Brain energy depletion in a rodent model of diffuse traumatic brain injury is not prevented with administration of sodium lactate

Abstract

Lactate has been identified as an alternative fuel for the brain in situations of increased energy demand, as following a traumatic brain injury (TBI). This study investigates the effect of treatment with sodium lactate (NaLac) on the changes in brain energy state induced by a severe diffuse TBI. Rats were assigned to one of the eight groups (n=10 per group): 1-sham, normal saline; 2-TBI, normal saline; 3-TBI, hypertonic saline; 4-TBI, 100mM NaLac, 5-TBI, 500 mM NaLac; 6-TBI, 1280 mM NaLac; 7-TBI, 2000 mM NaLac and 8-TBI-500 mM NaLac+magnesium sulfate. Cerebrums were removed 6h after trauma. Metabolites representative of the energy state (ATP, ATP-catabolites), N-acetylaspartate (NAA), antioxidant defenses (ascorbic acid, glutathione), markers of oxidative stress (malondialdehyde, ADP-ribose) and nicotinic coenzymes (NAD(+)) were measured by HPLC. TBI induced a marked decrease in the cerebral levels of ATP, NAA, ascorbic acid, glutathione and NAD(+) and a significant rise in the content of ATP-catabolites, malondialdehyde and ADP-ribose. These alterations were not ameliorated with NaLac infusion. We observed a significant reduction in cerebral NAD(+), an essential co-enzyme for mitochondrial lactate-dehydrogenase that converts lactate into pyruvate and thus replenishes the tricarboxylic acid cycle. These results suggest that the metabolic pathway necessary to consume lactate may be compromised following a severe diffuse TBI in rats.

Fig. 1

Bar graphs showing (A) the energy charge potential (ECP) and (B) the sum of ATP catabolites determined by HPLC in deproteinized extracts of the cerebral tissue obtained 6 hours after the head trauma. Rats were divided into eight groups as indicated in Table 1. When compared to the sham-operated group, TBI resulted in a significant decrease in ECP and a significant increase in the sum of ATP catabolites. No significant differences were detected among the TBI groups. Data are expressed as means ± standard error of the mean (SEM). Asterisk, * identifies statistical significance (p<0.05).

Fig. 2

Bar graphs showing (A) the GSH/GSSG ratio and (B) the concentration of Malondialdehyde (MDA) determined by HPLC in deproteinized extracts of the cerebral tissue obtained 6 hours after the head trauma. Rats were divided into eight groups as indicated in Table 1. Cerebral concentration of MDA is expressed as nmol/g w.w. When compared to the sham-operated group, TBI resulted in a significant decrease in the GSH/GSSG ratio and a significant increase in the cerebral content of MDA. No significant differences were detected among the TBI groups. Data are expressed as means ± standard error of the mean (SEM). * p<0.05.

Fig. 3

Bar graphs showing the levels of (A) ADP-ribose and (B) NAD⁺ determined by HPLC in deproteinized extracts of the cerebral tissue obtained 6 hours after the head trauma. Rats were divided into eight groups as indicated in Table 1. Cerebral concentration of ADP-ribose and NAD⁺ are expressed as nmol/g w.w. When compared to the sham-operated group, TBI resulted in a significant increase in the cerebral ADP-ribose content and a significant decrease in the cerebral NAD⁺ levels. No significant differences were detected among the TBI groups. Data are expressed as means ± standard error of the mean (SEM). * p<0.05.

Fig. 4

Schematic depiction of the metabolic pathway followed by exogenous lactate once it crosses the blood brain barrier. Lactate is primarily taken up by neurons through the high-affinity isoform monocarboxylate-2 transporter (MCT-2), predominantly localized at the synapses (thick arrow). The isoform MCT-4, mainly found on astrocytic end-feet, has a lower affinity for lactate (thin arrow). Once in the cytosol of neurons, lactate is transported into the mitochondrial matrix by a monocarboxylate (MCT) transporter. Lactate is converted into pyruvate by mitochondrial lactate-dehydrogenase (LDH, isoforms 1 or 3), an enzyme that does not require the coenzyme NAD⁺. LDH 1–3 is the first enzyme of the mitochondrial oxidative pathway and supplies pyruvate to the tricarboxylic acid (TCA) cycle. Eventually lactate oxidization will yield up to 18 ATPs per molecule.