Effect of centrally administered endothelin agonists on systemic and regional blood circulation in the rat: role of sympathetic nervous system

Advances in Therapies to Treat Neonatal Hypoxic-Ischemic Encephalopathy

Neonatal hypoxic-ischemic encephalopathy (HIE) is a condition that results in brain damage in newborns due to insufficient blood and oxygen supply during or after birth. HIE is a major cause of neurological disability and mortality in newborns, with over one million neonatal deaths occurring annually worldwide. The severity of brain injury and the outcome of HIE depend on several factors, including the cause of oxygen deprivation, brain maturity, regional blood flow, and maternal health conditions. HIE is classified into mild, moderate, and severe categories based on the extent of brain damage and resulting neurological issues. The pathophysiology of HIE involves different phases, including the primary phase, latent phase, secondary phase, and tertiary phase. The primary and secondary phases are characterized by episodes of energy and cell metabolism failures, increased cytotoxicity and apoptosis, and activated microglia and inflammation in the brain. A tertiary phase occurs if the brain injury persists, characterized by reduced neural plasticity and neuronal loss. Understanding the cellular and molecular aspects of the different phases of HIE is crucial for developing new interventions and therapeutics. This review aims to discuss the pathophysiology of HIE, therapeutic hypothermia (TH), the only approved therapy for HIE, ongoing developments of adjuvants for TH, and potential future drugs for HIE.

Oxidative stress: A target to treat Alzheimer's disease and stroke

Oxidative stress has been established as a well-known pathological condition in several neurovascular diseases. It starts with increased production of highly oxidizing free-radicals (e.g. reactive oxygen species; ROS and reactive nitrogen species; RNS) and becomes too high for the endogenous antioxidant system to neutralize them, which results in a significantly disturbed balance between free-radicals and antioxidants levels and causes cellular damage. A number of studies have evidently shown that oxidative stress plays a critical role in activating multiple cell signaling pathways implicated in both progression as well as initiation of neurological diseases. Therefore, oxidative stress continues to remain a key therapeutic target for neurological diseases. This review discusses the mechanisms involved in reactive oxygen species (ROS) generation in the brain, oxidative stress, and pathogenesis of neurological disorders such as stroke and Alzheimer's disease (AD) and the scope of antioxidant therapies for these disorders.

Sovateltide Mediated Endothelin B Receptors Agonism and Curbing Neurological Disorders

Neurological/neurovascular disorders constitute the leading cause of disability and the second leading cause of death globally. Major neurological/neurovascular disorders or diseases include cerebral stroke, Alzheimer’s disease, spinal cord injury, neonatal hypoxic-ischemic encephalopathy, and others. Their pathophysiology is considered highly complex and is the main obstacle in developing any drugs for these diseases. In this review, we have described the endothelin system, its involvement in neurovascular disorders, the importance of endothelin B receptors (ETBRs) as a novel potential drug target, and its agonism by IRL-1620 (INN—sovateltide), which we are developing as a drug candidate for treating the above-mentioned neurological disorders/diseases. In addition, we have highlighted the results of our preclinical and clinical studies related to these diseases. The phase I safety and tolerability study of sovateltide has shown it as a safe and tolerable compound at therapeutic dosages. Furthermore, preclinical and clinical phase II studies have demonstrated the efficacy of sovateltide in treating acute ischemic stroke. It is under development as a first-in-class drug. In addition, efficacy studies in Alzheimer’s disease (AD), acute spinal cord injury, and neonatal hypoxic-ischemic encephalopathy (HIE) are ongoing. Successful completion of these studies will validate that ETBRs signaling can be an important target in developing drugs to treat neurological/neurovascular diseases.

Central endothelin ETB receptor activation reduces blood pressure and catecholaminergic activity in the olfactory bulb of deoxycorticosterone acetate-salt hypertensive rats

Endothelins regulate catecholaminergic activity in the olfactory bulb (OB) in normotensive and hypertensive animals. Administration of an endothelin ET_A receptor antagonist decreases blood pressure in deoxycorticosterone acetate-salt (DOCA-salt) rats along with a reduction in tyrosine hydroxylase (TH) activity and expression. In the present work, we sought to establish the role of brain endothelin ET_B receptor on blood pressure regulation and its relationship with the catecholaminergic system within the OB of DOCA-Salt rats. Sprague-Dawley male rats were divided into control and DOCA-Salt groups. Blood pressure, heart rate and TH activity as well as neuronal nitric oxide synthase (nNOS) expression were assessed following IRL-1620 (selective endothelin ET_B receptor agonist) applied to be brain. IRL-1620 significantly reduced systolic, diastolic, and mean arterial pressure in DOCA-Salt hypertensive rats. It also decreased TH activity, TH total and phosphorylated forms expression as well as its mRNA in the OB of hypertensive animals. The expression of phospho-Ser1417-nNOS, which reflects nNOS activation, was significantly decreased in the of OB of DOCA-salt rats, but it was enhanced by IRL-1620. These findings suggest that DOCA-Salt hypertension depends on endogenous central endothelin ET_A receptor activity, rather than on ET_B, and that low endothelin ET_B stimulation is essential for blood pressure elevation in this animal model. The effect of endothelin ET_A receptor antagonism may also result from endothelin ET_B receptor overstimulation. The present study shows that endothelin receptors are involved in the regulation of TH in the OB and that such changes are likely implicated in the hemodynamic control and sympathetic outflow.

Sovateltide (IRL-1620) activates neuronal differentiation and prevents mitochondrial dysfunction in adult mammalian brains following stroke

The development of effective drugs for stroke is urgently required as it is the 2nd largest killer in the world and its incidence is likely to increase in the future. We have demonstrated cerebral endothelin B receptors (ETBR) as a potential target to treat acute cerebral ischemic stroke. However, the mechanism of ETBR mediated neural regeneration and repair remains elusive. In this study, a permanent middle cerebral artery occluded (MCAO) rat model was used. Sovateltide (an ETBR agonist) injected intravenously showed better survival and neurological and motor function improvement than control. Higher neuronal progenitor cells (NPCs) differentiation along with better mitochondrial morphology and biogenesis in the brain of sovateltide rats were noted. Exposure of cultured NPCs to hypoxia and sovateltide also showed higher NPC differentiation and maturation. This study shows a novel role of ETBR in NPCs and mitochondrial fate determination in cerebral ischemia, and in improving neurological deficit after stroke.

Sovateltide (IRL-1620) affects neuronal progenitors and prevents cerebral tissue damage after ischemic stroke

Stimulation of endothelin B receptors by its agonist IRL-1620 (INN, sovateltide) provides neuroprotection and neurological and motor function improvement following cerebral ischemia. We investigated the effect of sovateltide on stem and progenitor cells mediated neural regeneration and its effect on the cerebral tissue repair and restoration of neurological and motor function. Sovateltide (5 μg/kg) was injected intravenously in permanent middle cerebral artery occluded (MCAO) rats at 4, 6, and 8 h at days 0, 3, and 6. Neurological and motor function tests were carried out pre-MCAO and at day 7 post-MCAO. At day 7, significantly reduced expression of neuronal differentiation markers HuC/HuD and NeuroD1 was seen in MCAO + vehicle than sham rats. Sovateltide treatment upregulated HuC/HuD and NeuroD1 compared to MCAO + vehicle and their expression was similar to sham. Expression of stem cell markers Oct 4 and Sox 2 was similar in rats of all of the groups. Significantly reduced infarct volume and DNA damage with recovery of neurological and motor function was observed in sovateltide-treated MCAO rats. These results indicate that sovateltide initiates a regenerative response by promoting differentiation of neuronal progenitors and maintaining stem cells in an equilibrium following cerebral ischemic stroke.

Endothelin receptors in the brain modulate autonomic responses and arrhythmogenesis during acute myocardial infarction in rats

AIMS

Endothelin has been implicated in various processes in the brain, including the modulation of sympathetic responses. The present study examined the pathophysiologic role of brain endothelin-receptors in the setting of acute myocardial infarction, characterized by high incidence of ventricular tachyarrhythmias.

MAIN METHODS

We investigated the effects of intracerebroventricular administration of antagonists of endothelin-receptors ET_A, ET_B, or both, during a 24 h-observation period post-coronary ligation in (n = 70) rats. Continuous recording was performed via implanted telemetry transmitters, followed by arrhythmia-analysis and calculation of autonomic indices derived from heart rate variability. The regional myocardial electrophysiologic properties were assessed by monophasic action potentials and multi-electrode recordings.

KEY FINDINGS

Sympathetic-activity was decreased and vagal-activity was enhanced after intracerebroventricular ET_A-receptor blockade, thus attenuating regional myocardial repolarization inhomogeneity. As a result, the incidence of ventricular tachyarrhythmias was markedly lower in this group. Such effects were also observed after intracerebroventricular blockade of ET_B-, or both, ET_A- and ET_B-receptors, although to a lesser extent.

SIGNIFICANCE

ET_A-receptors in the brain modulate sympathetic and vagal responses and alter arrhythmogenesis during evolving myocardial necrosis in rats. These findings provide insights into arrhythmogenic mechanisms during acute myocardial infarction and call for further investigation on the role of endothelin in the central autonomic network.

Anti-apoptotic activity of ETB receptor agonist, IRL-1620, protects neural cells in rats with cerebral ischemia

Endothelin-B receptor agonist, IRL-1620, provides significant neuroprotection following cerebral ischemia in rats. Whether this neuroprotection is due to inhibition of apoptosis is unknown. IRL-1620-treated rats following permanent middle cerebral artery occlusion (MCAO) showed significant improvement in neurological and motor functions along with a decrease in infarct volume at 24 h (-81.3%) and day 7 (-73.0%) compared to vehicle group. Cerebral blood flow (CBF) significantly improved in IRL-1620-treated animals compared to vehicle by day 7 post MCAO. IRL-1620-treated rats showed an increase in phospho-Akt and decrease in Bad level 7 h post-occlusion compared to vehicle, while Akt and Bad expression was similar in cerebral hemispheres at 24 h post-MCAO. The phospho-Bad level was lower in vehicle- but not in IRL-1620-treated rats at 24 h. Anti-apoptotic Bcl-2 expression decreased, while pro-apoptotic Bax expression increased in vehicle-treated MCAO rats, these changes were attenuated (P < 0.01) by IRL-1620. Mitochondrial membrane-bound Bax intensity significantly decreased in IRL-1620 compared to vehicle-treated MCAO rats. IRL-1620 treatment reduced (P < 0.001) the number of TUNEL-positive cells compared to vehicle at 24 h and day 7 post MCAO. The results demonstrate that IRL-1620 is neuroprotective and attenuates neural damage following cerebral ischemia in rats by increasing CBF and reducing apoptosis.

Novel Approaches for the Treatment of Alzheimer's and Parkinson's Disease

Neurodegenerative disorders affect around one billion people worldwide. They can arise from a combination of genomic, epigenomic, metabolic, and environmental factors. Aging is the leading risk factor for most chronic illnesses of old age, including Alzheimer’s and Parkinson’s diseases. A progressive neurodegenerative process and neuroinflammation occur, and no current therapies can prevent, slow, or halt disease progression. To date, no novel disease-modifying therapies have been shown to provide significant benefit for patients who suffer from these devastating disorders. Therefore, early diagnosis and the discovery of new targets and novel therapies are of upmost importance. Neurodegenerative diseases, like in other age-related disorders, the progression of pathology begins many years before the onset of symptoms. Many efforts in this field have led to the conclusion that exits some similar events among these diseases that can explain why the aging brain is so vulnerable to suffer neurodegenerative diseases. This article reviews the current knowledge about these diseases by summarizing the most common features of major neurodegenerative disorders, their causes and consequences, and the proposed novel therapeutic approaches.

A novel neuroregenerative approach using ET(B) receptor agonist, IRL-1620, to treat CNS disorders

Endothelin B (ET(B)) receptors present in abundance the central nervous system (CNS) have been shown to have significant implications in its development and neurogenesis. We have targeted ET(B) receptors stimulation using a highly specific agonist, IRL-1620, to treat CNS disorders. In a rat model of cerebral ischemia intravenous administration IRL-1620 significantly reduced infarct volume and improved neurological and motor functions compared to control. This improvement, in part, is due to an increase in neuroregeneration. We also investigated the role of IRL-1620 in animal models of Alzheimer's disease (AD). IRL-1620 improved learning and memory, reduced oxidative stress and increased VEGF and NGF in Abeta treated rats. IRL-1620 also improved learning and memory in an aged APP/PS1 transgenic mouse model of AD. These promising findings prompted us to initiate human studies. Successful chemistry, manufacturing and control along with mice, rat and dog toxicological studies led to completion of a human Phase I study in healthy volunteers. We found that a dose of 0.6 microg/kg of IRL-1620 can be safely administered, three times every four hours, without any adverse effect. A Phase II clinical study with IRL-1620 has been initiated in patients with cerebral ischemia and mild to moderate AD.

Blockage of spinal endothelin A receptors attenuates bone cancer pain via regulation of the Akt/ERK signaling pathway in mice

Bone cancer pain (BCP) is a common source of pain in patients with advanced stage and metastatic cancer; however, existing treatment for this kind of pain remains deficient. Being closely related to sensory change and inflammatory pain in both the central and peripheral nervous systems, endothelin A receptor (ETAR) plays an essential role in pain processing. As a result, ETAR antagonist has been reported to alleviate both neuropathic and inflammatory pain. Thus far, the role of ETAR in the process of BCP is still ambiguous. In this study, by using a BCP mouse model, the analgesic effect and molecular mechanism of the ETAR antagonist BQ-123 was investigated. Pain sensation in the BCP mouse model was investigated by the number of spontaneous flinches (NSF) and pain withdrawal threshold (PWT), and the mechanism of BCP was assessed by measuring p-Akt, p-Akt/t-Akt, p-ERK-1/2 and p-ERK-1/2/t-ERK-1/2 levels in L_4-6 segments of the spinal cord. Our results demonstrated that BCP mice showed a higher NSF and a lower PWT score than Sham mice. In addition to the development of nociceptive sensitization, p-Akt, p-Akt/t-Akt, p-ERK-1/2 and p-ERK-1/2/t-ERK-1/2 were up-regulated correspondingly in L_4-6 segments of the spinal cord in BCP mice. BQ-123 treatment showed a promising analgesic effect, and the effect was correlated to the down-regulation of p-Akt, p-Akt/t-Akt, p-ERK-1/2 and p-ERK-1/2/t-ERK-1/2 in spinal cord cells. The results suggested that intrathecal administration of BQ-123 was able to relieve BCP in mice as a consequence of suppressing the Akt and ERK signalling pathways.

Endothelin-1 Induces Degeneration of Cultured Motor Neurons Through a Mechanism Mediated by Nitric Oxide and PI3K/Akt Pathway

Endothelin-1 (ET-1) is a vasoactive peptide produced by activated astrocytes and microglia and is implicated in initiating and sustaining reactive gliosis in neurodegenerative diseases. We have previously suggested that ET-1 can play a role in the pathophysiology of amyotrophic lateral sclerosis (ALS). Indeed, we reported that this peptide is abundantly expressed in reactive astrocytes in the spinal cord of SOD1-G93A mice and ALS patients and exerts a toxic effect on motor neurons (MNs) in an in vitro model of mixed spinal cord cultures enriched with reactive astrocytes. Here, we explored the possible mechanisms underlying the toxic effect of ET-1 on cultured MNs. We show that ET-1 toxicity is not directly caused by oxidative stress or activation of cyclooxygenase-2 but requires the synthesis of nitric oxide and is mediated by a reduced activation of the phosphoinositide 3-kinase pathway. Furthermore, we observed that ET-1 is also toxic for microglia, although its effect on MNs is independent of the presence of this type of glial cells. Our study confirms that ET-1 may contribute to MN death and corroborates the view that the modulation of ET-1 signaling might be a therapeutic strategy to slow down MN degeneration in ALS.

Stimulation of endothelin B receptors by IRL-1620 decreases the progression of Alzheimer's disease

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by severe cognitive impairment that ultimately leads to death. Endothelin (ET) and its receptors have been considered as therapeutic targets for AD. Recent studies in our lab have shown that stimulation of ETB receptors provide significant neuroprotection following Aβ1-40 administration. It is possible that IRL-1620 may be neuroprotective due to angiogenesis. However, the effect of IRL-1620 on neurovascular remodeling following Aβ1-40 administration has not been established. The purpose of this study was to determine the effect of stimulation of ETB receptors by IRL-1620 on vascular and neuronal growth factors after Aβ1-40 administration. Rats were treated with Aβ1-40 (day 1, 7 and 14) in the lateral cerebral ventricles using stereotaxically implanted cannula and received three intravenous injections of IRL-1620 (an ETB agonist), and/or BQ788 (an ETB antagonist) at 2-h interval on day 8; experiments were performed on day 15. Rats were sacrificed for estimation of brain ETB receptors, vascular endothelial growth factor (VEGF) and nerve growth factor (NGF) expression using immunofluorescence and Western blot. In the Morris swim task, amyloid-β (Aβ)-treated rats showed a significant (p<0.0001) impairment in spatial memory. Rats treated with IRL-1620 significantly (p<0.001) reduced the cognitive impairment induced by Aβ. BQ788 treatment completely blocked IRL-1620-induced improvement in cognitive impairment. IRL-1620 treatment enhanced the number of blood vessels labeled with VEGF compared to vehicle treatment. Additionally, cells showed increased (p<0.001) positive staining for NGF in IRL-1620-treated animals. ETB, VEGF and NGF protein expression significantly (p<0.001) increased in the brain of IRL-1620-treated rats as compared to vehicle. Pretreatment with BQ788 blocked the effects of IRL-1620, thus confirming the role of ETB receptors in the neurovascular remodeling actions of IRL-1620. Results of the present study demonstrate that IRL-1620 improves both acquisition (learning) and retention (memory) on the water maze task and enhances angiogenic and neurogenic remodeling. These findings indicate that the ETB receptor may be a novel therapeutic target for AD and other neurovascular degenerative disorders.

Endothelin ETA receptor antagonist reverses naloxone-precipitated opioid withdrawal in mice

Long-term use of opioids for pain management results in rapid development of tolerance and dependence leading to severe withdrawal symptoms. We have previously demonstrated that endothelin-A (ETA) receptor antagonists potentiate opioid analgesia and eliminate analgesic tolerance. This study was designed to investigate the involvement of central ET mechanisms in opioid withdrawal. The effect of intracerebroventricular administration of ETA receptor antagonist BQ123 on morphine and oxycodone withdrawal was determined in male Swiss Webster mice. Opioid tolerance was induced and withdrawal was precipitated by the opioid antagonist naloxone. Expression of ETA and ETB receptors, nerve growth factor (NGF), and vascular endothelial growth factor was determined in the brain using Western blotting. BQ123 pretreatment reversed hypothermia and weight loss during withdrawal. BQ123 also reduced wet shakes, rearing behavior, and jumping behavior. No changes in expression of vascular endothelial growth factor, ETA receptors, and ETB receptors were observed during withdrawal. NGF expression was unaffected in morphine withdrawal but significantly decreased during oxycodone withdrawal. A decrease in NGF expression in oxycodone- but not in morphine-treated mice could be due to mechanistic differences in oxycodone and morphine. It is concluded that ETA receptor antagonists attenuate opioid-induced withdrawal symptoms.

Endothelin modulates the cardiovascular effects of clonidine in the rat

Clonidine decreases mean arterial pressure (MAP) by acting as an α(2)-adrenergic receptor (AR) agonist in the central nervous system; it also acts on peripheral α-ARs to produce vasoconstriction. Endothelin (ET) has been shown to modulate the action of ARs. The present study was conducted to determine the involvement of ET in cardiovascular effects of clonidine. Intravenous administration of clonidine (10, 30 and 90μgkg(-1)) produced a dose-dependent decrease in MAP and heart rate (HR). Treatment with ET-1 (100, 300 and 900ngkg(-1)) significantly attenuated clonidine (10μgkg(-1)) induced fall in MAP and HR. Rats treated with ET-1 (900ngkg(-1)) showed an increase in MAP and HR after clonidine administration compared to untreated rats, while ET(A/B) antagonist, TAK-044 (1mgkg(-1)) and ET(A) antagonist, BMS-182874 (9mgkg(-1)) potentiated the hypotensive effect of clonidine. ET(B) receptor agonist, IRL-1620 (5μgkg(-1)) produced significant attenuation of clonidine induced fall in MAP and HR, while ET(B) receptor antagonist, BQ-788 (0.3mgkg(-1)), potentiated the hypotensive effect of clonidine. Prazosin (0.1mgkg(-1)) completely blocked ET-1 induced changes in cardiovascular effects of clonidine. Clonidine-induced contraction of rat abdominal aortic ring was potentiated by ET-1, which was completely blocked by prazosin. Clonidine produced an increase in ET(A) receptor expression in the brain and abdominal aorta while ET(B) receptors were not affected. It is concluded that ET enhances the responsiveness of vascular ARs to the constrictor effect of clonidine and ET antagonists potentiate the hypotensive effect of clonidine suggesting that a combination of ET antagonist with clonidine may be a useful option to treat hypertension.

Endothelin-1 as a neuropeptide: neurotransmitter or neurovascular effects?

Endothelin-1 (ET-1) is an endothelium-derived peptide that also possesses potent mitogenic activity. There is also a suggestion the ET-1 is a neuropeptide, based mainly on its histological identification in both the central and peripheral nervous system in a number of species, including man. A neuropeptide role for ET-1 is supported by studies showing a variety of effects caused following its administration into different regions of the brain and by application to peripheral nerves. In addition there are studies proposing that ET-1 is implicated in a number of neural circuits where its transmitter affects range from a role in pain and temperature control to its action on the hypothalamo-neurosecretory system. While the effect of ET-1 on nerve tissue is beyond doubt, its action on nerve blood flow is often ignored. Here, we review data generated in a number of species and using a variety of experimental models. Studies range from those showing the distribution of ET-1 and its receptors in nerve tissue to those describing numerous neurally-mediated effects of ET-1.

Endothelin type A receptor antagonist normalizes blood pressure in rats exposed to eucapnic intermittent hypoxia

We have reported that eucapnic intermittent hypoxia (E-IH) causes systemic hypertension, elevates plasma endothelin 1 (ET-1) levels, and augments vascular reactivity to ET-1 and that a nonspecific ET-1 receptor antagonist acutely lowers blood pressure in E-IH-exposed rats. However, the effect of chronic ET-1 receptor inhibition has not been evaluated, and the ET receptor subtype mediating the vascular effects has not been established. We hypothesized that E-IH causes systemic hypertension through the increased ET-1 activation of vascular ET type A (ET(A)) receptors. We found that mean arterial pressure (MAP) increased after 14 days of 7 h/day E-IH exposure (109 +/- 2 to 137 +/- 4 mmHg; P < 0.005) but did not change in sham-exposed rats. The ET(A) receptor antagonist BQ-123 (10 to 1,000 nmol/kg iv) acutely decreased MAP dose dependently in conscious E-IH but not sham rats, and continuous infusion of BQ-123 (100 nmol.kg(-1).day(-1) sc for 14 days) prevented E-IH-induced increases in MAP. ET-1-induced constriction was augmented in small mesenteric arteries from rats exposed 14 days to E-IH compared with those from sham rats. Constriction was blocked by the ET(A) receptor antagonist BQ-123 (10 microM) but not by the ET type B (ET(B)) receptor antagonist BQ-788 (100 microM). ET(A) receptor mRNA content was greater in renal medulla and coronary arteries from E-IH rats. ET(B) receptor mRNA was not different in any tissues examined, whereas ET-1 mRNA was increased in the heart and in the renal medulla. Thus augmented ET-1-dependent vasoconstriction via vascular ET(A) receptors appears to elevate blood pressure in E-IH-exposed rats.

Cardiovascular effects of centrally applied endothelin-11–31 and its relationship to endothelin-11–21 in rats

Endothelin-1(1-31) (ET-1(1-31)) is a novel member of the endothelin family, which comprises 31 amino acids and derived from the selective hydrolysis of big ET-1 by chymase. Although ET-1(1-31) has been reported to be involved in biological effects via direct or indirect (converting to ET-1(1-21)) mechanisms, the cardiovascular effects of central ET-1(1-31) are not fully identified. The present study was designed to comparatively investigate the cardiovascular effects of intracerebroventricular (icv) application of ET-1(1-31) or ET-1(1-21) in anesthetized rats. Injection (icv) of ET-1(1-31) (500 pmol) produced a biphasic blood pressure response: an initial increase (from 118+/-8 to 138+/-14 mmHg, P<0.05) followed by a sustained decrease in BP (from 118+/-8 to 58+/-9 mmHg, P<0.05), which was very similar to BP response to icv injection of big ET-1 (500 pmol) or ET-1(1-21) (25 pmol)(.) The cardiovascular effects of icv injection of ET-1(1-31) or ET-1(1-21) were completely antagonized by ET(A) receptor antagonist BQ123 but not ET(B) receptor antagonist BQ788. Furthermore, pretreatment with ET converting enzyme inhibitor phosphoramidon (10 nmol) abolished the cardiovascular effects evoked by icv injection of ET-1(1-31) or big ET-1. In conclusion, the current data showed that central ET-1(1-31) produced the similar cardiovascular effects as those of central ET-1(1-21), and suggesting that the central cardiovascular effects of ET-1(1-31) resulted from it converting to ET-1(1-21) and then activating ET(A) receptors.

Central effects of endothelin and its antagonists on sympathetic and cardiovascular regulation in SHR-SP

Intracerebroventricular injections of endothelin-1 (ET-1) are reported to cause dose-related increases in sympathetic nerve activity and blood pressure in anesthetized normotensive rats. These studies were performed to determine the following: which endothelin receptor, A or B, is involved in mediating sympathetic and cardiovascular effects of ET-1 injected centrally; whether central endothelin tonically participates in blood pressure regulation in normotensive rats; and whether the altered endothelin system in the central nervous system contributes to blood pressure elevation in hypertensive rats. ET-1, ET-A antagonist (BQ-123), or ET-B antagonist (RES-701-1) was injected into the lateral cerebral ventricle (i.c.v.) of urethane-anesthetized normotensive Wistar and Wistar-Kyoto (WKY) rats, spontaneously hypertensive rats (SHRs), and stroke-prone SHRs (SHR-SPs). In Wistar rats, i.c.v. injections of ET-1 (1, 5, 10 pmol) consistently increased sympathetic nerve activity, thereby elevating blood pressure in a dose-related manner. The pressor responses induced by i.c.v. ET-1 were abolished after intravenous pretreatment with phentolamine. Neither ET-A nor ET-B antagonist, when injected centrally, altered basal levels of sympathetic nerve activity, heart rate, or blood pressure in Wistar rats. However, sympathetic activation and pressor responses induced by i.c.v. injection of endothelin were completely abolished after i.c.v. pretreatment with ET-A antagonist but were unaffected after pretreatment with ET-B antagonist. Although i.c.v. injections of ET-1 increased sympathetic nerve activity and blood pressure in WKY rats, SHRs, and SHR-SPs, the magnitudes of these responses did not differ among these three groups. In contrast, i.c.v. injections of ET-A antagonist decreased sympathetic nerve activity, blood pressure, and heart rate only in SHR-SPs, but not in WKY rats and SHRs. In addition, the depressor effects of i.c.v. ET-A antagonist in SHR-SPs were ascertained while these rats were awake. In summary, i.c.v. injections of ET-1 increased sympathetic nerve activity and blood pressure via ET-A receptors but not via ET-B receptors. Central ET might tonically activate sympathetic nerve activity to thereby contribute to blood pressure elevation in SHR-SPs, but not in WKY rats and SHRs.