Cerebral O2 metabolism and cerebral blood flow in humans during deep and rapid-eye-movement sleep

It could be expected that the various stages of sleep were reflected in variation of the overall level of cerebral activity and thereby in the magnitude of cerebral metabolic rate of oxygen (CMRO2) and cerebral blood flow (CBF). The elusive nature of sleep imposes major methodological restrictions on examination of this question. We have now measured CBF and CMRO2 in young healthy volunteers using the Kety-Schmidt technique with 133Xe as the inert gas. Measurements were performed during wakefulness, deep sleep (stage 3/4), and rapid-eye-movement (REM) sleep as verified by standard polysomnography. Contrary to the only previous study in humans, which reported an insignificant 3% reduction in CMRO2 during sleep, we found a deep-sleep-associated statistically highly significant 25% decrease in CMRO2, a magnitude of depression according with studies of glucose uptake and reaching levels otherwise associated with light anesthesia. During REM sleep (dream sleep) CMRO2 was practically the same as in the awake state. Changes in CBF paralleled changes in CMRO2 during both deep and REM sleep.     

Body and brain temperature coupling: the critical role of cerebral blood flow

Direct measurements of deep-brain and body-core temperature were performed on rats to determine the influence of cerebral blood flow (CBF) on brain temperature regulation under static and dynamic conditions. Static changes of CBF were achieved using different anesthetics (chloral hydrate, CH; α-chloralose, αCS; and isoflurane, IF) with αCS causing larger decreases in CBF than CH and IF; dynamic changes were achieved by inducing transient hypercapnia (5% CO₂ in 40% O₂ and 55% N₂). Initial deep-brain/body-core temperature differentials were anesthetic-type dependent with the largest differential observed with rats under αCS anesthesia (ca. 2°C). Hypercapnia induction raised rat brain temperature under all three anesthesia regimes, but by different anesthetic-dependent amounts correlated with the initial differentials—αCS anesthesia resulted in the largest brain temperature increase (0.32 ± 0.08°C), while CH and IF anesthesia lead to smaller increases (0.12 ± 0.03 and 0.16 ± 0.05°C, respectively). The characteristic temperature transition time for the hypercapnia-induced temperature increase was 2–3 min under CH and IF anesthesia and ~4 min under αCS anesthesia. We conclude that both, the deep-brain/body-core temperature differential and the characteristic temperature transition time correlate with CBF: a lower CBF promotes higher deep-brain/body-core temperature differentials and, upon hypercapnia challenge, longer characteristic transition times to increased temperatures.     

α-Phenyl N-Tert-Butyl Nitrone (PBN) Increases the Cortical Cerebral Blood Flow by Inhibiting the Breakdown of Nitric Oxide in Anesthetized Rats

The effects of intravenous administration of a-phenyl N-tert-butyl nitrone (PBN) on cortical cerebral blood flow (CBF) were examined in Wistar rats under pentobarbital anesthesia and artificial ventilation. The cortical CBF in parietal cortex was measured by laser Doppler flowmetry. Intravenous administrations of 2 mg/kg and 20 mg/kg of PBN dose-dependently produced significant increases in cortical CBF and decreases in systemic blood pressure (BP). To examine whether these increased responses in cortical CBF produced by PBN were associated with the vasodilatation system of nitric oxide (NO), the NO synthase inhibitor L-N^G-nitroarginine (L-NOArg), which is an analog of L-arginine, was used to inhibit the NO-related-vasodilatative system. Since the PBN-induced responses in the cortical CBF were much attenuated in L-NOArg-treated rats (30 mg/kg, iv.), it was inferred that NO-related vasodilatation was strongly associated with the PBN-induced increase in cortical CBF.     

Regional cerebral blood flow measurement in rats with radioactive microspheres

The effect of the method of heart catheterization on the measurement of cerebral blood flow (CBF) with radioactive microspheres was evaluated during various experimental procedures in male Sprague-Dawley rats. Catheters were inserted into the left ventricle via the right carotid or right subclavian artery or directly into the left atrium for microsphere injections. CBF was measured in cerebral cortical and subcortical tissues under control anesthetized (70 % N₂O, 30 % O₂), hypoxic or hypercapnic test conditions. Under control conditions, CBF was similar in the right vs the left cerebral hemisphere in subclavian artery and atrial catheterized rats but was greater in the left vs the right cortex in carotid catheterized animals (p<.05). During hypoxia and hypercapnia CBF increased equally in both cerebral hemispheres in atrial catheterized rats. The increase in CBF was significantly attenuated in the cerebral hemisphere ipsilateral to carotid catheterization during hypoxia and hypercapnia, although the percentage increase in flow was similar in both hemispheres. The results indicate the limitations of measuring regional CBF changes under experimental test conditions in rats with a ligated carotid artery and suggest that atrial catheterization is the method of choice when comparable changes in CBF are desired in both cerebral hemispheres.     

Perivascular nitric oxide and superoxide in neonatal cerebral hypoxia-ischemia

Decreased cerebral blood flow (CBF) has been observed following the resuscitation from neonatal hypoxic-ischemic injury, but its mechanism is not known. We address the hypothesis that reduced CBF is due to a change in nitric oxide (NO) and superoxide anion O(2)(-) balance secondary to endothelial NO synthase (eNOS) uncoupling with vascular injury. Wistar rats (7 day old) were subjected to cerebral hypoxia-ischemia by unilateral carotid occlusion under isoflurane anesthesia followed by hypoxia with hyperoxic or normoxic resuscitation. Expired CO(2) was determined during the period of hyperoxic or normoxic resuscitation. Laser-Doppler flowmetry was used with isoflurane anesthesia to monitor CBF, and cerebral perivascular NO and O(2)(-) were determined using fluorescent dyes with fluorescence microscopy. The effect of tetrahydrobiopterin supplementation on each of these measurements and the effect of apocynin and N(omega)-nitro-L-arginine methyl ester (L-NAME) administration on NO and O(2)(-) were determined. As a result, CBF in the ischemic cortex declined following the onset of resuscitation with 100% O(2) (hyperoxic resuscitation) but not room air (normoxic resuscitation). Expired CO(2) was decreased at the onset of resuscitation, but recovery was the same in normoxic and hyperoxic resuscitated groups. Perivascular NO-induced fluorescence intensity declined, and O(2)(-)-induced fluorescence increased in the ischemic cortex after hyperoxic resuscitation up to 24 h postischemia. L-NAME treatment reduced O(2)(-) relative to the nonischemic cortex. Apocynin treatment increased NO and reduced O(2)(-) relative to the nonischemic cortex. The administration of tetrahydrobiopterin following the injury increased perivascular NO, reduced perivascular O(2)(-), and increased CBF during hyperoxic resuscitation. These results demonstrate that reduced CBF follows hyperoxic resuscitation but not normoxic resuscitation after neonatal hypoxic-ischemic injury, accompanied by a reduction in perivascular production of NO and an increase in O(2)(-). The finding that tetrahydrobiopterin, apocynin, and L-NAME normalized radical production suggests that the uncoupling of perivascular NOS, probably eNOS, due to acquired relative tetrahydrobiopterin deficiency occurs after neonatal hypoxic-ischemic brain injury. It appears that both NOS uncoupling and the activation of NADPH oxidase participate in the changes of reactive oxygen concentrations seen in cerebral hypoxic-ischemic injury.     

Isoflurane Anesthesia Initiated at the Onset of Reperfusion Attenuates Oxidative and Hypoxic-Ischemic Brain Injury

This study demonstrates that in mice subjected to hypoxia-ischemia (HI) brain injury isoflurane anesthesia initiated upon reperfusion limits a release of mitochondrial oxidative radicals by inhibiting a recovery of complex-I dependent mitochondrial respiration. This significantly attenuates an oxidative stress and reduces the extent of HI brain injury. Neonatal mice were subjected to HI, and at the initiation of reperfusion were exposed to isoflurane with or without mechanical ventilation. At the end of HI and isoflurane exposure cerebral mitochondrial respiration, H₂O₂ emission rates were measured followed by an assessment of cerebral oxidative damage and infarct volumes. At 8 weeks after HI navigational memory and brain atrophy were assessed. In vitro, direct effect of isoflurane on mitochondrial H₂O₂ emission was compared to that of complex-I inhibitor, rotenone. Compared to controls, 15 minutes of isoflurane anesthesia inhibited recovery of the compex I-dependent mitochondrial respiration and decreased H₂O₂ production in mitochondria supported with succinate. This was associated with reduced oxidative brain injury, superior navigational memory and decreased cerebral atrophy compared to the vehicle-treated HI-mice. Extended isoflurane anesthesia was associated with sluggish recovery of cerebral blood flow (CBF) and the neuroprotection was lost. However, when isoflurane anesthesia was supported with mechanical ventilation the CBF recovery improved, the event associated with further reduction of infarct volume compared to HI-mice exposed to isoflurane without respiratory support. Thus, in neonatal mice brief isoflurane anesthesia initiated at the onset of reperfusion limits mitochondrial release of oxidative radicals and attenuates an oxidative stress. This novel mechanism contributes to neuroprotective action of isoflurane. The use of mechanical ventilation during isoflurane anesthesia counterbalances negative effect of isoflurane anesthesia on recovery of cerebral circulation which potentiates protection against reperfusion injury.     

A benzodiazepine antagonist inhibits the cerebral metabolic and respiratory depressant effects of fentanyl

It is reported that benzodiazepines such as diazepam will stimulate the opiate receptor system and that B-carboline drugs, which are benzodiazepine antagonists, may interact with opiate receptors directly. The ability of 3-hydroxymethyl-B-carboline (3-HMC) to antagonize several parameters of fentanyl anesthesia was tested here in rats. Fentanyl (25 and 100 micrograms/kg iv) produced dose dependent depression of cerebral blood flow (CBF), measured by radioactive microspheres, and cerebral oxygen consumption (CMRO2). These effects were significantly inhibited by 10 mg/kg 3-HMC iv. To test for the specificity of this effect, 3-HMC was also given to rats ventilated with inspire concentrations of 2% halothane. Halothane depressed CMRO2 equally in 3-HMC and vehicle treated rats, indicating no significant effect of the benzodiazepine antagonist. Blood pressure was increased in 3-HMC compared to vehicle treated animals during both fentanyl and halothane anesthesia. CBF was increased in 3-HMC vs vehicle treated rats during halothane anesthesia but this could be accounted for by the elevated blood pressure and lack of cerebral autoregulation rather than a direct cerebrovascular effect. 3-HMC decreased the sleep time and respiratory depressant effects of fentanyl but enhanced the analgesic effects of the opiate, as measured by time to respond to a hot plate stimulus. These results indicate that 3-HMC has the ability to specifically antagonize fentanyl anesthesia. These effects may be produced by an action of 3-HMC at the benzodiazepine receptor and/or by an action of the B-carboline at opioid receptors.     

Effect of hypothermia on brain multi-parametric activities in normoxic and partially ischemic rats

Hypothermia, as well as anesthesia, are known to protect the brain against ischemia, hypoxia and other pathological damages. One of the mechanisms of this improvement could be by lowering brain function, and thereby lowering oxygen demand. We examined the effect of hypothermia on brain function and blood supply in awake and anesthetized rats and studied the interaction between partial ischemia and the responses to hypothermia. The brain function multiprobe (BFM) used enabled simultaneous measurements of cerebral blood flow (CBF), mitochondrial NADH redox state, extracellular K(+) concentration, DC potential and ECoG from the cerebral cortex in rats whose brain temperature was lowered by 5 degrees C. Hypothermia was induced in awake, anesthetized and brain ischemic-anesthetized rats. In anesthetized and ischemic-anesthetized rats, the time required for lowering the brain temperature by 5 degrees C was five times less than in the normal awake animals. No significant changes in CBF and NADH levels were found in response to hypothermia in the awake animals. In contrast, a significant decrease in extracellular K(+) concentration was recorded under hypothermia, probably due to the lower rate of depolarization. Hypothermia in anesthetized and in ischemic-anesthetized rats did not significantly affect the levels of mitochondrial NADH, CBF and extracellular K(+). Hypothermia under ischemia was expected to be more effective.     

C-fos expression in rat brain during heat stress

Rats, under urethane anesthesia, 0, 20, 40 or 80 min after the start of heat stress (42°C) were sacrificed for determination of c-fos expression in different brain regions. In situ hybridization and immunocytochemistry methods were used, respectively, for determination of c-fos mRNA and protein, respectively. In general, either colon temperature (T_CO), mean arterial pressure (MAP), local cerebral blood flow (CBF) or c-fos expression in different brain regions (including the preoptic area, supraoptic nuclei, paraventricular nuclei, thalamus, amygdala, nucleus tract solitarii, area postrema and ventrolateral medulla) increased at 20–40 min after the start of heat exposure. However, the heatstroke, which appears as profound decreases in both MAP and local CBF and increases in T_CO, was produced 80 min after heat stress. The c-fos expression was heavily induced in all these brain regions after the onset of heatstroke. The data suggest that c-fos expression in rat brain during heatstroke is associated with hyperthermia, arterial hypotension or cerebral ischemia.     

Effect of hypercapnia on cerebral blood flow and blood volume in pigs studied by positron emission tomography

Pigs are increasingly used as in vivo models in neuroscience, including studies using positron emission tomography. During anesthesia, cerebral blood flow (CBF) and cerebral blood volume (CBV) are mainly regulated by the partial pressure of CO2 (pCO2) in arterial blood. We sought to determine the effects of increased arterial pCO2 (hypercapnia) on CBF and CBV in anesthetized domestic pigs. We anesthetized 4 pigs and manipulated the tidal volume of the ventilator to different pCO2 levels. Baseline pCO2 was on average 6.5 kPa (n = 9 periods) and hypercapnia pCO2 ranged from 11 to 20 kPa, mean 18.5 kPa (n = 9 periods). Series of dynamic PET scans with H(2)15O (CBF measurements) and C15O (CBV measurements) were performed. CBF increased on average 54%, from mean 0.48 ml blood/min/ml brain tissue during normoxia to 0.74 ml blood/min/ml brain tissue during hypercapnia. CBV increased 41% from mean 0.061 ml blood/ml brain tissue (n = 6) during normoxia to 0.086 ml blood/ml brain tissue (n = 6) during hypercapnia. Our observations indicate that pCO2 levels have a major influence on porcine CBF and CBV and should be controlled in studies where a constant level is crucial.     

CEREBRAL BLOOD FLOW AND CEREBRAL METABOLIC RATES FOR OXYGEN, GLUCOSE, AND KETONE BODIES IN NEWBORN DOGS

Cerebral blood flow (CBF) and the cerebral metabolic rates for oxygen, glucose, acetoacetate, β-hydroxybutyrate and lactate were measured in 1- to 5-day old Beagle dogs under nitrous oxide anesthesia. CBF was determined by ¹³³Xe washout with mechanically integrated blood samples withdrawn simultaneously from a femoral artery and from the posterior one-third of the superior sagittal sinus. CBF and CMRO₂ in normocapnia (PaCO₂ 40 × 1 mm Hg) were 48 × 5 ml/100 g/min and 2.15 ml/100 g/min, respectively. There was a positive, linear relationship between CBF and PaCO₂, calculated for PaCO₂ values ranging from 26 to 70 mm Hg. Induced hypocapnia (PaCO₂ 31 × 1 mm Hg) or hypercapnia (PaCO₂ 58 × 2 mm Hg) did not alter the CMRO₂. Glucose and acetoacetate were taken up by the brain at all PaCO₂ levels examined; however, the cerebral uptake of glucose always exceeded the combined uptake of ketone bodies by more than a factor of ten. The cerebral metabolic rate for glucose (94.6 × 3.6 μmol/100 g/min) more than accounted for overall cerebral oxygen consumption, and yielded an oxygen:glucose ratio (mol:mol) of 5.1. Thus, as in adult animals, PaCO₂ is an important regulator of cerebral blood flow in puppies, and glucose is the major substrate for oxidative energy production in the immature brain. The oxidation of ketone bodies by the newborn dog brain accounts for not more than 6% of the in vivo cerebral oxygen consumption.     

腺苷在低氧适应家兔脑血流调节中的作用

本研究观察了腺苷在低氧适应家兔脑血流(CBF)调节中的作用。结果表明:缺氧时适应组CBF改变不明显,对照组CBF明显增加;缺氧时适应组脑腺苷的含量明显低于对照组,而脑腺苷酸的含量明显高于对照组;适应组脑微血管对腺苷的反应与对照组相近。提示缺氧时低氧适应家兔CBF改变不明显,同低氧适应后脑腺苷含量较低、腺苷酸含量较高有关。     

Interpretation of near-infrared spectroscopy signals: a study with a newly developed perfused rat brain model.

Using a newly developed perfused rat brain model, we examined direct effects of each change in cerebral blood flow (CBF) and oxygen metabolic rate on cerebral hemoglobin oxygenation to interpret near-infrared spectroscopy signals. Changes in CBF and total hemoglobin (tHb) were in parallel, although tHb showed no change when changes in CBF were small (< or =10%). Increasing CBF caused an increase in oxygenated hemoglobin (HbO(2)) and a decrease in deoxygenated hemoglobin (deoxy-Hb). Decreasing CBF was accompanied by a decrease in HbO(2), whereas changes in direction of deoxy-Hb were various. Cerebral blood congestion caused increases in HbO(2), deoxy-Hb, and tHb. Administration of pentylenetetrazole without increasing the flow rate caused increases in HbO(2) and tHb with a decrease in deoxy-Hb. There were no significant differences in venous oxygen saturation before vs. during seizure. These results suggest that, in activation studies with near-infrared spectroscopy, HbO(2) is the most sensitive indicator of changes in CBF, and the direction of changes in deoxy-Hb is determined by the degree of changes in venous blood oxygenation and volume.     

Nitric oxide and carbon dioxide in neurotoxicity induced by oxygen under pressure

Hyperbaric oxygen (HBO2) causes CO2 retention in the brain that leads to the increase in cerebral blood flow (CBF) by poorly understood mechanisms. We have tested the hypothesis that NO is implicated in CBF-responses to hypercapnia under hyperoxic conditions. Alert rats were exposed to HBO2 at 5 ata and blood flow in the striatum measured by H2 clearance every 10 min. Acetazolamide, the inhibitor of carbonic anhydrase, was used to increase brain PCO2. CBF responses to acetazolamide administration (30 mg/kg, i.p.) were assessed in rats breathing air at 1 ata or oxygen at 5 ata with and without NOS inhibition (L-NAME, 30 mg/kg, i.p.). In rats breathing air, acetazolamide increased CBF by 34 +/- 7.4% over 30 min and by 28 +/- 12% over 3 hours while NOS inhibition with L-NAME attenuated acetazolamide-induced cerebral vasodilatation. HBO2 at 5 ata reduced CBF during the first 30 min hyperoxia, after that CBF increased by 55 +/- 19% above pre-exposure levels. In acetazolamide-treated animals, no HBO, induced vasoconstricton was observed and striatal blood flow increased by 53 +/- 18% within 10 min of hyperbaric exposure. After NOS inhibition, cerebral vasodilatation in response to acetazolamide during HBO2 exposure was significantly attenuated. The study demonstrates that NO is implicated in acetazolamide (CO2)-induced cerebral hyperemia under hyperbaric oxygen exposure.     

A severe vicious cycle in uncontrolled subarachnoid hemorrhage: the effects on cerebral blood flow and hemodynamic responses upon intracranial hypertension

In subarachnoid hemorrhage (SAH), Cushing postulated that the increase in systemic arterial pressure (SAP) in response to elevation of intracranial pressure (ICP) was beneficial to cerebral perfusion. However, in uncontrolled SAH, the increased SAP may cause more bleeding into the subarachnoid space and further increase the ICP. We created an animal model to simulate SAH by connecting a femoral arterial catheter to the subarachnoid space. The global cerebral blood flow (CBF) was measured with a venous outflow method. The purposes were to observe the CBF change under the simulated SAH, and to evaluate the effects of an adrenergic blocker and a vasodilator. In addition, spectral analysis of the aortic pressure and flow was employed for the analysis of hemodynamic changes at various ICP levels. When the femoral arterial blood was allowed to flow into the subarachnoid space, the ICP was elevated. The Cushing response to increased ICP caused an increase in SAP. A vicious cycle was generated between ICP and SAP. The CBF under the vicious cycle was greatly depressed. The dog developed pulmonary edema (PE) within 5 mins. An alpha-adrenergic blocker (phentolamine) and a vasodilator (nitroprusside) were beneficial to the reduction of SAP and ICP, improvement of CBF, and prevention of PE. Hemodynamic analysis revealed that graded increases in ICP caused increases in SAP, total peripheral resistance, arterial impedance, and pulse reflection with decreases in stroke volume, cardiac output and arterial compliance. The hemodynamic changes may contribute to acute left ventricular failure that leads to pressure and volume loading in the lung circulation, and finally acute PE.     

Effects of sodium nitroprusside-induced hypotension on the cerebral and hindlimb blood flow in dogs

Sodium nitroprusside (SNP) has been commonly used as a vasodilator agent for deliberate hypotension with general anesthesia. The purpose of this study was to observe whether cerebral blood flow (CBF) was significantly reduced when SNP infusion was accomplished to decrease peripheral blood flows with systemic hypotension. We conducted the experiments in 15 pentobarbital-anesthetized dogs. CBF was measured in 7 dogs using a venous outflow method. Hindlimb blood flow (HBF) serving as a representative of the peripheral circulations was obtained by flow measurement in the femoral artery in 8 dogs. The systemic arteral pressure (SAP) was decreased stepwise (approximately 5 mmHg for each step) by adjusting the SNP infusion rate. During the systemic hypotension, the CBF remained fairly constant despite a marked decline in the mean SAP to 40 mmHg. The calculated cerebral vascular resistance was progressively decreased with the systemic hypotension. On the contrary, a reduction in the HBF was observed accompanying the fall in SAP. When the mean SAP was decreased to 50 mmHg, the HBF was only 46.3 +/- 7.6% of the control value. The calculated hindlimb vascular resistance was slightly elevated during the whole course of SNP-induced hypotension. The results reveal the disparity between the brain and hindlimb in the resistance and flow responses to SNP-induced hypotension. The constancy of CBF subserves adequate brain perfusion when deliberate hypotension is conducted for surgery in the peripheral organs.     

Noninvasive Optical Measurement of Cerebral Blood Flow in Mice Using Molecular Dynamics Analysis of Indocyanine Green

In preclinical studies of ischemic brain disorders, it is crucial to measure cerebral blood flow (CBF); however, this requires radiological techniques with heavy instrumentation or invasive procedures. Here, we propose a noninvasive and easy-to-use optical imaging technique for measuring CBF in experimental small animals. Mice were injected with indocyanine green (ICG) via tail-vein catheterization. Time-series near-infrared fluorescence signals excited by 760 nm light-emitting diodes were imaged overhead by a charge-coupled device coupled with an 830 nm bandpass-filter. We calculated four CBF parameters including arrival time, rising time and mean transit time of a bolus and blood flow index based on time and intensity information of ICG fluorescence dynamics. CBF maps were generated using the parameters to estimate the status of CBF, and they dominantly represented intracerebral blood flows in mice even in the presence of an intact skull and scalp. We demonstrated that this noninvasive optical imaging technique successfully detected reduced local CBF during middle cerebral artery occlusion. We further showed that the proposed method is sufficiently sensitive to detect the differences between CBF status in mice anesthetized with either isoflurane or ketamine–xylazine, and monitor the dynamic changes in CBF after reperfusion during transient middle cerebral artery occlusion. The near-infrared optical imaging of ICG fluorescence combined with a time-series analysis of the molecular dynamics can be a useful noninvasive tool for preclinical studies of brain ischemia.     

Nitric oxide mediates hypoxia-induced cerebral vasodilation in humans.

Nitric oxide (NO) plays a pivotal role in the regulation of peripheral vascular tone. Its role in the regulation of cerebral vascular tone in humans remains to be elucidated. This study investigates the role of NO in hypoxia-induced cerebral vasodilatation in young healthy volunteers. The effect of the NO synthase inhibitor N(G)-monomethyl-L-arginine (L-NMMA) on the cerebral blood flow (CBF) was assessed during normoxia and during hypoxia (peripheral O(2) saturation 97 and 80%, respectively). Subjects were positioned in a magnetic resonance scanner, breathing normal air (normoxia) or a N(2)-O(2) mixture (hypoxia). The CBF was measured before and after administration of L-NMMA (3 mg/kg) by use of phase-contrast magnetic resonance imaging techniques. Administration of L-NMMA during normoxia did not affect CBF. Hypoxia increased CBF from 1,049 +/- 113 to 1,209 +/- 143 ml/min (P < 0.05). After L-NMMA administration, the augmented CBF returned to baseline (1,050 +/- 161 ml/min; P < 0.05). Similarly, cerebral vascular resistance declined during hypoxia and returned to baseline after administration of L-NMMA (P < 0.05 for both). Use of phase-contrast magnetic resonance imaging shows that hypoxia-induced cerebral vasodilatation in humans is mediated by NO.     

P-450 epoxygenase and NO synthase inhibitors reduce cerebral blood flow response to N-methyl-D-aspartate

Epoxyeicosatrienoic acids are cerebral vasodilators produced in astrocytes by cytochrome P-450 epoxygenase activity. The P-450 inhibitor miconazole attenuates the increase in cerebral blood flow (CBF) elicited by glutamate. We evaluated whether epoxygenase activity is involved in the CBF response to activation of the N-methyl-D-aspartate (NMDA) receptor subtype by using two structurally distinct inhibitors, miconazole and N-methylsulfonyl-6-(2-propargyloxyphenyl) hexanamide (MS-PPOH), a selective epoxygenase substrate inhibitor. Drugs were delivered locally through microdialysis probes in striata of anesthetized rats. Local CBF was measured by hydrogen clearance and compared with CBF in contralateral striatum receiving vehicle. Microdialysis perfusion of NMDA doubled CBF and increased nitric oxide (NO) production estimated by recovery of labeled citrulline in the dialysate during labeled arginine infusion. Perfusion of miconazole or MS-PPOH blocked the increase in CBF without decreasing citrulline recovery. Perfusion of N(omega)-nitro-L-arginine decreased baseline CBF and inhibited the CBF response to NMDA. Perfusion of MS-PPOH did not inhibit the CBF response to sodium nitroprusside. We conclude that both the P-450 epoxygenase and NO synthase pathways are involved in the local CBF response to NMDA receptor activation, and that the signaling pathway may be more complex than simply NO diffusion from neurons to vascular smooth muscle.     

Inhibition of nitric oxide synthase does not alter dynamic cerebral autoregulation in humans

The aim of this study was to determine whether inhibition of nitric oxide synthase (NOS) alters dynamic cerebral autoregulation in humans. Beat-to-beat blood pressure (BP) and cerebral blood flow (CBF) velocity (transcranial Doppler) were measured in eight healthy subjects in the supine position and during 60 degrees head-up tilt (HUT). NOS was inhibited by intravenous NG-monomethyl-L-arginine (L-NMMA) infusion. Dynamic cerebral autoregulation was quantified by transfer function analysis of beat-to-beat changes in BP and CBF velocity. Pressor effects of L-NMMA on cerebral hemodynamics were compared with those of phenylephrine infusion. In the supine position, L-NMMA increased mean BP from 83+/-3 to 94+/-3 mmHg (P < 0.01). However, CBF velocity remained unchanged. Consequently, cerebrovascular resistance index (CVRI) increased by 15% (P < 0.05). BP and CBF velocity variability and transfer function gain at the low frequencies of 0.07-0.20 Hz did not change with L-NMMA infusion. Similar changes in mean BP, CBF velocity, and CVRI were observed after phenylephrine infusion, suggesting that increase in CVRI after L-NMMA was mediated myogenically by increase in arterial pressure rather than a direct effect of cerebrovascular NOS inhibition. During baseline tilt without L-NMMA, steady-state BP increased and CBF velocity decreased. BP and CBF velocity variability at low frequencies increased in parallel by 277% and 217%, respectively (P < 0.05). However, transfer function gain remained unchanged. During tilt with L-NMMA, changes in steady-state hemodynamics and BP and CBF velocity variability as well as transfer gain and phase were similar to those without L-NMMA. These data suggest that inhibition of tonic production of NO does not appear to alter dynamic cerebral autoregulation in humans.