Competition among oxidizable substrates in brains of young and adult rats. Dissociated cells

Glial Glutamine Homeostasis in Health and Disease

Glutamine is an essential cerebral metabolite. Several critical brain processes are directly linked to glutamine, including ammonia homeostasis, energy metabolism and neurotransmitter recycling. Astrocytes synthesize and release large quantities of glutamine, which is taken up by neurons to replenish the glutamate and GABA neurotransmitter pools. Astrocyte glutamine hereby sustains the glutamate/GABA-glutamine cycle, synaptic transmission and general brain function. Cerebral glutamine homeostasis is linked to the metabolic coupling of neurons and astrocytes, and relies on multiple cellular processes, including TCA cycle function, synaptic transmission and neurotransmitter uptake. Dysregulations of processes related to glutamine homeostasis are associated with several neurological diseases and may mediate excitotoxicity and neurodegeneration. In particular, diminished astrocyte glutamine synthesis is a common neuropathological component, depriving neurons of an essential metabolic substrate and precursor for neurotransmitter synthesis, hereby leading to synaptic dysfunction. While astrocyte glutamine synthesis is quantitatively dominant in the brain, oligodendrocyte-derived glutamine may serve important functions in white matter structures. In this review, the crucial roles of glial glutamine homeostasis in the healthy and diseased brain are discussed. First, we provide an overview of cellular recycling, transport, synthesis and metabolism of glutamine in the brain. These cellular aspects are subsequently discussed in relation to pathological glutamine homeostasis of hepatic encephalopathy, epilepsy, Alzheimer's disease, Huntington's disease and amyotrophic lateral sclerosis. Further studies on the multifaceted roles of cerebral glutamine will not only increase our understanding of the metabolic collaboration between brain cells, but may also aid to reveal much needed therapeutic targets of several neurological pathologies.

Effect of 2-deoxy-D-glucose on aminoacids metabolism in rats' cerebral cortex slices

We studied the effect of different concentrations of 2-deoxy-D-glucose on the L-[U-14C]leucine, L-[1-14C]leucine and [1-14C]glycine metabolism in slices of cerebral cortex of 10-day-old rats. 2-deoxy-D-glucose since 0.5 mM concentration has inhibited significantly the protein synthesis from L-[U-14C]leucine and from [1-14C]glycine in relation to the medium containing only Krebs Ringer bicarbonate. Potassium 8.0 mM in incubation medium did not stimulate the protein synthesis compared to the medium containing 2.7 mM, and at 50 mM diminishes more than 2.5 times the protein synthesis compared to the other concentration. Only at the concentration of 5.0 mM, 2-deoxy-D-glucose inhibited the CO2 production and lipid synthesis from L-[U-14C] leucine. This compound did not inhibit either CO2 production, or lipid synthesis from [1-14C]glycine. Lactate at 10 mM and glucose 5.0 mM did not revert the inhibitory effect of 2-deoxy-D-glucose on the protein synthesis from L-[U-14C]leucine. 2-deoxy-D-glucose at 2.0 mM did not show any effect either on CO2 production, or on lipid synthesis from L-[U-14C]lactate 10 mM and glucose 5.0 mM.

Oxidation of 14C-labeled substrates by hippocampal slice cultures

In vitro studies of glucose and lactate utilization have been performed in acute hippocampal slices or dissociated neurons and glia. While some studies concluded that lactate and glucose are equivalent substrates to support evoked synaptic activity, others showed decreased synaptic activity in the presence of lactate as compared to glucose. We found diminished neural activity in the presence of lactate in hippocampal slice cultures. We developed a method to examine the oxidation rates of 14C-labeled substrates by hippocampal slice cultures. The rate of 14CO2 production from either 14C-glucose or 14C-lactate remained unchanged for 6 h suggesting that slice cultures are metabolically stable. While the glucose oxidation rate saturated between 2.8 and 10 mM, lactate oxidation rate had not saturated at 10 mM. These data suggest that organotypic slice cultures provide a method to examine elements of cerebral metabolism in vitro.

Brain uptake and metabolism of ketone bodies in animal models

As a consequence of the high fat content of maternal milk, the brain metabolism of the suckling rat represents a model of naturally occurring ketosis. During the period of lactation, the rate of uptake and metabolism of the two ketone bodies, beta-hydroxybutyrate and acetoacetate is high. The ketone bodies enter the brain via monocarboxylate transporters whose expression and activity is much higher in the brain of the suckling than the mature rat. beta-Hydroxybutyrate and acetoacetate taken up by the brain are efficiently used as substrates for energy metabolism, and for amino acid and lipid biosynthesis, two pathways that are important for this period of active brain growth. Ketone bodies can represent about 30-70% of the total energy metabolism balance of the immature rat brain. The active metabolism of ketone bodies in the immature brain is related to the high activity of the enzymes of ketone body metabolism. Thus, the use of ketone bodies by the immature rodent brain serves to spare glucose for metabolic pathways that cannot be fulfilled by ketones such as the pentose phosphate pathway mainly. The latter pathway leads to the biosynthesis of ribose mandatory for DNA synthesis and NADPH which is not formed during ketone body metabolism and is a key cofactor in lipid biosynthesis. Finally, ketone bodies by serving mainly biosynthetic purposes spare glucose for the emergence of various functions such as audition, vision as well as more integrated and adapted behaviors whose appearance during brain maturation seems to critically relate upon active glucose supply and specific regional increased use.

Effect of alpha-ketoisocaproate and leucine on the in vivo oxidation of glutamate and glutamine in the rat brain

Leucine and alpha-ketoisocaproate (alpha-KIC) were perfused at increasing concentrations into rat brain hippocampus by microdialysis to mimic the conditions of maple syrup urine disease. The effects of elevated leucine or alpha-KIC on the oxidation of L-[U-14C]glutamate and L-[U-14C]glutamine in the brain were determined in the non-anesthetized rat. 14CO2 generated by the metabolic oxidation of [14C]glutamate and [14C]glutamine in brain was measured following its diffusion into the eluant during the microdialysis. Leucine and alpha-KIC exhibited differential effects on 14CO2 generation from radioactive glutamate on glutamine. Infusion of 0.5 mM alpha-KIC increased [14C]glutamate oxidation approximately 2-fold; higher concentrations of alpha-KIC did not further stimulate [14C]glutamate oxidation. The enhanced oxidation of [14C]glutamate may be attributed to the function of alpha-KIC as a nitrogen acceptor from [14C]glutamate yielding [14C]alpha-ketoglutarate, an intermediate of the tricarboxylic acid cycle. [14C]glutamine oxidation was not stimulated as much as [14C]glutamate oxidation and only increased at 10 mM alpha-KIC reflecting the extra metabolic step required for its oxidative metabolism. In contrast, leucine had no effect on the oxidation of either [14C]glutamate or [14C]glutamine. In maple syrup urine disease elevated alpha-KIC may play a significant role in altered energy metabolism in brain while leucine may contribute to clinical manifestations of this disease in other ways.

The role of glutamine and other alternate substrates as energy sources in the fetal rat lung type II cell

Glucose has been thought to be the primary substrate for energy metabolism in the developing lung; however, alternate substrates are used for energy metabolism in other organs. To examine the role of alternate substrates in the lung, we measured rates of oxidation of glutamine, glucose, lactate, and 3-hydroxybutyrate in type II pneumocytes isolated from d 19 fetal rat lungs by measuring the production of 14CO2 from labeled substrates. Glutamine had a rate of 24.36 +/- 4.51 nmol 14CO2 produced/ h/mg of protein (mean +/- SEM), whereas lactate had a significantly higher rate, 40.29 +/- 4.42. 3-Hydroxybutyrate had a rate of 14.91 +/- 1.93. The rate of glucose oxidation was 2.13 +/- 0.36, significantly lower than that of glutamine. To examine the interactions of substrates normally found in the intracellular milieu, we measured the effect of unlabeled substrates as competitors on labeled substrate. This identifies multiple metabolic compartments of energy metabolism. Glucose, but not lactate, inhibited the oxidation of glutamine, suggesting a compartmentation of tricarboxylic acid cycle activity, rather than simple dilution by glucose. Glucose and lactate had reciprocal inhibition. Our data suggest at least two separate compartments in the type II cells for substrate oxidation, one for glutamine metabolism and a second for glucose metabolism. In summary, we have documented that glutamine and other alternate substrates are oxidized preferentially over glucose for energy metabolism in the d 19 fetal rat lung type II pneumocyte. In addition, we have delineated some of the compartmentation that occurs within the developing type II cell, which may determine how these substrates are used.

Energy metabolism of adult astrocytes in vitro

In this study we established cultures of astrocytes from the forebrain of the adult rat. The homeostatic regulatory mechanisms of the aerobic and anaerobic pathways of energy metabolism in these cells showed that adult astrocytes express many of the regulatory properties previously demonstrated in neonatal astrocytes. Changes in mitochondrial respiration and ATP production were readily evident upon incubation with the relevant substrates. Inhibition of mitochondrial respiration led to a compensatory increase in anaerobic glycolysis as evidenced by an increased release of lactate. We assessed the role of cytosolic calcium in the regulation of the mitochondrial energy metabolism. Increases in cytosolic calcium concentration in response to ATP or stimulation of mechanical receptors were followed by depolarizations of the mitochondrial membrane potential, whose magnitude reflected the amplitude of the cytosolic calcium response. The changes in mitochondrial membrane potential were largely dependent on the presence of external calcium. These results provide the first evidence of a signalling mechanism in astrocytes by which changes in cytosolic calcium mediate changes in respiration, possibly through mitochondrial calcium uptake and subsequent activation of several mitochondrial dehydrogenases. This signalling pathway would thus ensure that energy demands due to changes in cytosolic calcium concentrations are met by increases in energy production through increases in mitochondrial oxidative phosphorylation.

Transport of 3-hydroxybutyrate by cultured rat brain astrocytes

It is well established that 3-hydroxybutyrate can serve as an energy source for the brain. Since substrate utilization may be regulated in part by transport across the cellular membrane, we investigated the uptake of 3-hydroxybutyrate by primary cultures of rat brain astrocytes. Measurement of the net uptake indicated a saturable system and a Lineweaver-Burke type plot was consistent with a single carrier-mediated mechanism with a Km of 6.03 mM and a Vmax of 32.7 nmol/30 seconds/mg protein. The rate of uptake at pH 6.2 was more than ten times the rate at pH 8.2, with the rate at pH 7.4 being intermediate between these values, suggesting the possibility of cotransport with H+ or exchange with OH- (antiport). Mersalyl had only a slight effect on the transport of 3-hydroxybutyrate, suggesting that sulfhydryl groups are not involved in the transport of this monocarboxylic acid. Phenylpyruvate and alpha-ketoisocaproate also attenuated the transport, but lactate had only a marginal effect. These results suggest that the utilization of 3-hydroxybutyrate as an energy source by astrocytes is regulated in part by carrier-mediated transport and that the uptake system is different from the lactate transport system.

Transport of L-lactate by cultured rat brain astrocytes

Several reports indicate that lactate can serve as an energy substrate for the brain. The rate of oxidation of this substrate by cultured rat brain astrocytes was 3-fold higher than the rate with glucose, suggesting that lactate can serve as an energy source for these cells. Since transport into the astrocytes may play an important role in regulating nutrient use by individuals types of brain cells, we investigated the uptake of L-[U-14C]lactate by primary cultures of rat brain astrocytes. Measurement of the net uptake suggested two carrier-mediated mechanisms and an Eadie-Hofstee type plot of the data supported this conclusion revealing 2 Km values of 0.49 and 11.38 mM and Vmax values of 16.55 and 173.84 nmol/min/mg protein, respectively. The rate of uptake was temperature dependent and was 3-fold higher at pH 6.2 than at 7.4, but was 50% less at pH 8.2. Although the lactate uptake carrier systems in astrocytes appeared to be labile when incubated in phosphate buffered saline for 20 minutes, the uptake process exhibited an accelerative exchange mechanism. In addition, lactate uptake was altered by several metabolic inhibitors and effectors. Potassium cyanide and alpha-cyano-4-hydroxycinnamate inhibited lactate uptake, but mersalyl had little or no effect. Phenylpyruvate, alpha-ketoisocaproate, and 3-hydroxybutyrate at 5 and 10 mM greatly attenuated the rate of lactate uptake. These results suggest that the availability of lactate as an energy source is regulated in part by a biphasic transport system in primary astrocytes.

Oxidation of very-long-chain fatty acids in rat brain: cerotic acid is beta-oxidized exclusively in rat brain peroxisomes

We studied the effect of sodium 2-[5-(4-chlorophenyl)pentyl]oxirane-2-carboxylate (POCA), a potent inhibitor of mitochondrial carnitine palmitoyltransferase I, on fatty acid oxidation by rat brain cells. In cultured glial cells as well as in dissociated brain cells from adult rats palmitic acid (16:0) oxidation was inhibited by about 85% of control values when 25 microM POCA was added to the medium, whereas no inhibition of cerotic acid (26:0) oxidation was observed. Furthermore, omission of carnitine from the culture medium resulted in a 57.7% decrease in palmitic acid oxidation in cultured glial cells, whereas cerotic acid oxidation was not influenced. These results indicate that rat brain peroxisomes contribute only little (about 15%) to palmitic acid oxidation and provide conclusive evidence that cerotic acid is oxidized exclusively in rat brain peroxisomes.

The metabolism of malate by cultured rat brain astrocytes

Since malate is known to play an important role in a variety of functions in the brain including energy metabolism, the transfer of reducing equivalents and possibly metabolic trafficking between different cell types; a series of biochemical determinations were initiated to evaluate the rate of 14CO2 production from L-[U-14C]malate in primary cultures of rat brain astrocytes. The 14CO2 production from labeled malate was almost totally suppressed by the metabolic inhibitors rotenone and antimycin A suggesting that most of malate metabolism was coupled to the electron transport system. A double reciprocal plot of the 14CO2 production from the metabolism of labeled malate revealed biphasic kinetics with two apparent Km and Vmax values suggesting the presence of more than one mechanism of malate metabolism in these cells. Subsequent experiments were carried out using 0.01 mM and 0.5 mM malate to determine whether the addition of effectors would differentially alter the metabolism of high and low concentrations of malate. Effectors studied included compounds which could be endogenous regulators of malate metabolism and metabolic inhibitors which would provide information regarding the mechanisms regulating malate metabolism. Both lactate and aspartate decreased 14CO2 production from 0.01 mM and 0.5 mM malate equally. However, a number of effectors were identified which selectively altered the metabolism of 0.01 mM malate including aminooxyacetate, furosemide, N-acetylaspartate, oxaloacetate, pyruvate and glucose, but had little or no effect on the metabolism of 0.5 mM malate. In addition, alpha-ketoglutarate and succinate decreased 14CO2 production from 0.01 mM malate much more than from 0.5 mM malate. In contrast, a number of effectors altered the metabolism of 0.5 mM malate more than 0.01 mM. These included methionine sulfoximine, glutamate, malonate, alpha-cyano-4-hydroxycinnamate and ouabain. Both the biphasic kinetics and the differential action of many of the effectors on the 14CO2 production from 0.01 mM and 0.5 mM malate provide evidence for the presence of more than one pool of malate metabolism in cultured rat brain astrocytes.

Induction of ketone body enzymes in glial cells

Ketone bodies serve a dual function in developing brain. They are important sources of energy for metabolism and serve as precursors for lipid synthesis. Astrocytes have two to three times higher activity than oligodendroglia for one of the enzymes involved in ketone body metabolism, 3-ketoacid-CoA transferase. Both glial cell types have similar levels of activity for beta-hydroxybutyrate dehydrogenase. Glucocorticoids and dibutytyl cAMP produce a significant stimulation of activity of both enzymes in astrocytes and oligodendroglia. However, the most striking induction in activity of the two enzymes is in the presence of hydrocortisone and sodium butyrate. There is a three- to eightfold stimulation with these effectors in both astrocytes and oligodendroglia. Thus, in brain the expression of ketone body enzyme activities is finely regulated by hormones and by agents that increase cAMP levels.

Glucose, glutamine and ketone body utilisation by resting and concanavalin A activated rat splenic lymphocytes

The utilisation of glucose, glutamine, acetoacetate and D-3-hydroxybutyrate were investigated over 72 h of incubation of rat splenic lymphocytes, with and without concanavalin A. Lymphocytes consumed both ketone bodies; acetoacetate was consumed preferentially. The ketone bodies reduced glucose consumption by 30-50%, but had little effect on lactate production. Glutamine uptake was concentration dependent up to 4 mM, and consumption was increased in the presence of concanavalin. Glutamine stimulated glucose consumption and lactate production in both resting and activated cells. Complete oxidation contributed 65% of glucose-derived ATP, but less than 40% of glutamine-derived ATP. Glutamine metabolism makes only a minor contribution to lymphocyte ATP generation.

Transport of 3-hydroxy[3-14C]butyrate by dissociated cells from rat brain

Recent studies suggest that the utilization of oxidizable substrates by the brain may be regulated in part by transport across the plasma membrane. Dissociated brain cells obtained by mechanical disruption of rat brain were used to measure the uptake of 3-hydroxy[3-14C]butyrate. Total uptake revealed two mechanisms (diffusion and a carrier-mediated system). A Lineweaver-Burk plot of the latter component yielded an apparent Km of 1.47 mM and a maximal velocity (Vmax) of 5 nmol.min-1.mg protein-1. The rates of uptake were temperature dependent and were significantly higher at pH 6.2 than at pH 7.4 or 8.2. Preloading the cells and increasing the intracellular concentration of 3-hydroxybutyrate using 12.5 and 25 mM increased the rate of uptake 143 and 206%, respectively, indicative of an accelerative exchange mechanism. Uptake was inhibited approximately 50% by (in mM) 10 phenylpyruvate, 10 alpha-ketoisocaproate, 10 KCN, and 1.5 NaAsO2. Uptake was also decreased by (in mM) 5 lactate, 5 methyl malonic acid, 1 alpha-cyano-4-hydroxycinnamate, and 1 mersalyl. Dissociated brain cells from 14- to 16-day-old rats accumulated 3-hydroxybutyrate at a rate more than two-fold greater than cells from either younger (2-day-old) or older (28-day-old and adult) animals. These data are consistent with the proposal that 3-hydroxybutyrate is taken up by the brain by both diffusion and a carrier-mediated transport system, and they support the hypothesis that transport at the cellular level contributes to the regulation of substrate utilization by the brain.

Serum effects on substrate oxidation by dissociated brain cells: possible sites of action

This report is an extension of recent studies indicating the presence of a factor in serum that preferentially inhibits 14CO2 production from labeled glucose. Experiments with dissociated cells revealed that the inhibitory effects of serum were only slightly changed over more than a 50-fold range in initial glucose concentration. Serum had no effect on the rate of glucose transport (uptake of 1,3[3H]2-deoxyglucose). The inhibitory effect of serum was greater on 14CO2 production from [6-14C]glucose than [1-14C]glucose. Other studies revealed that 14CO2 production from [1-14C]pyruvate was more than 5 times the rate obtained using [3-14C]pyruvate; however, the inhibitory effect of serum was much greater on the latter (20% vs 60% inhibition respectively) at 2 mM pyruvate and in the presence of 1% fetal bovine serum. Attempts to characterize the factor using Amicon filtration showed the highest inhibitory activity in a 10,000 mol. wt. fraction, although some inhibitory activity was found in commercial preparations of bovine serum albumin. Delipidation of serum had no effect. Based on these results, we postulate that the observed decrease in labeled CO2 production reflects the regulation of substrate utilization at the pyruvate carboxylase step by one or more factors in serum (with a mol. wt. of approximately 10,000).

Capacity for substrate utilization in oxidative metabolism by neurons, astrocytes, and oligodendrocytes from developing brain in primary culture

Neuron, astrocyte, and oligodendrocyte cultures which were established from developing rat brain were examined for their utilization of glucose, ketone bodies, and free fatty acids by oxidative processes. 14CO2 production was measured in these cells from [1-14C] or [6-14C]glucose; [1-14C]octanoate and [1-14C], [6-14C], or [16-14C]palmitate; and [3-14C]acetoacetate and D(-)-3-hydroxy[3-14C]butyrate. Pyruvate dehydrogenase (EC 1.2.4.1.) and 3-oxoacid-CoA transferase (EC 2.8.3.5) activities were found at high levels in each of the cell populations. Astrocytes and oligodendrocytes produced much more 14CO2 from [1-14C]glucose than from [6-14C]glucose, indicating substantial hexose monophosphate shunt activity. This process was not as active in neurons. All three cell populations readily utilized the ketone bodies for oxidative metabolism at rates 7-9 times greater than they utilized glucose. Only astrocytes were able to utilize fatty acids for 14CO2 production, and the rate of utilization was greater than that of the ketone bodies. We found that the metabolic patterns of these brain cells which were derived from the developing brain complement the nature of the diet of the suckling animal which is rich in fat and low in carbohydrate. They readily utilized the ketone bodies or fatty acids and spared glucose for processes that metabolites of fat cannot fulfill.

2-Oxo-[14C]glutarate is taken up by glutamatergic nerve terminals in the rat striatum

High affinity uptake of [14C]glutamate into rat striatal synaptosomes was reduced by 33% after bilateral cortical ablation. The lesion had no effect on striatal [14C]GABA uptake, but reduced 2-oxo-[14C]glutarate uptake by 67%. The results demonstrate the existence of a high-affinity uptake site for 2-oxoglutarate on glutamatergic nerve terminals and support the contention that this Krebs cycle intermediate may be used to replenish the neuronal pool of neurotransmitter glutamate. 2-Oxo-[14C]glutarate uptake may serve as a selective marker for glutamatergic neurones.

Competition of glycerol with other oxidizable substrates in rat brain

The rate of conversion of [1,3-14C]glycerol into 14CO2 was measured in the presence and absence of unlabelled alternative substrates in whole homogenates from the brains of young (4-6 and 18-20 days old) and adult rats. Unlabelled glucose decreased 14CO2 production from [1,3-14C]glycerol by about 40% at all ages studied. Unlabelled 3-hydroxybutyrate significantly decreased the 14CO2 production from both low (0.2 mM) and high (2.0 mM) concentrations of glycerol in 4-6- and 18-20-day-old rat pups. However, the addition of 3-hydroxybutyrate had no effect on the rate of 14CO2 production from 2.0 mM-glycerol in adult rats, suggesting that the interaction of 3-hydroxybutyrate with glycerol in adult rat brain is complex and may be related to the biphasic kinetics previously reported for glycerol oxidation. Unlabelled glutamine decreased the production of 14CO2 by brain homogenates from 18-20-day-old and adult rats, but not in 4-6-day-old rat pups. In the converse situation, the addition of unlabelled glycerol to whole brain homogenates had little effect on the rate of 14CO2 production from [6-14C]glucose, 3-hydroxy[3-14C]butyrate and [U-14C]glutamine, although some significant differences were noted. Collectively these results suggest that glycerol and these other substrates may be metabolized in separate subcellular compartments in brain such that the products of glucose, 3-hydroxybutyrate and glutamine metabolism can dilute the oxidation of glycerol, but the converse cannot occur. The data also demonstrate that there are complex age-related changes in the interaction of glycerol with 3-hydroxybutyrate and glutamine. The fact that glycerol oxidation was only partially suppressed by the addition of 1-5 mM-glucose, -3-hydroxybutyrate or -glutamine could also suggest that glycerol may be selectively utilized as an energy substrate in some discrete brain region.

Transport of 2-deoxy-D-glucose by dissociated brain cells

The characteristics of glucose transport into dissociated cells from rat brain were determined using [1,2-3H]2-deoxyglucose as substrate. The rate of net uptake exhibited biphasic saturation kinetics with increasing substrate concentration; two values each for Km (8.85 and 1.05 mM) and Vmax (20.41 +/- 5.99 nmol/min/mg protein) were obtained, indicating the presence of two transport systems. D-glucose competed with [1,2-3H]2-deoxyglucose as shown by increasing degrees of inhibition of uptake of labeled substrate with increasing concentrations of D-glucose. The presence of an accelerative exchange mechanism was demonstrated by enhanced rates of uptake of labeled substrate by cells pre-loaded with high levels of unlabeled 2-deoxyglucose. Transport was inhibited by cytochalasin B, phloretin and phloridzin in a manner suggesting that the system is sodium-independent. Transport was also inhibited by sodium cyanide, potassium cyanide and dinitrophenol, but not by sodium arsenite or ouabain. Insulin status of the animals had no effect on the rate of transport of this substrate. Net transport was significantly lower in neonatal (4-day-old) rats than in either older sucklings (14-16-day-old) or adult animals; no significant difference between the latter two groups was observed. These findings demonstrate that two carrier-mediated systems for glucose transport are present on the membranes of these mixed brain cells suggesting that the kinetic characteristics of glucose transport may differ between neurons and glial cells. The age change in transport rate may reflect age-associated glial cell proliferation and/or an age-dependent increase in the number of transporters per cell in one brain cell type.

Substrate oxidation by isolated rat brain mitochondria and synaptosomes

The rates of [6-14C]-glucose oxidation by reconstituted systems of cytosol and mitochondria or cytosol and synaptosomes were essentially the same as the rate of oxidation of [3-14C]-3-hydroxybutyrate. However, the rate of [U-14C]-glutamine oxidation by mitochondria was 2.5 times that by synaptosomes. The addition of glutamine (5 mM) caused a reduction in the rates of oxidation [6-14C]-glucose of 20% and 40% by mitochondria and synaptosomes, respectively. Conversely, the addition of glucose (5 mM) had little or no effect on the rate of [U-14C]-glutamine oxidation by either organelle. Amino-oxyacetate decreased [U-14C]-glutamine oxidation by mitochondria more than 35% but had little or no effect on the rate of glutamine oxidation by synaptosomes. When glucose (5 mM) was added to [3-14C]-3-hydroxybutyrate, the rates of oxidation by the mitochondria and synaptosomes were increased by 65% and 77%, respectively. However, in the reverse situation the addition of 3-hydroxybutyrate decreased [6-14C]-glucose oxidation by synaptosomes (35%) but did not decrease the rate by mitochondria. These results suggest that differences in the rates of substrate utilization by mitochondria and synaptosomes and differences in substrate interactions in these two subcellular organelles may contribute to metabolic compartmentation in the brain.

Decreased oxidation of labeled glucose by dissociated brain cells in the presence of fetal bovine serum

The effect of serum on the rate of substrate oxidation by dissociated brain cells in vitro was examined. At a serum protein concentration of approximately 0.55 milligram per milliliter, oxidation of [6-14C]glucose to 14CO2 was decreased more than 50 percent. Oxidation of [3-14C]-3-hydroxybutyrate and [U-14C]glutamine was decreased much less. Serum from cows, rats, horses, and humans produced similar effects, as did serum from young and old animals and from both sexes. The effect on [6-14C]glucose oxidation was proportional to serum protein concentration, and significant inhibitory activity was obtained with dialyzed serum. Heating (80 degrees C for 10 minutes) significantly reduced the inhibitory activity. These results suggest the presence of a factor in serum that can preferentially decrease glucose oxidation. Such a factor would have profound implications for metabolic regulation in vivo and for studies of cells in vitro in which serum is included in the growth medium.

Competition among oxidizable substrates in brains of young and adult rats. Whole homogenates

The rates of conversion into 14CO2 of D-(-)-3-hydroxy[3-14C]butyrate, [3-14C]acetoacetate, [6-14C]glucose and [U-14C]glutamine were measured in the presence and absence of unlabelled alternative oxidizable substrates in whole homogenates from the brains of young and adult rats. The addition of unlabelled glutamine resulted in decreased 14CO2 production from [6-14C]glucose in brain homogenates from both young and adult rats. In contrast, glucose had no effect on [U-14C]glutamine oxidation. In suckling animals, both 3-hydroxybutyrate and acetoacetate decreased the rate of oxidation of [6-14C]glucose, but in adults only 3-hydroxybutyrate had an effect, and to a lesser degree. The addition of unlabelled glucose markedly enhanced the rates of oxidation of both ketone bodies in adult brain tissue and had little or no effect in the young. The rate of production of 14CO2 from [U-14C]glutamine was increased by the addition of unlabelled ketone bodies in brain homogenates from young, but not from adult rats. In the converse situation, unlabelled glutamine added to 14C-labelled ketone bodies diminished 14CO2 production in young rats, but had no effect in adult animals. These results revealed a complex age-dependent pattern of interaction in which certain substrates apparently competed with each other, whereas an enhanced rate of 14CO2 production was found with others.