Local cerebral glucose utilization in controlled graded levels of hyperglycemia in the conscious rat

Local cerebral glucose utilization assayed by the [14C]deoxyglucose ([14C]DG) method and calculated by means of its operational equation with values for the rate constants and lumped constant determined in rats under physiological conditions remains relatively stable with variations in arterial plasma glucose concentration within the normoglycemic range. Large changes in arterial plasma glucose level may, however, significantly alter the values of these constants and lead to artifactual results. Values for the lumped constant have been measured and reported for a wide range of arterial plasma glucose concentrations ranging from hypoglycemia to hyperglycemia in the rat (Schuier et al., 1981; Suda et al., 1981; Pettigrew et al., 1983). In the present study we have redetermined the rate constants in rats with arterial plasma glucose levels clamped at approximately 350, 450, and 550 mg/dl (i.e., 19, 25, and 31 mM) by a glucose clamp technique. The rate constants for the transport of DG from plasma to brain, K1*, and its phosphorylation in tissue, k3*, were found to decline with increasing plasma glucose levels, while the rate constant for its transport back from brain to plasma, k*2, remained relatively unchanged from its value in normoglycemia. These rate constants were used together with the previously determined values for the lumped constants to calculate local rates of cerebral glucose utilization in three groups of rats in which arterial plasma glucose levels were clamped at approximately 350, 450, and 550 mg/dl (i.e., 19, 25, and 31 mM). Average glucose utilization in the brain as a whole was unchanged in hyperglycemia from the values calculated in normoglycemic rats with the standard normal set of constants. Changes in the rate of glucose utilization were found, however, in the hypothalamus, globus pallidus, and amygdala during hyperglycemia.     

Influence of plasma glucose concentration on lumped constant of the deoxyglucose method: effects of hyperglycemia in the rat

The lumped constant of the deoxyglucose method was determined by the steady-state, model-independent method in the brain of normal conscious rats with arterial plasma glucose concentrations varying from normoglycemia (i.e., 8 mM) to hyperglycemia (i.e., 31 mM). The lumped constant for brain was found to decrease very gradually with increasing arterial plasma glucose concentration from a value of approximately 0.45 in the midnormoglycemic range (i.e., 7-8 mM) to approximately 0.38 at 28-31 mM. 3-O-[14C]Methylglucose was used to assess the distribution of glucose within the brain structures in hyperglycemia; the results indicated that the glucose concentration, and therefore also the values for the lumped constant, remain relatively uniform in hyperglycemia with arterial plasma glucose concentrations as high as 34 mM. The values for the lumped constant for rat brain determined in the present studies were combined with those previously determined in this laboratory for hypoglycemia and normoglycemia by the same method to provide a single source for the values for the lumped constant to be used over the full range of arterial plasma glucose concentrations. In several rats the lumped constant for cephalic extracerebral tissues was also evaluated in parallel with those for the brain. The lumped constant for the cephalic extracerebral tissues was found to be about twice that for brain and to be unaffected by changes in arterial plasma glucose levels.     

Direct chemical measurement of the lambda of the lumped constant of the [14C]deoxyglucose method in rat brain: effects of arterial plasma glucose level on the distribution spaces of [14C]deoxyglucose and glucose and on lambda

The lumped constant in the operational equation of the 2-[14C]deoxyglucose (DG) method contains the factor lambda that represents the ratio of the steady-state tissue distribution spaces for [14C]DG and glucose. The lumped constant has been shown to vary with arterial plasma glucose concentration. Predictions based mainly on theoretical grounds have suggested that disproportionate changes in the distribution spaces for [14C]DG and glucose and in the value of lambda are responsible for these variations in the lumped constant. The influence of arterial plasma glucose concentration on the distribution spaces for DG and glucose and on lambda were, therefore, determined in the present studies by direct chemical measurements. The brain was maintained in steady states of delivery and metabolism of DG and glucose by programmed intravenous infusions of both hexoses designed to produce and maintain constant arterial concentrations. Hexose concentrations were assayed in acid extracts of arterial plasma and freeze-blown brain. Graded hyperglycemia up to 28 mM produced progressive decreases in the distribution spaces of both hexoses from their normoglycemic values (e.g., approximately -20% for glucose and -50% for DG at 28 mM). In contrast, graded hypoglycemia progressively reduced the distribution space for glucose and increased the space for [14C]DG. The values for lambda were comparatively stable in normoglycemic and hyperglycemic conditions but rose sharply (e.g., as much as 9-10-fold at 2 mM) in severe hypoglycemia.     

Autoradiographic determination of cerebral glucose content, blood flow, and glucose utilization in focal ischemia of the rat brain: influence of the plasma glucose concentration

Focal cerebral ischemia was produced by occlusion of the middle cerebral artery in rats. Cerebral blood flow measured with [14C]iodoantipyrine was severely reduced in the lateral portion of neostriatum. This area of dense ischemia was sharply demarcated against the surroundings. The adjacent cortex was perfused at one-third of normal, whereas blood flow in the medial neostriatum was only slightly reduced. This pattern of perfusion was independent of the plasma glucose concentration of the animal. In contrast, the glucose utilization calculated from the 2-[3H]deoxyglucose accumulation depended on the plasma glucose concentration. Enhanced glucose utilization was evident in the border areas surrounding the ischemic focus in normoglycemic animals. Neither acutely nor chronically diabetic animals had such an increase of metabolism in the borderzone. Moderately hyperglycemic rats had a narrow rim of enhanced glucose utilization immediately surrounding the ischemic core, whereas animals with plasma glucose values above 22 mmol/L had no such rim. In mild hypoglycemia (2-4 mmol/L), the glucose utilization was slightly enhanced in the border areas, but during severe hypoglycemia (less than 2.5 mmol/L), the glucose utilization declined gradually toward the ischemic core. Glucose content, and thereby the lumped constant (measured by 3-0-[14C]methylglucose) showed little regional variation, except in the ischemic core. These findings indicate that blood flow alterations after occlusion of the middle cerebral artery in rats are not influenced by the plasma glucose utilizations. In contrast, glucose utilization depends on a combination of plasma glucose concentration and blood flow instead of blood flow per se.     

Cerebral blood flow during chronic hypoglycemia in the rat

Regional cerebral blood flow (rCBF) was measured in normoglycemic and chronically hypoglycemic rats. Chronic hypoglycemia was produced by continuously infusing insulin for 6-7 days. During chronic hypoglycemia (plasma glucose = 1.97 mumol/ml), rCBF increased in all regions except the cerebellum and hypothalamus. Blood flow increases present during chronic hypoglycemia were not as great as those previously measured during acute hypoglycemia. Therefore, adjustments in the regulation of rCBF occurred during chronic hypoglycemia compared to acute hypoglycemia.     

Regional differences in brain glucose content in graded hypoglycemia

Graded hypoglycemia was induced with insulin in anesthetized and artificially ventilated rats. The brains were frozenin situ, and the regional glucose concentration was determined in different areas of the brain with the bioluminescent technique. In all nine brain structures analyzed, brain tissue glucose content assessed with the bioluminescent technique correlated closely with the plasma glucose levels; the tissue/plasma glucose concentration ratios approximating 0.3. There were, however, relatively marked regional differences. For example, whereas glucose concentrations in the neocortex, caudoputamen, hippocampus, and cerebellum were very low in rats having a plasma glucose concentration of less than 4 μmol/mL, higher glucose concentrations were present in these animals in the thalamus, hypothalamus, and brainstem. The lowest glucose content was found in the caudoputamen, which was depleted of glucose in animals with plasma levels below 3 μmol/mL. It is concluded that regional inhomogeneities in the glucose levels observed during hypoglycemia may, at least in part, explain differences in the vulnerability of different brain structures following reversible hypoglycemia.     

Modeling the dependence of hexose distribution volumes in brain on plasma glucose concentration: implications for estimation of the local 2-deoxyglucose lumped constant

The steady-state distribution volumes of glucose, 3-O-methylglucose, and 2-deoxyglucose (2DG) are known to change as the concentration of glucose in plasma ranges from hypo- to hyperglycemic values. Model estimates of the three distribution volumes were compared with distribution volume values experimentally measured in the brains of conscious rats as the concentration of glucose in plasma was varied from 2 to 28 mM. The dependence on plasma glucose concentration of the 2DG lumped constant, the factor that relates the phosphorylation rate of 2DG to the net rate of glucose utilization at unit specific radioactivity in the plasma, had been determined previously in separate series of experiments. The model was extended to incorporate this dependence of the lumped constant. In the model both the transport and the phosphorylation barriers were assumed to be single and saturable. The values of their respective half-saturation concentrations and the ratio of the two maximum velocities for glucose were assumed to be invariant over the entire range of plasma glucose concentration. Good agreement between measured and estimated values for the distribution volumes and the lumped constant was attained over the full range of plasma glucose concentration. The model estimates reflected the progressive transport limitation of the brain glucose content as plasma glucose levels were reduced to hypoglycemic values. The results also indicated that these changes should be evident in the time course of 2DG in brain following administration by bolus or continuous infusion, and thus that indexes of local lumped constant change could be derived from the time course data.     

In vivo measurements of brain glucose transport using the reversible Michaelis-Menten model and simultaneous measurements of cerebral blood flow changes during hypoglycemia.

Glucose is the major substrate that sustains normal brain function. When the brain glucose concentration approaches zero, glucose transport across the blood-brain barrier becomes rate limiting for metabolism during, for example, increased metabolic activity and hypoglycemia. Steady-state brain glucose concentrations in alpha-chloralose anesthetized rats were measured noninvasively as a function of plasma glucose. The relation between brain and plasma glucose was linear at 4.5 to 30 mmol/L plasma glucose, which is consistent with the reversible Michaelis-Menten model. When the model was fitted to the brain glucose measurements, the apparent Michaelis-Menten constant, Kt, was 3.3 +/- 1.0 mmol/L, and the ratio of the maximal transport rate relative to CMRglc, Tmax/CMRglc, was 2.7 +/- 0.1. This Kt is comparable to the authors' previous human data, suggesting that glucose transport kinetics in humans and rats are similar. Cerebral blood flow (CBF) was simultaneously assessed and constant above 2 mmol/L plasma glucose at 73 +/- 6 mL 100 g(-1) min(-1). Extrapolation of the reversible Michaelis-Menten model to hypoglycemia correctly predicted the plasma glucose concentration (2.1 +/- 0.6 mmol/L) at which brain glucose concentrations approached zero. At this point, CBF increased sharply by 57% +/- 22%, suggesting that brain glucose concentration is the signal that triggers defense mechanisms aimed at improving glucose delivery to the brain during hypoglycemia.     

Regional brain glucose metabolism in chemically-induced seizures in the rat

Measurement of regional brain glucose metabolism may give information concerning the mechanism of neuronal cell death developing after prolonged periods of epileptic activity. Regional brain glucose utilization was measured in paralyzed ventilated rats during seizures induced by L-allylglycine, kainic acid and bicuculline using the [14C]deoxyglucose method. Regional brain glucose concentration was measured in another series of rats, after similar periods of seizure activity, to permit a more accurate calculation of the lumped constant. In L-allylglycine-induced seizures regional brain glucose concentration did not vary from control values, so no correction of the lumped constant was necessary. Regional brain glucose utilization increased throughout the brain, the largest increase being in the hippocampus (control 36 +/- 6 mumol 100 g-1 min-1; seizure 120 +/- 12 mumol 100 g-1 min-1). In kainic acid-induced seizures, brain glucose concentration fell in the hippocampus, involving some correction of the lumped constant. Increases in glucose utilization were limited primarily to the hippocampus, with some involvement of the inferior colliculus. The ventral hippocampus showed the largest increase in glucose utilization (control 34 +/- 5 mumol 100 g-1 min-1; seizure 167 +/- 10 mumol 100 g-1 min-1). In bicuculline-induced seizures, in starved rats, brain glucose concentration fell in all regions investigated and no increase in regional glucose utilization was recorded. In L-allylglycine and kainic acid-induced seizures, the hippocampus, a region vulnerable to neuronal damage, shows the largest increase in glucose utilization. Studies involving bicuculline need further investigation, due to severe perturbation of brain and plasma glucose concentration.     

Brain glucose concentrations in patients with type 1 diabetes and hypoglycemia unawareness

Although it is well established that recurrent hypoglycemia leads to hypoglycemia unawareness, the mechanisms responsible for this are unknown. One hypothesis is that recurrent hypoglycemia alters brain glucose transport or metabolism. We measured steady-state brain glucose concentrations during a glucose clamp to determine whether subjects with type 1 diabetes and hypoglycemia unawareness may have altered cerebral glucose transport or metabolism after exposure to recurrent hypoglycemia. We compared 14 subjects with diabetes and hypoglycemia unawareness to 27 healthy control subjects. Brain glucose concentrations were measured under similar metabolic conditions using in vivo (1)H nuclear magnetic resonance (NMR) spectroscopy at 4 Tesla during a hyperglycemic clamp (plasma glucose = 16.7 mmol/l) with somatostatin and insulin. Subjects with type 1 diabetes and hypoglycemia unawareness had significantly higher brain glucose concentrations compared to that in controls under the same conditions (5.5 +/- 0.3 vs. 4.7 +/- 0.1 micromol/g wet weight, P = 0.016). These data suggest that changes in brain glucose transport or metabolism may occur as a result of recurrent hypoglycemia.     

Increase in glucose transporter densities of Glut3 and decrease of glucose utilization in rat brain after one week of hypoglycemia.

The present study addresses the question whether a chronic decrease of plasma glucose concentration for 1 week induces a global or local increase in glucose transporter densities Glut1 and Glut3 in the brain. To induce chronic hypoglycemia insulin was infused into rats by osmotic minipumps for 1 week resulting in a mean plasma glucose concentration of 3.1+/-0.5 mmol/l (control group: 8.1+/-0.5 mmol/l). Global and local densities of Glut1 and Glut3 glucose transporters were measured by immunoautoradiographic methods. The mean density of glucose transporters Glut1 remained unchanged, whereas the mean density of Glut3 increased slightly, although significantly. To determine whether the increased density of Glut3 is related to a change in glucose metabolism, the local cerebral metabolic rate of glucose (lCMR(glc)) was quantified by the 2-deoxyglucose method. Mean glucose utilization was decreased by 15%. Local analysis of transporter densities (Glut1 and Glut3) and glucose utilization showed a significant correlation between local glucose transporter densities (Glut1 and Glut3) and lCMR(glc) during hypoglycemia as already previously observed during normoglycemia. It is concluded that 1 week of hypoglycemia is a stimulus for the induction of additional glucose transporters Glut3 in the brain. These additional neuronal glucose transporters may support the maintenance of glucose utilization which is not completely maintained under these conditions.     

Quantification of glucose utilization in an experimental brain tumor model by the deoxyglucose method

Reevaluation of lumped and rate constants is necessary when Sokoloff's 2-deoxyglucose (DG) method is used to measure glucose utilization in pathological tissue. We describe here a modification of Sokoloff's lumped constant measurement that permits simultaneous estimation of both lumped and rate constants from a single animal experiment. A subcutaneous tumor model (AA ascites tumor) was used for measurement of these constants with a procedure similar to Sokoloff's that kept the plasma tracer concentration constant. Measured constants were as follows: lumped constant, 0.654 +/- 0.081; k1, 0.196 +/- 0.038 min-1; k2, 0.262 +/- 0.067 min-1; k3, 0.117 +/- 0.044 min-1. These constants were used to quantify glucose utilization in the implanted brain tumor. To test the validity of this method, we compared a fraction of the free DG pool calculated using the tumor constants with a fraction measured directly by chromatographic analysis of tissue samples from both subcutaneous tumor and implanted brain tumor. The values derived by chemical analysis agreed well with those predicted by the calculations. The value of k4 varied from 0.0031 +/- 0.0018 min-1 for the tumor tissue to 0.0214 +/- 0.0024 min-1 for tumors with a large necrotic center. This method would be especially useful when applied to xenograft human gliomas in nude mice for quantification of glucose utilization in human gliomas by means of positron emission tomography.     

Cerebral carbohydrate metabolism during hypoglycemia and anoxia in newborn rats

The cerebral metabolic responses to perinatal hypoglycemia and anoxia were studied in newborn rats given regular insulin (30 units per kilogram of body weight). Animals were observed for up to 2 hours with no apparent ill effects in spite of blood glucose concentrations of 0.75 mmol per liter. When exposed to 100% nitrogen at 37°C, hypoglycemic animal survived only one-tenth as long as littermate controls with normal blood glucose levels (4.7 mmol/L). Pretreatment of hypoglycemic rats with glucose (10 mmol/kg) 10 and 30 minutes prior to nitrogen exposure nearly completely reversed the anoxic vulnerability. Hypoglycemia led to progressive reductions in crebral glycogen and glucose; however, only glucose reverted to normal levels 20 minutes after systemic glucose administration. The glycolytic intermediates glucose 6-phosphate and lactate were also lower during hypoglycemia. Brain glucose levels below 0.1 mmol per kilogram were associated with a disrupted cerebral energy state, reflected by declines in phosphocreatine (33%) and adenosine triphosphate (ATP) (10%). Cerebral energy utilization (metabolic rate) was minimally reduced (?7.2%) by hypoglycemia and returned to the control value (2.36 mmol ～ P/kg/min) with glucose treatment. The cerebral energy reserves ATP, adenosine diphosphate, and phosphocreatine delined more rapidly and to a lower level in hypoglycemic rats subjected to 2 1/2 minutes of anoxia than in normoglycemic animals rendered similarly hypoxic. The findings suggest that decreased anoxic resistance of hypoglycemic newborn rats is not primarily a function of reduced brain glycogen or altered cerebral metabolic rate. The presence of endogenous cerebral glucose stores combined with continued circulating glucose (cerebrovascular perfusion) appear to be critical factors for maintaining perinatal hypoxic survival.     

Influence of MK-801 on brain extracellular calcium and potassium activities in severe hypoglycemia

The purpose of the present study was to examine the effect of blockade of N-methyl-D-aspartate (NMDA) receptors on the depolarization associated with severe hypoglycemia, which is commonly preceded by one or a few transient depolarizations reminiscent of cortical spreading depression (CSD). In the cerebral cortices of rats [K+]e and [Ca2+]e were measured with ion-selective microelectrodes. NMDA blockade was achieved by injection of MK801 in doses that block CSD. In control rats, the latency from the time point when blood glucose reached minimal levels to onset of ionic shifts was 33.2 +/- 3.5 min, and [K+]e rose from 3.2 +/- 0.2 to 55 +/- 5 mM. All variables remained unchanged in rats treated with MK801. In another four rats treated with MK801, [Ca2+]e declined from 1.06 +/- 0.22 to 0.12 +/- 0.02 mM. Plasma glucose measurements indicated that the cortex depolarized at a plasma glucose concentration between 0.7 and 0.8 mM, i.e., within a narrow range, suggesting a threshold phenomenon. In conclusion, activation of NMDA receptors seems of minor importance for hypoglycemic depolarization. The ionic transients that precede the persistent hypoglycemic depolarization are probably mediated by mechanisms distinct from those of electrically induced CSD.     

Effects of hypoglycemia on functional magnetic resonance imaging response to median nerve stimulation in the rat brain.

The authors studied the effects of a standardized mild-moderate hypoglycemic stimulus (glucose clamp) on brain functional magnetic resonance imaging (fMRI) responses to median nerve stimulation in anesthetized rats. In the baseline period (plasma glucose 6.6 +/- 0.3 mmol/L), the MR signal changes induced by median nerve activation were determined within a fixed region of the somatosensory cortex from preinfusion activation maps. Subsequently, insulin and a variable glucose infusion were administered to decrease plasma glucose. The goal was to produce a stable hypoglycemic plateau (2.8 +/- 0.2 mmol/L) for 30 minutes. Thereafter, plasma glucose was restored to euglycemic levels (6.0 +/- 0.3 mmol/L). In the early phase of insulin infusion (15 to 30 minutes), before hypoglycemia was reached (4.7 +/- 0.3 mmol/L), the activation signal was unchanged. However, once the hypoglycemic plateau was achieved, the activation signal was significantly decreased to 57 +/- 6% of the preinfusion value. Control regions in the brain that were not activated showed no significant changes in MR signal intensity. Upon return to euglycemia, the activation signal change increased to within 10% of the original level. No significant activation changes were noted during euglycemic hyperinsulinemic clamp experiments. The authors concluded that fMRI can detect alterations in cerebral function because of insulin-induced hypoglycemia. The signal changes observed in fMRI activation experiments were sensitive to blood glucose levels and might reflect increases in brain metabolism that are limited by substrate deprivation during hypoglycemia.     

Hyperglycemia in the reperfusion period hampers recovery from cerebral ischemia

Glucose treatment prior to cerebral ischemia is followed by similar metabolic and hemodynamic recovery ( Siemkowicz & Gjedde 1980), and normalisation of brain extracellular ions ( Siemkowicz & Hansen 1981). In view of this, the present study investigated whether post-ischemic hyperglycemia influenced recovery from cerebral ischemia.
In rats which received 50% glucose during a 10 min period of cerebral ischemia, and which had a plasma glucose level of 28.5 mM after 10 min of recirculation, recovery was inferior to that of rats receiving either 8% NaCl or 0.9% NaCl (and hence the rats were normoglycemic).
Furthermore, rats which had been rendered hyperglycemic (39 mM) prior to ischemia, and which had plasma glucose lowered to 15 mM by insulin treatment during ischemia, did not recover and died within 4 days. Conversely, rats with somewhat lower preischemic hyperglycemia (28 mM), and which had plasma glucose lowered to 12 mM by insulin treatment during ischemia, recovered as well as the normoglycemic rats.
In conclusion, preischemic and postischemic hyperglycemia is detrimental to recovery from cerebral ischemia.     

Hyperglycemia reduces the extent of cerebral infarction in rats

Although hyperglycemia is known to exacerbate neuronal injury in the setting of reversible brain ischemia, its effect on irreversible thrombotic infarction is less well understood. In this study, unilateral thrombotic infarction was induced photochemically in the parietal cortex of Wistar rats. Seven days later, brains were perfusion-fixed for light microscopy. Infarct areas were measured by computer-assisted planimetry on multiple coronal sections at 250-micron intervals; these data were integrated to yield infarct volumes. Fasted, normoglycemic rats were compared with hyperglycemic rats that had received 1.2-1.5 ml of 50% dextrose i.p. 15 minutes prior to the induction of infarction. Infarct volume averaged 12.5 +/- 4.0 mm3 (mean +/- SD) in rats (n = 14) with plasma glucose levels of 72-184 mg/dl; this differed statistically from the average volume of 9.3 +/- 3.3 mm3 observed in rats (n = 13) with elevated plasma glucose (range 264-607 mg/dl). Spearman rank correlation analysis confirmed a significant correlation of larger infarct volumes with lower plasma glucose levels. In contrast, rats receiving mannitol i.p. to produce an osmotic load comparable with that of the dextrose-pretreated animals showed larger infarct volumes than saline-treated controls. The small but definite beneficial effect of hyperglycemia in this end-arteriolar thrombotic infarction model is possibly attributable to improved local energy metabolism at the periphery of the lesion during the early period of lesion expansion.     

Modification of glutamate binding sites in newborn brain during hypoglycemia

We have shown that acute insulin-induced hypoglycemia leads to specific changes in the cerebral NMDA receptor-associated ion channel in the newborn piglet. The present study tests the hypothesis that exposure to acute hypoglycemia in the newborn will alter the glutamate binding site of both NMDA and kainate receptors. Studies were performed in 3-6 days-old piglets randomized to control (n=6) or hypoglycemic (n=6) groups. Hypoglycemia was maintained for 120 min using insulin infusion. Saturation binding assays were performed in cerebral cell membranes using (3)H-glutamate or (3)H-kainate to determine the characteristics of the glutamate binding sites of the NMDA and kainate receptors, respectively. The concentration of glucose in cerebral cortex was 10-fold less in hypoglycemic piglets than in controls (P<0.05). Brain ATP was not significantly decreased during hypoglycemia, but phosphocreatine decreased from control of 6.6 +/- 1.3 micromoles/g brain to 3.2 +/- 1.9 micromoles/g brain in hypoglycemic piglets. The B(max) for NMDA-displaceable (3)H-glutamate binding was 992 +/- 64 fmol/mg protein in hypoglycemic animals, significantly higher than the control value of 746 +/- 42 fmol/mg protein. However, the dissociation constant for glutamate was unchanged during hypoglycemia. The (3)H-kainate binding studies demonstrated no change in B(max) of high-affinity kainate receptors during hypoglycemia. In contrast, the affinity of the kainate receptor glutamate binding site significantly increased compared to control. Thus, acute hypoglycemia in the newborn piglet had specific effects on the glutamate binding sites of the NMDA and kainate receptors that could be due to alterations in cell membrane lipids or modification of receptor proteins.     

Effect of hypoglycemia on brain glycogen metabolism in vivo

The brain contains a small but significant amount of glycogen, which has long been considered to play an insignificant role in the brain. In this study, brain glycogen metabolism was measured using (13)C NMR spectroscopy at 9.4 T. Brain glycogen metabolism was modulated by hyperinsulinemia resulting in a net accumulation. The role of glycogen in maintaining brain function is unknown; one possibility is that it may serve as an endogenous glucose reservoir to protect the brain against severe hypoglycemia. To address this possibility, rats were subjected to insulin-induced moderate hypoglycemia and when the level of brain glucose approached zero, brain glycogen content began to decrease gradually, demonstrating utilization of this glucose reservoir. The brain glycogen signal never became undetectable, however, even during 2 hr of hypoglycemia. When plasma and brain glucose concentrations were restored, glycogen increased and the concentration exceeded the pre-hypoglycemic level by several-fold. The data suggest that brain glycogen can provide fuel for extended periods of time when glucose supply is inadequate. Furthermore, brain glycogen can rebound (super-compensate) after a single episode of hypoglycemia. We postulate that brain glycogen serves as an energy store during hypoglycemia and that it may participate in the creation of reduced physiological responses to hypoglycemia that are involved in a symptom often observed in patients with diabetes, hypoglycemia unawareness.     

Comparative regional analysis of 2-fluorodeoxyglucose and methylglucose uptake in brain of four stroke patients. With special reference to the regional estimation of the lumped constant

The glucose metabolic rate of the human brain can be measured with labeled deoxyglucose, using positron emission tomography, provided certain conditions are fulfilled. The original method assumed irreversible trapping of deoxyglucose metabolites in brain during the experimental period, and it further requires that a conversion factor between deoxyglucose and glucose, the "lumped constant," be known for the brain regions of interest. We examined the assumption of irreversible trapping of fluorodeoxyglucose metabolites in brain of four patients in 365 normal and 4 recently infarcted regions. The average net, steady-state rate of fluorodeoxyglucose (KD) accumulation in normal regions of the four patients was 0.025 ml g-1 min-1. We also examined the variability of the lumped constant. We first confirmed that methylglucose is not phosphorylated in the human brain. We then estimated the lumped constant from the regional distribution of labeled methylglucose in brain. The average (virtual) volume of distribution of labeled methylglucose in the normal regions was 0.46 ml g-1 and was the same in both gray and white matter structures. The average brain glucose content corresponding to this value was 1.3 mumol g-1, assuming a Michaelis constant (Kt) of 3.7 mM for glucose transport across the blood-brain barrier. The lumped constant varied insignificantly between 0.4 and 0.5 in most regions, with an overall average of 0.44. It did not vary significantly between the patients and was the same in gray and white matter structures, but was inversely related to the calculated metabolic rate. This observation indicates that metabolic rates calculated with a fixed lumped constant (e.g., 0.40) would be slightly underestimated at high metabolic rates and slightly overestimated at low metabolic rates. The average glucose metabolic rates of the 365 normal regions, in which gray matter regions prevailed by 20:1, was 32 mumol 100 g-1 min-1. The average glucose phosphorylation rate in white matter was 20 mumol 100 g-1 min-1 with a lumped constant of 0.45. In the recently infarcted areas, the lumped constants varied from 0.37 to 2.83, corresponding to glucose metabolic rates varying from 2 to 18 mumol 100 g-1 min-1. Two infarct types were identified. In one type, the phosphorylation-limited type, glucose content and the lumped constant were close to normal (1 mumol g-1 and 0.40, respectively). In the other, the transport/flow-limited type, the glucose content was low (0.2 mumol g-1), and the lumped constant in excess of unity. The evidence from the present study upholds the model of Sokoloff et al. in every detail.