Sarafotoxin S6c: an agonist which distinguishes between endothelin receptor subtypes

Cellular localization of endothelin receptor subtypes in the rat kidney following in vitro labelling

1. We have previously shown that [125I]-endothelin (ET) receptor binding is localized almost exclusively to the fenestrated endothelial cells of glomerular capillaries and peritubular capillaries in the rat kidney following systemic administration of the radioligand in vivo. Because of the lack of specific ET receptor binding in other glomerular and tubular structures following in vivo labelling, we undertook further studies, using electron microscopic autoradiography and ET receptor subtype selective ligands, to investigate whether other renal components also contain ET receptor binding and, if so, to determine the cellular localization of the ET receptor subtypes, ETA and ETB, following in vitro labelling. 2. At the electron microscopic level, ET binding sites were localized primarily to the fenestrated endothelium of glomerular and peritubular capillaries of the cortex, inner stripe of the outer medulla and the inner medulla. ET binding sites also occurred overlying renomedullary interstitial cells (RMIC) of the inner medulla. 3. The ETB receptor selective agonist, sarafotoxin 6c (S6c), abolished ET binding in the vascular endothelium throughout the kidney, while the ETA receptor selective antagonist, BQ123, was without effect. Both BQ123 and S6c partially inhibited the binding in the RMIC of the inner medulla. 4. These results indicate that ET receptor binding in the fenestrated endothelium in the glomerular capillaries and peritubular capillaries belongs mainly to the ETB subtype, whereas both ETA and ETB subtypes are present in the RMIC.

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 adrenocortical secretagogue effect is mediated by the B receptor in rats

We investigated the gene expression and localization of endothelin-1 (ET-1) receptor subtypes ET(A) and ET(B) in the rat adrenal cortex as well as their involvement in the corticosteroid secretagogue effect of ET-1 in vitro. Reverse transcription-polymerase chain reaction with primers specific for ET(A) and ET(B) cDNAs demonstrated the expression of both receptor genes in homogenates of adrenocortical tissue. However, in isolated zona glomerulosa and zona fasciculata cells, only ET(B) mRNA was detected. Autoradiographic examination of the selective displacement of 125I-ET-1 binding by BQ-123 and BQ-788 (specific ligands for ET(A) and ET(B), respectively) indicated that zona glomerulosa possesses both ET(A) and ET(B), whereas zona fasciculata is exclusively provided with ET(B). ET-1 enhanced in a concentration-dependent manner aldosterone and corticosterone secretions of dispersed zona glomerulosa and zona fasciculata cells, respectively. The ET(B) antagonist BQ-788 markedly reduced the secretory response of zona glomerulosa cells and completely suppressed that of zona fasciculata cells, whereas the ET(A) antagonist BQ-123 was ineffective. These findings indicate that in the rat, the adrenocortical secretagogue action of ET-1 is mediated by the ET(B) receptor subtype and that the ET(A) receptor is not directly involved in such an effect.

Endothelin-1-induced potentiation of human airway smooth muscle proliferation: an ETA receptor-mediated phenomenon

1. In this study the mitogenic effects in human cultured tracheal smooth muscle cells of endothelin-1 (ET-1), ET-3, and sarafotoxin S6c (S6c), the ETB receptor-selective agonist, were explored either alone or in combination with the potent mitogen, epidermal growth factor (EGF). 2. In confluent, growth-arrested human airway smooth, neither ET-1 (0.01 nM-1 microM) nor ET-3 (0.001 nM-1 microM) or S6c (0.01 nM-1 microM) induced cell proliferation, as assessed by [3H]-thymidine incorporation. In contrast, EGF (1.6 pM-16 nM) produced concentration-dependent stimulation of DNA synthesis (EC50 of about 0.06 nM). The maximum increase of about 60 fold above control, elicited by 16 nM EGF, was similar to that obtained with 10% foetal bovine serum (FBS). EGF (0.16-16 nM) also produced a concentration-dependent increase in cell counts, whereas ET-1 (1-100 nM) was without effect on this index of mitogenesis. 3. ET-1 (1-100 nM) potentiated EGF-induced proliferation of human tracheal smooth muscle cells. For example, ET-1 (100 nM), which alone was without significant effect, increased by 3.0 to 3.5 fold the mitogenic influence of EGF (0.16 nM). The potentiating effect of ET-1 on EGF-induced proliferation was antagonized by BQ-123 (3 microM), the ETA receptor antagonist, but was unaffected by the ETB receptor antagonist BQ-788 (10 microM). 4. Neither ET-3 (1-100 nM) nor S6c (1-100 nM) influenced the mitogenic effects of EGF (0.16-1.6 nM). 5. [125I]-ET-1 binding studies revealed that on average the ratio of ETA to ETB receptors in human cultured tracheal smooth muscle cells was 35:65 ( +/- 3; n = 4), confirming the predominance of the ETB receptor subtype in human airway smooth muscle. 6. These data indicate that ET-1 alone does not induce significant human airway smooth muscle cell proliferation. However, it potently potentiated mitogenesis induced by EGF, apparently via an ETA receptor-mediated mechanism. These findings suggest that ET-1, a mediator detected in increased amounts in patients with acute asthma, may potentiate the proliferative effects of mitogens and contribute to the airway smooth muscle hyperplasia associated with chronic severe asthma.

In vitro characterisation of Ro 46-8443, the first non-peptide antagonist selective for the endothelin ETB receptor

We describe here Ro 46-8443, the first non-peptide endothelin ETB receptor selective antagonist. It displays up to 2000-fold selectivity for ETB receptors both in terms of binding inhibitory potency and functional inhibition. The observed parallel rightward shift of concentration-response curves with different antagonist concentrations is consistent with a competitive binding mode. Since R0 46-8443 selectively inhibits ETB receptor mediated responses, it is a valuable tool for clarifying the role of ETB receptors in pathology.

ETB-type receptors mediate endothelin-stimulated contraction in the aortic vascular smooth muscle of the spiny dogfish shark, Squalus acanthias

Using various agonists, and the specific antagonist BQ-123, we have examined the sensitivity to endothelin of the vascular smooth muscle of the ventral aorta of the spiny dogfish shark, Squalus acanthias. Human endothelin-1 produced significant contraction of isolated vascular smooth muscle rings, with an EC50 of 10 nmol.l-1. The presence of an intact endothelium did not alter this response but the magnitude of the contraction was greater in rings with an intact endothelium. The response to 0.2 mumol.l-1 endothelin-1 was equivalent to that of 0.1 mmol.l-1 acetylcholine, and significantly greater than that to 80 mmol.l-1 KCl, suggesting high sensitivity even to the heterologous, mammalian peptide. The Hill plot of the contractile response was a straight line with a slope of 1.12, indicating that a single receptor was mediating the response. Endothelin-1, endothelin-3, and sarafotoxin S6c produced similar concentration-response curves, and the response to endothelin-1 was insensitive to the ETA-specific inhibitor BQ-123. Our data are consistent with the hypothesis that the receptor involved in the contractile response to endothelin of shark aortic vascular smooth muscle is of the ETB-rather than the ETA-type.

Necessity of dual blockade of endothelin ETA and ETB receptor subtypes for antagonism of endothelin-1-induced contraction in human bronchi

1. Endothelin (ET)-1 has been postulated to be involved in the development of obstructive airway diseases in man. In the present study, we attempted to characterize ET receptor subtypes mediating ET-1-induced contraction in human isolated bronchi. The ET receptor antagonists used in the present study were BQ-123 (ETA receptor-selective), BQ-788 (ETB receptor-selective) and BQ-928 (ETA/ETB dual). Sarafotoxin S6c (S6c) was also used as an ETB receptor-selective agonist. 2. In human bronchi, ET-1 and S6c (10(-12)M to 10(-7) M) produced concentration-dependent contraction with almost equal potency (pD2: 8.88 +/- 0.16 for ET-1 and 9.42 +/- 0.15 for S6c). The contraction induced by S6c was competitively antagonized by BQ-788 alone (1 and 10 microM) with a pKB value of 7.49 +/- 0.21, suggesting that the stimulation of ETB receptors causes a contraction of human bronchi. However, contrary to expectation, the concentration-response curves for ET-1 were not affected by BQ-788. The ET-1- and S6c-induced contractions were not affected by BQ-123 (10 microM). Thus, ET-1-induced contraction of human bronchi is not antagonized by BQ-123 alone or by BQ-788 alone. 3. Combined treatment with 10 microM BQ-123 and 10 microM BQ-788 significantly antagonized the contraction induced by ET-1 with a dose-ratio of 11. BQ-928 also significantly antagonized ET-1-induced contraction with a pKB value of 6.32 +/- 0.24. 4. The specific binding of [125I]-ET-1 to human bronchial membrane preparations was inhibited by BQ-123 (100 pM to 1 microM) by approximately 40%. Combination treatment with BQ-788 (100 pM to 1 microM) completely inhibited the BQ-123-resistant component of [125I]-ET-1 specific binding. 5. In conclusion, the present study demonstrates that BQ-788 alone cannot inhibit ET-1-induced contractions in human bronchi, although human bronchial ETB receptors are BQ-788-sensitive. Furthermore, it was shown that blockade of both receptor subtypes antagonizes ET-1-induced contraction, and that both receptor subtypes co-exist in human bronchial smooth muscles. These findings suggest that ETA receptors as well as ETB receptors are involved in ET-1-induced contraction in human bronchi. If ET-1 is involved in human airway diseases, dual blockade of ETA and ETB receptors may be necessary to treat the diseases.

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.

Enhanced coronary vasoconstriction to endothelin-B-receptor activation in experimental congestive heart failure

BACKGROUND

Endothelin (ET), a coronary vasoconstrictor, mediates its activity through the specific receptors ET-A and ET-B, which may demonstrate different activity under pathophysiological conditions. Thoracic inferior vena cava constriction (TIVCC) is an experimental model of congestive heart failure that is characterized by a decrease in cardiac output and an increase in circulating ET concentrations. The present study was designed to test the hypothesis that experimental heart failure altered coronary vascular responsiveness to ET-A- and ET-B-receptor stimulation in vivo.

METHODS AND RESULTS

ET-1 was infused at a rate of 2 ng/kg per minute into the left circumflex coronary artery in normal dogs (n = 5) and in dogs subjected to TIVCC (TIVCC dogs, n = 6). Similarly, sarafotoxin, an ET-B-receptor agonist, was infused at the same dosage in normal (n = 5) and TIVCC (n = 6) dogs. Intracoronary infusion of ET-1 significantly decreased coronary blood flow and increased coronary vascular resistance in normal dogs; this effect was significantly attenuated in TIVCC compared with normal dogs. The percent changes in coronary blood flow and coronary vascular resistance in the TIVCC compared with the normal dogs was -11 +/- 8% versus -48 +/- 7% (P < .01) and 29 +/- 10% versus 105 +/- 23% (P < .01), respectively. There was no significant effect on coronary blood flow, coronary vascular resistance, or coronary artery diameter in normal dogs that received an intracoronary infusion of sarafotoxin. In contrast, the administration of intracoronary sarafotoxin in TIVCC compared with normal dogs resulted in significant percent changes in coronary blood flow and coronary vascular resistance (-31 +/- 4% versus -7 +/- 3% [P < .001] and 53 +/- 12% versus 12 +/- 8% [P < .02], respectively).

CONCLUSIONS

The present study demonstrates an attenuated coronary vasoconstrictor response to ET-1 with an enhanced vasoconstrictor response to sarafotoxin and suggests an alteration in coronary ET receptor sensitivity in experimental heart failure.

Selective and non-selective ET antagonists reveal an ETB receptors mediated ET-1-induced behavioural effect in conscious rats

The injection of endothelin-1 (ET-1) (1 pmol/rat) into the dorsolateral periaqueductal gray (PAG) area of freely moving rats induced rotation along the long axis of the body (barrel-rolling). Preinjection (10 min before) of BQ-788 (an ETB receptor selective antagonist; 5 nmol) or bosentan (an ETA/ETB receptor non-selective antagonist; 10 nmol) to the PAG reduced the behavioral response to ET-1. In contrast, pretreatment with FR139317 (an ETA receptor selective antagonist; 5 nmol) did not affect the ET-1-induced barrel-rolling. These results suggest that barrel-rolling induced by microinjection of ET-1 into the PAG area is predominantly mediated via ETB-like receptors.

The distribution and density of receptor subtypes for endothelin-1 in peripheral lung of the rat, guinea-pig and pig

1. Quantitative autoradiographic studies were conducted to determine the distributions and densities of endothelin-A (ETA) and ETB receptor subtypes in peripheral lung alveolar wall tissue of the rat, guinea-pig and pig, with a view to assessing the potential suitability of these tissues as models for investigations of ET receptor function in human alveolar tissue. 2. High levels of specific [125I]-ET-1 binding were detected in peripheral lung components from all three species tested. In mature porcine alveolar wall tissue, specific binding increased in a time-dependent manner to a plateau, consistent with the previously described pseudo-irreversible binding of this ligand to a finite population of specific binding sites. 3. [125I]-ET-1 was associated specifically with both ETA and ETB binding site subtypes in alveolar wall tissue of foetal pig lung as early as 36 days gestation, raising the possibility of a functional role for ET-1 in lung development. In addition, both ETA and ETB binding site subtypes were detected in alveolar wall tissue and in peripheral airway smooth muscle of mature lung parenchyma from all three species. However, the binding subtype proportions differed in these tissues. For example, in porcine peripheral bronchial smooth muscle, ETA sites apparently predominated, whereas ETB sites constituted the major subtype detected in alveolar wall in this species. These data suggest significant shifts in ET receptor subtype expression at different levels in the respiratory tract. 4. ET binding site subtype proportions in the alveolar wall also differed markedly between species. In rat lung alveoli, ETA and ETB sites were detected in similar proportions (52 +/- 3% and 43 +/- 5% respectively). In contrast, in guinea-pig peripheral lung, ETB binding sites clearly predominated, constituting approximately 80% of total specific binding, with ETA sites accounting for only 12%. Porcine alveolar wall tissue also contained a mixture of these ET receptor subtypes, with ETA and ETB binding comprising 23 +/- 3% and 65 +/- 1% respectively of the total population of specific binding sites detected. These latter proportions are similar to values previously obtained in human peripheral lung tissue, suggesting that porcine lung might be a useful model of the human peripheral lung in subsequent studies of the functions of these pulmonary ET receptor subtypes.

Role of endothelin in mediating neurogenic plasma extravasation in rat dura mater

In addition to their potent vasoconstrictor properties, the endothelins (endothelin-1 and -3) may possess neurotransmitter/neuromediator and neuroendocrine actions. The aim of the present study was to evaluate the role of endothelins (ET) in mediating neurogenic inflammation of cephalic tissues in the rat. For this purpose, bosentan, a specific non-peptide mixed antagonist of ET receptors, was tested in rat models of neurogenic and non-neurogenic plasma extravasation in the dura mater and extracranial tissues (eyelid, conjunctiva, lip, tongue). Bosentan was effective for preventing neurogenic inflammation in the dura mater induced by unilateral electrical stimulation of the trigeminal ganglion or intravenous injection of capsaicin, whereas it was ineffective in extracranial tissues or after injection of substance P (non-neurogenic inflammation). The effect of nerve fiber stimulation on ET plasma concentrations in superior sagittal sinus was measured using selective radioimmunoassays for ET-1 and -3. Endothelin-3 concentration significantly increased after intravenous injection of capsaicin, whereas ET-1 levels remained unchanged. Competition binding assays on microsomal membranes from the trigeminal ganglion revealed a single class of binding sites with equal affinity for ET-1 and ET-3, suggesting a homogenous population of ETB receptors. The role of ETB receptors in mediating inflammation was evidenced by the lack of efficacy of a selective ETA receptor antagonist, in contrast to the full efficacy of a selective ETB receptor antagonist, for preventing neurogenic inflammation induced by unilateral stimulation of the trigeminal ganglion. The role of ETB receptors was finally confirmed by the observation that exogenous administration of the ETB receptor agonist sarafotoxin S6c also induced plasma protein extravasation in the dura mater. This extravasation was not a direct effect of ETB receptor stimulation, because it was inhibited by spantide, a selective tachykinin receptor antagonist. These data strongly suggest that ET, acting through ETB receptors, may play an important role in mediating neurogenic inflammation in the meninges of rats. Since the profile of activity of bosentan is similar to that of the 5-HT1D/B agonists, sumatriptan and ergot alkaloids, one may speculate that ET receptor antagonists might be potentially effective in the treatment of acute migraine attacks.

Influence of regional differences in ETA and ETB receptor subtype proportions on endothelin-1-induced contractions in porcine isolated trachea and bronchus

1. Quantitative autoradiographic studies were conducted to determine the distributions and densities of ETA and ETB binding site subtypes in porcine tracheal and bronchial smooth muscle. In addition, the roles of ETA and ETB receptors in endothelin-1-mediated contraction of these tissues were assessed. 2. Quantitative autoradiographic studies revealed that both ETA and ETB binding sites for [125I]-endothelin-1 were present in both bronchial and tracheal airway smooth muscle. However, the proportions of these sites were markedly different at these two levels within the respiratory tract. In tracheal smooth muscle, the proportions of ETA and ETB sites were 30 +/- 1% and 70 +/- 1% respectively, whereas in bronchial smooth muscle, these proportions were virtually reversed, being 73 +/- 2% and 32 +/- 8% respectively. 3. Endothelin-1 induced concentration-dependent contraction of porcine tracheal and bronchial airway smooth muscle. Endothelin-1 had similar potency (concentration producing 30% of the maximum carbachol contraction, Cmax) in trachea (22 nM; 95% confidence limits (c.l.), 9-55 nM; n = 9) and bronchus (22 nM; c.l., 9-55 nM; n = 6). Endothelin-1 also produced comparable maximal contractions in trachea (59 +/- 5% Cmax; n = 9) and bronchus (65 +/- 4% Cmax, n = 6). 4. In trachea, endothelin-1 induced contractions were not significantly inhibited by either the ETA receptor-selective antagonist, BQ-123 (3 microM) or the ETB receptor-selective antagonist, BQ-788 (1 microM). However, in the combined presence of BQ-123 and BQ-788, the concentration-effect curve to endothelin-1 was shifted to the right by 3.7 fold (n = 8; P = 0.01). 5. In bronchus, concentration-effect curves to endothelin-1 were shifted to the right by BQ-123 (3 microM; 4.3 fold; P < 0.05), but not by BQ-788 (1 microM). In the presence of both antagonists, concentration-effect curves to endothelin-1 were shifted by at least 6.7 fold (n = 6; P = 0.01). 6. Sarafotoxin S6c induced contraction in both tissue types, although the maximum contraction was greater in trachea (53 +/- 7% Cmax; n = 6) than in bronchus (21 +/- 5% Cmax; n = 6). BQ-788 (1 microM) markedly reduced sarafotoxin S6c potency in both trachea and bronchus (e.g. by 50 fold in trachea; c.l., 14-180; n = 6; P < 0.05). 7. These data demonstrate that the proportions of functional endothelin receptor subtypes mediating contraction of airway smooth muscle to endothelin-1, vary significantly at different levels in the porcine respiratory tract.

Cardiovascular responses mediated by two types of endothelin ET(B) receptor in spontaneously hypertensive and Wistar-Kyoto rats

This study shows the effects of a selective endothelin ET(B) receptor agonist, IRL 1720 {Ac-[Ala11,15]endothelin-1-(8-21)}, on cardiovascular responses in anesthetized spontaneously hypertensive rats and Wistar-Kyoto rats. Single intravenous bolus injection of IRL 1720 caused a dose-related short-lasting fall in blood pressure, left ventricular pressure and myocardial contractility. However, repeated intravenous bolus injection of 10(-5) mol/kg IRL 1720 produced a biphasic response consisting of an initial short-lasting decrease followed by a sustained increase in these parameters. The initial decrease was reduced, whereas the following increase was enhanced with the repeated injections of IRL 1720. The cardiovascular pressor response was not inhibited by the endothelin ET(A) receptor antagonist, FR139317 ((R)2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]carbonyl] amino-4-+methylpentanoyl] amino-3-[3-(1-methyl-1H-indolyl)]propionyl]amino -3- (2-pyridyl)propionic acid). The effects of IRL 1720 were qualitatively similar but more potent in spontaneously hypertensive rats than in Wistar-Kyoto rats. These results suggest the existence of two types of endothelin ET(B) receptor for IRL 1720: a tachyphylactic endothelin ET(B) receptor that mediates cardiovascular depressor responses and a less tachyphylactic endothelin ET(B) receptor that mediates pressor responses in the rat.

Pharmacological characterization of the ETA receptor in the vascular smooth muscle comparing its analogous distribution in the rat mesenteric artery and in the arterial mesenteric bed

The potency of ET-1, ET-2, and ET-3 to contract the isolated perfused rat arterial mesenteric bed was 2.73 +/- 0.57, 1.63 +/- 0.32, and 144 +/- 30 nM, respectively. The vasomotor effect of the ETs was slow in onset, persistent but reversible. Sarafotoxin S6b mimicked the ETs with a potency twofold lower than ET-1; sarafotoxin S6c and the C-terminal hexapeptide of ET-1 was inactive. ETH agonists such as IRL-1620 and AGETB-89 were inactive as vasoconstrictors within the range of concentrations examined. Minor chemical modifications of ET-1 amino acids residues in position 7 or 21 decreased significantly the peptide potency; ET-1 analogues with one or none of the disulfide bonds resulted inactive. The vasomotor effect of ETs was blocked in a competitive, reversible, and selective manner by FR 139317 and BQ-123, the latter being about threefold less potent than the former antagonist. The potency of FR 139317 was 20-fold higher to antagonize ET-3 than ET-1, and threefold higher to block ET-2 than ET-1. In strict analogy to FR 139317, BQ-123 was 12-fold more potent to antagonize ET-3 than ET-1, and fourfold more potent to antagonize ET-2 than ET-1. Upon removal of the endothelial cell layer, the vasomotor potency of ET-1 or the antagonist potency of FR 139317 remained unaltered, suggesting that the vasomotor receptors are localized in the arterial smooth muscles. The ET-1-induced vasomotor responses desensitized, an effect not crossed to noradrenaline (NA); perfusion with 10 microM indomethacin did not alter the vasomotor potency of ET-1, excluding the participation of eicosanoids in the arteriolar effects of ET-1. In isolated rings of the rat mesenteric artery, set to record isometric contractions of the circular muscular layer, the potency of the ETs and their structural analogues was as follows; ET-2 = ET-1 = sarafotoxin S6b > ET-3 > sarafotoxin S6c. The C-terminal hexapeptide of ET-1 and [Ala 1,3,11,15]ET-1 were inactive. The ET-1-induced vasoconstriction was antagonized in a concentration-dependent fashion by FR 139317. These results allow to conclude that the ETA receptors present in the arterial mesenteric circulation are localized in the vascular smooth muscle of the large-sized arteries as well as the smaller arterioles and precapillary vessels of the rat arterial mesenteric bed.

Pharmacological characterization of endothelin receptors in the rabbit iris sphincter muscle: suggestion for the presence of atypical receptors

Pharmacological profiles of endothelin (ET) receptors in the isolated rabbit iris sphincter were characterized. ET isopeptides caused dose-dependent contraction of the preparation. The respective EC50 values for ET-1, ET-2 and ET-3 were 39.4, 58.0 and 84.3 nM, so that ET-1 was twice as potent as ET-3. Sarafotoxin (SRTX) -b, an ET(A)/ET(B) non-selective agonist, caused very potent contraction with an EC50 of 1.13 nM. However, selective ET(B) receptor agonists SRTX-c and IRL 1620 showed no contractile activity up to 1 microM. BQ-123, a selective ET(A) receptor antagonist, shifted the dose-response curves of ET isopeptides to the right. The pA2 value for ET-1 was 5.52 with a slope of 1.06, which is not different from unity, and the pK(B) value for ET-2 was 5.06. Interestingly, very low doses of BQ-123 antagonized responses to ET-3 and SRTX-b, with a Schild plot slope of approximately 0.7 which is significantly different from unity, suggesting receptor heterogeneity. The abscissal intercepts of the Schild plots were -9.29 for ET-3 and -8.53 for SRTX-b. FR 139317, another ET(A) receptor antagonist, also preferentially antagonized responses to ET-3. RES-701-1, a selective ET(B) receptor antagonist, did not shift dose-response curves for ET-1 and ET-3. These results suggest that ET receptors in the rabbit iris sphincter muscle cannot be classified into the ET(A), ET(B) or ET(C) receptor subtypes, so far established. When compared to the established receptor subtypes, ET receptors in this preparation were quite different from the ET(B) receptor, but apparently showed a pharmacological profile most similar to the ET(A) receptor, suggesting the presence of heterogeneous and atypical ET(A) receptors.

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.

In vitro studies of the subtypes of endothelin (ET) receptors present in the rat testis, and of their involvement in the secretory response of Leydig cells to ET-1

The distribution of the endothelin (ET)-receptor subtypes ET(A) and ET(B) in the rat testis and their involvement in the secretory response of Leydig cells to ET-1 have been investigated by the use of specific ligands. Autoradiography showed that [125I]ET-1 binding was intense in the interstitial area of the testis, containing Leydig cells, and virtually absent in the walls of seminiferous tubules. Labelling was almost completely displaced by BQ-123, a selective ETA receptor antagonist, while sarafotoxin-6C and BQ-788, two specific ET(B) ligands, were ineffective. ET-1 concentration-dependently enhanced testosterone secretion of dispersed rat Leydig cells, and the response was suppressed by BQ-123, but not by BQ-788. Both antagonists per se did not affect either basal and hCG stimulated secretion of Leydig cells. Taken together our findings indicate that rat Leydig cells are mainly, provided with ETA, and that this ET-receptor subtype mediates their secretory response to ET-1.

Distribution of endothelin-1-receptor subtypes in rat portal vein

Endothelin-1 (ET-1), a potent vasoactive peptide, was first isolated from cultured porcine endothelial cells. Subsequent studies revealed the existence of two additional related peptides, ET-2 and ET-3, and at least two distinct ET-receptor subtypes, ETA (selective for ET-1) and ETB (nonselective for ET isopeptides). These isopeptides and receptors are widely distributed in many tissues and are involved in numerous biological responses. The aim of this study was to identify the eventual distribution of the two distinct endothelin-receptor subtypes in isolated endothelium-denuded rat portal vein rings (PVRs) and strips (PVSs). BQ-123 (0.6, 1, and 6 microM) and PD-145065 (0.06, 0.1, 0.6, and 6 microM) were used to differentiate the subtypes because they are selective antagonists for ETA and nonselective antagonists for ETA-ETB receptors, respectively. To characterize the ET receptors further, sarafotoxin-S6c (a selective ETB-receptor agonist) and IRL-1038 (a selective ETB-receptor antagonist) were used. In PVRs, cumulative additions of ET-1 (0.1-100 nM) caused graded and slow contractions and potentiated spontaneous rhythmic contractions. The EC50 values and maximal response to 100 nM of ET-1 were 2.72 nM and 0.75 g, respectively (n = 7). PVSs showed ET-1 EC50 values very similar to those of PVRs, but Emax values to 100 nM of ET-1 were significantly lower (Emax = 0.33 g; n = 7). Moreover, ET-1 clearly increased the amplitude and frequency of spontaneous contractions in both types of specimens, although these were greater in the PVSs. Thirty-minute incubation with the selective ETA-receptor antagonist BQ-123 blunted ET-1-induced effects in PVS specimens but only weakly antagonized ET-1-induced contractions in PVRs. In contrast, the nonselective ETA-ETB-receptor antagonist PD-145065 significantly shifted the ET-1 concentration-response curve to the right in PVRs and partially inhibited ET-1 effects in PVSs. Moreover, sarafotoxin-S6c (0.1-100 nM) contracted PVRs and PVSs in a similar manner to ET-1; its effects were antagonized by IRL-1038 only at the PVR level. The differences observed in PVR and PVS specimens in response to agonists and antagonists of ET confirmed the great heterogeneity of endothelin-sarafotoxin receptors. In our experimental models, functionally ETB-like (or non-ETA) receptors seem mostly to mediate vasoconstriction.

Identification of receptor binding and activation sites in endothelin-1 by use of site-directed mutagenesis

This study addresses the structural requirements for the intracellular processing and receptor binding properties of endothelin-1 (ET-1). Point mutants of preproendothelin-1 cDNA, with replacement of the codons for Lys9 of ET-1 by ones for Ala and Glu and of Ile20 and Trp21 by ones encoding Ala, were expressed in COS-7 cells. Competitive binding experiments on rat vascular smooth muscle cells (A-10), which were shown to be an ETA receptor-rich cell line, between [125I]ET-1 and synthetic ET-1, wild-type recombinant ET-1, and recombinant [Ala9]ET-1, [Glu9]ET-1, [Ala20]ET-1, and [Ala21]ET-1 yielded Ki values of 0.2 +/- 0.02, 0.2 +/- 0.02, 0.04 +/- 0.01, 1.4 +/- 0.2, 1.6 +/- 0.2, and > 50 nmol/L, respectively. In similar experiments with ETB receptor-rich human Girardi heart cells, the corresponding values were 0.2 +/- 0.03, 0.2 +/- 0.03, 0.2 +/- 0.04, 0.2 +/- 0.06, 1.4 +/- 0.4, and > 50 nmol/L. The ETA receptor-mediated contractile responses to [Glu9]ET-1 and [Ala20]ET-1, measured by using canine coronary artery rings, were decreased approximately fourfold to fivefold compared with the response produced by synthetic or wild-type recombinant ET-1, whereas [Ala9]ET-1 was found to be more potent, and [Ala21]ET-1 did not produce any contraction. These results demonstrate that Ile20 and Trp21 are involved in binding to both receptor subtypes. Of considerable interest was the observation that [Glu9]ET-1 also blunts the ETA receptor subtype-mediated contractile response to ET-1 stimulus.

Endothelin subtype B receptor-mediated calcium and contractile responses in small arteries of hypertensive rats

Endothelin-1 elicits vasoconstrictor responses through endothelin subtype A receptors, which are located on vascular smooth muscle cells, and vasodilator responses through endothelin subtype B receptors, which occur predominantly on endothelial cells. Endothelin subtype B receptors also may be present on vascular smooth muscle cells, in which they may mediate vasoconstriction. The aims of this study were to determine the presence of vascular smooth muscle vasoconstrictor endothelin subtype B receptors in mesenteric resistance arteries and to assess whether endothelin subtype B receptor-mediated responses differ between spontaneously hypertensive rats and Wistar-Kyoto rats. Contractile responses to the endothelin subtype B receptor agonist sarafotoxin S6c and endothelin-1 were measured simultaneously with [Ca2+]i in endothelium-denuded mesenteric resistance arteries from adult spontaneously hypertensive rats and Wistar-Kyoto rats. To simulate in vivo conditions matched as closely as possible to in vitro conditions, vessels were mounted in a vessel flow chamber in which intraluminal pressure was maintained at 60 mm Hg. Contraction was determined by video imaging to record lumen diameter, and [Ca2+]i was measured by the fura 2 method. Basal [Ca2+]i was significantly higher (P < .01) in hypertensive (170 +/- 4 nmol/L) compared with normotensive rats (134 +/- nmol/L). The endothelin subtype B receptor agonist sarafotoxin S6c increased [Ca2+]i in a concentration-dependent manner. Sarafotoxin S6c-induced [Ca2+]i and contractile responses were significantly lower in hypertensive compared with normotensive rats. These data demonstrate that endothelin subtype B receptors are present in vascular smooth muscle of small arteries and that endothelin subtype B receptor-mediated vasoconstriction occurs through intracellular calcium signaling pathways.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin at pathophysiological concentrations mediates coronary vasoconstriction via the endothelin-A receptor

BACKGROUND

Endothelin-1 (ET-1) is an endothelium-derived vasoconstrictor peptide. Controversy persists regarding the predominant ET receptor that mediates coronary vasoconstriction at pathophysiological concentrations. The aim of the present study was to test the hypothesis that ET mediates local coronary vasoconstriction via the ET-A receptor at low concentrations of exogenous ET-1 designed to mimic pathophysiological states compared with pharmacological concentrations.

METHODS AND RESULTS

ET-1 (group 1, n = 5) or sarafotoxin, a specific ET-B receptor agonist (group 3, n = 6) (each at 2 ng/kg per minute), was infused into the left circumflex coronary artery in the anesthetized dog. In group 2 dogs (n = 5), the same dose of ET-1 was infused with 4 micrograms/kg per minute of the specific ET-A receptor antagonist FR-139317. In group 4 (n = 5), the same dose of sarafotoxin was infused with 50 micrograms/kg per minute of the specific inhibitor of nitric oxide formation, NG-monomethyl-L-arginine (L-NMMA). No difference in hemodynamics, coronary blood flow (CBF), coronary vascular resistance (CVR), or coronary artery diameter (CAD) was observed at baseline between the groups. In group 1, intracoronary ET-1 significantly decreased CBF and CAD in association with an increase in CVR. The percentage decrease in CBF and CAD in the group that received ET-1 and the ET-A receptor antagonist (group 2) was significantly less than that in the group that received ET-1 alone (group 1) (-12 +/- 3% versus -48 +/- 6% [P < .001] and -4.6 +/- 0.8 versus 1.0 +/- 0.3 [P < .05], respectively). The administration of the ET-A receptor antagonist (group 2) abolished the ET-mediated increase in CVR (7 +/- 5% versus 105 +/- 21%, P < .005). There was no significant effect on CBF, CVR, or CAD in the group receiving sarafotoxin alone (group 3). The administration of L-NMMA and sarafotoxin (group 3). The administration of L-NMMA and sarafotoxin (group 4) resulted in a significant percentage decrease in CBF compared with the group that received sarafotoxin alone (-28 +/- 7% versus -8 +/- 2% [P < .05]).

CONCLUSIONS

The present study demonstrates that low concentrations of exogenous ET-1, which may mimic pathophysiological concentrations, result in coronary vasoconstriction mediated predominantly via the ET-A receptor because such vasoconstriction is significantly attenuated by blockade with FR-139317. The ET-B receptor may have a dual vasoconstrictive and vasodilatory effect.

Cardiac and renal endothelin-1 binding sites in streptozotocin-induced diabetic rats

The aim of this work was to study cardiac and renal endothelin binding sites during the progression of diabetes. Male Crl:CD (BR) rats were made diabetic by injection of streptozotocin (STZ, 45 mg kg-1 i.v.). Only rats with a glycaemia of 500 mg per 100 ml or higher, were used. The hearts were taken at 2, 4 or 6 weeks and kidneys at 2 and 6 weeks, after diabetes induction, for binding studies. In the heart, the number of Et-1 binding sites was significantly increased 2 weeks after STZ-induction of diabetes (449 +/- 13 vs. 345 +/- 18 fmol (mg protein) -1, in controls; p < 0.05) without modification of KD value (104 +/- 5 vs 101 +/- 7 pM). Comparable results were obtained 4 and 6 weeks after STZ-induction. In the kidney both the parameters were unchanged at all the times tested.

IN CONCLUSION

a specific increase in cardiac Et-1 binding sites, without change in affinity of the peptide, was found 2, 4 and 6 weeks after diabetes induction; while renal Et-1 binding sites were not modified.

Prostacyclin synthesis elicited by endothelin-1 in rat aorta is mediated by an ETA receptor via influx of calcium and is independent of protein kinase C

The purpose of this study was to characterize the receptor(s) and second messenger systems involved in prostacyclin (prostaglandin [PG] I2) synthesis elicited by endothelin (ET)-1 in the rat aorta. PGI2 synthesis, measured as immunoreactive 6-keto-PGF1 alpha, was assessed in aortic rings exposed to endothelin receptor agonists in the presence and absence of selective ETA and ETB receptor antagonists. ET-1, which has equal affinity for both endothelin receptor subtypes, and ET-3, a preferential ETB receptor agonist, enhanced 6-keto-PGF1 alpha synthesis in a time- and concentration-dependent manner. ET-1 was more potent than ET-3 in increasing 6-keto-PGF1 alpha synthesis. Moreover, the selective ETB receptor agonists IRL-1620 and sarafotoxin S6c did not significantly increase 6-keto-PGF1 alpha synthesis. Furthermore, ET-1-induced 6-keto-PGF1 alpha synthesis was attenuated by an ETA receptor antagonist, BQ-123, in a dose-dependent manner but not by an ETB receptor antagonist, BQ-788. Depletion of extracellular Ca2+ or addition of Ca2+ channel blockers (nifedipine, verapamil, SK&F 96365) attenuated ET-1-mediated 6-keto-PGF1 alpha synthesis, while a Ca2+ channel agonist, S(-)-Bay K 8644, potentiated this effect of ET-1. Selective protein kinase C inhibitors (bisindolylmaleimide I, calphostin C) did not alter ET-1-induced 6-keto-PGF1 alpha synthesis. These data suggest that PGI2 synthesis elicited by ET-1 in the rat aorta is mediated primarily through influx of extracellular Ca2+ via activation of an ETA receptor and is independent of protein kinase C.

Identification of ETB receptor subtypes using linear and truncated analogs of ET

Using canine spleen and lung membranes as model systems, we have shown the presence of subtypes of ETB receptors. This classification was done based on the binding profiles of various analogs of ET-1 that have been identified as ETB-selective. Saturation binding experiments performed with [125I] ET-3 and [125I] IRL-1260 (ETB-selective ligands) indicated that [125I] IRL-1620 labeled 80-90% of [125I] ET-3 binding sites in canine lung, whereas in canine spleen, the binding of [125I] IRL-1620 was 10-20% of [125I] ET-3 binding. In addition, competition binding experiments using ETB-selective agonists [Ala1,3,11,15]-ET-1 (also known as 4-Ala ET-1), N-acetyl-[Ala11,15] ET-1 (6-21) also known as BQ3020 and Suc-[Glu9, Ala11,15] ET-1 (8-21) also known as IRL-1620 indicated that all three ligands displaced [125I] ET-3 from canine lung membranes with approximately 500-1000 fold greater affinity than canine spleen membranes. On the other hand, ET-1, ET-3, S6a, S6b, and S6d displayed very similar IC50 values in both preparations, except S6c which was approximately 20 fold less potent in canine spleen compared to lung. These data indicate that ETB receptors present in canine spleen are different from those present in lung and that ETB-selective linear as well as truncated analogs of ET-1 are good tools to identify these subtypes of ETB receptors.

Heterogeneous endothelin receptors mediate relaxation and contraction in the guinea-pig ileum

IRL1620, a specific endothelin ETB receptor agonist, induced relaxation followed by contraction in the guinea-pig ileum, as did endothelin-1. Both components of the response were concentration-dependent in the range studied. Repeated administration of IRL1620 induced tachyphylaxis only of the contractile component, whereas endothelin-1 desensitized both components. BQ-123 (cyclo[D-Trp-D-Asp-Pro-D-Val-Leu]), a specific endothelin ETA receptor antagonist, did not inhibit the relaxation induced by either agonist, although it did inhibit the contraction induced by endothelin-1, but not by IRL1620. PD145065 (Ac-(D-Bhg-Leu-Asp-Ile-Ile-Trp) (D-Bhg = 5H-dibenzyl[a,d]cycloheptene-10,11-dihydroglycine)), a combined endothelin ETA/endothelin ETB receptor antagonist, inhibited the contractile effects of both endothelin-1 and IRL1620 and also inhibited the relaxation induced by IRL1620. Apamin, a Ca(2+)-activated K+ channel blocker, inhibited only the endothelin-1-induced relaxation. Our studies suggest that two endothelin ETB receptor subtypes mediate relaxation in the guinea-pig ileum: one is less sensitive to PD145065 but apamin-inhibitable, and the other is more sensitive to PD145065 but not apamin-inhibitable. Our results also suggest that both endothelin ETA and endothelin ETB receptor subtypes mediate contraction in the ileum.

Effects of endothelins on renin secretion from rat kidneys

Using a preparation of isolated rat kidneys perfused at constant renal artery pressure (80 mmHG) we investigated the role of endothelins in the regulation of renin release. Addition of three related endothelins (ET-1, ET-2, ET-3) at a concentration of 10 pmol L(-1) tended to enhance renin secretion rates. Higher doses (100 pmol L(-1), 1 nmol L(-1)) of different ETs such as the selective ETB receptor agonist sarafotoxin S6c (100 pmol L(-1), 1 nmol L(-1)) inhibited renin release and increased renal vascular resistance with similar potency. These effects of ETs were blunted when calcium ions were removed from the perfusate. Renin release activated by isoproterenol (10 nmol L(-1)) was also significantly reduced with ET-1, -2 and -3 (1 nmol L(-1)). BQ-123 (500 nmol L(-1)), a selective ETA receptor antagonist, only attenuated, whilst the non-selective ET receptor blocker bosentan (Ro 47-0203, 10 micro mol L(-1)) almost abolished the renal vasopressor and renin inhibitory action of ET-1 and sarafotoxin S6c. BQ-123 and bosentan alone did not affect either perfusate flow or basal renin secretion rates in isolated perfused kidneys. These findings indicate that all three ET peptides equipotently inhibit renin secretion from the kidneys. Most of the vasopressor and renin inhibitory effect of ETs is mediated by ETB rather than ETA receptors involving a calcium-dependent signal transduction mechanism. Moreover, our results suggest that intrarenally released ETs do not contribute to the regulation of renin secretion from isolated perfused rat kidneys.

The role of intracellular calcium and protein kinase C in endothelin-stimulated proliferation of rat type I astrocytes

The increased expression of immunoreactive endothelin-1 (ET-1) in reactive astrocytes and its mitogenic effects on astrocytes and glioma cell lines, have implicated endothelins in the development of reactive gliosis. In this study, an increase in DNA synthesis in rat type I astrocytes was observed after cultures were transiently exposed to ET-1 for 15 min, suggesting that early signal transduction events are essential and sufficient for the propagation of the ET-1-induced mitogenic signal. Prompt increases in inositol triphosphate (IP3) formation and [Ca2+]i were observed upon the addition of ET-1 to these cells. The ET-1-evoked increase in [Ca2+]i consisted of an initial peak which was preserved in Ca(2+)-free medium, and a sustained phase which was abolished in Ca(2+)-free medium and partly attenuated by nifedipine. ET-1 also increased the activity of membrane-associated protein kinase C (PKC) and induced the in vivo phosphorylation of the 85 kD MARCKS protein, an endogenous PKC-specific substrate. The ET-1-evoked increases in DNA synthesis, IP3, [Ca2+]i, membrane PKC, and 85 kD MARCKS protein phosphorylation in rat cortical astrocytes were prevented by either the selective endothelin ETA receptor antagonist, BQ-123, or the phospholipase C (PLC)-specific inhibitor, U-73122. However, the inhibition of PKC activity did not affect ET-1-induced DNA synthesis in rat cortical astrocytes. These results suggest that ET-1-induced IP3 and/or [CA2+]i responses, but not the activation of PKC, are essential for the growth-factor like actions of ET-1 in rat cortical astrocytes.

Endothelin-1 induces gene expression through stimulation of endothelin type A receptors in normal rat kidney cells

RNA blots of total cellular RNA isolated from quiescent and endothelin (ET-1)-stimulated normal rat kidney (NRK) cells demonstrated that ET-1 induced the expression of c-jun, jun B, and c-fos mRNA in a time-dependent manner with maximal expression of mRNA by 1 hr after the addition of ET-1. Five hundred picomolar ET-1 was sufficient to induce maximal mRNA expression. These data agreed with saturation experiments which demonstrated that maximal binding of [125I]ET-1 was achieved at concentrations greater than 100 pM. The Kd and Bmax values for [125I]ET-1 binding to NRK membranes were 20.5 pM and 22.2 fmol/mg protein, respectively. Competition experiments for the binding of [125I]ET-1 to NRK membranes demonstrated that ET-1 was a more potent inhibitor (Ki = 0.047 nM) than ET-3 (Ki = 10.8 nM). No specific binding of [125I]ET-3 (40 or 500 pM) to NRK membranes could be observed. The expression of c-jun, jun B, and c-fos mRNA was inhibited by the endothelin type A receptor (ET)-selective antagonist, BQ-123. Thus, these data demonstrate that ET-1 mediates the expression of immediate response gene mRNA in NRK cells via the ETA receptor. ET-1 stimulation of NRK cells also upregulated EGF receptors, providing a possible mechanism for ET-1 complementation of epidermal growth factor (EGF) mitogenicity in NRK cells.

Evaluation of influence of 24-hour cold preservation on endothelin production and on endothelin receptors in the bone vasculature

Endothelin-1 is a vasoactive peptide produced by the vascular endothelium. It is one of the most potent endogenous vascular smooth muscle constrictors. Two subtypes of the endothelin receptor have been cloned and sequenced and denoted endothelin-A and endothelin-B. The aim of this study was to define the influence of cold ischemia on the production of endothelin-1 and on the endothelin receptors. Two different preservation techniques (cold storage only and cold storage with microperfusion with University of Wisconsin solution) also were compared. The study was performed in an in vitro bone perfusion model to isolate the vascular endothelium from blood components. The production of endothelin-1 by the bone vasculature was not altered after 24 hours of cold ischemia. No contractions were observed with S6c, a selective endothelin-B agonist, and this effect was not influenced by cold ischemia. The response mediated by the endothelin-A receptor was increased significantly, an effect that was not influenced by preservation with University of Wisconsin solution. This latter finding was the only significant alteration in the vascular function detected in the in vitro model after 24 hours of cold ischemia. With regard to the pharmacologic properties of endothelin-1, this mediated response could be implicated in the pathogenesis of vasospasm.

Effects of the ETA/ETB receptor antagonist, bosentan on endothelin-1-induced myocardial ischaemia and oedema in the rat

1. The purpose of this study were to assess the role of ETB receptors in mediating endothelin-1 (ET-1)-induced myocardial ischaemia and oedema in rats and to study the inhibitory action of the novel nonpeptide ETA/ETB receptor antagonist, bosentan on these actions of ET-1. 2. Intravenous bolus injection of ET-1 (1 nmol kg-1) into anaesthetized rats produced marked ST segment elevation of the electrocardiogram without causing arrhythmias. ST segment elevation developed within 30-50 s and persisted for at least 30 min following injection of the peptide. 3. Pretreatment of the animals with bosentan (10 mg kg-1, i.v.) inhibited on average by 96% the ST segment elevation elicited by ET-1 (1 nmol kg-1) compared to the 82% inhibition observed with the ETA receptor-selective antagonist, FR 139317 (2.5 mg kg-1, i.v.). 4. Bolus injection of ET-1 (1 nmol kg-1, i.v.) to conscious chronically catheterized rats evoked a transient depressor response followed by a prolonged pressor effect. Corresponding to changes in blood pressure, a transient tachycardia and a sustained bradycardia were observed. ET-1 (1 nmol kg-1) enhanced albumin extravasation by 119 and 93% in the left ventricle and right atrium, respectively, as measured by the local extravascular accumulation of Evans blue dye. 5. Pretreatment of the animals with bosentan (10 mg kg-1) inhibited by 71 and 90% the depressor and pressor actions of ET-1 (1 nmol kg-1) and the accompanying tachycardia and bradycardia, respectively. FR 139317 (2.5 mg kg-1) attenuated the pressor response to ET-1 and accompanying bradycardia by 75%, without affecting the depressor action and accompanying tachycardia. ET-1-induced albumin extravasation was completely inhibited by bosentan (10 mg kg-1) both in the left ventricle and right atrium, compared to the 86% inhibition observed with FR 139317 (2.5 mg kg-1).6. Like ET-1, the ETB receptor-selective agonist, IRL 1620 (0.3 and 1 nmol kg-1, i.v.) also produced dose-dependent ST segment elevation in anaesthetized rats and enhanced albumin extravasation (up to141% of control) in the left ventricle and right atrium, respectively, in conscious rats. These effects ofIRL 1620 were completely prevented by bosentan (10 mg kg-1).7. These results indicate that ETB receptors, albeit to a lesser extent than ETA receptors, are also involved in mediating ET-1-induced myocardial ischaemia and oedema in the rat, and suggest the therapeutic potential for bosentan in the treatment of ischaemic myocardial diseases.

Developmental expression of endothelin receptors in cerebellar neurons differentiating in culture

We describe the identification and expression of endothelin (ET) receptor subtypes in differentiating cultured cerebellar neurons. Using [125I]ET-1 and the subtype-selective ligands BQ-123 and sarafotoxin 6c as selective ligands for the ETA and ETB receptors, respectively, we found that cerebellum from 8-day-old rats displayed only the ETB receptor subtype. We next cultivated cerebellar granule cell neurons to study ET receptor differentiation between 2 and 22 days in vitro. Using the above reagents, we found that while unlabeled ET-1 displayed monophasic competition curves, BQ-123 and sarafotoxin 6c displayed partial displacement curves, indicating the presence of both ETA and ETB receptors on these neurons. The proportion of ETB receptors gradually decreased from day 2 onwards the proportion of ETA receptors gradually increased. On days 2, 3, 4, and 5 of culture, the ETB:ETA receptor ratios were 90:10, 70:30, 60:40, and 40:60, respectively. There was no further change in receptor subtype ratio beyond day 5 and up to day 22. Northern blot analysis showed that ETB receptor message expression was 6.9-fold higher than that of ETA receptor expression on day 2, but steadily decreased with time, whereas ETA receptor message expression was minimal on day 2 and maximal by day 3 and 4. By day 7, receptor message was of equal abundance, which was in good agreement with the binding studies. This novel, developmentally regulated process predicts the existence of endogenous mediators of neuronal ET receptor expression.

Identification of endothelin receptor subtypes in rat retina using subtype-selective ligands

We investigate the existence of endothelin receptor subtypes using subtype selective ligands and the presence of immunoreactive (IR) endothelin (ET)-3 (IR-ET-3) by radioimmunoassay (RIA) in rat retina. Scatchard transformation of saturation binding experiments with [125I]ET-3 revealed specific binding sites with a Kd and Bmax values of 42 +/- 12 pM and 111 +/- 24 fmol/mg of protein, respectively. The Kd was similar to that obtain in previous studies using [125I]ET-1. However, the Bmax was 65% of that obtained with [125I]ET-1. Competitive experiments in the presence of the cyclic pentapeptide BQ123 (selective for ETA receptor) and Sarafotoxin 6C (selective for ETB receptor), demonstrated the existence of ETA and ETB receptors in a ratio of 35:65. The order of potency of ET family peptides was ET-3 = ET-1 > S6C for ETB receptor and ET-1 > ET-3 > BQ123 for ETA receptor. Cross-linking of [125I]ET-1 to retinal membranes with disuccinimidyl suberate and SDS-PAGE followed by autoradiography resulted in the labeling of two bands with apparent molecular masses of 52 and 34 kDa. Similar results were obtained using [125I]ET-3, suggesting that ETA and ETB receptors have similar molecular mass. The 34 kDa band is a proteolytic degradation product of the 52 kDa band. The concentration of IR-ET-3 was 1212 +/- 153 fmol/g wet weight in rat retina. All these data suggest that ETs may play a role in neurotransmission or neuromodulation in the retina, operating on both ETA and ETB receptor subtypes present in this tissue.

Endothelin ETA and ETB receptors cause vasoconstriction of human resistance and capacitance vessels in vivo

BACKGROUND

The role of endothelin ETB receptors in mediating vasoconstriction in humans is unclear. As yet, there have been no in vivo studies in resistance vessels, and in vitro data have been contradictory. We therefore investigated the function of ETB receptors in vivo in human forearm resistance and hand capacitance vessels using endothelin-1 as a nonselective agonist at ETA and ETB receptors and endothelin-3 and sarafotoxin S6c as selective agonists at the ETB receptor.

METHODS AND RESULTS

A series of single-blind studies were performed, each in six healthy men. Brachial artery infusion of endothelin-1 and endothelin-3 caused slow-onset dose-dependent forearm vasoconstriction. Although endothelin-3 caused significantly less forearm vasoconstriction than endothelin-1 at low doses, vasoconstriction was similar to the two isopeptides at the highest dose (60 pmol/min). Endothelin-3 caused transient forearm vasodilatation at this dose, whereas endothelin-1 showed only a nonsignificant trend toward causing early vasodilatation. Intra-arterial sarafotoxin S6c caused a progressive reduction in forearm blood flow, although less than that to endothelin-1 (P = .04). Dorsal hand vein infusion of sarafotoxin S6c caused local venoconstriction that was also less than that to endothelin-1 (P = .002).

CONCLUSIONS

Selective ETB receptor agonists cause constriction of forearm resistance and hand capacitance vessels in vivo in humans, suggesting that both ETA and ETB receptors mediate vasoconstriction. Hence, antagonists at both ETA and ETB receptors, or inhibitors of the generation of endothelin-1, may be necessary to completely prevent vasoconstriction to endogenously generated endothelin-1.

Endothelin-1 stimulates myristoylated alanine-rich C-kinase substrate (MARCKS) phosphorylation in rat cerebellar slices

Protein phosphorylation induced by endothelins has been studied using [32P]orthophosphate-prelabelled rat cerebellar slices. Endothelin-1 increased phosphorylation of an 87 kDa protein in a time-dependent manner (reaching a maximum effect at about 2.5 min) and with an EC50 equal to 93 +/- 32 nM. Endothelin-3 and sarafotoxin 6c induced similar levels of phosphorylation. Endothelin-1 also promoted [3H]inositol phosphate accumulation with similar EC50 (71 +/- 7.5 nM). The phosphoprotein of 87 kDa seems to be myristoylated alanine-rich C-kinase substrate (MARCKS) as demonstrated by acetic acid extraction. In addition, 12-O-tetradecanoylphorbol-13-acetate (TPA) increased 87 kDa protein phosphorylation while Ro-31-8220, a specific protein kinase inhibitor, inhibited both TPA and endothelin-induced 87 kDa protein phosphorylation. Therefore, it is concluded that protein kinase C is involved in the endothelin action on cerebellum.

The role of endothelin in experimental cerebral vasospasm

Endothelin (ET) may play a role in vasospasm after subarachnoid hemorrhage (SAH). The aim of our study was to test whether the systemic administration of bosentan, a nonpeptidic ET(A) and ETB receptor antagonist, could reverse vasospasm without inducing hypotension. In rabbits (single-hemorrhage model) and in dogs (double-hemorrhage model), SAH was induced; after vasospasm was established, the animals received intravenously either saline or a 30 mg/kg bolus of bosentan. The cross-sectional area of the basilar artery was analyzed by quantitative angiography. In rabbits (n = 13), bosentan reversed basilar vasospasm to the same extent as did an intravertebral injection of sodium nitroprusside. In dogs (n = 10), bosentan reversed only 52 +/- 10% of the vasospasm reversible by papaverine. Bosentan did not alter the heart rate or the arterial blood pressure in either the rabbits or the dogs. In the cerebrospinal fluid, SAH increased endothelin-1 (ET1) and big ET1 by 6 and 3.8 times, respectively; in the basilar artery, SAH increased ET1 concentration, big ET1 concentration, and ET-converting enzyme activity by 1.3, 2, and 2.7 times, respectively. In addition, a local involvement of ET was also suggested by the relaxing effect of bosentan on basilar artery rings from rabbits with SAH and not from control rabbits. Receptor binding studies performed on dog basilar arteries revealed a shift in the phenotype expression of ET receptors from the A to the B type after SAH. We conclude that ET plays a major role in SAH and that systemic ET blockade might selectively dilate spastic arteries.

Evidence for ETA and ETB receptors in rat skin and an investigation of their function in the cutaneous microvasculature

1. The relative contribution of ETA and ETB receptors in the response of rat skin to endothelins was investigated by use of the selective ETB agonist IRL-1620 and the selective ETA antagonist BQ-123. 2. Binding data suggest the presence of ETA and ETB receptors as preincubation with [Ala3,11,18Nle7]-endothelin-1 reduced ET-1 binding by approximately 40%. 3. Intradermal injection of endothelin-1 (ET-1, 1-10 pmol/site) and ET-3 (3-100 pmol/site) induced a dose-dependent decrease in local blood flow assessed by 133Xe clearance at test sites in rat skin. 4. The endothelin analogue [Ala3,11,18Nle7]-ET-1 (30-1000 pmol/site) induced significant vasoconstriction (P < 0.05) at the highest doses used and the selective ETB receptor agonist, IRL-1620 [Suc[Glu9,Ala11,15] endothelin (8-21)], (0.01-100 pmol/site) acted in a potent manner to induce a significant (P < 0.01) dose-dependent decrease in 133Xe clearance. 5. Co-injection with the selective ETA receptor antagonist, BQ-123 (1 nmol/site), completely abolished the vasoconstriction to ET-1 and partially to ET-3, but had no effect on IRL-1620-induced vasoconstriction. In addition, IRL-1620 responses were not altered at sites treated with submaximal doses of a nitric oxide synthase inhibitor or a prostaglandin synthase inhibitor. 6. ET-1 and IRL-1620 (100 fmol-1 pmol/site) did not induce oedema formation as measured by [125I]-albumin accumulation in the presence or absence of the vasodilator, calcitonin gene-related peptide (CGRP). ET-1 (1-3 pmol/site) inhibited substance P-induced oedema formation and this effect,suggested to be secondary to a vasoconstrictor effect, was significantly reversed by BQ-123 (1 nmol/site).7. The findings in this study indicate that there are ETA and ETB receptors in rat skin and agents which activate either receptor act to mediate a decrease in cutaneous blood flow, but have no effect on increased microvascular permeability.

Growth inhibitory properties of endothelin-1 in human hepatic myofibroblastic Ito cells. An endothelin B receptor-mediated pathway

Ito cells play a pivotal role in the development of liver fibrosis associated with chronic liver diseases. During this process, Ito cells acquire myofibroblastic features, proliferate, and synthesize fibrosis components. Considering the reported mitogenic properties of endothelin-1 (ET-1), we investigated its effects on the proliferation of human Ito cells in their myofibroblastic phenotype. Both ET receptor A (ETA: 20%) and ET receptor B (ETB: 80%) binding sites were identified, using a selective ETA antagonist, BQ 123, and a selective ETB agonist, sarafotoxin S6C (SRTX-C). ET-1 did not stimulate proliferation of myofibroblastic Ito cells. In contrast, ET-1 inhibited by 60% DNA synthesis and proliferation of cells stimulated with either human serum or platelet-derived growth factor -BB (PDGF-BB). PD 142893, a nonselective ETA/ETB antagonist totally blunted this effect. SRTX-C was as potent as ET-1, while BQ 123 did not affect ET-1-induced growth inhibition. Analysis of the intermediate steps leading to growth-inhibition by ET-1 revealed that activation of mitogen-activated protein kinase by serum or PDGF-BB was decreased by 50% in the presence of SRTX-C. In serum-stimulated cells, SRTX-C reduced c-jun mRNA expression by 50% whereas c-fos or krox 24 mRNA expression were not affected. We conclude that ET-1 binding to ETB receptors causes a potent growth inhibition of human myofibroblastic Ito cells, which suggests that this peptide could play a key role in the negative control of liver fibrogenesis. Our results also point out that, in addition to its well known promitogenic effects, ET-1 may also exert negative control of growth on specific cells.

Role of endothelin-1 in beagles with dehydromonocrotaline-induced pulmonary hypertension

BACKGROUND

Although plasma levels of endothelin-1 (ET-1) increase in patients with pulmonary hypertension (PH), its role in PH is unknown. We investigated the contribution of endogenous ET-1 to cardiopulmonary changes in beagles with dehydromonocrotaline (DMCT)-induced PH.

METHODS AND RESULTS

Eight 3-month-old beagles were given a single injection of 3 mg/kg DMCT via the right atrium. During the 8 weeks after injection, the mean pulmonary arterial pressure (PAP) and plasma ET-1 level increased significantly from 11.6 +/- 2.3 to 35.9 +/- 7.1 mm Hg and from 1.24 +/- 0.25 to 3.25 +/- 0.94 pg/mL, respectively. In controls, ET-1 infusion elevated the systemic arterial pressure (SAP) but did not alter PAP. In PH beagles, ET-1 infusion increased SAP, which was attenuated by FR139317 (an endothelin type [ET] A receptor antagonist), and produced a dose-dependent decrease in PAP, which was attenuated by RES-701-1 (an ETB receptor antagonist). In PH beagles, FR139317 infusion decreased PAP, and RES-701-1 infusion increased PAP. Sarafotoxin S6c (an ETB agonist) infusion decreased PAP in PH beagles.

CONCLUSIONS

These results suggest that endogenous ET-1 is elevated in PH disease and may mitigate PH by acting on ETB receptors.

Two different endothelin B receptor subtypes mediate contraction of the rabbit saphenous vein

To study endothelin receptor subtypes that mediate venous smooth muscle contraction, effects of some endothelin receptor agonists and antagonists on the rabbit lateral saphenous vein were examined and compared with those on the saphenous artery. In the artery, endothelin (ET)-1 elicited concentration-dependent contractions, while selective ETB-receptor agonists, IRL1620 (Suc-[Glu9,Ala11,15]ET-1(8-21)) and sarafotoxin 6c (S6c) had almost no effect. The ET-1-induced responses shifted in parallel to the right by BQ-123 (cyclo (-D-Trp-D-Asp-Pro-D-Val-Leu-)), an ETA-receptor antagonist, or PD142893 (Ac-D-Dip-Leu-Asp-Ile-Ile-Trp), an ETA/ETB-receptor antagonist, indicating the involvement of the ETA receptor in this response. In the saphenous vein, not only ET-1 and ET-3, but also ETB-receptor agonists, IRL1620, S6c and [Glu9]sarafotoxin 6b ([Glu9]S6b), produced concentration-dependent, BQ-123-insensitive contractions. PD142893 did not affect the ET-1-induced contraction, but it shifted greatly the IRL1620-induced concentration-response curve in parallel to the right. The major components of ET-3-, S6c- and [Glu9]S6b-induced contractions were resistant to PD142893. These results indicate that two different vasoconstrictive ETB-receptor subtypes, ETB1 (sensitive to IRL1620 and PD142893) and ETB2 (insensitive to IRL1620 and PD142893), are located on the smooth muscle of the saphenous vein.

Endothelin receptor in microvessels isolated from human meningiomas: quantification with radioluminography

1. We characterized specific 125I-endothelin-1 (125I-ET-1) binding sites in microvessels isolated from human meningiomas, using an in vitro quantitative receptor autoradiographic technique coupled to a radioluminographic imaging plate system. 2. This newly developed and highly sensitive method revealed high-affinity ET receptors present in pellet sections of the microvessels from all the meningiomas studied, regardless of histological subtypes (dissociation constant, 1.2 +/- 0.3 nM; maximum binding capacity, 185 +/- 56 fmol/mg; means +/- SE for nine tumors). 3. In five cases of meningiomas, ET-3 competed for 125I-ET-1 binding to microvessels from those tumors with a low affinity [50% inhibiting concentration (IC50) of 1.6 +/- 0.4 x 10(-6) M], and a selective ETB receptor agonist, sarafotoxin S6c, up to 10(-6) M, did not displace ET binding from the sections. 4. In the sections of microvessels from four other tumors, biphasic competition curves were obtained in the case of incubation in the presence of increasing concentrations of ET-3, with an IC50 of 1.1 +/- 0.2 x 10(-9) M for the high-affinity component and 1.6 +/- 0.3 x 10(-6) M for the low-affinity component, respectively. In addition, S6c competed for ET binding to those sections (IC50 = 2.3 +/- 0.2 x 10(-10) M) and 10(-6) M S6c displaced 30% of the control, corresponding to the high-affinity component of competition curves obtained in the presence of ET-3. 5. Our results suggest that (a) capillaries in human meningiomas express a large number of high-affinity ETA (non-ETB) receptors with a small proportion of ETB receptors, and (b) ET may have a role in neovascularization, tumor blood flow, and/or function of the blood-tumor barrier in meningioma tissues by interacting with specific receptors present on the surface of the endothelium.

Intraocular pressure response to intravitreal injection of endothelin-1 and the mediatory role of ETA receptor, ETB receptor, and cyclooxygenase products in rabbits

Endothelin-1 (ET-1) affects intraocular pressure (IOP) in rabbits. First, we studied IOP responses to the intravitreal injection of various doses of ET-1 ranging from 5 ng to 5 micrograms in unanesthetized rabbits, and observed a transient rise in IOP, from 0.5 to 2 h in duration, invariably followed by a prolonged IOP reduction, lasting for more than 72 h in rabbits treated with 0.5 microgram and 5 micrograms of ET-1. ET-1 (0.05 microgram and 0.15 microgram) resulted in a prolonged IOP reduction without an early IOP rise. Both IOP rise and reduction were significantly related to the dose of ET-1. A masked, randomized, study revealed that the intraperitoneal administration of indomethacin (50 mg/kg) prior to ET-1 injection significantly reduced the ocular hypertensive response, but not th ocular hypotensive response, to ET-1. The ETA receptor selective antagonist, 97-139 (155 micrograms) had no effect on IOP when used alone. However, when used in combination with 0.5 microgram of ET-1, 97-139 significantly inhibited both the IOP rise (0.5-2 h) and reduction (8-96 h) caused by ET-1. The ETB receptor selective agonist, sarafotoxin S6c (0.5 microgram), caused a sustained IOP reduction of 2 to 96 h in duration without the initial IOP rise. We also determined the concentration of prostaglandin (PG) E2 in the aqueous humor using radioimmunoassay techniques on samples obtained at 1 and 24 h after ET-1 injection, and examined the effects of pretreatment with indomethacin or 97-139 on PGE2 concentration.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of endothelins and sarafotoxin 6c in the rat isolated perfused lung

1. Endothelin (ET) receptors within the vasculature and airways were studied in a rat perfused lung model in which pulmonary perfusion pressure (PPP), pulmonary inflation pressure (PIP) and lung weight were continuously monitored. 2. The vascular potencies of ETs (ET-1 > ET-2 > ET-3) suggest an action via ETA receptors. This was confirmed by use of the antagonist, BQ123 (2 microM). The vasoconstrictor effects of sarafotoxin 6c (SX6C) also indicated the presence of ETB receptors. 3. Lung weight increases induced by ETs appeared to be a consequence of their vasoconstrictor potencies. The mixed ET receptor antagonist, bosentan (5 microM), markedly attenuated the responses of ET-1 and SX6C on PPP and lung weight, further implicating activation of both ETA and ETB receptors in these responses. 4. Endothelin-1 (ET-1) induced an accumulation of albumin-bound Evans blue dye in orthogradely perfused lungs. Retrograde perfusion attenuated the extravasation and increase in lung weight due to ET-1 but significantly augmented those induced by SX6C. 5. The bronchoconstrictor actions of ETs (ET-1 = ET-2 = ET-3) and SX6C suggest this is an ETB-mediated response. However SX6C was more potent than ETs and the dose-response curve was significantly steeper and achieved a higher maximum. 6. Indomethacin did not affect the vascular or bronchial responses to ET-1 or SX6C. 7. These findings indicate that rat pulmonary vasculature contains both ETA and ETB receptors. Retrograde perfusion suggests that ETB receptors are located arterially whereas ETA receptors are predominantly venous in distribution. Differences in the bronchoconstrictor potency of SX6C (compared to ETs) and the antagonism by bosentan may indicate ETB receptor heterogeneity in the airways.

Role of positions 9 and 10 in the endothelin molecule for biological activity and discrimination of receptor subtypes

The importance of residues 9 and 10 in endothelin-1 was assessed by studying the responses of the guinea-pig ileum to [Ala9]endothelin-1 and [Ala10]endothelin-1. Both analogues induced relaxation followed by contraction. [Ala9]Endothelin-1 showed similar ED50 values and maximum response to those of endothelin-1, whereas [Ala10]endothelin-1 showed a larger ED50 value and was a partial agonist. Endothelin-1 and [Ala10]endothelin-1 induced similar degrees of tachyphylaxis, whereas [Ala9]endothelin-1 induced very little tachyphylaxis, indicating that Lys9 is important for inducing tachyphylaxis. Apamin inhibited the relaxation induced by endothelin-1 and [Ala9]endothelin-1 but not that induced by [Ala10]endothelin-1. BQ-123 (cyclo[D-Trp-D-Asp-Pro-D-Val-Leu), a specific endothelin ETA receptor antagonist, inhibited [Ala9]endothelin-1-, but not [Ala10]endothelin-1-induced contraction. Cross-tachyphylaxis and additivity studies indicated that [Ala9]endothelin-1, like endothelin-1, acts at the endothelin ETA receptor, whereas [Ala10]endothelin-1 behaved as an endothelin ETB receptor agonist, like sarafotoxin S6c. Thus, the residue at position 10 plays a significant role in receptor activation and is a candidate for further exploration of receptor antagonism.

Characterization of endothelin receptor subtypes on airway smooth muscle cells

Endothelin-1 (ET-1) has constrictor and mitogenic effects on airway smooth muscle strips and cultured cells, respectively. This study addresses the type of the ET receptor subtype(s) present on ovine airway smooth muscle cells and the possibility of autocrine effects. The expression of the preproendothelin-1 gene was demonstrated by Northern analysis, and the medium obtained from these cells contained immunoreactive-ET-1. Competitive binding experiments between [125I]ET-1 and ET-1, ET-3, and two ET-receptor subtype selective-ligands, BQ-123 (ETA) and sarafotoxin S6c (ETB), yielded IC50 values of 1.1 +/- 0.1, 227 +/- 13, 12 +/- 1, and 194 +/- 21 nM, respectively. ET-3 also gave a limited number of higher affinity sites. In the presence of BQ-123 (1 microM), the binding of [125I]ET-1 was decreased by 80-85%, and the IC50 values with ET-1, ET-3, and S6c were 2.0 +/- 0.4, 3.6 +/- 0.6, and 1.1 +/- 0.9 nM, respectively. In similar experiments with 0.1 microM sarafotoxin S6c, the respective IC50 values for ET-1 and ET-3 were 2.4 +/- 0.4 and 300 +/- 20 nM. These results demonstrate that about 85 +/- 5% of ET-1 binding to airway smooth muscle cells is to ETA receptors and that these cells produce ET-1 in vitro.

Endothelin receptor subtypes in small arteries. Studies with FR139317 and bosentan

We studied the effects of the selective endothelin A receptor antagonist FR139317 and the combined endothelin A/endothelin B receptor antagonist bosentan in rat mesenteric arteries by using a video dimension analyzer. In endothelium-denuded arteries, increasing concentrations of endothelin-1 evoked a biphasic vasoconstriction. The first phase was observed at low concentrations (10(-16) to 10(-11) mol/L) of endothelin-1 and was relatively weak. However, the contractions characterizing the second phase, which occurred at higher concentrations (10(-10) to 3 x 10(-8) mol/L) of endothelin-1, were much stronger. FR139317 concentration-dependently shifted the second phase of the endothelin-1-induced contraction curve to the right without affecting the first phase. In contrast, bosentan inhibited both the first and the second phase. Even after the blockade of endothelin A receptor, increasing concentrations of the endothelin B receptor agonists endothelin-3 and sarafotoxin S6c still induced small contractions. Maximal contractions induced by single-bolus extraluminal application of endothelin-3 (10(-9) mol/L) or sarafotoxin S6c (3 x 10(-8) mol/L) were markedly more pronounced than responses induced by cumulative concentrations, suggesting endothelin B receptor downregulation upon repeated and sustained activation. The response induced by a single bolus of endothelin-3 (10(-9) mol/L) was antagonized by bosentan but not by FR139317, confirming that endothelin B receptors were involved. In endothelium-intact arteries half-maximally precontracted with norepinephrine, bosentan but not FR139317 inhibited the relaxations induced by intraluminally applied endothelin-3. (ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin-1 and its receptors A and B in human aldosterone-producing adenomas

Endothelin-1 stimulates aldosterone secretion by interacting with specific receptors. Accordingly, we wished to investigate endothelin-1, endothelin-A (ETA) receptor, and endothelin-B (ETB) receptor gene expression, localization, and properties in aldosterone-producing adenomas and in the normal human adrenal cortex. We carried out 125I-endothelin-1 displacement studies with cold endothelin-1, endothelin-3, the specific ETA antagonist BQ-123, and the specific ETB weak agonist sarafotoxin 6 C and coanalyzed data with the nonlinear iterative curve-fitting program LIGAND. We also studied gene expression with reverse transcription-polymerase chain reaction with specific primers for endothelin-1, ETA, and ETB complementary DNA. Normal adrenal cortices from consenting kidney cancer patients (n = 2) and aldosterone-producing adenomas (n = 4) were studied; for the latter, surrounding normal cortex and kidney biopsy tissue served as controls. To further localize the receptor subtypes, tissue sections were studied by autoradiography in the presence and absence of 500 nmol/L BQ-123, 100 nmol/L sarafotoxin 6 C, and 1 mumol/L cold endothelin-1. In all tissues examined, endothelin-1, ETA, and ETB messenger RNAs were easily detected. However, in aldosterone-producing adenomas, both receptors' genes were expressed at a higher level than in the kidney.(ABSTRACT TRUNCATED AT 250 WORDS)