Are arterial, muscle and working limb lactate exchange data obtained on men at altitude consistent with the hypothesis of an intracellular lactate shuttle?

OM-X®, a Fermented Vegetables Extract, Facilitates Muscle Endurance Capacity in Swimming Exercise Mice

The anti-fatigue effect was investigated of the probiotic supplement, OM-X®, on forced swimming capacity in mice. Mice were administered either vehicle (distilled water; DW) or OM-X® (85 mg/kg body weight) by gavage for 4 weeks. Forced swimming tests were conducted weekly using the Ishihara-modified Matsumoto swimming pool. The endurance swimming time of the final forced swimming exercise in mice fed with OM-X® group showed an approximately 2-fold increase compared with the vehicle control group. Biomedical parameters, including blood lactate, blood superoxide dismutase (SOD) activity, serum triacylglycerol (TG), hepatic total lipids (TL), TG and phospholipid (PL) were significantly lower in mice fed with OM-X® than those in the vehicle control group. Furthermore, the mRNA expression levels of carbamoyl phosphate synthetase 1 (Cps1) and arginase 1 (Arg1), in the urea cycle, were increased by OM-X® feeding. Thus, our findings suggest promotion of lipid metabolism and up-regulation of the urea cycle, at least in part, for the anti-fatigue effect mediated by OM-X®.

Oxygen consumption and usage during physical exercise: the balance between oxidative stress and ROS-dependent adaptive signaling

The complexity of human DNA has been affected by aerobic metabolism, including endurance exercise and oxygen toxicity. Aerobic endurance exercise could play an important role in the evolution of Homo sapiens, and oxygen was not important just for survival, but it was crucial to redox-mediated adaptation. The metabolic challenge during physical exercise results in an elevated generation of reactive oxygen species (ROS) that are important modulators of muscle contraction, antioxidant protection, and oxidative damage repair, which at moderate levels generate physiological responses. Several factors of mitochondrial biogenesis, such as peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α), mitogen-activated protein kinase, and SIRT1, are modulated by exercise-associated changes in the redox milieu. PGC-1α activation could result in decreased oxidative challenge, either by upregulation of antioxidant enzymes and/or by an increased number of mitochondria that allows lower levels of respiratory activity for the same degree of ATP generation. Endogenous thiol antioxidants glutathione and thioredoxin are modulated with high oxygen consumption and ROS generation during physical exercise, controlling cellular function through redox-sensitive signaling and protein-protein interactions. Endurance exercise-related angiogenesis, up to a significant degree, is regulated by ROS-mediated activation of hypoxia-inducible factor 1α. Moreover, the exercise-associated ROS production could be important to DNA methylation and post-translation modifications of histone residues, which create heritable adaptive conditions based on epigenetic features of chromosomes. Accumulating data indicate that exercise with moderate intensity has systemic and complex health-promoting effects, which undoubtedly involve regulation of redox homeostasis and signaling.

Hypoxia and exercise provoke both lactate release and lactate oxidation by the human brain

Lactate is shuttled between organs, as demonstrated in the Cori cycle. Although the brain releases lactate at rest, during physical exercise there is a cerebral uptake of lactate. Here, we evaluated the cerebral lactate uptake and release in hypoxia, during exercise and when the two interventions were combined. We measured cerebral lactate turnover via a tracer dilution method ([1-(13)C]lactate), using arterial to right internal jugular venous differences in 9 healthy individuals (5 males and 4 females), at rest and during 30 min of submaximal exercise in normoxia and hypoxia (F(i)o(2) 10%, arterial oxygen saturation 72 ± 10%, mean ± sd). Whole-body lactate turnover increased 3.5-fold and 9-fold at two workloads in normoxia and 18-fold during exercise in hypoxia. Although middle cerebral artery mean flow velocity increased during exercise in hypoxia, calculated cerebral mitochondrial oxygen tension decreased by 13 mmHg (P<0.001). At the same time, cerebral lactate release increased from 0.15 ± 0.1 to 0.8 ± 0.6 mmol min(-1) (P<0.05), corresponding to ∼10% of cerebral energy consumption. Concurrently, cerebral lactate uptake was 1.0 ± 0.9 mmol min(-1) (P<0.05), of which 57 ± 9% was oxidized, demonstrating that lactate oxidation may account for up to ∼33% of the energy substrate used by the brain. These results support the existence of a cell-cell lactate shuttle that may involve neurons and astrocytes.

Changes in HIF-1α protein, pyruvate dehydrogenase phosphorylation, and activity with exercise in acute and chronic hypoxia

Exercise under acute hypoxia elicits a large increase in blood lactate concentration ([La](b)) compared with normoxic exercise. However, several studies in humans show that with the transition to chronic hypoxia, exercise [La](b) returns to normoxic levels. Although extensively examined over the last decades, the muscle-specific mechanisms responsible for this phenomenon remain unknown. To assess the changes in skeletal muscle associated with a transition from acute to chronic hypoxia, CD-1 mice were exposed for 24 h (24H), 1 wk (1WH), or 4 wk (4WH) to hypobaric hypoxia (equivalent to 4,300 m), exercised under 12% O(2), and compared with normoxic mice (N) at 21% O(2). Since the enzyme pyruvate dehydrogenase (PDH) plays a major role in the metabolic fate of pyruvate (oxidation vs. lactate production), we assessed the changes in its activity and regulation. Here we report that when run under hypoxia, 24H mice exhibited the highest blood and intramuscular lactate of all groups, while the 1WH group approached N group values. Concomitantly, the 24H group exhibited the lowest PDH activity, associated with a higher phosphorylation (inactive) state of the Ser(232) residue of PDH, a site specific to PDH kinase-1 (PDK1). Furthermore, protein levels of PDK1 and its regulator, the hypoxia inducible factor-1α (HIF-1α), were both elevated in the 24H group compared with N and 1WH groups. Overall, our results point to a novel mechanism in muscle where the HIF-1α pathway is desensitized in the transition from acute to chronic hypoxia, leading to a reestablishment of PDH activity and a reduction in lactate production by the exercising muscles.

The concept of maximal lactate steady state: a bridge between biochemistry, physiology and sport science

The maximal lactate steady state (MLSS) is defined as the highest blood lactate concentration (MLSSc) and work load (MLSSw) that can be maintained over time without a continual blood lactate accumulation. A close relationship between endurance sport performance and MLSSw has been reported and the average velocity over a marathon is just below MLSSw. This work rate delineates the low- to high-intensity exercises at which carbohydrates contribute more than 50% of the total energy need and at which the fuel mix switches (crosses over) from predominantly fat to predominantly carbohydrate. The rate of metabolic adenosine triphosphate (ATP) turnover increases as a direct function of metabolic power output and the blood lactate at MLSS represents the highest point in the equilibrium between lactate appearance and disappearance both being equal to the lactate turnover. However, MLSSc has been reported to demonstrate a great variability between individuals (from 2-8 mmol/L) in capillary blood and not to be related to MLSSw. The fate of enhanced lactate clearance in trained individuals has been attributed primarily to oxidation in active muscle and gluconeogenesis in liver. The transport of lactate into and out of the cells is facilitated by monocarboxylate transporters (MCTs) which are transmembrane proteins and which are significantly improved by training. Endurance training increases the expression of MCT1 with intervariable effects on MCT4. The relationship between the concentration of the two MCTs and the performance parameters (i.e. the maximal distance run in 20 minutes) in elite athletes has not yet been reported. However, lactate exchange and removal indirectly estimated with velocity constants of the individual blood lactate recovery has been reported to be related to time to exhaustion at maximal oxygen uptake.

Effects of short-term training on plasma acid-base balance during incremental exercise in man

The present study examined the effect of short-term submaximal training on plasma acid-base balance during exercise. The influence of water and ion exchange between plasma, active muscles and erythrocytes in the response to training were also studied. The contributions of independent physicochemical variables (i.e. strong ion difference ([SID]), total concentration of weak acids ([Atot]) and PO2) to changes in arterial (a) and femoral venous (v) plasma [H+] were examined in six subjects (age 24+/-1.5 years; maximum oxygen consumption rate (VO2,max), 3.67+/-0.24 l min(-1)) during steady-state cycling for 15 min at each of 30, 65 and 75% of VO2,max before (pre) and after (post) training for 7 days on a cycle ergometer (2 h daily at 60 % VO2,max). The rise in [H+]a during exercise was attenuated post-training by 3 and 5 nequiv l(-1) (P<0.05) at 65 and 75% VO2,max, respectively, due first to less decrease in [SID]a, secondary to lower [Cl-]a and [Lac-]a; and second, to a reduction in [Atot]a, due to greater plasma volume and less plasma water flux (Jv) into leg muscle (P<0.05). The rise in [H+]v was also less in post-training by 4.5 and 6 nequiv l(-1) (P<0.05) at 65 and 75% VO2,max, respectively, and attributed solely to lower [Atot]v (P<0.05). Attenuation of exercise induced decreases in plasma [SID]a and [SID]v from rest to 75 % VO2,max was accompanied by reductions in erythrocyte Lac- and Cl- uptake (P<0.05), and smaller increases in erythrocyte K+ release (P<0.05). We conclude that the training-induced attenuation of the rise in plasma [H+]a and [H+]v during incremental exercise resulted from adaptive changes within muscles (less Lac- production and less water uptake) and erythrocytes (less uptake of Lac-, Cl- and K+), leading to greater [SID] and lower [Atot] in both arterial and femoral venous plasma.