Biomedical subjects

M Hargreaves

Publications and source records attributed to M Hargreaves.

At least 19 recordsLinked to original sources

PubMed2001

Adrenaline increases skeletal muscle glycogenolysis, pyruvate dehydrogenase activation and carbohydrate oxidation during moderate exercise in humans.

1. To evaluate the role of adrenaline in regulating carbohydrate metabolism during moderate exercise, 10 moderately trained men completed two 20 min exercise bouts at 58 +/- 2 % peak pulmonary oxygen uptake (V(O2,peak)). On one occasion saline was infused (CON), and on the other adrenaline was infused intravenously for 5 min prior to and throughout exercise (ADR). Glucose kinetics were measured by a primed, continuous infusion of 6,6-[(2)H]glucose and muscle samples were obtained prior to and at 1 and 20 min of exercise. 2. The infusion of adrenaline elevated (P < 0.01) plasma adrenaline concentrations at rest (pre-infusion, 0.28 +/- 0.09; post-infusion, 1.70 +/- 0.45 nmol l(-1); means +/- S.E.M.) and this effect was maintained throughout exercise. Total carbohydrate oxidation increased by 18 % and this effect was due to greater skeletal muscle glycogenolysis (P < 0.05) and pyruvate dehydrogenase (PDH) activation (P < 0.05, treatment effect). Glucose rate of appearance was not different between trials, but the infusion of adrenaline decreased (P < 0.05, treatment effect) skeletal muscle glucose uptake in ADR. 3. During exercise muscle glucose 6-phosphate (G-6-P) (P = 0.055, treatment effect) and lactate (P < 0.05) were elevated in ADR compared with CON and no changes were observed for pyruvate, creatine, phosphocreatine, ATP and the calculated free concentrations of ADP and AMP. 4. The data demonstrate that elevated plasma adrenaline levels during moderate exercise in untrained men increase skeletal muscle glycogen breakdown and PDH activation, which results in greater carbohydrate oxidation. The greater muscle glycogenolysis appears to be due to increased glycogen phosphorylase transformation whilst the increased PDH activity cannot be readily explained. Finally, the decreased glucose uptake observed during exercise in ADR is likely to be due to the increased intracellular G-6-P and a subsequent decrease in glucose phosphorylation.

Adult↗

PubMed2001

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

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

Adult↗

PubMed2001

Pre-exercise nutritional strategies: effects on metabolism and performance.

The goals of pre-exercise nutritional strategies are to optimise the availability of carbohydrate (CHO) and fluid. Ingestion of CHO 3-4 hr prior to exercise can increase liver and muscle glycogen stores and has been associated with enhanced endurance exercise performance. The metabolic effects of CHO ingestion persist for at least 6 hr. Although an increase in plasma insulin following CHO ingestion in the hour prior to exercise inhibits lipolysis and liver glucose output, and can lead to transient hypoglycemia during subsequent exercise, there is no convincing evidence that this is always associated with impaired exercise performance. Having said that, individual experience should inform individual practice. Interventions to increase plasma FFA availability prior to exercise have been shown to reduce CHO utilisation during exercise, but do not appear to have major ergogenic benefits. It is more difficult to hyperhydrate prior to exercise and although there has been interest in glycerol ingestion, to date research results have been equivocal. At the very least, athletes should ensure euhydration prior to exercise.

Dietary Carbohydrates↗

PubMed2001

Effect of prior exercise on glucose metabolism in trained men.

Several studies have demonstrated that oral glucose tolerance is impaired in the immediate postexercise period. A double-tracer technique was used to examine glucose kinetics during a 2-h oral glucose (75 g) tolerance test (OGTT) 30 min after exercise (Ex, 55 min at 71 +/- 2% of peak O(2) uptake) and 24 h after exercise (Rest) in endurance-trained men. The area under the plasma glucose curve was 71% greater in Ex than in Rest (P = 0.01). The higher glucose response occurred even though whole body rate of glucose disappearance was 24% higher after exercise (P = 0.04, main effect). Whole body rate of glucose appearance was 25% higher after exercise (P = 0.03, main effect). There were no differences in total (2 h) endogenous glucose appearance (R(a)E) or the magnitude of suppression of R(a)E, although R(a)E was higher from 15 to 30 min during the OGTT in Ex. However, the cumulative appearance of oral glucose was 30% higher in Ex (P = 0.03, main effect). There were no differences in glucose clearance rate or plasma insulin responses between the two conditions. These results suggest that adaptations in splanchnic tissues by prior exercise facilitate greater glucose output from the splanchnic region after glucose ingestion, resulting in a greater glycemic response and, consequently, a greater rate of whole body glucose uptake.

Adult↗

PubMed2001

Differential effects of exercise on insulin-signaling gene expression in human skeletal muscle.

Skeletal muscle insulin sensitivity is enhanced after acute exercise and short-term endurance training. We investigated the impact of exercise on the gene expression of key insulin-signaling proteins in humans. Seven untrained subjects (4 women and 3 men) completed 9 days of cycling at 63 +/- 2% of peak O(2) uptake for 60 min/day. Muscle biopsies were taken before, immediately after, and 3 h after the exercise bouts (on days 1 and 9). The gene expression of insulin receptor substrate-2 and the p85 alpha subunit of phosphatidylinositol 3-kinase was significantly higher 3 h after a single exercise bout, although short-term training ameliorated this effect. Gene expression of insulin receptor and insulin receptor substrate-1 was not significantly altered at any time point. These results suggest that exercise may have a transitory impact on the expression of insulin receptor substrate-2 and phosphatidylinositol 3-kinase; however, the predominant actions of exercise on insulin sensitivity appear not to reside in the transcriptional activation of the genes encoding major insulin-signaling proteins.

Exercise↗

PubMed2001

Effect of carbohydrate ingestion on glucose kinetics during exercise in the heat.

Six endurance-trained men [peak oxygen uptake (V(O(2))) = 4.58 +/- 0.50 (SE) l/min] completed 60 min of exercise at a workload requiring 68 +/- 2% peak V(O(2)) in an environmental chamber maintained at 35 degrees C (<50% relative humidity) on two occasions, separated by at least 1 wk. Subjects ingested either a 6% glucose solution containing 1 microCi [3-(3)H]glucose/g glucose (CHO trial) or a sweet placebo (Con trial) during the trials. Rates of hepatic glucose production [HGP = glucose rate of appearance (R(a)) in Con trial] and glucose disappearance (R(d)), were measured using a primed, continuous infusion of [6,6-(2)H]glucose, corrected for gut-derived glucose (gut R(a)) in the CHO trial. No differences in heart rate, V(O(2)), respiratory exchange ratio, or rectal temperature were observed between trials. Plasma glucose concentrations were similar at rest but increased (P < 0.05) to a greater extent in the CHO trial compared with the Con trial. This was due to the absorption of ingested glucose in the CHO trial, because gut R(a) after 30 and 50 min (16 +/- 5 micromol. kg(-1). min(-1)) was higher (P < 0.05) compared with rest, whereas HGP during exercise was not different between trials. Glucose R(d) was higher (P < 0.05) in the CHO trial after 30 and 50 min (48.0 +/- 6.3 vs 34.6 +/- 3.8 micromol. kg(-1). min(-1), CHO vs. Con, respectively). These results indicate that ingestion of carbohydrate, at a rate of approximately 1.0 g/min, increases glucose R(d) but does not blunt the rise in HGP during exercise in the heat.

Adult↗

PubMed2001

No effect of mild heat stress on the regulation of carbohydrate metabolism at the onset of exercise.

To investigate the influence of heat stress on the regulation of skeletal muscle carbohydrate metabolism, six active, but not specifically trained, men performed 5 min of cycling at a power output eliciting 70% maximal O2 uptake in either 20 degrees C (Con) or 40 degrees C (Heat) after 20 min of passive exposure to either environmental condition. Although muscle temperature (T(mu)) was similar at rest when comparing trials, 20 min of passive exposure and 5 min of exercise increased (P < 0.05) T(mu) in Heat compared with Con (37.5 +/- 0.1 vs. 36.9 +/- 0.1 degrees C at 5 min for Heat and Con, respectively). Rectal temperature and plasma epinephrine were not different at rest, preexercise, or 5 min of exercise between trials. Although intramuscular glycogen phosphorylase and pyruvate dehydrogenase activity increased (P < 0.05) at the onset of exercise, there were no differences in the activities of these regulatory enzymes when comparing Heat with Con. Accordingly, glycogen use in the first 5 min of exercise was not different when comparing Heat with Con. Similarly, no differences in intramuscular concentrations of glucose 6-phosphate, lactate, pyruvate, acetyl-CoA, creatine, phosphocreatine, or ATP were observed at any time point when comparing Heat with Con. These results demonstrate that, whereas mild heat stress results in a small difference in contracting T(mu), it does not alter the activities of the key regulatory enzymes for carbohydrate metabolism or glycogen use at the onset of exercise, when plasma epinephrine levels are unaltered.

Adult↗

PubMed2001

Exercise increases serum Hsp72 in humans.

Recent evidence suggests that heat shock proteins (Hsps) may have an important systemic role as a signal to activate the immune system. Since acute exercise is known to induce Hsp72 (the inducible form of the 70-kDa family of Hsp) in a variety of tissues including contracting skeletal muscle, we hypothesized that such exercise would result in the release of Hsp72 from stressed cells into the blood. Six humans (5 males, 1 female) ran on a treadmill for 60 minutes at a workload corresponding to 70% of their peak oxygen consumption. Blood was sampled from a forearm vein at rest (R), 30 minutes during exercise, immediately postexercise (60 minutes), and 2, 8, and 24 hours after exercise. These samples were analyzed for serum Hsp72 protein. In addition, plasma creatine kinase (CK) was measured at these time points as a crude marker of muscle damage. With the exception of the sample collected at 30 minutes, muscle biopsies (n = 5 males) were also obtained from the vastus lateralis at the time of blood sampling and analyzed for Hsp72 gene and protein expression. Serum Hsp72 protein increased from rest, both during and after exercise (0.13 0.10 vs 0.87+/-0.24 and 1.02+/-0.41 ng/mL at rest, 30 and 60 minutes, respectively, P < 0.05, mean SE). In addition, plasma CK was elevated (P < 0.05) 8 hours postexercise. Skeletal muscle Hsp72 mRNA expression increased 6.5-fold (P < 0.05) from rest 2 hours postexercise, and although there was a tendency for Hsp72 protein expression to be elevated 2 and 8 hours following exercise compared with rest, results were not statistically significant. The increase in serum Hsp72 preceded any increase in Hsp72 gene or protein expression in contracting muscle, suggesting that Hsp72 was released from other tissues or organs. This study is the first to demonstrate that acute exercise can increase Hsp72 in the peripheral circulation, suggesting that during stress these proteins may indeed have a systemic role.

Adult↗

PubMed2000

Effect of prolonged, submaximal exercise and carbohydrate ingestion on monocyte intracellular cytokine production in humans.

The present study was undertaken to examine the effect of exercise and carbohydrate (CHO) ingestion on intracellular monocyte cytokine production. Subjects performed 2 h of cycling at 70 % peak pulmonary O2 uptake (VO2,peak) while ingesting either an 8 % CHO beverage or a sweet placebo. Whole blood was incubated with (stimulated) or without (spontaneous) lipopolysaccharide (LPS) and surface stained for monocyte surface antigens. The cells were permeabilised, stained for intracellular cytokines and analysed using flow cytometry. Exercise had no effect on the number of monocytes spontaneously producing cytokines, but the number of stimulated IL-1alpha-, TNF-alpha- and IL-6-positive monocytes were elevated (P < 0.01) immediately post-exercise and 2 h post-exercise. These stimulated cells produced less (P < 0.05) TNF-alpha immediately post-exercise, and less (P < 0.05) TNF-alpha and IL-1alpha 2 h post-exercise. There was a small, but significant increase (P < 0.05) in the plasma IL-6 concentration immediately post-exercise. Exercise resulted in an elevation (P < 0.01) in the plasma adrenaline concentration in the placebo trial, and ingestion of CHO attenuated this increase. CHO ingestion had no effect on monocyte cytokine production, plasma IL-6 or circulating leukocyte numbers. These data suggest that circulating monocytes are not the origin of increased levels of plasma IL-6 during exercise: prolonged cycling exercise increased the number of monocytes producing cytokines upon stimulation, but these cells produced less cytokines post-exercise. In addition, attenuation of plasma adrenaline levels had no effect on plasma IL-6 or monocyte cytokine production.

Adult↗

PubMed2000

Skeletal muscle metabolism during exercise in humans.

1. Contracting skeletal muscle is able to use a number of intra- and extramuscular substrates to generate ATP during exercise. These include creatine phosphate (CP), muscle glycogen, blood-borne glucose, lactate and free fatty acids (FFA), derived from either adipose tissue or intramuscular triglyceride stores. 2. During high-intensity short-duration exercise, CP degradation and the breakdown of muscle glycogen to lactate are the major energy yielding pathways, although oxidative metabolism can make a significant contribution. The 'anaerobic' substrates are also important fuels during the transition from rest to steady state exercise. 3. The oxidative metabolism of carbohydrate and lipid supplies most, if not all, of the ATP during prolonged submaximal exercise. Muscle glycogen, blood glucose and FFA are the key fuels. The relative importance of the various substrates for exercise metabolism is primarily determined by exercise intensity and duration, although training status, dietary manipulation and environmental factors can modify the metabolic response to exercise.

Anaerobiosis↗

PubMed2000

Effect of acute plasma volume expansion on thermoregulation and exercise performance in the heat.

PURPOSE: We investigated the effects of acute plasma volume expansion on exercise performance in the heat. METHODS: Six moderately trained men cycled for 40 min at 64 +/- 2% peak pulmonary oxygen uptake (VO2peak) followed by an individual performance time trial, where subjects completed a set amount of work (267 +/- 15 kJ) in as little time as possible. Exercise trials were performed at 35 degrees C with a relative humidity of 40%. Subjects performed two exercise trials: one after 13.1 +/- 1% acute plasma volume expansion (PVE), which was achieved by the intravenous infusion of 8 mL x kg(-1) body weight of Hemaccel (35 g x L(-1) polygeline, 145 mmol x L(-1) Na+, and 145 mmol x L(-1) Cl-) and the other without prior treatment (CON). RESULTS: Core temperature, skin blood flow, and heart rate progressively increased (P < 0.05) during exercise, but no differences were observed between trials. Plasma glucose and lactate were similar at rest and during exercise, as was VO2 during exercise. Exercise performance was not influenced by plasma volume expansion (CON 17.5 +/- 0.4 min and PVE 17.1 +/- 0.2 min). CONCLUSION: These data suggest that, in moderately trained men, plasma volume expansion alone does not enhance thermoregulatory function and exercise performance during moderate intensity exercise in the heat.

Acclimatization↗

PubMed2000

Effect of carbohydrate or carbohydrate plus medium-chain triglyceride ingestion on cycling time trial performance.

This study examined the effectiveness of ingesting a carbohydrate or carbohydrate + medium-chain triglycerides (MCT) on metabolism and cycling performance. Eight endurance-trained men [peak O(2) uptake = 4.71 +/- 0.09 (SE) l/min] completed 35 kJ/kg as quickly as possible [time trial (TT)] while consuming 250 ml/15 min of either a 6% (wt/vol) carbohydrate solution (C), a 6% carbohydrate + 4.2% MCT solution (C+M), or a sweet placebo (P). Time to complete the set amount of work was reduced in both C and C+M compared with P by 7 and 5%, respectively (C: 166 +/- 7 min; C+M: 169 +/- 7 min; P: 178 +/- 11 min; P < 0.01). Plasma glucose concentration was maintained at or above resting values throughout both C and C+M trials but decreased (P < 0.05) below resting values in P at the completion of the TT. The estimated rate of carbohydrate oxidation was not different during the first 90 min of exercise but thereafter was reduced (P < 0.05) in P and was maintained in both C and C+M. These data demonstrate that carbohydrate ingestion during exercise improves 100-km TT performance compared with a sweet placebo, but the addition of MCT does not provide any further performance enhancement.

Adult↗

PubMed2000

Effects of exercise on GLUT-4 and glycogenin gene expression in human skeletal muscle.

To investigate the effect of exercise on GLUT-4, hexokinase, and glycogenin gene expression in human skeletal muscle, 10 untrained subjects (6 women and 4 men, 21.4 +/- 1.2 yr, 66.3 +/- 5.0 kg, peak oxygen consumption = 2.30 +/- 0.19 l/min) exercised for 60 min on a cycle ergometer at a power output requiring 73 +/- 4% peak oxygen consumption. Muscle samples were obtained by needle biopsy before, immediately after, and 3 h after exercise. Gene expression was quantified, relative to 29S ribosomal protein cDNA, by RT-PCR. GLUT-4 gene expression was increased immediately after exercise (1.7 +/- 0.4 vs. 0.9 +/- 0.3 arbitrary units; P < 0.05) and remained significantly higher than baseline 3 h after the end of exercise (2. 2 +/- 0.4 vs. 0.9 +/- 0.3 arbitrary units; P < 0.05). Hexokinase II gene expression was significantly higher than the resting value 3 h after the end of exercise (2.9 +/- 0.4 vs. 1.3 +/- 0.3 arbitrary units; P < 0.05). Exercise increased glycogenin mRNA more than twofold (2.8 +/- 0.6 vs. 1.2 +/- 0.2 arbitrary units; P < 0.05) 3 h after the end of exercise. For the first time, we report that a single bout of exercise is sufficient to cause upregulation of GLUT-4 and glycogenin gene expression in human skeletal muscle. Whether these increases, together with the associated increase in hexokinase II gene expression, lead to increased expression of these key proteins in skeletal muscle and contribute to the enhanced skeletal muscle glucose uptake, glycogen synthesis, and insulin action observed following exercise remains to be determined.

Adult↗

PubMed2000

Effect of carbohydrate ingestion on ammonia metabolism during exercise in humans.

The present study was undertaken to examine the effect of carbohydrate ingestion on plasma and muscle ammonia (NH(3) denotes ammonia and ammonium) accumulation during prolonged exercise. Eleven trained men exercised for 2 h at 65% peak pulmonary oxygen consumption while ingesting either 250 ml of an 8% carbohydrate-electrolyte solution every 15 min (CHO) or an equal volume of a sweet placebo. Blood glucose and plasma insulin levels during exercise were higher in CHO, but plasma hypoxanthine was lower after 120 min (1.7 +/- 0.3 vs. 2.6 +/- 0.1 micromol/l; P < 0. 05). Plasma NH(3) levels were similar at rest and after 30 min of exercise in both trials but were lower after 60, 90, and 120 min of exercise in CHO (62 +/- 9 vs. 76 +/- 9 micromol/l; P < 0.05). Muscle NH(3) levels were similar at rest and after 30 min of exercise but were lower after 120 min of exercise in CHO (1.51 +/- 0.21 vs. 2.07 +/- 0.23 mmol/kg dry muscle; P < 0.05; n = 5). These data are best explained by carbohydrate ingestion reducing muscle NH(3) production from amino acid degradation, although a small reduction in net AMP catabolism within the contracting muscle may also make a minor contribution to the lower tissue NH(3) levels.

Adenine Nucleotides↗

PubMed2000

Effect of fat adaptation and carbohydrate restoration on metabolism and performance during prolonged cycling.

For 5 days, eight well-trained cyclists consumed a random order of a high-carbohydrate (CHO) diet (9.6 g. kg(-1). day(-1) CHO, 0.7 g. kg(-1). day(-1) fat; HCHO) or an isoenergetic high-fat diet (2.4 g. kg(-1). day(-1) CHO, 4 g. kg(-1). day(-1) fat; Fat-adapt) while undertaking supervised training. On day 6, subjects ingested high CHO and rested before performance testing on day 7 [2 h cycling at 70% maximal O(2) consumption (SS) + 7 kJ/kg time trial (TT)]. With Fat-adapt, 5 days of high-fat diet reduced respiratory exchange ratio (RER) during cycling at 70% maximal O(2) consumption; this was partially restored by 1 day of high CHO [0.90 +/- 0.01 vs. 0.82 +/- 0.01 (P < 0.05) vs. 0.87 +/- 0.01 (P < 0.05), for day 1, day 6, and day 7, respectively]. Corresponding RER values on HCHO trial were [0. 91 +/- 0.01 vs. 0.88 +/- 0.01 (P < 0.05) vs. 0.93 +/- 0.01 (P < 0.05)]. During SS, estimated fat oxidation increased [94 +/- 6 vs. 61 +/- 5 g (P < 0.05)], whereas CHO oxidation decreased [271 +/- 16 vs. 342 +/- 14 g (P < 0.05)] for Fat-adapt compared with HCHO. Tracer-derived estimates of plasma glucose uptake revealed no differences between treatments, suggesting muscle glycogen sparing accounted for reduced CHO oxidation. Direct assessment of muscle glycogen utilization showed a similar order of sparing (260 +/- 26 vs. 360 +/- 43 mmol/kg dry wt; P = 0.06). TT performance was 30.73 +/- 1.12 vs. 34.17 +/- 2.48 min for Fat-adapt and HCHO (P = 0.21). These data show significant metabolic adaptations with a brief period of high-fat intake, which persist even after restoration of CHO availability. However, there was no evidence of a clear benefit of fat adaptation to cycling performance.

Adaptation, Physiological↗

PubMed2000

Fiber type-specific expression of GLUT4 in human skeletal muscle: influence of exercise training.

The fiber type-specific expression of skeletal muscle GLUT4 and the effect of 2 weeks of low-intensity training were investigated in 8 young untrained male subjects. Single muscle fibers were dissected from a vastus lateralis biopsy sample. Based on myosin heavy chain (MHC) expression, fibers were pooled into 3 groups (MHC I, MHC IIA, and MHC IIX), and the GLUT4 content of 15-40 pooled fibers was determined using SDS-PAGE and immunological detection. The GLUT4 content in pooled muscle fibers expressing MHC I was approximately 20% higher (P < 0.05) than that in muscle fibers expressing MHC IIA or MHC IIX. No difference in GLUT4 could be detected between fibers expressing MHC IIA or MHC IIX. Two weeks of exercise training increased (P < 0.05) the peak power output of the knee extensors by 13%, the maximal activities of citrate synthase and 3-hydroxyacyl-CoA dehydrogenase by 21 and 18%, respectively, and the GLUT4 protein content by 26% in a muscle homogenate. Furthermore, a 23% increase (P < 0.05) in GLUT4 was seen in fibers expressing the MHC I isoform after exercise training for 2 weeks. No change was seen in fibers expressing MHC IIA or MHC IIX. In conclusion, our data directly demonstrate that GLUT4 is expressed in a fiber type-specific manner in human skeletal muscle, although fiber type differences are relatively small. In addition, low-intensity exercise training recruiting primarily fibers expressing MHC I increased GLUT4 content in these fibers but not in fibers expressing MHC IIA or MHC IIX, indicating that GLUT4 protein content is related more to activity level of the fiber than to its fiber type, which is defined by expression of contractile protein.

Adult↗

PubMed1999

Glucose production during strenuous exercise in humans: role of epinephrine.

The increase in hepatic glucose production (HGP) that occurs during intense exercise is accompanied by a simultaneous increase in epinephrine, which suggests that epinephrine may be important in regulating HGP. To further investigate this, six trained men were studied twice. The first trial [control (Con)] consisted of 20 min of cycling at 40 +/- 1% peak oxygen uptake (VO2 peak) followed by 20 min at 80 +/- 2% VO2 peak. During the second trial [epinephrine (Epi)], subjects exercised for 40 min at 41 +/- 2% VO2 peak. Epinephrine was infused during the latter 20 min of exercise and resulted in plasma levels similar to those measured during intense exercise in Con. Glucose kinetics were measured using a primed, continuous infusion of [3-3H]glucose. HGP was similar at rest (Con, 11.0 +/- 0.5 and Epi, 11.1 +/- 0.5 micromol. kg-1. min-1). In Con, HGP increased (P < 0.05) during exercise to 41.0 +/- 5.2 micromol. kg-1. min-1 at 40 min. In Epi, HGP was similar to Con during the first 20 min of exercise. Epinephrine infusion increased (P < 0.05) HGP to 24.0 +/- 2.5 micromol. kg-1. min-1 at 40 min, although this was less (P < 0.05) than the value in Con. The results suggest that epinephrine can increase HGP during exercise in trained men; however, epinephrine during intense exercise cannot fully account for the rise in HGP. Other glucoregulatory factors must contribute to the increase in HGP during intense exercise.

Adult↗

PubMed1999

Effect of short-term training on mitochondrial ATP production rate in human skeletal muscle.

Seven untrained volunteers [3 men, 4 women, 20.1 +/- 2.0 (SD) yr, 66. 0 +/- 11.0 kg, 171 +/- 13 cm] participated in a 10-day cycle exercise training program. Resting muscle samples were obtained from vastus lateralis before and after 5 and 10 days of training. Mitochondrial ATP production rate (MAPR) was assayed in isolated mitochondria by using a bioluminescence technique and referenced to the activity of glutamate dehydrogenase in the muscle sample. MAPR increased 136 and 161% after 10 days of training for the mitochondrial substrate combinations pyruvate + palmitoyl-L-carnitine + alpha-ketoglutarate + malate and palmitoyl-L-carnitine + malate, respectively. Total muscle glutamate dehydrogenase and citrate synthase activity increased 53 and 16%, respectively, after 5 days but did not significantly increase further after 10 days. The results from the present study indicate that MAPR, measured by using the substrate combinations pyruvate + palmitoyl-L-carnitine + alpha-ketoglutarate + malate and palmitoyl-L-carnitine + malate, can rapidly increase in response to endurance training.

Adenosine Triphosphate↗