Overview

Although many aspects of human cerebral metabolism are common to the metabolism of other tissues and organs in the body, there are a few fundamental differences. First, the brain is an unusual organ in having the highest energy requirement by mass. Even though it constitutes less than 2% of body weight, the adult brain receives 25% of cardiac output at rest and uses 20% of the total energy produced by the body.¹ In children, the figures are even more impressive, with up to 50% of the energy consumption of the body being accounted for by the brain. Much of this energy allocation is devoted to activities connected to neural signaling, most of all (>50%) to the work of adenosine triphosphate (ATP)-driven ion pumps, particularly sodium-potassium adenosine triphosphatase (Na⁺,K⁺-ATPase), which maintains and restores the transmembrane Na⁺/K⁺ gradients that are repeatedly diminished by the propagation of action potentials and synaptic transmission.^2,3 Other signaling-related costs pertain to neurotransmitter synthesis and reuptake from the synaptic cleft and axoplasmic transport. The remainder of energy expenditure is due to so-called housekeeping activities, such as the synthesis of molecules for general cellular purposes.⁴ These energy demands necessitate that the brain have reliable mechanisms to adequately protect its supply of oxygen and glucose from blood and ensure that it is tightly matched with demand (i.e., the level of neural activity). Second, the metabolism of the brain is distinguished by the singular contribution of astrocytes. Third, the brain possesses a BBB. Fourth, the brain exhibits highly developed metabolic compartmentalization, which refers to the fact that astrocytes and neuronal cells are so metabolically specialized that certain substrates and synthesized products are restricted to a particular cell type or “compartment” even though they may be required for the function of another. As detailed later, this feature necessitates close interaction and trafficking of molecules between cells of different type.^5–8 More is said on this interdependence later.

Cerebral Metabolic Rate

The global cerebral metabolic rate (CMR) is conventionally expressed in terms of the consumption of glucose (CMRGlc) or oxygen (CMRO₂), which respectively measures 25 to 30 µmol/100 g per minute and 130 to 180 µmol/100 g per minute in a resting human adult.^9,10 CMRO₂ is considered a function of mitochondrial activity and can be calculated from CBF and the arteriovenous oxygen content difference. Notably, CMRGlc is considerably higher during the first few years of life because of rapid brain growth and myelination, with a gradual reduction to adult levels by the end of the second decade.¹¹ Good correlation between CBF, CMRO₂, and CMRGlc is seen during rest, with the ratio between CMRO₂ and CMRGlc being maintained at around 6 : 1. However, with neural activation, this relationship is altered.

The metabolism of the brain exhibits considerable variation on multiple levels: by region, activation state, cell type, and subcellular location. Regional variance is reflected in the local metabolic rates for oxygen and glucose (LCMRO₂ and LCMRGlc), as well as in the levels of some metabolic enzymes. On the whole, a wider and much higher range of LCMRGlc is exhibited by cerebral gray matter than by white matter, with the highest values recorded in physiologically more active areas such as the auditory cortex, inferior colliculus, and somatosensory cortex.^12,13 Consistent with this disparity, gray matter exceeds white matter in the activity of the key enzyme of mitochondrial energy metabolism, cytochrome oxidase.^14,15 At the cellular level, neuronal energy needs are estimated to greatly exceed that of glial cells, even though neurons are significantly outnumbered by glia.² The higher metabolic activity of neurons relative to glial cells is suggested by their higher mitochondrial density and expression of cytochrome oxidase,¹⁶ as well as by their heightened sensitivity to hypoxic or hypoglycemic injury. Within the neuron itself, there is further evidence of heterogeneity of metabolic activity, with dendrites and synaptic terminals having higher cytochrome oxidase activity than cell bodies and axons.¹⁶

Energy Capture and Transfer

ATP is the principal energy currency of all living cells, including neurons and glial cells. The energy of catabolic processes is captured in the two high-energy phosphate bonds of ATP, largely through the process of mitochondrial oxidative phosphorylation (Fig. 343-1). Only minor contributions derive from substrate-level phosphorylation in the glycolytic pathway and the tricarboxylic acid (TCA; also known as the citric acid or Kreb’s) cycle. In contrast, ATP utilization occurs mostly in the cytosol through cleavage of the terminal orthophosphate group of ATP by hydrolytic enzymes, collectively referred to as ATP hydrolases or “ATPases” for short, to liberate adenosine diphosphate (ADP). ADP can then undergo similar breakdown to release adenosine monophosphate (AMP). ATP can also be directly hydrolyzed to AMP and inorganic pyrophosphate (PP_i). All these hydrolytic reactions are energy producing, or exergonic, and are coupled to many energy-requiring, or endergonic, reactions that could not otherwise proceed because of unfavorable thermodynamics. The high energy demand in the brain results in fast cycling between ATP, ADP, and inorganic phosphate (P_i), which requires fast transport between the mitochondria and cytosol. When ATP utilization is rapid or ATP generation is inhibited, the brain is assisted in the maintenance of ATP supply by the following buffer systems:

FIGURE 343-1 The four stages of glucose metabolism. A, Stage 1, aerobic glycolysis. The main glycolytic regulatory enzymes are hexokinase (HK), pyruvate kinase (PK), and especially phosphofructokinase (PFK). B, Stage 2, acetyl coenzyme A (acetyl CoA) synthesis via the thiamin (B₁)-dependent pyruvate dehydrogenase (PDH) multienzyme complex. NAD⁺, oxidized form of nicotinamide adenine dinucleotide; NADH, oxidized form of NAD⁺ reduced by acceptance of electrons to NADH. C, Stage 3, Krebs (tricarboxylic acid) cycle. FADH₂, reduced form of flavin adenine dinucleotide. D, Stage 4, generation of energy by oxidative phosphorylation in the form of adenosine triphosphate (ATP; see text for details). Q, ubiquinone.

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Reaction 1, catalyzed by creatine kinase, yields ATP from phosphocreatine (PCr), a storage form of high-energy phosphate. PCr also functions as a shuttle for the transfer of high-energy phosphate from mitochondria to the cytosol. The importance of PCr in energy homeostasis is underscored by the fact that the total creatine pool (as Cr and PCr) in the brain is at least 3 times larger than the adenosine nucleotide pool (AMP, ADP, and ATP). Reaction 2, catalyzed by adenylate kinase, re-creates ATP from ADP. Although other nucleoside triphosphates, such as guanosine triphosphate (GTP), cytidine triphosphate, and uridine triphosphate (UTP), serve functions similar to those of ATP, their regeneration from the corresponding nucleoside diphosphates occurs at the expense of ATP.¹⁷ Therefore, directly or indirectly, ATP drives all endergonic reactions of the cell.

Choice of Metabolic Substrates

Although the brain harnesses energy from a variety of substrates, it relies predominantly on glucose. Indeed, measurement of cerebral arteriovenous levels of a range of substrates and their metabolic products in mature humans under normal steady-state conditions has established that glucose is the only energy substrate that is taken up by endothelial cells of the BBB in more than trivial amounts.¹⁸ The virtual parity of the respiratory quotient under normal conditions demonstrates the overwhelming but not complete consumption of this glucose by oxidation. The brain also obtains some energy from the metabolism of other substrates such as amino acids and endogenously produced carbohydrates and lactate. Recent studies have even suggested that glial-derived lactate may actually be the preferred fuel of neurons,^19,20 but on a whole-organ basis, no single metabolic process apart from glucose oxidation has the yield to support the intense activity of the brain for more than very brief periods. Therefore, with the brain possessing just low stores of glucose,⁸ moderate to severe hypoglycemia can result in rapid deterioration in the level of consciousness and essential cerebral functions.²¹ A notable exception occurs in fasting individuals and nursing babies, in whom the imposition of hypoglycemia and high fat metabolism can lead to the dominant use of ketone bodies by the brain for cellular fuel. In this regard, it is interesting that the rate of transport of ketone bodies across the BBB is the limiting step in terms of their cerebral metabolism.^22,23 However, even though the oxidation of ketone bodies may provide up to 75% of the total cerebral energy supply,²⁴ it is unable to serve as a complete replacement for the oxidation of glucose.

Metabolism of Glucose

Glucose and its metabolites occupy key positions at the intersection point of a number of essential catabolic and anabolic pathways. As discussed earlier, glucose is the main substrate for energy production, which occurs via glycolytic and TCA cycle metabolism (see Fig. 343-1). In addition, phosphorylated glucose can be condensed into glycogen to serve as the main energy reserve in the brain. Furthermore, glucose enters the synthetic pathways of three key neurotransmitters: glutamate, γ-aminobutyric acid (GABA), and acetylcholine, as well as that of a range of amino acids via TCA cycle intermediates. Finally, metabolism of glucose via the pentose phosphate pathway provides both ribose 5-phosphate for the synthesis of nucleotides and the reducing molecule nicotinamide adenine dinucleotide phosphate (NADPH) for lipid synthesis and antioxidant defense.

Glucose and Oxygen Delivery

Glucose is an extremely hydrophilic molecule, and its delivery to brain cells involves facilitated transport, which is tightly regulated in a cell- and region-specific manner by the glucose transporter proteins (GLUTs) of the solute carrier family 2 transport protein group.^25,26 Many of the 13 known GLUT isoforms have been identified in brain but, in most cases, have not been recognized to have any defined role in glucose transport.^27–30 Cerebral glucose uptake is predominantly mediated by GLUT1 and GLUT3. GLUT1 exists in two molecular weights of 45 and 55 kD as a result of differing extents of glycosylation. Glucose enters the brain through the 55-kD GLUT1 transporters of microvascular endothelial cells, which are variably distributed between the luminal and abluminal cell membranes, as well as a sizable intracellular pool, probably as a mechanism to modulate cerebral glucose uptake according to metabolic demands.^26,31 The 45-kD GLUT1 is usually localized to glial cells, the choroid plexus, and the ependyma²⁶ and has just limited expression in neurons, except in response to stress from conditions such as hypoxia or hypoglycemia.^32,33 Neurons express the higher affinity, higher capacity GLUT3 transporter.³⁴

In contrast to glucose, efficient delivery of oxygen from the atmosphere to the brain relies on stepwise diffusion from air to blood and then to cells. This process is mediated by the respiratory and circulatory systems. The oxygen-carrying capacity of blood is increased 65-fold by hemoglobin. The affinity of hemoglobin for oxygen is represented by a sigmoid oxygen-hemoglobin dissociation curve and is such that it avidly binds oxygen under the condition of the high oxygen tension of the pulmonary alveoli and releases it under the condition of low oxygen tension in cerebral tissues. The magnitude of the gradient in oxygen partial pressure between capillary blood and tissues creates the driving force for the diffusion of blood into tissues. Oxygen use, mainly by mitochondria, determines the oxygen partial pressure of the tissue, and hence this driving force is matched to metabolic needs. The oxygen-hemoglobin dissociation curve can be shifted to the left or right by changes in pH and temperature. Hence, acidosis and hyperthermia facilitate extraction of oxygen from blood by tissues.

At the average rate of global CBF (55 mL/100 g per minute), the oxygen extraction fraction (i.e., the proportion of oxygen extracted by the brain relative to the amount delivered to it in arterial blood) is approximately 0.5 (or 50%), as opposed to the glucose extraction fraction of about 0.1 (10%).¹⁰ Thus, the supply of glucose is typically far in excess of requirements, which leaves plenty of reserve for periods of high metabolic demand.

Energy Production from Glucose

Glycolysis is the main pathway for glucose metabolism (see Fig. 343-1) and takes place in the cytosol with the net formation of 2 equivalents of ATP and 2 equivalents of pyruvate from 1 equivalent of glucose:

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Depending on the redox state of the cell, the resultant pyruvate can take one of two paths. Under normal aerobic conditions, pyruvate and the reduced form of nicotinamide adenine dinucleotide (NADH) are taken up by mitochondria, where their oxidation by the TCA cycle and respiratory chain provide for the vast bulk of ATP production (see Fig. 343-1). In the absence of oxygen, reoxidation of NADH through the respiratory chain is blocked and must instead occur by the reductive conversion of pyruvate to lactate by lactate dehydrogenase (LDH), or else glycolysis cannot continue.

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By yielding lactate rather than pyruvate as the end product, anaerobic glycolysis leads to the extremely modest yield of only 2 ATP because lactate is a dead-end metabolic product. However, studies have demonstrated that even in a brain that is replete with oxygen, anaerobic glycolysis occurs in astrocytes during activation. The resulting lactate can be used by neurons as an energy substrate after conversion back to pyruvate by LDH (now acting in reverse).

Pyruvate enters the mitochondrial matrix to undergo stepwise oxidation in the TCA cycle to carbon dioxide and water, with much of the resulting release of energy made available to the mitochondrial respiratory chain as reducing equivalents. Usually, the pyruvate is converted to acetyl coenzyme A (acetyl CoA) by pyruvate dehydrogenase (see Fig. 343-1).

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Acetyl CoA then enters the TCA cycle, where the first step involves its condensation with oxaloacetate to yield citrate. In the next few steps, citrate is isomerized and oxidatively decarboxylated to yield α-ketoglutarate, which in turn is oxidatively decarboxylated to form succinate. The remaining steps involve the regeneration of oxaloacetate from succinate (see Fig. 343-1). With each round of the cycle, two carbon atoms enter as acetyl CoA and two carbon atoms leave as CO₂. Because an acetyl group is more reduced than CO₂, the completed cycle involves four oxidation-reduction reactions that give rise to three molecules of NADH and one of reduced flavin adenine dinucleotide (FADH₂). In addition, conversion of succinyl CoA to succinate involves the cleavage of an energy-rich thioester bond, which is coupled to the phosphorylation of guanosine diphosphate to form GTP. If the GTP is not used for protein synthesis or signal transduction, its γ-phosphate group can readily be transferred to ADP to form ATP.

The NADH and FADH₂ from each of the preceding stages in the oxidation of glucose are used by mitochondria for the generation of ATP by oxidative phosphorylation (see Fig. 343-1). The driving force of oxidative phosphorylation is the electron transfer potential of NADH or FADH₂ relative to O₂. Electrons donated by NADH are passed sequentially down a “respiratory” chain of three large protein complexes embedded in the inner mitochondrial membrane, from NADH-ubiquinone (Q) reductase (complex I) to cytochrome reductase (complex III) to cytochrome-c oxidase (complex IV). Transfer of electrons between complexes I and III and between complexes III and IV is accomplished by reduced ubiquinone (QH₂) and cytochrome c, respectively. In contrast to NADH, QH₂ is the entry point for electrons from FADH₂. At complex IV (cytochrome-c oxidase), the electrons are consumed in a reaction with oxygen, the final electron sink, to form water. This electron flow leads to translocation of H⁺ from the mitochondrial matrix to the intermembrane space, and the resultant concentration gradient creates a proton motive force or transmembrane electrochemical potential (Δψ_m) to drive an inner membrane–bound ATP synthase (complex V). In this fashion, oxidation of NADH and FADH₂ theoretically drives the formation of 3 and 2 ATP molecules, respectively. On tallying up the energy yield from each stage of the metabolism of glucose, it can be seen that a molecule of glucose that undergoes complete oxidation may theoretically yield up to 38 molecules of ATP, the majority of which are contributed by oxidative phosphorylation.

In each of the stages of generation of ATP from glucose, energy supply is coupled to energy demand by enzymes whose activities are subject to feedback regulation by one or more downstream ionic or molecular species. The main site of regulation of glycolysis is at the level of phosphofructokinase (PFK), an enzyme that catalyzes the ATP-dependent addition of a phosphate group to fructose 6-phosphate early in the pathway (see Fig. 343-1). Increases in levels of ATP (a fundamental end product of glucose metabolism), citrate (the first molecule generated in the Krebs cycle), and H⁺ (which is produced in numerous downstream reactions) indicate a relatively robust energy supply and exert a negative feedback effect on PFK, which dampens further glycolysis. Conversely, increases in AMP, cyclic AMP (cAMP), ADP, K⁺, NH₄⁺, and P_i tend to accompany a rundown of cellular energy and thereby stimulate the activity of PFK. At the stage of acetyl CoA synthesis, the pyruvate dehydrogenase complex is directly stimulated by increasing levels of its substrate pyruvate and inhibited by its end products acetyl CoA and NADH (see Fig. 343-1). It is also responsive to energy status, as shown by the [NADH]/[NAD⁺], [acetyl CoA]/[CoA] and [ATP]/[ADP] ratios. Increases in these ratios result in the phosphorylation and deactivation of pyruvate dehydrogenase. The rate of the TCA cycle is immediately dependent on the availability of the oxidized form of nicotinamide adenine dinucleotide (NAD⁺), which in turn is dependent on the availability of ADP and hence ultimately on the rate of utilization of ATP. The enzymes of the TCA cycle are also individually regulated. For example, the dehydrogenases of the TCA cycle are activated by Ca²⁺, which increases in concentration during the work of muscle contraction. The most important factor regulating the overall rate of oxidative phosphorylation is the level of ADP. As ADP levels rise (reflecting higher consumption or inadequate production of ATP), oxidative phosphorylation is stimulated; that is, the respiratory chain is activated by a need for ATP synthesis. Taken together, these complex and multiple regulatory mechanisms reflect the precise control of glucose metabolism in response to the prevailing cellular conditions.

Other Metabolic Fates of Glucose

Metabolic Contributions of Brain Structural Elements

Astrocytes

A growing body of research has resulted in a revision of the traditional view of astrocytes as a passive supporting act to neurons. The proliferation and development of astrocytes are indeed driven by trophic cues associated with neuronal activity, but the growth and survival of neurons are in turn dependent on astrocytes.⁴³ In terms of metabolism, astrocytes form a highly active compartment that is separate from that of neurons but with which it interacts in a dynamic and essential fashion. This interaction occurs via the narrow extracellular space (ECS) and involves the shuttling of ionic and molecular metabolites and neurotransmitters such as lactate and glutamate. Despite being essentially nonexcitable, astrocytes have also been implicated in the integration of neuronal input, modulation of synaptic activity, and even long-range signaling. Neurotransmitter-evoked elevations in astrocytic calcium can trigger the release of chemical transmitters that can cause sustained modulatory actions on neighboring neurons. Astrocytes are also involved in brain water homeostasis and induction of the BBB (see review by Bennaroch⁵).

Before considering the influence of astrocytes on cerebral metabolism, it is relevant to consider their structural relationship to each other and to other cells in the brain. In the mammalian brain, neurons make up no more than 50% of the cerebral cortical volume and in most regions are outnumbered 10 : 1 by astrocytes.⁴⁴ Each astrocyte typically defines a nonoverlapping three-dimensional domain and is polarized such that one or more astrocytic processes contact a capillary while many more are entwined within the neuropil and engage with hundreds to thousands of synapses. Where astrocytic processes abut capillaries, they are specialized into structures known as end-feet. These end-feet are so numerous that almost the entire surface of the capillaries is covered.⁴⁵ The potential significance of this interposition of astrocytes between neurons and the capillary (endothelium) has been recognized since the late 19th century.⁴⁴ It puts astrocytes in a special position to both sense neuronal signaling and capture glucose directly from the capillary, thereby permitting them to govern excitation-metabolism coupling. Another noteworthy feature of astrocytes is their ample connections with each other and the ECS through gap junctions and hemichannels composed mainly of connexin.^43,46,47

As alluded to before, astrocytes play a leading role in the flux of glucose into neurons for energy. The glucose taken up by astrocytes may have one of two primary fates: it may be converted to lactate via astrocytic glycolysis, or it may be converted via glycogenesis to the glucose storage polymer glycogen. By contrast, in adult neurons, aerobic glycolysis results in the formation of pyruvate, not lactate, and glycogen metabolism and storage normally do not occur. That glycogen, the storage form of glucose, is located almost entirely in astrocytes is one indication of the dominant position of astrocytes in the metabolic processing of glucose. However, it should be noted that glycogenolysis in astrocytes is dictated by specific neurotransmitters and neuromodulators, thus giving neurons tight rein over this energy store.⁴⁸ As elaborated on earlier, astrocytic glycolysis is also stimulated by neuronal activation, which leads to the production of lactate. It has been proposed that this lactate is secreted into the ECS and is taken up and oxidized by activated neurons as their main energy source. Notably, 1 molecule of lactate entering the TCA cycle to undergo oxidation can yield 17 ATP molecules under normoxic conditions, which is about half the energy production of aerobic glucose metabolism. Although controversial, the existence of astrocyte-neuron lactate shuttling is based on evidence. For instance, immunohistochemistry has identified a selective distribution of LDH isoforms, with neurons predominantly expressing LDH1, the form that is enriched in lactate-consuming tissues, and astrocytes expressing LDH5, the form that is enriched in lactate-producing tissues.⁶ Localization of the lactate monocarboxylate transporters MCT-1 and MCT-4 to astrocytes and MCT-2 to neurons demonstrates the capacity for lactate exchange between these cells.⁴⁹ When primary neuronal cultures are in the presence of lactate and glucose, they preferentially consume lactate as their oxidative substrate.

The fidelity and safety of glutamate-mediated neurotransmission are dependent on very efficient uptake and modification of glutamate by astrocytes (Fig. 343-2). They keep the extracellular glutamate concentration very low by the rapid Na⁺-dependent GLT1 (predominant in the cortex and hippocampus) and GLAST (predominant in the cerebellum) cotransporter-mediated removal of the neurotransmitter from the synapse, thereby enabling quick termination of glutamate signaling before excitotoxic neuronal injury can occur.⁵⁰ Astrocytes can handle this influx of glutamate in several different ways. They possess the necessary aminotransferases to transfer the α-amino group of glutamate to oxaloacetate or pyruvate to yield α-ketoglutarate and either aspartate or alanine. The resultant α-ketoglutarate can be oxidized for energy metabolism in the TCA cycle. Indeed, cultured astrocytes have been reported to use glutamate as an energy substrate even in the presence of glucose.⁵¹ Glutamate can also be converted into α-ketoglutarate by glutamate dehydrogenase–catalyzed oxidative deamination. However, the most common means by which astrocytes inactivate glutamate is by conversion to glutamine through the attachment of ammonium ions (see Fig. 343-2). This last reaction is endergonic and requires glutamine synthase, an enzyme limited to astrocytes, and serves the additional important function of removing ammonia.⁵²

FIGURE 343-2 Functions of glutamate in the brain. Glutamate, synthesized from recycled astrocytic glutamine or neuronal α-ketoglutarate (α-KG), undergoes neuroglial uptake and participates in the glutamine/glutamate cycle between neurons and astrocytes. It is involved in normal excitatory neurotransmission (and therefore ionic homeostasis and the pathogenesis of excitotoxicity); ammonia metabolism; the biosynthesis of proteins, fatty acids, and glutathione (GSH, an antioxidant); astrocytic energy metabolism; and the synthesis and actions of neurotransmitters such as glutamine and γ-aminobutyric acid (GABA).

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Being electrically “inert,” the glutamine can then be safely released to neurons for recycling into glutamate by neuronal mitochondrial glutaminase. This cycling between glutamine and glutamate is commonly referred to as the glutamine-glutamate shuttle (see Fig. 343-2). The sodium that is cotransported into astrocytes during the uptake of glutamate (glutamate/Na⁺ ratio, 1 : 3) stimulates the activity of Na⁺,K⁺-ATPase, which depletes intracellular ATP and in turn stimulates PFK, the principal rate-limiting enzyme of glycolysis.⁵³ The resulting net production by glycolysis of 2 molecules each of ATP and lactate per molecule of glucose would in theory supply enough energy for astrocytic uptake (1 ATP for the glutamate transporter) and conversion (1 ATP to drive amidation) of 1 molecule of glutamate to glutamine, thus leaving lactate in surplus of requirements.⁵⁴ Herein lies one possible explanation for the well-documented early rise in lactate in cerebral tissue during cortical activation, as occurs in seizures.^55,56 More importantly, this hypothesized stimulation of glycolysis by glutamate provides a mechanism for the observed coupling of excitation and metabolism in the brain. Cortical activation causes rapid glutamatergic signaling, which increases the glutamine/glutamate flux and therefore drives the increased astrocytic consumption of glucose by glycolysis. This logic is supported by evidence of coupling of the glutamate/glutamine shuttle to glucose energetics in the cerebral cortex in vivo.^57,58

Astrocytes are capable of long-range signaling and buffering functions through the use of their gap junctions to form a large intercellular network, or syncytium. For example, increases in the extracellular K⁺ concentration secondary to neuronal activity leads to the entry of K⁺ into astrocytes through strong inwardly rectifying K⁺ (K_ir) channels.^59,60 The resulting local depolarization is propagated electrotonically through the glial cell network via the gap junctions, which leads to the efflux of K⁺ at distant cell processes that are not experiencing the elevated K⁺ concentration. The high density of K⁺ channels on astrocytic end-feet allows K⁺ to be deposited in the perivascular space, where it can be recycled when neural activity ceases. This so-called spatial buffering of K⁺ is important because even modest efflux of K⁺ from neurons can lead to considerable changes in the concentration of K⁺ in the ECS, with potentially detrimental effects on maintenance of neuronal membrane potential, activation and inactivation of voltage-gated channels, synaptic transmission, and electrogenic transport of neurotransmitters. The gap junctions of astrocytes also contribute to the propagation of intercellular Ca²⁺ waves, probably by enhancing the release of ATP, as well as by providing an intercellular pathway. Astrocyte membrane depolarization by glutamate causes the mobilization of Ca²⁺ from the endoplasmic reticulum via the inositol triphosphate (IP₃) generated by the activation of metabotropic glutamate receptor 5.⁶¹ It has previously been thought that IP₃, Ca²⁺, or both are propagated across gap junctions to create a “Ca²⁺ wave.” More contemporary evidence suggests that the Ca²⁺ wave is propagated through the paracrine actions of ATP released from astrocytes on purinergic receptors.⁶² Release of glutamate from astrocytes has been associated with this calcium wave. Taken together, it is clear that an increase in neuronal depolarization is coupled to an increase in astrocytic depolarization, which in turn influences local metabolic and electrical activity via K⁺, H⁺, and Ca²⁺ ions.

Astrocytes are also involved in brain water homeostasis. They take up K⁺ through transporters, particularly Na⁺,K⁺-ATPase and Na⁺/K⁺/2Cl⁻ transporters. The resulting net ionic gain results in osmotic water uptake and slight swelling of the cell. Redistribution of this water is facilitated by the high density of aquaporin channels on perivascular end-feet. This excess metabolic water probably joins interstitial fluid in the perivascular spaces and is cleared in cerebrospinal fluid (CSF) or the lymphatics. There are separate mechanisms to clear metabolic water from neurotransmitter activation or ion uptake, some of which involve elevations in intracellular Ca²⁺ [Ca²⁺]_i.

Finally, yet another distinct role of astrocytes in cerebral metabolism is regulation of the development and function of the endothelial cells of the BBB. Astrocytes have important influences on the BBB, which plays a crucial role in cerebral metabolism (see later). First, contact of astrocytic foot processes with endothelial cells of the BBB upregulates the formation of tight junctions in the latter, mainly by inducing the production of occludin, an integral protein of tight junctions.^63,64 Second, astrocytes can also induce the expression in endothelial cells of membrane transporters and specialized enzymes such as γ-glutamyl transpeptidase.^65,66 Astrocytes submitted to hypoglycemic conditions may release factors that increase glucose uptake through the BBB.⁶⁷

Hemodynamics

The analogy between blood flow in an arterial system and current flow in an electrical circuit is a useful starting point in explaining the hemodynamic parameters that govern blood flow. Recall Ohm’s law, which states that the flow of current (I) through an electrical conductor is obtained by dividing the voltage drop between the ends (ΔV, or “voltage difference”) by the electrical resistance (R_e):

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Replacement of current flow with blood flow (Q), voltage difference with pressure difference (ΔP; i.e., pressure gradient between inflow and outflow), and electrical resistance with vascular resistance (R_v) yields the following equation:

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Empirically, this equation tells us that blood flow varies directly with blood pressure and inversely with vascular resistance. With regard to the cerebral circulation, it follows that CBF varies directly with cerebral perfusion pressure (CPP) and inversely with cerebrovascular resistance (CVR):

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CPP is itself defined as the difference between mean arterial pressure (MAP) and intracranial pressure (ICP):

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When ICP is constant, CPP varies directly with MAP. MAP is not simply the average of systolic (SP) and diastolic (DP) pressure but rather is defined as follows:

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There are a host of other physical factors that influence CBF, but the relationships are inherently complex and more difficult to define or measure than those already mentioned. Blood flowing through an artery most often exhibits laminar or streamlined flow whereby an infinite number of concentric laminae are formed and move in a parabolic distribution of velocities that is greatest in the center of the tube and zero immediately adjacent to the wall of the tube. Laminar flow is described by the following Hagen-Poiseuille law:

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Here, r refers to vessel radius, ΔP to pressure gradient, η to the coefficient of fluid viscosity, and L to vessel length.⁷³ In practical terms, L and η can usually be regarded as effectively constant, and the implication of the Hagen-Poiseuille law is that blood flow not only varies proportionally with the pressure gradient but also with the fourth power of the vessel radius. This provides an explanation for the clinical observation of the large change in blood flow that can occur with only small changes in vessel diameter. However, it only approximates real life because contrary to the key assumptions behind the Hagen-Poiseuille law, normal blood flow is not continuous but pulsatile, and blood vessels are not rigid and branchless tubes. In addition, if the rate of flow is continuously increased, there comes a point when resistance to flow increases sharply and the flow ceases to be laminar, instead forming a turbulent pattern. The situation in the brain is made even more complex by the operation of cerebrovascular autoregulation (discussed in detail later), the possible presence of arterial stenosis, and the diameter and extent of arterial collaterals.⁷⁴ The intracranial arteries of the circle of Willis represent the naturally existing site of collateral blood flow for the cerebral circulation. It should be noted, however, that when a major cerebral artery undergoes gradual occlusion, the extracranial arteries can also provide important collateral supply to the cerebral circulation via the ophthalmic, meningeal, and leptomeningeal arteries.

Under normal physiologic conditions, blood flow is regulated in the brain through changes in vascular resistance. By combining Equations 10 and 14, the significant relationship between vessel radius r and CVR is revealed:

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Indeed, the resistance of the cerebral circulation is subject to dynamic changes in the contractile state of vascular smooth muscle (VSM), most of all at the level of the penetrating precapillary arterioles; that is, the principal cerebral resistance vessels are those that arise perpendicularly from the pial arteries on the brain surface before penetrating the parenchyma. However, up to 50% of total CVR arises from smaller pial arteries (150 to 200 µm in diameter) and arteries of the circle of Willis.^73,75

Hemorheology

One of the principal influences on the flow behavior of a liquid is a property known as viscosity. Viscosity represents the internal friction or resistance of the particles in a liquid to the sliding or shear forces necessary for flow to occur. Sir Isaac Newton was the first to propose that for a liquid undergoing perfectly laminar flow, the shear stress τ between the laminae is proportional to the shear rate or velocity gradient δu/δz in the direction perpendicular to the laminae by a constant of proportionality, ε:

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For an archetypal Newtonian fluid such as water, ε is properly called the coefficient of viscosity but can be equated with fluid viscosity and is a constant that is dependent only on temperature. In contrast, the viscosity of a non-Newtonian fluid can undergo large variations as the shear rate changes.

The composition and environment of blood confer on itself complex and anomalous viscous properties.^76,77 Importantly, by being composed of a concentrated suspension of cells within proteinaceous plasma, blood is a particulate fluid. Furthermore, many of these cells are capable of altering their shape and forming physical interactions with each other or the glycocalyx on the endothelial wall.^78,79 From the foregoing, it is intuitive that blood viscosity is not only a function of plasma viscosity but also depends on the concentration of cells, one measure of the latter being the hematocrit. Less obvious is that such factors as the deformability and aggregability of erythrocytes and the adherence or nonadherence of leukocytes to the endothelium^80–82 can lead to marked deviations from Newton’s law. However, in blood vessels, where the internal diameter is very large in comparison to the size of the cells, blood of normal hematocrit approximates Newtonian behavior reasonably well.⁷⁷

The non-Newtonian behavior of blood is best demonstrated in its passage through the microcirculation. In large arteries, where the shear rate is low, calculations using Poiseuille’s equation yield an “apparent” viscosity that is much higher than expected. This has been explained by the tendency of erythrocytes to form clumps or rouleaux at low shear rates, thereby increasing resistance to flow. With the higher shear rates typically found in the microvasculature, the apparent viscosity falls because any cell aggregates are dispersed into single cells that stretch and align themselves with the axial and fastest moving laminae of the bloodstream, thus leaving a slower, cell-depleted zone of plasma along the vessel margin. This marginal zone is thought to progressively dilute the hematocrit as the caliber of the blood vessel is reduced and may be accentuated by plasma skimming or cell screening at branching points. As a result, when blood flows through progressively diminishing arterioles or capillaries with a diameter of 300 µm or smaller, there is a linear fall in apparent viscosity known as the Fahraeus-Lindqvist effect. Eventually, however, when blood reaches capillaries with a diameter that is less than that of an erythrocyte (6 to 8 µm), a steep rise in blood viscosity and an inversion phenomenon (i.e., reversal of the Fahraeus-Lindqvist effect) take place.^83,84 The anomaly represented by the Fahraeus-Lindqvist effect is of critical importance in counteracting the adverse influence of blood vessel geometry (number, length, and diameter) on resistance to microcirculatory blood flow.

Blood viscosity becomes even more critical to cerebral perfusion in pathologic states of low blood flow. Regardless of whether the cause of the low blood flow is a decrease in arterial perfusion pressure or an increase in vascular resistance, the accompanying decrease in shear stress causes an elevation in the hematocrit. Alternatively, the hematocrit could be raised by a disproportionate increase in postcapillary over precapillary resistance, which causes increased transcapillary fluid leakage. Either way, the increased apparent viscosity leads to a further retardation in blood flow, thus setting up the conditions for a vicious circle. This is the theoretical basis for the clinical use of hemodilution techniques as a means of attempting to improve cerebral perfusion in conditions such as ischemic stroke and cerebral vasospasm.

Relationship between Cerebral Blood Flow and Intracranial Pressure

CBF and ICP are related by the Monro-Kellie doctrine, the modern version of which states that because the intracranial space is fixed and its principal components, the brain parenchyma, blood, and CSF, are nearly incompressible, any change in the volume of one component must lead to a reciprocal change in volume of the other components or ICP must rise. Cerebral blood volume (CBV) takes up a significant proportion of intracranial volume. At any point in time, changes in CBV are determined by changes in arterial inflow relative to venous drainage and are therefore reliant on CBF. If intracranial compliance is low, a rise in CBF can increase ICP. However, as ICP rises, there must be a compensatory rise in MAP or CPP will fall (see Equations 11 and 12), with a deleterious effect on CBF.

Regulation of Cerebral Blood Flow

Unlike other organs, regulation of blood flow in the brain is distinguished by the influence of astrocytes and neurons. Extracerebral blood vessels receive a rich “extrinsic” supply of perivascular fibers from the parasympathetic (mainly the sphenopalatine, otic, and internal carotid) and sympathetic (mainly from the superior cervical) ganglia, as well as the sensory roots of the trigeminal ganglia. On entering the brain parenchyma, cerebral arteries lose this ganglionic nerve supply and instead acquire “intrinsic” innervation from parenchymal neurons. The best characterized intrinsic neural pathways that project to cortical blood vessels are those from the nucleus basalis, locus caeruleus, and raphe nucleus. With electrical or chemical stimulation of these areas, increases or decreases in cortical CBF occur. Anatomic studies have shown that neurons in these areas send projection fibers to cortical blood vessels, as well as to astrocytes. In fact, noradrenergic afferents from the locus caeruleus target mainly perivascular astrocytes. Changes in perivascular astrocytic [Ca²⁺]_i secondary to noradrenaline cause vasoconstriction of the adjacent arterioles.⁸⁵ Hence, arterial tone is influenced by astrocytes, as well as by neurons.

There are three principal components in the regulation of CBF. First, changes in perfusion pressure are capable of producing marked changes in CVR in a phenomenon that is dubbed autoregulation. Second, cerebral blood vessels exhibit changes in caliber in response to variations within certain ranges of PCO₂ and, to a lesser extent, PO₂. Third, the activity of the brain is linked to blood flow via so-called flow-metabolism coupling, where changes in cerebral metabolism resulting from neural stimulation are tied or “coupled” to corresponding changes in CBF. There has been much debate surrounding the explanation of CBF regulation, particularly in reference to the last characteristic, and multiple mechanisms have been described. These mechanisms involve an array of vasoactive mediators. The functions of these various molecules are not exclusive and involve pathways that frequently intersect with one another. It is beyond the scope of this chapter to elaborate on every single mediator, so the following discussion is confined to those considered to be of greatest relevance and importance in cerebrovascular homeostasis.