Modeling of brain metabolism and pyruvate compartmentation using (13)C NMR in vivo: caution required

Abstract

Two variants of a widely used two-compartment model were prepared for fitting the time course of [1,6-(13)C2]glucose metabolism in rat brain. Features common to most models were included, but in one model the enrichment of the substrates entering the glia and neuronal citric acid cycles was allowed to differ. Furthermore, the models included the capacity to analyze multiplets arising from (13)C spin-spin coupling, known to improve parameter estimates in heart. Data analyzed were from a literature report providing time courses of [1,6-(13)C2]glucose metabolism. Four analyses were used, two comparing the effect of different pyruvate enrichment in glia and neurons, and two for determining the effect of multiplets present in the data. When fit independently, the enrichment in glial pyruvate was less than in neurons. In the absence of multiplets, fit quality and parameter values were typical of those in the literature, whereas the multiplet curves were not modeled well. This prompted the use of robust statistical analysis (the Kolmogorov-Smirnov test of goodness of fit) to determine whether individual curves were modeled appropriately. At least 50% of the curves in each experiment were considered poorly fit. It was concluded that the model does not include all metabolic features required to analyze the data.

Figure 1

Schematic diagram of the proposed model. The two compartments represent neuronal and glial metabolism of pyruvate precursors. The major metabolic pathways in both brain cells are displayed: from glucogenic precursors (mostly blood-borne glucose) to pyruvate through glycolysis, the anaplerotic pathway via pyruvate carboxylase in the glial compartment, neuronal, and glial tricarboxylic acid cycle (TCA) cycle, the glutamate–glutamine cycle, and glutamate-α–ketoglutarate and oxaloacetate–aspartate exchange. Variables estimated from the nuclear magnetic resonance (NMR) data using the model include: α-KG, α-ketoglutarate; ASP, aspartate; FUM, fumarate; GLN, glutamine; GLU, glutamate; OAA, oxaloacetate; PYRg, pyruvate enrichment in glia; PYRn, pyruvate enrichment in neurons; Vg, glial tricarboxylic acid cycle rate; Vn, neuronal tricarboxylic acid cycle rate; Vnt, neurotransmitter exchange rate; Vpc, pyruvate carboxylase rate, which also represents the rate of loss of carbon from the system (bottom of left-hand side of figure); Vx, glutamate-α–ketoglutarate and oxaloacetate–aspartate exchange rate; ‘Vf', exchange rate between oxaloacetate and fumarate.

Figure 2

In vivo time courses of glutamate and glutamine ¹³C contents (μmol/min/g wet weight) at carbons 2, 3, and 4 during [1,6-¹³C₂]glucose infusion. The fitting from the model is represented by the continuous lines. The model used is the single-pyruvate compartment version.

Figure 3

In vivo time course of the glutamate singlet (4S) (open circles) and doublet (4D34) (filled circles) ¹³C content (μmol/min/g wet weight) during [1,6-¹³C₂]glucose infusion. The model fitting is represented by continuous (doublet 4D34) and dashed (singlet 4S) lines. The model used is the single-pyruvate version.