Cerebrospinal fluid endothelin-1 and endothelin-3 levels in normal and neurosurgical patients: a clinical study and literature review

Endothelin and the heart in health and diseases

Endothelin-1 (ET-1), a 21-amino acid peptide, was initially identified in 1988 as a potent vasoconstrictor and pressor substance isolated from the culture supernatant of porcine aortic endothelial cells. From human genomic DNA analysis, two other family peptides, ET-2 and ET-3, were found. They showed different effects and distribution, suggesting that each peptide may play separate roles in different organs. In the heart, ET-1 also causes positive inotropic and chronotropic responses and hypertrophic activity of the cardiomyocytes. ETs act via activation of two receptor subtypes, ET_A and ET_B receptors, both of which are coupled to various GTP-binding proteins depending on cell types. Endogenous ET-1 may be involved in progression of various cardiovascular diseases. ET antagonists are currently used clinically in the treatment for patients with pulmonary hypertension, and are considered to have further target diseases as heart failure, cardiac hypertrophy and other cardiac diseases, renal diseases, systemic hypertension, and cerebral vasospasm.

Evaluation of endothelin-1 and MMPs-2, -9, -14 in cerebrospinal fluid as indirect indicators of blood-brain barrier dysfunction in chronic canine hypothyroidism

Chronic canine hypothyroidism is associated with blood-brain barrier (BBB) disruption. We hypothesized that this change is mediated by endothelin-1(ET-1) and matrix metalloproteinases (MMP) -2, -9, and -14, as evidenced by increased concentrations of these proteins in cerebrospinal fluid (CSF) compared to controls. CSF from 18 dogs, 9 controls and 9 with experimentally induced hypothyroidism was collected before and 6, 12, and 18 months after induction of hypothyroidism. Concentrations of ET-1 using an ELISA kit, and for MMP-2, -9, and -14 using gelatinase zymography were measured in CSF. ET-1 was undetectable in CSF of control and hypothyroid dogs at all time-points. Constitutively expressed MMP-2 was detectable in CSF samples in all dogs at all time-points. No other MMPs were detectable in CSF. No differences in CSF concentrations of ET-1 and MMP-2, 9, and 14 were found between hypothyroid and euthyroid dogs. Therefore, ET-1 and MMP-2, 9, and 14 are unlikely to be primary mediators of BBB damage in chronically hypothyroid dogs.

Genetic risk factors for intracranial aneurysms: a meta-analysis in more than 116,000 individuals

OBJECTIVE

There is an urgent need to identify risk factors for sporadic intracranial aneurysm (IA) development and rupture. A genetic component has long been recognized, but firm conclusions have been elusive given the generally small sample sizes and lack of replication. Genome-wide association studies have overcome some limitations, but the number of robust genetic risk factors for IA remains uncertain.

METHODS

We conducted a comprehensive systematic review and meta-analysis of all genetic association studies (including genome-wide association studies) of sporadic IA, conducted according to Strengthening the Reporting of Genetic Association Studies and Human Genome Epidemiology Network guidelines. We tested the robustness of associations using random-effects and sensitivity analyses.

RESULTS

Sixty-one studies including 32,887 IA cases and 83,683 controls were included. We identified 19 single nucleotide polymorphisms associated with IA. The strongest associations, robust to sensitivity analyses for statistical heterogeneity and ethnicity, were found for the following single nucleotide polymorphisms: on chromosome 9 within the cyclin-dependent kinase inhibitor 2B antisense inhibitor gene (rs10757278: odds ratio [OR] 1.29; 95% confidence interval [CI] 1.21-1.38; and rs1333040: OR 1.24; 95% CI 1.20-1.29), on chromosome 8 near the SOX17 transcription regulator gene (rs9298506: OR 1.21; 95% CI 1.15-1.27; and rs10958409: OR 1.19; 95% CI 1.13-1.26), and on chromosome 4 near the endothelin receptor A gene (rs6841581: OR 1.22; 95% CI 1.14-1.31).

CONCLUSIONS

Our comprehensive meta-analysis confirms a substantial genetic contribution to sporadic IA, implicating multiple pathophysiologic pathways, mainly relating to vascular endothelial maintenance. However, the limited data for IA compared with other complex diseases necessitates large-scale replication studies in a full spectrum of populations, with investigation of how genetic variants relate to phenotype (e.g., IA size, location, and rupture status).

Current purpose and practice of hypertonic saline in neurosurgery: a review of the literature

OBJECTIVE

To review and summarize controversies and current concepts regarding the use of hypertonic saline during the perioperative period in neurosurgery.

METHODS

Relevant literature was searched on PubMed and Scopus electronic databases to identify all studies that have investigated the use of hypertonic saline in neurosurgery.

RESULTS

Fluid management during the course of neurosurgical practice has been debated at length, especially strategies to control intracranial pressure and small volume resuscitation. The goal of fluid therapy includes minimizing cerebral edema, preserving intravascular volume, and maintaining cerebral perfusion pressure. Mannitol is widely recognized as the gold standard for treating intracranial hypertension but can result in systemic hypotension. Thus, hypertonic saline provides volume expansion and may improve cerebral and systemic hemodynamics. Recently published prospective data, however, regarding the use of osmotic agents fails to establish clear guidelines in neurosurgical patients.

CONCLUSIONS

We suggest that hypertonic saline will emerge as an alternative to mannitol, especially for a long-term use or multiple doses are needed and lead to a great opportunity for collaborative research.

Endothelin-1 levels in plasma and cerebrospinal fluidfollowing subarachnoid haemorrhage

A serial measurement of endothelin-1(ET-1) levels in plasma, cisternal and ventricular cerebrospinal fluid(CSF) was performed in 16 patients with subarachnoid haemorrhage (SAH). The patients were classified as grade III or IV according to the clinical grade of Hunt and Hess, and computerised tomography(CT) was classified as Fisher's CT group 3. Cisternal and ventricular CSF and plasma were obtained from the patients on the day of operation days 0-3, days 5-8 and days 14-18 after SAH. ET-I concentration in each sample was quantified by sandwich-enzyme immunoassay. ET-I levels in plasma and CSF were the highest between days 0-3 and then decreased. The ET-I levels in the cisternal CSF were significantly higher during days 0-3(p<0. 01) and days 5-8(p<0. 01) than those in the ventricular CSF It is suggested that ET-I could play an important role in the early stages of the cerebral vasospasm.

Hypertonic resuscitation after severe injury: is it of benefit?

There is a wealth of preclinical data suggesting potential benefit from the administration of hypertonic solutions after severe injury with hypovolemic shock, including improved tissue perfusion, improved flow through the microcirculation, and modulation of the inflammatory response, which may mitigate subsequent organ failure. However, despite these potential advantages, clinical trials of hypertonic resuscitation early after injury have failed to demonstrate significant benefit for resuscitation of hemorrhagic shock, and although there is no difference in overall mortality, there appears to be a trend toward earlier mortality among those receiving hypertonic fluids. Likewise, for TBI there are data suggesting that hypertonic fluids should support cerebral perfusion and mitigate intracranial hypertension, yet the clinical trials of early administration to these patients have also failed to show benefit. Further study is warranted in this patient population, as a longer period of hypertonicity may be required to show a clinical effect. Assessment of long-term neurologic outcome in this patient population remains the gold standard in determining benefit.

Common variant near the endothelin receptor type A (EDNRA) gene is associated with intracranial aneurysm risk

The pathogenesis of intracranial aneurysm (IA) formation and rupture is complex, with significant contribution from genetic factors. We previously reported genome-wide association studies based on European discovery and Japanese replication cohorts of 5,891 cases and 14,181 controls that identified five disease-related loci. These studies were based on testing replication of genomic regions that contained SNPs with posterior probability of association (PPA) greater than 0.5 in the discovery cohort. To identify additional IA risk loci, we pursued 14 loci with PPAs in the discovery cohort between 0.1 and 0.5. Twenty-five SNPs from these loci were genotyped using two independent Japanese cohorts, and the results from discovery and replication cohorts were combined by meta-analysis. The results demonstrated significant association of IA with rs6841581 on chromosome 4q31.23, immediately 5' of the endothelin receptor type A with P = 2.2 × 10(-8) [odds ratio (OR) = 1.22, PPA = 0.986]. We also observed substantially increased evidence of association for two other regions on chromosomes 12q22 (OR = 1.16, P = 1.1 × 10(-7), PPA = 0.934) and 20p12.1 (OR = 1.20, P = 6.9 × 10(-7), PPA = 0.728). Although endothelin signaling has been hypothesized to play a role in various cardiovascular disorders for over two decades, our results are unique in providing genetic evidence for a significant association with IA and suggest that manipulation of the endothelin pathway may have important implications for the prevention and treatment of IA.

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

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

Hypertonic saline: a clinical review

Literature suggest that hypertonic saline (HTS) solution with sodium chloride concentration greater than the physiologic 0.9% can be useful in controlling elevated intracranial pressure (ICP) and as a resuscitative agent in multiple settings including traumatic brain injury (TBI). In this review, we discuss HTS mechanisms of action, adverse effects, and current clinical studies. Studies show that HTS administered during the resuscitation of patients with a TBI improves neurological outcome. HTS also has positive effects on elevated ICP from multiple etiologies, and for shock resuscitation. However, a prospective randomized Australian study using an aggressive resuscitation protocol in trauma patients showed no difference in amount of fluids administered during prehospital resuscitation, and no differences in ICP control or neurological outcome. The role of HTS in prehospital resuscitation is yet to be determined. The most important factor in improving outcomes may be prevention of hypotension and preservation of cerebral blood flow. In regards to control of elevated ICP during the inpatient course, HTS appears safe and effective. Although clinicians currently use HTS with some success, significant questions remain as to the dose and manner of HTS infusion. Direct protocol comparisons should be performed to improve and standardize patient care.

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.

Molecular mechanisms of early brain injury after subarachnoid hemorrhage

OBJECTIVES

Increasing body of experimental and clinical data indicates that early brain injury after initial bleeding largely contributes to unfavorable outcome after subarachnoid hemorrhage (SAH). This review presents molecular mechanisms underlying brain injury at its early stages after SAH.

METHODS

PubMed was searched using term 'subarachnoid hemorrhage' and key words referring to molecular and cellular pathomechanisms of SAH-induced early brain injury.

RESULTS

The authors reviewed intracranial phenomena and molecular agents that contribute to the early development of pathological sequelae of SAH in cerebral and vascular tissues, including cerebral ischemia and its interactions with injurious blood components, blood-brain barrier disruption, brain edema and apoptosis.

DISCUSSION

It is believed that detailed knowledge of molecular signaling pathways after SAH will serve to improve therapeutic interventions. The most promising approach is the protection of neurovascular unit including anti-apoptosis therapy.

Lumbar cerebrospinal fluid proteome in multiple sclerosis: characterization by ultrafiltration, liquid chromatography, and mass spectrometry

Neurological diseases, including multiple sclerosis (M.S.), often provoke changes in the functioning of the endothelial and epithelial brain barriers and give rise to disease-associated alterations of the cerebrospinal fluid (CSF) proteome. In the present study, pooled and ultrafiltered CSF of M.S. and non-M.S. patients was digested with trypsin and analyzed by off-line strong cation-exchange chromatography (SCX) coupled to on-line reversed-phase LC-ESI-MS/MS. In an alternative approach, the trypsin-treated subproteomes were analyzed directly by LC-ESI-MS/MS and gas-phase fractionation in the mass spectrometer. Taken together, both proteomic approaches in combination with a three-step evaluation process including the search engines Sequest and Mascot, and the validation software Scaffold, resulted in the identification of 148 proteins. Sixty proteins were identified in CSF for the first time by mass spectrometry. For validation purposes, the concentration of cystatin A was determined in individual CSF and serum samples of M.S. and non-M.S. patients using ELISA.

CEREBROVASCULAR DYSFUNCTION IS AN ATTRACTIVE TARGET FOR THERAPY IN HEAT STROKE¶

1. The aim of the present review is to summarize clinical observations and results of animal models that advance the knowledge of the attenuation of cerebrovascular dysfunction in the setting of heat stroke. It is a narrative review of selected published literature from Medline over the period 1959-2005. 2. All heat-stressed rodents, even under general anaesthesia, have hyperthermia, systemic inflammation, hypercoagulable state, arterial hypotension and tissue ischaemia and injury in multiple organs. These findings demonstrate that rodent heat stroke models can nearly mirror the full spectrum of human heat stroke. Experimental heat stroke fulfills the empirical triad used for the diagnosis of classical human heat stroke, namely hyperthermia, central nervous system alterations and a history of heat stress. 3. These physiological dysfunctions and survival during heat stroke can be improved by whole-body or brain cooling therapy adopted immediately after the onset of heat stroke. 4. However, in the absence of body or brain cooling, these heat stroke reactions can still be reduced by the following measures: (i) fluid replacement with 3% NaCl solution, 10% human albumin or hydroxyethyl starch; (ii) intravenous delivery of anti-inflammatory drugs, free radical scavengers or interleukin-1 receptor antagonists; (iii) hyperbaric oxygen therapy; or (iv) transplantation of human umbilical cord blood cells. 5. In addition, before initiation of heat stress, prior manipulations with one of the following measures was found to be able to protect against heat stroke reactions: (i) systemic delivery of alpha-tocopherol, mannitol, inducible nitric oxide synthase inhibitors, mu-opioid receptor antagonists, endothelin ETA receptor antagonists, serotoninergic nerve depletors or receptor antagonists, or glutamate receptor antagonists; or (ii) heat shock protein 72 preconditioning. 6. There is compelling evidence that cerebrovascular dysfunction is an attractive target for therapy in heat stroke.

Hypertonic fluid resuscitation from subarachnoid hemorrhage in rats: A comparison between small volume resuscitation and mannitol

OBJECTIVE

Death and severe morbidity after subarachnoid hemorrhage (SAH) are mainly caused by global cerebral ischemia through increased intracranial pressure (ICP) and decreased cerebral blood flow (CBF). We have recently demonstrated neuroprotective effects of small volume resuscitation (7.5% saline in combination with 6% dextran 70) in an animal model of SAH, leading to normalization of increased ICP, reduced morphological damage and improved neurological recovery. In the present study, we compared the concept of small volume resuscitation represented by two clinically licenced hypertonic-hyperoncotic saline solutions with the routinely used hyperosmotic agent-mannitol-and investigated their effects on ICP, CBF, neurological recovery and morphological damage after SAH in rats.

METHODS

60 dextran-resistant Wistar rats were subjected to SAH by an endovascular filament. ICP, MABP (mean arterial blood pressure) and bilateral local CBF were continuously recorded. All animals were randomly assigned to four groups: (I) NaCl 0.9% (4 ml/kg bw), (II) 7.5% NaCl+6% dextran 70 (4 ml/kg bw), (III) 7.2% NaCl+HES 200,000 (4 ml/kg bw) and (IV) 20% mannitol (9.33 ml/kg bw) given 30 min after SAH. Neurological deficits were assessed on days 1, 3 and 7 after SAH. The morphological damage was evaluated on day 7 after SAH.

RESULTS

The induction of SAH resulted in an immediate ICP increase to 46.6+/-3.2 mm Hg (mean+/-S.E.M.) and 29.6+/-1.3 (mean+/-S.E.M.) mm Hg 90 min post-SAH. While a treatment with both hypertonic saline solutions (II, III) decreased ICP as well as the 20% mannitol solution, only the group treated with hypertonic saline and dextran 70 (II) showed an increase of ipsilateral CBF for 20 min after the infusion and significantly more surviving neurons in the motorcortex and caudoputamen. Mortality was reduced from 60% (I) and 73% (III and IV), respectively, to 40% in group II.

CONCLUSION

Of all hypertonic solutions investigated, small volume resuscitation with NaCl 7.5% in combination with 6% dextran 70 evolved to be most effective in terms of reducing the initial harmful sequelae of SAH, leading to lowered ICP and less morphological damage after SAH in the rat.

Endothelin-1 in plasma, cisternal CSF and microdialysate following aneurysmal SAH

OBJECTIVE

Endothelin-1 (ET-1) is postulated to play an important role in the development of cerebral vasospasm (CVS) following SAH. This study was conducted to investigate the time course of ET-release in three different sources: CSF, plasma and microdialysate.

METHODS

In a prospective study ET-1-concentrations were measured in plasma, cisternal CSF and microdialysate in 20 patients with aneurysmal SAH for at least 8 days after hemorrhage.

RESULTS

ET-1 concentration in microdialysate was almost four times higher compared to CSF and plasma. (p<0.001) Only in CSF ET-1-release showed a significant increase over time with highest values on day 5 post ictus (p = 0.03). This was parallel to the increase of transcranial Doppler velocities. ET-1 in plasma and microdialysate did not change over time.

CONCLUSION

ET-1 may have a different biological function in different biological tissues. Only ET-1 in CSF seemed to be associated with CVS.

Endothelin-1 levels in plasma and cerebrospinal fluid of patients with cerebral vasospasm after aneurysmal subarachnoid hemorrhage

BACKGROUND

Plasma and cerebrospinal fluid (CSF) concentrations of endothelin-1 (ET-1) were measured in patients with subarachnoid hemorrhage (SAH) after aneurysmal rupture and compared with levels of ET-1 in volunteers. We analyze the relationship between levels of ET-1 in both CSF and plasma with the risk of developing cerebral vasospasm (CVS).

METHODS

Cerebrospinal fluid and blood samples were collected from 30 selected patients after SAH and from 10 healthy volunteers who were used as control. All samples were stored at -70 degrees C and the levels of ET-1 in CSF and blood were measured by using enzyme-linked immunosorbent assay and Western blot. All patients were submitted to angiography to confirm vasospasm.

RESULTS

From the 30 patients admitted at different days of SAH, 18 (60%) developed clinical CVS and 10 (33%) presented angiographic CVS. The levels of ET-1 in the CSF were significantly higher (P = .0001) in patients (1.618 +/- 1.05 fmol/mL) than in controls (0.365 +/- 0.328 fmol/mL). There was statistical difference (P < .05) in CSF levels of ET-1 between each group of the Hunt-Hess scale and controls. The mean plasma concentration of ET-1 was similar (P > .05) in the control group (1.531 +/- 0.753 fmol/mL) and in patients with SAH (1.920 +/- 1.15 fmol/mL).

CONCLUSIONS

These findings indicate that a significant rise in ET-1 levels in the CSF, but not in the plasma, occurs in patients who develop CVS after SAH. Our observation suggests that ET-1 might be involved in the pathogenesis of SAH-associated CVS.

Prior antagonism of endothelin-1A receptors alleviates circulatory shock and cerebral ischemia during rat heatstroke

In this study, we investigated the acute hemodynamic effects of an infusion of the endothelin-1 (ET-1)-A-selective receptor antagonists BQ-610 and BQ-123 in heatstroke rats with circulatory shock and cerebral ischemia. Heatstroke was induced by putting the anesthetized adult Sprague-Dawley rats into an ambient temperature of 42 degrees C. The moment in which the mean arterial pressure dropped irreversibly from the peak for an extent of 25 mmHg was taken as the onset of heatstroke. The interval between initiation of heat exposure and heatstroke onset was found to be about 80 min for rats treated with vehicle solution. When the animals were exposed to 42 degrees C for 80 min, hyperthermia, arterial hypotension, decrement of cardiac output (due to decreased stroke volume and decreased total peripheral resistance), increment of plasma ET-1 and tumor necrosis factor-alpha, and increment of cerebral ischemia and injury markers were manifested. Prior antagonism of ET-1 A receptors with BQ-610 (0.5 mg/kg, i.v.) or BQ-123 (1 mg/kg, i.v.), but not ET-1B receptors with BQ-788 (0.5 mg/kg, i.v.), 60 min before the initiation of heat exposure, appreciably alleviated hyperthermia, arterial hypotension, decreased cardiac output, increment of tumor necrosis factor-alpha, and increment of cerebral ischemia (e.g., glutamate and lactate/pyruvate ratio) and injury (e.g., glycerol) markers exhibited during heatstroke. The data indicates that ET-1A receptor antagonism may maintain appropriate levels of mean arterial pressure and cerebral circulation during heatstroke by reducing production of tumor necrosis factor-alpha.

Hypertonic Fluid Resuscitation from Subarachnoid Hemorrhage in Rats

OBJECTIVE

Increased intracranial pressure (ICP) and decreased cerebral blood flow leading to global cerebral ischemia are the primary causes of death after severe subarachnoid hemorrhage (SAH). Hypertonic saline has been demonstrated to exert neuroprotective properties after traumatic brain injury by osmotic mobilization of parenchymal water and improvement of microcirculation. We used a rat model to investigate the effects of hypertonic fluid resuscitation after SAH on ICP, cerebral blood flow, body weight, neurological recovery, and morphological damage.

METHODS

Sixty rats were subjected to SAH induced by an endovascular filament. ICP and local cerebral blood flow were recorded continuously. Animals were assigned to three groups: 1) NaCl 0.9%; 2) NaCl 7.5% (4 ml/kg); and 3) NaCl 7.5% plus 6% dextran 70 (4 ml/kg) given 30 minutes after SAH. Body weight and neurological deficits were assessed daily. Morphological damage was evaluated on Day 7.

RESULTS

SAH resulted in an immediate increase of ICP to approximately 60 mm Hg initially, and then to approximately 30 mm Hg for the next 90 minutes. Although NaCl 7.5% alone and in combination with dextran led to an immediate, significant, and lasting decrease of ICP to 15 to 20 mm Hg, only the combined therapy significantly increased body weight and improved neurological recovery. Furthermore, the group that received combined therapy exhibited significantly more surviving neurons in hippocampus, cortex, caudoputamen, and cerebellum. Mortality was reduced nonsignificantly, from approximately 65% in groups I and II to 35% in Group III.

CONCLUSION

Treatment with NaCl 7.5% plus 6% dextran 70 is significantly effective for reducing the initial harmful sequelae of SAH. The regimen resulted in lowered ICP, improved neurological recovery, and less morphological damage after SAH in the rat.

Attenuation of cerebral vasospasm by systemic administration of an endothelin-A receptor antagonist, TBC 11251, in a rabbit model of subarachnoid hemorrhage

Cerebral vasospasm is a major complication of subarachnoid hemorrhage (SAH) after the rupture of an intracranial aneurysm. Although the cause of cerebral vasospasm has not been fully established, several lines of evidence suggest that the vasoconstrictor peptide endothelin (ET) may play a crucial role. In the present study the potential of TBC 11251 (TBC), a newly developed ET(A) receptor antagonist, to prevent and/or reverse cerebral vasospasm was examined in a well-established rabbit model of SAH. Sixty-five New Zealand White rabbits were assigned to one of six groups. Experimental SAH was induced in rabbits comprising five of the groups by injecting autologous arterial blood into the cisterna magna. The treatment groups were as follows: 1) control (no SAH); 2) SAH only; 3) SAH + placebo at 24 and 36 hours (24/36); 4) SAH + TBC (24/36); 5) SAH + placebo twice daily (BID); and 6) SAH + TBC BID. All drug-treated animals received an intravenous dosage of 5 mg/kg TBC. After 48 hours, the animals were killed by intracardiac perfusion with fixative. The brainstems were removed and the basilar arteries (BAs) were prepared for histological examination. The cross-sectional area of each BA was measured using computer-assisted videomicroscopy by an investigator blind to the group from which it came. A one-way analysis of variance and paired group mean comparisons with the post-hoc Fisher least significant difference test were used for analysis of BA diameters and physiological parameters. The model provided reliable vasospasm, with the mean BA cross-sectional area constricting from 0.388 mm2 in the control group to 0.106 mm2 (27.4% of control) in the SAH only group. Treatment with TBC (24/36) after SAH (reversal protocol) produced a mean BA area of 0.175 mm2 (44.2% of control) which, although larger than the placebo group value of 0.135 mm2 (39.9% of control), was not statistically significant. However, treatment with TBC BID (prevention protocol) produced a mean BA area of 0.303 mm2 (78.1% of control) compared with the placebo BID value of 0.134 mm2 (34.6% of control); this effect was statistically significant (p < 0.01). There were no side effects noted and no differences in the mean arterial pressures between drug and placebo groups. These findings demonstrate that systemic administration of the ET(A) receptor antagonist TBC significantly attenuates cerebral vasospasm after SAH when given as a preventative therapy, and they provide additional support for the role of ET in the establishment of vasospasm.

Sustained arterial narrowing after prolonged exposure to perivascular endothelin

OBJECTIVE

The vasoconstrictor peptide endothelin-1 (ET-1) produces narrowing of cerebral arteries and has been implicated in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage. Little is known, however, regarding the physiological consequences of prolonged exposure of arterial wall to ET-1.

METHODS

In 30 rats, normal saline or 10(-8) mol/h of ET-1 was continuously applied for 3 or 5 days to the adventitial surface of the femoral artery in a Silastic cuff via an osmotic infusion pump. Vessels were examined for histopathological changes and luminal narrowing during ET-1 infusion (3 or 5 d) or at intervals from 2 to 9 days after infusion was stopped.

RESULTS

Marked arterial constriction (30-40% arterial diameter reduction) was present during continuous ET-1 infusion for 3 or 5 days. For both 3- and 5-day ET-1 infusions, significant reduction in arterial cross sectional area persisted up to 4 days after cessation of infusion, after which normal caliber returned. In arteries with persistent luminal narrowing after cessation of ET-1 infusion, light microscopic findings revealed morphological changes in the vessel wall similar to those observed in cerebral vasospasm after subarachnoid hemorrhage, with apparent increased collagen deposition in media and adventitia.

CONCLUSION

Continuous infusion of ET-1 produces reversible arterial narrowing that persists beyond the usual interval of physiological effect for this agent. Prolonged arterial constriction may produce physiological changes in arterial wall that act to maintain a narrowed lumen.

Temporal relationship between endothelin-1 concentrations and cerebral vasospasm in patients with aneurysmal subarachnoid hemorrhage

BACKGROUND AND PURPOSE

Endothelin 1 (ET-1) is a potent vasoconstrictor that may play a role in cerebral vasospasm following subarachnoid hemorrhage (SAH). However, data regarding its pathogenic role in the development of vasospasm are controversial. We planned a prospective, observational clinical study to investigate the temporal relationship between increased ET-1 production and cerebral vasospasm or other neurological sequelae after SAH.

METHODS

ET-1 levels in cerebrospinal fluid (CSF) were measured in 20 SAH patients from admission (within 24 hours from the bleeding) until day 7. Patients received a daily transcranial Doppler study and a neurological evaluation. On day 7, angiography was performed to verify the degree and extent of vasospasm. Patients were then classified as having (1) clinical vasospasm, (2) angiographic vasospasm, (3) no vasospasm, or (4) poor neurological condition without significant vasospasm (low Glasgow Coma Scale score [GCS]).

RESULTS

On admission, ET-1 levels were increased in the low-GCS group compared with the other groups (P=0.04). On day 4 ET-1 levels were not significantly different among groups, whereas on day 7 ET-1 levels were significantly increased in both the clinical vasospasm and low-GCS groups compared with the angiographic vasospasm and no vasospasm groups (P<0.005). Moreover, when the low-GCS group was excluded, there was a significant relationship between vasospasm grade and CSF ET-1 levels (R(2)=0.73).

CONCLUSIONS

CSF ET-1 levels were markedly elevated in patients with clinical manifestations of vasospasm (day 7) and with a poor neurological condition not related to vasospasm. However, ET-1 levels were low in clinical vasospasm patients before clinical symptoms were evident (day 4) and remained low in angiographic vasospasm patients throughout the study period. Thus, our data suggest that CSF ET-1 levels are increased in conditions of severe neuronal damage regardless whether this was due to vasospasm or to the primary hemorrhagic event. In addition, CSF ET-1 levels paralleled the neurological deterioration but were not predictive of vasospasm.

Inhibition of endothelin-converting enzyme activity in the rabbit basilar artery

OBJECTIVE

Endothelin (ET)-1 may be involved in the regulation of cerebrovascular resistance under pathological conditions, most notably during the development of vasospasm after subarachnoid hemorrhage. Blocking ET-converting enzyme activity may be a promising approach to the prevention of cerebral vasospasm after subarachnoid hemorrhage.

METHODS

In this study, the effects of several putative ET-converting enzyme inhibitors were investigated after intracisternal application in rabbits, to inhibit basilar artery contractions induced by big ET-1 (2 x 10(-6) mol/L).

RESULTS

In the group pretreated with [D-Val22]big ET-1[16-38] (2 x 10(-5) mol/L) (n = 8), the angiographically measured diameter of the basilar artery changed from 0.63 +/- 0.12 mm to 0.66 +/- 0.12 mm. In the control group (n = 8), the diameter of the basilar artery decreased from 0.71 +/- 0.13 mm to 0.57 +/- 0.15 mm. These results corresponded to an increase in vessel diameter of 5 +/- 10% in the treatment group and a decrease in vessel diameter of 20 +/- 16% in the control group (P = 0.014). In the group pretreated with captopril (2 x 10(-4) mol/L) (n = 8), the angiographically measured diameter of the basilar artery changed from 0.64 +/- 0.11 mm to 0.71 +/- 0.10 mm. These results corresponded to an increase in vessel diameter of 14 +/- 19% in the treatment group, compared with a decrease in vessel diameter of 20 +/- 16% in the control group (P = 0.014).

CONCLUSION

These results demonstrate that [D-Val22]big ET-1[16-38] and captopril act as highly potent ET-converting enzyme inhibitors, affecting big ET-1-induced contraction of the rabbit basilar artery.

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.

Characterization of the contractile and relaxant action of the endothelin-1 precursor, big endothelin-1, in the isolated rat basilar artery

The presence of functional endothelin converting enzyme (ECE) activity in basilar artery ring segments was investigated by measuring the contractile and relaxant effects of big endothelin (ET)-1. Under resting tension conditions cumulative application of big ET1-1 elicited a concentration-related contraction with the concentration-effect curve (CEC) shifted to the right against ET-1 by a factor of 31 and 29 in segments with the endothelium intact or mechanically removed, respectively. Preincubation with the ET(A) receptor antagonist, BQ123, induced an apparently parallel rightwards shift without affecting the maximum contraction. This shift was more pronounced for ET-1 than for big ET-1. With the putative ECE inhibitor phosphoramidon (10(-3) M) in the bath a small rightwards shift of the CEC for big ET-1 was observed in control segments and a more marked one in de-endothelialized segments. In segments precontracted with prostaglandin (PG) F(2alpha) big ET-1 induced a significant although transient relaxation whereas ET-1 did not. However, in the presence of BQ123 both ET-1 and big ET-1 elicited concentration-related relaxation with a significantly higher maximum effect obtained with big ET-1. The potency was 13 fold higher for ET-1, which is markedly less than that found for contraction. The results, therefore, suggest 1) the presence of functional ECE-activity in the rat basilar artery wall, and 2) differences in the functional ECE activity located in the endothelium and media.

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.

Pathophysiology of endothelin in the cardiovascular system

In this article, we review the basic pharmacological and biochemical features of endothelin and the pathophysiological roles of endothelin in cardiovascular diseases. Development of receptor antagonists has accelerated the pace of investigations into the pathophysiological roles of endogenous endothelin-1 in various diseases, e.g. chronic heart failure, renal diseases, hypertension, cerebral vasospasm, and pulmonary hypertension. In chronic heart failure, the expression of endothelin-1 and its receptors in cardiomyocytes is increased, and treatment with an endothelin receptor antagonist improves survival and cardiac function. Endothelin receptor antagonists also improve other cardiovascular diseases. These results suggest that the interference with endothelin pathway either by receptor blockade or by inhibition of endothelin converting enzyme may provide novel therapeutic drugs strategies for multiple disease states.

Blockade and reversal of endothelin-induced constriction in pial arteries from human brain

BACKGROUND AND PURPOSE

Substantial evidence now implicates endothelin (ET) in the pathophysiology of cerebrovascular disorders such as the delayed vasospasm associated with subarachnoid hemorrhage and ischemic stroke. We investigated the ET receptor subtypes mediating vasoconstriction in human pial arteries.

METHODS

ET receptors on human pial and intracerebral arteries were visualized with the use of autoradiography, and the subtypes mediating vasoconstriction were identified by means of wire myography.

RESULTS

ET-1 was more potent than ET-3 as a vasoconstrictor, indicating an ETA-mediated effect. Similarly, the selective ETB agonist sarafotoxin S6c had no effect on contractile action at concentrations up to 30 nmol/L. The nonpeptide ETA receptor antagonist PD156707 (3 to 30 nmol/L) caused a parallel rightward shift of the ET-1-induced response, yielding a pA2 of 9.2. Consistent with these results, PD156707 (30 nmol/L) fully reversed an established constriction in pial arteries induced by 1 nmol/L ET-1, while the selective ETB receptor antagonist BQ788 (1 micromol/L) had little effect. The calcium channel blocker nimodipine (0.3 to 3 micromol/L) significantly attenuated the maximum response to ET-1 in a concentration-dependent manner without changing potency. In agreement with the functional data, specific binding of [125I]PD151242 to ETA receptors was localized to the smooth muscle layer of pial and intracerebral blood vessels. In contrast, little or no [125I]BQ3020 binding to ETB receptors was detected.

CONCLUSIONS

These data indicate an important role for ETA receptors in ET-1-induced constriction of human pial arteries and suggest that ETA receptor antagonists may provide additional dilatory benefit in cerebrovascular disorders associated with raised ET levels.

Regulation of ET-1 biosynthesis in cerebral microvascular endothelial cells by vasoactive agents and PKC

Endothelin-1 (ET-1) is the most potent vasoconstrictor agent known. ET-1 is elevated in the cerebrospinal fluid following hemorrhage and brain injury and can compromise cerebral microvascular homeostasis. The modulation of ET-1 production by cerebral microvascular endothelial cells and the mechanism by which such changes take place are very important in our understanding of the pathological roles of ET-1. In the present study, we investigated the effects of vasoconstrictor agents that can be released from hemolyzed blood, cAMP-dependent dilators, and the role of protein kinase C (PKC) in the regulation of ET-1 production by piglet cerebral microvascular endothelial cells in culture. ET-1 was measured by RIA. 1) Cerebral microvascular endothelial cells synthesize and release ET-1 into the media; 2) 5-hydroxytryptamine (5-HT), lysophosphatidic acid (LPA), thromboxane analog U-46619, fetal bovine serum (20%), and phorbol 12-myristate 13-acetate significantly increase ET-1 production; 3) basal and vasoconstrictor agent-induced increases in ET-1 production by endothelial cells may be mediated via PKC; 4) cAMP-dependent vasodilators attenuate the basal production of ET-1 by cerebral microvessels; and 5) pretreatment of endothelial cells with a higher concentration of LPA, U-46619, or 5-HT counterbalances the cAMP-dependent dilator agent-induced reduction in basal ET-1 production. Therefore, by-products of hemolyzed blood can stimulate the production of ET-1 by a PKC-mediated mechanism. cAMP-dependent dilators can attenuate the vasoconstrictor agent-induced elevation in ET-1 production. These results suggest that cerebral microvascular homeostasis could be compromised by effects of interactions among vasoactive agents released during conditions injurious to the brain and they may further the understanding of potential contributions of hemolyzed blood clots to subarachnoid hemorrhage-induced vasospasm.

Endothelin and subarachnoid hemorrhage: an overview

INTRODUCTION

Delayed cerebral vasospasm occurring after subarachnoid hemorrhage (SAH) is still responsible for a considerable percentage of the morbidity and mortality in patients with aneurysms. It has been suggested that the pathogenesis of delayed cerebral vasospasm is related to a number of pathological processes, including endothelial damage and smooth muscle cell contraction resulting from spasmogenic substances generated during lysis of subarachnoid blood clots, changes in vascular responsiveness, and inflammatory or immunological reactions of the vascular wall. It has been recognized that the endothelium plays an important role in the regulation of the cerebral vascular tone. In 1988, endothelin (ET)-1, a potent vasoconstrictor, was isolated from cultured porcine aortic endothelial cells.

RESULTS

ET-1, which is one of three distinct isoforms of ETs (ET-1, ET-2, and ET-3), has a more marked effect on cerebral arteries than do the other two isoforms. Elevated levels of ETs have been demonstrated in the cerebrospinal fluid and plasma of patients after SAH and cerebral infarction. ETs act by at least three different receptor subtypes, the ET(A) receptor, which is localized in vascular smooth muscle cells and mediates vasoconstriction, and two different ET(B) receptor subtypes. The ET(B1) receptor subtype is present in vascular endothelial cells and mediates the endothelium-dependent vasodilation. The ET(B2) receptor subtype is present in smooth muscle cells causing vasoconstriction. ET-1 acts from the adventitial but not from the luminal side of cerebral arteries. In vivo and in vitro ET-1 causes a dose-dependent and long-lasting vasoconstriction, similar to cerebral vasospasm after SAH. The vasoconstriction caused by ET-1 can be reversed by selective ET(A) receptor antagonists or combined ET(A) and ET(B) receptor antagonists.

CONCLUSION

The results of current clinical and experimental investigations support the hypothesis that ET-1 is a major cause of cerebral vasospasm after SAH. Other studies indicate that SAH causes complex changes in the ET system and increased ET-1 levels after SAH, which are not solely responsible for the development of vasospasm but may occur after cerebral ischemia. Further investigations are therefore needed to clarify these different hypotheses.

Profiles of an intravenously available endothelin A-receptor antagonist, S-0139, for preventing cerebral vasospasm in a canine two-hemorrhage model

We examined the prophylactic effect of a novel nonpeptide endothelin (ET) A-receptor selective antagonist, S-0139, using a canine two-hemorrhage model and an ET-1-induced cerebral vasospasm model. The agent markedly prevented cerebral vasospasm in the canine two-hemorrhage model when given intracisternally or intravenously by continuous daily dosing. An efficacious intravenous method was to apply a relatively high initial dose followed by daily sustaining administration at a much lower dose, which alone would have been ineffective. The need for sustaining dosing may imply daily successive attacks of ETs in the cerebral vessel compartment after the introduction of autologous blood into the subarachnoid space. A small amount of S-0139 was detected from the cerebrospinal fluid (CSF) with an apparent lag time after its disappearance from the plasma following intravenous dosing of 0.83 mg/kg/min for 12 min, however, cerebral vasoconstriction induced by ET-1 dosing from the adventitial side was clearly inhibited during such a lag period. Moreover, its movement into the CSF was greatly enhanced after the application of autologous blood to the animals. From these results, we conclude that ET-1 play a major role in producing delayed cerebral vasospasm in this canine two-hemorrhage model, and S-0139 effectively antagonizes the action of ET-1 even by intravenous treatment because it moves easily into the cerebral vessel compartment from plasma.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Prevention and reversal of cerebral vasospasm by an endothelin-converting enzyme inhibitor, CGS 26303, in an experimental model of subarachnoid hemorrhage

Delayed cerebral ischemia due to cerebral vasospasm is a major cause of morbidity and mortality in patients with aneurysmal subarachnoid hemorrhage (SAH). Increasing evidence implicates the potent vasoconstrictor peptide endothelin (ET) in the pathophysiology of cerebral vasospasm. In the present study the authors examined the therapeutic value of blocking the production of ET-1 by inhibiting the conversion of its relatively inactive precursor, Big ET-1, to a physiologically active form. An inhibitor of ET-converting enzyme (ECE), CGS 26303, was injected intravenously after inducing SAH in New Zealand white rabbits. Injections of CGS 26303 were initiated either 1 hour after SAH (prevention protocol) or 24 hours after SAH (reversal protocol). One of three concentrations (3, 10, or 30 mg/kg) of CGS 26303 was injected twice daily, and all animals were killed by perfusion fixation 48 hours after SAH occurred. Basilar arteries were removed and sectioned, and their cross-sectional areas were measured in a blind manner by using computer-assisted videomicroscopy. Treatment with CGS 26303 attenuated arterial narrowing after SAH in both the prevention and reversal protocols. The protective effect of CGS 26303 achieved statistical significance at all dosages in the prevention protocol and at 30 mg/kg in the reversal protocol. These findings demonstrate that inhibiting the conversion of Big ET-1 to ET-1 via intravenous administration of an ECE inhibitor can be an effective strategy for limiting angiographic vasospasm after SAH. Moreover, the results demonstrate that treatment with the ECE inhibitor is capable of reducing vasospasm even when initiated after the process of arterial narrowing has begun. Finally, the results provide further support for the role of ET in the establishment of cerebral vasospasm. The ECE inhibitor CGS 26303 thus represents a promising therapeutic agent for the treatment of cerebral vasospasm following aneurysmal SAH.

Effects of intraluminal or extraluminal endothelin on perfused rabbit basilar arteries

The authors investigated selective intra- and extraluminal effects of endothelin (ET) on perfused basilar and extracranial arteries and also studied the interaction between ET and extraluminal oxyhemoglobin (oxyHb). The basilar, mesenteric, and femoral arteries were isolated from 23 Japanese White rabbits. After isolation of the intra- and extraluminal sides of the preparation, 3 x 10(-10) to 3 x 10(-8) mol/L of ET was administered intra- or extraluminally. After extraluminal pretreatment with 10(-5) mol/L oxyHb, 10(-5) mol/L N(G)-monomethyl-L-arginine (L-NMMA), or 10(-6) mol/L indomethacin, 10(-10) to 10(-8) mol/L of ET was administered intra- or extraluminally. Arterial contraction was evaluated by measuring the increase in the perfusion pressure gradient with a differential pressure gauge. Both intra- and extraluminal ET (10(-9) to 3 x 10(-8) mol/L) showed potent and dose-dependent vasoconstricting effects on basilar arteries (p < 0.01). The effect of ET on the basilar arteries was significantly greater than on the femoral or mesenteric arteries (both p < 0.01). The effect of intraluminal ET was enhanced by extraluminal oxyHb (p < 0.05) and L-NMMA (p < 0.01), but not by extraluminal indomethacin. Extraluminal oxyHb did not potentiate the contraction induced by extraluminal ET. These results indicate that the sensitivity of the basilar artery to intraluminal ET is greater than that of the femoral or mesenteric artery. Endothelin may act as a potent vasoconstrictor intra- as well as extraluminally under conditions such as subarachnoid hemorrhage in which oxyHb is present in the extraluminal space and endothelium-derived relaxing factors are inhibited.

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.

Molecular pharmacology and pathophysiological significance of endothelin

Since the discovery of the most potent vasoconstrictor peptide, endothelin, in 1988, explosive investigations have rapidly clarified much of the basic pharmacological, biochemical and molecular biological features of endothelin, including the presence and structure of isopeptides and their genes (endothelin-1, -2 and -3), regulation of gene expression, intracellular processing, specific endothelin converting enzyme (ECE), receptor subtypes (ETA and ETB), intracellular signal transduction following receptor activation, etc. ECE was recently cloned, and its structure was shown to be a single transmembrane protein with a short intracellular N-terminal and a long extracellular C-terminal that contains the catalytic domain and numerous N-glycosylation sites. In addition to acute contractile or secretory actions, endothelin has been shown to exert long-term proliferative actions on many cell types. In this case, intracellular signal transduction appears to converge to activation of mitogen-activated protein kinase. As a recent dramatic advance, a number of non-peptide and orally active receptor antagonists have been developed. They, as well as current peptide antagonists, markedly accelerated the pace of investigations into the true pathophysiological roles of endogenous endothelin-1 in mature animals; e.g., hypertension, pulmonary hypertension, acute renal failure, cerebral vasospasm, vascular thickening, cardiac hypertrophy, chronic heart failure, etc. Thus, the interference with the endothelin pathway by either ECE-inhibition or receptor blockade may provide an exciting prospect for the development of novel therapeutic drugs.

Systemic administration of an inhibitor of endothelin-converting enzyme for attenuation of cerebral vasospasm following experimental subarachnoid hemorrhage

The potent vasoconstrictor peptide, endothelin-1 (ET-1), has been implicated in the pathophysiology of cerebral vasospasm that occurs after subarachnoid hemorrhage (SAH). This peptide is synthesized as a large prepropeptide that requires a series of modifying steps for its activation. The last of these steps involves the proteolytic conversion of a relatively inactive propeptide, Big ET-1, to its active, 21-amino acid peptide form. The enzyme responsible for converting Big ET-1 to ET-1 is a metalloprotease called endothelin-converting enzyme (ECE). In the present study the authors examined the effects of a newly developed inhibitor of ECE on responses to ET peptides in the normal basilar artery and on pathophysiological constriction in the spastic basilar artery after SAH. In the first series of experiments the authors examined normal basilar arteries in the rabbit, which were exposed transclivally and measured on-line using videomicroscopy. Intravenous administration or topical application of an active inhibitor of ECE, CGS 26303, blocked vasoconstrictor responses to topically applied Big ET-1 but not to ET-1. In contrast, topical application of a structurally related compound that does not inhibit ECE, CGS 24592, was ineffective in blocking vasoconstriction that was elicited by a topical application of Big ET-1. These findings indicate that CGS 26303 when administered systemically is capable of blocking the conversion of Big ET-1 to ET-1 in the basilar artery without affecting the ability of the vessel to respond to ET-1. In the second series of experiments the authors examined the effects of the ECE inhibitor on cerebral vasospasm after experimental SAH. Intraperitoneal administration of CGS 26303 via osmotic minipumps significantly attenuated the delayed spastic response of the basilar artery to an intracisternal injection of autologous blood. This study provides the first evidence that systemic administration of an inhibitor of ECE is capable of preventing cerebral vasospasm after SAH. The results reinforce a growing body of evidence that ETs play a critical role in the development of spastic constriction after SAH. Moreover, the findings indicate that blocking the conversion of Big ET-1 to its active ET-1 form using CGS 26303 may represent a feasible strategy for ameliorating cerebral vasospasm.

Endothelin peptides in brain diseases

Recently, scientists interested in diseases of the human brain have paid much attention to the endothelin group of peptides. Under normal conditions they are found in some types of neurons and in endothelial cells of microvessels but not in glial cells. This review focuses on the endothelin peptides and their involvement in various brain diseases. Particular attention is paid to their expression in reactive astrocytes seen in many pathological conditions of the human brain. Endothelin-1 is a very potent vasoconstrictor which may be involved in the vasospasm occurring in subarachnoid haemorrhage. Intracerebral injection or application to cerebral arteries in animals will cause a focal necrosis, apparently due to severe vasoconstriction. Reactive astrocytes occurring in cases with infarcts, lacunae, Alzheimer's disease, progressive multifocal leukoencephalopathy (PML) and subacute sclerosing panencephalitis (SSPE) express endothelin-like immunoreactivity. Astrocytes in vitro may produce, store and release endothelins. To some extent astrocytes grown in vitro mimic reactive astrocytes in vivo since in cultures astrocytes are removed from their natural environment which may trigger reactive responses. Therefore, in vivo reactive astrocytes may produce, store and release endothelins just as in vitro. If endothelins are released from reactive astrocytes they may act as mitogens and may influence microcirculation by inducing vasoconstriction of intracerebral arterioles. In such ways endothelins may contribute to the final lesions seen in cases with infarcts, lacunae, traumatic conditions, Alzheimer's disease and inflammatory diseases of the brain.

Increased expression of endothelin B receptor mRNA following subarachnoid hemorrhage in monkeys

These studies tested the hypothesis that the cerebral vasospasm that follows subarachnoid hemorrhage (SAH) is due to alterations in endothelin (ET) and ET receptor expression. Eight monkeys underwent cerebral angiography and induction of SAH. Angiography was repeated 7 days later to confirm the presence of cerebral vasospasm, and animals were killed. RNA was isolated from right (vasospastic) and left (control) side middle cerebral arteries and surrounding cerebral cortex. The levels of prepro (PP) ET-1 (ppET-1) and ppET-3 and ETA and ETB receptor MRNAs were determined using a quantitative reverse transcriptase polymerase chain reaction-based assay. ET-1 peptide was also measured in CSF at baseline and after 7 days. Specific agonist binding to ETA and ETB receptors in both middle cerebral arteries and in surrounding brain cortex was measured in three animals by autoradiographic binding assays. Levels of ETB receptor mRNA were 3.4 +/- 2.2-fold higher in the right than in the left cerebral arteries (p < 0.01). There were no significant differences in the levels of ppET-1, ppET-3, or ETA receptor mRNA in cerebral arteries. ET-1 peptide was not elevated in CSF. Levels of ETA and ETB receptor mRNAs were 2.6 +/ 1.1- and 2.1 +/ 1.3-fold higher, respectively, in the right than in the left cerebral cortex, while the level of ppET-3 mRNA was 2.1 +/- 1.0-fold lower. There were no differences in ppET-1 mRNA levels between right and left cerebral cortex. Binding to ETA and ETB receptors in cerebral arteries and cortex did not differ significantly between right and left sides. These results do not support the hypothesis that overexpression of ET-1 is principal cause of vasospasm, but rather they suggest that SAH causes complex changes in the ET system that together are responsible for the cellular response to SAH.

Prevention of cerebral vasospasm after experimental subarachnoid hemorrhage by RO 47-0203, a newly developed orally active endothelin receptor antagonist

Since their discovery in 1988, endothelins have attracted scientific interest because of their extremely potent and long-lasting vasoconstrictive effects, similar to cerebral vasospasm in humans. In this study, the efficacy of the orally active endothelin receptor antagonist RO 47-0203 for prevention of cerebral vasospasm after experimental subarachnoid hemorrhage, using the two-hemorrhage dog model, was investigated. Twenty-eight beagle dogs were used in this laboratory experiment. Fourteen animals each were assigned to the treatment and control groups. In the treatment group, each dog received two single doses of 30 mg/kg RO 47-0203 orally per day. The diameter of the basilar artery decreased from 1.36 +/- 0.17 mm on Day 1 to 1.19 +/- 0.23 mm on Day 8 in the treatment group, whereas in the control group, the vessel diameter decreased from 1.48 +/- 0.19 mm on Day 1 to 1.02 +/- 0.22 mm on Day 8. These results corresponded to a decrease of vessel diameter of 13.1% +/- 11.2% in the treatment group and a decrease of vessel diameter of 30.7% +/- 12.4% in the control group (P < 0.001). Concentrations of endothelin-1 in cerebrospinal fluid significantly increased with time after subarachnoid hemorrhage. These results emphasize the important role of endothelin in the development of cerebral vasospasm and present first-time evidence that prevention of cerebral vasospasm can be achieved by the endothelin receptor antagonist RO 47-0203.