In vitro biological profile of a highly potent novel endothelin (ET) antagonist BQ-123 selective for the ETA receptor

E. Coli LPS-induced calcium signaling regulates the expression of hypoxia-inducible factor 1α in periodontal ligament fibroblasts in a non-hypoxia-dependent manner

Periodontitis, an inflammatory disease, can cause significant damage to the oral tissues which support the teeth. During the early development of periodontitis, periodontal ligament fibroblasts (PDLFs) undergo metabolic reprogramming regulated by hypoxia-inducible factor 1α (HIF-1α), which is strongly linked to the progression of inflammation. However, the precise mechanisms by which PDLFs regulate HIF-1α and its associated metabolic reprogramming during early inflammation remain unclear. This study illustrated that brief and low-dose exposure to Escherichia coli (E. coli) lipopolysaccharide (LPS) can serve as a non-hypoxic stimulus, effectively replicating early periodontal inflammatory reactions. This is evidenced by the upregulation of HIF-1α expression and the activation of HIF-1α-mediated crucial glycolytic enzymes, namely lactate dehydrogenase a, pyruvate kinase, and hexokinase 2, concomitant with an augmentation in the inflammatory response within PDLFs. We observed that the effects mentioned and their impact on macrophage polarization were notably attenuated when intracellular and extracellular stores of Ca2+ were depleted using BAPTA-AM and Ca2+-free medium, respectively. Mechanistically, our findings demonstrated that the transcriptional process of HIF-1α is regulated by Ca2+ during E. coli LPS stimulation, mediated through the signal transducer and activator of transcription 3 (STAT3) pathway. Additionally, we observed that the stabilization of intracellular HIF-1α proteins occurs via the endothelin (ET)-1-endothelin A receptor pathway, independent of hypoxia. Taken together, our research outcomes underscore the pivotal involvement of Ca2+ in the onset of early periodontitis by modulating HIF-1α and glycolysis, thereby presenting novel avenues for early therapeutic interventions.

Hyperglycemia Augments Endothelin-1-Induced Constriction of Human Retinal Venules

Purpose

Endothelin-1 (ET-1) is a potent vasoactive factor implicated in development of diabetic retinopathy, which is commonly associated with retinal edema and hyperglycemia. Although the vasomotor activity of venules contributes to the regulation of tissue fluid homeostasis, responses of human retinal venules to ET-1 under euglycemia and hyperglycemia remain unknown and the ET-1 receptor subtype corresponding to vasomotor function has not been determined. Herein, we addressed these issues by examining the reactivity of isolated human retinal venules to ET-1, and results from porcine retinal venules were compared.

Methods

Retinal tissues were obtained from patients undergoing enucleation. Human and porcine retinal venules were isolated and pressurized to assess diameter changes in response to ET-1 after exposure to 5 mM control glucose or 25 mM high glucose for 2 hours.

Results

Both human and porcine retinal venules exposed to control glucose developed similar basal tone and constricted comparably to ET-1 in a concentration-dependent manner. ET-1–induced constrictions of human and porcine retinal venules were abolished by ET_A receptor antagonist BQ123. During high glucose exposure, basal tone of human and porcine retinal venules was unaltered but ET-1–induced vasoconstrictions were enhanced.

Conclusions

ET-1 elicits comparable constriction of human and porcine retinal venules by activation of ET_A receptors. In vitro hyperglycemia augments human and porcine retinal venular responses to ET-1.

Translational Relevance

Similarities in vasoconstriction to ET-1 between human and porcine retinal venules support the latter as an effective model of the human retinal microcirculation to help identify vascular targets for the treatment of retinal complications in patients with diabetes.

Mouse connective tissue mast cell proteases tryptase and carboxypeptidase A3 play protective roles in itch induced by endothelin-1

Background

Itch is an unpleasant sensation that can be debilitating, especially if it is chronic and of non-histaminergic origin, as treatment options are limited. Endothelin-1 (ET-1) is a potent endogenous vasoconstrictor that also has the ability to induce a burning, non-histaminergic pruritus when exogenously administered, by activating the endothelin A receptor (ET_AR) on primary afferents. ET-1 is released endogenously by several cell-types found in the skin, including macrophages and keratinocytes. Mast cells express ET_ARs and can thereby be degranulated by ET-1, and mast cell proteases chymase and carboxypeptidase A3 (CPA3) are known to either generate or degrade ET-1, respectively, suggesting a role for mast cell proteases in the regulation of ET-1-induced itch. The mouse mast cell proteases (mMCPs) mMCP4 (chymase), mMCP6 (tryptase), and CPA3 are found in connective tissue type mast cells and are the closest functional homologs to human mast cell proteases, but little is known about their role in endothelin-induced itch.

Methods

In this study, we evaluated the effects of mast cell protease deficiency on scratching behavior induced by ET-1. To investigate this, mMCP knock-out and transgenic mice were injected intradermally with ET-1 and their scratching behavior was recorded and analyzed.

Results

CPA3-deficient mice and mice lacking all three proteases demonstrated highly elevated levels of scratching behavior compared with wild-type controls. A modest increase in the number of scratching bouts was also seen in mMCP6-deficient mice, while mMCP4-deficiency did not have any effect.

Conclusion

Altogether, these findings identify a prominent role for the mast cell proteases, in particular CPA3, in the protection against itch induced by ET-1.

Single-dose radiotherapy disables tumor cell homologous recombination via ischemia/reperfusion injury

Tumor cure with conventional fractionated radiotherapy is 65%, dependent on tumor cell-autonomous gradual buildup of DNA double-strand break (DSB) misrepair. Here we report that single-dose radiotherapy (SDRT), a disruptive technique that ablates more than 90% of human cancers, operates a distinct dual-target mechanism, linking acid sphingomyelinase-mediated (ASMase-mediated) microvascular perfusion defects to DNA unrepair in tumor cells to confer tumor cell lethality. ASMase-mediated microcirculatory vasoconstriction after SDRT conferred an ischemic stress response within parenchymal tumor cells, with ROS triggering the evolutionarily conserved SUMO stress response, specifically depleting chromatin-associated free SUMO3. Whereas SUMO3, but not SUMO2, was indispensable for homology-directed repair (HDR) of DSBs, HDR loss of function after SDRT yielded DSB unrepair, chromosomal aberrations, and tumor clonogen demise. Vasoconstriction blockade with the endothelin-1 inhibitor BQ-123, or ROS scavenging after SDRT using peroxiredoxin-6 overexpression or the SOD mimetic tempol, prevented chromatin SUMO3 depletion, HDR loss of function, and SDRT tumor ablation. We also provide evidence of mouse-to-human translation of this biology in a randomized clinical trial, showing that 24 Gy SDRT, but not 3×9 Gy fractionation, coupled early tumor ischemia/reperfusion to human cancer ablation. The SDRT biology provides opportunities for mechanism-based selective tumor radiosensitization via accessing of SDRT/ASMase signaling, as current studies indicate that this pathway is tractable to pharmacologic intervention.

Endothelin-1 (ET-1) induces resistance to bortezomib in human multiple myeloma cells via a pathway involving the ETB receptor and upregulation of proteasomal activity

PURPOSE

Bortezomib (BTZ) is used for the treatment of multiple myeloma (MM). However, a significant proportion of patients may be refractory to the drug. This study aimed to investigate whether the endothelin (ET-1) axis may act as an escape mechanism to treatment with bortezomib in MM cells.

METHODS

NCI-H929 and RPMI-8226 (human MM cell lines) were cultured with or without ET-1, BTZ, and inhibitors of the endothelin receptors. ET-1 levels were determined by ELISA, while the protein levels of its receptors and of the PI3K and MAPK pathways' components by western blot. Effects of ET-1 on cell proliferation were studied by MTT and on the ubiquitin proteasome pathway by assessing the chymotryptic activity of the 20S proteasome in cell lysates.

RESULTS

Endothelin receptors A and B (ETAR and ETBR, respectively) were found to be expressed in both cell lines, with the RPMI-8226 cells that are considered resistant to BTZ, expressing higher levels of ETBR and in addition secreting ET-1. Treatment of the NCI-H929 cells with ET-1 increased proliferation, while co-incubation of these cells with ET-1 and BTZ decreased BTZ efficacy with concomitant upregulation of 20S proteasomal activity. Si-RNA silencing or chemical blockade of ETBR abrogated the protective effects of ET-1. Finally, data suggest that the predominant signaling pathway involved in ET-1/ETBR-induced BTZ resistance in MM cells may be the MAPK pathway.

CONCLUSION

Our data suggest a possible role of the ET-1/ETBR axis in regulating the sensitivity of MM cells to BTZ. Thus, combining bortezomib with strategies to target the ET-1 axis could prove to be a novel promising therapeutic approach in MM.

Salvianolic Acid A, as a Novel ETA Receptor Antagonist, Shows Inhibitory Effects on Tumor in Vitro

Endothelin-1 (ET-1) autocrine and paracrine signaling modulate cell proliferation of tumor cells by activating its receptors, endothelin A receptor (ET_AR) and endothelin B receptor (ET_BR). Dysregulation of ET_AR activation promotes tumor development and progression. The potential of ET_AR antagonists and the dual-ET_AR and ET_BR antagonists as therapeutic approaches are under preclinical and clinical studies. Salvianolic acid A (Sal A) is a hydrophilic polyphenolic derivative isolated from Salvia miltiorrhiza Bunge (Danshen), which has been reported as an anti-cancer and cardio-protective herbal medicine. In this study, we demonstrate that Sal A inhibits ET_AR activation induced by ET-1 in both recombinant and endogenous ET_AR expression cell lines. The IC₅₀ values were determined as 5.7 µM in the HEK293/ET_AR cell line and 3.14 µM in HeLa cells, respectively. Furthermore, our results showed that Sal A suppressed cell proliferation and extended the doubling times of multiple cancer cells, including HeLa, DU145, H1975, and A549 cell lines. In addition, Sal A inhibited proliferation of DU145 cell lines stimulated by exogenous ET-1 treatment. Moreover, the cytotoxicity and cardio-toxicity of Sal A were assessed in human umbilical vein endothelial cells (HUVEC) and Human-induced pluripotent stem cell-derived cardiomyocytes (hiPS-CMs), which proved that Sal A demonstrates no cytotoxicity or cardiotoxicity. Collectively, our findings indicate that Sal A is a novel anti-cancer candidate through targeting ET_AR.

Discovery of Dual ETA/ETB Receptor Antagonists from Traditional Chinese Herbs through in Silico and in Vitro Screening

Endothelin-1 receptors (ETAR and ETBR) act as a pivotal regulator in the biological effects of ET-1 and represent a potential drug target for the treatment of multiple cardiovascular diseases. The purpose of the study is to discover dual ETA/ETB receptor antagonists from traditional Chinese herbs. Ligand- and structure-based virtual screening was performed to screen an in-house database of traditional Chinese herbs, followed by a series of in vitro bioassay evaluation. Aristolochic acid A (AAA) was first confirmed to be a dual ETA/ETB receptor antagonist based intracellular calcium influx assay and impedance-based assay. Dose-response curves showed that AAA can block both ETAR and ETBR with IC₅₀ of 7.91 and 7.40 μM, respectively. Target specificity and cytotoxicity bioassay proved that AAA is a selective dual ETA/ETB receptor antagonist and has no significant cytotoxicity on HEK293/ETAR and HEK293/ETBR cells within 24 h. It is a feasible and effective approach to discover bioactive compounds from traditional Chinese herbs using in silico screening combined with in vitro bioassay evaluation. The structural characteristic of AAA for its activity was especially interpreted, which could provide valuable reference for the further structural modification of AAA.

Study of breakthrough cancer pain in an animal model induced by endothelin-1

Cancer patients with bone metastases often suffer breakthrough pain. However, little progress has been made in the treatment of breakthrough pain and its associated mechanism(s) in the patient with cancer due to lacking of resembling and predictive animal models. We previously have demonstrated that endothelin-1 plays an important role in breakthrough cancer pain. In the present study, we have established an animal model of breakthrough cancer pain induced by endothelin-1. The animal model of breakthrough cancer pain is strictly followed the definition and meets the characteristics of breakthrough pain. The model is reliable, reproducible and easy to be produced. To our knowledge, this is the first report for establishing such an animal model. In addition, we also found that a selective ETA receptor antagonist BQ-123 could reverse endothelin-1 induced breakthrough pain. We further studied the characteristics of pain behaviors such as hind limb use score and voluntary wheel running as well as the electrophysiology of sciatic nerve fibers with the model. The murine model shows high resemblance compared to the breakthrough cancer pain in the patients with cancer clinically. It provides a platform for further study of the pathogenesis of breakthrough cancer pain and targeted intervention.

Regulation of vascular smooth muscle tone by adipose-derived contracting factor

Obesity and arterial hypertension, important risk factors for atherosclerosis and coronary artery disease, are characterized by an increase in vascular tone. While obesity is known to augment vasoconstrictor prostanoid activity in endothelial cells, less is known about factors released from fat tissue surrounding arteries (perivascular adipose). Using lean controls and mice with either monogenic or diet-induced obesity, we set out to determine whether and through which pathways perivascular adipose affects vascular tone. We unexpectedly found that in the aorta of obese mice, perivascular adipose potentiates vascular contractility to serotonin and phenylephrine, indicating activity of a factor generated by perivascular adipose, which we designated “adipose-derived contracting factor” (ADCF). Inhibition of cyclooxygenase (COX) fully prevented ADCF-mediated contractions, whereas COX-1 or COX-2-selective inhibition was only partially effective. By contrast, inhibition of superoxide anions, NO synthase, or endothelin receptors had no effect on ADCF activity. Perivascular adipose as a source of COX-derived ADCF was further confirmed by detecting increased thromboxane A₂ formation from perivascular adipose-replete aortae from obese mice. Taken together, this study identifies perivascular adipose as a novel regulator of arterial vasoconstriction through the release of COX-derived ADCF. Excessive ADCF activity in perivascular fat under obese conditions likely contributes to increased vascular tone by antagonizing vasodilation. ADCF may thus propagate obesity-dependent hypertension and the associated increased risk in coronary artery disease, potentially representing a novel therapeutic target.

Kinase-dependent activation of voltage-gated Ca2+ channels by ET-1 in pulmonary arterial myocytes during chronic hypoxia

Exposure to chronic hypoxia (CH) causes pulmonary hypertension. The vasoconstrictor endothelin-1 (ET-1) is thought to play a role in the development of hypoxic pulmonary hypertension. In pulmonary arterial smooth muscle cells (PASMCs) from chronically hypoxic rats, ET-1 signaling is altered, with the ET-1-induced change in intracellular calcium concentration (Δ[Ca(2+)](i)) occurring through activation of voltage-dependent Ca(2+) channels (VDCC) even though ET-1-induced depolarization via inhibition of K(+) channels is lost. The mechanism underlying this response is unclear. We hypothesized that activation of VDCCs by ET-1 following CH might be mediated by protein kinase C (PKC) and/or Rho kinase, both of which have been shown to phosphorylate and activate VDCCs. To test this hypothesis, we examined the effects of PKC and Rho kinase inhibitors on the ET-1-induced Δ[Ca(2+)](i) in PASMCs from rats exposed to CH (10% O(2), 3 wk) using the Ca(2+)-sensitive dye fura 2-AM and fluorescent microscopy techniques. We found that staurosporine and GF109203X, inhibitors of PKC, and Y-27632 and HA 1077, Rho kinase inhibitors, reduced the ET-1-induced Δ[Ca(2+)](i) by >70%. Inhibition of tyrosine kinases (TKs) with genistein or tyrphostin A23, or combined inhibition of PKC, TKs, and Rho kinase, reduced the Δ[Ca(2+)](i) to a similar extent as inhibition of either PKC or Rho kinase alone. The ability of PKC or Rho kinase to activate VDCCs in our cells was verified using phorbol 12-myristate 13-acetate and GTP-γ-S. These results suggest that following CH, the ET-1-induced Δ[Ca(2+)](i) in PASMCs occurs via Ca(2+) influx through VDCCs mediated primarily by PKC, TKs, and Rho kinase.

Calcitonin Gene-Related Peptide Terminates Long-Lasting Vasopressor Responses to Endothelin 1 In Vivo

Slow dissociation of endothelin 1 from its endothelin A receptors is responsible for the long-lasting vasoconstrictor effects of the peptide. We showed recently that calcitonin gene-related peptide selectively terminates long-lasting contractile responses to endothelin 1 in isolated rat mesenteric arteries. Here we assessed whether the antiendothelinergic effect of calcitonin gene-related peptide is vascular bed specific and may terminate long-lasting pressor responses to exogenous and locally produced endothelin 1 in vivo. Regional heterogeneity of the calcitonin gene-related peptide/endothelin A receptor cross-talk was explored in arteries isolated from various rat organs. Endothelin A receptor-mediated arterial contractions were terminated by calcitonin gene-related peptide in rat mesenteric, renal, and spermatic arteries but not in basilar, coronary, epigastric, gastric, splenic, and saphenous arteries. Endothelin A receptor antagonism only ended endothelin 1–induced contractions in spermatic arteries. In anesthetized rats, instrumented with Doppler flow probes to record regional blood flows, long-lasting pressor and vasoconstrictor responses to an intravenous bolus injection of endothelin 1 or big endothelin 1 were transiently reduced by sodium nitroprusside (NO donor) but terminated by intravenously administered calcitonin gene-related peptide. In conscious rats, calcitonin gene-related peptide but not sodium nitroprusside terminated prolonged (>60-minute) pressor responses to endothelin 1 but not those to intravenous infusion of phenylephrine. In conclusion, pressor responses to circulating and locally produced endothelin 1 that are resistant to endothelin receptor antagonism and NO can be terminated by a regionally selective effect of calcitonin gene-related peptide. Calcitonin gene related peptide receptor agonism may represent a novel strategy to treat endothelin 1–associated cardiovascular pathologies.

Nociceptive tolerance is improved by bradykinin receptor B1 antagonism and joint morphology is protected by both endothelin type A and bradykinin receptor B1 antagonism in a surgical model of osteoarthritis

Introduction

Endothelin-1, a vasoconstrictor peptide, influences cartilage metabolism mainly via endothelin receptor type A (ETA). Along with the inflammatory nonapeptide vasodilator bradykinin (BK), which acts via bradykinin receptor B1 (BKB1) in chronic inflammatory conditions, these vasoactive factors potentiate joint pain and inflammation. We describe a preclinical study of the efficacy of treatment of surgically induced osteoarthritis with ETA and/or BKB1 specific peptide antagonists. We hypothesize that antagonism of both receptors will diminish osteoarthritis progress and articular nociception in a synergistic manner.

Methods

Osteoarthritis was surgically induced in male rats by transection of the right anterior cruciate ligament. Animals were subsequently treated with weekly intra-articular injections of specific peptide antagonists of ETA and/or BKB1. Hind limb nociception was measured by static weight bearing biweekly for two months post-operatively. Post-mortem, right knee joints were analyzed radiologically by X-ray and magnetic resonance, and histologically by the OARSI histopathology assessment system.

Results

Single local BKB1 antagonist treatment diminished overall hind limb nociception, and accelerated post-operative recovery after disease induction. Both ETA and/or BKB1 antagonist treatments protected joint radiomorphology and histomorphology. Dual ETA/BKB1 antagonism was slightly more protective, as measured by radiology and histology.

Conclusions

BKB1 antagonism improves nociceptive tolerance, and both ETA and/or BKB1 antagonism prevents joint cartilage degradation in a surgical model of osteoarthritis. Therefore, they represent a novel therapeutic strategy: specific receptor antagonism may prove beneficial in disease management.

Acute effects of endothelin receptor antagonists on hepatic hemodynamics of cirrhotic and noncirrhotic rats

Hepatic and circulating endothelin-1 (ET-1) are increased in patients with cirrhosis and in cirrhotic animals. However, the distinct roles of ET receptor subtypes ETA and ETB in cirrhosis and portal hypertension (PHT) have not been clearly elucidated. Thus, we studied the effects of selective ET-1 antagonists (ETA-ant or ETB-ant) and nonselective ET-1 antagonist (ETA/B-ant) on hepatic hemodynamics in cirrhotic rats. Liver fibrosis and PHT were induced by complete bile duct ligation (BDL) in rats. Two weeks after BDL or sham surgery, hemodynamic responses were measured during intraportal infusion of incremental doses of the following ET-ants: (i) BQ-123, (ii) BQ-788, and (iii) bosentan. After equilibration with vehicle, doses of ET-ants were infused for 30 min periods, and steady-state systemic and hepatic hemodynamics, portal venous pressure (PVP), and hepatic blood flow (HBF) were measured. BDL induced significant PHT and elevated concentrations of plasma ET-1 compared with sham. ETA-ant decreased PVP of cirrhotic rats but had no effect on sham, whereas ETB-ant increased PVP in sham but had no effect in BDL. Nonselective ETA/B-ant decreased PVP of BDL similarly to ETA-ant. Both ETA-ant and ETB-ant decreased local HBF, whereas a nonselective antagonist did not change HBF in sham; however no significant changes were observed in HBF of BDL rats with any of the antagonists. These findings suggest ETA activation contributes to PHT in cirrhotic rats, whereas ETB-mediated portal depressor effects are attenuated in cirrhotic rats compared with noncirrhotic rats.

Stimuli of sensory-motor nerves terminate arterial contractile effects of endothelin-1 by CGRP and dissociation of ET-1/ET(A)-receptor complexes

Background

Endothelin-1 (ET-1), a long-acting paracrine mediator, is implicated in cardiovascular diseases but clinical trials with ET-receptor antagonists were not successful in some areas. We tested whether the quasi-irreversible receptor-binding of ET-1 (i) limits reversing effects of the antagonists and (ii) can be selectively dissociated by an endogenous counterbalancing mechanism.

Methodology/Principal findings

In isolated rat mesenteric resistance arteries, ET_A-antagonists, endothelium-derived relaxing factors and synthetic vasodilators transiently reduced contractile effects of ET-1 but did not prevent persistent effects of the peptide. Stimuli of peri-vascular vasodilator sensory-motor nerves such as capsaicin not only reduced but also terminated long-lasting effects of ET-1. This was prevented by CGRP-receptor antagonists and was mimicked by exogenous calcitonin gene-related peptide (CGRP). Using 2-photon laser scanning microscopy in vital intact arteries, capsaicin and CGRP, but not ET_A-antagonism, were observed to promote dissociation of pre-existing ET-1/ET_A-receptor complexes.

Conclusions

Irreversible binding and activation of ET_A-receptors by ET-1 (i) occur at an antagonist-insensitive site of the receptor and (ii) are selectively terminated by endogenously released CGRP. Hence, natural stimuli of sensory-motor nerves that stimulate release of endogenous CGRP can be considered for therapy of diseases involving ET-1.

Role of Ca(2+)/calmodulin-dependent protein kinase II in the regulation of the cardiac L-type Ca(2+) current during endothelin-1 stimulation

Endothelin-1 (ET-1) shows a positive inotropic effect on cardiac muscle. Although the L-type Ca(2+) current (I(Ca)) is one of the important determinants of cardiac excitation-contraction coupling, the effect of ET-1 on the I(Ca) is not always clear. The controversial results appear to be due to different patch-clamp methods. The present study measured the effect of ET-1 on the I(Ca) of rat ventricular myocytes using the perforated patch-clamp technique. The holding potential was set to -40 mV, and depolarization was applied every 10 s. ET-1 (10 nM) increased the I(Ca) in a monophasic manner. The current reached a steady state 15 min after the application of ET-1, when the measurement was done. Endothelin receptor subtype expression was also investigated using Western immunoblotting. ET(A)-receptor protein was expressed, but ET(B)-receptor protein was not expressed, in the cell membranes of rat ventricular myocytes. The effect of ET-1 on the I(Ca) was inhibited by a selective ET(A)-receptor antagonist, BQ-123, but not by a selective ET(B)-receptor antagonist, BQ-788. The effect was inhibited by protein kinase C (PKC) inhibitor chelerythrine and Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-93, but not by its inactive analog KN-92. The effect of ET-1 was also blocked by another CaMKII inhibitor, autocamtide-2-related inhibitory peptide. These results suggest that ET-1 increases the I(Ca) via the ET(A)-receptor-PKC-CaMKII pathway.

Comparative analyses of different surfactants on matrix-assisted laser desorption/ionization mass spectrometry peptide analysis

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) has been extensively used for proteomics and peptidomics analysis. Nevertheless, these analyses, when focused on low molecular mass proteins, show some limitation due to background interference from surfactant ions. Surfactants are routinely used as a solubilizing or denaturing agents for proteins and peptides. In this report, an evaluation and further comparison of the effects of an ionic surfactant, sodium dodecyl sulfate (SDS), and a non-ionic surfactant, tergitol, on MALDI-MS analyses of the amphipathic peptides, angiotensin and bradykinin, were carried out. At concentrations > or = 10 mmol L(-1), SDS deteriorates the MALDI spectral quality by reducing the signal and intensity of the analyte ions. In particular, it affects the hydrophobic peptide where the signal of surfactant-interfering ions suppresses the analyte ion signal. Whereas, the non-ionic surfactant, tergitol, improves the MALDI-MS analysis of peptide mixtures or hydrophobic peptides by reducing interference from the surfactant itself in positive ion mode analysis. Three-dimensional molecular modeling of two different peptides in complex to tergitol NP-40 and SDS were conducted in order to explain the molecular effects of both agents. In summary, while SDS must be removed from the sample solution to avoid interference of ions from SDS and suppression of analyte ion signal, tergitol at low concentrations may be used as an additive with sample solution for MALDI-MS analysis of peptides. Finally, molecular modeling analyses associated with docking were used in order to explain experimental biochemical data.

Vascular endothelial dysfunction with aging: endothelin-1 and endothelial nitric oxide synthase

To determine whether impaired endothelium-dependent dilation (EDD) in older adults is associated with changes in the expression of major vasoconstrictor or vasodilator proteins in the vascular endothelium, endothelial cells (EC) were obtained from the brachial artery and peripheral veins of 56 healthy men, aged 18-78 yr. Brachial artery EC endothelin-1 (ET-1) [0.99 +/- 0.10 vs. 0.57 +/- 0.10 ET-1/human umbilical vein EC (HUVEC) intensity, P = 0.01] and serine 1177 phosphorylated endothelial nitric oxide synthase (PeNOS) (0.77 +/- 0.09 vs. 0.44 +/- 0.07 PeNOS/HUVEC intensity, P < 0.05) (quantitative immunofluorescence) were greater, and EDD (peak forearm blood flow to intrabrachial acetylcholine) was lower (10.2 +/- 0.9 vs. 14.7 +/- 1.7 ml.100 ml(-1).min(-1), P < 0.05) in older (n = 18, 62 +/- 1 yr) vs. young (n = 15, 21 +/- 1 yr) healthy men. EDD was inversely related to expression of ET-1 (r = -0.39, P < 0.05). Brachial artery EC eNOS expression did not differ significantly with age, but tended to be greater in the older men (young: 0.23 +/- 0.03 vs. older: 0.33 +/- 0.07 eNOS/HUVEC intensity, P = 0.08). In the sample with venous EC collections, EDD (brachial artery flow-mediated dilation) was lower (3.50 +/- 0.44 vs. 7.68 +/- 0.43%, P < 0.001), EC ET-1 and PeNOS were greater (P < 0.05), and EC eNOS was not different in older (n = 23, 62 +/- 1 yr) vs. young (n = 27, 22 +/- 1 yr) men. EDD was inversely related to venous EC ET-1 (r = -0.37, P < 0.05). ET-1 receptor A inhibition with BQ-123 restored 60% of the age-related impairment in carotid artery dilation to acetylcholine in B6D2F1 mice (5-7 mo, n = 8; 30 mo, n = 11; P < 0.05). ET-1 expression is increased in vascular EC of healthy older men and is related to reduced EDD, whereas ET-1 receptor A signaling tonically suppresses EDD in old mice. Neither eNOS nor PeNOS is reduced with aging. Changes in ET-1 expression and bioactivity, but not eNOS, contribute to vascular endothelial dysfunction with aging.

Chronic endothelium-dependent regulation of arterial blood pressure by atrial natriuretic peptide: role of nitric oxide and endothelin-1

Atrial natriuretic peptide (ANP), via its guanylyl cyclase (GC)-A receptor, plays a key role in the regulation of arterial blood pressure (ABP) and volume. Endothelial-restricted deletion of GC-A in mice [endothelial cell (EC) GC-A knockout (KO)] resulted in hypervolemic hypertension, demonstrating that the endothelium participates in the hypotensive and hypovolemic actions of ANP. Published studies showed that ANP modulates the release of the vasoactive factors nitric oxide (NO) and endothelin-1 (ET-1) from cultured endothelia. Based on these observations, we examined the role of these endothelial factors in ANP-dependent vasodilatation (studied in isolated arteries) and chronic regulation of ABP (measured in awake mice by tail-cuff plethysmography). ANP induced concentration-dependent vasorelaxations of aortic, carotid, and pulmonary arteries. These responses were not different between control and EC GC-A KO mice, and were significantly enhanced after inhibition of NO synthase [by N(G)-nitro-L-arginine-methyl ester]. Intravenous administration of N(G)-nitro-L-arginine-methyl ester to conscious mice significantly increased ABP. The extent of these hypertensive reactions was similar in EC GC-A KO mice and control littermates (increases in systolic blood pressure by approximately 25 mm Hg). Conversely, antagonism of ET-1/endothelin-A receptors with BQ-123 reduced ABP significantly and comparably in both genotypes (by approximately 11 mm Hg). Finally, the vascular and tissue expression levels of components of the NO system and of immunoreactive ET-1 were not different in control and EC GC-A KO mice. We conclude that the endothelium, but not modulation of endothelial NO or ET-1, participates in the chronic regulation of ABP by ANP.

Dual Roles for Endothelin-B Receptors in Modulating Adjuvant-Induced Inflammatory Hyperalgesia in Rats

Injection of endothelin-1 (ET-1) into the plantar rat hindpaw causes acute pain at high concentrations and tactile sensitization at low concentrations. The pro-nociceptive actions are driven through ET(A) receptors for both levels of [ET-1], but the ET(B) receptors are only pro-nociceptive for allodynia from low [ET-1] and anti-nociceptive for pain from high [ET-1]. The goal of the present work was to discriminate the roles of the ET receptors in the acute hyperalgesia from inflammation by complete Freund's adjuvant (CFA, 20 mg/paw) into the rat hindpaw. Selective antagonists were injected l0 min before and then together with CFA. An ET(A) receptor antagonist, BQ-123, reduced CFA-induced thermal hyperalgesia (by up to 50%), as did an ET(B) receptor antagonist, BQ-788 (by up to 66%). BQ-123 and BQ-788 also delayed the onset (by 1.5 - 2 h) but insignificantly reduced the maximum degree of CFA-induced allodynia (~10%). Surprisingly, an ET(B) receptor agonist, IRL-1620, also reduced maximum thermal hyperalgesia induced by CFA, suppressed peak allodynia and delayed its occurrence by ~ 3 h. The latter actions of IRL-1620 were reversed by co-administration of BQ-788, naloxone hydrochloride and the peripherally restricted opiate receptor antagonist naloxone methiodide, and by antiserum against β-endorphin. These findings demonstrate an important role for endogenous ET-1 in acute inflammatory pain and a dual action of ET(B) receptors, including a pro-algesic action along with the important activation of a local analgesic pathway, implying that at least two different ET(B) receptors contribute to modulation of inflammatory pain.

Transcriptional regulation of monocyte chemotactic protein-1 release by endothelin-1 in human airway smooth muscle cells involves NF-kappaB and AP-1

BACKGROUND AND PURPOSE

Endothelin-1 (ET-1) is implicated in airway inflammation in asthma, but the mechanisms of its effects are poorly understood. We studied the effect of ET-1 on expression of the chemokine, monocyte chemotactic protein-1 (MCP-1), in primary cultures of human airway smooth muscle cells.

EXPERIMENTAL APPROACH

MCP-1 release was measured by elisa. Pharmacological antagonists/inhibitors, reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting were used to study ET receptors and kinase cascades. Transcriptional regulation was studied by real-time RT-PCR, transient transfection studies and chromatin immunoprecipitation assay. Major findings were confirmed in cells from three donors and mechanistic studies in cells from one donor.

KEY RESULTS

ET-1 increased MCP-1 release through an ET(A) and ET(B) receptor-dependent mechanism. ET-1 increased MCP-1 mRNA levels but not mRNA stability suggesting it was acting transcriptionally. ET-1 increased the activity of an MCP-1 promoter-reporter construct. Serial deletions of the MCP-1 promoter mapped ET-1 effects to a region between -213 and -128 base pairs upstream of the translation start codon, containing consensus sequences for activator protein-1 (AP-1) and nuclear factor-kappaB (NF-kappaB). ET-1 promoted binding of AP-1 c-Jun subunit and NF-kappaB p65 subunit to the MCP-1 promoter. Blocking the inhibitor of kappaB kinase-2 with 2-[(aminocarbonyl)amino]-5-[4-fluorophenyl]-3-thiophenecarboxamide (TPCA-1) decreased ET-1-stimulated MCP-1 production. p38 and p44/p42 mitogen-activated protein kinases were involved in upstream signalling.

CONCLUSIONS AND IMPLICATIONS

ET-1 regulated MCP-1 transcriptionally, via NF-kappaB and AP-1. The upstream signalling involved ET(A), ET(B) receptors, p38 and p44/p42 mitogen-activated protein kinases. These may be targets for novel asthma therapies.

Tumor-evoked sensitization of C nociceptors: a role for endothelin

Primary and metastatic cancers that effect bone are frequently associated with pain. Sensitization of primary afferent C nociceptors innervating tissue near the tumor likely contributes to the chronic pain and hyperalgesia accompanying this condition. This study focused on the role of the endogenous peptide endothelin-1 (ET-1) as a potential peripheral algogen implicated in the process of cancer pain. Electrophysiological response properties, including ongoing activity and responses evoked by heat stimuli, of C nociceptors were recorded in vivo from the tibial nerve in anesthetized control mice and mice exhibiting mechanical hyperalgesia following implantation of fibrosarcoma cells into and around the calcaneus bone. ET-1 (100 microM) injected into the receptive fields of C nociceptors innervating the plantar surface of the hind paw evoked an increase in ongoing activity in both control and tumor-bearing mice. Moreover, the selective ETA receptor antagonist, BQ-123 (3 mM), attenuated tumor-evoked ongoing activity in tumor-bearing mice. Whereas ET-1 produced sensitization of C nociceptors to heat stimuli in control mice, C nociceptors in tumor-bearing mice were sensitized to heat, and their responses were not further increased by ET-1. Importantly, administration of BQ-123 attenuated tumor-evoked sensitization of C nociceptors to heat. We conclude that ET-1 at the tumor site contributes to tumor-evoked excitation and sensitization of C nociceptors through an ETA receptor mediated mechanism.

G-protein-coupled receptors and melanoma

G-protein-coupled receptors (GPCR) are the largest family of receptors with over 500 members. Evaluation of GPCR gene expression in primary human tumors identified over-expression of GPCR in several tumor types. Analysis of cancer samples in different disease stages also suggests that some GPCR may be involved in early tumor progression and others may play a critical role in tumor invasion and metastasis. Currently, >50% of drug targets to various human diseases are based on GPCR. In this review, the relationships between several GPCR and melanoma development and/or progression will be discussed. Finally, the possibility of using one or more of these GPCR as therapeutic targets in melanoma will be summarized.

Endothelins are vascular-derived axonal guidance cues for developing sympathetic neurons

During development, sympathetic neurons extend axons along a myriad of distinct trajectories, often consisting of arteries, to innervate one of a large variety of distinct final target tissues. Whether or not subsets of neurons within complex sympathetic ganglia are predetermined to innervate select end-organs is unknown. Here we demonstrate in mouse embryos that the endothelin family member Edn3 (ref. 1), acting through the endothelin receptor EdnrA (refs 2, 3), directs extension of axons of a subset of sympathetic neurons from the superior cervical ganglion to a preferred intermediate target, the external carotid artery, which serves as the gateway to select targets, including the salivary glands. These findings establish a previously unknown mechanism of axonal pathfinding involving vascular-derived endothelins, and have broad implications for endothelins as general mediators of axonal growth and guidance in the developing nervous system. Moreover, they suggest a model in which newborn sympathetic neurons distinguish and choose between distinct vascular trajectories to innervate their appropriate end organs.

In vitro effect of cyclosporine-A on angiotensins secretion by glomerular cells

AIM

Cyclosporine-A (CyA) is used to control transplant rejections and to treat autoimmune diseases. We investigated the possibility that changes induced by CyA on endothelin 1 (ET), angiotensin I (AI) and angiotensin II (AII) concentrations recognize a common pathway through which different mechanisms operate.

METHODS

We measured ET, AI and AII concentrations, before and after either ET or CyA addition to the incubation medium of glomeruli of pig kidneys, isolated in vitro. The measurements were carried out with or without selective (ET(A) and ET(B)) or unselective ET(A)-ET(B) receptor inhibitors.

RESULTS

In the presence of CyA, AI and ET are positively correlated either when ET(B) receptors are blocked, or when both receptors are free, while this correlation becomes negative when ET(A) receptors alone are blocked. Adding ET to the medium, the correlations between AI and ET are negative when either ET(A), or ET(B) or both are blocked. The effects of CyA and ET are significant only during the first 2 h of incubation.

CONCLUSION

Cyclosporine-A recruits angiotensins and ET through ET(A) receptors, a mechanism possibly responsible of glomerular damage. This stimulation is time-dependent. Prevention of the renal damage from CyA should require selective ET(A) receptor blockade.

Intracellular mechanisms and receptor types for endothelin-1-induced positive and negative inotropy in mouse ventricular myocardium

We examined the intracellular mechanisms for endothelin-1-induced positive and negative inotropic components that coexist in the mouse ventricular myocardium using isolated ventricular tissue and myocytes from 4-week-old mice. In the presence of SEA0400, a specific inhibitor of the Na+-Ca2+ exchanger, endothelin-1 produced positive inotropy. Endothelin-1, when applied to cardiomyocytes in the presence of SEA0400, did not change the peak amplitude of the Ca2+ transient but increased intracellular pH and Ca2+ sensitivity of contractile proteins. On the other hand, in the presence of dimethylamiloride (DMA), a specific inhibitor of the Na+-H+ exchanger, endothelin-1 produced negative inotropy. In cardiomyocytes, in the presence of DMA, endothelin-1 produced a decrease in peak amplitude of the Ca2+ transient. In the presence of both DMA and SEA0400, endothelin-1 produced neither positive nor negative inotropy. Positive inotropy was blocked by BQ-123 and negative inotropy by BQ-788. These results suggested that endothelin-1-induced positive inotropy is mediated by ET(A) receptors, activation of the Na+-H+ exchanger and an increase in intracellular pH and Ca2+ sensitivity and that the negative inotropy is mediated by ET(B) receptors, activation of the Na+-Ca2+ exchanger and decrease in Ca2+ transient amplitude.

Fractional urinary excretion of endothelin-1 is reduced by acute ETB receptor blockade

Evidence suggests that urinary excretion of endothelin-1 (ET-1) reflects renal ET-1 production and is independent of systemic ET-1 activity. The influence of ET receptors on urinary ET-1 excretion has not been studied in humans, yet peritubular ETB receptors are abundant within the kidney. We have studied the effects of acute ETA and ETB receptor blockade with BQ-123 and BQ-788, respectively, on urinary ET-1 excretion in a randomized, placebo-controlled, double-blind study in 16 subjects with a wide range of GFRs (15–152 ml/min). Plasma ET-1 concentrations (pET-1) and urinary ET-1 excretion rate (uET-1) at baseline correlated inversely with GFR ( R2= 0.18 and 0.36, respectively, P < 0.01). However, changes in pET-1 after ET receptor antagonism were not related to changes in uET-1 ( R2= 0.007, P = 0.18). pET-1 increased only after BQ-788, alone or in combination with BQ-123, consistent with ETB receptor-mediated clearance of ET-1 from the circulation. uET-1 was reduced only after BQ-788 alone [−4.7 pg/min (SD 5.5), P < 0.01]. Because BQ-788 also reduced GFR, fractional excretion of ET-1 (FeET-1) was calculated. FeET-1 fell after BQ-788 alone [−41% (SD 26%), P < 0.01] or in combination with BQ-123 [−40% (SD 29%), P < 0.01]. FeET-1 was not altered by placebo or BQ-123 alone. In conclusion, urinary ET-1 excretion does not appear to relate to the pool of plasma ET-1. Because of the short duration of this study, it is unlikely that ET receptor blockade had significant effects on renal ET-1 production. Therefore, the reduction in FeET-1 after ETB blockade appears to indicate that renal excretion of ET-1 is at least partly facilitated by ETB receptor activation.

The endothelin system as a therapeutic target in cardiovascular disease: great expectations or bleak house?

There is considerable evidence that the potent vasoconstrictor endothelin-1 (ET-1) contributes to the pathogenesis of a variety of cardiovascular diseases. As such, pharmacological manipulation of the ET system might represent a promising therapeutic goal. Many clinical trials have assessed the potential of ET receptor antagonists in cardiovascular disease, the most positive of which have resulted in the licensing of the mixed ET receptor antagonist bosentan, and the selective ET(A) receptor antagonists, sitaxsentan and ambrisentan, for the treatment of pulmonary arterial hypertension (PAH). In contrast, despite encouraging data from in vitro and animal studies, outcomes in human heart failure have been disappointing, perhaps illustrating the risk of extrapolating preclinical work to man. Many further potential applications of these compounds, including resistant hypertension, chronic kidney disease, connective tissue disease and sub-arachnoid haemorrhage are currently being investigated in the clinic. Furthermore, experience from previous studies should enable improved trial design and scope remains for development of improved compounds and alternative therapeutic strategies. Although ET-converting enzyme inhibitors may represent one such alternative, there have been relatively few suitable compounds developed, and consequently, clinical experience with these agents remains extremely limited. Recent advances, together with an increased understanding of the biology of the ET system provided by improved experimental tools (including cell-specific transgenic deletion of ET receptors), should allow further targeting of clinical trials to diseases in which ET is involved and allow the therapeutic potential for targeting the ET system in cardiovascular disease to be fully realized.

Endothelins induce CCR7 expression by breast tumor cells via endothelin receptor A and hypoxia-inducible factor-1

Endothelin expression is increased in breast tumors and is associated with invasion and metastasis, whereas CCR7 expression by breast tumor cells may have a role in the organ specificity of breast cancer spread. In this article, we have analyzed whether endothelins influence breast tumor cell expression of the chemokine receptor CCR7. Stimulation of human breast tumor cell lines with endothelins increased cell surface expression of CCR7 via endothelin receptor A. The iron chelators desferrioxamine and cobalt chloride, which induce hypoxia-inducible factor (HIF)-mediated transcription, also increased CCR7 expression; transfection of a dominant-negative version of the HIF regulatory subunit, HIF-1alpha, into MCF-7 cells abolished CCR7 induction by endothelins, indicating that increased expression is due to HIF-1 stabilization. Endothelin stimulation promoted invasion toward the CCR7 ligands CCL19 and CCL21. Endothelin-mediated chemokine-independent invasion itself is dependent on CCR7 activity and could be abolished using a CCR7-neutralizing monoclonal antibody. In human breast carcinomas, mRNA expression of endothelins correlated with the level of CCR7 expression, both of which were associated with the presence of lymph node metastases. Expression of the CCR7 ligands CCL19 and CCL21 was also higher in breast cancer patients with lymph node involvement compared with those without, but expression of these chemokines did not correlate with endothelin expression. These data show that CCR7 may be regulated by the breast tumor microenvironment and further support the use of endothelin receptor antagonists in the treatment of invasive and metastatic breast cancer.

Endothelin receptor antagonists

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

Endothelin ETA receptor blockade potentiates morphine analgesia but does not affect gastrointestinal transit in mice

Development of analgesic tolerance and constipation remain a major clinical concern during long-term administration of morphine in pain management. Central endothelin mechanisms are involved in morphine analgesia and tolerance. The present study was conducted to investigate the effect of intracerebroventricular (i.c.v.) and peripheral administration of endothelin ET(A) receptor antagonist, BMS182874, and endothelin ET(B) receptor agonist, IRL1620, on morphine analgesia and changes in gastrointestinal transit in male Swiss Webster mice. Results indicate that morphine (6 mg/kg, s.c.) produced a significant increase in tail flick latency compared to control group. Pretreatment with BMS182874 (50 microg, i.c.v.) significantly enhanced morphine-induced analgesia, while IRL1620 (30 microg, i.c.v.) pretreatment did not affect tail-flick latency values. Changes in gastrointestinal transit were measured by percent of distance traveled by charcoal in the small intestine of gastrointestinal tract. Percent distance traveled in morphine (6 mg/kg, s.c.) treated mice (48.45+/-5.65%) was significantly lower (P<0.05) compared to control group (85.07+/-1.82%). Administration of BMS182874 centrally (50 mug, i.c.v.) or peripherally (10 mg/kg, i.p.) did not affect morphine-induced inhibition of gastrointestinal transit. Pretreatment with IRL1620 (30 microg, i.c.v., or 10 mg/kg, i.v.) also did not affect morphine-induced inhibition of gastrointestinal transit. This study demonstrates that endothelin ET(A) receptor antagonists delivered to the CNS enhance morphine analgesia without affecting gastrointestinal transit.

Endothelin-3 production by human rhabdomyosarcoma: a possible new marker with a paracrine role

Several autocrine and paracrine growth factor circuits have been found in human rhabdomyosarcoma cells. In this study we show that endothelin-3 (ET-3), a vasoactive peptide, is produced by human rhabdomyosarcoma cell lines, whereas it is not expressed by human sarcoma cell lines of non-muscle origin. We did not find evidence of a significant autocrine loop; nevertheless ET-3 produced by rhabdomyosarcoma cells can act as a paracrine factor, since it promotes migration of endothelial cells. Moreover ET-3 is present in plasma of mice bearing xenografts of human rhabdomyosarcoma cells, and may be potential new marker of the human rhabdomyosarcoma to be studied further.

Morphine tolerance does not develop in mice treated with endothelin-A receptor antagonists

Long-term use of morphine leads to development of antinociceptive tolerance. We provide evidence that central endothelin (ET) mechanisms are involved in development of morphine tolerance. In the present study, we investigated the effect of ET(A) receptor antagonists, BQ123 and BMS182874, on morphine antinociception and tolerance in mice. Mechanism of interaction of ET(A) receptor antagonists with morphine was investigated. BQ123 (3 microg, i.c.v.) and BMS182874 (50 microg, i.c.v.) significantly enhanced antinociceptive effect of morphine (P < 0.05), through an opioid-mediated effect. Treatment with a single dose of BQ123 (3 microg, i.c.v.) reversed tolerance to morphine antinociception in morphine-tolerant mice. BQ123 or BMS182874 did not affect naloxone binding in the brain. Therefore, ET(A) receptor antagonists did not bind directly to opioid receptors. [35S]GTPgammaS binding was stimulated by morphine and ET-1 in non-tolerant mice. Morphine- and ET-1-induced GTP stimulation was significantly lower (P < 0.05) in morphine-tolerant group (33% and 42%, respectively) compared to control group. BQ123 and BMS182874 did not activate binding in non-tolerant mice. BQ123 and BMS182874 significantly increased G protein activation in morphine-tolerant mice (96% and 86%, respectively; P < 0.05). These results provide evidence that uncoupling of G protein occurs in morphine-tolerant mice, and ET(A) antagonists promote coupling of G protein to its receptors, thereby restoring antinociceptive effect. These findings indicate that ET(A) receptor antagonists potentiate morphine antinociception and reverse antinociceptive tolerance in mice, through their ability to couple G proteins to opioid receptors.

Endothelin-1 increases intracellular Ca(2+) in rabbit pulmonary artery smooth muscle cells through phospholipase C

In freshly isolated rabbit pulmonary artery smooth muscle cells, endothelin (ET)-1 induced a transient increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) followed by a return to the initial [Ca(2+)](i). This response was not abolished by the voltage-dependent Ca(2+) channel blocker nicardipine or removal of Ca(2+) from the bath solution but was inhibited by ryanodine and thapsigargin. This finding suggested that the increase in [Ca(2+)](i) induced by ET-1 was attributable to release of Ca(2+) from ryanodine- and inositol 1,4,5-trisphosphate-sensitive intracellular Ca(2+) stores. The transient increase in [Ca(2+)](i) induced by ET-1 was also inhibited by pretreatment with antagonists of ET type A and B (ET(A) and ET(B)) receptors (BQ-123 and BQ-788, respectively). Furthermore, the ET(B) receptor agonist IRL-1620 induced an increase in [Ca(2+)](i) that was followed by a sustained increase in [Ca(2+)](i); the sustained increase in [Ca(2+)](i) was blocked by nicardipine. Using the nystatin-perforated patch-clamp technique, we found that IRL-1620 caused an increase in Ca(2+) current that was inhibited by addition of ET-1. ET-1 did not inhibit Ca(2+) current when cells were pretreated with BQ-123. These results suggested that when both receptor types are activated, the opposing responses lead to abolition of the sustained [Ca(2+)](i) increases induced by ET(B) receptor activation. Western blot analysis confirmed expression of ET(A) and ET(B) receptors. Finally, U-73122 inhibited the ET-1-induced [Ca(2+)](i) increase, indicating that phospholipase C was involved in modulation of the ET-1-induced [Ca(2+)](i) increase in rabbit pulmonary artery smooth muscle cells.

Endothelin-1-mediated increase in reactive oxygen species and NADPH Oxidase activity in hearts of aryl hydrocarbon receptor (AhR) null mice

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor characterized to play a role in detection and adaptation to environmental stimuli. Genetic deletion of the AhR results in cardiac hypertrophy that is mediated primarily by endothelin-1 (ET-1); ET-1 has been implicated in the elevation of reactive oxygen species (ROS) in the heart, which are thought to contribute to several cardiovascular disorders, including cardiac hypertrophy. Thus, we tested the novel hypothesis that ET-1 induces ROS in AhR null mice via ET(A) receptor activation. We first confirmed the presence of ROS in the hearts of AhR null mice by measuring superoxide (O2*-)-dependent oxidation of dihydroethidium. Ethidium fluorescence was increased 10-fold in the hearts of AhR null mice, compared to the wild type. Then, to elucidate whether ET-1 mediated the increase in ROS, mice were chronically treated with 100 ng/kg/day of the ET(A) receptor antagonist BQ-123. In AhR null mice, BQ-123 significantly reduced elevated plasma 8-isoprostane, a systemic end product of phospholipid oxidation by ROS, and cardiac thiobarbituric acid reactive substances (TBARS), a nonspecific assessment of ROS production. Furthermore, BQ-123 reduced both cardiac lucigenin chemiluminescence and cardiac mRNA expression of NAD(P)H oxidase subunits gp91phox, p47phox, and p67phox in AhR null mice below the levels observed in wild-type mice. These findings demonstrate that ET-1 activation of ET(A) receptors mediates an increase in ROS that is associated with cardiac hypertrophy in AhR null mice. In addition, the ET-1-mediated increase in ROS appears to be initiated via increased NAD(P)H oxidase activity.

Effects of acute systemic endothelin receptor blockade on cardiac electrophysiology in vivo

BQ-123, a selective endothelin-A receptor antagonist, has been demonstrated to suppress arrhythmias. However, the role of physiologic levels of endogenous endothelin-1 (ET-1) with respect to electrophysiologic properties of the heart is unknown. BQ-123 (0.45, 0.9, 1.8, 3.6, 7.2, and 14.4 microg/kg/min; n = 10) or saline (control, n = 5) was administered IV for 15 minutes of continuous-rate infusion at incremental doses to anesthetized normal pigs. BQ-123 had no effect on PR and QT interval, QRS duration, intraatrial and AV nodal conduction time as well as the atrial, AV nodal, and ventricular effective refractory periods. As compared with baseline, BQ-123 at 7.2 and 14.4 microg/kg/min caused an increase in heart rate (99 +/- 17 versus 110 +/- 14 and 118 +/- 14 bpm, respectively; P < 0.05), shortened sinus node recovery time (818 +/- 165 versus 641 +/- 69 and 609 +/- 74 milliseconds, respectively; P < 0.05) and decreased mean arterial pressure at 14.4 microg/kg/min (95 +/- 18 versus 80 +/- 11 mm Hg; P < 0.05). We conclude that in the normal pig, physiologic levels of ET-1 have no effect on conduction properties of atrial, AV nodal, or Purkinje fibers. However, antagonism of ET-1 by BQ-123 unmasks the effect of ET-1 on maintenance of vasomotor tone, which in turn may affect heart rate and sinus node automaticity in the intact pig.

Endothelins in breast tumour cell invasion

Endothelins are a family of small, structurally related, vasoactive peptides that have a great number of physiological roles in many tissues. The 'endothelin axis' consists of three 21 amino acid peptides (ET-1, ET-2 and ET-3), two G-protein-coupled receptors (ET-RA and ET-RB), and two activating peptidases or endothelin-converting enzymes (ECE-1 and ECE-2). There is increased expression of the endothelin axis in invasive breast cancer compared to the normal breast or non-invasive neoplastic tissue. Endothelin expression is associated with invading regions of tumours in patient biopsies and is more common in tumours with high histological grade and lymphovascular invasion, and there is increased systemic endothelin in patients with lymph node metastases compared to those without lymph node involvement. Stimulation of breast tumour cell lines with endothelins leads to an invasive phenotype in vitro. Over-expression of the endothelins and their receptors is insufficient to induce an invasive phenotype in benign cells, yet expression by tumour cells leads to markedly increased invasive ability indicating that endothelins act in concert with other factors--both autocrine and paracrine--including cytokines, matrix metalloproteinases and the activation of tumour-associated macrophages. The association between endothelins, poor prognosis and invasion may mean that the endothelin axis is a valid therapeutic target for the treatment of invasive breast cancer. This review summarises our current knowledge of endothelins in breast cancer invasion and discusses the potential further directions of such research as well as the possibility of anti-endothelin-based therapy of breast cancer.

A role for endothelin-2 and its receptors in breast tumor cell invasion

We have studied the role of endothelins (ET-1, ET-2 and ET-3) and ET receptors (ET-RA and ET-RB) in the invasive capacity of breast tumor cells, which express ET-1 and ET-2 as well as ET-RA and ET-RB. Of five human breast tumor cell lines tested, all expressed mRNAs for ET-1, ET-2, and ET-RB. ET-RA mRNA was expressed by four of five tumor cell lines. Breast tumor cells migrated toward ET-1 and ET-2 but not toward ET-3. Chemotaxis involved signaling via both receptors, and a pertussis toxin-sensitive p42/p44 mitogen-activated protein kinase (MAPK)-mediated pathway that could be inhibited by MAPK kinase (MEK)1/2 antagonists. Chemotaxis toward ETs did not involve p38 or stress-activated protein kinase (SAPK)/Jun N-terminal kinase (JNK) and was not inhibited by hypoxia. Incubation of tumor cells with ET-2 also increased chemotaxis toward the chemokines CXCL12 and CCL21. As well as inducing chemotaxis of tumor cells, ET-1 and ET-2 increased tumor cell invasion through Matrigel. Furthermore, stimulation of macrophage/tumor cell cocultures with ETs led to increased matrix metalloproteinase (MMP)-2 and -9 production by macrophages and a marked increase in invasion of tumor cells. Antagonism of either ET-RA or ET-RB decreased the invasion seen in ET-stimulated cocultures, as did a broad-spectrum MMP inhibitor. Immunohistochemical staining of human breast tumor sections showed increased ET and ET receptor protein expression by tumor cells in invasive ductal carcinoma compared with normal breast tissue or ductal carcinoma in situ. Furthermore, tumor cell ET and receptor expression was stronger at the invasive margin of invasive ductal carcinomas, in the lymphovascular space, and in lymph node metastases. ET expression often colocalized with ET-RB expression in all neoplastic tissue indicating a possible autocrine action of ETs. We suggest that expression of ETs and their receptors by human breast tumors, particularly in conjunction with a high macrophage infiltrate, may have a role in the progression of breast cancer and the invasion of tumor cells.

ET-1-associated vasomotion and vasospasm in lymphatic vessels of the guinea-pig mesentery

In vitro experiments were performed to investigate the actions of endothelin-1 (ET-1) on vasomotion and vasospasm in guinea-pig mesenteric lymphatics. ET-1 modulated lymphatic vasomotion independent of the endothelium, with lower concentrations (<or=10 nm) increasing lymphatic vasomotion and higher concentrations (>or=100 nm) causing vasospasm. ET-1-induced increases in vasomotion were accompanied by an increase in tonic [Ca2+]i. These actions were inhibited by the ETA receptor antagonist BQ-123 (1 microm), the phospholipase C (PLC) inhibitor U73122 (5 microm), removal of extracellular Ca2+, chelation of intracellular Ca2+ with BAPTA/AM (10 microm), the store Ca2+-ATPase inhibitor thapsigargin (1 microm), caffeine (10 mm) and the inositol 1,4,5-trisphosphate (IP3) receptor blocker heparin and 2-APB (30 microm). In contrast, the ETB receptor antagonist BQ-788 (1 microm), ryanodine (1 & 20 microm), pertussis toxin (PTx) or Cs+ had no significant actions on vasomotion or the magnitude of increase in tonic [Ca2+]i. ET-1-induced vasospasm was accompanied by a transient increase in smooth muscle [Ca2+]i followed by a sustained plateau, an action that was abolished by removal of extracellular Ca2+, but only marginally inhibited by nifedipine (1 microm). Caffeine (10 mm), SKF 96165 (30 microm) or U73122 (5 microm) together with nifedipine (1 microm) abolished ET-1-induced vasospasm and increase in [Ca2+]i. These results indicate that ET-1 increases lymphatic vasomotion by acting on smooth muscle ETA receptors and activation of G-protein-PLC-IP3 cascade, which is known to cause pacemaker Ca2+ release and resultant pacemaker potentials. High concentrations of ET-1 cause a failure in Ca2+ homeostasis causing vasospasm, triggered by excessive Ca2+ influx primarily through store-operated channels (SOCs) with l-Ca2+ voltage-operated channels (VOCs) also contributing, but to a much lesser extent.

Enrasentan, an antagonist of endothelin receptors

Endothelins are powerful vasoconstrictor agents produced by endothelial cells and identified by Yanagisawa et al. in 1988. Two types of receptors for endothelins have been identified: ET(A) receptors are located on smooth muscle cells of the vascular wall and are responsible for endothelin-induced vasoconstriction while ET(B) receptors are located on endothelial cells and induce these cells to release NO and prostacyclin. Moreover, these peptides not only cause a potent and prolonged vasoconstriction but are also known to enhance cell proliferation and to stimulate extracellular matrix accumulation. High levels of plasma or tissue endothelins have been found in patients with heart failure, diabetes, stroke, primary pulmonary hypertension, liver cirrhosis and other diseases. Given these effects of endothelins, blocking their receptors might be a new way to reduce blood pressure and to treat other illnesses. Accordingly, many endothelin antagonists have been developed and evaluated in animals and humans. Enrasentan is a mixed ET(A) and ET(B) receptor antagonist with a higher affinity for ET(A) receptors, although it cannot be considered a selective antagonist. In an animal model of hypertension and cardiac hypertrophy the drug has reduced blood pressure, prevented cardiac hypertrophy and preserved myocardial function. In rats with hyperinsulinemia and hypertension enrasentan normalized blood pressure and prevented cardiac and renal damage. In rats with stroke the drug reduced the ischemic area in the brain. Enrasentan has been added to conventional treatment in patients with heart failure (NYHA Class 2-3) and no addictive effect of the drug has been observed. This is in contrast with results obtained in animal models and still has not been explained. In conclusion, many possible clinical applications can be suggested for this drug, but further studies are necessary to better evaluate its therapeutic efficacy.

Endothelin-1 inhibits TNF alpha-induced iNOS expression in 3T3-F442A adipocytes

1. Endothelin-1 (ET-1) and tumor necrosis factor alpha (TNFalpha) by their action on adipocytes have been independently linked to the pathogenesis of insulino-resistance. In isolated adipocytes, TNFalpha induces the expression of the inducible nitric oxide synthase (iNOS). The purpose of the present work was, in the 3T3-F442A adipocyte cell line, to characterise TNFalpha-induced iNOS expression and to determine whether or not ET-1 could influence TNFalpha-induced iNOS expression and NO production. 2. In differentiated 3T3-F442A, treatment with TNFalpha (20 ng ml(-1)) induced the expression of a functional iNOS as demonstrated by nitrite assay, Western blot, reverse transcription-polymerase chain reaction and Northern blot analysis. TNFalpha-induced iNOS expression requires nuclear factor kappaB activation, but does not necessitate the activation of the PI-3 kinase/Akt and P38-MAP kinase pathways. 3. ET-1, but not ET-3, inhibited the TNFalpha-induced expression of iNOS protein and mRNA as well as nitrite production. The effects of ET-1 were blocked by a specific ETA (BQ123, pA(2) 7.4) but not by a specific ETB receptor antagonist (BQ788). 3T3-F442A adipocytes express the mRNAs for prepro-ET-1 and the ET-A receptor subtype, but not for the ET-B subtype. 4. The inhibitory effect of ET-1 was not affected by bisindolylmaleimide, SB 203580 or indomethacin, inhibitors of protein kinase C, p38-MAP kinase and cyclooxygenase, respectively, and was not associated with cAMP production. However, the effect of ET-1 was partially reversed by wortmannin, suggesting the involvement of PI3 kinase in the transduction signal of ET-1. 5. Differentiated 3T3-F442A adipocytes did not release ET-1 with or without exposure to TNFalpha, although the mRNA for preproET-1 was detected in both pre- and differentiated adipocytes. 6. Thus, these results confirm that adipocytes are a target for circulating ET-1 and demonstrate that the activation of the ETA receptor subtype can prevent TNFalpha-induced iNOS expression.

Insulin sensitivity and big ET-1 conversion to ET-1 after ETA- or ETB-receptor blockade in humans

Cardiovascular diseases are characterized by insulin resistance and elevated endothelin (ET)-1 levels. Furthermore, ET-1 induces insulin resistance. To elucidate this mechanism, six healthy subjects were studied during a hyperinsulinemic euglycemic clamp during infusion of (the ET-1 precursor) big ET-1 alone or after ET(A)- or ET(B)-receptor blockade. Insulin levels rose after big ET-1 with or without the ET(B) antagonist BQ-788 (P < 0.05) but were unchanged after the ET(A) antagonist BQ-123 + big ET-1. Infused glucose divided by insulin fell after big ET-1 with or without BQ-788 (P < 0.05). Insulin and infused glucose divided by insulin values were normalized by ET(A) blockade. Mean arterial blood pressure rose during big ET-1 with or without BQ-788 (P < 0.001) but was unchanged after BQ-123. Skeletal muscle, splanchnic, and renal blood flow responses to big ET-1 were abolished by BQ-123. ET-1 levels rose after big ET-1 (P < 0.01) in a similar way after BQ-123 or BQ-788, despite higher elimination capacity after ET(A) blockade. In conclusion, ET-1-induced reduction in insulin sensitivity and clearance as well as splanchnic and renal vasoconstriction are ET(A) mediated. ET(A)-receptor stimulation seems to inhibit the conversion of big ET-1 to ET-1.

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

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

Role of endothelin and endothelin A-type receptor in adaptation of the carotid body to chronic hypoxia

Chronic exposure in a low-PO(2) environment (i.e., chronic hypoxia, CH) elicits an elevated hypoxic ventilatory response and increased hypoxic chemosensitivity in arterial chemoreceptors in the carotid body. In the present study, we examine the hypothesis that changes in chemosensitivity are mediated by endothelin (ET), a 21-amino-acid peptide, and ET(A) receptors, both of which are normally expressed by O(2)-sensitive type I cells. Immunocytochemical staining showed incremental increases in ET and ET(A) expression in type I cells after 3, 7, and 14 days of CH (380 Torr). Peptide and receptor upregulation was confirmed in quantitative RT-PCR assays conducted after 14 days of CH. In vitro recordings of carotid sinus nerve activity after in vivo exposure to CH for 1-16 days demonstrated a time-dependent increase in chemoreceptor activity evoked by acute hypoxia. In normal carotid body, the specific ET(A) antagonist BQ-123 (5 microM) inhibited 11% of the nerve discharge elicited by hypoxia, and after 3 days of CH the drug diminished the hypoxia-evoked discharge by 20% (P < 0.01). This inhibitory effect progressed to 45% at day 9 of CH and to nearly 50% after 12, 14, and 16 days of CH. Furthermore, in the presence of BQ-123, the magnitude of the activity evoked by hypoxia did not differ in normal vs. CH preparations, indicating that the increased activity was the result of endogenous ET acting on an increasing number of ET(A). Collectively, our data suggest that ET and ET(A) autoreceptors on O(2)-sensitive type I cells play a critical role in CH-induced increased chemosensitivity in the rat carotid body.

Endothelin-3 induces both human and opossum gallbladder contraction mediated mainly by endothelin-B receptor subtype in vitro

BACKGROUND

Endothelins are produced by gallbladder epithelial cells, suggesting a role in the regulation of gallbladder function.

AIMS

To characterize the effect of endothelin-3 (ET-3) on human and Australian possum gallbladder contractility and identify the receptor(s) involved.

METHODS

Human and possum gallbladder muscle strips were exposed to cumulative concentrations of ET-3 (10 pmol/L-100 nmol/L). Strips were pretreated with either tetrodotoxin (TTX) (1 micro mol/L), the selective ET receptor antagonists BQ-123 (ET(A)), BQ-788 (ET(B)), alone or together, or the mixed ET antagonist tezosentan (all 1 micro mol/L). Maximal changes in tone were measured and expressed as percentage of carbachol (100 micro mol/L)-induced tone. ANOVA was used for statistical analysis.

RESULTS

Endothelin-3 induced a concentration-dependent increase in tone in both human and pos-sum strips (P < 0.05) and at 100 nmol/L represented 44.2 +/- 4.5% and 40.3 +/- 4.6% of carbachol-induced tone, respectively. The effect on human strips was TTX insensitive, whereas the possum concentration-response curve was shifted to the right. Individually, BQ-123 and BQ-788 shifted the human concentration-response curve to the right, but a greater inhibition by BQ-788 was achieved in the possum (P < 0.05). However, BQ-123 plus BQ-788 further reduced the ET-3 effect (P < 0.001) to a level comparable to that observed in the presence of tezosentan in both human and possum strips.

CONCLUSION

Endothelin-3 produces potent gallbladder contraction in vitro, acting mainly via ET(B) receptors and also interacting with ET(A)receptors. The receptors are located on the smooth muscle, but in possum gallbladder, neural receptors may also be involved. These findings suggest that ET-3 may regulate motility of possum and human gallbladder.

Vascular effects of newer cardiovascular drugs: focus on nebivolol and ACE-inhibitors

Alterations in the function and structure of the blood vessel wall account for most clinical events in the coronary and cerebrovascular circulation such as myocardial infarction and stroke. Cardiovascular drugs may exert beneficial effects on the vascular wall both at the level of the endothelium and vascular smooth muscle cells. Therefore, endothelial mediators, in particular nitric oxide (NO) and endothelin (ET), are of special interest. Drugs can modulate the expression and actions of NO, a vasodilator with antiproliferative and antithrombotic properties, and of ET, a potent vasoconstrictor and proliferative mitogenic agent. The most successful drugs in this context are statins and angiotensin-converting enzyme (ACE)-inhibitors. While statins increase the expression of NO synthase. ACE-inhibitors increase the release of NO via bradykinin-mediated mechanisms. Antioxidant properties of drugs are also important, as oxidative stress is crucial in atherosclerotic vascular disease. These properties may explain part of the effects of calcium antagonists and ACE-inhibitors. Indeed, angiotensin II stimulates NAD(P)H oxidases responsible for the formation of superoxide, which inactivates NO. ACE-Inhibitors thus increase the bioavailability of NO. Newer cardiovascular drugs such as nebivolol are able to directly stimulate NO release from the endothelium both in isolated arteries and in the human forearm circulation. ET receptor antagonists may exert beneficial effects in the vessel wall by preventing the effects of ET at its receptors and by reducing ET production. In summary, cardiovascular drugs have important effects on the vessel wall, which may be clinically relevant for the prevention and treatment of cardiovascular disease.

Endothelin-1 induces contraction of human and Australian possum gallbladder in vitro

BACKGROUND

Endothelin-1 (ET-1) stimulates guinea pig gallbladder (GB) muscle strip contractility; however, the role and source of ET-1 in the GB remains to be elucidated.

AIMS

To determine the effect of ET-1 on human and possum GB muscle strip contractility and evaluate whether ET-1 is present in GB tissue.

METHODS

GB muscle strips were mounted in organ baths to measure isometric tension. ET-1 was added cumulatively with and without pretreatment with the neural blocker tetrodotoxin (TTX) or the ET receptor antagonists BQ-123, BQ-788, and tezosentan. Immunohistochemical localization of ET was performed on freshly fixed and cultured GBs.

RESULTS

ET-1 induced concentration-dependent increases in tone in human and possum GB strips (p<0.05). This response was unaffected by BQ-123, BQ-788, and TTX but antagonized by BQ-123+BQ-788 in the human tissue only. Tezosentan (10(-4) mol/l) blocked the ET-1-induced response in human and possum GB strips (p<0.001). Although ET immunoreactivity was absent in freshly fixed possum GB, immunoreactivity was observed in the GB epithelium of freshly fixed human tissue and in both possum and human tissue following 24 h of organ culture.

CONCLUSION

ET-1 acts directly on human and possum GB smooth muscle producing contractions, possibly via ET-B receptors. ET may be present under pathophysiological conditions altering GB function.

Systemic ETA receptor antagonism with BQ-123 blocks ET-1 induced forearm vasoconstriction and decreases peripheral vascular resistance in healthy men

1. The effect on systemic haemodynamics of BQ-123, a selective endothelin A (ETA) receptor antagonist, was investigated in healthy men by giving, on separate occasions, ascending intravenous doses of 100, 300, 1000 and 3000 nmol min(-1) BQ-123, each for 15 min, in a randomized, placebo-controlled, double-blind study. The response of forearm blood flow to brachial artery infusion of endothelin-1 (ET-1; 5 pmol min(-1) for 90 min) was also studied using bilateral forearm plethysmography, after systemic pre-treatment, on separate occasions, with one of two doses of BQ-123 (300 and 1000 nmol min(-1) for 15 min) or placebo. 2. Systemic BQ-123 dose-dependently decreased systemic vascular resistance (P<0.01 for all doses vs placebo) and mean arterial pressure (P<0.05 for 300 nmol min(-1) and P<0.01 for 1000 and 3000 nmol min(-1)) during the 60 min following infusion. There were concurrent increases in heart rate and cardiac index. BQ-123, when infused systemically for 15 min, appeared to reach a maximum effect at 1000 nmol min(-1). 3. Intra-brachial ET-1 infusion, after pre-treatment with placebo, caused a slow onset progressive forearm vasoconstriction without systemic effects. This vasoconstriction was attenuated by pre-treatment with BQ-123 at 300 nmol min(-1) and abolished by BQ-123 at 1000 nmol min(-1) (P<0.01 vs placebo). 4. These effects occurred at concentrations of BQ-123 in the plasma (510+/-64 nmol l(-1)) that were ETA receptor selective, and were not accompanied by an increase in plasma ET-1 that would have indicated ETB receptor blockade. 5. We conclude that ETA-mediated vascular tone contributes to the maintenance of basal systemic vascular resistance and blood pressure in healthy men.