Insufficient supply of reducing equivalents to the respiratory chain in cerebral cortex during severe insulin-induced hypoglycemia in cats

Glyceraldehyde metabolism in mouse brain and the entry of blood-borne glyceraldehyde into the brain

D-Glyceraldehyde, a reactive aldehyde metabolite of fructose and glucose, is neurotoxic in vitro by forming advanced glycation end products (AGEs) with neuronal proteins. In Alzheimer's disease brains, glyceraldehyde-containing AGEs have been detected intracellularly and in extracellular plaques. However, little information exists on how the brain handles D-glyceraldehyde metabolically or if glyceraldehyde crosses the blood-brain barrier from the circulation into the brain. We injected [U-13C]-D-glyceraldehyde intravenously into awake mice and analyzed extracts of serum and brain by 13C nuclear magnetic resonance spectroscopy. 13C-Labeling of brain lactate and glutamate indicated passage of D-glyceraldehyde from blood to brain and glycolytic and oxidative D-glyceraldehyde metabolism in brain cells. 13C-Labeling of serum glucose and lactate through hepatic metabolism of [U-13C]-D-glyceraldehyde could not explain the formation of 13C-labeled lactate and glutamate in the brain. Cerebral glyceraldehyde dehydrogenase and reductase activities, leading to the formation of D-glycerate and glycerol, respectively, were 0.27-0.28 nmol/mg/min; triokinase, which phosphorylates D-glyceraldehyde to D-glyceraldehyde-3-phosphate, has been demonstrated previously at low levels. Thus, D-glyceraldehyde metabolism toward glycolysis could proceed both through D-glycerate, glycerol, and D-glyceraldehyde-3-phosphate. The aldehyde group of D-glyceraldehyde was overwhelmingly hydrated into a diol in aqueous solution, but the diol dehydration rate greatly exceeded glyceraldehyde metabolism and did not restrict it. We conclude that (1) D-glyceraldehyde crosses the blood-brain barrier, and so blood-borne glyceraldehyde could contribute to AGE formation in the brain, (2) glyceraldehyde is taken up and metabolized by brain cells. Metabolism thus constitutes a detoxification mechanism for this reactive aldehyde, a mechanism that may be compromised in disease states.

Age-related metabolic fatigue during low glucose conditions in rat hippocampus

Previous reports have indicated that with aging, intrinsic brain tissue changes in cellular bioenergetics may hamper the brain's ability to cope with metabolic stress. Therefore, we analyzed the effects of age on neuronal sensitivity to glucose deprivation by monitoring changes in field excitatory postsynaptic potentials (fEPSPs), tissue Po2, and NADH fluorescence imaging in the CA1 region of hippocampal slices obtained from F344 rats (1-2, 3-6, 12-20, and >22 months). Forty minutes of moderate low glucose (2.5 mM) led to approximately 80% decrease of fEPSP amplitudes and NADH decline in all 4 ages that reversed after reintroduction of 10 mM glucose. However, tissue slices from 12 to 20 months and >22-month-old rats were more vulnerable to low glucose: fEPSPs decreased by 50% on average 8 minutes faster compared with younger slices. Tissue oxygen utilization increased after onset of 2.5 mM glucose in all ages of tissue slices, which persisted for 40 minutes in younger tissue slices. But, in older tissue slices the increased oxygen utilization slowly faded and tissue Po2 levels increased toward baseline values after approximately 25 minutes of glucose deprivation. In addition, with age the ability to regenerate NADH after oxidation was diminished. The NAD(+)/NADH ratio remained relatively oxidized after low glucose, even during recovery. In young slices, glycogen levels were stable throughout the exposure to low glucose. In contrast, with aging utilization of glycogen stores was increased during low glucose, particularly in hippocampal slices from >22 months old rats, indicating both inefficient metabolism and increased demand for glucose. Lactate addition (20 mM) improved oxidative metabolism by directly supplementing the mitochondrial NADH pool and maintained fEPSPs in young as well as aged tissue slices, indicating that inefficient metabolism in the aging tissue can be improved by directly enhancing NADH regeneration.

Hypoglycemic injury to the immature brain

Despite the fact that hypoglycemia is an extremely common disorder of the newborn, consensus has been difficult to reach regarding definition, diagnosis, outcome, and treatment. With improved neuroradiologic techniques, such as MRI and PET scanning becoming increasingly available, studies to determine the correlation between hypoglycemia and outcome will help to clarify issues surrounding the effects of hypoglycemia on brain pathology. Long-term epidemiologic studies correlating the severity and duration of hypoglycemia with neurologic consequences are required, and can be complemented by appropriate parallel investigations in animal models of neonatal hypoglycemia.

Glucose metabolism in the developing brain

As in adults, glucose is the predominant cerebral energy fuel for the fetus and newborn. Studies in experimental animals and humans indicate that cerebral glucose utilization initially is low and increases with maturation with increasing regional heterogeneity. The increases in cerebral glucose utilization with advancing age occurs as a consequence of increasing functional activity and cerebral energy demands. The levels of expression of the 2 primary facilitative glucose transporter proteins in brain, GLUT1 (blood-brain barrier and glia) and GLUT3 (neuronal), display a similar maturational pattern. Alternate cerebral energy fuels, specifically the ketone bodies and lactate, can substitute for glucose, especially during hypoglycemia, thereby protecting the immature brain from potential untoward effects of hypoglycemia. Unlike adults, glucose supplementation during hypoxia-ischemia is protective in the immature brain, whereas hypoglycemia is deleterious. Accordingly, glucose plays a critical role in the developing brain, not only as the primary substrate for energy production but also to allow for normal biosynthetic processes to proceed.

The effect of hyperglycemia on cerebral metabolism during hypoxia-ischemia in the immature rat

Unlike adults, hyperglycemia with circulating glucose concentrations of 25-35 mM/L protects the immature brain from hypoxic-ischemic damage. To ascertain the effect of hyperglycemia on cerebral oxidative metabolism during the course of hypoxia-ischemia, 7-day postnatal rats underwent unilateral common carotid artery ligation followed by exposure to 8% O2 for 2 h at 37 degrees C. Experimental animals received 0.2 cc s.c. 50% glucose at the onset of hypoxia-ischemia, and 0.15 cc 25% glucose 1 h later to maintain blood glucose concentrations at 20-25 mM/L for 2 h. Control rat pups received equivalent concentrations or volumes of either mannitol or 1 N saline at the same intervals. The cerebral metabolic rate for glucose (CMRglc) increased from 7.1 (control) to 20.2 mumol 100 g-1 min-1 in hyperglycemic rats during the first hour of hypoxia-ischemia, 79 and 35% greater than the rates for saline-and mannitol-injected animals at the same interval, respectively (p < 0.01). Brain intracellular glucose concentrations were 5.2 and 3.0 mM/kg in the hyperglycemic rat pups at 1 and 2 h of hypoxia-ischemia, respectively; glucose levels were near negligible in mannitol- and saline-treated animals at the same intervals. Brain intracellular lactate concentrations averaged 13.4 and 23.3 mM/kg in hyperglycemic animals at 1 and 2 h of hypoxia-ischemia, respectively, more than twice the concentrations estimated for the saline- and mannitol-treated littermates. Phosphocreatine (PCr) and ATP decreased in all three experimental groups, but were preserved to the greatest extent in hyperglycemic animals. Results indicate that anaerobic glycolytic flux is increased to a greater extent in hyperglycemic immature rats than in normoglycemic littermates subjected to cerebral hypoxia-ischemia, and that the enhanced glycolysis leads to greater intracellular lactate accumulation. Despite cerebral lactosis, energy reserves were better preserved in hyperglycemic animals than in saline-treated controls, thus accounting for the greater resistance of hyperglycemic animals to hypoxic-ischemic brain damage.

Cerebral mitochondrial redox states during metabolic stress in the immature rat

The brain mitochondrial NAD+/NADH ratio, as a reflection of the oxidation-reduction (redox) state of cellular compartment, was determined under conditions of hypoxia, anoxia, hypoxia-ischemia, complete ischemia and hypoglycemia in immature rats. NAD+/NADH ratios were calculated from changes in the concentrations of specific oxidative substrates and calculated intracellular pH during cerebral metabolic stress. The results suggest that the use of the acetoacetate/beta-hydroxybutyrate substrate couple provides a more accurate prediction of the mitochondrial redox state under adverse conditions than use of the alpha-ketoglutarate/glutamate couple. It is possible that the mitochondrial oxidation seen with the latter substrate couple during cerebral metabolic stress might reflect a population of cells (neurons or glia) which are substrate-deprived relative to the rest of the brain in the setting of metabolic stress produced by oxygen deficiency.

Glucose, lactic acid, and perinatal hypoxic-ischemic brain damage

Investigations suggest that hyperglycemia, superimposed on hypoxia-ischemia or cerebral ischemia, accentuates brain damage in adult experimental animals and humans, but not in immature animals. Fundamental differences in the immature and adult brain, which account for the age-specific paradox, are discussed. Based on currently available data, we recommend that glucose supplementation not be curtailed during labor and delivery of asphyxiated human infants; on the contrary, glucose therapy may substantially reduce hypoxic-ischemic brain damage.

Glycolysis, oxidative metabolism, and brain potassium ion clearance

Studies were directed toward defining relationships between brain ion transport, glycolysis, and oxidative phosphorylation. This was done by examining the relative sensitivity to hypoxemia and to iodoacetate (IAA)-induced inhibition of glycolysis in rats anesthetized with pentobarbital. Both insults had minimal effects on K+o baseline. In response to neuronal activation, IAA increased the time required for K+o clearance from maximal values to half-recovery of baseline. Hypoxemia slowed the later phase of K+o clearance, when K+o was approaching "resting" levels. Hypoxemia produced greater declines in high-energy intermediates than did IAA, which indicated that the IAA effect was not due to a greater overall insult to metabolism and suggested a direct link between ATP produced by glycolysis and ion transport activity. These data demonstrate that K+o clearance requires energy from glycolysis and oxidative phosphorylation for different phases of the recovery process and that inhibition specific to glycolysis or oxidative phosphorylation may be temporally resolved within a single stimulus.

Cerebral oxygen monitoring with near infrared spectroscopy: clinical application to neonates

Near infrared spectroscopy is a new noninvasive optical method for bedside monitoring of cerebral oxygenation. It uses differential absorbance of near infrared light to assess relative changes in the oxidation-reduction state of cytochrome aa3, as well as changes in the amounts of oxyhemoglobin, deoxyhemoglobin, and blood volume in the monitored field. Although this technique is applicable to all ages and sizes of patients and to multiple clinical settings, the majority of clinical studies to date have focused on the neonate. These studies have demonstrated its potential for advancing neonatal care and in understanding how diseases and therapies affect cerebral oxygenation. This paper reviews the near infrared spectroscopy technique and summarizes its potential applications in the field of neonatal intensive care.

Regulation of oxidative phosphorylation in the mammalian cell

The cell is capable of maintaining a steady-state flux of energy from mitochondrial oxidative phosphorylation, producing ATP, to the cytosolic adenosinetriphosphatases (ATPases), performing work. Considerable effort has been devoted to investigating the individual mechanisms involved in these two processes. However, less effort has been directed toward learning how these reactions of energy metabolism interact through the cytosol to maintain the observed steady state in the intact cell. The "classical" model for the cytosolic interaction of these two processes involves the feedback of ATP hydrolysis products, ADP and Pi, from the ATPases to oxidative phosphorylation. This model is based on data from isolated mitochondria in which the rate of oxidative phosphorylation is controlled by the concentration of ADP and Pi. Yet, recent data from intact tissues with high oxidative phosphorylation capacities (i.e., heart, brain, and kidney) indicate that the cytosolic concentration of ADP and Pi do not change significantly with work. These data imply that this simple feedback model is not adequate to explain the regulation of energy metabolism in these tissues. Other sites within the oxidative phosphorylation process must be playing a regulatory role or the kinetics of ATP synthesis must be very different than currently believed to establish the steady state. This review covers the potential sites within oxidative phosphorylation which may be regulated through cytosolic transducers to result in the necessary feedback network regulating the steady-state flow of energy in the cell. These sites will include substrate delivery to the cytochrome chain, the processes involved in the phosphorylation of ADP to ATP, and the delivery of oxygen.

Improved brain metabolism with fructose 1-6 diphosphate during insulin-induced hypoglycemic coma

The effect of fructose 1-6 diphosphate (FDP) on brain metabolism and brain function was investigated in hypoglycemic rabbits. The electroencephalogram and differences in oxygen content of arterial and cerebral venous blood were used as indicators for brain metabolic activity. Hypoglycemic coma was induced and maintained for 1 hour by insulin administration. At the onset of isoelectric EEG, six rabbits were treated with FDP and five rabbits received 0.9% saline. The animals were killed by an overdose of barbiturate 60 minutes after hypoglycemic recovery with glucose. FDP-treated rabbits had lower arterial glucose concentration after 40 minutes of treatment (p less than .05) and a significantly greater difference between the oxygen content of arterial and venous blood after 40 minutes (p less than .01), and after 60 minutes (p less than .025) of FDP infusion than saline-treated rabbits. FDP-treated rabbits also had a lower cerebral glucose-oxygen index than did saline-treated rabbits (p less than .005, after 20 and 40 minutes of FDP infusion). FDP administration was followed by a return of EEG activity during hypoglycemia, whereas saline produced no such effect. After glucose infusion, EEG activity was improved in FDP-treated rabbits; in saline-treated rabbits, minimal or no EEG activity was observed. The data suggest the possibility that, at the doses given in this study, FDP is taken up and used as a metabolic substrate by the brain.

Cytosolic free calcium, NAD/NADH redox state and hemodynamic changes in the cat cortex during severe hypoglycemia

Using indo-1, a fluorescent Ca2+ indicator, in vivo fluorometric measurements were made of changes in cytosolic free Ca2+, NAD/NADH redox state, and hemodynamics directly from the cat cortex during and after severe insulin-induced hypoglycemia. Cytosolic free Ca2+ started to increase when the EEG became isoelectric, remained at a significantly high level (p less than 0.05) during the period of isoelectric EEG (IEEG), and recovered to the control level 6 min following an intravenous infusion of glucose. The NAD/NADH redox state oxidized significantly during IEEG and then recovered rapidly to the control level after the glucose infusion. Local cortical blood volume (LCBV) increased gradually during the progression of hypoglycemia, reaching the maximal level (146 +/- 7%) at the end of IEEG, and then started to recover. The mean transit time (MTT) through the cortical microcirculation was shortened during the IEEG (control: 3.84 +/- 0.41 s versus IEEG: 2.73 +/- 0.17 s, p less than 0.05), whereas it was prolonged during the 30-min recovery period (5.68 +/- 0.58 s, p less than 0.05). Local cortical blood flow calculated from the LCBV and MTT showed a twofold increase 5 min into IEEG (201 +/- 27% of control, p less than 0.05), recovered 15 min into the recovery period, and then decreased to 77% of control (p less than 0.05) by 30 min. The data support the hypothesis that hypoglycemic brain damage might be mediated by an elevation of cytosolic free calcium.

Acute ultrastructural response of hypoxic hypoxia with relative ischemia in the isolated brain

The acute cortical response to surgical brain isolation and subsequent extracorporal normoxic or 30 min hypoxic (PaO2 = 20 mm Hg) perfusions (hypoxic hypoxia with relative ischemia) was evaluated. Cerebral blood flow, arterial pH and CO2 were maintained constant during both perfusions; only the arterial oxygen content was changed. The isolated brain model used in this and previous investigations produces no qualitative ultrastructural changes in the neocortex following brain isolation and normoxic perfusion. However, the acute cortical structural response to 30 min of hypoxic hypoxia with relative ischemia demonstrated a number of important observations. Hypoxic hypoxia produced ultrastructural responses common to cerebral ischemia such as nuclear chromatin clumping, nucleolar condensation and cytoskeletal breakdown. Although neuronal abnormalities seen after 30 min of hypoxic hypoxia were similar to those acute neuronal changes observed following complete cerebral ischemia without recirculation, they differed three ways: (a) mitochondrial swelling and microvacuolation were observed in many cortical pyramidal neurons. (b) Glycogen particles within astroglial processes were observed even after a 30-min period of hypoxic hypoxia. (c) Perivascular astroglial swelling was minimal despite considerable perineuronal swelling. In contrast, incomplete cerebral ischemia produces mitochondrial changes similar to those in hypoxic hypoxia but also causes the depletion of tissue glycogen and perivascular glial swelling. Thus, hypoxic hypoxia with relative ischemia produces a unique acute ultrastructural response compared to either complete or incomplete cerebral ischemia.

Inhibition of glycolysis alters potassium ion transport and mitochondrial redox activity in rat brain

To examine the relationships between brain glycolysis, ion transport, and mitochondrial reduction/oxidation (redox) activity, extracellular potassium ion activity (K+0) and redox shifts of cytochrome oxidase (cytochrome a,a3) were recorded previous to and during superfusion of rat cerebral cortex with the glycolytic inhibitor iodoacetic acid (IAA). IAA produced oxidation of cytochrome a,a3, increased local oxygenation, increased K+0, and, in response to neuronal activation, slowed rates of K+0 reaccumulation. Rates of rereduction of cytochrome a,a3, after the oxidation of this cytochrome by stimulation, were also slowed by IAA. These effects of IAA demonstrate the dependence of K+0 reaccumulation on the integrity of glycolysis, support the concept that active processes are involved in brain ion transport, and suggest a link between ATP supplied by glycolysis and ion transport activity. These data are also compatible with the suggestion that residual dysfunctions after brain ischemia result from derangements in glycolytic functioning rather than from limitations in oxygen availability or oxidative metabolic activity.

Low-frequency oscillations of cortical oxidative metabolism in waking and sleep

To study the changes in cortical oxidative metabolism and blood volume during behavioral state transitions, we employed reflectance spectrophotometry of the cortical cytochrome c oxidase (cyt aa3) redox state and blood volume in unanesthetized cats implanted with bilateral cortical windows and EEG electrodes. Continuous oscillations in the redox state and blood volume (approximately 9/min) were observed during waking and sleep. These primarily metabolic oscillations of relatively high amplitude were usually synchronous in homotopic cortical areas, and persisted during barbiturate-induced electrocortical silence. Their mean amplitude and frequency did not vary across different behavioral/EEG states, although the mean levels of cyt aa3 oxidation and blood volume during rapid eye movement (REM) sleep significantly exceeded those during waking and slow-wave sleep. These data suggest the existence of a spontaneously oscillating metabolic phenomenon in cortex that is not directly related to neuroelectric activity. A superimposed increase in cortical oxidative metabolism and blood volume occurs during REM sleep. Experimental data concerning cerebral metabolism and blood flow that are obtained by clinical methods that employ relatively long sample acquisition times should therefore be interpreted with caution.

A simplified method for monitoring the cytochrome aa3 redox state in bilateral cortical areas of unanesthetized cats

We describe a versatile optical system that enables the simultaneous monitoring of the redox state of cytochrome c oxidase (cytochrome aa3) in two homologous cortical areas under chronically implanted windows in cats. A single light source, broad bandpass primary filters, light-conducting rods, and narrow-bandpass interference detecting filters are employed. We observed reproducible responses of the cytochrome redox state and blood volume to carotid occlusion and terminal anoxia during anesthesia, and to graded doses of pentobarbital in awake animals.

Regional cerebral and neural lobe blood flow during insulin-induced hypoglycemia in unanesthetized rats

The effects of hypoglycemia on regional cerebral blood flow (rCBF) were studied in awake restrained rats. The rats were divided into three groups consisting of a normoglycemic control group that received only saline, a hypoglycemic group A, which was given insulin 30 min before flow was measured, and a hypoglycemic group B, which was given insulin 90 and 30 min before flow was measured. Regional CBF was measured using 14C-iodoantipyrine. Mean plasma glucose was 8.76 mumol/ml in the control group, 2.63 mumol/ml in hypoglycemic group A, and 1.51 mumol/ml in hypoglycemic group B. Plasma epinephrine and norepinephrine concentrations increased to approximately 375% and 160%, respectively, of control values in hypoglycemic groups A and B. In the hypoglycemic group A, rCBF significantly increased in three brain regions. In the hypoglycemic group B, rCBF increased significantly in all brain regions measured, with the exception of the neural lobe, in which it decreased. The increase in rCBF ranged from 38% in the hypothalamus to 138% in the thalamus. Neural lobe blood flow significantly decreased by 31%. The neural lobe was the only brain region studied that is not protected by a blood-brain barrier. It may be sensitive to changes in the concentration of vasoactive agents in blood, such as epinephrine and norepinephrine.

Regional cerebral glucose utilization during insulin-induced hypoglycemia in unanesthetized rats

Regional cerebral glucose utilization (rCMRgl) was studied during insulin-induced hypoglycemia in unanesthetized rats. Rats were surgically prepared using halothane and nitrous oxide anesthesia and allowed 5 h to recover from the anesthesia before rCMRgl was measured. The rCMRgl was measured using [6-14C]glucose in a normoglycemic control group and two hypoglycemic groups, A (30 min after insulin injection) and B (2 h after insulin injection). The mean plasma glucose level was 7.03 mumol/ml in the normoglycemic group, 1.96 mumol/ml in hypoglycemic group A, and 1.40 mumol/ml in hypoglycemic group B. The rCMRgl in hypoglycemic group A decreased 8-18% in 17 brain regions measured; five changes were statistically significant. The rCMRgl in hypoglycemic group B decreased significantly in all but one of the brain regions measured; the decrease ranged from 15% in the pyramidal tract to 36% in the motor and auditory cortices. The rCMRgl in every brain region decreased when the plasma glucose level fell below 1.5-2.5 mumol/ml. No brain region could maintain rCMRgl at plasma glucose concentrations lower than predicted by regional glucose influx described in previous studies. Glucose utilization in all brain regions appears to be limited by the influx of glucose.