Beetroot juice supplementation reduces whole body oxygen consumption but does not improve indices of mitochondrial efficiency in human skeletal muscle.

KEY POINTS: Oral consumption of nitrate (NO3(-)) in beetroot juice has been shown to decrease the oxygen cost of submaximal exercise; however, the mechanism of action remains unresolved. We supplemented recreationally active males with beetroot juice to determine if this altered mitochondrial bioenergetics. Despite reduced submaximal exercise oxygen consumption, measures of mitochondrial coupling and respiratory efficiency were not altered in muscle. In contrast, rates of mitochondrial hydrogen peroxide (H2O2) emission were increased in the absence of markers of lipid or protein oxidative damage. These results suggest that improvements in mitochondrial oxidative metabolism are not the cause of beetroot juice-mediated improvements in whole body oxygen consumption. ABSTRACT: Ingestion of sodium nitrate (NO3(-)) simultaneously reduces whole body oxygen consumption (V̇O2) during submaximal exercise while improving mitochondrial efficiency, suggesting a causal link. Consumption of beetroot juice (BRJ) elicits similar decreases in V̇O2 but potential effects on the mitochondria remain unknown. Therefore we examined the effects of 7-day supplementation with BRJ (280 ml day(-1), ∼26 mmol NO3(-)) in young active males (n = 10) who had muscle biopsies taken before and after supplementation for assessments of mitochondrial bioenergetics. Subjects performed 20 min of cycling (10 min at 50% and 70% V̇O2 peak) 48 h before 'Pre' (baseline) and 'Post' (day 5 of supplementation) biopsies. Whole body V̇O2 decreased (P < 0.05) by ∼3% at 70% V̇O2 peak following supplementation. Mitochondrial respiration in permeabilized muscle fibres showed no change in leak respiration, the content of proteins associated with uncoupling (UCP3, ANT1, ANT2), maximal substrate-supported respiration, or ADP sensitivity (apparent Km). In addition, isolated subsarcolemmal and intermyofibrillar mitochondria showed unaltered assessments of mitochondrial efficiency, including ADP consumed/oxygen consumed (P/O ratio), respiratory control ratios and membrane potential determined fluorometrically using Safranine-O. In contrast, rates of mitochondrial hydrogen peroxide (H2O2) emission were increased following BRJ. Therefore, in contrast to sodium nitrate, BRJ supplementation does not alter key parameters of mitochondrial efficiency. This occurred despite a decrease in exercise V̇O2, suggesting that the ergogenic effects of BRJ ingestion are not due to a change in mitochondrial coupling or efficiency. It remains to be determined if increased mitochondrial H2O2 contributes to this response.

Omega-3 supplementation alters mitochondrial membrane composition and respiration kinetics in human skeletal muscle.

Studies have shown increased incorporation of omega-3 fatty acids into whole skeletal muscle following supplementation, although little has been done to investigate the potential impact on the fatty acid composition of mitochondrial membranes and the functional consequences on mitochondrial bioenergetics. Therefore, we supplemented young healthy male subjects (n = 18) with fish oils [2 g eicosapentaenoic acid (EPA) and 1 g docosahexanoic acid (DHA) per day] for 12 weeks and skeletal muscle biopsies were taken prior to (Pre) and following (Post) supplementation for the analysis of mitochondrial membrane phospholipid composition and various assessments of mitochondrial bioenergetics. Total EPA and DHA content in mitochondrial membranes increased (P < 0.05) ∼450 and ∼320%, respectively, and displaced some omega-6 species in several phospholipid populations. Mitochondrial respiration, determined in permeabilized muscle fibres, demonstrated no change in maximal substrate-supported respiration, or in the sensitivity (apparent Km) and maximal capacity for pyruvate-supported respiration. In contrast, mitochondrial responses during ADP titrations demonstrated an enhanced ADP sensitivity (decreased apparent Km) that was independent of the creatine kinase shuttle. As the content of ANT1, ANT2, and subunits of the electron transport chain were unaltered by supplementation, these data suggest that prolonged omega-3 intake improves ADP kinetics in human skeletal muscle mitochondria through alterations in membrane structure and/or post-translational modification of ATP synthase and ANT isoforms. Omega-3 supplementation also increased the capacity for mitochondrial reactive oxygen species emission without altering the content of oxidative products, suggesting the absence of oxidative damage. The current data strongly emphasize a role for omega-3s in reorganizing the composition of mitochondrial membranes while promoting improvements in ADP sensitivity.

Sex differences in the physiological responses to exercise-induced dehydration: consequences and mechanisms.

Physiological strain during exercise is increased by mild dehydration (∼1%-3% body mass loss). This response may be sex-dependent, but there are no direct comparative data in this regard. This review aimed to develop a framework for future research by exploring the potential impact of sex on thermoregulatory and cardiac strain associated with exercise-induced dehydration. Sex-based comparisons were achieved by comparing trends from studies that implemented similar experimental protocols but recruited males and females separately. This revealed a higher core temperature (Tc) in response to exercise-induced dehydration in both sexes; however, it seemingly occurred at a lower percent body mass loss in females. Although less clear, similar trends existed for cardiac strain. The average female may have a lower body water volume per body mass compared with males, and therefore the same percent body mass loss between the sexes may represent a larger portion of total body water in females potentially posing a greater physiological strain. In addition, the rate at which Tc increases at exercise onset might be faster in females and induce a greater thermoregulatory challenge earlier into exercise. The Tc response at exercise onset is associated with lower sweating rates in females, which is commonly attributed to sex differences in metabolic heat production. However, a reduced sweat gland sensitivity to stimuli, lower fluid output per sweat gland, and sex hormones promoting fluid retention in females may also contribute. In conclusion, the limited evidence suggests that sex-based differences exist in thermoregulatory and cardiac strain associated with exercise-induced dehydration, and this warrants future investigations.

Constitutive UCP3 overexpression at physiological levels increases mouse skeletal muscle capacity for fatty acid transport and oxidation.

Uncoupling protein 3 (UCP3) expression is directly correlated to fatty acid oxidation in skeletal muscle. UCP3 has been hypothesized to facilitate high rates of fatty acid oxidation, but evidence thus far is lacking. Our aim was to investigate the effects of UCP3 overexpression and ablation on fatty acid uptake and metabolism in muscle of mice having congenic backgrounds. In mice constitutively expressing the UCP3 protein (human form) at levels just over twofold higher than normal (230% of wild-type levels), indirect calorimetry demonstrated no differences in total energy expenditure (VO2), but a shift toward increased fat oxidation compared with wild-type (WT) mice. Metabolic efficiency (gram weight gain/kcal ingested) was similar between Ucp3 overexpressors, WT and Ucp3 (-/-) mice. In muscle of Ucp3-tg mice, plasma membrane fatty acid binding protein (FABPpm) content was increased compared with WT mice. Although hormone-sensitive lipase activity was unchanged across the genotypes, there were increases in carnitine palmitoyltransferase I, beta-hydroxyacylCoA dehydrogenase, and citrate synthase activities and decreases in intramuscular triacylglycerol in muscle of Ucp3-tg mice. There were no differences in muscle mitochondrial content. High-energy phosphates and total muscle carnitine and CoA were also greater in Ucp3-tg compared with WT mice. Taken together, the findings demonstrate an increased capacity for fat oxidation in the absence of significant increases in thermogenesis in Ucp3-tg mice. Findings from Ucp3 (-/-) mice revealed few differences compared with WT mice, consistent with the possibility of compensatory mechanisms. In conjunction with our observed increases in CoA and carnitine in muscle of Ucp3 overexpressors, the findings support the hypothesized role for Ucp3 in facilitating fatty acid oxidation in muscle.

Effect of aging on the buffering capacity of fast-twitch skeletal muscle.

The effects of aging on the in situ buffering capacity of fast-twitch muscle fibers was examined in the tibialis anterior (TA) and extensor digitorum longus (EDL) muscles of specific pathogen free Fischer 344 rats. Muscles were electrically stimulated with trains of impulses lasting 100 ms at a frequency of 80 Hz. Trains were delivered at a rate of 1 Hz for 1 min with the hindlimb circulation occluded. Muscle hydrogen ion (H+) release during stimulation was estimated from the accumulation of metabolites. The free [H+] was measured using an homogenate technique. Muscle buffering capacity (Slykes) was estimated as delta mmol H+/l muscle water/delta pH unit. Muscle pH was unaffected by age both at rest and following stimulation in the TA and EDL. H+ release and buffering capacity were significantly reduced in aged TA muscle but unaffected by age in the EDL. Reduced buffering through metabolic processes accounted for only a small portion of the lower buffering capacity in aged TA. Most of the decrease in buffering capacity appeared to be due to reduced protein buffering. Therefore, aged TA muscle was less able to buffer a given H+ load when compared to adult controls. A more rapid accumulation of H+ during intense stimulation may lead to a earlier onset of fatigue in the aged muscle. It is not clear why the EDL buffering capacity was unaffected by age when the fiber mass profiles of the EDL and TA muscles appear similar (approximately 80% fast glycolytic fibers). It is possible that alterations in activity patterns with aging could have differential effects on the two muscles. Detailed activity pattern and fiber mass analyses are required in adult and aged EDL and TA muscles of Fischer 344 rats to answer this question.

Carbohydrate metabolism during exercise in females: effect of reduced fat availability.

This study examined the effect of reduced plasma free fatty acid (FFA) availability on carbohydrate metabolism during exercise. Six untrained women cycled for 60 minutes at approximately 58% of maximum oxygen uptake after ingestion of a placebo (CON) or nicotinic acid (NA), 30 minutes before exercise (7.4 +/- 0.5 mg.kg(-1) body weight), and at 0 minutes (3.7 +/- 0.3 mg.kg(-1)) and 30 minutes (3.7 +/- 0.3 mg.kg(-1)) of exercise. Glucose kinetics were measured using a primed, continuous infusion of [6,6-(2)H] glucose. Plasma FFA (CON, 0.86 +/- 0.12; NA, 0.21 +/- 0.11 mmol.L(-1) at 60 minutes, P <.05) and glycerol (CON, 0.34 +/- 0.05; NA, 0.10 +/- 0.04 mmol.L(-1) at 60 minutes, P <.05) were suppressed throughout exercise. Mean respiratory exchange ratio (RER) during exercise was higher (P <.05) in NA (0.89 +/- 0.02) than CON (0.83 +/- 0.02). Plasma glucose and glucose production were similar between trials. Total glucose uptake during exercise was greater (P <.05) in NA (1,876 +/- 161 micromol.kg(-1)) than in CON (1,525 +/- 107 micromol.kg(-1)). Total fat oxidation was reduced (P <.05) by approximately 32% during exercise in NA. Total carbohydrate oxidized was approximately 42% greater (P <.05) in NA (412 +/- 40 mmol) than CON (290 +/- 37 mmol), of which, approximately 16% (20 +/- 10 mmol) could be attributed to glucose. Plasma insulin and glucagon were similar between trials. Catecholamines were higher (P <.05) during exercise in NA. In summary, during prolonged moderate exercise in untrained women, reduced FFA availability results in a compensatory increase in carbohydrate oxidation, which appears to be due predominantly to an increase in glycogen utilization, although there was a small, but significant, increase in whole body glucose uptake.

Skeletal muscle glycogen phosphorylase a kinetics: effects of adenine nucleotides and caffeine.

This study aimed to determine physiologically relevant kinetic and allosteric effects of P(i), AMP, ADP, and caffeine on isolated skeletal muscle glycogen phosphorylase a (Phos a). In the absence of effectors, Phos a had Vmax = 221 +/- 2 U/mg and Km = 5.6 +/- 0.3 mM P(i) at 30 degrees C. AMP and ADP each increased Phos a Vmax and decreased Km in a dose-dependent manner. AMP was more effective than ADP (e.g., 1 microM AMP vs. ADP: Vmax = 354 +/- 2 vs. 209 +/- 8 U/mg, and Km = 2.3 +/- 0.1 vs. 4.1 +/- 0.3 mM). Both nucleotides were relatively more effective at lower P(i) levels. Experiments simulating a range of contraction (exercise) conditions in which P(i), AMP, and ADP were used at appropriate physiological concentrations demonstrated that each agent singly and in combination influences Phos a activity. Caffeine (50-100 microM) inhibited Phos a (Km approximately 8-14 mM, approximately 40-50% reduction in activity at 2-10 mM P(i)). The present in vitro data support a possible contribution of substrate (P(i)) and allosteric effects to Phos a regulation in many physiological states, independent of covalent modulation of the percentage of total Phos in the Phos a form and suggest that caffeine inhibition of Phos a activity may contribute to the glycogen-sparing effect of caffeine.

Performance and metabolic responses to a high caffeine dose during prolonged exercise.

The present study examined whether a high caffeine dose improved running and cycling performance and altered substrate metabolism in well-trained runners. Seven trained competitive runners [maximal O2 uptake (VO2max) 72.6 +/- 1.5 ml.kg-1.min-1] completed four randomized and double-blind exercise trials at approximately 85% VO2max; two trials running to exhaustion and two trials cycling to exhaustion. Subjects ingested either placebo (PL, 9 mg/kg dextrose) or caffeine (CAF, 9 mg/kg) 1 h before exercise. Endurance times were increased (P less than 0.05) after CAF ingestion during running (PL 49.2 +/- 7.2 min, CAF 71.0 +/- 11.0 min) and cycling (PL 39.2 +/- 6.5 min, CAF 59.3 +/- 9.9 min). Plasma epinephrine concentration [EPI] was increased (P less than 0.05) with CAF before running (0.22 +/- 0.02 vs. 0.44 +/- 0.08 nM) and cycling (0.31 +/- 0.06 vs. 0.45 +/- 0.06 nM). CAF ingestion also increased [EPI] (P less than 0.05) during exercise; PL and CAF values at 15 min were 1.23 +/- 0.13 and 2.51 +/- 0.33 nM for running and 1.24 +/- 0.24 and 2.53 +/- 0.32 nM for cycling. Similar results were obtained at exhaustion. Plasma norepinephrine was unaffected by CAF at rest and during exercise. CAF ingestion also had no effect on respiratory exchange ratio or plasma free fatty acid data at rest or during exercise. Plasma glycerol was elevated (P less than 0.05) by CAF before exercise and at 15 min and exhaustion during running but only at exhaustion during cycling. Urinary [CAF] increased to 8.7 +/- 1.2 and 10.0 +/- 0.8 micrograms/ml after the running and cycling trials.(ABSTRACT TRUNCATED AT 250 WORDS)

Skeletal muscle phosphofructokinase activity examined under physiological conditions in vitro.

The in vitro activity of skeletal muscle phosphofructokinase (PFK) was determined over the full physiological range of citrate concentrations. Enzyme aggregation was enhanced with a crowding agent, as the regulatory properties of PFK are altered with dilution. Cuvette conditions simulated concentrations of effectors and substrates during rest, moderate aerobic exercise, and intense aerobic exercise in human skeletal muscle. Citrate inhibition was not eliminated with enhanced enzyme aggregation, but activity was improved at all citrate concentrations. Maximal PFK activity with no citrate present was 0.27 +/- 0.01 mumol.min-1.mg-1 protein with resting effectors and 1.64 +/- 0.07 and 7.15 +/- 0.52 mumol.min-1.mg-1 protein with moderate aerobic and intense aerobic effector levels, respectively. Under resting conditions, PFK activity decreased to 49% of maximal when citrate was increased from 0 to 0.15 mM and only a small further inhibition to 43% occurred at 0.5 mM. Citrate was a less potent inhibitor under both exercise conditions with the sharpest decline to 72-77% of maximal activity at 0.15 mM followed by a slower decline to 65-70 and 53% activity at 0.25 and 0.5 mM citrate, respectively. The present in vitro measurements predict that alterations in citrate around concentrations normally reported in resting and exercising muscle would have little effect on flux through PFK. Therefore, the generally accepted concept that citrate is a potent inhibitor of PFK in all physiological situations has been exaggerated.

Metabolic, catecholamine, and exercise performance responses to various doses of caffeine.

This study examined the exercise responses of well-trained endurance athletes to various doses of caffeine to evaluate the impact of the drug on exercise metabolism and endurance capacity. Subjects (n = 8) withdrew from all dietary sources of caffeine for 48 h before each of four tests. One hour before exercise they ingested capsules of placebo or caffeine (3, 6, or 9 mg/kg), rested quietly, and then ran at 85% of maximal O2 consumption to voluntary exhaustion. Blood samples for methylxanthine, catecholamine, glucose, lactate, free fatty acid, and glycerol analyses were taken every 15 min. Plasma caffeine concentration increased with each dose (P < 0.05). Its major metabolite, paraxanthine, did not increase between the 6 and 9 mg/kg doses, suggesting that hepatic caffeine metabolism was saturated. Endurance was enhanced with both 3 and 6 mg/kg of caffeine (increases of 22 +/- 9 and 22 +/- 7%, respectively; both P < 0.05) over the placebo time of 49.4 +/- 4.2 min, whereas there was no significant effect with 9 mg/kg of caffeine. In contrast, plasma epinephrine was not increased with 3 mg/kg of caffeine but was greater with the higher doses (P < 0.05). Similarly only the highest dose of caffeine resulted in increases in glycerol and free fatty acids (P < 0.05). Thus the highest dose had the greatest effect on epinephrine and blood-borne metabolites yet had the least effect on performance. The lowest dose had little or no effect on epinephrine and metabolites but did have an ergogenic effect. These results are not compatible with the traditional theory that caffeine mediates its ergogenic effect via enhanced catecholamines.

Epinephrine infusion enhances muscle glycogenolysis during prolonged electrical stimulation.

To determine the effects of epinephrine (EPI) infusion on muscle glycogenolysis and force production, the quadriceps muscles of both legs in six subjects were intermittently stimulated for 30 min. Contractions lasted 1.6 s (20 Hz) and were separated by 1.6 s of rest. EPI was infused (approximately 0.14 micrograms.kg body wt-1.min-1) in one leg during the last 15 min and the vastus lateralis was biopsied at rest (control leg only) and after 15, 18 (EPI leg only), and 30 min of stimulation. EPI infusion doubled the mole fraction of phosphorylase a (22.5 +/- 4.1 to 44.8 +/- 9.0%) and glycogenolysis (2.16 +/- 0.72 to 5.45 +/- 0.81 mmol glucosyl U.kg dry muscle wt-1.min-1) during stimulation. Muscle glucose 6-phosphate increased from 3.04 +/- 0.17 to 6.43 +/- 0.20 mmol/kg dry muscle wt, and lactate increased from 25.8 +/- 4.4 to 34.3 +/- 4.6 mmol/kg after 3 min of EPI infusion. Isometric force production was unaltered by EPI infusion. These results demonstrate a strong glycogenolytic effect of EPI infusion during prolonged electrical stimulation and suggest that the extra pyruvate formed was converted mainly to lactate. Exclusive anaerobic metabolism of the extra substrate would provide only a 10% increase in total ATP production, possibly accounting for the lack of improvement in force production. We suggest that the decrease in force production during prolonged electrical stimulation is related to decreased excitation of the contractile mechanism rather than inhibition of cross-bridge turnover caused by a shortage of energy or accumulation of hyproducts.

High physiological levels of epinephrine do not enhance muscle glycogenolysis during tetanic stimulation.

This study examined whether high physiological concentrations of epinephrine (EPI) would enhance muscle glycogenolysis during intense muscular contractions. Muscles of the rat hindlimb were perfused for 12 min at rest and 45 s of tetanic stimulation (1.0-Hz train rate, 100-ms train duration at 80 Hz) without EPI (control) or with 15 or 35 nM EPI. In the EPI groups the muscles were perfused with EPI for the last 2 min of rest perfusion and throughout stimulation. Glycogenolysis in the white gastrocnemius, red gastrocnemius, plantaris, and soleus muscles during stimulation was unaffected by the presence of EPI in the perfusion medium. In addition, muscle lactate and hindlimb lactate efflux were similar in EPI and control groups. It is concluded that EPI is not important for enhancing glycogenolysis in rat muscles composed predominantly of fast-twitch fibers during intense short-term tetanic stimulation.

Pyruvate ingestion for 7 days does not improve aerobic performance in well-trained individuals.

The purposes of the present studies were to test the hypotheses that lower dosages of oral pyruvate ingestion would increase blood pyruvate concentration and that the ingestion of a commonly recommended dosage of pyruvate (7 g) for 7 days would enhance performance during intense aerobic exercise in well-trained individuals. Nine recreationally active subjects (8 women, 1 man) consumed 7, 15, and 25 g of pyruvate and were monitored for a 4-h period to determine whether blood metabolites were altered. Pyruvate consumption failed to significantly elevate blood pyruvate, and it had no effect on indexes of carbohydrate (blood glucose, lactate) or lipid metabolism (blood glycerol, plasma free fatty acids). As a follow-up, we administered 7 g/day of either placebo or pyruvate, for a 1-wk period to seven, well-trained male cyclists (maximal oxygen consumption, 62.3 +/- 3.0 ml. kg(-1). min(-1)) in a randomized, double-blind, crossover trial. Subjects cycled at 74-80% of their maximal oxygen consumption until exhaustion. There was no difference in performance times between the two trials (placebo, 91 +/- 9 min; pyruvate, 88 +/- 8 min). Measured blood parameters (insulin, peptide C, glucose, lactate, glycerol, free fatty acids) were also unaffected. Our results indicate that oral pyruvate supplementation does not increase blood pyruvate content and does not enhance performance during intense exercise in well-trained cyclists.

Dietary carbohydrate, muscle glycogen content, and endurance performance in well-trained women.

This study examined the ability of well-trained eumenorrheic women to increase muscle glycogen content and endurance performance in response to a high-carbohydrate diet (HCD; approximately 78% carbohydrate) compared with a moderate-carbohydrate diet (MD; approximately 48% carbohydrate) when tested during the luteal phase of the menstrual cycle. Six women cycled to exhaustion at approximately 80% maximal oxygen uptake (VO(2 max)) after each of the randomly assigned diet and exercise-tapering regimens. A biopsy was taken from the vastus lateralis before and after exercise in each trial. Preexercise muscle glycogen content was high after the MD (625.2 +/- 50.1 mmol/kg dry muscle) and 13% greater after the HCD (709.0 +/- 44.8 mmol/kg dry muscle). Postexercise muscle glycogen was low after both trials (MD, 91.4 +/- 34.5; HCD, 80.3 +/- 19.5 mmol/kg dry muscle), and net glycogen utilization during exercise was greater after the HCD. The subjects also cycled longer at approximately 80% VO(2 max) after the HCD vs. MD (115:31 +/- 10:47 vs. 106:35 +/- 8:36 min:s, respectively). In conclusion, aerobically trained women increased muscle glycogen content in response to a high-dietary carbohydrate intake during the luteal phase of the menstrual cycle, but the magnitude was smaller than previously observed in men. The increase in muscle glycogen, and possibly liver glycogen, after the HCD was associated with increased cycling performance to volitional exhaustion at approximately 80% VO(2 max).

Effects of intense swimming and tetanic electrical stimulation on skeletal muscle ions and metabolites.

The purpose of this study was to compare changes in ions and metabolites in four different rat hindlimb muscles in response to intense swimming exercise in vivo (263 +/- 33 s) (SWUM), and to 5 min (300 s) of tetanic electrical stimulation of artificially perfused rat hindlimbs (STIM). With both swimming and electrical stimulation, soleus (SOL) contents of creatine phosphate (CP), ATP, and glycogen changed the least, whereas the largest decreases in these metabolites occurred in the white gastrocnemius (WG). Lactate (La-) accumulation and glycogen breakdown were significantly greater in SWUM hindlimb muscles compared with STIM. The high arterial La- concentration [( La-] = 20 meq.l-1) in SWUM may have contributed to elevated muscle [La-], whereas one-pass perfusion kept arterial [La-] below 2 meq.l-1 in STIM. In SWUM, intracellular [Na+] increased significantly in the plantaris (PL), red gastrocnemius (RG), and WG, but not in SOL. [Cl-] increased, and [K+], [Ca2+], and [Mg2+] decreased in all muscles. In STIM, intracellular [K+], [Mg2+], and [Ca2+] decreased significantly, whereas [Na+] and [Cl-] increased in all muscles. Differences in the magnitude of ion and fluid fluxes between groups can be explained by the different methods of hindlimb perfusion. In conclusion, STIM is a useful model of in vivo energy metabolism and permits mechanisms of transsarcolemmal ion movements to be studied.

Anaerobic energy provision in aged skeletal muscle during tetanic stimulation.

The effect of age on skeletal muscle anaerobic energy metabolism was investigated in adult (11 mo) and aged (25 mo) Fischer 344 rats. Hindlimb skeletal muscles innervated by the sciatic nerve were stimulated to contract with trains of supramaximal impulses (100 ms, 80 Hz) at a train rate of 1 Hz for 60 s, with an occluded circulation. Soleus, plantaris, and red and white gastrocnemius (WG) were sampled from control and stimulated limbs. All muscle masses were reduced with age (9-13%). Peak isometric tensions, normalized per gram of wet muscle, were lower throughout the stimulation in the aged animals (28%). The potential for anaerobic ATP provision was unaltered with age in all muscles, because resting high-energy phosphates and glycogen contents were similar to adult values. Anaerobic ATP provision during stimulation was unaltered by aging in soleus, plantaris, and red gastrocnemius muscles. In the WG, containing mainly fast glycolytic (FG) fibers, ATP and phosphocreatine contents were depleted less in aged muscle. In situ glycogenolysis and glycolysis were 90.0 +/- 4.8 and 69.3 +/- 2.6 mumol/g dry muscle (dm) in adult WG and reduced to 62.3 +/- 6.9 and 51.5 +/- 5.5 mumol/g dm, respectively, in aged WG. Consequently, total anaerobic ATP provision was lower in aged WG (224.5 +/- 20.9 mumol/g dm) vs. adult (292.6 +/- 7.6 mumol/g dm) WG muscle. In summary, the decreased tetanic tension production in aged animals was associated with a decreased anaerobic energy production in FG fibers. Reduced high-energy phosphate use and a greater energy charge potential after stimulation suggested that the energy demand was reduced in aged FG fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma and muscle amino acid and ammonia responses during prolonged exercise in humans.

Plasma and muscle amino acid (AA) and ammonia (NH3) responses were measured during prolonged submaximal exercise in humans. Increased NH3 production during submaximal exercise has been attributed to the activity of the purine nucleotide cycle, without consideration of any possible contribution from AA. Six men cycled at 75% of maximal O2 uptake until exhaustion on two occasions after 2.5 days of ingestion of a high-carbohydrate or mixed diet. Plasma samples (antecubital vein) and muscle biopsies (vastus lateralis) were obtained at rest and during exercise and analyzed for plasma and muscle NH3 and AA as well as muscle metabolites. There were no significant diet effects in these parameters, so the majority of results focus on the effects of exercise. Plasma and muscle NH3 increased significantly from the onset and continued to increase throughout exercise. The total and total essential [AA] of muscle were significantly increased at exhaustion, whereas both the plasma and muscle branched-chain AA contents were unchanged. This suggests that protein catabolism was occurring during exercise and the branched-chain AA were used for energy and NH3 production.

Skeletal muscle metabolism during high-intensity sprint exercise is unaffected by dichloroacetate or acetate infusion.

This study investigated whether increased provision of oxidative substrate would reduce the reliance on nonoxidative ATP production and/or increase power output during maximal sprint exercise. The provision of oxidative substrate was increased at the onset of exercise by the infusion of acetate (AC; increased resting acetylcarnitine) or dichloroacetate [DCA; increased acetylcarnitine and greater activation of pyruvate dehydrogeanse (PDH-a)]. Subjects performed 10 s of maximal cycling on an isokinetic ergometer on three occasions after either DCA, AC, or saline (Con) infusion. Resting PDH-a with DCA was increased significantly over AC and Con trials (3.58 +/- 0.4 vs. 0.52 +/- 0.1 and 0.74 +/- 0.1 mmol. kg wet muscle(-1). min(-1)). DCA and AC significantly increased resting acetyl-CoA (35.2 +/- 4.4 and 22.7 +/- 2.9 vs. 10.2 +/- 1.3 micromol/kg dry muscle) and acetylcarnitine (12.9 +/- 1.4 and 11.0 +/- 1.0 vs. 3.3 +/- 0.6 mmol/kg dry muscle) over Con. Resting contents of phosphocreatine, lactate, ATP, and glycolytic intermediates were not different among trials. Average power output and total work done were not different among the three 10-s sprint trials. Postexercise, PDH-a in AC and Con trials had increased significantly but was still significantly lower than in DCA trial. Acetyl-CoA did not increase in any trial, whereas acetylcarnitine increased significantly only in DCA. Exercise caused identical decreases in ATP and phosphocreatine and identical increases in lactate, pyruvate, and glycolytic intermediates in all trials. These data suggest that there is an inability to utilize extra oxidative substrate (from either stored acetylcarnitine or increased PDH-a) during exercise at this intensity, possibly because of O(2) and/or metabolic limitations.