Introduction
Patients with traumatic brain injury (TBI) routinely exhibit cerebral glucose uptake in excess of that expected by the low levels of oxygen consumption and lactate production, bringing into question the metabolic fate of glucose metabolized by the brain. Utilizing 13C-glucose labeling, prior studies have demonstrated increased flux through the pentose phosphate cycle (PPC) during times of cellular stress, such as hypothermia and cellular transformation. The PPC supplies substrates for macromolecular synthesis, DNA repair, and free radical scavenging. This study assessed the PPC following TBI utilizing 13C-labeling techniques.
Methods
13C-glucose (Cambridge Isotope Laboratories; Andover, MA) was infused for 60 min in 7 consented, severe-TBI patients (GCS ≤ 9; 6 males and 1 female) and 6 normal subjects (3 males and 3 females), mean ages 43. 8 and 36.8, respectively (p = 0.54). Arterial and jugular bulb blood sampled during infusion was analyzed for 13C-labeled isotopomers of lactate by gas chromatography coupled to mass spectroscopy. Lactate produced by the action of glycolysis can result in m0 or m2 lactate (lactate with zero or two carbon-13 atoms, respectively). One carbon-13 is lost as CO2 in the PPC, resulting in m1 lactate (only one carbon-13 atom). The product of lactate concentration and fractional abundance of these isotopomers determines blood levels. Arterial-venous differences determine cerebral PPC contribution. Finally, a previously derived formula (PPC = [m1/m2]/[3+{m1/m2}]) for PPC based on the ratio of m1 to m2 lactate yields the flux of glucose through the PPC relative to glycolysis.
Results
There was good enrichment of labeled glucose in arterial-venous blood (mean TBI 17.3% and normal 28.7%; p<0.0001) and incorporation into lactate, demonstrating metabolism of labeled substrate. The PPC was increased in TBI patients relative to normal subjects (23.58% vs. 8.04%, respectively; p=0.002). In the TBI patients there were trends towards correlation of the PPC with CMRO2 (r=0.77, p=0.1) and jugular lactate (r=0.9, p=0.08). There was no correlation with enrichment of labeled glucose for either TBI (r=0.4, p=0.5) or normal subjects (r=−0.37, p=0.5).
Conclusion
13C-labeling techniques allow detection and quantification of altered metabolic fluxes. Glucose consumed by the brain after TBI appears to be redirected towards alternate metabolic pathways. In particular, flux through the PPC is increased after TBI relative to normal. Whether this increased flux represents metabolic dysfunction or, alternatively, a physiologic response to injury is unclear. However, increased flux through the PPC supplies many substrates that may be critically important for minimizing secondary injury and initiating recovery. Foremost among these are reducing equivalents for prevention of oxidative injury and for lipid biosynthesis, and ribose for DNA repair, replication and transcription, and thus protein synthesis indirectly as well. Elucidating this altered metabolic milieu in the brain following TBI may open doors for novel metabolic support to prevent secondary injury and improve outcomes. In the future, the labeling technology utilized in this study could be used with 13C MR spectroscopy, which would allow more direct monitoring of brain metabolism in vivo and also add regional and anatomic detail to the findings.