Lactate metabolism: a new paradigm for the third millennium

Abstract

For much of the 20th century, lactate was largely considered a dead-end waste product of glycolysis due to hypoxia, the primary cause of the O2 debt following exercise, a major cause of muscle fatigue, and a key factor in acidosis-induced tissue damage. Since the 1970s, a 'lactate revolution' has occurred. At present, we are in the midst of a lactate shuttle era; the lactate paradigm has shifted. It now appears that increased lactate production and concentration as a result of anoxia or dysoxia are often the exception rather than the rule. Lactic acidosis is being re-evaluated as a factor in muscle fatigue. Lactate is an important intermediate in the process of wound repair and regeneration. The origin of elevated [lactate] in injury and sepsis is being re-investigated. There is essentially unanimous experimental support for a cell-to-cell lactate shuttle, along with mounting evidence for astrocyte-neuron, lactate-alanine, peroxisomal and spermatogenic lactate shuttles. The bulk of the evidence suggests that lactate is an important intermediary in numerous metabolic processes, a particularly mobile fuel for aerobic metabolism, and perhaps a mediator of redox state among various compartments both within and between cells. Lactate can no longer be considered the usual suspect for metabolic 'crimes', but is instead a central player in cellular, regional and whole body metabolism. Overall, the cell-to-cell lactate shuttle has expanded far beyond its initial conception as an explanation for lactate metabolism during muscle contractions and exercise to now subsume all of the other shuttles as a grand description of the role(s) of lactate in numerous metabolic processes and pathways.

Figures

Figure 1. Illustration of the essential elements of the hypothetical intracellular (intramuscular) lactate shuttle in comparison to the well-established malate–aspartate NAD+/NADH shuttle

Note that for purposes of clarity the well-established glycerol phosphate shuttle is not shown. Redrawn with permission from Gladden (2001). The H+ ions for pyruvate and lactate are inserted to emphasize that the MCT1 and presumably PYR symports a proton; MCT1 can transport both pyruvate and lactate. Note that operation of such an intracellular lactate shuttle would deliver both reducing equivalents and substrate for oxidation to mitochondria. Key components of this hypothesis are in bold lettering and/or red fill for comparison to the malate–aspartate shuttle in normal font and blue fill: a high activity of cytosolic LDH is considered to guarantee La− formation in the cytosol under virtually all conditions but especially during exercise; MCT1 has been reported to be present in mitochondrial membrane allowing La− transport from cytosol into mitochondria; LDH inside mitochondria is required to complete the intracellular shuttle by converting La− to pyruvate. The asterisk beside the mitochondrial LDH denotes that the presence of LDH inside mitochondria is disputed and that some investigators consider operation of such a shuttle to be thermodynamically unfeasible. MCT1: monocarboxylate transporter 1; PYR: the mitochondrial pyruvate transporter; ETC: electron transport chain; Shuttles: the malate–aspartate NAD+ /NADH shuttle and the glycerol phosphate shuttle, which is not shown.

Figure 2. Illustration of a simpler intracellular (intramuscular) lactate shuttle hypothesis originally proposed by Stainsby & Brooks (1990)

Note that the space above and to the right of the diagonal dashed line denotes sites that are remote from mitochondria and/or compartmentalized while the space down and to the left of the line denotes sites near mitochondria. La− is in large, bold lettering in the sites remote from mitochondria indicating that (a) [La−] should be highest here, and (b) [La−] is much greater than pyruvate concentration, especially during exercise. In this model, La− would be the predominant species diffusing from sites of glycolytic formation to low [La−] areas just outside mitochondrial membranes where La− would be converted back to pyruvate with delivery of NADH to the malate–aspartate (and glycerol phosphate) NAD+/NADH shuttles. This model does not require intramitochondrial LDH. MCT1 is shown because pyruvate might enter mitochondria via this transporter in addition to the traditional pyruvate carrier (PYR). LDH: lactate dehydrogenase; MCT1: monocarboxylate transporter 1; PYR: the mitochondrial pyruvate transporter; ETC: electron transport chain; Shuttles: the malate–aspartate NAD+/NADH shuttle and the glycerol phosphate shuttle, which is not shown for purposes of clarity. Redrawn with permission from Gladden (1996) from Handbook of Physiology, section 12, Exercise: Regulation and Integration of Multiple Systems, edited by Loring B. Rowell & John T. Shepherd, copyright 1996 by The American Physiological Society. Used by permission of Oxford University Press, Inc.

Figure 3. Illustration of the putative astrocyte–neuron lactate shuttle

The basic outline of the astrocyte–neuron lactate shuttle hypothesis is as follows. Blood glucose is a major energy substrate that can be taken up by both neurons and astrocytes via their specific glucose transporters (GLUT3 in neurons and GLUT1 in astrocytes); note that GLUT1 is also present in the plasma membrane of endothelial cells making up capillaries. Blood glucose may be more readily available to astrocytes because the surface of intraparenchymal capillaries is covered by specialized astrocytic end-feet. Release of the neurotransmitter, glutamate, at glutamatergic synapses leads to glutamate uptake into surrounding astrocytes via glutamate transporters GLT-1 and GLAST to terminate the action of glutamate on postsynaptic receptors. Glutamate entry into astrocytes is powered by the Na+ concentration gradient and current evidence suggests that one glutamate enters with three Na+ and one H+ while one K+ is simultaneously extruded. The resulting increase in intra-astrocytic [Na+] activates a glia-specific Na+–K+-ATPase α2 subunit. Glutamate is converted to glutamine by glutamine synthetase. Both the ATPase pump activation and the glutamine synthesis activate astrocytic glycolysis that is possibly compartmentalized with these processes; presence of LDH5, the muscle form of LDH, is argued to promote La− formation. The end result is La− accumulation and efflux into the extracellular fluid, facilitated by MCT1. Subsequently, La− is taken up into neurons via MCT2. Glutamine also diffuses from astrocytes into the extracellular fluid and on into neurons where it is used to resynthesize glutamate. La− taken up into neurons is preferentially converted to pyruvate, arguably because of pyruvate utilization as an aerobic fuel and the presence of LDH1, the heart form of LDH. In this hypothesis, the energy metabolism of neurons is largely aerobic with La− serving as the major fuel. See text and Table 2 for further details of the hypothesis, and Table 3 for concerns/questions about the hypothesis. GLUT1 and GLUT3: specific glucose transporters located in the membranes of brain endothelial cells and astrocytes (GLUT1), and neurons (GLUT3); MCT1 and MCT2: specific monocarboxylate transporters located in the membranes of brain endothelial cells and astrocytes (MCT1), and neurons (MCT2); Gluc: glucose; Gly: glycogen; Pyr−: pyruvate; La−: lactate; Glu: glutamate; Gln: glutamine; GS: glutamine synthetase; LDH1 and LDH5: specific forms of lactate dehydrogenase in neurons (LDH1) and astrocytes (LDH5); ECF: extracellular fluid; GLT-1 and GLAST: glutamate transporters; α2: glia-specific Na+–K+-ATPase subunit. Redrawn with permission from Pellerin (2003), Lactate as a pivotal element in neuron-glia metabolic cooperation, Neurochemistry International 43, 331–338. Used by permission of Elsevier.

Figure 4. Illustration of the proposed lactate–alanine shuttle between astrocytes and glutamatergic neurons

This diagram focuses on the recycling of glutamate and ammonia. Glutamate released by neurons as a neurotransmitter is taken up by astrocytes and incorporated with ammonia (NH4+) to form glutamine. The glutamine is released to be taken up by neurons to re-form glutamate with ammonia release. The ammonia is used to synthesize glutamate via glutamate dehydrogenase and this glutamate then transaminates pyruvate to alanine. The alanine can leave the neurons to be taken up by astrocytes and combined with 2-oxoglutarate to be transaminated back to glutamate with accompanying pyruvate formation. Pyruvate in the astrocytes forms La− that is released and taken up into neurons. In the neurons, La− is converted back to pyruvate thus completing the lactate–alanine shuttle. LDH: lactate dehydrogenase; AAT: alanine aminotransferase; GDH: glutamate dehydrogenase; GS: glutamine synthetase; Glnase: glutaminase; mit: mitochondrial; cyt: cytosolic; Pyr−: pyruvate; Ala: alanine; Glu: glutamate; 2-oxoglu: 2-oxoglutarate; Gln: glutamine. Redrawn with permission from Waagepetersen et al. (2000), a possible role of alanine for ammonia transfer between astrocytes and glutamatergic neurons, Journal of Neurochemistry 75, 471–479. Used by permission of Blackwell Publishing Ltd.

Figure 5. Illustration of the proposed peroxisomal lactate shuttle

Based on (McClelland et al. 2003), Salway (1999) and Baumgart et al. (1996). The outline of reactions displays the β-oxidation of very-long-chain fatty acids with acetyl-CoA release to the cytosol. Although not shown, shortened fatty acyl-CoA molecules could be released to the cytosol as well. Key elements of the peroxisomal lactate shuttle are highlighted in red indicating that NADH is reoxidized to NAD+ inside the peroxisome by the conversion of pyruvate to La−; La− then leaves the peroxisome via the monocarboxylate carrier, MCT2. In the cytosol, La− is converted back to pyruvate with concomitant conversion of NAD+ to NADH, thus delivering reducing equivalents from the peroxisome to the cytosol. The resulting pyruvate returns to the peroxisome via the MCT2 to continue the shuttle. This shuttle provides an avenue for NADH reoxidation in the peroxisome, a necessary process for the continuation of peroxisomal β-oxidation of fatty acids. E1: acyl-CoA oxidase; E2: enoyl-CoA hydratase; E3: L-3-hydroxyacyl-CoA dehydrogenase; E4: thiolase; E5: catalase; pLDH: LDH located inside the peroxisome; cLDH: LDH located in the cytosol, outside the peroxisome; Pyr−: pyruvate; La−: lactate.