Therapeutic effects of endothelin receptor antagonists in stroke

Intranasal Radioiodinated Ferulic Acid Polymeric Micelles as the First Nuclear Medicine Imaging Probe for ETRA Brain Receptor

INTRODUCTION

The aim of this work was to prepare a selective nuclear medicine imaging probe for the Endothelin 1 receptor A in the brain.

MATERIAL AND METHODS

Ferulic acid (an ETRA antagonist) was radiolabeled using 131I by direct electrophilic substitution method. The radiolabeled ferulic acid was formulated as polymeric micelles to allow intranasal brain delivery. Biodistribution was studied in Swiss albino mice by comparing brain uptake of 131I-ferulic acid after IN administration of 131I-ferulic acid polymeric micelles, IN administration of 131I-ferulic acid solution and IV administration of 131I-ferulic acid solution.

RESULTS

Successful radiolabeling was achieved with an RCY of 98 % using 200 μg of ferulic acid and 60 μg of CAT as oxidizing agents at pH 6, room temperature and 30 min reaction time. 131I-ferulic acid polymeric micelles were successfully formulated with the particle size of 21.63 nm and polydispersity index of 0.168. Radioactivity uptake in the brain and brain/blood uptake ratio for I.N 131I-ferulic acid polymeric micelles were greater than the two other routes at all periods.

CONCLUSION

Our results provide 131I-ferulic acid polymeric micelles as a hopeful nuclear medicine tracer for ETRA brain receptor.

Endothelin-1 (ET-1) promotes a proinflammatory microglia phenotype in diabetic conditions

Diabetes increases the risk and severity of cognitive impairment, especially after ischemic stroke. It is also known that the activation of the endothelin (ET) system is associated with cognitive impairment and microglia around the periinfarct area produce ET-1. However, little is known about the effect of ET-1 on microglial polarization, especially under diabetic conditions. We hypothesized that (i) ET-1 activates microglia to the proinflammatory M-1-like phenotype and (ii) hypoxia/ lipopolysaccharide (LPS) activates the microglial ET system and promotes microglial activation towards the M-1 phenotype in diabetic conditions. Microglial cells (C8B4) cultured under normal-glucose (25 mmol/L) conditions and diabetes-mimicking high-glucose (50 mmol/L) conditions for 48 h were stimulated with ET-1, cobalt chloride (200 μmol/L), or LPS (100 ng/mL) for 24 h. PPET-1, ET receptor subtypes, and M1/M2 marker gene mRNA expression were measured by RT-PCR. Secreted ET-1 was measured by ELISA. A high dose of ET-1 (1 μmol/L) increases the mRNA levels of ET receptors and activates the microglia towards the M1 phenotype. Hypoxia or LPS activates the ET system in microglial cells and shifts the microglia towards the M1 phenotype in diabetic conditions. These in vitro observations warrant further investigation into the role of ET-1-mediated activation of proinflammatory microglia in post-stroke cognitive impairment in diabetes.

Repurposing of Approved Cardiovascular Drugs against Ischemic Cerebrovascular Disease by Disease-Disease Associated Network-Assisted Prediction

Stroke is one of the leading causes of death and disability globally, while intravenous thrombolysis with recombinant tissue plasminogen activator remains the only Food and Drug Administration (FDA)-approved therapy for ischemic stroke. The attempts to develop new treatments for acute ischemic stroke meet costly and spectacularly disappointing results, which requires both long time and high costs, whereas repurposing of safe existing drugs to new indications provides a cost-effective and not time-consuming alternative. Vascular protection is a promising strategy for improving stroke outcome, as vascular function is critical to both cardiovascular diseases (CVD) and ischemic cerebrovascular disease (ICD). Vascular function related biological processes and pathways maybe the critical associations between CVD and ICD. In this study, a multi-database, in silico target identification, gene function enrichment, and network pharmacology analysis integration approach was proposed and applied to investigate the FDA-approved CVD drugs repurposing for ICD. A list of 119 candidate drugs can be obtained for further investigation of their potential in ICD treatment. As a pleiotropic drug with multi-target, carvedilol was set an example to investigate its promising potential for ICD therapy. Our results indicated that the mode of action of carvedilol for ICD treatment may tightly associated with vascular function regulation and the mechanism is multi-target and multi-signaling pathway related. The disease-disease association network-assisted prediction needs further investigations. In summary, the proposed methods herein may provide a promising alternative to inferring novel disease indications for known drugs.

Susceptibility of juvenile and adult blood-brain barrier to endothelin-1: regulation of P-glycoprotein and breast cancer resistance protein expression and transport activity

Background

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) play a critical role in keeping neurotoxic substances from entering the brain. We and others have previously reported an impact of inflammation on the regulation of adult blood–brain barrier (BBB) efflux transporters. However, studies in children have not been done. From the pediatric clinical perspective, it is important to understand how the central nervous system (CNS) and BBB drug efflux transporters differ in childhood from those of adults under normal and inflammatory conditions. Therefore, we examined and compared the regulation of P-gp and BCRP expression and transport activity in young and adult BBB and investigated the molecular mechanisms underlying inflammatory responses.

Methods

Rats at postnatal day (P) P21 and P84, corresponding to the juvenile and adult stages of human brain maturation, respectively, were treated with endothelin-1 (ET-1) given by the intracerebroventricular (icv) route. Twenty-four hours later, we measured P-gp and BCRP protein expression in isolated brain capillary by immunoblotting as well as by transport activity in vivo by measuring the unbound drug partitioning coefficient of the brain (K_p,uu,brain) of known efflux transporter substrates administered intravenously. Glial activation was measured by immunohistochemistry. The release of cytokines/chemokines (interleukins-1α, 1-β (IL-1β), -6 (IL-6), -10 (IL-10), monocyte chemoattractant protein (MCP-1/CCL2), fractalkine and tissue inhibitor of metalloproteinases-1 (TIMP-1)) were simultaneously measured in brain and serum samples using the Agilent Technology cytokine microarray.

Results

We found that juvenile and adult BBBs exhibited similar P-gp and BCRP transport activities in the normal physiological conditions. However, long-term exposure of the juvenile brain to low-dose of ET-1 did not change BBB P-gp transport activity but tended to decrease BCRP transport activity in the juvenile brain, while a significant increase of the activity of both transporters was evidenced at the BBB in the adult brain. Moreover, juvenile and adult brain showed differences in their expression profiles of cytokines and chemokines mediated by ET-1.

Conclusions

BBB transporter activity during neuroinflammation differs between the juvenile and adult brains. These findings emphasize the importance of considering differential P-gp and BCRP transport regulation mechanisms between adult and juvenile BBB in the context of neuroinflammation.

Endothelin-mediated calcium responses in supraoptic nucleus astrocytes influence magnocellular neurosecretory firing activity

In addition to their peripheral vasoactive effects, accumulating evidence supports an important role for endothelins (ETs) in the regulation of the hypothalamic magnocellular neurosecretory system, which produces and releases the neurohormones vasopressin (VP) and oxytocin (OT). Still, the precise cellular substrates, loci and mechanisms underlying the actions of ETs on the magnocellular system are poorly understood. In the present study, we combined patch-clamp electrophysiology, confocal Ca(2+) imaging and immunohistochemistry to study the actions of ETs on supraoptic nucleus (SON) magnocellular neurosecretory neurones and astrocytes. Our studies show that ET-1 evoked rises in [Ca(2+) ](i) levels in SON astrocytes (but not neurones), an effect largely mediated by the activation of ET(B) receptors and mobilisation of thapsigargin-sensitive Ca(2+) stores. The presence of ET(B) receptors in SON astrocytes was also verified immunohistochemically. ET(B) receptor activation either increased (75%) or decreased (25%) SON firing activity, both in VP and putative OT neurones, and these effects were prevented when slices were preincubated in glutamate receptor blockers or nitric oxide synthase blockers, respectively. Moreover, ET(B) -mediated effects in SON neurones were also prevented by a gliotoxin compound, and when changes in [Ca(2+) ](i) were prevented with bath-applied BAPTA-AM or thapsigargin. Conversely, intracellular Ca(2+) chelation in the recorded SON neurones failed to block ET(B) -mediated effects. In summary, our results indicate that ET(B) receptor activation in SON astrocytes induces the mobilisation of [Ca(2+) ](i) , likely resulting in the activation of glutamate and nitric oxide signalling pathways, evoking in turn excitatory and inhibitory SON neuronal responses, respectively. Taken together, our study supports an important role for astrocytes in mediating the actions of ETs on the magnocellular neurosecretory system.

Brain endothelial barrier passage by monocytes is controlled by the endothelin system

Homeostasis of the brain is dependent on the blood-brain barrier (BBB). This barrier tightly regulates the exchange of essential nutrients and limits the free flow of immune cells into the CNS. Perturbations of BBB function and the loss of its immune quiescence are hallmarks of a variety of brain diseases, including multiple sclerosis (MS), vascular dementia, and stroke. In particular, diapedesis of monocytes and subsequent trafficking of monocyte-derived macrophages into the brain are key mediators of demyelination and axonal damage in MS. Endothelin-1 (ET-1) is considered as a potent pro-inflammatory peptide and has been implicated in the development of cardiovascular diseases. Here, we studied the role of different components of the endothelin system, i.e., ET-1, its type B receptor (ET(B)) and endothelin-converting enzyme-1 (ECE-1) in monocyte diapedesis of a human brain endothelial cell barrier. Our pharmacological inhibitory and specific gene knockdown studies point to a regulatory function of these proteins in transendothelial passage of monocytes. Results from this study suggest that the endothelin system is a putative target within the brain for anti-inflammatory treatment in neurological diseases.

Vascular Protection Following Cerebral Ischemia and Reperfusion

Despite considerable research that has contributed to a better understanding of the pathophysiology of stroke, translation of this knowledge into effective therapies has largely failed. The only effective treatment for ischemic stroke is rapid recanalization of an occluded vessel by dissolving the clot with tissue plasminogen activator (tPA). However, stroke adversely affects vascular function as well that can cause secondary brain injury and limit treatment that depends on a patent vasculature. In middle cerebral arteries (MCA), ischemia/reperfusion (I/R) cause loss of myogenic tone, vascular paralysis, and endothelial dysfunction that can lead to loss of autoregulation. In contrast, brain parenchymal arterioles retain considerable tone during I/R that likely contributes to expansion of the infarct into the penumbra. Microvascular dysregulation also occurs during ischemic stroke that causes edema and hemorrhage, exacerbating the primary insult. Ischemic injury of vasculature is progressive with longer duration of I/R. Early postischemic reperfusion has beneficial effects on stroke outcome but can impair vascular function and exacerbate ischemic injury after longer durations of I/R. This review focuses on current knowledge on the effects of I/R on the structure and function of different vascular segments in the brain and highlight some of the more promising targets for vascular protection.

Endothelin-converting enzyme inhibitors for the treatment of subarachnoid hemorrhage-induced vasospasm

A burgeoning body of evidence suggests that endothelin-1 (ET-1), the most potent endogenous vasoconstrictor yet identified, may be critical in the pathophysiology of various cardiovascular diseases. The ET system may also be implicated in the pathogenesis of cerebral vasospasm after aneurysmal subarachnoid hemorrhage (SAH). Clinical studies have shown that the levels of ET-1 are increased in the cerebrospinal fluid (CSF) of patients following SAH, suggesting that ET-1-mediated vasoconstriction plays a major role in the development of vasospasm after SAH. The potential involvement of ETs in SAH-induced vasospasm has triggered considerable interest in developing therapeutic strategies that inhibit the biologic effects of ET. One promising approach to block the biosynthesis of ETs is suppressing the proteolytic conversion of the precursor peptide (big ET-1) to its vasoactive form (ET-1) using metalloprotease as endothelin-converting enzyme (ECE) inhibitor. To date, three types of ECE-1 inhibitors have been synthesized: dual ECE-1/neutral endopeptidase 24.11 (NEP) inhibitors, triple ECE-1/NEP/angiotensin-converting enzyme (ACE) inhibitors and selective ECE-1 inhibitors. The therapeutic effects of ECE-1 inhibitors on the prevention and reversal of SAH-induced vasospasm in animal studies are reviewed and discussed.

Effect of endothelin antagonist (TAK-044) on cerebral ischemic volume, oxidative stress markers and neurobehavioral parameters in the middle cerebral artery occlusion model of stroke in rats

Stroke causes brain injury in millions of people world wide each year. Despite the enormity of problem, currently there is no established therapy, which can restore the blood flow at infracted area and also improve the neurological deficit. The present study was carried out to investigate the effect of an endothelin antagonist (TAK-044) in middle cerebral artery (MCA) occlusion model of acute ischemic stroke in rats. Male Wistar rats were pretreated with TAK-044 (5 mg/kg, i.p.) for 7 days and thereafter subjected to focal ischemia by occlusion of MCA using intraluminal thread for two hours. 30 min after reperfusion the animals were subjected to diffusion-weighted imaging (DWI) for assessment of protective effect. Twenty-four hours later the motor performance was tested and subsequently the animals were sacrificed for estimation of markers of oxidative stress; malondialdehyde (MDA), glutathione (GSH) and superoxide dismutase (SOD). Control group received vehicle (saline) and similar experimental protocol was followed. In the TAK-044 pretreated group, percent hemispheric lesion area (% HLA) in DWI was significantly attenuated 17.5 +/- 0.5% as compared to control group 61.2 +/- 5.9%. Significant motor impairment, with significant elevated levels of MDA, decrease in GSH and SOD were observed in the vehicle treated MCA occluded rats. Pretreatment with TAK-044 prevented the motor impairment and significantly reversed the changes in markers of oxidative stress (MDA, GSH and SOD). In addition to well-known vasodilatory effect, TAK-044 has recently been documented to have antioxidant and anti-inflammatory properties. These effects can contribute to the protection afforded by TAK-044 in the present study.

A selective endothelin ETA receptor antagonist, SB 234551, improves cerebral perfusion following permanent focal cerebral ischemia in rats

In recent experimental studies, a selective antagonist of endothelin ET(A) receptors, SB 234551, improved neurological and histological outcome in both head trauma and transient focal cerebral ischemia. The present study was conducted to ascertain the degree to which hemodynamic alterations are responsible for this therapeutic effect in a model of permanent middle cerebral artery occlusion (MCAo) in rats. Anesthetized Sprague-Dawley rats were subjected to permanent MCAo by insertion of an intraluminal nylon suture coated with poly-L-lysine. The agent (SB 234551, 30 microg/kg/min = 1.8 mg/kg/h) or vehicle (PBS; 0.6 ml/h) was administered by i.v. infusion beginning 15 min after onset of MCAo and lasting for 23.75 h. Autoradiographic measurement of local cerebral blood flow (lCBF) was performed at 24 h. Physiological data were similar among groups. SB 234551 augmented perfusion by 1.7- to 1.8-fold in both the ischemic hemisphere and in the contralateral (non-ischemic) hemisphere when compared to vehicle-treated ischemic animals. In the ischemic hemisphere, the brain regions significantly benefited were those lying outside the zone of most dense ischemia (i.e., paramedian cortex and thalamus), while in the non-ischemic hemisphere all regions measured showed significant lCBF augmentation. This study demonstrates that SB 234551 therapy results in significant improvement of local cerebral perfusion in the ischemic as well as in the non-ischemic hemispheres after permanent MCAo.

Stimulation of cannabinoid (CB1) and prostanoid (EP2) receptors opens BKCa channels and relaxes ocular trabecular meshwork

Prostanoids and cannabinoids have ocular hypotensive and neuroprotective properties. The effect of the prostanoid AH13205 (EP2), the thromboxane-mimetic U46619, the cannabinoid (CB) agonists WIN55212-2 and CP 55,940, endothelin-1 (ET-1) and 8-bromo-cAMP on the membrane currents of trabecular meshwork (TM) cells were measured using the patch-clamp technique and compared to their effects on TM contractility. Previous studies show relaxation of TM to AH 13205 and other substances that elevate cAMP, while U46619 and endothelin-1 contract TM. This study shows that after contraction (100%) with carbachol (10(-6)m), the CB agonist CP 55,940 dose-dependently reduced contractility to 83+/-4% (n=9) (10(-6)m) and 61+/-10%, (n=7) (10(-5)m). In the presence of both the CB1 antagonist AM251 (10(-6)m) and CP 55,940 (10(-5)m), the contractile response to carbachol reached 84+/-3% (n=6) of the original level. In patch-clamp experiments, membrane permeable 8-bromo-cAMP (10(-4)m) had no effect on currents of TM cells. In contrast, AH 13205 and two cannabinoids reversibly enhanced outward current through high-conductance Ca(2+)-activated K(+) channels (BKCa, BK, maxi-K) to the following values (in % of the initial value at 100 mV): AH 13205 (10(-5)m): 200+/-28% (n=6), CP 55,940 (10(-6)m): 196+/-33% (n=7), CP 55,940 (10(-5)m): 484+/-113% (n=7), WIN55212-2 (10(-5)m): 205+/-41% (n=10). Iberiotoxin (10(-7)m) completely blocked these responses. The current response to CP 55,940 (10(-5)m) could be partially blocked by the CB1 antagonist AM251 (10(-6)m). Conversely, the contractile agents in this study either caused a transient reduction in outward current (ET-1(5x10(-8)m)) or had no effect (U46619 (10(-6)m)). We conclude that stimulation of EP2 and CB1 receptors in TM is coupled to the activation of BKCa channels via a non-diffusible second messenger cascade. This effect may contribute to the relaxant activity of EP2 and CB1 agonists in isolated TM strips, modulating ocular outflow.

ATP-induced inhibition of gap junctional communication is enhanced by interleukin-1 beta treatment in cultured astrocytes

Nucleotides are signaling molecules involved in variety of interactions between neurons, between glial cells as well as between neurons and glial cells. In addition, ATP and other nucleotides are massively released following brain insults, including inflammation, and may thereby be involved in mechanisms of cerebral injury. Recent concepts have shown that in astrocytes intercellular communication through gap junctions may play an important role in neuroprotection. Therefore, we have studied the effects of nucleotides on gap junction communication in astrocytes. Based on measurement of intercellular dye coupling and recording of junctional currents, the present study shows that ATP (10-100 microM) induces a rapid and a concentration-dependent inhibition of gap junction communication in cultured cortical astrocytes from newborn mice. Effects of agonists and antagonists of purinergic receptors indicate that the inhibition of gap junctional communication by ATP mainly involves the stimulation of metabotropic purinergic 1 (P2Y(1)) receptors. Pretreatment with the pro-inflammatory cytokine interleukin-1beta (10 ng/ml, 24 h), which has no effect by itself on gap junctional communication, increases the inhibitory effect of ATP and astrocytes become sensitive to uridine 5'-triphosphate (UTP). As indicated by the enhanced expression of P2Y(2) receptor mRNA, P2Y(2) receptors are responsible for the increased responses evoked by ATP and UTP in interleukin-1beta-treated cells. In addition, the effect of endothelin-1, a well-known inhibitor of gap junctional communication in astrocytes was also exacerbated following interleukin-1beta treatment. We conclude that ATP decreases intercellular communication through gap junctions in astrocytes and that the increased sensitivity of gap junction channels to nucleotides and endothelin-1 is a characteristic feature of astrocytes exposed to pro-inflammatory treatments.

Impact of the endothelin-A receptor antagonist BQ 610 on microcirculation in global cerebral ischemia and reperfusion

The role of endogenous endothelin-1 in mediating microcirculatory disturbances after global cerebral ischemia was investigated in Mongolian gerbils. The pial microcirculation was studied by intravital fluorescent microscopy before, during, and up to 3 h after occlusion of both carotid arteries for 15 min. Pretreatment was achieved with the peptidergic selective endothelin-A (ET-A) receptor antagonist BQ 610. The neurological outcome was assessed daily for up to 4 days. The antagonist attenuated postischemic leukocyte-endothelium interactions in postcapillary venules, in particular the number of rolling leukocytes was found to be reduced (13.0+/-9.4 x 100 microm(-1) min(-1) in the control vs. 2.0+/-2.5 in the experimental group, P<0.05). The local microvascular perfusion, measured by the arterio-venous transit time, was improved during reperfusion by BQ 610 (1.3+/-0.5 s in the control vs. 0.7+/-0.2 s in the experimental group, P<0.05). The neurological deficit was significantly reduced in animals treated with the ET-A antagonist (P<0.05). The inhibition of the postischemic inflammatory reaction and the reversal of the delayed hypoperfusion may account for the improved neurological outcome. These observations suggest that application of endothelin-A antagonists may be a useful approach to interfere with derangements in cerebral ischemia/reperfusion.

Selective blockade of endothelin-B receptors exacerbates ischemic brain damage in the rat

BACKGROUND AND PURPOSE

Endothelins act through 2 receptors, namely, ET(A) and ET(B). In the cerebral circulation, ET(A) mediates marked and prolonged vasoconstriction, and its blockade increases cerebral blood flow (CBF) and reduces ischemic brain damage. However, the role of ET(B) receptors remains unclear. In this study we examined, in rats, the kinetics of expression of ET(B) and the effects of ET(B) blockade on changes in CBF and brain damage after focal cerebral ischemia and N-methyl-D-aspartate (NMDA)-induced excitotoxic injury.

METHODS

Rats were subjected to transient (60 minutes) focal cerebral ischemia or cortical injection of NMDA. The selective ET(B) antagonist BQ-788 was injected intracerebroventricularly 30 minutes before and 30 minutes after the onset of ischemia. Cortical perfusion was monitored by laser-Doppler flowmetry. The volume of infarction or NMDA-induced cortical lesion was assessed at 24 hours after the insult. The reverse transcription-polymerase chain reaction technique was used to assess ET(B) expression.

RESULTS

Cerebral ischemia failed to alter the expression of ET(B) mRNA in both acute and chronic stages. Administration of BQ-788 resulted in an increase in infarction volume (178%; P<0.05) accompanied by a decrease in residual CBF (-26.7% versus control; P<0.01). In these animals we found a positive correlation between the volume of infarction and the severity of the decrease in CBF. NMDA-induced cortical lesions were not affected by the administration of BQ-788.

CONCLUSIONS

Our results suggest that the ET(B) antagonist BQ-788 induces deleterious effects that are mediated by the reduction of residual blood flow after ischemia and argue that the optimal therapeutic strategy in stroke would be to target the use of selective ET(A) antagonists and not mixed ET(A)/ET(B) antagonists.

Cortical expression of endothelin receptor subtypes A and B following middle cerebral artery occlusion in rats

This work aimed to define the spatial expression of endothelin A (ET(A)) and B (ET(B)) receptors in the cerebral cortex after permanent middle cerebral artery occlusion (MCAO) and to identify the phenotype of cells expressing ET(A) and ET(B) receptors. Cortical expression of ET(A) and ET(B) receptors was determined at the mRNA level by semi-quantitative reverse transcription-polymerase chain reaction and at the protein level by immunofluorescence staining, 12, 24 and 72 h after MCAO. Cells expressing endothelin receptors were phenotyped by double labelling with antibodies, anti-protein gene product (PGP9.5) and anti-ED1, towards neurons and activated microglia/macrophages, respectively. Both ET(A) and ET(B) receptor mRNA expressions increased significantly in the ipsilateral cortex in a time-dependent manner after MCAO. Robust expression of ET(A) receptors was noted in most neurons of the ischemic core and in several neurons in laminae 3 and 4 of the peri-infarct region 24 and 72 h after MCAO. ET(B) receptor immunoreactivity was observed in activated microglia/macrophages, beginning 24 h after MCAO. These results provide the first evidence that the action of endothelin during ischemia may be mediated by neuronal ET(A) receptors and activated microglia/macrophage ET(B) receptors. This differential localization of ET(A) and ET(B) receptors suggests that endothelin is involved in some complex neuron-glial interactions in addition to its vascular modulatory activity during ischemia.

Endothelin antagonism with bosentan: current status and future perspectives

Endothelin receptor antagonists have been proposed for the treatment of a variety of disorders in which the endothelins may act as pathogenic mediators, such as hypertension, congestive heart failure, and cerebral vasospasm. Bosentan is a nonpeptide competitive antagonist, which can be a good tool for studying the endothelin system. It is specific for the endothelin system and blocks the actions of endothelin at both mammalian receptors (A and B). Bosentan has recently moved into Phase III clinical trial. This review will attempt to overview the experimental and clinical effects of bosentan.

Ca(2+) influx through nonselective cation channels plays an essential role in endothelin-1-induced mitogenesis in C6 glioma cells

Ca(2+) channels activated by endothelin-1 (ET-1) in C6 glioma cells (C6 cells) were characterized using whole-cell patch-clamps and by monitoring the intracellular free Ca(2+) concentration ([Ca(2+)](i)), when administering Ca(2+) channel blockers such as LOE 908 and SK&F 96365. Using this methodology, the Ca(2+) channels involved in ET-1-induced mitogenesis were identified. The patch-clamp study and [Ca(2+)](i) monitoring showed that 10 nM ET-1 activated two types of Ca(2+)-permeable nonselective cation channels (NSCC); one was sensitive to LOE 908 but resistant to SK&F 96365 (NSCC-1) and the other was sensitive to both LOE 908 and SK&F 96365 (NSCC-2). Conversely, 0.1 nM ET-1 activated only NSCC-1.ET-1-induced mitogenesis in a concentration-dependent manner, with the maximum effect arising at concentrations > or =10 nM. LOE 908 completely suppressed the 10 nM ET-1-induced mitogenesis, whereas SK&F 96365 only partially suppressed it. The IC(50) values of these blockers for the ET-1-induced mitogenesis were similar to those for the 10 nM ET-1-induced increase in [Ca(2+)](i). In contrast, LOE 908 completely suppressed 0.1 nM ET-1-induced mitogenesis, whereas SK&F 96365 did not affect it.Collectively, these results demonstrate that the sustained increase in [Ca(2+)](i), via NSCC-1 and NSCC-2, may be essential for ET-1-induced mitogenesis in C6 cells. Moreover, the sensitivity of NSCC-1 to ET-1 is higher than that of NSCC-2 to ET-1.

Perinatal hypoxia-/ischemia-induced endothelin-1 mRNA in astrocyte-like and endothelial cells

Under pathological conditions in the adult CNS, such as ischemia, subarachnoid hemorrhage and Alzheimer's disease, endothelin (ET)-1- and -3-like immunoreactivities are elevated in astrocytes of the injured adult brain. However, it is not clear whether this is due to increased synthesis or increased binding of ET-1. Further, it is not known whether ET-1 expression is altered in the perinatal brain after cerebral hypoxia/ischemia (H/I). Here, we determined the sites of ET-1 expression in perinatal mouse brain after H/I injury by in situ hybridization using a probe specific for the ET-1 gene. Astrocyte-like cells, which do not normally express ET-1 mRNA, showed high levels of ET-1 mRNA expression. Endothelial cells of the capillaries and small vessels also showed an increased level of ET-1 mRNA. Our data suggest that ET-1 mRNA levels in the astrocyte-like cells and vascular endothelial cells are dynamically regulated by ischemia and may participate in perinatal ischemia-related neural damage.

Augmentation of endothelial function by endothelin antagonism in human saphenous vein conduits

OBJECT

Cerebral revascularization with saphenous vein (SV) conduits is used in the management of hard-to-treat lesions that require deliberate arterial occlusion and in selected patients with occlusive vascular disease. Endothelial dysfunction is thought to contribute to acute perioperative vasospasm and chronic graft atherosclerosis. In the present study the authors examined the contribution of the potent vasoconstrictor endothelin-1 (ET-1) to endothelial dysfunction in human SVs.

METHODS

The effects of an ET(A/B) receptor antagonist (bosentan), an ET(A) receptor antagonist (BQ-123), and an ET(B) receptor antagonist (BQ-788) on in vitro endothelium-dependent and -independent responses were studied in human SVs. Vascular segments were obtained in 34 patients who had undergone revascularization procedures, and isometric dose-response curves (DRCs) were constructed using the isolated tissue bath procedure as follows: 1) cumulative DRCs to norepinephrine; and 2) DRCs to acetylcholine (ACh) and sodium nitroprusside in the absence and presence of bosentan, BQ-123, or BQ-788. Maximal vasodilatory responses and sensitivity were compared between groups. In the presence of bosentan (Experiment 1) and BQ-123 or BQ-788 (Experiment 2), ACh responses were significantly augmented (percent maximum relaxation values: 7+/-2 [control] compared with 17+/-3 [bosentan], p < 0.002 [Experiment 1]; and 12+/-2 [control] compared with 29+/-2 [BQ-123] and 25+/-2 [BQ-788], p < 0.003 and p < 0.002, respectively [Experiment 2]). The sensitivity of SVs to ACh was unaffected by treatment. These beneficial effects were specific for the endothelium.

CONCLUSIONS

Blockade of ET receptors significantly improves endothelial function in SVs. Furthermore, these effects appear to be independently and maximally mediated by antagonism of either ET(A) or ET(B) receptors. Interventions aimed at improving endothelial function may serve to counter perioperative vasospasm and impede atherosclerosis in SVs used for revascularization procedures.

Plasma endothelin-1 levels neither increase nor correlate with neurological scores, stroke risk factors, or outcome in patients with ischemic stroke

BACKGROUND AND PURPOSE

Endothelins (ETs) are potent vasoconstrictors and may play a role in the pathophysiology of several diseases. Limited and controversial data exist on their role in human ischemic stroke. We planned a prospective, observational, and longitudinal clinical study to test whether ET-1 levels increase in various phases of ischemic stroke and whether the ET-1 levels correlate with neurological scores, stroke etiology, stroke risk factors, or final outcome.

METHODS

We measured plasma ET-1 levels with a sandwich-enzyme immunoassay method in 101 consecutive patients with ischemic stroke on admission and 1 week, 1 month, and 3 months after stroke and in 101 sex- and age-matched control subjects. At each sampling, the patients underwent a complete neurological evaluation. All stroke risk factors were recorded, an array of laboratory tests were performed, and the subtype of ischemic stroke was determined. The patients were contacted 3 years later for prognostic determination.

RESULTS

ET-1 levels in patients (2.4+/-1.3 pg/mL on admission, 2.2+/-1.4 pg/mL at 1 week, 2.1+/-1.4 pg/mL at 1 month, and 2.1+/-1.2 pg/mL at 3 months) were not different from those of the control subjects (2.2+/-0.9 pg/mL) at any time point. No correlation was found between the ET-1 levels and stroke etiology, stroke risk factors, stroke recurrence risk, age, sex, or neurological scores, except that ET-1 levels correlated with the use of warfarin and with body mass index.

CONCLUSIONS

Plasma ET-1 levels were normal in patients with ischemic stroke. Our findings cannot exclude a role of ETs in the pathophysiology of ischemic stroke because plasma levels might not accurately reflect intracerebral concentrations, but they also do not support the occurrence of a major plasma ET-1 level increase at any phase of stroke. Our patient population is the largest ever reported in whom ET-1 levels were measured, but it consisted of mild and moderately ill patients with stroke due to the study design, of which the aim was long-term observation, which excludes severely ill patients.

Distinct pharmacological properties of ET-1 and ET-3 on astroglial gap junctions and Ca(2+) signaling

Astrocytes represent a major target for endothelins (ETs), a family of peptides that have potent and multiple effects on signal transduction pathways and can be released by several cell types in the brain. In the present study we have investigated the involvement of different ET receptor subtypes on intercellular dye diffusion, intracellular Ca(2+) homeostasis, and intercellular Ca(2+) signaling in cultured rat astrocytes from hippocampus and striatum. Depending on the ET concentration and the receptor involved, ET-1- and ET-3-induced intracellular Ca(2+) increases with different response patterns. Both ET-1 and ET-3 are powerful inhibitors of gap junctional permeability and intercellular Ca(2+) signaling. The nonselective ET receptor agonist sarafotoxin S6b and the ET(B) receptor-selective agonist IRL 1620 mimicked these inhibitions. The ET-3 effects were only marginally affected by an ET(A) receptor antagonist but completely blocked by an ET(B) receptor antagonist. However, the ET-1-induced inhibition of gap junctional dye transfer and intercellular Ca(2+) signaling was only marginally blocked by ET(A) or ET(B) receptor-selective antagonists but fully prevented when these antagonists were applied together. The ET-induced inhibition of gap junction permeability and intercellular Ca(2+) signaling indicates that important changes in the function of astroglial communication might occur when the level of ETs in the brain is increased.

Endothelin and its antagonists in hypertension: can we foresee the future?

Endothelin-receptor antagonists may soon become a new therapeutic class of agents used to treat cardiovascular diseases. Although the first clinical trials are anxiously awaited to position this new family of compounds in the treatment of essential or secondary forms of hypertension, we dispose of an impressive amount of studies in which plasma endothelin levels have been measured, in addition to chronic preclinical studies that provide a general picture of what we can expect from these drugs. The experimental models that do respond to endothelin- receptor antagonists share vascular overexpression of endothelin, which seems directly linked with vascular hypertrophy of resistance arteries. In addition, salt sensitivity may represent an unbalanced condition between relaxing and constrictive properties of the renal endothelium that can respond favorably to endothelin blockade. Thus, African-American hypertensives may well be a responsive target population for the new drugs. In addition to blood pressure control, endothelin may also be involved in the evolution of end-organ damage by its potent vasoactive and vasoproliferative properties. The kidney, heart, large arteries and brain may therefore benefit from these drugs, but it is still unclear if this benefit goes beyond what can be attributed to the reduction of arterial pressure. Moreover, most studies performed so far have looked at prevention of end-organ damage, while very few have addressed the clinically relevant question of regression of lesions already installed by the disease process.

The many aspects of endothelins in ischemia-reperfusion injury: emergence of a key mediator

The endothelins (ETs) are regulatory peptides, distributed in many organ systems and producing potent physiological effects. They are the most powerful vasoconstrictive substances known today. They also act as promitogens. Many data supporting pathophysiological roles for ETs are reported, especially regarding diseases related to the vascular system, such as hypertension, pulmonary hypertension, preeclampsia, ischemic heart diseases, renal failure, subarachnoidal hemorrhage, and cerebral ischemia. The development of drugs blocking ET binding to its receptors (antagonists) and the biosynthesis of ETs (ECE inhibitors) presently attracts great interest in terms of establishing new treatments for diseases in which ETs are believed to be involved. Here we review the evidence supporting a role for ETs in the various etiologies related to ischemia-reperfusion injury, such as is found in heart disease, cerebral ischemia, and organ transplantation.

A novel endothelin antagonist, A-127722, attenuates ischemic lesion size in rats with temporary middle cerebral artery occlusion: a diffusion and perfusion MRI study

BACKGROUND AND PURPOSE

Endothelins (ETs) are potent vasoconstrictors. Plasma ET levels increase during acute brain ischemia and may worsen the ischemic damage. Diffusion-weighted MRI (DWI) and perfusion imaging (PI) are powerful tools for evaluation of acute cerebral ischemia. We studied the effects of A-127722, a novel ET(A)-selective ET antagonist, on cerebral ischemic lesion size using 2,3,5-triphenyltetrazolium chloride (TTC) staining postmortem, on acute ischemic lesion development with DWI, and on the cerebral circulation using PI.

METHODS

Twenty male Sprague-Dawley rats received either 5 mg/kg of A-127722 or vehicle (n=10 per group) intravenously 30 minutes and subcutaneously 4 hours after middle cerebral artery occlusion (MCAO). Whole-brain DWI and single-slice PI were done before initiation of treatment and repeated frequently thereafter up to 4 hours after MCAO. The animals were reperfused in the MRI scanner 90 minutes after the onset of MCAO. At 24 hours the animals were killed, and the brains were cut into six 2-mm-thick slices and stained with 2% TTC. Percent hemispheric lesion volume (%HLV) was calculated for each animal.

RESULTS

Physiological parameters, body weight, neurological scores, and premature mortality (2 versus 2) did not differ between the two groups. No hypotension, abnormal behavior, or other adverse effects were seen. TTC-derived %HLV was 25.3+/-5.6% for controls and 16.2+/-9.6% for treated animals (36% reduction, P<.02). Six animals in each group had successful reperfusion as shown by PI. Among these animals, %HLV was 23.2+/-3.1% for controls and 9.3+/-4.4% for treated animals (60% reduction, P=.0001). The beneficial effect of A-127722 was limited to animals in which successful reperfusion was demonstrated. No difference in PI-detected perfusion deficit size was observed between the groups. DWI did not demonstrate significant in vivo lesion size differences.

CONCLUSIONS

A-127722 significantly reduced ischemic lesion size in rats without observable adverse effects. It is not clear whether the effect was due to vasodilatation of collateral arterioles not detectable by PI or whether A-127722 has neuroprotective properties that are independent of vascular effects.

Endothelin-B receptors in cerebral resistance arterioles and their functional significance after focal cerebral ischemia in cats

In the cerebral circulation, endothelin-A receptor activation mediates marked prolonged vasoconstriction whereas endothelin-B (ETB) receptor activation effects dilation. In contrast to some peripheral vascular beds, ET(B) receptor-induced vasoconstriction has not yet been demonstrated in brain vessels. In this study in chloralose-anesthetized cats, with perivascular microapplications of ET(B) selective agonist (BQ-3020) and antagonist (BQ-788), we investigated whether ET(B) receptor-mediated constriction could be uncovered in cortical arterioles in vivo. In addition, we examined whether normal dilator response to ET(B) receptor activation is preserved in postischemic cerebral arterioles. The first microapplication of the selective ET(B) receptor agonist BQ-3020 (1 micromol/L) onto a pial cortical arteriole elicited marked dilation (caliber increased by 26.3 +/- 15.1% from preinjection baseline). A second application of BQ-3020 (10-minute interval) onto the same vessel failed to evoke any significant vasomotor response. Subsequent (third and fourth) adventitial microapplication of the ET(B) receptor agonist on the same arteriolar site effected a significant constriction of cerebral arterioles (-15.3 +/- 12.7% and -9.7 +/- 6.3% from preinjection baseline, respectively, at 20 and 30 minutes after the first application). The pial arterioles did not display tachyphylaxis to repeated applications of potassium (10 mmol/L). The perivascular application of the ET(B) receptor antagonist BQ-788 (0.001 to 1 micromol/L) had no effect on arteriolar caliber per se but blocked both BQ-3020-induced dilation (inhibitory concentration approximately 5 nmol/L) and vasoconstriction elicited by repeated activation of ET(B) receptors. After middle cerebral artery occlusion, most of the arterioles examined displayed a sustained dilation. The microapplication of BQ-3020 into the perivascular space surrounding postischemic dilated arterioles elicited a constriction of a similar magnitude to that induced by application of CSF (-17 +/- 7% and -17 +/- 7% from preinjection baseline, respectively). The adventitial microapplication of the ET(B) receptor antagonist (BQ-788, 0.1 micromol/L) on postocclusion dilated pial arterioles effected no change in the arteriolar caliber when compared with preinjection baseline. This BQ-788-induced response was significantly different from that induced by perivascular microinjection of CSF (P < 0.001, analysis of variance). These investigations indicate that (1) repeated activation of ET(B) receptors displays tachyphylaxis of the vasodilator response but also uncovers significant constriction of cerebral arterioles in vivo; (2) the ability of BQ-3020 to elicit dilation is lost within 30 minutes of induced focal ischemia; and (3) ET(B)-mediated contractile tone contributes in a small but significant manner in limiting postischemia dilation of cortical pial arterioles.

Effect of an endothelin-1 antagonist, BQ-485, on cerebral oxygen metabolism after complete global cerebral ischemia in dogs

Endothelin-1 (ET-1) plays an important role in the physiologic or pathophysiologic regulation of cerebral circulation. To evaluate the effects of the newly synthesized ETA receptor-selective antagonist, BQ-485 (N-perhydroazepin-l-ylcarbonyl-Leu-D-Trp-D-Trp-OH), on the cerebral metabolism of oxygen during the delayed cerebral hypoperfusion that follows global cerebral ischemia, we occluded the ascending aorta and caval veins of 10 beagle dogs for 12.5 min. The animals were randomized into two groups. BQ-485 was given directly into the carotid artery at 0.03 mg/kg per min for 30 min, starting 15 min after reperfusion in the treatment group (n = 5). Isotonic saline was infused in the control group (n = 5). A fiberoptic catheter was inserted into the superior sagittal sinus to monitor its oxygen saturation (SssO2) continuously. Arterial O2 content (CaO2), and sagittal sinus O2 content (CssO2) were monitored before and at 0.5, 1, 2, 4, 6 and 8 h after the ischemic insult. BQ-485 significantly prevented the expected decrease in SssO2 and increase in the cerebral O2 utilization coefficient at 4, 6 and 8 h after the ischemic insult (P < 0.05). Thus, BQ-485 ameliorated the mismatch between O2 supply and demand in the delayed hypoperfusion phase. We conclude that ET may be involved in the pathogenesis of delayed cerebral hypoperfusion after cardiac arrest.

Endothelin production links superoxide generation to altered opioid-induced pial artery vasodilation after brain injury in pigs

BACKGROUND AND PURPOSE

Traumatic brain injury conveys significant morbidity and mortality to infants and children. In the newborn pig, opioids contribute to pial artery vasconstriction after fluid percussion injury (FPI). FPI attenuates vasodilation and cGMP production by methionine enkephalin (Met) and leucine enkephalin (Leu) and reverse dynorphin (Dyn) from a dilator to a constrictor. Superoxide anion (O2-) production contributes to altered cerebral hemodynamics after FPI, and O2- scavengers partially restore decreased dilator responses after FPI. Endothelin-1 (ET-1), a purported mediator of cerebral vasospasm, has been suggested to alter nitric oxide function and cGMP concentration. The present study was designed to determine the contribution of ET-1 to altered opioid-induced dilation after FPI and the role of O2- in such altered responses.

METHODS

Injury of moderate severity (1.9 to 2.3 atm) was produced by the lateral FPI technique in anesthetized newborn pigs equipped with a closed cranial window. Superoxide dismutase (SOD)-inhibitable nitroblue tetrazolium (NBT) reduction was determined as an index of O2- generation.

RESULTS

FPI increased cerebrospinal fluid ET-1 from 20 +/- 2 to 93 +/- 6 pg/mL (approximately 10(-10) mol/L). Topical ET-1 (10(-10) mol/L) increased SOD-inhibitable NBT reduction from 1 +/- 1 to 16 +/- 3 pmol/mm2, similar to previously reported NBT reduction after FPI (14 +/- 2 pmol/mm2). BQ123 (10(-6) mol/L), an ET-1 antagonist, blunted the NBT reduction observed after FPI (4 +/- 1 pmol/mm2). Met produced pial vasodilation that was attenuated by FPI and partially restored by BQ123 pretreatment (7 +/- 1%, 11 +/- 1%, and 17 +/- 1% versus 3 +/- 1%, 6 +/- 1%, and 9 +/- 2% versus 5 +/- 1%, 9 +/- 1%, and 14 +/- 2% for 10(-10), 10(-8), and 10(-6) mol/L Met during control conditions, after FPI, and after FPI pretreated with BQ123, respectively). Met-induced dilation was associated with increased cerebrospinal fluid cGMP, and these biochemical changes were likewise blunted by FPI and partially restored by BQ123 (357 +/- 12, 455 +/- 15, 500 +/- 19, and 632 +/- 11 versus 264 +/- 4, 267 +/- 4, 295 +/- 12, and 305 +/- 15 versus 309 +/- 19, 432 +/- 11, 529 +/- 10, and 593 +/- 4 pg/mL for resting conditions, 10(-10), 10(-8), and 10(-6) mol/L Met during control conditions, after FPI, and after FPI pretreated with BQ123, respectively). Similar partial restoration of vascular and biochemical parameters was observed for Leu and Dyn.

CONCLUSIONS

These data show that ET-1, in concentrations similar to that present in cerebrospinal fluid after FPI, increases O2- production. These data also indicate the opioid-induced vasodilation and cGMP production are partially restored after FPI by ET-1 receptor blockade. These data suggest that ET-1 contributes to altered cerebral hemodynamics after FPI, at least in part, through elevated O2- production.

Involvement of sphingolipids in the endothelin-1 signal transduction mechanism in rat brain

In cerebral cortex, endothelin-1 (ET-1) evoked a decrease of 40% in sphingomyelin (SM) levels together with an increase in both ceramide and glycosphingolipid (GSL) levels (100 and 56% respectively). These facts indicate that ET-1 increases sphingomyelinase activity and, possibly, activates the synthesis of GSL. By contrast, in cerebellum ET-1 seems to activate the hydrolysis of both SM and GSL, since the peptide evoked a decrease (near 30%) of their levels concomitantly with an increased production of ceramides (200%). These ceramides are clearly different from those produced in cerebral cortex which come from the SM hydrolysis only. It is suggested that ETB receptor subtype is involved in these responses.