G. van Hall

Ingestion of branched-chain amino acids and tryptophan during sustained exercise in man: failure to affect performance.

1. An increased uptake of tryptophan in the brain may increase serotoninergic activity and recently has been suggested to be a cause of fatigue during prolonged exercise. The present study, therefore, investigates whether ingestion of tryptophan or the competing branched‐chain amino acids (BCAAs) affect performance. Ten endurance‐trained male athletes were studied during cycle exercise at 70‐75% maximal power output, while ingesting, ad random and double‐blind, drinks that contained 6% sucrose (control) or 6% sucrose supplemented with (1) tryptophan (3 g l‐1), (2) a low dose of BCAA (6 g l‐1) or (3) a high dose of BCAA (18 g l‐1). 2. These treatments greatly increased the plasma concentration of the respective amino acids. Using the kinetic parameters of transport of human brain capillaries, BCAA supplements were estimated to reduce brain tryptophan uptake at exhaustion by 8‐12%, while tryptophan ingestion caused a 7‐ to 20‐fold increase. Exercise time to exhaustion was not different between treatments (122 +/‐ 3 min). 3. The data suggest that manipulation of tryptophan supply to the brain either has no additional effect upon serotoninergic activity during prolonged exhaustive exercise or that manipulation of serotoninergic activity functionally does not contribute to mechanisms of fatigue.

Maximal muscular vascular conductances during whole body upright exercise in humans

That muscular blood flow may reach 2.5 l kg−1 min−1 in the quadriceps muscle has led to the suggestion that muscular vascular conductance must be restrained during whole body exercise to avoid hypotension. The main aim of this study was to determine the maximal arm and leg muscle vascular conductances (VC) during leg and arm exercise, to find out if the maximal muscular vasodilatory response is restrained during maximal combined arm and leg exercise. Six Swedish elite cross‐country skiers, age (mean ±s.e.m.) 24 ± 2 years, height 180 ± 2 cm, weight 74 ± 2 kg, and maximal oxygen uptake 5.1 ± 0.1 l min−1 participated in the study. Femoral and subclavian vein blood flows, intra‐arterial blood pressure, cardiac output, as well as blood gases in the femoral and subclavian vein, right atrium and femoral artery were determined during skiing (roller skis) at ∼76% of and at with different techniques: diagonal stride (combined arm and leg exercise), double poling (predominantly arm exercise) and leg skiing (predominantly leg exercise). During submaximal exercise cardiac output (26–27 l min−1), mean blood pressure (MAP) (∼87 mmHg), systemic VC, systemic oxygen delivery and pulmonary (∼4 l min−1) attained similar values regardless of exercise mode. The distribution of cardiac output was modified depending on the musculature engaged in the exercise. There was a close relationship between VC and in arms (r= 0.99, P < 0.001) and legs (r= 0.98, P < 0.05). Peak arm VC (63.7 ± 5.6 ml min−1 mmHg−1) was attained during double poling, while peak leg VC was reached at maximal exercise with the diagonal technique (109.8 ± 11.5 ml min−1 mmHg−1) when arm VC was 38.8 ± 5.7 ml min−1 mmHg−1. If during maximal exercise arms and legs had been vasodilated to the observed maximal levels then mean arterial pressure would have dropped at least to 75–77 mmHg in our experimental conditions. It is concluded that skeletal muscle vascular conductance is restrained during whole body exercise in the upright position to avoid hypotension.

Human skeletal muscle and erythrocyte proteins involved in acid-base homeostasis: adaptations to chronic hypoxia

Chronic hypoxia is accompanied by changes in blood and skeletal muscle acid‐base control. We hypothesized that the underlying mechanisms include altered protein expression of transport systems and the enzymes involved in lactate, HCO3− and H+ fluxes in skeletal muscle and erythrocytes. Immunoblotting was used to quantify densities of the transport systems and enzymes. Muscle and erythrocyte samples were obtained from eight Danish lowlanders at sea level and after 2 and 8 weeks at 4100 m (Bolivia). For comparison, samples were obtained from eight Bolivian natives. In muscle membranes there were no changes in fibre‐type distribution, lactate dehydrogenase isoforms, Na+,K+‐pump subunits or in the lactate‐H+ co‐transporters MCT1 and MCT4. The Na+–H+ exchanger protein NHE1 was elevated by 39 % in natives compared to lowlanders. The Na+‐HCO3− co‐transporter density in muscle was elevated by 47–69 % after 2 and 8 weeks at altitude. The membrane‐bound carbonic anhydrase (CA) IV in muscle increased in the lowlanders by 39 %, whereas CA XIV decreased by 23–47 %. Levels of cytosolic CA II and III in muscle and CA I and II in erythrocytes were unchanged. The erythrocyte lactate‐H+ co‐transporter MCT1 increased by 230–405 % in lowlanders and was 324 % higher in natives. The erythrocyte inorganic anion exchanger (Cl−‐HCO3− exchanger AE1) was increased by 149–228 %. In conclusion, chronic hypoxia induces dramatic changes in erythrocyte proteins, but only moderate changes in muscle proteins involved in acid‐base control. Together, these changes suggest a hypoxia‐induced increase in the capacity for lactate, HCO3− and H+ fluxes from muscle to blood and from blood to erythrocytes.

Human Skeletal Muscle Fatty Acid and Glycerol Metabolism During Rest, Exercise and Recovery

This study was conducted to investigate skeletal muscle fatty acid (FA) and glycerol kinetics and to determine the contribution of skeletal muscle to whole body FA and glycerol turnover during rest, 2 h of one‐leg knee‐extensor exercise at 65 % of maximal leg power output, and 3 h of recovery. To this aim, the leg femoral arterial‐venous difference technique was used in combination with a continuous infusion of [U‐13C]palmitate and [2H5]glycerol in five post‐absorptive healthy volunteers (22 ± 3 years). The influence of contamination from non‐skeletal muscle tissues, skin and subcutaneous adipose tissue, on FA and glycerol kinetics was studied by catheterization of the femoral vein in antegrade and retrograde directions. Substantially higher net leg FA and glycerol uptakes were observed with a retrograde compared to an antegrade catheter position, as a result of a much lower tracer‐calculated leg FA and glycerol release. The whole body FA rate of appearance (Ra) increased with exercise and decreased rapidly in recovery but stayed higher compared to pre‐exercise. The leg net FA uptake decreased immediately on cessation of exercise to near pre‐exercise level, but the tracer FA uptake and release decreased slowly and reached constant values after ≈1.5 h of recovery similar to pre‐exercise. Whole body FA reesterification (FA Rd ‐ FA oxidation; Rd, rate of disappearance) was ≈400 μmol min−1 at rest and during exercise, and increased during recovery to 495 μmol min−1. Leg FA reesterification was 17 μmol min−1 at rest and decreased to 9 μmol min−1 during recovery, due to a larger fraction of leg FA uptake being directed to oxidation. A net glycerol exchange across the leg could not be detected under all conditions, but a substantial leg glycerol uptake was observed, which was substantially higher during exercise. Total body skeletal muscle FA and glycerol uptake/release was estimated to account for 18–25 % of whole body Rd or Ra. In conclusion: (1) skeletal muscle FA and glycerol metabolism, using the leg arterial‐venous difference method, can only be studied if contamination from skin and subcutaneous adipose tissue is prevented; (2) whole body FA reesterification is unchanged when going from rest to exercise, but is increased during recovery; (3) in post‐absorptive man total body skeletal muscle contributes 17–24 % to whole body FA and glycerol turnover and FA reesterification at rest; (4) glycerol is taken up by skeletal muscle and the uptake increases many fold during exercise.

Deamination of amino acids as a source for ammonia production in human skeletal muscle during prolonged exercise.

1. The influence of pre‐exercise muscle glycogen content on ammonia production, adenine nucleotide breakdown and amino acid metabolism was investigated during prolonged exercise in six subjects having one leg with a normal and one leg with a low muscle glycogen content. One‐leg knee‐extensor exercise was performed for 90 min, at a workload of 60‐65% of the maximal power output, first with one leg and then with the other. 2. During exercise ammonia was released in gradually increasing amounts and plateaued after 1 h exercise at a rate of approximately 80 mumol min‐1. The total ammonia production was 9.1 +/‐ 0.4 and 9.5 +/‐ 1.4 mmol (kg dry muscle)‐1 in the normal and low glycogen content leg, respectively. 3. Levels of muscle phosphocreatine (PC), total adenine nucleotides and inosine monophosphate (IMP) were similar at rest and after 90 min of exercise. 4. Only minor differences were observed between rest and exercise and between legs for the muscle concentrations of glutamine, alanine and the branched‐chain amino acids. Muscle glutamate concentration decreased by 60‐70% within the first 10 min of exercise. Glutamate consumption over 90 min quantitatively equalled ammonia production. Most of the glutamate was consumed within the first 10 min of exercise, while ammonia production gradually increased during exercise. Therefore deamination of glutamate cannot be the main source of ammonia production during the later stage of exercise. 5. It is concluded that during prolonged one‐leg exercise at moderate intensity: (a) ammonia production is not affected by pre‐exercise muscle glycogen content, (b) ammonia production exceeds by far the breakdown of adenine nucleotides to IMP and therefore has to be derived from alternative sources, and (c) deamination of amino acids is a likely source of ammonia production during prolonged exercise.

Exercise economy does not change after acclimatization to moderate to very high altitude

For more than 60 years, muscle mechanical efficiency has been thought to remain unchanged with acclimatization to high altitude. However, recent work has suggested that muscle mechanical efficiency may in fact be improved upon return from prolonged exposure to high altitude. The purpose of the present work is to resolve this apparent conflict in the literature. In a collaboration between four research centers, we have included data from independent high‐altitude studies performed at varying altitudes and including a total of 153 subjects ranging from sea‐level (SL) residents to high‐altitude natives, and from sedentary to world‐class athletes. In study A (n=109), living for 20–22 h/day at 2500 m combined with training between 1250 and 2800 m caused no differences in running economy at fixed speeds despite low typical error measurements. In study B, SL residents (n=8) sojourning for 8 weeks at 4100 m and residents native to this altitude (n=7) performed cycle ergometer exercise in ambient air and in acute normoxia. Muscle oxygen uptake and mechanical efficiency were unchanged between SL and acclimatization and between the two groups. In study C (n=20), during 21 days of exposure to 4300 m altitude, no changes in systemic or leg VO2 were found during cycle ergometer exercise. However, at the substantially higher altitude of 5260 m decreases in submaximal VO2 were found in nine subjects with acute hypoxic exposure, as well as after 9 weeks of acclimatization. As VO2 was already reduced in acute hypoxia this suggests, at least in this condition, that the reduction is not related to anatomical or physiological adaptations to high altitude but to oxygen lack because of severe hypoxia altering substrate utilization. In conclusion, results from several, independent investigations indicate that exercise economy remains unchanged after acclimatization to high altitude.

Intramuscular fatty acid metabolism in contracting and non-contracting human skeletal muscle

The present study was undertaken to investigate the fate of blood‐borne non‐esterified fatty acids (NEFA) entering contracting and non‐contracting knee extensor muscles of healthy young individuals. [U‐13C]‐palmitate was infused into a forearm vein during 5 h of one‐legged knee extensor exercise at 40 % of maximal work capacity and the NEFA kinetics, oxidation and rate of incorporation into intramuscular triacylglycerol (mTAG) were determined for the exercising and the non‐exercising legs. During 4 h of one‐legged knee extensor exercise, mtag content decreased by 30 % (P < 0.05) in the contracting muscle, whereas it was unchanged in the non‐contracting muscle. The uptake of plasma NEFA, as well as the proportion directed towards oxidation, was higher in the exercising compared to the non‐exercising leg, whereas the rate of palmitate incorporation into mtag was fourfold lower (0.70 ± 0.14 vs. 0.17 ± 0.04 μmol (g dry wt)−1 h−1; P lt; 0.05), resulting in fractional synthesis rates of 1.0 ± 0.2 and 3.8 ± 0.9 % h−1 (P lt; 0.01) for the contracting and non‐contracting muscle, respectively. These findings demonstrate that mTAG in human skeletal muscle is continuously synthesised and degraded and that the metabolic fate of plasma NEFA entering the muscle is influenced by muscle contraction, so that a higher proportion is directed towards oxidation at the expense of storage in mTAG.

The re-establishment of the normal blood lactate response to exercise in humans after prolonged acclimatization to altitude

1 One to five weeks of chronic exposure to hypoxia has been shown to reduce peak blood lactate concentration compared to acute exposure to hypoxia during exercise, the high altitude ‘lactate paradox’. However, we hypothesize that a sufficiently long exposure to hypoxia would result in a blood lactate and net lactate release from the active leg to an extent similar to that observed in acute hypoxia, independent of work intensity. 2 Six Danish lowlanders (25–26 years) were studied during graded incremental bicycle exercise under four conditions: at sea level breathing either ambient air (0 m normoxia) or a low‐oxygen gas mixture (10 % O2 in N2, 0 m acute hypoxia) and after 9 weeks of acclimatization to 5260 m breathing either ambient air (5260 m chronic hypoxia) or a normoxic gas mixture (47 % O2 in N2, 5260 m acute normoxia). In addition, one‐leg knee‐extensor exercise was performed during 5260 m chronic hypoxia and 5260 m acute normoxia. 3 During incremental bicycle exercise, the arterial lactate concentrations were similar at sub‐maximal work at 0 m acute hypoxia and 5260 m chronic hypoxia but higher compared to both 0 m normoxia and 5260 m acute normoxia. However, peak lactate concentration was similar under all conditions (10.0 ± 1.3, 10.7 ± 2.0, 10.9 ± 2.3 and 11.0 ± 1.0 mmol l−1) at 0 m normoxia, 0 m acute hypoxia, 5260 m chronic hypoxia and 5260 m acute normoxia, respectively. Despite a similar lactate concentration at sub‐maximal and maximal workload, the net lactate release from the leg was lower during 0 m acute hypoxia (peak 8.4 ± 1.6 mmol min−1) than at 5260 m chronic hypoxia (peak 12.8 ± 2.2 mmol min−1). The same was observed for 0 m normoxia (peak 8.9 ± 2.0 mmol min−1) compared to 5260 m acute normoxia (peak 12.6 ± 3.6 mmol min−1). Exercise after acclimatization with a small muscle mass (one‐leg knee‐extensor) elicited similar lactate concentrations (peak 4.4 ± 0.2 vs. 3.9 ± 0.3 mmol l−1) and net lactate release (peak 16.4 ± 1.8 vs. 14.3 mmol l−1) from the active leg at 5260 m chronic hypoxia and 5260 m acute normoxia. 4 In conclusion, in lowlanders acclimatized for 9 weeks to an altitude of 5260 m, the arterial lactate concentration was similar at 0 m acute hypoxia and 5260 m chronic hypoxia. The net lactate release from the active leg was higher at 5260 m chronic hypoxia compared to 0 m acute hypoxia, implying an enhanced lactate utilization with prolonged acclimatization to altitude. The present study clearly shows the absence of a lactate paradox in lowlanders sufficiently acclimatized to altitude.

Mechanisms of activation of muscle branched-chain alpha-keto acid dehydrogenase during exercise in man.

1. Exercise leads to activation (dephosphorylation) of the branched‐chain alpha‐keto acid dehydrogenase (BCKADH). Here we investigate the effect of low pre‐exercise muscle glycogen content and of branched‐chain amino acid (BCAA) ingestion on the activity of BCKADH at rest and after 90 min of one‐leg knee‐extensor exercise at 65% maximal one‐leg power output in five subjects. 2. Pre‐exercise BCAA ingestion (308 mg BCAAs (kg body wt)‐1) caused an increased muscle BCAA uptake, a higher intramuscular BCAA concentration and activation of BCKADH both at rest (9 +/‐ 1 versus 25 +/‐ 5% for the control and BCAA test, respectively) and after exercise (27 +/‐ 4 versus 54 +/‐ 7%). 3. At rest the percentage active BCKADH was not different, 6 +/‐ 2% versus 5 +/‐ 1%, in the normal and low glycogen content leg (392 +/‐ 21 and 147 +/‐ 34 mumol glycosyl units (g dry muscle)‐1, respectively). The post‐exercise BCKADH activity was higher in the low (46 +/‐ 2%) than in the normal glycogen content leg (26 +/‐ 2%). 4. It is concluded that: (1) the mechanism of activation by BCAA ingestion probably involves an increase of the muscle BCAA concentration; (2) BCKADH activation caused by exercise and BCAA ingestion are additive; (3) low pre‐exercise muscle glycogen content augments the exercise‐induced BCKADH activation without an increase in muscle BCAA concentration; and (4) the mechanism of BCKADH activation via BCAA ingestion and low muscle glycogen content are different.

Muscle protein degradation and amino acid metabolism during prolonged knee-extensor exercise in humans.

The aim of this study was to investigate whether prolonged one-leg knee-extensor exercise enhances net protein degradation in muscle with a normal or low glycogen content. Net amino acid production, as a measure of net protein degradation, was estimated from leg exchange and from changes in the concentrations of amino acids that are not metabolized in skeletal muscle. Experiments were performed at rest and during one-leg knee-extensor exercise in six subjects having one leg with a normal glycogen content and the other with a low glycogen content. Exercise was performed for 90 min at a workload of 60-65% of maximal one-leg power output, starting either with the normal-glycogen or the low-glycogen leg, at random. The net production of threonine, lysine and tyrosine and the sum of the non-metabolized amino acids were 9-20-fold higher (P<0.05) during exercise of the normal-glycogen leg than at rest. Total amino acid production was also 10-fold higher during exercise compared with that at rest (difference not significant). The net production rates of threonine, glycine and tyrosine and of the sum of the non-metabolized amino acids were about 1.5-2.5-fold higher during exercise with the leg with a low glycogen content compared with the leg with a normal glycogen content (P<0.05). Total amino acid production was 1.5-fold higher during exercise for the low-glycogen leg compared with the normal-glycogen leg (difference not significant). These data indicate that prolonged one-leg knee-extensor exercise leads to a substantial increase in net muscle protein degradation, and that a lowering of the starting muscle glycogen content leads to a further increase. The carbon atoms of the branched-chain amino acids (BCAA), glutamate, aspartate and asparagine, liberated by protein degradation, and the BCAA and glutamate extracted in increased amounts from the blood during exercise, are used for the synthesis of glutamine and for tricarboxylic-acid cycle anaplerosis.