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Key Points: Rodent studies suggest muscle fibre type-specific insulin response in the recovery from exercise.  The current study investigates muscle fibre type-specific insulin action in the recovery from exercise in healthy subjects.  In type I and type II muscle fibres, key proteins in glucose metabolism are similarly regulated by insulin during recovery from exercise.  Our findings imply that both type I and type II muscle fibres contribute to the phenomenon of increased insulin sensitivity in the recovery from a single bout of exercise in humans., Abstract: Human skeletal muscle consists of slow-twitch (type I) and fast-twitch (type II) muscle fibres. Muscle insulin action, regulating glucose uptake and metabolism, is improved following a single exercise bout. Rodent studies suggest that this phenomenon is confined to specific muscle fibre types. Whether this phenomenon is also confined to specific fibre types in humans has not been described. To investigate this, nine healthy men underwent a euglycaemic hyperinsulinaemic clamp (EHC) in the recovery from a single bout of one-legged knee-extensor exercise. Pools of type I and type II fibres were prepared from muscle biopsies taken in the rested and prior exercised leg before and after the EHC. AMPK γ3 and TBC1D4 - two key proteins regulating muscle insulin action following exercise - were higher expressed in type II than type I fibres. However, phosphor-regulation of TBC1D4 was similar between fibre types when related to the total amount of TBC1D4 protein. The activating dephosphorylation of glycogen synthase was also similar in the two fibre types. Thus, insulin-induced regulation of key proteins important for transport and intracellular flux of glucose towards glycogen storage in the recovery from exercise, does not differ between fibre types. In conclusion, the insulin-sensitizing effect of a single bout of exercise includes both type I and type II fibres in human skeletal muscle. This may be an important observation for future pharmacological strategies targeting muscle insulin sensitivity in humans., (© 2020 The Authors. The Journal of Physiology © 2020 The Physiological Society.)

Key Points: Increased insulin action is an important component of the health benefits of exercise, but its regulation is complex and not fully elucidated. Previous studies of insulin-stimulated GLUT4 translocation to the skeletal muscle membrane found insufficient increases to explain the increases in glucose uptake. By determination of leg glucose uptake and interstitial muscle glucose concentration, insulin-induced muscle membrane permeability to glucose was calculated 4 h after one-legged knee-extensor exercise during a submaximal euglycaemic-hyperinsulinaemic clamp. It was found that during submaximal insulin stimulation, muscle membrane permeability to glucose in humans increases twice as much in previously exercised vs. rested muscle and outstrips the supply of glucose, which then becomes limiting for glucose uptake. This methodology can now be employed to determine muscle membrane permeability to glucose in people with diabetes, who have reduced insulin action, and in principle can also be used to determine membrane permeability to other substrates or metabolites., Abstract: Increased insulin action is an important component of the health benefits of exercise, but the regulation of insulin action in vivo is complex and not fully elucidated. Previously determined increases in skeletal muscle insulin-stimulated GLUT4 translocation are inconsistent and mostly cannot explain the increases in insulin action in humans. Here we used leg glucose uptake (LGU) and interstitial muscle glucose concentration to calculate insulin-induced muscle membrane permeability to glucose, a variable not previously possible to quantify in humans. Muscle membrane permeability to glucose, measured 4 h after one-legged knee-extensor exercise, increased ∼17-fold during a submaximal euglycaemic-hyperinsulinaemic clamp in rested muscle (R) and ∼36-fold in exercised muscle (EX). Femoral arterial infusion of N G -monomethyl l-arginine acetate or ATP decreased and increased, respectively, leg blood flow (LBF) in both legs but did not affect membrane glucose permeability. Decreasing LBF reduced interstitial glucose concentrations to ∼2 mM in the exercised but only to ∼3.5 mM in non-exercised muscle and abrogated the augmented effect of insulin on LGU in the EX leg. Increasing LBF by ATP infusion increased LGU in both legs with uptake higher in the EX leg. We conclude that it is possible to measure functional muscle membrane permeability to glucose in humans and it increases twice as much in exercised vs. rested muscle during submaximal insulin stimulation. We also show that muscle perfusion is an important regulator of muscle glucose uptake when membrane permeability to glucose is high and we show that the capillary wall can be a significant barrier for glucose transport., (© 2019 The Authors. The Journal of Physiology © 2019 The Physiological Society.)

Key Points: A single bout of exercise is capable of increasing insulin sensitivity in human skeletal muscle. Whether this ability is affected by training status is not clear. Studies in mice suggest that the AMPK-TBC1D4 signalling axis is important for the increased insulin-stimulated glucose uptake after a single bout of exercise. The present study is the first longitudinal intervention study to show that, although exercise training increases insulin-stimulated glucose uptake in skeletal muscle at rest, it diminishes the ability of a single bout of exercise to enhance muscle insulin-stimulated glucose uptake. The present study provides novel data indicating that AMPK in human skeletal muscle is important for the insulin-sensitizing effect of a single bout of exercise., Abstract: Not only chronic exercise training, but also a single bout of exercise, increases insulin-stimulated glucose uptake in skeletal muscle. However, it is not well described how adaptations to exercise training affect the ability of a single bout of exercise to increase insulin sensitivity. Rodent studies suggest that the insulin-sensitizing effect of a single bout of exercise is AMPK-dependent (presumably via the α 2 β 2 γ 3 AMPK complex). Whether this is also the case in humans is unknown. Previous studies have shown that exercise training decreases the expression of the α 2 β 2 γ 3 AMPK complex and diminishes the activation of this complex during exercise. Thus, we hypothesized that exercise training diminishes the ability of a single bout of exercise to enhance muscle insulin sensitivity. We investigated nine healthy male subjects who performed one-legged knee-extensor exercise at the same relative intensity before and after 12 weeks of exercise training. Training increased V ̇ O 2 peak and expression of mitochondrial proteins in muscle, whereas the expression of AMPKγ3 was decreased. Training also increased whole body and muscle insulin sensitivity. Interestingly, insulin-stimulated glucose uptake in the acutely exercised leg was not enhanced further by training. Thus, the increase in insulin-stimulated glucose uptake following a single bout of one-legged exercise was lower in the trained vs. untrained state. This was associated with reduced signalling via confirmed α 2 β 2 γ 3 AMPK downstream targets (ACC and TBC1D4). These results suggest that the insulin-sensitizing effect of a single bout of exercise is also AMPK-dependent in human skeletal muscle., (© 2018 The Authors. The Journal of Physiology © 2018 The Physiological Society.)

Key Points: Regulation of autophagy in human muscle in many aspects differs from the majority of previous reports based on studies in cell systems and rodent muscle. An acute bout of exercise and insulin stimulation reduce human muscle autophagosome content. An acute bout of exercise regulates autophagy by a local contraction-induced mechanism. Exercise training increases the capacity for formation of autophagosomes in human muscle. AMPK activation during exercise seems insufficient to regulate autophagosome content in muscle, while mTORC1 signalling via ULK1 probably mediates the autophagy-inhibiting effect of insulin. Studies in rodent muscle suggest that autophagy is regulated by acute exercise, exercise training and insulin stimulation. However, little is known about the regulation of autophagy in human skeletal muscle. Here we investigate the autophagic response to acute one-legged exercise, one-legged exercise training and subsequent insulin stimulation in exercised and non-exercised human muscle. Acute one-legged exercise decreased (P<0.01) lipidation of microtubule-associated protein 1A/1B-light chain 3 (LC3) (∼ 50%) and the LC3-II/LC3-I ratio (∼ 60%) indicating that content of autophagosomes decreases with exercise in human muscle. The decrease in LC3-II/LC3-I ratio did not correlate with activation of 5'AMP activated protein kinase (AMPK) trimer complexes in human muscle. Consistently, pharmacological AMPK activation with 5-aminoimidazole-4-carboxamide riboside (AICAR) in mouse muscle did not affect the LC3-II/LC3-I ratio. Four hours after exercise, insulin further reduced (P<0.01) the LC3-II/LC3-I ratio (∼ 80%) in muscle of the exercised and non-exercised leg in humans. This coincided with increased Ser-757 phosphorylation of Unc51 like kinase 1 (ULK1), which is suggested as a mammalian target of rapamycin complex 1 (mTORC1) target. Accordingly, inhibition of mTOR signalling in mouse muscle prevented the ability of insulin to reduce the LC3-II/LC3-I ratio. In response to 3 weeks of one-legged exercise training, the LC3-II/LC3-I ratio decreased (P<0.05) in both trained and untrained muscle and this change was largely driven by an increase in LC3-I content. Taken together, acute exercise and insulin stimulation reduce muscle autophagosome content, while exercise training may increase the capacity for formation of autophagosomes in muscle. Moreover, AMPK activation during exercise may not be sufficient to regulate autophagy in muscle, while mTORC1 signalling via ULK1 probably mediates the autophagy-inhibiting effect of insulin., (© 2015 The Authors. The Journal of Physiology © 2015 The Physiological Society.)

It is well known that exercise has a major impact on substrate metabolism for many hours after exercise. However, the regulatory mechanisms increasing lipid oxidation and facilitating glycogen resynthesis in the post-exercise period are unknown. To address this, substrate oxidation was measured after prolonged exercise and during the following 6 h post-exercise in 5´-AMP activated protein kinase (AMPK) α2 and α1 knock-out (KO) and wild-type (WT) mice with free access to food. Substrate oxidation was similar during exercise at the same relative intensity between genotypes. During post-exercise recovery, a lower lipid oxidation (P < 0.05) and higher glucose oxidation were observed in AMPKα2 KO (respiratory exchange ratio (RER) = 0.84 ± 0.02) than in WT and AMPKα1 KO (average RER = 0.80 ± 0.01) without genotype differences in muscle malonyl-CoA or free-carnitine concentrations. A similar increase in muscle pyruvate dehydrogenase kinase 4 (PDK4) mRNA expression in WT and AMPKα2 KO was observed following exercise, which is consistent with AMPKα2 deficiency not affecting the exercise-induced activation of the PDK4 transcriptional regulators HDAC4 and SIRT1. Interestingly, PDK4 protein content increased (63%, P < 0.001) in WT but remained unchanged in AMPKα2 KO. In accordance with the lack of increase in PDK4 protein content, lower (P < 0.01) inhibitory pyruvate dehydrogenase (PDH)-E1α Ser(293) phosphorylation was observed in AMPKα2 KO muscle compared to WT. These findings indicate that AMPKα2 regulates muscle metabolism post-exercise through inhibition of the PDH complex and hence glucose oxidation, subsequently creating conditions for increased fatty acid oxidation., (© 2015 The Authors. The Journal of Physiology © 2015 The Physiological Society.)

Key Points: Acute glucagon-like peptide-1 (GLP-1) infusion reversed the high fat diet-induced microvascular insulin resistance that occurred after both 5 days and 8 weeks of a high fat diet intervention. When GLP-1 was co-infused with insulin it had overt effects on whole body insulin sensitivity as well as insulin-mediated skeletal muscle glucose uptake after 5 days of a high fat diet, but not after 8 weeks of high fat diet intervention. Acute GLP-1 infusion did not have an additive effect to that of insulin on microvascular recruitment or skeletal muscle glucose uptake in the control group. Here we demonstrate that GLP-1 potently increases the microvascular recruitment in rat skeletal muscle but does not increase glucose uptake in the fasting state. Thus, like insulin, GLP-1 increased the microvascular recruitment but unlike insulin, GLP-1 had no direct effect on skeletal muscle glucose uptake., Abstract: Acute infusion of glucagon-like peptide-1 (GLP-1) has potent effects on blood flow distribution through the microcirculation in healthy humans and rats. A high fat diet induces impairments in insulin-mediated microvascular recruitment (MVR) and muscle glucose uptake, and here we examined whether this could be reversed by GLP-1. Using contrast-enhanced ultrasound, microvascular recruitment was assessed by continuous real-time imaging of gas-filled microbubbles in the microcirculation after acute (5 days) and prolonged (8 weeks) high fat diet (HF)-induced insulin resistance in rats. A euglycaemic hyperinsulinaemic clamp (3 mU min(-1)  kg(-1) ), with or without a co-infusion of GLP-1 (100 pmol l(-1) ), was performed in anaesthetized rats. Consumption of HF attenuated the insulin-mediated MVR in both 5 day and 8 week HF interventions which was associated with a 50% reduction in insulin-mediated glucose uptake compared to controls. Acute administration of GLP-1 restored the normal microvascular response by increasing the MVR after both 5 days and 8 weeks of HF intervention (P < 0.05). This effect of GLP-1 was associated with a restoration of both whole body insulin sensitivity and increased insulin-mediated glucose uptake in skeletal muscle by 90% (P < 0.05) after 5 days of HF but not after 8 weeks of HF. The present study demonstrates that GLP-1 increases MVR in rat skeletal muscle and can reverse early stages of high fat diet-induced insulin resistance in vivo., (© 2015 The Authors. The Journal of Physiology © 2015 The Physiological Society.)

In skeletal muscle hormone-sensitive lipase (HSL) has long been accepted to be the principal enzyme responsible for lipolysis of intramyocellular triacylglycerol (IMTG) during contractions. However, this notion is based on in vitro lipase activity data, which may not reflect the in vivo lipolytic activity. We investigated lipolysis of IMTG in soleus muscles electrically stimulated to contract ex vivo during acute pharmacological inhibition of HSL in rat muscles and in muscles from HSL knockout (HSL-KO) mice. Measurements of IMTG are complicated by the presence of adipocytes located between the muscle fibres. To circumvent the problem with this contamination we analysed intramyocellular lipid droplet content histochemically. At maximal inhibition of HSL in rat muscles, contraction-induced breakdown of IMTG was identical to that seen in control muscles (P < 0.001). In response to contractions IMTG staining decreased significantly in both HSL-KO and WT muscles (P < 0.05). In vitro TG hydrolase activity data revealed that adipose triglyceride lipase (ATGL) and HSL collectively account for ∼98% of the TG hydrolase activity in mouse skeletal muscle, other TG lipases accordingly being of negligible importance for lipolysis of IMTG. The present study is the first to demonstrate that contraction-induced lipolysis of IMTG occurs in the absence of HSL activity in rat and mouse skeletal muscle. Furthermore, the results suggest that ATGL is activated and plays a major role in lipolysis of IMTG during muscle contractions.

Deacetylases such as sirtuins (SIRTs) convert NAD to nicotinamide (NAM). Nicotinamide phosphoribosyl transferase (Nampt) is the rate-limiting enzyme in the NAD salvage pathway responsible for converting NAM to NAD to maintain cellular redox state. Activation of AMP-activated protein kinase (AMPK) increases SIRT activity by elevating NAD levels. As NAM directly inhibits SIRTs, increased Nampt activation or expression could be a metabolic stress response. Evidence suggests that AMPK regulates Nampt mRNA content, but whether repeated AMPK activation is necessary for increasing Nampt protein levels is unknown. To this end, we assessed whether exercise training- or 5-amino-1-β-D-ribofuranosyl-imidazole-4-carboxamide (AICAR)-mediated increases in skeletal muscle Nampt abundance are AMPK dependent. One-legged knee-extensor exercise training in humans increased Nampt protein by 16% (P < 0.05) in the trained, but not the untrained leg. Moreover, increases in Nampt mRNA following acute exercise or AICAR treatment (P < 0.05 for both) were maintained in mouse skeletal muscle lacking a functional AMPK α2 subunit. Nampt protein was reduced in skeletal muscle of sedentary AMPK α2 kinase dead (KD), but 6.5 weeks of endurance exercise training increased skeletal muscle Nampt protein to a similar extent in both wild-type (WT) (24%) and AMPK α2 KD (18%) mice. In contrast, 4 weeks of daily AICAR treatment increased Nampt protein in skeletal muscle in WT mice (27%), but this effect did not occur in AMPK α2 KD mice. In conclusion, functional α2-containing AMPK heterotrimers are required for elevation of skeletal muscle Nampt protein, but not mRNA induction. These findings suggest AMPK plays a post-translational role in the regulation of skeletal muscle Nampt protein abundance, and further indicate that the regulation of cellular energy charge and nutrient sensing is mechanistically related.

Fatty acids (FAs) as fuel for energy utilization during exercise originate from different sources: FAs transported in the circulation either bound to albumin or as triacylglycerol (TG) carried by very low density lipoproteins and FAs from lipolysis of muscle TG stores. Despite a high rate of energy expenditure during high intensity exercise the total FA oxidation is suppressed to below that observed during moderate intensity exercise. Although this has been known for many years, the mechanisms behind this phenomenon are still not fully elucidated. A failure of adipose tissue to deliver sufficient FAs to exercising muscle has been proposed, but evidence is emerging that factors within the muscle might be of more importance. The high rate of glycolysis during high intensity exercise might be the 'driving force' via the increased production of acetyl-CoA, which in turn is trapped by carnitine. This will lead to decreased availability of free carnitine for long chain FA transport into mitochondria. This review summarizes our present view on how FA metabolism is regulated during exercise with a special focus on the limitations in FA oxidation in the transition from moderate to high intensity exercise in humans.

Here the hypothesis that skeletal muscle Ca(2+)-calmodulin-dependent kinase II (CaMKII) expression and signalling would be modified by endurance training was tested. Eight healthy, young men completed 3 weeks of one-legged endurance exercise training with muscle samples taken from both legs before training and 15 h after the last exercise bout. Along with an approximately 40% increase in mitochondrial F(1)-ATP synthase expression, there was an approximately 1-fold increase in maximal CaMKII activity and CaMKII kinase isoform expression after training in the active leg only. Autonomous CaMKII activity and CaMKII autophosphorylation were increased to a similar extent. However, there was no change in alpha-CaMKII anchoring protein expression with training. Nor was there any change in expression or Thr(17) phosphorylation of the CaMKII substrate phospholamban with training. However, another CaMKII substrate, serum response factor (SRF), had an approximately 60% higher phosphorylation at Ser(103) after training, with no change in SRF expression. There were positive correlations between the increases in CaMKII expression and SRF phosphorylation as well as F(1)ATPase expression with training. After training, there was an increase in cyclic-AMP response element binding protein phosphorylation at Ser(133), but not expression, in muscle of both legs. Taken together, skeletal muscle CaMKII kinase isoform expression and SRF phosphorylation is higher with endurance-type exercise training, adaptations that are restricted to active muscle. This may contribute to greater Ca(2+) mediated regulation during exercise and the altered muscle phenotype with training.

We investigated if acute endurance-type exercise interacts with insulin-stimulated activation of atypical protein kinase C (aPKC) and insulin signalling to peptide chain elongation in human skeletal muscle. Four hours after acute one-legged exercise, insulin-induced glucose uptake was approximately 80% higher (N = 12, P < 0.05) in previously exercised muscle, measured during a euglycaemic-hyperinsulinaemic clamp (100 microU ml(-1)). Insulin increased (P < 0.05) both insulin receptor substrate (IRS)-1 and IRS-2 associated phosphatidylinositol (PI)-3 kinase activity and led to increased (P < 0.001) phosphorylation of Akt on Ser(473) and Thr(308) in skeletal muscle. Interestingly, in response to prior exercise IRS-2-associated PI-3 kinase activity was higher (P < 0.05) both at basal and during insulin stimulation. This coincided with correspondingly altered phosphorylation of the extracellular-regulated protein kinase 1/2 (ERK 1/2), p70S6 kinase (P70S6K), eukaryotic elongation factor 2 (eEF2) kinase and eEF2. aPKC was similarly activated by insulin in rested and exercised muscle, without detectable changes in aPKC Thr(410) phosphorylation. However, when adding phosphatidylinositol-3,4,5-triphosphate (PIP3), the signalling product of PI-3 kinase, to basal muscle homogenates, aPKC was more potently activated (P = 0.01) in previously exercised muscle. Collectively, this study shows that endurance-type exercise interacts with insulin signalling to peptide chain elongation. Although protein turnover was not evaluated, this suggests that capacity for protein synthesis after acute endurance-type exercise may be improved. Furthermore, endurance exercise increased the responsiveness of aPKC to PIP3 providing a possible link to improved insulin-stimulated glucose uptake after exercise.

The aim of the study was to examine local muscle metabolism in response to graded exercise when the involved muscle mass is too small to elicit marked hormonal changes and local blood flow restriction. Nine healthy overnight fasted male subjects performed knee extension exercise with both thighs kicking at 25% of maximal power (Wmax) for 45 min (23+/-1% of pulmonary) followed by 35 min of kicking with one thigh at 65% and the other at 85% W(max) (40+/-1% ). Primed constant infusion of [U-13C] palmitate and [2H5]glycerol was carried out. Blood was sampled from a femoral artery and both femoral veins, and thigh blood flow was determined by thermodilution. Muscle biopsies were obtained from m. vastus lateralis of both thighs. From rest through exercise at 25, 65 and 85% Wmax the thigh blood flow (0.3+/-0.1, 2.5+/-0.2, 3.5+/-0.2, 4.1+/-0.3 l min(-1)) and oxygen uptake (0.02+/-0.01, 0.27+/-0.03, 0.48+/-0.04, 0.55+/-0.05 l min(-1)) increased (P<0.05). The plasma fatty acids oxidized in the thigh (5+/-1, 114+/-15, 162+/-30, 180+/-31 micromol min(-1)) increased (P<0.05) with exercise intensity, whereas the total thigh fat oxidation (19+/-6, 312+/-64, 356+/-93, 323+/-120 micromol min(-1)) increased (P<0.05) from rest, but remained unchanged through exercise. The thigh glycerol uptake (1+/-1, 16+/-4, 24+/-10, 39+/-8 micromol min(-1)) increased significantly from rest through exercise at 25-65 and 85% Wmax, respectively. Glucose uptake and glycogen breakdown always increased with exercise intensity. In conclusion, in the presence of a high blood flow and oxygen supply and only small hormonal changes, total fat oxidation in muscle increases from rest to light exercise, but then remains constant with exercise intensity up to heavy exercise. However, with increasing exercise intensity, oxidation of plasma free fatty acids increases and accordingly oxidation of other fat sources decreases. These findings are in contrast to whole body measurements performed during graded exercise involving a large muscle mass during which fat oxidation peaks at around 60% of .

Ca2+ signalling is proposed to play an important role in skeletal muscle function during exercise. Here, we examined the expression of multifunctional Ca2+-calmodulin-dependent protein kinases (CaMK) in human skeletal muscle and show that CaMKII and CaMKK, but not CaMKI or CaMKIV, are expressed. Furthermore, the effect of exercise duration and intensity on skeletal muscle CaMKII activity and phosphorylation of downstream targets was examined. Eight healthy men exercised at approximately 67% of peak pulmonary O2 uptake(VO2peak) with muscle samples taken at rest and after 1, 10, 30, 60 and 90 min of exercise. Ten other men exercised for three consecutive 10 min bouts at 35%, 60% and 85% VO2peak with muscle samples taken at rest, at the end of each interval and 30 min post-exercise. There was a rapid and transient increase in autonomous CaMKII activity and CaMKII phosphorylation at Thr287 in skeletal muscle during exercise. Furthermore, the phosphorylation of phospholamban (PLN) at Thr17, which was identified as a CaMKII substrate in skeletal muscle, was rapidly (< 1 min) increased by exercise, and remained phosphorylated 5-fold above basal level during 90 min of exercise. The phosphorylation of serum response factor at Ser103, a putative CaMKII substrate, was higher after 30 min of exercise. PLN phosphorylation at Thr17 was higher with increasing exercise intensities. These data indicate that CaMKII is the major multifunctional CaMK in skeletal muscle and its activation occurs rapidly and is sustained during continuous exercise, with the activation being greater during intense exercise.

5'AMP-activated protein kinase (AMPK) is an energy sensor activated by perturbed cellular energy status such as during muscle contraction. Activated AMPK is thought to regulate several key metabolic pathways. We used sex comparison to investigate whether AMPK signalling in skeletal muscle regulates fat oxidation during exercise. Moderately trained women and men completed 90 min bicycle exercise at 60% VO2peak. Both AMPK Thr172 phosphorylation and alpha2AMPK activity were increased by exercise in men (approximately 200%, P < 0.001) but not significantly in women. The sex difference in muscle AMPK activation with exercise was accompanied by an increase in muscle free AMP (approximately 164%, P < 0.01), free AMP/ATP ratio (159%, P < 0.05), and creatine (approximately 42%, P < 0.001) in men but not in women (NS), suggesting that lack of AMPK activation in women was due to better maintenance of muscle cellular energy balance compared with men. During exercise, fat oxidation per kg lean body mass was higher in women than in men (P < 0.05). Regression analysis revealed that a higher proportion of type 1 muscle fibres (approximately 23%, P < 0.01) and a higher capillarization (approximately 23%, P < 0.05) in women than in men could partly explain the sex difference in alpha2AMPK activity (r = -0.54, P < 0.05) and fat oxidation (r = 0.64, P < 0.05) during exercise. On the other hand, fat oxidation appeared not to be regulated via AMPK. In conclusion, during prolonged submaximal exercise at 60% VO2peak, higher fat oxidation in women cannot be explained by higher AMPK signalling but is accompanied by improved muscle cellular energy balance in women probably due to sex specific muscle morphology.

Protein synthesis in skeletal muscle is known to decrease during contractions but the underlying regulatory mechanisms are unknown. Here, the effect of exercise on skeletal muscle eukaryotic elongation factor 2 (eEF2) phosphorylation, a key component in protein translation machinery, was examined. Eight healthy men exercised on a cycle ergometer at a workload eliciting approximately 67% peak pulmonary oxygen consumption (VO2 peak) with skeletal muscle biopsies taken from the vastus lateralis muscle at rest as well as after 1, 10, 30, 60 and 90 min of exercise. In response to exercise, there was a rapid (i.e. < 1 min) 5- to 7-fold increase in eEF2 phosphorylation at Thr56 that was sustained for 90 min of continuous exercise. The in vitro activity of skeletal muscle eEF2 kinase was not altered by exercise indicating that the increased activity of eEF2 kinase to eEF2 is not mediated by covalent mechanisms. In support of this, the increase in AMPK activity was temporally unrelated to eEF2 phosphorylation. However, skeletal muscle eEF2 kinase was potently activated by Ca(2)(+)-calmodulin in vitro, suggesting that the higher eEF2 phosphorylation in working skeletal muscle is mediated by allosteric activation of eEF2 kinase by Ca(2)(+) signalling via calmodulin. Given that eEF2 phosphorylation inhibits eEF2 activity and mRNA translation, these findings suggest that the inhibition of protein synthesis in contracting skeletal muscle is due to the Ca(2)(+)-induced stimulation of eEF2 kinase.

Atypical protein kinase C (aPKC) and extracellular signal-regulated kinase (ERK) are emerging as important signalling molecules in the regulation of metabolism and gene expression in skeletal muscle. Exercise is known to increase activity of aPKC and ERK in skeletal muscle but the effect of exercise intensity hereon has not been studied. Furthermore, the relationship between activity and phosphorylation of the two enzymes during exercise is unknown. Nine healthy young men exercised for 30 min on a bicycle ergometer on two occasions. One occasion consisted of three consecutive 10 min bouts of 35, 60 and 85% of peak pulmonary oxygen uptake V(O(2 peak)) and the second of one 30 min bout at 35% of V(O(2 peak)). Both trials also included 30 min recovery. Muscle biopsies were obtained from the vastus lateralis muscle before and after each exercise bout. Exercise increased muscle aPKC activity at 35% V(O(2 peak)), whereupon no further increase was observed at higher exercise intensities. Activation of aPKC was not accompanied by increased phosphorylation of aPKC Thr(410/403). ERK1/2 activity increased in a similar pattern to aPKC, reaching maximal activity at 35% V(O(2 peak)), whereas ERK1 Thr(202)/Tyr(204) and ERK2 Thr(183)/Tyr(185) phosphorylation increased with increasing exercise intensity. Thus, aPKC and ERK1/2 activity in muscle during exercise did not correspond to phosphorylation of sites on aPKC or ERK1/2, respectively, which are considered important for their activation. It is concluded that assessment of aPKC and ERK1/2 activity in muscle using phosphospecific antibodies did not reflect direct activity measurements on immunoprecipitated enzyme in vitro. Thus, estimation of enzyme activity during exercise by use of phosphospecific antibodies should not be performed uncritically. In addition, increase in muscle activity of aPKC or ERK1/2 during exercise is not closely related to energy demands of the muscle but may serve other regulatory or permissive functions in muscle.

Hormone-sensitive lipase (HSL) catalyses the hydrolysis of myocellular triacylglycerol (MCTG), which is a potential energy source during exercise. Therefore, it is important to elucidate the regulation of HSL activity in human skeletal muscle during exercise. The main purpose of the present study was to investigate the role of 5'AMP-activated protein kinase (AMPK) in the regulation of muscle HSL activity and Ser565 phosphorylation (the presumed AMPK target site) in healthy, moderately trained men during 60 min bicycling (65%). Alpha2AMPK activity during exercise was manipulated by studying subjects with either low (LG) or high (HG) muscle glycogen content. HSL activity was distinguished from the activity of other neutral lipases by immunoinhibition of HSL using an anti-HSL antibody. During exercise a 62% higher (P < 0.01) alpha2AMPK activity in LG than in HG was paralleled by a similar difference (61%, P < 0.01) in HSL Ser565 phosphorylation but without any difference between trials in HSL activity or MCTG hydrolysis. HSL activity was increased (117%, P < 0.05) at 30 min of exercise but not at 60 min of exercise. In both trials, HSL phosphorylation on Ser563 (a presumed PKA target site) was not increased by exercise despite a fourfold increase (P < 0.001) in plasma adrenaline. ERK1/2 phosphorylation was increased by exercise in both trials (P < 0.001) and was higher in LG than in HG both at rest and during exercise (P = 0.06). In conclusion, the present study suggests that AMPK phosphorylates HSL on Ser565 in human skeletal muscle during exercise with reduced muscle glycogen. Apparently, HSL Ser565 phosphorylation by AMPK during exercise had no effect on HSL activity. Alternatively, other factors including ERK may have counterbalanced any effect of AMPK on HSL activity.

In this study, the hypothesis that the release of interleukin (IL)-6 from human muscle is linked to exercise intensity and muscle glucose uptake was investigated. In the overnight fasted state, seven healthy males performed knee extension exercise, kicking with both legs, each at 25 % of maximal power (W(max)) for 45 min (eliciting 23 +/- 1 % of pulmonary maximal oxygen uptake, V(O2,max)) and then simultaneously with one leg at 65 % and the other leg at 85 % W(max) for 35 min (40 +/- 1 % of pulmonary V(O2,max)). Blood was sampled from a femoral artery and both femoral veins, and blood flow was determined by thermodilution. Thigh plasma flow (0.15 +/- 0.01, 1.4 +/- 0.2, 2.0 +/- 0.1 and 2.3 +/- 0.2 l min(-1) thigh(-1) at rest and 25 %, 65 % and 85 % W(max), respectively) and thigh oxygen uptake (0.02 +/- 0.01, 0.27 +/- 0.03, 0.48 +/- 0.04 and 0.55 +/- 0.05 l min(-1) thigh(-1) at rest and 25 %, 65 % and 85 % W(max), respectively) increased with increasing exercise intensity (P < 0.05). Also, thigh IL-6 release (0.4 +/- 0.1, 1.3 +/- 0.5, 1.5 +/- 0.6 and 2.5 +/- 0.7 ng min(-1) thigh(-1) at rest and 25 %, 65 % and 85 % W(max), respectively) and thigh glucose uptake (0.05 +/- 0.01, 0.3 +/- 0.05, 0.75 +/- 0.16, 1.07 +/- 0.15 mmol min(-1) thigh(-1) at rest and 25 %, 65 % and 85 % W(max), respectively) increased with increasing exercise intensity (P < 0.05). During the last 35 min of exercise, arterial catecholamine concentrations were higher (P < 0.05) than at rest and during low-intensity exercise. During exercise, thigh IL-6 release was positively related to both thigh glucose uptake (P < 0.001) and thigh glucose delivery (P < 0.005), but not to thigh glucose extraction. Thigh IL-6 release was also positively related to arterial plasma adrenaline concentration. The pre-exercise muscle glycogen concentration tended to correlate with the arteriovenous IL-6 concentration difference at rest, and the postexercise glycogen concentration was inversely correlated with IL-6 release during the final 35 min of exercise. In conclusion, the study indicates that IL-6 release from human muscle is positively related to exercise intensity, arterial adrenaline concentration and muscle glucose uptake. This supports the hypothesis that IL-6 may be linked to the regulation of glucose homeostasis during exercise. The observation of a relationship between IL-6 release and muscle glycogen store both at rest and after exercise suggests that IL-6 may act as a carbohydrate sensor.

1. This study was carried out to test the hypothesis that the greater fat oxidation observed during exercise after adaptation to a high-fat diet is due to an increased uptake of fat originating from the bloodstream. 2. Of 13 male untrained subjects, seven consumed a fat-rich diet (62 % fat, 21 % carbohydrate) and six consumed a carbohydrate-rich diet (20 % fat, 65 % carbohydrate). After 7 weeks of training and diet, 60 min of bicycle exercise was performed at 68 +/- 1 % of maximum oxygen uptake. During exercise [1-(13)C]palmitate was infused, arterial and venous femoral blood samples were collected, and blood flow was determined by the thermodilution technique. Muscle biopsy samples were taken from the vastus lateralis muscle before and after exercise. 3. During exercise, the respiratory exchange ratio was significantly lower in subjects consuming the fat-rich diet (0.86 +/- 0.01, mean +/- S.E.M.) than in those consuming the carbohydrate-rich diet (0.93 +/- 0.02). The leg fatty acid (FA) uptake (183 +/- 37 vs. 105 +/- 28 micromol min(-1)) and very low density lipoprotein-triacylglycerol (VLDL-TG) uptake (132 +/- 26 vs. 16 +/- 21 micromol min(-1)) were both higher (each P < 0.05) in the subjects consuming the fat-rich diet. Whole-body plasma FA oxidation (determined by comparison of (13)CO(2) production and blood palmitate labelling) was 55-65 % of total lipid oxidation, and was higher after the fat-rich diet than after the carbohydrate-rich diet (13.5 +/- 1.2 vs. 8.9 +/- 1.1 micromol min(-1) kg(-1); P < 0.05). Muscle glycogen breakdown was significantly lower in the subjects taking the fat-rich diet than those taking the carbohydrate-rich diet (2.6 +/- 0.5 vs. 4.8 +/- 0.5 mmol (kg dry weight)(-1) min(-1), respectively; P < 0.05), whereas leg glucose uptake was similar (1.07 +/- 0.13 vs. 1.15 +/- 0.13 mmol min(-1)). 4. In conclusion, plasma VLDL-TG appears to be an important substrate source during aerobic exercise, and in combination with the higher plasma FA uptake it accounts for the increased fat oxidation observed during exercise after fat diet adaptation. The decreased carbohydrate oxidation was apparently due to muscle glycogen sparing and not to diminished plasma glucose uptake.

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 (VO2,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. Following caffeine ingestion serum fatty acid and glycerol concentration increased (P< or =0.05) at rest, suggesting enhanced adipose tissue lipolysis. In addition circulating adrenaline concentration was increased (P< or =0.05) at rest following caffeine ingestion and this, as well as leg noradrenaline spillover, was elevated (P< or =0.05) above placebo values during exercise. Caffeine resulted in a modest increase (P< or =0.05) in leg vascular resistance, but no difference was found in leg blood flow. Arterial lactate and glucose concentrations were increased (P< or =0.05) by caffeine, while the rise in plasma potassium was dampened (P< or =0.05). 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. 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.

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 alpha-isoforms of AMPK during and after exercise. Healthy human subjects performed bicycle exercise on two separate occasions at either a low ( approximately 50% maximum rate of O2 uptake (VO2,max) for 90 min) or a high ( approximately 75% VO2,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 alpha2-AMPK isoform immediately after high intensity exercise, whereas no activation was observed after low intensity exercise. The activation of alpha2-AMPK was totally reversed 3 h after exercise. In contrast, alpha1-AMPK was not activated during either of the two exercise trials. 4. The in vitro AMP dependency of alpha2-AMPK was significantly greater than that of alpha1-AMPK ( approximately 3- vs. approximately 2-fold). 5. We conclude that in humans activation of alpha2-AMPK during exercise is dependent upon exercise intensity. The stable activation of alpha2-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 alpha1-AMPK activation makes this kinase complex a less likely candidate.

1. The present study examined the regulation of human skeletal muscle AMP deamination during intense exercise and quantified muscle accumulation and release of purines during and after intense exercise. 2. Seven healthy males performed knee extensor exercise at 64.3 W (range: 50-70 W) to exhaustion (234 s; 191-259 s). In addition, on two separate days the subjects performed exercise at the same intensity for 30 s and 80 % of exhaustion time (mean, 186 s; range, 153-207 s), respectively. Muscle biopsies were obtained from m.v. lateralis before and after each of the exercise bouts. For the exhaustive bout femoral arterio-venous concentration differences and blood flow were also determined. 3. During the first 30 s of exercise there was no change in muscle adenosine triphosphate (ATP), inosine monophosphate (IMP) and ammonia (NH3), although estimated free ADP and AMP increased 5- and 45-fold, respectively, during this period. After 186 s and at exhaustion muscle ATP had decreased (P < 0.05) by 15 and 19 %, respectively, muscle IMP was elevated (P < 0. 05) from 0.20 to 3.65 and 5.67 mmol (kg dry weight)-1, respectively, and muscle NH3 had increased (P < 0.05) from 0.47 to 2.55 and 2.33 mmol (kg d.w.)-1, respectively. The concentration of H+ did not change during the first 30 s of exercise, but increased (P < 0.05) to 245.9 nmol l-1 (pH 6.61) after 186 s and to 374.5 nmol l-1 (pH 6. 43) at exhaustion. 4. Muscle inosine and hypoxanthine did not change during exercise. In the first 10 min after exercise the muscle IMP concentration decreased (P < 0.05) by 2.96 mmol (kg d.w.)-1 of which inosine and hypoxanthine formation could account for 30 %. The total release of inosine and hypoxanthine during exercise and 90 min of recovery amounted to 1.07 mmol corresponding to 46 % of the net ATP decrease during exercise or 9 % of ATP at rest. 5. The present data suggest that AMP deamination is inhibited during the initial phase of intense exercise, probably due to accumulation of orthophosphate, and that lowered pH is an important positive modulator of AMP deaminase in contracting human skeletal muscle in vivo. Furthermore, formation and release of purines occurs mainly after intense exercise and leads to a considerable loss of nucleotides.

1. The effects of unaccustomed eccentric exercise on exercise metabolism during a subsequent bout of graded concentric exercise were investigated in seven healthy male subjects. Arterial and bilateral femoral venous catheters were inserted 2 days after eccentric exercise of one thigh (eccentric thigh) and blood samples were taken before and during graded two-legged concentric knee-extensor exercise. Muscle biopsies were obtained from the eccentric and control vastus lateralis before (rest) and after (post) the concentric exercise bout. 2. Maximal knee-extensor concentric exercise capacity was decreased by an average of 23 % (P < 0.05) in the eccentric compared with the control thigh. 3. The resting muscle glycogen content was lower in the eccentric thigh than in the control thigh (402 +/- 30 mmol (kg dry wt)-1 vs. 515 +/- 26 mmol (kg dry wt)-1, means +/- s.e.m., P < 0.05), and following the two-legged concentric exercise this difference substantially increased (190 +/- 46 mmol (kg dry wt)-1 vs. 379 +/- 58 mmol (kg dry wt)-1, P < 0.05) despite identical power and duration of exercise with the two thighs. 4. There was no measurable difference in glucose uptake between the eccentric and control thigh before or during the graded two-legged concentric exercise. Lactate release was higher from the eccentric thigh at rest and, just before termination of the exercise bout, release of lactate decreased from this thigh (suggesting decreased glycogenolysis), whereas no decrease was found from the contralateral control thigh. Lower glycerol release from the eccentric thigh during the first, lighter part of the exercise (P < 0.05) suggested impaired triacylglycerol breakdown. 5. At rest, sarcolemmal GLUT4 glucose transporter content and glucose transport were similar in the two thighs, and concentric exercise increased sarcolemmal GLUT4 content and glucose transport capacity similarly in the two thighs. 6. It is concluded that in muscle exposed to prior eccentric contractions, exercise at a given power output requires a higher relative workload than in undamaged muscle. This increases utilization of the decreased muscle glycogen stores, contributing to decreased endurance.

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.

1. Unaccustomed eccentric exercise decreases whole-body insulin action in humans. To study the effects of one-legged eccentric exercise on insulin action in muscle and systemically, the euglycaemic clamp technique combined with arterial and bilateral femoral venous catheterization was used. Seven subjects participated in two euglycaemic clamps, performed in random order. One clamp was preceded 2 days earlier by one-legged eccentric exercise (post-eccentric exercise clamp (PEC)) and one was without the prior exercise (control clamp (CC)). 2. During PEC the maximal insulin-stimulated glucose uptake over the eccentric thigh was marginally lower when compared with the control thigh, (11.9%, 64.6 +/- 10.3 vs. 73.3 +/- 10.2 mumol kg-1 min-1, P = 0.08), whereas no inter-thigh difference was observed at a submaximal insulin concentration. The glycogen concentration was lower in the eccentric thigh for all three clamp steps used (P < 0.05). The glucose transporter GLUT4 protein content was on average 39% lower (P < 0.05) in the eccentric thigh in the basal state, whereas the maximal activity of glycogen synthase was identical in the two thighs for all clamp steps. 3. The glucose infusion rate (GIR) necessary to maintain euglycaemia during maximal insulin stimulation was lower during PEC compared with CC (15.7%, 81.3 +/- 3.2 vs. 96.4 +/- 8.8 mumol kg-1 min-1, P < 0.05). 4. Our data show that 2 days after unaccustomed eccentric exercise, muscle and whole-body insulin action is impaired at maximal but not submaximal concentrations. The local effect cannot account for the whole-body effect, suggesting the release of a factor which decreases insulin responsiveness systemically.

1. Ten untrained young men ingested a carbohydrate-rich diet (65 energy percent (E%) carbohydrate, T-CHO) and ten similar subjects a fat-rich diet (62 E% fat, T-FAT) while endurance training was performed 3-4 times a week for 7 weeks. For another 8th week of training both groups ingested the carbohydrate-rich diet (T-CHO and T-FAT/CHO). 2. Maximal oxygen uptake increased by 11% (P < 0.05) in both groups after 7 and 8 weeks. Time to exhaustion at 81% of pre-training maximal oxygen uptake increased significantly from a mean (+/- S.E.M.) of 35 +/- 4 min to 102 +/- 5 and 65 +/- 7 min in T-CHO and T-FAT, respectively, after 7 weeks (P < 0.05, T-CHO vs. T-FAT). After 8 weeks, endurance remained unchanged in T-CHO but increased (P < 0.05) to 77 +/- 9 min in T-FAT/CHO which, however, was still less (P < 0.05) than in T-CHO. 3. Muscle glycogen breakdown rate during exercise was halved by endurance training equally in both T-CHO and T-FAT after 7 and 8 weeks, and muscle glycogen stores at exhaustion were not depleted in any group. 4. During exercise after 7 weeks, the respiratory exchange ratio (RER) was unchanged in T-CHO (0.88 +/- 0.01) compared with pre-training but decreased (P < 0.05) to 0.82 +/- 0.02 in T-FAT. After 8 weeks, RER in both T-CHO and T-FAT/CHO was approximately 0.87. 5. During exercise, plasma noradrenaline concentration and heart rate were higher in T-FAT than in T-CHO both at 7 and at 8 weeks. 6. It is concluded that ingesting a fat-rich diet during an endurance training programme is detrimental to improvement in endurance. This is not due to a simple lack of carbohydrate fuel, but rather to suboptimal adaptations that are not remedied by short-term increased carbohydrate availability. Furthermore, the study suggests that the decrease in RER usually seen after training when exercising at the same absolute intensity as before training can be prevented by a carbohydrate-rich diet.

1. The present study examined how uptake of lactate and H+ in resting muscle is affected by blood flow, arterial lactate concentration and muscle metabolism. 2. Six males subjects performed intermittent arm exercise in two separate 32 min periods (Part I and Part II) and in one subsequent 20 min period in which one leg knee-extensor exercise was also performed (Part III). The exercise was performed at various intensities in order to obtain different steady-state arterial blood lactate concentrations. In the inactive leg, femoral venous blood flow (draining about 7.7 kg of muscles) was measured and femoral arterial and venous blood was collected frequently. Biopsies were taken from m. vastus lateralis of the inactive leg at rest and 10 and 30 min into both Part I and Part II as well as 10 min into recovery from Part II. 3. The arterial plasma lactate concentrations were 7, 9 and 16 mmol l-1 after 10 min of Parts I, II and III, respectively, and the corresponding arterial-venous difference (a-vdiff) for lactate in the resting leg was 1.3, 1.4 and 2.0 mmol l-1. The muscle lactate concentration was 2.8 mmol (kg wet wt)-1 after 10 min of Part I and remained constant throughout the experiment. During Parts I and II, a-vdiff lactate decreased although the arterial lactate concentration and plasma-muscle lactate gradient were unaltered throughout each period. Thus, membrane transport of lactate decreased during each period. 4. Blood flow in the inactive leg was about 2-fold higher during arm exercise compared to the rest periods, resulting in a 2-fold higher lactate uptake. Thus, lactate uptake by inactive muscles was closely related to blood flow. 5. Throughout the experiment a-vdiff for actual base excess and for lactate were of similar magnitude. Thus, in inactive muscles lactate uptake appears to be coupled to the transport of H+.

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.

1. The effect of elevated muscle glycogen on anaerobic energy production, and glycogenolytic and glycolytic rates was examined in man by using the one-legged knee extension model, which enables evaluation of metabolism in a well-defined muscle group. 2. Six subjects performed very intense exercise to exhaustion (EX1) with one leg with normal glycogen (control) and one with a very high concentration (HG). With each leg, the exhaustive exercise was repeated after 1 h of recovery (EX2). Prior to and immediately after each exercise bout, a muscle biopsy was taken from m. vastus lateralis of the active leg for determination of glycogen, lactate, creatine phosphate (CP) and nucleotide concentrations. Measurements of leg blood flow and femoral arterial-venous differences for oxygen content, lactate, glucose, free fatty acids and potassium were performed before and regularly during the exhaustive exercises. 3. Muscle glycogen concentration prior to EX1 was 87.0 and 176.8 mmol (kg wet wt)-1 for the control and HG leg, respectively, and the decreases during exercise were 26.3 (control) and 25.6 (HG) mmol (kg wet wt)-1. The net glycogen utilization rate was not related to pre-exercise muscle glycogen concentration. Muscle lactate concentration at the end of EX1 was 18.8 (control) and 16.1 (HG) mmol (kg wet wt)-1, and the net lactate production (including lactate release) was 26.5 (control) and 23.6 (HG) mmol (kg wet wt)-1. Rate of lactate production was unrelated to initial muscle glycogen level. Time to exhaustion for EX1 was the same for the control leg (2.82 min) and HG leg (2.92 min). 4. Muscle glycogen concentration before EX2 was 14 mmol (kg wet wt)-1 lower than prior to EX1. During EX2 the muscle glycogen decline of 19.6 mmol (kg wet wt)-1 for the control leg was less than for the HG leg (26.2 mmol (kg wet wt)-1). The muscle lactate concentrations at the end of EX2 were about 7-8 mmol (kg wet wt)-1 lower compared to EX1, and the net lactate production was reduced by 40%. The exercise time during EX2 was 0.35 min shorter for the control leg, while no difference was observed for the HG leg. 5. Total reduction in ATP and CP was similar during the four exercise bouts, while a higher accumulation of inosine monophosphate (IMP) occurred during EX2 for the control leg (0.72 mmol (kg wet wt)-1) compared to the HG leg (0.20 mmol (kg wet wt)-1).(ABSTRACT TRUNCATED AT 400 WORDS)

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)