Glucose is the obligate energetic fuel for the mammalian brain, and most studies of cerebral energy metabolism assume that the majority of cerebral glucose utilization fuels neuronal activity via oxidative metabolism, both in the basal and activated state. Glucose transporter (GLUT) proteins deliver glucose from the circulation to the brain: GLUT1 in the microvascular endothelial cells of the blood-brain barrier (BBB) and glia; GLUT3 in neurons. Lactate, the glycolytic product of glucose metabolism, is transported into and out of neural cells by the monocarboxylate transporters (MCT): MCT1 in the BBB and astrocytes and MCT2 in neurons. The proposal of the astrocyte-neuron lactate shuttle hypothesis suggested that astrocytes play the primary role in cerebral glucose utilization and generate lactate for neuronal energetics, especially during activation. Since the identification of the GLUTs and MCTs in brain, much has been learned about their transport properties, that is capacity and affinity for substrate, which must be considered in any model of cerebral glucose uptake and utilization. Using concentrations and kinetic parameters of GLUT1 and -3 in BBB endothelial cells, astrocytes, and neurons, along with the corresponding kinetic properties of the MCTs, we have successfully modeled brain glucose and lactate levels as well as lactate transients in response to neuronal stimulation. Simulations based on these parameters suggest that glucose readily diffuses through the basal lamina and interstitium to neurons, which are primarily responsible for glucose uptake, metabolism, and the generation of the lactate transients observed on neuronal activation. Keywords: glucose and lactate; glucose transporter proteins; mathematical modeling; monocarboxylate transporters; neurons and astrocytes; substrate delivery and metabolism IntroductionThe central dogma of cerebral energy metabolism is that glucose is the obligate energetic fuel of the mammalian brain and the only substrate able to completely sustain neural activity (Siesjo, 1978). Furthermore, it has traditionally been assumed that the majority of cerebral glucose utilization fuels neuronal activity via oxidative metabolism, both in the basal and activated state (Sokoloff et al, 1977). Rates of cerebral blood flow directly relate to measurements of cerebral oxygen consumption, generating the concept of the 'flow-metabolism couple' (Sokoloff, 1976;Sokoloff et al, 1977). The introduction of neuroimaging techniques to study cerebral metabolism revealed an 'uncoupling' between cerebral oxygen consumption, blood flow, and glucose utilization during brain activation (Fox and Raichle, 1986; Fox et al, 1988), with the suggestion of regional stimulation of oxidative glycolysis during neuronal activation. The temporal relationship between the release of lactate and the onset of neuronal activation, the source of the lactate, that is neuronal or glial, and its subsequent diffusion and disposal are all matters of considerable debate. Initial studies by Prichard et al (1991) assu...

Glucose is the principle energy source for the mammalian brain. Delivery of glucose from the blood to the brain requires transport across the endothelial cells of the blood‐brain barrier and into the neurons and glia. The facilitative glucose transporter proteins mediate these processes. The primary isoforms in brain are GLUT1, detected at high concentrations as a highly glycosylated form, (55 kDa) in blood‐brain barrier, and also as a less glycosylated, 45 kDa form, present in parenchyma, predominantly glia; GLUT3 in neurons; and GLUT5 in microglia. The rest of the transporter family, GLUTs 2, 4, and 7, have also been detected in brain but at lower levels of expression and confined to more discrete regions. All of the transporters probably contribute to cerebral glucose utilization, as part of overall metabolism and metabolic interactions among cells. We discuss the properties, regulation, cell‐specific location, and kinetic characteristics of the isoforms, their potential contributions to cerebral metabolism, and several experimental paradigms in which alterations in energetic demand and/or substrate supply affect glucose transporter expression. GLIA 21:2–21, 1997. © 1997 Wiley‐Liss, Inc.

Dehydroascorbic acid (DHA) is rapidly taken up by cells and reduced to ascorbic acid (AA). Using the Xenopus laevis oocyte expression system we examined transport of DHA and AA via glucose transporter isoforms GLUT1-5 and SGLT1. The apparent K m of DHA transport via GLUT1 and GLUT3 was 1.1 ؎ 0.2 and 1.7 ؎ 0.3 mM, respectively. High performance liquid chromatography analysis confirmed 100% reduction of DHA to AA within oocytes. GLUT4 transport of DHA was only 2-4-fold above control and transport kinetics could not be calculated. GLUT2, GLUT5, and SGLT1 did not transport DHA and none of the isoforms transported AA. Radiolabeled sugar transport confirmed transporter function and identity of all cDNA clones was confirmed by restriction fragment mapping. GLUT1 and GLUT3 cDNA were further verified by polymerase chain reaction. DHA transport activity in both GLUT1 and GLUT3 was inhibited by 2-deoxyglucose, D-glucose, and 3-O-methylglucose among other hexoses while fructose and L-glucose showed no inhibition. Inhibition by the endofacial inhibitor, cytochalasin B, was non-competitive and inhibition by the exofacial inhibitor, 4,6-O-ethylidene-␣-glucose, was competitive. Expressed mutant constructs of GLUT1 and GLUT3 did not transport DHA. DHA and 2-deoxyglucose uptake by Chinese hamster ovary cells overexpressing either GLUT1 or GLUT3 was increased 2-8-fold over control cells. These studies suggest GLUT1 and GLUT3 isoforms are the specific glucose transporter isoforms which mediate DHA transport and subsequent accumulation of AA. Ascorbate (AA)1 is transported across cellular membranes by two distinct mechanisms. Ascorbate itself is transported by a sodium-dependent saturable transporter which has not been isolated (1-8). Ascorbate outside cells can be oxidized to dehydroascorbic acid (DHA), which is transported by a different mechanism (7, 9 -14). Once within cells, dehydroascorbic acid is immediately reduced to ascorbate by both chemical and protein mediated processes (15-18).Dehydroascorbic acid is structurally similar to glucose.Therefore, DHA entry has been proposed to be mediated by glucose transporters (12,13,19,20). Despite investigations in several cell types, this hypothesis has not been proven. The ideal means to verify it is to express glucose transporters using an expression system, and to study DHA transport activity. If any transporters were active, transport kinetics could be characterized only under conditions of 100% internal reduction to ascorbate, consistent with DHA transport into cells being ratelimiting (7). If internal DHA reduction were incomplete, kinetics could not be calculated. Although one study characterized DHA transport by expressed GLUT1 (21), there were a number of flaws in this report. Experiments were performed using mixtures of ascorbic acid and ascorbic acid oxidase instead of pure DHA as substrate. There was insufficient data about internal DHA reduction at each external DHA concentration, and calculations of high affinity transport were based on incorrect mathematical assumptions...

Glucose metabolism is vital to most mammalian cells, and the passage of glucose across cell membranes is facilitated by a family of integral membrane transporter proteins, the GLUTs. There are currently 14 members of the SLC2 family of GLUTs, several of which have been the focus of this series of reviews. The subject of the present review is GLUT3, which, as implied by its name, was the third glucose transporter to be cloned (Kayano T, Fukumoto H, Eddy RL, Fan YS, Byers MG, Shows TB, Bell GI. J Biol Chem 263: [15245][15246][15247][15248] 1988) and was originally designated as the neuronal GLUT. The overriding question that drove the early work on GLUT3 was why would neurons need a separate glucose transporter isoform? What is it about GLUT3 that specifically suits the needs of the highly metabolic and oxidative neuron with its high glucose demand? More recently, GLUT3 has been studied in other cell types with quite specific requirements for glucose, including sperm, preimplantation embryos, circulating white blood cells, and an array of carcinoma cell lines. The last are sufficiently varied and numerous to warrant a review of their own and will not be discussed here. However, for each of these cases, the same questions apply. Thus, the objective of this review is to discuss the properties and tissue and cellular localization of GLUT3 as well as the features of expression, function, and regulation that distinguish it from the rest of its family and make it uniquely suited as the mediator of glucose delivery to these specific cells.neurons; sperm; preimplantation embryo; white blood cells GLUCOSE METABOLISM IS VITAL to most mammalian cells, and the passage of glucose across cell membranes is facilitated by a family of integral membrane transporter proteins, the GLUTs. There are currently 14 members of the SLC2 family of GLUTs, several of which have been the focus of this series of reviews. The subject of the present review is GLUT3 which, as implied by its name, was the third glucose transporter to be cloned (62) and was originally designated as the neuronal glucose transporter. Together with GLUT1, -2, and -4, it comprises the Class 1 group of transporters (For review see Refs. 15,81,121). With the cloning of GLUT3, it became apparent that the brain did not rely exclusively on GLUT1 and that GLUT3 was highly and specifically expressed by neurons. Thus, GLUT3 became the third facilitative glucose transporter isoform with unique characteristics suited for cell-specific expression and function. GLUT2 is ideally suited for expression in liver and pancreas due to its high K m for glucose; GLUT4 and its translocation from intracellular vesicles to the cell surface facilitates insulin-stimulated glucose uptake in insulin-sensitive cells: muscle and fat. The overriding question that drove the early work in GLUT3 in the brain was: why would neurons need a separate glucose transporter isoform; what is it about GLUT3 that specifically suits the needs of the highly metabolic and oxidative neuron with its high glucose demand?...