Pharmacological heterogeneity of both endothelin ETA- and ETB-receptors in the human saphenous vein

The interaction between endothelin-1 and nitric oxide in the vasculature: new perspectives

Nitric oxide (NO) and endothelin-1 (ET-1) are natural counterparts in vascular function, and it is becoming increasingly clear that an imbalance between these two mediators is a characteristic of endothelial dysfunction and is important in the progression of vascular disease. Here, we review classical and more recent data that suggest that ET-1 should be regarded as an essential component of NO signaling. In particular, we review evidence of the role of ET-1 in models of acute and chronic NO synthase blockade. Furthermore, we discuss the possible mechanisms by which NO modulates ET-1 activity. On the basis of these studies, we suggest that NO tonically inhibits ET-1 function, and in conditions of diminished NO bioavailability, the deleterious effects of unmitigated ET-1 actions result in vasoconstriction and eventually lead to vascular remodeling and dysfunction.

Involvement of endothelin B receptors in the endothelin-3-induced increase of ghrelin and growth hormone in Holstein steers

The present study was designed to determine the dose-dependent effects of endothelin-3 (ET-3) on the secretion of ghrelin and growth hormone (GH) and characterize the receptors involved in these effects. Eight Holstein steers were randomly assigned to receive intravenous bolus injections of vehicle (0.1% bovine serum albumin in saline), bovine ET-3 (0.1, 0.4, 0.7 and 1.0microg/kg), IRL1620 (selective ET(B) receptor agonist, 2.0microg/kg), [d-Lys(3)]-GHRP-6 (GH secretagogue receptor type 1a [GHS-R1a] antagonist, 20.0microg/kg) and bovine ET-3 (1.0microg/kg) combined with [d-Lys(3)]-GHRP-6 (20.0microg/kg), respectively. Blood samples were collected at -30, -15, 0, 5, 10, 15, 20, 25, 30, 35, 40, 50 and 60min relative to injection time. Concentrations of acyl ghrelin, total ghrelin (acyl and des-acyl ghrelin) and GH in plasma were analyzed by a double antibody radioimmunoassay system. Concentrations of acyl and total ghrelin were significantly increased by ET-3 in a dose-dependent manner. Concentrations of GH were markedly elevated by administration of 0.4, 0.7 and 1.0microg/kg of ET-3, and the effect of 0.7microg/kg was greater than that of 1.0microg/kg. The minimum effective dose of ET-3 in the secretion of ghrelin and GH was 0.4microg/kg. IRL 1620 mimicked the effects of ET-3 on the secretion of ghrelin and GH in plasma. ET-3-induced elevation of plasma GH was blocked by [d-Lys(3)]-GHRP-6. These results indicate that ET-3 dose-dependently stimulates ghrelin release, and ET(B) receptors involve in these processes. Moreover, this study shows that endogenous ghrelin response to ET-3 increases GH secretion through GHS-R1a.

Effect of chronic and selective endothelin receptor antagonism on microvascular function in type 2 diabetes

Vascular dysfunction, which presents either as an increased response to vasoconstrictors or an impaired relaxation to dilator agents, results in worsened cardiovascular outcomes in diabetes. We have established that the mesenteric circulation in Type 2 diabetes is hyperreactive to the potent vasoconstrictor endothelin-1 (ET-1) and displays increased nitric oxide-dependent vasodilation. The current study examined the individual and/or the relative roles of the ET receptors governing vascular function in the Goto-Kakizaki rat, a mildly hyperglycemic, normotensive, and nonobese model of Type 2 diabetes. Diabetic and control rats received an antagonist to either the ET type A (ETA; atrasentan; 5 mg x kg(-1) x day(-1)) or type B (ET(B); A-192621; 15 or 30 mg x kg(-1) x day(-1)) receptors for 4 wk. Third-order mesenteric arteries were isolated, and vascular function was assessed with a wire myograph. Maximum response to ET-1 was increased in diabetes and attenuated by ETA antagonism. ETB blockade with 15 mg/kg A-192621 augmented vasoconstriction in controls, whereas it had no further effect on ET-1 hyperreactivity in diabetes. The higher dose of A-192621 showed an ETA-like effect and decreased vasoconstriction in diabetes. Maximum relaxation to acetylcholine (ACh) was similar across groups and treatments. ETB antagonism at either dose had no effect on vasorelaxation in control rats, whereas in diabetes the dose-response curve to ACh was shifted to the right, indicating a decreased relaxation at 15 mg/kg A-192621. These results suggest that ETA receptor blockade attenuates vascular dysfunction and that ETB receptor antagonism exhibits differential effects depending on the dose of the antagonists and the disease state.

Effect of chronic endothelin receptor antagonism on cerebrovascular function in type 2 diabetes

Diabetes increases the risk of stroke and contributes to poor clinical outcomes in this patient population. Myogenic tone of the cerebral vasculature, including basilar arteries, plays a key role in controlling cerebral blood flow. Increased myogenic tone is ameliorated with ET receptor antagonism in Type 1 diabetes. However, the role of endothelin-1 (ET-1) and its receptors in cerebrovascular dysfunction in Type 2 diabetes, a common comorbidity in stroke patients, remains poorly elucidated. Therefore, we hypothesized that 1) cerebrovascular dysfunction occurs in the Goto-Kakizaki (GK) model of Type 2 diabetes, and 2) pharmacological antagonism of ETA receptors ameliorates, while ETB receptor blockade augments vascular dysfunction. GK or control rats were treated with antagonists to either ETA (atrasentan, 5 mg.kg(-1).day(-1)) or ETB (A-192621, 15 or 30 mg.kg(-1).day(-1)) receptors for 4 wk and vascular function of basilar arteries was assessed using a wire myograph. GK rats exhibited increased sensitivity to ET-1. ET(A) receptor antagonism caused a rightward shift, indicating decreased sensitivity in diabetes, while it increased sensitivity to ET-1 in control rats. Endothelium-dependent relaxation was impaired in diabetes. ETA receptor blockade restored relaxation to control values in the GK animals with no significant effect in Wistar rats and ETB blockade with 30 mg.kg(-1).day(-1) A-192621 caused paradoxical constriction in diabetes. These studies demonstrate that cerebrovascular dysfunction occurs and may contribute to altered regulation of myogenic tone and cerebral blood flow in diabetes. While ETA receptors mediate vascular dysfunction, ETB receptors display differential effects. These results underscore the importance of ETA/ETB receptor balance and interactions in cerebrovascular dysfunction in diabetes.

Endothelin-1 inhibits apoptosis through a sphingosine kinase 1-dependent mechanism in uterine leiomyoma ELT3 cells

Uterine leiomyomas, or fibroids, are the most common tumors of the myometrium. The ELT3 cell line, derived from Eker rat leiomyoma, has been successfully used as a model for the study of leiomyomas. We have demonstrated previously the potent mitogenic properties of the peptidic hormone endothelin (ET)-1 in this cell line. Here we investigated the antiapoptotic effect of ET-1 in ELT3 cells. We found that 1) serum starvation of ELT3 cells induced an apoptotic process characterized by cytochrome c release from mitochondria, caspase-3/7 activation, nuclei condensation and DNA fragmentation; 2) ET-1 prevented the apoptotic process; and 3) this effect of ET-1 was fully reproduced by ETB agonists. In contrast, no antiapoptotic effect of ET-1 was observed in normal myometrial cells. A pharmacological approach showed that the effect of ET-1 on caspase-3/7 activation in ELT3 cells was not dependent on phosphatidylinositol 3-kinase, ERK1/2, or phospholipase D activities. However, inhibitors of sphingosine kinase-1 (SphK1), dimethylsphingosine and threo-dihydrosphingosine, reduced the effect of ET-1 by about 50%. Identical results were obtained when SphK1 expression was down-regulated in ELT3 cells transfected with SphK1 small interfering RNA. Furthermore, serum starvation induced a decrease in SphK1 activity that was prevented by ET-1 without affecting the level of SphK1 protein expression. Finally, sphingosine 1-phosphate, the product of SphK activity, was as efficient as ET-1 in inhibiting serum starvation-induced caspase-3/7 activation. Together, these results demonstrate that ET-1 possesses a potent antiapoptotic effect in ELT3 cells that involves sphingolipid metabolism through the activation of SphK1.

L-type calcium channels in the renal microcirculatory response to endothelin

The signaling pathways of endothelin (ET)-1-mediated vasoconstriction in the renal circulation have not been elucidated but appear to be distinct between ETAand ETBreceptors. The purpose of this study was to determine the role of L-type Ca2+channels in the vasoconstrictor response to ET-1 and the ETBreceptor agonist sarafotoxin 6c (S6c) in the rat kidney. Renal blood flow (RBF) was measured with an ultrasonic flow probe in anesthetized rats, and a microcatheter was inserted into the renal artery for drug infusion. All rats were given vehicle (0.9% NaCl) or three successive bolus injections (1, 10, and 100 pmol) of ET-1 or S6c at 30-min intervals ( n = 6 in each group). ET-1 and S6c produced dose-dependent decreases in RBF. The Ca2+channel blocker nifedipine (1.5 μg) significantly attenuated the RBF response only at the highest doses of ET-1 and S6c. In the isolated blood-perfused juxtamedullary nephron preparation, Ca2+channel blockade with diltiazem had a very small inhibitory effect on ET-1-induced decreases in afferent arteriolar diameter only at the lowest concentrations of ET-1. In vascular smooth muscle cells isolated from preglomerular vessels, ET-1 produced a typical biphasic Ca2+response, whereas S6c had no effect on cytosolic Ca2+. Furthermore, Ca2+channel blockade (diltiazem or Ni2+) had no effect on the peak or sustained increase in cytosolic Ca2+produced by ET-1. These results support the hypothesis that L-type Ca2+channels play only a minor role in the constrictor responses to ET-1 in the renal microcirculation.

Increased contractility of diabetic rabbit corpora smooth muscle in response to endothelin is mediated via Rho-kinase beta

Corpus cavernosum smooth muscle (CCSM) from rabbits made diabetic for 6 months as a result of alloxan injection exhibited increased sensitivity (3vs 9 nM EC(50)) and generated 20-50% greater force to endothelin-1 (ET-1) compared to CCSM from normal rabbits. In contrast, the force produced by the CCSM in response to KCl and phenylephrine was not significantly altered in diabetic CCSM. The increased ET-1 sensitivity is associated with a two to three-fold upregulation of ET receptor A at both mRNA and protein levels in diabetic CCSM. ET-1-induced CCSM contraction is largely dependent upon Rho-kinase (ROK), since it is almost completely blocked by Y-27632 (a highly selective ROK inhibitor). Furthermore, expression of ROKbeta isoform is selectively upregulated in CCSM from diabetic rabbits. Thus, an increased CCSM tone, modulated by sensitization of the endothelin-mediated contractile pathway via ROK, may be a key component of the molecular mechanism of diabetes-induced erectile dysfunction.

Endothelin receptor subtype antagonist activity of S-0139 in various isolated rabbit and canine arteries

Vascular responses to endothelin peptides have been proposed to be mainly mediated via subtypes of the endothelin receptor, endothelin ET(A1), endothelin ET(B1), and endothelin ET(B2). The antagonist activity of 27-O-3-[2-(3-carboxy-acryloylamino)-5-hydroxyphenyl]acryloyloxy myricerone, sodium salt (S-0139) at these endothelin receptor subtypes was evaluated using isolated rabbit femoral, pulmonary, and mesenteric arteries. S-0139 competitively antagonized the endothelin-1-induced contraction mediated by the endothelin ET(A1) receptor in endothelium-denuded rabbit femoral arteries with a pA(2) value of 8.6+/-0.1. Endothelin ET(B2) receptor-mediated contraction induced by sarafotoxin S6c in endothelium-denuded rabbit pulmonary arteries was also inhibited by S-0139 with a pA(2) value of 5.6+/-0. 1. The pA(2) value of S-0139 for the endothelin ET(B1) receptor, evaluated from the endothelin-3-induced relaxant response in endothelium-intact rabbit mesenteric arteries, was 6.2+/-0.2. In isolated canine basilar, coronary, mesenteric and renal arteries, endothelin-1 caused concentration-dependent contractions with EC(50) values of 0.49+/-0.07, 0.61+/-0.25, 0.92+/-0.21 and 1.18+/-0.24 nM, respectively. S-0139 antagonized the endothelin-1-induced contraction in these arteries with pA(2) values of 8.0+/-0.1, 7. 6+/-0.2, 7.6+/-0.2 and 7.6+/-0.1, respectively. These results suggest that S-0139 is a potent and selective endothelin ET(A1) receptor antagonist, and that the contractions induced by endothelin-1 in canine basilar, coronary, mesenteric and renal arteries are mediated mainly via the endothelin ET(A1) receptor subtype.

Distribution of endothelin receptors in saphenous veins of African Americans: implications of racial differences

The relative density of endothelin-receptor subtypes A and B (ET(A) and ET(B), respectively) on endothelial and smooth muscle cells is the major determinant of the contractile response to endothelin-1 (ET-1). To investigate the effects of race on the distribution of ET receptors, the endothelin-receptor subtypes on membrane fractions prepared from saphenous veins obtained from African-American patients undergoing coronary bypass surgery were analyzed. Similar studies were repeated with endothelium-denuded samples to study the role of endothelium- and smooth muscle-derived ET(B) receptors. Competitive-binding experiments on membrane fractions by using [125I]-ET-1 and unlabeled ligands, ET-1, ET-3, sarafotoxin-6-c, and BQ-123 yielded two classes of binding sites on endothelium-intact vessels from both female and male subjects. In women, the maximal binding capacities were 91+/-6 and 67+/-13 fmol/mg protein for ET(A) and ET(B) receptors, respectively; the corresponding values in men were 178+/-19 and 127+/-13 fmol/mg protein. Similar experiments performed with endothelium-denuded saphenous veins indicated the presence of both receptor subtypes on vascular smooth muscle, in contrast to our earlier report on the presence of only ET(A) receptors on vessels obtained from white Americans. Our findings demonstrate that the ratio and the density of ET receptors are different in black and white Americans.

Endothelium-independent vascular relaxation mediating ETB receptor in rabbit mesenteric arteries

Vascular response mediating endothelin (ET)B receptor was studied using isolated rabbit mesenteric arteries. ET-1 (0.1-30 nM) caused a concentration-dependent contraction, whereas ET-3 >100 nM caused only weak contraction. Up to 1 microM of sarafotoxin S6c showed no contraction. In arteries precontracted with phenylephrine, ET-3 (0. 03-1 nM) caused a concentration-dependent relaxation, which was not affected by endothelium denudation. The ET-3-induced relaxation was antagonized by BQ-788 and PD-142893 but not by BQ-123 in the endothelium-denuded arteries. Treatment with indomethacin but not with NG-nitro-L-arginine methyl ester completely inhibited the relaxation. ET-3 stimulated the release of 6-keto-PGF1alpha and PGE2 from the endothelium-denuded arteries. ET-3 also significantly increased cAMP content but not cGMP content in the arteries. Radioligand-binding studies using serial sections of the artery revealed the expression of not only ETA but also ETB receptors in the smooth muscle layer of the arteries. These results suggest that ET-3 activates ETB receptor in smooth muscle cells of rabbit mesenteric artery, producing vasodilator prostaglandins from arachidonic acid probably via a catalysis of cyclooxygenase, which accumulates cAMP in subendothelial tissues and produces relaxations.

Endothelin B receptors on human endothelial and smooth-muscle cells show equivalent binding pharmacology

We have described the pharmacologic profiles of endothelin B receptors in human endothelial cells and vascular and nonvascular smooth-muscle cells. First, by amplifying endothelin B receptor numbers through the use of phosphoramidon and intact cell-binding techniques, we demonstrated the presence of these receptors in human umbilical vein endothelial cells (100% endothelin B receptors), human aortic smooth-muscle cells (22% endothelin B, 78% endothelin A receptors), and human bronchial smooth-muscle cells (55% endothelin B, 45% endothelin A receptors) by using [125I]-endothelin-1 radioligand binding. The typical binding profiles of the endothelin B receptors were established through competition binding curve analysis with endothelin-1, endothelin-3, sarafotoxin 6c, and the endothelin A receptor-selective antagonist BQ-123. In the presence of BQ-123, a diverse group of antagonists, including PD 142893, BQ-788, SB 209670, and Ro 47-0203, were used to probe for binding differences indicative of multiple endothelin B-receptor subtypes. The results indicate a rank order of potency for the antagonists of BQ-788 > SB 209670 > PD 142893 > Ro 47-0203 for each cell line, and that between any of these human cell lines, measurements of [125I]-endothelin-1-binding antagonism for each of the four test compounds differed by less than twofold. Although this study cannot discount the possibility of more than one endothelin B-receptor subtype in humans, it does indicate that these tissues express receptors that show equivalent binding pharmacology.

Different endothelins stimulate cytokine production by peritoneal macrophages and microglial cell line

Endothelins (ETs), potent vasoconstricting peptides, are produced by macrophages upon stimulation and may participate in the amplification or regulation of the inflammatory response. However, it is not clear whether ETs can act in an autocrine manner on macrophages and which role they play in relationship with other cytokines. To address these issues, we studied the effects of ETs on the production of inflammatory cytokines by mouse peritoneal macrophages or by a retrovirus-transformed microglial cell line. Here, we report that ET-2, but not ET-1 or ET-3, is able to stimulate the production of interleukin-1 (IL-1) and interleukin-6 (IL-6) by peptone-elicited mouse macrophages (pMO). In contrast, ET-3 and ET-1, but not ET-2, are active on microglial cells. No tumour necrosis factor-alpha (TNF-alpha) or nitric oxide (NO) were detected in the supernatants of ET-stimulated cultures. The activity of ET-2 on pMO was time and dose dependent and was inhibited by the addition of ETA and ETB receptor antagonists, BQ123 and IRL1038, respectively. In addition, when pMO were stimulated by interferon-gamma (IFN-gamma) in the presence of ET-2, a significant inhibition of IL-6 and IL-1 production was observed compared with the effects of the same doses of IFN-gamma or ET-2 used separately. The inhibition was specifically due to the activity of ET-2, since it was reversed by the addition of BQ123 or IRL1038. Similar results were seen when the content of NO in the supernatants of pMO stimulated by IFN-gamma plus ET-2 was evaluated. These results suggest that ETs may possess both a pro-inflammatory action on macrophages from different tissues and a regulatory activity on IFN-gamma.

Analysis of two pharmacologically predicted endothelin B receptor subtypes by using the endothelin B receptor gene knockout mouse

1. This study was performed to clarify whether the endothelin (ET) receptor subtypes mediating two pharmacologically heterogeneous response to ETH receptor agonists in normal mice are the product(s) of a single ETB receptor gene. 2. Vasodilator responses to sarafotoxin S6c (S6c) in the thoracic aorta and contractile responses to ET-1 and IRL1620 in the stomach were examined in tissues from normal and ETB receptor gene knockout mice, in the absence and presence of an ETA receptor antagonist, BQ-123, or an ETA/ETB receptor antagonist, PD142893. 3. In the normal mouse aorta precontracted with phenylephrine, S6c (0.1-100 nM) caused concentration-dependent relaxations (pD2 = 8.4). BQ-123 had no effect on these responses. However, PD142893 almost abolished the relaxations induced by 0.1-300 nM S6c. 4. In aortae taken from ETB receptor gene knockout mice, S6c up to 1 microM failed to cause relaxations, confirming that ETB receptors are involved in mediating this response. 5. In normal mouse gastric fundus, 0.1 nM-1 microM ET-1, S6c or IRL1620 caused dose-dependent, BQ-123-insensitive contractions, which were much more resistant to PD142893 than S6c-induced relaxations of the aorta. The pD2 values for S6c in the absence and presence of PD142893 (10 microM) were 8.12 +/- 0.11 and 7.70 +/- 0.11, respectively. 6. In the gastric fundus of the ETB receptor gene knockout mouse, S6c and IRL1620 caused no contractions. ET-1 (0.1 nM-1 microM) caused contractions sensitive to both BQ-123 and PD142893, indicating that only ETA receptors mediate ET-1-induced contractions of the knockout mouse gastric fundus. 7. Since both the PD142893-sensitive vasodilator response of the aorta and the PD142893-resistant contractile response of the gastric fundus to S6c were completely absent in the ETB receptor gene knockout mouse, we conclude that the two pharmacologically heterogeneous responses to S6c are mediated by receptors derived from the same ETB receptor gene.

Influence of endothelin ET(A) and ET(B) receptor antagonists on endothelin-induced contractions of the guinea pig isolated gall bladder

The receptors mediating guinea pig gall bladder (GPGB) contractions induced by endothelin-1 (ET-1) and related peptides were characterized using various ET receptor antagonists. As all ET-receptor agonists used, except sarafotoxin S6c (SRTX), failed to induce a clear-cut maximal response at the highest concentration tested (i.e. 100 nM), their potencies are expressed in terms of a CK50 (i.e. the concentration causing 50% of the response to 80 mM KCl). ET-1 (CK50 0.8 nM) was equipotent to ET-2 and SRTX (selective ET(B) receptor agonist), but more potent than ET-3 (5-fold) or IRL 1620 (selective ET(B) receptor agonist). BQ-123 (0.3 microM, peptidic ET(A) receptor antagonist) did not alter responses to ET-1, ET-3 or SRTX. BQ-788 (1 microM, peptidic ET(B) receptor antagonist) reduced the potency of ET-3 (9-fold at the CK50 level) and SRTX ( > 20-fold), but not ET-1. SRTX responses were unaffected by RES-701-1 (3 microM, peptidic ET(B) receptor antagonist). The combination BQ-123 (0.3 microM) plus BQ-788 (1 microM) did not modify responses to ET-1, inhibited SRTX responses similarly to BQ-788 alone and abolished ET-3 responses. Bosentan (1 microM, non-peptidic ET(A)/ET(B) receptor antagonist) reduced the potency of ET-1 (15-fold). ET-3 (9-fold) and SRTX (4-fold). In rat aorta, the antagonists blocked ET-1-induced contractions (BQ-123 and bosentan) or SRTX-induced endothelium-dependent relaxations (BQ-788, RES-701-1 and bosentan). Thus, the GPGB expresses both ET(A) and ET(B) receptors. As BQ-123 only blocked responses to ET-3 in the presence of BQ-788, there appears to be cross-talk between both receptor types. Also, the binding sites of ET-1 and ET-3 on the ET(A) receptor may not coincide entirely, as BQ-123, even in presence of BQ-788, did not affect ET-1-induced contractions.

Effect of ETA receptor antagonists on cardiovascular responses induced by centrally administered sarafotoxin 6b: role of sympathetic nervous system

The present study was carried out to investigate the cardiovascular effects of centrally administered SRT6b in saline, BQ123 and BMS182874 pretreated male Sprague-Dawley rats, using a radioactive microsphere technique. SRT6b (100 ng, ICV) produced a transient increase (40%) in blood pressure at 5 min followed by a sustained decrease (-42%) at 30 and 60 min in control rats. Total peripheral resistance and heart rate were not significantly altered. Cardiac output increased (16%) at 5 min and decreased 30 and 60 min following SRT6b administration. Central venous pressure was not affected by SRT6b. Regional blood flow and vascular resistance did not change at 5 min following administration of SRT6b. However, a significant decrease in blood flow to the brain, heart, kidneys, liver, spleen, gastrointestinal tract and mesentery and pancreas was observed 30 and 60 min following administration of SRT6b in control (saline treated) rats. Pretreatment with ETA selective receptor antagonists, BQ123 (10 micrograms, ICV) or BMS182874 (50 micrograms, ICV) significantly attenuated the pressor and depressor effects of centrally administered SRT6b. SRT6b induced decrease in blood flow was completely blocked by pretreatment with BQ123 or BMS182874. ET-1 (100 ng, ICV) produced an increase followed by a decrease similar to SRT6b. Reserpine (5 mg/kg, IP) pretreatment attenuated the cardiovascular effects of ET-1. Role of sympathetic nervous system was determined by measuring splanchnic nerve activity. SRT6b when administered in the lateral cerebral ventricle did not produce any significant effect at 5 min, however, a significant decrease in sympathetic nerve activity was observed 30 min after its administration. It is concluded that centrally administered SRT6b produces significant changes in systematic and regional blood circulation which can be completely blocked by ETA receptor antagonist. The cardiovascular effects of centrally administered SRT6b appear to be mediated through the sympathetic nervous system.