The Metabolism of [3- 13 C]Lactate in the Rat Brain Is Specific of a Pyruvate Carboxylase-Deprived Compartment Anne-Karine Bouzier, Eric Thiaudiere, Marc Biran, Richard Rouland, Paul Canioni, and Michel Merle Laboratoire de Re ´sonance Magne ´tique des Syste `mes Biologiques, UMR 5536, Centre National de la Recherche Scientifique–Universite ´ Victor Segalen, Bordeaux, France Abstract: Lactate metabolism in the adult rat brain was investigated in relation with the concept of lactate traf- ficking between astrocytes and neurons. Wistar rats were infused intravenously with a solution containing either [3- 13 C]lactate (534 mM) or both glucose (750 mM) and [3- 13 C]lactate (534 mM). The time courses of both the concentration and 13 C enrichment of blood glucose and lactate were determined. The data indicated the occur- rence of [3- 13 C]lactate recycling through liver gluconeo- genesis. The yield of glucose labeling was, however, reduced when using the glucose-containing infusate. Af- ter a 20-min or 1-h infusion, perchloric acid extracts of the brain tissue were prepared and subsequently ana- lyzed by 13 C- and 1 H-observed/ 13 C-edited NMR spec- troscopy. The 13 C labeling of amino acids indicated that [3- 13 C]lactate was metabolized in the brain. Based on the alanine C3 enrichment, lactate contribution to brain me- tabolism amounted to 35% under the most favorable conditions used. By contrast with what happens with [1- 13 C]glucose metabolism, no difference in glutamine C2 and C3 labeling was evidenced, indicating that lactate was metabolized in a compartment deprived of pyruvate carboxylase activity. This result confirms, for the first time from an in vivo study, that lactate is more specifically a neuronal substrate. Key Words: 13 C-NMR—Lactate— Brain metabolism—Neurons—Astrocytes. J. Neurochem. 75, 480 – 486 (2000). It is recognized that blood glucose is the main energy substrate for the adult brain (Sokoloff, 1989). Glucose carbons can reach neurons through at least three different ways: (a) After crossing the blood– brain barrier, glucose diffuses to cells (neurons and astrocytes) through the extracellular space. (b) Glucose is transported across endothelial cells and astrocyte end-feets and reaches the neurons after diffusion inside the astrocytes. According to this way, membrane crossing is facilitated by different transporters (GLUT-1 and GLUT-3) (Vannucci et al., 1998). (c) Glucose is metabolized inside the astrocytes in another substrate, which is then transferred to the neu- rons. Several in vitro studies suggest that this substrate could be lactate (Dringen et al., 1993; Pellerin and Mag- istretti, 1994; Larrabee, 1995; Waagepetersen et al., 1998). Actually, it has been shown that lactate supports synaptic function in brain slices in the absence of glucose (Schurr et al., 1988) and also allows neuron protection and recovery after hypoxia (Schurr et al., 1997). The presence of different isoforms of monocarboxylate trans- porters on astrocytes and neurons (MCT1 and MCT2) (Bro ¨er et al., 1997, 1999) and a specific cellular distri- bution of lactate dehydrogenase (EC 1.1.1.27) isoen- zymes (LDH1 and LDH5) (Bittar et al., 1996) sustain the concept of an astrocyte–neuron lactate shuttle for energy supply to neurons (Pellerin and Magistretti, 1994). Because of a limited entry across the blood– brain barrier, blood lactate is generally supposed not to be a significant substrate for adult brain metabolism under normal resting conditions. The monocarboxylate trans- porter MCT1 is, however, present on vascular endothe- lial cells (Leino et al., 1999), and its kinetic characteris- tics [K m for L-lactate in the millimolar range (Jackson and Halestrap, 1996; Bro ¨ er et al., 1997)] make possible a substantial entry of lactate in brain under high lacticemia conditions. In fact, protection by lactate of cerebral func- tion during hypoglycemia has been demonstrated (Maran et al., 1994; Veneman et al., 1994; King et al., 1997). Using favorable conditions for lactate entry into brain therefore offers the opportunity of investigating the spec- ificity of its metabolism in this tissue. Indeed, by using a labeled molecule, lactate metabolism can be distin- guished from that of glucose, when both substrates gen- erate the same intermediate (pyruvate). The aim of the present study was to analyze the in vivo lactate metabolism in the adult rat brain. After infusion of [3- 13 C]lactate to the animals, the incorporation of 13 C into the amino acids extracted from brain was analyzed Received January 5, 2000; revised manuscript received March 13, 2000; accepted March 16, 2000. Address correspondence and reprint requests to Dr. M. Merle at Laboratoire de Re ´sonance Magne ´tique des Syste `mes Biologiques, UMR 5536, CNRS–Universite ´ Victor Segalen, case 93, 146 rue Le ´o- Saignat, 33076 Bordeaux, France. E-mail: merle@rmsb.u-bordeaux2.fr Abbreviation used: ppm, parts per million. 480 Journal of Neurochemistry Lippincott Williams & Wilkins, Inc., Philadelphia © 2000 International Society for Neurochemistry