Acute cerebral ischaemia: concurrent changes in cerebral blood flow, energy metabolites, pH, and lactate measured with hydrogen clearance and 31P and 1H nuclear magnetic resonance spectroscopy. I. Methodology

High-sensitivity THz-ATR imaging of cerebral ischemia in a rat model

The fast label-free detection of the extent and degree of cerebral ischemia has been the difficulty and hotspot for precise and accurate neurosurgery. We experimentally demonstrated that the fresh cerebral tissues at different ischemic stages within 24 hours can be well distinguished from the normal tissues using terahertz (THz) attenuated total reflection (ATR) imaging system. It was indicated that the total reflectivity of THz wave for ischemic cerebral tissues was lower than that for normal tissues. Especially, compared to the images stained with 2,3,5-triphenyl tetrazolium chloride (TTC), the ischemic tissues can be detected using THz wave with high sensitivity as early as the ischemic time of 2.5 hours, where THz images showed the ischemic areas became larger and diffused as the ischemic time increasing. Furthermore, the THz spectroscopy of cerebral ischemic tissues at different ischemic times was obtained in the range of 0.5-2.0 THz. The absorption coefficient of ischemic tissue increased with the increase of ischemic time, whereas the refractive index decreased with prolonging the ischemic time. Additionally, it was found from hematoxylin and eosin (H&E) staining microscopic images that, with the ischemic time increasing, the cell size and cell density of the ischemic tissues decreased, whereas the intercellular substance of the ischemic tissues increased. The result showed that THz recognition mechanism of the ischemia is mainly based on the increase of intercellular substance, especially water content, which has a stronger impact on absorption of THz wave than that of cell density. Thus, THz imaging has great potential for recognition of cerebral ischemia and it may become a new method for intraoperative real-time guidance, recognition in situ, and precise excision.

Real-time influence of intracellular acidification and Na+ /H+ exchanger inhibition on in-cell pyruvate metabolism in the perfused mouse heart: A 31 P-NMR and hyperpolarized 13 C-NMR study

Disruption of acid-base balance is linked to various diseases and conditions. In the heart, intracellular acidification is associated with heart failure, maladaptive cardiac hypertrophy, and myocardial ischemia. Previously, we have reported that the ratio of the in-cell lactate dehydrogenase (LDH) to pyruvate dehydrogenase (PDH) activities is correlated with cardiac pH. To further characterize the basis for this correlation, these in-cell activities were investigated under induced intracellular acidification without and with Na+ /H+ exchanger (NHE1) inhibition by zoniporide. Male mouse hearts (n = 30) were isolated and perfused retrogradely. Intracellular acidification was performed in two ways: (1) with the NH4 Cl prepulse methodology; and (2) by combining the NH4 Cl prepulse with zoniporide. 31 P NMR spectroscopy was used to determine the intracellular cardiac pH and to quantify the adenosine triphosphate and phosphocreatine content. Hyperpolarized [1-13 C]pyruvate was obtained using dissolution dynamic nuclear polarization. 13 C NMR spectroscopy was used to monitor hyperpolarized [1-13 C]pyruvate metabolism and determine enzyme activities in real time at a temporal resolution of a few seconds using the product-selective saturating excitation approach. The intracellular acidification induced by the NH4 Cl prepulse led to reduced LDH and PDH activities (-16% and -39%, respectively). This finding is in line with previous evidence of reduced myocardial contraction and therefore reduced metabolic activity upon intracellular acidification. Concomitantly, the LDH/PDH activity ratio increased with the reduction in pH, as previously reported. Combining the NH4 Cl prepulse with zoniporide led to a greater reduction in LDH activity (-29%) and to increased PDH activity (+40%). These changes resulted in a surprising decrease in the LDH/PDH ratio, as opposed to previous predictions. Zoniporide alone (without intracellular acidification) did not change these enzyme activities. A possible explanation for the enzymatic changes observed during the combination of the NH4 Cl prepulse and NHE1 inhibition may be related to mitochondrial NHE1 inhibition, which likely negates the mitochondrial matrix acidification. This effect, combined with the increased acidity in the cytosol, would result in an enhanced H+ gradient across the mitochondrial membrane and a temporarily higher pyruvate transport into the mitochondria, thereby increasing the PDH activity at the expense of the cytosolic LDH activity. These findings demonstrate the complexity of in-cell cardiac metabolism and its dependence on intracellular acidification. This study demonstrates the capabilities and limitations of hyperpolarized [1-13 C]pyruvate in the characterization of intracellular acidification as regards cardiac pathologies.

Metabolic fingerprints discriminating severity of acute ischemia using in vivo high-field 1 H magnetic resonance spectroscopy

Despite the improving imaging techniques, it remains challenging to produce magnetic resonance (MR) imaging fingerprints depicting severity of acute ischemia. The aim of this study was to evaluate the potential of the overall high-field ¹ H MR Spectroscopy (¹ H-MRS) neurochemical profile as a metabolic signature for acute ischemia severity in rodent brains. We modeled global ischemia with one-stage 4-vessel-occlusion (4VO) in rats. Vascular structures were assessed immediately by magnetic resonance angiography. The neurochemical responses in the bilateral cortex were measured 1 h after stroke onset by ¹ H-MRS. Then we used Partial-Least-Squares discriminant analysis on the overall neurochemical profiles to seek metabolic signatures for ischemic severity subgroups. This approach was further tested on neurochemical profiles of mouse striatum 1 h after permanent middle cerebral artery occlusion, where vascular blood flow was monitored by laser Doppler. Magnetic resonance angiography identified successful 4VO from controls and incomplete global ischemia (e.g., 3VO). ¹ H-MR spectra of rat cortex after 4VO showed a specific metabolic pattern, distinct from that of respective controls and rats with 3VO. Partial-Least-Squares discriminant analysis on the overall neurochemical profiles revealed metabolic signatures of acute ischemia that may be extended to mice after permanent middle cerebral artery occlusion. Fingerprinting severity of acute ischemia using neurochemical information may improve MR diagnosis in stroke patients.

Spin-lock imaging of early tissue pH changes in ischemic rat brain

We have previously reported that the dispersion of spin-lattice relaxation rates in the rotating frame (R_1ρ ) of tissue water protons at high field can be dominated by chemical exchange contributions. Ischemia in brain causes changes in tissue pH, which in turn may affect proton exchange rates. Amide proton transfer (APT, a form of chemical exchange saturation transfer) has been shown to be sensitive to chemical exchange rates and able to detect pH changes non-invasively following ischemic stroke. However, the specificity of APT to pH changes is decreased because of the influence of several other factors that affect magnetization transfer. R_1ρ is less influenced by such confounding factors and thus may be more specific for detecting variations in pH. Here, we applied a spin-locking sequence to detect ischemic stroke in animal models. Although R_1ρ images acquired with a single spin-locking amplitude (ω₁ ) have previously been used to assess stroke, here we use ΔR_1ρ , which is the difference in R_1ρ values acquired with two different locking fields to emphasize selectively the contribution of chemical exchange effects. Numerical simulations with different exchange rates and measurements of tissue homogenates with different pH were performed to evaluate the specificity of ΔR_1ρ to detect tissue acidosis. Spin-lock and APT data were acquired on five rat brains after ischemic strokes induced via middle cerebral artery occlusions. Correlations between these data were analyzed at different time points after the onset of stroke. The results show that ΔR_1ρ (but not R_1ρ acquired with a single ω₁ ) was significantly correlated with APT metrics consistent with ΔR_1ρ varying with pH.

Multiparametric magnetic resonance imaging of acute experimental brain ischaemia

Ischaemia is a condition in which blood flow either drops to zero or proceeds at severely decreased levels that cannot supply sufficient oxidizable substrates to maintain energy metabolism in vivo. Brain, a highly oxidative organ, is particularly susceptible to ischaemia. Ischaemia leads to loss of consciousness in seconds and, if prolonged, permanent tissue damage is inevitable. Ischaemia primarily results in a collapse of cerebral energy state, followed by a series of subtle changes in anaerobic metabolism, ion and water homeostasis that eventually initiate destructive internal and external processes in brain tissue. (31)P and (1)H NMR spectroscopy were initially used to evaluate anaerobic metabolism in brain. However, since the early 1990s (1)H Magnetic Resonance Imaging (MRI), exploiting the nuclear magnetism of tissue water, has become the key method for assessment of ischaemic brain tissue. This article summarises multi-parametric (1)H MRI work that has exploited diffusion, relaxation and magnetisation transfer as 'contrasts' to image ischaemic brain in preclinical models for the first few hours, with a view to assessing evolution of ischaemia and tissue viability in a non-invasive manner.

Dicholine salt of succinic acid, a neuronal insulin sensitizer, ameliorates cognitive deficits in rodent models of normal aging, chronic cerebral hypoperfusion, and beta-amyloid peptide-(25-35)-induced amnesia

BACKGROUND

Accumulated evidence suggests that insulin resistance and impairments in cerebral insulin receptor signaling may contribute to age-related cognitive deficits and Alzheimer's disease. The enhancement of insulin receptor signaling is, therefore, a promising strategy for the treatment of age-related cognitive disorders. The mitochondrial respiratory chain, being involved in insulin-stimulated H2O2 production, has been identified recently as a potential target for the enhancement of insulin signaling. The aim of the present study is to examine: (1) whether a specific respiratory substrate, dicholine salt of succinic acid (CS), can enhance insulin-stimulated insulin receptor autophosphorylation in neurons, and (2) whether CS can ameliorate cognitive deficits of various origins in animal models.

RESULTS

In a primary culture of cerebellar granule neurons, CS significantly enhanced insulin-stimulated insulin receptor autophosphorylation. In animal models, CS significantly ameliorated cognitive deficits, when administered intraperitoneally for 7 days. In 16-month-old middle-aged C57Bl/6 mice (a model of normal aging), CS enhanced spatial learning in the Morris water maze, spontaneous locomotor activity, passive avoidance performance, and increased brain N-acetylaspartate/creatine levels, as compared to the age-matched control (saline). In rats with chronic cerebral hypoperfusion, CS enhanced spatial learning, passive avoidance performance, and increased brain N-acetylaspartate/creatine levels, as compared to control rats (saline). In rats with beta-amyloid peptide-(25-35)-induced amnesia, CS enhanced passive avoidance performance and increased activity of brain choline acetyltransferase, as compared to control rats (saline). In all used models, CS effects lasted beyond the seven-day treatment period and were found to be significant about two weeks following the treatment.

CONCLUSION

The results of the present study suggest that dicholine salt of succinic acid, a novel neuronal insulin sensitizer, ameliorates cognitive deficits and neuronal dysfunctions in animal models relevant to age-related cognitive impairments, vascular dementia, and Alzheimer's disease.

Evolving Paradigms for Neuroprotection: Molecular Identification of Ischemic Penumbra

Ischemic penumbra defines the existence of tissue at risk of infarction and which is, hence, potentially salvageable and the target for current stroke reperfusion and neuroprotective therapies. Penumbral tissue evolves toward irreversibly damaged tissue at different rates in individual stroke patients yielding different therapeutic windows depending on the individual duration of risk of infarction of this tissue. An accurate identification of the penumbra is then necessary in order to individualize the window of opportunity for therapeutic interventions. Imaging techniques, although helpful, may not give the most accurate information as to the existence of penumbra given that the threshold for identification of penumbra varies depending on the technique used. A better identification of the true penumbral tissue might be based on the cascade of molecular events that are responsible for the evolution of the penumbra toward infarcted tissue. Multiple penumbras can be defined in molecular terms taking into account which vessel is occluded, the time of evolution of the ischemia, the degree of the ischemia, and the sensitivity to ischemia of the different cells. Future studies are necessary to clarify whether the enhancement of cytoprotective mechanisms, and/or the block of cytotoxic mechanisms confirming the existence of penumbra at different times of ischemic evolution, are effective neuroprotective strategies.

Molecular Identification of the Ischemic Penumbra

Review of results of experimental and clinical studies indicates that the penumbra of physiologically impaired but potentially salvageable tissue surrounding the central core of focal cerebral ischemia that develops shortly after onset of major conducting vessel occlusion is complex and dynamic with severity and duration thresholds for hypoxic stress and injury that are specific to tissue site, cell type, molecular pathway or gene expression investigated and efficiency of collateral or residual flow and reperfusion. Imaging methods that have been utilized in vivo to identify penumbra and predict response to reperfusion and other protective therapies include magnetic resonance spectroscopy, diffusion- and perfusion-MRI as well as positron emission tomography. However, resolution of focal lesions characterized by lactic acidosis or cellular edema does not predict tissue survival, and imaging thresholds for resuscitation after reperfusion have not been determined experimentally. HSP-70 stress protein induction represents an endogenous protective mechanism that occurs in penumbra but not core neurones. A robust protective effect has been demonstrated during focal ischemia in transgenic mice overexpressing HSP-70 perhaps by suppressing early cytochrome c release. Delayed manganese mediated striatal neurodegeneration can be detected with T1 MRI after brief episodes of transient focal ischemia. Future studies may define endogenous cytotoxic and cytoprotective molecular penumbras that can be exploited to improve outcome after temporary focal ischemia.

Comparative study of the FAIR technique of perfusion quantification with the hydrogen clearance method

Arterial spin labeling magnetic resonance methods, including flow-sensitive alternating inversion recovery (FAIR), are becoming increasingly common for the noninvasive quantification of cerebral blood flow (CBF). This report compares the FAIR method with hydrogen clearance. The latter is an established, invasive technique for CBF measurement in animals. Paired readings of CBF were obtained in gerbils to maximize the degree of spatial and temporal correspondence between methods. Flow-sensitive alternating inversion recovery (50 averages, 6.7-minute measurement time) and hydrogen clearance measurements were made concurrently. Cerebral blood flow values measured by both techniques displayed an initial decrease because of the injurious effects of electrode insertion and subsequent recovery. Mixed model regression analysis, structural equations modeling, and a simple concordance correlation coefficient analysis were performed. No evidence of a marked systematic bias in the FAIR measurements was found; mixed model regression analysis yielded relative bias estimates of 0.4 (confidence interval: 3.0, 3.9) mL. 100 g-1. min-1 and -3.7 (-12.1, 4.7) mL. 100 g-1. min-1 at 20 and 100 mL. 100 g-1. min-1, respectively. The principal limitation of the FAIR technique was the magnitude of the random measurement error (imprecision), which had a standard deviation on the order of 10 mL. 100 g-1. min-1.

Prediction of adverse outcome with cerebral lactate level and apparent diffusion coefficient in infants with perinatal asphyxia

PURPOSE

To compare the predictive value for adverse outcome of quantitative cerebral lactate level and of apparent diffusion coefficient (ADC) in infants with perinatal asphyxia in the early postnatal period.

MATERIALS AND METHODS

Lactate-choline ratios determined with proton magnetic resonance (MR) spectroscopy and ADC determined with diffusion MR imaging in basal ganglia and thalami in 26 full-term neonates (age range, 1-10 days) were compared with severity of acute hypoxic-ischemic encephalopathy and long-term clinical outcome. Differences in metabolites between outcome groups were evaluated with the nonparametric Kruskal-Wallis test and the Dunn test. Logistic regression was performed to examine the predictive value of each metabolite for differentiating normal from abnormal or fatal clinical outcome. The likelihood ratio test was used to assess the statistical significance of each metabolite.

RESULTS

Logistic regression confirmed that lactate-choline ratio could be used to differentiate normal (n = 5) from abnormal (n = 14) or fatal (n = 6) outcome (P <.001). The probability of an adverse outcome exceeded 95% for a lactate-choline ratio of 1.0. Even when analyses were restricted to the early postnatal period, lactate-choline ratio was still a significant predictor of adverse outcome (P =.001). Although ADC images were useful in clinical examination of these infants, quantitative ADCs were not predictive of outcome (P =.82).

CONCLUSION

Higher lactate-choline ratios in basal ganglia and thalami of infants with perinatal asphyxia were predictive of worse clinical outcomes. Absolute ADC in the same brain regions did not indicate a statistically significant relationship with clinical outcome. Cerebral lactate level is useful in identifying infants who would benefit from early therapeutic intervention.

How can we measure substrate, metabolite and neurotransmitter concentrations in the human brain?

Cerebral injury and disease is associated with fundamental derangements in metabolism, with changes in the concentration of important substrates (e.g. glucose), metabolites (e.g. lactate) and neurotransmitters (e.g. glutamate and y-aminobutyric acid) in addition to changes in oxygen utilization. The ability to measure these substances in the human brain is increasing our understanding of the pathophysiology of trauma, stroke, epilepsy and tumours. There are several techniques in clinical practice already in use and new methods are under evaluation. Such techniques include the use of cerebral probes (e.g. microdialysis. voltammetry and spectrophotometry) and functional imaging (e.g. positron emission tomography and magnetic resonance spectroscopy). This review describes these techniques in terms of their principles and clinical applications.

Use of spin echo T(2) BOLD in assessment of cerebral misery perfusion at 1.5 T

Inadequate blood supply relative to metabolic demand, a haemodynamic condition termed as misery perfusion, often occurs in conjunction with acute ischaemic stroke. Misery perfusion results in adaptive changes in cerebral physiology including increased cerebral blood volume (CBV) and oxygen extraction ratio (OER) to secure substrate supply for the brain. It has been suggested that the presence of misery perfusion may be an indication of reversible ischaemia, thus detection of this condition may have clinical impact in acute stroke imaging. The ability of single spin echo T(2) to detect misery perfusion in the rat brain at 1.5 T owing to its sensitivity to blood oxygenation level dependent (BOLD) contrast was studied both theoretically and experimentally. Based on the known physiology of misery perfusion, tissue morphometry and blood relaxation data, T(2) behaviour in misery perfusion was simulated. The interpretation of these computations was experimentally assessed by quantifying T(2) in a rat model for cerebral misery perfusion. CBF was quantified with the H(2) clearance method. A drop of CBF from 58+/-8 to 17+/-3 ml/100 g/min in the parieto-frontal cortex caused shortening of T(2) from 66.9+/-0.4 to 64.6+/-0.5 ms. Under these conditions, no change in diffusion MRI was detected. In contrast, the cortex with CBF of 42+/-7 ml/100 g/min showed no change in T(2). Computer simulations accurately predicted these T(2) responses. The present study shows that the acute drop of CBF by 70% causes a negative BOLD that is readily detectable by T(2) MRI at 1.5 T. Thus BOLD may serve as an index of misery perfusion thus revealing viable tissue with increased OER.

Effects of blood sugar level on rat transient focal brain ischemia consecutively observed by diffusion-weighted EPI and (1)H echo planar spectroscopic imaging

The effects of blood sugar level on transient focal brain ischemia were examined by consecutive diffusion-weighted EPI and (1)H echo planar spectroscopic imaging. A remote-controlled rat intraluminal suture middle cerebral artery occlusion (MCAO) model was prepared. Animals were divided into three experimental groups: control, 1 g/kg, and 2 g/kg glucose groups (n = 6 for each). Saline or glucose was infused intraperitoneally 30 min prior to MCAO. The glucose-loaded groups showed increased lactate accumulation and marked decreases in average diffusion coefficient in the ischemic region during 40-min MCAO. These changes were correlated with blood sugar levels at the onset of MCAO. After reperfusion, all rats in the control and 1 g/kg groups recovered from the ischemic changes, but three rats with marked hyperglycemia in the 2 g/kg group showed irreversible changes. The adverse effects of hyperglycemia on transient focal brain ischemia were clearly demonstrated by sequential 2D images. Magn Reson Med 42:895-902, 1999.

Imaging of acute cerebral ischemia

Until recently, there was no efficacious treatment for acute cerebral ischemia. As a result, the role of neuroimaging and the radiologist was peripheral in the diagnosis and management of this disease. The demonstration of efficacy using thrombolysis has redefined this role, with the success of intervention becoming increasingly dependent on timely imaging and accurate interpretation. The potential benefits of intervention have only begun to be realized. In this State-of-the-Art review of imaging of acute stroke, the role of imaging in the current and future management of stroke is presented. The role of computed tomography is emphasized in that it is currently the most utilized technique, and its value has been demonstrated in prospective clinical trials. Magnetic resonance techniques are equally emphasized in that they have the potential to provide a single modality evaluation of tissue viability and vessel patency in an increasingly rapid evaluation.

Proton NMR studies of brain metabolism

As a result of several technical developments that have taken place over the past few years, it is now possible to obtain 1 H spectra of very high quality from localized regions of the human brain. 1 H spectroscopy provides scope for detecting a wide range of metabolites, and offers spatial resolution that is superior to that available with other nuclei. The animal and clinical studies that have so far been reported indicate that abnormal 1 H spectra are associated with a variety of disorders of the brain. Among the metabolites of interest are lactate and N -acetylaspartate. The signal from lactate can provide information about abnormal glycolytic metabolism, for example in brain tumours and cerebrovascular disease. N -Acetylaspartate is believed to be located primarily in neurons, and its signal could prove to be particularly useful as a non-invasive marker for neurons.

Use of brain lactate levels to predict outcome after perinatal asphyxia

Perinatal asphyxia is an important cause of neurological disability, but early prediction of outcome can be difficult. We performed proton magnetic resonance spectroscopy (MRS) and global cerebral blood flow measurements by xenon-133 clearance in 16 infants with evidence of perinatal asphyxia. Cerebral blood flow was determined daily in the first 3 days after birth in seven cases. Proton MRS was performed in 11 infants within the first week (mean 3.7 days), the rest within the first month (mean 22.2 days), and all had a scan around 3 months of age. Four infants died neonatally, three showed neurological deficits and the rest seemed to be progressing normally at neurodevelopmental follow-up at 1 year of age. A significant correlation was found between initial brain lactate levels and severe outcome (p = 0.0003) just as between cerebral hyperperfusion (mean cerebral blood flow (CBF) 86 ml(100 g)-1 min-1), (p = 0.02) and outcome. The diagnostic and prognostic implications of early MRS and CBF are predictive of poor outcome in severely asphyxiated infants.

Normal and abnormal calcium homeostasis in neurons: a basis for the pathophysiology of traumatic and ischemic central nervous system injury

Clinical recovery after central nervous system (CNS) trauma or ischemia may be limited by a neural injury process that is triggered and perpetuated at the cellular level, rather than by a lesion amenable to surgical repair. It is widely thought that one such process, a fundamental pathological mechanism initiated by CNS injury, is a disruption of cellular Ca2+ homeostasis. Because of the critical role of Ca2+ ions in regulating innumerable cellular functions, this major homeostatic disturbance is thought to trigger neuronal and axonal degeneration and produce clinical disability. We review those aspects of normal and pathological Ca2+ homeostasis in neurons that relate to neurodegeneration and to the application of neuroprotective strategies for the treatment of CNS injury. In particular, we examine the contribution of Ca(2+)-permeable ionic channels, Ca2+ pumps, intracellular Ca2+ stores, intracellular Ca2+ buffering systems, and the roles of secondary, Ca(2+)-dependent processes in neurodegeneration. A number of hypotheses linking Ca2+ ions and Ca2+ permeable channels to neurotoxicity are discussed with an emphasis on strategies for lessening Ca(2+)-related damage. A number of these strategies may have a future role in the treatment of traumatic and ischemic CNS injury.

Non-invasive neurochemical analysis of focal excitotoxic lesions in models of neurodegenerative illness using spectroscopic imaging

Water-suppressed chemical shift magnetic resonance imaging was used to detect neurochemical alterations in vivo in neurotoxin-induced rat models of Huntington's and Parkinson's disease. The toxins were: N-methyl-4-phenylpyridinium (MPP+), aminooxyacetic acid (AOAA), 3-nitropropionic acid (3-NP), malonate, and azide. Local or systemic injection of these compounds caused secondary excitotoxic lesions by selective inhibition of mitochondrial respiration that gave rise to elevated lactate concentrations in the striatum. In addition, decreased N-acetylaspartate (NAA) concentrations were noted at the lesion site over time. Measurements of lactate washout kinetics demonstrated that t1/2 followed the order: 3-NP approximately MPP+ >> AOAA approximately malonate, which parallels the expected lifetimes of the neurotoxins based on their mechanisms of action. Further increases in lactate were also caused by intravenous infusion of glucose. At least part of the excitotoxicity is mediated through indirect glutamate pathways because lactate production and lesion size were diminished using unilateral decortectomies (blockade of glutamatergic input) or glutamate antagonists (MK-801). Lesion size and lactate were also diminished by energy repletion with ubiquinone and nicotinamide. Lactate measurements determined by magnetic resonance agreed with biochemical measurements made using freeze clamp techniques. Lesion size as measured with MR, although larger by 30%, agreed well with lesion size determined histologically. These experiments provide evidence for impairment of intracellular energy metabolism leading to indirect excitotoxicity for all the compounds mentioned before and demonstrate the feasibility of small-volume metabolite imaging for in vivo neurochemical analysis.

Dissociation between lactate accumulation and acidosis in middle cerebral artery-occluded rats assessed by 31P and 1H NMR metabolic images under a 2-T magnetic field

The relationships among tissue edema, lactate accumulation, and intracellular pH in middle cerebral artery (MCA)-occluded rats were investigated with multiecho 1H magnetic resonance imaging and spatially resolved metabolic images constructed by 1H and 31P nuclear magnetic resonance (NMR) chemical shift imaging (CSI). For the effective and sensitive detection of NMR signals from the brain, outer volume suppression (OVS), reduced k-space sampling and proton irradiation were incorporated into the CSI sequences. The consecutive three measurements of calculated T2 image, lactate image, and pH image which were required for 3.75 h were repeated for four cycles of 1-16 h after MCA occlusion. Tissue edema and lactate accumulation in the infarcted region were gradually and consistently increased during the 15-h observation period. In contrast, severe acidosis was already detected on the first pH image (2-4.7 h after MCA occlusion); thereafter, the degree of acidosis became milder and showed no further progression. The dissociation between the time courses of the lactate accumulation and pH decrease was clearly demonstrated by the NMR metabolic images. Acid-base balance in cerebral infarction might be affected not only by lactate production but also by complicated interactions with tissue edema and some other factors.

Mapping of lactate and N-acetyl-L-aspartate predicts infarction during acute focal ischemia: in vivo 1H magnetic resonance spectroscopy in rats

The time course, anatomic distribution, and extent of changes in cerebral lactate, N-acetyl-L-aspartate (NAA), and other metabolite levels determined by three-dimensional in vivo 1H magnetic resonance spectroscopy and single-voxel spectral analysis after middle cerebral artery occlusion in rats. Increased lactate was detected in the central ischemic region within 1.3 hours after the onset of permanent occlusion (n = 22) or 0.5 hour after the onset of 1 hour of temporary occlusion and then reperfusion (n = 8). Permanent occlusion resulted in persistent lactate elevation and a 25.4 +/- 4.1% reduction in the NAA peak after 1.3 hours; NAA was almost completely depleted after 24 hours. Results also demonstrated delayed depletion of all other magnetic resonance spectroscopy-visible 1H metabolites, including creatine, choline, and glutamate, after permanent occlusion. After 1 hour of temporary focal ischemia, lactate returned to nearly normal levels within 0.4 hour after the onset of reperfusion; at 72 hours, a recurrent increase in lactate and a new decrease in NAA were observed, suggesting delayed tissue injury. Histological analysis, performed in 10 rats, demonstrated infarcts that corresponded in distribution to regions of NAA depletion at 72 hours. These findings indicate that lactate elevation is a sensitive early marker of ischemia; however, temporary recovery of lactate accumulation after reperfusion did not predict sustained metabolic recovery. In contrast, NAA depletion within 1.3 hours after the onset of ischemia identified central ischemic regions that were destined for infarction. Potential clinical applications include selection and monitoring of therapeutic intervention, as well as prediction of outcome, in patients with acute stroke.

Dynamic monitoring of cerebral metabolites during and after transient global ischemia in rats by quantitative proton NMR spectroscopy in vivo

Localized proton NMR spectroscopy was used to dynamically monitor alterations of cerebral metabolites before, during, and after a 10 min period of global forebrain ischemia in anesthetized rats. Metabolic assessment was based on user-independent determination of absolute brain concentrations at a nominal temporal resolution of 1.6 min. While the concentrations of N-acetyl aspartate (neuronal marker), creatines, cholines, and myo-inositol (glial marker) remained constant, ischemia induced a rapid decline of brain glucose. One hour after reperfusion, glucose recovered to 4.1 +/- 2.2 mmol/kg wet weight significantly above the basal value of 2.3 +/- 1.3 mmol/kg wet weight. Mirroring glucose depletion, lactate increased from 1.0 +/- 0.6 to 13.5 +/- 1.5 mmol/kg wet weight 10-15 min after the onset of ischemia. During reperfusion lactate clearance was characterized by a first-order rate constant of 0.03/min. The time courses of glucose and lactate reflect the rapid onset of anaerobic glycolysis during states of critically diminished blood flow. Assuming complete ischemia the production of lactate from glucose and cerebral glycogen stores yields a brain glycogen concentration of 4.7 +/- 0.9 mmol glycosyl unit/kg wet weight. Elevation of brain glucose during early reperfusion suggests a transient mismatch of glucose uptake and consumption during the first 1-2 hours post ischemia.

In vivo 31P NMR studies of orientation effects upon rat brain metabolism during mild hypoxia

Energy metabolites in rat brain under the same level of hypoxia were monitored by 31P NMR in both horizontal and vertical magnets. The changes in PCr, Pi, and pHi in the vertical setting toward the end of hypoxia were significantly larger and the recovery in the horizontally held animals was more complete. The results demonstrated quantitatively that the stress of the alignment is superimposed on the stress of hypoxia in the vertical magnet.

31P-NMR study of transient ischemia in rat hippocampal slices in vitro

Intracellular high energy phosphates (HEP) were monitored in rat hippocampal slices in vitro by 31P-NMR during continuous superfusion, no flow and reperfusion in order to model the changes which occur during cerebral ischemia and reperfusion in vivo. With continuous superfusion, stable intracellular HEP resonance signals were observed for over 4 h. When superfusion was stopped, there were rapid decreases in pH and phosphocreatine levels followed by slower loss of ATP. These changes are similar to those observed during cerebral ischemia in vivo by 31P-NMR. Upon reperfusion, the pH returned to normal, but the extent of HEP recovery depended on the length of time superfusion was halted. Following a 10 min ischemic period HEP levels returned to greater than 90% of preischemic values, while following a 16 min ischemic period there was only 60% recovery. Superfusion with low calcium, high magnesium medium significantly improved the recovery of HEP following 16 min of ischemia to 80% of preischemic levels. These data support the hypothesis that calcium influx during and following ischemia can disrupt energy metabolism in the hippocampus, and that magnesium can have a protective action on cellular energy status, perhaps by further blocking calcium influx.

Localized proton MR spectroscopy of the brain in children

Small-voxel (3.0-8.0 cm3), magnetic resonance (MR) imaging-guided proton MR spectroscopy was performed in 54 patients (aged 6 days to 19 years) with intracranial masses (n = 16), neurodegenerative disorders (n = 34), and other neurologic diseases (n = 4) and in 23 age-matched control subjects without brain disease. A combined short TE (18 msec) stimulated-echo acquisition mode (STEAM) and long TE (135 and/or 270 msec) spin-echo point-resolved spatially localized spectroscopy (PRESS) protocol, using designed radio-frequency pulses, was performed at 1.5 T. STEAM spectra revealed short T2 and/or strongly coupled metabolites; prominent resonances were obtained from N-acetyl aspartate (NAA), choline-containing compounds (Cho), and total creatine (tCr). Lactate was well resolved with the long TE PRESS sequence. Intracranial tumors were readily differentiated from cerebrospinal fluid (CSF) collections. All tumors showed low NAA, high Cho, and reduced tCr levels. Neurodegenerative disorders showed low or absent NAA levels and enhanced mobile lipid, glutamate and glutamine, and inositol levels, consistent with neuronal loss, gliosis, demyelination, and amino acid neurotoxicity. Preliminary experience indicates that proton MR spectroscopy can contribute in the evaluation of central nervous system abnormalities of infants and children.

Controllable graded cerebral ischaemia in the gerbil: studies of cerebral blood flow and energy metabolism by hydrogen clearance and 31P NMR spectroscopy

A technique for remotely controlling the degree of carotid artery occlusion in the gerbil model of cerebral ischaemia has been developed. The technique relies on manually adjustable nylon snares around the carotid arteries, in conjunction with a computer-based monitoring system, to control the degree of occlusion. This has allowed us to determine the dependence of energy metabolism (as assessed by 31P NMR spectroscopy) on blood flow in greater detail than was possible in our previous studies. Data obtained show that energy changes first appear at flows of 25-30 mL/100 g/min, while at flows below 20 mL/100 g/min there is a major derangement of energy metabolism. The model was used to determine the sensitivity of cerebral energy metabolism to reduced cerebral blood flow under normothermic conditions and in mild hypothermia (30 degrees C). Hypothermia had a protective effect in that energy metabolites were maintained at flows significantly below the normothermic threshold.

Cerebral metabolic studies in vivo by combined 1H/31P and 1H/13C NMR spectroscopic methods

Intracellular pH and ammonium ion concentration are potent modulators of cerebral amino acid metabolism. Furthermore, intracellular acidosis and hyperammonemia accompany conditions such as ischemic encephalopathy and seizures and may contribute to the pathological sequelae observed. In vivo NMR spectroscopy permits multiple, non-destructive measurements of important cerebral metabolic intermediates in the same animal. We describe here the use of 1H, and 31P NMR spectroscopy to investigate the effects of acute changes in intracellular pH and ammonium ions on cerebral glutamate, glutamine, and lactate levels in vivo. We then show how 1H NMR can be used to indirectly follow the flow of 13C label from [1-13C] glucose into the cerebral glutamate pool, allowing us to measure cerebral TCA activity in normal and chronically hyperammonemic rats. Male Sprague-Dawley rats (160-210 gm), fasted 24-hours, were tracheotomized, paralyzed and ventilated on 30% O2/70% N2O. NMR spectroscopy was performed at a field strength of 8.4 Tesla using a Bruker AM-360 wide bore spectrometer. An elliptical surface-coil (8 x 12 mm) was double-tuned to either the 1H and 31P or 1H and 13C frequencies. After retraction of extracranial tissues, the coil was positioned over the skull 2 mm posterior to the bregma. Tail arteries and veins were cannulated allowing periodic measurements of PO2, pCO2, pH and glucose in arterial blood and intravenous infusions. Respiratory acidosis was induced in rats by the addition of CO2 to the ventilation gas mixture. Arterial pCO2 increased within 5 min from a pre-hypercarbic value of 36.4 +/- 6.1 mm Hg to 200-220 mm Hg and was maintained at this level for over 1 hour. Hypercarbia led to rapid cerebral acidification. Intracellular pH decreased from 7.18 +/- 0.08 (pre-hypercarbic period) to 6.68 +/- 0.06 (n = 4) at 10 min and remained stable throughout the NMR observation period. Glutamate decreased to 53 +/- 4% of control after 60 min of hypercarbia, while glutamine increased to 126 +/- 7% of control. Acute hyperammonemia was produced by a programmed intravenous infusion of 250 mM ammonium acetate, which rapidly raised and maintained the concentration of ammonium ions in the blood at approximately 500 microM. Shortly after the start of the infusion (10-20 min), the levels of glutamine and lactate rose continuously throughout the experiment, reaching levels of 170 +/- 25% and 260 +/- 60% of control, respectively (n = 12) after 50 min. Glutamate decreased during the same time interval to 80 +/- 4% of control (n = 12).(ABSTRACT TRUNCATED AT 400 WORDS)

NMR-spectroscopic investigation of cerebral reanimation after prolonged ischemia

The severity of brain injury following interruption of blood flow depends on a number of ischemic and post-ischemic variables. The most important ischemic variables are the duration of ischemia, the amount of residual blood flow, the type and depth of anesthesia, brain glucose content and temperature. Among the post-ischemic factors the no-reflow phenomenon, edema and a variety of biochemical disturbances are of particular importance. Due to the complex interaction of these factors irreversible brain injury usually occurs after less than 10 min cerebrocirculatory arrest in normothermia. However, the safe ischemia time of the brain can be substantially extended when appropriate therapeutic measures are used to alleviate post-ischemic injury. NMR-spectroscopy is particularly suited for the analysis of this process. Recording of 31P, 1H and 19F spectra allow the continuous non-invasive assessment of such basic parameters as brain energy state, tissue pH, the content of lactate and blood flow (using Freon-23 as an inert tracer). In addition, information is obtained about changes in the content of phosphomonoesters and -diesters, glutamate, glutamine, aspartate and N-acetyl aspartate. These measurements can be combined with in vivo electrophysiological and post-mortem biochemical investigations for the further refinement of functional/metabolic monitoring. We have used this approach to study the potentials of post-ischemic resuscitation after one hour complete ischemia of the normothermic cat brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of hypoglycemia on changes of brain lactic acid and intracellular pH produced by ischemia

Previous investigators have attributed the fall of brain intracellular pH (pHi) produced by ischemia to accumulation of lactic acid. The goal of the present experiments was to examine the hypothesis that the acidosis produced by cerebral ischemia is due to accumulation of lactic acid. The present experiments inhibited lactic acid production by lowering glucose availability using insulin-induced hypoglycemia. The adverse effects of hypoglycemia were prevented by the prior elevation of beta-hydroxybutyric acid and acetoacetic acid induced by a high lipid diet. Brain pHi and lactic acid were measured by 31P and 1H NMR. The results showed that insulin-induced hypoglycemia markedly inhibits production of lactic acid, but has no effect on brain pHi during ischemia. These findings suggest that, at least under some conditions, the acidosis produced by cerebral ischemia is not due to accumulation of lactic acid.

The effect of nimodipine on high-energy phosphates and intracellular pH during cerebral ischemia

Experimental and clinical studies suggest that the calcium channel blocker nimodipine may reduce cerebral ischemic injury. Using rapid acquisition phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy, we examined the effect of nimodipine on cerebral energy metabolism during severe ischemia in gerbils. High-energy phosphates and intracellular pH were characterized at baseline and at 2-min intervals following bilateral common carotid artery (CCA) ligation. Serial forebrain spectroscopy was continued until phosphocreatine (PCr) and adenosine triphosphate (ATP) resonances disappeared. Controls (n = 10) were compared to gerbils receiving intraperitoneal nimodipine 30 min prior to carotid ligation, at the following doses: 0.5 mg/kg (n = 8), 1.0 mg/kg (n = 10), 2.0 mg/kg (n = 8), or 4.0 mg/kg (n = 4). In the control group, PCr and ATP peaks were undetectable after a mean of 5.4 +/- 0.47 min following CCA ligation. Compared with controls, the mean time for depletion of high-energy phosphates following carotid ligation was prolonged at nimodipine doses of 0.5 mg/kg and 1.0 mg/kg, but the differences did not reach statistical significance. In the 2.0 mg/kg group, however, ATP was preserved until 9.8 +/- 1.0 min following the onset of ischemia, significantly longer than the control group (p = 0.005, Mann-Whitney test). Nimodipine had no effect on the time course or severity of intracellular acidosis. In this model of severe ischemia, relatively high doses of nimodipine slowed the depletion of high-energy phosphates without altering intracellular acidosis. This suggests that nimodipine may provide cerebral protection by directly altering ischemic cellular metabolism.

NMR spectroscopy: current status and future possibilities

Nuclear magnetic resonance (NMR) spectroscopy is now established as a non-invasive method of studying metabolism in living systems, ranging from cellular suspensions to man. With respect to clinical applications, recent developments include the successful implementation of new techniques for spatial localisation, and in particular the acquisition of excellent 1H spectra from selected regions of the human brain. Localised 1H spectroscopy opens the way to monitoring a wide range of compounds that are inaccessible to 31P NMR, and should add considerably to the information that is available from 31P studies. NMR spectroscopy does, however, have its limitations, which arise primarily from the fact that it is an insensitive technique. This lack of sensitivity limits the spatial resolution for metabolic studies, and means that metabolites must be present at fairly high concentrations in order to produce detectable signals. In this article, we illustrate the scope and limitations of NMR spectroscopy by describing a few examples of studies undertaken on animals and humans.

Diffusion-weighted imaging studies of cerebral ischemia in gerbils. Potential relevance to energy failure

BACKGROUND AND PURPOSE

Diffusion-weighted magnetic resonance imaging has been shown to be particularly suited to the study of the acute phase of cerebral ischemia in animal models. The studies reported in this paper were undertaken to determine whether this technique is sensitive to the known ischemic thresholds for cerebral tissue energy failure and disturbance of membrane ion gradients.

METHODS

Diffusion-weighted images of the gerbil brain were acquired under two sets of experimental conditions: as a function of cerebral blood flow after controlled graded occlusion of the common carotid arteries (partial ischemia), as a function of time following complete bilateral carotid artery occlusion (severe global ischemia), and on deocclusion after 60 minutes of ischemia.

RESULTS

During partial cerebral ischemia, the diffusion-weighted images remained unchanged until the cerebral blood flow was reduced to 15-20 ml.100 g-1.min-1 and below, when image intensity increased as the cerebral blood flow was lowered further. This is similar to the critical flow threshold for maintenance of tissue high-energy metabolites and ion homeostasis. After the onset of severe global cerebral ischemia, diffusion-weighted image intensity increased gradually after a delay of approximately 2.5 minutes, consistent with complete loss of tissue adenosine triphosphate and with the time course of increase in extracellular potassium. This hyperintensity decreased on deocclusion following 60 minutes of ischemia.

CONCLUSIONS

The data suggest that diffusion-weighted imaging is sensitive to the disruption of tissue energy metabolism or a consequence of this disruption. This raises the possibility of imaging energy failure noninvasively. In humans, this could have potential in visualizing brain regions where energy metabolism is impaired, particularly during the acute phase following stroke.

Early time course of N-acetylaspartate, creatine and phosphocreatine, and compounds containing choline in the brain after acute stroke. A proton magnetic resonance spectroscopy study

BACKGROUND AND PURPOSE

The early time course after acute stroke of cerebral N-acetylaspartate, creatine and phosphocreatine, and compounds containing choline was studied in vivo by means of localized water-suppressed proton magnetic resonance spectroscopy.

METHODS

Eight patients with acute stroke were studied serially in the acute phase, 1 week after, and 2-4 weeks after the onset of clinical symptoms. Ten healthy volunteers served as controls. A stimulated echo (STEAM) sequence was used for measurement of the brain metabolites in a volume of interest located within the infarcted area as visualized by magnetic resonance imaging. For quantification, the unsaturated water signal was used as the internal standard. Regional cerebral blood flow in the infarcted area was measured relative to a symmetrically located unaffected area by means of single-photon emission computed tomographic scanning, using 99mTc-labeled d,l-hexamethylenepropyleneamine oxime as the flow tracer.

RESULTS

Relative regional cerebral blood flow was considerably reduced in the infarcted area in the acute phase. After 1 week, hyperemia was seen in all but one patient. The N-acetylaspartate content was significantly reduced, with the loss appearing to occur between 6 and 24 hours after the stroke incident. The reduction in N-acetylaspartate content was greater in the central part than in the peripheral part of the infarcted area. Creatine and phosphocreatine were also reduced in the infarcted area, whereas no significant change was seen in the choline content.

CONCLUSIONS

Assuming that N-acetylaspartate content reflects neuronal survival or loss, our results may suggest that treatment procedures with restoration of blood flow to severely ischemic areas should be initiated within the first 6 hours after stroke onset.

Cerebral lactate production and blood flow in acute stroke

Eight stroke patients were examined serially in the acute phase and 1 week and 2-4 weeks after stroke with water-suppressed proton magnetic resonance spectroscopy. The time courses of lactate level and regional cerebral blood flow were studied. A high lactate level was found in the acute phase. The lactate content decreased to barely detectable levels during the following 3 weeks, while regional blood flow increased during this period. The inverse relationship between lactate level and cerebral blood flow suggests that lactate plays no substantial role in the vasodilatation underlying the hyperemia that follows reperfusion. The amount of lactate present in the acute phase reflects the severity of ischemia in the affected region. The lactate level was still above normal in the subacute phase with hyperemia, suggesting lactate production through aerobic glycolysis. Thus, the lactate level in the subacute phase probably does not reflect the degree of anaerobic glycolysis in hypoxic neuronal tissue.

Effects of hyperglycemia on the time course of changes in energy metabolism and pH during global cerebral ischemia and reperfusion in rats: correlation of 1H and 31P NMR spectroscopy with fatty acid and excitatory amino acid levels

The effects of hyperglycemia on the time course of changes in cerebral energy metabolite concentrations and intracellular pH were measured by nuclear magnetic resonance (NMR) spectroscopy in rats subjected to temporary complete brain ischemia. Interleaved 31P and 1H NMR spectra were obtained every 5 min before, during, and for 2 h after a 30-min bilateral carotid occlusion preceded by permanent occlusion of the basilar artery. The findings were compared with free fatty acid and excitatory amino acid levels as well as with cations and water content in funnel-frozen brain specimens. One hour before occlusion, nine rats received 50% glucose (12 ml/kg i.p.) and five received 7% saline (12 ml/kg i.p.). Before ischemia, there were no differences in cerebral metabolite levels or pH between hyperglycemic rats and controls. During the carotid occlusion, the lactate/N-acetylaspartate (Lac/NAA) peak ratio was higher (0.73-1.48 vs. 0.56-0.82; p less than 0.05) and pH was lower (less than 6.0 vs. 6.45 +/- 0.05; p less than 0.05) in the hyperglycemic rats than in the controls. Phosphocreatine and adenosine triphosphate were totally depleted in both groups. Within 5-15 min after the onset of reperfusion, the Lac/NAA peak ratio increased further in all rats; however, only in extremely hyperglycemic rats (serum glucose greater than 960 mg/dl) did the lactic acidosis progress rather than recover later during reperfusion. Total free fatty acid and excitatory amino acid levels, but not cation concentration or water content, in brain correlated with serum glucose levels during and after ischemia and with NMR findings after 2 h of reperfusion. Although profound hyperglycemia (serum glucose of 970-1,650 mg/dl) appears to be associated with progression of anaerobic glycolysis and failure of cerebral energy metabolism to recover after temporary complete brain ischemia and with postischemic excitotoxic and lipolytic reactions thought to participate in delayed cellular injury, severe hyperglycemia (490-720 mg/dl) was associated with recovery of energy metabolism.

Effect of mannitol on local cerebral blood flow after temporary complete cerebral ischemia in rats

The effects of pretreatment with mannitol on local cerebral blood flow (CBF) after permanent or temporary global cerebral ischemia were evaluated with 14C-iodoantipyrine autoradiography in rats under halothane-N2O endotracheal anesthesia. Blood pressure, pulse rate, arterial blood gas levels, and electroencephalographic (EEG) tracings were monitored throughout the experiments. After permanent occlusion of the basilar artery and both external carotid and pterygopalatine arteries, severe global ischemia was induced by permanent occlusion of the common carotid arteries (CCA's) or by a 30-minute temporary CCA occlusion followed by 5 minutes of reperfusion. Intravenous mannitol (25%, 1 gm/kg) or saline solution was administered 5 minutes before occlusion of the CCA's. Cerebral blood flow was measured in 24 anatomical regions. The EEG tracings flattened within 2 to 3 minutes after the onset of ischemia, and no recovery was observed during reperfusion. In the mannitol-treated rats and the saline-treated controls, autoradiographic studies after permanent occlusion showed no CBF in the forebrain or cerebellum, although brain-stem and spinal cord CBF values were normal. After 5 minutes of reperfusion, CBF in the cortex, basal ganglia, and white matter was 100% to 200% higher in mannitol-treated rats and 50% to 100% higher in saline-injected rats than in the nonischemic anesthetized control group. Heterogeneously distributed areas of no-reflow were seen in all saline-injected rats but were observed in none of the mannitol-treated rats. Pretreatment with mannitol prevented postischemic obstruction of the microcirculation during 5 minutes of recirculation after 30 minutes of severe temporary ischemia, but the EEG signals did not recover. Further studies of the functional and morphological responses to longer periods of postischemic recirculation are needed to verify the extent to which these mannitol-induced effects are protective.

Quantitative P-31 MR spectroscopy of the liver in alcoholic cirrhosis

To determine the cause of reduced urea synthesis in cirrhosis, absolute concentrations of phosphorus metabolites in the human liver have been measured in vivo with magnetic resonance (MR) spectroscopy. One-dimensional chemical shift imaging was used to obtain phosphorus-31 spectra from five healthy volunteers and five patients with alcoholic cirrhosis. A reference standard included in all studies enabled the calculation of absolute concentrations. In contrast to hepatic metabolite ratios, absolute concentrations reveal that in the cirrhotic patients, concentrations of adenosine triphosphate (ATP) were significantly reduced and concentrations of phosphomonoesters slightly reduced. Intracellular pH was unchanged. Histologic evidence suggests that the amount of ATP per cell was unchanged and could not account for the reduced urea production. Instead, urea synthesis depends on the functional liver cell mass, which was reduced by 31% in alcoholic cirrhosis. Quantitative in vivo P-31 MR spectroscopy of liver has potential clinical applications and can supplement the more generally used P-31 metabolite ratios.

Cell and tissue responses of a murine tumour to phthalocyanine-mediated photodynamic therapy

Mice bearing a subcutaneously growing tumour (Colo 26) were injected intravenously with the photosensitiser chloroaluminum sulphonated phthalocyanine (5 mg/kg) 24 h prior to irradiating the tumour with laser light (675 nm; 50mW, 100 J/tumour). Energy status of the tumour, as assessed by the loss of high energy phosphates in the 31P-nuclear magnetic resonance spectra, was altered dramatically following treatment, such that the ATP fell to undetectable levels within 1 h of light irradiation. However, assessment of the clonogenic capacity of neoplastic cells isolated from dissociated tumours showed that these rapid changes in cellular metabolism were not reflected in similar rapid changes in cell viability. Reductions in clonogenic capacity, which fell to less than 0.1% of control values at 24h postirradiation, closely mirrored those resulting from the cessation of vascular perfusion. Evaluation of tumour blood flow, using the technique of hydrogen washout, showed that the treatment protocol evoked a gradual and selective reduction in flow within the tumour resulting in complete vascular stasis by approximately 5 h after treatment. The results indicate that while chloroaluminum sulphonated phthalocyanine-mediated photodynamic therapy caused early metabolic damage in neoplastic cells, loss of viability paralleled the induction of complete inhibition of vascular flow in the tumour.

Effect of hypoxia on cerebral metabolites measured by proton nuclear magnetic resonance spectroscopy in rats

Proton nuclear magnetic resonance spectroscopy is a unique method to monitor noninvasively the concentrations of cerebral metabolites. N-Acetyl-L-aspartate, the concentration of which is assumed to be stable during hypoxia, has been used to form ratios with lactate. To determine the stability of the signal from N-acetyl-L-aspartate, we used a model of graded hypoxia in rats to monitor the percentage changes from baseline of the peak heights for lactate, lipids, and N-acetyl-L-aspartate. Anesthetized adult rats were exposed sequentially to 15% and 10% O2 while proton nuclear magnetic resonance spectra were collected with a surface coil in a 7-T 89-mm-bore spectrometer. Brain lactate concentration was either increased by feeding or infusion of glucose (n = 9) or lowered by fasting (n = 7). After death the brains were removed and frozen, and the water- and lipid-soluble compounds were extracted to identify the origin of the signals. We analyzed the data both as the percentage change from baseline for heights of the lactate (1.33 ppm), lipids (1.5 ppm), and N-acetyl-L-aspartate (2.02 ppm) peaks and as the ratios of heights of the 1.33 and 2.02 and the 1.5 and 2.02 ppm peaks. Both hypoxic episodes caused a 45% decrease from baseline in the 2.02 ppm peak. During the second hypoxic episode, the 1.33:2.02 ppm peak height ratio increased significantly in hyperglycemic rats (p less than 0.05) but was unchanged in hypoglycemic rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral blood flow in experimental ischemia assessed by 19F magnetic resonance spectroscopy in cats

We evaluated a 19F magnetic resonance spectroscopic technique that detects Freon-23 washout as a means of measuring cerebral blood flow in halothane-anesthetized adult cats during and after transient cerebral ischemia produced by vascular occlusion. The experiments were performed to test the ability of this recently developed method to detect postischemic flow deficits. Results were consistent with postischemic hypoperfusion. The method also proved valuable for measuring small residual flow during vascular occlusion. Our experiments indicate that this method provides simple, rapid, and repeatable flow measurements that can augment magnetic resonance examinations of cerebral metabolic parameters in the study of ischemia.

31 phosphorous nuclear magnetic resonance spectroscopy of rat brain with temporary global cerebral ischemia--methodology

31 P Magnetic Resonance Spectroscopy offers a technique for measuring intracellular pH and the high energy phosphates Phosphocreatine (PCr) and ATP in vivo. The 2-vessel occlusion model of temporary global cerebral ischaemia in the rat was adopted in this technique. This approach gave reliable results for intracellular pH and valuable information about PCr and ATP with an accurate time resolution for the changes during ischaemia and early reperfusion. The method may be a useful tool to evaluate possible benefits from different kinds of pre- and post ischaemic drug intervention.

Relationship between plasma glucose, brain lactate, and intracellular pH during cerebral ischemia in gerbils

The dose-response relation between plasma glucose and brain lactate and the relation of these parameters to intracellular pH during severe cerebral ischemia have not been well characterized over a wide range of plasma glucose levels. Experiments to delineate these relations in the gerbil model of global ischemia were performed by using phosphorus-31 nuclear magnetic resonance spectroscopy to measure intracellular pH and a new method to measure brain lactate. Ischemia increased final brain lactate linearly 4 mumol/g for every 100 mg/dl increase in plasma glucose up to 650 mg/dl (p = 0.0001, r2 = 0.9); beyond 650 mg/dl, saturation of the glucose transport-glycolysis system occurred. Plasma glucose correlated better with ischemic intracellular pH than did brain lactate. However, when brain lactate levels are compared with intracellular pH during ischemia, the relation may be threshold rather than linear. A narrow transition zone, during which ischemic intracellular pH decreased precipitously with increasing brain lactate, was observed between 17 and 22 mumol/g; below 17 mumol/g, intracellular pH remained stable at 6.8-6.9, whereas above 22 mumol/g, intracellular pH decreased maximally to about 6.2. The marked decrease in intracellular pH that occurs when brain lactate surpasses 17 mumol/g suggests that this sudden drop in intracellular pH may account for the "lactate threshold" for increased cerebral ischemic damage.

Cerebral energy metabolism in experimental canine hydrocephalus

Cerebral energy metabolism, its relationship to the stage and extent of hydrocephalus, and the effect of cerebrospinal fluid (CSF) removal on it were studied in experimental canine hydrocephalus produced by intracisternal injection of kaolin by using phosphorus-31 (P-31) magnetic resonance (MR) spectroscopy and MR imaging. P-31 MR spectra were serially obtained before and after CSF removal, maximally on eight occasions over a period of nearly 5 h. There was a decrease in the ratio of creatine phosphate to inorganic phosphate, used as an indicator of the bioenergetic state, in acute and subacute stages of hydrocephalus as compared with the control. An animal in the subacute stage, when periventricular edema was most prominent, exhibited the most predominent decrease in this ratio at 14 days after hydrocephalic insult. The recovery of the ratio toward the control level was seen in the chronically hydrocephalic animal. There was no change in adenosine triphosphate (ATP) levels in any stage of hydrocephalus. Serial spectra obtained after the withdrawal of ventricular CSF showed no change in the bioenergetic state of the brain in any stage of hydrocephalus. There was no relationship between either the extent of hydrocephalus or the ventricular CSF pressure and the change in the bioenergetic state or the levels of any of the phosphorus compounds. These findings may indicate the alteration of the mitochondrial energy metabolism in hydrocephalus, which may explain the mechanism of hydrocephalic syndrome.

Restoration of energy metabolism and resolution of oedema following profound ischaemia

Cerebral ischaemia was produced in 2 groups of gerbils by occlusion of the common carotid arteries for 30 minutes, resulting in cerebral oedema. In group 1 cerebral oedema was measured by specific gravity microgravimetry, and in group 2 brain metabolism and blood flow were measured by 31P and 1H NMR spectroscopy and hydrogen clearance respectively. In group 1 the brain water content did not return to control levels by 180 minutes of reperfusion. Energy metabolism, determined by 31P NMR spectroscopy returned to control by 12 minutes, intracellular pH (pHi) by 20 minutes, and lactate, determined by 1H NMR spectroscopy, by 50 minutes. There was a lag of about 10 minutes before lactate began to be cleared from the brain. We suggest that while pHi is low, Na+/H+ exchange will negate the Na+ extrusion driven by the Na+/K+ ATPase. When pHi approaches normal there will be a net extrusion of Na+, taking osmotic water with it, and presumably with passive washout of lactate. This may be the cause of the initial delay in lactate clearance.

NMR spectroscopic investigation of the recovery of energy and acid-base homeostasis in the cat brain after prolonged ischemia

The effects of 1 h of complete global ischemia on the recovery of high-energy phosphates, intracellular pH (pHi), and lactate in the cat brain in vivo was investigated by 31P and 1H NMR spectroscopy. Ischemia led to a decrease in creatine phosphate (CrP), nucleoside triphosphates (NTP), and pHi, while inorganic phosphate and lactate increased. Intracellular pH decreased from a control value of 7.07 +/- 0.04 to 6.17 +/- 0.12 after 1 h of ischemia (N = 7). The degree of metabolic recovery after recirculation was variable. In three animals CrP and NTP were detected within 4 min and NTP increased to greater than or equal to 90% of control within 1 h; these levels were maintained for the 3 h of observation. In four other animals, CrP and NTP reached only 20 to 80% of control; however, high-energy phosphates decreased and lactate increased spontaneously between 1 and 2.5 h. Immediately following recirculation, pHi decreased further by an average of 0.3 units. The rate of recovery of cerebral pHi was slower than that of PCr and NTP for the majority of animals. Recovery of pHi was not detected for an average of 32 min after recirculation--by this time, NTP had attained 80 +/- 10% of their preischemic level. Recovery of pHi (and lactate) was not observed in two animals where PCr and NTP recovered transiently to only 30-43% of the preischemic level. Recovery of cerebral pHi was markedly heterogeneous in one animal, since two Pi peaks were detected shortly after recirculation.(ABSTRACT TRUNCATED AT 250 WORDS)