Synergistic inhibition by BQ-123 and BQ-788 of endothelin-1-induced contractions of the rabbit pulmonary artery

The Regulation of Endothelin-1 in Pregnancies Complicated by Gestational Diabetes: Uncovering the Vascular Effects of Insulin

Gestational diabetes mellitus (GDM) is a condition of pregnancy defined by new-onset hyperglycemia. GDM is associated with impaired maternal endothelial and vascular reactivity. Endothelin-1 (ET-1) is a potent vasoconstrictor that contributes to endothelial dysfunction, however, its abundance and actions in GDM are unclear. Maternal plasma was obtained from pregnancies complicated by GDM (n = 24) and gestation-matched controls (n = 42); circulating ET-1 levels were assessed by ELISA. Human omental arteries from healthy pregnancies and those complicated by GDM were dissected from omental fat biopsies and collected at cesarean section. mRNA expression of ET-1 and its receptors, ETA and ETB, in addition to vascular cell adhesion molecule-1 (VCAM1) and intercellular adhesion molecule-1 (ICAM1) were assessed by qPCR (n = 28). Using wire myography, we investigated vascular constriction to ET-1 (10-11-10-4 M) in omental arteries from pregnancies complicated by GDM, compared to gestation-matched controls (n = 7). GDM cases were stratified by clinical management, diet intervention (n = 5), or insulin treatment (n = 6). Additionally, arteries from healthy pregnancies were treated with insulin (1 mU/mL (n = 7) and 10 mU/mL (n = 5)) or vehicle control. Vasoactive response to ET-1 was measured via wire myography. Circulating ET-1 levels and mRNA expression of the ET-1 system in omental arteries were not found to be significantly different between pregnancies complicated by GDM compared to healthy controls. However, we found insulin treatment during pregnancy and in ex vivo models reduced ET-1 vasoconstriction of maternal vasculature in GDM. These data suggest insulin may improve vascular function in GDM, however, further investigation is needed to define the role of ET-1 in pregnancy.

Early manifestations and differential gene expression associated with photoreceptor degeneration in Prom1-deficient retina

Retinitis pigmentosa (RP) and macular dystrophy (MD) are characterized by gradual photoreceptor death in the retina and are often associated with genetic mutations, including those in the prominin-1 (Prom1) gene. Prom1-knockout (KO) mice recapitulate key features of these diseases including light-dependent retinal degeneration and constriction of retinal blood vessels. The mechanisms underlying such degeneration have remained unclear, however. We here analysed early events associated with retinal degeneration in Prom1-KO mice. We found that photoreceptor cell death and glial cell activation occur between 2 and 3 weeks after birth. Whereas gene expression was not affected at 2 weeks, the expression of several genes was altered at 3 weeks in the Prom1-KO retina, with the expression of that for endothelin-2 (Edn2) being markedly upregulated. Expression of Edn2 was also induced by light stimulation in Prom1-KO mice reared in the dark. Treatment with endothelin receptor antagonists attenuated photoreceptor cell death, gliosis and retinal vessel stenosis in Prom1-KO mice. Our findings thus reveal early manifestations of retinal degeneration in a model of RP/MD and suggest potential therapeutic agents for these diseases. This article has an associated First Person interview with the first author of the paper.

Hypoxia-induced pulmonary hypertension-Utilizing experiments of nature

An increase in pulmonary artery pressure is a common observation in adult mammals exposed to global alveolar hypoxia. It is considered a maladaptive response that places an increased workload on the right ventricle. The mechanisms initiating and maintaining the elevated pressure are of considerable interest in understanding pulmonary vascular homeostasis. There is an expectation that identifying the key molecules in the integrated vascular response to hypoxia will inform potential drug targets. One strategy is to take advantage of experiments of nature, specifically, to understand the genetic basis for the inter-individual variation in the pulmonary vascular response to acute and chronic hypoxia. To date, detailed phenotyping of highlanders has focused on haematocrit and oxygen saturation rather than cardiovascular phenotypes. This review explores what we can learn from those studies with respect to the pulmonary circulation. LINKED ARTICLES: This article is part of a themed issue on Risk factors, comorbidities, and comedications in cardioprotection. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.1/issuetoc.

Roles of Endothelin B Receptors and Endothelial Nitric Oxide Synthase in the Regulation of Pulmonary Hemodynamic in Cirrhotic Rats

INTRODUCTION

Hepatopulmonary syndrome and portopulmonary hypertension are common complications of liver disorders. This study aimed to determine roles of ET-B receptors and endothelial-derived NO synthase in the regulation of pulmonary hemodynamic in cirrhotic rats.

METHODS

Male Sprague-Dawley rats were divided into the Sham and common bile duct ligation (CBDL) groups. After 28 days, animals were anesthetized, and the right ventricle, femoral artery, and vein cannulated. Then, intravenous injection of BQ-788 (a selective ET-B receptor antagonist) and L-NAME (eNOS inhibitor) were performed sequentially.

RESULTS

After the first injection of BQ-788, the right ventricular systolic pressure (RVSP) and mean arterial systemic pressure increased only in the Sham group. L-NAME increased RVSP in the Sham and CBDL groups, whereas mean arterial systemic pressure elevated only in the Sham group significantly. Reinjection of BQ-788 increased RVSP in the Sham group, whereas it decreased RVSP in the CBDL group. Both plasma NO metabolites and lung endothelin-1 increased in the CBDL group.

CONCLUSION

ET-B receptors on the endothelial cells play roles in the regulation of pulmonary and systemic vascular tone in normal condition through the NO-mediated pathway, whereas ET-B receptors on the smooth muscle cells have a role in the pulmonary vascular tone in liver cirrhosis.

Endothelin-1-Rho kinase interactions impair lung structure and cause pulmonary hypertension after bleomycin exposure in neonatal rat pups

Bronchopulmonary dysplasia (BPD) is the chronic lung disease associated with premature birth, characterized by impaired vascular and alveolar growth. In neonatal rats bleomycin decreases lung growth and causes pulmonary hypertension (PH), which is poorly responsive to nitric oxide. In the developing lung, through Rho kinase (ROCK) activation, ET-1 impairs endothelial cell function; however, whether ET-1-ROCK interactions contribute to impaired vascular and alveolar growth in experimental BPD is unknown. Neonatal rats were treated daily with intraperitoneal bleomycin with and without selective ETA (BQ123/BQ610) and ETB (BQ788) receptor blockers, nonselective ET receptor blocker (ETRB) (bosentan), or fasudil (ROCK inhibitor). At day 14, lungs were harvested for morphometrics, and measurements of Fulton's index (RV/LV+S), medial wall thickness (MWT), and vessel density. Lung ET-1 protein and ROCK activity (phospho-MYPT-1:total MYPT-1 ratio) were also measured by Western blot analysis. Bleomycin increased lung ET-1 protein expression by 65%, RV/LV+S by 60%, mean linear intercept (MLI) by 212%, and MWT by 140% and decreased radial alveolar count (RAC) and vessel density by 40 and 44%, respectively (P < 0.01 for each comparison). After bleomycin treatment, fasudil and bosentan partially restored RAC and vessel density and decreased MLI, RV/LV+S, and MWT to normal values. Bleomycin increased ROCK activity by 120%, which was restored to normal values by bosentan but not selective ETRB. We conclude that ET-1-ROCK interactions contribute to decreased alveolar and vascular growth and PH in experimental BPD. We speculate that nonselective ETRB and ROCK inhibitors may be effective in the treatment of infants with BPD and PH.

Endothelin(A)-endothelin(B) receptor cross talk in endothelin-1-induced contraction of smooth muscle

The efficacy of selective endothelin (ET) receptor antagonists may be limited by a functional interaction between the ET(A) and ET(B) receptors. This interaction, also termed "cross talk", is characterized by the dependency of the inhibition of an ET-1 response due to antagonism of one ET receptor subtype upon concomitant antagonism of the other ET receptor subtype. Although a reduction in ET(A)-ET(B) receptor cross talk would presumably increase the efficacy of selective ET receptor antagonists, an approach that accomplishes this aim is largely absent due to a lack of mechanistic understanding. Toward this goal, we evaluated the characteristics and potential dependencies of cross talk in smooth muscle. Smooth muscle was adopted as an exemplar not only because cross talk is widely reported in this tissue type, thereby allowing numerous comparisons, but also significant controversy surrounds the use of selective versus nonselective ET receptor antagonists in ET-1-related pathophysiologies involving smooth muscle. Based on this evaluation, we suggest that ET(A)-ET(B) receptor cross talk is a dynamic process directed by either or both ET receptor subtypes and expressed to varying magnitudes depending on the ET-1 and selective ET receptor antagonist concentrations, tone due to intraluminal pressure/stretch, agonists acting at receptors other than the ET(A)/ET(B) receptors, and endothelial/epithelial function. It is speculated that ET(A)-ET(B) receptor cross talk occurs through signal transduction pathways along with changes at the receptor level. Pharmacologic intervention of the signaling pathways could increase the therapeutic efficacy of ET receptor antagonists.

Functional vascular smooth muscle-like cells derived from adult mouse uterine mesothelial cells

In mammalian visceral organs, vascular smooth muscle cells (VSMCs) originate from an epithelial-to-mesenchymal transition (EMT) of embryonic mesothelial cells (MCs). The ability of adult MCs to recapitulate EMT and to acquire smooth muscle (SM) markers upon provasculogenic culture suggested they might retain embryonic vasculogenic differentiation potential. However, it remains unknown whether adult MCs-derived SM-like cells may acquire specific vascular SM lineage markers and the functionality of differentiated contractile VSMCs. Here, we describe how a gentle trypsinization of adult mouse uterine cords could selectively detach their outermost uterine mesothelial layer cells. As other MCs; uterine MCs (UtMCs) uniformly expressed the epithelial markers β-catenin, ZO-1, E-cadherin, CD54, CD29, and CK18. When cultured in a modified SM differentiation media (SMDM) UtMCs initiated a loss of epithelial characteristics and gained markers expression of EMT (Twist, Snail, and Slug), stem and progenitor (Nanog, Sox2, C-kit, Gata-4, Isl-1, and nestin), SM (α-SMA, calponin, caldesmon, SM22α, desmin, SM-MHC, and smoothelin-B) and cardiac (BMP2, BMP4, ACTC1, sACTN, cTnI, cTnT, ANF, Cx43, and MLC2a). UtMCs repeatedly subcultured in SMDM acquired differentiated VSM-like characteristics and expressed smoothelin-B in the typical stress-fiber pattern expression of contractile VSMCs. Relevantly, UtMCs-derived VSM-like cells could generate "mechanical force" to compact collagen lattices and displayed in diverse degree voltage (K(+)) and receptor (endothelin-1, oxytocin, norepinephrine, carbachol and vasopressin)-induced [Ca(2+)](i) rises and contraction. Thus, we show for the first time that UtMCs could recapitulate in vitro differentiative events of early cardiovascular differentiation and transdifferentiate in cells exhibiting molecular and functional characteristics of VSMCs.

The role of endothelin and endothelin antagonists in traumatic brain injury: a review of the literature

OBJECTIVES

To date, there is increasing evidence for the role of endothelins in the pathophysiological development of cerebral vasospasms associated with a variety of neurological diseases, e.g., stroke and subarachnoid hemorrhage. In contrast, only little is known regarding the role of endothelins in impaired cerebral hemodynamics after traumatic brain injury. Therapeutic work in blocking the endothelin system has led to the discovery of a number of antagonists potentially useful in restoring cerebral blood flow after traumatic brain injury, potentially reducing the detrimental effects of secondary brain injury. Therefore, the present work provides an overview of background topics such as structures and biosynthesis of endothelins, different types as well as potential mechanisms and sites of action. In addition, the role of age for the effects of endothelins on cerebral hemodynamics after traumatic brain injury is discussed.

RESULTS

Description of data supporting the role of the endothelins play in a host of neurological deficits.

CONCLUSIONS

Endothelin antagonists may be effective as novel treatments for various neuropathologies.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Regulation of cerebral blood flow

The control of cerebral blood flow is complex, and only beginning to be elucidated. Studies have identified three key regulatory paradigms. The first is cerebral pressure autoregulation, which maintains a constant flow in the face of changing cerebral perfusion pressure. Flow-metabolism coupling refers to the brains ability to vary blood flow to match metabolic activity. An extensive arborization of perivascular nerves also serves to modulate cerebral blood flow, so-called neurogenic regulation. Central to these three paradigms are two cell types: endothelium and astrocytes. The endothelium produces several vasoactive factors that are germane to the regulation of cerebral blood flow: nitric oxide, endothelium-dependent hyperpolarization factor, the eicosanoids, and the endothelins. Astrocytic foot processes directly abut the blood vessels, and play a key role in regulation of cerebral blood flow. Lastly, new research has been investigating cell-cell communication at the microvascular level. Several lines of evidence point to the ability of the larger proximal vessels to coordinate vasomotor responses downstream.

Endothelin(A)-endothelin(B) receptor cross-talk and endothelin receptor binding

BACKGROUND AND OBJECTIVE

The magnitude of inhibition of an endothelin (ET)-1 response by selective blockade of the ET(A) or ET(B) receptors can be limited by apparent compensation mediated by the unblocked receptor. While the mechanism underlying this functionally defined interaction, or 'cross-talk', is not clear, binding studies suggest an interaction between the ET receptor subtypes.

KEY FINDINGS

These binding studies are reviewed and suggest that, in general, they support the hypothesis that ET(A) and ET(B) receptor activation of intracellular signalling pathways influence ET-1 binding to these receptor subtypes.

SUMMARY

However, the relationship of these binding studies to functional effects and, thus, functional ET(A)-ET(B) receptor cross-talk, remains largely untested.

Pharmacological characteristics of endothelin receptors on sheep rectal blood vessels

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

Endothelin receptor antagonists in preclinical models of pulmonary hypertension

Pulmonary hypertension (PH), a chronic disorder of the pulmonary vasculature, is characterized by progressive elevation in pulmonary artery pressure and the ultimate development of right-sided heart failure and death. Being a rapidly progressive disease with limited therapeutic options, the pathogenesis of PH is complex and multifactorial. The pathogenesis may result from a combination of vasoconstriction, inward vascular wall remodelling and in situ thrombosis that involves dysfunction of underlying cellular pathways and mediators. Among these, the activation of endothelin (ET) system has been shown to be important in the development and perpetuation of PH. Endothelin-1 (ET-1), a potent vasoconstrictor and mitogen, exerts its biological effects by binding to two G-protein-coupled receptor isoforms, endothelin A (ETA) receptor and endothelin B (ETB) receptor. These two receptors are nonredundant and unique because of distinct localization, unique binding locations and affinities for the endothelin peptide and activation of distinct signalling pathways. Importantly, there is now substantial evidence that direct antagonism of ET receptors that can block either ETA- or ETA- and ETB receptors can be beneficial for the treatment of PH in both preclinical and clinical setting. This review provides an overview of endothelin biology, various preclinical models that have been widely used to investigate the pathophysiology of PH as well as the individual roles of the ET receptors (ETA and ETB) and their regulation in disease pathogenesis. We also review current data on the use of selective and nonselective ET receptor antagonism in the preclinical PH models.

Endothelin receptor blockade in the management of pulmonary arterial hypertension: selective and dual antagonism

Various treatments approved by the United States Food and Drug Administration for the management of pulmonary arterial hypertension (PAH) target three of the many pathways implicated in the development of PAH: prostacyclin-, endothelin-1 (ET-1)-, and nitric oxide-mediated pathways. The objectives of this manuscript are to provide background information on the role of ET-1 in the pathogenesis of PAH, to provide theoretical considerations for the advantages and disadvantages of dual vs single endothelin receptor antagonists (ERAs) for the management of PAH, and to describe the clinical study results from randomized, double-blind, placebo-controlled trials for the various ERAs. ET receptors (ET(A) and ET(B)) have different densities and distributions throughout the body and are dynamically regulated, such that blockade of ET(A) and ET(B) receptors may have different results in normal vs pathological conditions. Although differences in biological effects can be found in studies of isolated cells, blood vessels and animal models, clinical treatment studies have not identified clear differences in efficacy among the various ERAs. The main differences appear to be in safety profiles, with a greater frequency of serum liver function abnormalities occurring with the available dual ET(A)/ET(B) antagonist, and possibly higher rates of peripheral edema noted with selective ET(A) agents. Head-to-head studies will be necessary to resolve the question of whether single vs dual blockade produces better clinical results with fewer side effects in patients with PAH.

The endothelin system as a therapeutic target in cardiovascular disease: great expectations or bleak house?

There is considerable evidence that the potent vasoconstrictor endothelin-1 (ET-1) contributes to the pathogenesis of a variety of cardiovascular diseases. As such, pharmacological manipulation of the ET system might represent a promising therapeutic goal. Many clinical trials have assessed the potential of ET receptor antagonists in cardiovascular disease, the most positive of which have resulted in the licensing of the mixed ET receptor antagonist bosentan, and the selective ET(A) receptor antagonists, sitaxsentan and ambrisentan, for the treatment of pulmonary arterial hypertension (PAH). In contrast, despite encouraging data from in vitro and animal studies, outcomes in human heart failure have been disappointing, perhaps illustrating the risk of extrapolating preclinical work to man. Many further potential applications of these compounds, including resistant hypertension, chronic kidney disease, connective tissue disease and sub-arachnoid haemorrhage are currently being investigated in the clinic. Furthermore, experience from previous studies should enable improved trial design and scope remains for development of improved compounds and alternative therapeutic strategies. Although ET-converting enzyme inhibitors may represent one such alternative, there have been relatively few suitable compounds developed, and consequently, clinical experience with these agents remains extremely limited. Recent advances, together with an increased understanding of the biology of the ET system provided by improved experimental tools (including cell-specific transgenic deletion of ET receptors), should allow further targeting of clinical trials to diseases in which ET is involved and allow the therapeutic potential for targeting the ET system in cardiovascular disease to be fully realized.

Dual ET(A)/ET(B) vs. selective ET(A) endothelin receptor antagonism in patients with pulmonary hypertension

Since the identification of endothelin as a key mediator in the pathogenesis of several diseases, including pulmonary arterial hypertension (PAH), the pharmacologic control of the activated endothelin system with endothelin receptor antagonists (ETRA) has been a major therapeutic achievement for the treatment of patients with PAH. To date, dual ET(A)/ET(B) and selective ET(A) receptor antagonists have clinically been evaluated. To answer the question of whether selective or dual ETRA is preferable in patients with PAH, experimental and clinical data with relevance to the pulmonary circulation are reviewed in this article. Whereas experimental and clinical data provide unambiguous evidence that ET(A) receptors mediate the detrimental effects of ET-1, such as vasoconstriction and cell proliferation, the elucidation of the role of ET(B) receptors has been more complex. It has been shown that there is a subpopulation of ET(B) receptors on smooth muscle cells and fibroblasts mediating vasoconstriction and proliferation. On the contrary, there is clear evidence that endothelial ET(B) receptors continue to mediate vasodilation, vasoprotection and ET-1 clearance despite the pathology associated with pulmonary hypertension. More difficult to assess is the net effect of these mechanisms in patients to be treated with ETRA. When considering the available data from controlled clinical trials, nonselectivity does not appear to carry a relevant clinical benefit for the treatment of patients with PAH when compared with selective ET(A) receptor antagonism.

Difference in the characteristics of ETA-receptor-stimulated response between rat small pulmonary and renal arteries

We investigated the difference in the characteristics of endothelin-1 (ET-1)-induced contraction and the responses of intracellular Ca(2+) concentration ([Ca(2+)](i)) between rat small pulmonary artery and renal artery. ET-1 (30 nM) failed to elicit any contraction in renal arteries pretreated with 3 microM BQ-123, an ETA blocker. However, in the pulmonary artery a combination of BQ-123 and BQ-788, an ETB blocker (5 microM each), only partially inhibited the ET-1-induced contraction (by 25%). To focus on the ETA receptor, in the presence of 5 microM BQ-788, nitric oxide donors (sodium nitroprusside and (+/-)-S-nitroso-N-acetylpenicillamine) and forskolin reduced both the ET-1-induced contraction and increase in [Ca(2+)](i) in both pulmonary and renal arteries. However, the effects were stronger in the renal than in the pulmonary artery. ET-1-induced increase in [Ca(2+)](i) was only partially attenuated by 10 microM verapamil (to 81% of control) in pulmonary arteries but was reduced to 56.1% of control in renal arteries. Our results provide evidence that ET-1 may activate ET receptor(s) insensitive to both BQ-123 and BQ-788 in rat small pulmonary artery, at least under these conditions. Furthermore, the effects of relaxants such as L-type Ca(2+) channel blocker and nitric oxide donors on the ET-1-induced contraction were studied.

Endothelin-1 increases intracellular Ca(2+) in rabbit pulmonary artery smooth muscle cells through phospholipase C

In freshly isolated rabbit pulmonary artery smooth muscle cells, endothelin (ET)-1 induced a transient increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) followed by a return to the initial [Ca(2+)](i). This response was not abolished by the voltage-dependent Ca(2+) channel blocker nicardipine or removal of Ca(2+) from the bath solution but was inhibited by ryanodine and thapsigargin. This finding suggested that the increase in [Ca(2+)](i) induced by ET-1 was attributable to release of Ca(2+) from ryanodine- and inositol 1,4,5-trisphosphate-sensitive intracellular Ca(2+) stores. The transient increase in [Ca(2+)](i) induced by ET-1 was also inhibited by pretreatment with antagonists of ET type A and B (ET(A) and ET(B)) receptors (BQ-123 and BQ-788, respectively). Furthermore, the ET(B) receptor agonist IRL-1620 induced an increase in [Ca(2+)](i) that was followed by a sustained increase in [Ca(2+)](i); the sustained increase in [Ca(2+)](i) was blocked by nicardipine. Using the nystatin-perforated patch-clamp technique, we found that IRL-1620 caused an increase in Ca(2+) current that was inhibited by addition of ET-1. ET-1 did not inhibit Ca(2+) current when cells were pretreated with BQ-123. These results suggested that when both receptor types are activated, the opposing responses lead to abolition of the sustained [Ca(2+)](i) increases induced by ET(B) receptor activation. Western blot analysis confirmed expression of ET(A) and ET(B) receptors. Finally, U-73122 inhibited the ET-1-induced [Ca(2+)](i) increase, indicating that phospholipase C was involved in modulation of the ET-1-induced [Ca(2+)](i) increase in rabbit pulmonary artery smooth muscle cells.

Multiple actions of halothane on contractile response to noradrenaline in isolated mesenteric resistance arteries

Halothane, a volatile anaesthetic, produces systemic hypotension and significantly alters organ blood flow. Isometric force was recorded in isolated rat small mesenteric arteries to investigate its action on contractile response to noradrenaline, the sympathetic neurotransmitter. Halothane (1-5%) enhanced contractile response to noradrenaline in the endothelium-intact arteries, but had little influence in the endothelium-denuded arteries. However, halothane consistently inhibited the noradrenaline response in the endothelium-denuded arteries pretreated with ryanodine (10 microM). The enhancement of the contractile response to noradrenaline in the endothelium-intact arteries was unaffected by treatment with N(G)-nitro L-arginine, tetraethylammonium, apamin, charybdotoxin, indomethacin, diclofenac, nordihydroguaiaretic acid, BQ-123, BQ-788, losartan, ketanserin, or superoxide dismutase. Halothane prolonged vasorelaxation after washout of noradrenaline in the endothelium-denuded arteries. Both ryanodine and vanadate (0.1-0.3 mM), a putative inhibitor of the plasma membrane Ca2+-ATPase, also prolonged the vasorelaxation. Halothane still prolonged the vasorelaxation in the ryanodine-treated arteries, but not in the vanadate-treated arteries. Halothane decreased the pD2 value for the pCa-force relation in the beta-escin-permeabilised, endothelium-denuded arteries. Halothane appears to influence contractile response to noradrenaline through multiple actions including endothelium-dependent enhancing, endothelium-independent enhancing, and endothelium-independent inhibitory actions. Nitric oxide, endothelium-derived hyperpolarising factor, cyclooxygenase products, lipoxygenase products, endothelin-1, angiotensin-II, serotonin, and superoxide anions are not involved in the endothelium-dependent enhancement. The endothelium-independent enhancement is presumably due to its ability to stimulate Ca2+ release from the ryanodine-sensitive intracellular stores, while the endothelium-independent inhibition is due, at least in part, to depressed Ca2+-activation of contractile proteins. Halothane may inhibit the plasma membrane Ca2+-ATPase of vascular smooth muscle cells.

Vascular reactivity of mesenteric arteries and veins to endothelin-1 in a murine model of high blood pressure

We characterized vascular reactivity to endothelin-1 (ET-1) in mesenteric vessels from DOCA-salt hypertensive and SHAM control mice and assessed the effect that endothelial-derived vasodilators have on ET-1-induced vasoconstriction. Changes in the diameter of unpressurized small mesenteric arteries and veins (100- to 300-microm outside diameter) were measured in vitro using computer-assisted video microscopy. Veins were more sensitive than arteries to the contractile effects of ET-1. There was a decrease in arterial maximal responses (E(max)) compared to veins, this effect was larger in DOCA-salt arteries. The selective ET(B) receptor agonist, sarafotoxin 6c (S6c), contracted DOCA-salt and SHAM veins but did not contract arteries. The ET(B) receptor antagonist, BQ-788 (100 nM), but not the ET(A) receptor antagonist, BQ-610 (100 nM), blocked S6c responses. BQ-610 partially inhibited responses to ET-1 in mesenteric veins from DOCA-salt and SHAM mice while BQ-788 did not affect responses to ET-1. Co-administration of both antagonists inhibited responses to ET-1 to a greater extent than BQ-610 alone suggesting a possible functional interaction between ET(A) and ET(B) receptors. Responses to ET-1 in mesenteric arteries were completely inhibited by BQ-610 while BQ-788 did not affect arterial responses. Nitric oxide synthase inhibition potentiated ET-1 responses in veins from SHAM but not DOCA-salt mice. There was a prominent role for ET-mediated nitric oxide release in DOCA-salt but not SHAM arteries. In summary, these studies showed a differential regulation of ET-1 contractile mechanisms between murine mesenteric arteries and veins.

Prostanoid receptor expression by human airway smooth muscle cells and regulation of the secretion of granulocyte colony-stimulating factor

The prostanoid receptors on human airway smooth muscle cells (HASMC) that augment the release by IL-1β of granulocyte colony-stimulating factor (G-CSF) have been characterized and the signaling pathway elucidated. PCR of HASM cDNA identified products corresponding to EP2, EP3, and EP4receptor subtypes. These findings were corroborated at the protein level by immunocytochemistry. IL-1β promoted the elaboration of G-CSF, which was augmented by PGE2. Cicaprost (IP receptor agonist) was approximately equiactive with PGE2, whereas PGD2, PGF2α, and U-46619 (TP receptor agonist) were over 10-fold less potent. Neither SQ 29,548 nor BW A868C (TP and DP1receptor antagonists, respectively) attenuated the enhancement of G-CSF release evoking any of the prostanoids studied. With respect to PGE2, the EP receptor agonists 16,16-dimethyl PGE2(nonselective), misoprostol (EP2/EP3selective), 17-phenyl-ω-trinor PGE2(EP1selective), ONO-AE1-259, and butaprost (both EP2selective) were full agonists at enhancing G-CSF release. AH 6809 (10 μM) and L-161,982 (2 μM), which can be used in HASMC as selective EP2and EP4receptor antagonists, respectively, failed to displace to the right the PGE2concentration-response curve that described the augmented G-CSF release. In contrast, AH 6809 and L-161,982 in combination competitively antagonized PGE2-induced G-CSF release. Augmentation of G-CSF release by PGE2was mimicked by 8-BrcAMP and abolished in cells infected with an adenovirus vector encoding an inhibitor protein of cAMP-dependent protein kinase (PKA). These data demonstrate that PGE2facilitates G-CSF secretion from HASMC through a PKA-dependent mechanism by acting through EP2and EP4prostanoid receptors and that effective antagonism is realized only when both subtypes are blocked concurrently.

E-ring 8-isoprostanes are agonists at EP2- and EP4-prostanoid receptors on human airway smooth muscle cells and regulate the release of colony-stimulating factors by activating cAMP-dependent protein kinase

8-Isoprostanes are bioactive lipid mediators formed via the nonenzymatic peroxidation of arachidonic acid by free radicals and reactive oxygen species. However, their cognate receptors, biological actions, and signaling pathways are poorly studied. Here, we report the effect of a variety of E- and Falpha-ring 8-isoprostanes on the release of granulocyte/macrophage colony-stimulating factor (GM-CSF) and granulocyte colony-stimulating factor (G-CSF) from human airway smooth muscle (HASM) cells stimulated with interleukin-1beta (IL-1beta). The elaboration of GM-CSF and G-CSF by IL-1beta was inhibited and augmented, respectively, in a concentration-dependent manner by 8-iso-prostaglandin (PG) E(1) and 8-iso-PGE(2), but not by 8-iso-PGF(1alpha), 8-iso-PGF(2alpha), and 8-iso-PGF(3)alpha. AH 6809 (6-isopropoxy-9-oxoxanthine-2-carboxylic acid), an EP(1)-/EP(2)-/DP-receptor blocking drug, antagonized the inhibitory effect of 8-iso-PGE(1) and 8-iso-PGE(2) on GM-CSF output with an affinity consistent with an interaction at prostanoid receptors of the EP(2)-subtype. In contrast, the facilitation by 8-iso-PGE(1) and 8-iso-PGE(2) of G-CSF release was unaffected by AH 6809 and the selective EP(4)-receptor antagonist L-161,982 [4'-[3-butyl-5-oxo-1-(2-trifluoromethyl-phenyl)-1,5-dihydro-[1,2,4]triazol-4-ylmethyl]-biphenyl-2-sulfonic acid (3-methyl-thiophene-2-carbonyl)-amide]. However, when used in combination, AH 6809 and L-161,982 displaced 5-fold to the right the 8-iso-PGE and 8-iso-PGE concentration-response curves. The opposing (1)effect of E-ring (2)8-isoprostanes on GM-CSF and G-CSF release was mimicked by 8-bromo-cAMP and abolished in cells infected with an adenovirus vector encoding an inhibitor protein of cAMP-dependent protein kinase (PKA). Together, these data demonstrate that E-ring 8-isoprostanes regulate the secretion of GM-CSF and G-CSF from HASM cells by a cAMP- and PKA-dependent mechanism. Moreover, antagonist studies revealed that 8-iso-PGE(1) and 8-iso-PGE(2) act solely via EP(2) -receptors to inhibit GM-CSF release, whereas both EP(2)- and EP(4)-receptor subtypes positively regulate G-CSF output.

The Endothelin/Sarafotoxin-Induced Increase of the Proliferation of Undifferentiated and DMSO-Differentiated GEM-81 Goldfish Erythrophoroma Cells is Mediated by ETB Receptors

Endothelins (ETs) and sarafotoxins (SRTXs) have been reported to exert ET(B)-mediated effects on vertebrate pigment cells. GEM-81 cell line, a red pigment cell-derived cutaneous tumor of the teleost Carassius auratus, expresses ET(B) receptors and can be differentiated with 1.5% DMSO treatment, thus constituting an useful model to investigate ET and SRTX effects on cultured fish pigment cells. Our aim was to characterize the pharmacology and biological effects mediated by ET receptors in DMSO-differentiated and undifferentiated cells. ET subtype receptors and their respective Ki values in both cell types were determined by competitive binding assays using (125)I ET-1 and BQ-485 (an ET(A) antagonist) or BQ-788 (an ET(B) antagonist). BQ-788, but not BQ-485, significantly reduced (125)I-ET-1 binding in both cell types, with similar low (Ki > nM) affinities. To determine the proliferation effects of ETs/SRTXs, cells were treated for 72 h with the hormones, and counted in a hemocytometer. The proliferation assays were repeated for SRTX S6c in the presence or absence of BQ-788. The results demonstrated that, with the exception of ET-1 (biphasic effect) and ET-3 (no significant effect) in undifferentiated GEM-81 cells, all the tested hormones induced increases in the proliferation of both types of cells. The hormones were equipotent in DMSO-differentiated cells, which exhibited increased sensitivity to ETs, but not to SRTXs, as compared with undifferentiated cells. The BQ-788 antagonistic effect was also exerted on the proliferation responses to SRTX S6c. These results corroborate the long and important evolutionary history of the ET/SRTX receptor system in vertebrate pigment cells.

Role of endothelin ETB receptor in the pathogenesis of monocrotaline-induced pulmonary hypertension in rats

We investigated the role of endothelin ETB receptor in the development of monocrotaline-induced pulmonary hypertension, by using the spotting-lethal (sl) rat, which carries a naturally occurring deletion in the endothelin ETB receptor gene. Three weeks after injection of saline or monocrotaline (60 mg/kg, s.c.), hemodynamics, cardiac hypertrophy and endothelin-1 levels in right ventricle were determined. Monocrotaline produced a marked pulmonary hypertension associated with increases in right ventricular pressure and hypertrophy, pulmonary arterial medial thickening and the endothelin-1 levels. The monocrotaline-induced alterations tended to be enhanced in ETB-deficient homozygous rats, compared with cases in wild-type rats. The treatment with selective ETA receptor antagonist ABT-627 [2R-(4-methoxyphenyl)-4S-(1,3-benzodioxol-5-yl)-1-(N,N-di(n-butyl)aminocarbonyl-methyl)-pyrrolidine-3R-carboxylic acid] for 3 weeks (10 mg/kg/day, twice daily) almost completely suppressed the monocrotaline-induced pulmonary hypertension and related organ damage both in ETB-deficient and wild-type animals to the same levels. Thus, we suggest that the antagonism of the ETA receptor is essential for the protection from monocrotaline-induced pulmonary hypertension, irrespective of the presence of the ETB receptors, although a protective role of ETB receptor-mediated action in the pathogenesis of this disease model cannot be ruled out.

Roles of Endothelin ETA and ETB Receptors in the Pathogenesis of Monocrotaline-Induced Pulmonary Hypertension

The functional roles of endothelin ETA and ETB receptors in the development of monocrotaline (MCT)-induced pulmonary hypertension were investigated using MCT-treated rats in the absence or presence of a daily administration of A-192621, a selective ETB receptor antagonist, ABT-627, a selective ETA receptor antagonist, or a combination of both drugs. Four weeks after the injection of saline or MCT (60 mg/kg, s.c.), cardiac hypertrophy, right ventricular systolic pressure and morphologic changes of pulmonary arteries were evaluated. Compared with the control animals, MCT produced marked pulmonary hypertension associated with increases in right ventricular pressure and hypertrophy, and pulmonary arterial medial thickening. These MCT-induced alterations were markedly suppressed by daily treatment with ABT-627 for 4 weeks (10 mg/kg/d, twice daily), whereas treatment with A-192621 significantly aggravated the above MCT-induced pathologic changes. The blockade of both receptor subtypes by a combination of A-192621 and ABT-627 also significantly improved the MCT-induced pathologic changes, to the same extent as with ABT-627 administration. Thus, an exaggerated response to MCT during ETB receptor blockade also seems to be mediated by ETA receptor activation, thereby suggesting that ETA receptor-mediated action is exclusively contributive to the pathogenesis of MCT-induced pulmonary hypertension, although we cannot rule out a protective role of ETB receptor-mediated action.

BQ-788, a selective endothelin ET(B) receptor antagonist

We describe characteristics of a selective endothelin (ET) ET(B) receptor antagonist, BQ-788 [N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D-1-methoxycarbonyltryptophanyl-D-norleucine], which is widely used to demonstrate the role of endogenous or exogenous ETs in vitro and in vivo. In vitro, BQ-788 potently and competitively inhibited (125)I-labeled ET-1 binding to ET(B) receptors in human Girrardi heart cells (hGH) with an IC(50) of 1.2 nM, but only poorly inhibited the binding to ET A receptors in human neuroblastoma cell line SK-N-MC cells (IC(50), 1300 nM). In isolated rabbit pulmonary arteries, BQ-788 showed no agonistic activity up to 10 microM and competitively inhibited the vasoconstriction induced by an ET(B)-selective agonist (pA(2), 8.4). BQ-788 also inhibited several bioactivities of ET-1, such as bronchoconstriction, cell proliferation, and clearance of perfused ET-1. Thus, it is confirmed that BQ-788 is a potent, selective ET(B) receptor antagonist. In vivo, in conscious rats, BQ-788, 3 mg/kg/h, i.v., completely inhibited a pharmacological dose of ET-1- or sarafotoxin6c (S6c) (0.5 nmol/kg, i.v.)-induced ET(B) receptor-mediated depressor, but not pressor responses. Furthermore, BQ-788 markedly increased the plasma concentration of ET-1, which is considered an index of potential ET(B) receptor blockade in vivo. In Dahl salt-sensitive hypertensive (DS) rats, BQ-788, 3 mg/kg/h, i.v., increased blood pressure by about 20 mm Hg. It is reported that BQ-788 also inhibited ET-1-induced bronchoconstriction, tumor growth and lipopolysaccharide-induced organ failure. These data suggest that BQ-788 is a good tool for demonstrating the role of ET-1 and ET(B) receptor subtypes in physiological and/or pathophysiological conditions.

Endothelin receptor function in mesenteric veins from deoxycorticosterone acetate salt-hypertensive rats

OBJECTIVES

To identify the receptors by which endothelin-1 (ET-1) increases venomotor tone in hypertension.

METHODS

Vascular reactivity to ET-1 and the selective endothelin receptor subtype B (ET(B)) agonist, sarafotoxin 6c (S6c), was studied in mesenteric blood vessels from deoxycorticosterone acetate (DOCA-salt) hypertensive and normotensive control rats. The diameter of small (< or = 280 microm) mesenteric arteries and veins was monitored in vitro using computer-assisted video microscopy. Contractions of mesenteric arteries (< or= 250 microm diameter) were also studied, using a myograph. ET-1 mRNA levels were measured in mesenteric arteries and veins using real-time RT-PCR techniques.

RESULTS

ET-1-induced contractions were reduced in arteries of DOCA-salt hypertensive rats compared with those of normotensive control rats; S6c produced negligible contractions in arteries from both groups. ET-1 concentration-responses curves in arteries measured using video microscopy or a myograph were similar. ET-1 and S6c caused veins to contract, and there were no differences between responses to these agonists in tissues from DOCA-salt hypertensive rats or normotensive control rats. Studies using ET(A) and ET(B) receptor antagonists indicated that ET-1-induced venoconstriction was mediated by ET(A) receptors. Potassium chloride concentration-response curves were similar in arteries and veins from normotensive control rats and DOCA-salt hypertensive rats. ET-1 mRNA levels in DOCA-salt hypertensive rat arteries or veins were not different from those in normotensive control rat arteries and veins.

CONCLUSIONS

These data indicate that ET-1 reactivity is maintained in mesenteric veins, but not arteries, in DOCA-salt hypertension. Therefore, the sustained increase in venomotor tone mediated by ET(A) receptors that is known to occur in vivo in DOCA-salt hypertensive rats is not caused by direct venoconstriction.

Endothelin-1 binding to endothelin receptors in the rat anterior pituitary gland: possible formation of an ETA-ETB receptor heterodimer

1. Interaction in the recognition of endothelin-1 (ET-1), a typical bivalent ET receptor-ligand, between ETA and ETB receptors was investigated in the rat anterior pituitary gland, using our quantitative receptor autoradiographic method with tissue sections preserving the cell-membrane structure and ET receptor-related compounds. 2. In saturation binding studies with increasing concentrations (0.77-200 pM) of 125I-ET-1 (nonselective bivalent radioligand), 125I-ET-1 binding to the rat anterior pituitary gland was saturable and single with a KD of 71 pM and a Bmax of 120 fmol mg(-1). When 1.0 microM BQ-123 (ETA antagonist) was added to the incubation buffer, binding parameters were 8.3 pM of KD and 8.0 fmol mg(-1) of Bmax, whereas 10 nM sarafotoxin S6c (ETB agonist) exerted little change in these binding parameters (KD, 72 pM; Bmax, 110 fmol mg(-1)). 3. Competition binding studies with a fixed amount (3.8 pM) of 125 I-ET-1 revealed that when 1.0 microM BQ-123 was present in the incubation buffer, ETB receptor-related compounds such as sarafotoxin S6c, ET-3, IRL1620 (ETB agonist), and BQ-788 (ETB antagonist) competitively inhibited 125I-ET-1 binding with K(i)s of 140,18,350 pM, and 14 nM, respectively, however, these compounds were not significant competitors for 125I-ET-1 binding in the case of absence of BQ-123. 4. In cold-ligand saturation studies with a fixed amount (390 pM) of 125I-IRL 1620 (ETB radioligand), IRL1620 bound to a single population of the ETB receptor, and no change was observed in binding characteristics in the presence of 1.0 microM BQ-123. 125I-IRL1620 binding was competitively inhibited by ET-1 and ET-3 in the absence of BQ-123, with K(i)s of 20 and 29 pM, respectively, the affinities being much the same as those of 29 nM, in the presence of 1.0 microM BQ-123. 5. Two nonbivalent ETA antagonists, BQ-123 and PD151242, were highly sensitive and full competitors for 125I-ET-1 binding (5.0 pM), in the presence of 10 nM sarafotoxin S6c. 6. Taken together with the present finding that mRNAs encoding the rat ETA and the ETB receptors are expressed in the anterior pituitary gland, we tentatively conclude that although there are ETA and ETB receptors with a functional binding capability for ET receptor-ligands, the ETB receptor does not independently recognize ET-1 without the aid of the ETA receptor. If this thesis is tenable, then ET-1 can bridge between the two receptors to form an ETA-ETB receptor heterodimer.

Combined endothelin receptor blockade evokes enhanced vasodilatation in patients with atherosclerosis

Endothelin (ET)-1 causes vasoconstriction via ET(A) and ET(B) receptors located on vascular smooth muscle cells and vasodilatation via ET(B) receptors on endothelial cells. Studies in vitro indicate an upregulation of ET(B) receptors in atherosclerosis. The present study investigated the vascular effects evoked by endogenous ET-1 in atherosclerotic patients. Forearm blood flow (FBF) was measured with venous occlusion plethysmography in 10 patients with atherosclerosis and in 10 healthy control subjects during intra-arterial infusion of selective ET receptor antagonists. The ET(B) receptor antagonist BQ788 evoked a significant increase in FBF (31+/-13%) in the patients, whereas a 20+/-9% reduction was observed in the control subjects. The ET(A) receptor antagonist BQ123 combined with BQ788 evoked a marked increase in FBF (102+/-25%) in the patients compared with no effect in the control subjects (-3+/-9%, P<0.001 versus patients). The ET(A) receptor antagonist BQ123 increased FBF to a similar degree in patients (39+/-11%) as in control subjects (41+/-11%). The increase in FBF evoked by selective ET(A) receptor blockade was significantly (P<0.05) less than that evoked by combined ET(A)/ET(B) receptor blockade in the atherosclerotic patients. These observations suggest an enhanced ET-1-mediated vascular tone in atherosclerotic patients, which is at least partly due to increased ET(B)-mediated vasoconstriction.

ET(A) and ET(B) receptors modulate the proliferation of human pulmonary artery smooth muscle cells

We determined the distribution of ET(A) and ET(B) receptors in pulmonary arteries from pulmonary hypertensive patients and control subjects, using in vitro autoradiography, and investigated their role in mediating the proliferative effects of endothelin-1 (ET-1) on distal human pulmonary artery smooth muscle cells (PASMCs). Distal arteries possessed more medial [(125)I]-ET-1 binding sites (105 +/- 10 versus 45 +/- 6 amol/mm(2); p < 0.001) and a greater proportion of ET(B) receptors than proximal arteries (36 +/- 3% versus 3 +/- 1%; p < 0.001). Receptor density in distal arteries and lung parenchyma was twofold greater (p < 0.05) in pulmonary hypertensive patients than in control subjects. ET-1 (10(-9)-10(-7) mol/L) stimulated DNA synthesis (147 +/- 10% of control subjects; p < 0.05) and attenuated the antiproliferative action of cicaprost and forskolin on PASMCs, these effects being mediated via ET(A) and ET(B) receptors. Serum-stimulated proliferation was attenuated by inhibiting either endogenous ET-1 release with phosphoramidon (10(-5) mol/L) or its action with PD145065 (10(-5) mol/L). Cicaprost (10(-10)-10(-7) mol/L) inhibited ET-1 release from PASMCs (49 +/- 16% of control after 24 h; p < 0.001) and increased intracellular cAMP levels, whereas ET(B) receptor stimulation selectively reduced cAMP levels. In conclusion, ET(A) and ET(B) receptors are differentially distributed in human pulmonary arteries. Both receptors promote the proliferation of PASMCs in vitro and may contribute to vascular remodeling in pulmonary hypertension.

Influence of indomethacin on effects of endothelin-1 on guinea pig isolated rings of common bile duct and sphincter of Oddi

The effects of endothelin-1 on motility of guinea pig extra-hepatic biliary tract portions were studied. Endothelin-1 (< or =100 nM) failed to contract rings of hepatic, cystic, proximal or distal common bile ducts, or choledochal or papillary halves of sphincter of Oddi. At 100 nM, endothelin-1 or sarafotoxin S6c (selective endothelin ET(B) receptor agonist) inhibited contractions of choledochal (but not papillary) sphincter of Oddi to carbachol (1 microM) by 63+/-5 and 45+/-9%, respectively. In distal common bile duct, indomethacin (5.6 microM) unmasked potent contractile effects of endothelin-1 [EC(50) 7.8 (5.5-11.1) nM; E(MAX) 80+/-6% of response to 80 mM KCl] and enhanced the contractile potency of carbachol (585-fold at EC(50) level), but not cholecystokinin C-terminal octapeptide. Inhibition of cholinergic responsiveness of the choledochal sphincter of Oddi by endothelin-1 was reduced by BQ-123 (1 microM; endothelin ET(A) receptor antagonist; cyclo[DTrp-DAsp-Pro-DVal-Leu]) and abolished by either BQ-123 plus BQ-788 (1 microM; endothelin ET(B) receptor antagonist; N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D-1-methoxycarboyl-D-norleucine) or indomethacin. Thus, eicosanoids of the cyclo-oxygenase pathway (i.e. prostanoids) suppress endothelin-1-induced contractions of distal common bile duct and mediate endothelin ET(A) and ET(B) receptor-dependent inhibition of cholinergic responsiveness of the choledochal portion of the sphincter of Oddi.

In depth pharmacological characterization of endothelin B receptors in the rat middle cerebral artery

Whereas the endothelin A receptor is generally believed to mediate vasoconstriction; the endothelin B receptor seems elusive; both dilative and constrictive responses have been reported. Using the in vitro arteriograph, a method allowing compartmentalized study of vessel segments, segments of rat middle cerebral artery were cannulated with micropipettes, pressurized and luminally perfused. Vessel diameters were evaluated using a microscope equipped with an imaging system. Both intra- and extraluminal applications of endothelin-1 produced constriction. Intraluminal administration of a selective endothelin B receptor agonist sarafotoxin 6c in precontracted cerebral arteries and in the presence of the endothelin A receptor blocker FR139317 caused vasodilation in a concentration-dependent manner. Inhibition of nitric oxide synthase significantly reduced the dilation induced by sarafotoxin 6c, whereas inhibition of cyclooxygenase and endothelium-derived hyperpolarizing factor did not.

Endothelin-1 induces contraction of human and Australian possum gallbladder in vitro

BACKGROUND

Endothelin-1 (ET-1) stimulates guinea pig gallbladder (GB) muscle strip contractility; however, the role and source of ET-1 in the GB remains to be elucidated.

AIMS

To determine the effect of ET-1 on human and possum GB muscle strip contractility and evaluate whether ET-1 is present in GB tissue.

METHODS

GB muscle strips were mounted in organ baths to measure isometric tension. ET-1 was added cumulatively with and without pretreatment with the neural blocker tetrodotoxin (TTX) or the ET receptor antagonists BQ-123, BQ-788, and tezosentan. Immunohistochemical localization of ET was performed on freshly fixed and cultured GBs.

RESULTS

ET-1 induced concentration-dependent increases in tone in human and possum GB strips (p<0.05). This response was unaffected by BQ-123, BQ-788, and TTX but antagonized by BQ-123+BQ-788 in the human tissue only. Tezosentan (10(-4) mol/l) blocked the ET-1-induced response in human and possum GB strips (p<0.001). Although ET immunoreactivity was absent in freshly fixed possum GB, immunoreactivity was observed in the GB epithelium of freshly fixed human tissue and in both possum and human tissue following 24 h of organ culture.

CONCLUSION

ET-1 acts directly on human and possum GB smooth muscle producing contractions, possibly via ET-B receptors. ET may be present under pathophysiological conditions altering GB function.

The endothelin system in septic and endotoxin shock

The view of the endothelium as a passive barrier has gradually changed as a number of endothelium-derived substances have been discovered. Substances like nitric oxide, prostaglandins and endothelins have potent and important properties, involving not only the circulation as such but also the response to stimuli like inflammation and trauma. The endothelin system, discovered in 1988, has not only strong vasoconstrictor properties, but also immunomodulating, endocrinological and neurological effects exerted through at least two types of receptors. Septic shock, a condition with high mortality, is associated with vast cardiovascular changes, organ dysfunction with microcirculatory disturbances and dysoxia. In the experimental setting, endotoxaemia resembles these changes and is, as well as septic shock, accompanied by a pronounced increase in plasma endothelin levels. The pathophysiology in septic and endotoxin shock remains to be fully elucidated, but several studies indicate that endothelial dysfunction is one contributing mechanism. Activation of the endothelin system is associated with several pathological conditions complicating septic shock, such as acute respiratory distress syndrome, cardiac dysfunction, splanchnic hypoperfusion and disseminated intravascular coagulation. Through the development of both selective and nonselective endothelin receptor antagonists, the endothelin system has been the object of a large number of studies during the last decade. This review highlights systematically the findings of previous studies in the area. It provides strong indications that the endothelin system, apart from being a marker of vascular injury, is directly involved in the pathophysiology of septic and endotoxin shock. Interventions with endothelin receptor antagonists during septic and endotoxin shock have so far only been done in animal studies but the results are interesting and promising.

Effect of a novel bifunctional endothelin receptor antagonist, IRL 3630A, on guinea pig respiratory mechanics

This study characterized the in vitro pharmacological properties of a newly developed endothelin receptor antagonist, N-butanesulfonyl-[N-(3, 5-dimethylbenzoyl)-N-methyl-3-[4-(5-isoxazolyl)-phenyl]-(D)- alanyl]-( L)-valineamide sodium salt (IRL 3630A), and its in vivo effects on respiratory mechanics were determined. IRL 3630A showed highly balanced affinities to human endothelin ET(A) and ET(B) receptors, giving apparent K(i) values of 1.5 and 1.2 nM, respectively. This compound also potently antagonized the endothelin-1-induced intracellular Ca(2+) increases in both embryonic bovine tracheal (EBTr) cells expressing endothelin ET(A) receptors and human Girardi heart (hGH) cells expressing endothelin ET(B) receptors. In guinea pig isolated tracheas having both endothelin ET(A) and ET(B) receptors, IRL 3630A greatly inhibited endothelin-1-induced contraction (pA(2)=7.1), which was partially or scarcely suppressed by the endothelin ET(A) receptor antagonist cyclo[-(D)-Trp-(D)-Asp-(L)-Pro-(D)-Val-(L)-Leu-] (BQ-123) or the endothelin ET(B) receptor antagonist N-(3, 5-dimethylbenzoyl)-N-methyl-3-(4-phenyl)-(D)-phenylalanyl-(L)-t ryptop han (IRL 2500), respectively. Bolus i.v. injections of IRL 3630A administered into anaesthetized guinea pigs at 10 and 30 microg/kg inhibited endothelin-1 (1.3 microg/kg)-induced changes in respiratory resistance and compliance in a dose dependent manner, whereas both sodium 2-benzo[1, 3]dioxol-5-yl-4-(4-methoxy-phenyl)-4-oxo-3-(3,4, 5-trimethoxy-benzyl)-but-2-enoate (an endothelin ET(A) receptor antagonist: PD 156707) and IRL 2500 at doses of up to 30 microg/kg did not affect endothelin-1-induced changes in respiratory mechanics, reflecting the in vitro results. IRL 3630A is thus an effective bifunctional endothelin receptor antagonist, and will be useful in clarifying the role of endothelin in pulmonary diseases such as bronchial asthma.

Antiadrenergic effects of endothelin-1 on the L-type Ca2+ current in dog ventricular myocytes

The effect of endothelin-1 (ET-1) on L-type Ca2+ current (I(Ca)) and the interaction of ET-1 with beta-adrenergic stimulation were studied in dog ventricular myocytes by means of a whole-cell patch-clamp technique. ET-1 (10(-8) M) had no effect on the baseline I(Ca), but at 10(-9)-10(-7) M, it inhibited the isoproterenol (ISO)-induced increase in I(Ca). The maximal inhibition induced by ET-1 at 3 x 10(-8) M was approximately 30%, and the median inhibitory (IC50) value of ET-1 was 1.1 x 10(-9) M. The inhibitory action of ET-1 (10(-8) M) on the ISO-induced increase in I(Ca) was markedly attenuated by the ET(A) antagonist FR139317 (10(-6) M) and was partially inhibited by the ET(B) antagonist BQ-788 (10(-6) M). The inhibitory action of ET-1 was totally inhibited by the nonselective ET-receptor antagonist, TAK-044 (10(-6) M). These results indicate that ET-1 exerts an antiadrenergic effect on the ISO-induced increase in I(Ca), which is mediated mainly by ET(A), but activation of ET(B) receptors might contribute to the effect of ET-1 to a lesser extent.

Localization and function of ET-1 and ET receptors in small arteries post-myocardial infarction: upregulation of smooth muscle ET(B) receptors that modulate contraction

Endothelin-1 (ET-1) has been implicated as a mediator of increased vascular tone during development of heart failure post-myocardial infarction (MI). In the present study, expression and pharmacology of ET-1 and its receptors were studied in small mesenteric arteries from rats at 5 and 12 weeks after coronary artery ligation for induction of MI, or sham-operation. In vessels from sham-operated and 5 week post-MI rats preproET-1mRNA, immunoreactive (ir) ET-1, ET(B) receptor mRNA and irET(B) receptor were confined to the endothelium, while ET(A) receptor mRNA was distributed throughout the media. At 12 weeks post-MI, preproET-1 and irET(A) receptor localization was similar but ET(B) receptor mRNA and immunoreactivity were detectable in the media, as well as the endothelium. The ET-1 concentration-response curve (CRC) was progressively shifted to the right in pressurized third generation mesenteric arteries from 5 and 12 week post-MI rats relative to sham-operated rats, with no change in the maximum. The ET(A) receptor antagonist BQ-123 (10(-6) M) induced a rightward shift of the ET-1 CRC in all vessels. Desensitization of ET(B) receptors, by exposure to SRTX S6c (3x10(-8) M), had no effect on the ET-1 CRC in vessels from 5 week post-MI or sham-op rats but induced a leftward shift in vessels from 12 week post-MI rats. These results identify the endothelium as the primary site of ET-1 synthesis in small arteries and the ET(A) receptor as mediating the effects of ET-1 in these vessels. However, ET(B) receptor expression increases in vascular smooth muscle post-MI and is linked to mechanisms that inhibit the contractile response to ET-1.

Regional modulation of cyclic nucleotides by endothelin-1 in rat pulmonary arteries: direct activation of G(i)2-protein in the main pulmonary artery

The ability of endothelin-1 (ET-1) to modulate the cyclic nucleotides, guanosine 3' 5' cyclic monophosphate (cyclic GMP) and adenosine 3' 5' cyclic monophosphate (cyclic AMP) was assessed in the main elastic pulmonary elastic artery (4 - 5 mm i.d.) and the small muscular pulmonary arteries (150 - 200 micrometer i.d.) of the rat. ET-1 caused an increase in cyclic GMP in the larger vessels but had no effect in the smaller arteries. The increase in cyclic GMP was not dependent on an intact endothelium and was inhibited by the ET(A)-receptor antagonist FR139137 (1 microM). ET-1 caused a decrease in cyclic AMP in the main pulmonary arteries, an effect that was partially blocked by FR139317 but not influenced by the ET(B)-receptor antagonist BQ-788 (1 microM) or removal of the vascular endothelium. In contrast, ET-1 caused an increase in cyclic AMP in the small vessels, an effect that was blocked by BQ-788 but unaffected by FR139317. In the main pulmonary arteries, ET-1 caused enhanced incorporation of radiolabelled ADP-ribose by cholera toxin into G(i)2 in the main pulmonary artery, an indicator of its receptor-mediated activation. In summary, we have shown that in the small muscular pulmonary artery of the rat, (where ET(B) mediated vasoconstriction prevails), there is an ET(B)-mediated increase in cyclic AMP with no net effect on cyclic GMP levels. In the large arteries, (where vasoconstriction is mediated via the ET(A) receptor), there is an ET(A)-mediated increase in cyclic GMP (endothelium independent) and an ET(A)-mediated (endothelium independent) decrease in cyclic AMP.

ET-1-induced pulmonary vasoconstriction shifts from ET(A)- to ET(B)-receptor-mediated reaction after preconstriction

Endothelin-1 (ET-1) has been reported to induce pulmonary vasoconstriction via either ET(A) or ET(B) receptors, and vasorelaxation after ET-1 injection has been observed. Our study investigated the effects of ET-1 in isolated rabbit lungs, which were studied at basal tone (part I) and after preconstriction (U-46619; part II). Pulmonary arterial pressure (PAP) and lung weight gain were monitored continuously. In part I, ET-1 (10(-8) M; n = 6; control) was injected after pretreatment with the ET(A)-receptor antagonist BQ-123 (10(-6) M; n = 6) or the ET(B)-receptor antagonist BQ-788 (10(-6) M; n = 6). The same protocol was carried out in part II after elevation of pulmonary vascular tone. ET-1 induced an immediate PAP increase (DeltaPAP 4.3 +/- 0.4 mmHg at 10 min) that was attenuated by pretreatment with BQ-123 (P < 0.05 at 10 min and P < 0.01 thereafter) and that was more pronounced after BQ-788 (P < 0.01 at 10 min and P < 0.001 thereafter). In part II, ET-1 induced an immediate rise in PAP with a maximum after 5 min (DeltaPAP 6.3 +/- 1.4 mmHg), leveling off at DeltaPAP 3.2 +/- 0.2 mmHg after 15 min. Pretreatment with BQ-123 failed to attenuate the increase. BQ-788 significantly reduced the peak pressure at 5 min (0.75 +/- 0.4 mmHg; P < 0.001) as well as the plateau pressure thereafter (P < 0.01). We conclude that ET-1 administration causes pulmonary vasoconstriction independent of basal vascular tone, and, at normal vascular tone, the vasoconstriction seems to be mediated via ET(A) receptors. BQ-788 treatment resulted in even more pronounced vasoconstriction. After pulmonary preconstriction, ET(A) antagonism exerted no effects on PAP, whereas ET(B) antagonism blocked the PAP increase. Therefore, ET-1-induced pulmonary vasoconstriction is shifted from an ET(A)-related to an ET(B)-mediated mechanism after pulmonary vascular preconstriction.

Does blockade of endothelinB1-receptor activation increase endothelinB2/endothelinA receptor-mediated constriction in the rabbit basilar artery?

The purpose of this study was to investigate whether endothelin (ET)-1 activation of ETB1 receptors influences the relative magnitude of ETA/ETB2 receptor-mediated ET-1 constriction in the rabbit basilar artery. Initial challenge of ET-1-constricted vessels with BQ610, an ETA-receptor antagonist, resulted in approximately 60% relaxation, and subsequent addition of BQ788, an ETB1/2-receptor antagonist, relaxed the remaining constriction. To test whether blockade of ETB1 receptors influenced the relative magnitude of ETA/ETB2 receptor-mediated constriction, ET-1-constricted vessels were exposed to RES-701-1, an ETB1-receptor antagonist, before challenge with BQ610 or BQ788. RES-701-1 enhanced the ET-1 constriction by approximately 60%, consistent with blockade of ETB1 receptor-mediated endothelium-dependent relaxation. In ET-1-constricted vessels treated with RES-701-1, BQ610 challenge resulted in complete relaxation, whereas BQ788 was without effect. However, when 10 nM acetylcholine was added to RES-701-1-treated ET-1-constricted vessels, (a) BQ610 challenge resulted in only approximately 30% relaxation, and subsequent BQ788 addition relaxed the remaining constriction; and (b) BQ788 challenge resulted in approximately 35% relaxation, and subsequent BQ610 addition relaxed the remaining constriction. Acetylcholine induced approximately 10% relaxation of RES-701-1-treated ET-1-constricted vessels. It is speculated that a dynamic relation exists between ETA and ETB2 receptor-mediated constriction, such that ET-1-induced ETB2 receptor-mediated constriction of the basilar artery is dependent on ETB1 receptor activation and, in the absence of this activation, the constriction reverts to completely ETA receptor mediated.

Vasoconstrictor effect of endothelin-1 in human skin: role of ETA and ETB receptors

The aim of this project was to investigate the role of ETA and ETB receptors in the mediation of endothelin (ET)-1-induced vasoconstriction in human skin. This information should provide important insights into the design of pharmacological intervention against skin vasospasm induced by ET-1 in peripheral vascular disease or surgical trauma. Vasoconstriction in response to intra-arterial drug infusion in isolated perfused human skin flaps (8 x 18 cm) derived from dermolipectomy specimens was assessed by studying changes in skin perfusion and perfusion pressure under constant flow rate in each drug treatment (n = 4). It was observed that ET-1 (10(-10) to 10(-8) M) and norepinephrine (NE, 10(-8) to 10(-5) M) caused skin vasoconstriction in a concentration-dependent manner, with the vasoconstrictor potency of ET-1 approximately 200-fold higher than NE. The ETA-receptor antagonist BQ-123 but not the ETB-receptor antagonist BQ-788 blocked the vasoconstrictor effect of ET-1. This observation was confirmed by studying skin perfusion using the dermofluorometry technique. In addition, ETB-receptor agonists BQ-3020 and sarafotoxin S6c (10(-9) to 10(-6) M) did not evoke skin vasoconstriction. BQ-3020 also did not elicit skin vasoconstriction even in the presence of 10(-5) M of Nomega-nitro-L-arginine methyl ester and indomethacin. Furthermore, results from saturable and competitive ET-1 radioligand membrane receptor binding assays revealed that high-affinity and capacity binding sites are predominantly the ETA receptor subtype in endothelium-denuded skin arteries and veins of 0.5-1.5 mm diameter, with an ETA-to-ETB receptor ratio of 83:17 in arteries (n = 5) and 78:22 in veins (n = 7). Results from the present functional and radioligand receptor binding studies clearly indicate that ET-1 is a very potent vasoconstrictor in human skin and its vasoconstrictor effect is primarily mediated by ETA receptors, with no significant participation from ETB receptors.

Radioiodinated endothelin-1: a radiotracer for imaging endothelin receptor distribution and occupancy

Endothelin (ET) is one of the most potent vasoconstrictors known. Recently, ET has been implicated in various diseases, e.g., acute renal failure and congestive heart failure, which present the possibility of treating such diseases with endothelin receptor antagonists. However, establishing the dosages for these antagonists may be difficult because no convenient physiologic indicator of action exists, and because of complexities in receptor function. Two receptor subtypes have been identified for which selective antagonists have been reported (e.g., BQ-123 for the ETA receptor and BQ-788 for the ETB receptor). Of the three natural peptide hormones (ET-1, ET-2, and ET-3), ET-1 exhibits high affinity for both subtypes of receptor. Using the selective peptide antagonists, and a nonpeptide antagonist with relatively balanced affinity for the two subtypes (L-749,329), we have characterized the interactions of [125I]ET-1 with its receptors in vivo (in rat). BQ-123, BQ-788, and L-749,329 inhibited binding consistent with binding to a single receptor site. However, the sum of inhibition by the selective antagonist was greater than 100% (as defined by inhibition with L-749,329), which suggests (a) lower in vivo selectivity than determined in vitro and/or (b) receptor subtype interactions. The latter explanation is supported, in part, by in vitro autoradiographic studies as well as studies in isolated tissues and cells. We synthesized ET-1 labeled with I-123 and obtained images of receptor distribution in both rat and rhesus monkey and have demonstrated our ability to visualize, via planar, noninvasive imaging, the occupancy of endothelin receptor by antagonists in both kidney and lung. [123I]ET-1 can therefore be used to determine clinical dosages of antagonist needed for receptor saturation.

Delayed loss of ETB receptor-mediated vasorelaxation after cold lesion of the rat parietal cortex

The aim of this study was to investigate the involvement of endothelins (ET) in brain injury. The effect of ET was studied in the isolated basilar artery (BA) taken from control, sham-operated, and cold-lesioned rats. Cold lesion was induced by application of a precooled (-78 degrees C) copper cylinder (outer diameter 5 mm) for 60 seconds to the intact dura over the parietal cortex. After precontraction with prostaglandin (PG) F2alpha, ET-3 (10(-10) to 10(-8) mol/L) dilated BA with a pD2 (negative log of the half-maximal concentration) of 9.06+/-0.031 (mean +/- SD) and a maximal effect (Emax) of 1.64+/-1.0 mN at 3 x 10(-9) mol/L in sham-operated animals. This dilation was reduced 24 and 48 hours after cold lesion by 33% and 73%, respectively, at 3 x 10(-9) mol/L. The effects of acetylcholine (10(-8) to 10(-4) mol/L) and sodium nitroprusside (10(-3) mol/L) were unaltered. Activation of the ETB receptor in thoracic aorta by the specific agonist IRL 1620 also resulted in a reduced dilation (51% by 48 hours after cold lesion). Reverse transcriptase-polymerase chain reaction of the BA showed unaltered expression of mRNA for the ETB receptor after cold lesion whereas ETB immunoreactivity in BA and in its intraparenchymal arteries was reduced at 24 and 48 hours. In contrast to the reduction of ET-3-induced dilation, the constrictor effects of ET-1 and ET-3 were retained after cold lesion. Endothelin-1 (10(-12) to 10(-6) mol/L) dose-dependently contracted segments of untreated control BA segments under resting conditions with a pD2 of 8.03+/-0.22 and an Emax of 6.35+/-0.70 mN. Further evidence that the constrictor ability of BA was not influenced by cold lesion is given by the unaltered response to 124 mmol/L K+ and 10(-6) mol/L serotonin. We conclude that the ETB receptor of BA after cold lesion is downregulated specifically, apparently at the posttranscriptional level. Because the ETB-mediated dilation in thoracic aorta was also reduced, downregulation of the ETB receptor apparently is not restricted to cerebral arteries. The nitric oxide-cyclic guanosine monophosphate system in BA is, however, intact.

Reduced reactivity of the middle cerebral artery and its large branches after cold lesion

The aim of this study was to measure vascular reactivity in the isolated middle cerebral artery (MCA) after brain injury. Segments of MCA were prepared from control, sham-operated, and cold-lesioned rats. Cold lesion was induced by application of a precooled (-78 degrees C) copper cylinder (diameter 5 mm) for 60 sec to the intact dura over the parietal cortex. Endothelin-1 (ET-1) (10(-12) to 3 x 10(-7) M) induced a dose-dependent contraction with a pD2 (-log10 EC50) of 8.36+/-0.12 (mean+/-SEM) and an Emax (maximal response) of 2.41+/-0.15 mN (millinewton) at 10(-7) M in sham-operated animals under resting conditions. This maximum contraction induced by 10(-7) M ET-1 was significantly (p < 0.05) reduced 24 and 48 h after cold lesion by 41% and 30%, respectively. After precontraction with 10(-5) M prostaglandin (PG) F2alpha, ET-3 (10(-12) to 10(-8) M) relaxed the MCA with an Emax of 0.42+/-0.07 mN at 10(-8) M and a pD2 of 9.20+/-0.19 in sham-operated animals. This relaxation was reduced 24 and 48 h after cold lesion by 19% and 62% at 10(-8) M, respectively. Concentration-effect curves for bradykinin (BK, 10(-8) to 10(-5) M) in uridine triphosphate (UTP, 10(-4) M)-precontracted MCA segments revealed relaxation with a pD2 of 7.08+/-0.10 and an Emax of 0.65+/-0.06 mN at 10(-6) M in sham-treated animals. This effect of BK was reduced by 35% and 20% at 10(-6) M 24 and 48 h after cold lesion, respectively. In addition, the contractile responses to 124 mM K+, 10(-5) M PGF2alpha and the dilation induced by 10(-3) sodium nitroprusside (SNP) were reduced in MCA segments taken 24 and 48 h after lesion compared with shams. We conclude that attenuation of ET effects can be explained, at least in part, by tachyphylaxis to ETs. The unspecific reduction of vascular reactivity may result from spreading depression.

Arrhythmogenic effects induced by coronary conversion of pulmonary big endothelin to endothelin: aggravation of this phenomenon in heritable hyperlipidemia

OBJECTIVES

We investigated whether endogenous pulmonary big endothelin has arrhythmogenic properties under normal conditions and in heritable hyperlipidemia.

BACKGROUND

Endothelin (ET), one of the most potent vasoconstrictors, is known to induce ventricular arrhythmias. It is unclear, however, whether its precursor, big endothelin, released from the lung, contributes to arrhythmogenesis.

METHODS

In a lung-heart model in which a Langendorff heart is serially perfused with the effluent from the isolated lung of the same animal, we evaluated arrhythmias in control and in Watanabe heritable hyperlipidemic (WHHL) rabbits.

RESULTS

In both controls (n=12) and WHHL (n=8), serial perfusion evoked a decrease in coronary flow (controls, -11+/-3%; WHHL, -25+/-6%) and a fourfold increase of ventricular extrasystoles (VES) (controls, 40.7+/-8; WHHL, 40.2+/-5 VES/40 min, p < 0.05). However, WHHL developed more and longer nonsustained ventricular tachycardias (VT) compared with controls (incidence, 1.38+/-1.1 vs. 0.33+/-0.5 VT/40 min, p < 0.05; length, 14.36+/-3.1 vs. 7.25+/-1.5 beats/VT, p < 0.05). Arrhythmias were not ischemia-induced because corresponding mechanical flow reduction had no arrhythmogenic effect (n=6 in controls and WHHL). Although vasoconstriction disappeared entirely, arrhythmias were only partly suppressed by ET(A) antagonists (BQ-123, 2 micromol/liter; A-127722, 20 micromol/liter). The ET-converting enzyme inhibitor phosphoramidon (50 micromol/liter) completely suppressed arrhythmias and vasoconstriction. The ET(B) antagonists (IRL-1038, 4 micromol/liter; IRL-1025, 5 micromol/liter) had no effect (n=6).

CONCLUSIONS

Endogenous pulmonary big ET produces arrhythmogenic effects that are aggravated in heritable hyperlipidemia. These effects, requiring coronary conversion of big ET into ET, are partly ET(A)-mediated and ET(B)-independent.