Four days of simulated shift work reduces insulin sensitivity in humans

AIM

The aim of this study was to investigate the effects of 4 consecutive simulated night shifts on glucose homeostasis, mitochondrial function and central and peripheral rhythmicities compared with a simulated day shift schedule.

METHODS

Seventeen healthy adults (8M:9F) matched for sleep, physical activity and dietary/fat intake participated in this study (night shift work n = 9; day shift work n = 8). Glucose tolerance and insulin sensitivity before and after 4 nights of shift work were measured by an intravenous glucose tolerance test and a hyperinsulinaemic euglycaemic clamp respectively. Muscles biopsies were obtained to determine insulin signalling and mitochondrial function. Central and peripheral rhythmicities were assessed by measuring salivary melatonin and expression of circadian genes from hair samples respectively.

RESULTS

Fasting plasma glucose increased (4.4 ± 0.1 vs. 4.6 ± 0.1 mmol L^-1 ; P = .001) and insulin sensitivity decreased (25 ± 7%, P < .05) following the night shift, with no changes following the day shift. Night shift work had no effect on skeletal muscle protein expression (PGC1α, UCP3, TFAM and mitochondria Complex II-V) or insulin-stimulated pAkt Ser473, pTBC1D4Ser318 and pTBC1D4Thr642. Importantly, the metabolic changes after simulated night shifts occurred despite no changes in the timing of melatonin rhythmicity or hair follicle cell clock gene expression across the wake period (Per3, Per1, Nr1d1 and Nr1d2).

CONCLUSION

Only 4 days of simulated night shift work in healthy adults is sufficient to reduce insulin sensitivity which would be expected to increase the risk of T2D.

Altered stress hormone response following acute exercise during prostate cancer treatment

Exercise training reduces the side effects of cancer treatments; however, the stress hormone response to acute exercise during prostate cancer (PCa) treatment is unclear. The study purpose was to examine the effects of acute exercise on circulating cortisol, epinephrine (Epi), and norepinephrine (NE) concentrations during PCa treatment with and without androgen deprivation therapy (ADT). Men with PCa (n = 11), with PCa on ADT (n = 11), and with non-cancer controls (n = 8) had blood samples for stress hormones collected before and immediately (0 hour), 2 hours, and 24 hours after 45 minutes of intermittent cycling at 60% of peak wattage. NE increased by 385% (P < .001) at 0 hour and remained elevated at 2 hours (P < .05) with no group differences. Overall, cortisol significantly increased at 0 hour (36%, P < .012) and then significantly decreased below baseline at 2 hours (-24%, P < .001) before returning to resting levels at 24 hours. Cortisol levels during ADT were 32% lower than PCa (P = .006) with no differences vs controls. Epi increased immediately after exercise more in controls (817%, P < .001) than with ADT (700%) and PCa (333%) patients, and both cancer groups' absolute levels were attenuated relative to controls (ADT: -54%, PCa: -52%, P = .004). Compared with age-matched controls, PCa and ADT patients exhibited similar stress hormone responses with acute exercise for NE and cortisol but an attenuated EPI response that suggests altered adrenal function. Future studies should examine the physical stress of multiple exercise bouts to verify these findings and to explore the functional hormonal effects, such as immune and metabolic responses, during cancer treatment.

Xanthine oxidase inhibition attenuates skeletal muscle signaling following acute exercise but does not impair mitochondrial adaptations to endurance training

The aim of this research was to examine the impact of the xanthine oxidase (XO) inhibitor allopurinol on the skeletal muscle activation of cell signaling kinases' and adaptations to mitochondrial proteins and antioxidant enzymes following acute endurance exercise and endurance training. Male Sprague-Dawley rats performed either acute exercise (60 min of treadmill running, 27 m/min, 5% incline) or 6 wk of endurance training (5 days/wk) while receiving allopurinol or vehicle. Allopurinol treatment reduced XO activity to 5% of the basal levels (P < 0.05), with skeletal muscle uric acid levels being almost undetectable. Following acute exercise, skeletal muscle oxidized glutathione (GSSG) significantly increased in allopurinol- and vehicle-treated groups despite XO activity and uric acid levels being unaltered by acute exercise (P < 0.05). This suggests that the source of ROS was not from XO. Surprisingly, muscle GSSG levels were significantly increased following allopurinol treatment. Following acute exercise, allopurinol treatment prevented the increase in p38 MAPK and ERK phosphorylation and attenuated the increase in mitochondrial transcription factor A (mtTFA) mRNA (P < 0.05) but had no effect on the increase in peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), nuclear respiratory factor-2, GLUT4, or superoxide dismutase mRNA. Allopurinol also had no impact on the endurance training-induced increases in PGC-1α, mtTFA, and mitochondrial proteins including cytochrome c, citrate synthase, and β-hydroxyacyl-CoA dehydrogenase. In conclusion, although allopurinol inhibits cell signaling pathways in response to acute exercise, the inhibitory effects of allopurinol appear unrelated to exercise-induced ROS production by XO. Allopurinol also has little effect on increases in mitochondrial proteins following endurance training.

Exercise increases skeletal muscle GLUT4 gene expression in patients with type 2 diabetes

The aim of the study was to determine the effect of a single bout of exercise on GLUT4 gene expression in muscle of patients with type 2 diabetes (T2D) and control subjects, matched for age and body mass index. Nine patients with T2D and nine control subjects performed 60 min of cycling exercise at ~55% peak power (W(max) ). Skeletal muscle biopsies were obtained at baseline, immediately post and 3-h post exercise. GLUT4 mRNA expression increased (p < 0.05) to a similar extent immediately post exercise in control (~60%) and T2D (~66%) subjects, and remained elevated (p < 0.05) 3-h post exercise with no differences between groups. Similarly, p-AMP-activated protein kinase, p38 mitogen-activated kinase and proliferator-activated receptor gamma co-activator-alpha mRNA expression were increased (p < 0.05) post exercise, and were not different between the groups. In conclusion, a single bout of exercise increased skeletal muscle GLUT4 mRNA expression in patients with T2D to a similar extent as in control subjects.

S-glutathionylation of troponin I (fast) increases contractile apparatus Ca2+ sensitivity in fast-twitch muscle fibres of rats and humans

Oxidation can decrease or increase the Ca2+ sensitivity of the contractile apparatus in rodent fast-twitch (type II) skeletal muscle fibres, but the reactions and molecular targets involved are unknown. This study examined whether increased Ca2+ sensitivity is due to S-glutathionylation of particular cysteine residues. Skinned muscle fibres were directly activated in heavily buffered Ca2+ solutions to assess contractile apparatus Ca2+ sensitivity. Rat type II fibres were subjected to S-glutathionylation by successive treatments with 2,2′-dithiodipyridine (DTDP) and glutathione (GSH), and displayed a maximal increase in pCa50 (−log10 [Ca2+] at half-maximal force) of ∼0.24 pCa units, with little or no effect on maximum force or Hill coefficient. Partial similar effect was produced by exposure to oxidized gluthathione (GSSG, 10 mM) for 10 min at pH 7.1, and near-maximal effect by GSSG treatment at pH 8.5. None of these treatments significantly altered Ca2+ sensitivity in rat type I fibres. Western blotting showed that both the DTDP–GSH and GSSG–pH 8.5 treatments caused marked S-glutathionylation of the fast troponin I isoform (TnI(f)) present in type II fibres, but not of troponin C (TnC) or myosin light chain 2. Both the increased Ca2+ sensitivity and glutathionylation of TnI(f) were blocked by N-ethylmaleimide (NEM). S-nitrosoglutathione (GSNO) also increased Ca2+ sensitivity, but only in conditions where it caused S-glutathionylation of TnI(f). In human type II fibres from vastus lateralis muscle, DTDP–GSH treatment also caused similar increased Ca2+ sensitivity and S-glutathionylation of TnI(f). When the slow isoform of TnI in type I fibres of rat was partially substituted (∼30%) with TnI(f), DTDP–GSH treatment caused a significant increase in Ca2+ sensitivity (∼0.08 pCa units). TnIf in type II fibres from toad and chicken muscle lack Cys133 present in mammalian TnIf, and such fibres showed no change in Ca2+ sensitivity with DTDP–GSH nor any S-glutathionylation of TnI(f) (latter examined only in toad). Following 40 min of cycling exercise in human subjects (at ∼60% peak oxygen consumption), TnI(f) in vastus lateralis muscle displayed a marked increase in S-glutathionylation (∼4-fold). These findings show that S-glutathionylation of TnI(f), most probably at Cys133, increases the Ca2+ sensitivity of the contractile apparatus, and that this occurs in exercising humans, with likely beneficial effects on performance.

High-dose antioxidant vitamin C supplementation does not prevent acute exercise-induced increases in markers of skeletal muscle mitochondrial biogenesis in rats

High doses of the antioxidant vitamin C prevent the increases in skeletal muscle mitochondrial biogenesis after exercise training. Since exercise training effects rely on the acute stimulus of each exercise bout, we examined whether vitamin C supplementation also attenuates the increases in skeletal muscle metabolic signaling and mitochondrial biogenesis in response to an acute exercise bout. Male Sprague-Dawley rats performed 60 min of treadmill running (27 m/min, 5% grade) or remained sedentary. For 7 days before this, one-half of the rats received water containing 500 mg/kg body wt vitamin C. Acute exercise significantly ( P < 0.05) increased the phosphorylation of p38 MAPK, AMP-activated kinase-α, and activating transcription factor (ATF)-2 and the ratio of oxidized to total glutathione (GSSG/TGSH) in the gastrocnemius. However, vitamin C had no effect on these increases. Similarly, vitamin C did not prevent the exercise-induced increases in peroxisome proliferator-activated receptor-γ coactivator-1α, nuclear respiratory factor (NRF)-1, NRF-2, mitochondrial transcription factor A, glutathione peroxidase-1, MnSOD, extracellular SOD, or glucose transporter 4 ( P < 0.05) mRNA after exercise. Surprisingly, vitamin C supplementation significantly increased the basal levels of GSSG/TGSH, NRF-1, and NRF-2 mRNA and basal ATF-2 phosphorylation. In summary, despite other studies in rats showing that vitamin C supplementation prevents increases in skeletal muscle mitochondrial biogenesis and antioxidant enzymes with exercise training, vitamin C had no affect on the acute exercise-induced increases of these markers.

Local hindlimb antioxidant infusion does not affect muscle glucose uptake during in situ contractions in rat

There is evidence that reactive oxygen species (ROS) contribute to the regulation of skeletal muscle glucose uptake during highly fatiguing ex vivo contraction conditions via AMP-activated protein kinase (AMPK). In this study we investigated the role of ROS in the regulation of glucose uptake and AMPK signaling during low-moderate intensity in situ hindlimb muscle contractions in rats, which is a more physiological protocol and preparation. Male hooded Wistar rats were anesthetized, and then N-acetylcysteine (NAC) was infused into the epigastric artery (125 mg·kg−1·h−1) of one hindlimb (contracted leg) for 15 min before this leg was electrically stimulated (0.1-ms impulse at 2 Hz and 35 V) to contract at a low-moderate intensity for 15 min. The contralateral leg did not receive stimulation or local NAC infusion (rest leg). NAC infusion increased ( P < 0.05) plasma cysteine and cystine (by ∼360- and 1.4-fold, respectively) and muscle cysteine (by 1.5-fold, P = 0.001). Although contraction did not significantly alter muscle tyrosine nitration, reduced (GSH) or oxidized glutathione (GSSG) content, S-glutathionylation of protein bands at ∼250 and 150 kDa was increased ( P < 0.05) ∼1.7-fold by contraction, and this increase was prevented by NAC. Contraction increased ( P < 0.05) skeletal muscle glucose uptake 20-fold, AMPK phosphorylation 6-fold, ACCβ phosphorylation 10-fold, and p38 MAPK phosphorylation 60-fold, and the muscle fatigued by ∼30% during contraction and NAC infusion had no significant effect on any of these responses. This was despite NAC preventing increases in S-glutathionylation with contraction. In conclusion, unlike during highly fatiguing ex vivo contractions, local NAC infusion during in situ low-moderate intensity hindlimb contractions in rats, a more physiological preparation, does not attenuate increases in skeletal muscle glucose uptake or AMPK signaling.

Central role of nitric oxide synthase in AICAR and caffeine-induced mitochondrial biogenesis in L6 myocytes

5-Aminoimidazole-4-carboxamide-ribonucleoside (AICAR) and caffeine, which activate AMP-activated protein kinase (AMPK) and cause sarcoplasmic reticulum calcium release, respectively, have been shown to increase mitochondrial biogenesis in L6 myotubes. Nitric oxide (NO) donors also increase mitochondrial biogenesis. Since neuronal and endothelial NO synthase (NOS) are calcium dependent and are also phosphorylated by AMPK, we hypothesized that NOS inhibition would attenuate the activation of mitochondrial biogenesis in response to AICAR and caffeine. L6 myotubes either were not treated (control) or were exposed acutely or for 5 h/day over 5 days to 100 microM of N(G)-nitro-L-arginine methyl ester (L-NAME, NOS inhibitor), 100 microM S-nitroso-N-acetyl-penicillamine (SNAP) (NO donor) +/- 100 microM L-NAME, 2 mM AICAR +/- 100 microM L-NAME, or 5 mM caffeine +/- 100 microM L-NAME (n = 12/treatment). Acute AICAR administration increased (P < 0.05) phospho- (P-)AMPK, but also increased P-CaMK, with resultant chronic increases in peroxisome proliferator-activated receptor-gamma coactivator-1 alpha (PGC-1 alpha), cytochrome-c oxidase (COX)-1, and COX-4 protein expression compared with control cells. NOS inhibition, which had no effect on AICAR-stimulated P-AMPK, surprisingly increased P-CaMK and attenuated the AICAR-induced increases in COX-1 and COX-4 protein. Caffeine administration, which increased P-CaMK without affecting P-AMPK, increased COX-1, COX-4, PGC-1 alpha, and citrate synthase activity. NOS inhibition, surprisingly, greatly attenuated the effect of caffeine on P-CaMK and attenuated the increases in COX-1 and COX-4 protein. SNAP increased all markers of mitochondrial biogenesis, and it also increased P-AMPK and P-CaMK. In conclusion, AICAR and caffeine increase mitochondrial biogenesis in L6 myotubes, at least in part, via interactions with NOS.

Differential attenuation of AMPK activation during acute exercise following exercise training or AICAR treatment

Short-term exercise training in humans attenuates AMP-activated protein kinase (AMPK) activation during subsequent exercise conducted at the same absolute workload. Short-term 5-aminoimidazole-4-carboxyamide-ribonucleoside (AICAR) administration in rats mimics exercise training on skeletal muscle in terms of increasing insulin sensitivity, mitochondrial enzymes, and GLUT4 content, but it is not known whether these adaptations are accompanied by reduced AMPK activation during subsequent exercise. We compared the effect of 10 days of treadmill training (60 min/day) with 10 days of AICAR administration (0.5 mg/g body weight ip) on subsequent AMPK activation during 45 min of treadmill exercise in male Sprague-Dawley rats. Compared with nonexercised control rats, acute exercise significantly (P < 0.05) increased AMPKalpha Thr172 phosphorylation (p-AMPKalpha; 1.6-fold) and ACCbeta Ser218 phosphorylation (p-ACCbeta; 4.9-fold) in the soleus and p-ACCbeta 2.2-fold in the extensor digitorum longus. Ten days of exercise training abolished the increase in soleus p-AMPKalpha and attenuated the increase in p-ACCbeta (nonsignificant 2-fold increase). Ten days of AICAR administration also attenuated the exercise-induced increases in AMPK signaling in the soleus although not as effectively as 10 days of exercise training (nonsignificant 1.3-fold increase in p-AMPKalpha; significant 3-fold increase in p-ACCbeta). The increase in skeletal muscle 2-deoxyglucose uptake during exercise was greater after either 10 days of exercise training or AICAR administration. In conclusion, 10 days of AICAR administration substantially mimics the effect of 10 days training on attenuating skeletal muscle AMPK activation in response to subsequent exercise.

NOS isoform-specific regulation of basal but not exercise-induced mitochondrial biogenesis in mouse skeletal muscle

Nitric oxide is a potential regulator of mitochondrial biogenesis. Therefore, we investigated if mice deficient in endothelial nitric oxide synthase (eNOS-/-) or neuronal NOS (nNOS-/-) have attenuated activation of skeletal muscle mitochondrial biogenesis in response to exercise. eNOS-/-, nNOS-/- and C57Bl/6 (CON) mice (16.3 +/- 0.2 weeks old) either remained in their cages (basal) or ran on a treadmill (16 m min(-1), 5% grade) for 60 min (n = 8 per group) and were killed 6 h after exercise. Other eNOS-/-, nNOS-/- and CON mice exercise trained for 9 days (60 min per day) and were killed 24 h after the last bout of exercise training. eNOS-/- mice had significantly higher nNOS protein and nNOS-/- mice had significantly higher eNOS protein in the EDL, but not the soleus. The basal mitochondrial biogenesis markers NRF1, NRF2alpha and mtTFA mRNA were significantly (P< 0.05) higher in the soleus and EDL of nNOS-/- mice whilst basal citrate synthase activity was higher in the soleus and basal PGC-1alpha mRNA higher in the EDL. Also, eNOS-/- mice had significantly higher basal citrate synthase activity in the soleus but not the EDL. Acute exercise increased (P< 0.05) PGC-1alpha mRNA in soleus and EDL and NRF2alpha mRNA in the EDL to a similar extent in all genotypes. In addition, short-term exercise training significantly increased cytochrome c protein in all genotypes (P< 0.05) in the EDL. In conclusion, eNOS and nNOS are differentially involved in the basal regulation of mitochondrial biogenesis in skeletal muscle but are not critical for exercise-induced increases in mitochondrial biogenesis in skeletal muscle.

Effect of nitric oxide synthase inhibition on mitochondrial biogenesis in rat skeletal muscle

The purpose of this study was to determine whether nitric oxide synthase (NOS) inhibition decreased basal and exercise-induced skeletal muscle mitochondrial biogenesis. Male Sprague-Dawley rats were assigned to one of four treatment groups: NOS inhibitor N(G)-nitro-l-arginine methyl ester (l-NAME, ingested for 2 days in drinking water, 1 mg/ml) followed by acute exercise, no l-NAME ingestion and acute exercise, rest plus l-NAME, and rest without l-NAME. The exercised rats ran on a treadmill for 53 +/- 2 min and were then killed 4 h later. NOS inhibition significantly (P < 0.05; main effect) decreased basal peroxisome proliferator-activated receptor-gamma coactivator 1beta (PGC-1beta) mRNA levels and tended (P = 0.08) to decrease mtTFA mRNA levels in the soleus, but not the extensor digitorum longus (EDL) muscle. This coincided with significantly reduced basal levels of cytochrome c oxidase (COX) I and COX IV mRNA, COX IV protein and COX enzyme activity following NOS inhibition in the soleus, but not the EDL muscle. NOS inhibition had no effect on citrate synthase or beta-hydroxyacyl CoA dehydrogenase activity, or cytochrome c protein abundance in the soleus or EDL. NOS inhibition did not reduce the exercise-induced increase in peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha) mRNA in the soleus or EDL. In conclusion, inhibition of NOS appears to decrease some aspects of the mitochondrial respiratory chain in the soleus under basal conditions, but does not attenuate exercise-induced mitochondrial biogenesis in the soleus or in the EDL.

Effect of exercise intensity and hypoxia on skeletal muscle AMPK signaling and substrate metabolism in humans

We compared in human skeletal muscle the effect of absolute vs. relative exercise intensity on AMP-activated protein kinase (AMPK) signaling and substrate metabolism under normoxic and hypoxic conditions. Eight untrained males cycled for 30 min under hypoxic conditions (11.5% O(2), 111 +/- 12 W, 72 +/- 3% hypoxia Vo(2 peak); 72% Hypoxia) or under normoxic conditions (20.9% O(2)) matched to the same absolute (111 +/- 12 W, 51 +/- 1% normoxia Vo(2 peak); 51% Normoxia) or relative (to Vo(2 peak)) intensity (171 +/- 18 W, 73 +/- 1% normoxia Vo(2 peak); 73% Normoxia). Increases (P < 0.05) in AMPK activity, AMPKalpha Thr(172) phosphorylation, ACCbeta Ser(221) phosphorylation, free AMP content, and glucose clearance were more influenced by the absolute than by the relative exercise intensity, being greatest in 73% Normoxia with no difference between 51% Normoxia and 72% Hypoxia. In contrast to this, increases in muscle glycogen use, muscle lactate content, and plasma catecholamine concentration were more influenced by the relative than by the absolute exercise intensity, being similar in 72% Hypoxia and 73% Normoxia, with both trials higher than in 51% Normoxia. In conclusion, increases in muscle AMPK signaling, free AMP content, and glucose disposal during exercise are largely determined by the absolute exercise intensity, whereas increases in plasma catecholamine levels, muscle glycogen use, and muscle lactate levels are more closely associated with the relative exercise intensity.

L-Arginine infusion increases glucose clearance during prolonged exercise in humans

Nitric oxide synthase (NOS) inhibition has been shown in humans to attenuate exercise-induced increases in muscle glucose uptake. We examined the effect of infusing the NO precursor L-arginine (L-Arg) on glucose kinetics during exercise in humans. Nine endurance-trained males cycled for 120 min at 72+/-1% Vo(2 peak) followed immediately by a 15-min "all-out" cycling performance bout. A [6,6-(2)H]glucose tracer was infused throughout exercise, and either saline alone (Control, CON) or saline containing L-Arg HCL (L-Arg, 30 g at 0.5 g/min) was confused in a double-blind, randomized order during the last 60 min of exercise. L-Arg augmented the increases in glucose rate of appearance, glucose rate of disappearance, and glucose clearance rate (L-Arg: 16.1+/-1.8 ml.min(-1).kg(-1); CON: 11.9+/- 0.7 ml.min(-1).kg(-1) at 120 min, P<0.05) during exercise, with a net effect of reducing plasma glucose concentration during exercise. L-Arg infusion had no significant effect on plasma insulin concentration but attenuated the increase in nonesterified fatty acid and glycerol concentrations during exercise. L-Arg infusion had no effect on cycling exercise performance. In conclusion, L-Arg infusion during exercise significantly increases skeletal muscle glucose clearance in humans. Because plasma insulin concentration was unaffected by L-Arg infusion, greater NO production may have been responsible for this effect.

Effects of the nitric oxide donor, sodium nitroprusside, on resting leg glucose uptake in patients with type 2 diabetes

AIMS/HYPOTHESIS

Nitric oxide (NO) has been implicated as an important signalling molecule in the contraction-mediated glucose uptake pathway and may represent a novel strategy for blood glucose control. The current study sought to determine whether acute infusion of the NO donor, sodium nitroprusside (SNP), increases leg glucose uptake at rest in patients with type 2 diabetes.

METHODS

Fifteen male patients with type 2 diabetes (aged 54+/-4 years, mean+/-SD) were entered into a randomised, cross-over design study, examining the effect of a 30-min intra-femoral infusion of SNP on leg glucose uptake. Comparison was made with a 30-min infusion of verapamil, titrated to elicit similar leg blood flow responses to SNP. Leg blood flow was measured by thermodilution in the femoral vein, and leg glucose uptake was calculated as the product of leg blood flow and the femoral arterio-venous (A-V) glucose concentration gradient.

RESULTS

The two drugs increased leg blood flow to a similar extent (p=0.50). Both leg A-V glucose concentration gradient (SNP 0.12+/-0.05, verapamil -0.06+/-0.04 mmol/l; mean+/- SEM, p=0.03) and leg glucose uptake (SNP 0.17+/-0.09, verapamil -0.09+/-0.06 mmol/min; p=0.03) were higher with the SNP treatment than with verapamil. These results occurred independently of any significant difference in plasma insulin concentration between drugs (p=0.56).

CONCLUSIONS/INTERPRETATION

Acute infusion of SNP resulted in greater glucose uptake relative to verapamil. NO may therefore be an important mediator of peripheral glucose disposal and a potential therapeutic target in patients with type 2 diabetes.

5'-aminoimidazole-4-carboxyamide-ribonucleoside-activated glucose transport is not prevented by nitric oxide synthase inhibition in rat isolated skeletal muscle

1. The nucleoside intermediate 5'-aminoimidazole-4-carboxyamide-ribonucleoside (AICAR) activates skeletal muscle AMP-activated protein kinase (AMPK) and increases glucose uptake. The AMPK phosphorylates neuronal nitric oxide synthase (nNOS)mu in skeletal muscle fibres. There is evidence that both AMPK and nNOSmu may be involved in the regulation of contraction-stimulated glucose uptake. 2. We examined whether both AICAR- and contraction-stimulated glucose uptake were mediated by NOS in rat skeletal muscle. 3. Rat isolated epitrochlearis muscles were subjected in vitro to electrically stimulated contractions for 10 min and/or incubated in the presence or absence of AICAR (2 mmol/L) or the NOS inhibitor NG-monomethyl-L-arginine (L-NMMA; 100 micromol/L). 4. Muscle contraction significantly (P < 0.05) altered the metabolic profile of the muscle. In contrast, AICAR and L-NMMA had no effect on the metabolic profile of the muscle, except that AICAR increased muscle 5'-aminoimidazole-4-carboxyamide-ribonucleotide (ZMP) and AICAR content. Nitric oxide synthase inhibition caused a small but significant (P < 0.05) reduction in basal 3-O-methylglucose transport, which was observed in all treatments. 5'-Aminoimidazole-4-carboxyamide-ribonucleoside significantly increased (P < 0.05) glucose transport above basal, with NOS inhibition decreasing this slightly (increased by 209% above basal compared with 184% above basal with NOS inhibition). Contraction significantly increased glucose transport above basal, with NOS inhibition substantially reducing this (107% increase vs 31% increase). 5'-Aminoimidazole-4-carboxyamide-ribonucleoside plus contraction in combination were not additive on glucose transport. 5. These results suggest that NO plays a role in basal glucose uptake and may regulate contraction-stimulated glucose uptake. However, NOS/nitric oxide do not appear to be signalling intermediates in AICAR-stimulated skeletal muscle glucose uptake.

Progressive increase in human skeletal muscle AMPKalpha2 activity and ACC phosphorylation during exercise

The effect of prolonged moderate-intensity exercise on human skeletal muscle AMP-activated protein kinase (AMPK)alpha1 and -alpha2 activity and acetyl-CoA carboxylase (ACCbeta) and neuronal nitric oxide synthase (nNOSmu) phosphorylation was investigated. Seven active healthy individuals cycled for 30 min at a workload requiring 62.8 +/- 1.3% of peak O(2) consumption (VO(2 peak)) with muscle biopsies obtained from the vastus lateralis at rest and at 5 and 30 min of exercise. AMPKalpha1 activity was not altered by exercise; however, AMPKalpha2 activity was significantly (P < 0.05) elevated after 5 min (approximately 2-fold), and further elevated (P < 0.05) after 30 min (approximately 3-fold) of exercise. ACCbeta phosphorylation was increased (P < 0.05) after 5 min (approximately 18-fold compared with rest) and increased (P < 0.05) further after 30 min of exercise (approximately 36-fold compared with rest). Increases in AMPKalpha2 activity were significantly correlated with both increases in ACCbeta phosphorylation and reductions in muscle glycogen content. Fat oxidation tended (P = 0.058) to increase progressively during exercise. Muscle creatine phosphate was lower (P < 0.05), and muscle creatine, calculated free AMP, and free AMP-to-ATP ratio were higher (P < 0.05) at both 5 and 30 min of exercise compared with those at rest. At 30 min of exercise, the values of these metabolites were not significantly different from those at 5 min of exercise. Phosphorylation of nNOSmu was variable, and despite the mean doubling with exercise, statistically significance was not achieved (P = 0.304). Western blots indicated that AMPKapproximately 2 was associated with both nNOSmu and ACCbeta consistent with them both being substrates of AMPKalpha2 in vivo. In conclusion, AMPKalpha2 activity and ACCbeta phosphorylation increase progressively during moderate exercise at approximately 60% of VO(2 peak) in humans, with these responses more closely coupled to muscle glycogen content than muscle AMP/ATP ratio.

AMPK signaling in contracting human skeletal muscle: acetyl-CoA carboxylase and NO synthase phosphorylation

AMP-activated protein kinase (AMPK) is a metabolic stress-sensing protein kinase responsible for coordinating metabolism and energy demand. In rodents, exercise accelerates fatty acid metabolism, enhances glucose uptake, and stimulates nitric oxide (NO) production in skeletal muscle. AMPK phosphorylates and inhibits acetyl-coenzyme A (CoA) carboxylase (ACC) and enhances GLUT-4 translocation. It has been reported that human skeletal muscle malonyl-CoA levels do not change in response to exercise, suggesting that other mechanisms besides inhibition of ACC may be operating to accelerate fatty acid oxidation. Here, we show that a 30-s bicycle sprint exercise increases the activity of the human skeletal muscle AMPK-alpha1 and -alpha2 isoforms approximately two- to threefold and the phosphorylation of ACC at Ser(79) (AMPK phosphorylation site) approximately 8.5-fold. Under these conditions, there is also an approximately 5.5-fold increase in phosphorylation of neuronal NO synthase-mu (nNOSmu;) at Ser(1451). These observations support the concept that inhibition of ACC is an important component in stimulating fatty acid oxidation in response to exercise and that there is coordinated regulation of nNOSmu to protect the muscle from ischemia/metabolic stress.

Effect of carbohydrate ingestion on glucose kinetics and muscle metabolism during intense endurance exercise

There has been recent interest in the potential performance and metabolic effects of carbohydrate ingestion during exercise lasting approximately 1 h. In this study, 13 well-trained men ingested in randomized order either a 6% glucose solution (CHO trial) or a placebo (Con trial) during exercise to exhaustion at 83+/-1% peak oxygen uptake. In six subjects, vastus lateralis muscle was sampled at rest, at 32 min, and at exhaustion, and in six subjects, glucose kinetics was determined by infusion of [6,6-(2)H]glucose in both trials and ingestion of [6-(3)H]glucose in the CHO trial. Of the 84 g of glucose ingested during exercise in the CHO trial, only 22 g appeared in the peripheral circulation. This resulted in a small (12 g) but significant (P<0.05) increase in glucose uptake without influencing carbohydrate oxidation, muscle glycogen use, or time to exhaustion (CHO: 68.1+/-4.1 min; Con: 69.6+/-5.5 min). Decreases in muscle phosphocreatine content and increases in muscle inosine monophosphate and lactate content during exercise were similar in the two trials. Although endogenous glucose production during exercise was partially suppressed in the CHO trial, it remained significantly above preexercise levels throughout exercise. In conclusion, only 26% of the ingested glucose appeared in the peripheral circulation. Glucose ingestion increased glucose uptake and partially reduced endogenous glucose production but had no effect on carbohydrate oxidation, muscle metabolism, or time to exhaustion during exercise at 83% peak oxygen uptake.

Nitric oxide synthase inhibition reduces leg glucose uptake but not blood flow during dynamic exercise in humans

Nitric oxide (NO) appears to play a role in contraction-stimulated glucose uptake in isolated rodent skeletal muscle; however, no studies have examined this question in humans. Seven healthy men completed two 30-min bouts of supine cycling exercise at 60 +/- 2% peak pulmonary oxygen uptake (VO2 peak), separated by 90 min of rest. The NO synthase inhibitor N(G)-monomethyl-L-arginine ([L-NMMA]; total dose 5 mg/kg body weight) or saline (control) were administered via the femoral artery for the final 20 min of exercise in a randomized blinded crossover design. L-Arginine (5 mg/kg body weight) was co-infused during the final 5 min of each exercise bout. Leg blood flow (LBF) was measured by thermodilution in the femoral vein, and leg glucose uptake was calculated as the product of LBF and femoral arteriovenous (AV) glucose difference. L-NMMA infusion significantly (P < 0.05) reduced leg glucose uptake compared with control (48 +/- 12% lower at 15 min, mean +/- SE). The reduction in glucose uptake was due solely to a decrease in AV glucose difference, as there was no effect of L-NMMA infusion on LBF during exercise. Co-infusion of L-arginine restored glucose uptake during L-NMMA infusion to levels similar to control. These results indicate that NO production contributes substantially to exercise-mediated skeletal muscle glucose uptake in humans independent of skeletal muscle blood flow.

Fluid ingestion does not influence intense 1-h exercise performance in a mild environment

PURPOSE

It is generally recommended that fluid be ingested during exercise at a rate that prevents body mass loss and prevents dehydration. It is, however, not known whether these recommendations are valid during intense endurance exercise in a mild environment. The purpose of this study was to examine the effect of fluid ingestion volume on heart rate (HR), rectal temperature, plasma electrolytes, and performance during intense endurance exercise at 21 degrees C.

METHODS

Eight well-trained men (26+/-1 yr; 79.6+/-3.5 kg; VO2peak = 5.05+/-0.17 L.min(-1) ; mean+/-SEM) cycled for 45 min at 80+/-1% VO2peak while receiving either no fluid replacement (NF), a volume of water that prevented body mass loss (FR-100 = 1.47+/-0.05 L), or 50% of this volume (FR-50 = 0.72+/-0.03 L). The 45-min exercise bout was followed immediately by a 15-min "all-out" performance ride.

RESULTS

NF was associated with a 1.9+/-0.0% body mass loss, while FR-50 and FR-100 resulted in losses of 1.0 = 0.1% and 0.0+/-0.1%, respectively. Although values tended to be higher in NF, fluid ingestion had no significant effect on HR or rectal temperature during exercise. Reductions in plasma volume and increases in plasma sodium and potassium concentrations during exercise were largely unaffected by fluid ingestion. RPE increased to a similar extent during exercise in the three trials while a mild increase in the degree of stomach bloating/fullness was evident in FR-100. Work completed during the 15-min performance ride was similar in the three trials (NF: 273+/-8, FR-50: 267+/-8, FR-100: 269+/-9 kJ).

CONCLUSIONS

There appears to be little benefit from ingesting water during intense 1-h cycling exercise in mild environmental conditions since such ingestion has no significant effect on HR, body temperature, plasma volume, plasma electrolytes, or performance.

Influence of ingested fluid volume on physiological responses during prolonged exercise

The effect of different rates of fluid ingestion on heart rate, rectal temperature, plasma electrolytes, hormones and performance was examined during prolonged strenuous exercise conducted at 21 degrees C. Seven well-trained males (24 +/- 1 yr; 68.6 +/- 2.9 kg; VO2 peak = 4.69 +/- 0.17 L min-1; mean +/- SEM) cycled for 2 h at 69 +/- 1% VO2 peak while receiving either no fluid replacement (NF), a volume of water estimated to prevent body weight loss (FR-100 = 2.32 +/- 0.10 L 2 h-1) or 50% of this volume (FR-60 = 1.16 +/- 0.05 L 2 h-1). The 2-h exercise bout was followed by a ride to exhaustion at a workload estimated to be 90% VO2 peak. After 2 h of exercise, NF was associated with a 3.2 +/- 0.1% weight loss, while FR-50 and FR-100 resulted in losses of 1.8 +/- 0.1 and 0.1 +/- 0.1%, respectively. Compared with FR-100, heart rate and rectal temperature were elevated (P < 0.05) during the second hour of exercise in NF, with FR-50 intermediate. Reductions in plasma volume during exercise were greater in NF and FR-50, compared with FR-100 and plasma sodium concentration was elevated in NF, decreased slightly in FR-100, with FR-50 intermediate. Plasma renin activity, aldosterone and atrial natriuretic peptide increased to similar extents in the three trials. Plasma vasopressin remained unchanged for FR-100, increased for NF, with intermediate values for FR-50. Exercise time to exhaustion at 90% VO2-peak was longer in FR-100 (328 +/- 93 s) than NF (171 +/- 75 s) with FR-50 (248 +/- 107 s) not significantly different from either FR-100 or NF. In conclusion, the responses of heart rate, rectal temperature, plasma sodium, and vasopressin during, and performance following, prolonged cycling exercise conducted at 21 degrees C are related to the amount of fluid ingested (i.e. the degree of dehydration).

Accumulated oxygen deficit during supramaximal all-out and constant intensity exercise

Two studies were conducted to test the validity of an all-out procedure for the assessment of the maximal accumulated oxygen deficit (AOD). Subjects in study 1 (N = 9; VO2max = 57 +/- 3 ml.kg-1.min-1 [+/- SEM]) completed three supramaximal efforts on a cycle ergometer. Exhaustive exercise during an all-out isokinetic procedure (mean intensity of 149% VO2max) was compared with constant intensity exercise at approximately 110% and 125% VO2max. Subjects in study 2 (N = 12; VO2max = 55 +/- 3 ml.kg-1.min-1) completed a constant intensity test to exhaustion at approximately 110% VO2max and a 90 s all-out test on a Monark friction loaded cycle ergometer (mean intensity of 143% VO2max). The AOD within each study were not significantly different (study 1:43.9, 44.1, and 42.0 ml.kg-1 for the 110%, 125%, and all-out tests; study 2: 52.1 and 51.2 ml.kg-1 for the 110% and all-out tests, respectively; P > 0.05). The total amount of work was significantly greater the longer the test, the additional work being attributed to aerobic processes. The rate of both aerobic and anaerobic energy production in the first 30 s of exercise was directly related to exercise intensity and the protocol used. The results indicate that an all-out procedure provides a valid estimate of the maximal AOD and shows potential for a more complete assessment of anaerobic ability as traditional indices of high intensity exercise performance are also obtained.

Carbohydrate feedings and exercise performance: effect of initial muscle glycogen concentration

To determine whether the ergogenic benefits of carbohydrate (CHO) feedings are affected by preexercise muscle glycogen levels, eight cyclists performed four self-paced time trials on an isokinetic ergometer over a simulated distance of 70 km. Trials were performed under the following preexercise muscle glycogen and beverage conditions: 1) high glycogen (180.2 +/- 9.7 mmol/kg wet wt) with a CHO beverage (HG-CHO), 2) high glycogen (170.2 +/- 10.4 mmol/kg wet wt) with a non-CHO beverage (HG-NCHO), 3) low glycogen (99.8 +/- 6.0 mmol/kg wet wt) with a CHO beverage (LG-CHO), and 4) low glycogen (109.7 +/- 5.3 mmol/kg wet wt) with a non-CHO beverage (LG-NCHO). The CHO drink (ingested at the onset of exercise and every 10 km thereafter) provided 116 +/- 6 g CHO/trial and prevented the decline in serum glucose observed during both NCHO trials. Performance times ranged from 117.93 +/- 1.44 (HG-CHO) to 122.91 +/- 2.46 min (LG-NCHO). No intertrial differences (P > 0.05) were observed for O2 consumption (75% of maximal O2 consumption), power output (237 W), or self-selected pace (8.44 min/5 km) during the initial 71-79% of exercise. Over the final 14% of the time trial, power output and pace (231 W and 8.62 min/5 km) were similar for the HG-CHO, HG-NCHO, and LG-CHO conditions, but both variables were significantly lower during the LG-NCHO trial (198 W and 9.67 min/5 km, P < 0.05 vs. all other trials).(ABSTRACT TRUNCATED AT 250 WORDS)

Reduced training volume and intensity maintain aerobic capacity but not performance in distance runners

It has recently been shown that a 70% reduction in training volume, while maintaining training intensity, results in the maintenance of VO2 max and 5 km running performance in distance runners. The purpose of this study was to examine the effects of a 4 wk reduction in training volume and intensity in distance runners. Ten well-conditioned males (VO2max = 63.4 +/- 1.3 ml.kg-1 x min-1) underwent 4 wks of base training (BT) at their accustomed training distance (71.8 +/- 3.6 km.wk-1) and intensity (76% of total distance > 70% VO2max). Training volume (-66%), frequency (-50%), and intensity (all running < 70% VO2max) were then decreased for a 4 wk reduced training period (RT). Treadmill VO2max was unchanged with RT (p > 0.05) as were resting plasma volume, estimated from haemoglobin and haematocrit levels, and resting heart rate (HR). Submaximal treadmill exercise VO2 (l.min-1), ventilation and HR were also unchanged, however, submaximal exercise RER and blood lactate accumulation following 4 mins at 95% VO2max (8.39 vs 9.89 mmol.l-1) were significantly elevated by RT (p < 0.05). Estimated percent body fat also increased (10.4% vs 11.8%) (p < 0.05). Five km race completion time significantly increased from 16.6 +/- 0.3 mins at week 4 of BT to 16.8 +/- 0.3 mins (12 seconds) at week 4 of RT. Nine of the 10 subjects were slower after RT. It is concluded that aerobic capacity was maintained in these runners, despite the combined reduction in training volume and intensity. However, it appears that training intensity during RT is important for the maintenance of 5 km running performance.

Psychological effects during reduced training volume and intensity in distance runners

Improved mood state has ben linked with reduced training (RT). However, RT has been measured only as a reduction in training volume, either maintaining or not considering the intensity of training employed. To investigate the effects of a 4-week reduction in both training volume and intensity on running performance and mood state, 10 well-trained adult male runners trained for 4 weeks at baseline training distance and pace (BT), followed by 4 weeks training reduced in volume by 66%, with intensity diminished so that all workouts were below 70% VO2max. Subjects completed the Profile of Mood States (POMS) before BT (PREBT), before RT (PRERT), and after RT (POSRT). They ran 5 km time trials PRERT and POSRT. Comparisons were made for the positive mood state of Vigor with an average of the values for the 5 negative mood states (NM) of the POMS. Eight of 10 changes from PRERT to POSRT were toward more negative mood. Nine of 10 runners required more time for the 5 km run POSRT than PRERT. Overall, 7 of 10 runners exhibited both an increase in 5 km time and a change toward more negative mood state (p < .004). However, the magnitudes of these changes were unrelated. These results suggest that mood state and running performance may be linked. Moreover, because mood state did not improve, this study suggests that studies dealing with the topic of reduced training should be specific with regard to the influence of both training intensity and volume.

Day to day variation in time trial cycling performance

In an attempt to assess the reproducibility of laboratory cycling performance, eight well-trained (VO2max = 4.6 +/- 0.2 l.min-1) male cyclists completed 12 trials involving 4 successive performance rides at each of three total work outputs (approximately 1600, 200, and 14 kilojoules, respectively). These trials, designated as long, medium, and short trials (LT, MT, ST), represented exercise bouts of 105.12 +/- 0.41, 12.03 +/- 0.17 and 0.55 +/- 0.11 minutes, respectively. The trials, conducted on a computerized cycle ergometer in an isokinetic mode, were separated by a minimum of 72 hrs. All trials for each subject were completed at the same time of day. In all trials, subjects were allowed to select the pace in order to complete the ride in the shortest possible time. The mean coefficient of variation (CV) for performance time in each trial was: LT = +/- 1.01%, MT = +/- 0.95%, and ST = +/- 2.43%, respectively. The CV for performance time in ST was significantly greater than the CV in either LT or MT. In LT, performance time was significantly faster, and the mean % VO2max was significantly higher in trial 4 versus trials 1-3. There was no order effect in the MT or ST rides. The CV for mean VO2 (l.min-1), mean % VO2max, and RER during the LT rides were +/- 3.02%, +/- 3.64%, and +/- 3.53%, respectively. These data suggest that trained cyclists have the ability to reproduce endurance performance with a CV of approximately 1.0% in a time-trial protocol.(ABSTRACT TRUNCATED AT 250 WORDS)

Time course of glycogen accumulation after eccentric exercise

This study examined the time course of glycogen accumulation in skeletal muscle depleted by concentric work and subsequently subjected to eccentric exercise. Eight men exercised to exhaustion on a cycle ergometer [70% of maximal O2 consumption (VO2max)] and were placed on a carbohydrate-restricted diet. Approximately 12 h later they exercised one leg to subjective failure by repeated eccentric action of the knee extensors against a resistance equal to 120% of their one-repetition maximum concentric knee extension force (ECC leg). The contralateral leg was not exercised and served as a control (CON leg). During the 72-h recovery period, subjects consumed 7 g carbohydrate.kg body wt-1.day-1. Moderate soreness was experienced in the ECC leg 24-72 h after eccentric exercise. Muscle biopsies from the vastus lateralis of the ECC and CON legs revealed similar glycogen levels immediately after eccentric exercise (40.2 +/- 5.2 and 47.6 +/- 6.4 mmol/kg wet wt, respectively; P greater than 0.05). There was no difference in the glycogen content of ECC and CON legs after 6 h of recovery (77.7 +/- 7.9 and 85.1 +/- 4.9 mmol/kg wet wt, respectively; P greater than 0.05), but 18 h later, the ECC leg contained 15% less glycogen than the CON leg (90.2 +/- 8.2 vs. 105.8 +/- 8.9 mmol/kg wet wt; P less than 0.05). After 72 h of recovery, this difference had increased to 24% (115.8 +/- 8.0 vs. 153.0 +/- 12.2 mmol/kg wet wt; P less than 0.05). These data confirm that glycogen accumulation is impaired in eccentrically exercised muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

Fluid replacement after dehydration: influence of beverage carbonation and carbohydrate content

This investigation evaluated the effects of beverage carbonation and carbohydrate (CHO) content on fluid replacement following exercise/thermal dehydration. On four occasions separated by at least 7 days, eight healthy men cycled at 50% of VO2max in a hot environmental chamber (40 degrees C, 40% relative humidity) until a weight loss of 4.12 +/- 0.22% was attained. In the subsequent four hours, subjects ingested one of four solutions at 15-min intervals. The total volume ingested equalled that lost during dehydration. The solutions were administered in randomized order and varied in their carbonation and carbohydrate (CHO) content: 1. CK: carbonated 10% glucose-fructose solution, 2. NCK: non-carbonated 10% glucose-fructose solution, 3. CNK: carbonated non-caloric solution, and 4. NCNK: non-carbonated non-caloric solution. Plasma volume changes, total plasma protein concentration, plasma osmolality, and the plasma glucose concentration were determined at rest before and after dehydration, and at 30, 90, 150, and 240 min of recovery. Plasma volume changes and the plasma protein concentration were not different (p greater than 0.05) between treatments. Values for the plasma glucose concentration and the change in plasma osmolality were significantly elevated when CHO beverages were ingested when compared with non-CHO beverage ingestion. Five-min cycling bouts were performed at 70% of VO2max before and after dehydration and at 60, 120, 180, and 240 min of rehydration. The respiratory exchange ratio was elevated in both of the CHO treatments when compared with both of the non-CHO treatments at 60, 120, 180 and 240 min of rehydration.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of drink carbonation on the gastric emptying characteristics of water and flavored water

The intent of this study was to determine whether adding carbonation to either water or a low calorie sport drink would affect gastric emptying (GE). Fifteen subjects rode for 20 minutes on a cycle ergometer at 55% of max VO2. After 5 minutes of exercise, the subjects ingested 5.5 ml/kg body weight of a test solution: water (W), carbonated water (CW), and a low calorie sport drink in both a carbonated (C2C) and noncarbonated (2C) form. At the end of each ride, the stomach was emptied through gastric aspiration. The results indicate that carbonation has no effect on GE. However, the type of drink did have an effect on GE, as both 2C and C2C emptied from the stomach at a slower rate than either W or CW. Subjective ratings of gastrointestinal comfort were similar for both carbonated and noncarbonated forms, and at no time did the subjects report discomfort. The results were independent of the exercise challenge, as exercise intensity, heart rate, and ratings of perceived exertion did not differ between experimental trials. It is concluded that carbonation does not affect the GE characteristics of a drink taken during submaximal exercise, but the flavoring system of the low calorie beverage decreased the rate of GE by as much as 25% when compared to water.