Studies on the effects of lactate transport inhibition, pyruvate, glucose and glutamine on amino acid, lactate and glucose release from the ischemic rat cerebral cortex

Diet supplementation with sodium pyruvate increases sleep time and lifespan in Drosophila model

Sodium pyruvate is a natural metabolite commonly used in biological fields, including cell culture. This study investigated the effects of sodium pyruvate on the lifespan and other physiological characters of Drosophila melanogaster, by measuring feeding, fecundity, and spontaneous activity. The results indicated that 0.2 mol/L of sodium pyruvate increased the median lifespan of female flies by 8.33%. Moreover, the group sleep duration of female flies significantly increased by 53.98% when exposed to the sodium pyruvate concentration. However, the intake of sodium pyruvate did not significantly affect the fecundity or food intake of female flies. Our results also show that the effect of extending lifespan and increasing sleep time was dose-dependent and sex-specific. Our data provides the role of sodium pyruvate as an insect culture additive by enhancing survival.

PPM1K mediates metabolic disorder of branched-chain amino acid and regulates cerebral ischemia-reperfusion injury by activating ferroptosis in neurons

Ischemic stroke is a neurological disorder caused by vascular stenosis or occlusion, accounting for approximately 87% of strokes. Clinically, the most effective therapy for ischemic stroke is vascular recanalization, which aims to rescue neurons undergoing ischemic insults. Although reperfusion therapy is the most effective treatment for ischemic stroke, it still has limited benefits for many patients, and ischemia-reperfusion (I/R) injury is a widely recognized cause of poor prognosis. Here, we aim to investigate the mechanism of protein phosphatase Mg2+/Mn2+ dependent 1 K (PPM1K) mediates metabolic disorder of branched-chain amino acids (BCAA) by promoting fatty acid oxidation led to ferroptosis after cerebral I/R injury. We established the I/R model in mice and used BT2, a highly specific BCAA dehydrogenase (BCKD) kinase inhibitor to promote BCAA metabolism. It was further verified by lentivirus knocking down PPM1K in neurons. We found that BCAA levels were elevated after I/R injury due to dysfunctional oxidative degradation caused by phosphorylated BCKD E1α subunit (BCKDHA). Additionally, the level of phosphorylated BCKDHA was determined by decreased PPM1K in neurons. We next demonstrated that BCAA could induce oxidative stress, lipid peroxidation, and ferroptosis in primary cultured cortical neurons in vitro. Our results further showed that BT2 could reduce neuronal ferroptosis by enhancing BCAA oxidation through inhibition of BCKDHA phosphorylation. We further found that defective BCAA catabolism could induce neuronal ferroptosis by PPM1K knockdown. Furthermore, BT2 was found to alleviate neurological behavior disorders after I/R injury in mice, and the effect was similar to ferroptosis inhibitor ferrostatin-1. Our findings reveal a novel role of BCAA in neuronal ferroptosis after cerebral ischemia and provide a new potential target for the treatment of ischemic stroke.

Metabolic alterations induced by ischemia in primary cultures of astrocytes: merging 13C NMR spectroscopy and metabolic flux analysis

Disruption of brain energy metabolism is the hallmark of cerebral ischemia, a major cause of death worldwide. Astrocytes play a key role in the regulation of brain metabolism and their vulnerability to ischemia has been described. Aiming to quantify the effects of an ischemic insult in astrocytic metabolism, primary cultures of astrocytes were subjected to 5 h of oxygen and glucose deprivation in a bioreactor. Flux distributions, before and after ischemia, were estimated by metabolic flux analysis using isotopic information and the consumption/secretion rates of relevant extracellular metabolites as constraints. During ischemia and early recovery, 30% of cell death was observed; several metabolic alterations were also identified reflecting a metabolic response by the surviving cells. In the early recovery ( approximately 10 h), astrocytes up-regulated glucose utilization by 30% and increased the pentose phosphate pathway and tricarboxylic acid cycle fluxes by three and twofold, respectively. Additionally, a two to fivefold enhancement in branched-chain amino acids catabolism suggested the importance of anaplerotic molecules to the fast recovery of the energetic state, which was corroborated by measured cellular ATP levels. Glycolytic metabolism was predominant in the late recovery. In summary, this work demonstrates that changes in fluxes of key metabolic pathways are implicated in the recovery from ischemia in astrocytes.

Simultaneous efflux of endogenous D-ser and L-glu from single acute hippocampus slices during oxygen glucose deprivation

D-serine and L-glutamate play crucial roles in excitotoxicity through N-methyl-D-aspartate receptor coactivation, but little is known about the temporal profile of efflux during cerebral ischemia. We utilized a newly designed brain slice microperfusion device coupled offline to capillary electrophoresis laser-induced fluorescence to monitor dynamic efflux of endogenous D-ser and L-glu in response to oxygen glucose deprivation (OGD) in single acute hippocampus slices. Efflux profiles with 2-min temporal resolution in response to 24-min OGD show that efflux of D-ser slightly precedes efflux of L-glu by one 2-min sampling interval. Thus both coagonists are available to activate NMDA receptors by the time when glu is released. The magnitude of D-ser efflux relative to baseline values is, however, less than that for L-glu. Peak efflux during OGD, expressed as pre-OGD baseline values, was as follows: D-ser 254% +/- 24%, L-glu 1,675% +/- 259%, L-asp 519% +/- 128%, and L-thr 313% +/- 33%. L-glutamine efflux was shown to decrease significantly in response to OGD. The microperfusion/CE-LIF approach shows several promising attributes for studying endogenous chemical efflux from single, acute brain slices.

Transient benefits but lack of protection by sodium pyruvate after 2-hour middle cerebral artery occlusion in the rat

Sodium pyruvate has shown protective effects in various experimental models of brain ischemia. The main holdup of this drug is that most of the benefits are reported with a very narrow time window for intervention. Here we investigated whether pyruvate could protect the brain against ischemic damage using a model of 2-hour middle cerebral artery occlusion in the rat. The time course of blood pyruvate after i.p. administration of sodium pyruvate (400 mg/kg) was studied. Animals were treated with the drug or with vehicle 45 min after reperfusion following 2-hour ischemia. Tissue ATP content was determined 5 and 10 h after ischemia onset, and infarct volume was measured at days 1 and 2. The neurological score was evaluated before and after treatment in the different experimental groups. Pyruvate prevented the drop of cortical ATP induced by ischemia in the ipsilateral cortex and ameliorated the neurological deficit at 5 h after the onset of ischemia, supporting some beneficial effects of the treatment. However, these effects were not sustained at 10 h. Furthermore, pyruvate failed to significantly reduce infarct volume and the neurological deficit at 24 and 48 h, in spite of some trend to smaller infarction after pyruvate administration. Therefore, under the present experimental conditions, systemic administration of sodium pyruvate at 3 h after the beginning of ischemia exerted only a transient benefit but not a persistent protection against brain damage.

Involvement of lactate in glucose metabolism and glucosensing function in selected tissues of rainbow trout

The aim of this study was to obtain evidence in rainbow trout for a role of lactate in glucose homeostasis as well as in the function of glucosensing tissues. In a first set of experiments, trout were injected, either (1)intraperitoneally (N=8) with 5 ml kg–1 of Cortland saline alone (control) or saline containing l-(+)-lactate (22.5 mg kg–1 or 45 mg kg–1), oxamate (22.5 mg kg–1) or d-glucose (500 mg kg–1),or (2) intracerebroventricularly (N=11) with 1 μl 100 g–1 body mass of Cortland saline alone (control) or containing d-glucose (400 μg μl–1) or l-(+)-lactate (400 μg μl–1), with samples being obtained 6 h after treatment. In a second set of experiments,hypothalamus, hindbrain and Brockmann bodies were incubated in vitrofor 1 h at 15°C in modified Hanks' medium containing 2, 4 or 8 mmol l–1l-(+)-lactate alone (control) or with 50 mmol l–1 oxamate, 1 mmol l–1 DIDS, 1 mmol l–1 dichloroacetate, 10 mmol l–12-deoxy-d-glucose, 1 mmol l–1α-cyano-4-hydroxy cinnamate or 10 mmol l–1d-glucose. The response of parameters assessed (metabolite levels,enzyme activities and glucokinase expression) in tissues provided evidence for(1) a role for lactate in the regulation of glucose homeostasis through changes not only in brain regions but also in liver energy metabolism, which are further reflected in changes in plasma levels of metabolites; (2) the possible presence in trout brain of an astrocyte–neuron lactate shuttle similar to that found in mammals; and (3) the lack of capacity of lactate to mimic in vitro (but not in vivo) glucose effects in fish glucosensing regions.

Metabolic and functional profiling of the ischemic/reperfused rat retina

We quantitatively tracked the recovery in amino acid labeling and cation channel functionality within distinct retinal elements for up to 2 weeks after an ischemic insult. Pattern recognition analysis of multiple amino acid and agmatine (a cation channel probe; 1-amino-4-guanidobutane; AGB) immunocytochemical patterns was used to classify all neural elements within the retina. This classification was spatially complete and with single-cell resolution. By 48 hours of reperfusion the amino acid labeling pattern of virtually all cell populations had returned to near preischemic levels, with the exception of glutamine and alanine levels, which remained significantly higher in many cell populations. Classification resulted in a total of 18 statistically separable theme classes (including neurons, glia, and extraretinal classes), a reduction of 10 theme classes from the normal retina (Sun et al. [ 2007a, b] J Comp Neurol, this issue). In addition to the known selective losses of amacrine cell types within the inner nuclear layer, we now demonstrate a selective loss of theme classes representing cone bipolar cells within the bipolar cell population. While there was a recovery in the amino acid labeling pattern, there were persistent cation channel gating anomalies (as reflected by AGB labeling) within several theme classes, including the theme class representing all the remaining rod bipolar cells, suggesting aberrant neuronal function secondary to metabolic insult.

Lactate uptake contributes to the NAD(P)H biphasic response and tissue oxygen response during synaptic stimulation in area CA1 of rat hippocampal slices

Synaptic train stimulation (10 Hz x 25 s) in hippocampal slices results in a biphasic response of NAD(P)H fluorescence indicating a transient oxidation followed by a prolonged reduction. The response is accompanied by a transient tissue PO(2) decrease indicating enhanced oxygen utilization. The activation of mitochondrial metabolism and/or glycolysis may contribute to the secondary NAD(P)H peak. We investigated whether extracellular lactate uptake via monocarboxylate transporters (MCTs) contributes to the generation of the NAD(P)H response during neuronal activation. We measured the effect of lactate uptake inhibition [using the MCT inhibitor alpha-cyano-4-hydroxycinnamate (4-CIN)] on the NAD(P)H biphasic response, tissue PO(2) response, and field excitatory post-synaptic potential in hippocampal slices during synaptic stimulation in area CA1 (stratum radiatum). The application of 4-CIN (150-250 micromol/L) significantly decreased the reduction phase of the NAD(P)H response. When slices were supplemented with 20 mmol/L lactate in 150-250 micromol/L 4-CIN, the secondary NAD(P)H peak was restored; whereas 20 mmol/L pyruvate supplementation did not produce a recovery. Similarly, the tissue PO(2) response was decreased by MCT inhibition; 20 mmol/L lactate restored this response to control levels at all 4-CIN concentrations. These results indicate that lactate uptake via MCTs contributes significantly to energy metabolism in brain tissue and to the generation of the delayed NAD(P)H peak after synaptic stimulation.

Brain metabolism and extracellular space diffusion parameters during and after transient global hypoxia in the rat cortex

Hypoxia results in both reversible and irreversible changes in the brain extracellular space (ECS). This study utilized microdialysis to monitor changes in the energy-related metabolites lactate, pyruvate, glucose and glutamate in the rat cortex before, during and after 30-min transient global hypoxia, induced in anesthetized rats by reducing inspired oxygen to 6% O(2) in nitrogen. Changes in metabolite levels were compared with ECS diffusion parameters calculated from diffusion curves of tetramethylammonium applied by iontophoresis. Significant increases in lactate concentration and the lactate/pyruvate ratio, as well as decreased glucose levels, were found in the cortex immediately after the induction of hypoxia. Following recovery to ventilation with air, extracellular lactate and glucose levels and the lactate/pyruvate ratio returned to control levels within 40, 20 and 30 min, respectively. Glutamate levels started to increase 20-30 min after the onset of hypoxia and returned to prehypoxic values within 30-40 min of reoxygenation. The ECS volume fraction alpha decreased by about 5% from 0.18+/-0.01 during the first 20-25 min of hypoxia; after 25 min alpha dropped a further 22% to 0.14+/-0.01. Within 10 min of reoxygenation, alpha returned to control values, then increased to 0.20+/-0.01 and remained at this level until the end of the experiment. The observed 22% decrease in alpha markedly influences dialysate levels measured during hypoxia. In our study, the complete posthypoxic recovery of cortical metabolite levels and ECS diffusion properties suggests that metabolic enzymes and related cellular components (e.g., mitochondria) may tolerate prolonged hypoxic periods and recover to prehypoxic values.

Fuelling cerebral activity in exercising man

The metabolic response to brain activation in exercise might be expressed as the cerebral metabolic ratio (MR; uptake O2/glucose + 1/2 lactate). At rest, brain energy is provided by a balanced oxidation of glucose as MR is close to 6, but activation provokes a 'surplus' uptake of glucose relative to that of O2. Whereas MR remains stable during light exercise, it is reduced by 30% to 40% when exercise becomes demanding. The MR integrates metabolism in brain areas stimulated by sensory input from skeletal muscle, the mental effort to exercise and control of exercising limbs. The MR decreases during prolonged exhaustive exercise where blood lactate remains low, but when vigorous exercise raises blood lactate, the brain takes up lactate in an amount similar to that of glucose. This lactate taken up by the brain is oxidised as it does not accumulate within the brain and such pronounced brain uptake of substrate occurs independently of plasma hormones. The 'surplus' of glucose equivalents taken up by the activated brain may reach approximately 10 mmol, that is, an amount compatible with the global glycogen level. It is suggested that a low MR predicts shortage of energy that ultimately limits motor activation and reflects a biologic background for 'central fatigue'.

Energy Substrate-Supplemented Resuscitation Affects Brain Monocarboxylate Transporter Levels and Gliosis in a Rat Model of Hemorrhagic Shock

BACKGROUND

Monocarboxylate (MC)-supplemented resuscitation has been shown to attenuate cellular injury after hemorrhagic shock. However, little is known about its effect on the central nervous system. The brain can use MCs such as lactate, pyruvate, and beta-hydroxybutyrate as energy substrates. The transit of MCs into the central nervous system is facilitated by the monocarboxylate transporters (MCTs), and their blockage can exacerbate neuronal damage. We examined the expression of MCT1 and markers specific for activation of astroglia and microglia in the brains of rats subjected to hemorrhagic shock and resuscitation. The hypothesis was that resuscitation with MC-based fluids would be accompanied by MCT1 up-regulation and glial response.

METHODS

Rats (n = 30) were subjected to volume-controlled hemorrhage. Test groups included: sham, no resuscitation, resuscitation with normal saline, resuscitation with racemic lactated Ringer's solution, resuscitation with pyruvate Ringer's solution, and resuscitation with beta-hydroxybutyrate-containing ketone Ringer's solution. Plasma levels of MC were measured serially. The brains were investigated using GFAP, CD11b, CD43, MCT1, and GLUT1 immunohistochemistry.

RESULTS

Rats resuscitated with MC-containing fluids had increased levels of MCT1 in brain endothelial cells and neuropil compared with sham rats. Enhanced staining was localized to the choroid plexus, astrocytic end feet, and white matter structures. None of the resuscitation treatment induced astrocytic hyperplasia, and pyruvate Ringer's solution and ketone Ringer's solution resuscitation led to hypertrophy of astrocytes.

CONCLUSION

In hemorrhagic shock, resuscitation with MC-based fluids increased brain MCT1 level and led to activation of astrocytes. Enhanced MC trafficking could be an essential route for energy supply to neurons under adverse circulatory conditions.

Formation of extracellular glutamate from glutamine: exclusion of pyroglutamate as an intermediate

A 4.6-fold increase in interstitial glutamate was observed following the reverse microdialysis of 5 mM glutamine into the rat hippocampus. Two possible mechanisms of glutamine hydrolysis were examined: (a) an enzymatic glutaminase activity and (b) a non-enzymatic mechanism. Injection of 14C-glutamine at the site of microdialysis followed by microdialysis with artificial cerebral spinal fluid allowed isolation of 14C-glutamine (63%), 14C-glutamate (14%), and a compound tentatively identified as pyroglutamate (22%). In this study, we determined if non-enzymatic pyroglutamate formation from glutamine contributed to the synthesis of glutamate. Pyroglutamate is in chemical equilibrium with glutamate, although under physiological conditions, the chemical equilibrium is strongly in the direction of pyroglutamate. In vitro stability studies indicated that 14C-glutamine and 14C-pyroglutamate are not subject to significant non-enzymatic breakdown at pH 6.5-7.5 at 37 degrees C for up to 8 h. Reverse microdialysis with 1 mM pyroglutamate did not increase interstitial glutamate levels. Following injection of 14C-pyroglutamate and microdialysis, radioactivity was recovered in 14C-pyroglutamate (88%) and 14C-glutamine (11%). Less than 1% of the radioactivity was recovered as glutamate. Our data do not support a role of pyroglutamate as an intermediate in the formation of extracellular glutamate following the infusion of glutamine. However, it confirms that pyroglutamate, a known constituent in brain, is actively metabolized in brain cells and contributes to glutamine in the interstitial space.

Glucose transport to the brain: a systems model

Glucose transport to the brain involves sophisticated interactions of solutes, transporters, enzymes, and cell signaling processes, within an intricate spatial architecture. The dynamics of the transport are influenced by the adaptive nature of the blood-brain barrier (BBB), the semi-impermeable membranes of brain capillaries. As both the gate and the gatekeeper between blood-borne nutrients and brain tissue, the BBB helps govern brain homeostasis. Glucose in the blood must cross the BBB's luminal and abluminal membranes to reach neural tissue. A robust representation of the glucose transport mechanism can highlight a target for brain therapeutic intervention, help characterize mechanisms behind several disease phenotypes, or suggest a new delivery route for drugs. The challenge for researchers is understanding the relationships between influential physiological variables in vivo, and using that knowledge to predict how alterations or interventions affect glucose transport. This paper reviews factors influencing glucose transport and approaches to representing blood-to-brain glucose transport including in vitro, in vivo, and kinetic models. Applications for different models are highlighted, while their limitations in answering arising questions about the human in vivo BBB lead to a discussion of an alternate approach. A developing complex systems simulation is introduced, initiating a single platform to represent the dynamics of glucose transport across the adapting human blood-brain barrier.

Interleukin-1β potentiates neuronal injury in a variety of injury models involving energy deprivation

The purpose of this study was to develop a suitable in vitro model system to study the biochemical pathway(s) by which interleukin-1beta (IL-1beta) contributes to the pathogenesis of cerebral ischemia. Thus, the effect of IL-1beta on a number of injury paradigms associated with energy deprivation was investigated using murine mixed cortical cell cultures. While IL-1beta by itself was not neurotoxic, pre-treatment-but not concurrent or post-treatment-with this cytokine potentiated neuronal injury induced by depriving cultures of either oxygen, glucose, or both oxygen and glucose. Cytotoxicity was abolished by an IL-1beta-neutralizing antibody. Together, these results demonstrate the establishment of reliable and reproducible in vitro models that will now allow detailed investigation of the cellular and molecular mechanisms relating to IL-1beta-mediated neuronal cell death.

Monocarboxylate transport inhibition alters retinal function and cellular amino acid levels

We assessed the effect of the in vivo application of monocarboxylate transport inhibitors on retinal function and amino acid immunocytochemistry. We wanted to determine the impact that altered aerobic metabolite availability has on retinal function and the characteristics of amino acid shunting into metabolic pools. Electroretinograms were collected from anaesthetized rats at various times after intravitreal injection of the monocarboxylate transport inhibitors alpha-cyano-4-hydroxycinnamate (4-CIN; 2 micro L, 0.1-10 mm) or p-(dipropylsulphamoyl)benzoic acid (probenecid; 1-10 mm). Changes in retinal function were compared with quantitative amino acid immunocytochemical changes in retinas harvested 20 and 40 min after either 4-CIN or vehicle treatment. The injection of 4-CIN resulted in a dose-dependent reduction of the ON-bipolar cell P2 wave amplitude (20-80%) and delay in its implicit time. The phototransduction sensitivity was mildly reduced whereas the ON-bipolar cell P2 sensitivity was unaffected. Probenecid induced functional changes similar to those observed with 4-CIN. We also mapped the amino acid alterations within specific cell classes induced by 4-CIN application. All neurones displayed a reduced glutamate content averaging 48%; reduced GABA (31%) and glycine (28%) were found within amacrine cells and glutamine was reduced in all cell classes except photoreceptor and Müller cells. All cell classes in the retina demonstrated increases in aspartate (57%), whereas leucine (24%) and ornithine (21%) were only significantly increased in photoreceptor and bipolar cells. The reduction in glutamate immunolabelling in specific retinal cell classes was mirrored by an increase in aspartate levels at these locations. In addition, attenuated glutamine immunolabelling also closely matched the spatial pattern observed for glutamate. Our immunocytochemical analysis provides evidence that monocarboxylate transport inhibition induces a shift in the equilibrium of glutamate transamination reactions involving aspartate throughout the retina whereas photoreceptor and bipolar cells also use glutamate transamination reactions involving ornithine and leucine. The distribution pattern of glutamine secondary to monocarboxylate inhibition suggests that this amino acid is a major precursor for glutamate throughout the retina.

Sodium-coupled neutral amino acid (System N/A) transporters of the SLC38 gene family

The sodium-coupled neutral amino acid transporters (SNAT) of the SLC38 gene family resemble the classically-described System A and System N transport activities in terms of their functional properties and patterns of regulation. Transport of small, aliphatic amino acids by System A subtypes (SNAT1, SNAT2, and SNAT4) is rheogenic and pH sensitive. The System N subtypes SNAT3 and SNAT5 also countertransport H(+), which may be key to their operation in reverse, and have narrower substrate profiles than do the System A subtypes. Glutamine emerges as a favored substrate throughout the family, except for SNAT4. The SLC38 transporters undoubtedly play many physiological roles including the transfer of glutamine from astrocyte to neuron in the CNS, ammonia detoxification and gluconeogenesis in the liver, and the renal response to acidosis. Probing their regulation has revealed additional roles, and recent work has considered SLC38 transporters as therapeutic targets in neoplasia.

Blood-mediated scavenging of cerebrospinal fluid glutamate

The maintenance of brain extracellular glutamate (Glu) at levels below its excitotoxic threshold is performed by Glu transporters present on glia and neurons as well as on brain capillary endothelial cells which remove brain Glu into blood. The feasibility of accelerating the naturally occurring brain-to-blood Glu efflux was studied using paradigms based on the fate of Glu present in the cerebrospinal fluid or infused into the brain ventricles and monitored before, during, and after decreasing blood Glu levels with pyruvate and oxaloacetate, the respective Glu co-substrates of the blood resident enzymes glutamate-pyruvate transaminase and glutamate-oxaloacetate transaminase. Results from cerebroventricular perfusions with [3H]Glu, intracerebroventricular injections of [3H]Glu, and measurements of the basal CSF Glu levels point out to the same conclusion that the intravenous administration of pyruvate and oxaloacetate which decreases blood Glu levels accelerates the brain-to-blood Glu efflux. We conclude that the brain extracellular Glu levels can be controlled in part by the blood Glu levels. The results may provide not only a rational explanation for the inhibition of Glu release and neuroprotective effects of parentally administered pyruvate in hemorrhagic shock and forebrain ischemia but could also outline a potential strategy for the removal of excess Glu in various neurodegenerative disorders.

Correlating retinal function and amino acid immunocytochemistry following post-mortem ischemia

We wanted to determine the characteristics associated with electrophysiological and neurochemical changes secondary to ischemic insult as well as correlate these electrophysiological and neurochemical changes. A Ganzfeld source was used to elicit electroretinograms in anesthetized adult Sprague-Dawley rats. Following baseline recordings, one eye was removed for control quantitative amino acid immunocytochemistry, and ischemic insult was induced by cervical dislocation. Following the induction of ischemia, a single electroretinogram signal was collected at 1, 2, 4, 6, 8, 16, 32 or 64 min, after which the eye was removed for immunocytochemistry. The post-receptoral b-wave was undetectable after 1 min post-ischemia, whereas phototransduction declined more gradually and persisted for up to 16 min post-mortem. Both phototransduction saturated amplitude and sensitivity decayed with a similar time course (tc=3.06 (2.73, 3.48) versus 3.29 (2.61, 4.62)min). Significant elevation of amino acid neurotransmitter levels was not observed until 6 min post-mortem. Between 8 and 16 min post-ischemia, glutamate and GABA were significantly accumulated in neurons and Müller cells (p<0.05). Beyond 16 min, the neurotransmitter elevation in neurons and Müller cells was relatively attenuated. Aspartate immunoreactivity was significantly elevated at 4 and 6 min post-ischemia in neurons, prior to a change in any other amino acid. Moreover, of the amino acids assessed the post-ischemic change in aspartate immunoreactivity showed the best correlation with phototransduction decay (r2=0.68). Our findings show that complete impairment of phototransduction coincides with the accumulation of amino acid neurotransmitter. The correlation of aspartate immunoreactivity and phototransduction provides evidence of heightened glutamate oxidation during ischemic insult.

Quantitative on-line monitoring of hippocampus glucose and lactate metabolism in organotypic cultures using biosensor technology

Quantitative glucose and lactate metabolism was assessed in continuously perfused organotypic hippocampal slices under control conditions and during exposure to glutamate and drugs that interfere with aerobic and anaerobic metabolism. On-line detection was possible with a system based on slow perfusion rates, a half-open (medium/air interface) tissue chamber and a flow injection analytic system equipped with biosensors for glucose and lactate. Under basal conditions about 50% of consumed glucose was converted to lactate in hippocampal slice cultures. Using medium containing lactate (5 mm) instead of glucose (5 mm) significant lactate uptake was observed, but this uptake was less than the net uptake of lactate equivalents in glucose-containing medium. Glucose deprivation experiments suggested lactate efflux from glycogen stores. The effects of drugs compromising or stimulating energy metabolism, i.e. 2-deoxyglucose, 3-nitropropionic acid, alpha-cyano-4-hydroxycinnamate, l-glutamate, d-asparate, ouabain and monensin, were tested in this flow system. The data show that maintaining Na+ and K+ gradients consumed much of the energy but do not support the hypothesis that l-glutamate stimulates glycolysis in hippocampal slice cultures.

Energy metabolism, stress hormones and neural recovery from cerebral ischemia/hypoxia

All the advancements in the understanding of the molecular and cellular processes leading to the great investments in developing neuroprotection against cerebral ischemic/hypoxic damage cannot obscure the simple fact that exhaustion of energy supplies is still at the basis of this disorder. Much has been investigated and postulated over the years about the quick collapse of energy metabolism that follows oxygen and glucose deprivation in the brain. Anaerobic glycolysis, recognized as a pathway of paramount importance in keeping energy supplies, although, at bare minimum, has also presented a dilemma-a significant increase in lactate production during ischemia/hypoxia (IH). The dogma of lactate as a useless end product of anaerobic glycolysis and its postulated role as a detrimental player in the demise of the ischemic cell has persisted for the past quarter of a century. This persistence is due to, at least in part, the well-documented phenomenon termed "the glucose paradox of cerebral ischemia," the unexplained aggravation of postischemic neuronal damage by preischemic hyperglycemia. Recent studies have questioned the deleterious effect of lactic acid, while others even have offered the possibility that this monocarboxylate serves as an aerobic energy substrate during recovery from IH. Reviewed here are studies published over the past few years along with some key older papers on the topic of energy metabolism and recovery of neural tissue from IH. New insights gained from both in vitro and in vivo studies on energy metabolism of the ischemic/hypoxic brain should improve our understanding of this key metabolic process and the chances of protecting this organ from the consequences of energy deprivation.

Evidence for swelling-induced adenosine and adenine nucleotide release in rat cerebral cortex exposed to monocarboxylate-containing or hypotonic artificial cerebrospinal fluids

Recent reports have described a swelling-induced release of adenosine triphosphate (ATP) from a variety of non-nervous system cell types, which may be involved in the regulatory volume decrease (RVD) response. The present study examined the effects of swelling induced by applications of hypotonic or monocarboxylic acid containing artificial cerebrospinal fluid (aCSF) on the release of adenosine nucleotides and adenosine from the in vivo rat cerebral cortex using a cortical cup technique. Hypotonic aCSF (25mM NaCl) elicited a significant increase in adenosine, but not adenine nucleotide, release. Applications of sodium L-lactate, pyruvate, or acetate (all 20mM) evoked increases in adenine nucleotides but not adenosine. D-Lactate (20mM) enhanced adenosine and ATP release. Inhibition of the plasma membrane monocarboxylate transporter with cyano-4-hydroxycinnamate (4-CIN, 2mM) blocked the effects of L-lactate on purine release. These in vivo results demonstrate that osmoregulatory processes in cortical cells evoke an efflux of adenine nucleotides and/or adenosine. In that these purines activate a variety of receptors, it is possible that they may function as autocrine or paracrine signaling agents, facilitating volume regulation and enhancing local blood flow.

Further studies on the effects of topical lactate on amino acid efflux from the ischemic rat cortex

A rat four-vessel cerebral occlusion model was used to examine the effects of D-lactate and oxamate, a lactate dehydrogenase inhibitor, on cortical window superfusate levels of amino acids, glucose and L-lactate. Superfusate levels of aspartate, glutamate, taurine, GABA and phosphoethanolamine rose during ischemia and then declined during reperfusion. Glycine and alanine levels tended to increase during reperfusion, whereas glutamine levels were lower. Serine levels were not altered. Glucose levels declined rapidly during ischemia and recovered during reperfusion. Lactate levels were sustained during ischemia and increased during reperfusion. Unlike L-lactate, which attenuated ischemia/reperfusion (I/R) evoked amino acid release (J.W. Phillis, D. Song, L.L. Guyot, M.H. O'Regan, Lactate reduces amino acid release and fuels recovery of function in the ischemic brain, Neurosci. Lett. 272 (1999) 195-198), topical application of D-lactate (20 mM), which is not used as an energy substrate, enhanced the I/R release of aspartate, glutamate, GABA and taurine into cortical superfusates, and also elevated L-lactate levels above those in the controls. Glucose levels were not altered. Oxamate (20 mM) application elevated the pre-ischemia levels of alanine, glycine and GABA and those of GABA during ischemia. Levels of all amino acids, with the exception of phosphoethanolamine, were elevated during reperfusion. Oxamate, an inhibitor of lactate dehydrogenases 1 and 5, did not alter the pattern of efflux of glucose and L-lactate. In the presence of oxamate, L-lactate (20 mM) failed to inhibit amino acid release. The failure of D-lactate to attenuate amino acid release confirms the inability of this isomer to act as a metabolic substrate. The oxamate data indicate that inhibition of lactate dehydrogenase is detrimental to the viability of cortical cells during I/R, even though extracellular lactate levels are elevated. The pre-ischemia increases in alanine and glycine are suggestive of elevations in pyruvate as a result of the block of its conversion to lactate, with transamination reactions converting pyruvate to form these amino acids. In summary, the results further substantiate the concept of a role for L-lactate as a cerebral energy substrate.