Postischemic attenuation of cerebral artery reactivity is increased in the presence of tissue plasminogen activator

Review of studies on dynamic cerebral autoregulation in the acute phase of stroke and the relationship with clinical outcome

Acute stroke is associated with high morbidity and mortality. In the last decades, new therapies have been investigated with the aim of improving clinical outcomes in the acute phase post stroke onset. However, despite such advances, a large number of patients do not demonstrate improvement, furthermore, some unfortunately deteriorate. Thus, there is a need for additional treatments targeted to the individual patient. A potential therapeutic target is interventions to optimize cerebral perfusion guided by cerebral hemodynamic parameters such as dynamic cerebral autoregulation (dCA). This narrative led to the development of the INFOMATAS (Identifying New targets FOr Management And Therapy in Acute Stroke) project, designed to foster interventions directed towards understanding and improving hemodynamic aspects of the cerebral circulation in acute cerebrovascular disease states. This comprehensive review aims to summarize relevant studies on assessing dCA in patients suffering acute ischemic stroke, intracerebral haemorrhage, and subarachnoid haemorrhage. The review will provide to the reader the most consistent findings, the inconsistent findings which still need to be explored further and discuss the main limitations of these studies. This will allow for the creation of a research agenda for the use of bedside dCA information for prognostication and targeted perfusion interventions.

Circulating tPA contributes to neurovascular coupling by a mechanism involving the endothelial NMDA receptors

The increase of cerebral blood flow evoked by neuronal activity is essential to ensure enough energy supply to the brain. In the neurovascular unit, endothelial cells are ideally placed to regulate key neurovascular functions of the brain. Nevertheless, some outstanding questions remain about their exact role neurovascular coupling (NVC). Here, we postulated that the tissue-type plasminogen activator (tPA) present in the circulation might contribute to NVC by a mechanism dependent of its interaction with endothelial N-Methyl-D-Aspartate Receptor (NMDAR). To address this question, we used pharmacological and genetic approaches to interfere with vascular tPA-dependent NMDAR signaling, combined with laser speckle flowmetry, intravital microscopy and ultrafast functional ultrasound in vivo imaging. We found that the tPA present in the blood circulation is capable of potentiating the cerebral blood flow increase induced by the activation of the mouse somatosensorial cortex, and that this effect is mediated by a tPA-dependent activation of NMDAR expressed at the luminal part of endothelial cells of arteries. Although blood molecules, such as acetylcholine, bradykinin or ATP are known to regulate vascular tone and induce vessel dilation, our present data provide the first evidence that circulating tPA is capable of influencing neurovascular coupling (NVC).

Reperfusion Changes After Stroke and Practical Approaches for Neuroprotection

Reperfusion is the first line of care in a growing number of eligible acute ischemic stroke patients. Early reperfusion with thrombolytic drugs and endovascular mechanical devices is associated with improved outcome and lower mortality rates compared with natural history. Reperfusion is not without risk, however, and may result in reperfusion injury, which manifests in hemorrhagic transformation, brain edema, infarct progression, and neurologic worsening. In this article, the functional and structural changes and underlying molecular mechanisms of ischemia and reperfusion are reviewed. The pathways that lead to reperfusion injury and novel neuroprotective strategies with endogenous properties are discussed.

Improved Reperfusion and Vasculoprotection by the Poly(ADP-Ribose)Polymerase Inhibitor PJ34 After Stroke and Thrombolysis in Mice

Benefits from thrombolysis with recombinant tissue plasminogen activator (rt-PA) after ischemic stroke remain limited due to a narrow therapeutic window, low reperfusion rates, and increased risk of hemorrhagic transformations (HT). Experimental data showed that rt-PA enhances the post-ischemic activation of poly(ADP-ribose)polymerase (PARP) which in turn contributes to blood-brain barrier injury. The aim of the present study was to evaluate whether PJ34, a potent PARP inhibitor, improves poor reperfusion induced by delayed rt-PA administration, exerts vasculoprotective effects, and finally increases the therapeutic window of rt-PA. Stroke was induced by thrombin injection (0.75 UI in 1 μl) in the left middle cerebral artery (MCA) of male Swiss mice. Administration of rt-PA (0.9 mg kg^-1) or saline was delayed for 4 h after ischemia onset. Saline or PJ34 (3 mg kg^-1) was given intraperitoneally twice, just after thrombin injection and 3 h later, or once, 3 h after ischemia onset. Reperfusion was evaluated by laser Doppler, vascular inflammation by immunohistochemistry of vascular cell adhesion molecule-1 (VCAM-1) expression, and vasospasm by morphometric measurement of the MCA. Edema, cortical lesion, and sensorimotor deficit were evaluated. Treatment with PJ34 improved rt-PA-induced reperfusion and promoted vascular protection including reduction in vascular inflammation (decrease in VCAM-1 expression), HT, and MCA vasospasm. Additionally, the combined treatment significantly reduced brain edema, cortical lesion, and sensorimotor deficit. In conclusion, the combination of the PARP inhibitor PJ34 with rt-PA after cerebral ischemia may be of particular interest in order to improve thrombolysis with an extended therapeutic window.

tPA Modulation of the Blood-Brain Barrier: A Unifying Explanation for the Pleiotropic Effects of tPA in the CNS

The plasminogen activation (PA) system is best known for its role in fibrinolysis. However, it has also been shown to regulate many nonfibrinolytic functions in the central nervous system (CNS). In particular, tissue-type plasminogen activator (tPA) is reported to have pleiotropic activities in the CNS, regulating events such as neuronal plasticity, excitotoxicity, and cerebrovascular barrier integrity, whereas urokinase-type plasminogen activator is mainly associated with tissue remodeling and cell migration. It has been suggested that the role tPA plays in controlling barrier integrity may provide a unifying mechanism for the reported diverse, and often opposing, functions ascribed to tPA in the CNS. Here we will review the possibility that the pleiotropic effects reported for tPA in physiologic and pathologic processes in the CNS may be a consequence of its role in the neurovascular unit in regulation of cerebrovascular responses and subsequently parenchymal homeostasis. We propose that this might offer an explanation for the ongoing debate regarding the neurotoxic versus neuroprotective roles of tPA.

Cerebral Vascular Disease and Neurovascular Injury in Ischemic Stroke

The consequences of cerebrovascular disease are among the leading health issues worldwide. Large and small cerebral vessel disease can trigger stroke and contribute to the vascular component of other forms of neurological dysfunction and degeneration. Both forms of vascular disease are driven by diverse risk factors, with hypertension as the leading contributor. Despite the importance of neurovascular disease and subsequent injury after ischemic events, fundamental knowledge in these areas lag behind our current understanding of neuroprotection and vascular biology in general. The goal of this review is to address select key structural and functional changes in the vasculature that promote hypoperfusion and ischemia, while also affecting the extent of injury and effectiveness of therapy. In addition, as damage to the blood-brain barrier is one of the major consequences of ischemia, we discuss cellular and molecular mechanisms underlying ischemia-induced changes in blood-brain barrier integrity and function, including alterations in endothelial cells and the contribution of pericytes, immune cells, and matrix metalloproteinases. Identification of cell types, pathways, and molecules that control vascular changes before and after ischemia may result in novel approaches to slow the progression of cerebrovascular disease and lessen both the frequency and impact of ischemic events.

Effects of Early Post-Ischemic Reperfusion and tPA on Cerebrovascular Function and Nitrosative Stress in Female Rats

Stroke is a major health issue in women. Our previous studies in male rats showed decreased myogenic tone in middle cerebral arteries (MCAs) after ischemia and reperfusion (I/R), while tone in parenchymal arterioles (PAs) was increased. This vascular response may aggravate stroke damage in males by limiting reperfusion; however, the effect in females is not known. The current study investigated the effect of I/R and tissue plasminogen activator (tPA) on myogenic tone and reactivity of MCAs and PAs in female rats. Nitrosative stress by peroxynitrite and recruitment of inflammatory neutrophils to the microvasculature were also studied. Female rats were subjected to 2-h MCA filament occlusion (n = 16) or sham surgery (n = 17) and given tPA (1 mg/kg, i.v) or vehicle followed by 30-min reperfusion. Myogenic tone and reactivity were measured in isolated and pressurized MCAs and PAs from the same animals. Cerebrovascular F-actin, 3-nitrotyrosine (3-NT, peroxynitrite marker), and intravascular neutrophils were quantified. Myogenic tone and constriction to the nitric oxide synthase inhibitor Nω-nitro-L-arginine were decreased in MCAs but unchanged in PAs after I/R with no effect of tPA. F-actin and 3-NT expression were unaffected by I/R or tPA. Our study showed that MCAs from females, similar to what has been seen in males, are dilated after I/R and have decreased myogenic tone while tone in PAs was unchanged. Increased small vessel resistance may contribute to decreased reperfusion and worse outcome after stroke.

Sustained mitochondrial functioning in cerebral arteries after transient ischemic stress in the rat: a potential target for therapies

The objective of the present study was to determine whether mitochondrial function in the cerebral vasculature is maintained after transient middle cerebral artery (MCA) occlusion (tMCAO) in rats. Sprague-Dawley rats were exposed to 90 min of tMCAO followed by 4 or 48 h of reperfusion. MCAs from ischemic (ipsilateral) and nonischemic (contralateral) sides were compared with control MCAs from sham-operated rats. We determined 1) vasoreactivity to diazoxide (DZ; a mitochondrial ATP-activated K(+) channel opener), ACh, bradykinin (BK), serotonin, and sodium nitroprusside; 2) levels of mitochondrial and nonmitochondrial proteins and mitochondrial DNA; and 3) vascular levels of tetramethylrhodamine ethyl ester (an indicator of mitochondrial membrane potential). All dilator responses, including those with DZ, were intact 4 h post-tMCAO. Dilator responses to ACh, BK, and sodium nitroprusside were reduced in ipsilateral MCAs at 48 h compared with contralateral MCAs, but DZ responses were comparable with control MCAs. Surprisingly, contralateral responses to ACh, BK, and serotonin were reduced compared with control MCAs at 48 h. Ipsilateral vasodilation to DZ at 48 h was eliminated by endothelial denudation and endothelial nitric oxide synthase (eNOS) inhibition but was only reduced in control MCAs. Mitochondrial proteins, phosphorylated eNOS, mitochondrial DNA, and mitochondrial membrane potential were higher in ipsilateral compared with contralateral MCAs. In conclusion, contrary to conventional wisdom, mitochondria remain functional for at least 48 h after severe ischemic stress in MCAs, and DZ-induced dilation is preserved due to maintained mitochondrial mass, probably in the endothelium, and eNOS signaling. Our findings support the concept that functioning vascular mitochondria are an unexpected target for novel stroke therapies.

Hyperglycemia, acute ischemic stroke, and thrombolytic therapy

Ischemic stroke is a leading cause of disability and is considered now the fourth leading cause of death. Many clinical trials have shown that stroke patients with acute elevation in blood glucose at onset of stroke suffer worse functional outcomes, longer in-hospital stay, and higher mortality rates. The only therapeutic hope for these patients is the rapid restoration of blood flow to the ischemic tissue through intravenous administration of the only currently proven effective therapy, tissue plasminogen activator (tPA). However, even this option is associated with the increased risk of intracerebral hemorrhage. Nonetheless, the underlying mechanisms through which hyperglycemia (HG) and tPA worsen the neurovascular injury after stroke are not fully understood. Accordingly, this review summarizes the latest updates and recommendations about the management of HG and coadministration of tPA in a clinical setting while focusing more on the various experimental models studying (1) the effect of HG on stroke outcomes, (2) the potential mechanisms involved in worsening the neurovascular injury, (3) the different therapeutic strategies employed to ameliorate the injury, and finally, (4) the interaction between HG and tPA. Developing therapeutic strategies to reduce the hemorrhage risk with tPA in hyperglycemic setting is of great clinical importance. This can best be achieved by conducting robust preclinical studies evaluating the interaction between tPA and other therapeutics in order to develop potential therapeutic strategies with high translational impact.

Alteplase treatment does not increase brain injury after mechanical middle cerebral artery occlusion in the rat

Recanalization of an occluded vessel with recombinant tissue plasminogen activator is an effective strategy for treating acute ischemic stroke. Recombinant tissue plasminogen activator is administered as alteplase, a formulation containing many excipients including L-arginine, the substrate for nitric oxide production. Most studies fail to compare the effects of alteplase on brain injury to its L-arginine carrier solution. This study aimed to verify the previously reported detrimental effects of alteplase after cerebral ischemia and delineate the contribution of L-arginine. Male Wistar rats, subjected to 90 minutes of intraluminal middle cerebral artery occlusion (MCAO), were administered alteplase, the carrier solution or saline upon reperfusion. Neither alteplase nor the carrier affected cerebral blood flow (CBF) restoration throughout the first 60 minutes of reperfusion. Alteplase treatment was associated with increased mortality after MCAO. Twenty-four hours after MCAO, neurologic function and infarct volume did not differ between rats treated with alteplase, the carrier solution, or saline. Irrespective of treatment group, infarct volume was correlated with CBF during reperfusion, neuroscore, and peri-infarct depolarizations. These results suggest that alteplase treatment, independent of thrombolysis, does not cause increased ischemic injury compared with its appropriate carrier solution, supporting the continued use of alteplase in eligible ischemic stroke patients.

Transient focal cerebral ischemia induces long-term cerebral vasculature dysfunction in a rodent experimental stroke model

Constriction and dilation of large arteries of brain regulates cerebral vascular resistance and cerebral microvascular pressure, which play key roles in regulation of cerebral circulation. We investigated the effect of ischemic stroke on vascular reactivity of middle cerebral artery (MCA) using a rat transient focal cerebral ischemia model. Focal cerebral ischemia was induced by 1 hour MCA occlusion followed by reperfusion. MCAs were dissected from ischemic or contralateral hemisphere at 2 days or 2 weeks post reperfusion and mounted on 2 glass micropipettes for assessment of vascular reactivity. MCAs from brains of sham surgeries were used as control. At 2 days post reperfusion, a significant alteration of myogenic reactivity was found in MCAs dissected from both ischemic and non-ischemic hemispheres, which could still be identified at 2 weeks after reperfusion. Phenylephrine (PE) induced remarkable vasoconstriction in MCAs from animals that underwent sham surgery. No significant alteration of vasoconstrictive response to PE was found in MCAs isolated from either ischemic or contralateral hemisphere at 2 days or 2 weeks after ischemic stroke, as compared with MCAs from sham animals. Acetylcholine (ACh) induced mild dilation in normal MCAs, which was reversed in MCAs from both ischemic and non-ischemic hemispheres at 2 weeks after ischemic stroke. Sodium nitroprusside (SNP) induced vasodilation in MCAs from animals with sham operation, which was diminished in MCAs from both ischemic and non-ischemic hemisphere at 2 days and 2 weeks after ischemic stroke. These results demonstrated that focal cerebral ischemia could induce long-term global cerebral vasculature dysfunction.

Treatment with tPA Predicts Better Outcome Even if MCA Occlusion Persists

Background and hypothesis

Functional improvement after middle cerebral artery ischaemia seems to depend on recanalization of large-vessel occlusion as early as possible. The only approved medical treatment for acute stroke is early IV tissue plasminogen activator administration. However, while some patients do not benefit from quick recanalization, others recover despite persistent middle cerebral artery occlusion. We wondered whether there are different effects of tissue plasminogen activator treatment on large artery and small artery reopening.

Methods

We enrolled 55 acute stroke patients who showed persisting middle cerebral artery occlusion evidenced by transcranial colour-coded duplex ultrasonography in follow-up examination within 48 h postonset of middle cerebral artery stroke syndromes (mean 30·8 ± 5·4 h after admission). Twenty-two of 55 had been treated with tissue plasminogen activator and 33/55 had been treated without tissue plasminogen activator. We compared neurological (National Institutes of Health Stroke Scale) and functional (modified Rankin Scale) scores at baseline, after seven-days, and then after two-months. Risk factors, previous stroke prophylaxis, as well as clinical baseline characteristics were analysed to exclude significant differences between both groups.

Results

Despite later admission to hospital (tissue plasminogen activator patients 1·6 ± 0·66 h vs. non-tissue plasminogen activator patients 7·4 ± 5·84 h; P < 0·001), there was no significant difference between both groups concerning demographic data, severity of symptoms on admission, risk factors, stroke prophylaxis, as well as basic laboratory values (international normalized ratio, leucocyte count, C-reactive protein) blood pressure and body temperature on admission. Irrespective of Doppler findings demonstrating persistent middle cerebral artery occlusion in all 55 patients, there was a significant neurological and functional improvement in tissue plasminogen activator patients compared to non-tissue plasminogen activator patients. Tissue plasminogen activator patients had a mean improvement on National Institutes of Health Stroke Scale within the first seven-days of 2·8 points, while non-tissue plasminogen activator patients deteriorated by 2·2 points ( P < 0·001). Concerning modified Rankin Scale tissue plasminogen activator-treated patients showed a mean improvement within the first seven-days of 0·5 points, while non-tissue plasminogen activator patients deteriorated by 0·3 points ( P = 0·019). A favourable overall short-term clinical course (i.e. improvement on National Institutes of Health Stroke Scale >3 points and/or modified Rankin Scale >1 point) was found in 36·4% of tissue plasminogen activator patients and in 6·1% of non-tissue plasminogen activator patients ( P = 0·0047). At two-months follow-up, patients still showed a median modified Rankin Scale of 4 points after tissue plasminogen activator treatment and 5 points after non-tissue plasminogen activator treatment ( P = 0·023).

Conclusion

Although the prognosis of patients with persisting middle cerebral artery occlusion after tissue plasminogen activator administration is known to be poor, patients do better if treated with tissue plasminogen activator vs. those who could not be treated – mainly for late presentation. This may be due to sufficient small vascular territory recanalization despite persistence of large artery occlusion after tissue plasminogen activator treatment.

Cerebral blood flow alteration in neuroprotection following cerebral ischaemia

The best neuroprotectant for acute ischaemic stroke would always be the rapid return of oxygen and glucose to physiological levels. This is currently provided by thrombolysis which restores blood flow to the ischaemic region. The attempt to confer neuroprotection by targeting the brain parenchyma has shown promise in experimental stroke models, but has unequivocally failed to translate to the clinic. Neuroprotective therapy primarily targets the biochemical cascade that produces cell death following cerebral ischaemia. However, these agents may also alter signal transduction that controls cerebral blood flow, for example glutamate, which may affect the outcome after ischaemia. In these cases, neuroprotection may potentially be due to the improved access to oxygen and glucose rather than biochemical prevention of cell death. Improvement in cerebral blood flow is an important but often overlooked effect of neuroprotective therapy, analogous to the protective effects of drug-induced hypothermia. This short review will discuss cerebral blood flow alteration and protection of the brain in the context of ischaemic preconditioning, oxygen sensing and thrombolysis. Future neuroprotection studies in cerebral ischaemia require stringent monitoring of cerebral blood flow, plus other physiological parameters. This will increase the chances that any protection observed may be able to translate to human therapy.

Vascular Protection Following Cerebral Ischemia and Reperfusion

Despite considerable research that has contributed to a better understanding of the pathophysiology of stroke, translation of this knowledge into effective therapies has largely failed. The only effective treatment for ischemic stroke is rapid recanalization of an occluded vessel by dissolving the clot with tissue plasminogen activator (tPA). However, stroke adversely affects vascular function as well that can cause secondary brain injury and limit treatment that depends on a patent vasculature. In middle cerebral arteries (MCA), ischemia/reperfusion (I/R) cause loss of myogenic tone, vascular paralysis, and endothelial dysfunction that can lead to loss of autoregulation. In contrast, brain parenchymal arterioles retain considerable tone during I/R that likely contributes to expansion of the infarct into the penumbra. Microvascular dysregulation also occurs during ischemic stroke that causes edema and hemorrhage, exacerbating the primary insult. Ischemic injury of vasculature is progressive with longer duration of I/R. Early postischemic reperfusion has beneficial effects on stroke outcome but can impair vascular function and exacerbate ischemic injury after longer durations of I/R. This review focuses on current knowledge on the effects of I/R on the structure and function of different vascular segments in the brain and highlight some of the more promising targets for vascular protection.

Glial and neuronal control of brain blood flow

Blood flow in the brain is regulated by neurons and astrocytes. Knowledge of how these cells control blood flow is crucial for understanding how neural computation is powered, for interpreting functional imaging scans of brains, and for developing treatments for neurological disorders. It is now recognized that neurotransmitter-mediated signalling has a key role in regulating cerebral blood flow, that much of this control is mediated by astrocytes, that oxygen modulates blood flow regulation, and that blood flow may be controlled by capillaries as well as by arterioles. These conceptual shifts in our understanding of cerebral blood flow control have important implications for the development of new therapeutic approaches.

The contribution of L-arginine to the neurotoxicity of recombinant tissue plasminogen activator following cerebral ischemia: a review of rtPA neurotoxicity

Alteplase is the only drug licensed for acute ischemic stroke, and in this formulation, the thrombolytic agent recombinant tissue plasminogen activator (rtPA) is stabilized in a solution of L-arginine. Improved functional outcomes after alteplase administration have been shown in clinical trials, along with improved histological and behavioral measures in experimental models of embolic stroke. However, in animal models of mechanically induced ischemia, alteplase can exacerbate ischemic damage. We have systematically reviewed the literature of both rtPA and L-arginine administration in mechanical focal ischemia. The rtPA worsens ischemic damage under certain conditions, whereas L-arginine can have both beneficial and deleterious effects dependent on the time of administration. The interaction between rtPA and L-arginine may be leading to the production of nitric oxide, which can cause direct neurotoxicity, altered cerebral blood flow, and disruption of the neurovascular unit. We suggest that alternative formulations of rtPA, in the absence of L-arginine, would provide new insight into rtPA neurotoxicity, and have the potential to offer more efficacious thrombolytic therapy for ischemic stroke patients.

Decreased vasoconstrictor responses in remote cerebral arteries after focal brain ischemia and reperfusion in the rat, in vitro

The effects of brain ischemia and reperfusion on smooth muscle function in remote cerebral and peripheral arteries are hardly known. Maximum vasoconstrictions (E(max)) caused by 120mmol/l KCl and 5-HT in endothelium-denuded ring preparations were measured in ischemic and control cerebral arteries of rats after a 1-h right middle cerebral artery occlusion followed by 0-min (I/NR) or 2-3-min (I/SR) reperfusion, and in peripheral arteries after I/SR. Surprisingly, vasoconstrictions to 5-HT and 120mmol/lK(+) were attenuated in remote brain vessels after I/SR, i.e. in the contralateral middle cerebral artery and the basilar artery, while I/NR depressed E(max) of 5-HT and high KCl only in the ischemic middle cerebral artery. Pretreatment with N-(2-mercaptopropionyl) glycine (MPG, 100mg/kg i.p.), a free radical scavenger, fully prevented the impairment of vasomotor function in the middle cerebral artery on both sides after I/SR. Moreover, vasomotor functions were normal in the coronary, renal and pulmonary arteries after I/SR. In conclusion, focal cerebral ischemia and reperfusion impaired vasoconstrictor responses in remote brain arteries of rats by a mechanism involving free radicals. The lack of similar effects in peripheral vessels indicates poor defence of brain arteries against remote injury caused by reactive oxygen species-dependent mechanisms.

Tissue plasminogen activator-mediated PDGF signaling and neurovascular coupling in stroke

The use of tissue plasminogen activator (tPA) as a thrombolytic treatment in ischemic stroke is limited largely due to concerns for hemorrhagic complications. The underlying mechanisms are still unknown, but evidence is beginning to emerge that tPA interacts with key regulators of the neurovascular unit (NVU), and that these interactions may contribute to the undesirable side effects associated with the use of tPA in ischemic stroke. Understanding these connections and tPA's normal function within the NVU may offer new insights into future therapeutic approaches.

Diet-induced obesity causes cerebral vessel remodeling and increases the damage caused by ischemic stroke

Hypertension, elevated fasting blood glucose and plasma insulin develop in rats fed a high fat (HF) diet. Our goal was to assess the effects of obesity, beginning in childhood, on the adult cardiovascular system. We hypothesized that rats fed a HF diet would have larger ischemic cerebral infarcts and middle cerebral artery (MCA) remodeling. Three-week-old male Sprague Dawley rats were fed a HF (obese) or control diet for 10 weeks. Cerebral ischemia was induced by MCA occlusion (MCAO). MCA structure was assessed by pressure myography and cerebral vessel matrix metalloproteinase (MMP) activity and expression and collagen levels were measured in vessels from rats that did not undergo MCAO. The cerebral infarct was greater in the obese rats than the control (46.0+/-2.1 vs 28.0+/-7.5% of the hemisphere infarcted, obese vs control p<0.05). The MCAs from obese rats had smaller lumens (232+/-7.2 vs 254+/-7.8 microm obese vs control p<0.05) and thicker walls (19.6+/-0.8 vs 17.8+/-0.9 microm obese vs control p<0.05) and were less compliant than MCAs from control rats. MMP-2 activity and collagen I expression were increased in vessels from obese rats and MMP-13 expression was reduced. These results suggest that obesity, beginning in childhood, causes inward vessel remodeling with a concomitant increase in vessel stiffness due to increased collagen deposition. These changes in MCA structure may be responsible for the increase in the ischemic damage after MCAO.

Neutrophils contribute to intracerebral haemorrhages after treatment with recombinant tissue plasminogen activator following cerebral ischaemia

BACKGROUND AND PURPOSE

Polymorphonuclear neutrophils (PMNs) contribute to the vascular damage caused by transient cerebral ischaemia. Here we have evaluated the role of PMNs in intracerebral haemorrhage (ICH) induced in a model of thrombolysis with recombinant tissue plasminogen activator (t-PA) during the acute phase of cerebral ischaemia.

EXPERIMENTAL APPROACH

The middle cerebral artery (MCA) of male spontaneously hypertensive rats was occluded for 1 h followed by reperfusion and, 5 h later, infusion of thrombolytic products (generated in vitro by t-PA on autologous clots). Effects of pretreatment (before the MCA occlusion) with vinblastine (4 days before; 0.5 mg.kg(-1)), monoclonal anti-neutrophil antibody (mAbRP3; 12 h, 0.3 mg.kg(-1)) or saline on ICH, neutrophil infiltration, MCA vascular reactivity and brain infarct volume were assessed, 24 h after the beginning of reperfusion.

KEY RESULTS

Depletion of circulating neutrophils significantly reduced t-PA-induced ICH (vinblastine, 4.6 +/- 1.0; mAbRP3, 5.2 +/- 1.0 vs. saline, 10.8 +/- 2.7 haemorrhages; P < 0.05). This depletion was associated with a decrease in cerebral infiltration by neutrophils and a decrease of endothelium-dependent, vascular dysfunction in isolated MCA, induced by the ischaemia/reperfusion and t-PA treatment. Brain infarct volume was significantly decreased after vinblastine treatment (159 +/- 13 mm(3) vs. 243 +/- 16 mm(3) with saline; P < 0.01) but not after depletion with mAbRP3 (221 +/- 22 mm(3)).

CONCLUSIONS AND IMPLICATIONS

Our results showed that pharmacological depletion of PMNs prevented t-PA-induced ICH, in parallel with a decrease in cerebral infiltration by PMNs and a decreased endothelial dysfunction in cerebral blood vessels.

Poststroke neuronal rescue and synaptogenesis mediated in vivo by protein kinase C in adult brains

Global cerebral ischemia/hypoxia, as can occur during human stroke, damages brain neural networks and synaptic functions. The recently demonstrated protein kinase C (PKC) activation-induced synaptogenesis in rat hippocampus suggested the potential of PKC-mediated antiapoptosis and synaptogenesis during conditions of neurodegeneration. Consequently, we examined the effects of chronic bryostatin-1, a PKC activator, on the cerebral ischemia/hypoxia-induced impairment of synapses and neurotrophic activity in the hippocampal CA1 area and on hippocampus-dependent spatial learning and memory. Postischemic/hypoxic bryostatin-1 treatment effectively rescued ischemia-induced deficits in synaptogenesis, neurotrophic activity, and spatial learning and memory. These results highlight a neuroprotective signaling pathway, as well as a therapeutic strategy with an extended time window for reducing brain damage due to stroke by activating particular PKC isozymes.

Key role of tissue plasminogen activator in neurovascular coupling

The increase in blood flow evoked by synaptic activity is essential for normal brain function and underlies functional brain imaging signals. Nitric oxide, a vasodilator released by NMDA receptor activation, is critical for the flow increase, but the factors linking NMDA receptor activity to nitric oxide-dependent hyperemia are poorly understood. Here, we show that tissue plasminogen activator (tPA), a serine protease implicated in NMDA receptor signaling, is required for the flow increase evoked by somatosensory stimulation. tPA acts by facilitating neuronal nitric oxide release, but this effect does not involve enhancement of NMDA currents or the associated intracellular Ca(2+) rise. Rather, the evidence suggests that tPA controls NMDA-dependent nitric oxide synthesis by influencing the phosphorylation state of neuronal nitric oxide synthase. These findings unveil a previously unrecognized role of tPA in vital homeostatic mechanisms coupling NMDA receptor signaling with nitric oxide synthesis and local cerebral perfusion.

Intact female stroke-prone hypertensive rats lack responsiveness to mineralocorticoid receptor antagonists

Data from the Framingham Heart Study suggest that women may be more sensitive to the deleterious cardiovascular remodeling effects of aldosterone. Previous studies from our laboratory have shown that chronic treatment with spironolactone, a mineralocorticoid receptor (MR) antagonist, decreases ischemic cerebral infarct size and prevents remodeling of the middle cerebral artery (MCA) in male spontaneously hypertensive stroke-prone rats (SHRSP). Therefore, we hypothesized that MR antagonism would reduce ischemic infarct size and prevent MCA remodeling in female SHRSP. Six-week-old female SHRSP were treated for 6 wk with spironolactone (25 or 50 mg.kg(-1).day(-1)) or eplerenone (100 mg.kg(-1).day(-1)) and compared with untreated controls. At 12 wk, cerebral ischemia was induced for 18 h using the intraluminal suture occlusion technique, or the MCA was isolated for analysis of passive structure using a pressurized arteriograph. MR antagonism had no effect on infarct size or passive MCA structure in female SHRSP. To study the potential effects of estrogen, the above experiments were repeated in bilaterally ovariectomized (OVX) female SHRSP treated with spironolactone (25 mg.kg(-1).day(-1)). Infarct size and vessel structure in OVX SHRSP were not different from control SHRSP. Spironolactone had no effect on infarct size in OVX SHRSP. However, MCA lumen and outer diameters were increased in spironolactone-treated OVX SHRSP, suggesting an effect of estrogen. Cerebral artery MR expression, assessed by Western blotting, was increased in female, compared with male, SHRSP. These studies highlight an apparent sexual dimorphism of MR expression and activity in the cerebral vasculature from hypertensive rats.

Spironolactone improves structure and increases tone in the cerebral vasculature of male spontaneously hypertensive stroke-prone rats

BACKGROUND

Previous studies show that ischemic cerebral infarct size is related to cerebral vessel structure. Spironolactone, a mineralocorticoid receptor antagonist, decreases ischemic cerebral infarct size in male spontaneously hypertensive stroke-prone rats (SHRSP). Therefore, we hypothesized that chronic spironolactone treatment would improve cerebral artery structure in the SHRSP.

METHODS

Six-week-old male SHRSP were treated with spironolactone (2.5 mg/day) for 6 weeks and were compared to untreated control SHRSP and normotensive Wistar Kyoto (WKY) rats. Using a pressurized arteriograph, structural measurements of the middle cerebral artery (MCA) were taken under passive (calcium-free), zero-flow conditions. Myogenic tone was calculated from active and passive measurements taken at 75 and 125 mmHg. Mean arterial pressure was measured using radiotelemetry.

RESULTS

Myogenic tone was increased only at 75 mmHg in the spironolactone-treated SHRSP compared to control rats. The MCA lumen and outer diameters were increased in the spironolactone-treated SHRSP compared to control SHRSP, but were not different from WKY rats, indicating a decrease in vascular remodeling. There was no effect of spironolactone on blood pressure, suggesting that this is a blood pressure-independent effect.

CONCLUSION

Increased myogenic tone and lumen diameter in the spironolactone-treated SHRSP may be responsible for the protective role of spironolactone in ischemic stroke.

The plasminogen activator system modulates sympathetic nerve function

Sympathetic neurons synthesize and release tissue plasminogen activator (t-PA). We investigated whether t-PA modulates sympathetic activity. t-PA inhibition markedly reduced contraction of the guinea pig vas deferens to electrical field stimulation (EFS) and norepinephrine (NE) exocytosis from cardiac synaptosomes. Recombinant t-PA (rt-PA) induced exocytotic and carrier-mediated NE release from cardiac synaptosomes and cultured neuroblastoma cells; this was a plasmin-independent effect but was potentiated by a fibrinogen cleavage product. Notably, hearts from t-PA–null mice released much less NE upon EFS than their wild-type (WT) controls (i.e., a 76.5% decrease; P < 0.01), whereas hearts from plasminogen activator inhibitor-1 (PAI-1)–null mice released much more NE (i.e., a 275% increase; P < 0.05). Furthermore, vasa deferentia from t-PA–null mice were hyporesponsive to EFS (P < 0.0001) but were normalized by the addition of rt-PA. In contrast, vasa from PAI-1–null mice were much more responsive (P < 0.05). Coronary NE overflow from hearts subjected to ischemia/reperfusion was much smaller in t-PA–null than in WT control mice (P < 0.01). Furthermore, reperfusion arrhythmias were significantly reduced (P < 0.05) in t-PA–null hearts. Thus, t-PA enhances NE release from sympathetic nerves and contributes to cardiac arrhythmias in ischemia/reperfusion. Because the risk of arrhythmias and sudden cardiac death is increased in hyperadrenergic conditions, targeting the NE-releasing effect of t-PA may have valuable therapeutic potential.

Estrogen restores postischemic sensitivity to the thromboxane mimetic U46619 in rat pial artery

The objectives of the study were to (1) characterize the dose-response relationship to the TXA2 analog, U46619 (0.01, 0.1, and 1 micromol/L) after global cerebral ischemia, (2) determine whether chronic 17beta-estradiol (E2) replacement alters this relationship, and (3) determine if E2's mechanisms are transduced through cognate estrogen receptors. Rats were assigned to five groups (n=6): placebo-implanted ovariectomized (OVX) females, OVX plus chronic E2 (CE), OVX plus acute E2 (AE), OVX plus chronic E2 plus the estrogen receptor inhibitor ICI 182,780 (CEI), and OVX plus acute E2 plus ICI 182,780 (AEI). Rats were anesthetized, intubated, cannulated (femoral artery and vein), fitted with a closed cranial window, and subjected to 15-min reversible forebrain ischemia (4-vessel occlusion, 4-VO) and 60 mins of reperfusion. Arterial blood gases, intrawindow pressure, and temperature were controlled. Vessel diameter was measured before and 5 mins after superfusion of each concentration of U46619. Compared with preischemic responses, contractile response to U46619 was depressed at all concentrations after ischemia in the OVX group. In the chronic E2 and acute E2 groups, contractile response to 1 micromol/L of U46619 was normalized to near baseline values. However, in the CEI and the AEI groups, postischemic vasoconstriction was similar to that observed in the OVX rats. We conclude that E2 targets the cerebral microvasculature to preserve postischemic pial artery reactivity and that the effect is receptor mediated. Restoration of normal constriction to vascular agonists may be an important mechanism by which E2 protects the vasculature and diminishes tissue damage after ischemia.

Extending the Time Window for Thrombolysis: Evidence from Acute Stroke Trials

Data from intravenous tissue plasminogen activator studies have shown rapidly diminishing clinical benefit beyond 3 hours when noncontrast CT is used for treatment triage. Newer trials, such as the Desmoteplase in Acute Ischemic Stroke trial, have now successfully pushed the time window out to 9 hours using the concept of penumbral imaging and treatment of the perfusion-diffusion mismatch. Advanced imaging with CT or MR imaging protocols is providing a means for rational physiologic selection and outcomes assessment in stroke treatment.

Plasminogen activators contribute to age-dependent impairment of NMDA cerebrovasodilation after brain injury

Previous studies have observed that fluid percussion brain injury (FPI) impaired NMDA induced pial artery dilation in an age-dependent manner. This study was designed to investigate the contribution of plasminogen activators to impaired NMDA dilation after FPI in newborn and juvenile pigs equipped with a closed cranial window. In the newborn pig, NMDA (10(-8), 10(-6) M) induced pial artery dilation was reversed to vasoconstriction following FPI, but pretreatment with the plasminogen activator inhibitor PAI-1 derived hexapeptide (EEIIMD) (10(-7) M) prevented post injury vasoconstriction (9 +/- 1 and 16 +/- 1, vs. -6 +/- 2 and-11 +/- 3, vs. 5 +/- 1 and 9 +/- 1% for responses to NMDA 10(-8), 10(-6) M prior to FPI, after FPI, and after FPI in EEIIMD pretreated animals, respectively). In contrast, in the juvenile pig, NMDA dilation was only attenuated following FPI and EEIIMD pretreatment partially prevented such inhibition (9 +/- 1 and 16 +/- 1 vs. 2 +/- 1 and 4 +/- 1 vs. 5 +/- 1 and 7 +/- 1% for responses to NMDA prior to FPI, after FPI, and after FPI in EEIIMD pretreated animals, respectively). Additionally, EEIIMD blunted age-dependent pial artery vasoconstriction following FPI. EEIIMD blocked dilation to the plasminogen activator agonists uPA and tPA while responses to SNP and papaverine were unchanged. Pretreatment with suPAR, which blocked dilation to uPA, elicited effects on pial artery diameter and NMDA vascular activity post FPI similar to that observed with EEIIMD. These data show that EEIIMD and suPAR partially prevented FPI induced alterations in NMDA dilation and reductions in pial artery diameter. EEIIMD and suPAR are efficacious and selective inhibitors of plasminogen activator induced dilation. These data suggest that plasminogen activators contribute to age-dependent impairment of NMDA induced dilation following FPI.

Altered NO function contributes to impairment of uPA and tPA cerebrovasodilation after brain injury

Urokinase (uPA) and tissue plasminogen activator (tPA) are serine proteases implicated in fibrinolysis, but their role in the regulation of the cerebrovascular response to brain trauma has not been investigated. This study was designed to (1) characterize the cerebrovascular activity of uPA and tPA, (2) investigate the role of nitric oxide (NO) in uPA and tPA vascular activity, and (3) characterize the effect of fluid percussion brain injury (FPI) on vascular responses to uPA and tPA. The closed cranial window technique in chloralose anesthetized newborn pigs was used to measure pial artery diameter and collect CSF for radioimmunoassay (RIA) of cGMP concentration. Topical uPA (10(-9), 10(-7) M) elicited pial artery dilation that was blunted by the NO synthase inhibitor, L-NNA (10(-6) M) (8 +/- 1% and 13 +/- 1 vs. 3 +/- 1% and 7 +/- 2%, respectively). Vasodilation in response to uPA was associated with an increase in CSF cGMP concentration (645 +/- 20, 865 +/- 39 and 1088 +/- 33 fmol/mL cGMP for control, uPA 10(-9), 10(-7) M, respectively). Similar data were obtained for tPA. Pial artery dilation to uPA was blunted following FPI (7 +/- 1% and 12 +/- 1% vs. 3 +/- 1% and 6 +/- 1%, respectively), while uPA-associated release of cGMP was blocked (677 +/- 45, 909 +/- 53, and 1110 +/- 55 vs. 283 +/- 10, 316 +/- 18, and 333 +/- 26 fmol/mL for control, uPA 10(-9), 10(-7) M before and after FPI, respectively). Similar data were obtained for tPA. These data show that uPA and tPA produce pial artery dilation in an NO-dependent manner. FPI blunted uPA and tPA induced pial artery dilation as well as the associated release of cGMP. These data suggest therefore that altered NO function contributes to the impairment of uPA and tPA cerebrovasodilation after brain injury.

The fibrinolytic system attenuates vascular tone: effects of tissue plasminogen activator (tPA) and aminocaproic acid on renal microcirculation

1. The renal medulla is a major source of plasminogen activators (PA), recently shown to induce vasodilation in vitro. Treatment with PA inhibitors has been associated with renal dysfunction, suggesting compromised renal microvasculature. We investigated the impact of the PA inhibitor epsilon amino-caproic acid (EACA) upon vascular tone in vitro, and studied the effect of both tPA and EACA upon intrarenal hemodynamics in vivo. 2. In vitro experiments were carried out in isolated aortic rings and with cultured vascular smooth muscle cells. Studies of renal microcirculation and morphology were conducted in anesthetized Sprague-Dawley rats. 3. In isolated aortic rings, EACA (but not the other inhibitors of the fibrinolytic system PAI-1 or alpha-2 antiplasmin) reduced the half-maximal effective concentration of phenylephrine (PE) required to induce contraction (from 32 nm in control solution to 2 and 0.1 nm at EACA concentrations of 1 and 10 microm, respectively). Using reteplase (retavase) in the same model, we also provide evidence that the vasoactivity of tPA is in part kringle-dependent. In cultured vascular smooth muscle cells, Ca(2+) internalization following PE was enhanced by EACA, and retarded by tPA. 4. In anesthetized rats, EACA (150 mg x kg(-1)) did not affect systemic blood pressure, total renal or cortical blood flow. However, the outer medullary blood flow declined 12+/-2% below the baseline (P<0.03). By contrast, tPA (2 mg x kg(-1)), transiently increased outer medullary blood flow by 8+/-5% (P<0.02). Fibrin microthrombi were not found within the renal microvasculature in EACA-treated animals. 5. In conclusion, both fibrinolytic and antifibrinolytic agents modulate medullary renal blood flow with reciprocal effects of vasodilation (PA) and vasoconstriction (EACA). In vitro studies suggest that these hemodynamic responses are related to direct modulation of the vascular tone.

In vitro and in vivo effects of tPA and PAI-1 on blood vessel tone

Tissue type plasminogen activator (tPA) is a key enzyme in the fibrinolytic cascade. In this paper we report that tPA contains 2 independent epitopes that exert opposite effects on blood vessel tone. Low concentrations of tPA (1 nM) inhibit the phenylephrine (PE)–induced contraction of isolated aorta rings. In contrast, higher concentrations (20 nM) stimulate the contractile effect of PE. The 2 putative vasoactive epitopes of tPA are regulated by the plasminogen activator inhibitor-1 (PAI-1) and by a PAI-1–derived hexapeptide that binds tPA. TNK-tPA, a tPA variant in which the PAI-1 docking site has been mutated, stimulates PE-induced vasoconstriction at all concentrations used. The stimulatory, but not the inhibitory, effect of tPA on the contraction of isolated aorta rings was abolished by anti–low-density lipoprotein receptor–related protein/α2-macroglobulin receptor (LRP) antibodies. Administering tPA or TNK-tPA to rats regulates blood pressure and cerebral vascular resistance in a dose-dependent mode. In other in vivo experiments we found that the vasopressor effect of PE is more pronounced in tPA knockout than in wild-type mice. Our findings draw attention to a novel role of tPA and PAI-1 in the regulation of blood vessel tone that may affect the course of ischemic diseases.

Failure of ischemic neuroprotection by potentiators of gamma-aminobutyric acid

INTRODUCTION

Potentiators of inhibitory neurotransmission may provide a neuroprotective effect on cerebral tissue exposed to ischemia, without inducing toxic side effects. Topiramate and vigabatrin enhance the action of gamma-aminobutyric acid (GABA), and each has side effect profiles known to be well tolerated through their clinical use as anticonvulsant medications. We assessed the potential benefit through GABA activation by these drugs on infarct size and functional recovery following focal cerebral ischemia in mice.

METHODS

Silicon-coated suture was advanced through the internal carotid artery of 89 halothane-anesthetized mice to temporarily occlude the right middle cerebral artery for either 45 minutes (topiramate), or 120 minutes (vigabatrin). Animals were treated either at the time of reperfusion with topiramate (100 mg/kg, 40 mg/kg, or saline control), or two hours before arterial occlusion with vigabatrin, (1000 mg/kg, 500 mg/kg, or saline control). Neurological outcome was measured 24 hours after ischemia using a 28-point functional examination score. Infarct volume was estimated by summing area maps of stained slices of infarcted hemispheres.

RESULTS

Functional examination scores at 24 hours were similar between the high dose topiramate group, the low dose topiramate group, and the control group. Similarly, no differences were noted between examination scores of high dose vigabatrin, low dose vigabatrin, and control. Consistent sized right hemisphere infarcts were noted within each group on histological examination. Mean infarct volumes did not differ between groups treated with high dose topiramate, low dose topiramate, or control. Infarct volumes of animals treated with saline control were slightly larger than that of high dose vigabatrin and low dose vigabatrin groups, but the difference did not reach significance.

CONCLUSION

Treatment with these two potentiators of GABA did not result in significant differences in outcome following focal cerebral ischemia, by either functional or histological measures. These results do not support a substantial neuroprotective role of GABA following ischemia in this mouse suture model.

Effects of ischemia and myogenic activity on active and passive mechanical properties of rat cerebral arteries

Passive (papaverine induced) and active (spontaneous pressure induced) biomechanical properties of ischemic and nonischemic rat middle cerebral arteries (MCAs) were studied under pressurized conditions in vitro. Ischemic (1 h of occlusion), contralateral, and sham-operated control MCAs were isolated from male Wistar rats (n = 22) and pressurized using an arteriograph system that allowed control of transmural pressure (TMP) and measurement of lumen diameter and wall thickness. Three mechanical stiffness parameters were computed: overall passive stiffness (beta), pressure-dependent modulus changes (E(inc,p)), and smooth muscle cell (SMC) activity-dependent changes (E(inc,a)). The beta-value for ischemic vessels was increased compared with sham vessels (13.9 +/- 1.7 vs. 9.1 +/- 1.4, P < 0.05), indicating possible short-term remodeling due to ischemia. E(inc,p) increased with pressure in the passive vessels (P < 0.05) but remained relatively constant in the active vessels for all vessel types, indicating that pressure-induced SMC contractile activity (i.e., myogenic reactivity) in cerebral arteries leads to the maintenance of a constant elastic modulus within the autoregulatory pressure range. E(inc,a) increased with pressure for all conditions, signifying that changes in stiffness are influenced by SMC activity and vascular tone.