Characterization of endothelin receptors and localization of 125I-endothelin-1 binding sites in human umbilical artery

Comparison of relaxant effects of nifedipine and NS11021 on isolated umbilical arteries of healthy and preeclamptic pregnant women

OBJECTIVES

Potassium (K⁺) channel openers and calcium (Ca²⁺) channel blockers are currently used to treat acute severe hypertension in pregnancy. We aimed to investigate the vasorelaxant effect of NS11021, a potent and specific big-conductance Ca²⁺-activated K⁺ (BKCa) channel activator, and to compare it with the vasorelaxant effect of nifedipine on human umbilical arteries (HUAs) isolated from healthy and preeclamptic pregnants.

STUDY DESIGN

A total of 29 HUAs were isolated immediately after delivery from 14 healthy and 15 preeclamptic pregnant with severe features. The concentration-dependent relaxation responses were obtained to nifedipine and NS11021 on HUAs precontracted with endothelin-1 (ET-1) (10^-8 M) in an isolated tissue bath.

RESULTS

Both nifedipine and NS11021 caused concentration-dependent relaxation responses in HUAs from healthy and preeclamptic pregnants. While the maximum responses (Emax) and pD₂ values of nifedipine did not change significantly in both groups, the Emax and pD₂ values of NS11021 were significantly decreased in the preeclampsia group (Emax ± SEM; %75.57 ± 4.53 and %43.75 ± 14.00 and pD₂ ± SEM; 6.92 ± 0.26 and 5.24 ± 0.53 respectively, p < 0.05). In addition, the pD₂ value of NS11021 was not significantly different from that of nifedipine in the control group, but decreased significantly in the preeclampsia group (pD₂ ± SEM 7.1 ± 0.41 and 5.2 ± 0.53, p < 0.05, respectively).

CONCLUSIONS

Efficacy and potency of NS11021 decreased in HUAs from preeclamptic pregnants. Also, NS11021 is less potent than nifedipine in the preeclampsia group. BKCa channels may have a role in the pathogenesis of preeclampsia, however, further experimental studies are needed to elucidate that.

Comparison of Vascular Responses to Vasoconstrictors in Human Placenta in Preeclampsia between Preterm and Later Term

BACKGROUND

Placental blood vessels play important roles in maternal-fetal circulation. Although pathologic mechanisms of preeclampsia are unclear, it is known that placental vascular dysfunction could contribute to pregnant hypertension. However, placental micro-vessel function or dysfunction at preterm has not been investigated.

METHODS

Human placentas from normal and preeclamptic pregnancies at preterm and term were obtained. Placental micro-vessels were used for determining vascular tension and responses to various vasoconstrictors as well as intracellular calcium store capability. It was the first time to show vascular responses in placental arteries to angiotensin II, endothelin-1, and other vascular drugs at preterm.

RESULTS

Compared to the control, placental vascular contractile responses to angiotensin II and caffeine were significantly decreased, while placental vascular responses to KCl, endothelin-1, and bradykinin were not significantly altered in the later term group in preeclampsia. In comparison of placental micro-vessel tension between the preterm and later term, caffeine- and serotonin-induced vascular contractions were significantly weaker in the preterm than that in the later term. On the contrary, vascular response to angiotensin II was increased in the preterm preeclampsia, while KCl-, endothelin-1, and bradykinin-mediated placental vessel responses in the preterm preeclampsia were similar to that in later term preeclampsia.

CONCLUSION

New data showed that micro-vessel responses to angiotensin II and serotonin, not endothelin- 1 or bradykinin, were significantly reduced in the human placentas at preterm, and intracellular Ca2+ store capacity was damaged too, providing important information on possible contributions of placental vascular dysfunction to pregnant hypertension.

How is the human umbilical artery regulated?

The purpose of this review is to present an update of the main mechanisms involved in the physiological regulation of contraction and relaxation of the human umbilical artery (HUA) smooth muscle cells. A literature review was performed based on the analysis of papers available on PubMed. The most important and relevant studies regarding the regulation of the HUA are presented in this article. The vascular smooth muscle is a highly specialized structure, whose main function is to regulate the vascular tonus. This is controlled by a balance between the cellular signaling pathways that mediate contraction and relaxation. The cells responsible for the contractile property of this muscle are the smooth muscle cells (SMC), and an excellent source of these cells is the HUA, involved in fetoplacental circulation. Since the umbilical blood vessels are not innervated, the HUA tonus is modulated by vasoactive substances that regulate the contractile process. The main vasoactive substances that induce contraction are serotonin, histamine, thromboxane, bradykinin, endothelin 1 and prostaglandin F2α, that are linked to the activation of proteins G_q and G_i/0 . On the other hand, the main vasorelaxation mechanisms are the activation of adenyl and guanil cyclases, potassium channels and the inhibition of calcium channels. The SMC from the HUA allow the study of different cellular mechanisms and their functions. Therefore, these cells are an important tool to study the mechanisms regulating the contractility of this artery, allowing to detect potential therapeutic targets to treat HUA disorders (gestational hypertension and pre-eclampsia).

Functional differences between the arteries perfusing gas exchange and nutritional membranes in the late chicken embryo

The chicken extraembryonic arterial system comprises the allantoic arteries, which irrigate the gas exchange organ (the chorioallantoic membrane, CAM) and the yolk sac (YS) artery, which irrigates the nutritional organ (the YS membrane). We compared, using wire myography, the reactivity of allantoic and YS arteries from 19-day chicken embryos (total incubation 21 days). The contractions induced by KCl, the adrenergic agonists norepinephrine (NE, nonselective), phenylephrine (α1), and oxymetazoline (α2), electric field stimulation (EFS), serotonin, U46619 (TP receptor agonist), and endothelin (ET)-1 and the relaxations induced by acetylcholine (ACh), sodium nitroprusside (SNP, NO donor), forskolin (adenylate cyclase activator), and isoproterenol (β-adrenergic agonist) were investigated. Extraembryonic allantoic arteries did not show α-adrenergic-mediated contraction (either elicited by exogenous agonists or EFS) or ACh-induced (endothelium-dependent) relaxation, whereas these responses were present in YS arteries. Interestingly, the intraembryonic segment of the allantoic artery showed EFS- and α-adrenergic-induced contraction and ACh-mediated relaxation. Moreover, glyoxylic acid staining showed the presence of catecholamine-containing nerves in the YS and the intraembryonic allantoic artery, but not in the extraembryonic allantoic artery. Isoproterenol- and forskolin-induced relaxation and ET-1-induced contraction were higher in YS than in allantoic arteries, whereas serotonin- and U46619-induced contraction and SNP-induced relaxation did not significantly differ between the two arteries. In conclusion, our study demonstrates a different pattern of reactivity in the arteries perfusing the gas exchange and the nutritional membranes of the chicken embryo.

Comparison of human ETA and ETB receptor signalling via G-protein and β-arrestin pathways

AIMS

To determine the pharmacology of ET(A)- and ET(B)-mediated β-arrestin recruitment and compare this to established human pharmacology of these receptors to identify evidence for endothelin receptor biased signalling and pathway specific blockade by antagonists.

MAIN METHODS

The ability of ET-1, ET-2, ET-3, sarafotoxin 6b and sarafotoxin 6c to activate ET(A) and ET(B)-mediated β-arrestin recruitment was determined in CHO-K1 cells. Affinities were obtained for ET(A) selective (BQ123, sitaxentan, ambrisentan), ET(B) selective (BQ788) and mixed (bosentan) antagonists using ET-1 and compared to affinities obtained in competition experiments in human heart and by Schild analysis in human saphenous vein. Agonist dependence of affinities was compared for BQ123 and BQ788 in the ET(A) and ET(B) β-arrestin assays respectively.

KEY FINDINGS

For β-arrestin recruitment, order of potency was as expected for the ET(A) (ET-1≥ET-2>>ET-3) and ET(B) (ET-1=ET-2=ET-3) receptors. However, at the ET(A) receptor sarafotoxin 6b and ET-3 were partial agonists. Antagonism of ET peptides by selective and mixed antagonists appeared non-competitive. BQ123, but not BQ788, exhibited agonist-dependent affinities. Bosentan was significantly more effective an inhibitor of β-arrestin recruitment mediated by ET(A) compared to the ET(B) receptor. In the ET(A) vasoconstrictor assay, ET-1, ET-2 and S6b were equipotent, full agonists and antagonists tested behaved in a competitive manner, although affinities were lower than predicted from the competition binding experiments in left ventricle.

SIGNIFICANCE

These data suggest that the pharmacology of ET(A) and ET(B) receptors linked to G-protein- and β-arrestin mediated responses was different and bosentan appeared to show bias, preferentially blocking ET(A) mediated β-arrestin recruitment.

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.

Characterisation of constrictor endothelin receptors in the human internal thoracic artery and saphenous vein

We studied the endothelin receptors mediating contraction in the human saphenous vein (SV) and internal thoracic artery (ITA). In the SV, the ET(A)-receptor antagonist BQ123 (1 microM) did not significantly shift the ET-1 concentration-response curve but did cause a parallel shift in the ITA. In the SV, the ET(A)-receptor agonist sarafotoxin 6b (S6b) produced a monophasic concentration-response curve that was antagonised biphasically by BQ123 (0.1-1 microM). In the ITA, S6b was an ineffective agonist with contractions seen only at 3 x 10(-9) M upward. The ET(B)-receptor agonist sarafotoxin 6c (S6c) caused constrictions in only 74% of SV rings and 42% of ITA rings. In the tissues that did respond, S6c caused a monophasic concentration-response curve with a lower maximal response than ET-1. The ET(B) antagonist BQ788 did not antagonise the responses to ET-1 in either the SV or the ITA but did antagonise the responses to S6c in the SV. The results from this study suggest that mainly ET(A) receptors mediate the contractile responses in the human SV and ITA. There is also evidence for an ET(B)-mediated response, although the contractions were much smaller than those elicited by ET-1. We also conclude that the ET(A) receptors mediating responses in these human vessels are atypical because of the different effects of BQ123 on the two vessels.

Atypical receptors mediate the response to endothelin-1 and sarafotoxin S6b in the human umbilical artery

The receptors mediating smooth muscle response to endothelin-1 and sarafotoxin S6b in the human umbilical artery were investigated in vitro. Both agonists induced contractions that were unaffected by the endothelin ET(B) receptor antagonist BQ 788 (10(-9), 10(-8), 10(-7) M). The non-selective endothelin ET(A/B) receptor antagonist PD 142893 (10(-7) M) decreased the contraction induced by endothelin-1. PD 142893 (10(-9) M) enhanced the contraction induced by sarafotoxin S6b whereas higher concentrations had no effect. Removing the endothelium did not affect the antagonising action of PD 142893 on endothelin-1-induced contractions while the enhancement of the sarafotoxin S6b-induced contraction was abolished. Sarafotoxin S6b induced relaxation in segments precontracted by 5-hydroxytryptamine and exposed to the endothelin ET(A) receptor antagonist BQ 123 (10(-7) M) and PD 142893 (10(-9) M) abolished this relaxation. These endothelial receptors seem neither to be classical endothelin ET(A) nor endothelin ET(B) receptors and they are not activated by endothelin-1.

Influence of ETR-p1/f1 antisense peptide on endothelin-induced constriction in rat renal arcuate arteries

1. This study set out to examine the endothelin receptor subtypes mediating vasoconstriction in the rat renal arcuate artery. This was done in isolated vessels 120-200 microns in diameter, incubated with a selective agonist and the novel 'antisense' peptide to part of the human endothelinA receptor. 2. Groups of vessels (n = 6) were incubated with increasing concentrations of endothelin-1 (ET-1), from 1 to 100 nM, which caused a 65% maximal contraction at the highest dose with an pEC50 of 8.16 +/- 0.11 M. By contrast, in six other vessels sarafotoxin 6c over the same dose range gave a minimal contraction (around 5% of maximum). 3. Preincubation of six vessels with the antisense peptide ETR p1/f1 at 1 microM had no effect on the ET-1 induced vasoconstriction, in terms of displacement of the concentration-response curve or the maximal tension achieved by the agonist. In the six vessels exposed to 4 microM ETR p1/f1, there was a significant shift of the concentration-response curve and a lower pEC50 at 7.78 +/- 0.09 M (P < 0.05). At the highest concentrations of ETR p1/f1, there was a marked suppression of all responses to ET-1, which at the maximal concentrations tested, 0.1 microM, only reached some 10% of the maximal achievable contraction. 4. Increasing ET-1 concentrations up to 2 microM in vessels incubated with 40 microM ETR-p1/f1 showed that the blockade could be overcome and that the relationship was shifted to the right (P < 0.001) by approximately one log unit with a pEC50 of 7.13 +/- 0.11 M. A Schild plot of the data indicated the antagonist to be acting competitively at a single population of receptors. 5. At the highest concentrations tested, 40 microM, ETR-p1/f1 had no effect on noradrenaline-induced contractions, indicating a lack of non-specific actions. 6. Together, these data suggest that at the rat renal arcuate artery the endothelinA receptor is the predominant functional receptor mediating contraction. Furthermore, this study has shown the potential usefulness of this novel type of 'antisense' peptide in blocking receptor activation.

Comparison of the contractile effects and binding kinetics of endothelin-1 and sarafotoxin S6b in rat isolated renal artery

1. To date, only two mammalian endothelin (ET) receptors, termed ETA and ETB, have been cloned, sequenced and characterized. However, several functional studies of isolated blood vessels suggest that ET-1-induced contractions may be mediated by multiple ETA receptors. In this study, the ETA receptors in renal arteries isolated from Wistar rats were characterized by isometric tension recording and radioligand binding techniques. 2. ET-1, sarafotoxin S6b (StxS6b) and ET-3 produced concentration-dependent contraction with similar response maxima in endothelium-denuded arteries, whereas the ETB receptor-selective agonist StxS6c was inactive. ET-1 and StxS6b were equipotent and 30 times more potent than ET-3. This agonist profile, together with the findings that the ETA receptor-selective antagonists, BQ-123 and FR-139317 caused concentration-dependent, rightward shifts of the concentration-effect curves to each agonist indicated that ET-1-induced contractions in rat renal artery were mediated via ETA receptors. 3. BQ-123 and FR-139317 were both significantly more potent inhibitors of contractions induced by StxS6b or ET-3 than of responses to ET-1, raising the possibility that a component of ET-1-induced contraction was mediated through atypical, BQ-123 (or FR-139317)-insensitive ETA receptors. However, in competition binding studies, specific [125I]-ET-1 and [125I]-StxS6b binding to rat renal artery sections was completely abolished by BQ-123 in a manner consistent with an action at a single site. Thus, competition binding studies did not provide any supportive evidence of the existence of a BQ-123-insensitive ETA receptor. 4. Additional studies revealed marked differences in the kinetics of [125I]-ET-1 and [125I]-StxS6b binding. Following a 3 h period of association of [125I]-ET-1 with its receptors, no significant dissociation of receptor-bound [125I]-ET-1 was observed during a 4 h washout period. In stark contrast, dissociation studies revealed that specific [125I]-StxS6b binding to ETA receptors was reversible (t0.5diss, 100 min). A series of association binding studies were also consistent with the specific binding of [125I]-ET-1 and [125I]-StxS6b being irreversible and reversible processes, respectively. 5. Thus, differences in BQ-123 potency against ET-1 and StxS6b-induced contractions in rat renal arteries might be due to differences in the kinetics of agonist binding, rather than due to the existence of atypical ETA receptors.

Characterization of endothelin receptors in the human umbilical artery and vein

The aim of the present study was to characterize pharmacologically endothelin receptors that are present in human umbilical vessels. 2. Endothelin-1 (ET-1) and endothelin-2 (ET-2) are potent stimulants of both the human umbilical artery (pEC50 7.9 and 7.5) and vein (pEC50 8.1 and 8.0). Endothelin-3 (ET-3) is inactive on the artery but contracts the vein (pEC50 7.6). IRL1620 is inactive in both vessels. The order of potency of agonists is suggestive of a typical ET(A) receptor in the artery (ET-1 = ET-2 > > ET-3) and a mixture of ET(A) and ET(B) receptors in the vein (ET-1 = ET-2 > or = ET-3). 3. The selective ET(A) receptor antagonist, BQ123, competitively inhibits the effect of ET-1 in the human umbilical artery (pA2 6.9), while in the vein, only a mixture of BQ123 and BQ788 (a selective ET(B) antagonist) weakly displaces to the right of the cumulative concentration-response curve to ET-1. Contractions induced by ET-3 in the vein are inhibited by BQ788 (pA2 7.6), but not by BQ123. 4. Inhibition of Ca2+ channels by nifedipine (0.1 microM) is accompanied by a significant decrease of the maximal response to ET-1 by 40% in the artery and by 30% in the vein. The response of the vein to ET-3 is almost abolished by nifedipine. 5. The results indicate that: (i) endothelins contract the human isolated umbilical artery via stimulation of an ET(A) receptor type; (ii) the contraction induced by ET-1 in the vein is mediated by both ET(A) and ET(B) receptors, while ET-3 stimulates the ET(B) receptor; (iii) the contribution of Ca2+ channels to the contraction mediated by the ET(B) receptor appears to be more important than to that mediated by the ET(A) receptor.

Failure of BQ123, a more potent antagonist of sarafotoxin 6b than of endothelin-1, to distinguish between these agonists in binding experiments

1. In homogenates of human saphenous vein, [125I]-ET-1 and [125I]-S6b each labelled a single population of high affinity binding sites with K(D) values of 0.64 +/- 0.11 nM and 0.55 +/- 0.08 nM respectively. Hill slopes were close to one. However, the density of receptors labelled by [125I]-ET-1 was significantly greater than that by [125I]-S6b (187.6 +/- 23.0 compared to 91.7 +/- 23.6 fmol mg-1 protein, P < 0.02). 2. BQ123, an ET(A-)selective antagonist, inhibited specific [125I]-ET-1 and [125I]-S6b binding with equal affinity. BQ123 competed in a biphasic manner for both [125I]-ET-1 (0.1 nM) and [125I]-S6b (0.1 nM) with ET(A) K(D) values of 0.55 +/- 0.17 nM and 0.52 +/- 0.02 nM and ET(B) K(D) values of 14.4 +/- 2.60 microM and 11.2 +/- 0.31 microM respectively. S6b monophasically inhibited 0.1 nM [125I]-ET-1 (K(D) 1.16 +/- 0.9 nM) but competed for 0.25 nM [125I]-ET-1 in a biphasic manner (K(D) high affinity site 1.99 +/- 0.84 nM, K(D) low affinity site 0.68 +/- 0.63 microM, ratio 67% : 33%). 3. BQ123 antagonized the vasoconstrictor responses of ET-1 with a pK(B) value of 6.47 whereas BQ123 exhibited 50 fold higher affinity against S6b-mediated vasoconstriction with a pK(B) value of 8.18. Regression slopes were 0.80 +/- 0.13 and 1.08 +/- 0.11 respectively. 4. In desensitization experiments, S6b (300 nM) did not contract preparations which were no longer responsive to ET-1 whereas a small contraction to ET-1 (300 nM) was obtained in preparations rendered unresponsive to S6b. 5. Medial sections of non-diseased human aorta, which express only ET(A) receptors, were used to compare dissociation rates of the two agonists. The time course for the dissociation of [125I]-ET-1 and [125I]-S6b was similar with 20-30% of each ligand dissociating at 4 h. 6. These data suggest that whilst BQ123, in common with other endothelin antagonists, is a much more potent blocker of S6b contractile responses than of ET-1 contractile responses, this is not reflected by the equal affinity of BQ123 determined in competition binding experiments against both [125I]-ET-1 and [125I]-S6b. This discrepancy in antagonist potency is probably not due to a marked difference in the rate of dissociation of [125I]-ET-1 and [125I]-S6b from endothelin receptors. One possible explanation is that ET-1 is activating an additional population of receptors which may have lower affinity for BQ123. This is suggested by the discrepancy in receptor density identified by [125I]-ET-1 and [125I]-S6b.

Endothelin receptors in human coronary artery and aorta

1. ETA and ETB-selective and non-selective ligands were used to define the endothelin receptors in the media (vascular smooth muscle layer) of human aorta and coronary artery. Saturation experiments with iodinated endothelin-1 (ET-1), endothelin-2 and sarafotoxin 6b (S6b) identified high affinity binding sites in aorta (KD [125I]-ET-1 0.33 +/- 0.02 nM (n = 9), KD [125I]-ET-2 1.04 +/- 0.23 nM (n = 5), KD [125I]-S6b 0.15 +/- 0.01 nM (n = 9 +/- s.e.mean)) and coronary artery (KD [125I]-ET-1 0.43 +/- 0.10 nM, KD [125I]-ET-2 0.71 +/- 0.17 nM, KD [125I]-S6b 0.27 +/- 0.03 nM (n = 3 +/- s.e.mean)). Hill coefficients (nH) approached unity in each case. 2. No specific binding was detectable with [125I]-ET-3 (4 pM-4 nM) in aorta. Unlabelled ET-3 competed monophasically with [125I]-ET-1 in aorta (KD, 8.21 +/- 1.62 nM, compared to unlabelled ET-1 KD, 0.60 +/- 0.20 nM) (n = 3 +/- s.e.mean). In coronary artery, the KD and Bmax values calculated from [125I]-ET-3 saturation experiments were 2.13 +/- 1.39 nM and 20.6 +/- 12.9 fmol mg-1 protein, respectively (n = 3 +/- s.e.mean). 3. ETA antagonists competed monophasically for [125I]-ET-1 (100 pM) binding sites with nanomolar or subnanomolar affinity in the aorta (KD BQ123, 0.47 +/- 0.13 nM; KD FR139317, 0.40 +/- 0.10 nM; KD PD151242, 2.09 +/- 0.48 nM) and coronary artery (KD FR139317, 0.41 +/- 0.13 nM; KD PD151242, 3.60 +/- 0.74 nM) (n = 3 +/- s.e.mean). However, two site fits were preferred on analysis of competition experiments with ETB-selective agonists versus [125I]-ET-1 in coronary artery (BQ3020: KDETA 0.96 +/- 0.14 microM, KD ETB 1.34 +/- 1.08 nM and sarafotoxin 6c: KD ETA 1.15 +/- 0.14 microM, KD ETB 1.77 +/- 0.72 nM) (n = 3 +/- s.e.mean). The selectivity of the agonists for ETB receptors (700 fold) was lower than reported in other species. 4. Sarafotoxin 6b (2 pM-2 microM) completely inhibited [125I]-ET-1 (100 pM) binding in aorta (KD 1.36 +/- 0.22 nM) (n = 3 +/- s.e.mean). The non-peptide compounds Ro462005 and bosentan, competed with [125I]-ET-1 binding in coronary artery with KD values of 0.19 +/- 0.04 microM and 2.94 +/- 0.95 nM, respectively (n = 3 +/- s.e.mean). 5. Inhibition of [125I]-ET-2 and [125I]-S6b binding by FR139317 was similar to the inhibition of [125I]-ET-1 binding in both arteries, being monophasic with KD values in the same range. 6. ETA receptors in coronary artery media were detected by [125I]-PD151242 (KD 0.23 +/- 0.04 nM, Bmax 10.1 +/- 1.2 fmol mg-1 protein) (n = 3 +/- s.e.mean). [125I]-BQ3020, an ETB-selective radioligand, indicated the presence of a smaller population of ETB receptors in this tissue (KD 0.60 +/- 0.31 nM, Bmax 4.5 +/- 2.1 fmol mg-1 protein) (n = 3 +/- s.e.mean). 7. Autoradiography with [125I]-PD151242 and [125I]-BQ3020 confirmed the predominance of ETA receptors in the media of both arteries. 8. The results of this study indicate that ETA receptors predominate in the vascular smooth muscle of human cardiac arteries, with a small and variable population of ETB receptors detectable in the coronary artery.

The endothelin system and its potential as a therapeutic target in cardiovascular disease

Endothelin (ET)-1, an endothelium-derived peptide, is the most potent vasoconstrictor agent described to date. ET-1 also has positive inotropic and chronotropic effects in the heart and is a co-mitogen in both cardiac and vascular myocytes. The major elements of the system involved in formation of ET-1 and its isopeptides, as well as the receptors mediating their effects, have been cloned and characterised. Antagonists of the ET receptors are now available, and selective inhibitors of the ET-converting enzymes are being developed. Early studies using receptor antagonists support the involvement of ET-1 in the pathophysiology of several cardiovascular diseases. The relative merits of ET-converting enzyme inhibitors and receptor antagonists for the treatment of cardiovascular disease are discussed.

Vasoactive peptides and uterine vessels

Endothelins and atrial natriuretic peptide (ANP) are vasoactive peptides with effects on the human uterine and umbilical arteries. Endothelin (ET) contracts the vascular smooth muscle. Both ETA- and non-ETA-non-ETB-receptors seem to be involved. Autoradiography reveals binding of ET to vascular smooth muscle. ANP counteracts the contractile effects of angiotensin II in the human uterine artery. Head-down tilt results in elevation of plasma ANP in healthy pregnant women, while the same manoeuvre induces down-regulation of the reninangiotensin-aldosterone system in non-pregnant women and patients suffering from pre-eclampsia.

Reduced contractile effect of endothelin-1 and noradrenalin in human umbilical artery from pregnancies with abnormal umbilical artery flow velocity waveforms

This study on the human umbilical artery was undertaken in order to elucidate possible correlations between changes in response to vasoactive substances in vitro and abnormal umbilical artery flow velocity waveforms in vivo associated with preeclampsia and intrauterine growth retardation. The vascular reactivity to endothelin-1, noradrenalin, serotonin, the thromboxane A2 analogue U46619, substance P and prostacyclin was determined in umbilical artery segments from 13 normal pregnancies and 29 pregnancies complicated with preeclampsia and/or intrauterine growth retardation with normal or abnormal umbilical flow velocity waveforms. The contractile effect in vitro of endothelin-1 and noradrenalin was reduced in segments from pregnancies complicated by abnormal umbilical flow velocity waveforms in vivo. No differences were detected in the contractile effect of serotonin and U46619, or in the relaxatory effect of substance P and prostacyclin. In conclusion, endothelin-1- and noradrenalin-related mechanisms could be involved in the abnormal umbilical flow velocity waveforms associated with preeclampsia and intrauterine growth retardation.

Endothelin ETA and ETB mRNA and receptors expressed by smooth muscle in the human vasculature: majority of the ETA sub-type

1. We measured the ratio of ETA and ETB sub-types in the media (containing mainly smooth muscle) of human cardiac arteries (aorta, pulmonary and coronary), internal mammary arteries and saphenous veins. 2. In saturation experiments, [125I]-endothelin-1 ([125I]-ET-1) bound with high affinity to the media of each vessel (n = 3 individuals or homogenate preparations +/- s.e. mean): coronary artery, KD = 0.14 +/- 0.02 nM, Bmax = 71.0 +/- 21.0 fmol mg-1 protein; pulmonary artery, KD = 0.85 +/- 0.25 nM, Bmax = 15.2 +/- 10.3 fmol mg-1 protein; aorta, KD = 0.51 +/- 0.02 nM, Bmax = 9.4 +/- 4.4 fmol mg-1 protein; internal mammary artery. KD = 0.34 +/- 0.31 nM, Bmax = 2.0 +/- 0.5 fmol mg-1 protein and saphenous vein, KD = 0.28 +/- 0.05 nM, Bmax = 52.8 +/- 1.0 fmol mg-1 protein. In each vessel, over the concentration-range tested, Hill slopes were close to unity and a one site fit was preferred to a two site model. 3. In competition binding assays, the ETA selective ligand, BQ123 inhibited the binding of 0.1 nM [125I]-ET-1 to the media in a biphasic manner. In each case, a two site fit was preferred to a one or three site model: coronary artery, KDETA = 0.85 +/- 0.03 nM, KDETB = 7.58 +/- 2.27 microM, ratio = 89:11%; pulmonary artery, KDETA = 0.27 +/- 0.05 nM, KDETB = 24.60 +/- 5.34 microM, ratio = 92:8%; aorta, KDETA = 0.80 +/- 0.40 nM, KDETB = 2.67 +/- 2.60 microM ratio = 89:11%; saphenous vein, KDETA = 0.55 +/- 0.17 nM, KDETB = 14.4 +/- 0.26 microM, 85:15% (n = 3 individuals or homogenate preparations +/- s.e. mean). BQ123 showed up to 18000 fold selectivity for the ETA over the ETB sub-type. The ETA-selective ligand, [125I]-PD151242 labelled 85% of the receptors detected by a fixed concentration of [125I]-ET-1 in media of internal mammary artery, measured by quantitative autoradiography. In contrast, the density of ETB receptors detected with [125I]-BQ3020 was 7.0 +/- 1.5 amol mm-2, representing about 8% of [125I]-ET-1. 4. A single band corresponding to the expected position for mRNA encoding the ETA receptor (299 base pairs) was found in the media in each of the five vessels (n = 3 individuals) using reverse transcript as epolymerase chain reaction assays. A single band corresponding to the ETB sub-type (428 base pairs) was also always detected.5. 35S-labelled antisense probes to ETA and ETB hybridised to the media of epicardial coronary arteries as well as intramyocardial vessels, confirming the presence of mRNA encoding both sub-types in the vascular smooth muscle of the vessel wall.6 Although mRNA for both receptors was detected, competition binding using BQ123 demonstrated that the majority (at least 85%) of ET receptors present in smooth muscle are the ETA sub-type. These results provide further support for the hypothesis that the ETA sub-type is the receptor that must be blocked in humans to produce a beneficial vasodilatation in pathophysiological conditions where there is an increase in peptide concentration or receptor density.

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.

Different endothelin receptors involved in endothelin-1- and sarafotoxin S6B-induced contractions of the human isolated coronary artery

1. Endothelin receptors, that mediate contraction of the human isolated coronary artery, were characterized by use of a number of agonists and antagonists. Contraction induced by the non-selective agonists, endothelin (ET)-1 and sarafotoxin S6b, was compared in endothelium-intact and endothelium-denuded ring segments. The effects of ET-1 and BQ-123 (an ETA receptor antagonist) were investigated both in ring segments and in spirally cut strips. Lastly, the effect of phosphoramidon was studied on contraction induced by big-ET-1. 2. The order of agonist potency (pD2) in endothelium-intact coronary artery ring segments was: ET-1 (8.27) approximately sarafotoxin S6b (8.16) > big-ET-1 (< 7.1) approximately ET-3 (< 6.9). [Ala1,3,11,15]ET-1 (ETB receptor agonist) caused significant contraction only at 1 microM, whereas 0.3 microM big-ET-3 had no effect. Removal of the endothelium in ring segments did not affect the contractile response to ET-1 or to sarafotoxin S6b. 3. After a full concentration-response curve had been obtained to ET-1 or sarafotoxin S6b, further contractions of the endothelium-intact coronary artery segments could only be achieved by applying ET-1 in segments exposed to sarafotoxin S6b, and not the reverse. 4. BQ-123 (0.1 microM) antagonized contractions of endothelium-intact ring segments induced by sarafotoxin S6b (pKB 7.86). Only 10 microM BQ-123 antagonized contractions induced by ET-1 (pKB 5.75). FR139317 was also more potent against sarafotoxin S6b (pKB 8.24-8.47) than against ET-1 (pKB 6.11). [Ala1,3,11,15]ET-1 (1 microM) had no effect on the contractile response to ET-1 or to sarafotoxin S6b. 5. In strip preparations with intact endothelium, the pD2 of ET-l increased to 9.04 =/- 0.16 (vs.8.50 +/- 0.07 in rings), and BQ-123 (1 microM) caused a rightward shift of the ET-l induced concentration response curve (pKB 6.62 vs. 5.75 in rings).6. Contractile responses to big-ET-1 of endothelium-intact coronary artery segments were attenuated in the presence of phosphoramidon (100 microM), indicating conversion of big-ET-1 to ET-1 within the coronary artery segment.7. The present study indicates that ET-1 and sarafotoxin S6b contract the human isolated coronary artery via different receptors, which can probably be best characterized as subtypes of the ETA receptor.Furthermore, it is demonstrated that the type of preparation (ring or strip) may affect the potency of ET-1 as an agonist and of BQ-123 as an antagonist.

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.

Characterization of endothelin (ET) receptors in the isolated gall bladder of the guinea-pig: evidence for an additional ET receptor subtype

1. We have characterized the receptors mediating contractions induced by endothelin-1 (ET-1), ET-2, ET-3 and the ETB-selective receptor agonists, sarafotoxin 6c (SX6c), IRL 1620, BQ-3020, [Ala1,3,11,15]ET-1 and ET (16-21) in strips of the isolated gall bladder of the guinea-pig (GPGB). We used as antagonists BQ-123 (ETA receptor selective) and PD 145065 (ETA/ETB receptor non-selective). 2. ET-1, ET-2 and ET-3 (10(-10) M to 3 x 10(-7) M) caused similar slowly-developing concentration-dependent contractions of the GPGB. Contractile effects induced by ET-1, ET-2 or ET-3 (at 3 x 10(-7) M) were also similar (230 +/- 25, 241 +/- 7 and 287 +/- 37% of that to histamine at 5 x 10(-6) M, n = 7, 6, 12, respectively). However, the threshold concentration for ET-1 or ET-2 was 10(-10) M whereas it was 3 x 10(-9) M for ET-3. 3. SX6c (10(-10) M to 3 x 10(-7) M) also caused slowly-developing concentration-dependent contractions at a threshold concentration of 10(-10) M (n = 16). However, the contraction caused by SX6c at 3 x 10(-7) M was 116 +/- 9% of that to histamine at 5 x 10(-6) M, which was half of that induced by the same concentration of the ET isopeptides. The contraction induced by IRL 1620 at 3 x 10-7 M (n = 9) was 43 +/- 9% of that to histamine at 5 x 10-6 M, which was one fifth of that produced by the same concentration of ET-1. Contractions induced by BQ-3020 or [Ala1,3,11,15]ET-I at 3 x 10-7 M were even less than those produced by IRL 1620. ET (16-21) was inactive up to 10-5 M. Addition of a concentration of 3 x 10-7 M of ET-1 to tissues with developed contractions induced by the bolus addition of 3 x 10-7 M SX6c caused a further contraction of the GPGB to the level observed with ET-1 alone at 3 x 10-7M (n = 8).4. BQ-123 (10-5 M) did not affect the concentration-response curve to ET-1 and the contraction induced by 3 x 10- M was also not affected (n = 5; 239 +/- 19% of histamine at 5 x 10-6 M). PD 145065(10-5 M) shifted the ET-1 concentration-response curve to the right and the contraction induced by ET-1at 3 x 10-7 M was inhibited by 15% (n = 6; NS). A higher concentration of BQ-123 (10-4 M) caused a significant shift to the right of the ET-1 concentration-response curve similar to that caused by PD 145065 (10-s M) and caused a 24% (n = 6; NS) inhibition of the contractions induced by ET-1 at 3 x 10-7 M. PD 145065 (10-4 M) abolished contractions induced by ET-1 (up to 10- M) and inhibited the response to ET-1 at 3 x 10-7 M by 52% (n = 4; P< 0.05).5. Contractions induced by ET-3 were more sensitive to inhibition by the antagonists. BQ-123 (10-6,10-5 or 10-4 M) inhibited responses to 3 x 10-7 M ET-3 by 66, 71 and 83%, respectively (n = 5, 5, 3;P< 0.05). PD 145065 (10-6, 10-5 or 10-4 M) attenuated more strongly than did BQ-123 the contractions induced by ET-3. For instance, the contractions caused by ET-3 at 3 x 10-7 M were decreased by 73 and 80% (n = 5, 5; P<0.05) in the presence of PD 145065 (10-6 or 10-5 M, respectively). PD 145065(10-4 M) completely abolished contractions to ET-3 (n = 4; up to 3 x 10-7M).6. Contractions induced by SX6c, especially those observed at concentrations lower than 10-8 M, were attenuated by BQ-123 (up to 10-4 M). PD 145065 (10-5M) shifted to the right the concentration response curve to SX6c and inhibited by 38% (P<0.05) the contractions induced by 3 x 10-7M.However, the contractions induced by a bolus addition of a high concentration of SX6c (3 x 10-7 M)and the subsequent addition of an identical concentration of ET-1 on top of SX6c were not affected byBQ-123 (10-6 or 10-5 M).7. These results suggest that ETB receptors are involved in the contractions induced by endothelins in the GPGB. However, SX6c and other selective ETB agonists produced only half or less than half of the contractile response induced by non-selective agonists. In addition, the responses to ET-1 but not to ET-3, were insensitive to the antagonist action of BQ-123 at 10-5 M whereas BQ-123 or PD 145065 at 10-5 M strongly antagonized contractions induced by ET-3. Finally, BQ-123 at 10-4 M inhibited contractions to ET-1 and SX6c. Thus, within the GPGB there may well be additional ET receptor(s) not conforming to the established ETA/ETB receptor subtype classification, as well as ETB receptors.

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.