Bente Kiens

Skeletal muscle substrate utilization during submaximal exercise in man: effect of endurance training.

1. The influence of training‐induced adaptations in skeletal muscle tissue on the choice between carbohydrates (CHO) and lipids as well as the extra‐ vs. intracellular substrate utilization was investigated in seven healthy male subjects performing one‐legged knee‐extension exercise. In each subject one of the knee extensors was endurance trained for eight weeks, whereafter the trained (T) and non‐trained (NT) thighs were investigated a week apart. 2. The activity of beta‐hydroxy‐acyl‐coenzyme A dehydrogenase (HAD) and capillary density in the knee extensors were significantly larger in T than in NT. 3. During dynamic knee‐extension exercise, performed at the same absolute intensity for 2 h, femoral venous blood flow was lower in T than in NT (P < 0.05), but oxygen uptake was similar. 4. Respiratory quotient (RQ) values over the exercising thigh, averaging 0.81 (T) vs. 0.91 (NT; P < 0.05) indicated that a shift towards a larger fat combustion occurred with endurance training. 5. Both free fatty acids (FFA) and serum triacylglycerol contributed to the utilization of fat in NT and T muscles with no significant contribution from muscle fibre triacylglycerol. 6. At high plasma FFA concentrations net uptake of FFA plateaued in NT but not in T muscles. 7. The findings suggest that FFA uptake in exercising muscle is a saturable process and that the transport capacity is enhanced by training. The lower CHO utilization in the T leg was mainly a function of the glycogenolysis of the muscle being reduced. Hormones such as insulin, noradrenaline and adrenaline are unlikely to play a role in this shift as differences in plasma levels during T and NT leg exercise were small and insignificant, implying that local structural and functional adaptations of the training muscle are crucial for the observed shifts in the metabolic response to exercise.

Isoform-specific and exercise intensity-dependent activation of 5'-AMP-activated protein kinase in human skeletal muscle.

1 5′‐AMP‐activated protein kinase (AMPK) has been suggested to play a key role in the regulation of metabolism in skeletal muscle. AMPK is activated in treadmill‐exercised and electrically stimulated rodent muscles. Whether AMPK is activated during exercise in humans is unknown. 2 We investigated the degree of activation and deactivation of α‐isoforms of AMPK during and after exercise. Healthy human subjects performed bicycle exercise on two separate occasions at either a low (∼50% maximum rate of O2 uptake (V̇O2,max) for 90 min) or a high (∼75% V̇O2,max for 60 min) intensity. Biopsies from the vastus lateralis muscle were obtained before and immediately after exercise, and after 3 h of recovery. 3 We observed a 3‐ to 4‐fold activation of the α2‐AMPK isoform immediately after high intensity exercise, whereas no activation was observed after low intensity exercise. The activation of α2‐AMPK was totally reversed 3 h after exercise. In contrast, α1‐AMPK was not activated during either of the two exercise trials. 4 The in vitro AMP dependency of α2‐AMPK was significantly greater than that of α1‐AMPK (∼3‐ vs.∼2‐fold). 5 We conclude that in humans activation of α2‐AMPK during exercise is dependent upon exercise intensity. The stable activation of α2‐AMPK, presumably due to the activation of an upstream AMPK kinase, is compatible with a role for this kinase complex in the regulation of skeletal muscle metabolism during exercise, whereas the lack of stable α1‐AMPK activation makes this kinase complex a less likely candidate.

Carbohydrates and fat for training and recovery

An important goal of the athletes everyday diet is to provide the muscle with substrates to fuel the training programme that will achieve optimal adaptation for performance enhancements. In reviewing the scientific literature on post-exercise glycogen storage since 1991, the following guidelines for the training diet are proposed. Athletes should aim to achieve carbohydrate intakes to meet the fuel requirements of their training programme and to optimize restoration of muscle glycogen stores between workouts. General recommendations can be provided, preferably in terms of grams of carbohydrate per kilogram of the athletes body mass, but should be fine-tuned with individual consideration of total energy needs, specific training needs and feedback from training performance. It is valuable to choose nutrient-rich carbohydrate foods and to add other foods to recovery meals and snacks to provide a good source of protein and other nutrients. These nutrients may assist in other recovery processes and, in the case of protein, may promote additional glycogen recovery when carbohydrate intake is suboptimal or when frequent snacking is not possible. When the period between exercise sessions is  <8 h, the athlete should begin carbohydrate intake as soon as practical after the first workout to maximize the effective recovery time between sessions. There may be some advantages in meeting carbohydrate intake targets as a series of snacks during the early recovery phase, but during longer recovery periods (24 h) the athlete should organize the pattern and timing of carbohydrate-rich meals and snacks according to what is practical and comfortable for their individual situation. Carbohydrate-rich foods with a moderate to high glycaemic index provide a readily available source of carbohydrate for muscle glycogen synthesis, and should be the major carbohydrate choices in recovery meals. Although there is new interest in the recovery of intramuscular triglyceride stores between training sessions, there is no evidence that diets which are high in fat and restricted in carbohydrate enhance training.

Anaerobic energy production and O2 deficit-debt relationship during exhaustive exercise in humans.

1. Eight subjects performed one‐legged, dynamic, knee‐extensor exercise, first at 10 W followed by 10 min rest, then at an intense, exhaustive exercise load (65 W) lasting 3.2 min. After 60 min recovery, exercise was performed for 8‐10 min each at 20, 30, 40 and 50 W. Measurements of pulmonary oxygen uptake, heart rate, blood pressure, leg blood flow, and femoral arterial‐venous differences of oxygen content and lactate were performed as well as determination of ATP, creatine phosphate (CP) inosine monophosphate (IMP) and lactate concentrations on biopsy material from the quadriceps muscle before and immediately after the intense exercise, and at 3, 10 and 60 min into recovery. 2. Individual linear relations (r = 0.95‐1.00) between the power outputs for submaximal exercise and oxygen uptakes (leg and pulmonary) were used to estimate the energy demand during intense exercise. Pulmonary and leg oxygen deficits determined as the difference between energy demand and oxygen uptake were 0.46 and 0.48 l (kg active muscle)‐1, respectively. Limb and pulmonary oxygen debts (oxygen uptake during 60 min of recovery ‐ pre‐exercise oxygen uptake) were 0.55 and 1.65 l (kg active muscle)‐1, respectively. 3. During the intense exercise, muscle [ATP] decreased by 30% and [CP] by 60% from resting concentrations of 6.2 and 22.4 mmol (kg wet wt)‐1, respectively, and [IMP] increased to 1.1 mmol (kg wet wt)‐1. Muscle [lactate] increased from 2 to 28.1 mmol (kg wet wt)‐1, and the concomitant net lactate release was 14.8 mmol (kg wet wt)‐1 or about 1/3 of the total net lactate production. During recovery 70% of the accumulated lactate was released to the blood, and the nucleotides and CP returned to about 40 and 85% of pre‐exercise values at 3 and 10 min of recovery, respectively. 4. Total reduction in ATP and CP (and elevation of IMP) during the intense exercise amounted to 16.4 mmol ATP (kg wet wt)‐1, which together with the lactate production accounted for 83.1 mmol ATP (kg wet wt)‐1. In addition 6‐8 mmol ATP (kg wet wt)‐1 are made available related to accumulation of glycolytic intermediates including pyruvate (and alanine). Estimated leg oxygen deficit corresponded to an ATP production of 94.7 mmol ATP kg‐1; this value included 3.1 mmol kg‐1 related to unloading of HbO2 and MbO2.(ABSTRACT TRUNCATED AT 400 WORDS)

AMP-activated protein kinase (AMPK) β1β2 muscle null mice reveal an essential role for AMPK in maintaining mitochondrial content and glucose uptake during exercise

AMP-activated protein kinase (AMPK) β1 or β2 subunits are required for assembling of AMPK heterotrimers and are important for regulating enzyme activity and cellular localization. In skeletal muscle, α2β2γ3-containing heterotrimers predominate. However, compensatory up-regulation and redundancy of AMPK subunits in whole-body AMPK α2, β2, and γ3 null mice has made it difficult to determine the physiological importance of AMPK in regulating muscle metabolism, because these models have normal mitochondrial content, contraction-stimulated glucose uptake, and insulin sensitivity. In the current study, we generated mice lacking both AMPK β1 and β2 isoforms in skeletal muscle (β1β2M-KO). β1β2M-KO mice are physically inactive and have a drastically impaired capacity for treadmill running that is associated with reductions in skeletal muscle mitochondrial content but not a fiber-type switch. Interestingly, young β1β2M-KO mice fed a control chow diet are not obese or insulin resistant but do have impaired contraction-stimulated glucose uptake. These data demonstrate an obligatory role for skeletal muscle AMPK in maintaining mitochondrial capacity and contraction-stimulated glucose uptake, findings that were not apparent in mice with single mutations or deletions in muscle α, β, or γ subunits.

Utilization of skeletal muscle triacylglycerol during postexercise recovery in humans

The utilization of muscle triacylglycerols was studied during and after prolonged bicycle ergometer exercise to exhaustion in eight healthy young men. Two days before exercise and in the postexercise recovery period, subjects were fed a carbohydrate-rich diet (65-70% of energy from carbohydrates). Exercise decreased muscle glycogen concentrations from 533 +/- 18 to 108 +/- 10 mmol/kg dry wt, whereas muscle triacylglycerol concentrations were unaffected (49 +/- 5 before vs. 49 +/- 8 mmol/kg dry wt after exercise). During the first 18 h after exercise, muscle glycogen concentrations were restored to 409 +/- 20 mmol/kg dry wt. In contrast, muscle triacylglycerol concentrations decreased (P < 0.05) to a nadir of 38 +/- 5 mmol/kg dry wt, and muscle lipoprotein lipase activity increased by 72% compared with values before exercise. Pulmonary respiratory exchange ratio values of 0.80-0.82 indicated a relatively high fractional lipid combustion despite the high carbohydrate intake. From 18 to 42 h of recovery, muscle glycogen synthesis was slow and muscle triacylglycerol concentrations and lipoprotein lipase activity were restored to the preexercise values. It is concluded that muscle triacylglycerol concentrations are not diminished during exhaustive glycogen-depleting exercise. However, in the postexercise recovery period, muscle glycogen resynthesis has high metabolic priority, resulting in postexercise lipid combustion despite a high carbohydrate intake. It is suggested that muscle triacylglycerols, and probably very low density lipoprotein triacylglycerols, are important in providing fuel for muscle metabolism in the postexercise recovery period.The utilization of muscle triacylglycerols was studied during and after prolonged bicycle ergometer exercise to exhaustion in eight healthy young men. Two days before exercise and in the postexercise recovery period, subjects were fed a carbohydrate-rich diet (65-70% of energy from carbohydrates). Exercise decreased muscle glycogen concentrations from 533 ± 18 to 108 ± 10 mmol/kg dry wt, whereas muscle triacylglycerol concentrations were unaffected (49 ± 5 before vs. 49 ± 8 mmol/kg dry wt after exercise). During the first 18 h after exercise, muscle glycogen concentrations were restored to 409 ± 20 mmol/kg dry wt. In contrast, muscle triacylglycerol concentrations decreased ( P < 0.05) to a nadir of 38 ± 5 mmol/kg dry wt, and muscle lipoprotein lipase activity increased by 72% compared with values before exercise. Pulmonary respiratory exchange ratio values of 0.80-0.82 indicated a relatively high fractional lipid combustion despite the high carbohydrate intake. From 18 to 42 h of recovery, muscle glycogen synthesis was slow and muscle triacylglycerol concentrations and lipoprotein lipase activity were restored to the preexercise values. It is concluded that muscle triacylglycerol concentrations are not diminished during exhaustive glycogen-depleting exercise. However, in the postexercise recovery period, muscle glycogen resynthesis has high metabolic priority, resulting in postexercise lipid combustion despite a high carbohydrate intake. It is suggested that muscle triacylglycerols, and probably very low density lipoprotein triacylglycerols, are important in providing fuel for muscle metabolism in the postexercise recovery period.

Palmitate transport and fatty acid transporters in red and white muscles

We performed studies 1) to investigate the kinetics of palmitate transport into giant sarcolemmal vesicles, 2) to determine whether the transport capacity is greater in red muscles than in white muscles, and 3) to determine whether putative long-chain fatty acid (LCFA) transporters are more abundant in red than in white muscles. For these studies we used giant sarcolemmal vesicles, which contained cytoplasmic fatty acid binding protein (FABPc), an intravesicular fatty acid sink. Intravesicular FABPcconcentrations were sufficiently high so as not to limit the uptake of palmitate under conditions of maximal palmitate uptake (i.e., 4.5-fold excess in white and 31.3-fold excess in red muscle vesicles). All of the palmitate taken up was recovered as unesterified palmitate. Palmitate uptake was reduced by phloretin (-50%), sulfo- N-succinimidyl oleate (-43%), anti-plasma membrane-bound FABP (FABPpm, -30%), trypsin (-45%), and when incubation temperature was lowered to 0°C (-70%). Palmitate uptake was also reduced by excess oleate (-65%), but not by excess octanoate or by glucose. Kinetic studies showed that maximal transport was 1.8-fold greater in red vesicles than in white vesicles. The Michaelis-Menten constant in both types of vesicles was ∼6 nM. Fatty acid transport protein mRNA and fatty acid translocase (FAT) mRNA were about fivefold greater in red muscles than in white muscles. FAT/CD36 and FABPpm proteins in red vesicles or in homogenates were greater than in white vesicles or homogenates ( P < 0.05). These studies provide the first evidence of a protein-mediated LCFA transport system in skeletal muscle. In this tissue, palmitate transport rates are greater in red than in white muscles because more LCFA transporters are available.We performed studies 1) to investigate the kinetics of palmitate transport into giant sarcolemmal vesicles, 2) to determine whether the transport capacity is greater in red muscles than in white muscles, and 3) to determine whether putative long-chain fatty acid (LCFA) transporters are more abundant in red than in white muscles. For these studies we used giant sarcolemmal vesicles, which contained cytoplasmic fatty acid binding protein (FABPc), an intravesicular fatty acid sink. Intravesicular FABPc concentrations were sufficiently high so as not to limit the uptake of palmitate under conditions of maximal palmitate uptake (i.e., 4.5-fold excess in white and 31.3-fold excess in red muscle vesicles). All of the palmitate taken up was recovered as unesterified palmitate. Palmitate uptake was reduced by phloretin (-50%), sulfo-N-succinimidyl oleate (-43%), anti-plasma membrane-bound FABP (FABPpm, -30%), trypsin (-45%), and when incubation temperature was lowered to 0 degrees C (-70%). Palmitate uptake was also reduced by excess oleate (-65%), but not by excess octanoate or by glucose. Kinetic studies showed that maximal transport was 1.8-fold greater in red vesicles than in white vesicles. The Michaelis-Menten constant in both types of vesicles was approximately 6 nM. Fatty acid transport protein mRNA and fatty acid translocase (FAT) mRNA were about fivefold greater in red muscles than in white muscles. FAT/CD36 and FABPpm proteins in red vesicles or in homogenates were greater than in white vesicles or homogenates (P < 0.05). These studies provide the first evidence of a protein-mediated LCFA transport system in skeletal muscle. In this tissue, palmitate transport rates are greater in red than in white muscles because more LCFA transporters are available.

Effect of muscle acidity on muscle metabolism and fatigue during intense exercise in man.

1. The aim of this study was to examine the effect of muscle pH on muscle metabolism and development of fatigue during intense exercise. 2. Seven subjects performed intense exhaustive leg exercise on two occasions: with and without preceding intense intermittent arm exercise leading to high or moderate (control) blood lactate concentrations (HL and C, respectively). Prior to and immediately after each exercise bout, a muscle biopsy was taken from m. vastus lateralis of the active leg. Leg blood flow was measured and femoral arterial and venous blood samples were collected before and frequently during the exhaustive exercises. 3. The duration of the exercise was shorter in HL than in C (3.46 +/‐ 0.28 vs. 4.67 +/‐ 0.55 min; means +/‐ S.E.M.; P < 0.05). Before exercise muscle pH was the same in C and HL (7.17 vs. 7.10), but at the end of exercise muscle pH was lower in HL than in C (6.82 vs. 6.65; P < 0.05). The release of potassium during exercise was higher (P < 0.05) in HL compared with C, but the arterial and femoral venous plasma potassium concentrations were the same at exhaustion in HL and C. 4. Muscle lactate concentration was higher in HL compared with C (3.7 +/‐ 0.4 vs. 1.6 +/‐ 0.2 mmol (kg wet weight)‐1; P < 0.05), but the same at exhaustion (26.5 +/‐ 2.7 vs. 25.4 +/‐ 2.4 mmol (kg wet weight)‐1). Total release of lactate in HL was lower than in C (18.7 +/‐ 4.5 vs. 50.4 +/‐ 11.0 mmol; P < 0.05), but rate of lactate production was not different (9.0 +/‐ 1.0 vs. 10.2 +/‐ 1.3 mmol (kg wet weight)‐1 min‐1). The rate of muscle glycogen breakdown was the same in C and HL (8.1 +/‐ 1.2 vs. 8.2 +/‐ 1.0 mmol (kg wet weight)‐1 min‐1). 5. The present data suggest that elevated muscle acidity does not reduce muscle glycogenolysis/glycolysis and is not the only cause of fatigue during intense exercise in man. Instead, accumulation of potassium in muscle interstitium may be an important factor in the development of fatigue.

Caffeine ingestion does not alter carbohydrate or fat metabolism in human skeletal muscle during exercise

1 This study examined the effect of ingesting caffeine (6 mg kg−1) on muscle carbohydrate and fat metabolism during steady‐state exercise in humans. Young male subjects (n= 10) performed 1 h of exercise (70 % maximal oxygen consumption (V̇O2,max)) on two occasions (after ingestion of placebo and caffeine) and leg metabolism was quantified by the combination of direct Fick measures and muscle biopsies. 2 Following caffeine ingestion serum fatty acid and glycerol concentration increased (P≤ 0.05) at rest, suggesting enhanced adipose tissue lipolysis. 3 In addition circulating adrenaline concentration was increased (P≤ 0.05) at rest following caffeine ingestion and this, as well as leg noradrenaline spillover, was elevated (P≤ 0.05) above placebo values during exercise. 4 Caffeine resulted in a modest increase (P≤ 0.05) in leg vascular resistance, but no difference was found in leg blood flow. 5 Arterial lactate and glucose concentrations were increased (P≤ 0.05) by caffeine, while the rise in plasma potassium was dampened (P≤ 0.05). 6 There were no differences in respiratory exchange ratio or in leg glucose uptake, net muscle glycogenolysis, leg lactate release or muscle lactate, or glucose 6‐phosphate concentration. Similarly there were no differences between treatments in leg fatty acid uptake, glycerol release or muscle acetyl CoA concentration. 7 These findings indicate that caffeine ingestion stimulated the sympathetic nervous system but did not alter the carbohydrate or fat metabolism in the monitored leg. Other tissues must have been involved in the changes in circulating potassium, fatty acids, glucose and lactate.

Effects of insulin and exercise on muscle lipoprotein lipase activity in man and its relation to insulin action.

The effects of exercise and a physiological increase in plasma insulin concentration on muscle lipoprotein lipase activity (mLPLA), leg exchange of glucose, and serum lipoprotein levels were investigated in healthy young men. During euglycemic hyperinsulinemia (n = 7) at 44 mU.liter-1, m-LPLA in non-exercised muscle decreased from 30 +/- 7.4 mU.g-1 wet weight (w.w.) (mean +/- SE) to 19 +/- 3.3 (P less than 0.05). Furthermore, the decrease in m-LPLA correlated closely (r = 0.97, P less than 0.05) with the increase in leg glucose uptake. Moreover, basal m-LPLA correlated with the insulin-induced increase in leg glucose uptake (r = 0.93, P less than 0.05). In the control group (n = 6) in which saline was infused in place of insulin and glucose, m-LPLA in nonexercised muscle did not change with time. No change in m-LPLA was observed immediately after one-legged knee extension exercise, but 4 h after exercise m-LPLA was higher (P less than 0.05) in the exercised thigh (47 +/- 17.8 mU.g-1 w.w.) compared with the contralateral nonexercised thigh (29 +/- 6.3 mU.g-1 w.w.). This difference was not found 8 h after exercise. The triacylglycerol content of serum lipoproteins decreased during insulin infusion. It is concluded that in contrast to the effect on adipose tissue, physiological concentrations of insulin decrease m-LPLA in proportion to the effect of insulin on muscle glucose uptake, while muscle contractions cause a local, delayed, and transient increase in m-LPLA. Further-more, basal m-LPLA is an indicator of muscle insulin sensitivity.