Comparison of Lactate and Glucose Metabolism in Cultured Neocortical Neurons and Astrocytes Using ¹³C-NMR Spectroscopy

Abstract

In cerebral cortical neurons, synthesis of the tricarboxylic acid (TCA) cycle-derived amino acids, glutamate and aspartate as well as the neurotransmitter of these neurons, γ-aminobutyrate (GABA), was studied incubating the cells in media containing 0.5 mM [U-¹³C]glucose in the absence or presence of glutamine (0.5 mM). Lyophilized cell extracts were analyzed by ¹³C nuclear magnetic resonance (NMR) spectroscopy and HPLC. The present findings were compared to results previously obtained using 1.0 mM [U-¹³C]lactate as the labeled substrate for the neurons. Regardless of the amino acids studied, incubation periods of 1 and 4 h resulted in identical amounts of ¹³C incorporated. Furthermore, the metabolism of lactate was studied under analogous conditions in cultured cerebral cortical astrocytes. The incorporation of ¹³C from lactate into glutamate was much lower in the astrocytes than in the neurons. In cerebral cortical neurons the total amount of ¹³C in GABA, glutamate and aspartate was independent of the labeled substrate. The enrichment in glutamate and aspartate was, however, higher in neurons incubated with lactate. Thus, lactate appears to be equivalent to glucose with regard to its access to the TCA cycle and subsequent labeling of glutamate, aspartate and GABA. It should be noted, however, that incubation with lactate in place of glucose led to lower cellular contents of glutamate and aspartate. The presence of glutamine affected the metabolism of glucose and lactate differently, suggesting that the metabolism of these substrates may be compartmentalized.

Energy Metabolism in Astrocytes: High Rate of Oxidative Metabolism and Spatiotemporal Dependence on Glycolysis/Glycogenolysis

“…However, direct comparison of relative rates of glucose and lactate oxidation are difficult to interpret because of the metabolic consequences of administering 'loading' (versus tracer) doses of labeled lactate and the many metabolic steps and metabolite pools between the labeled precursor and metabolites that must be analyzed to draw firm conclusions. Waagepetersen et al (1998) concluded that lactate is less efficient The expected precursor-product relationship (larger isotope enrichment in the precursor than in the product) is observed, since labeling of C4 in glutamine is secondary to [4-13 C]glutamate uptake by astrocytes and glutamine formation in these cells (from Gruetter et al, 2001). (B) [2-13 C]Acetate leads to an increase in glutamine labeling in C4 (filled circles), primarily due to net synthesis of glutamine in astrocytes, which with a normal precursor-product relationship is followed by slower labeling of C4 in glutamate (open squares), secondary to glutamine uptake by neurons and its conversion to glutamate (From Lebon et al, 2002).…”

The Ketogenic Diet and Brain Metabolism of Amino Acids: Relationship to the Anticonvulsant Effect

“…Instead, they may convert some pyruvate to lactate and release the latter to the extracellular fluid, from which neurons extract it and oxidize it as a fuel. Neurons can respire on lactate (102), but they may require glucose as a substrate if they are to maintain large internal pools of glutamate and aspartate (125). Astrocytic release of lactate and subsequent neuronal oxidation may constitute a mechanism by which neuronal and metabolic activity are effectively coupled (33,66).…”

Regulation of glial metabolism studied by 13C‐NMR

“…In favour of a role of aspartate synthesis in astrocytes, 3 h incubation of astrocytes with pathological concentrations of ammonia and inhibition of GS with MSO increased de novo aspartate synthesis by 115-130 and 158-165% of control, respectively, and even to 200-290% after concomitant administration. 48 In favour of a proposed role of aspartate similar to that of alanine with regards to the alanine-lactate shuttle between neurons and astrocytes, exogenous glutamine stimulates aspartate synthesis from [U- 13 C 6 ]glucose in cortical neurons, 156 and aspartate nitrogen is transferred to alanine in astrocytes. 173 Furthermore, the uptake of aspartate from the incubation medium of cultured astrocytes is as rapid as that of glutamate, 215 and both compounds are utilized more rapidly than all other amino acids.…”

“…21,90 Nonetheless, neuronal lactate utilization dominated. 92,116,117,156 Also glutamate is taken up very efficiently by astrocytes 163,164 at a rate of approx. 0.5 mmol/h/mg, which almost corresponds to the average value of glucose uptake ( Table 3).…”