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

Anti-hypotensive treatment and endothelin blockade synergistically antagonize exercise fatigue in rats under simulated high altitude

Rapid ascent to high altitude causes illness and fatigue, and there is a demand for effective acute treatments to alleviate such effects. We hypothesized that increased oxygen delivery to the tissue using a combination of a hypertensive agent and an endothelin receptor A antagonist drugs would limit exercise-induced fatigue at simulated high altitude. Our data showed that the combination of 0.1 mg/kg ambrisentan with either 20 mg/kg ephedrine or 10 mg/kg methylphenidate significantly improved exercise duration in rats at simulated altitude of 4,267 m, whereas the individual compounds did not. In normoxic, anesthetized rats, ephedrine alone and in combination with ambrisentan increased heart rate, peripheral blood flow, carotid and pulmonary arterial pressures, breathing rate, and vastus lateralis muscle oxygenation, but under inspired hypoxia, only the combination treatment significantly enhanced muscle oxygenation. Our results suggest that sympathomimetic agents combined with endothelin-A receptor blockers offset altitude-induced fatigue in rats by synergistically increasing the delivery rate of oxygen to hypoxic muscle by concomitantly augmenting perfusion pressure and improving capillary conductance in the skeletal muscle. Our findings might therefore serve as a basis to develop an effective treatment to prevent high-altitude illness and fatigue in humans.

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

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

Amelioration of hypoperfusion after traumatic brain injury by in vivo endothelin-1 knockout

Endothelin 1 (ET-1) is one of the most powerful vasoconstrictors in the brain. Its expression is upregulated after traumatic brain injury (TBI) and is a major factor in the ensuing hypoperfusion. Attenuation of ET-1 effects has been mainly achieved by blockade of its receptors. The result of a direct blockade of ET-1 mRNA synthesis is not known. We used the Marmarou's model to inflict injury to male Sprague-Dawley rats injected with antisense ET-1 oligodeoxynucleotides (ODNs) before injury. Laser Doppler flowmetry in noninjured rats (2 groups, i.e., untreated and animals that received cODNs) revealed a constant cerebral blood flow of approximately 14 mL.min-1.100 g-1, whereas the values from injured animals pretreated with control ODNs (cODNs) or from animals subjected to TBI alone were approximately 8.0 mL.min-1.100 g-1 during the 18-48 h time period post-TBI. After antisense ET-1 ODNs pretreatment, however, cerebral blood flow in injured animals was approximately 17 mL.min-1.100 g-1 during the 6-48 h time period. Antisense ET-1 ODNs-treated animals also had 19%-29% larger microvessel cross-sectional area and approximately one-third less ET-1 immunoreactivity in the 50-75% range after injury than did cODNs-treated animals after TBI. The results indicate that this direct in vivo approach is an effective therapeutic intervention for the restoration of cerebral blood flow after TBI.

Calponin control of cerebrovascular reactivity: therapeutic implications in brain trauma

Calponin (Cp) is an actin-binding protein first characterized in chicken gizzard smooth muscle (SM). This review discusses the role of Cp in mediating SM contraction, the biochemical process by which Cp facilitates SM contraction and the function of Cp in the brain. Recent work on the role of Cp in pathological states with emphasis on traumatic brain injury is also discussed. Based on past and present data, the case is presented for targeting Cp for novel genetic and pharmacological therapies aimed at improving outcome following traumatic brain injury (TBI).

A small-molecule-inducible Nrf2-mediated antioxidant response provides effective prophylaxis against cerebral ischemia in vivo

The transcription factor nuclear factor erythroid 2-related factor 2 (Nrf2) coordinates expression of genes required for free radical scavenging, detoxification of xenobiotics, and maintenance of redox potential. Previously, activation of this pleiotropic response was neuroprotective in cell culture models that simulate components of stroke damage. However, the role of Nrf2 in limiting stroke damage in vivo remained unclear. We report that Nrf2 activation protects the brain from cerebral ischemia in vivo. Acute (1-3 d) intracerebroventricular or intraperitoneal pretreatment with tert-butylhydroquinone (tBHQ), an Nrf2 activity inducer, reduced cortical damage and sensorimotor deficit at 24 h and even 1 month after ischemia-reperfusion in rats. Cortical glutathione levels robustly increased with tBHQ administration to rats and Nrf2-expressing mice, but not Nrf2(-/-) mice. Basal and inducible activities of antioxidant/detoxification enzymes in Nrf2(-/-) mice were reduced when compared with Nrf2(+/+) controls. Interestingly, larger infarcts were observed in Nrf2(-/-) mice at 7 d after stroke, but not at 24 h, suggesting that Nrf2 may play a role in shaping the penumbra well after the onset of ischemia. Neuronal death caused by a "penumbral" model of stroke, using intracortical endothelin-1 microinjection, was attenuated by tBHQ administration to Nrf2(+/+), but not to Nrf2(-/-) mice, confirming the Nrf2-specific action of tBHQ in vivo. We conclude that Nrf2 plays a role in modulating ischemic injury in vivo. Accordingly, Nrf2 activation by small molecule inducers may be a practical preventative treatment for stroke-prone patients.

Intraspinal application of endothelin results in focal ischemic injury of spinal gray matter and restricts the differentiation of engrafted neural stem cells

Previous data have shown that pluripotent stem cells engrafted into the contused spinal cord differentiate only along an astrocytic lineage. The unknown restrictive cues appear to be quite rigid as even neuronal-restricted precursors fail to differentiate to the mature potential they exhibit in vitro after similar grafting into the contused spinal cord. It has been hypothesized that this potent lineage restriction is, in part, the result of the significant loss of both gray and white matter observed following spinal contusion, which elicits a massive acute inflammatory response and is manifested chronically by dramatic cystic cavitation. To evaluate the gray matter component, we developed a clinically relevant model of focal gray matter ischemic injury using the potent vasoconstrictor endothelin (ET-1) and characterized the differentiation of pluripotent stem cells transplanted into this atraumatic vascular SCI. Results demonstrate that low dose ET-1 microinjection into cervical spinal gray matter results in an inflammatory response that is temporally comparable to that observed following traumatic SCI, as well as chronic gray matter loss, but without significant cystic cavitation or white matter degeneration. However, despite the preservation of host spinal parenchyma, no elaboration of neuronal phenotypes was observed from engrafted stem or precursor cells. These results suggest that a common pathologic component responsible for this lineage restriction exists between contusive SCI and ET-1 mediated focal ischemic SCI.

Cocaine- and amphetamine-regulated transcript (CART) peptide: a vasoactive role in the cerebral circulation

Cocaine- and amphetamine-regulated transcript (CART) peptides are known to be involved in the stress response and have been implicated in the regulation of the cardiovascular system. We evaluated the direct vasoactive properties of CART in the cerebral circulation and its potential mechanisms of action. Penetrating cerebral arterioles, isolated from male Sprague-Dawley rats, were cannulated using a concentric micropipette setup, pressurized and perfused. The vascular response to intraluminal and extraluminal CART peptide was characterized. The endothelium dependence of this response was assessed by means of the endothelial light-dye injury model. The nonspecific endothelin receptor antagonist PD-145065, the ET(A)-specific antagonist BQ-123, the ET(B)-specific antagonist BQ-788, and the inhibitor of endothelin-converting enzyme phosphoramidon were used to characterize the involvement of the endothelin pathway in the vascular response to CART peptide. Extraluminal and intraluminal application of CART peptide (0.1 nm to 1 micromol/L) evoked a long-lasting dose-dependent constriction of isolated penetrating cerebral arterioles to approximately 80% of resting myogenic tone. Disruption of the endothelium by the endothelial light/dye injury model resulted in the abolition of this response (P<0.05). Extraluminal administration of PD-145065, BQ-123, and phosphoramidon blocked the constriction response to CART peptide (P<0.01). The ET(B) antagonist, BQ-788, did not alter the constriction response to CART peptide. Cocaine- and amphetamine-regulated transcript peptide is a potent vasoconstrictor in the cerebral circulation. Its direct vasoactive properties are endothelium-dependent and are mediated by ET(A), not ET(B), endothelin receptors.

ET-1 induces cortical spreading depression via activation of the ETA receptor/phospholipase C pathway in vivo

Recently, it has been shown that brain topical superfusion of endothelin (ET)-1 at concentrations around 100 nM induces repetitive cortical spreading depressions (CSDs) in vivo. It has remained unclear whether this effect of ET-1 is related to a primary neuronal/astroglial effect, such as an increase in neuronal excitability or induction of interastroglial calcium waves, or a penumbra-like condition after vasoconstriction. In vitro, ET-1 regulates interastroglial communication via combined activation of ET(A) and ET(B) receptors, whereas it induces vasoconstriction via single activation of ET(A) receptors. We have determined the ET receptor profile and intracellular signaling pathway of ET-1-induced CSDs in vivo. In contrast to the ET(B) receptor antagonist BQ-788 and concentration dependently, the ET(A) receptor antagonist BQ-123 completely blocked the occurrence of ET-1-induced CSDs. The ET(B) receptor antagonist did not increase the efficacy of the ET(A) receptor antagonist. Direct stimulation of ET(B) receptors with the selective ET(B) agonist BQ-3020 did not trigger CSDs. The phospholipase C (PLC) antagonist U-73122 inhibited CSD occurrence in contrast to the protein kinase C inhibitor Gö-6983. Our findings indicate that ET-1 induces CSDs through ET(A) receptor and PLC activation. We conclude that the induction of interastroglial calcium waves is unlikely the primary cause of ET-1-induced CSDs. On the basis of the receptor profile, likely primary targets of ET-1 mediating CSD are either neurons or vascular smooth muscle cells.

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

BACKGROUND AND PURPOSE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Cyclooxygenase inhibitors attenuate endothelin ET(B) receptor-mediated contraction in human temporal artery

It is well documented that endothelin ET(B) receptor-mediated contraction develops in artery segments incubated in culture and that the reaction is augmented by proinflammatory cytokines, but little is known of the mechanisms involved. Segments of human temporal artery were incubated in organ culture for 2 days in the absence or presence of interleukin-1 beta (IL-1 beta), with or without nonsteroidal anti-inflammatory drugs, glucocorticoids or a nitric oxide synthase inhibitor. Thereafter, contractions were induced by the selective endothelin ET(B) receptor agonist, sarafotoxin S6c. Acetylsalicylic acid, indomethacin, nimesulide and rofecoxib were all effective in eliminating the increase in endothelin ET(B) receptor-mediated contraction induced by interleukin-1 beta, but only indomethacin and rofecoxib significantly reduced the spontaneous development of this reaction in cultured arteries. Dexamethasone and methylprednisolone augmented the reaction, and the nitric oxide synthase inhibitor had no effect. The results clearly indicate a role for cyclooxygenase, most likely cyclooxygenase-2, in endothelin ET(B) receptor-mediated contraction in this preparation.

Hypoxic modulation of striatal lesions induced by administration of endothelin-1

Levels of endothelin-1 (ET-1), a potent endogenous vasoconstrictor, are elevated in plasma and cerebrospinal fluid (CSF) following cerebral ischemia and reperfusion injury. The present study sought insight into the potential differential vasoactive effects on the cerebral vasculature and resultant neural damage of ET-1 during normoxic vs. ischemic conditions and upon reperfusion. Under normoxic conditions, intrastriatal stereotaxic injection of exogenous ET-1 (40 pmol) induced a significant (P<0.05) reduction (</=29+/-12%) in the regional (striatal) cerebral blood flow measured by Laser Doppler flowmetry (CBF(LDF)) for up to 40 min in halothane-anesthetized male Long-Evans rats. Intrastriatal injection of ET-1 10 min after the onset of hypoxia (12% O(2), balance N(2)) tended to blunt, but not significantly, the striatal CBF(LDF) responses to the 35 min period of hypoxia. ET-1 given during reoxygenation significantly (P<0.05) reduced striatal CBF(LDF), which was similar to the effect of ET-1 during normoxia. ET-1-induced infarction when administered prior to hypoxia, but not during or post-hypoxia, was significantly (P<0.05) exacerbated compared to infarction of ET-1 without hypoxia. These results suggest that exogenous ET-1 administered into the brain parenchyma can induce an infarction associated with modulation of CBF(LDF) during the normoxic or reoxygenation period, but not during the hypoxic period and that the increased release of ET-1 in any pathological phase of cerebral ischemia contributes to irreversible neural damage with associated hemodynamic disturbances.

Cytokine enhancement of endothelin ET(B) receptor-mediated contraction in human temporal artery

Segments of human temporal artery were incubated in organ culture for 2 days in the absence or presence of cytokines. Thereafter, contractions were induced by the selective endothelin ET(B) receptor agonist sarafotoxin S6c, a peptide that does not induce contraction in fresh human temporal artery. Interleukin-1beta was most potent in increasing the sarafotoxin-induced contraction in cultured segments. Tumour necrosis factor (TNF)-alpha increased the magnitude of contraction to a similar degree, but at a higher dose. A significant increase was also induced by interferon-gamma, but not by interleukin-6 at the concentrations used. The results suggest that endothelin ET(B) receptor-mediated contraction can be enhanced by pro-inflammatory cytokines in a concentration-dependent manner, and this may have relevance for pathophysiological conditions where inflammation and vasoactivity are important.

Differential glial and vascular expression of endothelins and their receptors in rat brain after neurotrauma

We characterized the time-course, intensity of expression and cellular origin of components of the endothelin (ET) system in the rat brain after a standardized neurotrauma (cryogenic lesion of the parietal cortex). ET mRNAs were expressed at sham level after neurotrauma, whereas immunoreactivity for ET-1 was enhanced in glia and endothelium of the lesioned hemisphere and both hippocampi. The number of ET-3 positive mononuclear cells in the lesion perimeter increased starting at 24h after injury. At 48h after neurotrauma, ET-receptor immunoreactivity was increased in astrocytes. In basilar artery endothelium, ETB-immunoreactivity was reduced at 48h to 72h recovering at 7 days whereas ETA-receptor and ET-peptide immunoreactivities were not altered. In summary, neurotrauma leads to a multicellular stimulation of endothelins in the brain along with a delayed selective loss of vascular ETB-receptors. These changes seem to be posttranscriptional and cell type specific. They favor vasoconstriction increasing the risk of late vasospasm and ischemia.

A rat model of endothelin-3-induced middle cerebral artery occlusion with controlled reperfusion

Surge hyperemia and mechanical damage to the cerebrovascular endothelium may serve to exacerbate the neuropathological outcome in animal models of focal cerebral ischemia. We have modified an existing model of endothelin-1-induced middle cerebral artery (MCA) occlusion to enable controlled reperfusion without damage to the cerebral vasculature. Endothelin-1 (ET-1) and endothelin-3 (ET-3) were injected via a double-injection cannula into brain parenchyma adjacent to the MCA of anesthetized rats to produce focal cerebral ischemia. ET-1 and ET-3 produced large ischemic lesions that were restricted to those cortical and subcortical structures supplied by the MCA. The volume of ischemic damage produced by 100 pmol of ET-1 and ET-3 was similar. The endothelin-A (ET(A)) receptor antagonist FR139317 (3 or 30 nmol) injected 10 min after ET-1 did not significantly alter the volume of damage. By contrast, the lesion produced by ET-3 was completely inhibited by FR139317 at the 10 min time-point. FR139317 partially attenuated the ET-3-induced lesion when administered 30 min post-occlusion, but injection 90 min following ET-3 produced a lesion not different to that produced by ET-3 alone. These findings were supported by laser Doppler flowmetry which determined FR139317 induces reperfusion when injected 10 or 90 min following ET-3. ET-3-induced MCA occlusion is therefore amenable to reversal by the ET(A) receptor antagonist FR139317, and this model may offer a means to investigate the neuropathology of reperfusion without the procedure-related artifacts associated with some reperfusion models.

Drug development for stroke: importance of protecting cerebral white matter

Multiple pharmacological mechanisms have been identified over the last decade which can protect grey matter from ischaemic damage in experimental models. A large number of drugs targeted at neurotransmitter receptors and related mechanisms involved in ischaemic damage have advanced to clinical trials in stroke and head injury based on their proven ability to reduce grey matter damage in animal models. The outcome to date of the clinical trials of neuroprotective drugs has been disappointing. Although the failure to translate preclinical pharmacological insight into therapy is multifactorial, we propose that the failure to ameliorate ischaemic damage to white matter has been a major factor. The recent development of quantitative techniques to assess ischaemic damage to cellular elements in white matter, both axons and oligodendrocytes, allows rigorous evaluation of pharmacologic mechanisms which may protect white matter in ischaemia. Such pharmacological approaches provide therapeutic opportunities which are both additional or alternatives to those currently being evaluated in man.

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

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

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

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

Subarachnoid haemorrhage: what happens to the cerebral arteries?

1. Subarachnoid haemorrhage (SAH) is a unique disorder and a major clinical problem that most commonly occurs when an aneurysm in a cerebral artery ruptures, leading to bleeding and clot formation. Subarachnoid haemorrhage results in death or severe disability of 50-70% of victims and is the cause of up to 10% of all strokes. Delayed cerebral vasospasm, which is the most critical clinical complication that occurs after SAH, seems to be associated with both impaired dilator and increased constrictor mechanisms in cerebral arteries. Mechanisms contributing to development of vasospasm and abnormal reactivity of cerebral arteries after SAH have been intensively investigated in recent years. In the present review we focus on recent advances in our knowledge of the roles of nitric oxide (NO) and cGMP, endothelin (ET), protein kinase C (PKC) and potassium channels as they relate to SAH. 2. Nitric oxide is produced by the endothelium and is an important regulator of cerebral vascular tone by tonically maintaining the vasculature in a dilated state. Endothelial injury after SAH may interfere with NO production and lead to vasoconstriction and impaired responses to endothelium-dependent vasodilators. Inactivation of NO by oxyhaemoglobin or superoxide from erythrocytes may also occur in the subarachnoid space after SAH. 3. Nitric oxide stimulates activity of soluble guanylate cyclase in vascular muscle, leading to intracellular generation of cGMP and relaxation. Subarachnoid haemorrhage appears to cause impaired activity of soluble guanylate cyclase, resulting in reduced basal levels of cGMP in cerebral vessels and often decreased responsiveness of cerebral arteries to NO. 4. Endothelin is a potent, long-lasting vasoconstrictor that may contribute to the spasm of cerebral arteries after SAH. Endothelin is present in increased levels in the cerebrospinal fluid of SAH patients. Pharmacological inhibition of ET synthesis or of ET receptors has been reported to attenuate cerebral vasospasm. Production of and vasoconstriction by ET may be due, in part, to the decreased activity of NO and formation of cGMP. 5. Protein kinase C is an important enzyme involved in the contraction of vascular muscle in response to several agonists, including ET. Activity of PKC appears to be increased in cerebral arteries after SAH, indicating that PKC may be critical in the development of cerebral vasospasm. Recent evidence suggests that PKC activation may occur in cerebral arteries after SAH as a result of decreased negative feedback influence of NO/cGMP. 6. Cerebral arteries are depolarized after SAH, possibly due to decreased activity of potassium channels in vascular muscle. Decreased basal activation of potassium channels may be due to several mechanisms, including impaired activity of NO (and/or cGMP) or increased activity of PKC. Vasodilator drugs that produce hyperpolarization, such as potassium channel openers, appear to be unusually effective in cerebral arteries after SAH. 7. Thus, endothelial damage and reduced activity of NO may contribute to cerebral vascular dysfunction after SAH. Potassium channels may represent an important therapeutic target for the treatment of cerebral vasospasm after SAH.

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

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