Twelve weeks of endurance training increases FFA mobilization and reesterification in postmenopausal women.

We examined the effects of exercise intensity and training on rates of lipolysis, plasma free fatty acid (FFA) appearance (R(a)), disappearance (R(d)), reesterification (R(s)), and oxidation (R(oxP)) in postmenopausal (PM) women. Ten sedentary but healthy women (55 ± 0.6 yr) completed 12 wk of supervised endurance exercise training on a cycle ergometer [5 days/wk, 1 h/day, 65% peak oxygen consumption (Vo(2peak))]. Flux rates were determined by continuous infusion of [1-(13)C]palmitate and [1,1,2,3,3-(2)H(5)]glycerol during 90 min of rest and 60 min of cycle ergometer exercise during one pretraining exercise trial [65% Vo(2peak) (PRE)] and two posttraining exercise trials [at power outputs that elicited 65% pretraining Vo(2peak) (absolute training; ABT) and 65% posttraining Vo(2peak) (relative training; RLT)]. Initial body weights (68.2 ± 4.5 kg) were maintained over the course of study. Training increased Vo(2peak) by 16.3 ± 3.9% (P < 0.05) (Zarins ZA, Wallis GA, Faghihnia N, Johnson ML, Fattor JA, Horning MA and Brooks GA. Metabolism 58: 9: 1338-1346, 2009). Glycerol R(a) and R(d) were elevated in the RLT trial (P < 0.05), but not the ABT trial after training. Rates of plasma FFA R(a), R(d), and R(oxP) were elevated during the ABT compared with PRE trial (P < 0.05). FFA R(s) accounted for most (50-70%) of R(d) during exercise; training reduced FFA R(s) during ABT, but not RLT compared with PRE. We conclude that, despite the large age-related decrease in metabolic scope in PM women, endurance training increases the capacities for FFA mobilization and oxidation during exercises of a given power output. However, after menopause, total lipid oxidation capacity remains low, with reesterification accounting for most of FFA R(d).

Lactate shuttles in nature.

Once thought to be the consequence of oxygen lack in contracting skeletal muscle, the glycolytic product lactate is formed and utilized continuously under fully aerobic conditions. "Cell-cell" and "intracellular lactate shuttle" concepts describe the roles of lactate in the delivery of oxidative and gluconeogenic substrates, as well as in cell signalling. Examples of cell-cell shuttles include lactate exchanges between white-glycolytic and red-oxidative fibres within a working muscle bed, between working skeletal muscle and heart, and between tissues of net lactate release and gluconeogenesis. Lactate exchange between astrocytes and neurons that is linked to glutamatergic signalling in the brain is an example of a lactate shuttle supporting cell-cell signalling. Lactate uptake by mitochondria and pyruvate-lactate exchange in peroxisomes are examples of intracellular lactate shuttles. Lactate exchange between sites of production and removal is facilitated by monocarboxylate transport proteins, of which there are several isoforms, and, probably, also by scaffolding proteins. The mitochondrial lactate-pyruvate transporter appears to work in conjunction with mitochondrial lactate dehydrogenase, which permits lactate to be oxidized within actively respiring cells. Hence mitochondria function to establish the concentration and proton gradients necessary for cells with high mitochondrial densities (e.g. cardiocytes) to take up and oxidize lactate. Arteriovenous difference measurements on working cardiac and skeletal muscle beds as well as NMR spectral analyses of these tissues show that lactate is formed and oxidized within the cells of formation in vivo. Glycolysis and lactate oxidation within cells permits high flux rates and the maintenance of redox balance in the cytosol and mitochondria. Other examples of intracellular lactate shuttles include lactate uptake and oxidation in sperm mitochondria and the facilitation of beta-oxidation in peroxisomes by pyruvate-lactate exchange. An ancient origin to the utility of lactate shuttling is implied by the observation that mitochondria of Saccharomyces cerevisiae contain flavocytochrome b(2), a lactate-cytochrome c oxidoreductase that couples lactate dehydrogenation to the reduction of cytochrome c. The presence of cell-cell and intracellular lactate shuttles gives rise to the notion that glycolytic and oxidative pathways can be viewed as linked, as opposed to alternative, processes, because lactate, the product of one pathway, is the substrate for the other.

Recovery of (13)CO2 during rest and exercise after [1-(13)C]acetate, [2-(13)C]acetate, and NaH(13)CO3 infusions.

For estimating the oxidation rates (Rox) of glucose and other substrates by use of (13)C-labeled tracers, we obtained correction factors to account for label dilution in endogenous bicarbonate pools and TCA cycle exchange reactions. Fractional recoveries of (13)C label in respiratory gases were determined during 225 min of rest and 90 min of leg cycle ergometry at 45 and 65% peak oxygen uptake (VO(2 peak)) after continuous infusions of [1-(13)C]acetate, [2-(13)C]acetate, or NaH(13)CO(3). In parallel trials, [6,6-(2)H]glucose and [1-(13)C]glucose were given. Experiments were conducted after an overnight fast with exercise commencing 12 h after the last meal. During the transition from rest to exercise, CO(2) production increased (P < 0.05) in an intensity-dependent manner. Significant differences were observed in the fractional recoveries of (13)C label as (13)CO(2) at rest (NaH(13)CO(3), 77.5 +/- 2.8%; [1-(13)C]acetate, 49.8 +/- 2.4%; [2-(13)C]acetate, 26.1 +/- 1.4%). During exercise, fractional recoveries of (13)C label from [1-(13)C]acetate, [2-(13)C]acetate, and NaH(13)CO(3) were increased compared with rest. Magnitudes of label recoveries during both exercise intensities were tracer specific (NaH(13)CO(3), 93%; [1-(13)C]acetate, 80%; [2-(13)C]acetate, 65%). Use of an acetate-derived correction factor for estimating glucose oxidation resulted in Rox values in excess (P < 0.05) of glucose rate of disappearance during hard exercise. We conclude that, after an overnight fast: 1) recovery of (13)C label as (13)CO(2) from [(13)C]acetate is decreased compared with bicarbonate; 2) the position of (13)C acetate label affects carbon dilution estimations; 3) recovery of (13)C label increases in the transition from rest to exercise in an isotope-dependent manner; and 4) application of an acetate correction factor in glucose oxidation measurements results in oxidation rates in excess of glucose disappearance during exercise at 65% of VO(2 peak). Therefore, bicarbonate, not acetate, correction factors are advocated for estimating glucose oxidation from carbon tracers in exercising men.

The effects of endurance training and exhaustive exercise on mitochondrial enzymes in tissues of the rat (Rattus norvegicus).

The aim of the present study was to ascertain the effects of training and exhaustive exercise on mitochondrial capacities to oxidize pyruvate, 2-oxoglutarate, palmitoylcarnitine, succinate and ferrocytochrome c in various tissues of the rat. Endurance capacity was significantly increased (P<0.01) by an endurance training program over a period of 5-6 weeks. The average run time to exhaustion was 214.2+/-23.8 min for trained rats in comparison with 54.5+/-11.7 min for their untrained counterparts. Oxidative capacities were reduced in liver (P<0.05) and brown adipose tissue (P<0.05) as a result of endurance training. On the contrary, the oxidative capacity of skeletal muscle was slightly increased and that of heart almost unaffected except for the oxidation of palmitoylcarnitine, which was significantly reduced (P<0.05) as a result of training.

Autoregulation of glucose production in men with a glycerol load during rest and exercise.

Related to hepatic autoregulation we evaluated hypotheses that 1) glucose production would be altered as a result of a glycerol load, 2) decreased glucose recycling rate (Rr) would result from increased glycerol uptake, and 3) the absolute rate of gluconeogenesis (GNG) from glycerol would be positively correlated to glycerol rate of disappearance (R(d)) during a glycerol load. For these purposes, glucose and glycerol kinetics were determined in eight men during rest and during 90 min of leg cycle ergometry at 45 and 65% of peak O2 consumption (.VO2 (peak)). Trials were conducted after an overnight fast, with exercise commencing 12 h after the last meal. Subjects received a continuous infusion of [6,6-(2)H(2)]glucose, [1-(13)C]glucose, and [1,1,2,3,3-(2)H(5)]glycerol without (CON) or with an additional 1,000 mg (rest: 20 mg/min; exercise: 40 mg/min) of [2-(13)C]- or unlabeled glycerol added to the infusate (GLY). Infusion of glycerol dampened glucose Rr, calculated as the difference between [6,6-(2)H(2)]- and [1-(13)C]glucose rates of appearance (R(a)), at rest [0.35 +/- 0.12 (CON) vs. 0.12 +/- 0.10 mg. kg(-1). min(-1) (GLY), P < 0.05] and during exercise at both intensities [45%: 0.63 +/- 0.14 (CON) vs. 0.04 +/- 0.12 (GLY); 65%: 0.73 +/- 0.14 (CON) vs. 0.04 +/- 0.17 mg. kg(-1). min(-1) (GLY), P < 0.05]. Glucose R(a) and oxidation were not affected by glycerol infusion at rest or during exercise. Throughout rest and both exercise intensities, glycerol R(d) was greater in GLY vs. CON conditions (rest: 0.30 +/- 0.04 vs. 0.58 +/- 0.04; 45%: 0.57 +/- 0.07 vs. 1.19 +/- 0.04; 65%: 0.73 +/- 0.06 vs. 1.27 +/- 0.05 mg. kg(-1). min(-1), CON vs. GLY, respectively). Differences in glycerol R(d) (DeltaR(d)) between protocols equaled the unlabeled glycerol infusion rate and correlated with plasma glycerol concentration (r = 0.97). We conclude that infusion of a glycerol load during rest and exercise at 45 and 65% of .VO2(peak) 1) does not affect glucose R(a) or R(d), 2) blocks glucose Rr, 3) increases whole body glycerol R(d) in a dose-dependent manner, and 4) results in gluconeogenic rates from glycerol equivalent to CON glucose recycling rates.

Chronically and acutely exercised rats: biomarkers of oxidative stress and endogenous antioxidants.

The responses to oxidative stress induced by chronic exercise (8-wk treadmill running) or acute exercise (treadmill running to exhaustion) were investigated in the brain, liver, heart, kidney, and muscles of rats. Various biomarkers of oxidative stress were measured, namely, lipid peroxidation [malondialdehyde (MDA)], protein oxidation (protein carbonyl levels and glutamine synthetase activity), oxidative DNA damage (8-hydroxy-2'-deoxyguanosine), and endogenous antioxidants (ascorbic acid, alpha-tocopherol, glutathione, ubiquinone, ubiquinol, and cysteine). The predominant changes are in MDA, ascorbic acid, glutathione, cysteine, and cystine. The mitochondrial fraction of brain and liver showed oxidative changes as assayed by MDA similar to those of the tissue homogenate. Our results show that the responses of the brain to oxidative stress by acute or chronic exercise are quite different from those in the liver, heart, fast muscle, and slow muscle; oxidative stress by acute or chronic exercise elicits different responses depending on the organ tissue type and its endogenous antioxidant levels.

Intra- and extra-cellular lactate shuttles.

The "lactate shuttle hypothesis" holds that lactate plays a key role in the distribution of carbohydrate potential energy that occurs among various tissue and cellular compartments such as between: cytosol and mitochondria, muscle and blood, blood and muscle, active and inactive muscles, white and red muscles, blood and heart, arterial blood and liver, liver and other tissues such as exercising muscle, intestine and portal blood, portal blood and liver, zones of the liver, and skin and blood. Studies on resting and exercising humans indicate that most lactate (75-80%) is disposed of through oxidation, with much of the remainder converted to glucose and glycogen. Lactate transport across cellular membranes occurs by means of facilitated exchange along pH and concentration gradients involving a family of lactate transport proteins, now called monocarboxylate transporters (MCTs). Current evidence is that muscle and other cell membrane lactate transporters are abundant with characteristics of high Km and Vmax. There appears to be long-term plasticity in the number of cell membrane transporters, but short-term regulation by allosteric modulation or phosphorylation is not known. In addition to cell membranes, mitochondria also contain monocarboxylate transporters (mMCT) and lactic dehydrogenase (mLDH). Therefore, mitochondrial monocarboxylate uptake and oxidation, rather than translocation of transporters to the cell surfaces, probably regulate lactate flux in vivo. Accordingly, the "lactate shuttle" hypothesis has been modified to include a new, intracellular component involving cytosolic to mitochondrial exchange. The intracellular lactate shuttle emphasizes the role of mitochondrial redox in the oxidation and disposal of lactate during exercise and other conditions.

Endurance training, expression, and physiology of LDH, MCT1, and MCT4 in human skeletal muscle.

To evaluate the effects of endurance training on the expression of monocarboxylate transporters (MCT) in human vastus lateralis muscle, we compared the amounts of MCT1 and MCT4 in total muscle preparations (MU) and sarcolemma-enriched (SL) and mitochondria-enriched (MI) fractions before and after training. To determine if changes in muscle lactate release and oxidation were associated with training-induced changes in MCT expression, we correlated band densities in Western blots to lactate kinetics determined in vivo. Nine weeks of leg cycle endurance training [75% peak oxygen consumption (VO(2 peak))] increased muscle citrate synthase activity (+75%, P < 0.05) and percentage of type I myosin heavy chain (+50%, P < 0.05); percentage of MU lactate dehydrogenase-5 (M4) isozyme decreased (-12%, P < 0.05). MCT1 was detected in SL and MI fractions, and MCT4 was localized to the SL. Muscle MCT1 contents were consistent among subjects both before and after training; in contrast, MCT4 contents showed large interindividual variations. MCT1 amounts significantly increased in MU, SL, and MI after training (+90%, +60%, and +78%, respectively), whereas SL but not MU MCT4 content increased after training (+47%, P < 0.05). Mitochondrial MCT1 content was negatively correlated to net leg lactate release at rest (r = -0.85, P < 0.02). Sarcolemmal MCT1 and MCT4 contents correlated positively to net leg lactate release at 5 min of exercise at 65% VO(2 peak) (r = 0.76, P < 0.03 and r = 0. 86, P < 0.01, respectively). Results support the conclusions that 1) endurance training increases expression of MCT1 in muscle because of insertion of MCT1 into both sarcolemmal and mitochondrial membranes, 2) training has variable effects on sarcolemmal MCT4, and 3) both MCT1 and MCT4 participate in the cell-cell lactate shuttle, whereas MCT1 facilitates operation of the intracellular lactate shuttle.

Endurance training increases gluconeogenesis during rest and exercise in men.

The hypothesis that endurance training increases gluconeogenesis (GNG) during rest and exercise was evaluated. We determined glucose turnover with [6,6-(2)H]glucose and lactate incorporation into glucose by use of [3-(13)C]lactate during 1 h of cycle ergometry at two intensities [45 and 65% peak O(2) consumption (VO(2 peak))] before and after training [65% pretraining VO(2 peak)], same absolute workload (ABT), and 65% posttraining VO(2 peak), same relative intensity (RLT). Nine males (178.1 +/- 2.5 cm, 81.8 +/- 3.3 kg, 27.4 +/- 2.0 yr) trained for 9 wk on a cycle ergometer 5 times/wk for 1 h at 75% VO(2 peak). The power output that elicited 66.0 +/- 1.1% of VO(2 peak) pretraining elicited 54.0 +/- 1.7% posttraining. Rest and exercise arterial glucose concentrations were similar before and after training, regardless of exercise intensity. Arterial lactate concentration during exercise was significantly greater than at rest before and after training. Compared with 65% pretraining, arterial lactate concentration decreased at ABT (4.75 +/- 0.4 mM, 65% pretraining; 2.78 +/- 0.3 mM, ABT) and RLT (3.76 +/- 0.46 mM) (P < 0.05). At rest after training, the percentage of glucose rate of appearance (R(a)) from GNG more than doubled (1.98 +/- 0.5% pretraining; 5.45 +/- 1.3% posttraining), as did the rate of GNG (0.11 +/- 0.03 mg x kg(-1) x min(-1) pretraining, 0.24 +/- 0.06 mg x kg(-1) x min(-1) posttraining). During exercise after training, %glucose R(a) from GNG increased significantly at ABT (2.3 +/- 0.8% at 65% pre- vs. 7.6 +/- 2.1% posttraining) and RLT (6.1 +/- 1.5%), whereas GNG increased almost threefold (P < 0.05) at ABT (0.24 +/- 0.08 mg x kg(-1) x min(-1) 65% pre-, and 0.71 +/- 0.18 mg x kg(-1) x min(-1) posttraining) and RLT (0.75 +/- 0.26 mg x kg(-1) x min(-1)). We conclude that endurance training increases gluconeogenesis twofold at rest and threefold during exercise at given absolute and relative exercise intensities.

Women at altitude: carbohydrate utilization during exercise at 4,300 m.

To evaluate the hypothesis that exposure to high altitude would reduce blood glucose and total carbohydrate utilization relative to sea level (SL), 16 young women were studied over four 12-day periods: at 50% of peak O(2) consumption in different menstrual cycle phases (SL-50), at 65% of peak O(2) consumption at SL (SL-65), and at 4,300 m (HA). After 10 days in each condition, blood glucose rate of disappearance (R(d)) and respiratory exchange ratio were measured at rest and during 45 min of exercise. Glucose R(d) during exercise at HA (4.71 +/- 0.30 mg. kg(-1). min(-1)) was not different from SL exercise at the same absolute intensity (SL-50 = 5.03 mg. kg(-1). min(-1)) but was lower at the same relative intensity (SL-65 = 6.22 mg. kg(-1). min(-1), P < 0.01). There were no differences, however, when glucose R(d) was corrected for energy expended (kcal/min) during exercise. Respiratory exchange ratios followed the same pattern, except carbohydrate oxidation remained lower (-23.2%, P < 0.01) at HA than at SL when corrected for energy expended. In women, unlike in men, carbohydrate utilization decreased at HA. Relative abundance of estrogen and progesterone in women may partially explain the sex differences in fuel utilization at HA, but subtle differences between menstrual cycle phases at SL had no physiologically relevant effects.

Active muscle and whole body lactate kinetics after endurance training in men.

We evaluated the hypotheses that endurance training decreases arterial lactate concentration ([lactate](a)) during continuous exercise by decreasing net lactate release () and appearance rates (R(a)) and increasing metabolic clearance rate (MCR). Measurements were made at two intensities before [45 and 65% peak O(2) consumption (VO(2 peak))] and after training [65% pretraining VO(2 peak), same absolute workload (ABT), and 65% posttraining VO(2 peak), same relative intensity (RLT)]. Nine men (27.4 +/- 2.0 yr) trained for 9 wk on a cycle ergometer, 5 times/wk at 75% VO(2 peak). Compared with the 65% VO(2 peak) pretraining condition (4.75 +/- 0.4 mM), [lactate](a) decreased at ABT (41%) and RLT (21%) (P < 0.05). decreased at ABT but not at RLT. Leg lactate uptake and oxidation were unchanged at ABT but increased at RLT. MCR was unchanged at ABT but increased at RLT. We conclude that 1) active skeletal muscle is not solely responsible for elevated [lactate](a); and 2) training increases leg lactate clearance, decreases whole body and leg lactate production at a given moderate-intensity power output, and increases both whole body and leg lactate clearance at a high relative power output.

Cardiac and skeletal muscle mitochondria have a monocarboxylate transporter MCT1.

To evaluate the potential role of monocarboxylate transporter-1 (MCT1) in tissue lactate oxidation, isolated rat subsarcolemmal and interfibrillar cardiac and skeletal muscle mitochondria were probed with an antibody to MCT1. Western blots indicated presence of MCT1 in sarcolemmal membranes and in subsarcolemmal and interfibrillar mitochondria. Minimal cross-contamination of mitochondria by cell membrane fragments was verified by probing for the sarcolemmal protein GLUT-1. In agreement, immunolabeling and electron microscopy showed mitochondrial MCT1 in situ. Along with lactic dehydrogenase, the presence of MCT1 in striated muscle mitochondria permits mitochondrial lactate oxidation and facilitates function of the "intracellular lactate shuttle."

Muscle net glucose uptake and glucose kinetics after endurance training in men.

We evaluated the hypotheses that alterations in glucose disposal rate (R(d)) due to endurance training are the result of changed net glucose uptake by active muscle and that blood glucose is shunted to working muscle during exercise requiring high relative power output. We studied leg net glucose uptake during 1 h of cycle ergometry at two intensities before training [45 and 65% of peak rate of oxygen consumption (VO(2 peak))] and after training [65% pretraining VO(2 peak), same absolute workload (ABT), and 65% posttraining VO(2 peak), same relative workload (RLT)]. Nine male subjects (178.1 +/- 2.5 cm, 81.8 +/- 3.3 kg, 27.4 +/- 2.0 yr) were tested before and after 9 wk of cycle ergometer training, five times a week at 75% VO(2 peak). The power output that elicited 66.0 +/- 1.1% of VO(2 peak) before training elicited 54.0 +/- 1.7% after training. Whole body glucose R(d) decreased posttraining at ABT (5.45 +/- 0.31 mg. kg(-1). min(-1) at 65% pretraining to 4.36 +/- 0.44 mg. kg(-1). min(-1)) but not at RLT (5.94 +/- 0.47 mg. kg(-1). min(-1)). Net glucose uptake was attenuated posttraining at ABT (1.87 +/- 0.42 mmol/min at 65% pretraining and 0.54 +/- 0.33 mmol/min) but not at RLT (2.25 +/- 0. 81 mmol/min). The decrease in leg net glucose uptake at ABT was of similar magnitude as the drop in glucose R(d) and thus could explain dampened glucose flux after training. Glycogen degradation also decreased posttraining at ABT but not RLT. Leg net glucose uptake accounted for 61% of blood glucose flux before training and 81% after training at the same relative (65% VO(2 peak)) workload and only 38% after training at ABT. We conclude that 1) alterations in active muscle glucose uptake with training determine changes in whole body glucose kinetics; 2) muscle glucose uptake decreases for a given, moderate intensity task after training; and 3) hard exercise (65% VO(2 peak)) promotes a glucose shunt from inactive tissues to active muscle.

Endurance training increases fatty acid turnover, but not fat oxidation, in young men.

We examined the effects of exercise intensity and a 10-wk cycle ergometer training program [5 days/wk, 1 h, 75% peak oxygen consumption (VO2 peak)] on plasma free fatty acid (FFA) flux, total fat oxidation, and whole body lipolysis in healthy male subjects (n = 10; age = 25.6 +/- 1.0 yr). Two pretraining trials (45 and 65% of VO2 peak) and two posttraining trials (same absolute workload, 65% of old VO2 peak; and same relative workload, 65% of new VO2 peak) were performed by using an infusion of [1-13C]palmitate and [1,1,2,3, 3-2H]glycerol. An additional nine subjects (age 25.4 +/- 0.8 yr) were treated similarly but were infused with [1,1,2,3,3-2H]glycerol and not [1-13C]palmitate. Subjects were studied postabsorptive for 90 min of rest and 1 h of cycling exercise. After training, subjects increased VO2 peak by 9.4 +/- 1.4%. Pretraining, plasma FFA kinetics were inversely related to exercise intensity with rates of appearance (Ra) and disappearance (Rd) being significantly higher at 45 than at 65% VO2 peak (Ra: 8.14 +/- 1.28 vs. 6.64 +/- 0.46, Rd: 8. 03 +/- 1.28 vs. 6.42 +/- 0.41 mol. kg-1. min-1) (P

Respiratory gas-exchange ratios during graded exercise in fed and fasted trained and untrained men.

We evaluated the hypotheses that endurance training increases relative lipid oxidation over a wide range of relative exercise intensities in fed and fasted states and that carbohydrate nutrition causes carbohydrate-derived fuels to predominate as energy sources during exercise. Pulmonary respiratory gas-exchange ratios [(RER) = CO2 production/O2 consumption (VO2)] were determined during four relative, graded exercise intensities in both fed and fasted states. Seven untrained (UT) men and seven category 2 and 3 US Cycling Federation cyclists (T) exercised in the morning in random order, with target power outputs of 20 and 40% peak VO2 (VO2 peak) for 2 h, 60% VO2 peak for 1.5 h, and 80% VO2 peak for a minimum of 30 min after either a 12-h overnight fast or 3 h after a standardized breakfast. Actual metabolic responses were 22 +/- 0.33, 40 +/- 0.31, 59 +/- 0.32, and 75 +/- 0.39% VO2 peak. T subjects showed significantly (P < 0.05) decreased RER compared with UT subjects at absolute workloads when fed and fasted. Fasting significantly decreased RER values compared with the fed state at 22, 40, and 59% VO2 peak in T and at 40 and 59% VO2 peak in UT subjects. Training decreased (P < 0.05) mean RER values compared with UT subjects at 22% VO2 peak when they fasted, and at 40% VO2 peak when fed or fasted, but not at higher relative exercise intensities in either nutritional state. Our results support the hypothesis that endurance training enhances lipid oxidation in men after a 12-h overnight fast at low relative exercise intensities (22 and 40% VO2 peak). However, a training effect on RER was not apparent at high relative exercise intensities (59 and 75% VO2 peak). Because most athletes train and compete at exercise intensities >40% maximal VO2, they will not oxidize a greater proportion of lipids compared with untrained subjects, regardless of nutritional state.

Role of mitochondrial lactate dehydrogenase and lactate oxidation in the intracellular lactate shuttle.

To evaluate the potential role of mitochondrial lactate dehydrogenase (LDH) in tissue lactate clearance and oxidation in vivo, isolated rat liver, cardiac, and skeletal muscle mitochondria were incubated with lactate, pyruvate, glutamate, and succinate. As well, alpha-cyano-4-hydroxycinnamate (CINN), a known monocarboxylate transport inhibitor, and oxamate, a known LDH inhibitor were used. Mitochondria readily oxidized pyruvate and lactate, with similar state 3 and 4 respiratory rates, respiratory control (state 3/state 4), and ADP/O ratios. With lactate or pyruvate as substrates, alpha-cyano-4-hydroxycinnamate blocked the respiratory response to added ADP, but the block was bypassed by addition of glutamate (complex I-linked) and succinate (complex II-linked) substrates. Oxamate increased pyruvate (approximately 10-40%), but blocked lactate oxidation. Gel electrophoresis and electron microscopy indicated LDH isoenzyme distribution patterns to display tissue specificity, but the LDH isoenzyme patterns in isolated mitochondria were distinct from those in surrounding cell compartments. In heart, LDH-1 (H4) was concentrated in mitochondria whereas LDH-5 (M4) was present in both mitochondria and surrounding cytosol and organelles. LDH-5 predominated in liver but was more abundant in mitochondria than elsewhere. Because lactate exceeds cytosolic pyruvate concentration by an order of magnitude, we conclude that lactate is the predominant monocarboxylate oxidized by mitochondria in vivo. Mammalian liver and striated muscle mitochondria can oxidize exogenous lactate because of an internal LDH pool that facilitates lactate oxidation.

Evaluation of exercise and training on muscle lipid metabolism.

To evaluate the hypothesis that endurance training increases intramuscular triglyceride (IMTG) oxidation, we studied leg net free fatty acid (FFA) and glycerol exchange during 1 h of cycle ergometry at two intensities before training [45 and 65% of peak rate of oxygen consumption (V(O2) peak)] and after training [65% pretraining V(O2) peak, same absolute workload (ABT), and 65% posttraining V(O2) peak, same relative intensity (RLT)]. Nine male subjects (178.1 +/- 2.5 cm, 81.8 +/- 3.3 kg, 27.4 +/- 2.0 yr) were tested before and after 9 wk of cycle ergometer training, five times per week at 75% V(O2) peak. The power output that elicited 66.1 +/- 1.1% of V(O2) peak before training elicited 54.0 +/- 1.7% after training due to a 14.6 +/- 3.1% increase in V(O2) peak. Training significantly (P < 0.05) decreased pulmonary respiratory exchange ratio (RER) values at ABT (0.96 +/- 0.01 at 65% pre- vs. 0.93 +/- 0.01 posttraining) but not RLT (0.95 +/- 0.01). After training, leg respiratory quotient (RQ) was not significantly different at either ABT (0.98 +/- 0.02 pre- vs. 0.98 +/- 0.03 posttraining) or RLT (1.01 +/- 0.02). Net FFA uptake was increased at RLT but not ABT after training. FFA fractional extraction was not significantly different after training or at any exercise intensity. Net glycerol release, and therefore IMTG lipolysis calculated from three times net glycerol release, did not change from rest to exercise or at ABT but decreased at the same RLT after training. Muscle biopsies revealed minor muscle triglyceride changes during exercise. Simultaneous measurements of leg RQ, net FFA uptake, and glycerol release by working legs indicated no change in leg FFA oxidation, FFA uptake, or IMTG lipolysis during leg cycling exercise that elicits 65% pre- and 54% posttraining V(O2) peak. Training increases working muscle FFA uptake at 65% V(O2) peak, but high RER and RQ values at all work intensities indicate that FFA and IMTG are of secondary importance as fuels in moderate and greater-intensity exercise.

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

The "Lactate Shuttle" Hypothesis posits that lactate removal requires exchange among producing and consuming cells. The "Intra-cellular Lactate Shuttle" hypothesis posits that lactate exchange occurs among compartments within cells, and that mitochondria are the major sites of cellular lactate disposal. Thus, cells with high mitochondrial densities (cardiocytes, myocytes, hepatocytes) are those which participate in lactate clearance. The model of an Intracellular Lactate Shuttle recognizes that the Keq for LDH is 3.6 x 10(4) M-1; thus, glycolysis results in cytosolic lactate production regardless of the intracellular PO2. The model also requires presence of a mitochondrial monocarboxylate transporter (MCT) that allows uptake of lactate as well as pyruvate, and intra-mitochondrial LDH whose function is linked to the ETC, and which permits lactate-->pyruvate conversion and oxidation. Recently, we have shown that liver, heart and muscle mitochondria readily oxidize lactate and contain LDH and MCT1. Accordingly, we have concluded that lactate is the predominant monocarboxylate oxidized by mitochondria in vivo. The model of an "Intra-cellular Lactate Shuttle" is consistent with many of the observations on men at sea level and altitude. The observations include: oxidation is the primary fate of lactate disposal during rest and exercise; lactate production and oxidation occur simultaneously within resting and working muscle; increasing [lactate]a increases muscle lactate extraction, and that by increasing SaO2 acclimatization reduces blood [lactate].