Pathophysiological role of endothelin revealed by the first orally active endothelin receptor antagonist

Reversal of endothelin-1 release by stimulation of endothelial alpha2-adrenoceptor contributes to cerebral vasorelaxation

Agonists acting on the vascular endothelium can modulate the release of a number of factors that interact with the surrounding smooth muscle cells and influence their tone. One such factor is the vasoconstricting agent endothelin-1 (ET-1), which has been implicated in several disease states, including stroke. However, very little is known about the physiological role of ET-1 in the cerebral circulation. We demonstrate that activation of alpha2-adrenoceptors in human pial artery endothelial cells reduces both constitutive and agonist-stimulated release of immunoreactive ET-1. That this has physiological relevance is supported by our demonstration that in segments of rabbit middle cerebral arteries, alpha2-adrenoceptor activation reduces the release of endothelium-derived ET-1 and causes an endothelium-dependent relaxation. The adrenoceptor-dependent relaxation was not blocked by combined addition of indomethacin and N omega-nitro-L-arginine in 25 mmol/L KCl-depolarizing physiological solution but was selectively antagonized by a subthreshold concentration of exogenous ET-1. Our data suggest that activation of endothelial alpha2-adrenoceptor would favor a decrease in ET-1 production and possibly promote vascular relaxation.

Nonspecific endothelin-receptor antagonist blunts monocrotaline-induced pulmonary hypertension in rats

Endothelin-1 (ET-1), a potent vasoactive and mitogenic peptide, has been implicated in the pathogenesis of several forms of pulmonary hypertension. We hypothesized that nonspecific blockade of ET receptors would blunt the development of monocrotaline (MCT)-induced pulmonary hypertension in rats. A single dose of the nonspecific ET blocker bosentan (100 mg/kg) given to intact rats by gavage completely blocked the pulmonary vasoconstrictor actions of Big ET-1 and partially blunted hypoxic pulmonary vasoconstriction. After 3 wk, MCT-injected (105 mg/kg sc) rats gavaged once daily with bosentan (200 mg/kg) had lower right ventricular (RV) systolic pressure (RVSP), RV-to-body weight (RV/BW) and RV-to-left ventricular (LV) plus septal (S) weight [RV/(LV+S)] ratios and less percent medial thickness of small pulmonary arteries than control MCT-injected rats. Lower dose bosentan (100 mg/kg) had no effect on these parameters after MCT or saline injection. Bosentan raised plasma ET-1 levels but had no effect on lung ET-1 levels. Bosentan (200 mg/kg) also had no effect on wet-to-dry lung weight ratios 6 days after MCT injection. When given during the last 10 days, but not the first 11 days of a 3-wk period after MCT injection, bosentan reduced RV/(LV+S) compared with MCT-injected controls. We conclude that ET-1 contributes to the pathogenesis of MCT-induced pulmonary hypertension and acts mainly during the later inflammatory rather than the acute injury phase after injection.

Endothelin-A receptors mediate vasoconstriction of capillaries in the spiral ligament

The purpose of this study was to determine whether endothelin-1 (ET-1), endothelin-2 (ET-2) or endothelin-3 (ET-3) alter the vascular diameter of capillaries in the spiral ligament. Changes in vascular tone were measured in capillaries from the isolated spiral ligament in vitro. Capillaries were occluded on one end and opened on the other end. Red blood cells trapped in the capillaries served as markers for a luminal volume defined by the red cell itself, the capillary wall and the occluder. Movement of the red cell toward the open end was taken as evidence for vasoconstriction and movement of the red cell toward the occluder was taken as evidence for vasodilation. The inner diameter of the capillaries was 7.0 microm and decreased maximally by a factor of 0.8 in response to ET-1 and ET-2 (both 10(-8) M). Vasoconstriction induced by ET-1 and ET-2 was concentration-dependent in the range between 10(-12) and 10(-8) M whereas ET-3 (10(-8) M) had no effect. The EC50s for ET-1 and ET-2 were 1.2 x 10(-10) M and 1.4 x 10(-9) M, respectively. Thus, the potency order was ET-1 > ET-2 >> ET-3. Vasoconstriction induced by ET-1 and ET-2 was completely inhibited by the competitive antagonist 10(-6) M BQ-123 (cyclic D-Asp-L-Pro-D-Val-L-Leu-D-Trp). Vasoconstriction induced by ET-1 or ET-2 continued for more than 1 min after removal of agonist from the perfusate. Rapid vasodilation of capillaries preconstricted by ET-1 was observed in response to 10(-3) M sodium nitroprusside. Sodium nitroprusside, however, had no significant effect on the vascular diameter of resting capillaries. These results demonstrate that capillaries in the spiral ligament can constrict and the endothelin-mediated vasoconstriction occurs via ET(A) receptors.

Reduction of renal mass is lethal in mice lacking vimentin. Role of endothelin-nitric oxide imbalance

Modulation of vascular tone by chemical and mechanical stimuli is a crucial adaptive phenomenon which involves cytoskeleton elements. Disruption, by homologous recombination, of the gene encoding vimentin, a class III intermediate filament protein mainly expressed in vascular cells, was reported to result in apparently normal phenotype under physiological conditions. In this study, we evaluated whether the lack of vimentin affects vascular adaptation to pathological situations, such as reduction of renal mass, a pathological condition which usually results in immediate and sustained vasodilation of the renal vascular bed. Ablation of 3/4 of renal mass was constantly lethal within 72 h in mice lacking vimentin (Vim-/-), whereas no lethality was observed in wild-type littermates. Death in Vim-/- mice resulted from end-stage renal failure. Kidneys from Vim-/- mice synthesized more endothelin, but less nitric oxide (NO), than kidneys from normal animals. In vitro, renal resistance arteries from Vim-/- mice were selectively more sensitive to endothelin, less responsive to NO-dependent vasodilators, and exhibited an impaired flow (shear stress)- induced vasodilation, which is NO dependent, as compared with those from normal littermates. Finally, in vivo administration of bosentan, an endothelin receptor antagonist, totally prevented lethality in Vim-/- mice. These results suggest that vimentin plays a key role in the modulation of vascular tone, possibly via the tuning of endothelin-nitric oxide balance.

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.

Additional hypotensive effect of endothelin-1 receptor antagonism in hypertensive dogs under angiotensin-converting enzyme inhibition

BACKGROUND

Endothelin-1 (ET-1) may play a role in hypertension. ET-1 receptor antagonism by bosentan lowers blood pressure in hypertension. We evaluated whether the effect of bosentan is still observed under ACE inhibitors (ACEI).

METHODS AND RESULTS

Thirty anesthetized and 18 conscious hypertensive dogs were studied randomly. Anesthetized dogs were divided into 4 groups: group 1 received cumulative doses of bosentan (bolus+30-minute infusion: 0.1 mg/kg+/-0.23 mg/kg per hour to 3 mg/kg+/-7 mg/kg per hour); group 2, the same dose-responses after 1 mg/kg enalaprilat; group 3, the vehicle after enalaprilat; and group 4, the dose responses to bosentan followed by enalaprilat. The conscious dogs were divided into 3 groups: group 5 received 2 cumulative doses of bosentan; group 6, the vehicle; and group 7, enalaprilat alone. In groups 1 and 2, bosentan produced dose-related decreases (P=.0001) in left ventricular systolic pressure and mean aortic pressure (AOP). In group 1, bosentan decreased mean AOP by 22%. In group 2, enalaprilat decreased mean AOP by 25% (from 173+/-26 to 130+/-25 mm Hg; P<.005); an additional 18% decrease was obtained with bosentan, the mean AOP reaching 98+/-21 mm Hg (P<.01). In group 3, the effect of enalaprilat alone was a 22% decrease in mean AOP (P<.005). The additive effect of the bosentan-ACEI association was also observed in group 4. In group 5, bosentan reduced mean AOP by 20% (P<.005), whereas mean AOP remained unchanged in group 6. The effect of ACEI alone (group 7) was similar to that of bosentan.

CONCLUSIONS

Bosentan produces an additional hypotensive effect to that of ACEI, which opens new therapeutic perspectives.

Source and cause of endothelin-1 release into cerebrospinal fluid after subarachnoid hemorrhage

Despite years of research, delayed cerebral vasospasm remains a serious complication of subarachnoid hemorrhage (SAH). Recently, it has been proposed that endothelin-1 (ET-1) mediates vasospasm. The authors examined this hypothesis in a series of experiments. In a primate model of SAH, serial ET-1 levels were measured in samples from the perivascular space by using a microdialysis technique and in cerebrospinal fluid (CSF) and plasma during the development and resolution of delayed vasospasm. To determine whether elevated ET-1 production was a direct cause of vasospasm or acted secondary to ischemia, the authors also measured ET-1 levels in plasma and CSF after transient cerebral ischemia. To elucidate the source of ET-1, they measured its production in cultures of endothelial cells and astrocytes exposed to oxyhemoglobin (10 microM), methemoglobin (10 microM), or hypoxia (11% oxygen). There was no correlation between the perivascular levels of ET-1 and the development of vasospasm or its resolution. Cerebrospinal fluid and plasma levels of ET-1 were not affected by vasospasm (CSF ET-1 levels were 9.3 +/- 2.2 pg/ml and ET-1 plasma levels were 1.2 +/- 0.6 pg/ml) before SAH and remained unchanged when vasospasm developed (7.1 +/- 1.7 pg/ml in CSF and 2.7 +/- 1.5 pg/ml in plasma). Transient cerebral ischemia evoked an increase of ET-1 levels in CSF (1 +/- 0.4 pg/ml at the occlusion vs. 3.1 +/- 0.6 pg/ml 4 hours after reperfusion; p < 0.05), which returned to normal (0.7 +/- 0.3 pg/ml) after 24 hours. Endothelial cells and astrocytes in culture showed inhibition of ET-1 production 6 hours after exposure to hemoglobins. Hypoxia inhibited ET-1 release by endothelial cells at 24 hours (6.4 +/- 0.8 pg/ml vs. 0.1 +/- 0.1 pg/ml, control vs. hypoxic endothelial cells; p < 0.05) and at 48 hours (6.4 +/- 0.6 pg/ml vs. 0 +/- 0.1 pg/ml, control vs. hypoxic endothelial cells; p < 0.05), but in astrocytes hypoxia induced an increase of ET-1 at 6 hours (1.5 +/- 0.6 vs. 6.4 +/- 1.1 pg/ml, control vs. hypoxic astrocytes; p < 0.05). Endothelin-1 is released from astrocytes, but not endothelial cells, during hypoxia and is released from the brain after transient ischemia. There is no relationship between ET-1 and vasospasm in vivo or between ET-1 and oxyhemoglobin, a putative agent of vasospasm, in vitro. The increase in ET-1 levels in CSF after SAH from a ruptured intracranial aneurysm appears to be the result of cerebral ischemia rather than reflecting the cause of cerebral vasospasm.

Binding characteristics of [125I]TTA 386, ETA-selective antagonist

The data presented in this manuscript describe the binding characteristics of the ETA-selective antagonist, [125I]TTA 386 (hexamethylenelmino carbonyl-Leu-Tri-Ala-beta-Ala-Tyr-Phe). This radioligand bound with high affinity and specificity to cloned human ETA receptors and rat mesenteric artery ETA receptors. The apparent dissociation constants (KdS) and maximum binding capacities were 1.0 nM and 8.5 pmol/mg for cloned human ETA receptors and 0.8 nM and 170 fmol/mg for rat mesenteric artery membranes respectively. Binding of [125I]TTA 386 was fast reaching equilibrium by 45 min and 15 min for human ETA and rat mesenteric artery membrane, respectively. Addition of excess unlabeled ligand resulted in the dissociation of bound radioligand from both preparations. Competition of [125I]TTA 386 binding by unlabeled ET-1, ET-3, TTA 386 and BQ123 revealed appropriate ETA pharmacology. This radioligand did not display any binding to cloned human ETB receptors.

Endothelin-1: possible implications in pulmonary vascular disease

The vasoactive properties of endothelin-1 (ET-1) in the animal model very with the tone of the pulmonary vessels, the dose level of ET-1, and the maturation of the vessels. The action of ET-1 is mediated by endothelium-derived nitric oxide, prostaglandins, and electrolytes. Plasma levels of ET-1 are elevated in pulmonary hypertension in both animals and humans. ET-1 antagonists may prove useful in treating pulmonary hypertension in children and adults.

Endogenous endothelin modulates blood pressure, plasma volume, and albumin escape after systemic nitric oxide blockade

To assess whether acute nitric oxide (NO) blockade could unmask the vascular actions of endogenous endothelin, we tested the effects of the endothelin type A/type B (ET(A)/ET(B)) receptor antagonist bosentan and the selective ET(A) antagonist FR 139317 on blood pressure, plasma volume, and albumin escape after inhibition of NO synthesis with N(G)-nitro-L-arginine methyl ester (L-NAME). Conscious, chronically catheterized rats received L-NAME in the absence and presence of 17.4 micromol/kg (10 mg/kg) bosentan or 3.8 micromol/kg (2.5 mg/kg I.V., 10 minutes before L-NAME) FR 139317. Red blood cell volume and plasma volume were determined with chromium-51-tagged erythrocytes and iodine-125-labeled albumin, respectively. L-NAME (0.46 to 7.42 micromol/kg [0.125 to 2 mg/kg]) induced a dose-dependent increase in blood pressure, which was attenuated by 60% and 48% with bosentan and FR 139317, respectively (P<.01). L-NAME (7.42 micromol/kg) also increased hematocrit. This effect was associated with an increase in total-body albumin escape, which is reflected by a 14% reduction in plasma volume. Red blood cell volume remained unchanged. L-NAME promoted albumin escape primarily in the lung, heart, liver, kidney, and gastrointestinal tract. Both bosentan and FR 139317 markedly reduced these effects of L-NAME. Furthermore, L-NAME increased plasma levels of immunoreactive endothelin-1 from 8.6+/-0.4 (n=10) to 14.7+/-1.4 pg/mL (n=9, P<.01). These results demonstrate that the pressor response, losses in plasma volume, and increase in albumin escape observed after inhibition of NO synthesis are in part the consequence of unmasking the actions of endogenous endothelin, which are mediated predominantly via ET(A) receptors. These findings suggest a role for endogenous endothelin in the regulation of vascular functions in conditions when NO formation by endothelial cells is impaired.

Activation of renin synthesis is dependent on intact nitric oxide production

The present study investigated whether or not nitric oxide (NO) synthesis mediates mechanisms regulating activation of renin formation. Studies were performed on afferent arterioles freshly isolated from the rat kidney. We have shown previously that this preparation is a useful model to study regulation of renin synthesis and secretion. The expression of renin mRNA was assessed by ribonuclease protection assay, and total renin content and renin secretion by radioimmunoassay. In afferent arterioles isolated from rats treated with the angiotensin-converting enzyme inhibitor ramipril, renin mRNA levels, total renin content and renin secretion were increased threefold compared to untreated controls. Inhibition of NO-synthase by NG-nitro-L-arginine methyl ester (L-NAME) in the ramipril-treated rats, abolished the increase in renin mRNA levels, total renin content and renin secretion. In other animals furosemide, a diuretic acting on macula densa cells, activated renin synthesis to a level similar to that found in the ramipril-treated group. Addition of L-NAME to the furosemide-treated rats suppressed the increases in renin mRNA levels, total renin content and renin secretion, suggesting that NO acts on renin activation by a mechanism independent of angiotensin II. In separate experiments, the inhibitory effect of L-NAME on the activation of renin secretion was abolished when afferent arterioles were treated with nicardipine, an L-type Ca2+ channel blocker, suggesting that the suppression of renin activation during NO inhibition is due to increased Ca2+ entry. Since endothelin is a potent mediator of Ca2+ influx and an inhibitor of renin release, we tested whether or not endothelin could be involved in the inhibitory effect of L-NAME on renin secretion. Application of the endothelin receptor antagonist, bosentan, in vitro mimicked the effect of nicardipine. In addition, bosentan coadministered with L-NAME in vivo blunted the inhibitory effect of L-NAME and restored the increases in renin mRNA level, synthesis and secretion. These data indicate that the physiological mechanism(s) regulating activation of renin synthesis and secretion are impaired during NO inhibition, probably because of increased Ca2+ influx. This increase in calcium flux is mediated at least partially by the action of endothelin.

Structure-activity relationships of N2-aryl-3-(isoxazolylsulfamoyl)-2-thiophenecarboxamides as selective endothelin receptor-A antagonists

We report here that N2-aryl-3-(isoxazolylsulfamoyl)-2-thiophenecarboxamides are potent and selective small molecule ETA receptor antagonists. The aryl group was subjected to extensive structural modification. With monosubstitution, the para position was most useful in increasing potency, with methyl being preferred. With disubstitution, 2,4-disubstitution further enhanced activity with methyl or cyano groups being preferred at the 2-position. In this series, a benzo-[d][1,3]dioxole group is equivalent to a 4-methyl group in in vitro activity and afforded the compounds with both in vivo activity and moderate half-lives.

New concepts in the pathogenesis of portal hypertension: hepatic wounding and stellate cell contractility

The pathogenesis of portal hypertension is multifactorial, and appears to result from interplay between fixed and dynamically modulable elements; the stellate cell is a newly recognized example of the latter. This perisinusoidal, pericyte-like cell has contractile features that are most prominent after liver injury, concomitant with their activation. These data imply an exaggerated contractile phenotype in the cirrhotic liver. This cell may contribute to increased intrahepatic portal hypertension via perisinusoidal constriction of the sinusoid or by contraction of fibrous extracellular matrix rich in type I collagen with concomitant disruption of lobular architecture. Endothelins and NO play a major role in the modulation of stellate cell contractility, and are therefore important in the pathogenesis of intrahepatic portal hypertension. These new data provide potential areas for therapeutic intervention in this clinical entity.

Contraction of human airway smooth muscle by endothelin-1 and IRL 1620: effect of bosentan

We have examined whether 4-tert-butyl-N-[6-(2-hydroxy-ethoxy)-2,2'-bipyrimidin-4-yl]-benzen esulfonamide (bosentan; endothelin ET(A/B) receptor antagonist) and (R)2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]carbonyl] amino-4-methylpentanoil]amino-3-[3-(1-methyl-1H-indoyl)]prop ionyl]amino-3-(2-pyridyl) propionic acid (FR 139317; endothelin ET(A) receptor antagonist) inhibit contractions of human airway smooth muscle induced by endothelin-1 or Suc-[Glu9,Ala(11,15)]enthothelin-1-(8-21) (IRL 1620; endothelin ET(B) receptor agonist). Endothelin-1 and IRL 1620 were equipotent. Bosentan and FR 139317 (each 10 microM) produced a small shift in response curves to endothelin-1 (1.6- and 1.5-fold, respectively). However, bosentan was more potent against contractions elicited by IRL 1620 (10 microM, 11.2-fold shift) suggesting that these agonists exhibit different kinetic interactions with endothelin receptors or implying an interaction with a novel endothelin ET(B) receptor subtype in human airways.

Bosentan, a novel synthetic mixed-type endothelin receptor antagonist, attenuates acute gastric mucosal lesions induced by indomethacin and HCl in the rat: role of endogenous endothelin-1

Endothelin-1 has been reported to be responsible for gastric mucosal damage in various experimental models. We evaluated the role of endogenous endothelin-1 in the pathogenesis of gastric mucosal damage induced by indomethacin and HCl in the rat. Rats were given indomethacin (25 mg/kg) subcutaneously, and 15 min later, 0.2N HCl intragastrically. Gastric mucosal damage, gastric endogenous endothelin-1, and gastric mucosal hemodynamics were measured. The effects of bosentan, a mixed endothelin receptor antagonist, on gastric mucosal integrity and hemodynamics were assessed. Gastric endogenous endothelin-1 was significantly elevated at 20 min, gastric mucosal blood flow began to decrease significantly at 25 min, and gastric damage occupied 52.2% of the total glandular mucosa at 135 min after injection of indomethacin. Intragastric pretreatment with bosentan (5, 10, 30, and 60 mg/kg) significantly attenuated gastric damage, to 26.1%, 7.7%, 3.6%, and 1.6%, respectively, of the total glandular mucosa. Bosentan (60 mg/kg) prevented the initial decrease of blood flow and, even at 135 min, improved blood flow and hemoglobin oxygen saturation significantly. We suggest that indomethacin-induced endogenous endothelin-1 diminishes gastric mucosal blood flow and tissue oxygenation and ultimately causes gastric damage. Endogenous endothelin-1 may play an important role in the pathogenesis of the acute gastric mucosal lesions induced by indomethacin and HCl.

Structure-activity relationships in a series of orally active gamma-hydroxy butenolide endothelin antagonists

The design of potent and selective non-peptide antagonists of endothelin-1 (ET-1) and its related isopeptides are important tools defining the role of ET in human diseases. In this report we will describe the detailed structure-activity relationship (SAR) studies that led to the discovery of a potent series of butenolide ETA selective antagonists. Starting from a micromolar screening hit, PD012527, use of Topliss decision tree analysis led to the discovery of the nanomolar ET(A) selective antagonist PD155080. Further structural modifications around the butenolide ring led directly to the subnanomolar ETA selective antagonist PD156707, IC50's = 0.3 (ET(A)) and 780 nM (ET(B)). This series of compounds exhibited functional activity exemplified by PD156707. This derivative inhibited the ETA receptor mediated release of arachidonic acid from rabbit renal artery vascular smooth muscle cells with an IC50 = 1.1 nM and also inhibited the ET-1 induced contraction of rabbit femoral artery rings (ETA mediated) with a pA2 = 7.6. PD156707 also displayed in vivo functional activity inhibiting the hemodynamic responses due to exogenous administration of ET-1 in rats in a dose dependent fashion. Evidence for the pH dependence of the open and closed tautomerization forms of PD156707 was demonstrated by an NMR study. X-ray crystallographic analysis of the closed butenolide form of PD156707 shows the benzylic group located on the same side of the butenolide ring as the gamma-hydroxyl and the remaining two phenyl groups on the butenolide ring essentially orthogonal to the butenolide ring. Pharmacokinetic parameters for PD156707 in dogs are also presented.

New non-peptide endothelin-A receptor antagonists: synthesis, biological properties, and structure-activity relationships of 5-(dimethylamino)-N-pyridyl-,-N-pyrimidinyl-,-N-pyridazinyl-, and -N-pyrazinyl-1-naphthalenesulfonamides

Use of automated synthesis led to the discovery of several 6-membered nitrogen heterocycles as replacements for the N-isoxazolyl substituent present in the 1-naphthalenesulfonamides endothelin-A (ETA) antagonist 5-(dimethylamino)-N-(3,4-dimethyl-5-isoxazolyl)-1-naphthalenesu lfo namides (BMS 182874). In each of these heterocycles, a small substituent such as halogen para to the position of attachment to the sulfonamide nitrogen atom was found to be advantageous for ETA receptor affinity. Of these heterocycles, 2-pyrazines offered the greatest scope for improving receptor affinity. Optimization of the substituents at the 3- and 5-positions in the pyrazine ring led to potent, ETA-selective compounds such as 5-(dimethylamino)-N-(5-chloro-3-methoxy-2-pyrazinyl)-1- naphthalenesulfonamides (7m, ETA pIC50 8.1). When dosed orally at 10 mg/kg to conscious, normotensive rats infused with big ET-1, compounds such as 7m showed significant inhibition of the pressor response with a duration of effect lasting for the 5-h course of the experiment.

Presence of contractile endothelin-A and dilatory endothelin-B receptors in human cerebral arteries

OBJECTIVE

The aim of the present study was to elucidate the endothelin receptor subtypes responsible for the endothelin-induced vasomotor responses of human cerebral arteries.

METHODS

Human cerebral arteries with endothelium were mounted in in vitro tissue baths, and the vascular responses to endothelin-1 (ET-1) and sarafotoxin 6c (a selective ETB agonist) were studied in the presence or absence of endothelin blockers, bosentan (Ro 47-0203), a novel nonpeptide ETA and ETB receptor antagonist, and FR139317, a selective ETA receptor antagonist. The presence of messenger ribonucleic acid encoding the human ETA and ETB receptors in human cerebral arteries with intact endothelium and in segments denuded of endothelium was studied by the use of reverse transcriptase-polymerase chain reaction.

RESULTS

ET-1 induced concentration-dependent contraction of human cerebral arteries; the pEC50 value was 9.4 +/- 0.2. The vasoconstriction was significantly antagonized both by bosentan and by FR139317. The pA2 values were 7.2 +/- 0.4 and 7.4 +/- 0.4, respectively. Sarafotoxin 6c failed to cause contraction of human cerebral arteries. In precontracted vessels, however, sarafotoxin 6c induced dilatation that was significantly inhibited by bosentan (10 mumol/L), resulting in a pA2 value of 6.0 +/- 0.2. Furthermore, messenger ribonucleic acid encoding the human ETA and ETB receptors was detected in human cerebral arteries both with and without endothelium.

CONCLUSION

The ET-1-induced vasoconstriction of human cerebral arteries is primarily mediated by the ETA receptor, whereas the sarafotoxin 6c-induced vasodilatation seems to be mediated via the ETB receptor.

Combined inhibition of endothelin and angiotensin II receptors blocks volume load-induced cardiac hormone release

Volume expansion has been shown to increase plasma atrial natriuretic peptide (ANP) levels, but the precise role of paracrine and autocrine factors in stretch-induced cardiac hormone release is not clear. In the present study, we report the effects of endothelin (ET) and angiotensin receptor (AT receptor) antagonists on baseline and atrial stretch-induced immunoreactive ANP (IR-ANP) and immunoreactive N-terminal ANP (IR-NT-ANP) release in vivo by using BQ-123 (ETA receptor antagonist), bosentan (ETA and ETB receptor antagonist), and losartan (AT1 receptor antagonist). Intravenous administration of BQ-123 had no significant effect on baseline hemodynamics in conscious rats, whereas bosentan (10 mg/kg) and losartan (10 mg/kg) decreased slightly (4 to 7 mm Hg, P < .05 to .001) the mean arterial pressure. Both the ETA receptor antagonist BQ-123 and ETA/ETB receptor antagonist bosentan decreased plasma ANP and NT-ANP responses to volume load (P < .05 to .001), whereas the AT1 receptor antagonist losartan had no significant effect on this response. The relative increase in plasma IR-ANP corresponding to a 3 mm Hg increase in right atrial pressure was 2.7-fold in the vehicle-treated group. BQ-123 (0.3 and 1.0 mg/kg) decreased this response 2.5- and 2.1-fold (P < .05); bosentan (3 and 10 mg/kg), 1.7-fold (P < .001) and 1.9-fold (P < .05); and bosentan (10 mg/kg)+losartan (10 mg/kg), 1.6-fold (P < .001). The responses in plasma IR-NT-ANP decreased simultaneously. These results indicate that combined inhibition of ETA/B and AT1 receptors almost completely blocks ANP response to acute volume load. Therefore, our study shows that endogenous paracrine and/or autocrine factors liberated in response to atrial wall stretch rather than myocyte stretch itself are responsible for the activation of ANP peptide secretion in response to acute volume load. Our results also show that ETA receptors are more important in the regulation of mechanical stretch-induced changes in cardiac hormone secretion than AT1 receptors.

Inhibitors of endothelin

With the advent of the first generation of both selective and nonselective endothelin antagonists being a relatively recent event, the manifold therapeutic potentials of these compounds are only now being explored clinically. Undoubtedly, numerous clinical utilities for these compounds will soon be realized.

Acute and short-term effects of the nonpeptide endothelin-1 receptor antagonist bosentan in humans

In recent years, evidence from various animals experiments has accumulated that emphasizes the role of endothelin-1 in the pathophysiology of several cardiovascular diseases, including congestive heart failure. The recent advent of potent antagonists of this system now allows the assessment of the involvement of endothelin-1 in the maintenance of vascular tone in animals and humans. We report hemodynamic data from two trails in patients with chronic severe congestive heart failure (i.e., reduced left ventricular ejection fraction of < 30%, elevated resting pulmonary capillary wedged pressure > 15 mmHg, and/or reduced cardiac index of 2.5 L/min/m2 or less) who were treated with the mixed endothelin-type A and type B-receptor antagonist bosentan. In the first study, the acute effect of bosentan (300 mg, intravenous) on hemodynamics and neurohormones was investigated. Bosentan was well tolerated and significantly improved impaired hemodynamics due to systemic and venous vasodilation. In the second, trial, bosentan was given orally (0.5 g bid) for 14 days, in addition to conventional triple treatment for congestive heart failure, including digitalis, angiotensin-converting enzyme inhibitors, and diuretics. Cardiac hemodynamics were monitored during the first 24 hours of treatment, and measurements were repeated during the last day of bosentan therapy. Bosentan was well tolerated in these patients as well, and hemodynamic measures were compatible with an additional effect of bosentan after 2 weeks. However, there was a slight increase in heart rate as well. Our result underline the importance of endogenously generated endothelin-1 in congestive heart failure and suggest a potential benefit of endothelin antagonism in such patients. However, long-term studies are needed to establish whether chronic endothelin antagonism has beneficial clinical effects and is capable of improving survival and/or symptoms in severe heart failure patients who remain symptomatic despite standard triple therapy.

The role of endothelins in the paracrine control of the secretion and growth of the adrenal cortex

Endothelins (ETs) are a family of vasoactive peptides (ET-1, ET-2, and ET-3) mainly secreted by vascular endothelium and widely distributed in the various body systems, where they play major autocrine/paracrine regulatory functions, acting via two subtypes of receptors (ETA and ETB): Adrenal cortex synthesizes and releases ETS and expresses both ETA and ETB. Zona glomerulosa possesses both ETA and ETB, whereas zona fasciculata/reticularis is almost exclusively provided with ETB. ETS exert a strong mineralocorticoid and a less intense glucocorticoid secretagogue action, mainly via ETB receptors. ETS also appear to enhance the growth and steroidogenic capacity of zona glomerulosa and to stimulate its proliferative activity. This trophic action of ETS is likely to be mediated mainly by ETA receptors. The intraadrenal release of ETS undergoes a multiple regulation, with the rise in blood flow rate and the local release of nitric oxide being the main stimulatory factors. Data are also available that indicate that ETS may also have a role in the pathophysiology of primary aldosteronism caused by adrenal adenomas and carcinomas.

Endothelins and cardiac hypertrophy

Cardiac hypertrophy is a commonly observed complication of a variety of cardiovascular diseases. As well as mechanical stresses such as pressure overload, several humoral factors may contribute to the development of cardiac hypertrophy. In the early 1990s, endothelin-1 (ET-1) was found to induce cardiomyocyte hypertrophy in cultured neonatal rat cardiomyocytes. Furthermore, evidence is accumulating to date to support the postulate that ET-1 may function as an autocrine/paracrine factor in the development of cardiac hypertrophy in vivo as well as in vitro In this review article, I discuss physiological and pathophysiological roles of ET-1 and its related peptides in cardiac hypertrophy associated with various cardiovascular diseases.

Endothelin-1 induces GM-CSF, IL-6 and IL-8 but not G-CSF release from a human bronchial epithelial cell line (BEAS-2B)

Endothelin (ET) is a powerful vasoconstrictor and bronchoconstrictor peptide that may be involved in the pathogenesis of bronchial asthma. We have investigated the effect of ET on the secretion of IL-6, IL-8, GM-CSF and G-CSF in a bronchial epithelial cell line (BEAS-2B). Incubation of BEAS-2B cells with ET-1 (10(-13) to 10(-7) M) for 4 h caused dose-related increases in the release of IL-8 (68% increase above control, P < 0.001) and IL-6 (43% increase above control, P < 0.001), compared to untreated control cells. After 48 h incubation, ET-1 also increased the release of IL-8 by 35% (P < 0.001) and GM-CSF by 38% (P < 0.01). ET-1 had no significant effect on G-CSF release. ET-1 did not induce cell proliferation at 24 or 48 h. Since ET-immunoreactive materials are expressed in epithelial cells in asthma, it is possible that ET-1 of epithelial origin may act in a paracrine or autocrine fashion on airway epithelial ET receptors to stimulate IL-8, IL-N6 and GM-CSF release. Thus, ET-1 may play a role in the regulation of the cytokine responses involved in inflammation of the airway mucosa.

Molecular pharmacology and pathophysiological significance of endothelin

Since the discovery of the most potent vasoconstrictor peptide, endothelin, in 1988, explosive investigations have rapidly clarified much of the basic pharmacological, biochemical and molecular biological features of endothelin, including the presence and structure of isopeptides and their genes (endothelin-1, -2 and -3), regulation of gene expression, intracellular processing, specific endothelin converting enzyme (ECE), receptor subtypes (ETA and ETB), intracellular signal transduction following receptor activation, etc. ECE was recently cloned, and its structure was shown to be a single transmembrane protein with a short intracellular N-terminal and a long extracellular C-terminal that contains the catalytic domain and numerous N-glycosylation sites. In addition to acute contractile or secretory actions, endothelin has been shown to exert long-term proliferative actions on many cell types. In this case, intracellular signal transduction appears to converge to activation of mitogen-activated protein kinase. As a recent dramatic advance, a number of non-peptide and orally active receptor antagonists have been developed. They, as well as current peptide antagonists, markedly accelerated the pace of investigations into the true pathophysiological roles of endogenous endothelin-1 in mature animals; e.g., hypertension, pulmonary hypertension, acute renal failure, cerebral vasospasm, vascular thickening, cardiac hypertrophy, chronic heart failure, etc. Thus, the interference with the endothelin pathway by either ECE-inhibition or receptor blockade may provide an exciting prospect for the development of novel therapeutic drugs.

Systemic hypertension induced by hepatic overexpression of human preproendothelin-1 in rats

Endothelin-1 (ET-1) has been implicated in the regulation of vascular tone in various pathological conditions. To examine the effect of in vivo overexpression of the peptide in rats, we prepared recombinant adenovirus stocks encoding the human preproET-1 cDNA (Ad.ET-1) or Escherichia coli lacZ (Ad.betaGal), each driven by cytomegalovirus early promoter. Ad.ET-1 or Ad.betaGal was injected into the caudal vein of rats and the animals were studied under anesthesia 96 h later. Hepatic overexpression of the virus-derived human ET-1 mRNA was accompanied by a 13-fold elevation of liver ET-1 content in the Ad.ET-1 group. Circulating plasma ET-1 levels in the Ad.ET-1 group were sixfold higher than those in the Ad.betaGal group. Mean arterial blood pressure was increased by 28 mmHg in the Ad.ET-1 group as compared with the Ad.betaGal group. In the Ad.ET-1 group, intravenous infusion of the ET(A) receptor antagonist FR 139317 reduced the blood pressure to levels seen in the Ad.betaGal group, whereas the same antagonist did not significantly alter the blood pressure in the Ad.betaGal group. Intravenous infusion of the ET(B) receptor antagonist BQ-788 caused a small but significant increase in blood pressure in both groups. These findings demonstrate that endogenous overexpression of preproET-1, accompanied by an elevation of plasma ET-1 concentrations to the levels seen in pathophysiological states, can cause systemic hypertension through the activation of the ETA receptor.

Contractile effect of big endothelin-1 and its conversion to endothelin-1 in rabbit cerebral arteries

The effect of big endothelin-1 (big ET-1) and its conversion to endothelin-1 (ET-1) in rabbit cerebral arteries were examined. Big ET-1 and ET-1 induced concentration-dependent contractions in the basilar artery; ET-1 was approximately 8 times more potent than big ET-1. The metalloprotease inhibitor phosphoramidon (30 mumol/l) almost abolished the contractile response to big ET-1, whereas the ET-1-induced contraction was unaffected. Removal of the endothelium did not attenuate the big ET-1-induced contraction. ET-1 was approximately 14 times more potent than endothelin-3 (ET-3) to elicit contraction. The contractions induced by big ET-1, ET-1 and ET-3 were all inhibited by ET(A) receptor antagonist BQ 123 (3 mumol/l). The ET(B) receptor antagonist IRL 1038 (3 mumol/l) had no effect on the contractile responses to big ET-1 and ET-1, but produced a small inhibition of the ET-3-induced contraction. Formation of ET-1 was demonstrated in membrane fractions of cerebral arteries incubated with big ET-1 as measured by high pressure liquid chromatography followed by radioimmunoassay. These results suggest that externally applied big ET-1 is converted to ET-1 by a phosphoramidon-sensitive "endothelin converting enzyme" present in the vascular smooth muscle cells. The ET-1 formed subsequently mediates the big ET-1-induced contraction by activation of mainly ET(A) receptors, although a small contribution of ET(B) receptors cannot be excluded.

Systemic administration of an inhibitor of endothelin-converting enzyme for attenuation of cerebral vasospasm following experimental subarachnoid hemorrhage

The potent vasoconstrictor peptide, endothelin-1 (ET-1), has been implicated in the pathophysiology of cerebral vasospasm that occurs after subarachnoid hemorrhage (SAH). This peptide is synthesized as a large prepropeptide that requires a series of modifying steps for its activation. The last of these steps involves the proteolytic conversion of a relatively inactive propeptide, Big ET-1, to its active, 21-amino acid peptide form. The enzyme responsible for converting Big ET-1 to ET-1 is a metalloprotease called endothelin-converting enzyme (ECE). In the present study the authors examined the effects of a newly developed inhibitor of ECE on responses to ET peptides in the normal basilar artery and on pathophysiological constriction in the spastic basilar artery after SAH. In the first series of experiments the authors examined normal basilar arteries in the rabbit, which were exposed transclivally and measured on-line using videomicroscopy. Intravenous administration or topical application of an active inhibitor of ECE, CGS 26303, blocked vasoconstrictor responses to topically applied Big ET-1 but not to ET-1. In contrast, topical application of a structurally related compound that does not inhibit ECE, CGS 24592, was ineffective in blocking vasoconstriction that was elicited by a topical application of Big ET-1. These findings indicate that CGS 26303 when administered systemically is capable of blocking the conversion of Big ET-1 to ET-1 in the basilar artery without affecting the ability of the vessel to respond to ET-1. In the second series of experiments the authors examined the effects of the ECE inhibitor on cerebral vasospasm after experimental SAH. Intraperitoneal administration of CGS 26303 via osmotic minipumps significantly attenuated the delayed spastic response of the basilar artery to an intracisternal injection of autologous blood. This study provides the first evidence that systemic administration of an inhibitor of ECE is capable of preventing cerebral vasospasm after SAH. The results reinforce a growing body of evidence that ETs play a critical role in the development of spastic constriction after SAH. Moreover, the findings indicate that blocking the conversion of Big ET-1 to its active ET-1 form using CGS 26303 may represent a feasible strategy for ameliorating cerebral vasospasm.

Action and binding of the endothelin antagonist bosentan on astrocytes of cultured rat central nervous system. Electrophysiological and autoradiographic studies

Our autoradiographic studies demonstrate that astrocytes in explant cultures of rat central nervous system possess binding sites for the first orally active, mixed, nonpeptide endothelin receptor antagonist [3H] bosentan. Binding of [3H]bosentan was inhibited by unlabelled bosentan and endothelin-1 at high concentrations, suggesting specific binding of the antagonist. Electrophysiological studies have revealed that bosentan reversibly blocked the depolarizations by endothelin but not by angiotensin II, indicating that the antagonist specifically antagonizes the action of endothelin on the glial membrane. This is consistent with biochemical studies from other laboratories demonstrating that bosentan did not interfere with binding of angiotensin II. The availability of bosentan, a potent and selective endothelin receptor antagonist should help to elucidate the role of endothelin on astrocyte function.

Increase in hepatic tissue blood flow by teprenone

The major objective of the present study was to evaluate mechanisms by which teprenone, a gastric mucosal protecting agent, increases hepatic mucosal blood flow using male Sprague-Dawley rats. Hepatic and gastric blood flow was measured using a laser blood flow meter after administration of teprenone, dissolved in Tween 80, into the inferior vena cava. Teprenone itself increased hepatic and gastric blood flow. It also increased hepatic and gastric blood flow in rats with acute hepatic disorders due to carbon tetrachloride (CCL4) and improved histological changes, such as inflammatory cell infiltration and fatty changes in the liver. The fact that blood endothelin (ET) concentrations increased after administration of teprenone suggest that teprenone has great affinity for ET beta receptors and shows ET beta-receptor antagonist-like effects. Hepatic blood flow decreased after administration of N-nitro-L-arginine methyl ester, a nitric oxide (NO) synthetase inhibitor, suggesting that teprenone increase NO activity. Teprenone was thought to increase hepatic and gastric blood flow by different mechanisms, because it increased gastric mucosal prostaglandin E2 concentrations.

Effects of an angiotensin-converting enzyme inhibitor, a calcium antagonist, and an endothelin receptor antagonist on renal afferent arteriolar structure

Narrowed afferent arteriolar diameter in young, spontaneously hypertensive rats (SHR) may be a contributor to later development of high blood pressure. Thus, treatment that causes dilation of the afferent arterioles in SHR may inhibit the redevelopment of high blood pressure when treatment is withdrawn. We treated SHR with an ACE inhibitor (cilazapril, 5 to 10 mg/kg per day, high; 1 mg/kg per day, low), a calcium antagonist (mibefradil, 20 to 30 mg/kg per day), and an endothelin receptor antagonist (bosentan, 100 mg/kg per day) from age 4 to 20 weeks. Untreated SHR and Wistar-Kyoto rats were also investigated. At 20 weeks, the rats were killed, and morphology of the afferent arterioles was studied. Other SHR (untreated, high cilazapril, low cilazapril, mibefradil) were treated in exactly the same way and then followed to 32 weeks without treatment. The morphometric studies showed that cilazapril increased the lumen diameter in the afferent arterioles and decreased the media-lumen ratio in a dose-dependent manner. On withdrawal of cilazapril treatment, the reduction in blood pressure persisted. Mibefradil tended to increase afferent arteriolar diameter, whereas it did not alter media-lumen ratio. The persistent effect on blood pressure was only moderate after withdrawal of mibefradil. Bosentan had no effect on renal afferent arteriolar structure or blood pressure. In conclusion, cilazapril was more effective than mibefradil in altering afferent arteriolar structure and caused the most persistent effect on blood pressure after treatment withdrawal. The association of increased afferent arteriolar diameter and lower blood pressure level after withdrawal of treatment may suggest a pathogenic role for afferent arteriolar diameter in the development of high blood pressure in SHR.

Inhibition by bosentan, an endothelin antagonist, of the hypersensitivity to Ca2+ channel activator evoked by salt-loading in basilar artery of stroke-prone spontaneously hypertensive rats

High salt diet dramatically decreases the life time of spontaneously hypertensive stroke-prone rats (SHRSP). This has been related to an increase in the incidence of stroke. We have investigated the influence of high salt diet on the reactivity to the Ca2+ channel activator Bay K 8644 of basilar artery isolated from SHRSP. The results show that the sensitivity of basilar artery to Bay K 8644 was increased by salt load and that this hypersensitivity was blunted by bosentan, an ETA/ETB antagonist.

BIBP 3226, the first selective neuropeptide Y1 receptor antagonist: a review of its pharmacological properties

Based on the assumption that the pharmacophoric groups interacting with the Y1 receptor are located in the C-terminal part of neuropeptide Y, low molecular weight compounds with high affinity and selectivity for the Y1 receptor were designed and synthesized. The prototype BIBP 3226 possesses affinity for the Y1 receptor in the nanomolar range. In addition, this compound is selective displaying rather low affinity for Y2, Y3, Y4 and a set of 60 other receptors. Both biochemical and pharmacological studies showed that BIBP 3226 behaves as a competitive antagonist. Using BIBP 3226 it was possible to investigate the role of NPY and/or Y1 receptors in blood pressure regulation. The interesting observation was that antagonism to Y1 receptors had no major influence on the basal blood pressure but attenuated stress induced hypertension. This strongly supports the hypothesis that NPY is mainly released during stress involving intense sympathetic nervous system activation. Moreover, BIBP 3226 can be used to characterize NPY receptor subtypes. For instance, we were able to show that presynaptic NPY receptors mediating catecholamine release do not solely belong to the Y2 subtype, but that presynaptic Y1 receptors also exist. In conclusion, BIBP 3226 has been shown to be an important tool for the elucidation of the physiological role of Y1 receptors in the cardiovascular system.

Endothelin and calcium antagonists in the skin microcirculation of patients with coronary artery disease

BACKGROUND

Endothelin, a potent endothelium-derived vasoconstrictor peptide, is elevated in coronary artery disease (CAD); however, its pathophysiological role is uncertain. Calcium antagonists are widely used in patients with CAD. Using laser Doppler flowmetry, we investigated the influence of two endothelin antagonists and the calcium antagonist diltiazem on endogenous and exogenous endothelin in the skin microcirculation of CAD patients and healthy control subjects.

METHODS AND RESULTS

Both endothelin antagonists and diltiazem applied intradermally induced vasodilation in CAD patients, which was more pronounced with the ETA/ETB antagonist than with the ETA antagonist or diltiazem. Exogenous endothelin led to profound vasoconstriction in CAD patients and healthy volunteers. Both endothelin antagonists and diltiazem blunted the vasoconstriction to exogenous endothelin in CAD patients and young healthy volunteers and less so in old healthy volunteers. However, compared with both endothelin antagonists, a 10-times-higher dose of diltiazem was required. Systemic diltiazem (240 mg, slow release) attenuated endothelin-induced vasoconstriction in CAD patients. Neurogenic vasodilation to exogenous endothelin was inhibited by both endothelin antagonists.

CONCLUSIONS

This study demonstrates that endogenous endothelin of CAD patients contributes to the regulation of vascular tone in the skin microcirculation not only through ETA receptors but also possibly through ETB receptors. Diltiazem inhibited endothelin-induced vasoconstriction, but endothelin antagonists were slightly more effective. Thus, endothelin antagonists represent potent new tools to interfere with the vascular effects of endothelin in CAD patients. Future studies must confirm these findings in other areas of the circulation.

Characterization of [125I]-endothelin-1 and [125I]-BQ3020 binding to rat cerebellar endothelin receptors

1. We performed radioligand binding experiments on rat cerebellar homogenates using [125I]-endothelin-1 ¿[1251]-ET-1¿ and [125I]-BQ3020 to examine the pharmacology of endothelin receptors in rat brain. Saturation experiments demonstrated a single population of binding sites with high affinity for both radioligands ([125I]-ET-1, pKd = 8.94 +/- 0.17; [125I]-BQ3020, pKd = 9.18 +/- 0.14 nM; mean +/- s.e.mean). However, [125I]-BQ3020 only recognised approximately one third the number of endothelin receptors measured with [125I]-ET-1. 2. Saturation binding experiments with [125I]-PD151242 revealed high affinity binding to a single population of ETA receptors in the cerebellar homogenates (pKd = 9.95 +/- 0.14; Bmax = 30 +/- 15 fmol mg-1 protein). 3. Competition experiments were performed with ligands that are either non-selective for endothelin receptor subtypes. The rat cerebellar endothelin receptor displayed a high affinity for endothelin-1 (ET-1), endothelin-3 (ET-3) and sarafotoxin-S6c (STX-6c) although the affinity for ET-3 was slightly higher than the affinity for ET-1 using both radioligands. The selective ETA antagonists, BQ123, BMS-182,874 and JKC-301 all displayed low affinities at the endothelin receptors. In contrast the selective ETB agonists, IRL1620 and [Ala1,3,11,15]ET-1 and the selective ETB antagonist, BQ-788 had moderate affinities at the endothelin receptor, in the low nanomolar range. The ETB agonist, BQ3020, had approximately 10 fold higher affinity than IRL1620 and [Ala1,3,11,15]ET-1 at the rat cerebellar endothelin receptors. The non-selective antagonists, Ro-46,2005, Ro-47,0203 and PD-142,893 displayed moderate affinities at the cerebellar receptor. 4. Since [125I]-BQ3020 recognises only a fraction of the [125I]-ET-1 binding sites, the majority of the endothelin receptors in the cerebellum cannot be classed as ETB. Although [125I]-PD151242 was able to detect ETA receptors in the rat cerebellar homogenates, the small population of ETA receptors (2% of the total endothelin population as measured with [125I]-ET-1) could not account for the non-ETB receptor population. We conclude that the rat brain cerebellar receptor has a profile similar to the ETB1 receptor as it has a high affinity for ET-1, ET-3, STX-6c and was moderately sensitive to PD-142,893. However, as the ETB ligands BQ-788, IRL1620 and [Ala1,3,11,15]ET-1 have only a moderate affinity for the rat cerebellar endothelin receptor and since ET-3 has a higher affinity as compared to ET-1, our findings suggest that the rat cerebellum contains predominately ETc receptors.

The effects of bosentan on cerebral blood flow and histopathology following middle cerebral artery occlusion in the rat

The involvement of endothelins in the cerebrovascular events which follow a focal ischemic insult in the rat was explored in the present study. Intravenous (i.v.) administration of bosentan (3, 15 and 30 mg/kg), an endothelin ETA and ETB receptor antagonist, prior to middle cerebral artery occlusion in the rat did not significantly alter cortical perfusion in these rats. A 62 +/- 3% reduction in laser doppler flow was observed 10 min after middle cerebral artery occlusion in the vehicle-treated group compared to a 49 +/- 5% reduction in laser doppler flow in the group receiving 15 mg/kg bosentan. Pre-treatment with intravenous bosentan (15 mg/kg) prior to middle cerebral artery occlusion in the rat also failed to elicit significant alterations in the reduction in regional cerebral blood flow (frontal cortex; 81 +/- 13 ml/100 g/min) and subsequent hemispheric volume of ischemic damage observed (94 +/- 9 mm3) compared to the vehicle treated animals (68 +/- 9 ml/100 g/min, 113 +/- 5 mm3, respectively). Minimal changes were also observed in these endpoints, when a 15 mg/kg dose of bosentan was administered following middle cerebral artery occlusion. In conclusion bosentan failed to expose a major role for endothelins in focal ischemic pathology in the rat.

Endothelin receptor mediated constriction and dilatation in feline cerebral resistance arterioles in vivo

The receptors mediating the cerebrovascular actions of endothelins have been examined in feline cerebral resistance arterioles in vivo. The adventitial microapplication of the endothelin ETA receptor antagonist BQ-123 (cyclo D-aspartate-D-tryptophan-L-leucine-D-valine-L-proline) (0.1-10 microM) per se had minimal effect on cerebral resistance arterioles examined. The adventitial microapplication of endothelin-1 (10 nM) elicited a marked vasoconstriction of cerebral resistance arterioles (-29.1 +/- 1.9% from pre-injection baseline). The endothelin-1 induced vasoconstriction was attenuated, in a dose dependent manner, by the adventitial co-application of BQ-123 and endothelin-1 (estimated IC50 0.7 microM). The adventitial microapplication of the endothelin ETB receptor agonist BQ-3020 N-acetyl[Ala11,Ala15]ET-1 (6-21)) (0.001-1 microM) effected a dose dependent vasodilatation (EC50 30 nM, maximum response 25 +/- 5% from pre-injection baseline). The magnitude of the vasodilatation elicited by BQ-3020 (100 nM and 1 microM) was dependent on the pre-injection calibre of the arterioles examined. The intracarotid infusion (via the lingual artery) of BQ-3020 (0.5-500 pmol/min) had no significant effect on the calibre of cerebral resistance arterioles. These results suggest that the peptide endothelin ETB receptor agonist fails to gain access to the cerebrovascular endothelin ETB receptors following its intraluminal administration. These investigations indicate that endothelin ETA receptors mediate vasoconstriction and endothelin ETB receptors mediate vasodilatation in feline cerebral resistance arterioles in vivo.

Bosentan ameliorates cyclosporin A-induced hypertension in rats and primates

Cyclosporine-induced hypertension is a major problem in transplant therapy. The pathophysiology of this disease is unclear. Cyclosporine increases endothelin synthesis and release, which may contribute to this hypertension. We examined the effects of chronic endothelin receptor blockade with the novel nonpeptide endothelin receptor antagonist bosentan in two animal models of cyclosporine-induced hypertension. Cyclosporine was administered daily to female Wistar rats (10 mg/kg per day SC for 30 days) and marmosets (30 mg/kg per day PO for 20 days). Control rats received vehicle. Tail-cuff systolic pressure was significantly elevated in the cyclosporine-treated animals before the last week of treatment. Bosentan (100 mg/kg) in arabic gum or arabic gum alone was given daily to the rats by gavage during the last 5 days of cyclosporine treatment and to the marmosets for the last 7 days of cyclosporine treatment. Tail-cuff systolic pressure was measured daily during bosentan treatment. Bosentan but not gum alone significantly lowered blood pressure in the cyclosporine-hypertensive rats from 134 +/- l to 122 +/- 3 mm Hg (P<.Ol) and in the cyclosporine-hypertensive marmosets from 156 +/- 2 to 139+/- 4 mm Hg (P<.Ol). There were no differential effects on plasma creatinine concentration, endothelin concentration, or end-organ weights. Bosentan had no effect in the vehicle-treated rats. These data provide further evidence to support a role for endothelin in cyclosporine-induced hypertension and demonstrate the effectiveness of endothelin receptor antagonism as a novel treatment in cyclosporine-induced hypertension.

Discovery and optimization of a novel class of orally active nonpeptidic endothelin-A receptor antagonists

A novel class of endothelin-A receptor ligands was discovered by high-throughput screening. Lead structure optimization led to highly potent antagonists which can be synthesized in a short sequence. The compounds are endothelin-A-selective, are orally available, and show a long duration of action.