The effect of hypercapnia upon intracellular pH in the brain, evaluated by the bicarbonate-carbonic acid method and from the creatine phosphokinase equilibrium

Ultrasensitive Dual Fluorophore-Conjugated Carbon Dots for Intracellular pH Sensing in 3D Tumor Models

The dysregulation of pH has been linked to the onset of chronic conditions, such as cancer and neurological diseases. Consequently, the development of a highly sensitive tool for intracellular pH sensing is imperative to investigate the interplay between pH and the biochemical changes accompanying disease pathogenesis. Here, we present the development of a ratiometric fluorescent nanoprobe, NpRhoDot, designed for precisely measuring pH levels. We demonstrate its efficacy in sensitively reporting intracellular pH in monolayer A549 lung cancer cells, primary fibroblast cells, and 3D tumor spheroids derived from the DLD-1 colorectal adenocarcinoma cell line. NpRhoDot leverages a novel design, where stable carbon dots are functionalized with a pH-responsive ratiometric fluorescent probe comprising a naphthalimide-rhodamine moiety, NpRho1. This design confers NpRhoDot with the high pH sensitivity characteristics of organic fluorescent probes, along with excellent photostability up to 1 h and biocompatibility of carbon dots. Through one-photon and two-photon fluorescence microscopy, we validate the reliability of NpRhoDot for biosensing intracellular pH in monolayer and three-dimensional tumor models from pH 4 to 7.

Creatine mapping of the brain at 3T by CEST MRI

PURPOSE

To assess the feasibility of CEST-based creatine (Cr) mapping in brain at 3T using the guanidino (Guan) proton resonance.

METHODS

Wild type and knockout mice with guanidinoacetate N-methyltransferase deficiency and low Cr and phosphocreatine (PCr) concentrations in the brain were used to assign the Cr and protein-based arginine contributions to the GuanCEST signal at 2.0 ppm. To quantify the Cr proton exchange rate, two-step Bloch-McConnell fitting was used to fit the extracted CrCEST line-shape and multi-B1 Z-spectral data. The pH response of GuanCEST was simulated to demonstrate its potential for pH mapping.

RESULTS

Brain Z-spectra of wild type and guanidinoacetate N-methyltransferase deficiency mice show a clear Guan proton peak at 2.0 ppm at 3T. The CrCEST signal contributes ∼23% to the GuanCEST signal at B1  = 0.8 μT, where a maximum CrCEST effect of 0.007 was detected. An exchange rate range of 200-300 s-1 was estimated for the Cr Guan protons. As revealed by the simulation, an elevated GuanCEST in the brain is observed when B1 is less than 0.4 μT at 3T, when intracellular pH reduces by 0.2. Conversely, the GuanCEST decreases when B1 is greater than 0.4 μT with the same pH drop.

CONCLUSIONS

CrCEST mapping is possible at 3T, which has potential for detecting intracellular pH and Cr concentration in brain.

The exchange rate of creatine CEST in mouse brain

PURPOSE

To estimate the exchange rate of creatine (Cr) CEST and to evaluate the pH sensitivity of guanidinium (Guan) CEST in the mouse brain.

METHODS

Polynomial and Lorentzian line-shape fitting (PLOF) were implemented to extract the amine, amide, and Guan CEST signals from the brain Z-spectrum at 11.7T. Wild-type (WT) and knockout mice with the guanidinoacetate N-methyltransferase deficiency (GAMT-/- ) that have low Cr and phosphocreatine (PCr) concentrations in the brain were used to extract the CrCEST signal. To quantify the CrCEST exchange rate, a two-step Bloch-McConnell (BM) fitting was used to fit the CrCEST line-shape, B1 -dependent CrCEST, and the pH response with different B1 values. The pH in the brain cells was altered by hypercapnia to measure the pH sensitivity of GuanCEST.

RESULTS

Comparison between the Z-spectra of WT and GAMT-/- mice suggest that the CrCEST is between 20% and 25% of the GuanCEST in the Z-spectrum at 1.95 ppm between B1  = 0.8 and 2 μT. The CrCEST exchange rate was found to be around 240-480 s-1 in the mouse brain, which is significantly lower than that in solutions (∼1000 s-1 ). The hypercapnia study on the mouse brain revealed that CrCEST at B1  = 2 μT and amineCEST at B1  = 0.8 μT are highly sensitive to pH change in the WT mouse brain.

CONCLUSIONS

The in vivo CrCEST exchange rate is slow, and the acquisition parameters for the CrCEST should be adjusted accordingly. CrCEST is the major contribution to the opposite pH-dependence of GuanCEST signal under different conditions of B1 in the brain.

Simultaneous detection of metabolite concentration changes, water BOLD signal and pH changes during visual stimulation in the human brain at 9.4T

This study presents a method to directly link metabolite concentration changes and BOLD response in the human brain during visual stimulation by measuring the water and metabolite signals simultaneously. Therefore, the metabolite-cycling (MC) non-water suppressed semiLASER localization technique was optimized for functional 1H MRS in the human brain at 9.4 T. Data of 13 volunteers were acquired during a 26:40 min visual stimulation block-design paradigm. Activation-induced BOLD signal was observed in the MC water signal as well as in the NAA-CH3 and tCr-CH3 singlets. During stimulation, glutamate concentration increased 2.3 ± 2.0% to a new steady-state, while a continuous increase over the whole stimulation period could be observed in lactate with a mean increase of 35.6 ± 23.1%. These increases of Lac and Glu during brain activation confirm previous findings reported in literature. A positive correlation of the MC water BOLD signal with glutamate and lactate concentration changes was found. In addition, a pH decrease calculated from a change in the ratio of PCr to Cr was observed during brain activation, particularly at the onset of the stimulation.

pH Mapping of Skeletal Muscle by Chemical Exchange Saturation Transfer (CEST) Imaging

Magnetic resonance imaging (MRI) is extensively used in clinical and basic biomedical research. However, MRI detection of pH changes still poses a technical challenge. Chemical exchange saturation transfer (CEST) imaging is a possible solution to this problem. Using saturation transfer, alterations in the exchange rates between the solute and water protons because of small pH changes can be detected with greater sensitivity. In this study, we examined a fatigued skeletal muscle model in electrically stimulated mice. The measured CEST signal ratio was between 1.96 ppm and 2.6 ppm in the z-spectrum, and this was associated with pH values based on the ratio between the creatine (Cr) and the phosphocreatine (PCr). The CEST results demonstrated a significant contrast change at the electrical stimulation site. Moreover, the pH value was observed to decrease from 7.23 to 7.15 within 20 h after electrical stimulation. This pH decrease was verified by 31P magnetic resonance spectroscopy and behavioral tests, which showed a consistent variation over time.

Normobaric hyperoxia stimulates superoxide and nitric oxide production in the caudal solitary complex of rat brain slices

Central CO₂-chemosensitive neurons in the caudal solitary complex (cSC) are stimulated not only by hypercapnic acidosis, but by hyperoxia as well. While a cellular mechanism for the CO₂ response has yet to be isolated, previous data show that a redox-sensitive mechanism underlies neuronal excitability to hyperoxia. However, it remains unknown how changes in Po₂ affect the production of reactive oxygen and nitrogen species (RONS) in the cSC that can lead to increased cellular excitability and, with larger doses, to cellular dysfunction and death. To this end, we used fluorescence microscopy in real time to determine how normobaric hyperoxia increases the production of key RONS in the cSC. Because neurons in the region are CO₂ sensitive, we also examined the potential effects of CO₂ narcosis, used during euthanasia before brain slice harvesting, on RONS production. Our findings show that normobaric hyperoxia (0.4 → 0.95 atmospheres absolute O₂) increases the fluorescence rates of fluorogenic dyes specific to both superoxide and nitric oxide. Interestingly, different results were seen for superoxide fluorescence when CO₂ narcosis was used during euthanasia, suggesting long-lasting changes in superoxide production and/or antioxidant activity subsequent to CO₂ narcosis before brain slicing. Further research needs to distinguish whether the increased levels of RONS reported here are merely increases in oxidative and nitrosative signaling or, alternatively, evidence of redox and nitrosative stress.

Brain tissue stiffness is a sensitive marker for acidosis

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We introduce tissue stiffness as a sensitive indicator for pathophysiological changes to CNS tissue.

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We applied atomic force microscopy to investigate tissue stiffness.

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We found that CO₂ overexposure-induced acidosis changes brain properties.

Background

Carbon dioxide overdose is frequently used to cull rodents for tissue harvesting. However, this treatment may lead to respiratory acidosis, which potentially could change the properties of the investigated tissue.

New method

Mechanical tissue properties often change in pathological conditions and may thus offer a sensitive generic readout for changes in biological tissues with clinical relevance. In this study, we performed force-indentation measurements with an atomic force microscope on acute cerebellar slices from adult rats to test if brain tissue undergoes changes following overexposure to CO₂ compared to other methods of euthanasia.

Results

The pH significantly decreased in brain tissue of animals exposed to CO₂. Concomitant with the drop in pH, cerebellar grey matter significantly stiffened. Tissue stiffening was reproduced by incubation of acute cerebellar slices in acidic medium.

Comparison with existing methods

Tissue stiffness provides an early, generic indicator for pathophysiological changes in the CNS. Atomic force microscopy offers unprecedented high spatial resolution to detect such changes.

Conclusions

Our results indicate that the stiffness particularly of grey matter strongly correlates with changes of the pH in the cerebellum. Furthermore, the method of tissue harvesting and preparation may not only change tissue stiffness but very likely also other physiologically relevant parameters, highlighting the importance of appropriate sample preparation.

Amide proton signals as pH indicator for in vivo MRS and MRI of the brain-Responses to hypercapnia and hypothermia

Using proton MRS and MRI of mouse brain at 9.4T, this work provides the first in vivo evidence of pH-dependent concurrent changes of three amide signals and related metabolic responses to hypercapnia and hypothermia. During hypercapnia, amide proton MRS signals of glutamine at 6.8-6.9ppm and 7.6ppm as well as of unspecific compounds at 8.1-8.3ppm increase by at least 50% both at 37°C and 22°C. These changes reflect a reduced proton exchange with water. They are strongly correlated with intracellular pH which ranges from 6.75±0.10 to 7.13±0.06 as determined from a shift in creatine phosphokinase equilibrium. In MRI, saturation transfer from aliphatic as well as aromatic and/or amide protons alters slightly during hypercapnia and significantly during hypothermia. The asymmetry in magnetization transfer ratios decreased slightly during hypercapnia and hypothermia. Regardless of pH or temperature, saturation transfer from aliphatic protons between -2 and -4ppm frequency offset to water protons is significantly greater than that from aromatic/amide protons at corresponding offsets between +2 and +4ppm. Irradiation of aliphatic compounds at -3.5ppm frequency offset from water predominantly saturates lipids and water associated with myelin. Taken together, the results indicate that, for the B1 power used in this study, dipolar coupling between aliphatic and water protons rather than proton exchange is the dominant factor in Z-spectra and magnetization transfer ratio asymmetry of the brain in vivo.

Ambient CO2, fish behaviour and altered GABAergic neurotransmission: exploring the mechanism of CO2-altered behaviour by taking a hypercapnia dweller down to low CO2 levels

Recent studies suggest that projected rises of aquatic CO2 levels cause acid–base regulatory responses in fishes that lead to altered GABAergic neurotransmission and disrupted behaviour, threatening fitness and population survival. It is thought that changes in Cl− and HCO3− gradients across neural membranes interfere with the function of GABA-gated anion channels (GABAA receptors). So far, such alterations have been revealed experimentally by exposing species living in low-CO2 environments, like many oceanic habitats, to high levels of CO2 (hypercapnia). To examine the generality of this phenomenon, we set out to study the opposite situation, hypothesizing that fishes living in typically hypercapnic environments also display behavioural alterations if exposed to low CO2 levels. This would indicate that ion regulation in the fish brain is fine-tuned to the prevailing CO2 conditions. We quantified pH regulatory variables and behavioural responses of Pangasianodon hypophthalmus, a fish native to the hypercapnic Mekong River, acclimated to high-CO2 (3.1 kPa) or low-CO2 (0.04 kPa) water. We found that brain and blood pH was actively regulated and that the low-CO2 fish displayed significantly higher activity levels, which were reduced after treatment with gabazine, a GABAA receptor blocker. This indicates an involvement of the GABAA receptor and altered Cl− and HCO3− ion gradients. Indeed, Goldman calculations suggest that low levels of environmental CO2 may cause significant changes in neural ion gradients in P. hypophthalmus. Taken together, the results suggest that brain ion regulation in fishes is fine-tuned to the prevailing ambient CO2 conditions and is prone to disruption if these conditions change.

Embryonic common snapping turtles (Chelydra serpentina) preferentially regulate intracellular tissue pH during acid-base challenges

The nests of embryonic turtles naturally experience elevated CO2 (hypercarbia), which leads to increased blood PCO2 and a respiratory acidosis resulting in reduced blood pH [extracellular pH (pHe)]. Some fishes preferentially regulate tissue pH [intracellular pH (pHi)] against changes in pHe; this has been proposed to be associated with exceptional CO2 tolerance and has never been identified in amniotes. As embryonic turtles may be CO2 tolerant based on nesting strategy, we hypothesized that they preferentially regulate pHi, conferring tolerance to severe acute acid-base challenges. This hypothesis was tested by investigating pH regulation in common snapping turtles (Chelydra serpentina) reared in normoxia then exposed to hypercarbia (13kPa PCO2) for 1h at three developmental ages, 70 and 90% of incubation, and in yearlings. Hypercarbia reduced pHe but not pHi, at all developmental ages. At 70% of incubation, pHe was depressed by 0.324 pH units while pHi of brain, white muscle, and lung increased; heart, liver, and kidney pHi remained unchanged. At 90% of incubation, pHe was depressed by 0.352 pH units but heart pHi increased with no change in pHi of other tissues. Yearling exhibited a pHe reduction of 0.235 pH units but had no changes in pHi of any tissues. The results indicate common snapping turtles preferentially regulate pHi during development, but the degree of the response is reduced throughout development. This is the first time preferential pHi regulation has been identified in an amniote. These findings may provide insight into the evolution of acid-base homeostasis during development of amniotes, and vertebrates in general.

Impact of fasting on human brain acid-base homeostasis using natural abundance (13) C and (31) P MRS

PURPOSE

To use (13) C magnetic resonance spectroscopy (MRS) and (31) P MRS to develop a direct assay for regional [HCO3-] in the human brain and to define brain pH and physiological response of [HCO3-] to fasting.

MATERIALS AND METHODS

Seven healthy subjects underwent MRS examinations on a 1.5T MRI scanner. Subjects were well fed with repeated examinations performed after 4 and 12 hours of fasting. Proton noise decoupling (13) C MRS were acquired using pulse and acquired acquisition while (31) P MRS were acquired using a 2D chemical shift imaging method with relaxation time (TR) of 2 seconds.

RESULTS

Fasting brain bicarbonate concentrations (6.7 ± 2.5 mM for 12-hour fasting, P = 0.002 and 8.3 ± 2.1 mM for 4-hour fasting, P = 0.015) are significantly reduced compared to fed state (11.6 ± 1.3 mM). However, no significant difference in brain pH was observed, confirming the critical role of pCO2 in intracerebral pH homeostasis.

CONCLUSION

We demonstrated that the intracellular HCO3- in human brain is readily modified by diet but appears to have no measurable effect on cerebral pH. Natural abundance (13) C can provide useful information relevant to human brain pH homeostasis by providing information for HCO3-.

Distribution of pH changes in mouse neonatal hypoxic-ischaemic insult

We assessed the distribution in brain pH after neonatal hypoxic-ischaemic insult and its correlation with local injury. Postnatal day 7 mice were injected with neutral red and underwent left carotid occlusion and exposure to 8% oxygen. Images captured from the cut surface of snap-frozen brain were used to calculate the pH from the blue-green absorbance ratios. Carotid occlusion alone had no effect, but combined with hypoxia caused rapid, biphasic pH decline, with the first plateau at 15-30 min, and the second at 60-90 min. The ipsilateral dorsal cortex, hippocampus, striatum and thalamus were most affected. Contralateral pH initially showed only 30% of the ipsilateral decline, becoming more acidotic with increasing duration. Systemic blood analysis revealed, compared with hypoxia alone, that combined insult caused a 63% decrease in blood glucose (1.3 ± 0.2 mM), a 2-fold increase in circulating lactate (17.7 ± 2.9 mM), a reduction in CO(2) to 1.9 ± 0.1 kPa and a drop in pH (7.26 ± 0.06). Re-oxygenation resulted in the normalisation of systemic changes, as well as a global alkaline rebound in brain pH at 4-6 h. A topographic comparison of brain injury showed only a partial correlation with pH changes, with the severest injury occurring in the ipsilateral hippocampus and sparing acidic parts of the contralateral cortex.

Sulfur amino acid-free diet results in increased glutamate in human midbrain: a pilot magnetic resonance spectroscopic study

OBJECTIVE

This pilot study was designed to determine if metabolic effects in different brain regions (left and right parietal lobes, midbrain) caused by 3 d of food consumption without methionine or cysteine could be detected by proton magnetic resonance spectroscopy.

METHODS

Healthy individuals 18 to 36 y old (n = 8) were studied by magnetic resonance spectroscopy after receiving a diet with adequate sulfur amino acids (SAAs) or with zero SAA for 3 d. Pulse sequences were used to selectively measure glutathione (GSH), and linear combination modeling of spectra was used to measure other high-abundance brain metabolites and expressed relative to creatine (Cr).

RESULTS

Although dietary SAAs are required to maintain GSH, the 3-d SAA insufficiency resulted in no significant change in GSH/Cr in the three brain regions. Principal component analysis of 16 metabolites measured by linear combination modeling showed that the metabolic pattern in the midbrain, but not in the parietal lobes, was distinguished according to the dietary SAAs. Multivariate statistical analysis showed that the major discriminating factors were signals of glutamate/Cr, (glutamate + glutamine)/Cr, and myoinositol/Cr. Correlation analyses between midbrain metabolites and GSH-related metabolites in plasma showed that midbrain glutamate/Cr had an inverse correlation with plasma cystine.

CONCLUSION

The data show that magnetic resonance spectroscopy is a non-invasive tool suitable for nutritional assessment and suggest that nutritional imbalance caused by 3 d of SAA-free food more selectively affects the midbrain than the parietal lobes.

The effect of aminosulfonate buffers on the light responses and intracellular pH of goldfish retinal horizontal cells

Retinal horizontal cell feedback acts as a gain control at the first synapse in the visual system and generates center-surround receptive fields in the outer retina. One model of feedback proposes that elevation of protons in the photoreceptor synaptic cleft produces feedback. Most evidence supporting the proton model has depended on the effect of proton buffers, in particular aminosulfonates, but these agents could potentially have effects other than external pH regulation. We therefore determined if the effects of aminosulfonates on horizontal cell rollback, an indicator of feedback, were consistent with external proton buffering. Intracellular recording from horizontal cells in isolated goldfish retina revealed that rollback was blocked only by aminosulfonates with an acid dissociation constant suited for buffering at the pH (7.5) of the Ringer's solution. In isolated goldfish horizontal cells, aminosulfonates, even those that did not block rollback, altered intracellular pH. This suggests that the effect of aminosulfonates on rollback is not because of changing intracellular pH. Measures of both intracellular and extracellular pH revealed that treatment with either glutamate or kainate resulted in acidification. As glutamate produced both internal and external acidification, intracellular and extracellular horizontal cell pH would be expected to increase in response to light, a change consistent with the proton model of feedback.

Hypercapnia causes cellular oxidation and nitrosation in addition to acidosis: implications for CO2 chemoreceptor function and dysfunction

Cellular mechanisms of CO2 chemoreception are discussed and debated in terms of the stimuli produced during hypercapnic acidosis and their molecular targets: protons generated by the hydration of CO2 and dissociation of carbonic acid, which target membrane-bound proteins and lipids in brain stem neurons. The CO2 hydration reaction, however, is not the only reaction that CO2 undergoes that generates molecules capable of modifying proteins and lipids. Molecular CO2 also reacts with peroxynitrite (ONOO-), a reactive nitrogen species (RNS), which is produced from nitric oxide (*NO) and superoxide (*O2-). The CO2/ONOO- reaction, in turn, produces additional nitrosative and oxidative reactive intermediates. Furthermore, protons facilitate additional redox reactions that generate other reactive oxygen species (ROS). ROS/RNS generated by these redox reactions may act as additional stimuli of CO2 chemoreceptors since neurons in chemosensitive areas produce both *NO and *O2- and, therefore, ONOO-. Perturbing *NO, *O2-, and ONOO- activities in chemosensitive areas modulates cardiorespiration. Moreover, neurons in at least one chemosensitive area, the solitary complex, are stimulated by cellular oxidation. Together, these data raise the following two questions: 1) do pH and ROS/RNS work in tandem to stimulate CO2 chemoreceptors during hypercapnic acidosis; and 2) does nitrosative stress and oxidative stress contribute to CO2 chemoreceptor dysfunction? To begin considering these two issues and their implications for central chemoreception, this minireview has the following three goals: 1) summarize the nitrosative and oxidative reactions that occur during hypercapnic acidosis and isocapnic acidosis; 2) review the evidence that redox signaling occurs in chemosensitive areas; and 3) review the evidence that neurons in the solitary complex are stimulated by cellular oxidation.

Brain energetics and tolerance to anoxia in deep hypothermia

The remarkable time-resolution enhancement by deep lethargic hypothermia (15 degrees C rectal temperature, "cold narcosis," "anesthesia by internal cold") of metabolic events in the rat brain after oxygen deprivation has been exploited to monitor metabolic changes by in vivo (31)P-NMR. A correlation was established between the bioenergetic status of the brain and physiological descriptors of tolerance (survival and revival times) determined in parallel experiments with large series of animals. Spectral peak integrals were transformed into absolute concentrations by comparison to biochemically determined time series of data obtained in freeze-trapping experiments conducted under identical conditions. Serial spectra were used to reconstruct the time-course kinetics of intracellular brain pH and of concentration changes of inorganic phosphate, phosphocreatine, ATP, and ADP. Both the biochemical and NMR time series of data were simultaneously fitted by a set of exponential kinetic equations accounting for relationships imposed by the Lohmann and adenylate kinase reactions. Depletion profiles were then computed for a number of descriptors of brain energy status (energy charge, phosphorylation potential, total adenylate, and primary energy stores expressed as the sum of high-energy phosphate-bond equivalents). The results contribute to the understanding of the role of brain energetics in tolerance to oxygen deprivation.

Oxidative metabolism, apoptosis and perinatal brain injury

Perinatal hypoxic-ischaemic injury (HII) is a significant cause of neurodevelopmental impairment and disability. Studies employing 31P magnetic resonance spectroscopy to measure phosphorus metabolites in situ in the brains of newborn infants and animals have demonstrated that transient hypoxia-ischaemia leads to a delayed disruption in cerebral energy metabolism, the magnitude of which correlates with the subsequent neurodevelopmental impairment. Prominent among the biochemical features of HII is the loss of cellular ATP, resulting in increased intracellular Na+ and Ca2+, and decreased intracellular K+. These ionic imbalances, together with a breakdown in cellular defence systems following HII, can contribute to oxidative stress with a net increase in reactive oxygen species. Subsequent damage to lipids, proteins, and DNA and inactivation of key cellular enzymes leads ultimately to cell death. Although the precise mechanisms of neuronal loss are unclear, it is now clear both apoptosis and necrosis are the significant components of cell death following HII. A number of different factors influence whether a cell will undergo apoptosis or necrosis, including the stage of development, cell type, severity of mitochondrial injury and the availability of ATP for apoptotic execution. This review will focus on some pathological mechanisms of cell death in which there is a disruption to oxidative metabolism. The first sections will discuss the process of damage to oxidative metabolism, covering the data collected both from human infants and from animal models. Following sections will deal with the molecular mechanisms that may underlie cerebral energy failure and cell death in this form of brain injury, with a particular emphasis on the role of apoptosis and mitochondria.

Neuronal sensitivity to hyperoxia, hypercapnia, and inert gases at hyperbaric pressures

As ambient pressure increases, hydrostatic compression of the central nervous system, combined with increasing levels of inspired Po2, Pco2, and N2 partial pressure, has deleterious effects on neuronal function, resulting in O2 toxicity, CO2 toxicity, N2 narcosis, and high-pressure nervous syndrome. The cellular mechanisms responsible for each disorder have been difficult to study by using classic in vitro electrophysiological methods, due to the physical barrier imposed by the sealed pressure chamber and mechanical disturbances during tissue compression. Improved chamber designs and methods have made such experiments feasible in mammalian neurons, especially at ambient pressures <5 atmospheres absolute (ATA). Here we summarize these methods, the physiologically relevant test pressures, potential research applications, and results of previous research, focusing on the significance of electrophysiological studies at <5 ATA. Intracellular recordings and tissue Po2 measurements in slices of rat brain demonstrate how to differentiate the neuronal effects of increased gas pressures from pressure per se. Examples also highlight the use of hyperoxia (<or=3 ATA O2) as a model for studying the cellular mechanisms of oxidative stress in the mammalian central nervous system.

Extra- and Intracellular pH in the Brain During Ischaemia, Related to Tissue Lactate Content in Normo- and Hypercapnic rats

The objective of the present study was to assess the relationship between the amount of lactate accumulated during complete ischaemia and the ensuing changes in extra- and intracellular pH (pHe and pHi, respectively). The preischaemic plasma glucose concentration of anaesthetized rats was varied by administration of glucose or insulin, pHe was determined in neocortex with ion-sensitive microelectrodes, and tissue lactate and CO2 contents were measured, tissue CO2 tension being known from separate experiments. The experiments were carried out in both normocapnic [arterial CO2 tension (PaCO2) approximately 40 mm Hg] and hypercapnic (PaCO2 approximately 80 mm Hg) animals. Irrespective of the preischaemic CO2 tension, DeltapHe was linearly related to tissue lactate content. Depending on the preischaemic glucose concentration, DeltapHe varied from <0.4 to >1.4 units. The results thus fail to confirm previous results that the changes in pHe describe two plateau functions (DeltapHe approximately 0.5 and 1.1, respectively), with a transition zone at tissue lactate contents of 17 - 20 mmol kg-1. Changes in pHi given in this study are based on the assumption of a uniform intracellular space. The pHi changed from a normal value of approximately 7.0 to 6.5, 6.1 and 5.8 at tissue lactate contents of 10, 20 and 30 mmol kg-1. The intrinsic (non-bicarbonate) buffer capacity, derived from these figures, was 23 mmol kg-1 pH-1. Some differences in pH and in HCO3- concentration between extra- and intracellular fluids persisted in the ischaemic tissue. These differences were probably caused by a persisting membrane potential in the ischaemic cells.

Brain N-acetylaspartate is elevated in Pelizaeus-Merzbacher disease with PLP1 duplication

OBJECTIVE

To assess alterations in brain metabolites of patients with Pelizaeus-Merzbacher disease (PMD) with the proteolipid protein gene 1 (PLP1) duplications using quantitative proton MRS.

METHODS

Five unrelated male Japanese patients with PMD with PLP1 duplications were analyzed using automated proton brain examination with the point resolved spectroscopy technique (repetition and echo time of 5,000 and 30 msec). Localized spectra in the posterior portion of the centrum semiovale were acquired, and absolute metabolite concentrations were calculated using the LCModel.

RESULTS

Absolute concentrations of N-acetylaspartate (NAA), creatine (Cr), and myoinositol (MI) were increased by 16% (p < 0.01), 43% (p < 0.001), and 31% (p < 0.01) in patients with PMD as compared with age-matched controls. There was no statistical difference in choline concentration.

CONCLUSION

The increased concentration of NAA, which could not be detected by previous relative quantitation methods, suggests two possibilities: axonal involvement secondary to dysmyelination, or increased cell population of oligodendrocyte progenitors. Elevated Cr and MI concentrations may reflect the reactive astrocytic gliosis. Our study thus emphasizes the importance of absolute quantitation of metabolites to investigate the disease mechanism of the dysmyelinating disorders of the CNS.

Changes of cerebral blood flow, oxygenation, and oxidative metabolism during graded motor activation

In the present studies fMRI and a hypercapnic calibration procedure were used to monitor simultaneous changes in cerebral blood flow (CBF), cerebral blood oxygenation, and cerebral metabolic rate of oxygen (CMRO(2)) during activation in the sensorimotor cortex. In the first set of experiments seven volunteers performed bilateral, self-paced finger tapping and in the second set of experiments six volunteers performed bilateral finger tapping with six different frequencies (0.5-3 Hz). During the latter task relative CBF and BOLD signal intensity changes varied linearly as a function of stimulus frequency. In good agreement with recent PET and fMRI data increases in CMRO(2) were smaller than the corresponding changes in CBF during self-paced finger tapping and at all levels of graded motor activation. At a single level of activation and during graded activation there was a positive linear relationship between CBF and CMRO(2) with ratios of approximately 3:1. Comparable proportionality constants have been found in the visual cortex and primary sensory cortex, indicating similarities between the relationship of CBF and CMRO(2) in various cortical regions.

Brain proton magnetic resonance spectroscopy and imaging in children exposed to cocaine in utero

OBJECTIVE

The effects of prenatal cocaine exposure have been examined using neurobehavioral and brain structural evaluations; however, no study has examined the effects of prenatal cocaine on brain metabolism. Proton magnetic resonance spectroscopy ((1)H-MRS) is a noninvasive method to examine the biochemistry of various brain regions. The purpose of this study was to examine the possible neurotoxic effects of prenatal cocaine exposure on the developing brain using (1)H-MRS.

METHODS

Cocaine-exposed children (n = 14) and age-matched unexposed control participants (n = 12) were evaluated with MRI and localized (1)H-MRS. Metabolite concentrations of N-acetyl-containing compounds (NA), total creatine (Cr), choline-containing compounds, myoinositol, and glutamate + glutamine were measured in the frontal white matter and striatum.

RESULTS

Despite an absence of structural abnormalities in either group, children exposed to cocaine in utero had significantly higher Cr (+13%) in the frontal white matter. NA, primarily a measure of N-acetyl aspartate and neuronal content, was normal in both regions examined by (1)H-MRS. Normal NA suggests no significant neuronal loss or damage in the 2 brain regions examined in children exposed to cocaine prenatally.

CONCLUSIONS

Consistent with findings in abstinent adult cocaine users, we found increased Cr in the frontal white matter, with normal NA in children exposed to cocaine. These findings suggest the need to investigate further possible abnormalities of energy metabolism in the brain of children exposed to cocaine in utero. In addition, this study demonstrates the feasibility of using (1)H-MRS to investigate the effects of prenatal drug exposure on the developing brain.

Metabolic effects of 1-methyl-4-phenylpyridinium (MPP(+)) in primary neuron cultures

Disruption of mitochondrial function has been proposed as an action of 1-methyl-4-phenylpyridinium (MPP(+)) that is responsible for its toxicity. In order to characterize effects of MPP(+) on energy metabolism in primary culture neurons, we monitored levels of several metabolites in cultured rat cerebellar granule cells exposed to MPP(+). The toxin produced a rapid concentration-dependent reduction in intracellular phosphocreatine (PCr), amounting to a 50-80% decrease within 30-60 min at 50 microM, that was maintained through the 1 week exposure interval examined. In contrast, ATP levels remained comparable to those of untreated neurons for approximately 4 days, at that time a 50% reduction in ATP was observed in association with a decrease in cell viability. Acute decreases in PCr were accompanied by increases in creatine such that the total creatine levels were maintained. Lactate levels in the culture medium were significantly increased (from 4.5 to 6.0 mM) within 6 hr after addition of MPP(+), with a concentration dependence similar to that observed for the reduction in PCr. Increased lactate production in the presence of MPP(+) coincided with a more rapid depletion of glucose in the culture medium. MPP(+) induced a rapid and sustained decrease in intracellular pH calculated from the creatine kinase equilibrium, and this acidification is considered primarily responsible for the observed decrease in PCr. These studies provide direct evidence that toxic concentrations of MPP(+) have acute effects on energy metabolism in primary culture neurons, consistent with an increased dependence on glycolysis to meet metabolic demand, but indicate that toxicity is not associated with overt, immediate failure to maintain cellular ATP.

High-resolution CMR(O2) mapping in rat cortex: a multiparametric approach to calibration of BOLD image contrast at 7 Tesla

The blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) method, which is sensitive to vascular paramagnetic deoxyhemoglobin, is dependent on regional values of cerebral metabolic rate of oxygen utilization (CMR(O2)), blood flow (CBF), and volume (CBV). Induced changes in deoxyhemoglobin function as an endogenous contrast agent, which in turn affects the transverse relaxation rates of tissue water that can be measured by gradient-echo and spin-echo sequences in BOLD fMRI. The purpose here was to define the quantitative relation between BOLD signal change and underlying physiologic parameters. To this end, magnetic resonance imaging and spectroscopy methods were used to measure CBF, CMR(O2), CBV, and relaxation rates (with gradient-echo and spin-echo sequences) at 7 Tesla in rat sensorimotor cortex, where cerebral activity was altered pharmacologically within the autoregulatory range. The changes in tissue transverse relaxation rates were negatively and linearly correlated with changes in CBF, CMR(O2), and CBV. The multiparametric measurements revealed that CBF and CMR(O2) are the dominant physiologic parameters that modulate the BOLD fMRI signal, where the ratios of (deltaCMR(O2)/CMR(O2)/(deltaCBF/ CBF) and (deltaCBV/CBV)/(deltaCBF/CBF) were 0.86 +/- 0.02 and 0.03 +/- 0.02, respectively. The calibrated BOLD signals (spatial resolution of 48 microL) from gradient-echo and spin-echo sequences were used to predict changes in CMR(O2) using measured changes in CBF, CBV, and transverse relaxation rates. The excellent agreement between measured and predicted values for changes in CMR(O2) provides experimental support of the current theory of the BOLD phenomenon. In gradient-echo sequences, BOLD contrast is affected by reversible processes such as static inhomogeneities and slow diffusion, whereas in spin-echo sequences these effects are refocused and are mainly altered by extravascular spin diffusion. This study provides steps by which multiparametric MRI measurements can be used to obtain high-spatial resolution CMR(O2) maps.

Hypercarbia and mild hypothermia, only when not combined, improve postischemic bioenergetic recovery in neonatal rat brain slices

In the immature brain, postischemic metabolism may be influenced beneficially by the effect of inducing hypercarbia or hypothermia. With use of 31P nuclear magnetic resonance spectroscopy, intracellular pH (pHi) and cellular energy metabolites in ex vivo neonatal rat cerebral cortex were measured before, during, and after substrate and oxygen deprivation in in vitro ischemia. Early postischemic hypothermia (fall in temperature -3.2 +/- 1.0 degrees C) delayed the normalization of pHi after ischemia by inducing an acid shift in pHi (P < 0.01). Postischemic hypercarbia (Krebs-Henseleit bicarbonate buffer equilibrated with 10% carbon dioxide in oxygen) and hypothermia induced separate, but potentially additive, reversible decreases in pHi, each of approximately -0.16 pH unit (P < 0.05). When these postischemic perturbations were applied in isolation, there was significant improvement of approximately 20% in the recovery of beta-ATP (P < 0.05). In combination, however, hypercarbia and hypothermia worsened recovery in ATP by approximately 20% (P < 0.05). In control tissue, which had not been exposed to ischemia, ATP content was also significantly reduced by co-administration of the two treatments (P < 0.05), an effect that persisted even after discontinuing the perturbing conditions. Therefore, in this vascular-independent neonatal preparation, early postischemic modulation of metabolism by hypercarbia or hypothermia appears to confer improved bioenergetic recovery, but only if they are not administered together.

Reduced ventilator pressure and improved P/F ratio during percutaneous arteriovenous carbon dioxide removal for severe respiratory failure

OBJECTIVE

To evaluate the effect of percutaneous arteriovenous carbon dioxide removal (AVCO2R) on ventilator pressures and P/F ratio in a clinically relevant large-animal model of severe respiratory failure.

SUMMARY BACKGROUND DATA

AVCO2R was developed as a simple arteriovenous shunt with a commercially available low-resistance gas exchange device of sufficient surface area for near-total CO2 removal. With an AV shunt 10% to 15% of cardiac output, AVCO2R allows a reduction in ventilator airway pressures without hypercapnia or the complex circuitry and monitoring required for conventional ECMO.

METHODS

AVCO2R was applied to a new, clinically relevant large-animal model of severe respiratory failure created by smoke inhalation and cutaneous flame bum injury. Adult sheep (n = 9, 38+/-6 kg) received a 40% total body surface area, third-deinsufflation. After injury, all animals were placed on volume-controlled mechanical ventilation to achieve PaO2 > 60 mmHg and PacO2 < 40 mmHg. Animals were placed on AVCO2R within 40 to 48 hours of injury when the PaO2/FiO2 was <200. Animals underwent cannulation of the carotid artery and jugular vein with percutaneous 10F arterial and 14F venous cannulas. Shunt flow was continuously monitored using an ultrasonic flow probe and calculated as a percentage of cardiac output.

RESULTS

AVCO2R flows of 800 to 900 ml/min (11% to 13% cardiac output) achieved 77 to 104 ml/min of CO2 removal (95% to 97% total CO2 production) while maintaining normocapnia. Significant reductions in ventilator settings were tidal volume, 421.3+/-39.8 to 270.0+/-6.3 ml; peak inspiratory pressure, 24.8+/-2.4 to 13.7+/-0.7 cm H2O; minute ventilation, 12.7+/-1.4 to 6.2+/-0.8 L/min; respiratory rate, 25.4+/-1.3 to 18.4+/-1.8 breaths/min; and FiO2, 0.88+/-0.1 to 0.39+/-0.1. The P/F ratio increased from 151.5+/-40.0 at baseline to 320.0+/-17.8 after 72 hours.

CONCLUSIONS

Percutaneous AVCO2R allows near-total CO2 removal and significant reductions in ventilator pressures with improvement in the P/F ratio.

The effect of alpha-phenyl-N-tert-butyl nitrone on bioenergetic state in substantia nigra following flurothyl-induced status epilepticus in rats

Status epilepticus (SE), i.e. ongoing seizures of more than 30 min duration, gives rise to bilateral pan-necrotic lesions of the substantia nigra, pars reticulata (SNPR). These are known to be preceded by an initial increase, followed by a depression of metabolic rate, and by failure of the bioenergetic state, suggesting mitochondrial dysfunction. We have previously shown that the spin trap alpha-phenyl-N-tert-butyl nitrone (PBN) prevents the lesions caused by 45 min of SE from occurring, in spite of ongoing seizure activity. In this article, we demonstrate that PBN, given 30 min before seizure induction, reduces or prevents the decrease in ATP concentration and adenylate energy charge, without significantly reducing the amount of lactate accumulated, or the decrease in intracellular pH (pHi). The results suggest that the spin trap nitrone preserves the structural and functional integrity of SNPR neurons by protecting the mitochondria against oxidative damage.

Unrecognized severe postoperative hypercapnia: a case of apneic oxygenation

Transcutaneous pulse oximetry is increasingly being used to supplant arterial blood gas measurement as a means to monitor oxygenation. Previous studies have demonstrated that, despite inadequate ventilation, oxygenation can be maintained during delivery of supplemental oxygen by a process known as diffusion respiration. In this setting, severe hypercapnia and acidosis rapidly develop. This case report demonstrates that pulse oximetry is an unreliable means to monitor adequacy of ventilation. A 75-year-old woman in good health suffered a fracture of the right hip that necessitated arthroplasty. During postoperative recovery, she remained unresponsive while receiving 100% oxygen through an endotracheal tube; mechanical ventilation was not used. Pulse oximetry indicated a blood oxygen saturation of 94 to 96%; however, results of blood gas studies 3 1/2 hours postoperatively revealed profound hypercapnia (arterial carbon dioxide tension, 265 mm Hg) and acidosis (pH, 6.65) but confirmed normal oxygen levels (arterial oxygen tension, 213 mm Hg). Assisted ventilation resulted in normalization of the blood gases and an improved level of consciousness. The patient was then transferred to Mayo Clinic Rochester and had an uneventful recovery.

Brain parenchyma PO2, PCO2, and pH during and after hypoxic, ischemic brain insult in dogs

OBJECTIVES

1) The investigation of fiberoptic PO2, PCO2, and pH sensor technology as a monitor of brain parenchyma during and after brain injury, and 2) the comparison of brain parenchyma PO2, PCO2, and pH with intracranial pressure during and after hypoxic, ischemic brain insult.

DESIGN

Prospective, controlled, animal study in an acute experimental preparation.

SETTING

Physiology laboratory in a university medical school.

SUBJECTS

Fourteen mongrel dogs (20 to 35 kg), anesthetized, room-air ventilated.

INTERVENTIONS

Anesthesia was induced with thiopental and maintained after intubation using 1% to 1.5% halothane in room air (FiO2 0.21). Mechanical ventilation was established to maintain end-tidal PCO2 approximately 35 torr (-4.7 kPa). Intravenous, femoral artery, and pulmonary artery catheters were placed. The common carotid arteries were surgically exposed, and ultrasonic blood flow probes were applied. A calibrated intracranial pressure probe was placed through a right-side transcranial bolt, and a calibrated intracranial chemistry probe with optical sensors for PO2, PCO2, and pH was placed through a left-side bolt into brain parenchyma. Brain insult was induced in the experimental group (n = 6) by hypoxia (FiO2 0.1), ischemia (bilateral carotid artery occlusion), and hypotension (mean arterial pressure [MAP] approximately 40 mm Hg produced with isoflurane approximately 4%). After 45 mins, carotid artery occlusion was released, FiO2 was reset to 0.21, and anesthetic was returned to halothane (approximately 1.25%). The control group (n = 5) had the same surgical preparation and sequence of anesthetic agent exposure but no brain insult.

MEASUREMENTS AND MAIN RESULTS

Monitored variables included brain parenchyma PO2, PCO2, and pH, which were monitored at 1-min intervals, and intracranial pressure, MAP, arterial hemoglobin oxygen saturation (by pulse oximetry), end-tidal PCO2, and carotid artery blood flow rate, for which data were collected at 15-min intervals for 7 hrs. Arterial and mixed venous blood gas analyses were done at approximately 1-hr intervals. Baseline data agreed closely with other published results: brain parenchyma PO2 of 27 +/- 7 (SD) torr (3.6 +/- 0.9 kPa); brain parenchyma PCO2 of 69 +/- 12 torr (9.2 +/- 1.6 kPa); and brain parenchyma pH of 7.13 +/- 0.09. Postcalibration data were accurate, indicating stability and durability over several hours. In six experiments, during the brain insult, brain parenchyma PO2 decreased to 16 +/- 2 torr (2.1 +/- 0.3 kPa), brain parenchyma PCO2 increased to 105 +/- 44 torr (14 +/- 5.9 kPa) (p < .05), and brain parenchyma pH decreased to 6.75 +/- 0.08 (p < .05). Intracranial pressure (ICP) remained nearly constant (baseline 16 +/- 6 to 14 +/- 5 mm Hg at the end of the brain insult). Cerebral perfusion pressure (CPP = MAP - ICP) decreased (baseline 95 +/- 15 to 28 +/- 8 mm Hg; p < .05). On release of brain insult stresses, ICP increased to 30 +/- 9 mm Hg and CPP increased to 71 +/- 19 mm Hg (p < .05). A biphasic recovery was observed for brain parenchyma pH, which had the slowest recovery of the monitored variables. On average, brain parenchyma pH gradually returned toward baseline, and was no longer significantly different from baseline 3 hrs after release of insult stresses. Brain parenchyma PCO2 continued to decrease rapidly after brain insult and then remained approximately 52 +/- 10 torr (approximately 6.9 +/- 1.3 kPa) (p < .05). Brain parenchyma PO2 increased from a minimum at the end of brain insult to a maximum of 43 +/- 17 torr (5.7 +/- 2.3 kPa) within 1.25 hrs (p < .05), and then gradually decreased to approximately 35 +/- 10 torr (approximately 4.7 +/- 1.3 kPa). Cerebral perfusion pressure gradually decreased as ICP increased 3 to 5 hrs after insult.

CONCLUSIONS

Intracranial chemistry probes with optical sensors demonstrated stable, reproducible monitoring of brain parenchyma PO2, PCO2, and pH in dogs for periods lasting > 8 hrs. Significant changes in brain p

Cerebral lactate production in relation to intracranial pressure, cranial computed tomography findings, and outcome in patients with severe head injury

Severe head injury is frequently associated with focal or global disturbances of cerebral blood flow and metabolism. Routine monitoring of intracranial pressure (ICP) and cerebral perfusion pressure (CPP) in these patients does not provide information about critically reduced local or global cerebral blood flow. Measurements of cerebral lactate difference, Lactate-Oxygen-Index (LOI) and cerebral oxygen extraction were evaluated for advanced monitoring by comparing these parameters with ICP, cranial computed tomography (CCT) findings, and outcome in a group of severely head-injured patients. In 21 patients with severe brain trauma (GCS < or = 8), arterial as well as jugular venous lactate levels and oxygen saturation were measured in vitro every 6 h after admission of patients to the intensive care unit (ICU) throughout the acute course of treatment. Arterial blood pressure, blood gases, and ICP were assessed by standard monitoring measurements. CCT was performed initially after admission of the patients to the hospital, during the acute period in the ICU, if indicated, and 10 to 14 days after trauma. Outcome was classified according to the Glasgow outcome scale (GOS) at six months after injury. Data were averaged in each patient for every day after trauma and over the entire monitoring period. Resulting values were tested for correlation by regression analysis. Additionally, the data of the group of patients with normal to minimally elevated mean ICP (ICP < 20 mmHg, n = 12) were compared to those of the patients with increased mean ICP (ICP > 20 mmHg, n = 9). The cerebral lactate difference in all patients on the day of trauma was significantly increased as compared to the later period (0.20 vs. 0.11-0.07 mmol/l, p < 0.05), but was not different with high or normal to minimally elevated ICP. In patients with intracranial hypertension, the cerebral lactate difference remained significantly increased from the first to the fifth day after injury, whereas it normalized in this period in the group with normal to minimally elevated ICP. Averaged over the acute course, patients with increased ICP had significantly higher mean lactate differences (0.18 +/- 0.16 vs. 0.067 +/- 0.025 mmol/l, p = 0.001) and higher mean LOIs (0.072 +/- 0.071 vs. 0.028 +/- 0.013, p = 0.011). There was a significant correlation of increased mean cerebral lactate difference to poor outcome (r = 0.46, p = 0.035). Cerebral oxygen extraction in all patients tended to increase on the day of trauma (36.7% vs. 29.2% to 31.5% during the subsequent course), but this difference was not significant. The initial degree of brain swelling, classified by CCT according to Marshall et al. (1991), showed no correlation with cerebral lactate differences, ICP, O2-extraction, or outcome. Neither was there a correlation of cerebral oxygen extraction to ICP nor to outcome. In conclusion, the severity of brain trauma and outcome of patients was reflected by increased cerebral lactate production. Unchanged values of global cerebral oxygen extraction suggest that the regulatory mechanisms of brain oxygen supply were not impaired by trauma. Measurements of cerebral lactate differences and brain oxygen extraction may contribute to advanced monitoring in severe head injury.

In vivo proton magnetic resonance spectroscopy of the normal aging human brain

The effect of age on brain metabolite concentrations was evaluated using localized proton magnetic resonance spectroscopy. This technique allows in vivo measurements of N-acetyl compounds (NA), total creatine (CR), choline-containing compounds (CHO), myo-inositol (MI), glutamate and glutamine (GLX), as well as the percentage of cerebrospinal fluid (CSF) and the brain water content within the brain region studied. Frontal gray matter and frontal white matter brain regions were examined in 36 normal healthy volunteers (19-78 years of age). Using a rigorous absolute quantitation method, with an external reference and atrophy correction, we found relatively stable concentrations of NA, a neuronal marker. In contrast, CR, CHO, MI, and the percentage of CSF increased in the gray matter with age. However, the brain water content decreased significantly with age (r = -0.72; p < 0.0001). No significant age-related changes in metabolite concentrations, CSF or brain water content were observed in the white matter regions. These findings demonstrate that biochemical alterations are associated with aging in the frontal gray matter. There might be an increase in the brain density as indicated by increased metabolite concentrations and decreased brain water content with aging.

Paradoxical mitochondrial oxidation in perinatal hypoxic-ischemic brain damage

Measurements of cytoplasmic and mitochondrial markers of the oxidation-reduction (redox) state of brain tissue were conducted in a perinatal animal model of cerebral hypoxia-ischemia to ascertain underlying biochemical mechanisms whereby ischemia (reduced oxygen and substrate supply) causes brain damage. Seven-day postnatal rats underwent unilateral common carotid artery ligation followed by exposure to 8% oxygen at 37 degrees C for 3 h. During the course of hypoxia-ischemia, the rat pups were quick frozen in liquid nitrogen and their brains processed for the enzymatic, fluorometric measurement of cerebral metabolites necessary for the calculation of intracellular pH and cytoplasmic and mitochondrial redox states. The results showed an early mitochondrial reduction followed by re-oxidation during the course of hypoxia-ischemia. The oxidation reflected a partial depletion in accumulated reducing equivalents and coincides temporally with the duration of hypoxia-ischemia required to convert selective neuronal necrosis into cerebral infarction. The findings suggest that perinatal cerebral hypoxia-ischemia is characterized more by a limitation of substrate than of oxygen supply to the brain, which may explain why glucose supplementation of the immature animal improves neuropathologic outcome, in contrast to adults.

Effect of D- and L-1,3-butanediol isomers on glycolytic and citric acid cycle intermediates in the rat brain

DL-1,3-butanediol (DL-BD) is an ethanol dimer which affords cerebral protection in various experimental models of hypoxia and ischemia but its mechanism of action is unknown. DL-BD is a ketogenic alcohol and it has been proposed that its protective effect was accomplished through cerebral utilization of ketone bodies. Since DL-BD is a racemic, its metabolic effects could be due to D, L or both isomers. The effects of equimolar doses of DL-, D- and L-BD (25 mmol/Kg) on cerebral metabolism were studied by measuring the cortical levels of the main glycolytic (glycogen, glucose, glucose 6-phosphate, fructose 1,6-diphosphate, pyruvate and lactate) and citric acid cycle (citrate, alpha-ketoglutarate and L-malate) intermediates. The two BD isomers exerted different effects on cerebral metabolism. Unlike L-BD, D- and DL-BD treatments resulted in a slight (+10%) but significant increase in citrate level whereas L-BD treatment led to significant reduction in pyruvate (-12%) and lactate (-24%) levels. These effects were apparently not linked to hyperketonemia, since DL-BHB treatment, which mimicked hyperketonemia induced by DL-BD, had no effect on cerebral metabolites but might be due to intracerebral metabolism of BD.

Proton magnetic resonance spectroscopy of the left medial temporal and frontal lobes in chronic schizophrenia: preliminary report

Proton magnetic resonance spectroscopy (MRS) was performed in 30 medicated schizophrenic patients and 30 normal subjects. Two groups, each containing 15 schizophrenic patients and 15 age-and sex-matched normal subjects, received MRS examinations for different volumes of interest, either the frontal lobe or the medial temporal lobe. Schizophrenic patients showed a decrease in the ratios of N-acetylaspartate (NAA)/choline-containing compounds (Cho) and NAA/creatine-phosphocreatine (Cr). The patients also showed an increase in the ratio of Cho/Cr in the left medial temporal lobe but not in the left frontal lobe. The age at onset of illness correlated positively with the ratios of NAA/Cho and NAA/Cr in the medial temporal lobe. No significant correlation was observed between the ratios of NAA/Cho, NAA/Cr, or Cho/Cr in the left medial temporal and frontal lobes and clinical symptomatology as assessed by the Scale for the Assessment of Negative Symptoms and the Positive and Negative Syndrome Scale.

Permissive hypercapnia

Traditional practice of mechanical ventilation includes tactics to reduce lung injury, such as avoidance of excessive airway pressure, patient distress, and tidal volume. Gas exchange objectives have received priority, however, and a degree of lung injury has been accepted as inevitable. The current trend toward increasing use of permissive hypercapnia is based on the recognition that lung injury induced by mechanical ventilation may be reduced by compensated hypercapnia with few serious adverse effects and contraindications.

Permissive hypercapnia as a ventilatory strategy in burned children: effect on barotrauma, pneumonia, and mortality

OBJECTIVE

To document the incidence of barotrauma, pneumonia, and respiratory death associated with a mechanical ventilation protocol based on permissive hypercapnia in pediatric burn patients.

DESIGN

Retrospective review.

MATERIALS AND METHODS

Patients were managed using a mechanical ventilation protocol based on permissive hypercapnia, tolerating moderate (pH > 7.20) respiratory acidosis to keep inflating pressures below 40 cm H2O.

MAIN RESULTS

Over a 2.5-year interval, 54 burned children (11% of 495 acute admissions) with an average age of 6.5 years (range 5 weeks to 17 years), average burn size of 44% (range 0 to 98%), and median burn size of 46% required mechanical ventilatory support for an average of 12.5 days (range 1 to 56 days). Inhalation injury was diagnosed in 34 (63%) of the children and 72% percent were admitted within 24 hours of injury. Overt barotrauma occurred in 5.6% of the patients, pneumonia in 32%, and respiratory death in 0%.

CONCLUSIONS

A conventional ventilation protocol based on permissive hypercapnia is associated with acceptable rates of barotrauma and pneumonia. The low incidence of respiratory death associated with this strategy suggests that it also minimizes ventilator-induced lung injury.

The influence of acidosis on hypoglycemic brain damage

The objective of the study was to explore whether hypoglycemic brain damage is affected by super-imposed acidosis. To that end, animals with insulin-induced hypoglycemic coma, defined in terms of a negative DC potential shift, massive release of K+, or cellular uptake of Ca2+, were exposed to excessive hypercapnia (PaCO2 approximately 200 or approximately 300 mm Hg) during the last 25 min of the 30-min coma period. Animals were allowed to survive for 7 days before their brains were fixed by perfusion, and the cell damage was assessed by light microscopy. Other animals were analyzed with respect to changes in extracellular pH (pHe) or extracellular K+ or Ca2+ concentrations (K+e and Ca2+e, respectively). The total CO2 content (TCO2) was also measured to allow derivation of intracellular pH (pHi). The increase in PaCO2 to 190 +/- 15 and 312 +/- 23 mm Hg (means +/- SD) reduced the pHe from a predepolarization value of approximately 7.4 and a postdepolarization value (after the first 5 min of coma) of approximately 7.3 to 6.8 and 6.7, respectively. The corresponding mean pHi values were 6.7 and 6.5. The hypercapnia did not alter the K+e, which rose to 50-60 mM at the onset of hypoglycemic coma, but it increased the Ca2+e from approximately 0.05 to 0.10-0.16 mM. Normocapnic animals with induced hypoglycemic coma of 30-min duration showed the expected neuronal lesions in the neocortex, hippocampus, and caudoputamen. Hypercapnia clearly aggravated this damage, particularly in the caudoputamen, subiculum, and CA1 region of the hippocampus, and caused additional damage to cells in the CA3 region and piriform cortex. A rise in CO2 tension from approximately 200 to 300 mm Hg did not further aggravate the damage. The results thus demonstrate that relative moderate acidosis aggravates damage that is believed to be mostly neuronal, sparing glia cells and vascular tissue.

Proton magnetic resonance spectroscopy in suspected vascular ischemic parkinsonism

Up to now the existence of "vascular parkinsonism" has been doubtful because conclusive clinicopathologic studies are lacking. The objective of the present magnetic resonance spectroscopy (MRS) study is to detect metabolic signs as a reflect of ischemic lesions which could be responsible for the clinical features of vascular parkinsonism. Proton MRS of the brain was performed in 12 patients suspected of vascular parkinsonism on clinical grounds and ischemic score, and in a control group of 15 patients with idiopathic Parkinson's disease. The MR spectra were measured in the striatum and deep white matter. MRS did not demonstrate metabolic evidence for the existence of ischemia (elevated lactate) or cell loss (decreased N-acetyl-aspartate levels) in patients suspected of vascular parkinsonism. Several explanations for our findings are discussed.

Regulation of intra- and extracellular pH in the rat brain in acute hypercapnia: a re-appraisal

Recent results have demonstrated that intracellular pH (pHi) in nerve and glial cells is not regulated back to normal during CO2 exposure if extracellular pH (pHe) is reduced. This raises the question about regulation of pHi and pHe in vivo. In order to successively reduce pHe we exposed animals to incremental increases in CO2 tension (11, 27.5, 42.5%) and studied regulation of pHi during the first 90 min of hypercapnia. Extracellular pH, as well as Na+, K+, and Cl- concentrations, were also measured, as were whole tissue contents of Na+, K+, and Cl-. At all CO2 tensions studied, pHe slowly increased during CO2 exposure. In animals breathing 11% CO2 (delta pHe approximately 0.2 units), pHi increased slowly. However, in animals exposed to 27.5% CO2 or 42.5% CO2 (delta pHe > 0.4 units), no regulation of pHi was observed. Extracellular HCO3- concentrations increased substantially already during the first 15 min of hypercapnia (not significant in animals breathing 42.5% CO2), and then gradually rose. These increases were accompanied by a decrease in Cl- and an increase in Na+ concentration, K+ concentration remaining constant. The total tissue content of these ions remained constant, suggesting that extracellular HCO3- concentration increases by Cl-/HCO3- antiport and/or by Na+.2HCO3- symport, the HCO3- emanating from intracellular sources. The results challenge the dogma of the supremacy of mechanisms regulating pHi, and suggest that brain cells, possibly astrocytes, regulate pHe at the expense of their own pH homeostasis. By inference, we further conclude that regulation of pHi normally occurs only if pHe is first regulated back close to normal value.

Effect of 1,3-butanediol on cerebral energy metabolism. Comparison with beta-hydroxybutyrate

Previous studies have shown that 1,3-butanediol (BD) has beneficial effects in experimental models of hypoxia or ischemia but the mechanism by which it exerts its protective effects remains unknown. BD is converted in the body to beta-hydroxybutyrate (BHB) and it has been proposed that its effects were linked to its ketogenic effect. The effects of BD (25 and 50 mmol/kg) on cerebral energy metabolism of rats were studied by measuring the cerebral level of energy metabolites and by evaluating the cerebral metabolic rate according to the Lowry's method. BD induced an increase in [cortical glucose]/[plasma glucose] ratio which was associated with a decrease in lactate level and an increase in glucose and glycogen stores. In contrast, BHB treatment which mimicked hyperketonemia equivalent to BD did not modify cerebral glycolysis metabolites. Calculation of the energy reserve flux after decapitation showed that BD did not reduce the cerebral metabolic rate excluding a protective effect due to a depressant, barbiturate-like, action. These results suggest that BD induces a reduction of cerebral glycolytic rate. However, the effect is not linked to hyperketonemia but might be due to intracerebral conversion of BD to BHB.

Prolonged antinociception following carbon dioxide anesthesia in the laboratory rat

In the laboratory rat, inhalation (30 s) of high (> 70%) CO2 concentrations resulted in short-term (1-3 min) anesthesia, followed by a prolonged (up to 60 min) mild antinociception. Exposure to 100% CO2 resulted in significant thermal (hot-plate, 52 degrees, and tail-flick) and mechanical (tail-pinch, 886 g force) antinociception. Control animals, placed in the same chamber filled with air, showed no such effects. Rats exposed to 70% CO2 exhibited effects on the hot plate comparable to those seen after inhalation of 100% CO2, indicating that the response is not due to CO2-induced hypoxia. Additionally, recovery from halothane-induced anesthesia of comparable duration did not result in antinociception, confirming that anesthesia alone is not sufficient to produce the effect. Pretreatment with the opiate antagonist naltrexone (0.1-10 mg/kg i.p.) did not diminish the CO2-induced antinociception, suggesting that endogenous opioids are not obligatory in the mechanism of this response. Furthermore, hypophysectomy abolished hot-plate antinociception in animals exposed to 100% CO2 while sham-treated controls exhibited a pattern of hot-plate responses similar to that reported above. Taken together, these findings show that: (1) recovery from CO2-induced anesthesia results in a prolonged mild antinociception, detectable with thermal and mechanical nociceptive tests; and (2) this response may represent a novel from of environmentally induced antinociception, mediated by a non-opiate hormonal substance.

Phosphorus-31 magnetic resonance spectra reveal prolonged intracellular acidosis in the brain following subarachnoid hemorrhage

Subarachnoid hemorrhage may be complicated by cerebral ischemia which, though reversible initially, can progress to an irreversible neurological deficit. 31P magnetic resonance spectroscopy, which can determine intracellular pH and thus detect areas of ischemia noninvasively, was applied to 10 patients on 30 occasions, at various times after subarachnoid hemorrhage. In 5 of them, there were focal areas of the brain in which the intracellular pH was reduced to < 6.8 compared with the normal range of 7.05 +/- 0.05. Consciousness was impaired in 4 of these patients. Repeat studies in these 4 patients showed that intracellular pH remained abnormally low for several days but eventually returned toward normal. The return of intracellular pH to normal paralleled an improvement in clinical condition in each case. In the fifth patient with lowered regions of intracellular pH, there had been an impaired level of consciousness and a transient focal deficit prior to the single study. In the other 5 patients there were no areas of reduced pHi even though in 3 of them there was intraventricular or cisternal blood shown on brain computerized tomography. In 2 of these 3 patients there were no abnormal neurological signs at the time of the magnetic resonance study. The third patient had a dense and persistent hemiparesis. The remaining two patients had no abnormal neurological signs at any stage. We suggest that the areas of acidosis may reflect ischemia which is potentially reversible. Since the technique is noninvasive, sequential 31P magnetic resonance spectroscopy of the brain offers a method of detecting cerebral ischemia and, more importantly, of assessing methods of treatment.

Metabolic assessment of a neuron-enriched fraction of rat cerebrum using high-resolution 1H and 13C NMR spectroscopy

This study explored the utility of 1H and 13C magnetic resonance spectroscopy to study a neuron-enriched preparation made from rat cerebrum. The preparation contained high concentrations of N-acetylaspartate and gamma-aminobutyric acid and low concentrations of glutamine, indicating that it was in fact rich in neuronal cytosol. This was confirmed by immunohistochemical studies with antibodies to neuronal and glial markers. A method of metabolite quantification based on the creatine signal yielded metabolite concentrations similar to those of rat cerebrum, whereas concentrations based on the metabolite/protein ratio were five times lower, suggesting that much protein in the preparation was not associated with functioning cytoplasm. The metabolic competence of the preparation was assessed by quantitative measurements of its ability to convert 1-13C-glucose into lactate, glutamate, aspartate, and other metabolites under well oxygenated conditions for 30 min. Calculated from the creatine standard, the mean glycolytic rate was the same as in a synaptosomal preparation studied under similar conditions and the same as rat cerebrum in vivo. Tricarboxylic acid cycle flux occurred at half the rate observed in the synaptosomal preparation and 16% of the basal cerebral metabolic rate in vivo.

Recovery of mitochondrial and plasma membrane function following hypoglycemic coma: coupling of ATP synthesis, K+ transport, and changes in extra- and intracellular pH

The primary objective of the present study was to evaluate the recovery of plasma and mitochondrial membrane functions after 30 min of hypoglycemic coma and to establish whether a lingering accumulation of free fatty acids (FFAs) delays the recovery. A secondary objective was to study whether production of metabolic acids following glucose infusion leads to a fall in intracellular pH (pHi). Phosphocreatine, creatine, ATP, ADP, and AMP, as well as glycogen, glucose, lactate, pyruvate, and FFAs of rat brain cortex and caudoputamen were measured, and "free" ADP was calculated from the creatine kinase equilibrium. Extracellular pH (pHe) and K+ concentration (K+e) were measured with ion-sensitive microelectrodes, and pHi was derived by the CO2 method. Glucose injection was followed by resumption of oxidative phosphorylation within approximately 2 min and by an equally rapid restoration of normal K+e levels. These functions recovered although tissue FFAs remained elevated for at least 7-8 min. Tissue lactate content increased only moderately and production of metabolic acids did not lead to intracellular acidosis. After 15 min of recovery, pHi was moderately increased, although pHe fell toward 7.0. It is speculated that the dissociation between intra- and extra-cellular pH is compatible with an up-regulation of an Na+/H+ antiporter, e.g., by phosphorylation.

Regulation of intracellular pH in single rat cortical neurons in vitro: a microspectrofluorometric study

Intracellular pH (pHi) and the mechanisms of pHi regulation in cultured rat cortical neurons were studied with microspectrofluorometry and the pH-sensitive fluorophore 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein. Steady-state pHi was 7.00 +/- 0.17 (mean +/- SD) and 7.09 +/- 0.14 in nominally HCO3(-)-free and HCO3(-)-containing solutions, respectively, and was dependent on extracellular Na+ and Cl-. Following an acid transient, induced by an NH1 prepulse or an increase in CO2 tension, pHi decreased and then rapidly returned to baseline, with an average net acid extrusion rate of 2.6 and 2.8 mmol/L/min, in nominally HCO3(-)-free and HCO3(-)-containing solutions, respectively. The recovery was completely blocked by removal of extracellular Na+ and was partially inhibited by amiloride or 5-N-methyl-N-isobutylamiloride. In most cells pHi recovery was completely blocked in the presence of harmaline. The recovery of pHi was not influenced by addition of 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) or removal of Cl-. The rapid regulation of pHi seen following a transient alkalinization was not inhibited by amiloride or by removal of extracellular Na+, but was partially inhibited by DIDS and by removal of extracellular Cl-. The results are compatible with the presence of at least two different pHi-regulating mechanisms: an acid-extruding Na+/H+ antiporter, possibly consisting of different subtypes, and a passive Cl-/HCO3- exchanger, mediating loss of HCO3- from the cell.

Intracellular pH regulation in cultured rat astrocytes in CO2/HCO3(-)-containing media

We studied the regulation of intracellular pH (pHi) and the mechanisms of pHi regulation in cultured rat astrocytes using microspectrofluorometry and the pH-sensitive fluorophore 2',7'-bis(carboxyethyl-)-5,6-carboxyfluorescein. Control pHi was 7.00 +/- 0.02 in HCO3(-)-containing solutions at an extracellular pH of 7.35. Addition of 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS) or amiloride decreased pHi, as did removal of extracellular Na+, while removal of extracellular Cl- was followed by an increase in pHi. Following exposure to an acid transient induced by increasing the CO2 content from 5 to 15%, pHi rapidly returned to base line, with an average initial rate of recovery of 0.10 pH units min-1 (corresponding to a mean acid extrusion rate of 6.3 +/- 0.36 mmolo l-1 min-1). Regulation of pHi was impaired when either amiloride or DIDS was added or Cl- was removed. This inhibition was enhanced when both DIDS and amiloride were present, and pHi regulation was completely blocked in the absence of extracellular Na+. The rapid regulation of pHi normally seen following a transient alkalinisation was not inhibited by amiloride or removal of Na+, but was partially inhibited by DIDS and by the absence of extracellular Cl-. The results are compatible with the presence of at least three different pHi-regulating mechanisms: a Na+/H+ antiporter, a Na(+)-dependent HCO3-/Cl- exchanger (both regulating pHi during a transient acidification), and a passive Cl-/HCO3- exchanger (regulating pHi during transient alkalinisation). The results fail to provide firm evidence of the presence of an electrogenic Na+/HCO3- symporter.