Brain intracellular pH and metabolism during hypercapnia and hypocapnia in the new-born lamb

Hypercapnia Causes Injury of the Cerebral Cortex and Cognitive Deficits in Newborn Piglets

In critically ill newborns, exposure to hypercapnia (HC) is common and often accepted in neonatal intensive care units to prevent severe lung injury. However, as a "safe" range of arterial partial pressure of carbon dioxide levels in neonates has not been established, the potential impact of HC on the neurodevelopmental outcomes in these newborns remains a matter of concern. Here, in a newborn Yorkshire piglet model of either sex, we show that acute exposure to HC induced persistent cortical neuronal injury, associated cognitive and learning deficits, and long-term suppression of cortical electroencephalogram frequencies. HC induced a transient energy failure in cortical neurons, a persistent dysregulation of calcium-dependent proapoptotic signaling in the cerebral cortex, and activation of the apoptotic cascade, leading to nuclear deoxyribonucleic acid fragmentation. While neither 1 h of HC nor the rapid normalization of HC was associated with changes in cortical bioenergetics, rapid resuscitation resulted in a delayed onset of synaptosomal membrane lipid peroxidation, suggesting a dissociation between energy failure and the occurrence of synaptosomal lipid peroxidation. Even short durations of HC triggered biochemical responses at the subcellular level of the cortical neurons resulting in altered cortical activity and impaired neurobehavior. The deleterious effects of HC on the developing brain should be carefully considered as crucial elements of clinical decisions in the neonatal intensive care unit.

The influence of carbon dioxide on cerebral metabolism and oxygen consumption: combining multimodal monitoring with dynamic systems modelling

Hypercapnia increases cerebral blood flow. The effects on cerebral metabolism remain incompletely understood although studies show an oxidation of cytochrome c oxidase, Complex IV of the mitochondrial respiratory chain. Systems modelling was combined with previously published non-invasive measurements of cerebral tissue oxygenation, cerebral blood flow, and cytochrome c oxidase redox state to evaluate any metabolic effects of hypercapnia. Cerebral tissue oxygen saturation and cytochrome oxidase redox state were measured with broadband near infrared spectroscopy and cerebral blood flow velocity with transcranial Doppler ultrasound. Data collected during 5-min hypercapnia in awake human volunteers were analysed using a Fick model to determine changes in brain oxygen consumption and a mathematical model of cerebral hemodynamics and metabolism (BrainSignals) to inform on mechanisms. Either a decrease in metabolic substrate supply or an increase in metabolic demand modelled the cytochrome oxidation in hypercapnia. However, only the decrease in substrate supply explained both the enzyme redox state changes and the Fick-calculated drop in brain oxygen consumption. These modelled outputs are consistent with previous reports of CO2 inhibition of mitochondrial succinate dehydrogenase and isocitrate dehydrogenase. Hypercapnia may have physiologically significant effects suppressing oxidative metabolism in humans and perturbing mitochondrial signalling pathways in health and disease.

Assessment of human brain temperature by 1H MRS during visual stimulation and hypercapnia

Brain temperature is determined by the interplay between the cerebral metabolic rate of oxygen (CMRO2) and cerebral blood flow (CBF). In this study, single-voxel 1H nuclear MRS, with an accuracy of +/-0.2 degrees C for temperature determination, was used at 3 T to measure human brain temperature during visual stimulation (which increases both CBF and CMRO2) and hypercapnia (which increases CBF only). Visual stimulation had no detectable effect on brain temperature in the parenchyma showing blood oxygenation level dependent activation. Hypercapnia, leading to an increase in the end tidal CO2 by 8 +/- 2 mm Hg above the baseline, caused a short-lasting decrease in brain temperature of 0.30 +/- 0.33 degrees C. These results indicate that increased CBF may be a key factor, bringing about a small decrease in brain temperature during brain activation. However, the increase in CBF is not sufficient to lower brain temperature in the presence of a concomitant increase in endogenous heat production.

Mechanisms of injury to the newborn brain

Preterm and ill term infants are at risk for brain injury and subsequent neurodevelopmental delay as a result of many perinatal factors. Outlined in this article are the basic science mechanisms by which hypoxia, hypocapnia, and hypercapnia may result in neuronal injury in the newborn brain.

Hypercapnia-induced modifications of neuronal function in the cerebral cortex of newborn piglets

There is significant controversy over the effects of hypercapnia on the human newborn brain. Previous studies have shown that 1 h of an arterial CO2 pressure (Paco2) of 80 mm Hg alters brain cell membrane Na+K+-ATPase enzyme activity in the cerebral cortex of newborn piglets. The present study tests the hypothesis that hypercapnia (either a Paco2 of 65 or 80 mm Hg) results in decreased energy metabolism and alters neuronal nuclear enzyme activity and protein expression, specifically Ca++/calmodulin-dependent kinase (CaMK) IV activity, phosphorylation of cAMP response element binding protein (CREB), and expression of apoptotic proteins in cortical neuronal nuclei of newborn piglets. Studies were performed in 20 anesthetized normoxic piglets ventilated at either a Paco2 of 65 mm Hg, 80 mm Hg, or 40 mm Hg for 6 h. Energy metabolism was documented by ATP and phosphocreatine (PCr) levels. Results show ATP and PCr levels were significantly lower in the hypercapnic groups than the normocapnic. CaMK IV activity, phosphorylated CREB density, and Bax protein expression were all significantly higher in the hypercapnic groups than the normocapnic group. Bcl-2 protein was similar in all three groups, making the ratio of Bax/Bcl-2 significantly higher in the hypercapnic groups than in the normocapnic group. We conclude that hypercapnia alters neuronal energy metabolism, increases phosphorylation of transcription factors, and increases the expression of apoptotic proteins in the cerebral cortex of newborn piglets and therefore may be deleterious to the newborn brain.

Ventral medulla pHi measured in vivo by 31P NMR is not regulated during hypercapnia in anesthetized rat

Chemoreceptors in the ventral medulla contribute to the respiratory response to hypercapnia. Do they 'sense' intracellular pH (pHi)? We measured pHi in the ventral medulla or cortex (control) using 31P-NMR obtained via a novel 3 x 5 mm2 surface coil in anesthetized rats breathing air or 7% CO2. During air breathing over 240 min, pHi decreased slightly from 7.13 +/- 0.02 to 7.05 +/- 0.02 (SEM; n = 5; 2 cortex, 3 ventral medulla). During 180 min of hypercapnia, cortical pHi (n = 4) decreased from 7.17 +/- 0.02 to 6.87 +/- 0.01 by 90 min and recovered by 150 min. Ventral medulla pHi showed no such regulation. It decreased from 7.11 +/- 0.02 to 6.88 +/- 0.02 at 90 min and recovered only after cessation of hypercapnia (n = 5), results consistent with pHi being the chemoreceptor stimulus. However, non-chemoreceptor neurons that contribute to our medullary NMR signal also do not appear to regulate pHi in vitro. Regional differences in pHi regulation between cortex and ventral medulla may be due to both chemosensitive and non-chemosensitive neurons.

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)

Changes in high-energy phosphates in rat skeletal muscle during acute respiratory acidosis

We used 31P magnetic resonance spectroscopy to study changes in phosphorus metabolite concentrations in rat skeletal muscle during respiratory acidosis (14 and 20% inspired CO2) and recovery. As intracellular pH fell (from 7.05 to 6.75 after 20 min of 20% CO2), intracellular [P(i)] increased by up to 50% while phosphocreatine concentration decreased by up to 8%. The sum of all intracellular phosphates remained constant. [ADP] decreased by up to 40% in accordance with the creatine kinase equilibrium but the phosphorylation potential [ATP]/([ADP][P(i)]) was preserved as a result of increased [P(i)]. This adjustment may be a mechanism for maintaining mitochondrial ATP synthesis despite low pH. Eventually this increase in cellular [P(i)] could lead to slow efflux of P(i) from the skeletal muscle cell contributing to the hyperphosphataemia of acute respiratory acidosis.

New noninvasive techniques for assessing brain oxygenation and hemodynamics

Infants who require intensive care are at considerable risk of death or long-term neurodevelopmental disability. Therefore, noninvasive methods have been sought for assessing the structure and function of the brain in the immediate newborn period. The major aims are to investigate the prevalence and mechanisms of brain-damaging lesions, to test preventive strategies and treatment, and to assign the prognosis of the infants. Several methods have proved their worth, e.g., ultrasound imaging, electroencephalography, including the testing of evoked potentials, and Doppler ultrasonography for measuring flow velocity in intracerebral vessels.

Cerebral intracellular pH regulation during hypercapnia in unanesthetized rats: a 31P nuclear magnetic resonance spectroscopy study

The energy metabolism and the brain intracellular pH regulation under arterial CO2 tensions of 25-90 mm Hg were investigated in unanesthetized spontaneously breathing rats by in vivo phosphorus nuclear magnetic resonance spectroscopy (31P NMR). The 31P brain spectra, recorded with a high resolution spectrometer (AM 400 Brucker), allowed repeated non-invasive measurements of cerebral pH (pHi), phosphocreatine (PCr), inorganic phosphate (Pi) and adenosine triphosphate (ATP) levels in 15 rats breathing a gas mixture containing 21% O2, N2, and a varied percentage of CO2. The pHi decreased significantly when the paCO2 was increased by hypercapnia. The percentage of pH regulation, estimated from the linear regression analysis of pHi versus the logarithm of the paCO2 was 78%. This result indicates that spontaneously breathing unanesthetized animals have better pHi regulation under hypercapnia investigated than that estimated for higher levels of hypercapnia in previous studies on unanesthetized animals, suggesting that there is a threshold for this highly efficient regulation. Furthermore, there were no significant correlations between the PCr, ATP and Pi levels and the paCO2 levels during hypercapnia. This indicates that physiological variations of the CO2 tension in the blood, and consequently in the brain parenchyma, have little effect on cerebral energy metabolism in unanesthetized spontaneously breathing animals.

Preservation of single-flash visual evoked potentials at very low cerebral oxygen delivery in preterm infants

Single-flash visual evoked potentials (VEPs) were recorded in 32 preterm infants (mean gestational age: 29 weeks) during extreme physiologic conditions within the first day of life. The VEP configuration was normal in all patients at the onset of the investigation. Hypoxic episodes (PaO2 less than 3 kPa) caused rapid and consistent attenuation of the VEP mostly with an instantaneous recovery after normalization of PaO2. In contrast, VEP amplitude and latency were unaffected during episodes with low cerebral blood flow (4.5 ml/100 gm/min) and correspondingly low oxygen delivery to the brain (1 ml/100 gm/min), severe hypocapnia (PaCO2 1.6 kPa), and severe arterial hypotension (MABP 10 mm Hg), provided that the arterial oxygen tension was greater than 5 kPa. Absence of N1 was observed soon after the development of severe intracranial hemorrhage; however, this abnormality was short in duration. We conclude that the neurons generating VEPs are supported sufficiently during extreme physiologic episodes, except during severe hypoxia. The recovery time may be proportional to the cerebral insult.

Acid homeostasis following partial ischemia in neonatal brain measured in vivo by 31P and 1H nuclear magnetic resonance spectroscopy

The purpose of this study was to investigate neonatal brain energy metabolism, acid, and lactate homeostasis in the period immediately following partial ischemia. Changes in brain buffering capacity were quantified by measuring mean intracellular brain pH, calculated from the chemical shift of Pi, in response to identical episodes of hypercarbia before and after ischemia. In addition, the relationship between brain buffer base deficit and intracellular pH was compared during and following ischemia. Thus, in vivo 31P and 1H nuclear magnetic resonance spectra were obtained from the brains of seven newborn piglets exposed to sequential episodes of hypercarbia, partial ischemia, and a second episode of hypercarbia in the postischemic recovery period. For the first episode of hypercarbia, brain buffering was similar to values reported for adult animals of other species (percentage pH regulation = 54 +/- 16%). During ischemia, the brain base deficit per unit change in pH was -19 +/- 5 mM/pH unit, which is similar to values reported for adult rats. By 20-35 min postischemia, brain acidosis partly resolved in spite of a net increase in lactate concentration. Therefore, the consumption of lactate could not explain acid homeostasis in the first 35 min following ischemia. We conclude that H+/HCO3- or other proton equivalent translocation mechanisms must be sufficiently developed in piglet brain to support acid regulation. This is surprising, because a substantial body of evidence implies these processes would be less active in immature brain. The second episode of hypercarbia, from 35 to 65 min postischemia, resulted in a smaller decrease in brain pH compared with the first episode, a result indicating an increase in brain buffering capacity (percentage pH regulation = 79 +/- 29%). This was associated with a parallel decrease in brain lactate content, and therefore acid regulation could be attributed to either continued ion translocation or the consumption of lactate. A mild decrease in brain pH and content of energy metabolites was observed, a finding suggesting that the metabolic consequences of severe postischemic hypercarbia are neither particularly dangerous or beneficial.

Biochemical consequences of phosphorylcholines low pK with special relevance to human semen: passive pH regulation

1. The significant difference in pK between phosphorylcholine (PCh) and inorganic phosphate (Pi) of over 1 pH unit results in PCh being a source of acid buffering potential and an actual sink for protons when it is hydrolyzed. Phosphorylethanolamine (PEth) can fill the same role. 2. Both semen and tissues having substantial anaerobic metabolism among which are those of fetal origin can face circumstances of non-pathological acidosis. 3. We demonstrate from the literature that all of those tissues with the two understandable exceptions of erythrocytes and striated muscle, have high levels of PEth and/or PCh. 4. Since these phosphomonoesters can serve as lipid metabolites as well as low-pK fixed-phosphate buffer it appears they can serve a dual function in these tissues. 5. Their concentration would vary depending on the physiological and biochemical conditions present and we would call this a "Swiss army knife" theory of metabolic function.

Magnetic resonance and near infrared spectroscopy for investigation of perinatal hypoxic-ischaemic brain injury

Hypoxic-ischaemic injury to the brain is an important cause of perinatal death and seems to be the commonest cause of permanent neurodevelopmental disability in newborn infants who survive after intensive care. If this type of brain injury is to be prevented and treatment put on a rational basis, non-invasive methods are required for defining its mechanisms. This review has considered two such methods: magnetic resonance spectroscopy and near infrared spectroscopy. Magnetic resonance spectroscopy is used to measure, in brain tissue, the concentrations of the 'high energy' phosphorus metabolites that are dependent for their synthesis on the processes of oxidative phosphorylation. Intracellular pH can also be measured. Normal maturational changes in the brain have been defined and abnormalities detected in a range of conditions where hypoxic-ischaemic injury was suspected to have occurred. In laboratory animals the acute effects of curtailment of oxygen supply to the brain ('primary' energy failure) have been observed, and the effects of two commonly used treatments, infusions of sodium bicarbonate and glucose, have been tested. After resuscitation of newborn infants from severe intrapartum asphyxia, a latent period has often been noted before energy failure became detectable. This 'secondary' energy failure is due to a variety of damaging reactions initiated by the acute hypoxicischaemic episode and reperfusion of the brain. It is possible that in the future irreversible injury to brain cells following the episode may be prevented or ameliorated by the prompt use of cerebroprotective agents. The extent of abnormalities detected by magnetic resonance spectroscopy has prognostic implications: evidence of severe energy failure in the first days of life was regularly associated with subsequent death or with severe neurodevelopmental impairments. Many technical developments in magnetic resonance spectroscopy are under way, particularly employing proton (1H) spectroscopy, which will allow the intracerebral concentrations of a wide range of metabolites, including neurotransmitters, to be measured. The combination of spectroscopy with magnetic resonance imaging will permit quantitative data to be obtained from selected volumes within the brain. Near infrared spectroscopy is used to make observations at the cotside of the intracerebral concentrations of the chromophores oxyhaemoglobin, deoxyhaemoglobin, and oxidised cytochrome aa3, and it therefore provides information complementary to that obtained by magnetic resonance spectroscopy. Measurements can also be made of cerebral blood flow, cerebral blood volume, and other haemodynamic indices; in addition, the rea

Brain metabolism and intracellular pH during ischaemia: effects of systemic glucose and bicarbonate administration studied by 31P and 1H nuclear magnetic resonance spectroscopy in vivo in the lamb

Brain metabolism and intracellular pH were studied during and after episodes of incomplete cerebral ischaemia in lambs under sodium pentobarbitone anaesthesia. 31P and 1H magnetic resonance spectroscopy was used to monitor brain pHi and brain concentrations of inorganic phosphate (Pi), phosphocreatine (PCr), beta-nucleoside triphosphate (beta NTP), and lactate. Simultaneous measurements were made of arterio-cerebral venous concentration differences (AVDs) for oxygen, glucose, and lactate. Cerebral ischaemia was induced by a combination of bilateral carotid clamping and hypotension, and the acute effects of systemic administration of glucose and sodium bicarbonate were examined. The molar ratio of glucose to oxygen uptake by the brain (6G/O2) increased above unity during cerebral ischaemia. Statistically significant AVDs for lactate were not observed. Cerebral ischaemia was associated with a reduction in brain pHi PCr/Pi ratio, and an increase in brain lactate. No effect of arterial plasma glucose on brain lactate concentration or brain pHi was evident during cerebral ischaemia or in the postischaemic period. Administration of sodium bicarbonate systemically in the postischaemic period was associated with a rise in arterial and brain tissue PCO2. A fall in brain pHi occurred which was attributable in part to coincidental brain lactate accumulation. The increase in brain lactate measured by 1H nuclear magnetic resonance in vivo during ischaemia was insufficient to account for the change in buffer base calculated to have occurred from previous estimates of brain buffering capacity.

Brain metabolism and intracellular pH during ischaemia and hypoxia: an in vivo 31P and 1H nuclear magnetic resonance study in the lamb

Brain metabolism and intracellular pH were studied during and after episodes of ischaemia and hypoxia-ischaemia in lambs anaesthetised with sodium pentobarbitone. 31P and 1H magnetic resonance spectroscopy methods were used to monitor brain pHi and brain concentrations of Pi, phosphocreatine (PCr), beta--nucleoside triphosphate (beta NTP), and lactate. Simultaneous measurements were made of cerebral blood flow and cerebral oxygen and glucose consumption. Cerebral ischaemia sufficient to reduce oxygen delivery to 75% of control values was associated with a fall in brain pHi and increase in brain Pi. Progressively severe hypoxia-ischaemia was associated with a progressive fall in brain pHi, PCr, and beta NTP and increase in brain Pi. In two animals the increase in brain lactate during hypoxia-ischaemia measured by 1H nuclear magnetic resonance (NMR) could be quantitatively accounted for by the increased net uptake of glucose by the brain in relation to oxygen, but was insufficient to account for the concomitant acidosis according to previous estimates of brain buffering capacity. In four animals brain pHi, PCr, Pi, and beta NTP had returned to normal 1 h after the hypoxic-ischaemic episode. In one animal brain pHi had reverted to normal at a time when 1H NMR indicated persistent elevation of brain lactate.