In vivo bioassay of endothelium-derived relaxing factor

Mechanisms Associated with the Adverse Vascular Consequences of Rapid Posthypothermic Rewarming and Their Therapeutic Modulation in Rats

We previously demonstrated that rapid posthypothermic rewarming in noninjured animals was capable of damaging cerebral arterioles both at endothelial and smooth muscle levels. Such adverse consequences could be prevented with antioxidants, suggesting the involvement of free radicals. In this study, we further investigate the mechanisms associated with free radicals production by using two radical scavengers, superoxide dismutase (SOD) and catalase. Employing rats, the cerebral vascular response was evaluated at 2, 3, and 4 hours after onset of hypothermia. Before rapid rewarming, SOD treatment, but not catalase, preserved the NO-mediated dilation induced by acetylcholine (ACh). On the contrary, catalase preserved the hypercapnia-induced relaxation of the smooth muscle cells, whereas SOD offered only partial protection. Adding SOD to catalase treatment offered no additional benefit. These results suggest that rapid posthypothermic rewarming impairs ACh- and hypercapnia-induced vasodilation through different subcellular mechanisms. In the case of diminished vascular response to ACh, it appears to act on the endothelial front primarily by superoxide anions, as evidenced by its full preservation after SOD treatment. In terms of impaired dilation to hypercapnia, hydrogen peroxide and/or its derivatives are the likely candidates in targeting the smooth muscle cells. The partial protection of SOD to hypercapnia-induced dilation is believed to be the reduced amount of superoxide that would otherwise spontaneously dismutate to produce hydrogen peroxide. Although SOD exerts some indirect influence on the hydrogen peroxide production downstream, catalase apparently has no influence on upstream superoxide production.

Therapeutic targeting of the axonal and microvascular change associated with repetitive mild traumatic brain injury

Recent interest in mild traumatic brain injury (mTBI) has increased the recognition that repetitive mTBI occurring within the sports and military settings can exacerbate the adverse consequences of the initial injury. While multiple studies have recently reported the pathological, metabolic, and functional changes associated with repetitive mTBI, no consideration has been given to the development of therapeutic approaches to attenuate these abnormalities. In this study, we used the model of repetitive impact acceleration insult previously reported by our laboratory to cause no initial structural and functional changes, yet evoke dramatic change following second insult of the same intensity. Using this model, we employed established neuroprotective agents including FK506 and hypothermia that were administered 1 h after the second insult. Following either therapeutic intervention, changes of cerebral vascular reactivity to acetylcholine were assessed through a cranial window. Following the completion of the vascular studies, the animals were prepared to access the numbers of amyloid precursor protein (APP) positive axons, a marker of axonal damage. Following repetitive injury, cerebral vascular reactivity was dramatically preserved by either therapeutic intervention or the combination thereof compared to control group in which no intervention was employed. Similarly, APP density was significantly lower in the therapeutic intervention group compared in controls. Although the individual use of FK506 or hypothermia exerted significant protection, no additive benefit was found when both therapies were combined. In sum, the current study demonstrates that the exacerbated pathophysiological changes associated with repetitive mTBI can be therapeutically targeted.

Role of endothelial nitric oxide in cerebrovascular regulation

Endothelial nitric oxide (NO) plays important roles in the vascular system. Animal models that show vascular dysfunction demonstrate the protective role of endothelial NO dependent pathways. This review focuses on the role of endothelial NO in the regulation of cerebral blood flow and vascular tone. We will discuss the importance of NO in cerebrovascular function using animal models with altered endothelial NO production under normal, ischemic and reperfusion conditions, as well as in hyperoxia. Pharmacological and genetic manipulations of the endothelial NO system demonstrate the essential roles of endothelial NO synthase in maintenance of vascular tone and cerebral perfusion under normal and pathological conditions.

The combination of either tempol or FK506 with delayed hypothermia: implications for traumatically induced microvascular and axonal protection

Following traumatic brain injury (TBI), inhibition of reactive oxygen species and/or calcineurin can exert axonal and vascular protection. This protection proves optimal when these strategies are used early post-injury. Recent work has shown that the combination of delayed drug administration and delayed hypothermia extends this protection. Here we revisit this issue in TBI using the nitroxide antioxidant Tempol, or the immunophilin ligand FK506, together with delayed hypothermia, to determine their effects upon cerebral vascular reactivity and axonal damage. Animals were subjected to TBI and treated with Tempol at 30 or 90 min post-injury, or 90 min post-injury with concomitant mild hypothermia (33°C). Another group of animals were treated in the same fashion with the exception that they received FK506. Cranial windows were placed to assess vascular reactivity over 6 h post-injury, when the animals were assessed for traumatically induced axonal damage. Vasoreactivity was preserved by early Tempol administration; however, this benefit declined with time. The coupling of hypothermia and delayed Tempol, however, exerted significant vascular protection. The use of early and delayed FK506 provided significant vascular protection which was not augmented by hypothermia. The early administration of Tempol provided dramatic axonal protection that was not enhanced with hypothermia. Early and delayed FK506 provided significant axonal protection, although this protection was not enhanced by delayed hypothermia. The current investigation supports the premise that Tempol coupled with hypothermia extends its benefits. While FK506 proved efficacious with early and delayed administration, it did not provide either increased vascular or axonal benefit with hypothermia. These studies illustrate the potential benefits of Tempol coupled to delayed hypothermia. However, these findings do not transfer to the use of FK506, which in previous studies proved beneficial when coupled with hypothermia. These divergent results may be a reflection of the different animal models used and/or their associated injury severity.

Combinational therapy using hypothermia and the immunophilin ligand FK506 to target altered pial arteriolar reactivity, axonal damage, and blood-brain barrier dysfunction after traumatic brain injury in rat

This study evaluated the utility of combinational therapy, coupling delayed posttraumatic hypothermia with delayed FK506 administration, on altered cerebral vascular reactivity, axonal injury, and blood-brain barrier (BBB) disruption seen following traumatic brain injury (TBI). Animals were injured, subjected to various combinations of hypothermic/FK506 intervention, and equipped with cranial windows to assess pial vascular reactivity to acetylcholine. Animals were then processed with antibodies to the amyloid precursor protein and immunoglobulin G to assess axonal injury and BBB disruption, respectively. Animals were assigned to five groups: (1) sham injury plus delayed FK506, (2) TBI, (3) TBI plus delayed hypothermia, (4) TBI plus delayed FK506, and (5) TBI plus delayed hypothermia with FK506. Sham injury plus FK506 had no impact on vascular reactivity, axonal injury, or BBB disruption. Traumatic brain injury induced dramatic axonal injury and altered pial vascular reactivity, while triggering local BBB disruption. Delayed hypothermia or FK506 after TBI provided limited protection. However, TBI with combinational therapy achieved significantly enhanced vascular and axonal protection, with no BBB protection. This study shows the benefits of combinational therapy, using posttraumatic hypothermia with FK506 to attenuate important features of TBI. This suggests that hypothermia not only protects but also extends the therapeutic window for improved FK506 efficacy.

The adverse pial arteriolar and axonal consequences of traumatic brain injury complicated by hypoxia and their therapeutic modulation with hypothermia in rat

This study examined the effect of posttraumatic hypoxia on cerebral vascular responsivity and axonal damage, while also exploring hypothermia's potential to attenuate these responses. Rats were subjected to impact acceleration injury (IAI) and equipped with cranial windows to assess vascular reactivity to topical acetylcholine, with postmortem analyses using antibodies to amyloid precursor protein to assess axonal damage. Animals were subjected to hypoxia alone, IAI and hypoxia, IAI and hypoxia before induction of moderate hypothermia (33 degrees C), IAI and hypoxia induced during hypothermic intervention, and IAI and hypoxia initiated after hypothermia. Hypoxia alone had no impact on vascular reactivity or axonal damage. Acceleration injury and posttraumatic hypoxia resulted in dramatic axonal damage and altered vascular reactivity. When IAI and hypoxia were followed by hypothermic intervention, no axonal or vascular protection ensued. However, when IAI was followed by hypoxia induced during hypothermia, axonal and vascular protection followed. When this same hypoxic insult followed the use of hypothermia, no benefit ensued. These studies show that early hypoxia and delayed hypoxia exert damaging axonal and vascular consequences. Although this damage is attenuated by hypothermia, this follows only when hypoxia occurs during hypothermia, with no benefit found if the hypoxic insult proceeds or follows hypothermia.

The long-term microvascular and behavioral consequences of experimental traumatic brain injury after hypothermic intervention

Traumatic brain injury (TBI) has been demonstrated to induce cerebral vascular dysfunction that is reflected in altered responses to various vasodilators. While previous reports have focused primarily on the short-term vascular alterations, few have examined these vascular changes for more than 7 days, or have attempted to correlate these alterations with any persisting behavioral changes or potential therapeutic modulation. Accordingly, we evaluated the long-term microvascular and behavioral consequences of experimental TBI and their therapeutic modulation via hypothermia. In this study, one group was injured with no treatment, another group was injured and 1 h later was treated with 120 min of hypothermia followed by slow rewarming, and a third group was non-injured. Animals equipped with cranial windows for visualization of the pial microvasculature were challenged with various vasodilators, including acetylcholine, hypercapnia, adenosine, pinacidil, and sodium nitroprusside, at either 1 or 3 weeks post-TBI. In addition, all animals were tested for vestibulomotor tasks at 1 week post-TBI, and animals surviving for 3 weeks post-TBI were tested in a Morris water maze (MWM). The results of this investigation demonstrated that TBI resulted in long-term vascular dysfunction in terms of altered vascular reactivity to various vasodilators, which was significantly improved with the use of a delayed 120-min hypothermic treatment. In contrast, data from the MWM task indicated that injured animals revealed persistent deficits in the spatial memory test performance, with hypothermia exerting no protective effects. Collectively, these data illustrate that TBI can evoke long-standing brain vascular and spatial memory dysfunction that manifest different responses to hypothermic intervention. These findings further illustrate the complexity of TBI and highlight the fact that the chosen hypothermic intervention may not necessarily exert a global protective response.

Role of hydrogen peroxide and the impact of glutathione peroxidase-1 in regulation of cerebral vascular tone

Although arachidonic acid (AA) has diverse vascular effects, the mechanisms that mediate these effects are incompletely defined. The goal of our study was to use genetic approaches to examine the role of hydrogen peroxide (H2O2), glutathione peroxidase (Gpx1, which degrades H2O2), and CuZn-superoxide dismutase (SOD1, which produces H2O2 from superoxide) in mediating and in determining vascular responses to AA. In basilar arteries in vitro, AA produced dilation in nontransgenic mice, and this response was reduced markedly in transgenic mice overexpressing Gpx1 (Gpx1 Tg) or in those genetically deficient in SOD1. For example, AA (1 nmol/L to 1 mumol/L) dilated the basilar artery and this response was reduced by approximately 90% in Gpx1 Tg mice (P<0.01), although responses to acetylcholine were not altered. Dilation of cerebral arterioles in vivo in response to AA was inhibited by approximately 50% by treatment with catalase (300 U/mL) (P<0.05) and reduced by as much as 90% in Gpx1 Tg mice compared with that in controls (P<0.05). These results provide the first evidence that Gpx1 has functional effects in the cerebral circulation, and that AA-induced vascular effects are mediated by H2O2 produced by SOD1. In contrast, cerebral vascular responses to the endothelium-dependent agonist acetylcholine are not mediated by H2O2.

The role of oxidative stress and NADPH oxidase in cerebrovascular disease

The study of reactive oxygen species (ROS) and oxidative stress remains a very active area of biological research, particularly in relation to cellular signaling and the role of ROS in disease. In the cerebral circulation, oxidative stress occurs in diverse forms of disease and with aging. Within the vessel wall, ROS produce complex structural and functional changes that have broad implications for regulation of cerebral perfusion and permeability of the blood-brain barrier. These oxidative-stress-induced changes are thought to contribute to the progression of cerebrovascular disease. Here, we highlight recent findings in relation to oxidative stress in the cerebral vasculature, with an emphasis on the emerging role for NADPH oxidases as a source of ROS and the role of ROS in models of disease.

Reactive oxygen species: influence on cerebral vascular tone

Reactive oxygen species have multiple effects on vascular cells. Defining the sources and the impact of the various reactive oxygen species within the vessel wall has emerged as a major area of study in vascular biology. This review will focus on recent findings related to effects of reactive oxygen species on cerebral vascular tone. Effects of superoxide radical, hydrogen peroxide, and the reactive nitrogen species peroxynitrite are summarized. Although higher concentrations may be important for cerebral vascular biology in disease, relatively low concentrations of reactive oxygen species may function as signaling molecules involved with normal regulation of cerebral vascular tone. The mechanisms by which reactive oxygen species affect vascular tone may be quite complex, and our understanding of these processes is increasing. Additionally, the role of reactive oxygen species as mediators of endothelium-dependent relaxation is addressed. Finally, the consequences of the molecular interactions of superoxide with nitric oxide and arachidonic acid are discussed.

Nitric oxide and the cerebral circulation

BACKGROUND

Nitric oxide (NO) is a potent vasodilator that was initially described as the mediator of endothelium-dependent relaxation (endothelium-derived relaxing factor, EDRF). It is now known that NO is produced by a variety of other cell types.

SUMMARY OF REVIEW

Endothelium produces NO (EDRF) under basal conditions and in response to a variety of vasoactive stimuli in large cerebral arteries and the cerebral microcirculation. Endothelium-dependent relaxation is impaired in the presence of several pathophysiological conditions. This impairment may contribute to cerebral ischemia or stroke. Activation of glutamate receptors appears to be a major stimulus for production of NO by neurons. Neuronally derived NO may mediate local increases in cerebral blood flow during increases in cerebral metabolism. NO synthase-containing neurons also innervate large cerebral arteries and cerebral arterioles on the brain surface. Activation of parasympathetic fibers that innervate cerebral vessels produces NO-dependent increases in cerebral blood flow. Increases in cerebral blood flow during hypercapnia also appear to be dependent on production of NO. Astrocytes may release some NO constitutively, but astrocytes and microglia can release relatively large quantities of NO after induction of NO synthase in response to endotoxin or some cytokines. Expression of inducible NO synthase, perhaps in response to local production of cytokines, may exert cytotoxic effects in brain during or after ischemia.

CONCLUSIONS

Because endothelium, neurons, and glia can all produce NO in response to some stimuli, the influence of NO on the cerebral circulation appears to be very important. Under normal conditions, constitutively produced NO influences basal cerebral vascular tone and mediates vascular responses to a diverse group of stimuli. The inducible form of NO synthase produces much greater amounts of NO that may be an important mediator of cytotoxicity in brain.

Cerebral blood flow and cerebrovascular reactivity after inhibition of nitric oxide synthesis in conscious goats

1. The role of nitric oxide in the cerebral circulation under basal conditions and after vasodilator stimulation was studied in instrumented, conscious goats, by examining the action of inhibiting endogenous nitric oxide production with NG-nitro-L-arginine methyl ester (L-NAME). 2. In 6 unanaesthetized goats, blood flow to one brain hemisphere (electromagnetically measured), systemic arterial blood pressure and heart rate were continuously recorded. L-NAME (35 mg kg-1 by i.v. bolus) decreased resting cerebral blood flow by 43 +/- 3%, increased mean arterial pressure by 21 +/- 2%, and decreased heart rate by 41 +/- 2%; cerebrovascular resistance increased by 114 +/- 13% (P < 0.01); the immediate addition of i.v. infusion of L-NAME (0.15-0.20 mg kg-1 during 60-80 min) did not significantly modify these effects. Cerebral blood flow recovered at 72 h, arterial pressure and cerebrovascular resistance at 48 h, and heart rate at 6 days after L-NAME treatment. 3. A second treatment with L-NAME scheduled as above reproduced the immediate haemodynamic effects of the first treatment, which (except bradycardia) reversed with L-arginine (200-300 mg kg-1 by i.v. bolus). 4. Acetylcholine (0.01-0.3 micrograms), sodium nitroprusside (3-100 micrograms) and diazoxide (0.3-9 mg), injected into the cerebral circulation of 5 conscious goats, produced dose-dependent increases in cerebral blood flow, and decreases in cerebrovascular resistance; sodium nitroprusside (30 and 100 micrograms) also caused hypotension and tachycardia. 5. The reduction in cerebrovascular resistance from resting levels (in absolute values) to lower doses,but not to the highest dose, of acetylcholine was diminished, to sodium nitroprusside was increased, and to diazoxide was unaffected after L-NAME, compared to control conditions. The effects on cerebrovascular resistance to acetycholine normalized within 24 h and to sodium nitroprusside within 48 h after L-NAME treatment.6. This study provides information about the evolution of the changes in cerebral blood flow and cerebrovascular reactivity after inhibition of endogenous nitric oxide in conscious animals. The results suggest: (a) endogenous nitric oxide is involved in regulation of the cerebral circulation by producing a resting vasodilator tone, (b) the cerebral vasodilatation to acetylcholine is mediated, at least in part, by nitric oxide release, and (c) inhibition of nitric oxide production induces supersensitivity of cerebral vasculature to nitrovasodilators.

Effects of cilazapril on cerebral vasodilatation in hypertensive rats

Endothelium-dependent dilatation of cerebral arterioles is impaired during chronic hypertension. The goal of this study was to determine the effects of an angiotensin converting enzyme inhibitor, cilazapril, on endothelium-dependent dilatation in pial arterioles. Four-month-old Wistar-Kyoto (WKY) rats and stroke-prone spontaneously hypertensive rats (SHRSP) received cilazapril in their drinking water (500 mg/L) for 3 to 6 months. Treatment with cilazapril reduced mean arterial pressure in both WKY rats and SHRSP and had no significant effect on baseline diameter of pial arterioles measured with a cranial window. Responses to bradykinin and A23187, but not to nitroglycerin and adenosine, were impaired in SHRSP. Cilazapril did not affect responses to bradykinin (3 x 10(-7) M) and A23187 (10(-5) M) in WKY rats but significantly increased cerebral vasodilatation in response to bradykinin (52 +/- 4% vs 27 +/- 5%) and A23187 (19 +/- 3% vs 8 +/- 3%) in SHRSP. Cilazapril also tended to increase dilator responses to nitroglycerin and adenosine in SHRSP. In another group of SHRSP, treatment with cilazapril for 4 days produced a moderate reduction in blood pressure and increased cerebral vasodilatation in response to bradykinin, A23187, and adenosine. Topical application of the active form of cilazapril (cilazaprilat) for 40 minutes also increased cerebral vasodilatation in response to bradykinin, A23187, and nitroglycerin in SHRSP. The data indicate that an angiotensin converting enzyme inhibitor enhances cerebral vasodilatation in response to endothelium-dependent agonists in SHRSP and may also increase responses to endothelium-independent agonists.

Does nitric oxide mediate the increases in cerebral blood flow elicited by hypercapnia?

The endothelium-derived relaxing factor (EDRF), probably nitric oxide (NO) or a closely related compound (EDRF/NO), is a potent vasodilator that appears to regulate vascular tone in several vascular beds. I have investigated whether EDRF/NO is also involved in the regulation of the cerebral circulation--in particular, whether EDRF/NO participates in the increases in cerebral blood flow elicited by hypercapnia. Rats were anesthetized with halothane, 1-2% (vol/vol), paralyzed, and artificially ventilated. Arterial pressure was monitored and blood gases were controlled. Cerebral blood flow was continuously monitored through a cranial window over the sensory cortex by a laser-Doppler probe. The window was superfused with Ringer's solution (pH 7.3-7.4 at 37 degrees C). During superfusion with Ringer's solution, hypercapnia (PCO2 = 55.8 +/- 0.8 mmHg) increased cerebral blood flow by 121 +/- 6% (n = 27; P less than 0.001; analysis of variance). Topical superfusion with the NO synthase inhibitors N omega-nitro-L-arginine (1 mM) attenuated the cerebrovasodilation by 93 +/- 6% (n = 8). In contrast, the vasodilation elicited by topical papaverine (1 mM) was not affected by N omega-nitro-L-arginine (n = 10). Application of N omega-nitro-D-arginine (1 mM) did not affect the cerebrovasodilation elicited by hypercapnia (P greater than 0.05; n = 8). N omega-Methyl-L-arginine (1 mM) attenuated the cerebrovasodilation elicited by hypercapnia by 44 +/- 4% (n = 8; P less than 0.001), an effect completely reversed by coapplication of L-arginine (10 mM; P greater than 0.05; n = 13). These findings indicate that the powerful effects of CO2 on the cerebral circulation are mediated by arginine-derived EDRF/NO. EDRF/NO is an important molecular signal whose actions may also include the regulation cerebral circulation.

Regulation of cerebral blood vessels by humoral and endothelium-dependent mechanisms. Update on humoral regulation of vascular tone

Recent studies suggest that humoral and endothelium-dependent mechanisms may play an important role in the cerebral circulation. Angiotensin may acutely and chronically increase resistance of large cerebral arteries and reduce cerebral microvascular pressure without changing cerebral blood flow. We hypothesize that the brain may sense reductions in microvascular pressure and initiate compensatory neurohumoral responses to raise arterial pressure. Vasopressin appears to play an important role in regulation of production of cerebrospinal fluid and brain fluid volume. Vasopressin also may be protective when intracranial pressure is elevated. Endothelium-dependent mechanisms also may have important influences on tone of cerebral vessels. Synthesis of the endothelium-derived relaxing factor nitric oxide, or a nitric oxide-containing compound, appears to influence both basal tone and responses of large cerebral arteries to acetylcholine in vivo. Large cerebral arteries dilate in response to increased blood flow in vivo, and this response may be mediated in part by release of a humoral factor by endothelium. Endothelium-dependent responses of cerebral arterioles to receptor- and nonreceptor-mediated agonists are impaired during chronic hypertension. The mechanism of impairment of endothelium-dependent responses of cerebral arterioles appears to involve production of an endothelium-derived contracting factor.