Inhibition, by 2-oxo acids that accumulate in maple-syrup-urine disease, of lactate, pyruvate, and 3-hydroxybutyrate transport across the blood-brain barrier

Energy Expenditure in Chilean Children with Maple Syrup Urine Disease (MSUD)

INTRODUCTION

Maple syrup urine disease (MSUD) is an autosomal recessive disorder caused by a blockage of branched-chain keto acid of BCAA (branched-chain keto acid dehydrogenase, BCKDH) leading to neurological damage induced by accumulation of leucine and metabolites. MSUD expenditure and energy requirement information is limited.

OBJECTIVE

To determine if basal/total energy expenditure (BEE/TEE) is comparable between different determination methods and if values agree with recommendations of energy in MSUD children, and whether they relate to nutritional status.

METHODS

Case-control study between MSUD (n = 16) and healthy children (n = 11) aged 6-18 years. Current nutritional status, physical activity level, body composition by DEXA and BEE/TEE by indirect calorimetry (BEEr) and predictive equations (FAO/WHO/ONU - WHO - and Schofield) were assessed; STATA 2013 (p < 0.05).

RESULTS

When comparing the energy expenditure variables, there was no significant difference between groups. Moreover, compared to BEEr, equations underestimate according to BEE WHO and Schofield, respectively (P = 0.00; 0.02). The WHO equation had lower average calorie difference, greater concordance correlation and association with indirect calorimetry compared to the Schofield equation for both groups, being the best predictor of the BEE for MSUD group.

CONCLUSION

Energy recommendations for MSUD children are according to energy expenditure; thus the use of WHO equation is a clinically and statistically feasible tool for its determination.

Regulation of gonadotropin secretion by monitoring energy availability

Nutrition is a principal environmental factor influencing fertility in animals. Energy deficit causes amenorrhea, delayed puberty, and suppression of copulatory behaviors by inhibiting gonadal activity. When gonadal activity is impaired by malnutrition, the signals originating from an undernourished state are ultimately conveyed to the gonadotropin-releasing hormone (GnRH) pulse generator, leading to suppressed secretion of GnRH and luteinizing hormone (LH). The mechanism responsible for energetic control of gonadotropin release is believed to involve metabolic signals, sensing mechanisms, and neuroendocrine pathways. The availabilities of blood-borne energy substrates such as glucose, fatty acids, and ketone bodies, which fluctuate in parallel with changes in nutritional status, act as metabolic signals that regulate the GnRH pulse generator activity and GnRH/LH release. As components of the specific sensing system, the ependymocytes lining the cerebroventricular wall in the lower brainstem integrate the information derived from metabolic signals to control gonadotropin release. One of the pathways responsible for the energetic control of gonadal activity consists of noradrenergic neurons from the solitary tract nucleus in the lower brainstem, projecting to the paraventricular nucleus of the hypothalamus. Further studies are needed to elucidate the mechanisms underlying energetic control of reproductive function.

Metabolic and histologic effects of sodium pyruvate treatment in the rat after cortical contusion injury

This study determined the effects of intraperitoneal sodium pyruvate (SP) treatment on the levels of circulating fuels and on cerebral microdialysis levels of glucose (MD(glc)), lactate (MD(lac)), and pyruvate (MD(pyr)), and the effects of SP treatment on neuropathology after left cortical contusion injury (CCI) in rats. SP injection (1000 mg/kg) 5 min after sham injury (Sham-SP) or CCI (CCI-SP) significantly increased arterial pyruvate (p < 0.005) and lactate (p < 0.001) compared to that of saline-treated rats with CCI (CCI-Sal). Serum glucose also increased significantly in CCI-SP compared to that in CCI-Sal rats (p < 0.05), but not in Sham-SP rats. MD(pyr) was not altered after CCI-Sal, whereas MD(lac) levels within the cerebral cortex significantly increased bilaterally (p < 0.05) and those for MD(glc) decreased bilaterally (p < 0.05). MD(pyr) levels increased significantly in both Sham-SP and CCI-SP rats (p < 0.05 vs. CCI-Sal) and were higher in left/injured cortex of the CCI-SP group (p < 0.05 vs. sham-SP). In CCI-SP rats the contralateral MD(lac) decreased below CCI-Sal levels (p < 0.05) and the ipsilateral MD(glc) levels exceeded those of CCI-Sal rats (p < 0.05). Rats with a single low (500 mg/kg) or high dose (1000 mg/kg) SP treatment had fewer damaged cortical cells 6 h post-CCI than did saline-treated rats (p < 0.05), but three hourly injections of SP (1000 mg/kg) were needed to significantly reduce contusion volume 2 weeks after CCI. Thus, a single intraperitoneal SP treatment increases circulating levels of three potential brain fuels, attenuates a CCI-induced reduction in extracellular glucose while increasing extracellular levels of pyruvate, but not lactate, and can attenuate cortical cell damage occurring within 6 h of injury. Enduring (2 week) neuronal protection was achieved only with multiple SP treatments within the first 2 h post-CCI, perhaps reflecting the need for additional fuel throughout the acute period of increased metabolic demands induced by CCI.

Brain amino acid requirements and toxicity: the example of leucine

Glutamic acid is an important excitatory neurotransmitter of the brain. Two key goals of brain amino acid handling are to maintain a very low intrasynaptic concentration of glutamic acid and also to provide the system with precursors from which to synthesize glutamate. The intrasynaptic glutamate level must be kept low to maximize the signal-to-noise ratio upon the release of glutamate from nerve terminals and to minimize the risk of excitotoxicity consequent to excessive glutamatergic stimulation of susceptible neurons. The brain must also provide neurons with a constant supply of glutamate, which both neurons and glia robustly oxidize. The branched-chain amino acids (BCAAs), particularly leucine, play an important role in this regard. Leucine enters the brain from the blood more rapidly than any other amino acid. Astrocytes, which are in close approximation to brain capillaries, probably are the initial site of metabolism of leucine. A mitochondrial branched-chain aminotransferase is very active in these cells. Indeed, from 30 to 50% of all alpha-amino groups of brain glutamate and glutamine are derived from leucine alone. Astrocytes release the cognate ketoacid [alpha-ketoisocaproate (KIC)] to neurons, which have a cytosolic branched-chain aminotransferase that reaminates the KIC to leucine, in the process consuming glutamate and providing a mechanism for the "buffering" of glutamate if concentrations become excessive. In maple syrup urine disease, or a congenital deficiency of branched-chain ketoacid dehydrogenase, the brain concentration of KIC and other branched-chain ketoacids can increase 10- to 20-fold. This leads to a depletion of glutamate and a consequent reduction in the concentration of brain glutamine, aspartate, alanine, and other amino acids. The result is a compromise of energy metabolism because of a failure of the malate-aspartate shuttle and a diminished rate of protein synthesis.

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.

Lactate as a pivotal element in neuron-glia metabolic cooperation

Lactate has been considered for a long time as a metabolic waste and/or a sign of hypoxia in the central nervous system. Nevertheless, clear evidence that lactate can constitute an adequate energy substrate for brain tissue has been provided as early as in the 1950s with the pioneering work of McIlwain in brain slices. Over the years, several studies using different approaches have confirmed that lactate is efficiently oxidized by brain cells in vitro. Moreover, lactate has been shown under certain circumstances to have a neuroprotective effect and support neuronal activity. Similar confirmation of lactate utilization in vivo as well as putative neuroprotection in various excitotoxic models has been provided. Lactate was even shown to restore cognitive performance upon an hypoglycemic episode in humans. More recently, it was proposed that lactate could be produced by astrocytes and released in the extracellular space to form a pool readily available for neurons in case of high energy demands. Several elements support the concept of a lactate shuttle between astrocytes and neurons in the central nervous system. Among them, the description of specific monocarboxylate transporters found on both astrocytes and neurons is an important observation consistent with this concept. Interestingly, lactate shuttles between different cell types within the same organ have been described outside the central nervous system, notably in muscle and testis. Thus, lactate is emerging as a valuable intercellular exchange molecule in different systems including the brain where it might be an essential element of neuron-glia metabolic interactions.

Expression of monocarboxylic acid transporters (MCT) in brain cells. Implication for branched chain alpha-ketoacids transport in neurons

The alpha-ketoisocaproic acid (KIC) is a short branched-chain monocarboxylate, which accumulates in neural cells. It plays an important role in maintaining nitrogen balance in the brain, a process of a great importance for shuttling of glutamine and glutamate between astrocytes and neurons. Higher accumulation of KIC in isolated cerebral cortex neurons at lower external pH, as well as sensitivity of this process to alpha-cyano-4-hydroxycinnamate indicate an involvement of a transporter, belonging to the family of monocarboxylate transporters (MCT).The expression of MCT1 and MCT2 isoforms in the brain cells was studied using reverse transcriptase-polymerase chain reaction (RT-PCR) method. The mRNA coding MCT1 was detected in astrocytes, brain endothelial cells, tumour cells (neuroblastoma and glioma) and in cortex neurons of newborn rats, but not in adult ones. MCT2, which is less abundant isoform than MCT1, was expressed in astrocytes, in brain endothelial cells and at low level in newborn rats' neurons, being absent in neurons from adult brain.The observed sensitivity of KIC accumulation towards SH-groups reagents did not fit to the known characteristics of MCT1 and MCT2. Therefore, the change of MCT expression during brain development, as well as lack of MCT1 and MCT2 in neurons of adults, point to another MCT isoform being involved in alpha-ketoisocaproic acid accumulation. This could be either one of other known MCT isoforms or a new member of family MCT, specific towards branched chain alpha-ketoacids.

Pyruvate improves cerebral metabolism during hemorrhagic shock

Pyruvate (PYR) improves cellular and organ function hypoxia and ischemia by stabilizing the reduced nicotinamide adenine dinucleotide redox state and cytosolic ATP phosphorylation potential. In this in vivo study, we evaluated the effects of intravenous pyruvate on neocortical function, indexes of the cytosolic redox state, cellular energy state, and ischemia during a prolonged (4 h) controlled arterial hemorrhage (40 mmHg) in swine. Thirty minutes after the onset of hemorrhagic shock, sodium PYR (n = 8) was infused (0.5 g x kg(-1) x h(-1)) to attain arterial levels of 5 mM. The volume and osmotic effects were matched with 10% NaCl [hypertonic saline (HTS)] (n = 8) or 0.9% NaCl [normal saline (NS)] (n = 8). During the hemorrhage protocol, the time to peak hemorrhage volume was significantly delayed in the PYR group compared with the HTS and NS groups (94 +/- 5 vs. 73 +/- 6 and 72 +/- 4 min, P < 0.05). In addition to the early onset of the decompensatory phase of hemorrhagic shock, the complete return of the hemorrhage volume during decompensatory shock resulted in the death of five and four animals, respectively, in the HTS and NS groups. In contrast, in the PYR group, reinfusion of the hemorrhage volume was slower and all animals survived the 4-h hemorrhage protocol. During hemorrhage, the PYR group also exhibited improved cerebral cortical metabolic and function status. PYR slowed and reduced the rise in neocortical microdialysis levels of adenosine, inosine, and hypoxanthine and delayed the loss of cerebral cortical biopsy ATP and phosphocreatine content. This improvement in energetic status was evident in the improved preservation of the electrocorticogram in the PYR group. PYR also prevented the eightfold increase in the excitotoxic amino acid glutamate observed in the HTS group. The findings show that PYR administered after the onset of hemorrhagic shock markedly improves cerebral metabolic and functional status for at least 4 h.

Human brain beta-hydroxybutyrate and lactate increase in fasting-induced ketosis

Ketones are known to constitute an important fraction of fuel for consumption by the brain, with brain ketone content generally thought to be low. However, the recent observation of 1-mmol/L levels of brain beta-hydroxybutyrate (BHB) in children on the ketogenic diet suggests otherwise. The authors report the measurement of brain BHB and lactate in the occipital lobe of healthy adults using high field (4-T) magnetic resonance spectroscopy, measured in the nonfasted state and after 2- and 3-day fasting-induced ketosis. A 9-mL voxel located in the calcarine fissure was studied, detecting the BHB and lactate upfield resonances using a 1H homonuclear editing sequence. Plasma BHB levels also were measured. The mean brain BHB concentration increased from a nonfasted level of 0.05 +/- 0.05 to 0.60 +/- 0.26 mmol/L (after second day of fasting), increasing further to 0.98 +/- 0.16 mmol/L (after the third day of fasting). The mean nonfasted brain lactate was 0.69 +/- 0.17 mmol/L, increasing to 1.47 +/- 0.22 mmol/L after the third day. The plasma and brain BHB levels correlated well (r = 0.86) with a brain-plasma slope of 0.26. These data show that brain BHB rises significantly with 2- and 3-day fasting-induced ketosis. The lactate increase likely results from ketones displacing lactate oxidation without altering glucose phosphorylation and glycolysis.

Brain lactate uptake increases at the site of impact after traumatic brain injury

Although glucose is the main carbohydrate energy substrate for the normal brain, several studies published over the last 10 years now challenge this assumption. The activated brain increases its metabolism to meet increased energy demands by glycolysis after injury. In vitro studies now show that lactate alone can serve as an energy source to maintain synaptic function. In this study, we used 14C-lactate to test the hypothesis that blood lactate is acutely taken up by the injured brain, after fluid percussion injury (FPI) in the rat. 50 microCi radioactive lactate was injected i.v. immediately after FPI, in injured and sham rats. After 30 min, the brain was removed, frozen, and cut into 20 microm sections for autoradiography. Uptake of 14C-label was mainly concentrated at the injury site (2.5 times greater) although uninjured brain also took up the 14C-label. This increased concentration of radioactive lactate at the injury site suggests that the injured brain may use the lactate as an energy source.

Expression of monocarboxylate transporter mRNAs in mouse brain: support for a distinct role of lactate as an energy substrate for the neonatal vs. adult brain

Under particular circumstances like lactation and fasting, the blood-borne monocarboxylates acetoacetate, beta-hydroxybutyrate, and lactate represent significant energy substrates for the brain. Their utilization is dependent on a transport system present on both endothelial cells forming the blood-brain barrier and on intraparenchymal brain cells. Recently, two monocarboxylate transporters, MCT1 and MCT2, have been cloned. We report here the characterization by Northern blot analysis and by in situ hybridization of the expression of MCT1 and MCT2 mRNAs in the mouse brain. In adults, both transporter mRNAs are highly expressed in the cortex, the hippocampus and the cerebellum. During development, a peak in the expression of both transporters occurs around postnatal day 15, declining rapidly by 30 days at levels observed in adults. Double-labeling experiments reveal that the expression of MCT1 mRNA in endothelial cells is highest at postnatal day 15 and is not detectable at adult stages. These results support the notion that monocarboxylates are important energy substrates for the brain at early postnatal stages and are consistent with the sharp decrease in blood-borne monocarboxylate utilization after weaning. In addition, the observation of a sustained intraparenchymal expression of monocarboxylate transporter mRNAs in adults, in face of the seemingly complete disappearance of their expression on endothelial cells, reinforces the view that an intercellular exchange of lactate occurs within the adult brain.

Assessment of valproic acid serum-cerebrospinal fluid transport by microdialysis

The systemic disposition and serum-cerebrospinal fluid (CSF) translocation of valproic acid (VPA) were examined in rats after administration of VPA as a bolus, as a continuous infusion, or with probenecid. VPA in CSF was monitored continuously by in vivo microdialysis. Both prolonged VPA infusion and probenecid pretreatment increased the Km for saturable VPA elimination and decreased intrinsic hepatic clearance, perhaps due to competition of probenecid or accumulated VPA metabolites for glucuronidation or depletion of hepatic UDP-glucuronic acid. The CSF/serum VPA ratio increased rapidly initially, then decreased with time throughout the remainder of the experiment in all three groups. This time- and/or concentration-dependent behavior suggested that the rate of CSF penetration increased disproportionately with increasing serum VPA and could be described by a kinetic model incorporating a concentration-dependent first-order rate constant for VPA influx into CSF. Under all experimental conditions, the VPA efflux from CSF appeared to be saturable; an increase in the Michaelis constant for efflux was observed following probenecid pretreatment and during VPA infusion, suggesting competitive inhibition of transport by probenecid and derived metabolites of VPA, respectively. The mechanisms responsible for asymmetric VPA transport between serum and CSF, in particular the apparent concentration-dependent change in the rate constant governing CSF penetration, remain to be elucidated.

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.

Astrocyte metabolism of [15N]glutamine: implications for the glutamine-glutamate cycle

The metabolism of glutamine was studied in cultured astrocytes by incubating these cells with [2-15N]-glutamine and using gas chromatography-mass spectrometry to quantitate the transfer of 15N to other amino acids. We found that astrocytes simultaneously synthesize and consume [2-15N]glutamine, with the respective synthetic and utilization rates being approximately equal (ca. 13.0 nmol min-1 mg protein-1). Considerable 15N was transferred to alanine and a significant amount to the essential amino acids leucine, tyrosine, and phenylalanine, the latter process denoting active reamination of cognate ketoacids. A net export of alanine into the medium was noted. Astrocyte glutamine utilization appeared to be mediated via both the phosphate-activated glutaminase (PAG) pathway and the glutamine aminotransferase pathway, the activity of which was about half that of PAG. The glutamine concentration in the incubation medium determined whether net synthesis or utilization of this amino acid occurred. When glutamine was omitted from the medium, net synthesis occurred. When it was present at a high (5 mM) level, net consumption was observed. At a physiologic (0.5 mM) concentration, neither net synthesis nor consumption was noted, although the 15N data indicated that glutamine was actively metabolized. An implication of this work is that astrocytes clearly are capable of both synthesizing and utilizing glutamine, and current concepts of a glutamate-glutamine cycle functioning stoichiometrically between astrocytes and neurons may be an oversimplification.

The kinetics of transport of lactate and pyruvate into rat hepatocytes. Evidence for the presence of a specific carrier similar to that in erythrocytes

Time courses of L-lactate and pyruvate uptake into isolated rat hepatocytes were measured in a citrate-based medium to generate a pH gradient (alkaline inside), by using the silicone-oil-filtration technique at 0 degrees C to minimize metabolism. At low concentrations of lactate and pyruvate (0.5 mM), transport was inhibited by over 95% by 5 mM-alpha-cyano-4-hydroxycinnamate, whereas at higher concentrations (greater than 10 mM) a significant proportion of transport could not be inhibited. The rate of this non-inhibitable transport was linearly related to the substrate concentration, was less with pyruvate than with L-lactate, and appeared to be due to diffusion of undissociated acid. Uptake of D-lactate was not inhibited by alpha-cyano-4-hydroxycinnamate and occurred only by diffusion. Kinetic parameters for the carrier-mediated transport process were obtained after correction of the initial rates of uptake of lactate and pyruvate in the absence of 5 mM-alpha-cyano-4-hydroxycinnamate by that in the presence of inhibitor. Under the conditions used, the Km values for L-lactate and pyruvate were 2.4 and 0.6 mM respectively and the Ki for alpha-cyano-4-hydroxycinnamate as a competitive inhibitor was 0.11 mM. Km values for the transport of L-lactate and pyruvate into rat erythrocytes under similar conditions were 3.0 and 0.96 mM. The Vmax. of lactate and pyruvate transport into hepatocytes at 0 degrees C was 3 nmol/min per mg of protein. Carrier-mediated transport of 0.5 mM-L-lactate was inhibited by 0.2 mM-p-chloromercuribenzenesulphonate (greater than 90%), 0.5 mM-quercetin (80%), 0.6 mM-isobutylcarbonyl-lactyl anhydride (70%) and 0.5 mM-4,4'-di-isothiocyanostilbene-2,2'-disulphonate (50%). A similar pattern of inhibition of lactate transport is seen in erythrocytes. It is suggested that the same or a similar carrier protein exists in both tissues. The results also show that L-lactate transport into rat hepatocytes is very rapid at physiological temperatures and is unlikely to restrict the rate of its metabolism. Differences between our results and those of Fafournoux, Demigne & Remesy [(1985) J. Biol. Chem. 260, 292-299] are discussed.

Regional kinetic constants for blood-brain barrier pyruvic acid transport in conscious rats by the monocarboxylic acid carrier

The present investigation using labeled pyruvate describes the regional distribution and kinetics of the monocarboxylic acid carrier at the blood-brain barrier of conscious rats. The experimental procedure involved the arterial injection of a single bolus of 200 microliter containing [1-14C]pyruvate, [3H]water, and varying concentrations of unlabeled pyruvate into the common carotid via an indwelling externalized catheter. The hemisphere ipsi-lateral to the injection and rostral to the midbrain was removed and dissected into five regions. A kinetic analysis revealed no significant regional differences in Km values with an overall average of 1.37 mM. However, there was regional variation in the density of the monocarboxylic acid carrier as indicated by varied levels of the kinetic constant Vmax. The cortex showed the highest Vmax value of 0.42 +/- 0.08 mumol/min/g whereas values for the caudate/putamen, thalamus/hypothalamus, and remaining portion of hemisphere ranged significantly lower at 0.22-0.27 mumol/min/g. The Vmax for the hippocampus was intermediate at 0.37 +/- 0.12 mumol/min/g. The nonsaturable carrier described kinetically by KD had an overall average of 0.034 ml/min/g. The present study confirms quantitatively previous results suggesting a variable regional distribution of the monocarboxylic acid carrier.

Regional ketone body utilization by rat brain in starvation and diabetes

The rate of ketone body (beta-hydroxybutyrate and acetoacetate) metabolism was measured in individual cerebral structures of fed, starved, and diabetic rats. This was done by infusing beta-[3-14C]hydroxybutyrate intravenously and measuring the incorporation of 14C into brain by quantitative autoradiography. The capacity of the brain to use ketone bodies, expressed as plasma clearance, increased in starvation and diabetes by approximately 50-60%. Plasma clearance was near maximal after 2 days starvation and was not significantly increased after 4 days starvation, 6 days of diabetes or 28 days of diabetes. In all situations the ketone bodies provided only a modest amount of fuel for brain energy metabolism; 3.2% after 2 days starvation and 6.5 and 9.9% after 6 and 28 days of diabetes. The fraction of their energy requirement which the various structures could derive from the ketone bodies differed widely. In general the telencephalon made greatest use of ketone bodies, whereas the hindbrain used least. There was no correlation between the energy requirement of structures (estimated from glucose use in fed rats) and the fraction of energy they could derive from ketone bodies.

Blood-brain transfer of galactose in experimental galactosemia, with special reference to the competitive interaction between galactose and glucose

The interaction between glucose and galactose during transport across the cerebral capillary endothelium was studied in anesthetized rats. Although galactose is present in the diet of suckling mammals and is a potential substrate for brain metabolism in adult mammals, its effect on glucose transport in adult rats is unknown. A kinetic model was formulated to analyze the effect of chronically elevated galactose levels on glucose transport in adult rats. The analysis indicated that galactose and glucose compete for the same transport mechanism in the cerebral capillary endothelium. The Tmax of glucose and galactose were both about 380 mumol 100 g-1 min-1 and the Kt of galactose (30 mM) was about three times that of glucose (10 mM). During prolonged galactosemia in adult rats, neither the Tmax, nor the Kt of either competitor changed substantially when compared with rats subjected to acute galactosemia. At 10 mM galactose in plasma in rats with acute galactosemia, the inhibition of glucose transport, simulated a 25% reduction of plasma glucose, and in rats with chronic galactosemia a 20% reduction. This moderate effect is in contrast to the effect of galactose in suckling rats in which 10 mM galactose in plasma reduced the glucose transport to a level corresponding to a 50% reduction of the plasma glucose concentration.

Does pyruvate prevent acrylamide neurotoxicity? Implications for disease pathogenesis

We used the prototype environmental neurotoxin, acrylamide monomer, to evaluate the hypothesis that neurotoxin-induced nerve fiber degeneration results from inactivation of axonal glycolytic enzymes. Treating intoxicated rats with sodium pyruvate, we hypothesized, would bypass the putative neurotoxin-induced blockade in glycolysis, thus ameliorating neurobehavioral and morphologic measures of neurotoxicity. After establishing that pyruvate itself did not affect behavior, we examined its effects on acrylamide-intoxicated animals. Pyruvate treatment had a significant effect on only one of eight neurobehavioral measures, though others showed similar trends. A morphologic observation of lumbar dorsal root ganglion cell bodies and peripheral nerves failed to show an effect of pyruvate. Those results suggested that inactivation of glycolytic enzymes alone is not a sufficient explanation of pathogenesis.

Biochemical modulation of blood-brain barrier permeability

Hydrophilic substrates necessary for brain function cross the capillary by facilitated diffusion. The facilitation has many features in common with enzyme-catalyzed reactions and is probably subserved by protein entities in the endothelial wall. The proteins act as receptors, recognizing substrate molecules, and as translocators, giving the molecules access to an aqueous path through the endothelium. These receptor-translocators can be saturated, and the transport is subject to competitive inhibition by substrate analogs. Thus, amino acids inhibit the transport of each other, and galactose can inhibit glucose transport in suckling rats. The proteins can be induced, as in the case of ketone transport in starvation, and repressed, as in the case of glucose transport in hyperglycemia. In rats with hyperglycemia for three weeks, the maximum glucose transport capacity of the blood-brain barrier decreased from 400 to 290 mumol/hg/min. An important result of the description is the understanding that rigid distinctions between the function of receptors, translocators, and enzymes is impossible. Understanding of the biochemical properties of facilitated diffusion may help explain a variety of symptoms in many 'inborn errors of metabolism'. This understanding has followed greater, recent insights into the general properties of the blood-brain barrier (45,46,47).