Impairment of endothelium-dependent relaxation in human basilar artery after subarachnoid hemorrhage

Relevance of Porcine Stroke Models to Bridge the Gap from Pre-Clinical Findings to Clinical Implementation

In the search of animal stroke models providing translational advantages for biomedical research, pigs are large mammals with interesting brain characteristics and wide social acceptance. Compared to rodents, pigs have human-like highly gyrencephalic brains. In addition, increasingly through phylogeny, animals have more sophisticated white matter connectivity; thus, ratios of white-to-gray matter in humans and pigs are higher than in rodents. Swine models provide the opportunity to study the effect of stroke with emphasis on white matter damage and neuroanatomical changes in connectivity, and their pathophysiological correlate. In addition, the subarachnoid space surrounding the swine brain resembles that of humans. This allows the accumulation of blood and clots in subarachnoid hemorrhage models mimicking the clinical condition. The clot accumulation has been reported to mediate pathological mechanisms known to contribute to infarct progression and final damage in stroke patients. Importantly, swine allows trustworthy tracking of brain damage evolution using the same non-invasive multimodal imaging sequences used in the clinical practice. Moreover, several models of comorbidities and pathologies usually found in stroke patients have recently been established in swine. We review here ischemic and hemorrhagic stroke models reported so far in pigs. The advantages and limitations of each model are also discussed.

Cerebral artery myogenic reactivity: The next frontier in developing effective interventions for subarachnoid hemorrhage

Aneurysmal subarachnoid hemorrhage (SAH) is a devastating cerebral event that kills or debilitates the majority of those afflicted. The blood that spills into the subarachnoid space stimulates profound cerebral artery vasoconstriction and consequently, cerebral ischemia. Thus, once the initial bleeding in SAH is appropriately managed, the clinical focus shifts to maintaining/improving cerebral perfusion. However, current therapeutic interventions largely fail to improve clinical outcome, because they do not effectively restore normal cerebral artery function. This review discusses emerging evidence that perturbed cerebrovascular "myogenic reactivity," a crucial microvascular process that potently dictates cerebral perfusion, is the critical element underlying cerebral ischemia in SAH. In fact, the myogenic mechanism could be the reason why many therapeutic interventions, including "Triple H" therapy, fail to deliver benefit to patients. Understanding the molecular basis for myogenic reactivity changes in SAH holds the key to develop more effective therapeutic interventions; indeed, promising recent advancements fuel optimism that vascular dysfunction in SAH can be corrected to improve outcome.

Timing of Mean Transit Time Maximization is Associated with Neurological Outcome After Subarachnoid Hemorrhage

PURPOSE

Computed tomography perfusion (CTP) has gained significant relevance for the radiological screening of patients at risk of developing delayed cerebral ischemia (DCI) after aneurysmal subarachnoid hemorrhage (aSAH). Particularly, the impact of MTT_PEAK, i.e., the maximal mean transit time value in a series of CTP measurements, for the prediction of long-term outcome has recently been demonstrated by our group. Complementing this recent work, the present study investigated how the timing of MTT_PEAK affected the long-term outcome after aneurysmal subarachnoid hemorrhage.

METHODS

CTP examinations from 103 patients with clinical deterioration attributed to DCI after aSAH were retrospectively analyzed for time interval between SAH ictus and onset of MTT_PEAK in association with modified Rankin Scale (mRS) 23.1 months after SAH.

RESULTS

Patients with unfavorable outcome (mRS > = 2) suffered significant earlier MTT_PEAK onsets than patients with favorable outcome (mRS = 0 and 1). MTT_PEAK within the first week was associated with significantly higher mRS scores compared to later MTT_PEAK. Timing of MTT_PEAK together with the value of MTT_PEAK and initial World Federation of Neurosurgical Societies (WFNS) grade was a significant predictor for an unfavorable outcome (mRS > = 2).

CONCLUSIONS

The current findings suggest a presumably higher vulnerability of the brain to early microcirculatory impairments after aSAH and highlight that timing of MTT elevations could be considered for the identification of patients at increased risk for poor neurological outcome due to DCI.

Mechanisms of acute brain injury after subarachnoid hemorrhage

Brain injury after subarachnoid hemorrhage (SAH) is a biphasic event with an acute ischemic insult at the time of the initial bleed and secondary events such as cerebral vasospasm 3 to 7 days later. Although much has been learned about the delayed effects of SAH, less is known about the mechanisms of acute SAH-induced injury. Distribution of blood in the subarachnoid space, elevation of intracranial pressure, reduced cerebral perfusion and cerebral blood flow (CBF) initiates the acute injury cascade. Together they lead to direct microvascular injury, plugging of vessels and release of vasoactive substances by platelet aggregates, alterations in the nitric oxide (NO)/nitric oxide synthase (NOS) pathways and lipid peroxidation. This review will summarize some of these mechanisms that contribute to acute cerebral injury after SAH.

Cerebral microvasculature is an early target of subarachnoid hemorrhage

Most subarachnoid hemorrhage (SAH) patients exhibit clinical signs of cerebral ischemia at admission but no angiographic vasospasm. Consequently, the source of early cerebral ischemia is not understood. Parenchymal microvessels may contribute to early cerebral ischemia, but the low resolution of current imaging has prevented their analysis in SAH patients. Animal studies demonstrated that early after SAH structure and function of parenchymal vessels are compromised to the level that may very well contribute to early ischemia. We review these studies.

Endothelin-1 and endothelin receptor gene variants and their association with negative outcomes following aneurysmal subarachnoid hemorrhage

Aneurysmal subarachnoid hemorrhage (aSAH) is a devastating disease that affects approximately 30,000 people a year in the United States. Delayed cerebral ischemia (DCI) and cerebral vasospasm (CV) are common complications after aSAH. In addition, aSAH patients have a high risk of poor long-term outcomes. Endothelin-1 (ET-1), a potent vasoconstrictor, or its two types of receptors, ET receptor A (ETA) and ET receptor B (ETB), may play a role in the pathogenesis of DCI and CV. Genetic variations within the ET-1, ETA, or ETB genes may also account for variance observed in the outcomes of aSAH patients. The purpose of this study was to describe the distribution of the Lys198Asn polymorphism, a known functional SNP in the ET-1 gene, and tagging SNPs of the ET-1, ETA, and ETB genes in individuals recovering from aSAH. This study also investigated the relationships among the ET polymorphisms, DCI, and global functional outcomes measured at 3 and 6 months after aSAH. Participants included individuals aged 18-75 years with a diagnosis of aSAH. There was a trend found between the variant allele of an ET-1 SNP (rs6912834) and angiographic vasospasm. There were also associations found between two ETB SNPs (rs9574124 and rs3027111) and poor outcomes as measured by the Glasgow Outcome scale at 3 months. These findings support the role of ET-1 and ETB in recovery following aSAH.

The importance of early brain injury after subarachnoid hemorrhage

Aneurysmal subarachnoid hemorrhage (aSAH) is a medical emergency that accounts for 5% of all stroke cases. Individuals affected are typically in the prime of their lives (mean age 50 years). Approximately 12% of patients die before receiving medical attention, 33% within 48 h and 50% within 30 days of aSAH. Of the survivors 50% suffer from permanent disability with an estimated lifetime cost more than double that of an ischemic stroke. Traditionally, spasm that develops in large cerebral arteries 3-7 days after aneurysm rupture is considered the most important determinant of brain injury and outcome after aSAH. However, recent studies show that prevention of delayed vasospasm does not improve outcome in aSAH patients. This finding has finally brought in focus the influence of early brain injury on outcome of aSAH. A substantial amount of evidence indicates that brain injury begins at the aneurysm rupture, evolves with time and plays an important role in patients' outcome. In this manuscript we review early brain injury after aSAH. Due to the early nature, most of the information on this injury comes from animals and few only from autopsy of patients who died within days after aSAH. Consequently, we began with a review of animal models of early brain injury, next we review the mechanisms of brain injury according to the sequence of their temporal appearance and finally we discuss the failure of clinical translation of therapies successful in animal models of aSAH.

Early micro vascular changes after subarachnoid hemorrhage

During the last decade much effort has been invested in understanding the events that occur early after SAH. It is now widely accepted that these early events not only participate in the early ischemic injury but also set the stage for the pathogenesis of delayed vasospasm. That early cerebral ischemia occurs after SAH is documented in both experimental SAH and in human autopsy studies; however, angiographic evidence for vasoconstriction early after SAH is lacking and the source of early ischemic injury is therefore unclear. Recently, the cerebral microvasculature has been identified as an early target of SAH. Changes in the anatomical structure of cerebral microvessels, sufficient to cause functional deficits, are found early after experimental SAH. These changes may explain cerebral ischemia in human in the absence of angiographic evidence of large vessel vasoconstriction. This paper summarizes known alterations in cerebral microvasculature during the first 48 h after SAH.

Pharmacologically augmented S-nitrosylated hemoglobin improves recovery from murine subarachnoid hemorrhage

BACKGROUND AND PURPOSE

S-nitrosylated hemoglobin (S-nitrosohemoglobin) has been implicated in the delivery of O(2) to tissues through the regulation of microvascular blood flow. This study tested the hypothesis that enhancement of S-nitrosylated hemoglobin by ethyl nitrite inhalation improves outcome after experimental subarachnoid hemorrhage (SAH).

METHODS

A preliminary dosing study identified 20 ppm ethyl nitrite as a concentration that produced a 4-fold increase in S-nitrosylated hemoglobin concentration with no increase in methemoglobin. Mice were subjected to endovascular perforation of the right anterior cerebral artery and were treated with 20 ppm ethyl nitrite in air, or air alone for 72 hours, after which neurologic function, cerebral vessel diameter, brain water content, cortical tissue Po(2), and parenchymal red blood cell flow velocity were measured.

RESULTS

At 72 hours after hemorrhage, air- and ethyl nitrite-exposed mice had similarly sized blood clots. Ethyl nitrite improved neurologic score and rotarod performance; abated SAH-induced constrictions in the ipsilateral anterior, middle cerebral, and internal carotid arteries; and prevented an increase in ipsilateral brain water content. Ethyl nitrite inhalation increased red blood cell flow velocity and cortical tissue Po(2) in the ipsilateral cortex with no effect on systemic blood pressure.

CONCLUSIONS

Targeted S-nitrosylation of hemoglobin improved outcome parameters, including vessel diameter, tissue blood flow, cortical tissue Po(2), and neurologic function in a murine SAH model. Augmenting endogenous Po(2)-dependent delivery of NO bioactivity to selectively dilate the compromised cerebral vasculature has significant clinical potential in the treatment of SAH.

Metamorphosis of subarachnoid hemorrhage research: from delayed vasospasm to early brain injury

Delayed vasospasm that develops 3–7 days after aneurysmal subarachnoid hemorrhage (SAH) has traditionally been considered the most important determinant of delayed ischemic injury and poor outcome. Consequently, most therapies against delayed ischemic injury are directed towards reducing the incidence of vasospasm. The clinical trials based on this strategy, however, have so far claimed limited success; the incidence of vasospasm is reduced without reduction in delayed ischemic injury or improvement in the long-term outcome. This fact has shifted research interest to the early brain injury (first 72 h) evoked by SAH. In recent years, several pathological mechanisms that activate within minutes after the initial bleed and lead to early brain injury are identified. In addition, it is found that many of these mechanisms evolve with time and participate in the pathogenesis of delayed ischemic injury and poor outcome. Therefore, a therapy or therapies focused on these early mechanisms may not only prevent the early brain injury but may also help reduce the intensity of later developing neurological complications. This manuscript reviews the pathological mechanisms of early brain injury after SAH and summarizes the status of current therapies.

Fundamental increase in pressure-dependent constriction of brain parenchymal arterioles from subarachnoid hemorrhage model rats due to membrane depolarization

Intracerebral (parenchymal) arterioles are morphologically and physiologically unique compared with pial arteries and arterioles. The ability of subarachnoid hemorrhage (SAH) to induce vasospasm in large-diameter pial arteries has been extensively studied, although the contribution of this phenomenon to patient outcome is controversial. Currently, little is known regarding the impact of SAH on parenchymal arterioles, which are critical for regulation of local and global cerebral blood flow. Here diameter, smooth muscle intracellular Ca(2+) concentration ([Ca(2+)](i)), and membrane potential measurements were used to assess the function of intact brain parenchymal arterioles isolated from unoperated (control), sham-operated, and SAH model rats. At low intravascular pressure (5 mmHg), membrane potential and [Ca(2+)](i) were not different in arterioles from control, sham-operated, and SAH animals. However, raising intravascular pressure caused significantly greater membrane potential depolarization, elevation in [Ca(2+)](i), and constriction in SAH arterioles. This SAH-induced increase in [Ca(2+)](i) and tone occurred in the absence of the vascular endothelium and was abolished by the L-type voltage-dependent calcium channel (VDCC) inhibitor nimodipine. Arteriolar [Ca(2+)](i) and tone were not different between groups when smooth muscle membrane potential was adjusted to the same value. Protein and mRNA levels of the L-type VDCC Ca(V)1.2 were similar in parenchymal arterioles isolated from control and SAH animals, suggesting that SAH did not cause VDCC upregulation. We conclude that enhanced parenchymal arteriolar tone after SAH is driven by smooth muscle membrane potential depolarization, leading to increased L-type VDCC-mediated Ca(2+) influx.

Escape of intraluminal platelets into brain parenchyma after subarachnoid hemorrhage

Platelet aggregates are present in parenchymal vessels as early as 10 min after experimental subarachnoid hemorrhage (SAH). Structural injury to parenchymal vessel walls and depletion of collagen-IV (the major protein of basal lamina) occur in a similar time frame. Since platelets upon activation release enzymes which can digest collagen-IV, we investigated the topographic relationship between platelet aggregates, endothelium, and basal lamina after SAH produced by endovascular perforation, using triple immunofluorescence and confocal microscopy with deconvolution. The location of platelet aggregates in relation to zymography-detected active collagenase was also examined. As reported previously, most cerebral vessels profiles contained platelets aggregates at 10 min after SAH. High-resolution three-dimensional image analysis placed many platelets at the ab-luminal (basal) side of endothelium at 10 min, and others either within the vascular basal lamina or in nearby parenchyma. By 24 h post hemorrhage, large numbers of platelets had entered the brain parenchyma. The vascular sites of platelet movement were devoid of endothelium and collagen-IV. Collagenase activity colocalized with vascular platelet aggregates. Our data demonstrate that parenchymal entry of platelets into brain parenchyma begins within minutes after hemorrhage. Three-dimensional analysis suggests that platelet aggregates initiate or stimulate local disruption of endothelium and destruction of adjacent basal lamina after SAH.

Upregulation of estrogen receptor α and mediation of 17β-estradiol vasoprotective effects via estrogen receptor α in basilar arteries in rats after experimental subarachnoid hemorrhage

Object

The authors previously demonstrated that 17β-estradiol benzoate (E2) treatment prevents subarachnoid hemorrhage (SAH)–induced cerebral vasospasm and preserves endothelial nitric oxide synthase (eNOS) in male rats. Changes in the expression of estrogen receptor (ER) subtypes ERα and -β and their roles in the E2-mediated preservation of eNOS in SAH remain unknown. In the present study the effects of SAH on the expression of ERα and -β in the cerebral arteries were clarified, and the receptor roles in the E2-mediated preservation of eNOS expression in SAH were differentiated.

Methods

A 2-hemorrhage SAH model was induced by 2 autologous blood injections into the cisterna magna of adult male rats. The effect of SAH on ERα and -β expression was evaluated. Other rats subcutaneously received implanted Silastic tubes containing corn oil with E2 and daily injections of various doses of an ERα- (methyl-piperidinopyrazole [MPP]) or ERβ-selective antagonist (R,R-tetrahydrochrysene) after the first hemorrhage. The protein levels of ERα, ERβ, eNOS, and inducible nitric oxide synthase (iNOS) from basilar arteries were examined using Western blot analysis, and their mRNAs were evaluated by reverse transcription–polymerase chain reaction.

Results

The ERα but not the ERβ was upregulated in the basilar artery after SAH. Treatment with MPP eliminated E2-mediated effects in SAH, relieved cerebral vasospasm, preserved eNOS expression, and suppressed iNOS expression.

Conclusions

Estrogen receptor α is upregulated in the basilar artery after SAH. Note that E2 exerts its protective effects through ERα-dependent pathways to relieve cerebral vasospasm and preserve eNOS expression. A selective ERα agonist may be the drug of choice for the treatment of patients with SAH.

Attenuation of experimental subarachnoid hemorrhage--induced cerebral vasospasm by the adenosine A2A receptor agonist CGS 21680

OBJECT

Impaired endothelium-dependent relaxation is present in vasospastic cerebral vessels after subarachnoid hemorrhage (SAH) and may result from deficient production of endothelial nitric oxide synthase (eNOS) or increased production and/or activity of inducible NOS (iNOS). Accumulating evidence demonstrates that adenosine A2A receptors increase the production of NO by human and porcine arterial endothelial cells, which in turn leads to vasodilation. This study was designed to examine the effects of an adenosine A2A receptor agonist, (2(4-[2-carboxyethyl]phenyl)ethylamino)-5'-N-ethylcarboxamidoadenosine (CGS 21680), in the prevention of SAH-induced vasospasm.

METHODS

. Experimental SAH was induced in Sprague-Dawley rats by injecting 0.3 ml of autologous blood into the cisterna magna of each animal. Intraperitoneal injections of CGS 21680 or vehicle were administered 5 minutes and 24 hours after induction of SAH. The degree of vasospasm was determined by averaging measurements of cross-sectional areas of the basilar artery (BA) 48 hours after SAH. Expression of eNOS and iNOS in the BA was also evaluated. Prior to perfusion-fixation, there were no significant differences among animals in the control and treated groups in any physiological parameter that was recorded. The CGS 21680 treatment significantly attenuated SAH-induced vasospasm. Induction of iNOS mRNA and protein in the BA by the SAH was significantly diminished by administration of CGS 21680. The SAH-induced suppression of eNOS mRNA and protein was also relieved by the CGS 21680 treatment.

CONCLUSIONS

This is the first evidence that adenosine A2A receptor agonism is effective in preventing SAH-induced vasospasm without significant complications. The beneficial effect of adenosine A2A receptor agonists may be, at least in part, related to the prevention of augmented expression of iNOS and the preservation of normal eNOS expression following SAH. Adenosine A2A receptor agonism holds promise in the treatment of cerebral vasospasm following SAH and merits further investigation.

PERSISTENCE OF THE NITRIC OXIDE-DEPENDENT VASODILATORPATHWAY OF CEREBRAL VESSELS AFTEREXPERIMENTAL SUBARACHNOID HEMORRHAGE

OBJECTIVE

Efficiency of the treatment of cerebral vasospasm (CVS) after subarachnoid hemorrhage (SAH) by interfering with the nitric oxide-cyclic guanosine monophospate (cGMP) pathway seems to be inconsistent. So far, it remains unclear whether or not insufficient access to the drugs or impaired reactivity of the vessels is responsible for this inconsistency. Therefore, the aim of the present investigation was to characterize this pathway on cerebral arteries during CVS.

METHODS

CVS was induced using the rat double hemorrhage model and was determined by magnetic resonance perfusion weighted imaging. Rats were sacrificed on Day 3 and Day 5 after SAH. Immunohistochemical staining of the basilar artery for endothelial nitric oxide synthases and the alpha- and beta-subunits of the soluble guanylate cyclase was performed. Basilar artery ring segments on Day 5 were used for measurement of isometric force. Concentration effect curves for acetylcholine, sodium nitroprusside, and 8-bromo-cGMP were constructed and compared by maximum effect and pD2.

RESULTS

The immunohistochemical expression of endothelial nitric oxide synthase was comparable in all groups. The soluble guanylate cyclase alpha- and beta-subunits were significantly diminished on Day 3, but recovered by Day 5. The relaxation attributable to acetylcholine and 8-bromo-cGMP was virtually identical in controls and during CVS. Relaxation attributable to sodium nitroprusside, however, was significantly enhanced after SAH (maximum effect, control: 88 +/- 12%; Day 5: 117 +/- 26%).

CONCLUSION

The present investigations suggest the persistence of endothelium-, nitric oxide-, and cGMP-dependent relaxation during CVS. Therefore, the treatment of CVS interfering with this pathway seems not to be limited by alterations inside the vessel wall.

Effects of acute treatment with statins on cerebral autoregulation in patients after aneurysmal subarachnoid hemorrhage

OBJECT

The authors previously have demonstrated that acute treatment with pravastatin after aneurysmal subarachnoid hemorrhage (SAH) can ameliorate vasospasm-related delayed ischemic neurological deficits (DINDs). In the current study, they test the hypothesis that these effects are associated with improvement in indices describing autoregulation of cerebral blood flow.

METHODS

In this double-blind study, 80 patients between the ages of 18 and 84 years who had aneurysmal SAH were randomized equally to receive either 40 mg of oral pravastatin or placebo once daily for up to 14 days (medication was started 1.8 x 1.3 days after ictus). Autoregulation was measured using a daily transient hyperemic response test (THRT) on transcranial Doppler ultrasonography (800 measurements in 80 patients), and data were compared between the pravastatin and placebo groups and between patients with or without vasospasm, DINDs, or unfavorable outcome. Measurement of autoregulation also was performed using the pressure-reactivity index, a moving correlation coefficient between mean arterial and intracranial pressures (Days 0-5, 132 measurements in 32 patients). There was no difference in baseline autoregulation indices between the trial groups. The members of the pravastatin group not only had a shorter duration of impaired autoregulation but also had stronger transient hyperemic response ratios (THRRs) bilaterally. A negative correlation existed between the mean flow velocity in the middle cerebral artery and THRRs. Onset of DINDs occurred when bilateral autoregulation failed. On Days 3, 4, and 5, the pressure-reactivity index correlated significantly with ipsilateral impaired autoregulation.

CONCLUSIONS

The neuroprotective effects of acute treatment with pravastatin following aneurysmal SAH are associated with enhancement of autoregulation. A routine and daily assessment of cerebral autoregulation by using the THRT may help identify patients at high risk of DINDs.

The effect of an adenosine A1 receptor agonist in the treatment of experimental subarachnoid hemorrhage-induced cerebrovasospasm

BACKGROUND

Adenosine is a potent vasodilator and an important modulator of cardiovascular function. It has been postulated that nitric oxide (NO) is involved in adenosine-induced vasodilation. This study was designed to examine the effect of an adenosine A1 agonist, N6-cyclopentyladenosine (CPA), in the prevention of subarachnoid haemorrhage (SAH)-induced vasospasm. Method. Experimental SAH was induced in Sprague-Dawley rats by injecting 0.3 mL autogenous blood into the cisterna magna. Intraperitoneal injections of CPA (0.003 mg/kg), or vehicle were administered 5 min and 24 hours after induction of SAH. The degree of vasospasm was determined by averaging the cross sectional areas of the basilar artery 2 days after SAH. Expressions of endothelial nitric oxide synthase (eNOS) and inducible nitric oxide synthase (iNOS) in basilar artery were evaluated. Findings. There were no significant differences among the control and treated groups in physiological parameters recorded before sacrifice. When compared with animals in the control group, cross-sectional area of basilar arteries areas in the SAH only, SAH plus vehicle and SAH plus CPA groups were reduced by 19% (p < 0.01), 22% (p < 0.01), and 9% (p = 0.133), respectively. The cross-sectional areas of the CPA-treated group differed significantly from those of the SAH only and SAH plus vehicle group (p < 0.05). Induction of iNOS-mRNA and protein in basilar artery by SAH was not significantly diminished by CPA. The SAH-induced suppression of eNOS-mRNA and protein were relieved by CPA treatment. Conclusions. This is the first evidence to show an adenosine A1 receptor agonist is effective in partially preventing SAH-induced vasospasm without significant cardiovascular complications. The mechanisms of adenosine A1 receptor agonists in attenuating SAH-induced vasospasm may be, in part, related to preserve the normal eNOS expression after SAH. Inability in reversing the increased iNOS expression after SAH may lead to the incomplete anti-spastic effect of CPA.

The effect of 17β-estradiol in attenuating experimental subarachnoid hemorrhage–induced cerebral vasospasm

Object

Sex differences in the outcome of aneurysmal subarachnoid hemorrhage (SAH) are controversial, and the potential influence of estradiol on vasodilation is unclear. In the present study the authors evaluate the effect and possible mechanism of 17β-estradiol (E2) on SAH-induced vasospasm in a two-hemorrhage rodent model of SAH.

Methods

A 30-mm Silastic tube filled with E2 in corn oil (0.3 mg/ml) was subcutaneously implanted in male rats. Serum levels of E2 were measured on Days 0, 1, 2, 3, 4, and 7 postimplantation. The degree of vasospasm was determined by averaging the cross-sectional areas of the basilar artery (BA) 7 days after the first SAH. Expressions of endothelial nitric oxide synthase (eNOS) and inducible NOS (iNOS) in the BA were also evaluated.

Serum levels of E2 in the E2-treated rats were at physiological levels (56–92 pg/ml) and were significantly higher than those in the control and vehicle-treated groups. Treatment with E2 significantly (p < 0.01) attenuated SAH-induced vasospasm. Induction of iNOS messenger (m)RNA and protein in the BA by SAH was significantly diminished by the E2 treatment but not by vehicle treatment. The SAH-induced suppression of eNOS mRNA and protein was relieved by E2 treatment.

Conclusions

These results suggest that continuous treatment with E2 at physiological levels prevents cerebral vasospasm following SAH. The beneficial effect of E2 may be in part related to the prevention of augmentation of iNOS expression and the preservation of normal eNOS expression after SAH. Treatment with E2 holds therapeutic promise in the treatment of cerebral vasospasm following SAH and merits further investigation.

Brain and cancer: the protective role of erythropoietin

Erythropoietin (Epo) is a pleiotropic agent, that is to say, it can act on several cell types in different ways. An independent system Epo/Epo receptor (EpoR) was detected in brain, leading to the hypothesis that this hormone could be involved in cerebral functions. Epo/EpoR expression changes during ontogenesis, thus indicating the importance of this system in neurodevelopment. Moreover, the hypoxia-induced production of Epo in the adult brain suggests that it could exert a neurotrophic and neuroprotective effect in case of brain injury. Epo could also influence neurotransmission, inducing neurotransmitters (NT) release. Epo therapy in anemic cancer patients is still a controversial issue, because of its possible action as a growth and an angiogenic factor. In our speculative hypothesis Epo could be involved in a "two steps process" that, after a neovascularization phase, leads to its down regulation. Moreover, Epo-activated signaling pathways could be modulated as possible targets to interfere in neoplastic cells cycle. In conclusion, treatment with rHuEpo could change therapeutical perspectives in different pathological conditions, such as central nervous system (CNS) diseases, but further studies are needed to clarify its physiopathological activities in different clinical fields.

Induction of iNOS restricts functional activity of both eNOS and nNOS in pig cerebral artery

The aim of the study was to investigate the effect of iNOS expression on eNOS and nNOS functional activity in porcine cerebral arteries. iNOS was induced in pig basilar arteries using lipopolysaccharide (LPS). Arteries expressing iNOS generated NO and relaxed when challenged with L-arginine (30 microM), an effect that was reduced by treatment with dexamethasone (coincubated with LPS) and prevented by the iNOS inhibitor 1400 W (administered 10 min prior to precontraction). eNOS was activated by A23187 and was found to be impaired in arteries that had iNOS induced (A23187 1 microM relaxation: control 110+/-8%, LPS-treated 50+/-16% ; p<0.05, N=5-6). This was due mainly to reduced formation of NO by A23187 (NO concentration in response to A23187 1 microM: control 25+/-6 nM, LPS-treated 0.8+/-1.2 nM; p<0.001, N=5-6), in addition to a small reduction in the vasodilator response to the NO-donors NOC-22 and SIN-1. Cerebral vasodilation produced by stimulation of intramural nitrergic nerves was impaired in arteries that had iNOS induced, and this was reversed by 1400 W (control 23+/-4% relaxation, LPS-treated 11+/-1% relaxation, LPS plus 1400 W 10 microM treated 25+/-2% relaxation; p<0.01 for control versus LPS, N=6). It is concluded that the induction of iNOS in cerebral arteries reduces NO-mediated vasodilation initiated by eNOS and by nNOS, primarily by modulation of NO formation.

Phosphodiesterases in the vascular system

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers involved in the intracellular signal transduction of a variety of extracellular stimuli in several tissues. In the vascular system, these nucleotides play important roles in the regulation of vascular tone and in the maintenance of the mature contractile phenotype in smooth muscle cells. Given that cyclic nucleotide signaling regulates a wide variety of cellular functions, it is not surprising that cyclic nucleotide phosphodiesterases (PDEs). In paticular, the accumulating data showing that there are a large number of different PDE isozymes have triggered an equally large increase in interest about these enzymes. At least 11 different gene families of PDEs are currently known to exist in mammalian tissues. Most families contain several distinct genes, and many of these genes are expressed in different tissues as functionally unique alternative splice variants. This article reviews many of the important aspects about the structure, cellular localization, and regulation of each family of PDEs. Particular emphasis is placed on new information obtained in the last few years about vascular disease. The development of novel methods to deliver more potent and selective PDE inhibitors to individual cell types and subcellular locations will lead to new therapeutic uses for this class of drugs in diseases of the vascular system.

Endothelial dysfunction in a primate model of cerebral vasospasm

OBJECT

Although abnormalities in the control of endothelial vasomotility have been reported in both experimental and clinical studies, the mechanism of the endothelial dysfunction that occurs following subarachnoid hemorrhage (SAH) remains unclear. Because of the absence of previous in vivo studies of endothelial function in cerebral vessels in response to SAH or cerebral vasospasm, the authors investigated endothelium-dependent responses in an established primate model of vasospasm after SAH. Endothelial function was assessed by examining vascular responses to intracarotid injections of various drugs known to act via the endothelium. Drugs that have a rapid total body clearance were selected so that their pharmacological effects would be limited to the cerebral circulation after an intracarotid infusion.

METHODS

Seventeen adult male cynomolgus monkeys were used. Cerebrovascular endothelium-dependent responses were examined in control animals and in animals with SAH 7, 14, and 21 days after placement of a subarachnoid clot around the right middle cerebral artery. Cortical cerebral blood flow (CBF) and cerebrovascular resistance (CVR) were recorded continuously during 5-minute intracarotid infusions of 5% dextrose vehicle, acetylcholine, histamine, bradykinin, or Calcimycin. In control animals the intracarotid infusion of acetylcholine produced a significant (7.8 +/- 9.5%) increase in CBF and a 9.3 +/- 8.7% reduction in CVR in comparison with a control infusion of dextrose vehicle. The responses to acetylcholine disappeared in animals 7 days post-SAH, specifically in the subset of animals in which arteriography confirmed the presence of vasospasm. Infusion of Calcimycin produced no significant changes in CBF or CVR in control animals, but resulted in a significant reduction in CBF and increase in CVR in animals 7 days after SAH and in animals with vasospasm. An infusion of histamine or bradykinin had no significant effect on CBF or CVR.

CONCLUSIONS

An intracarotid infusion of acetylcholine, but not one of histamine, bradykinin, or Calcimycin, produced a measurable physiological response in the normal primate cerebrovasculature. Cerebral vasospasm that occurred after SAH produced a pathophysiological effect similar to the endothelial denudation shown in the in vitro experiments of Furchgott and Zawadzki, in which acetylcholine constricted the vessels via activation of receptors on smooth-muscle cells. Changes in vascular responses to acetylcholine and Calcimycin in animals with vasospasm, compared with control animals, provide evidence that endothelial dysfunction plays a key role in the development and/or sustenance of vasospasm after SAH.

An overview of new pharmacological treatments for cerebrovascular dysfunction after experimental subarachnoid hemorrhage

Cerebral vasospasm and the resulting cerebral ischemia occurring after subarachnoid hemorrhage (SAH) are still responsible for the considerable morbidity and mortality in patients affected by cerebral aneurysms. Mechanisms contributing to the development of vasospasm, abnormal reactivity of cerebral arteries and cerebral ischemia after SAH have been intensively investigated in recent years. It has been suggested that the pathogenesis of vasospasm is related to a number of pathological processes, including endothelial damage, smooth muscle cell contraction resulting from spasmogenic substances generated during lyses of subarachnoid blood clots, changes in vascular responsiveness and inflammatory or immunological reactions of the vascular wall. A great deal of experimental and clinical research has been conducted in an effort to find ways to prevent these complications. However, to date, the main therapeutic interventions remain elusive and are limited to the manipulation of systemic blood pressure, alteration of blood volume or viscosity, and control of arterial dioxide tension. Even though no single pharmacological agent or treatment protocol has been identified which could prevent or reverse these deadly complications, a number of promising drugs have been investigated. Among these is the hormone erythropoietin (EPO), the main regulator of erythropoiesis. It has recently been found that EPO produces a neuroprotective action during experimental SAH when its recombinant form (rHuEPO) is systemically administered. This topic review collects the relevant literature on the main investigative therapies for cerebrovascular dysfunction after aneurysmal SAH. In addition, it points out rHuEPO, which may hold promise in future clinical trials to prevent the occurrence of vasospasm and cerebral ischemia after SAH.

The induction and detection in vitro of iNOS in the porcine basilar artery

The expression of iNOS in vascular tissues has an adverse effect on vascular responses to vasoconstrictors and NO-mediated vasodilators. The development of a simple method for detecting the iNOS expression by functional means would be extremely useful. Here we describe a method for inducing iNOS in the porcine basilar artery followed by the detection of iNOS protein by immunocytochemical means and the characterisation of functional responses to U46619 and L-arginine. Porcine basilar arteries were treated with LPS (1, 10 and 100 microg/ml) for between 5 and 18 h at 37 degrees C. Inducible NOS protein was expressed in a concentration-dependent manner in the endothelial and smooth muscle cells after 5 h and persisted for 18 h. Vessels treated with LPS showed a time-dependent reduction in contractile function in response to U46619 (10 nM) reaching significance at the 18-h time point. Moreover, a similar time-dependent increase in the vasodilator response to exogenously applied L-arginine (30 microM) was observed at both 5- and 18-h time points. These effects of LPS at the 18-h time point were prevented by the incubation of vessels with dexamethasone (100 microM) in addition to LPS. The vasodilator response to L-arginine was prevented with the incubation with and in the presence of the inhibitor of inducible NOS, 1400W (10 microM) in addition to LPS. These results show that iNOS protein can be expressed in porcine cerebral arteries and that the iNOS is functional. The assessment of contractile function and responses to L-arginine using single concentrations is a rapid and effective method for establishing whether functional iNOS is present in porcine cerebral arteries.

The pleiotropic effects of erythropoietin in the central nervous system

Erythropoietin (Epo) is a hydrophobic sialoglycoproteic hormone produced by the kidney and responsible for the proliferation, maturation, and differentiation of the precursors of the erythroid cell line. Human recombinant erythropoietin (rHuEpo) is used to treat different types of anemia, not only in uremic patients but also in newborns with anemia of prematurity, in patients with cancer-related anemia or myeloproliferative disease, thalassemias, bone marrow transplants, or those with chronic infectious diseases. The pleiotropic functions of Epo are well known. It has been shown that this hormone can modulate the inflammatory and immune response, has direct hemodynamic and vasoactive effects, could be considered a proangiogenic factor because of its interaction with vascular endothelial growth factor, and its ability to stimulate mitosis and motility of endothelial cells. The multifunctional role of Epo has further been confirmed by the discovery in the central nervous system of a specific Epo/Epo receptor (EpoR) system. Both Epo and EpoR are expressed by astrocytes and neurons and Epo is present in the cerebrospinal fluid (CSF). Therefore, novel functions of Epo, tissue-specific regulation, and the mechanisms of action have been investigated. In this review we have tried to summarize the current data on the role of Epo on brain function. We discuss the different sites of cerebral expression and mechanisms of regulation of Epo and its receptor and its role in the development and maturation of the brain. Second, we discuss the neurotrophic and neuroprotective function of Epo in different conditions of neuronal damage, such as hypoxia, cerebral ischemia, and subarachnoid hemorrhage, and the consequent possibility that rHuEpo therapy could soon be used in clinical practice to limit neuronal damage induced by these diseases.

Acute and delayed vasoconstriction after subarachnoid hemorrhage: local cerebral blood flow, histopathology, and morphology in the rat basilar artery

The decreased local cerebral blood flow (LCBF) and cerebral ischemia that occur after subarachnoid hemorrhage (SAH) may be caused by acute and/or delayed vasospasm. In 36 Sprague-Dawley (350-450 g) rats SAH was induced by transclival puncture of the basilar artery. Mean arterial blood pressure (MABP), LCBF, intracranial pressure (ICP), and cerebral perfusion pressure (CPP) were measured in all rats for 30 min before and 60 min after SAH was induced. One set of control (n : 7) and experimental animals (n : 7) was sacrificed after the 60 min of initial post-hemorrhage measurements were recorded. Four days after SAH induction, LCBF and MABP were measured again for 60 min in subgroups of surviving experimental rats (n : 7) and control rats (n : 7). Histopathologic and morphologic examinations of the basilar artery were performed in each subgroup. There was a sharp drop in LCBF just after SAH was induced (55.50 +/- 11.46 mlLD/min/100 g and 16.1 +/- 3.6 mlLD/min/100 g for baseline and post-SAH, respectively; p < 0.001). The flow then gradually increased but had not returned to pre-SAH values by 60 min (p < 0.05). At 4 days after SAH induction, although LCBF was lower than that observed in the control group and pre-SAH values, it was not significantly different from either of these flow rates (p > 0.05). ICP (baseline 7.05 +/- 0.4 mmHg) increased acutely to 75.2 +/- 7.1 mmHg, but returned to normal levels by 60 min after SAH. CPP (baseline 84.5 +/- 6.3 mmHg) dropped accordingly (to 18.6 +/- 3.1 mmHg), and then increased, reaching 72.2 +/- 4.9 mmHg at 60 min after SAH (p > 0.05). Examinations of the arteries revealed decreased inner luminal diameter and distortion of the elastica layer in the early stage. LCBF in nonsurviver rats (n : 8) was lower than that in the animals that survived (p < 0.01). At 4 days post-hemorrhage, the rats' basilar arteries showed marked vasculopathy. The findings showed that acute SAH alters LCBF, ICP, and CPP, and that decreased LCBF affects mortality rate. Subsequent vasculopathy occurs in delayed fashion, and this was observed at 4 days after the hemorrhage event.

Microvascular endothelial dysfunction and its mechanism in a rat model of subarachnoid hemorrhage

After subarachnoid hemorrhage (SAH), large cerebral arteries are prone to vasospasm. Using a rat model of SAH, we examined whether cortical microvessels demonstrate vasomotor changes that may make them prone to spasm and whether endothelial dysfunction may account for any observed changes. Two days after percutaneous catheterization into the cisterna magna, 0.3 mL of autologous blood was injected into the subarachnoid space. The brain tissue was harvested 20 min later, and microvessels were dissected from the parietal cortex. Vasomotor responses to the thromboxane analog U46619, the protein kinase C agonist phorbol acetate, endothelin-1, adenosine diphosphate, nitroprusside, and isoproterenol were examined in vitroin cerebral arterioles from the control, sham-operated, and SAH animals. Endothelial nitric oxide synthase (NOS3) messenger RNA and protein concentration was measured by northern and western blotting, respectively. Arterioles from the SAH animals demonstrated attenuated dilation to the endothelium-dependent dilator adenosine diphosphate and accentuated constriction to endothelin-1, while responses to the other agents tested were unchanged. NOS3 protein concentration was decreased, but NOS3 messenger RNA was increased after SAH. After SAH, cortical arterioles demonstrate endothelial dysfunction, which may be the basis for microvascular spasm. This is in part related to decreased NOS3, which occurs despite an increase in its transcription.

IMPLICATIONS

Acute microvascular endothelial dysfunction may occur after subarachnoid hemorrhage and contribute to microvascular spasm.

Subarachnoid haemorrhage-induced sympathoexcitation in rats is reversed by bosentan or sodium nitroprusside

1. The roles played by nitric oxide (NO) and endothelin (ET) in the genesis of sympathetic nervous activation following experimental subarachnoid haemorrhage was investigated using spectral analysis of blood pressure rhythms. 2. Subarachnoid haemorrhage was induced in conscious rats by injecting 0.3 mL homologous blood via a catheter placed along the surface of the brain and directed towards the circle of Willis. Three hours after the insult and after sympathetic activation was evident, animals received either an acute injection of the ET antagonist bosentan (5 mg/kg, i.v.; n = 7), an infusion of the NO donor sodium nitroprusside (SNP; 18 microg/h; n = 7) or no treatment (n = 7). 3. Three hours following the induction of subarachnoid haemorrhage, the mid-frequency components of systolic blood pressure were markedly elevated, indicating a pronounced sympathoexcitation. However, blood pressure and heart rate levels remained unchanged at this time. In the absence of treatment, the mid-frequency components of blood pressure remained elevated for a subsequent 2 h. Treatment with a non-hypotensive dose of SNP reversed the sympathoexcitation within 1 h. Treatment with bosentan was also effective in reducing the mid-frequency oscillations in blood pressure associated with subarachnoid haemorrhage. 4. Our results indicate that subarachnoid haemorrhage is associated with an acute activation of the sympathetic nervous system. The degree of sympathoexcitation can be reversed by the use of either bosentan or SNP.

Effective improvement of the cerebral vasospasm after subarachnoid hemorrhage with low-dose nitroglycerin

In the rabbit subarachnoid hemorrhage (SAH) model, the sensitivity of spastic basilar arteries to nitric oxide (NO) was enhanced whereas the endothelial function to release/produce NO became impaired, as described previously. We assumed from these results that low-dose NO might selectively dilate spastic arteries without influencing normal or systemic blood vessels; therefore, we investigated whether exogenous low-dose NO effectively improves cerebral vasospasm. Low-dose NO was derived from a small size of the tape containing nitroglycerin, which is not invasive and is clinically available. The experimental SAH was induced by injecting autologous blood into the cisterna magna of the rabbit. Experiments were performed on the following three groups: (a) SAH group with nitroglycerin tape (nitroglycerin group), (b) SAH group with placebo tape (placebo group), and (c) saline group injected with saline instead of blood. The tape containing 0.675 mg nitroglycerin was applied once daily for 2 days onto the skin area of the rabbit's ear. Angiograms were performed once before cisternal injection of blood and/or saline and again on day 2. On day 2 the basilar artery was isolated and sliced into 2-mm ring preparations. Relaxations of the basilar artery to acetylcholine, sodium nitroprusside, and calcium ionophore A23187, as well as the contractile responses to serotonin and endothelin- 1, were measured. The diameter of the basilar artery on day 2 was reduced to 69.6+/-2.2% (n = 7) before the injection of autologous blood. The angiographic vasospasm of the basilar artery was partially but significantly (p<0.0001) improved to the percentage diameter of 89.4+/-1.4% (n = 7) by the application of low-dose nitroglycerin, which did not affect the systemic blood pressure and heart rate. In the basilar artery preparations harvested from SAH rabbits on day 2, the impaired acetylcholine-induced endothelium-dependent relaxation was partially but significantly (p<0.001) improved in the nitroglycerin group. However, this group remained unaffected in the increased sensitivity to nitroglycerin and the contractile responses to serotonin and endothelin-1. Low-dose nitroglycerin tape effectively improved the cerebral vasospasm after SAH without any significant changes in the systemic circulation and would be one of the useful and noninvasive treatments for cerebral vasospasm. The results seem to be partially affected by the effective dilation of the spastic artery and the improvement of the impaired endothelium-dependent relaxation with low-dose NO.

Impaired cerebral vasodilator responses to NO and PDE V inhibition after subarachnoid hemorrhage

Subarachnoid hemorrhage (SAH) is associated with impaired nitric oxide (NO)-mediated cerebral vasodilatation. We tested the hypothesis that SAH causes alterations in the production of, hydrolysis of, or responsiveness to cGMP in the rat basilar artery in vivo. Rats were injected with saline or autologous blood into the cisterna magna. Two days later, effects of vasoactive drugs on basilar artery diameter were examined using a cranial window preparation. Vasodilator responses to ACh, sodium nitroprusside (SNP), and low concentrations (</=10(-5) M) of zaprinast, an inhibitor of phosphodiesterase V (PDE V), were impaired in SAH rats (P < 0.05). In contrast, vasodilator responses to adenosine and 8-BrcGMP were similar in control and SAH rats. Vasoconstrictor responses to 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one, an inhibitor of soluble guanylate cyclase, were unaffected by SAH. In the presence of zaprinast (10(-5)-10(-4) M), responses to ACh and SNP were equivalent in control and SAH rats. Thus an increased rate of cGMP hydrolysis by PDE V may be a major factor contributing to the impairment of NO-mediated cerebral vasodilatation after SAH.

Etiology of cerebral vasospasm

Cerebral vasospasm is a gradual onset and prolonged constriction of the cerebral arteries in the subarachnoid space after subarachnoid hemorrhage. The principal cause is the surrounding blood clot. The significance of vasospasm is that flow through the constricted arteries may be reduced sufficiently to cause cerebral infarction. Subarachnoid blood clot is sufficient to cause vasospasm; it does not require additional arterial injury, intracranial hypertension or brain infarction, although these elements are often coexistent. The blood released at the time of aneurysmal rupture into the alien subarachnoid environment is an extraordinarily complex mix of cellular and extracellular elements that evolves as clotting occurs; cells disintegrate; local inflammation, phagocytosis and repair take place; severe constriction alters the metabolism and structure of the arterial wall as well as the balance of vasoconstrictor and dilator substances produced by its endothelium, neurogenic network and perhaps smooth muscle cells.

Extrinsic pathway of blood coagulation and thrombin in the cerebrospinal fluid after subarachnoid hemorrhage

OBJECTIVE

The involvement of thrombin in the pathophysiology of subarachnoid hemorrhage (SAH) was investigated by comparing thrombin expression and extrinsic pathway activation in the cerebrospinal fluid (CSF) and blood of patients with SAH with the neurological grades, outcome, and presence of delayed cerebral vasospasm.

METHODS

Blood and CSF samples were obtained from 38 patients with SAH on Days 3 through 5, 7 through 9, and 12 through 14 after the onset of SAH. CSF samples were also obtained from control patients. Thrombin-antithrombin III complex, prothrombin fragment F1 +2, tissue factor, and tissue factor pathway inhibitor were analyzed using enzyme-linked immunosorbent assay.

RESULTS

No markers in the blood or CSF were correlated with neurological grades and outcome. Thrombin-antithrombin III complex and prothrombin fragment F1 +2 levels were significantly higher in the CSF of patients with SAH than in the blood or the CSF of control patients and were significantly higher in patients with vasospasm than in patients without vasospasm on Days 7 through 9. Tissue factor levels were significantly higher in the CSF of patients with SAH than in the blood, but the levels were close to those in the CSF of control patients. Tissue factor pathway inhibitor levels in the CSF of patients with SAH and control patients were under the detection limit.

CONCLUSION

Thrombin in the blood may not reflect the pathophysiology of SAH. Imbalance between tissue factor and tissue factor pathway inhibitor in the CSF may tend to thrombin generation under normal physiological conditions and also after SAH. Thrombin in the CSF may be involved in the pathophysiology of vasospasm.

Effects of oxyhemoglobin on vasoconstriction in response to 5-hydroxytryptamine in isolated, perfused canine basilar arteries

OBJECTIVE

Oxyhemoglobin (OxyHb) is thought to be a critical trigger in the pathogenesis of cerebral vasospasm after aneurysmal subarachnoid hemorrhage. We investigated whether extraluminally applied OxyHb influenced vascular responses to intraluminally applied vasoactive agents in isolated, perfused, canine basilar arteries.

METHODS

The steel cannula insertion method was used to examine vascular responses to intraluminally applied 5-hydroxytryptamine (5-HT) receptor agonists, i.e., 5-HT, 5-carboxamidotryptamine (selective for 5-HT1 receptors), and alpha-methyl-5-hydroxytryptamine (selective for 5-HT2 receptors), potassium chloride, and acetylcholine, before and after extraluminal treatment with OxyHb.

RESULTS

Extraluminal application of 2.5 x 10(-5) mol/L OxyHb immediately induced a transient elevation of the basal perfusion pressure, which gradually decreased and then stabilized at a level slightly higher than the control level. Each 5-HT agonist induced dose-dependent vasoconstriction. The potencies of the agonists were not very different, but the efficacies varied, i.e., alpha-methyl-5-hydroxytryptamine > 5-HT > 5-carboxamidotryptamine. Each response was strongly inhibited by ketanserin (a selective 5-HT2 receptor antagonist), indicating that each agonist induces vasoconstriction mediated by 5-HT2 receptors. The vasoconstriction in response to each 5-HT receptor agonist was consistently potentiated by treatment with OxyHb (2.5 x 10(-5) mol/L). 5-HT receptor agonist-induced constrictions after OxyHb treatment were much more markedly inhibited by ketanserin, compared with those before OxyHb treatment. Acetylcholine-induced constrictions were enhanced by OxyHb, but KCl-induced constrictions were significantly decreased by OxyHb.

CONCLUSION

It is suggested that OxyHb enhancement of constrictions in response to 5-HT receptor agonists may be mediated by increased sensitivity of 5-HT2 receptors, in addition to actions in the endothelium, in canine basilar arteries. This potentiated vasoconstrictor mechanism may be partially implicated in cerebral vasospasm after subarachnoid hemorrhage.

Responses of human basilar and other isolated arteries to novel nitric oxide donors

The adducts of nitric oxide (NO), diethylamine/NO (DEA/NO) and diethylenetriamine/NO (DETA/NO), are new NO donors that spontaneously release NO in aqueous solutions. These donors may have therapeutic advantages over sodium nitroprusside (SNP), which depends on metabolism to yield NO. This study was performed to define and compare the pharmacodynamic properties of these NO donors on isolated rings of human, canine, and porcine basilar arteries and further to compare canine and porcine common carotid arteries precontracted with KCl. The median effective concentration (EC50) and the basic effect of 100 microM were determined for each NO donor. On basilar arteries, DEA/NO was the most potent but the maximal dilatation produced by 100 microM did not persist for 60 min, whereas that of DETA/NO and SNP did. DETA/NO was more potent than SNP on all three species of basilar arteries but was the least potent on peripheral (carotid) arteries. Methylene blue in equimolar concentrations significantly inhibited the vasorelaxant effects of DEA/NO and DETA/NO, suggesting a common mechanism of action. Of the NO donors studied, the pharmacodynamic properties of DETA/NO seemed most relevant clinically as a cerebrovascular dilator in being more potent than SNP while producing sustained vasorelaxation.

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.

Endothelial-mediated dilations following severe controlled cortical impact injury in the rat middle cerebral artery

The mechanisms associated with dysfunction of the cerebral vasculature following head trauma have not yet been fully elucidated. In an attempt to shed more light on the matter, we investigated the endothelial-mediated dilations in the rat middle cerebral artery (MCA) following severe traumatic brain injury (TBI). Rats were subjected to severe controlled cortical impact injury (CCI; 5 m/s, 130 ms duration, 3 mm deformation) over the right parietal cortex. At 24 h postinjury, ipsilateral segments of MCA and corresponding contralateral segments were isolated, mounted in a vessel chamber, and pressurized. The responses to 2 methylthio-ATP (2MeSATP), a selective agonist for the P2Y1 purinoceptors, N(omega)-nitro-L-arginine (L-NAME), an NO synthase inhibitor, and S-nitroso-N-acetylpenicillamine (SNAP), an exogenous NO donor, were determined. 2MeSATP elicited concentration dependent dilations in all MCAs studied. Ipsilateral MCAs harvested following TBI or sham-TBI, showed similar maximum dilations to 2MeSATP [70 +/- 4% (n = 17) and 72 +/- 6% (n = 13), respectively]. However, TBI reduced the concentration of 2MeSATP necessary to elicit one-half of the maximum dilation (EC50) from 15 to 9 nM (p < 0.05). Inhibition of NO synthase with 10(-5) M L-NAME abolished the dilation to 2MeSATP in both TBI and sham-TBI MCAs. The constriction to L-NAME was significantly reduced in TBI MCAs compared to sham vessels. Dilations to SNAP, an NO donor, were not altered by TBI indicating that the mechanisms of dilation involving NO in the vascular smooth muscle were not affected. Unlike other pathological conditions which often diminish endothelial-mediated responses, severe TBI enhanced the sensitivity to 2MeSATP without altering the maximum response.

The effect of subarachnoid hemorrhage on mechanisms of vasodilation mediated by cyclic adenosine monophosphate

OBJECT

This study was designed to determine whether subarachnoid hemorrhage (SAH) affects the function of the K+ channels responsible for relaxation of canine cerebral arteries in response to adenylate cyclase activation.

METHOD

The effect of K+ channel inhibitors on the arterial relaxation response to forskolin, a direct adenylate cyclase activator, was studied in rings of basilar arteries obtained from normal dogs and dogs in which SAH was induced (double-hemorrhage model). The levels of adenosine 3',5'-cyclic monophosphate (cAMP) were measured using the radioimmunoassay technique. In rings with the endothelium removed, relaxation induced by forskolin was not affected by SAH. The relaxation response to forskolin was reduced by charybdotoxin (10(-7) mol/L), a selective Ca++-activated K+ channel inhibitor, in normal arteries and arteries subjected to autologous blood injection. This inhibitory effect of charybdotoxin was significantly greater in arteries involved in SAH than in normal vessels. The relaxation response to forskolin was reduced by 4-aminopyridine (10(-3) mol/L), a delayed rectifier K+ channel inhibitor, only in arteries involved in SAH. In contrast, the relaxation response to forskolin was not affected by glyburide (10(-5) mol/L), an adenosine 5'-triphosphate-sensitive K+ channel inhibitor, in both normal and SAH arteries. Forskolin (3 x 10(-7) mol/L) produced an approximately 10-fold increase in levels of cAMP. The basal values and increased levels of cAMP detected after stimulation with forskolin were no different in normal arteries and those exposed to SAH.

CONCLUSIONS

These results demonstrate that formation of cAMP and the relaxation response to adenylate cyclase activation are not affected by SAH. However, in diseased arteries, K+ channels assume a more important role in the mediation of relaxation response to forskolin, indicating that SAH may change the mechanisms responsible for vasodilation induced by cAMP.

Acute vasoconstriction after subarachnoid hemorrhage

OBJECTIVE

Decreased cerebral blood flow (CBF) and cerebral ischemia occurring immediately after subarachnoid hemorrhage (SAH) may be caused by acute microvascular constriction. However, CBF can also be influenced by changes in intracranial pressure (ICP) and cerebral perfusion pressure (CPP). The goal of these experiments was to assess the significance of acute vasoconstriction after SAH and its relationship to changes in CBF, ICP, CPP, and extracellular glutamate concentrations.

METHODS

Three experiments were performed using the endovascular filament technique to produce SAH. In the first experiment, CBF, ICP, and CPP were measured for 60 minutes after SAH (n = 21) and were correlated with the 24-hour mortality rate. In the second experiment, rats undergoing SAH (n = 23) or a sham procedure (n = 7) were perfused 60 minutes after SAH for measurement of the circumference and wall thickness of the internal carotid and anterior cerebral arteries and correlation with CBF, ICP, and CPP. In the third experiment (n = 11), extracellular glutamate concentrations determined by hippocampal and cortical microdialysis and high performance liquid chromatography were correlated with physiological changes.

RESULTS

CBF reductions to less than 40% of baseline for 60 minutes after SAH predicted 24-hour mortality with 100% accuracy and were used to define "lethal" SAH. In contrast, ICP and CPP 60 minutes after SAH were not correlated with the mortality rate. The vascular circumference was significantly smaller in lethal than in sublethal SAH or sham-operated rats (P < 0.001). Vessel measurements were correlated with both CBF and hemorrhage size (P < 0.01). Extracellular glutamate concentration increased to 600% of baseline after lethal SAH in both hippocampus and cortex and was inversely correlated with CBF (r = 0.9, P < 0.001) but did not increase after sublethal SAH.

CONCLUSION

Acute vasoconstriction after SAH occurs independently of changes in ICP and CPP and is associated with decreased CBF, larger hemorrhage size, persistent elevations of extracellular glutamate, and poor outcome. Acute vasoconstriction seems to contribute directly to ischemic brain injury after SAH. Further evaluations of pharmacological agents with the potential to reverse acute vasoconstriction may increase CBF and improve outcome.

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.

The effects of intraventricular/periventricular blood on cerebral 3',5'-cyclic adenosine monophosphate concentration and cerebrovascular reactivity in newborn pigs

This study investigated the effects of intraventricular/periventricular blood on cerebral cAMP production and cortical cerebrovascular reactivity. Under halothane and N2O anesthesia, 3 mL of either autologous blood or artificial cerebrospinal fluid (CSF) were injected into the left caudate nucleus; volume was adequate to result in extrusion of fluid or blood into the lateral ventricles of 1-2-d-old piglets. Twenty-four hours later, a closed cranial window was implanted over the left parietal cortex. Pial arteriolar responses to vasodilator and vasoconstrictor stimuli were monitored. Before the application of vasoactive agents, cortical periarachioid CSF was collected for cAMP measurement. Pial arteriolar responses to topical application of endothelin-1 (10(-9) and 10(-8) M) and to leukotriene C4 (10(-10) and 10(-9) M) were similar between the two groups. However, pial arteriolar responses to topical application of cAMP-mediated vasodilators, prostaglandin E2 (10(-6) and 10(-5) M), and histamine (10(-6) and 10(-5) M), respectively, were markedly reduced in the blood group when compared with the artificial CSF (control) group. Mean CSF cAMP level in the blood group was significantly lower than the control group (199 +/- 31 versus 1092 +/- 238 fmol/mL, p = 0.0006). We conclude that in newborn pigs intraventricular/periventricular blood results in a marked reduction of CSF cAMP concentration and attenuation of the cerebrovascular responses to cAMP-mediated vasodilators on the cortical surface remote from the site of blood or hematoma.

Impairment of the modulatory role of nitric oxide on the endothelin-1-elicited contraction of cerebral arteries: a pathogenetic factor in cerebral vasospasm after subarachnoid hemorrhage?

OBJECTIVE

Nitric oxide (NO) and endothelin-1 (ET-1) are two endothelium-derived factors probably involved in the pathogenesis of cerebral vasospasm after subarachnoid hemorrhage (SAH). Our aim was twofold, i.e., to ascertain whether endothelial and nonendothelial NO modulates the contractile response of cerebral arteries to ET-1 and to analyze whether this relationship might be impaired after experimental SAH.

METHODS

Rings of middle cerebral artery from goats in the control group and from goats with SAH were set up for isometric tension recordings. SAH was induced 3 days before the experiments by infusion of 10 ml of autologous arterial blood through a catheter previously inserted into the subarachnoid space (basal cistern). In goats in the control group, the response to ET-1 was obtained as follows: 1) in control arteries (unrubbed and nonincubated arteries); 2) in rubbed arteries (arteries in which the endothelium was mechanically removed); 3) during incubation with NG-nitro-L-arginine (L-NOArg) alone or plus L- or D-arginine; and 4) in rubbed arteries plus incubation with L-NOArg. In goats with SAH, that response was obtained in control arteries, rubbed arteries, and during incubation with L-NOArg. Specimens of middle cerebral artery were processed for transmission electron microscopy study.

RESULTS

In goats in the control group, ET-1 elicited concentration-dependent contraction of the middle cerebral artery that was significantly potentiated after endothelium denudation or during incubation with L-NOArg. The latter effect was reversed by L-arginine but not by D-arginine. Combined endothelium denudation and incubation with L-NOArg produced a contractile response to ET-1 significantly higher than that induced by each treatment separately. Hyperreactivity to ET-1 was observed in goats with SAH. Endothelium denudation did not alter the enhanced response to ET-1, but it was further significantly increased after incubation with L-NOArg.

CONCLUSION

These results demonstrate that an ET-1-NO interaction exists in control cerebral arteries in such a way that endothelial and nonendothelial NO partially counteract the contractile response to ET-1 and that although SAH did not modify the effect of nonendothelial NO, the absence of endothelial NO after SAH may contribute to the hyperreactivity of cerebral arteries to ET-1 and, thereby, to the development of cerebral vasospasm.

Characterization of the potassium channels involved in EDHF-mediated relaxation in cerebral arteries

1. In the presence of NG-nitro-L-arginine (L-NOARG, 0.3 mM) and indomethacin (10 microM), the relaxations induced by acetylcholine and the calcium (Ca) ionophore A23187 are considered to be mediated by endothelium-derived hyperpolarizing factor (EDHF) in the guinea-pig basilar artery. 2. Inhibitors of adenosine 5'-triphosphate (ATP)-sensitive potassium (K)-channels (KATP; glibenclamide, 10 microM), voltage-sensitive K-channels (Kv; dendrotoxin-1, 0.1 microM or 4-aminopyridine, 1 mM), small (SKCa; apamin, 0.1 microM) and large (BKCa; iberiotoxin, 0.1 microM) conductance Ca-sensitive K-channels did not affect the L-NOARG/indomethacin-resistant relaxation induced by acetylcholine. 3. Synthetic charybdotoxin (0.1 microM), an inhibitor of BKCa and Kv, caused a rightward shift of the concentration-response curve for acetylcholine and reduced the maximal relaxation in the presence of L-NOARG and indomethacin, whereas the relaxation induced by A23187 was not significantly inhibited. 4. A combination of charybdotoxin (0.1 microM) and apamin (0.1 microM) abolished the L-NOARG/ indomethacin-resistant relaxations induced by acetylcholine and A23187. However, the acetylcholine-induced relaxation was not affected by a combination of iberiotoxin (0.1 microM) and apamin (0.1 microM). 5. Ciclazindol (10 microM), an inhibitor of Kv in rat portal vein smooth muscle, inhibited the L-NOARG/ indomethacin-resistant relaxations induced by acetylcholine and A23187, and the relaxations were abolished when ciclazindol (10 microM) was combined with apamin (0.1 microM). 6. Human pial arteries from two out of four patients displayed an L-NOARG/indomethacin-resistant relaxation in response to substance P. This relaxation was abolished in both cases by pretreatment with the combination of charybdotoxin (0.1 microM) and apamin (0.1 microM), whereas each toxin had little effect alone. 7. The results suggest that Kv, but not KATP and BKCa, is involved in the EDHF-mediated relaxation in the guinea-pig basilar artery. The synergistic action of apamin and charybdotoxin (or ciclazindol) could indicate that both Kv and SKCa are activated by EDHF. However, a single type of K-channel, which may be structurally related to Kv and allosterically regulated by apamin, could also be the target for EDHF.

Modulation by the endothelium of the inhibitory effects of pinacidil and nimodipine on endothelin-induced contraction in cerebral arteries

The effects of pinacidil and nimodipine on endothelin-1-induced contractions in isolated cerebral arteries with and without endothelium were compared. The sensitivity to endothelin-1 was increased (0.5 log units) in the rabbit basilar artery after removal of the endothelium. The nitric oxide synthase inhibitor N omega-nitro-L-arginine (0.1 mM) also increased the sensitivity to endothelin-1 (0.6 log units) in basilar arteries with endothelium, whereas N omega-nitro-D-arginine (0.1 mM) and indomethacin (3 microM) had no effect, indicating that withdrawal of endothelium-derived nitric oxide may account for the enhancement of the endothelin-1-induced contraction after endothelial denudation. Pinacidil (1 microM) shifted the concentration-response curve for endothelin-1 to the right without affecting the maximal response in arteries without endothelium, but had no effect on the endothelin-1-induced contraction in vessels with endothelium. Nimodipine (1 microM) reduced the maximal endothelin-1-induced contraction by approximately 50% in both the presence and absence of endothelium, whereas the sensitivity to endothelin-1 was reduced only in vessels without endothelium. Incubation in "calcium-free" medium reduced the maximal endothelin-1-induced contraction by 69% and 80% in vessels with and without endothelium, respectively. In human pial arteries with endothelium, pinacidil did not affect the endothelin-1-induced contraction, whereas nimodipine and exposure to "calcium-free" solution reduced the maximal response by 31% and 74% respectively. The results show that, in the rabbit, pinacidil and to a lesser extent nimodipine preferentially act on cerebral arteries with disrupted endothelium, indicating that vasoactive factors liberated from the endothelium may modify the effect of a vasodilator.