Metabolism of Lactate in the Rat Brain During the Early Neonatal Period

The metabolism of lactate in isolated cells from early neonatal rat brain has been studied. In these circumstances, lactate was mainly oxidized to CO2, although a significant portion was incorporated into lipids (78% sterols, 4% phosphatidylcholine, 2% phosphatidylethanolamine, and 1% phosphatidylserine). The rate of lactate incorporation into CO2 and lipids was higher than those found for glucose and 3-hydroxybutyrate. Lactate strongly inhibited glucose oxidation through the pyruvate dehydrogenase-catalyzed reaction and the tricarboxylic acid cycle while scarcely affecting glucose utilization by the pentose phosphate pathway. Lipogenesis from glucose was strongly inhibited by lactate without relevant changes in the rate of glycerol phosphate synthesis. These results suggest that lactate inhibits glucose utilization at the level of the pyruvate dehydrogenase-catalyzed reaction, which may be a mechanism to spare glucose for glycerol and NADPH synthesis. The effect of 3-hydroxybutyrate inhibiting lactate utilization only at high concentrations of 3-hydroxybutyrate suggests that before ketogenesis becomes active, lactate may be the major fuel for the neonatal brain. (-)-Hydroxycitrate and aminooxyacetate markedly inhibited lipogenesis from lactate, suggesting that the transfer of lactate carbons through the mitochondrial membrane is accomplished by the translocation of both citrate and N-acetylaspartate.     

CONTROL OF PYRUVATE AND β-HYDROXYBUTYRATE UTILIZATION IN RAT BRAIN MITOCHONDRIA AND ITS RELEVANCE TO PHENYLKETONURIA AND MAPLE SYRUP URINE DISEASE

—The effects of the amino acids (phenylalanine, valine, leucine and isoleucine) which accumulate in phenylketonuria (PKU) and maple syrup urine disease (MSUD), and their analogue α-keto acids (phenylpyruvate, α-keto isovalerate, α-keto isocaproate, α-keto-β-Me valerate) have been studied on rat brain mitochondrial respiration. Both phenylpyruvate and α-keto isocaproate specifically inhibited the oxidation of pyruvate plus malate and β-hydroxybutyrate plus malate by rat brain mitochondria in the presence of ADP. However, no inhibitory effects of similar concentrations of phenylpyruvate or α-keto isocaproate were observed on the isolated semipurified pyruvate or β-hydroxybutyrate dehydrogenases from rat brain mitochondria. The transport of pyruvate and β-hydroxybutyrate across the brain mitochondrial membrane was studied by both uptake and exchange of radioactively labelled substrates. Both these processes were inhibited by phenylpyruvate and α-ketoisocaproate. The results are interpreted as providing evidence for both pyruvate and β-hydroxybutyrate translocases across the brain mitochondrial membrane, and that the inhibition of these systems by phenylpyruvate and α-keto isocaproate may be important lesions in phenylketonuria and maple syrup urine disease respectively.     

Lactate Modulates the Activity of Primary Cortical Neurons through a Receptor-Mediated Pathway

Lactate is increasingly described as an energy substrate of the brain. Beside this still debated metabolic role, lactate may have other effects on brain cells. Here, we describe lactate as a neuromodulator, able to influence the activity of cortical neurons. Neuronal excitability of mouse primary neurons was monitored by calcium imaging. When applied in conjunction with glucose, lactate induced a decrease in the spontaneous calcium spiking frequency of neurons. The effect was reversible and concentration dependent (IC₅₀ ∼4.2 mM). To test whether lactate effects are dependent on energy metabolism, we applied the closely related substrate pyruvate (5 mM) or switched to different glucose concentrations (0.5 or 10 mM). None of these conditions reproduced the effect of lactate. Recently, a G_i protein-coupled receptor for lactate called HCA1 has been introduced. To test if this receptor is implicated in the observed lactate sensitivity, we incubated cells with pertussis toxin (PTX) an inhibitor of G_i-protein. PTX prevented the decrease of neuronal activity by L-lactate. Moreover 3,5-dyhydroxybenzoic acid, a specific agonist of the HCA1 receptor, mimicked the action of lactate. This study indicates that lactate operates a negative feedback on neuronal activity by a receptor-mediated mechanism, independent from its intracellular metabolism.     

3-Hydroxybutyrate Aids the Recovery of the Energy State from Aglycaemic Hypoxia of Adult but Not Neonatal Rat Brain Slices

Abstract: The level of phosphocreatine (PCr) and the intracellular pH (pH_i) of superfused cortical brain slices from adult or 10-day-old rats were monitored using ³¹P NMR. When the glucose in the superfusing medium was replaced by 3-hydroxybutyrate (3HB), there was a significant reduction in PCr of the adult but not the neonatal slices. The level of PCr of the adult slices was reduced by a greater amount by aglycaemic hypoxia compared with the neonatal brain slices and pH_i was decreased by the same amount. After aglycaemic hypoxia, the levels of PCr of the neonatal slices recovered to the same extent when perfused with glucose or 3HB alone or a mixture of glucose and 3HB. The recovery of the PCr was significantly more in the neonatal than the adult brain slices with glucose alone after aglycaemic hypoxia, whereas pH_i returned to control levels in both tissue types and with all substrates. The relative recovery of the PCr of the adult slices after aglycaemic hypoxia was the same with either 3HB or glucose. However, if glucose and 3HB were applied together, recovery of PCr was significantly improved compared with glucose alone.     

Monocarboxylates and glucose utilization as energy substrates in rat brain slices under selective glial poisoning – a 31P NMR study

We have investigated effects of various energy substrates including glucose, lactate and pyruvate on the recovery of the high energy phosphate levels after high-K⁺ stimulation in rat brain slices by using ³¹P NMR. It was found that lactate, pyruvate and glucose almost equally supported the recovery of phosphocreatine (PCr) levels after high-K⁺ stimulation (60 mM, 8 min) in artificial cerebrospinal fluid (ACSF). In iodoacetic acid (IAA) and fluorocitrate (FC)-pretreated slices, whereas glucose was unable to be utilized, the recovery of the PCr level after high-K⁺ stimulation in ACSF containing lactate was completely abolished, the recovery of the PCr in ACSF containing pyruvate was unaffected. These results indicate that neurons themselves can utilize pyruvate as an exogenous energy substrate, but not lactate, without glial support. In intact brain, glucose may be metabolized to pyruvate in glial cells and then transported to neurons as an energy substrate. These suggest an astrocyte-neuron pyruvate shuttle mechanism of the brain energy metabolism in vivo.We also investigated the effect of ischemic-preconditioning in FC-pretreated slices, which showed that the PCr levels recovered substantially in ACSF containing lactate after high-K⁺ stimulation. This indicates that after the preconditioning, such as ischemia, neurons themselves acquired the ability to utilize lactate as an energy substrate.     

Inhibition of lactate glucogneogenesis in rat kidney by dichloroacetate.

1. Sodium dichloroacetate (1mM) inhibited glucose production from L-lactate in kidney-cortex slices from fed, starved or alloxan-diabetic rates. In general gluconeogenesis from other substrates was no inhibited. 2. Sodium dichloracetate inhibited glucose production from L-lactate but no from pyruvate in perfused isolated kidneys from normal or alloxan-diabetic rats. 3. Sodium dichloroacetate is an inhibitor of the pyruvate dehydrogenase kinase reaction and it effected conversion of pyruvate dehydrogenase into its its active (dephosphorylated) form in kidney in vivo. In general, pyruvate dehydrogenase was mainly in the active form in kidneys perfused or incubated with L-lactate and the inhibitory effect of dichloroacetate on glucose production was not dependent on activation of pyruvate dehydrogenase. 4. Balance data from kidney slices showed that dichloroacetate inhibits lactate uptake, glucose and pyruvate production from lactate, but no oxidation of lactate. 5. The mechanism of this effect of dichloroactetate on glucose production from lactate has not been fully defined, but evidence suggests that it may involve a fall in tissue pyruvate concentration and inhibition of pyruvate carboxylation.     

乳酸在中枢神经系统中的作用及其与相关疾病的关系

一直以来,乳酸在脑中被视作代谢废物,对其功能认识严重滞后。近年来,越来越多的证据表明,乳酸在多种生理与病理过程中扮演重要角色。在神经细胞中,星形胶质细胞是产生和释放乳酸的主要细胞源,该细胞通过有氧糖酵解过程生成乳酸,随后经跨膜通道释放至胞外进入神经元为其供能。在中枢神经系统中,乳酸对稳态调节发挥着十分重要的作用。乳酸主要通过两种途径,即代谢途径(作为能量底物)与信号途径(作为信号分子)调控神经元的功能活动,广泛参与神经元能量代谢、兴奋性、可塑性、学习记忆及神经系统发育等生理过程调节,亦参与抑郁行为、阿尔兹海默病(AD)和脑损伤等病理过程的调节。在脑组织中,存在着乳酸特异性受体(GPR81),乳酸与其结合后调控胞内的第二信使。此外,还发现乳酸可通过未知受体调节神经元的兴奋性以及作为信号分子的其他作用。本文就乳酸作为能量底物和信号分子及其参与相关神经疾病的研究进展进行阐述,旨在为相关中枢神经系统疾病防治提供新思路。     

Energy Metabolism in Rat Brain: Inhibition of Pyruvate Decarboxylation by 3-Hydroxybutyrate in Neonatal Mitochondria

Abstract: The effect of 3-hydroxybutyrate on pyruvate decarboxylation by neonatal rat brain mitochondria and synaptosomes was investigated. The rate of [1 -¹⁴C]pyruvate decarboxylation (1 mm final concentration) by brain synaptosomes derived from 8-day-old rats was inhibited by 10% in the presence of 2 mm -d ,l -3-hydroxybutyrate and by more than 20% in the presence of 20 mm -d ,l -3-hydroxybutyrate. The presence of 2 mm -l ,d -3-hydroxybutyrate did not affect the rate of [1-¹⁴T]pyruvate decarboxylation (1 mm final concentration) by brain mitochondria; however, at a concentration of 20 mm -d ,l -3-hydroxybutyrate, a marked inhibition was seen in preparations from both 8-day-old (35% inhibition) and 21-day-old (24% inhibition) but not in those from adult rats. Although the presence of 100 mm -K⁺ in the incubation medium stimulated the rate of pyruvate decarboxylation by approximately 50% compared with the rate in the presence of 1 mm -K⁺, the presence of 20 mm -d ,l -3-hydroxybutyrate still caused a marked inhibition in both media (1 and 100 mm -K⁺). The presence of 20 mm -d ,l -3-hydroxybutyrate during the incubation caused an approximately 20% decrease in the level of the active form of the pyruvate dehydrogenase complex in brain mitochondria from 8-day-old rats. The concentrations of ATP, ADP, NAD⁺, NADH, acetyl CoA, and CoA were measured in brain mitochondria from 8-day-old rats incubated in the presence of 1 mm -pyruvate alone or 1 mm -pyruvate plus 20 mm -d ,l -3-hydroxybutyrate. Neither the ATP/ADP nor the NADH/NAD⁺ ratio showed significant changes. The acetyl CoA/CoA ratio was significantly increased by more than twofold in the presence of 3-hydroxybutyrate. The possible mechanisms and physiological significance of 3-hydroxybutyrate inhibition of pyruvate decarboxylation in neonatal rat brain mitochondria are discussed.     

The depolarizing action of GABA in cultured hippocampal neurons is not due to the absence of ketone bodies

Two recent reports propose that the depolarizing action of GABA in the immature brain is an artifact of in vitro preparations in which glucose is the only energy source. The authors argue that this does not mimic the physiological environment because the suckling rats use ketone bodies and pyruvate as major sources of metabolic energy. Here, we show that availability of physiologically relevant levels of ketone bodies has no impact on the excitatory action of GABA in immature cultured hippocampal neurons. Addition of β-hydroxybutyrate (BHB), the primary ketone body in the neonate rat, affected neither intracellular calcium elevation nor membrane depolarizations induced by the GABA-A receptor agonist muscimol, when assessed with calcium imaging or perforated patch-clamp recording, respectively. These results confirm that the addition of ketone bodies to the extracellular environment to mimic conditions in the neonatal brain does not reverse the chloride gradient and therefore render GABA hyperpolarizing. Our data are consistent with the existence of a genuine "developmental switch" mechanism in which GABA goes from having a predominantly excitatory role in immature cells to a predominantly inhibitory one in adults.     

Effects of pH on β-hydroxybutyrate transport in rat erythrocytes and thymocytes

Entry of β-hydroxybutyrate into erythrocytes and thymocytes is facilitated by a carrier (C), as judged from temperature dependence, saturation kinetics, stereospecificity, competition with lactate and pyruvate, and inhibition by moderate concentrations of methylisobutylxanthine, phloretin, or α-cyanocinnamate. We studied the dependence of influx and efflux on internal and external pH and [β-hydroxybutyrate]. Lowering external pH from 8.0 to 7.3 to 6.6 enhanced influx into erythrocytes by lowering entry $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ from 29 to 16 to 10 mM, entry $\text{V}$ being independent of external pH. Lowering external pH inhibited efflux. At low external pH, external β-hydroxybutyrate enhanced efflux slightly. At high external pH, external β-hydroxybutyrate inhibited efflux. Internal acidification inhibited influx and internal alkalization enhanced influx. Internal β-hydroxybutyrate (βHB) enhanced influx more in acidified than alkalized cells. These data are compatible with coupled βHB^?/OH^? exchange, βHB^? and OH^? competing for influx, C : OH^? moving faster than C : βHB^?, empty C being immobile. They are also compatible with coupled βHB^?/H⁺ copermeation, empty C moving inward faster than H⁺ : C : βHB^?, H⁺ : C being immobile, and C : βHB^? (without H⁺) being so unstable as not to be formed in significant amounts (relative to C, H⁺ : C, and H⁺ : C : βHB^?).     

Effects of dichloroacetate on the metabolism of glucose, pyruvate, acetate, 3-hydroxybutyrate and palmitate in rat diaphragm and heart muscle in vitro and on extraction of glucose, lactate, pyruvate and free fatty acids by dog heart in vivo

1. The extractions of glucose, lactate, pyruvate and free fatty acids by dog heart in vivo were calculated from measurements of their arterial and coronary sinus blood concentration. Elevation of plasma free fatty acid concentrations by infusion of intralipid and heparin resulted in increased extraction of free fatty acids and diminished extractions of glucose, lactate and pyruvate by the heart. It is suggested that metabolism of free fatty acids by the heart in vivo, as in vitro, may impair utilization of these substrates. These effects of elevated plasma free fatty acid concentrations on extractions by the heart in vivo were reversed by injection of dichloroacetate, which also improved extraction of lactate and pyruvate by the heart in vivo in alloxan diabetes. 2. Sodium dichloroacetate increased glucose oxidation and pyruvate oxidation in hearts from fed normal or alloxan-diabetic rats perfused with glucose and insulin. Dichloroacetate inhibited oxidation of acetate and 3-hydroxybutyrate and partially reversed inhibitory effects of these substrates on the oxidation of glucose. In rat diaphragm muscle dichloroacetate inhibited oxidation of acetate, 3-hydroxybutyrate and palmitate and increased glucose oxidation and pyruvate oxidation in diaphragms from alloxan-diabetic rats. Dichloroacetate increased the rate of glycolysis in hearts perfused with glucose, insulin and acetate and evidence is given that this results from a lowering of the citrate concentration within the cell, with a consequent activation of phosphofructokinase. 3. In hearts from normal rats perfused with glucose and insulin, dichloroacetate increased cell concentrations of acetyl-CoA, acetylcarnitine and glutamate and lowered those of aspartate and malate. In perfusions with glucose, insulin and acetate, dichloroacetate lowered the cell citrate concentration without lowering the acetyl-CoA or acetylcarnitine concentrations. Measurements of specific radioactivities of acetyl-CoA, acetylcarnitine and citrate in perfusions with [1-(14)C]acetate indicated that dichloroacetate lowered the specific radio-activity of these substrates in the perfused heart. Evidence is given that dichloroacetate may not be metabolized by the heart to dichloroacetyl-CoA or dichloroacetylcarnitine or citrate or CO(2). 4. We suggest that dichloroacetate may activate pyruvate dehydrogenase, thus increasing the oxidation of pyruvate to acetyl-CoA and acetylcarnitine and the conversion of acetyl-CoA into glutamate, with consumption of aspartate and malate. Possible mechanisms for the changes in cell citrate concentration and for inhibitory effects of dichloroacetate on the oxidation of acetate, 3-hydroxybutyrate and palmitate are discussed.     

Influence of lactose and lactate on growth and β-galactosidase activity of potential probiotic Propionibacterium acidipropionici

Dairy propionibacteria are microorganisms of interest for their role as starters in cheese technology and as well as their functions as probiotics. Previous studies have demonstrated that Propionibacterium acidipropionici metabolize lactose by a β-galactosidase that resists the gastrointestinal transit and the manufacture of a Swiss-type cheese, so that could be considered for their inclusion in a probiotic product assigned to intolerant individuals. In the present work we studied the effect of the sequential addition of lactose and lactate as first or second energy sources on the growth and β-galactosidase activity of P. acidipropionici Q4. The highest β-galactosidase activity was observed in a medium containing only lactate whereas higher final biomass was obtained in a medium with lactose. When lactate was used by this strain as a second energy source, a marked increase of the intracellular pyruvate level was observed, followed by lactate consumption and increase of specific β-galactosidase activity whereas lactose consumption became negligible. On the contrary, when lactose was provided as second energy source, lactic acid stopped to be metabolized, a decrease of the intracellular pyruvate concentration was observed and β-galactosidase activity sharply returned to a value that resembled the observed during the growth on lactose alone. Results suggest that the relative concentration of each substrate in the culture medium and the intracellular pyruvate level were decisive for both the choice of the energetic substrate and the β-galactosidase activity in propionibacteria. This information should be useful to decide the most appropriate vehicle to deliver propionibacteria to the host in order to obtain the highest β-galactosidase activity.     

Obligate role for ketone body oxidation in neonatal metabolic homeostasis

To compensate for the energetic deficit elicited by reduced carbohydrate intake, mammals convert energy stored in ketone bodies to high energy phosphates. Ketone bodies provide fuel particularly to brain, heart, and skeletal muscle in states that include starvation, adherence to low carbohydrate diets, and the neonatal period. Here, we use novel Oxct1(-/-) mice, which lack the ketolytic enzyme succinyl-CoA:3-oxo-acid CoA-transferase (SCOT), to demonstrate that ketone body oxidation is required for postnatal survival in mice. Although Oxct1(-/-) mice exhibit normal prenatal development, all develop ketoacidosis, hypoglycemia, and reduced plasma lactate concentrations within the first 48 h of birth. In vivo oxidation of (13)C-labeled β-hydroxybutyrate in neonatal Oxct1(-/-) mice, measured using NMR, reveals intact oxidation to acetoacetate but no contribution of ketone bodies to the tricarboxylic acid cycle. Accumulation of acetoacetate yields a markedly reduced β-hydroxybutyrate:acetoacetate ratio of 1:3, compared with 3:1 in Oxct1(+) littermates. Frequent exogenous glucose administration to actively suckling Oxct1(-/-) mice delayed, but could not prevent, lethality. Brains of newborn SCOT-deficient mice demonstrate evidence of adaptive energy acquisition, with increased phosphorylation of AMP-activated protein kinase α, increased autophagy, and 2.4-fold increased in vivo oxidative metabolism of [(13)C]glucose. Furthermore, [(13)C]lactate oxidation is increased 1.7-fold in skeletal muscle of Oxct1(-/-) mice but not in brain. These results indicate the critical metabolic roles of ketone bodies in neonatal metabolism and suggest that distinct tissues exhibit specific metabolic responses to loss of ketone body oxidation.     

Lactate Dehydrogenase Activity is Inhibited by Methylmalonate in vitro

Methylmalonic acidemia (MMAemia) is an inherited metabolic disorder of branched amino acid and odd-chain fatty acid metabolism, involving a defect in the conversion of methylmalonyl-coenzyme A to succinyl-coenzyme A. Systemic and neurological manifestations in this disease are thought to be associated with the accumulation of methylmalonate (MMA) in tissues and biological fluids with consequent impairment of energy metabolism and oxidative stress. In the present work we studied the effect of MMA and two other inhibitors of mitochondrial respiratory chain complex II (malonate and 3-nitropropionate) on the activity of lactate dehydrogenase (LDH) in tissue homogenates from adult rats. MMA potently inhibited LDH-catalyzed conversion of lactate to pyruvate in liver and brain homogenates as well as in a purified bovine heart LDH preparation. LDH was about one order of magnitude less sensitive to inhibition by MMA when catalyzing the conversion of pyruvate to lactate. Kinetic studies on the inhibition of brain LDH indicated that MMA inhibits this enzyme competitively with lactate as a substrate (K _i=3.02±0.59 mM). Malonate and 3-nitropropionate also strongly inhibited LDH-catalyzed conversion of lactate to pyruvate in brain homogenates, while no inhibition was observed by succinate or propionate, when present in concentrations of up to 25 mM. We propose that inhibition of the lactate/pyruvate conversion by MMA contributes to lactate accumulation in blood, metabolic acidemia and inhibition of gluconeogenesis observed in patients with MMAemia. Moreover, the inhibition of LDH in the central nervous system may also impair the lactate shuttle between astrocytes and neurons, compromising neuronal energy metabolism.S. R. Mirandola and E. N. Maciel contributed equally to this work.     

The specificity and metabolic implications of the inhibition of pyruvate transport in isolated mitochondria and intact tissue preparations by alpha-Cyano-4-hydroxycinnamate and related compounds.

1. Effects of alpha-cyano-4-hydroxycinnamate and alpha-cyanocinnamate on a number of enzymes involved in pyruvate metabolism have been investigated. Little or no inhibition was observed of any enzyme at concentrations that inhibit completely mitochondrial pyruvate transport. At much higher concentrations (1 mM) some inhibition of pyruvate carboxylase was apparent. 2. Alpha-Cyano-4-hydroxycinnamate (1-100 muM) specifically inhibited pyruvate oxidation by mitochondria isolated from rat heart, brain, kidney and from blowfly flight muscle; oxidation of other substrates in the presence or absence of ADP was not affected. Similar concentrations of the compound also inhibited the carboxylation of pyruvate by rat liver mitochondria and the activation by pyruvate of pyruvate dehydrogenase in fat-cell mitochondria. These findings imply that pyruvate dehydrogenase, pyruvate dehydrogenase kinase and pyruvate carboxylase are exposed to mitochondrial matrix concentrations of pyruvate rather than to cytoplasmic concentrations. 3. Studies with whole-cell preparations incubated in vitro indicate that alpha-cyano-4-hydroxycinnamate or alpha-cyanocinnamate (at concentrations below 200 muM) can be used to specifically inhibit mitochondrial pyruvate transport within cells and thus alter the metabolic emphasis of the preparation. In epididymal fat-pads, fatty acid synthesis from glucose and fructose, but not from acetate, was markedly inhibited. No changes in tissue ATP concentrations were observed. The effects on fatty acid synthesis were reversible. In kidney-cortex slices, gluconeogenesis from pyruvate and lactate but not from succinate was inhibited. In the rat heart perfused with medium containing glucose and insulin, addition of alpha-cyanocinnamate (200 muM) greatly increased the output and tissue concentrations of lactate plus pyruvate but decreased the lactate/pyruvate ratio. 4. The inhibition by cyanocinnamate derivatives of pyruvate transport across the cell membrane of human erythrocytes requires much higher concentrations of the derivatives than the inhibition of transport across the mitochondrial membrane. Alpha-Cyano-4-hydroxycinnamate appears to enter erythrocytes on the cell-membrane pyruvate carrier. Entry is not observed in the presence of albumin, which may explain the small effects when these compounds are injected into whole animals.     

The Development of Enzymes of Energy Metabolism in the Brain of a Precocial (Guinea Pig) and Non-Precocial (Rat) Species

Abstract: Key enzymes of ketone body metabolism (3-hydroxybutyrate de-hydrogenase, 3-oxo-acid: CoA transferase, acetoacetyl-CoA thiolase) and glucose metabolism (hexokinase, lactate dehydrogenase, pyruvate dehydrogenase, citrate synthase) have been measured in the brains of foetal, neonatal and adult guinea pigs and compared to those in the brains of neonatal and adult rats. The activities of the guinea pig brain ketone-body-metabolising enzymes remain relatively low in activity throughout the foetal and neonatal periods, with only slight increases occurring at birth. This contrasts with the rat brain, where three- to fourfold increases in activity occur during the suckling period (0–21 days post partum), followed by a corresponding decrease in the adult. The activities of the hexokinase (mitochondrial and cytosolic), pyruvate dehydrogenase, lactate dehydrogenase and citrate synthase of guinea pig brain show marked increases in the last 10–15 days before birth, so that at birth the guinea pig possesses activities of these enzymes similar to the adult state. This contrasts with the rat brain where these enzymes develop during the late suckling period (10–15 days after birth). The development of the enzymes of aerobic glycolytic metabolism correlate with the onset of neurological competence in the two species, the guinea pig being a "precocial" species born neurologically competent and the rat being a "non-precocial" species born neurologically immature. The results are discussed with respect to the enzymatic activities required for the energy metabolism of a fully developed, neurologically competent mammalian brain and its relative sensitivity to hypoxia.     

A metabolic switch in brain: glucose and lactate metabolism modulation by ascorbic acid

In this review, we discuss a novel function of ascorbic acid in brain energetics. It has been proposed that during glutamatergic synaptic activity neurons preferably consume lactate released from glia. The key to this energetic coupling is the metabolic activation that occurs in astrocytes by glutamate and an increase in extracellular [K⁺]. Neurons are cells well equipped to consume glucose because they express glucose transporters and glycolytic and tricarboxylic acid cycle enzymes. Moreover, neuronal cells express monocarboxylate transporters and lactate dehydrogenase isoenzyme 1, which is inhibited by pyruvate. As glycolysis produces an increase in pyruvate concentration and a decrease in NAD⁺/NADH, lactate and glucose consumption are not viable at the same time. In this context, we discuss ascorbic acid participation as a metabolic switch modulating neuronal metabolism between rest and activation periods. Ascorbic acid is highly concentrated in CNS. Glutamate stimulates ascorbic acid release from astrocytes. Ascorbic acid entry into neurons and within the cell can inhibit glucose consumption and stimulate lactate transport. For this switch to occur, an ascorbic acid flow is necessary between astrocytes and neurons, which is driven by neural activity and is part of vitamin C recycling. Here, we review the role of glucose and lactate as metabolic substrates and the modulation of neuronal metabolism by ascorbic acid.     

CEREBRAL METABOLIC AND CIRCULATORY CHANGES INDUCED BY HYPOXIA IN STARVED RATS

In order to study cerebral metabolic and circulatory effects of hypoxia under conditions of restricted glucose supply, the arterial P_o2, was reduced to 25–30mm Hg in artificially ventilated and lightly anaesthetized rats that were starved for 24 or 48 h prior to experiments. Arterial glucose concentrations, that were initially around 6μmol g^-1, were significantly reduced after 15min of hypoxia, and decreased to 50^o of control after 30min. In animals studied after 30min of hypoxia (24 h of starvation), cerebral blood flow had increased 4-fold and there was a moderate (25%) rise in cerebral oxygen consumption. During the course of hypoxia, cerebral cortical concentrations of glucose fell to low values. In spite of this, concentrations of pyruvate and lactate rose with time, and the sum of citric acid cycle intermediates (citrate, α-ketoglutarate, fumarate. malate and oxaloacetate) increased. Changes in amino acids were dominated by a fall in aspartate and a rise in alanine concentration. There was a moderate reduction in phosphocreatine and a slight rise in ADP concentration, but concentrations at ATP and AMP were unchanged. The changes observed are similar to those previously obtained in fed animals. It is concluded that even if blood glucose concentrations fall to 3μmol g^-1, and cerebral energy flux is maintained, substrate supply is sufficient to cover the energy requirements of the tissue. Hypoxia was accompanied by increases in the lactate/pyruvate and β-hydroxybutyrate acetoacetate ratios of blood. In the tissue, NADH/NAD⁺ ratios derived from the lactate, malate and β-hydroxybutyrate dehydrogenase systems rose, while that derived from the glutamate dehydrogenase reaction fell. It is concluded that the latter system is not well suited for estimating mitochondrial redox changes in brain tissue.     

Biophysical models of intrinsic homeostasis: Firing rates and beyond

In view of ever-changing conditions both in the external world and in intrinsic brain states, maintaining the robustness of computations poses a challenge, adequate solutions to which we are only beginning to understand. At the level of cell-intrinsic properties, biophysical models of neurons permit one to identify relevant physiological substrates that can serve as regulators of neuronal excitability and to test how feedback loops can stabilize crucial variables such as long-term calcium levels and firing rates. Mathematical theory has also revealed a rich set of complementary computational properties arising from distinct cellular dynamics and even shaping processing at the network level. Here, we provide an overview over recently explored homeostatic mechanisms derived from biophysical models and hypothesize how multiple dynamical characteristics of cells, including their intrinsic neuronal excitability classes, can be stably controlled.     

Inhibition of lipogenesis by halothane in isolated rat liver cells.

1. Halothane at clinically effective concentrations [2.5 and 4% (v/v) of the gas phase of the incubation flask] was found to inhibit significantly lipogenesis from endogenous substrates, e.g., glycogen, or from added lactate plus pyruvate. This was accompanied by a decrease in the ratio of the free [NAD+]/[NADH] of the mitochondrion and the cytoplasm, as shown by the [3-hydroxybutyrate]/[acetoacetate] ratio and the [lactate]/[pyruvate] ratio. 2. Acetoacetate or pyruvate decreased the inhibitory effect of halothane and restored lipogenesis to control rates. They were reduced rapidly by 3-hydroxybutyrate dehydrogenase or lactate dehydrogenase respectively, with the concomitant oxidation of NADH and the generation of NAD+. 3. These results suggest that the mechanism by which halothane inhibits lipogenesis from glycogen or lactate is by inhibition of the oxidation of NADH; this results in inhibition of flux of carbon through pyruvate dehydrogenase and a shortage of acetyl-CoA for fatty acid synthesis. Thus when NADH acceptors are added in the presence of halothane, the concentration of mitochondrial NAD+ is raised so that the flux of carbon through pyruvate dehydrogenase increases and lipogenesis is restored.