[1‐¹³C]Glucose metabolism in rat cerebellar granule cells and astrocytes in primary culture

Abstract

The metabolism of [ l -'3C]glucose in rat cerebellum astrocytes and granule cells was investigated using I3C-and 'H-NMR spectroscopy. Near homogeneous primary cultures of each cell type were incubated with [ 1 -13C]g1ucose, under the same conditions. Analysing the relative I3C enrichments of metabolites in spectra of cell perchloric acid extracts, on the one hand, the I3C-*H spin-coupling patterns in 'H-NMR spectra of cell medium lactate and the 13C-13C spin-coupling patterns in l3C-NMR spectra of purified cell glutamate, on the other hand, showed significant differences, between the two cell types, in the activity of various metabolic ways. First, the carbon flux through the oxidative branch of the hexose monophosphate shunt, which leads to unenriched lactate, was found higher in granule cells than in astrocytes. Second, although the specific I3C enrichment of lactate was higher in astrocytes than in granule cells, the fraction of '3C-enriched acetyl-CoA entering the citric acid cycle was more than twice as high in granule cells as in astrocytes. Lactate C3 and acetylCoA C2 enrichments were very similar in granule cells, whereas acetyl-CoA C2 enrichment was 60% lower than that of lactate C3 in astrocytes. These results can be explained by the fact that granule cells used almost exclusively the exogenous glucose to fuel the citric acid cycle, whereas astrocytes used concomitantly glucose and other carbon sources. Last, in the case of granule cells, glutamate C2 and C3 enrichments were equivalent; the carbon flux through the pyruvate carboxylase route was evaluated to be around 15% of the carbon flux through the citrate synthetase route. In astrocytes, glutamate C2 enrichment was higher than that of C3, which could be explained by a pyruvate carboxylase activity much more active in these cells than in granule cells.In the brain, the concept of the existence of two compartmentalized citric acid cycles has emerged from studies on neurotransmitter amino acids (Berl et al., 1961 ;Van den Berg et al., 1969; Hertz, 1979). The cellular basis of the metabolic compartmentation has been more specifically apprehended by analyzing the pools of glutamate and glutamine in brain cells. Effectively, the glial or neuronal nature of cells containing a small high-turnover pool, on the one hand, or a large slow-turnover pool of glutamate, on the other hand, has been demonstrated by histochemical and biochemical localization of enzymes (glutamine synthetase and glutaminase) and transport carriers involved in glutamate and glutamineCorrespondence to

A mathematical model of compartmentalized neurotransmitter metabolism in the human brain

“…Pyruvate carboxylase (PC, EC 6.4.1.1) transfers label from pyruvate C3 to OAA C3 in the glial compartment. In this model, reverse flux from OAA to fumarate was neglected, on the basis of previous reports suggesting a substantial difference in labeling at the C1 and C4 position of OAA (31), which is supported by differential labeling of Glu C2 and C3 in astrocytes (41). Neglecting any backflux to fumarate underestimates the pyruvate carboxylase flux when assessed from the differential labeling of the C2 and C3 positions in Glu and Gln.…”

Mathematical Modelling of the Citric Acid Cycle for the Analysis of Glutamine Isotopomers from Cerebellar Astrocytes Incubated with [1‐13C]glucose

“…Primary cultures of 8-day-old rat cerebellar astrocytes were performed as previously described [8]. The culture medium, Dulbecco's modified Eagle's medium (DMEM) supple-mented with 10% fetal calf serum, was replaced twice a week.…”

“…The model included also the possibility of orientation-conserved transfer of the four-carbon citric acid cycle intermediates. It was also applied to our previous data concerning glutamate kotopomers from granule cells incubated with [I-"Clglucose [8].…”

In vivo 13C NMR studies of compartmentalized cerebral carbohydrate metabolism

“…This calculation assumed a P:O ratio of 2.5:1, when assuming a P:O ratio of 3:1 as in previous studies (Sibson et al, 1998), almost 12 ATP are generated per glutamine molecule, which is approximately a third as efficient as complete glucose oxidation, yet five-six times as efficient as anaerobic glycolysis alone. Although differences exist as to the magnitude of the flux through pyruvate carboxylase, the labeling pattern is different between glutamine and Glu (Gruetter et al, 1998a;Gruetter et al, 2001;Lapidot and Gopher, 1994;Martin et al, 1993), which implies substantial contribution of pyruvate carboxylase to the flux through glutamine synthetase. Even the lowest reported value of 0.04 mol/(g min) results in an ATP generation of 0.4 mol/(g min), which corresponds to 50-75% of the ATP needed for Glu uptake and conversion to glutamine (Gruetter et al, 1998a;Gruetter et al, 2001).…”