Lactate Provides Metabolic Substrate Support and Attenuates Ischemic Brain Injury in Mice, Revealed by 1H-13C Nuclear Magnetic Resonance Metabolic Technique

<b>Background/Objectives:</b> Lactate, classically considered a metabolic byproduct of anaerobic glycolysis, is implicated in ischemic acidosis and neuronal injury. The recent evidence highlights its potential role in sustaining metabolic networks and neuroprotection. This study investigates lactate's compensatory mechanisms in ischemic brain injury by analyzing post-ischemic metabolic enrichments and inter-regional metabolite correlations. <b>Methods:</b> Dynamic metabolic profiling was conducted using ¹³C-labeled glucose combined with ¹H-¹³C NMR spectroscopy to quantify the metabolite enrichment changes in a murine cerebral ischemia model (<i>n</i> = 8). In vivo validation included intracerebroventricular pH-neutral lactate infusion in ischemic mice to assess the behavioral, electrophysiological, and mitochondrial outcomes. In vitro, HT22 hippocampal neurons underwent oxygen-glucose deprivation (OGD) with pH-controlled lactate supplementation (1 mM), followed by the evaluation of neuronal survival, mitochondrial membrane potential, and glycolytic enzyme expression. <b>Results:</b> NMR spectroscopy revealed a 30-50% reduction in most cerebral metabolites post-ischemia (<i>p</i> < 0.05), while the quantities of lactate and the related three-carbon intermediates remained stable or increased. Correlation analyses demonstrated significantly diminished inter-metabolite coordination post-ischemia, yet lactate and glutamate maintained high metabolic activity levels (r > 0.80, <i>p</i> < 0.01). Lactate exhibited superior cross-regional metabolic mobility compared to those of the other three-carbon intermediates. In vivo, lactate infusion improved the behavioral/electrophysiological outcomes and reduced mitochondrial damage. In the OGD-treated neurons, pH-neutral lactate (7.4) reduced mortality (<i>p</i> < 0.05), preserved the mitochondrial membrane potential (<i>p</i> < 0.05), and downregulated the glycolytic enzymes (HK, PFK, and PKM; <i>p</i> < 0.01), thereby attenuating H⁺ production. <b>Conclusions:</b> Under ischemic metabolic crisis, lactate and the three-carbon intermediates stabilize as critical substrates, compensating for global metabolite depletion. pH-neutral lactate restores energy flux, modulates the glycolytic pathways, and provides neuroprotection by mitigating acidotoxicity.