L-arginine dilates rat pial arterioles by nitric oxide-dependent mechanisms and increases blood flow during focal cerebral ischaemia

L-arginine-induced regional cerebral blood flow increase is abolished after transient focal cerebral ischemia in the rat

We investigated the L-arginine-induced, regional cerebral blood flow (rCBF) enhancement after different durations of transient focal cerebral ischemia in the rat to determine if L-arginine increases rCBF after transient focal cerebral ischemia. Focal ischemia (5 minutes and 20 minutes) followed by 90 minutes of reperfusion was induced in a normotensive rat suture-model. Regional cerebral blood flow in both hemispheres was measured by laser-Doppler-flowmetry. Reactivity of rCBF to L-arginine (300 mg/kg) was measured 45 minutes after reperfusion, and hypercapnia 90 minutes after reperfusion. The effect of D-arginine and pretreatment with the nitric oxide (NO) synthase inhibitor N(omega)-nitro-L-arginine (L-NA) (10 mg/kg) was examined in additional groups. Hypercapnia and L-arginine increased rCBF in sham operated controls and on the nonischemic hemispheres. D-arginine did not. Twenty-minute long ischemia significantly reduced the response to L-arginine (control side: 115 +/- 5.9%; ischemic side: 107 +/- 6.1%, n = 7) and hypercapnia, 5 minutes of ischemia did not. N(omega)-nitro-L-arginine pretreatment partly restored the L-arginine-induced rCBF increase. Thus, rCBF increase caused by L-arginine in the reperfusion period was unaffected by 5 minutes of ischemia, but reduced by 20 minutes of ischemia. The restoration after pretreatment with L-NA may be caused by attenuated production of cytotoxic substances, e.g., NO and related compounds.

Manipulation of cerebrovascular resistance during internal carotid artery occlusion by intraarterial verapamil

Occlusion of the internal carotid artery (ICA) results in acute cerebral hypotension. We hypothesized that during acute cerebral hypotension, in addition to physiological autoregulation, further arteriolar relaxation is possible by pharmacological means. We tested the feasibility of using intracarotid verapamil, a calcium channel blocker, to decrease the cerebrovascular resistance (CVR) and augment cerebral blood flow (CBF) at low postocclusion distal ICA pressures (PICA). Eleven patients undergoing trial occlusion of ICA were enrolled. Distal ICA or stump pressure, hemispheric CBF, and CVR were determined before and after carotid occlusion. During ICA occlusion, CBF and other physiological variables were determined before and after intracarotid verapamil. Two patients were excluded from the study. Carotid occlusion (n = 9) significantly decreased PICA (mean +/- SD, from 82 +/- 22 to 46 +/- 11 mm Hg, P = 0.001) and CBF (from 42 +/- 11 to 33 +/- 11 mL.100 g-1.min-1, P < 0.05). During occlusion, after intracarotid verapamil (3.9 +/- 1.6 mg), hemispheric CBF tended to increase from 31 +/- 11 to 35 +/- 14 mL.100 g-1.min-1 (P = 0.067). However, the percent increase in CBF after verapamil was a linear function of PICA (y = 1.01 x -32, n = 9, r2 = 0.84, P = 0.006). The decrease in CBF during carotid occlusion suggests that near maximal cerebral autoregulatory vasodilation had occurred, although our results indicate that it may be feasible to further augment CBF by pharmacological means during acute cerebral hypotension.

IMPLICATIONS

When the internal carotid artery is occluded during neurosurgical procedures, there may be a significant reduction in cerebral perfusion. The authors have demonstrated that the intraarterial administration of verapamil increases cerebral blood flow as a linear function of cerebral artery pressure. Intracarotid injection of vasodilators may augment cerebral blood flow during acute cerebral hypotension.

Interrelationships between astrocyte function, oxidative stress and antioxidant status within the central nervous system

Astrocytes have, until recently, been thought of as the passive supporting elements of the central nervous system. However, recent developments suggest that these cells actually play a crucial and vital role in the overall physiology of the brain. Astrocytes selectively express a host of cell membrane and nuclear receptors that are responsive to various neuroactive compounds. In addition, the cell membrane has a number of important transporters for these compounds. Direct evidence for the selective co-expression of neurotransmitters, transporters on both neurons and astrocytes, provides additional evidence for metabolic compartmentation within the central nervous system. Oxidative stress as defined by the excessive production of free radicals can alter dramatically the function of the cell. The free radical nitric oxide has attracted a considerable amount of attention recently, due to its role as a physiological second messenger but also because of its neurotoxic potential when produced in excess. We provide, therefore, an in-depth discussion on how this free radical and its metabolites affect the intra and intercellular physiology of the astrocyte(s) and surrounding neurons. Finally, we look at the ways in which astrocytes can counteract the production of free radicals in general by using their antioxidant pathways. The glutathione antioxidant system will be the focus of attention, since astrocytes have an enormous capacity for, and efficiency built into this particular system.

The role of nitric oxide in the pathophysiology of thromboembolic stroke in the rat

Although nitric oxide (NO) has been shown to play an important role in the pathophysiology of cerebral ischemia, its contribution to the pathogenesis of experimentally induced thromboembolic stroke is unknown. In this study, we pharmacologically manipulated NO levels in the acute post-thrombotic stage and determined the effects on behavior and histopathology. The following drugs were used: nitro-L-arginine-methyl ester (L-NAME), a non-specific endothelial and neuronal nitric oxide synthase (eNOS and nNOS) inhibitor, 3-bromo-7-nitroindazole (7-NI), a specific inhibitor for nNOS, the NO precursor, exogenous L-arginine and the NO-donor, 3-morpholino-sydnonimine (SIN-1). Male Wistar rats (n = 76) were randomly assigned to receive vehicle or drug immediately after common carotid artery thrombosis (CCAT). Regional measurements of cortical NOS activity using the [3H]L-arginine to [3H]L-citrulline conversion assay were decreased 1 h after treatment with L-NAME and 7-NI by 50 and 65%, respectively; hippocampal NOS activity was reduced with L-NAME by 35% and with 7-NI by 65%. L-NAME significantly worsened forelimb placing as compared to other groups. 7-NI accelerated sensorimotor recovery. Water maze retention deficits were noted 48 h after CCAT and these were exacerbated by L-NAME treatment. Histopathological protection was conferred in the hippocampus by 7-NI and SIN-1; conversely, L-NAME increased neuronal injury in the contralateral cortex. L-arginine had no effect on these outcomes. In conclusion, both structural and functional consequences of CCAT can be aggravated by limiting endothelial NO production in the acutely post-thrombotic brain. In contrast, inhibition of nNOS and infusion of an NO donor has a beneficial effect on pathology.

L-arginine and superoxide dismutase prevent or reverse cerebral hypoperfusion after fluid-percussion traumatic brain injury

To determine whether treatment with L-arginine or superoxide dismutase (SOD) would prove effective in reducing cerebral hypoperfusion after traumatic brain injury (TBI), we measured cerebral blood flow (CBF) using laser Doppler flowmetry (LDF) in rats treated before or after moderate (2.2 atm) fluid-percussion (FP) TBI. Rats were anesthetized with isoflurane and prepared for midline FP TBI and then for LDF by thinning the calvaria using an air-cooled drill. Rats were then randomly assigned to receive sham injury, sham injury plus L-arginine (100 mg/kg, 5 min after sham TBI), TBI plus 0.9% NaCl, TBI plus L-arginine (100 mg/kg, 5 min post-TBI), TBI plus SOD (24,000 U/kg pre-TBI + 1600 units/kg/min for 15 min after TBI), or TBI plus SOD and L-arginine. A second group of rats received TBI plus saline, L-, or D-arginine (100 mg/kg, 5 min after-TBI). After treatment and TBI or sham injury, CBF was measured continuously using LDF for 2 h and CBF was expressed as a percent of the preinjury baseline for 2 h after TBI. Rats treated with saline or D-arginine exhibited significant reductions in CBF that persisted throughout the monitoring period. Rats treated with L-arginine alone or in combination with SOD exhibited no decreases in CBF after TBI. CBF in the SOD-treated group decreased significantly within 15 min after TBI but returned to baseline levels by 45 min after TBI. These studies indicate that L-arginine but not D-arginine administered after TBI prevents posttraumatic hypoperfusion and that pretreatment with SOD will restore CBF after a brief period of hypoperfusion.

L-arginine infusion increases basal but not activated cerebral blood flow in humans

Nitric oxide is a potent vasodilator. Infusion of its precursor, L-arginine, results in increased cerebral blood flow (CBF) in experimental animals. We examined the effects of L-arginine infusion on CBF in humans using positron emission tomography and the quantitative H2(15)O method. Six subjects received 500 ml of 0.9% NaCl solution, and six subjects received an infusion of L-arginine (16.7 mg/kg/min; 500 mg/kg). Before and after the i.v. infusion, paired CBF measurements were performed at baseline and with vibrotactile stimulation of the right hand. In scans performed without vibrotactile stimulation, mean whole-brain CBF increased from 34.9 +/- 3.7 ml 100 g-1 min-1 to 38.2 +/- 4.4 ml 100 g-1 min-1. (9.5%; p < 0.005) after L-arginine infusion. The temporal pattern of CBF changes differed from that of plasma growth hormone and insulin levels and of arterial pH. In contrast, in the saline group, mean whole-brain CBF did not change significantly (35.8 +/- 5.9 ml 100 g-1 min-1 to 35.9 +/- 6.4 ml 100 g-1 min-1; 0.3%). Vibrotactile stimulation produced significant focal increases in CBF, which were unaffected by L-arginine infusion. L-arginine infusion was associated with an increase in plasma L-citrulline, a byproduct of nitric oxide synthesis.

Inducible nitric oxide synthase gene expression in vascular cells after transient focal cerebral ischemia

BACKGROUND AND PURPOSE

We investigated whether inducible nitric oxide synthase (iNOS) is expressed after transient cerebral ischemia and, if so, we sought to define the temporal profile and cellular localization of the expression and the role of iNOS in the mechanism of ischemic brain injury.

METHODS

The middle cerebral artery in rats was occluded for 2 hours by an intraluminal filament. The occurrence of transient ischemia and reperfusion was confirmed by laser-Doppler flowmetry (n = 5). iNOS message in the ischemic neocortex was determined by reverse-transcription polymerase chain reaction. iNOS enzymatic activity was assessed by citrulline assay. The cellular localization of iNOS expression was determined by immunohistochemistry.

RESULTS

iNOS mRNA was maximally expressed in postischemic brain at 12 hours and was not present at 4 days (n = 3 per time point). iNOS mRNA was not observed in the contralateral cerebral cortex. iNOS enzymatic activity developed in the postischemic brain between 12 and 24 hours (P < .05) and subsided at 4 days (n = 4 to 8 per time point). iNOS immunoreactivity in the ischemic region was restricted to the wall of capillaries and of larger blood vessels at 12 to 24 hours. In regions of early necrosis, inflammatory cells were iNOS positive. Treatment with the iNOS inhibitor aminoguanidine (n = 5; 100 mg/kg IP, BID for 4 days), starting 6 hours after ischemia, reduced infarct size in neocortex by 36 +/- 7% in comparison with vehicle-treated controls (n = 5) (P < .05).

CONCLUSIONS

Transient focal ischemia leads to iNOS expression in postischemic brain. However, the spatial and temporal patterns of expression differ from those occurring in permanent ischemia: iNOS is induced earlier and predominantly in vascular cells rather than in neutrophils. Thus, the temporal profile and localization of postischemic iNOS expression depend on the nature of the ischemic insult. The finding that aminoguanidine reduces infarct size adds further support to the hypothesis that postischemic iNOS expression contributes to ischemic brain damage.

L-arginine does not improve cortical perfusion or histopathological outcome in spontaneously hypertensive rats subjected to distal middle cerebral artery photothrombotic occlusion

The potential of nitric oxide (NO) to influence positively or negatively the outcome of mechanically induced focal cerebral ischemia is still controversial. Recent evidence suggests that NO of vascular origin, whether synthesized from exogenously administered L-arginine (L-Arg) or from NO donor compounds, is beneficial but that of neuronal origin is not. However, the therapeutic potential of NO to ameliorate stroke induced by arterial thrombosis has not been reported. We assessed the therapeutic effect of L-Arg administration in spontaneously hypertensive rats (SHR) subjected to permanent photothrombotic occlusion of the distal middle cerebral artery (dMCA). The ipsilateral carotid artery was left unligated to enhance L-Arg delivery into the putative penumbral region. Local CBF (LCBF) was assessed at 30 min by the [14C]iodoantipyrine technique (n = 9), while histological infarct volumes and index of peripheral ischemic cell change were determined at 3 days (n = 7). Rats (n = 9) given 300 mg/kg L-Arg at 18 and 3 h before photothrombotic dMCA occlusion and at 5 min afterward displayed no significant differences in LCBF compared with animals (n = 8) injected with water (the carrier vehicle) and similarly irradiated. Infarct volumes were also similar, being 37.0 +/- 9.7 mm3 (SD) in the vehicle-treated and 49.1 +/- 17.2 mm3 (SD) in the L-Arg-treated groups (both n = 7), as were assessments of ischemic neuronal density in the penumbra. In contrast, L-Arg administered intravenously in a dose of 300 mg/kg to nonischemic SHR (n = 5) increased cortical CBF by approximately 75% during a 70-min observation period. We conclude that thrombotic processes superimposed upon cerebral ischemia may facilitate tissue reactions that offset the potentially beneficial effect of L-Arg, and this caveat must be considered when proposing L-Arg for clinical treatment of focal thrombotic stroke.

L-arginine decreases infarct size in rats exposed to environmental tobacco smoke

This study examined the effects of L-arginine on myocardial infarct size, hemodynamics, and vascular reactivity in environmental tobacco smoke (ETS)-exposed and non-ETS-exposed rats. We previously demonstrated that exposure to ETS increased myocardial infarct size in a rat model of ischemia and reperfusion. If reduced reperfusion was caused by endothelial cell damage and increased vascular tone, L-arginine (ARG) would increase nitric oxide and better protect the heart. Sixty Sprague-Dawley rats were randomly divided into four groups: ETS or non-ETS (control) with and without ARG (2.25% ARG in drinking water). The ETS groups were exposed to passive smoking (4 Marlboro cigarettes per 15 minutes, 6 hours a day) for 6 weeks. After 6 weeks, all rats were subjected to 35 minutes of left coronary artery occlusion and 120 minutes of reperfusion, with hemodynamic monitoring. Aortic rings were harvested to evaluate vascular reactivity. Average air nicotine, carbon monoxide, and total particulate concentrations were 1304 +/- 215 microgram/m3, 78 +/- 2.0 ppm, and 31 +/- .7 mg/m3 (mean +/- SEM) for the ETS-exposed rats. Infarct size (infarct mass/risk area x 100%) increased with ETS exposure but decreased significantly in the ETS-with-ARG group compared with the ETS-without-ARG group (42% +/- 6% vs 64% +/- 6%, mean +/- SEM; p = 0.043). The benefit of ARG was dependent on ETS exposure (ETS x ARG interaction, p = 0.043). There were no significant differences between groups in heart rate, systolic pressure, and rate-pressure product. ARG significantly decreased myocardial infarct size after ischemia and reperfusion in ETS-exposed rats. Neither the adverse effects of ETS on infarct size nor the blockage of this effect by ARG appears to be the result of ETS-induced alterations in hemodynamics.

Nitric oxide production affects cerebral perfusion and metabolism after deep hypothermic circulatory arrest

BACKGROUND

Use of deep hypothermic circulatory arrest (DHCA) in infant cardiac surgery is associated with reduced cerebral perfusion and metabolism during the recovery period. We investigated the impairment of nitric oxide production as a possible cause.

METHODS

A group of 1-week-old piglets underwent normothermic cardiopulmonary bypass (group A); three other groups (B, C, and D; n = 6 per group) underwent 60 minutes of DHCA at 18 degrees C and 60 minutes of rewarming. The animals were then treated as follows: Groups A and B received L-omega-nitro-arginine-methyl-ester (L-NAME, 50 mg.kg-1); group C, saline solution; and group D, L-arginine (600 mg.kg-1).

RESULTS

In group A, global cerebral blood flow decreased to 37.3% +/- 4.2% of baseline after L-NAME administration (p < 0.005). In group B, global cerebral blood flow decreased to 44.6% +/- 4.4% of baseline after DHCA and 28.9% +/- 3.4% after L-NAME administration (p < 0.001). Following L-arginine treatment after DHCA (group D), global cerebral blood flow increased from 43.8% +/- 3.0% of baseline to 61.6% +/- 9.1% (p < 0.05); cerebral oxygen metabolism increased from 1.93 +/- 0.16 mL.min-1.100 g-1 after DHCA to 2.42 +/- 0.25 mL.min-1.100 g-1 (p < 0.05).

CONCLUSIONS

Tonal production of nitric oxide is impaired in the brain after DHCA and is partly responsible for the circulatory and metabolic changes observed. Stimulation of nitric oxide production (L-arginine) significantly improved recovery of cerebral blood flow and cerebral oxygen metabolism after DHCA.

Treatment of focal cerebral ischemia with synthetic oligopeptide corresponding to lectin domain of selectin

BACKGROUND AND PURPOSE

Synthetic oligopeptides with amino acid sequences of the lectin domain of selectin block selectin-mediated cell adhesion in vitro, which may be applied to a therapeutic intervention to attenuate acute inflammatory reactions. To evaluate the efficacy of such treatment against ischemic brain injury, the effects of administering a selectin oligopeptide that selectively blocks selectin-mediated cell adhesion on histological outcome and on cerebral blood flow (CBF) were studied in models of rodent focal cerebral ischemia.

METHODS

Spontaneously hypertensive rats were anesthetized with halothane. Permanent focal cerebral ischemia was induced by tandem left middle cerebral artery (MCA) and common carotid artery (CCA) occlusion. Focal cerebral ischemia with partial reperfusion was introduced by reperfusing the CCA after 2 hours of tandem MCA/CCA occlusion. A synthetic oligopeptide (amino acid residues 23-30 from N terminal) of E-selectin was dissolved in physiological saline and was injected intravenously at a dosage of 2 mg/kg or 10 mg/kg before artery occlusion. Control animals received equivalent volumes of physiological saline or 10 mg/kg of synthetic oligopeptide with a scrambled amino acid sequence. Twenty-four hours after the occlusion, seven coronal brain slices were stained with 2,3,5-triphenyltetrazolium chloride, and the volume of ischemic injury was calculated. In a separate set of animals, regional CBF was monitored with laser-Doppler flowmetry at the dorsolateral cerebral cortex during 2-hour ischemia and 30 minutes after partial reperfusion.

RESULTS

The volume of ischemic injury did not differ among groups in permanent ischemia. In ischemia with partial reperfusion, 10 mg/kg selectin oligopeptide, but not the same dosage of scrambled oligopeptide, significantly reduced the volume of ischemic injury (95 +/- 13, 73 +/- 11, 55 +/- 6, and 111 +/- 14 mm3 for saline [n = 11]; 2 mg/kg [n = 10] and 10 mg/kg [n = 16] selectin oligopeptide and 10 mg/kg scrambled oligopeptide [n = 6], respectively; P < .01 by one-way ANOVA followed by the Tukey test). Laser-Doppler flowmetry demonstrated a larger increase in CBF after reperfusion of the CCA in the 10-mg/kg selectin oligopeptide group.

CONCLUSIONS

Our data demonstrate that administration of a synthetic oligopeptide corresponding to the lectin domain of selectin decreases the size of ischemic injury after transient, but not after permanent, focal cerebral ischemia as evaluated at 24 hours after onset of ischemia. These effects were associated with an improved CBF at the dorsolateral cerebral cortex after partial reperfusion.

Role of nitric oxide in the maintenance of resting cerebral blood flow during chronic hypertension

The influence of nitric oxide (NO) on basal vascular tone varies with different hypertensive models or vascular beds. The goal of the present study was to examine the role of NO in the maintenance of resting cerebral blood flow (CBF) during chronic hypertension. In 9-10 months old Wistar-Kyoto (WKY) rats (n=47) and spontaneously hypertensive rats (SHR;n=47) anesthetized with pentobarbital sodium (60 mg/kg i.p.), regional CBF of the right parietal cortex was monitored by laser-Doppler flowmetry. Reductions in CBF in response to intravenous infusion of the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME; 1, 3, 10, and 30 mg/kg) were similar between WKY rats (17 +/- 6 approximately 43 +/- 6%; means +/- SE) and SHR (15 +/- 6 approximately 48 +/- 6%) while arterial blood pressure was maintained on the baseline level by controlled hemorrhage. Effects of L-NAME (3 mg/kg i.v.) on arterial blood pressure and CBF were almost completely inhibited by L-arginine (300 mg/kg i.v.), but not by D-arginine (300 mg/kg i.v.). In addition, intravenous infusion of L-arginine (300 mg/kg) alone did not affect resting CBF in both WKY rats and SHR. Thus, these findings suggest that 1) NO plays an important role in the maintenance of resting CBF in both normotensive and chronically hypertensive rats and 2) the contribution of NO to the maintenance of resting CBF is not altered during chronic hypertension.

Cerebral vasodilatation in the cat involves nitric oxide from parasympathetic nerves

The recent description of the synthesis of nitric oxide (NO) in vivo and its putative role in endothelial-dependent vasodilatation has led to considerable interest in possible roles for the molecule. In this study the distribution, origin and some aspects of the physiology of NO found in the cerebral circulation is examined. A rich supply of nerve fibres displaying immunoreactivity against nitric oxide synthase (NOS) could be seen in the adventitia of the cat cerebral vessels. There was a large number of NOS-positive cells in the sphenopalatine ganglion and these constituted a sub-population of the vasoactive intestinal polypeptide (VIP) positive cells. Blockade of NOS activity with NG-nitro-L-arginine methyl ester (L-NAME) significantly attenuated the cerebral vasodilator response to facial nerve stimulation when applied locally to the cortex. Intravenous administration of L-NAME did not affect the VII-elicited cerebral vasodilator response. It may be concluded that the parasympathetic nerves innervating the cerebral circulation contain NOS and furthermore, since blockade of NOS reduces parasympathetic vasodilatation, that NO is one of the transmitters in this system.

L-arginine ameliorates cerebral blood flow and metabolism and decreases infarct volume in rats with cerebral ischemia

Effects of L-arginine, 300 mg/kg, i.p., on the regional cerebral blood flow (rCBF), brain metabolism, and infarct volume were examined in spontaneously hypertensive rats subjected to occlusion of both left middle cerebral artery and left common carotid artery. Rats treated with L-arginine had higher rCBF, determined by hydrogen clearance method, in the ischemic core (7 +/- 1 ml/100 g/min, mean +/- S.E.M.) and penumbral regions (16 +/- 2) than did rats treated with saline (5 +/- 0 and 7 +/- 1, respectively). Simultaneously, L-arginine attenuated metabolic derangement in the ischemic tissue at 60 min, i.e. well maintained adenosine triphosphate (ATP) in ischemic region (1.29 +/- 0.07 mmol/kg in L-arginine group vs. 1.05 +/- 0.06 in saline group), and also close to normal levels in ATP (2.61 +/- 0.02 mmol/kg vs. 2.45 +/- 0.05), glucose (2.29 +/- 0.12 mmol/kg vs. 1.80 +/- 0.17) and lactate (1.63 +/- 0.10 mmol/kg vs. 2.24 +/- 0.21) in periischemic region. In another experiment, the effects of L-arginine on rCBF in the subcortical regions and on infarct volume were evaluated. L-arginine, compared with saline, increased rCBF by 8 ml/100 g/min in the ischemic side and reduced infarct volume by 29% at 24 h of ischemia. These findings support that L-arginine may be potentially useful for the treatment of acute cerebral ischemia.

L-arginine-induced conducted signals alter upstream arteriolar responsivity to L-arginine

Our purpose was to determine whether L-arginine was involved in vascular communication between downstream and upstream locations within a defined microvascular region. Arteriolar diameter was measured for the branches along a transverse arteriole in the superfused cremaster of anesthetized (pentobarbital sodium, 70 mg/kg i.p.) hamsters (N = 53). The upstream branch arterioles dilated significantly to locally applied L-arginine (100 mumol/L pipette concentration) only if the downstream branches (approximately 1400 microns away) were preexposed. With exposure order downstream to upstream, diameter change was last branch, -3.8 +/- 1.5% (of baseline); third, +58.1 +/- 27%; first, +92 +/- 26% (n = 5); with exposure order upstream to downstream: first branch, -0.4 +/- 3%; third, +5 +/- 11%; last, -5.6 +/- 7.5% (n = 4). Thus, downstream preexposure to L-arginine altered the responsivity upstream to locally applied L-arginine. Downstream-applied L-arginine also induced a conducted vasodilation (+17.8 +/- 2.8%; n = 14) 1327 +/- 166 microns upstream. This response was completely blocked by simultaneous sucrose (600 mOsm), halothane (0.0345%), or N omega-nitro-L-arginine (L-NNA, 100 mumol/L) exposure to the feed vessel (second micropipette) midway between the downstream site of L-arginine exposure and the upstream observation site. An acetylcholine-induced conducted vasodilation (+18.1 +/- 2.6%, n = 8) was also completely blocked by sucrose, halothane, or L-NNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of central inhibition of nitric oxide synthase on focal cerebral ischemia in rats

We have investigated whether central inhibition of nitric oxide synthase (NOS) could modify the tissue damage of focal cerebral ischemia produced by occlusion of the middle cerebral artery (MCA) in rats. NG-Nitro-L-arginine methyl ester (L-NAME) was administered intracerebroventricularly at two doses 15 min prior to occlusion of the MCA, as well as 4 and 24 h following occlusion. After the injection of L-NAME, the catalytic activity of the constitutive NOS, considered to be mainly neuronal, was effectively suppressed in the subcortical gray matter bilaterally, but not in the ischemic territory. Seven days after the MCA occlusion, the brains were evaluated for histopathologic damage. High-dose administration of L-NAME (120 micrograms/kg 15 min prior to MCA occlusion, followed by 150 micrograms/kg 4 and 24 h after occlusion) produced an enlargement of the infarct area and increased the volume of ischemic damage. These results indicate that extensive inhibition of NOS by a central route can increase the cerebral infarct size in focal ischemia even if NOS is not inhibited in the ischemic tissue and suggest that NO may also play a potentially beneficial role as well as a neurodestructive role in the pathophysiological mechanisms of focal cerebral ischemia.

Ex vivo demonstration of nitric oxide in the rat brain: effects of intrastriatal endothelin-1 injection

Nitric oxide (NO) is a novel transmitter with multiple functions in endothelium and neuronal tissue. In particular, it has been implicated in the pathogenesis of neurodegenerative diseases. The aim of the present study was to demonstrate the ex vivo detection of NO in basal conditions and after ET-1 intrastriatal injection by means of electron paramagnetic resonance (EPR) spectroscopy using locally injected hemoglobin (Hb) as a NO trapping agent. The extent of neostriatal damage after Hb and ET-1 injections was assessed by means of immunocytochemistry with a monoclonal antibody against dopamine and cAMP-phosphoprotein M(r) 32 (DARPP-32), which is considered a marker of striatal intrinsic neurons. In the absence of local Hb injection, no signal related to endogenous NO was detected in the neostriatum, suggesting that endogenous NO trapping agents are not sufficiently concentrated to allow NO detection with the present technique. Instead, 1 h after Hb injection, a clear nitrosyl-Hb signal can be detected in neostriatal homogenates. ET-1, a powerful vasoconstrictor agent, was used to cause neuronal loss in the neostriatum. No change in nitrosyl-Hb signal was observed in neostriatal 1 h after ET-1 injection, whereas an almost 3-fold increase in the signal intensity was present 24 h after ET-1 injection. The analysis of neostriatal damage showed that Hb injection did not cause either significant damage of striatal tissue or potentiation of ET-1-induced lesions. In conclusion, the present technique allows ex vivo detection of NO in the brain. The delayed increase in NO observed after ET-1 injection indicates that this molecule may participate in the development of slowly progressive neuronal damage occurring at late post-ischemic times.

Cerebrovascular alterations in mice lacking neuronal nitric oxide synthase gene expression

Nitric oxide (NO) is known to mediate increases in regional cerebral blood flow elicited by CO2 inhalation. In mice with deletion of the gene for neuronal NO synthase (NOS), CO2 inhalation augments cerebral blood flow to the same extent as in wild-type mice. However, unlike wild-type mice, the increased flow in mutants is not blocked by the NOS inhibition, N omega-nitro-L-arginine, and CO2 exposure fails to increase brain levels of cGMP. Topical acetylcholine elicits vasodilation in the mutants which is blocked by N omega-nitro-L-arginine, indicating normal functioning of endothelial NOS. Moreover, immunohistochemical staining for endothelial NOS is normal in the mutants. Thus, following loss of neuronal NOS, the cerebral circulatory response is maintained by a compensatory system not involving NO.

Nitric oxide in the central nervous system

The majority of the data on nitric oxide (NO) in the central nervous system (CNS) relies on histochemical and immunohistochemical evidence concerning the distribution of the nitric oxide synthase (NOS), its inhibition by specific antagonists and its co-localization with the receptor enzyme guanylate cyclase (GC) in the same functional region. All three isoforms, endothelial (eNOS), neural (nNOS) and macrophage type inducible (iNOS), are of importance to the normal and pathological function of the CNS. In nNOS gene deleted mice eNOS seems to contribute to the maintenance of neuronal function. NO may contribute to synaptic plasticity as a retrograde mediator that is released by postsynaptic NMDA-receptor activation. Microglia contains membrane-bound inducible iNOS that may be important in host defence function. Glia and pericytes surrounding the blood vessels contain GC that is stimulated by NO released from endothelium and nerve endings. Excessive production of highly reactive NO may be responsible for the neurotoxicity mediated by NMDA receptors that contributes to the symptomatology of strokes and neurodegenerative diseases. Moreover, after initial stimulation by cytokines, large amounts of NO produced by iNOS in the microglia (brain-based macrophages) may cause cellular damage.

Neuroprotection by peptide growth factors against anoxia and nitric oxide toxicity requires modulation of protein kinase C

Basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF) are neuroprotective during anoxia and nitric oxide (NO) toxicity. Signal transduction systems that modulate protein kinase C (PKC) also can modulate the toxic effects of anoxia and NO. We therefore examined whether PKC was involved in the protective effects of bFGF and EGF during anoxia and NO toxicity. Down-regulation or inhibition of PKC activity before anoxia or NO exposure prevented hippocampal neuronal degeneration. Yet, this protective effect of inhibition of PKC activity was not present with the coadministration of growth factors. Combined inhibition of PKC activity and application of bFGF or EGF lessened the protective mechanisms of the growth factors. In addition, the protective ability of the growth factors was lost during anoxia and NO exposure with the activation of PKC, suggesting that at least a minimal degree of PKC activation is necessary for growth factor protection. Although modulation of PKC activity may be a necessary prerequisite for protection against anoxia and NO toxicity by bFGF and EGF, only inhibition of PKC activity, rather than application of the growth factors, was protective following exposure to NO. These results suggest that the mechanism of protection by bFGF and EGF during anoxia and NO toxicity appears initially to be dependent on a minimum degree of PKC activation, but that other signal transduction pathways independent of PKC also may mediate protection by peptide growth factors.

Different mechanisms of L-arginine induced dilation of brain arterioles in normotensive and hypertensive rats

We evaluated the response of pial arterioles to L-arginine in anesthetized normotensive rats and spontaneously hypertensive rats equipped with a closed cranial window. Topical application of 10(-6)-10(-4) mol/l L-arginine, which is known to be the endogenous substrate for the synthesis of nitric oxide, induced dose-dependent arteriolar vasodilation. The response was more pronounced in hypertensive than in normotensive rats (at the concentration of 10(-4) mol/l L-arginine: 18.3 +/- 3.3% vs. 6.7 +/- 1.7%, respectively, means +/- S.E.). The stereoisomer D-arginine had no effect in hypertensive rats. Topical application of the nitric oxide synthase inhibitor N-nitro-L-arginine converted L-arginine-induced dilation to constriction in normotensive and hypertensive rats. The cyclooxygenase inhibitor indomethacin (5 micrograms/ml cerebrospinal fluid) also blocked the dilation in both strains. Photochemical endothelial injury blocked L-arginine-induced dilation in normotensive rats, but only partly antagonized the response in hypertensive animals. Intravenous or topical pretreatment with the free radical scavenger superoxide dismutase significantly reduced the dilating response to 10(-4) mol/l L-arginine in hypertensive rats. Superoxide dismutase did not significantly change the response to L-arginine in normotensive animals. It is concluded that nitric oxid formation in the endothelium and liberation of cyclooxygenase products cause L-arginine-induced dilation in normotensive rats. While nitric oxide and cyclooxygenase products are also involved in L-arginine-induced dilation in spontaneously hypertensive rats, superoxide radicals contribute to the enhanced response in this strain. This mechanism appears to be specific for the hypertensive animals and is only partly dependent on an intact endothelium.

Regional differences in cerebral vasomotor control by nitric oxide

Regional differences in the role of nitric oxide in cerebral vasomotor control were investigated with a nitric oxide synthesis inhibitor, NG-monomethyl-L-arginine, or a precursor of nitric oxide, L-arginine using both dog cerebral angiography for the larger artery study and rat isolated arterioles for the microcirculation study. NG-monomethyl-L-arginine (10 mumol) constricted the dog cerebral arteries, by 15.6%, 17.5%, and 27.3% in the middle cerebral, anterior cerebral, and basilar arteries, respectively. The greater constriction of the basilar artery did not reach statistical significance. However, L-arginine (100 mumol) produced significantly greater dilation of basilar arteries than the middle cerebral or anterior cerebral (31.3% vs. 16.7% or 13.1%). NG-monomethyl-L-arginine at 10(-3) M constricted rat arterioles originating from basilar arteries significantly more than the middle cerebral arteries (23% vs. 14%). L-arginine at 10(-3) M dilated rat arterioles from basilar arteries significantly more than from the middle cerebral artery (24 vs. 11%). These findings suggest that the roles of nitric oxide in vasomotor control differs by region in the brain, and it may be greater in vessels of the posterior than of the anterior circulation.

Different pial arteriolar responses to acetylcholine in the newborn and juvenile pig

Using the closed cranial window technique, the present study was designed to test the hypothesis that the pial arteriolar response to acetylcholine is age dependent. In newborn pigs (1-5 days old) pretreated with the phosphodiesterase inhibitor isobutyl methyl xanthine (IBMX), acetylcholine (10(-5) M) produced pial arteriolar constriction with no change in CSF cyclic GMP (cGMP) that was blocked by indomethacin (5 mg/kg i.v.). In contrast, in indomethacin- and IBMX-treated juvenile pigs (3-4 weeks old), acetylcholine (10(-) M) increased the pial arteriolar diameter by 17 +/- 1% and increased CSF cGMP by 2.1 +/- 0.3-fold. Similar vascular and biochemical changes for acetylcholine were observed in juvenile pigs pretreated with only IBMX. In the absence of IBMX, acetylcholine produced modest pial constriction in juvenile pigs. In the IBMX-pretreated juvenile pigs, L-nitroarginine (LNA; 10(-6) M) decreased pial arteriolar diameter by 15 +/- 2% and blocked acetylcholine-induced dilation and associated changes in CSF cGMP. A23187, a calcium ionophore, and sodium nitroprusside (SNP) elicited similar dilation and changes in CSF cGMP in both age groups. LNA blocked A23187 dilation, but SNP dilation was unchanged. L-Arginine (10(-3) M) partially restored acetylcholine- and A23187-induced dilation to indomethacin- and LNA-pretreated juvenile pigs. These data show that acetylcholine produces dilation in the juvenile pig through the production of the putative endothelium-derived relaxing factor (EDRF) nitric oxide but does not do so in the new born period. We speculate that contributions of EDRF to the acetylcholine-induced changes in pial arteriolar diameter develop with age.

Dual role of nitric oxide in focal cerebral ischemia

The importance of nitric oxide (NO) in the pathophysiology of cerebral ischemia was examined following middle cerebral artery occlusion in rats. A significant increase in infarct size developed following inhibition of NO synthase (NOS) activity by L-arginine analogues whereas intravenous L-arginine dose-dependently decreased infarct volume in the same models. Protection after L-arginine administration was associated with enhanced blood flow within the perinfarct zone as demonstrated by simultaneous recording of rCBF and electrocorticogram activity within subjacent brain. Selective NOS inhibition by 7-nitroindazole (7-NI) significantly reduced infarct volume at doses of 25 and 50 mg kg and in amounts that did not decrease the response of pial vessels to topical acetylcholine. Together these data suggest that enhanced NO production within the cerebrovasculature protects brain tissue during focal ischemia via hemodynamic mechanisms whereas neuronal overproduction may facilitate or mediate neurotoxicity. Recent data using transgenic animals lacking NOS activity support the latter conclusion.

The NOS inhibitor, 7-nitroindazole, decreases focal infarct volume but not the response to topical acetylcholine in pial vessels

We examined whether 7-nitroindazole (7-NI), a putative inhibitor of neuronal nitric oxide synthase (nNOS), decreases cerebral infarction 24 h after proximal middle cerebral artery (MCA) occlusion. In preliminary experiments, we determined that 7-NI (25, 50, and 100 mg/kg i.p.) decreased nitric oxide synthase (NOS) activity within cerebral cortex by 40-60% when measured up to 120 min, but not 240 min after administration. At 25 or 50 mg/kg, 7-NI did not alter the systemic arterial blood pressure or the dilation of pial arterioles after topical acetylcholine (10 and 100 microM). To examine the effect of 7-NI on infarct size, 55 Sprague-Dawley halothane-anesthetized rats were subjected to proximal MCA occlusion (modified Tamura method). Five minutes after occlusion, 7-NI (25 or 50 mg/kg i.p.) or vehicle was injected. Animals treated with 25 or 50 mg/kg showed 25 and 27% reductions in infarct volume, respectively. Coadministration of L-arginine (300 mg/kg i.p.) plus 7-NI (25 mg/kg i.p.) reversed the effect. If, indeed, the effects of 7-NI are mediated by inhibition of nNOS activity, these results suggest that enzymatic products of the neuronal isoform promote ischemic injury and that they do so at least within the first few hours after permanent occlusion. The results also emphasize the importance of developing strategies to selectively inhibit the neuronal isoform inasmuch as we observed previously that administering the less selective NOS inhibitor, N omega-nitro-L-arginine (L-NA), in the same model either caused no change or increased the volume of ischemic injury.

Nitric oxide regulates cerebral arteriolar tone in rats

BACKGROUND AND PURPOSE

Although cerebral penetrating arterioles are main regulators of the brain microcirculation, little is known about the effect of endothelium-derived relaxation factor on these vessels. This study examined the effects of nitric oxide synthase inhibitors on the spontaneous tone of isolated rat cerebral arterioles.

METHODS

Intraparenchymal penetrating arterioles (53 to 102 microns in passive diameter) isolated from Sprague-Dawley rats were cannulated with glass pipettes and subjected to 60 mm Hg of intraluminal pressure. The diameter response to intraluminal and extraluminal treatments was observed with an inverted microscope.

RESULTS

Extraluminal application of Nw-nitro-L-arginine (10(-5) mol/L) contracted the arterioles to 63.9 +/- 2.8% (P < .05) of the control diameter. This contracting effect was stereospecific and easily reversed by L-arginine dose dependently (10(-3), 10(-2) mol/L) but not by D-arginine. Intraluminally applied Nw-nitro-L-arginine also induced a similar degree of contraction. Another nitric oxide synthase inhibitor, NG-monomethyl L-arginine (10(-5), 10(-4) mol/L), applied extraluminally induced a dose-dependent contraction to 77.5 +/- 6.6% and 68.6 +/- 5.4% of the control (P < .05), which was also reversed by L-arginine. L-Arginine alone did not significantly affect vessel diameter, however. Treatment with indomethacin, a cyclooxygenase inhibitor, dilated the vessel to 115.2 +/- 7% (P < .05) but did not change the constricting effect of Nw-nitro-L-arginine.

CONCLUSIONS

Nw-Nitro-L-arginine and NG-monomethyl L-arginine produce substantial contraction in isolated brain arterioles, suggesting that nitric oxide of brain arterioles is continuously produced within the vessel wall. The dilatory effect of indomethacin appears to be independent of the vasoconstriction induced by nitric oxide synthase inhibitor. In these vessels, the effect of nitric oxide synthase inhibitors is not mediated by an indomethacin-sensitive mechanism. A balance probably exists between factors tending to constrict these arterioles and the elaboration of nitric oxide from endothelial cells, which tends to dilate them. The production of nitric oxide from isolated vessels indicates that parenchymal and vessel wall sources of nitric oxide are probably important in brain microcirculatory regulation.

Effects of cerebral ischemia in mice deficient in neuronal nitric oxide synthase

The proposal that nitric oxide (NO) or its reactant products mediate toxicity in brain remains controversial in part because of the use of nonselective agents that block NO formation in neuronal, glial, and vascular compartments. In mutant mice deficient in neuronal NO synthase (NOS) activity, infarct volumes decreased significantly 24 and 72 hours after middle cerebral artery occlusion, and the neurological deficits were less than those in normal mice. This result could not be accounted for by differences in blood flow or vascular anatomy. However, infarct size in the mutant became larger after endothelial NOS inhibition by nitro-L-arginine administration. Hence, neuronal NO production appears to exacerbate acute ischemic injury, whereas vascular NO protects after middle cerebral artery occlusion. The data emphasize the importance of developing selective inhibitors of the neuronal isoform.

Anti-ischaemic efficacy of a nitric oxide synthase inhibitor and a N-methyl-D-aspartate receptor antagonist in models of transient and permanent focal cerebral ischaemia

1. We have recently developed a new model of transient focal ischaemia in the rat utilising topical application of endothelin-1 to the left middle cerebral artery (MCA). In order to validate this approach the present study assessed the neuroprotective efficacy of the NMDA receptor antagonist dizocilpine (MK-801) in the endothelin-1 model. The anti-ischaemic efficacy of the nitric oxide (NO) synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) was subsequently evaluated, and contrasted with its efficacy against permanent focal ischaemia, to determine the utility of the endothelin-1 model for identification of novel pharmacoprotective agents. 2. MK-801 (0.12 mg kg-1 bolus, 108 micrograms kg-1 h-1 infusion i.v., either 1 or 2.5 h pre-transient MCA occlusion (MCAO)) induced hypotension that persisted for approximately 1.5 h so that mean arterial blood pressure (MABP) at the time of MCAO was significantly lower in the 1 h group compared with control (MABP: 86 +/- 11, 68 +/- 6 and 84 +/- 4 mmHg (mean +/- s.d.) for saline, 1 h MK-801 and 2.5 h MK-801 groups respectively). The 2.5 h pretreatment schedule resulted in significant reduction (71%) in the volume of hemispheric damage (assessed 4 h post onset of ischaemia) while the 1 h pretreatment schedule did not (volumes of hemispheric damage: 59 +/- 38, 51 +/- 51 and 17 +/- 28 mm3 for saline, 1 h and 2.5 h MK-801 groups). 3. Thus the considerable neuroprotective effect of MK-801 in the endothelin-1 model of transient focal cerebral ischaemia was highly sensitive to drug-induced hypotension. This result is in contrast to previous studies of permanent MCAO where MK-801-induced hypotension did not compromise its neuroprotective action.4. L-NAME (3 mg kg-1, i.v. 30 min pre-MCAO) moderately, but significantly, reduced (16%) the volume of ischaemic damage 4 h post-permanent MCA occlusion, whereas the 29% reduction in volume of damage achieved in the model of transient focal ischaemia did not attain significance due to the greater variability associated with this model. L-NAME did not significantly alter MABP in either model.5. The modest neuroprotection achieved with NO synthase inhibition suggests NO is of relatively minor importance as a mediator of neurotoxicity following permanent focal cerebral ischaemia. In addition the comparable efficacy of L-NAME against transient focal ischaemia suggests the presence of reperfusion does not enhance the contribution of NO to neuronal injury in the acute (4 h) phase following a focal ischaemic insult.

Failure to prevent selective CA1 neuronal death and reduce cortical infarction following cerebral ischemia with inhibition of nitric oxide synthase

We investigated the putative role of nitric oxide in the expression of neuronal injury following both transient severe forebrain ischemia (CA1 neuronal injury) and transient or permanent middle cerebral artery occlusion (neocortical pannecrosis). Using the four-vessel occlusion model and increasing doses of N-omega-nitro-L-arginine, 2-40 mg/kg, we were unable to demonstrate any reduction in the percentage of CA1 cells injured following 10 min of transient severe forebrain ischemia followed by seven days of reperfusion. Higher doses proved toxic insofar as they increased the mortality following the ischemic insult. Saline-treated animals (n = 8) had 77 +/- 10% CA1 injury while those treated with 2 mg/kg of nitro-arginine i.v. had 80 +/- 7% (n = 7), and those with 10 mg/kg i.v. had 78 +/- 11% (n = 8). Two of five rats given 20 mg/kg i.v., three of eight given 40 mg/kg i.v., and two of six given 10 mg/kg i.v. followed by 3 x 10 mg/kg i.p., died. Of those treated with high-dose nitro-arginine and which survived ischemia and seven days' reperfusion, no significant reduction in CA1 injury was detected. Wistar rats and spontaneously hypertensive rats treated with either saline or nitro-arginine i.v. were exposed to 2 h of transient middle cerebral artery occlusion followed by 22 h of reperfusion. There were seven animals in each group. Wistars treated with saline had 198 +/- 67 mm3 (mean +/- S.D.) of neocortical infarction, and those treated with 10 m/kg of nitro-arginine i.v. had 199 +/- 93 mm3. Spontaneously hypertensive rats, transiently ischemic, treated with saline had 164 +/- 25 mm3 of infarct volume, while those treated with 2 mg/kg i.v. had 151 +/- 53 mm3, and those treated with 10 mg/kg i.v. had 145 +/- 29 mm3. Animals treated with 40 mg/kg i.v. had a nonsignificantly larger mean infarct volume (191 +/- 81 mm3). High dose nitro-arginine caused an increase in hypertension in the spontaneously hypertensive rats and increased the severity of focal ischemia as measured by intra-ischemic regional cerebral blood flows. A final group of seven spontaneously hypertensive rats underwent permanent middle cerebral artery occlusion and repeated dosing with N-omega-nitro-L-arginine i.p. In these animals an infarct volume of 234 +/- 60 mm3 was observed, which was again not statistically different from saline-treated controls (208 +/- 43 mm3, n = 7).(ABSTRACT TRUNCATED AT 400 WORDS)

Electron paramagnetic resonance study on nitric oxide production during brain focal ischemia and reperfusion in the rat

The production of nitric oxide (NO) during brain focal ischemia and reperfusion was measured using diethyldithiocarbamate (DETC)/Fe-citrate, NO trapping reagents, and electron paramagnetic resonance spectroscopy. The NO production is potentiated after 5 min of ischemia, and is continued during 60 min of ischemia. During the reperfusion period after 60 min of ischemia, NO was also produced. However, its production during reperfusion was not observed when the ischemia time was less than 15 min. The NO signal during reperfusion after 60 min of ischemia decreased after 15 min of reperfusion. These results suggest that NO production during ischemia is a physiological reaction for increasing cerebral blood flow, while NO production during reperfusion may be related to cellular damage.

Regulation of baseline vascular resistance in the canine diaphragm by nitric oxide

1. The role played by nitric oxide (NO) in the regulation of blood flow to the canine isolated hemidiaphragm was evaluated by determining (a) the effects of the L-arginine analogues NG-nitro-L-arginine methyl ester (L-NAME), NG-nitro-L-arginine (L-NOARG), and argininosuccinic acid (ArgSA) on baseline vascular resistance and of the latter two agents on endothelium-dependent (acetylcholine, ACh) and endothelium independent (sodium nitroprusside, SNP) vasodilatation; (b) the effects of L- and D-arginine on baseline vascular resistance; and (c) the effects of L-glutamine, an inhibitor of intracellular recycling of L-citrulline to L-arginine, on baseline resistance and on the response to ACh and SNP. 2. L-NAME, L-NOARG and ArgSA (6 x 10(-4) M final concentration) increased baseline diaphragmatic vascular resistance to a similar extent (28.6 +/- 4.2%, 26.7 +/- 4.3% and 32.8 +/- 4.6% respectively). L-NOARG and ArgSA reversed the vasodilator effect of ACh but not of SNP. 3. L- and D-arginine had no effect on vascular resistance. 4. L-Glutamine (10(-3) M) increased baseline vascular resistance by 10 +/- 1.9% (P < 0.05) but did not alter responses to either ACh or SNP. 5. Basal NO release plays a role in the regulation of baseline diaphragmatic vascular resistance. L-Arginine analogues tested potently and specifically inhibited this process. Moreover, extracellular L-arginine appears to have no effect on baseline diaphragmatic vascular resistance.

Nitric oxide synthase inhibition and cerebrovascular regulation

There is increasing evidence that nitric oxide (NO) is an important molecular messenger involved in a wide variety of biological processes. Recent data suggest that NO is also involved in the regulation of the cerebral circulation. Thus, NO participants in the maintenance of resting cerebrovascular tone and may play an important role in selected vasodilator responses of the cerebral circulation. Furthermore, evidence has been presented suggesting that NO participates in the mechanisms of cerebral ischemic damage. Despite the widespread attention that NO has captured in recent years and the large number of studies that have been published on the subject, there is considerable controversy regarding the role of this agent in cerebrovascular regulation and in ischemic damage. In this paper the results of investigations on NO and the cerebral circulation are reviewed and the evidence for and against a role of NO is critically examined.

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.

Nitric oxide donors increase blood flow and reduce brain damage in focal ischemia: evidence that nitric oxide is beneficial in the early stages of cerebral ischemia

We studied whether administration of nitric oxide (NO) donors reduces the ischemic damage resulting from middle cerebral artery (MCA) occlusion in spontaneously hypertensive rats (SHRs). In halothane-anesthetized and ventilated SHRs, the MCA was occluded. CBF was monitored using a laser-Doppler flowmeter. Three to five minutes after MCA occlusion, the NO donors sodium nitroprusside (SNP; 3 mg/kg/h) or 3-morpholino-sydnonimine (SIN 1; 1.5-6 mg/kg/h) were administered into the carotid artery for 60 min. As a control, the effect of papaverine (3.6 mg/kg/h), a vasodilator that acts independently of NO, was also studied. The hypotension evoked by these agents was counteracted by intravenous infusion of phenylephrine. At the end of the infusion, rats were allowed to recover. Stroke size was determined 24 h later in thionin-stained sections. In sham occluded rats, SNP (n = 5), SIN 1 (n = 5), and papaverine (n = 5) produced comparable increases in CBF (p > 0.05 from vehicle). After MCA occlusion, SNP (n = 5) and SIN 1 (n = 5), but not papaverine (n = 5), enhanced the recovery of CBF (p < 0.05 from vehicle) and reduced the size of the infarct by 28 +/- 12 and 32 +/- 7%, respectively (mean +/- SD; p < 0.05 from vehicle). To determine whether NO donors could act by inhibiting platelet aggregation, we studied the effect of SNP on collagen-induced platelet aggregation. Intracarotid administration of SNP (3 mg/kg/h for 60 min) did not affect platelet aggregation to collagen, suggesting that the protective effect of NO donors was not due to inhibition of platelet function. We conclude that NO donors increase CBF to the ischemic territory and reduce the tissue damage resulting from focal ischemia. The protective effect may result from an increase in CBF to the ischemic territory, probably the ischemic penumbra. These findings suggest that NO donors may represent a new therapeutic strategy for the management of acute stroke.

Involvement of calcitonin gene-related peptide (CGRP) and nitric oxide (NO) in the pial artery dilatation elicited by cortical spreading depression

The aim of the present study was to examine whether the initial transient arterial dilatation during cortical spreading depression (CSD) was mediated by the release of calcitonin gene-related peptide (CGRP) and/or nitric oxide (NO). This question is of interest as the initial phase of CSD appears to be a model of events occurring during functional hyperemia and during the first period of classic migraine. Using an open cranial window technique, pial arterial diameter in the parietal cortex of cats was recorded with an image splitting method. Employing micropuncture technique, perivascularly applied CGRP8-37 did not alter the resting diameter of pial arteries but antagonized concentration dependently (5 x 10(-9)-10(-6) M) the dilatation (35%) due to 5 x 10(-8) M CGRP. NG-Nitro-L-Arginine (NOLAG, 10(-4) M) also had no effect on resting diameter of pial arteries, indicating that their resting tone is neither mediated by a continuous release of CGRP nor of NO. CSD was triggered by a remote intracortical injection of KCl (150 mM) and recorded by a microelectrode placed adjacent to the artery under investigation. CSD elicited a transient negative DC shift which was accompanied by a peak dilatation of 44 +/- 5.2% (S.E.M.). This dilatation was reduced by approximately 50% during topical application of 10(-7) M CGRP8-37 and 10(-4) M NOLAG each. A 75% inhibition of the CSD-induced dilatation was found during simultaneous application of both compounds. These data indicate that the initial dilatation during CSD is mediated, at least in part, by a release of CGRP and NO.(ABSTRACT TRUNCATED AT 250 WORDS)

L-arginine infusion promotes nitric oxide-dependent vasodilation, increases regional cerebral blood flow, and reduces infarction volume in the rat

BACKGROUND AND PURPOSE

We previously reported that L-arginine infusion increased pial vessel diameter by nitric oxide-dependent mechanisms, improved regional cerebral blood flow (rCBF) distal to middle cerebral artery (MCA) occlusion, and reduced infarction volume in spontaneously hypertensive rats when administered intraperitoneally before and after MCA occlusion. In this report we extend our findings (1) by examining the time course of L-arginine on rCBF and pial vessel diameter under basal conditions and on rCBF after MCA occlusion and (2) by reproducing the protective effect of L-arginine on infarct volume when given intravenously immediately after the onset of MCA occlusion in both normotensive and hypertensive models of focal cerebral ischemia.

METHODS

Changes in pial vessel diameter (closed cranial window) and rCBF (laser-Doppler flowmetry) were measured over time after L-arginine infusion into anesthetized Sprague-Dawley rats. rCBF was also measured distal to MCA occlusion in a brain region showing rCBF reductions in the range of 80% of baseline. The effects of infusing L-arginine (300 mg/kg for 10 minutes beginning 5 minutes after occlusion) were assessed on infarction volume in Sprague-Dawley rats after proximal MCA occlusion and in spontaneously hypertensive rats after common carotid artery plus distal MCA occlusion.

RESULTS

L-Arginine (300 mg/kg IV) elevated rCBF by 20% when measured in the dorsolateral cortex of Sprague-Dawley rats and caused L-nitroarginine-methyl ester-inhibitable increases in pial vessel diameter. L-Arginine (> or = 30 mg/kg IV) increased blood flow distal to MCA occlusion by 50%. These effects were sustained throughout the observation period (70 to 105 minutes). Changes in mean arterial blood pressure were not observed. L-Arginine (300 mg/kg IV) reduced infarction volume by 35% and 28% in Sprague-Dawley and spontaneously hypertensive rats, respectively, when examined 24 hours after vessel occlusion.

CONCLUSIONS

These studies extend our previous findings by demonstrating that exogenous L-arginine induces sustained rCBF increases in normal brain as well as in a marginally perfused brain region distal to MCA occlusion. Our data in Sprague-Dawley rats support the conclusion that L-arginine-induced increases in rCBF can decrease infarction volume. We conclude that nitric oxide-mediated mechanisms increase rCBF and decrease infarction volume after MCA occlusion in both normotensive and hypertensive animals.

The complex role of nitric oxide in the pathophysiology of focal cerebral ischemia

Nitrogen monoxide (NO) has recently emerged as an important mediator of cellular and molecular events which impacts the pathophysiology of cerebral ischemia. Although tempting to ask whether NO is "good or bad" for cerebral ischemia, the question underestimates the complexities of NO chemistry and physiology as well as oversimplifies the pathophysiology of focal cerebral ischemia. Important vascular and neuronal actions of NO have been defined which both enhance tissue survival and mediate cellular injury and death, and these will be reviewed. Strategies which modify NO synthesis and/or metabolism may someday assume therapeutic importance, but not until the tissue compartments generating NO, the activities of the enzymes that are inducibly and constitutively expressed, and the redox state of NO during the stages of ischemic injury, are defined with greater precision. Our knowledge of these processes is rudimentary. This review will summarize the evidence from animal models which supports an emerging role for NO in ischemic pathophysiology. Important aspects of NO synthesis and inhibitors of this process will also be discussed.

Basic fibroblast growth factor dilates rat pial arterioles

Basic fibroblast growth factor (bFGF) is a polypeptide that promotes the survival and differentiation of brain neurons, glia, and endothelial cells. It has been shown recently that intravenously administered bFGF lowers blood pressure by systemic vasodilation; this effect is mediated, in part, by nitric oxide (NO)-dependent mechanisms. In the current study, we directly evaluated the effect of bFGF on pial arterioles of pentobarbital-anesthetized Sprague-Dawley rats (n = 18) using the closed cranial window technique. Basic FGF (5-200 ng/ml) produced dose-dependent vasodilation; maximal vessel diameter (approximately 120% of control) was reached at 100 ng/ml. No vasodilation was found when bFGF was heat inactivated, or preincubated with blocking antibody. Moreover, bFGF-induced vasodilation was attenuated by coadministration of the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME), consistent with an NO-dependent mechanism. These results suggest that bFGF may play an important role in the regulation of cerebrovascular tone and cerebral blood flow.

Diminished effect of inhibition of nitric oxide synthase on regional cerebral vascular resistance in conscious and in isoflurane anesthetized rats during hemorrhage

Effects of a nitric oxide (NO) synthase inhibitor on regional cerebral vascular resistance (rCVR) and regional cerebral blood flow (rCBF) were studied during a severe hemorrhage in conscious and in isoflurane anesthetized groups of rats. Half of each group was infused with NG-nitro-L-arginine-methyl ester (L-NAME), a NO synthase inhibitor, at a rate of 2 mg.kg-1.min-1 for 30 min. Half of the L-NAME infused and half of the normal saline infused rats were bled to reduce the mean arterial blood pressure (MAP) to 44-49 mmHg. rCBF was measured using [14C]iodoantipyrine. rCVR was calculated as the ratio of MAP to rCBF. In the conscious non-hemorrhagic rats, L-NAME markedly increased rCVR in all the brain regions that we studied. In the conscious rats without L-NAME treatment, hemorrhage decreased rCVR in most of the brain regions. With L-NAME treatment in this group, hemorrhage increased rCVR only in the rostral part of the brain. Isoflurane decreased rCVR in most of the brain regions except the cortical area. L-NAME markedly increased rCVR in all the brain regions that we studied in the isoflurane anesthetized rats. In the isoflurane anesthetized rats, hemorrhage did not reduce rCVR in any of the brain regions. In the isoflurane anesthetized hemorrhagic rats, L-NAME did not significantly affect rCVR in any of the brain regions that we studied. We found that L-NAME increased rCVR to a greater extent in the non-hemorrhagic rats than in the hemorrhagic rats in both the conscious and in the isoflurane anesthetized rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Nitric oxide protects against cellular damage and cytotoxicity from reactive oxygen species

Nitric oxide, NO, which is generated by various components of the immune system, has been presumed to be cytotoxic. However, NO has been proposed to be protective against cellular damage resulting during ischemia reperfusion. Along with NO there is often concomitant formation of superoxide/hydrogen peroxide, and hence a synergistic relationship between the cytotoxic effects of nitric oxide and these active oxygen species is frequently assumed. To study more carefully the potential synergy between NO and active oxygen species in mammalian cell cytotoxicity, we utilized either hypoxanthine/xanthine cell cytotoxicity, we utilized either hypoxanthine/xanthine oxidase (a system that generates superoxide/hydrogen peroxide) or hydrogen peroxide itself. NO generation was accomplished by the use of a class of compounds known as "NONOates," which release NO at ambient temperatures without the requirement of enzyme activation or biotransformation. When Chinese hamster lung fibroblasts (V79 cells) were exposed to hypoxanthine/xanthine oxidase for various times or increasing amounts of hydrogen peroxide, there was a dose-dependent decrease in survival of V79 cells as measured by clonogenic assays. However, in the presence of NO released from (C2H5)2N[N(O)NO]-Na+ (DEA/NO), the cytotoxicity resulting from superoxide or hydrogen peroxide was markedly abrogated. Similarly, primary cultures of rat mesencephalic dopaminergic cells exposed either to hydrogen peroxide or to hypoxanthine/xanthine oxidase resulted in the degradation of the dopamine uptake and release mechanism. As was observed in the case of the V79 cells, the presence of NO essentially abrogated this peroxide-mediated cytotoxic effect on mesencephalic cells.

Endothelium-derived vasoactive factors and regulation of the cerebral circulation

Vasoactive factors produced and released by endothelium exert a powerful influence on vascular tone in the cerebral circulation. Endothelium-derived relaxing factor (EDRF), which has been identified as nitric oxide (NO) or an NO-containing compound, is produced under basal conditions in cerebral blood vessels. EDRF mediates endothelium-dependent relaxation in response to a number of stimuli in the cerebral circulation. The influence of NO on the cerebral circulation appears to be particularly important and complex because both neurons and glia, in addition to endothelium, produce NO in response to some stimuli. Neuronally derived NO may mediate local vasodilation in response to increased neuronal activity. In addition to EDRF, cerebral endothelium may produce other relaxing factors, including prostacyclin, endothelium-derived hyperpolarizing factor, and oxygen-derived free radicals. Several pathophysiological conditions are associated with impaired endothelium-dependent responses that may involve the decreased production of EDRF and release of endothelium-derived contracting factors, such as the cyclooxygenase products of arachidonic acid and endothelin. The release of endothelin, an extremely potent and long-lasting vasoconstrictor peptide, may contribute to vasospasm after subarachnoid hemorrhage.

Physiological mechanisms and pharmacological regulation of acute collateral perfusion

1. Pre-existing collateral vessels provide an alternative route for blood flow following acute arterial occlusion. 2. In a novel model of acute arterial occlusion in the isolated rabbit ear vascular bed the development and maintenance of collateral perfusion has an obligatory dependence on endothelium-derived relaxing factor (EDRF) activity. 3. This endothelium-dependent mechanism is severely impaired in dietary-induced hypercholesterolaemia, leading to substantially reduced collateral perfusion. 4. Collateral perfusion may also be influenced by vasodilators, specifically in the rabbit ear model the potassium channel opener BRL 38227 greatly enhances perfusion, sodium nitroprusside (an exogenous analogue of EDRF) has early but limited beneficial effects while verapamil is without effect.