Endothelin in human brain and pituitary gland: comparison with rat

Endothelin-1 axes in the framework of predictive, preventive and personalised (3P) medicine

Endothelin-1 (ET-1) is involved in the regulation of a myriad of processes highly relevant for physical and mental well-being; female and male health; in the modulation of senses, pain, stress reactions and drug sensitivity as well as healing processes, amongst others. Shifted ET-1 homeostasis may influence and predict the development and progression of suboptimal health conditions, metabolic impairments with cascading complications, ageing and related pathologies, cardiovascular diseases, neurodegenerative pathologies, aggressive malignancies, modulating, therefore, individual outcomes of both non-communicable and infectious diseases such as COVID-19. This article provides an in-depth analysis of the involvement of ET-1 and related regulatory pathways in physiological and pathophysiological processes and estimates its capacity as a predictor of ageing and related pathologies,a sensor of lifestyle quality and progression of suboptimal health conditions to diseases for their targeted preventionand as a potent target for cost-effective treatments tailored to the person.

Investigation of the Molecular Mechanisms by Which Endothelin-3 Stimulates Preadipocyte Growth

Endothelins induce many biological responses, and they are composed of three peptides: ET-1, ET-2, and ET-3. Reports have indicated that ET-1 regulates cell proliferation, adipogenesis, and other cell responses and that ET-3 stimulates the growth of gastrointestinal epithelial cells and melanocytes. However, the signalling pathways of ET3 that mediate the growth of fat cells are still unclear. Using 3T3-L1 white preadipocytes, we found that ET-3 induced increases in both cell number and BrdU incorporation. Pretreatment with an ETAR antagonist (but not an ETBR antagonist) blocked the ET-3-induced increases in both cell number and BrdU incorporation. Additionally, BQ610 suppressed the ET-3-induced increases in phosphorylation of AMPK, c-JUN, and STAT3 proteins, and pretreatment with specific inhibitors of AMPK, JNK/c-JUN, or JAK/STAT3 prevented the ET-3-induced increases in phosphorylation of AMPK, c-JUN, and STAT3, respectively. Neither p38 MAPK inhibitor nor PKC inhibitor altered the effects of ET-3 on cell growth. These data suggest that ET-3 stimulates preadipocyte growth through the ETAR, AMPK, JNK/c-JUN, and STAT3 pathways. Moreover, ET-3 did not alter HIB1B brown preadipocyte and D12 beige preadipocyte growth, suggesting a preadipocyte type-dependent effect. The results of this study may help explain how endothelin mediates fat cell activity and fat cell-associated diseases.

Role of CINC-1 and CXCR2 receptors on LPS-induced fever in rats

The classic model of fever induction is based on the administration of lipopolysaccharide (LPS) from Gram-negative bacteria in experimental animals. LPS-induced fever results in the synthesis/release of many mediators that assemble an LPS-fever cascade. We have previously demonstrated that cytokine-induced neutrophil chemoattractant (CINC)-1, a Glu-Leu-Arg (ELR) + chemokine, centrally administered to rats, induces fever and increases prostaglandin E₂ in the cerebrospinal fluid. We now attempt to investigate the involvement of CINC-1 and its functional receptor CXCR2 on the fever induced by exogenous and endogenous pyrogens in rats. We also investigated the effect of reparixin, an allosteric inhibitor of CXCR1/CXCR2 receptors, on fever induced by either systemic administration of LPS or intracerebroventricular injection of CINC-1, as well as TNF-α, IL-1β, IL-6, or ET-1, known mediators of febrile response. Our results show increased CINC-1 mRNA expression in the liver, hypothalamus, CSF, and plasma following LPS injection. Moreover, reparixin administered right before CINC-1 or LPS abolished the fever induced by CINC-1 and significantly reduced the response induced by LPS. In spite of these results, reparixin does not modify the fever induced by IL-1β, TNF-α, and IL-6, but significantly reduces ET-1-induced fever. Therefore, it is plausible to suggest that CINC-1 might contribute to LPS-induced fever in rats by activating CXCR2 receptor on the CNS. Moreover, it can be hypothesized that CINC-1 is placed upstream TNF-α, IL-1β, and IL-6 among the prostaglandin-dependent fever-mediator cascade and amidst the prostaglandin-independent synthesis pathway of fever.

Endothelins in inflammatory neurological diseases

Endothelins were discovered more than thirty years ago as potent vasoactive compounds. Beyond their well-documented cardiovascular properties, however, the contributions of the endothelin pathway have been demonstrated in several neuroinflammatory processes and the peptides have been reported as clinically relevant biomarkers in neurodegenerative diseases. Several studies report that endothelin-1 significantly contributes to the progression of neuroinflammatory processes, particularly during infections in the central nervous system (CNS), and is associated with a loss of endothelial integrity at the blood brain barrier level. Because of the paucity of clinical trials with endothelin-1 antagonists in several infectious and non-infectious neuroinflammatory diseases, it remains an open question whether the 21 amino acid peptide is a mediator/modulator rather than a biomarker of the progression of neurodegeneration. This review focuses on the potential roles of endothelins in the pathology of neuroinflammatory processes, including infectious diseases of viral, bacterial or parasitic origin in which the synthesis of endothelins or its pharmacology have been investigated from the cell to the bedside in several cases, as well as in non-infectious inflammatory processes such as neurodegenerative disorders like Alzheimers Disease or central nervous system vasculitis.

Neuroprotective Effect of IRL-1620, an Endothelin B Receptor Agonist, on a Pediatric Rat Model of Middle Cerebral Artery Occlusion

Objective: The purpose of this study was to determine the potential neuroprotective effect of endothelin B (ET_B) receptor agonist IRL-1620 treatment in a pediatric model of ischemic stroke. Design: A prospective, animal model study. Setting: An experimental laboratory. Subjects: Three-month-old male Wistar Han rats. Interventions: The rats underwent permanent middle cerebral artery occlusion (MCAO). At 2, 4, and 6 h post MCAO, they were treated with saline, IRL-1620 (5 μg/kg, IV), and/or ET_B antagonist BQ788 (1 mg/kg, IV). Measurements and Main Results: The rats were evaluated over the course of 7 days for neurological and motor deficit, cerebral blood flow (CBF), and infarct volume. Young rats treated with IRL-1620 following MCAO improved significantly in neurological and motor assessments as compared to the vehicle-treated group, as measured by neurological score (P = 0.00188), grip test (P < 0.0001), and foot-fault error (P = 0.0075). CBF in the infarcted hemisphere decreased by 45-50% in all groups immediately following MCAO. After 7 days, CBF in the infarcted hemisphere of the IRL-1620 group increased significantly (P = 0.0007) when compared to the vehicle-treated group (+2.3 ± 23.3 vs. -45.4 ± 10.2%). Additionally, infarct volume was significantly reduced in IRL-1620-treated rats as compared to vehicle-treated rats (P = 0.0035, 41.4 ± 35.4 vs. 115.4 ± 40.9 mm³). Treatment with BQ788 blocked the effects of IRL-1620. Conclusions: IRL-1620 significantly reduced neurological and motor deficit as well as infarct volume while increasing CBF in a pediatric rat model of cerebral ischemia. These results indicate that selective ET_B receptor stimulation may provide a novel therapeutic strategy in the treatment of pediatric ischemic stroke as has been demonstrated in adult ischemic stroke.

Endothelin

The endothelins comprise three structurally similar 21-amino acid peptides. Endothelin-1 and -2 activate two G-protein coupled receptors, ETA and ETB, with equal affinity, whereas endothelin-3 has a lower affinity for the ETA subtype. Genes encoding the peptides are present only among vertebrates. The ligand-receptor signaling pathway is a vertebrate innovation and may reflect the evolution of endothelin-1 as the most potent vasoconstrictor in the human cardiovascular system with remarkably long lasting action. Highly selective peptide ETA and ETB antagonists and ETB agonists together with radiolabeled analogs have accurately delineated endothelin pharmacology in humans and animal models, although surprisingly no ETA agonist has been discovered. ET antagonists (bosentan, ambrisentan) have revolutionized the treatment of pulmonary arterial hypertension, with the next generation of antagonists exhibiting improved efficacy (macitentan). Clinical trials continue to explore new applications, particularly in renal failure and for reducing proteinuria in diabetic nephropathy. Translational studies suggest a potential benefit of ETB agonists in chemotherapy and neuroprotection. However, demonstrating clinical efficacy of combined inhibitors of the endothelin converting enzyme and neutral endopeptidase has proved elusive. Over 28 genetic modifications have been made to the ET system in mice through global or cell-specific knockouts, knock ins, or alterations in gene expression of endothelin ligands or their target receptors. These studies have identified key roles for the endothelin isoforms and new therapeutic targets in development, fluid-electrolyte homeostasis, and cardiovascular and neuronal function. For the future, novel pharmacological strategies are emerging via small molecule epigenetic modulators, biologicals such as ETB monoclonal antibodies and the potential of signaling pathway biased agonists and antagonists.

Role of endothelin-1 in mediating changes in cardiac sympathetic nerve activity in heart failure

Heart failure (HF) is associated with increased sympathetic nerve activity to the heart (CSNA), which is directly linked to mortality in HF patients. Previous studies indicate that HF is associated with high levels of plasma endothelin-1 (ET-1), which correlates with the severity of the disease. We hypothesized that blockade of endothelin receptors would decrease CSNA. The effects of intravenous tezosentan (a nonselective ETA and ETB receptor antagonist) (8 mg·kg(-1)·h(-1)) on resting levels of CSNA, arterial pressure, and heart rate were determined in conscious normal sheep (n = 6) and sheep with pacing-induced HF (n = 7). HF was associated with a significant decrease in ejection fraction (from 74 ± 2% to 38 ± 1%, P < 0.001) and a significant increase in resting levels of CSNA burst incidence (from 56 ± 11 to 87 ± 2 bursts/100 heartbeats, P < 0.01). Infusion of tezosentan for 60 min significantly decreased resting mean aterial pressure (MAP) in both normal and HF sheep (-8 ± 4 mmHg and -4 ± 3 mmHg, respectively; P < 0.05). This was associated with a significant decrease in CSNA (by 25 ± 26% of control) in normal sheep, but there was no change in CSNA in HF sheep. Calculation of spontaneous baroreflex gain indicated significant impairment of the baroreflex control of HR after intravenous tezosentan infusion in normal animals but no change in HF animals. These data suggest that endogenous levels of ET-1 contribute to the baseline levels of CSNA in normal animals, but this effect is absent in HF.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Endogenous ghrelin released in response to endothelin stimulates growth hormone secretion in cattle

The purpose of this study was to evaluate whether circulating ghrelin and growth hormone (GH) concentrations in cattle are regulated by endothelin-1 (ET-1), endothelin-3 (ET-3), and secretin. Six Holstein steers (242+/-1 d old, 280.5+/-4.4 kg body weight [BW]; mean+/-SEM) were allocated randomly in an incomplete Latin square design to receive each of 4 treatment compounds (vehicle, ET-1, ET-3, and secretin) with 1-d intervals between successive treatments. The treatment compounds were injected intravenously via a catheter inserted into the external jugular vein of each steer. Blood was sampled from the indwelling catheter at -30, -15, 0, 5, 10, 15, 20, 30, 45, 60, 90, 120, 150, and 180 min. Plasma ghrelin and GH responses to the treatment compounds were measured by a double-antibody radioimmunoassay system. Data were analyzed by using a MIXED procedure of SAS, version 9.1. Plasma acyl ghrelin, total ghrelin, and GH concentrations were increased by both ET-1 and ET-3 injection (ET-1 injection: 311+/-15 pg/mL vs 245+/-15 pg/mL, 2.4+/-0.2 ng/mL vs 1.61+/-0.05 ng/mL, 4.73+/-0.92 ng/mL vs 1.17+/-0.09 ng/mL for acyl ghrelin, total ghrelin, and GH, respectively; ET-3 injection: 337+/-27 pg/mL vs 245+/-15 pg/mL, 2.6+/-0.1 ng/mL vs 1.61+/-0.05 ng/mL, 5.56+/-0.97 ng/mL vs 1.17+/-0.09 ng/mL for acyl ghrelin, total ghrelin, and GH, respectively; P<0.01). Ghrelin and GH concentrations were not changed by secretin injection throughout the experimental periods. These results indicate that ET-1 and ET-3 stimulate ghrelin and GH secretion in cattle and demonstrate for the first time that endogenous ghrelin released in response to endothelin injection stimulates GH secretion in vivo in cattle.

Cellular distribution of the endothelin system in the human brain

The vasoconstrictor endothelin-1 (ET-1) may also act as a neuropeptide. ET-1 is formed by the catalytic action of endothelin-converting enzyme-1 (ECE-1) on big ET-1 and its cellular actions are mediated via ET(A) and ET(B) receptors. Although localisation of these components in rodent brain has been extensively investigated, no single study has mapped their distribution in human brain. Here we describe the localisation of ET-1 mRNA, ET-1, ECE-1, ET(A) and ET(B) receptors within 24 human brain regions. In situ RT-PCR has previously detected ET-1 mRNA in 22 areas (excluding the post-central gyrus and pineal gland), and ET-1 immunoreactivity was visualised in cells of all regions. Using specific antibodies we have immunolocalised ECE-1 and ET(B) receptors in cells of 24 areas, and ET(A) receptors in nine regions (choroidal epithelial cells, neurones in the diencephalon, hippocampus, amygdaloid, dentate nucleus, Purkinje cells of the cerebellum, flocculo-nodular lobe and vermis). ET-1 mRNA, ET-1, ECE-1 and ET(B) receptors were observed in cortical pyramidal cells, neurones (brainstem, basal nuclei, thalamus, insula and claustrum, limbic region), cells in the anterior pituitary gland; nerve cell processes in the pars nervosa; pinealocytes and choroidal epithelial cells. Only ET-1 mRNA, ET-1, ECE-1, and ET(B) receptors were visualised in cerebral capillary endothelial cells. The presence of ET-1 mRNA, ECE-1 and ET-1 in 22 brain regions confirms ET expression and processing in human brain. The localisation of ET-1 and ET(B) receptors suggests receptor-mediated action akin to a neurotransmitter role for ET-1.

Endothelin system: the double-edged sword in health and disease

The endothelin system consists of two G-protein-coupled receptors, three peptide ligands, and two activating peptidases. Its pharmacological complexity is reflected by the diverse expression pattern of endothelin system components, which have a variety of physiological and pathophysiological roles. In the vessels, the endothelin system has a basal vasoconstricting role and participates in the development of diseases such as hypertension, atherosclerosis, and vasospasm after subarachnoid hemorrhage. In the heart, the endothelin system affects inotropy and chronotropy, and it mediates cardiac hypertrophy and remodeling in congestive heart failure. In the lungs, the endothelin system regulates the tone of airways and blood vessels, and it is involved in the development of pulmonary hypertension. In the kidney, it controls water and sodium excretion and acid-base balance, and it participates in acute and chronic renal failure. In the brain, the endothelin system modulates cardiorespiratory centers and the release of hormones. More advanced functional analysis of the endothelin system awaits not only additional pharmacological studies using highly specific endothelin antagonists but also the generation of genetically altered rodent models with conditional loss-of-function and gain-of-function manipulations.

Endothelin and dopamine release

Endothelins and endothelin receptors are widespread in the brain. There is increasing evidence that endothelins play a role in brain mechanisms associated with behaviour and neuroendocrine regulation as well as cardiovascular control. We review the evidence for an interaction of endothelin with brain dopaminergic mechanisms. Our work has shown that particularly endothelin-1 and ET(B) receptors are present at significant levels in typical brain dopaminergic regions such as the striatum. Moreover, lesion studies showed that ET(B) receptors are present on dopaminergic neuronal terminals in striatum and studies with local administration of endothelins into the ventral striatum showed that activation of these receptors causes dopamine release, as measured both with in vivo voltammetry and behavioural methods. While several previous studies have focussed on the possible role of very high levels of endothelins in ischemic and pathological mechanisms in the brain, possibly mediated by ET(A) receptors, we propose that physiological levels of these peptides play an important role in normal brain function, at least partly by interacting with dopamine release through ET(B) receptors.

Essential role for endothelin ET(B) receptors in fever induced by LPS (E. coli) in rats

1. The influence of endothelin receptor antagonists on febrile responses to E. coli lipopolysaccharide (LPS), interleukin-1beta (IL-1beta), tumour necrosis factor-alpha (TNF-alpha) and endothelin-1 (ET-1) was assessed in conscious rats. 2. Intravenous (i.v.) LPS (5.0 microg kg(-1)) markedly increased rectal temperature to a peak of 1.30 degrees C over baseline at 2.5 h. Pretreatment with the mixed endothelin ET(A)/ET(B) receptor antagonist bosentan (10 mg kg(-1), i.v.) or the selective endothelin ET(B) receptor antagonist BQ-788 (N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D -1-methoxycarboyl-D-norleucine; 3 pmol, into a lateral cerebral ventricle-i.c.v.) reduced the peak response to LPS to 0.90 and 0.75 degrees C, respectively. The selective endothelin ET(A) receptor antagonist BQ-123 (cyclo[D-Trp-D-Asp-Pro-D-Val-Leu]; 3 pmol, i.c.v.) was ineffective. 3. Increases in temperature caused by IL-1beta (180 fmol, i.c.v.), TNF-alpha (14.4 pmol, i.c.v.) or IL-1beta (150 pmol kg(-1), i.v.) were unaffected by BQ-788 (3 pmol, i.c.v.). 4. Central injection of endothelin-1 (0.1 to 3 fmol, i.c.v.) caused slowly-developing and long-lasting increases in rectal temperature (starting 2 h after administration and peaking at 4-6 h between 0.90 and 1.15 degrees C) which were not clearly dose-dependent. The response to endothelin-1 (1 fmol, i.c.v.) was prevented by BQ-788, but not by BQ-123 (each at 3 pmol, i.c.v.). Intraperitoneal pretreatment with the cyclo-oxygenase inhibitor indomethacin (2 mg kg(-1)), which partially reduced LPS-induced fever, did not modify the hyperthermic response to endothelin-1 (3 fmol, i.c.v.). 5. Therefore, central endothelin(s) participates importantly in the development of LPS-induced fever, via activation of a prostanoid-independent endothelin ET(B) receptor-mediated mechanism possibly not situated downstream from IL-1beta or TNF-alpha in the fever cascade.

Endothelin induces dopamine release from rat striatum via endothelin-B receptors

The aim of the present study was to determine whether local administration of endothelin induces the release of dopamine in the rat striatum and to characterize and localize endothelin receptors in this brain region. Local injection of endothelin-1 (10 pmol) into the ventral striatum of urethane-anaesthetized rats caused an increase of 8 microM in the extracellular concentration of dopamine as measured by in vivo chronoamperometry. The peak increase in dopamine concentration occurred within 5 min of endothelin injection. Injection of the selective endothelin-B receptor agonist [Ala1.3,11.15]endothelin-1 (10 pmol) also caused an increase in extracellular dopamine concentration, suggesting that endothelin is acting at the endothelin-B receptor to elicit its effect. In rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway, the response to local injection of endothelin-1 (10 pmol) was significantly inhibited on the lesioned side as compared to the non-lesioned side. In contrast, pretreatment of the rats with the N-methyl-D-aspartate receptor antagonist dizocilpine maleate (5 mg/kg, i.p.) or the nitric oxide synthase inhibitor NG-nitro-L-arginine (3 mg/kg, i.p.) did not alter the endothelin-induced release of dopamine. In binding studies, addition of endothelin-1 displaced [125I]endothelin-1 with a Ki of 220 pM. The endothelin-B receptor antagonist BQ788 displaced [125I]endothelin-1 with a Ki of 120 nM, whereas the endothelin-A receptor antagonist BQ123 produced only a 25% displacement at 10 microM, suggesting that endothelin receptors in the striatum are of the endothelin-B subtype. In rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal dopamine system, [125I]endothelin-1 binding was reduced by 53% in lesioned striatum compared to non-lesioned striatum, with no difference in the Kd. These data provide evidence that endothelin acts on a homogeneous population of endothelin-B receptors within the striatum to cause the release of dopamine and that a significant proportion of these receptors is located on dopaminergic neurons.

The endothelin system in cardiovascular physiology and pathophysiology

Endothelin-1, a member of a novel family of regulatory peptides, is the most potent vasoconstrictor and pressor substance known. Endothelin-1 is a 21-amino-acid endothelium-derived peptide causing uniquely sustained vasoconstriction. In addition, endothelin-1 has pronounced effects on the coronary, renal and cerebral circulations, enhances responses to other vasoconstrictors, and is comitogenic. Recent studies have shown that the endothelins are essential for normal fetal development, and that endothelin-1 plays an important physiological role in the regulation of basal vascular tone and blood pressure in healthy humans. There is now also a wealth of evidence suggesting that endothelin-1 is a key mediator in a range of cardiovascular diseases associated with sustained vasoconstriction, such as chronic heart failure, and with vasospasm, such as subarachnoid haemorrhage. In addition, endothelin-1 appears to act in opposition to nitric oxide to promote the atherosclerotic process. There are a large number of oral and intravenously active endothelin antagonists entering clinical development and a number of clinical studies, particularly with endothelin receptor antagonists, are now under way. Such studies are beginning to define the role of the endothelins in cardiovascular disease and to confirm the potential of the endothelin system as an important new therapeutic target.

Effects of endothelin-1 on the rat pituitary-adrenocortical axis under basal and stressful conditions

Endothelins (ETs) and their receptor subtypes A and B (ETA and ETB) are expressed in the various components of the mammalian hypothalamo-pituitary-adrenal (HPA) axis, but their involvement in the functional regulation of HPA is controversial. To gain insight into this topic, we have investigated the effects of ET-1 and/or the specific antagonists of ETA and ETB receptors (BQ-123 and BQ-788, respectively) on the plasma concentrations of ACTH, corticosterone and aldosterone of non-stressed (control) and ether- or cold-stressed rats. The study of the effects of the administration of the two ET-receptor antagonists alone could provide informations about the possible action of endogenous ETs on the HPA axis. Exogenous ET-1 increased ACTH, corticosterone and aldosterone blood levels in control rats, as well as evoked a sizable enhancement of the HPA axis response to ether stress and a marked depression of the response to cold stress. BQ-123 and BQ-788 did not prevent the stimulatory effect of exogenous ET-1 in control rats, but when administered alone, raised the plasma concentrations of ACTH, corticosterone and aldosterone. Both ET-receptor antagonists magnified the HPA axis response to ether and cold stresses, but their effect was not counteracted by exogenous ET-1. Although very difficult to interpret, our present findings allow us to conclude that endogenous ETs play a role in the maintenance of the basal activity of rat HPA axis acting through ETA and ETB receptor subtypes, which are partially insensitive to BQ-123 and BQ-788. Conversely, the involvement of ETs in the modulation of the HPA axis responses to various stresses is very doubtful.

Hemodynamic responses to endothelin-1 and endothelin antagonists microinjected into the nucleus tractus solitarius in rats

The role of endothelin-1 (ET-1) within the nucleus tractus solitarius (NTS) in central cardiovascular control was investigated by local microinjections of ET-1 and ET-receptor antagonists. In urethane-anesthetized Sprague-Dawley rats, a unilateral microinjection of ET-1 (1.0, 3.3, and 10.0 pmol) into the NTS significantly increased arterial pressure, left ventricular systolic pressure, and dP/dt(max) in a dose-dependent manner, and slightly decreased heart rate in a dose-independent manner. The pressor effect lasted >90 min. In normotensive rats, neither PD147953, a selective ETA-receptor antagonist, nor PD142893, a mixed ETA- and ETB-receptor antagonist, microinjected into the NTS elicited any changes in arterial pressure or heart rate. The pressor and bradycardic effects evoked by microinjection of ET-1 into the NTS could be blocked by local pretreatment with PD147953 and completely eliminated by intravenous pretreatment with the ganglionic blocker hexamethonium. The arterial baroreflex sensitivity was almost totally suppressed by microinjection of ET-1 (3.3 pmol) in alpha-chloralose-anesthetized Sprague-Dawley rats. A similar pattern of changes in the hemodynamic variables was elicited by microinjection of ET-1 (3.3 pmol) into the NTS in spontaneously hypertensive rats (SHRs) compared with Wistar-Kyoto (WKY) rats. In SHRs, microinjection of PD142893 did not elicit any changes in arterial pressure or heart rate. These results suggest that ET-1 modulates reflex control of hemodynamics by activation of autonomic nerve via ETA receptors in the NTS, and that the responsiveness of SHRs to ET-1 or PD142893 is similar to that of WKY rats.

Physiological role of brain endothelin in the central autonomic control: from neuron to knockout mouse

Although endothelin (ET) was discovered as a potent vascular endothelium-derived constricting peptide, its presumed physiological and pathophysiological roles are now considered much more diverse than originally though. Endothelin in the brain is thought to be deeply involved in the central autonomic control and consequent cardiorespiratory homeostasis, possibly as a neuromodulator or a hormone that functions locally in an autocrine/paracrine manner or widely through delivery by the cerebrospinal fluid (CSF). This notion is based on the following lines of evidence. (1) Mature ET, its precursors, converting enzymes, and receptors all are detected at strategic sites in the central nervous system (CNS), especially those controlling the autonomic functions. (2) The ET is present in the CSF at concentrations higher than in the plasma. (3) There is a topographical correspondence of ET and its receptors in the CNS. (4) The ET is released by primary cultures of hypothalamic neurons. (5) When ET binds to its receptors, intracellular calcium channels. (6) An intracerebroventricular or topical application of ET to CNS sites elicits a pattern of cardiorespiratory changes accompanied by responses of vasomotor and respiratory neurons. (7) Recently generated knockout mice with disrupted genes encoding ET-1 exhibited, along with malformations in a subset of the tissues of neural crest cell lineage, cardiorespiratory abnormalities including elevation of arterial pressure, sympathetic overactivity, and impairment of the respiratory reflex. Definitive evidence is expected from thorough analyses of knockout mice by applying conventional experimental methods.

Behavioural, autonomic and endocrine responses associated with C-fos expression in the forebrain and brainstem after intracerebroventricular infusions of endothelins

Endothelins are a range of peptides (endothelin-1, endothelin-2, and endothelin-3) well known to act peripherally as powerful cardiovascular-regulating agents. Recently, they have been shown to be localized in CSN, where they may act as central neurotransmitters. A variety of putative roles has been ascribed to them in the CNS. To identify those regions of the brain capable of responding to these peptides, the expression of c-fos (an immediate-early gene), has been used to map patterns of activation following intracerebroventricular (i.c.v.) infusions of endothelins in Lister-hooded rats. This has been correlated with changes in heart rate, core temperature and plasma corticosterone levels. Endothelin-3 i.c.v. (50 pmol) decreased both heart rate and core temperature (both recorded by telemetry). This effect lasted for about 30-45 min. Endothelin-1 (10 pmol) or endothelin-3 (50 pmol) i.c.v. induced c-fos expression in the specific regions in the forebrain and brainstem. Strong expression was found in the septum, bed nucleus of the stria terminalis, parvicellular paraventricular nucleus, the central nucleus of the amygdala, dorsal motor nucleus of the vagus and solitary nucleus. There was less marked c-fos expression in other areas of the basal forebrain, such as the organum vasculosum of the lamina terminals, median preoptic nucleus, supraoptic nucleus and the magnocellular. There are two classes of endothelin receptor (A and B). An endothelin-A receptor antagonist, BQ-123, abolished c-fos expression in all structures in the forebrain and brainstem following endothelin-1 infusions. However, an endothelin-B agonist (TetraAla endothelin-1) did not induce discernible c-fos expression in the forebrain or brainstem. These results suggest that the endothelin-A receptor is responsible for endothelin-dependent c-fos induction in the brain. Interactions between endothelins and angiotensin II were also studied. The pattern of c-fos induced by endothelin-3 and angiotensin II was different (particularly in the anteroventral region of the third ventricle). Furthermore, prior infusions of endothelin-3 interfered with the expression of c-fos induced by subsequent angiotensin II, and also suppressed the latter's dipsogenic effect. These results show that endothelin-3 and angiotensin II interact at both behavioural and cellular levels, and that endothelins may play significant roles in the central control of fluid balance and autonomic activity.

Identification of endothelin receptor subtype (ETB) in human cerebral cortex using subtype-selective ligands

Specific endothelin (ET) binding sites were characterized in membranes prepared from human cerebral cortices using binding assay and cross-linking analysis. The presence of immunoreactive (IR) ET-1 was studied by radioimmunoassay. Saturation binding experiments revealed that the KD and Bmax for 125I-ET-1 and 125I-ET-3 to membranes from gray matter were 25 +/- 6 pM and 115 +/- 15 fmol/mg of protein and 24 +/- 5 pM and 108 +/- 13 fmol/mg of protein, respectively. Similar results were obtained for white matter. In the presence of 10 nM sarafotoxin-6c, which is selective for ETB receptors, 125I-ET-1 and 125I-ET-3 binding was totally abolished. However, in the presence of 1 microM BQ123, which is selective for ETA receptors, both bindings were not affected. These results suggest that the human cerebral cortex contains only ETB receptors. Cross-linking of 125I-ET-1 and 125I-ET-3 to membranes with disuccinimidyl suberate resulted in the labeling of two bands of 48 and 31 kDa. Concentrations of IR-ET-1 in the gray and white matter were 7.0 +/- 3.2 and 2.5 +/- 1.7 fmol/g wet weight, respectively. The demonstration of high-affinity ETB receptors and the presence of IR-ET-1 suggest that the peptide may act as a neurotransmitter or neuromodulator in the human cerebral cortex.

Regional differences in endothelin converting enzyme activity in rat brain: inhibition by phosphoramidon and EDTA

1. It has been demonstrated previously that conversion of big endothelin-1 (bET-1) to endothelin-1 (ET-1) is inhibited in vitro and in vivo by phosphoramidon. In addition, ET-1 binding sites and mRNA have been shown within the brain. Here we expand upon our previous observation that rat brain contains phosphoramidon-inhibitable endothelin converting enzyme (ECE) and show that this activity is not uniformly distributed throughout the brain. 2. ECE activity was detected by a bioassay which depended upon the 10,000 fold difference in potency between bET-1 and ET-1 as stimulants of guanosine 3':5'-cyclic monophosphate (cyclic GMP) accumulation in kidney epithelial (PK1) cells of the pig. Data were confirmed by specific enzyme-linked immunosorbent assay (ELISA) employing antibody directed against ET-1/3(17-21). 3. Following homogenization of the whole brain and ultracentrifugation the 100,000 g pellet contained greater than 4 times more ECE activity than the cytosol. Washing of the pellet with KCl (1 M) and extraction with the detergent CHAPS (20 mM) revealed a phosphoramidon-inhibitable ECE within the residual particulate fraction (nominally classified as the cytoskeletal fraction). Phosphoramidon (IC50, approx. 5 microM) or EDTA inhibited the conversion of bET-1 to ET-1 by the cytoskeletal fraction of rat brain by more than 60%.2+ 4. Following dissection of rat brain into olfactory bulb, cerebral cortex, striatum, hippocampus, cerebellum, midbrain (including thalamus), hypothalamus and medulla oblongata (including pons) the greatest ECE was detected in the hypothalamus and medulla oblongata.After fractionation, the ECE-activities in the cytoskeletal fractions prepared from the hypothalamus or medulla oblongata were inhibited concentration-dependently by phosphoramidon or EDTA, with maximum inhibitions of>80% and >70%, respectively.5. These data show that rat brain contains a phosphoramidon- and EDTA-inhibitable ECE which maybe similar to that present in endothelial cells. The localization of this enzyme correlates with published reports of immunoreactive-ET-l, ET-1-binding sites, and messenger RNA for ET-1 in the rat brain, and suggests the presence of the entire synthetic pathway for ET-1.

Human brain contains a metalloprotease that converts big endothelin-1 to endothelin-1 and is inhibited by phosphoramidon and EDTA

Incubation of big endothelin-1 (bET-1) with protein derived from the detergent-extracted 100,000 g pellet prepared from human brain tissue resulted in the formation of endothelin-1 (ET-1) at a rate of 90 fmol mg-1 protein min-1. This formation was inhibited in a concentration-dependent manner by either phosphoramidon or EDTA, with half-maximal inhibitory concentrations of 2 and 20 microM, respectively. No conversion of big endothelin-3 (bET-3) to endothelin-3 (ET-3) was detected under the same conditions. These results show the presence in the human brain of a metalloprotease-like enzymatic activity which selectively converts bET-1 and ET-1. Together with earlier reports of mRNA for ET-1 this suggests the presence of the entire synthetic pathway for ET-1 in human brain.