg/min) or vehicle was infused for 30 min. Whereas vehicle infusion had no appreciable effect, a dose-related reversal of the pressor response to big ET-1 (11-100%) was observed by the end of the ABT-627 infusion. Blood samples were assayed for plasma concentrations of ABT-627; peak levels ranged from 9 +/- 2 to 937 +/- 168 ng/ml. In a separate group of pigs, the highest dose of ABT-627 produced only a modest reversal of the hypertensive response to an infusion of angiotensin II (300 ng/kg/min). Additional results indicate that the vasoconstrictor effects produced by sarafotoxin 6C (0.03 and 0.3 nmol/kg), an agonist of the ET(B) receptor, are not blocked by treatment with ABT-627 (10 microg/kg/min). However, complete blockade of the S6C response could be achieved using the ET(B) antagonist, A-192621 (0.33 mg/kg/min). Our results define the dose-response relation for the ET(A)-receptor antagonist ABT-627 in the vasculature of the domestic pig and suggest the presence of an ET(B)-receptor subtype that mediates vasoconstriction in this species.

Direct determination of endothelin receptor antagonist levels in plasma using a scintillation proximity assay

An assay using scintillation proximity bead technology has been developed suitable for the quantitation of endothelin (ET) receptor antagonists in preclinical and clinical samples of plasma. The assay measures the competitive inhibition of radiolabelled ET-1 binding to ET(A) receptor membranes bound to wheat germ agglutinin (WGA)-coated scintillation proximity assay (SPA) beads in the presence of plasma containing A-127722, a potent orally active, ET(A) selective ET antagonist. The assay requires as little as 50 microl plasma and no extraction procedure is needed. The SPA methodology eliminates the need for the separation of bound from free ligand. Using this method, A-127722 could be directly quantified in rat plasma with a detection limit of 1 ng/ml.

Endothelins and the lung

Since endothelins were discovered by Yanasigawa in 1988 it has been recognised that they may have an important role in lung pathophysiology. Despite their biological importance as vasoconstrictors the physiological role of endothelin has not yet been defined within the lungs. This review explores their role in acute and chronic disease. During acute inflammation and ischaemia-reperfusion injury cytokines may induce release of endothelin. This is important in the realm of acute lung injury and during surgical procedures such as cardiopulmonary operations including lung resections and transplantation. Complications of surgery including primary organ failure resulting in poor gas exchange as well as increased pulmonary vascular resistance have been linked to the presence of excessive endothelin. Endothelin may have an important role in transplantation biology. The complex process leading to successful lung transplantation includes optimising the donor with brain death, harvesting the lungs, managing acute and chronic rejection, and protecting the vital organs from toxic effects of immunosuppressants. During chronic disease processes, the mitotic action of endothelin may be important in vascular and airway remodelling by means of smooth muscle cell proliferation. We also explore recent advances in drug development, animal models and future directions for research.

Stuttering priapism in a liver transplant patient with toxic levels of fk506

This is the first report of stuttering priapism in a liver transplant patient with toxic levels of the immunosuppressive agent FK506. To date, stuttering priapism has only been reported in patients with sickle cell disease and is not currently listed in the toxicity profile of FK506 or cyclosporine, a drug with a similar mechanism of action. The erections resolved when the FK506 levels normalized. We review the possible mechanisms by which FK506 may have caused these erections.

Pharmacological characterization of endothelin receptor subtypes in the guinea-pig prostate gland

Experiments have been conducted to investigate the actions of endothelins on the guinea-pig prostate gland. Saturation experiments with [125I]-endothelin-1 (2-800 pM) in guinea-pig prostatic homogenates indicated the presence of high affinity binding sites with an equilibrium dissociation constant (KD) of 230+/-50 pM, a maximum number of binding sites (Bmax) of 52+/-16 fmol mg(-1) protein or 269+/-61 fmol g(-1) tissue and a Hill coefficient (nH) of 1.01+/-0.03 (n = 3). Competition experiments revealed that binding of [125I]-endothelin-1 (20 pM) was inhibited with the following order of potency: endothelin-1 >>BQ-788 (N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methyl-Leu-D-Trp[1-+ ++CO2CH3-D-Nle-ONa])> BQ-123 (cyclo-D-Asp-L-Pro-D-Val-Leu-D-Trp) > or = sarafotoxin S6c. At concentrations with negligible influence on smooth muscle tone, endothelin-1, endothelin-2 and sarafotoxin S6b (1 nM-0.1 microM) produced concentration-dependent potentiation of the contractions evoked by electrical field stimulation with trains of 20 pulses at 10 Hz every 50 s, 0.5 ms pulse width and a dial setting of 60 V. In contrast, the endothelin ET(B) receptor-preferring agonist endothelin-3 (1 nM- 1 microM) was much less potent, and the endothelin ET(B) receptor-selective agonists sarafotoxin S6c and BQ-3020 (Ac-[Ala11,15]-endothelin-1 (6-21)), up to 1 microM, were without effect. The endothelin ET(A) receptor antagonist BQ-123 (1 microM) markedly inhibited the potentiation induced by endothelin-1, endothelin-2 and sarafotoxin S6b while the endothelin ET(B) receptor antagonist BQ-788 (1 microM) was less effective. While our binding data indicates the presence of ET(A) and ET(B) binding sites in the guinea-pig prostate, the endothelin-induced facilitation of neurotransmission to the prostatic smooth muscle is mediated largely via activation of endothelin receptors of the ET(A) subtype.