Autoradiographic measurement of cerebral lactate transport rate constants in normal and activated conditions

Brain energy metabolism measured by (13)C magnetic resonance spectroscopy in vivo upon infusion of [3-(13)C]lactate

The brain uses lactate produced by glycolysis as an energy source. How lactate originated from the blood stream is used to fuel brain metabolism is not clear. The current study measures brain metabolic fluxes and estimates the amount of pyruvate that becomes labeled in glial and neuronal compartments upon infusion of [3-(13)C]lactate. For that, labeling incorporation into carbons of glutamate and glutamine was measured by (13)C magnetic resonance spectroscopy at 14.1 T and analyzed with a two-compartment model of brain metabolism to estimate rates of mitochondrial oxidation, glial pyruvate carboxylation, and the glutamate-glutamine cycle as well as pyruvate fractional enrichments. Extracerebral lactate at supraphysiological levels contributes at least two-fold more to replenish the neuronal than the glial pyruvate pools. The rates of mitochondrial oxidation in neurons and glia, pyruvate carboxylase, and glutamate-glutamine cycles were similar to those estimated by administration of (13)C-enriched glucose, the main fuel of brain energy metabolism. These results are in agreement with primary utilization of exogenous lactate in neurons rather than astrocytes.

Cerebral dynamics and metabolism of hyperpolarized [1-(13)C]pyruvate using time-resolved MR spectroscopic imaging

Dynamic hyperpolarized [1-(13)C]pyruvate metabolic imaging in the normal anesthetized rat brain is demonstrated on a clinical 3-T magnetic resonance imaging scanner. A 12-second bolus injection of hyperpolarized [1-(13)C]pyruvate is imaged at a 3-second temporal resolution. The observed dynamics are evaluated with regard to cerebral blood volume (CBV), flow, transport, and metabolic exchange with the cerebral lactate pool. A model for brain [1-(13)C]lactate, based on blood-brain transport kinetics, CBV, and the observed pyruvate dynamics is described.

Metabolic imaging in the anesthetized rat brain using hyperpolarized [1-13C] pyruvate and [1-13C] ethyl pyruvate

Formulation, polarization, and dissolution conditions were developed to obtain a stable hyperpolarized solution of [1-(13)C]-ethyl pyruvate. A maximum tolerated concentration and injection rate were determined, and (13)C spectroscopic imaging was used to compare the uptake of hyperpolarized [1-(13)C]-ethyl pyruvate relative to hyperpolarized [1-(13)C]-pyruvate into anesthetized rat brain. Hyperpolarized [1-(13)C]-ethyl pyruvate and [1-(13)C]-pyruvate metabolic imaging in normal brain is demonstrated and quantified in this feasibility and range-finding study.

Imaging brain activation: simple pictures of complex biology

Elucidation of biochemical, physiological, and cellular contributions to metabolic images of brain is important for interpretation of images of brain activation and disease. Discordant brain images obtained with [(14)C]deoxyglucose and [1- or 6-(14)C]glucose were previously ascribed to increased glycolysis and rapid [(14)C]lactate release from tissue, but direct proof of [(14)C]lactate release from activated brain structures is lacking. Analysis of factors contributing to images of focal metabolic activity evoked by monotonic acoustic stimulation of conscious rats reveals that labeled metabolites of [1- or 6-(14)C]glucose are quickly released from activated cells as a result of decarboxylation reactions, spreading via gap junctions, and efflux via lactate transporters. Label release from activated tissue accounts for most of the additional [(14)C]glucose consumed during activation compared to rest. Metabolism of [3,4-(14)C]glucose generates about four times more [(14)C]lactate compared to (14)CO(2) in extracellular fluid, suggesting that most lactate is not locally oxidized. In brain slices, direct assays of lactate uptake from extracellular fluid demonstrate that astrocytes have faster influx and higher transport capacity than neurons. Also, lactate transfer from a single astrocyte to other gap junction-coupled astrocytes exceeds astrocyte-to-neuron lactate shuttling. Astrocytes and neurons have excess capacities for glycolysis, and oxidative metabolism in both cell types rises during sensory stimulation. The energetics of brain activation is quite complex, and the proportion of glucose consumed by astrocytes and neurons, lactate generation by either cell type, and the contributions of both cell types to brain images during brain activation are likely to vary with the stimulus paradigm and activated pathways.

Fuelling cerebral activity in exercising man

The metabolic response to brain activation in exercise might be expressed as the cerebral metabolic ratio (MR; uptake O2/glucose + 1/2 lactate). At rest, brain energy is provided by a balanced oxidation of glucose as MR is close to 6, but activation provokes a 'surplus' uptake of glucose relative to that of O2. Whereas MR remains stable during light exercise, it is reduced by 30% to 40% when exercise becomes demanding. The MR integrates metabolism in brain areas stimulated by sensory input from skeletal muscle, the mental effort to exercise and control of exercising limbs. The MR decreases during prolonged exhaustive exercise where blood lactate remains low, but when vigorous exercise raises blood lactate, the brain takes up lactate in an amount similar to that of glucose. This lactate taken up by the brain is oxidised as it does not accumulate within the brain and such pronounced brain uptake of substrate occurs independently of plasma hormones. The 'surplus' of glucose equivalents taken up by the activated brain may reach approximately 10 mmol, that is, an amount compatible with the global glycogen level. It is suggested that a low MR predicts shortage of energy that ultimately limits motor activation and reflects a biologic background for 'central fatigue'.

Modeling cerebral arteriovenous lactate kinetics after intravenous lactate infusion in the rat

Venous-arterial lactate differences across the brain during lactate infusion in rats were studied, and the fate of lactate was described with a mathematical model that includes both cerebral and extracerebral kinetics. Ultrafiltration was used to sample continuously and simultaneously arterial and venous blood. Subsequent application of flow injection analysis and biosensors allowed the measurement of glucose and lactate concentrations every minute. Because of the high temporal resolution, arteriovenous lactate kinetics could be modeled in individual experiments. The existence of both a cerebral lactate sink and a lactate exchangeable compartment, representing approximately 24% of brain volume, was thus modeled.

Cerebral metabolism during upper and lower body exercise

When continuation of exercise calls for a "will," the cerebral metabolic ratio of O2 to (glucose + lactate) decreases, with the largest reduction (30-50%) at exhaustion. Because a larger effort is required to exercise with the arms than with the legs, we tested the hypothesis that the reduction in the cerebral metabolic ratio would become more pronounced during arm cranking than during leg exercise. The cerebral arterial-venous differences for blood-gas variables, glucose, and lactate were evaluated in two groups of eight subjects during exhaustive arm cranking and leg exercise. During leg exercise, exhaustion was elicited after 25 +/- 6 (SE) min, and the cerebral metabolic ratio was reduced from 5.6 +/- 0.2 to 3.5 +/- 0.2 after 10 min and to 3.3 +/- 0.3 at exhaustion (P < 0.05). Arm cranking lasted for 35 +/- 4 min and likewise decreased the cerebral metabolic ratio after 10 min (from 6.7 +/- 0.4 to 5.0 +/- 0.3), but the nadir at exhaustion was only 4.7 +/- 0.4, i.e., higher than during leg exercise (P < 0.05). The results demonstrate that exercise decreases the cerebral metabolic ratio when a conscious effort is required, irrespective of the muscle groups engaged. However, the comparatively small reduction in the cerebral metabolic ratio during arm cranking suggests that it is influenced by the exercise paradigm.

Increased lactate/pyruvate ratio augments blood flow in physiologically activated human brain

The factors regulating cerebral blood flow (CBF) changes in physiological activation remain the subject of great interest and debate. Recent experimental studies suggest that an increase in cytosolic NADH mediates increased blood flow in the working brain. Lactate injection should elevate NADH levels by increasing the lactate/pyruvate ratio, which is in near equilibrium with the NADH/NAD(+) ratio. We studied CBF responses to bolus lactate injection at rest and in visual stimulation by using positron-emission tomography in seven healthy volunteers. Bolus lactate injection augmented the CBF response to visual stimulation by 38-53% in regions of the visual cortex but had no effect on the resting CBF or the whole-brain CBF. These lactate-induced CBF increases correlated with elevations in plasma lactate/pyruvate ratios and in plasma lactate levels but not with plasma pyruvate levels. Our observations support the hypothesis that an increase in the NADH/NAD(+) ratio activates signaling pathways to selectively increase CBF in the physiologically stimulated brain regions.

NADH augments blood flow in physiologically activated retina and visual cortex

The mechanism(s) that increase retinal and visual cortex blood flows in response to visual stimulation are poorly understood. We tested the hypothesis that increased transfer of electrons and protons from glucose to cytosolic free NAD(+), reducing it to NADH, evoked by increased energy metabolism, fuels redox-signaling pathways that augment flow. The near-equilibrium between free cytosolic NADH/NAD(+) and lactate/pyruvate ratios established by lactate dehydrogenase predicts that transfer of additional electrons and protons from injected lactate to NAD(+) will augment the elevated blood flows in stimulated retina and cortex, whereas transfer of electrons and protons from NADH to injected pyruvate will attenuate the elevated flows. These predictions were tested and confirmed in rats. Increased flows evoked by stimulation also were prevented by inhibition of nitric oxide synthase. These findings support an important role for cytosolic free NADH in fueling a signaling cascade that increases *NO production, which augments blood flow in photostimulated retina and visual cortex.

Evidence for a lactate pool in the rat brain that is not used as an energy supply under normoglycemic conditions

Lactate derived from glucose can serve as an energy source in the brain. However, it is not certain how much lactate, directly taken from the blood circulation, may replace glucose as an energy source. This study aimed to estimate the uptake, release, and utilization of lactate entering the brain from the blood circulation. The change in cerebral venous-arterial glucose and lactate differences after lactate infusions in the anesthetized rat were measured. Ultrafiltration probes were placed in the aorta and in the jugular vein, and connected to a flow injection analysis system with biosensors for glucose and lactate. Measurements were taken every minute. Lactate efflux was observed at baseline, whereas an influx of lactate was seen during lactate infusion. Immediately after the infusion there was a net efflux of lactate from the brain. The results suggest that the majority of lactate moving into the brain is not used as an energy substrate, and that lactate does not replace glucose as an energy source. Instead, the authors propose the concept of a lactate pool in the brain that can be filled and emptied in accordance with the blood lactate concentration, but which is not used as an energy supply for cerebral metabolism.

Monitoring arterio-venous differences of glucose and lactate in the anesthetized rat with or without brain damage with ultrafiltration and biosensor technology

Continuous monitoring of arterio-venous glucose and lactate differences may serve as a diagnostic tool to assess normal brain function and brain pathology. We describe a method and some results obtained with arterio-venous measurements of glucose and lactate in the blood of the halothane-anesthetized rat and after brain injury. The method is based on low flow rate ultrafiltration for continuous collection of blood filtrate combined with flow injection analysis and biosensors for the detection of glucose and lactate. We measured the glucose and lactate concentration every minute in the jugular vein and the aorta at control conditions and during and after inflation of an embolectomy-balloon for 2 min. Net cerebral lactate efflux and glucose uptake was seen under control conditions and at low blood lactate levels. During brain injury both lactate release and glucose uptake were reduced and there was a net lactate influx at high arterial lactate levels. These results indicate that the flux of lactate in and out of the brain is not only dependent on the lactate concentration in the brain, but on blood levels as well, possibly because of bi-directional flux through the monocarboxylate transporter type 1.

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

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

The effects of morphine on blood-brain barrier disruption caused by intracarotid injection of hyperosmolar mannitol in rats

This study was performed to evaluate whether morphine could alter the degree of disruption of the blood-brain barrier (BBB) caused by hyperosmolar mannitol. Under isoflurane anesthesia, rats in a control group were infused with 25% mannitol into the internal carotid artery before measuring the transfer coefficient (Ki) of (14)C-alpha-aminoisobutyric acid. Infusion of morphine 3 mg/kg in the small-dose morphine group and 10 mg/kg in the large-dose morphine group was completed, 10 min before administering mannitol. There were no statistical differences in systemic blood pressures between these three groups of animals. In the control group, the Ki of the ipsilateral cortex where mannitol was injected, increased to 4.6 times that of the contralateral cortex (19.5 +/- 8.5 vs 4.2 +/- 1.2 microL. g(-1). min(-1), P < 0.002). The Ki of the ipsilateral cortex of the small-dose morphine group was 13.5 +/- 7.6 microL. g(-1). min(-1). The Ki of the ipsilateral cortex of the large-dose morphine group was 9.2 +/- 4.5 microL. g(-1). min(-1) and was smaller than that of control animals (P < 0.05). There was no significant difference in the Ki of the contralateral cortex among the three groups. In conclusion, morphine attenuated BBB disruption induced by hyperosmolar solution without significant effects on systemic blood pressure.

IMPLICATIONS

Our study suggests that morphine may be effective in reducing the blood-brain barrier disruption by hyperosmolar mannitol without significant effects on systemic blood pressure.

Rapid efflux of lactate from cerebral cortex during K+ -induced spreading cortical depression

Rapid transport of lactate from activated brain regions to blood, perhaps reflecting enhanced metabolite trafficking, would prevent local trapping of labeled metabolites of [6-14C]glucose and cause underestimation of calculated CMRglc. Because the identities of glucose metabolites lost from activated structures and major routes of their removal are not known, arteriovenous differences across brains of conscious normoxic rats for derivatives of [6-14C]glucose were determined under steady-state conditions in blood during K+ -induced spreading cortical depression. Lactate was identified as the major labeled product lost from brain. Its entry to blood was detected within 2 minutes after a pulse of [6-14C]glucose, and it accounted for 96% of the 14C lost from brain within approximately 8 minutes. Lactate efflux corresponded to 20% of glucose influx, but accounted for only half the magnitude of underestimation of CMRglc when [14C]glucose is the tracer, suggesting extensive [14C]lactate trafficking within brain. [14C]Lactate spreading within brain is consistent with (1) relatively uniform pattern labeling of K+ -treated cerebral cortex by [6-14C]glucose contrasting heterogeneous labeling by [14C]deoxyglucose, and (2) transport of 14C-labeled lactate and inulin up to 1.5 and 2.4 mm, respectively, within 10 minutes. Thus, newly synthesized lactate exported from activated cells rapidly flows to blood and probably other brain structures.

The effects of pentobarbital on blood-brain barrier disruption caused by intracarotid injection of hyperosmolar mannitol in rats

This study was performed to evaluate both the effects of pentobarbital on disruption of the blood-brain barrier (BBB) by hyperosmolar mannitol and the relationship between its effect on blood pressure and the integrity of the BBB. Under isoflurane anesthesia, rats in the control group were infused with 25% mannitol into the internal carotid artery before measuring the transfer coefficient (Ki) of 14C alpha-aminoisobutyric acid. Ten minutes before the administration of mannitol, rats received an infusion of pentobarbital: 20 mg/kg in the small-dose group and 50 mg/kg in the large-dose group. In another group of animals (hydralazine group), hydralazine was administered to maintain the mean arterial blood pressure (MAP) at 65 mm Hg during the experimental period. The MAP of the control group (113 +/- 14 mm Hg) was significantly higher (P < 0.002) than that of the small-dose pentobarbital group (78 +/- 13 mm Hg) or the large-dose pentobarbital group (68 +/- 14 mm Hg). In the control group, the Ki of the cortex ipsilateral to the mannitol injection was increased to 4.5 times that of the contralateral cortex (14.5 +/- 7.7 vs 3.2 +/- 0.6 microL x g(-1) x min(-1); P < 0.002). The Ki of the ipsilateral cortex of the small-dose pentobarbital group was 9.7 +/- 5.6 microL x g(-1) x min(-1). The Ki of the ipsilateral cortex of the large-dose pentobarbital group was 5.5 +/- 2.9 microL x g(-1) x min(-1), and lower (-9.0 microL x g(-1) x min(-1)) than that of the control animals (P < 0.05). There was no significant difference in the Ki of the contralateral cortex among any of the three groups of animals. At the same MAP, the Ki of the ipsilateral cortex of the large-dose pentobarbital group was lower (-4.3 microL x g(-1) x min(-1)) than that of the hydralazine group (9.8 +/- 4.6 microL x g(-1) x min(-1)) (P < 0.05). Pentobarbital attenuated the BBB disruption induced by hyperosmolar mannitol. This may be attributed, at least in part, to the blood pressure effect of pentobarbital.

IMPLICATIONS

When the blood-brain barrier (BBB) was disrupted by a hyperosmolar solution, pentobarbital attenuated the degree of leakage of the BBB. Systemic hypotension caused by pentobarbital played a significant role in decreasing the leakage. Our study suggests that when the BBB is disrupted, pentobarbital may be effective in protecting the BBB. Furthermore, systemic blood pressure plays an important role in determining the degree of disruption.

Behind the scenes of functional brain imaging: a historical and physiological perspective

At the forefront of cognitive neuroscience research in normal humans are the new techniques of functional brain imaging: positron emission tomography and magnetic resonance imaging. The signal used by positron emission tomography is based on the fact that changes in the cellular activity of the brain of normal, awake humans and laboratory animals are accompanied almost invariably by changes in local blood flow. This robust, empirical relationship has fascinated scientists for well over a hundred years. Because the changes in blood flow are accompanied by lesser changes in oxygen consumption, local changes in brain oxygen content occur at the sites of activation and provide the basis for the signal used by magnetic resonance imaging. The biological basis for these signals is now an area of intense research stimulated by the interest in these tools for cognitive neuroscience research.

Oxidative glucose metabolism in rat brain during single forepaw stimulation: a spatially localized 1H[13C] nuclear magnetic resonance study

In the alpha-chloralose-anesthetized rat during single forepaw stimulation, a spatially localized 1H[13C] nuclear magnetic resonance spectroscopic method was used to measure the rate of cerebral [C4]-glutamate isotopic turnover from infused [1,6-(13)C]glucose. The glutamate turnover data were analyzed using a mathematical model of cerebral glucose metabolism to evaluate the tricarboxylic acid (TCA) cycle flux (V(TCA)). During stimulation the value of V(TCA) in the sensorimotor region increased from 0.47 +/- 0.06 (at rest) to 1.44 +/- 0.41 micromol x g(-1) x min(-1) (P < 0.01) in the contralateral hemispheric compartment (24 mm3) and to 0.65 +/- 0.10 micromol x g(-1) x min(-1) (P < 0.03) in the ipsilateral side. Each V(TCA) value was converted to the cerebral metabolic rates of glucose oxidation (oxidative-CMR(glc)) and oxygen consumption (CMR(O2)). These rates were corrected for partial-volume based on activation maps obtained by blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI). The percent increase and the absolute value of oxidative-CMR(glc) in the activated regions are similar to values reported previously for total-CMR(glc) using the same activation paradigm. This indicates that the large majority of energy required for brain activation, in going from the resting to an activated state, is supplied by glucose oxidation. The level of activity during stimulation is relevant to awake animals because the oxidative-CMR(glc) (1.05 +/- 0.28 micromol x g(-1) x min(-1); current study) is in the range of total-CMR(glc) previously reported for awake rats undergoing physiologic activation (0.7-1.4 micromol x g(-1) x min(-1)). It is concluded that oxidative glycolysis is the main source of energy for increased brain activity and a positive BOLD fMRI signal-change occurs in conjunction with a large increase in CMR(O2).

Stimulated release of lactate in freely moving rats is dependent on the uptake of glutamate

1. Physiological stimulation of neuronal activity induces an increase in extracellular lactate. Experiments were designed to determine the role of the reuptake of neuronally released glutamate in lactate delivery to the extracellular compartment. 2. In vivo microdialysis was used in freely moving rats. The lactate concentration in striatal dialysate was assayed using an enzyme-based on-line assay at 1 min intervals. Drugs were given locally through the dialysis probe. 3. The extracellular concentration of lactate, determined using the zero net flux method, was 346 +/- 21 microM. 4. Induced grooming caused a maximal increase in lactate concentration in striatal dialysate of 58 +/- 10%. 5. Administration of 100 microM glutamate caused a transient increase in dialysate lactate concentration of 72 +/- 17%. 6. A 20 min infusion of the glutamate uptake blockers beta-D,L-threohydroxyaspartate (THA) or pirrolidine-2-4-dicarboxylate (PDC) produced an increase in basal lactate, which was sustained in response to THA and transient in response to PDC. 7. Grooming induced during the infusion of PDC produced no significant increase in lactate. 8. Grooming induced after the infusion of the glutamate uptake blockers gave rise to a reduced increase in lactate. 9. These results support the hypothesis that stimulated release of lactate is dependent on the uptake of glutamate.

The mechanisms controlling physiologically stimulated changes in rat brain glucose and lactate: a microdialysis study

1. This study is concerned with the supply of metabolic substrates for neuronal metabolism. Experiments were carried out to investigate whether mechanisms demonstrated in cultured astrocytes also occurred in vivo; these were cAMP-mediated breakdown of glycogen and glutamate uptake-stimulated release of lactate. 2. In vivo microdialysis was used in freely moving rats. Lactate and glucose in the dialysate were assayed using enzyme-based on-line assays. Drugs were given locally through the dialysis probe. Regional cerebral blood flow was measured using the hydrogen clearance method. 3. There was an increase in dialysate glucose in response to the beta-adrenoceptor agonist isoprenaline and to 8-bromo-cAMP, an analogue of cAMP, the second messenger of beta-adrenoceptor stimulation. The effect of isoprenaline was blocked by the antagonist propranolol. Isoprenaline had no effect on dialysate lactate, which was increased by the glutamate uptake blocker beta-D,L-threohydroxyaspartate (THA). 4. Physiological stimulation of neuronal activity produced an increase in both lactate and glucose. The increase in lactate was depressed in the presence of THA but was unaffected by propranolol. The increase in glucose was blocked by propranolol. Regional cerebral blood flow was increased by physiological stimulation but was unaffected by propranolol. 5. These results demonstrate that physiologically stimulated increases in glucose and lactate in the brain are mediated by different mechanisms.

Increased blood-brain permeability with hyperosmolar mannitol increases cerebral O2 consumption and O2 supply/consumption heterogeneity

This study was performed to evaluate whether increasing the permeability of the blood-brain barrier by unilateral intracarotid injection of hyperosmolar mannitol would alter O2 consumption and the O2 supply/consumption balance in the ipsilateral cortex. Rats were anesthetized with 1.4% isoflurane using mechanical ventilation. Retrograde catheterization of a unilateral external carotid artery was performed to administer 25% mannitol at a rate of 0.25 ml/kg/s for 30 s. The blood-brain barrier transfer coefficient (K(i) of 14C-alpha aminoisobutyric acid was measured in one group (N = 7) after administering mannitol. Regional cerebral blood flow (rCBF), regional arterial and venous O2 saturation and O2 consumption were measured in another group using a 14C-iodoantipyrine autoradiographic technique and microspectrophotometry (N = 7). Vital signs were similar before and after administering mannitol. K(i) was significantly higher in the ipsilateral cortex (IC) (22.3 +/- 8.4 microliters/g/min) than in the contralateral cortex (CC) (4.4 +/-1.1). rCBF was similar between the IC (105 +/- 21 ml/g/min) and the CC (93 +/- 20). Venous O2 saturation was lower in the IC (43 +/- 7%) than in the CC (55 +/- 4%). The coefficient of variation (100 x SD/mean) of venous O2 saturation was significantly elevated in the IC (32.3) compared with the CC (18.2), indicating increased heterogeneity of O2 supply/consumption balance. O2 consumption was higher in the IC (9.6 +/- 3.0 ml O2/100 g/min) than in the CC (6.7 +/- 1.5). Our data suggested that increasing permeability of the blood-brain barrier increased cerebral O2 consumption and the heterogeneity of local O2 supply/consumption balance.

Cerebral transport and metabolism of 1-11C-D-glucose during stepped hypoglycemia

Attempts to measure blood-to-brain glucose transport and cerebral glucose metabolism with 11C-glucose have been hampered by methods that require jugular venous sampling or do not adequately account for the efflux of labeled metabolites from the brain. We performed eight positron emission tomography studies with 1-11C-D-glucose in macaques at arterial plasma glucose concentrations of 8.43 to 1.51 mumol ml-1 (152-27 mg dl-1) using a model that includes a fourth rate constant to account for regional egress of all 11C-metabolites. Values for blood-to-brain glucose influx, cerebral glucose metabolism, and brain free glucose concentration agreed closely with values obtained in mammals by other investigators. Values for net extraction fraction corresponded closely to simultaneously measured arteriovenous values. We demonstrated that utilization of a model that includes a fourth rate constant to account for regional egress of all 11C-metabolites with positron emission tomography and 1-11C-D-glucose provides accurate measurements of blood-to-brain glucose transport and cerebral glucose metabolism in vivo without need for jugular venous sampling, even under conditions of severe hypoglycemia.

Labeling of metabolic pools by [6-14C]glucose during K(+)-induced stimulation of glucose utilization in rat brain

[6-14C]Glucose is the tracer sometimes recommended to assay cerebral glucose utilization (CMRglc) during transient or brief functional activations, but when used to study visual stimulation and seizures in other laboratories, it underestimated CMRglc. The metabolic fate of [6-14C]glucose during functional activation of cerebral metabolism is not known, and increased labeling of diffusible metabolites might explain underestimation of CMRglc and also reveal trafficking of metabolites. In the current studies cerebral cortex in conscious rats was unilaterally activated metabolically by KCl application, and CMRglc was determined in activated and contralateral control cortex with [6-14C]glucose or 2-[14C]deoxy-glucose ([14C]DG) over a 5- to 7-min interval. Local 14C concentrations were determined by quantitative autoradiography. Labeled precursor and products were measured bilaterally in paired cortical samples from funnel-frozen brains. Left-right differences in 14C contents were small with [6-14C]glucose but strikingly obvious in [14C]DG autoradiographs. CMRglc determined with [6-14C]glucose was slightly increased in activated cortex but 40-80% below values obtained with [14C]DG. [14C]Lactate was a major metabolite of [6-14C]glucose in activated but not control cortex and increased proportionately with unlabeled lactate. These results demonstrate significant loss of labeled products of [6-14C]glucose from metabolically activated brain tissue and indicate that [14C]DG is the preferred tracer even during brief functional activations of brain.

Optimizing the measurement of regional cerebral glucose consumption with [6-14C]glucose

[6-14C]Glucose is used to trace the cerebral metabolic rate of glucose (CMRGlc) in vivo in experiments lasting 5-10 min. Initially 14C is trapped in intermediary metabolite pools. Subsequently 14C is lost as a function of time and metabolic rate, primarily as 14CO2. Experiments were designed to evaluate the rate of 14C lost as 14CO2 or as [14C]lactate from brain labeled with [6-14C]glucose during times up to 15 min. CMRGlc was measured during 5, 7.5, 10 and 15 min in 60 brain areas. At longer times the loss of 14C was reflected by lower apparent values of brain CMRGlc. Arteriovenous measurements across brain revealed no significant loss of [14C]lactate in normal rats or rats with bicuculline-induced seizures. It was concluded that the primary form in which 14C was lost was as 14CO2. As expected, the rate of 14CO2 loss was greater in structures with high metabolic rates. The data were analyzed to determine the parameters necessary to rectify the data so that uniform values of CMRGlc were obtained up to 15 min. Tables were made to predict the degree of 14C loss as well as the 14C-metabolites/[6-14C]glucose ratio as a function of time and metabolic rate. These tables can be used to plan the maximum and minimum experimental times for optimal results.

Regional cerebral oxidative and total glucose consumption during rest and activation studied with positron emission tomography

The relationship between regional oxidative and total rCMRglc in five healthy volunteers in activated and non-activated areas of the brain has been investigated with positron emission tomography (PET). The tracers [1-11C]-D-glucose and [2-18F]2-fluoro-2-deoxy-D-glucose were used. A previous study has shown that the former may be used to measure the rate of glucose oxidation while the latter tracer is used to measure the total rate of glucose consumption. Regional activation was performed (voluntary finger movements). Use of a computerized brain atlas enabled comparison between the regional oxidative and total rCMRglc in each volume element of the brain for the group of subjects. The values of total and oxidative rCMRglc, when calculated for each volume element of the brain and displayed in a scatter plot, were found to be symmetrically grouped around a straight line which passes close to the origin. The slope of this line varied between the subjects. This indicates that, on the average, the fraction of non-oxidative glucose utilization is constant within each subject, regardless of the value of rCMRglc and, further, that the fraction of non-oxidative glucose utilization varies between subjects. The total and oxidative CMRglc in the activated left hand area were 23.4 +/- 0.9% (mean +/- SEM) and 11.7 +/- 0.3%, respectively, higher than in the contralateral homologous non-activated area. Our interpretation of the difference is that regional activation increases the fraction of non-oxidative glucose consumption. This interpretation is supported by a previous PET study using [15O]O2, and by studies using MRS technique.

Regional blood-brain lactate influx

Regional blood-to-brain lactate transport was studied in chloral hydrate anesthetized rats using the single pass, dual-label, indicator fractionation, right atrial injection method. Lactate influx was resolved into two components, a saturable, stereospecific (to the L-enantiomer) component and a non-saturable, non-stereospecific diffusional component. The saturable component was found to have a low efficiency and moderate capacity with transport affinity coefficients between 6 and 14 mM and transport maxima of 23-40 mumol/100 g/min in the various brain regions. Lactate transport was not inhibited by probenecid. The diffusional component was determined from D-lactate influx measurements and the regional linear diffusion coefficients ranged from 0.020 to 0.036 ml/g/min. At the usual levels of plasma lactate (1-1.5 mM) these two influx components were about equal. The relative contribution of the non-stereospecific diffusional component was increased at higher plasma lactate concentrations. Lactate clearance, estimated by the total apparent permeability x surface area products was between 6 and 8 ml/100 g/min.

Decrease of glucose in the human visual cortex during photic stimulation

Localized proton NMR spectroscopy was used to study cerebral metabolism in the visual cortex of healthy adults during rest and photic stimulation. Basal lactate levels showed considerable interindividual differences ranging from below detectability (less than 0.3 mM) to about 1 mM without consistent alteration during photic stimulation. Local brain glucose levels were significantly reduced (approximately 50%) during the entire period of photic stimulation and recovered to resting levels (approximately 0.8 mM) within 10 min after the end of stimulation. This decrease reflects the establishment of a new equilibrium due to enhanced delivery (blood flow) and enhanced consumption. The absence of lactate accumulation supports the hypothesis of a rapid efflux of lactate from brain tissue under activated conditions.

Lactate and pH change in close correlation in the extracellular space of the rat brain during cortical spreading depression

pH sensitive microcelectrodes were used in combination with microdialysis (MD) technique to measure extracellular pH (pHe) and extracellular lactate (lace) within the cortex of rat brains during cortical spreading depression (SD). SD was induced by local K(+)-application and identified by DC recordings. It was accompanied by an extracellular acidification of 0.34 +/- 0.06 pH units and by a 2.8 +/- 0.80 fold increase of lace; the recovery of pHe took place within three phases, that of lace within 2 phases. The recovery of both parameters was complete about 45 min after the onset of SD. We conclude that the changes of lace and pHe are closely related. This indicates both lactate and protons to be transported in parallel.