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1999 Circulation and energy metabolism in the brain / Donald D. Clarke and Louis Sokoloff Donald Dudley Clarke PhD Fordham University, [email protected]

Louis Sokoloff

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Recommended Citation Clarke, Donald Dudley PhD and Sokoloff, Louis, "Circulation and energy metabolism in the brain / Donald D. Clarke and Louis Sokoloff" (1999). Chemistry Faculty Publications. 81. https://fordham.bepress.com/chem_facultypubs/81

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C H A P T E R

Circulation and Ene,rgy Metabolism of the Brain

Donald D. Clarke and Louis Sokoloff

INTERMEDIARY METABOLISM 638 are correlated with blood and cerebrospinal ATP production in brain is highly regulated 638 fluid chemical changes 647 Glycogen is a dynamic but limited energy store in Brain samples are removed for biochemical brain 639 analyses 648 Brain glycolysis is regulated mainly by hexokinase Radioisotope incorporation can identify and and phosphof ructokinase 641 measure routes of metabolism 648 The pyruvate dehydrogenase complex plays a key Oxygen utilization in the cortex is measured by role in regulating oxidation 644 polarographic techniques 648 Energy output and oxygen consumption are Arteriovenous differences identify substances associated with high rates of enzyme activity in consumed or produced by brain 648 t he Krebs cycle 644 Combining cerebral blood flow and arteriovenous The pentose shunt, also termed the hexose differences permits measurement of rates of monophosphate pathway, is active in brain 645 consumption or production of substances by Glutamate in brain is compartmented into brain 649 separate pools 645 REGULATION OF CEREBRAL METABOLIC DIFFERENCES BETWEEN IN VITRO AND IN RATE 6SO VIVO BRAIN METABOLISM 646 The brain consumes about one-fifth of total body In contrast to cells of other tissues. individual oxygen utilization 650 nerve cel ls do not function autonomously 647 The main energy-demanding functions of the The blood-brain ba rrier selectively limits the rates brain are those of ion flux related to excitation of transfer of soluble substances between and conduction 651 blood and brain 647 Continuous cerebral circulation is absolutely required to provide sufficient oxygen 651 CEREBRAL ENERGY METABOLISM Local rates of cerebral blood flow and metabolism IN VIVO 647 can be measured by autoradiography and are Behavioral and central nervous system physiology coupled to local brain function 652

Basic Neurochemistry: Mo lecular, Cellular and Medical Aspects, 6th Ed., ed ited by G. J. Siegel et a!. Published by Lippincott-Raven Publishers, Philadelphia, 1999. Correspondence to Donald D. Clarke, Chemistry Department, Fordham University, Bronx, New York 10458. 638 Part Five Metabolism

SUBSTRATES OF CEREBRAL CEREBRAL METABOLIC RATE IN VARIOUS METABOLISM 656 PHYSIOLOGICAL STATES 662 Normally, the substrates are glucose and Cerebral metabolic rate is determined locally by oxygen and the products are carbon dioxide functional activity in discrete regions 662 and water 656 Metabolic rate and nerve conduction are related In brain, glucose utilization is obligatory 657 directly 662 The brain utilizes ketones in states of ketosis 660 It is difficult to define metabolic equivalents of consciousness, mental work and sleep 664 AGE AND DEVELOPMENT INFLUENCE CEREBRAL ENERGY METABOLISM 661 CEREBRAL ENERGY METABOLISM IN Metabolic rate increases during early PATHOLOGICAL STATES 665 development 661 Psychiatric disorders may produce effects related Metabolic rate declines and plateaus after to anxiety 665 maturation 661 Coma and systemic metabolic diseases depress Tissue pathology, but not aging, produces brain metabolism 666 secondary changes in metabolic rate 661 Measurement of local cerebral energy metabolism in humans 667

The biochemical pathways ofenergy metabolism in phosphate bonds (-P) generated during aerobic the brain are in most respects like those of other tis­ metabolism of a single glucose molecule. About sues, but special conditions peculiar to the central !So/o of brain glucose is converted to lactate and nervous system in vivo limit full expression of its does not enter the Krebs cycle, also termed the bioclhemical potentialities. In no tissue are the dis­ citric acid cycle. However, this might be crepancies between in vivo and in vitro properties matched by a corresponding uptake of ketone greater or the extrapolations from in vitro data to bodies. The total net gain of-Pis 33 equivalents conclusions about in vivo metabolic functions more per mole of glucose utilized. The steady-state hazardous. Valid identification of normally used concentration of ATP is high and represents the substrates and products of cerebral energy sum of very rapid synthesis and utilization. On metabolism, as well as reliable estimations of their average, half of the terminal phosphate groups rates of utilization and production, can be obtained turn over in about 3 sec; this is probably much only in the intact animal; in vitro studies identify faster in certain regions [2]. The level of-Pis pathways ofintermediary metabolism, mechanisms kept constant by regulation of ADP phosphoryl­ and potential rather than actual performance. ation in relation to ATP hydrolysis. The active Although the brain is said to be unique adenylyl kinase reaction, which forms equivalent among tissues in its high rate of oxidative amounts of ATP and AMP from ADP, prevents metabolism, the overall cerebral metabolic rate any great accumulation of ADP. Only a small for 0 2 (CMR02) is of the same order as the un­ amount of AMP is present under steady-state stressed heart and renal cortex [I). Regional conditions; thus, a relatively small decrease in fluxes in the brain may greatly exceed CMR02, ATP may lead to a relatively large increase in however, and these are closely coupled to AMP, which is a positive modulator of many re­ changes in metabolic demand. actions that lead to increased ATP synthesis. Such an amplification factor provides a sensitive control for maintenance of ATP levels [3). Be­ INTERMEDIARY METABOLISM tween 37 and 42°C, the brain metabolic rate in­ creases about So/o per degree. The concentration of creatine phosphate ATP production in brain is highly regulated (CRP) in brain is even higher than that of ATP, and creatine phosphokinase (CPK) is extremely Oxidative steps of carbohydrate metabolism active. The CRP level is exquisitely sensitive to normally contribute 36 of the 38 high-energy changes in oxygenation, providing - P for ADP Chapter 31 Brain Circulation and Metabolism 639

, phosphorylation and, thus, maintaining ATP The enzymes which synthesize and catab­ levels. The CPK system also may function in reg­ olize glycogen in other tissues are found in ulating mitochondrial activity. In neurons with a brain also, but their kinetic and regulatory very heterogeneous mitochondrial distribution, properties do differ [5). Glycogen metabolism the CRP shuttle may play a critical role in energy in brain, unlike in other tissues, is controlled transport [4]. The BB isoenzyme ofCPK is char­ locally. It is isolated from the tumult of sys­ acteristic of, but not confined to, brain. Thus, its temic activity, evidently by the blood-brain presence in body fluids does not necessarily in­ barrier (BBB). Although glucocorticoid hor­ dicate disruption of neural tissue. mones that penetrate the BBB increase glyco­ gen turnover, circulating protein hormones Glycogen is a dynamic but limited and biogenic amines have no effect. Beyond the energy store in brain BBB, cells are sensitive to local amine concen­ trations; drugs that cross the BBB and modify Although present in relatively low concentration local amine concentrations or membrane re­ in brain (3.3 mmoVkg brain in rat), glycogen is a ceptors thus cause metabolic changes (see unique energy reserve that requires no energy Chap. 32). (ATP) for initiation of its metabolism. As with Separate systems for the synthesis and glucose, glycogen levels in brain appear to vary degradation of glycogen provide a greater degree with plasma glucose concentrations. Biopsies of control than if glycogen were degraded by have shown that human brain contains much more glycogen than rodent brain, but the effects simply reversing its synthesis (Fig. 31-1). The of anesthesia and pathological changes in the amount of glucose-6-phosphate ( G6P), the ini­ biopsied tissue may have contributed. Glycogen tial synthetic substrate, usually varies inversely granules are seen in electron micrographs of glia with the rate of brain glycolysis because of and neurons ofimmature animals but only in as­ greater facilitation of the phosphofructokinase trocytes of adults. Barbiturates decrease brain step relative to transport and phosphorylation of metabolism and increase the number ofgranules glucose. Thus, a decline in G6P during energy seen, particularly in astrocytes of synaptic re­ need slows glycogen formation. gions; however, biochemical studies show that The glucosyl group of uridine diphospho­ neurons do contain glycogen and that enzymes glucose (UDP-glucose) is transferred to the ter­ for its synthesis and metabolism are present in minal glucose of the nonreducing end of an synaptosomes. Astrocyte glycogen may form a amylose chain in an a -1 ,4-glycosidic linkage store of carbohydrate made available to neurons (Fig. 31-1). This reaction, catalyzed by glycogen by still undefined mechanisms. Associated with synthetase (GS), is rate-controlling for glycogen the granules are enzymes involved in glycogen synthesis [5). In brain, as in other tissues, GS oc­ synthesis and, perhaps, degradation. The in­ curs in both a phosphorylated (D) form, which creased glycogen found in areas of brain injury depends on G6P as a positive modulator, and a may be due to glial changes or to decreased uti­ dephosphorylated, independent (I) form sensi­ lization during tissue preparation. tive to, but not dependent on, the modulator. The accepted role of glycogen is that of a Although in brain the I form of GS requires no carbohydrate reserve utilized when glucose falls stimulator, it has a relatively low affinity for below need. However, rapid, continual break­ UDP-glucose. At times of increased energy de­ down and synthesis of glycogen occur at a rate of mand, not only is there a change from the D to 19 IJ.moVkglmin. This is about 2% of the normal the I form but also an I form with even lower glycolytic flux in brain and is subject to elaborate affinity for substrate develops. Inhibition of control mechanisms. This suggests that, even glycogen synthesis is enhanced, and this in­ under steady-state conditions, local carbohy­ creases the availability of G6P for energy needs. drate reserves are important for brain function. Goldberg and O'Toole [5) hypothesize that the I However, if glycogen were the sole supply, nor­ form in brain is associated with inhibition of mal glycolytic flux in brain would be maintained glycogen synthesis under, conditions of energy for less than 5 min. demand, whereas the D form causes a relatively 640 Part Five Metabolism

Phosphoglucomutase 14

Glucose-6-P ------~ ---"?"-""'--+> Uridine diphosphoglucose 65 104 UTP P-P

UDPG Glycogen transglucosidase (Glycogen synthetase) 430

UDP

Glycogen -+------(1 ,4-glucosyl units) 2250 Branching enzyme

FIGURE 31-1. Glycogen metabo li sm in brain. Enzyme data from mouse brain ho mogenates. Numerals below each enzyme represent maximal velocity WmaJ at 38°C in millimo les per kilogram wet weight per minute; metabolite con­ centrations from quick-frozen adult mouse brain are in micromoles per kilogram wet weigh t. P-P, pyrophosphate . (Metabolic data from [41 ); enzyme data from [42]) .

small regulated synthesis under resting condi­ reedy that the conversion from phosphorylase tions. Regulation of the D form may reduce b to a is a control point of glycogenolysis in glycogen formation in brain to approximately vivo. Norepinephrine and, probably, dopamine 5% of its potential rate. In liver, where large activate glycogenolysis through cAMP; but amounts of glycogen are synthesized and de­ epinephrine, vasopressin and angiotensin II do graded, the I form of GS is associated with glyco­ so by another mechanism, possibly involving gen formation. At present, it appears that the Ca 2 + or a Ca 2 + -mediated proteolysis of phos­ two tissues use the same biochemical apparatus phorylase kinase. in different ways to bring about their different Hydrolysis of the u-1,4-glycoside linkages overall metabolic patterns. leaves a limit dextrin that turns over at only half Under steady-state conditions, it is proba­ the rate of the outer chains (see also Chap. 42). ble that Jess than 10% of phosphorylase in brain The debrancher enzyme that hydrolyzes the u- (Fig. 31-1) is in the unphosphorylated b form 1,6-glycoside linkages may be rate-limiting if the (requiring AMP), which is inactive at the very entire glycogen granule is utilized. Because one low AMP concentrations present normally. product of this enzyme is free glucose, approxi­ When a steady state is disturbed, there may be mately one glucose molecule for every 11 of G6P an extremely rapid conversion of enzyme to the is released if an entire glycogen molecule is de­ a form, which is active at low AMP. Brain phos­ graded (Fig. 31-1). a-Glucosidase, also termed phorylase b kinase is activated indirectly by acid maltase, is a lysosomal enzyme whose pre­ cAMP and by the molar concentrations of Ca2 + cise function in glycogen metabolism is not released during neuronal excitation (see Chap. known. In Pompe's disease, which is the heredi­ 22). The endoplasmic reticulum of brain, like tary absence of ex -glucosidase, glycogen accumu­ muscle, is capable of taking up Ca 2 + to termi­ lates in brain as well as elsewhere (see Chap. 42). nate its stimulatory effect. These reactions pro­ The steady-state concentration of glycogen is vide energy from glycogen during excitation regulated precisely by the coordination of syn­ and when cAMP-forming systems are acti­ thetic and degradative processes through enzy­ vated. It has not been possible to confirm di- matic regulation at several metabolic steps [6]. Chapter 31 Brain Circulation and Metabolism 641

Brain glycolysis is regulated mainly by phosphorylation by hexokinase. The reaction is hexokinase and phosphofructokinase essentially irreversible and is a key point in the regulation of carbohydrate metabolism in brain. Aerobic and anaerobic glycolysis have been de­ The electrophoretically slow-moving (type I) fined historically as the amount of lactate pro­ isoenzyme of hexokinase is characteristic of duced under conditions of "adequate" oxygen brain. In most tissues, hexokinase may be solu­ and no oxygen, respectively. More recently, gly­ ble and may exist in the cytosol or be attached colysis refers to the Embden-Meyerhoff glycolytic firmly to mitochondria. Under conditions in sequence from glucose, or glycogen glucosyl, to which no special effort is made to stop pyruvate. Glycolytic flux is defined indirectly: it is metabolism while isolating mitochondria, 80 to the rate at which glucose must be utilized to pro­ 90% of brain hexokinase is bound. In the live duce the observed rate of ADP phosphorylation. steady state, however, when availability of sub­ Figure 31-2 outlines the flow of glycolytic strate keeps up with metabolic demand and end substrates in brain. Glycolysis first involves products are removed, an equilibrium exists be-

Hexokinase Phosphohexose isomerase Phosphofructokinase 27.5 230 36 Glucose 7 • Gluc~~e-6-P +------.- Fruct~~e-6-P 1ATP 1540 I ATP ADP

Aldolase ADP Lactic dehydrogenase 12 140 _.------Fructose-1 ,6-diP Py3ru_9v~te 7 • Lactate 120 I "" 2280;770' NADH NAD+ 1 Triose phosphate isomerase ATP Dihydroxyacetone-P +-----Glyceraldehyde-3-P 1~ OE Pyruvate kinase NADH 165 P; ADP GlyceroP dehydrogenase 3.8 ~ Glyceraldehyde-3-P NAD+ NAD+ Phosphoenolpyruvate dehydrogenase 3.5' 131 Glyceroi-3-P 48' 2 Enolase NADH H 0~ 62 Glyceratephosphokinase 1140 2-Phosphoglycerate 3-P-glycerate < 7 "\ 1 ,3-Diphosphoglycerate 2.8' Phosphoglyceromu1ase 25' <1' 218 ATP ADP

FIGURE 31-2. Glycolysis in brain. Enzyme and metabolic data expressed as in Figure 31-1 . *Ten-day-old mouse brain [43]. (Data from [44].) 642 Part Five Metabolism tween the soluble and the bound enzyme. Bind­ of liver and brain can be related to the function ing changes the kinetic properties of hexokinase of liver as a carbohydrate storehouse for the and its inhibition by G6P so that the bound en­ body, whereas brain metabolism is adapted for zyme on mitochondria is more active. The ex­ rapid carbohydrate utilization for energy needs. tent of binding is inversely related to the ATP: In glycolysis, G6P is the substrate of phospho­ ADP ratio, so conditions in which energy utiliza­ hexose isomerase. This is a reversible reaction, tion exceeds supply shift the solubilization equi­ with a small free energy change and a 5:1 equi­ librium to the bound form and produce a greater librium ratio in brain that favors G6P. potential capacity for initiating glycolysis to Fructose-6-phosphate is the substrate of meet the energy demand. This mechanism al­ phosphofructokinase, a key regulatory enzyme lows ATP to function both as the substrate of the controlling glycolysis [3]. The other substrate is enzyme and, at another site, as a regulator to de­ MgATP 2 ~ . Like other regulatory reactions, it is crease ATP production through its influence on essentially irreversible. It is modulated by a large enzyme binding. It also confers preference on number of metabolites and cofactors, whose con­ glucose in the competition for the MgATP 2 ~ centrations under different metabolic conditions generated by mitochondrial oxidative phospho­ have a great effect on glycolytic flux. Prominent rylation. Thus, a process that will sustain ATP among these are availability of - P and citrate production continues at the expense of other concentrations. Brain phosphofructokinase is in­ uses of energy. Because energy reserves are ex­ hibited by ATP, Mg2+ and citrate and stimulated hausted rapidly postmortem, it is not surprising by NH!, K+, PO~~. 5'-AMP, 3',5',-cAMP, ADP that brain hexokinase is bound almost entirely. and fructose-1,6-bisphosphate. The significance of reversible binding of When oxygen is admitted to cells metaboliz­ other enzymes to mitochondria is not clear. The ing anaerobically, utilization of 0 2 increases, measured glycolytic flux, when compared with whereas utilization of glucose and production of the maximal velocity of hexokinase, indicates lactate drop; this is known as the Pasteur effect. that in the steady state the hexokinase reaction is Modulation of the phosphofructokinase reaction inhibited 97%. Brain hexokinase is inhibited by can account direcdy for the Pasteur effect. In the its product G6P, to a lesser extent by ADP and al­ steady state, A TP and citrate concentrations in losterically by 3-phosphoglycerate and several brain apparently are sufficient to keep phospho­ nucleoside phosphates, including cAMP and fructokinase relatively inhibited as long as the free ATP4 ~ . The ratio of ATP to Mg> + also may concentration of positive modulators, or disin­ have a regulatory action. In addition to acting on hibitors, is low. When the steady state is dis­ enzyme kinetics, G6P solubilizes hexokinase, turbed, activation of this enzyme produces an in­ thus reducing the efficiency of the enzyme when crease in glycolytic flux, which takes place almost the reaction product accumulates. The sum total as fast as events changing the internal"milieu. of these mechanisms is a fine tuning of the activ­ Fructose-1,6-bisphosphate is split by brain ity of the initial enzyme in glycolysis in response aldolase to glyceraldehyde-3-phosphate and di­ to changes in the cellular environment. Gluco­ hydroxyacetone phosphate. Dihydroxyacetone kinase, also termed low-affinity hexokinase, a phosphate is the common substrate for both major component of liver hexokinase, has not glycerophosphate dehydrogenase, an enzyme been found in brain. active in reduced nicotinamide adenine dinu­ G6P represents a branch point in cleotide (NADH) oxidation and lipid pathways metabolism because it is a common substrate for (see Chap. 3), and triose phosphate isomerase, enzymes involved in glycolytic, pentose phos­ which maintains an equilibrium between dihy­ phate shunt and glycogen-forming pathways. droxyacetone phosphate and glyceraldehyde-3- There is also slight but detectable G6Pase activ­ phosphate; the equilibrium strongly favors accu­ ity in brain, the significance of which is not clear. mulation of dihydroxyacetone phosphate. The liver requires this enzyme to convert glyco­ After the reaction with glyceraldehyde-3- gen to glucose. The differences between the hex­ phosphate dehydrogenase, glycolysis in brain okinases and the modes of glycogen metabolism proceeds through the usual steps. Brain enolase, Chapter 31 Brain Circulation and Metabolism 643 also termed D-2-phosphoglycerate hydrolyase, sis that participates in cytoplasmic oxidation of which catalyzes dehydration of 2-phosphoglyc­ NADH. This enzyme reduces dihydroxyacetone erate to phosphoenolpyruvate, is present as two phosphate to glycerol-3-phosphate, oxidizing related dimers, one of which (-y) is associated NADH in the process. Under hypoxic condi­ specifically with neurons and the other (a) with tions, a.-glycerophosphate and lactate increase glia. The neuronal subunit is identical to the initially at comparable rates, although the neuron-specific protein 14-3-2. Immunocyto­ amount of lactate produced greatly exceeds that chemical determination of the enolases makes of a-glycerophosphate. The relative concentra­ them useful in determining neuron:glia ratios in tions of the oxidized and reduced substrates of tissue samples, but neuron-specific enolase is these reactions indicate much higher local con­ not confined to neural tissue. Brain phospho­ centrations ofNADH in brain than are found by enolpyruvate kinase controls an essentially irre­ gross measurements. In fact, the relative propor­ versible reaction that requires not only Mg2 + , as tions of oxidized and reduced substrates of the do several other glycolytic enzymes, but also K+ reactions that are linked to the pyridine nu­ or Na+. This step also maybe regulatory. cleotides may be a better indicator oflocal redox Brain tissue, even when at rest and well oxy­ states (NAD +/NADH) in brain than the direct genated, produces a small amount of lactate, measurement of pyridine nucleotides them­ which is removed in the venous blood, account­ selves [3,7]. ing for 13% of the pyruvate produced by glycol­ An aspect of glucose metabolism that has ysis. The measured lactate concentration in led to much confusion is the observation that la­ brain depends on success in rapidly arresting beled glucose appears in carbon dioxide much brain metabolism prior to tissue processing. Five more slowly than might be suggested from an lactate dehydrogenase isoenzymes are present in examination of the glycolytic pathway plus the adult brain; the one that electrophoretically citric acid cycle [8]. Glucose flux is 0.5 to 1.0 moves most rapidly toward the anode, termed jJ.mol/min/g wet weight of brain in a variety of band 1, predominates. This isoenzyme is gener­ species. The concentration of glycolytic plus ally higher in tissues that are more dependent on Krebs cycle intermediates is 2 IJ.mol/g. Hence, aerobic processes for energy; the slower moving the intermediates might turn over every 2 to 4 isoenzymes are relatively higher in tissues such min and 14C02 production might reach a steady as white skeletal muscle, which is better adapted state in 5 to 10 min. This is not seen experimen­ to function at suboptimal oxygen levels. The dis­ tally. Also, large amounts of radioactivity are tribution of lactate dehydrogenase isoenzymes trapped in amino acids related to the Krebs cycle in various brain regions, layers of the retina, (70 to 80%) from 10 to 30 min after a glucose in­ brain neoplasms and brain tissue cultures and jection. This is due to high transaminase activity during development indicates that their synthe­ in comparison with flux through the Krebs cycle, sis might be controlled by tissue oxygen concen­ and amino acids made by transamination of cy­ trations. Lactate dehydrogenase functions in the cle intermediates behave as if they are part of the cytoplasm to oxidize NADH, which accumulates cycle. When pools of these amino acids as a result of the activity of glyceraldehyde-3- ( - 20 IJ.mol/g) are added to the Krebs cycle com­ phosphate dehydrogenase in glycolysis. This ponents plus glycolytic intermediates, the calcu­ permits glycolytic ATP production to continue lated time for 14C02 evolution is increased by a under anaerobic conditions. Lactate dehydroge­ factor of 10, in agreement with experimental re­ nase also functions under aerobic conditions be­ sults. cause NADH cannot easily penetrate mitochon­ In contrast, in liver, amino acids related to drial membranes. Oxidation of NADH in the the Krebs cycle are present at much lower cytoplasm depends on this reaction and on the steady-state values, and approximately 20% of activity of shuttle mechanisms that transfer re­ the radioactivity from administered glucose is ducing equivalents to mitochondria. trapped in these amino acids shortly after injec­ Glycerol phosphate dehydrogenase is an­ tion. Thus, ignoring the radioactivity trapped in other enzyme indirectly associated with glycoly- amino acids has a relatively small effect on esti- 644 Part Five Metabolism mates of glycolytic fluxes in liver but makes an acetyl-CoA produced within it, but there is ef­ enormous difference in brain. Immature brain flux of its condensation product, citrate. Acetyl­ resembles liver more nearly in this respect. The CoA can then be formed from citrate in the cy­ relationship of the Krebs cycle to glycolysis un­ tosol by ATP citrate lyase. The acetyl moiety of dergoes a sharp change during development, co­ acetylcholine is formed in a compartment, pre­ incident with the metabolic compartmentation sumably the synaptosome, with rapid glucose of amino acid metabolism characteristic of adult turnover. The cytosol of cholinergic endings is brain. rich in citrate lyase, and it is possible that citrate shuttles the acetyl-CoA from the mitochondrial The pyruvate dehydrogenase complex compartment to the cytosol. During hypoxia or plays a key role in regulating oxidation hypoglycemia, acetylcholine synthesis can be in­ hibited by failure of the acetyl-CoA supply. Pyruvate dehydrogenase has an activity of 14 nmol/min/mg protein in rat brain and controls Energy output and oxygen consumption the rate of pyruvate entry into the Krebs cycle as are associated with high rates of acetyl coenzyme A (acetyl-CoA). Pyruvate dehy­ enzyme activity in the Krebs cycle drogenase, or decarboxylase, is part of a mito­ chondrial multienzyme complex that also in­ The actual flux through the Krebs cycle depends cludes the enzymes lipoate acetyltransferase and on glycolysis and acetyl-CoA production, which lipoamide dehydrogenase; the coenzymes thi­ can "push" the cycle, the possible control at sev­ amine pyrophosphate, lipoic acid, CoA and eral enzymatic steps of the cycle and the local flavine; and nicotinamide adenine dinu­ ADP concentration, which is a prime activator cleotides. It is inactivated by being phospho­ of the mitochondrial respiration to which the rylated at the decarboxylase moiety by a tightly Krebs cycle is linked. The steady-state concen­ bound Mg2+ /ATP2- -dependent protein kinase tration of citrate in brain is about one-fifth that and activated by being dephosphorylated by a of glucose. This is relatively high compared with loosely bound Mg2+ - and Ca2+ -dependent glycolytic intermediates or isocitrate. phosphatase. About half the brain enzyme is As in other tissues, there are two isocitrate usually active. Pyruvate protects the complex dehydrogenases in brain. One is active primarily against inactivation by inhibiting the kinase. in the cytoplasm and requires nicotinamide ade­ ADP is a competitive inhibitor of Mg2+ for the nine dinucleotide phosphate (NADP+) as cofac­ inactivating kinase. Under conditions of greater tor; the other, bound to mitochondria and re­ metabolic demand, increases in pyruvate and quiring NAD+, is the enzyme that participates in ADP and decreases in acetyl-CoA and ATP make the citric acid cycle. The NAD+ -linked enzyme the complex more active. Pyruvate dehydroge­ catalyzes an essentially irreversible reaction, has nase is inhibited by NADH, decreasing forma­ allosteric properties, is inhibited by A TP and tion of acetyl-CoA during hypoxia and allowing NADH and may be stimulated by ADP. The more pyruvate to be reduced by lactate dehydro­ function ofcytoplasmic NADP+ isocitrate dehy­ genase, thus forming the NAD+ necessary to drogenase is uncertain, but it has been postu­ sustain glycolysis. Pyruvate dehydrogenase de­ lated that it supplies the NADPH necessary for fects do occur in several mitochondrial enzyme­ many reductive synthetic reactions. The rela­ deficiency states (see below and Morgan-Hughes tively high activity of this enzyme in immature [9], also Chap. 42). brain and white matter is consistent with such a Although acetylcholine synthesis normally role. a-Ketoglutarate (a-KG) dehydrogenase, is controlled by the rate of choline uptake and which oxidatively decarboxylates a-KG, requires choline acetyltransferase activity (see Chap. II), the same cofactors as does the pyruvate decar­ the supply of acetyl-CoA can be limiting under boxylation step. adverse conditions. Choline uptake is, however, Succinate dehydrogenase, the enzyme that independent of acetyl-CoA concentration. The catalyzes the oxidation of succinate to fumarate, mitochondrial membrane is not permeable to the is bound tightly to the mitochondrial mem- Chapter 31 Brain Circulation and Metabolism 645 brane. In brain, succinate dehydrogenase also been found in isolated nerve endings. The pen­ may have a regulatory role when the steady state lose pathway has relatively high activity in devel­ is disturbed. Isocitrate and succinate concentra­ oping brain, reaching a peak during myelina­ tions in brain are affected little by changes in the tion. Its main contribution is probably to flux of the citric acid cycle as long as an adequate produce the NADPH needed for reductive reac­ glucose supply is available. The highly unfavor­ tions necessary for lipid synthesis (see Chap. 3). able free energy change of the malate dehydroge­ Shunt enzymes and metabolic flux are found in nase reaction is overcome by the rapid removal synaptosomes. Although the capacity of the of oxaloacetate, which is maintained at low con­ pathway, as determined using nonphysiological centrations under steady-state conditions by the electron acceptors, remains constant through­ condensation reaction with acetyl-CoA [6]. out the rat life span, activity with physiological Malic dehydrogenase is one of several en­ acceptors could not be detected in middle-aged zymes in the citric acid cycle present in both the (18 months) and older animals. It seems that the cytoplasm and mitochondria. The function of shunt serves as a reserve pathway for use under the cytoplasmic components of these enzyme ac­ such stresses as the need for increased lipid syn­ tivities is not known, but they may assist in the thesis or repair or reduction of oxidative toxins. transfer of hydrogen equivalents from the cyto­ The shunt pathway also provides pentose for nu­ plasm into mitochondria. cleotide synthesis; however, only a small fraction The Krebs cycle functions as an oxidative of the activity of this pathway would be required. process for energy production and as a source of As with glycogen synthesis, turnover in the pen­ various amino acids, for example, glutamate, glu­ lose phosphate pathway decreases under condi­ tamine, "(-aminobutyrate (GABA), aspartate and tions of increased energy need, for example, dur­ asparagine. To export net amounts of a-KG or ing and after high rates of stimulation. Pentose oxaloacetate from the Krebs cycle, the supply of phosphate flux apparently is regulated by the dicarboxylic acids must be replenished. The ma­ concentrations of G6P, NADP+, glyceralde­ jor route for this seems to be the fixation of C02 hyde-3-phosphate and fructose-6-phosphate. to pyruvate or other substrates at the three-car­ Since transketolase, an enzyme in this pathway, bon level. Thus, the C02 fixation rate sets an up­ requires thiamine pyrophosphate as a cofactor, per limit at which biosynthetic reactions can oc­ poor myelin maintenance in thiamine deficiency cur. In studies of acute ammonia toxicity in cats, may reflect failure of this pathway to provide this has been estimated as 0.15 IJ.mol/g wet weight sufficient NADPH for lipid synthesis [6]. brain/min, or approximately 10% of the flux through the citric acid cycle (see below). Liver Glutamate in brain is compartmented seems to have ten times the capacity of brain for into separate pools C02 fixation, as is appropriate for an organ geared to making large quantities of protein for export The pools that subserve different metabolic path­ [10]. In brain, pyruvate carboxylase, which cat­ ways for glutamate equilibrate with each other alyzes C02 fixation, appears to be largely an astro­ only slowly. This compartmentation is a vital fac­ cytic enzyme. Pyruvate dehydrogenase seems to tor in the separate regulation of special functions be the rate-limiting step for the entry of pyruvate of glutamate (see Chap. 15) and GABA (see into the Krebs cycle from glycolysis. Chap. 16), such as neurotransmission, and of general functions, such as protein biosynthesis. The pentose shunt, also termed the Glutamate metabolism in brain shows at least hexose monophosphate pathway, is two distinct pools; in addition, the Krebs cycle in­ active in brain termediates associated with these pools are dis­ tinctly compartmented. Mathematical models to Under basal conditions 5 to 8% of brain glucose fit data from radiotracer experiments that re­ is likely to be metabolized via the pentose shunt quire separate Krebs cycles to satisfy the hypothe­ in the adult monkey and 2.3% in the rat [8]. ses of compartmentation have been developed. A Both shunt enzymes and metabolic flux have key assumption of current models is that GABA 646 Part Five Metabolism is metabolized at a site different from its synthe­ substrates, which are not taken up into brain ef­ sis. The best fit of kinetic data is obtained when ficiently, appear to be more readily taken up or glutamate from a small pool actively converted to activated, or both, in glia. This is believed to lead glutamine flows back to a larger pool (8 f).mol/g), to the observed abnormal glutamine/glutamate which is converted to GABA. Of possible rele­ ratio. Similarly, metabolic inhibitors, like fluoro­ vance to this is the finding that glutamate decar­ acetate, appear to act selectively in glia and to boxylase (GAD) is localized at or near nerve ter­ produce their neurotoxic action without marked minals, whereas GABA transaminase, the major inhibition of the overall Krebs cycle flux in brain. degradative enzyme, is mitochondrial. This difference in behavior has led to suggestions Evidence points to an inferred small pool of that acetate and fluoroacetate may be useful glutamate (2 fJ.mol/g) as probably glial. Gluta­ markers for the study of glial metabolism by au­ mate released from nerve endings appears to be toradiography [II]. taken up by glia or neurons, converted to glu­ A nonuniform distribution of metabolites in tamine and recycled to glutamate and GABA living systems is a widespread occurrence. Steady­ (see Chaps. 5 and 15). Various estimates of the state concentrations of GABA vary from 2 to 10 proportion of glucose carbon that flows through mM in discrete brain regions, and it has been esti­ the GABA shunt have been made, but the most mated that GABA may be as high as 50 mM in definitive experiments show this to be about nerve terminals. Observations in brain indicate the 10% of the total glycolytic flux. While this may existence of pools of metabolites with half-lives of seem small, that portion of the Krebs cycle flux many hours for mixing, which is most unusual. used for energy production, including ATP syn­ The discovery of subcellular, morphological com­ thesis and maintenance of ionic gradients, does partmentation, that is, different populations of not require C02 fixation, while the portion used mitochondria in cerebral cortex that have distinc­ for biosynthesis of amino acids does. Recycling tive enzyme complements, may provide a some­ the carbon skeleton of some of the glutamate re­ what better perspective by which to visualize such leased in neurotransmission through glutamine a separation of metabolic function [ 12]. and GABA to succinate diminishes the need for In addition to the phasic release of both ex­ dicarboxylic acids to replenish intermediates of citatory and inhibitory transmitters, there may be the Krebs cycle when export of ct-KG to make a continuous tonic release of GABA, dependent amino acids takes place. only on the activity of the enzyme responsible for It is difficult to get good estimates of the ex­ its synthesis and independent of the depolariza­ tent of C02 fixation in brain; the maximum ca­ tion of the presynaptic membrane. Such in­ pability measured during ammonia stress, when hibitory neurons could act tonically by constantly glutamine increases rapidly, suggests that C02 maintaining an elevated threshold in the excita­ fixation occurs at 0.15 f.Lmol/g/min in cat and tory neurons so that the latter would start firing 0.33 J.l.mol!glmin in rat; this is about the same when a decrease occurred in the continuous re­ rate as for the GABA shunt. lease of GABA acting on them. This is consistent For comparison, only about 2o/o of the glu­ with a known correlation between the inhibition cose flux in whole brain goes toward lipid syn­ of GAD and the appearance of convulsions after thesis and approximately 0.3% is used for protein certain drug treatments. GABA is depleted by synthesis. Thus, turnover of neurotransmitter some convulsant drugs and elevated by others. amino acids is a major biosynthetic activity in brain. Metabolic compartmentation of glutamate DIFFERENCES BETWEEN IN VITRO usually is observed when labeled ketogenic sub­ AND IN VIVO BRAIN METABOLISM strates are administered to animals. It is interest­ ing that acetoacetate and 13-hydroxybutyrate do In addition to the usual differences between in not show this effect, apparently because ketone vitro and in vivo studies that pertain to all tissues, bodies are a normal substrate for brain and are there are two unique conditions that pertain taken up in all kinds of cells. Acetate and similar only to the central nervous system. Chapter 31 Brain Circulation and Metabolism 647

In contrast to cells of other tissues, yield quantitative results. Some require such individual nerve cells do not function minimal operative procedures on the labora­ autonomously tory animal that no anesthesia is required, and there is no interference with the tissue except They are generally so incorporated into a com­ for the effects of the particular experimental plex neural network that their functional activity condition being studied. Some of these tech­ is integrated with that of various other parts of niques are applicable to normal, conscious hu­ the central nervous system and with somatic tis­ man subjects, and consecutive and comparative sues. In addition, neurons and adjacent glia are studies can be made repeatedly in the same sub­ linked in their metabolism. Any procedure that ject. Other methods are more traumatic and ei­ interrupts the structural and functional integrity ther require the animal to be killed or involve of the network inevitably would alter quantita­ such extensive surgical intervention and tissue tively and, perhaps, qualitatively its normal damage that the experiments approach an in metabolic behavior. vitro experiment carried out in situ. All, how­ ever, are capable of providing specific and use­ The blood-brain barrier selectively limits ful information. the rates of transfer of soluble substances between blood and brain Behavioral and central nervous system This barrier discriminates among various po­ physiology are correlated with blood tential substrates for cerebral metabolism (see and cerebrospinal fluid chemical Chap. 32). The substrate function is confined changes to those compounds in the blood that are not only suitable substrates for cerebral enzymes The simplest way to study the metabolism of the but also can penetrate from blood to brain at central nervous system in vivo is to correlate rates adequate to support the considerable en­ spontaneous or experimentally produced alter­ ergy demands of brain. Substances that can be ations in the chemical composition of the blood, readily oxidized by brain slices, minces or ho­ spinal fluid or both with changes in cerebral mogenates in vitro and that are utilized effec­ physiological functions or gross central nervous tively in vivo when formed endogenously system-mediated behavior. The level of con­ within the brain often are incapable of support­ sciousness, the reflex behavior or the electroen­ ing cerebral energy metabolism and function cephalogram (EEG) generally is used to monitor when present in the blood because of restricted the effects of chemical changes on functional passage through the BBB. The in vitro tech­ and metabolic activities of brain. Such methods niques establish only the existence and poten­ first demonstrated the need for glucose as a sub­ tial capacity of the enzyme systems required for strate for cerebral energy metabolism; hypo­ the use of a given substrate; they do not define glycemia produced by insulin or other means al­ the extent to which such a pathway actually is tered various parameters of cerebral function used in vivo. This can be done only in intact an­ that could not be restored to normal by admin­ imals, and it is this aspect of cerebral istering substances other than glucose. metabolism with which this part of the chapter The chief virtue of these methods is their is concerned. simplicity, but they are gross and nonspecific and do not distinguish between direct effects of the agent on cerebral metabolism and those sec­ CEREBRAL ENERGY METABOLISM ondary to changes produced initially in somatic IN VIVO tissues. Also, negative results are often inconclu­ sive, for there always remain questions of insuf­ Numerous methods have been used to study ficient dosage, inadequate cerebral circulation the metabolism of the brain in vivo; these vary and delivery to the tissues or impermeability of in complexity and in the degree to which they the BBB. 648 Part Five Metabolism

Brain samples are removed for tation. These methods have been used effectively biochemical analyses in studies of amine and neurotransmitter syn­ thesis and metabolism, lipid metabolism, pro­ The availability of analytical chemical techniques tein synthesis, amino acid metabolism and the makes it possible to measure specific metabolites distribution of glucose carbon through the vari­ and enzyme activities in brain tissue at selected ous biochemical pathways present in brain. times during or after exposure of the animal to an Radioisotope incorporation methods are experimental condition. This approach has been particularly valuable for studies of intermediary very useful in studies of the intermediary metabolism, which generally are not feasible by metabolism of brain. It has permitted estimation most other in vivo techniques. They are without of the rates of flux through the various steps of es­ equal for the qualitative identification of the tablished metabolic pathways and the identifica­ pathways and routes of metabolism. They suffer, tion of control points in the pathways where reg­ however, from a disadvantage: only one set of ulation may be exerted. Such studies have helped measurements per animal is possible because the to define more precisely changes in energy animal must be killed. Quantitative interpreta­ metabolism associated with altered cerebral tions often are confounded by the problems of functions, for example, those produced by anes­ compartmentation. Also, all too frequently, they thesia, convulsions or hypoglycemia. While such are misused; unfortunately, quantitative conclu­ methods require killing animals and analyzing sions are drawn incorrectly based on radioactiv­ tissues, in effect, they are in vivo methods since ity data without appropriate consideration of the they attempt to describe the state of the tissue specific activities of precursor pools. while it is still in the animal at the moment of killing. They have encountered their most serious Oxygen utilization in the cortex is problems with regard to this point. Postmortem measured by polarographic techniques changes in brain are extremely rapid and not al­ ways retarded completely even by the most rapid The 0 2 electrode has been used for measuring the freezing techniques available. These methods amount of 0 2 consumed locally in the cerebral have proved to be very valuable, nevertheless, cortex in vivo [13). The electrode is applied to the particularly in the area of energy metabolism. surface of the exposed cortex, and the local partial

pressure for 0 2 (P02 ) is measured continuously Radioisotope incorporation can identify before and during occlusion of the blood flow to and measure routes of metabolism the local area. During occlusion, P02 falls linearly as 0 2 is consumed by tissue metabolism, and the The technique of administering radioactive pre­ rate of fall is a measure of the rate of 0 2 con­ cursors followed by the chemical separation and sumption locally in the cortex. Repeated mea­ assay of products in the tissue has added greatly surements can be made successively in the animal, to the armamentarium for studying cerebral and the technique has been used to demonstrate metabolism in vivo. Labeled precursors are ad­ the increased 0 2 consumption of the cerebral cor­ ministered by any of a variety of routes; at se­ tex and the relation between changes in the EEG lected later times the brain is removed, the pre­ and the metabolic rate during convulsions [13). cursor and the various products of interest are The technique is limited to measurements in the isolated and the radioactivity and quantity of the cortex and, of course, to 0 2 utilization. compounds in question are assayed. Such tech­ niques facilitate identification of metabolic Arteriovenous differences identify routes and rates of flux through various steps of substances consumed or produced by a pathway. In some cases, comparison of specific brain activities of the products and precursors has led to the surprising finding of higher specific activ­ The primary functions of the circulation are to ities in products than in precursors. This is con­ replenish the nutrients consumed by the tissues clusive evidence of the presence of compartmen- and to remove the products of their metabolism. Chapter 31 Brain Circulation and Metabolism 649

These functions are reflected in the composition puncture. Other common laboratory animals are of the blood traversing the tissue. Substances less suitable because extensive communication taken up by a tissue from the blood are higher in between cerebral and extracerebral venous beds is concentration in the arterial inflow than in the present and uncontaminated representative ve­ venous outflow, and the converse is true for sub­ nous blood is difficult to obtain from the cere­ stances released by the tissue. The convention is brum without major surgical intervention. In to subtract the venous concentration from the these cases, one can sample blood from the con­ arterial concentration so that a positive arteri­ fluence of the sinuses, also termed the torcular ovenous difference represents net uptake and a herophili, even though it does not contain fully negative difference, net release. In nonsteady representative blood from the brainstem and states, as after a perturbation, there may be tran­ some of the lower portions of the brain. sient arteriovenous differences that reflect The chief advantages of these methods are changes in tissue concentrations and re-equili­ their simplicity and applicability to unanes­ bration of the tissue with the blood. In steady thetized humans. They permit the qualitative states, in which it is presumed that the tissue identification of the ultimate substrates and concentration remains constant, positive and products of cerebral metabolism. They have no negative arteriovenous differences mean net applicability, however, to those intermediates consumption or production of the substance by that are formed and consumed entirely within the tissue, respectively. Zero arteriovenous dif­ brain without being exchanged with blood or to ferences indicate neither consumption nor pro­ those substances that are exchanged between duction. brain and blood with no net flux in either direc­ This method is useful for all substances in tion. Furthermore, they provide no quantifica­ blood that can be assayed with enough accuracy, tion of the rates of utilization or production be­ precision and sensitivity to enable the detection cause arteriovenous differences depend not only of arteriovenous differences. The method is use­ on the rates of consumption or production by ful only for tissues from which mixed represen­ the tissue but also on blood flow (see below). tative venous blood can be sampled. Arterial Blood flow affects all of the arteriovenous differ­ blood has essentially the same composition ences proportionately, however, and compari­ throughout and can be sampled from any artery. son of the arteriovenous differences of various In contrast, venous blood is specific for each tis­ substances obtained from the same samples of sue, and to establish valid arteriovenous differ­ blood reflects their relative rates of utilization or ences the venous blood must represent the total production. outflow or the flow-weighted average of all of the venous outflows from the tissue under study, Combining cerebral blood flow and uncontaminated by blood from any other tissue. arteriovenous differences permits It is not possible to fulfill this condition for many measurement of rates of consumption tissues. or production of substances by brain The method is fully applicable to the brain, particularly in humans, in whom the anatomy of In a steady state, the tissue concentration of any venous drainage is favorable for such studies. substance utilized or produced by brain is pre­ Representative cerebral venous blood, with no sumed to remain constant. When a substance is more than approximately 3o/o contamination exchanged between brain and blood, the differ­ with extracerebral blood, is readily obtained from ence in its steady state of delivery to brain in the the superior bulb of the internal jugular vein in arterial blood and removal in the venous blood humans. The venipuncture can be made percuta­ must be equal to the net rate of its utilization or neously under local anesthesia; therefore, mea­ production by brain. This relation can be ex­ surements can be made during conscious states pressed as follows: undistorted by the effects of general anesthesia. Using this method with monkeys is similar, al­ CMR = CBF(A - V) though the vein must be exposed surgically before 650 Part Five Metabolism where (A - V) is the difference in concentration of inhalation is required to approach equilib­ in arterial and cerebral venous blood, CBF is the rium. At the end of this interval, the N20 con­ rate of cerebral blood flow in volume of blood centration in brain tissue is about equal to the per unit time and cerebral metabolic rate (CMR) cerebral venous blood concentration. Because is the steady-state rate of utilization or produc­ the method requires sampling of both arterial tion of the substance by brain. and cerebral venous blood, it lends itself readily If both the rate of cerebral blood flow and to the simultaneous measurement of arteriove­ the arteriovenous difference are known, then the nous differences of substances involved in cere­ net rate of utilization or production of the sub­ bral metabolism. This method and its modifica­ stance by brain can be calculated. This has been tions have provided most of our knowledge of the basis of most quantitative studies of cerebral the rates of substrate utilization or product for­ metabolism in vivo. mation by brain in vivo. The most reliable method for determining cerebral blood flow is the inert gas method of Kety and Schmidt [14]. Originally, it was de­ REGULATION OF CEREBRAL signed for use in studies of conscious, unanes­ METABOLIC RATE thetized humans, and it has been employed most widely for this purpose; but it also has been The brain consumes about one-fifth of adapted for use in animals. The method is based total body oxygen utilization on the Fick principle, which is an equivalent of the law of conservation of matter; and it utilizes The brain is metabolically one of the most active low concentrations of a freely diffusible, chemi­ of all organs in the body. This consumption of cally inert gas as a tracer substance. The original 0 2 provides the energy required for its intense gas was nitrous oxide, but subsequent modifica­ physicochemical activity. The most reliable data tions have substituted other gases, such as 85Kr, on cerebral metabolic rate have been obtained in 79 Kr or hydrogen, which can be measured more humans. Cerebral 0 2 consumption in normal, conveniently in blood. During a period of in­ conscious, young men is approximately 3.5 halation of 15% N20 in air, for example, timed ml/100 g brain/min (Table 31-1); the rate is sim­ arterial and cerebral venous blood samples are ilar in young women. The rate of 0 2 consump­ withdrawn and analyzed for their N 20 contents. tion by an entire brain of average weight (1,400 The cerebral blood flow in milliliters per 100 g of g) is then about 49 ml 0 2/min. The magnitude of brain tissue per minute can be calculated from this rate can be appreciated more fully when it is the following equation: compared with the metabolic rate of the whole body. An average man weighs 70 kg and con- CBF= 100!\ V(1)tn[A(t)- V(t)] dt where A(t) and V(t) are the arterial and cerebral CEREBRAL BLOOD FLOW AND venous blood concentrations of N,O, respec- METABOLIC RATE IN A NOR- lively, at any time t; V(T) is concentration of MAL YOUNG ADULT MAN• N 20 in venous blood at the end of the period of Per 100 g Per whole inhalation, that is, time T; X. is the partition coef­ of brain brain ficient for N20 between brain tissue and blood; t Function tissue (1,400g) is variable time in minutes; T is total period of Cerebral blood flow 57 798 inhalation ofN20, usually 10 min or more; and (mVmin) fl[A(t) - V(t)] dt is the integrated arteriove­ Cerebral 0 2 consumption 3.5 49 nous difference in N20 concentrations over the (mVmin) total period of inhalation. Cerebral glucose utilization 5.5 77 The partition coefficient for N20 is approx­ (mg/min) imately 1 when equilibrium has been achieved a Based on data derived from the literature, in Sokoloff between blood and brain tissue; at least 10 min [16). Chapter 31 Brain Circulation and Metabolism 651 sumes about 250 ml 0 2/min in the basal state. mainly excitation and conduction, and these are Therefore, the brain, which represents only reflected in the unceasing electrical activity of about 2% of total body weight, accounts for 20% the brain. The electrical energy ultimately is de­ of the resting total body 0 2 consumption. In rived from chemical processes, and it is likely children, the brain takes up an even larger frac­ that most of the energy consumption of the tion, as much as 50% in the middle of the first brain is used for active transport of ions to sus­ decade of life [15]. tain and restore the membrane potentials dis­ 0 2 is utilized in the brain almost entirely for charged during the processes of excitation and the oxidation of carbohydrate [16]. The energy conduction (see Chaps. 5 and 6). equivalent of the total cerebral metabolic rate Not all of the 0 2 consumption of the brain is, therefore, approximately 20 W, or 0.25 is used for energy metabolism. The brain con­ kcal/min. If it is assumed that this energy is uti­ tains a variety of oxidases and hydroxylases that lized mainly for the synthesis of high-energy function in the synthesis and metabolism of a phosphate bonds, that the efficiency of the en­ number of neurotransmitters. For example, ty­ ergy conservation is approximately 20% and that rosine hydroxylase is a mixed-function oxidase the free energy of hydrolysis of the terminal that hydroxylates tyrosine to 3,4-dihydroxy­ phosphate of ATP is approximately 7 kcal!mol, phenylalanine (DOPA), and dopamine 13-hy­ then this energy expenditure can be estimated to droxylase hydroxylates dopamine to form nor­ support the steady turnover of close to 7 mmol, epinephrine. Similarly, tryptophan hydroxylase or approximately 4 X 1021 molecules, of ATP hydroxylates tryptophan to form 5-hydroxy­ per minute in the entire human brain. The brain tryptophan in the pathway of serotonin synthe­ normally has no respite from this enormous en­ sis. The enzymes are oxygenases, which utilize ergy demand. Cerebral 0 2 consumption contin­ molecular 0 2 and incorporate it into the hy­ ues unabated day and night. Even during sleep droxyl group of the hydroxylated products. 0 2 there is only a relatively small decrease in cere­ also is consumed in the metabolism of these bral metabolic rate; indeed, it may even be in­ monoamine neurotransmitters, which are creased in rapid eye movement (REM) sleep (see deaminated oxidatively to their respective alde­ below). hydes by monoamine oxidases. All of these en­ zymes are present in brain, and the reactions cat­ The main energy-demanding functions alyzed by them use 0 2 • However, the total of the brain are those of ion flux related turnover rates of these neurotransmitters and to excitation and conduction the sum total of the maximal velocities of all ox­ idases involved in their synthesis and degrada­ The brain does not do mechanical work, like that tion can account for only a very small, possibly of cardiac and skeletal muscle, or osmotic work, immeasurable, fraction of the total 0 2 consump­ as the kidney does in concentrating urine. It does tion of brain. not have the complex energy-consuming metabolic functions of the liver nor, despite the Continuous cerebral circulation is synthesis of some hormones and neurotransmit­ absolutely required to provide sufficient ters, is it noted for its biosynthetic activities. oxygen Considerable emphasis has been placed on the extent of macromolecular synthesis in the cen­ Not only does the brain utilize 0 2 at a very rapid tral nervous system, an interest stimulated by the rate, but it is absolutely dependent on uninter­ recognition that there are some proteins with rupted oxidative metabolism for maintenance of short half-lives in brain. However, these repre­ its functional and structural integrity. There is a sent relatively small numbers of molecules, and large Pasteur effect in brain tissue, but even at its in fact, the average protein turnover and the rate maximal rate anaerobic glycolysis is unable to of protein synthesis in mature brain are slower provide sufficient energy. Since the 0 2 stored in than in most other tissues, except perhaps mus­ brain is extremely small compared with its rate cle. Clearly, the functions of nervous tissues are of utilization, the brain requires continuous re- 652 Part Five Metabolism

plenishment of its 0 2 by the circulation. If cere­ and low pH, which are products of metabolic ac­ bral blood flow is interrupted completely, con­ tivity, tend to dilate the blood vessels and in­ sciousness is lost within less than 10 sec, or the crease cerebral blood flow; changes in the oppo­ amount of time required to consume the 0 2 site direction constrict the vessels and decrease contained within the brain and its blood con­ blood flow [ 17]. Cerebral blood flow is regulated tent. Loss of consciousness as a result of anox­ through such mechanisms to maintain home­ emia, caused by anoxia or asphyxia, takes only a ostasis of these chemical factors in the local tis­ little longer because of the additional 0 2 present sue. The rates of production of these chemical in the lungs and the still-circulating blood. The factors depend on the rates of energy average critical level of 0 2 tension in brain tis­ metabolism; therefore, cerebral blood flow is ad­ sues, below which consciousness and the normal justed to the cerebral metabolic rate [ 17]. EEG pattern are invariably lost, lies between 15 and 20 mm Hg. This seems to be so whether the Local rates of cerebral blood flow and tissue anoxia is achieved by lowering the cerebral metabolism can be measured by blood flow or the arterial oxygen content. Cessa­ autoradiography and are coupled to tion of cerebral blood flow is followed within a local brain function few minutes by irreversible pathological changes within the brain, readily demonstrated by mi­ The rates of blood flow and metabolism pre­ croscopic anatomical techniques. In medical sented in Table 31-1 and discussed above repre­ crises, such as cardiac arrest, damage to the brain sent the average values in the brain as a whole. occurs earliest and is most decisive in determin­ The brain is not homogeneous, however; it is ing the degree of recovery. composed of a variety of tissues and discrete Cerebral blood flow must be able to main­ structures that often function independently or tain the avaricious appetite of the brain for 0 2• even inversely with respect to one another. The average rate of blood flow in the human There is little reason to expect that their perfu­ brain as a whole is approximately 57 ml/100 g sion and metabolic rates would be similar. In­ tissue/min (see Table 31-1). For a whole brain deed, experiments clearly indicate that they are this amounts to almost 800 ml/min, or approxi­ not. Local cerebral blood flow in laboratory ani­ mately 15% of the total basal cardiac output. mals has been determined from the local tissue This must be maintained within relatively nar­ concentrations, measured by a quantitative au­ row limits, for the brain cannot tolerate any ma­ toradiographic technique, and from the total jor drop in its perfusion. A fall in cerebral blood history of the arterial concentration of a freely flow to half its normal rate is sufficient to cause diffusible, chemically inert, radioactive tracer in­ loss of consciousness in normal, healthy, young troduced into the circulation [18]. The results men. There are, fortunately, numerous reflexes reveal that blood-flow rates vary widely and other physiological mechanisms to sustain throughout the brain, with average values in adequate le_;els of arterial blood pressure at the gray matter approximately four to five times head level, such as the baroreceptor reflexes, and those in white matter [18]. to maintain cerebral blood flow, even when arte­ A method has been devised to measure rial pressure falls in times of stress for example, glucose consumption in discrete functional and autoregulation. There are also mechanisms to structural components of the brain in intact, adjust cerebral blood flow to changes in cerebral conscious laboratory animals [19]. This metabolic demand. method also employs quantitative autoradiog­ Regulation of cerebral blood flow is raphy to measure local tissue concentrations achieved mainly by control of the tone or the de­ but utilizes 2-deoxy-o-[14C]glucose as the gree of constriction, or dilation, of the cerebral tracer. The local tissue accumulation of vessels. This in turn is controlled mainly by local [ 14C]deoxyglucose as [ 14C]deoxy-G6P in a chemical factors, such as PaC02, Pa02, pH and given interval of time is related to the amount others still unrecognized. High PaC02, low Pa02 of glucose that has been phosphorylated by Chapter 31 Brain Circulation and Metabolism 653

A

Plasma Brain Tlsaue Precursor Pool Metabolic Products

[14C]Giycoli£ids [ 14C]Deoxyglycog~ C]Giycoproteins

[14C]UDPDG 1l [14C]Deoxyglucose -1-Phosphate

kj k; 1l [14C]Oeoxyglucose [14C]Deoxyglucose -[14C]Deoxyglucose 6-Phosphate (Cj.) t: k2 (C~) : (C;,.) &I ' c ' 2! Total Tissue 14f Concentration= c; = C~ + C~ Ill ' k, k3 : Glucose::;::::: 8iii ~Glucose~ Glucose 6-Phosphate (Cp) ko (C.) •, ' ' ' (CM) --- .. ~.-- ' ' ' t- ' ' ' t ' - ' ' t :' C02 + H20

FIGURE 31-3. Theoretical basis of radioactive deoxyglucose method for measurement of local cerebral glucose uti­ lization. A: Theoretical model. Ci represents the totai'4C concentration in a single homogeneous tissue of the brain. C~ and Cp represent the concentrations of ['4C}deoxyglucose and glucose in the arterial plasma, respectively; C£ and CE represent their respective concentrations in the tissue pools that serve as substrates for hexokinase. CM represents the concentration of (14()deoxyglucose-6-phosphate in the tissue. k; k2 and k3 represent the rate constants for car­ rier-mediated transport of [14C)deoxyglucose from plasma to tissue, for carrier-mediated transport back from tissue to plasma and for phosphorylation by hexokinase, respectively. k 1, k2 and k3 are the equivalent rate constants for glu­ cose. [14C]Deoxyglucose and glucose share and compete for the carrier that transports them both between plasma and tissue and for hexokinase, which phosphorylates them to their respective hexose-6-phosphates. Dashed arrow represents the possibility of glucose-6-phosphate hydrolysis by glucos.e-6-phosphatase activity, if any. UDPDG, UDP­ deoxyglucose. (Figure continues on next page.} hexokinase over the same interval, and the rate oxyglucose and glucose. The method is based of glucose consumption can be determined on a kinetic model of the biochemical behavior from the [ 14C] deoxy-G6P concentration by ap­ of 2-deoxyglucose and glucose in brain. The propriate consideration of (i) the relative con­ model (diagrammed in Fig. 31-3) has been an­ centrations of [ 14C]deoxyglucose and glucose alyzed mathematically to derive an operational in the plasma, (ii) their rate constants for trans­ equation that presents the variables to be mea­ port between plasma and brain tissue and (iii) sured and the procedure to be followed to de­ the kinetic constants of hexokinase for de- termine local cerebral glucose utilization. 654 Part Five Metabolism

B General equation for measurement of reaction rates with tracers: Labeled Product Formed in Interval of Time, 0 to T Rate of reaction= [ Isotope effect J [ Integrated specific activity J correction factor of precursor

Operational equation of [ 1.C]deoxyglucose method: labeled Product formed in Interval of Time, 0 to T

Total ••c in tissue ••c in precursor at timeT remaining in tissue at time T

~

c: V. K. 0 C, 0 C, ,____,___, Isotope effect Integrated plasma Correction for lag in tissue correction specific activity equilibration with plasma factor

FIGURE 31·3. (Continued.) B: Functional anatomy of the operational equation of the radioactive deoxyglucose method. Trepresents the time at the termination of the experimental period;>.. equals the ratio of the distribution space of deoxyglucose in the tissue to that of glucose; ¢ equals the fraction of glucose that, once phosphorylated, contin­ ues down the glycolytic pathway; K~, V~ and Km, Vm represent the familiar Michaelis-Menten kinetic constants of hex­ okinase for deoxyglucose and glucose, respectively. Other symbols are the same as those defined in A. (From [19].)

To measure local glucose utilization, a pulse concentrations of (14C]deoxy-G6P. The autora­ of ("C]deoxyglucose is administered intra­ diographs, therefore, are pictorial representations venously at time zero and timed arterial blood of the relative rates of glucose utilization in all of samples are drawn for determination of the the structural components of the brain. plasma [ 14CJdeoxyglucose and glucose concentra­ Autoradiographs of the striate cortex in tions. At the end of the experimental period, usu­ monkey in various functional states are illus­ ally about 45 min, the animal is decapitated, the trated in Figure 31-4. This method has demon­ brain is removed and frozen and 20-f.1m-thick strated that local cerebral consumption of glu­ brain sections are autoradiographed on X-ray film cose varies as widely as blood flow throughout along with calibrated (14C]methyl-methacrylate the brain (Table 31-2). Indeed, in normal ani­ standards. Local tissue concentrations of 14C are mals, there is remarkably close correlation be­ determined by quantitative densitometric analysis tween local cerebral blood flow and glucose con­ of the autoradiographs. From the time courses of sumption (20]. Changes in functional activity the arterial plasma ( 14C]deoxyglucose and glucose produced by physiological stimulation, anesthe­ concentrations and the final tissue 14C concentra­ sia or deafferentation result in corresponding tions, determined by quantitative autoradiogra­ changes in blood flow and glucose consumption phy, local glucose utilization can be calculated by (21] in the structures involved in the functional means of the operational equation for all compo­ change. The (14C]deoxyglucose method for the nents of the brain identifiable in the autoradio­ measurement of local glucose utilization has graphs. The procedure is designed so that the au­ been used to map the functional visual pathways toradiographs reflect mainly the relative local and to identify the locus of the visual cortical Chapter 31 Brain Circulation and Metabolism 655

A FIGURE 31-4. Autoradiograms of coronal brain sections from rhesus monkeys at the level of the striate cortex. A: Animal with normal binoc­ ular vision. Note the laminar distribu­ tion of the density; the dark band corresponds to layer IV. B: Animal with bilateral visual deprivation. Note the almost uniform and reduced rela­ tive density, especially the virtual dis­ appearance of the dark band corre­ sponding to layer IV. C: Animal with right eye occluded. The half-brain on the left represents the left hemi­ sphere contralateral to the occluded eye. Note the alternate dark and light striations, each approximately 0.3 to 0.4 mm in width, representing the ocular dominance columns. These columns are most apparent in the dark lamina corresponding to layer IV but extend through the entire thick­ ness of the cortex. Arrows point to regions of bilateral asymmetry, where the ocular dominance columns are absent. These are pre­ sumably areas that normally have only monocu lar input. The one on the left, contralateral to the oc­ cluded eye, has a continuous dark lamina corresponding to layer IV that is completely absent on the side ipsi­ lateral to the occluded eye. These re­ >----< gions are believed to be the loci of the cortical representations of the 5.0mm blind spots. (From [21).) 656 Part Five Metabolism

REPRESENTATIVE VALUES• SUBSTRATES OF CEREBRAL FOR LOCAL CEREBRAL GLU­ METABOLISM COSE UTILIZATION IN THE NORMAL CONSCIOUS ALBINO Normally, the substrates are glucose and oxygen and the products are carbon dioxide and water Monkeyo: In contrast to most other tissues, which exhibit Gray matter considerable flexibility with respect to the nature Visual cortex 107 :!:: 6 59 :!: 2 Auditory cortex 162 :!: 5 79 :!: 4 of the foodstuffs extracted and consumed from Parietal cortex 112:!: 5 47 :!: 4 the blood, the normal brain is restricted almost Sensorimotor cortex 120 ± 5 44 ± 3 exclusively to glucose as the substrate for its en­ Thalamus: lateral nucleus 116 "' 5 54 "' 2 ergy metabolism. Despite long and intensive ef­ Thalamus: ventral nucleus 109 "' 5 43 "' 2 forts, the only incontrovertible and consistently Medial geniculate body 131 "' 5 65 "' 3 Lateral geniculate body 96 "' 5 39 ± 1 positive arteriovenous differences demonstrated Hypothalamus 54 ± 2 25 ± 1 for the human brain under normal conditions Mammillary body 121 "'5 57::!: 3 have been for glucose and oxygen [16]. Negative Hippocampus 79 ± 3 39 ± 2 arteriovenous differences, significantly different Amygdala 52 "' 2 25 "' 2 from zero, have been found consistently only for Caudate putamen 110 ± 4 52 "' 3 Nucleus accumbens 82 "' 3 36 ± 2 C02, although water, which has never been mea­ Globus pallidus 58 "' 2 26 ± 2 sured, also is produced. Pyruvate and lactate Substantia nigra 58 :!:: 3 29 :!:: 2 production have been observed occasionally, Vestibular nucleus 128 "' 5 66"' 3 certainly in aged subjects with cerebral vascular Cochlear nucleus 113 ± 7 51 ± 3 insufficiency but also irregularly in subjects with Superior olivary nucleus 133 ::!:: 7 63 :!:: 4 Inferior colliculus 197 :!:: 10 103 ::!:: 6 normal oxygenation of the brain. Superior colliculus 95 :!:: 5 55 :!:: 4 In the normal in vivo state, glucose is the Pontine gray matter 62 ::!::: 3 28 ::!:: 1 only significant substrate for energy metabolism Cerebellar cortex 57::!:: 2 31 "' 2 in the brain. Under normal circumstances, no Cerebellar nuclei 100 ± 4 45 ± 2 other potential energy-yielding substance has White matter been found to be extracted from the blood in Corpus callosum 40 ± 2 11 ± 1 Internal capsule 33 ± 2 13 ± 1 more than trivial amounts. The stoichiometry of Cerebellar white matter 37 ± 2 12 ± 1 glucose utilization and 0 2 consumption is sum­ Weighted average for whole brain marized in Table 31-3. The normal, conscious 68 ± 3 36 ± 1 human brain consumes oxygen at a rate of 156 fLmol/100 g tissue/min. C02 production is the a Values are the means plus or minus standard errors from measurements made in ten rats or seven monkeys. same, leading to a respiratory quotient (RQ) of b From Sokoloff and co4 workers [19]. 1.0, further evidence that carbohydrate is the ul­ c From Kennedy and co4 workers [22]. timate substrate for oxidative metabolism. 0 2 consumption and C02 production are equivalent to a rate of glucose utilization of 26 fLmol/100 g representation of the retinal "blind spot" in the tissue/min, assuming 6 J.Lmol of 0 2 consumed brain of rhesus monkey [21] (Fig. 31-4). These and of C02 produced for each micromole of glu­ results establish that local energy metabolism in cose completely oxidized to C02 and H20. The the brain is coupled to local functional activity actual glucose utilization measured is, however, and confirm the long-held belief that local cere­ 31 fLmol/100 glmin, which indicates that glucose bral blood flow is adjusted to metabolic demand consumption not only is sufficient to account for in local tissue. The method has been applied to total 0 2 consumption but is in excess by 5 humans by use of2-[18F]-fluoro-2-deoxy-o-glu­ fLmol/100 glmin. For complete oxidation of glu­

cose and positron emission tomography (PET), cose, the theoretical ratio of 0 2: glucose utiliza­ with similar results (see Chap. 54). tion is 6.0; the excess glucose utilization is re- Chapter 31 Brain Circulation and Metabolism 657

RELATIONSHIP BETWEEN This does not imply that the pathways of CEREBRAL OXYGEN CON­ glucose metabolism in the brain lead, like com­ SUMPTION AND GLUCOSE bustion, directly and exclusively to production UTILIZATION IN A NORMAL of C02 and H20. Various chemical and energy YOUNG ADULT MAN transformations occur between uptake of the primary substrates glucose and 0 2 and liberation Valuea of the end products C02 and H20. Compounds 0 2 consumption (j.LmoV1 00 g brain 156 derived from glucose or produced through the tissue/min) energy made available from glucose catabolism Glucose utilization {j.LmoV1 00 g 31 brain tissue/min) are intermediates in the process. Glucose carbon is incorporated, for example, into amino acids, 0 2 :glucose ratio (moVmol) protein, lipids and glycogen. These are turned Glucose equivalent of 0 2 consumption 26' {1-Lmol glucose/1 00 g brain tissue/min) over and act as intermediates in the overall path­

C02 production (j.LmoV1 00 g brain 156 way from glucose to C02 and H20. There is clear tissue/min) evidence from studies with [ 14C]glucose that not Cerebral respiratory quotient (RQ) 0.97 all of the glucose is oxidized directly and that at any given moment some of the C0 being pro­ From Sokoloff [16[. 2 a Values are the median of values reported in the litera­ duced is derived from sources other than glucose ture. that enter the brain at the same moment or just b Calculated on the basis of 6 mol of 0 2 required for prior to that moment. That 0 and glucose con­ complete oxidation of 1 mol of glucose. 2 sumption and C02 production are essentially in stoichiometric balance and that no other energy­ sponsible for a measured ratio of only 5.5 f.Lmol laden substrate is taken from the blood means, 0 2/f.Lmol glucose. The fate of the excess glucose is however, that the net energy made available to unknown, but it probably is distributed in part in the brain ultimately must be derived from the lactate, pyruvate and other intermediates of car­ oxidation of glucose. It should be noted that this bohydrate metabolism, each released from the is the situation in the normal state; as discussed brain into the blood in insufficient amounts to be later, other substrates may be used in special cir­ detectable as significant arteriovenous differ­ cumstances or in abnormal states. ences. Some of the glucose must be utilized not for the production of energy but for synthesis of In brain, glucose utilization is obligatory the chemical constituents of brain. Some oxygen is used for oxidation of sub­ The brain normally derives almost all of its en­ stances not derived from glucose, for example, in ergy from the aerobic oxidation of glucose, but the synthesis and metabolic degradation of this does not distinguish between preferential monoamine neurotransmitters, as mentioned and obligatory utilization of glucose. Most tis­ above. The amount of 0 2 used for these pro­ sues are largely facultative in their choice of sub­ cesses is, however, extremely small and is unde­ strates and can use them interchangeably more tectable in the presence of the enormous 0 2 con­ or less in proportion to their availability. This sumption used for carbohydrate oxidation. does not appear to be so in the brain. Except in The combination of a cerebral RQ of unity, some unusual and very special circumstances, an almost stoichiometric relationship between only the aerobic utilization of glucose is capable 0 2 uptake and glucose consumption and the ab­ of providing the brain with sufficient energy to sence of any significant arteriovenous difference maintain normal function and structure. The for any other energy-rich substrate is strong evi­ brain appears to have almost no flexibility in its dence that the brain normally derives its energy choice of substrates in vivo. This conclusion is from the oxidation of glucose. Thus, cerebral derived from the following evidence. metabolism is unique because no other tissue, except for testis [23], relies only on carbohydrate Glucose deprivation is followed rapidly by aber­ for energy. rations of cerebral function. Hypoglycemia, pro- 658 Part Five Metabolism duced by excessive insulin or occurring sponta­ bohydrate, presumably derived from the en­ neously in hepatic insufficiency, is associated dogenous carbohydrate stores of the brain. The with changes in mental state ranging from mild, effects are clearly the result of hypoglycemia and subjective sensory disturbances to coma, the not of insulin in the brain. In all cases, the be­ severity depending on both the degree and the havioral, functional and cerebral metabolic ab­ duration of the hypoglycemia. The behavioral normalities associated with insulin hypo­ effects are paralleled by abnormalities in EEG glycemia are reversed rapidly and completely by patterns and cerebral metabolic rate. The EEG the administration of glucose. The severity of the pattern exhibits increased prominence of slow, effects is correlated with the degree of hypo­ high-voltage &rhythms, and the rate of cerebral glycemia and not the insulin dosage, and the ef­ oxygen consumption falls. In studies of the ef­ fects of insulin can be prevented completely by fects of insulin hypoglycemia in humans [24], it the simultaneous administration of glucose with was observed that when arterial glucose fell from the insulin. a normal concentration of70 to 100 mg/100 ml Similar effects are observed in hypo­ to an average of 19 mg/100 ml, subjects became glycemia produced by other means, such as hep­ confused and cerebral oxygen consumption fell atectomy. Inhibition of glucose utilization at the to 2.6 ml/100 glmin, or 79% of normal. When phosphohexose isomerase step with pharmaco­ arterial glucose fell to 8 mg/100 ml, a deep coma logical doses of2-deoxyglucose also produces all ensued and cerebral 0 2 consumption decreased of the cerebral effects of hypoglycemia despite an even further to 1.9 ml/100 glmin (Table 31-4). associated elevation in blood glucose content. These changes are not caused by insuffi­ cient cerebral blood flow, which actually in­ In hypoglycemia, substrates other than glucose creases slightly during coma. In the depths of may be utilized. The hypoglycemic state pro­ coma, when the blood glucose content is very vides convenient test conditions to determine low, there is almost no measurable cerebral up­ whether a substance is capable ofsubstituting for take of glucose from the blood. Cerebral 0 2 con­ glucose as a substrate of cerebral energy sumption, although reduced, is still far from metabolism. If it can, its administration during negligible; and there is no longer any stoichio­ hypoglycemic shock should restore conscious­ metric relationship between glucose and 0 2 up­ ness and normal cerebral electrical activity with­ takes by the brain, evidence that the 0 2 is utilized out raising the blood glucose concentration. Nu­ for the oxidation of other substances. The cere­ merous potential substrates have been tested in bral RQ remains approximately l, however, in­ humans and animals. Very few can restore nor­ dicating that these other substrates are still car- mal cerebral function in hypoglycemia, and of

EFFECTS OF INSULIN HYPOGLYCEMIA ON CEREBRAL CIRCULATION AND METABOLISM IN HUMANS• Insulin-induced hypoglycemia Insulin-induced Control without coma hypoglycemic coma Arterial blood Glucose concentration (mg%) 74 19 0 2 content (vol%) 17.4 17.9 16.6 Mean blood pressure (mm Hg) 94 86 93 Cerebral circulation Blood flow (mV1 00 g/min) 58 61 63 0 2 consumption (mV1 00 g/min) 3.4 2.6 1.9 Glucose consumption (mg/1 00 g/min) 4.4 2.3 0.8 Respiratory quotient 0.95 1.10 0.92

" from Kety et al. [24}. Chapter 31 Brain Circulation and Metabolism 659 these all but one appear to operate through a va­ glycogenolysis and the elevation of blood glu­ riety of mechanisms to raise the blood glucose cose concentration. Glutamate, arginine, concentration rather than by serving directly as a glycine, GABA and succinate also act through substrate (Table 31-5). adrenergic effects that raise glucose concentra­ Mannose appears to be the only substance tions of the blood [16]. that can be utilized by the brain directly and It should be noted, however, that failure to rapidly enough to restore or maintain normal restore normal cerebral function in hypo­ function in the absence of glucose [25]. It tra­ glycemia is not synonymous with an inability of verses the BBB and is converted to mannose-6- the brain to utilize the substance. Many of the phosphate. This reaction is catalyzed by hexoki­ substances that have been tested and found inef­ nase as effectively as the phosphorylation of fective are compounds normally formed and uti­ glucose. The mannose-6-phosphate is then con­ lized within the brain and are normal intermedi­ verted to fructose-6-phosphate by phospho­ ates in its intermediary metabolism. Lactate; mannose isomerase, which is active in brain tis­ pyruvate; fructose-1,6-bisphosphate; acetate; 13- sue. Through these reactions mannose can enter hydrox:ybutyrate; and acetoacetate can be uti­ directly into the glycolytic pathway and replace lized by brain slices, homogenates or cell-free glucose. fractions; and the enzymes for their metabolism Maltose also is effective occasionally in are present in brain. Enzymes for the restoring normal behavior and EEG activity in metabolism of glycerol or ethanol, for example, hypoglycemia but only by raising the blood glu­ may not be present in sufficient amounts. For cose concentration through its conversion to other substrates, for example, n-13-hydroxybu­ glucose by maltase activity in blood and other tyrate and acetoacetate, the enzymes are ade­ tissues [16]. Epinephrine is effective at produc­ quate but the substrate is not available to the ing arousal from insulin coma, but this is brain because of inadequate blood levels or re­ achieved through its well-known stimulation of stricted transport through the BBB.

EFFECTIVENESS OF VARIOUS SUBSTANCES IN PREVENTING OR REVERSING THE EFFECTS OF HYPOGLYCEMIA OR GLUCOSE DEPRIVATION ON CEREBRAL FUNCTION AND METABOLISM• Substance Comments Epinephrine Raises blood glucose concentration Maltose Converted to glucose and raises blood glucose concentration Man nose Directly metabolized and enters glycolytic pathway Partially or occasionally Glutamate Occasionally effective by raising blood glucose effective Arginine concentration Glycine p-Aminobenzoate Succinate Ineffective Glycerol Some of these substances can be metabolized Ethanol to various extents by brain tissue and conceivably lactate could be effective if it were not for the Glyceraldehyde blood-brain barrier Hexose diphosphates Fumarate Acetate [3-Hydroxybutyrate Galactose Lactose Insulin

aSummarized from the literature, in Sokoloff [ 16] 660 Part Five Metabolism

Nevertheless, nervous system function in Cerebral utilization of ketone bodies ap­ the intact animal depends on substrates supplied pears to follow passively their concentrations in by the blood, and no satisfactory, normal, en­ arterial blood [27]. In normal adults, ketone dogenous substitute for glucose has been found. concentrations are very low in blood and cere­ Glucose, therefore, must be considered essential bral utilization of ketones is negligible. In ke­ for normal physiological behavior of the central totic states resulting from starvation; fat-feed­ nervous system. ing or ketogenic diets; diabetes; or any other condition that accelerates the mobilization and The brain utilizes ketones in states of catabolism of fat, cerebral utilization of ketones ketosis is increased more or less in direct proportion to the degree of ketosis [27]. Significant utiliza­ In special circumstances, the brain may fulfill its tion of ketone bodies by the brain is, however, nutritional needs partly, although not com­ normal in the neonatal period. The newborn pletely, with substrates other than glucose. Nor­ infant tends to be hypoglycemic but becomes mally, there are no significant cerebral arteri­ ketotic when it begins to nurse because of the ovenous differences for o-13-hydroxybutyrate high fat content of the mother's milk. When and acetoacetate, which are "ketone bodies" weaned onto the normal, relatively high-carbo­ formed in the course of the catabolism of fatty hydrate diet, the ketosis and cerebral ketone acids by liver. Owen and coworkers [26] ob­ utilization disappear. Studies have been carried served, however, that when human patients out mainly in the infant rat, but there is evi­ were treated for severe obesity by complete fast­ dence that the situation is similar in the human ing for several weeks, there was considerable up­ infant. take of both substances by the brain. If one as­ The first two enzymes in the pathway of ke­ sumed that the substances were oxidized tone utilization are o-13-hydroxybutyrate dehy­ completely, their rates of utilization would have drogenase and acetoacetyl-succinyl-CoA trans­ accounted for more than 50% of the total cere­ ferase. These exhibit a postnatal pattern of bral oxygen consumption, more than that ac­ development that is well adapted to the nutri­ counted for by the glucose uptake. o-13-hydroxy­ tional demands of the brain. At birth, the activ­ butyrate uptake was several times greater than ity of these enzymes in the brain is low; activity that of acetoacetate, a reflection of its higher rises rapidly with the ketosis that develops with concentration in the blood. The enzymes re­ the onset of suckling, reaches its peak just before sponsible for their metabolism, o-13-hydroxy­ weaning and then gradually declines after wean­ butyrate dehydrogenase, acetoacetate-succinyl­ ing to normal adult rates of approximately one­ CoA transferase and acetoacetyl-CoA-thiolase, third to one-fourth the maximal rates attained are present in brain tissue in sufficient amounts [27,28]. to convert them into acyl-CoA and to feed them It should be noted that o-13-hydroxybu­ into the tricarboxylic acid cycle at a sufficient tyrate is incapable of maintaining or restoring rate to satisfy the metabolic demands of the normal cerebral function in the absence of glu­ brain [27]. cose in the blood. This suggests that, although it Under normal circumstances, that is, am­ can partially replace glucose, it cannot fully sat­ ple glucose and few ketone bodies in the blood, isfy the cerebral energy needs in the absence of the brain apparently does not oxidize ketones some glucose consumption. One explanation in any significant amounts. In prolonged star­ may be that the first product ofo-13-hydroxybu­ vation, the carbohydrate stores of the body are tyrate oxidation, acetoacetate, is metabolized exhausted and the rate of gluconeogenesis is in­ further by its displacement of the succinyl moi­ sufficient to provide glucose fast enough to ety of succinyl-CoA to form acetoacetyl-GoA. A meet the requirements of the brain; blood ke­ certain rate of glucose utilization may be essen­ tone concentrations rise as a result of the rapid tial to drive the tricarboxylic cycle, to provide fat catabolism. The brain then apparently turns enough succinyl-CoA to permit the further oxi­ to the ketone bodies as the source of its energy dation of acetoacetate and, hence, to pull along supply. the oxidation ofo-13-hydroxybutyrate. Chapter 31 Brain Circulation and Metabolism 661

AGE AND DEVELOPMENT from the middle of the first decade of life to old INFLUENCE CEREBRAL ENERGY age are summarized in Table 31-6. By 6 years of METABOLISM age, cerebral blood flow and oxygen consump­ tion already have attained high rates, and they Metabolic rate increases during early decline thereafter to the rates of normal young development adulthood [15]. Cerebral oxygen consumption of5.2 ml/100 g brain tissue/min in a 5- to 6-year­ The energy metabolism of the brain and the blood old child corresponds to total oxygen consump­ flow that sustains it vary considerably from birth tion by the brain ofapproximately 60 ml!min, or to old age. Data on the cerebral metabolic rate ob­ more than 50% of the total body basal oxygen tained directly in vivo are lacking for the early post­ consumption, a proportion markedly greater natal period, but the results of in vitro measure­ than that occurring in adulthood. The reasons ments in animal brain preparations and inferences for the extraordinarily high cerebral metabolic drawn from cerebral blood flow measurements in rates in children are unknown, but presumably intact animals [29] suggest that cerebral oxygen they reflect the extra energy requirements for the consumption is low at birth, rises rapidly during biosynthetic processes associated with growth the period of cerebral growth and development and development. and reaches a maximal level at about the time mat­ uration is completed. This rise is consistent with Tissue pathology, but not aging, the progressive increase in the levels of a number produces secondary changes in of enzymes of oxidative metabolism in the brain. metabolic rate The rate ofblood flow in different structures of the brain reaches peak levels at different times, de­ Whole-brain cerebral blood flow and oxygen pending on the maturation rate of the particular consumption normally remain essentially un­ structure. In structures that consist predominantly changed between young adulthood and old age. of white matter, the peaks coincide roughly with In a population of normal elderly men in their maximal rates of myelination. From these peaks, eighth decade of life, who were selected carefully blood flow and, probably, cerebral metabolic rate for good health and freedom from all disease in­ decline to the levels characteristic of adulthood. cluding vascular disease, both blood flow and oxygen consumption were not significantly dif­ Metabolic rate declines and plateaus ferent from rates of normal young men SO years after maturation younger (see Table 31-6) [30]. In a comparable group of elderly subjects, who differed only by Reliable quantitative data on the changes in cere­ the presence ofobjective evidence of minimal ar­ bral circulation and metabolism in humans teriosclerosis, cerebral blood flow was signifi-

AGE AND SENILITY•

Cerebral Cerebral 0 2 Cerebral venous Age blood flow consumption 0 2 tension life period and condition (years) (ml/100 g/min) (ml/100 g/min) (mmHg) Childhood fib 1Q6b 5.2b Normal young adulthood 21 62 3.5 38 Aged Normal elderly 71 ' 58 3.3 36 Elderly with minimal arteriosclerosis 73b 48b 3.2 33b,c Elderly with senile psychosis 72' 48b,c 2.7b.c 33b.c •From Kennedy and Sokoloff [151 and Sokoloff [301. bStatistically significant difference from normal young adult (p < 0.05). cStatistically significant difference from normal elderly subjects (p < 0.05). 662 Part Five Metabolism cantly lower. It had reached a point at which the function [21]. For example, diminished visual or oxygen tension of the cerebral venous blood de­ auditory input depresses glucose utilization in clined, an indication of relative cerebral hypoxia. all components of the central visual or auditory However, normal cerebral oxygen consumption pathways, respectively (Fig. 31-4). Focal seizures was maintained through extraction of larger increase glucose utilization in discrete compo­ than normal proportions of the arterial blood nents of the motor pathways, such as the motor oxygen. In senile, psychotic patients with arte­ cortex and the basal ganglia (Fig. 31-5). riosclerosis, cerebral blood flow was no lower Convulsive activity, induced or sponta­ but cerebral oxygen consumption had declined. neous, often has been employed as a method of These data suggest that aging per se need not increasing electrical activity of the brain (see lower cerebral oxygen consumption and blood Chap. 37). Davies and Remand [13] used the flow but that, when blood flow is reduced, it is oxygen electrode technique in the cerebral cor­ probably secondary to vascular disease, which tex of cat and found increases in oxygen con­ produces cerebral vascular insufficiency and sumption during electrically induced or drug­ chronic relative hypoxia in the brain, or to an­ induced convulsions. Because the increased other tissue pathology that decreases function, oxygen consumption either coincided with or such as dementia (see Chap. 46). Because arte­ followed the onset of convulsions, it was con­ riosclerosis and Alzheimer's disease are so preva­ cluded that the elevation in metabolic rate was lent in the aged population, most individuals the consequence of the increased functional ac­ probably follow the latter pattern. However, age­ tivity produced by the convulsive state (see related changes in local regulation are possible Chaps. 37 and 54). and are the subject of research (see Chap. 30). Metabolic rate and nerve conduction are related directly CEREBRAL METABOLIC RATE IN VARIOUS PHYSIOLOGICAL STATES The [ 14C] deoxyglucose method has defined the nature and mechanisms of the relationship be­ tween energy metabolism and functional activity Cerebral metabolic rate is determined in nervous tissues. Studies in the superior cervi­ locally by functional activity in discrete cal ganglion of the rat have shown almost a di­ regions rect relationship between glucose utilization in the ganglion and spike frequency in the afferent In organs such as heart or skeletal muscle that fibers from the cervical sympathetic trunk [31]. perform mechanical work, increased functional A spike results from the passage of a finite cur­ activity clearly is associated with increased rent ofNa + into the cell and ofK+ out of the cell, metabolic rate. In nervous tissues outside the ion currents that degrade the ionic gradients re­ central nervous system, electrical activity is an sponsible for the resting membrane potential. almost quantitative indicator of the degree of Such degradation can be expected to stimulate functional activity; and in structures such as Na,K-ATPase activity to restore the ionic gradi­ sympathetic ganglia and postganglionic axons, ents to normal, and such ATPase activity in tum increased electrical activity produced by electri­ would stimulate energy metabolism. Indeed, cal stimulation definitely is associated with in­ Mata et al. [32] have found that, in the posterior creased utilization of oxygen. Within the central pituitary in vitro, stimulation of glucose utiliza­ nervous system, local energy metabolism also is tion due to either electrical stimulation or open­ correlated closely with the level of local func­ ing of Na + channels in the excitable membrane tional activity. Studies using the [14C]deoxyglu­ by veratridine is blocked by ouabain, a specific cose method have demonstrated pronounced inhibitor ofNa,K-ATPase activity (see Chap. 5). changes in glucose utilization associated withal­ Most, if not all, of the stimulated energy tered functional activity in discrete regions of the metabolism associated with increased functional central nervous system specifically related to that activity is confined to the axonal terminals FIGURE 31-5. Local glucose utiliza~ tion during penicillin~induced focal seizures. Penicillin was applied to the hand and face area of the left motor cortex of a rhesus monkey. The left side of the brain is on the left in each of the autoradiograms in the figure. Numbers are rates of local cerebral glucose uti­ lization in micromoles per 100 g tissue per minute. Note the following: upper left motor cortex in region of penicillin application and corresponding region of contralateral motor cortex; lower left ipsilateral and contralateral motor cortical regions remote from area of penicillin applications; upper right, ip­ silateral and contralateral putamen and globus pallidus; lower right, ipsilateral and contralateral thalamic nuclei and substant ia nigra. (From [21 }.) 664 Part Five Metabolism

100

Means ±SEM (n) =Number of animals 80 c E 0 0 "' 60 0 "'E 6 c: 0 ~ ~ :s 40 Q) (12) "'0 ":J Dorsal root a ganglion 20

0 0 5 10 15 Frequency of stimulation (Hz)

FIGURE 31·6. Effects of electrical stimulation of sciatic nerve on glucose utilization in the terminal zones in the dor­ sal horn of the spinal cord and in the cell bodies in the dorsal root ganglion. (From [331.) rather than to the cell bodies in a functionally ac­ fashionable to attribute a high demand for men­ tivated pathway (Fig. 31-6) [33]. Astrocytes also tal effort to the process of problem solving in contribute to the increased metabolism [34]. mathematics. Nevertheless, there appears to be no increased energy utilization by the brain dur­ It is difficult to define metabolic ing such processes. From resting levels, total equivalents of consciousness, mental cerebral blood flow and oxygen consumption re­ work and sleep main unchanged during the exertion of the mental effort required to solve complex arith­ Mental work. Convincing correlations between metical problems [35]. It may be that the as­ cerebral metabolic rate and mental activity have sumptions that relate mathematical reasoning to been obtained in humans in a variety of patho­ mental work are erroneous, but it seems more logical states of altered consciousness [35]. Re­ likely that the areas that participate in the pro­ gardless of the cause of the disorder, graded re­ cesses of such reasoning represent too small a ductions in cerebral oxygen consumption are fraction of the brain for changes in their ftmc­ accompanied by parallel graded reductions in tional and metabolic activities to be reflected in the degree of mental alertness, all the way to pro­ the energy metabolism of the brain as a whole. found coma (Table 31-7). It is difficult to define or even to conceive of the physical equivalent of Sleep. Physiological sleep is a naturally occur­ mental work. A common view equates concen­ ring, periodic, reversible state of unconscious­ trated mental effort with mental work, and it is ness; and the EEG pattern in deep, slow-wave Chapter 31 Brain Circulation and Metabolism 665

RELATIONSHIP BETWEEN relatively narrow range under physiological con­ LEVEL OF CONSCIOUSNESS ditions. There are, however, a number of patho­ logical states of the nervous system and other or­ gans that affect the functions of the brain either directly or indirectly, and some of these have Cerebral Cerebral 0 2 blood flow consumption profound effects on cerebral metabolism. Level of (ml/100 (ml/100 consciousness g/min) g/min) Mentally alert 54 3.3 Psychiatric disorders may produce Normal young men effects related to anxiety Mentally confused 48 2.8 Brain tumor In general, disorders that alter the quality of Diabetic acidosis mentation but not the level of consciousness Insulin (described in Chaps. 51 and 52), such as the hypoglycemia Cerebral functional neuroses, psychoses and psy­ arteriosclerosis chotomimetic states, have no apparent effect on Comatose 57 2.0 the average blood flow and oxygen consumption Brain tumor of the brain as a whole. Thus, no gross changes in Diabetic coma either function are observed in schizophrenia Insulin coma [35] or LSD intoxication (Table 31-8) [35]. Anesthesia There is still uncertainty about the effects of anx­ • From Sokoloff [35]. iety, mainly because of the difficulties in evaluat­ ing quantitatively the intensity. It is generally sleep is characterized by high-voltage, slow believed that ordinary degrees of anxiety, rhythms very similar to those often seen in or "nervousness," do not affect the cerebral pathological comatose states. As found in the metabolic rate but that severe anxiety, or pathological comatose states, cerebral glucose "panic," may increase cerebral oxygen con­ metabolism is depressed more or less uniformly sumption [35]. This may be related to the level throughout the brain of rhesus monkeys in of epinephrine circulating in the blood. Small stages 2 to 4 of normal sleep studied by the doses of epinephrine that raise heart rate and [ 14 C]deoxyglucose method [36]. There are no cause some anxiety do not alter cerebral blood comparable data available for the state of para­ flow and metabolism, but large doses that are doxical sleep, also termed REM sleep, or for nor­ sufficient to raise the arterial blood pressure mal sleep in humans. During the stages of sleep cause significant increases in both. as studied by PET scanning in humans, regional blood flow measurements suggest selective deac­ tivation in certain regions of association cortex CEREBRAL BLOOD FLOW AND during slow-wave sleep and selective activation METABOLIC RATE IN in other regions during REM sleep [37]. Re­ SCHIZOPHRENIA AND IN gional metabolic measurements show activation NORMAL YOUNG MEN DUR­ in the same regions in association with rapid eye ING LSD-INDUCED movements during REM sleep as with saccadic PSYCHOTOMIMETIC STATE• eye movements during wakefulness [38]. Cerebral Cerebral 0 2 blood blow consumption Condition (ml/100 g/min) (ml/100 g/min) CEREBRAL ENERGY METABOLISM IN PATHOLOGICAL STATES Normal 67 3.9 LSD intoxication 68 3.9 The cerebral metabolic rate of the brain as a Schizophrenia 72 4.0 whole is normally fairly stable and varies over a • From Sokoloff [35]. 666 Part Five Metabolism

Coma and systemic metabolic diseases In a number of conditions, the causes of de­ depress brain metabolism pression of both consciousness and the cerebral metabolic rate are unknown and must, by exclu­ Coma is correlated with depression of cerebral sion, be attributed to intracellular defects in the oxygen consumption; progressive reductions in brain. Anesthesia is one example. Cerebral oxygen the level of consciousness are paralleled by corre­ consumption always is reduced in the anes­ sponding graded decreases in cerebral metabolic thetized state regardless of the anesthetic agent rate (Table 31-7). There are almost innumerable used, whereas blood flow may or may not be de­ derangements that can lead to depression of con­ creased and may even be increased. This reduc­ sciousness. Table 31-9 includes only a few typical tion is the result of decreased energy demand and examples that have been studied by the same not insufficient nutrient supply or a block of in­ methods and by the same or related groups of in­ tracellular energy metabolism. There is evidence vestigators. Metabolic encephalopathy is dis­ that general anesthetics interfere with the balance cussed in detail in Chapter 38. of excitatory and inhibitory synaptic transmis­ Inadequate cerebral nutrient supply leads sion, particularly by an agonist action at GABA­ to decreases in the level of consciousness, rang­ ergic inhibitory synapses, thus reducing neuronal ing from confusional states to coma. The nutri­ interaction, functional activity and, consequently, tion of the brain can be limited by lowering the metabolic demands (see Chap. 16). oxygen or glucose concentrations of arterial Several metabolic diseases with broad sys­ blood, as in anoxia or hypoglycemia, or by im­ temic manifestations also are associated with dis­ pairment of their distribution to the brain turbances of cerebral function. Diabetes mellitus, through lowering cerebral blood flow, as in brain when allowed to progress to states of acidosis and tumors. Consciousness is then depressed, pre­ ketosis, leads to mental.confusion and, ultimately, sumably because of inadequate supplies of sub­ deep coma, with parallel proportionate decreases strate to support the energy metabolism neces­ in cerebral oxygen consumption (see Table 31-9) sary to sustain function of brain. [39]. The abnormalities usually are reversed com-

CEREBRAL BLOOD FLOW AND METABOLIC RATE IN HUMANS WITH VARIOUS DISORDERS AFFECTING MENTAL STATE•

Cerebral blood flow Cerebral 0 2 consumption Condition Mental state (ml/100 g/min) (ml/100 g/min) Normal Alert 54 3.3 Increased intracranial pressure Coma 34b 2.5b (brain tumor) Insulin hypoglycemia Arterial glucose level 74 mg/100 ml Alert 58 3.4 19 mg/100 ml Confused 61 2.6'> 8 mg/100 ml Coma 63 1.9b Thiopental anesthesia Coma 60b 2.1b Convulsive state Before convulsion Alert 58 3.7 After convulsion Confused 37b 3.1b Diabetes Acidosis Confused 45b 2.7b Coma Coma 65b 1.7b Hepatic insufficiency Coma 33b 1.7b a All studies listed were carried out by Kety and/or his associates, employing the same methods. For references, see Kennedy and Sokoloff [15] and Sokoloff [35]. b Denotes statistically significant difference from normal level (p < 0.05). Chapter 31 Brain Circulation and Metabolism 667

,. pletely by adequate insulin therapy. The cause of (see Chap. 38). The chemical basis of the func­ the coma or depressed cerebral metabolic rate is tional and metabolic disturbances in the brain in not known. Deficiency of cerebral nutrition can­ this condition also remains undetermined. not be implicated because the blood glucose con­ In the comatose states associated with these centration is elevated and cerebral blood flow and systemic metabolic diseases, there is depression of oxygen supply are more than adequate. Neither is both conscious mental activity and cerebral en­ insulin deficiency, presumably the basis of the sys­ ergy metabolism. From the available evidence, it temic manifestations of the disease, a likely cause is impossible to distinguish which, if either, is the of the cerebral abnormalities since no absolute re­ primary change. It is more likely that the depres­ quirement of insulin for cerebral glucose utiliza­ sions of both functions, although well correlated tion or metabolism has been demonstrated. Keto­ with each other, are independent reflections of a sis may be severe in this disease, and there is more general impairment of neuronal processes evidence that a rise in the blood concentration of by some unknown factors incident to the disease. at least one of the ketone bodies, acetoacetate, can cause coma in animals. In studies of human dia­ Measurement of local cerebral energy betic acidosis and coma, a significant correlation metabolism in humans between the depression of cerebral metabolic rate and the degree of ketosis has been observed, but Most of the in vivo measurements of cerebral en­ there is an equally good correlation with the de­ ergy metabolism described above, and all of those gree of acidosis [39]. Hyperosmolarity itself may in humans, were made in the brain as a whole cause coma. Ketosis, acidosis, hyperosmolarity or and represent the mass-weighted average of the a combination may be responsible for the distur­ metabolic activities in all of the component bances in cerebral function and metabolism. structures of the brain. The average, however, of­ Coma occasionally is associated with severe ten obscures transient and local events in the in­ impairment of liver function, or hepatic insuffi­ dividual components, and it is not surprising that ciency (see Chap. 38). In human patients in hep­ many of the studies of altered cerebral function, atic coma, cerebral metabolic rate is depressed both normal and abnormal, have failed to markedly (see Table 31-9). Cerebral blood flow demonstrate corresponding changes in energy also is depressed moderately but not sufficiently metabolism (see Table 31-8). The [14C]de­ to lead to limiting supplies of glucose and oxygen. oxyglucose method [19] has made it possible to Blood ammonia usually is elevated in hepatic measure glucose utilization simultaneously in all coma, and significant cerebral uptake of ammo­ of the components of the central nervous system, nia from the blood is observed. Ammonia toxic­ and it has been used to identify regions withal­ ity, therefore, has been suspected as the basis for tered functional and metabolic activities in a va­ cerebral dysfunction in hepatic coma. Because riety of physiological, pharmacological and ammonia can, through glutamic dehydrogenase pathological states [21]. As originally designed, activity, convert o:-KG to glutamate by reductive the method utilized autoradiography of brain amination, it has been suggested that ammonia sections for localization, which precluded its use might thereby deplete o:-KG and, thus, slow the in humans. However, later developments with Krebs cycle (see Chaps. 15 and 38). The correla­ PET [40] made it possible to adapt it for human tion between the degree of coma and blood am­ use, which is described fully in Chapter 54. monia is far from convincing, however, and coma has been observed in the absence of an increase in blood ammonia concentration. Although ammo­ REFERENCES nia may be involved in the mechanism of hepatic coma, the mechanism remains unclear, and other 1. Maker, H. S., and Nicklas, W. Biochemical re­ causal factors probably are involved. sponses of body organs to hypoxia and ischemia. Depression of mental functions and of the In E. D. Robin (ed.), Extrapulmonary Manifesta­ cerebral metabolic rate has been observed in as­ tions of Respiratory Disease. New York: Dekker, sociation with kidney failure and uremic coma 1978, pp. 107-150. 668 Part Five Metabolism

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