Labeling of Metabolic Pools by [6-¹⁴C]Glucose during K⁺-Induced Stimulation of Glucose Utilization in Rat Brain

Abstract

[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]deoxyglucose ([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.

Trafficking of Glucose, Lactate, and Amyloid-β from the Inferior Colliculus through Perivascular Routes

“…Discordant results obtained with [6- 14 C]glucose and [ 14 C]deoxyglucose were hypothesized to arise from upregulation of glycolysis, with increased [ 14 C]lactate production and release from activated structures (Fig. 1A) (Collins et al, 1987; Ackermann and Lear, 1989; Lear and Ackermann, 1989; Lear, 1990), a concept supported by rapid efflux of [ 14 C]lactate to blood during spreading cortical depression (Adachi et al, 1995; Cruz et al, 1999). Spreading and release of [ 14 C]glucose-derived label during acoustic activation of the inferior colliculus is reduced by inhibition of lactate transporters and astrocytic gap junctions (Cruz et al, 2007), supporting the importance of lactate release and implicating astrocytes in metabolite dispersal.…”

“…However, incomplete product trapping due to rapid release from activated tissue of labeled metabolites derived from [1- or 6- 14 C]glucose (Fig. 1A) causes underestimation (≥50%) of functional activation in different brain structures under normal stimulatory conditions compared to parallel studies with [ 14 C]deoxyglucose, which is metabolized mainly to [ 14 C]deoxyglucose-6-phosphate that is trapped intracellularly and not further metabolized by the glycolytic pathway (Sokoloff et al, 1977); undetected losses of labeled glucose metabolites also affects interpretation of results obtained under pathophysiological conditions, e.g., when excitatory activity is not reflected by label accumulation (Collins et al, 1987; Ackermann and Lear, 1989; Adachi et al, 1995; Cruz et al, 1999; Cruz et al, 2007). Discordant results obtained with [6- 14 C]glucose and [ 14 C]deoxyglucose were hypothesized to arise from upregulation of glycolysis, with increased [ 14 C]lactate production and release from activated structures (Fig.…”

Artifact versus reality—How astrocytes contribute to synaptic events

“…Further discussion of this important topic is beyond the scope of this review. However, it is important to acknowledge that multiple potential routes of gliotransmitter-independent neuroglia signaling exist and that activation of the astrocytic Na + /K + ATPase will provide metabolic support synaptic transmission by release of lactate (Adachi et al 1995). In summary, the astroglial cradle may not only isolate synapses, but also control synaptic strength by dynamic control of extracellular K + , and at the same time assist with local delivery of energy substrate to active synapses.…”

Contributions of glycogen to astrocytic energetics during brain activation

“…2). During spreading depression, labeled and unlabeled lactate are quickly released from brain to venous blood in amounts equal to about 20% of the 14 C-labeled and unlabeled glucose entering the brain in the same arteriovenous samples (Cruz et al 1999; Adachi et al 1995). This lactate efflux accounted for about half the magnitude of the underestimation of CMR glc with [6- 14 C] glucose (reflecting mainly oxidative metabolism) compared with [ 14 C]DG (reflecting total glucose utilization at the hexokinase step).…”