Comparative effects of the two endothelin ETA receptor antagonists, BQ-123 and FR139317, on endothelin-1-induced contraction in guinea-pig iliac artery

Pharmacological characteristics of endothelin receptors on sheep rectal blood vessels

Haemorrhoids is associated with high blood flow of the anorectal region. The question of whether pharmacological manipulation of vascular supply can relieve the symptoms of haemorrhoids has been raised. In order to undertake this type of clinical investigation, it is first essential to gain a better understanding of the properties of vascular receptors that may regulate blood flow into anal cushions and haemorrhoids. Due to the limited availability of human anorectal specimens and the good reliability of sheep tissue as an experimental model of human anorectal diseases, we studied the properties of endothelin receptors in sheep rectal artery (SRA) and vein (SRV), the vessels contributing to the blood flow of haemorrhoidal plexus, using isometric tension recordings. We found that endothelin-1 and sarafotoxin 6a were very potent constrictor agents in both SRA and SRV. The selective ET(A) receptor antagonist PD156707 (100 nM) produced a parallel rightward displacement of ET-1-induced contractions in both vessels and abolished sarafotoxin 6a-induced contractions in the SRA. PD156707 (3 μM) practically abolished contractions to ET-1 in the SRA, suggesting that the response is entirely mediated by ET(A) receptors. While, the selective ET(B) receptor antagonist BQ788 (100 nM) caused no significant change in ET-1-induced contractions in both vessels, a minor role for ET(B) receptor subtype to responses to sarafotoxin 6a in the artery was suggested.

Mechanisms involved in the effects of endothelin-1 in pig prostatic small arteries

Since endothelin-1 (ET-1) is involved in prostatic disorders, the current study investigated the mechanisms underlying the ET-1-induced effects in pig prostatic small arteries. The experiments were performed in rings mounted in microvascular myographs containing physiological saline solution at 37oC for isometric force recordings. On basal tension, ET-1 (0.1-30 nM) evoked concentration-dependent contractions, which were enhanced by endothelium removal. ET-1 contractions were inhibited by blockade of endothelin ETA and ETB receptors, extracellular Ca2+ removal and blockade of voltage-dependent (L-type)- and non-voltage-dependent-Ca2+ channels. On endothelium intact rings precontracted with noradrenaline, the ETB endothelin receptor agonist BQ3020 promoted a concentration-dependent relaxation which was reduced by blockade of ETB receptors, nitric oxide synthase, guanylyl cyclase and prostanoids synthesis. Endothelium removal abolished its relaxant response and unmasked a BQ3020-induced contraction. Tetraethylammonium and 4-aminopyridine, blockers of non-selective K+ channels and voltage-dependent K+ (Kv) channels, respectively, inhibited the relaxations to BQ3020. Iberiotoxin, apamin and glibenclamide, blockers of large and small Ca2+-activated- and ATP-dependent- K+ channels, respectively, failed to modify these responses. These data suggest that ET-1 promotes contraction of pig prostatic small arteries by activating vascular smooth muscle contractile endothelin ETA and ETB receptors coupled to extracellular Ca2+ entry, via voltage-dependent (L-type)- and non-voltage-dependent Ca2+ channels, also being due to intracellular Ca2+ mobilization. In addition, a population of endothelial ETB receptors mediates vasorelaxation via NO-cGMP pathway, vasodilator cyclooxygenase product(s) and Kv channels.

Induction of endothelin-2 expression by luteinizing hormone and hypoxia: possible role in bovine corpus luteum formation

The pattern and regulation of endothlin-2 (EDN2) expression and its putative roles in bovine ovaries were investigated. EDN2 mRNA was determined in corpus luteum (CL) and during folliculoluteal transition induced by GnRH in vivo. EDN2 was elevated only in the early CL and was not present in older CL. In the young CL, EDN2 mRNA was identified mainly in luteal cells but not endothelial cells that expressed the EDN1 gene. Similarly, in preovulatory follicles, EDN2 was expressed in the granulosa cells (GCs) and not in the vascular theca interna. LH and hypoxia are two major stimulants of CL formation. Therefore, GCs were cultured with bovine LH, under hypoxic conditions. GCs incubated with bovine LH resulted in increased EDN2 mRNA 42 h later. CoCl2, a hypoxia-mimicking agent, elevated EDN2 in GCs in a dose-dependent manner. Incubation of the human GC line (Simian virus 40 large T antigen) under low oxygen tension (1%) augmented EDN2 6 and 24 h later. In these two cell types, along with EDN2, hypoxia augmented VEGF. EDN2 induced in GCs changes that characterize the developing CL: cell proliferation as well as up-regulation of vascular endothelial growth factor and cyclooxygenase-2 (mRNA and protein levels). Human chorionic gonadotropin also up-regulated these two genes. Small interfering RNA targeting EDN-converting enzyme-1 effectively reduced its mRNA levels. This treatment, expected to lower the mature EDN2 peptide production, inhibited VEGF mRNA levels and GC numbers. Together these data suggest that elevated EDN2 in the early bovine CL, triggered by LH surge and hypoxia, may facilitate CL formation by promoting angiogenesis, cell proliferation, and differentiation.

Functional characterization and expression of endothelin receptors in rat carotid artery: involvement of nitric oxide, a vasodilator prostanoid and the opening of K+ channels in ETB-induced relaxation

We aimed to functionally characterize endothelin (ET) receptors in the rat carotid artery. mRNA and protein expressions of both ETA and ETB receptors, evaluated by reverse transcription-polymerase chain reaction (RT-PCR) and Western immunoblotting, were detected in carotid segments. Immunohistochemical assays showed that ETB receptors are expressed in the endothelium and smooth muscle cells, while ETA receptors are expressed only in the smooth muscle cells. In endothelium-denuded vessels, levels of ETB receptor mRNA were reduced. Vascular reactivity experiments, using standard muscle bath procedures, showed that ET-1 induces contraction in endothelium-intact and -denuded carotid rings in a concentration-dependent manner. Endothelial removal enhanced ET-1-induced contraction. BQ123 and BQ788, selective antagonists for ETA and ETB receptors, respectively, produced concentration-dependent rightward displacements of the ET-1 concentration-response curves. IRL1620, a selective agonist for ETB receptors, induced a slight vasoconstriction that was abolished by BQ788, but not affected by BQ123. IRL1620-induced contraction was augmented after endothelium removal. ET-1 concentration dependently relaxed phenylephrine-precontracted rings with intact endothelium. The relaxation was augmented in the presence of BQ123, reduced in the presence of BQ788 and completely abolished after endothelium removal. IRL1620 induced vasorelaxation that was abolished by BQ788 and endothelium removal, but not affected by BQ123. Preincubation of intact rings with N(G)-nitro-L-arginine methyl ester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), indomethacin or tetraethylammonium (TEA) reduced IRL1620-induced relaxation. The combination of L-NAME, indomethacin and TEA completely abolished IRL1620-induced relaxation while sulfaphenazole did not affect this response. 4-aminopyridine (4-AP), but not apamin, glibenclamide or charybdotoxin, reduced IRL1620-induced relaxation. The major finding of this work is that it firstly demonstrated functionally the existence of both ETA and ETB vasoconstrictor receptors located on the smooth muscle of rat carotid arteries and endothelial ETB receptors that mediated vasorelaxation via NO-cGMP pathway, vasodilator cyclooxygenase product(s) and the activation of voltage-dependent K+ channels.

Conversion of big endothelin-1 and characterization of its contractile effects on isolated human placental arteries

OBJECTIVES

To study the conversion of human big endothelin-1 (bigET-1) to endothelin-1 (ET-1) and to characterize contractile ET-1 receptors in human placental arteries.

METHODS

BigET-1 was incubated with artery membranes and the formation of ET-1 was investigated. ET-1 and bigET-1-induced contractile responses were studied in the absence or presence of the metalloprotease inhibitor phosphoramidon, the ET(A)-receptor antagonist BQ 123, or the ETB-receptor antagonists IRL 1038 and RES 701-1.

RESULTS

The artery membranes hydrolysed bigET-1 to ET-1 through a partly phosphoramidon-sensitive pathway. The contractile responses to ET-1 and bigET-1 were similar, with pEC50% values of 8.1 +/- 0.2 and 7.8 +/- 0.1, respectively (NS; n = 17). Phosphoramidon decreased pEC50% for bigET-1-evoked contractions (p < 0.05; n = 8), without affecting the response to ET-1. A Schild plot of BQ 123 effects on ET-1 and bigET-1-induced contractions resulted in identical pA2 values and a slope of 0.56 +/- 0.2 and 0.47 +/- 0.01, respectively. IRL 1038 and RES 701-1 did not affect the contractile responses.

CONCLUSION

BigET-1-evoked contractions in isolated human placental arteries depend on a rapid and metalloprotease-dependent hydrolytic conversion to ET-1, which in turn causes a, mainly ETA-receptor-mediated, contraction.

Age-related changes in contractile responses of rabbit lower urinary tract to endothelin

PURPOSE

As there are significant amounts of endothelin (ET) receptors in the mammalian urinary tract, we investigated the pharmacological properties and localization of ET receptors in the rabbit lower urinary tract as a function of age.

MATERIALS AND METHODS

The characteristics of ET receptors in bladder dome, trigone and urethra of 6 weeks and 6 months old male rabbits were determined using muscle bath and autoradiographic techniques.

RESULTS

ET-1 produces significant contractile responses in smooth muscle strips from bladder dome, trigone, and urethra in both 6 weeks and 6 months old rabbits. Although there was no significant difference in the maximum contractile response of urethral muscle strips to ET-1 between 6 weeks and 6 months old rabbits, the maximum responses to ET-1 were higher in both bladder dome and trigone of 6 weeks than 6 months old rabbits. A selective ETA receptor antagonist, BQ 610, shifted the concentration response curve to ET-1 to the right without decreasing maximal contractile responses in all regions from both age groups, whereas a selective ETB receptor antagonist, IRL 1038, had no significant effect on the contractile response in these tissues. Autoradiographic studies indicate that both ET receptor subtypes are expressed in bladder dome, trigone, and urethra with the ETA subtype being located only in the smooth muscle layers and the ETB subtype being located in both the urothelial and smooth muscle layers.

CONCLUSION

Our data indicate the presence of region- and age-dependent differences in the contractile properties of ET receptors in the male rabbit lower urinary tract. Although both ETA and ETB receptor subtypes are present in the smooth muscle layers, the ETA receptor is the sub-type that is primarily involved in the mediation of contractions.

Endothelin receptor-mediated Ca2+ mobilization and contraction in bovine oviductal arteries: comparison with noradrenaline and potassium

1. The effects of endothelin-1 (ET-1) were studied in bovine oviductal arteries and compared to those of noradrenaline (NA) and high K+ (K+). The influence of endothelium, the receptor subtypes involved, and the mechanisms of Ca2+ mobilization were assessed. 2. ET-1 (0.1-300 nM) induced concentration-dependent contractions with a potency of 10(3) and 10(2) times higher than NA (0.1 microM-0.1 mM) and K+ (9.5-119 mM), respectively. Removal of endothelium or NG-nitro-L-arginine (L-NOARG, 0.1 mM) pretreatment did not affect responses to either ET-1 or K+, whereas the NA response was significantly increased. Indomethacin (1 microM) had no effect on either of these agonists. 3. The rank order of potency for the ET isopeptides was: ET-1 = ET-2 > ET-3. The ETA receptor-selective agonist, sarafotoxin 6c (S6c), had no effect. The ETA receptor-selective antagonist, BQ-123, showed a competitive antagonism on the ET-1 response (pA2 value of 6.58 +/- 0.01), whereas contractions to ET-3 were completely abolished by BQ-123 at 0.1 microM. 4. Concentration-response curves to both ET-1 and NA were shifted to the right and their maximum response reduced to approximately 56% and 65% of controls, respectively, under 30 min of incubation in Ca(2+)-free solution, whereas responses to K+ were almost abolished by this treatment. Contractions to both NA (30 microM) and ET-1 (30 nM) were maximally inhibited after 10 min of extracellular Ca2+ deprivation. 5. Contractions to ET-1 were more potently inhibited by nickel (Ni2+, 0.3 mM), whereas nifedipine (1 microM) and cadmium (Cd2+, 0.1 mM) induced only a slight effect. In contrast, opposite effects were found for both NA and K+. 6. Treatment with ryanodine (100 microM) and caffeine (10 mM) in Ca(2+)-free solution reduced the tension measured 5 min after NA (30 microM) and ET-1 (30 nM) addition, but the sustained response (tension at 25 min) remained unaffected. 7. Calphostin C (1 microM), a specific protein kinase C (PKC) inhibitor, reduced the maximum contractile response to ET-1 by about 50% without significantly affecting its pD2 value. 8. These results suggest that ET-1 acts in bovine oviductal arteries by directly activating a homogenous population of ETA receptors in smooth muscle, without endothelial modulation. Several Ca2+ activation mechanisms seem to be involved in the contractile action of the peptide, including: (1) extracellular Ca2+ entrance through Ni(2+)-sensitive and L-type Ca2+ channels; (2) intracellular Ca2+ release from a ryanodine-sensitive Ca2+ store; and (3) sensitization of the contractile machinery to Ca2+ via PKC.

Characterization of endothelin receptors in the cerebral vasculature and their lack of effect on spreading depression

The changes in cerebral blood flow that accompany spreading depression are well-described, as are parallel changes in cellular activity, with a wave of hyperemia followed by a prolonged oligemic phase. In this study, a cat model of the CBF changes associated with spreading depression and in vitro pharmacology were used to determine if there is a role for the powerful peptide vasoconstrictor endothelin in this response. For the pharmacological studies, the middle cerebral artery was harvested from cats postmortem. For the physiological studies, cats were anesthetized with halothane induction and alpha-chloralose (60 mg/kg, intraperitoneal loading; 20 mg/kg i.v. 2-h maintenance). CBF was monitored continuously in the parietal cortex using laser Doppler flowmetry (CBFLDF) after exposure of the dura mater. The in vitro work demonstrated that endothelin-1 (ET-1) mediates a strong and potent contraction of cerebral vessels. Both the selective ETA receptor antagonist FR139317 and the combined ETA and ETB receptor antagonist Bosentan caused a rightward shift of the concentration-response curve without attenuation of the maximum effect. The calculated pA2 values were 6.28 and 6.90, respectively. The slope did not differ from unity, suggesting that the ET-1-mediated contraction is evoked by a single population of ETA receptors, which were effectively antagonized by these compounds. Spreading depression was induced with a needle stick injury to the cortex. Local administration of the endothelin antagonists FR139317 (10 microM) and Bosentan (10 microM) did not affect resting blood flow in the cortex. Induction of spreading depression following local administration of FR139317 and Bosentan resulted in responses no different from those in control cortex. These data demonstrate that endothelin does not play a significant role in the vasoconstrictor portion of the CBF change seen in spreading depression, nor does it affect resting flow. Since it is widely held that spreading depression, or a very similar mechanism, underlies the aura phase of migraine, it may be suggested from these studies that endothelin antagonists are unlikely to be useful in migraine.

Endothelin ETA receptors mediate human uterine smooth muscle contraction

Receptors mediating endothelin-induced contraction of myometrium were investigated in the human uterus. Endothelin-1 and endothelin-3 (10 pM to 0.3 microM) caused concentration-dependent contraction of myometrial strips. Endothelin-1 was approximately ten times more potent than endothelin-3, with pD2 values of 8.24 and 7.20, respectively. By contrast, two endothelin ETB receptor selective agonist, BQ 3020 (N-acetyl-[Ala11,15]endothelin-1-(6-21) and sarafotoxin 6c (up to 0.3 microM), did not induce contraction of human myometrium. The endothelin ETA receptor selective antagonist, FR139317 (1-hexahydroazepino-CO-Leu-D-Trp(CH3)-D-(2-pyridyl)alanine) (0.1, 0.3 and 1 microM), competitively antagonized the endothelin-1-elicited contraction, with a pA2 value of 7.10, whereas another endothelin ETA receptor-selective blocking drug, BQ 123 [cyclo(-D-Trp-D-Asp-Pro-D-Val-Leu)], behaved as a non-competitive antagonist. Pretreatment of myometrial strips with an endothelin ETB receptor selective antagonist, IRL 1038 ([Cys11-Cys15]endothelin-1-(11-21)), had no effect on contractions induced by endothelin-1. All these data indicate that only endothelin ETA receptors mediate endothelin-1-induced contractions of human myometrium.

Characterization of type A endothelin receptors in cultured human myometrial cells

The aim of the present study was to characterize endothelin (ET)-receptors in human myometrial cells in culture. 125I-labeled ET-1 binding to myometrial cells was specific and saturable, with a dissociation constant of 64.2 +/- 12.8 pM. Competition binding studies showed the following order of potency: ET-1 > ET-3, which is consistent with the presence of the ETA receptor subtype. FR-139317 and BQ-123, two ETA antagonists, both inhibited 125I-ET-1 binding. BQ-123 only elicited a partial inhibition. The fraction resistant to BQ-123 did not represent the ETB receptor subtype, since no specific 125I-ET-3 binding could be detected. ET-1 and ET-3 were found to stimulate [3H]inositol phosphate (IP) accumulation in cultured myometrial cells, with corresponding half-maximal effective concentration values of 0.26 +/- 0.04 and 87 +/- 17 nM, respectively. Both ETA antagonists inhibited ET-1-induced accumulation of [3H]IP. BQ-123 was only a partial inhibitor, whereas FR-139317 totally suppressed ET-1-stimulated production of [3H]IP. We conclude that human myometrial cells in culture exclusively possess ETA receptor subtypes coupled to phospholipase C.

Characterization of endothelin receptors in rat renal artery in vitro

1. The aim of this study was to investigate the function and characteristics of endothelin receptors in rat main branch renal artery in vitro. 2. Endothelin(ET)-1 (mean EC50 = 9.8 nM) was approximately 12 fold more potent than ET-3 (mean EC50 = 120 nM) as a contractile agonist and produced a greater maximum response. In contrast, neither of the ETB receptor-selective agonists, alanine[1,3,11,15]ET-1 nor sarafotoxin S6c, (0.1 nM-1 microM), induced any contractile effect, or any relaxant effect in endothelium-intact preparations pre-contracted with the thromboxane A2 mimetic, U-46619. Sarafotoxin S6c (30 nM) also failed to induce any further contraction in tissues pre-contracted with an EC50 concentration of ET-1. 3. The ETA receptor-selective antagonist, BQ123, behaved as a weak and variable antagonist of the contractile effects of ET-1 (mean pA2 estimates in the range 5.8-6.3). In contrast, BQ123 antagonized ET-3 with a potency (mean pA2 = 7.6) consistent with its affinity for ETA receptors. Co-incubation of BQ123 (3 microM) with the putative ETB receptor-selective antagonist, IRL1038 (10 microM), produced no greater antagonism of ET-1 responses than was induced by BQ123 (3 microM) alone. 4. In conclusion, ETB receptors do not appear to be present in rat main branch renal artery. The contractile effects of ET-3 in this tissue seem to be mediated by ETA receptors. While ETA receptors partly mediate the contractile effects of ET-1, these data raise the possibility that a population of novel BQ123-insensitive endothelin receptors may also contribute to this response.

Endothelin receptor antagonism

Following the original report by Yanagisawa et al. (1988) more than 7 years ago, compelling evidence that ET plays an important role in the local regulation of smooth muscle tone and cell growth has been reported. In addition, many studies point to a significant role for endothelin in nonvascular function. The investigation of the endothelin system has been greatly advanced in the last 2 to 3 years through significant advances in the development of potent and selective ET receptor antagonists. These agents have proven to be essential tools for elucidating the biological significance of the ET system, leading to the realization that antagonism of the ET system may have significant therapeutic potential. As emphasized in this review, the importance of chronic blockade of the ET system may be a critical aspect of future research in this exciting area. Confounding issues remain the lack of information about the role of the ETB receptor, the apparent pharmacological evidence for additional ET receptor subtypes, and species variation in the tissue distribution of ET isoforms and receptor subtypes. Along with the greater ability to understand the endothelin system provided by potent and selective pharmacological agents, is the important contribution of modern molecular biology techniques, highlighted by the insights gained from recent reports of results from ET gene disruption studies. Kurihara et al. (1994) found that ET-1-deficient homozygous mice die at birth of apparent respiratory failure secondary to severe craniofacial abnormalities. Subsequently, Yanagisawa's laboratory has presented and published a series of complementary gene disruption studies. First, Hosoda et al. (1994) demonstrated remarkably, that ETA receptor knockout mice bear morphological abnormalities nearly identical to ET-1 knockout mice. Second, they found that disruption of the ET-3 peptide and ETB receptor genes result in homozygous mice that share identical phenotypic traits (i.e., coloration changes and aganglionic megacolon) which are similar to a previously known natural mutation, the Piebald-Lethal mouse (Hosoda et al., 1994; Baynash et al., 1994). This phenotype has a human corollary known as Hirschsprung's Disease and it is now known that the disease, though multigenic, results from a missense mutation of the ETB receptor gene in some individuals (Puffenberger et al., 1994). Taken together these data indicate that the endothelin system is essential to correct embryonic neural crest development, a completely novel finding within the superfamily of guanine-protein-linked receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Endothelin ETA receptor antagonist, BQ-123, blocks the vasoconstriction induced by sarafotoxin 6b in the heart but not in other vascular beds

1. The cardiovascular effects of SRT6b in control and BQ-123, a specific ETA receptor antagonist, pretreated rats were determined in anesthetized rats using a radioactive microsphere technique. 2. Infusion of SRT6b produced an increase in blood pressure and total peripheral resistance, decrease in cardiac output and stroke volume, and no change in heart rate of control or BQ-123 treated rats. 3. SRT6b induced a decrease in blood flow to the heart which was completely blocked by BQ-123 pretreatment. The decrease in blood flow to other organs by SRT6b was not affected by BQ-123 pretreatment. 4. This study indicates that ET receptors in the coronary blood vessels are of a different type (neither ETA nor ETB) to those in other vascular beds.

The current endothelin receptor classification: time for reconsideration?

The possible involvement of endothelins in a variety of diseases has attracted the attention of many pharmacologists in search of a novel therapeutic approach. The rapid development of endothelin research has resulted in the molecular characterization and pharmacological recognition of ETA and ETB receptors, and in the development of compounds selective for these receptors. However, the characterization of receptors in various assays has shown that a number of effects are mediated by receptors that do not fit the present criteria for ETA or ETB receptors. In this article, Willem Bax and Pramod Saxena address endothelin receptors in general, and atypical receptors in particular.

Biological activities of [Thr2]sarafotoxin-b, a synthetic analogue of sarafotoxin-b

The 21 amino acid sarafotoxins (SRTX) c and d/e as well as endothelin-3 (ET-3) are known to be less toxic and weaker pharmacologically than the other isopeptides SRTX-a, SRTX-b and ET-1. Since SRTX-c, SRTX-d/e and ET-3 possess a Thr instead of a Ser at position 2, we investigated the possibility that this mutation could be responsible for the observed biological differences. Here we show that the synthetic [Thr2]SRTX-b has indeed a lower vasoconstriction efficacy (approximately 35%) in the rabbit aorta, but it is nearly as potent as SRTX-b in toxicity tests and in influencing contraction of the rat uterus. Using monoclonal antibodies directed against the structurally related endothelin-1, we also show that the antigenicity of the analogue is comparable to that of SRTX-b, suggesting that the overall structure of the two peptides is similar, despite the substitution at position 2. We suggest that the Thr2 substitution contributes to the lower activity of the 'weak' peptides in some systems; however, additional substitutions found in the 'weak' peptides of the ET/SRTX family most probably contribute to their low pharmacological activity.

Characterisation of the endothelin receptor mediating contraction of human pulmonary artery using BQ123 and Ro 46-2005

We have characterised the endothelin receptor mediating contraction of human isolated pulmonary artery. Endothelin-1 induced a concentration-dependent contraction of human endothelium-denuded pulmonary artery (EC50 5.6 nM). In contrast, endothelin-3 produced only a small contraction (approximately 12% of maximum endothelin-1 response) at the highest concentration tested (1 microM). The ETB receptor-selective agonist, sarafotoxin S6c (0.1 nM to 1 microM) did not cause contraction of human pulmonary artery. Pretreatment of human pulmonary artery with BQ123 (1-10 microM), an ETA receptor-selective blocking drug, resulted in a concentration-dependent, surmountable antagonism of endothelin-1-induced contractions (apparent pKB 6.6-7.0). Schild analyses yielded a shallow slope (0.58), which was significantly less than unity and, consequently, the calculated pA2 (8.1) was greater than the individual pKB values. Pretreatment of human pulmonary artery with Ro 46-2005 (30 microM), a non-peptide. non-selective endothelin receptor-blocking drug, resulted in a surmountable antagonism of endothelin-1-induced contractions (apparent pKB 5.5). In conclusion, endothelin-1-induced contraction of human pulmonary artery appears to be mediated predominantly via ETA receptors, although the shallow Schild slope observed with BQ123 indicates possible receptor heterogeneity.

A selective endothelin ETA antagonist, BQ-123, inhibits 125I-endothelin-1 (125I-ET-1) binding to human meningiomas and antagonizes ET-1-induced proliferation of meningioma cells

1. We studied the effects of BQ-123, a selective ETA receptor antagonist, on 125I-endothelin-1 (125I-ET-1) binding to cell surface receptors in surgically exercised human meningiomas and on ET-1-induced DNA synthesis in cultured human meningioma cells in vitro, using a quantitative receptor autoradiographic technique with radioluminography and 3H-thymidine incorporation, respectively. 2. All of the human meningiomas expressed high-affinity binding sites for 125I-ET-1, regardless of differences in histological subtypes (Kd = 2.6 +/- 0.2 nM, Bmax = 374 +/- 93 fmol/mg; mean +/- SE; n = 9). 3. BQ-123 competed for 125I-ET-1 binding to sections of meningiomas with IC50S of 3.2 +/- 0.9 x 10(-7) M, and 10(-4) M BQ-123 displaced 80% of the binding. 4. ET-1 significantly stimulated DNA synthesis in cultured human meningioma cells, up to 170% of the basal level in the presence of 10(-9) M ET-1. BQ-123 inhibited ET-1 (10(-9) M)-induced DNA synthesis in meningioma cells, in a dose-dependent manner, and 10(-5) M BQ-123 reduced it to 120% of the basal level. 5. The number of meningioma cells determined after 4 days in culture was dose dependently increased in the presence of ET-1 (10(-9) and 10(-7) M). The growth rate of meningioma cells, incubated with 10(-9) M ET-1, was reduced by 50% in the presence of 10(-7) M BQ-123.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences between endothelin receptors mediating contraction of guinea-pig aorta and pig coronary artery

Endothelin receptors mediating contraction were characterized and compared in rings from guinea-pig thoracic aorta and pig left circumflex coronary artery. In guinea-pig aorta, the following rank order of agonist potencies was found (mean EC50 value, nM): endothelin-1 (5.0) = endothelin-2 (5.5) > vasoactive intestinal contractor (VIC; 11.0) > sarafotoxin S6b (39.8) > [Ala3,11]endothelin-1 (121) > sarafotoxin S6a (> 150) > endothelin-3 (> 500). [Ala1,3,11,15] Endothelin-1, endothelin-(16-21), sarafotoxin S6c and sarafotoxin S6d were neither agonists nor antagonists at concentrations up to 1, 10, 3 and 1 microM, respectively. Cyclo-(D-Trp-D-Asp-Pro-D-Val-Leu) (BQ-123; 0.1-1 microM) behaved as a competitive antagonist of endothelin-1 (pA2 7.4 +/- 0.1, slope factor 0.91 +/- 0.17, n = 4). In pig coronary artery, all endothelins and sarafotoxins were agonists, except for endothelin-(16-21). Sarafotoxin S6c, [Lys4]sarafotoxin S6c, [Nle6]sarafotoxin S6c and [Ala1,3,11,15]endothelin-1 acted as partial agonists (Emax about 40% of that of endothelin-1). The rank order of agonist potencies was: sarafotoxin S6c (1.5) = [Lys4]sarafotoxin S6c (1.5) > [Nle6]sarafotoxin S6c (6.7) > or = sarafotoxin S6a (7.5) > or = endothelin-1 (12.6) > or = sarafotoxin S6b (14.8) > or = VIC (18.3) = endothelin-2 (19.3) > or = [Ala1,3,11,15]endothelin-1 (41.7) > or = [Ala3,11]endothelin-1 (55.2) > endothelin-3 (96.8) > sarafotoxin S6d (> 200). Endothelin-(16-21) was neither agonist nor antagonist at 10 microM. The concentration-response curves of endothelin-3 and sarafotoxin S6a were biphasic, consisting of a higher sensitivity (40-45% of the total effect) and a lower sensitivity component. BQ-123 (0.1-1 microM) did not alter the concentration-response curve of endothelin-1.(ABSTRACT TRUNCATED AT 250 WORDS)