Journal of Applied Physiology
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

J Appl Physiol 91: 2282-2288, 2001;
8750-7587/01 $5.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF) Free
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Saunders, P. U.
Right arrow Articles by Febbraio, M. A.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Saunders, P. U.
Right arrow Articles by Febbraio, M. A.
Vol. 91, Issue 5, 2282-2288, November 2001

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

P. U. Saunders1, M. J. Watt2, A. P. Garnham2, L. L. Spriet3, M. Hargreaves2, and M. A. Febbraio1

1 Department of Physiology, The University of Melbourne, Parkville, Victoria 3010; 2 School of Health Sciences, Deakin University, Burwood, Victoria 3125, Australia; and 3 Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada L8N 3Z5


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

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°C (Con) or 40°C (Heat) after 20 min of passive exposure to either environmental condition. Although muscle temperature (Tmu) was similar at rest when comparing trials, 20 min of passive exposure and 5 min of exercise increased (P < 0.05) Tmu in Heat compared with Con (37.5 ± 0.1 vs. 36.9 ± 0.1°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 Tmu, 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.

glycogen phosphorylase; pyruvate dehydrogenase; temperature


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

EXERCISE IN THE HEAT INCREASES intramuscular glycogen utilization (14-16), pyruvate (24) and lactate accumulation (14, 15, 24, 27, 40), and whole body carbohydrate (CHO) oxidation in the absence of any changes in glucose oxidation (18). In addition, contracting limb respiratory quotient (RQ) is higher when comparing exercise- and dehydration-induced hyperthermia with similar exercise in a euhydrated state (17). In contrast, others (26, 38, 39) have not observed increased glycogenolysis when comparing exercise in the heat with that in a cooler environment. The studies by Nielsen et al. (26) and Young et al. (39) had subjects commence exercise with different glycogen levels when comparing treatments, whereas the study by Yaspelkis et al. (38) used mildly acclimatized subjects and subjected them to a mild heat stress. Taken together, the literature suggests that both glycogen breakdown and CHO oxidation are augmented by heat stress during submaximal exercise, provided the magnitude of the heat stress is severe enough to markedly increase body temperature and subjects commence exercise with similar intramuscular glycogen levels (for review see Ref. 11).

The two key enzymes that regulate CHO metabolism are glycogen phosphorylase (Phos) and pyruvate dehydrogenase (PDH). Phos catalyzes the rate-limiting step of glycogenolysis, converting one glucosyl unit from glycogen and Pi to glucose 1-phosphate. PDH, a multienzyme complex that catalyzes the oxidative decarboxylation of pyruvate to acetyl-CoA, controls the amount of acetyl-CoA able to enter the tricarboxylic acid cycle (22). Both Phos and PDH exist in an active a form and a less active b form. Transformation between these two forms is controlled by kinases and phosphatases. Phosphorylase kinase catalyzes phosphorylation of Phos to its active a form, whereas dephosphorylation of Phos back to its less active b form is catalyzed by phosphorylase phosphatase (33). It has been demonstrated that increased levels of Ca2+ cause transformation (initial activation to a form) of Phos, setting a potential upper limit for glycogenolytic flux (4, 31). Posttransformation (regulation of activation once in a form) of Phos occurs via the availability of substrates (Pi and glycogen) and the presence of positive allosteric modulators such as free AMP (22, 28, 30). PDH kinase and PDH phosphatase regulate the activity state of PDH. PDH kinase phosphorylates and inactivates the complex, whereas PDH phosphatase dephosphorylates and activates the complex (22, 28, 29). The activity of PDH is determined by the amount of the complex in the a form (PDHa) (33).

Although an increase in glycogen use and CHO oxidation during exercise and heat stress is a often observed, the regulatory mechanisms for such an observation are poorly understood (11). It has been demonstrated that the augmented increase in contracting muscle temperature (12, 34) and circulating epinephrine (13, 36) contribute to the exaggerated CHO use observed during exercise in the heat. However, the effects of temperature and/or circulating epinephrine on either Phos or PDH activity have not been widely studied. Theoretically, a rise in temperature would increase the rate of key enzymatic reactions due to the Q10 effect (two- to threefold increase in reaction rates for every 10°C rise in temperature) (11); however, the effect of muscle temperature in vivo on either Phos or PDH activation has not been directly measured. Although our laboratory has recently demonstrated that increased circulating epinephrine increases PDH activation and glycogen use (36), epinephrine infusion does not always result in an increase in glycogenolysis (6, 23, 37). Apart from the direct effects of temperature and/or epinephrine on Phos and PDH activity, it is also important to note that heat stress increases creatine phosphate (PCr) degradation (15), which in turn increases free Pi, a substrate for Phos and pyruvate formation (24), which inhibits PDH kinase, allowing PDH to remain in the a form.

Studies that have demonstrated an effect of heat stress on CHO metabolism during exercise (14, 15, 17-19, 27) have been performed at a moderate intensity [60-70% maximal oxygen uptake (VO2 max)]. Importantly, at this exercise intensity, both Phos and PDH are fully activated within 1 min of the onset of exercise (22). Therefore, it is likely that, during exercise in the heat, enzymatic regulation of CHO metabolism would occur in the transition from rest to exercise. With this is mind, the aim of this study was to examine the effect of heat stress on the activation of Phos and PDH at the onset of moderate-intensity exercise. We hypothesized that exercise in the heat would increase the activation of both enzymes, thereby providing a mechanism for an increase in both glycogenolysis and CHO oxidation.


    METHODS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Subjects. Six healthy, active male subjects volunteered to participate in this study. Their age, body mass, VO2 max, and 70% VO2 max workload were 26.0 ± 5.0 (SD) yr, 75.0 ± 6.7 kg, 3.55 ± 0.39 l/min, and 180 ± 30 W, respectively. Subjects participated in some form of regular weekly aerobic exercise but were not highly endurance trained. Subjects were informed of all possible risks involved in participation, and written consent was received from all subjects before participation. The study was approved by the Human Research Ethics Committees of The University of Melbourne and Deakin University.

Experimental protocol. VO2 max was determined during an incremental cycling test to volitional exhaustion on an electrically braked cycle ergometer (Lode, Groningen, The Netherlands) at 20-22°C. At least 7 days after the VO2 max test, subjects performed one of two experimental trials. The experimental trials consisted of cycling for 5 min at a power output corresponding to 70% VO2 max at either 20°C (Con) or 40°C (Heat) (35% relative humidity for both trials). The order of the two trials was randomized between subjects, and trials were separated by at least 7 days. Subjects were asked to consume their normal diet and to abstain from exercise, caffeine, and alcohol for 24 h before the two experimental trials. To ensure subjects were in the same nutritional state for both trials, dietary records were kept for 24 h before their first trial and replicated during the same period before the second trial. To avoid the confounding effects of natural heat acclimatization, all trials were conducted in the winter months.

On the day of the experimental trials, subjects arrived at the laboratory in the morning after an overnight fast and rested in a supine position for 30 min. During this time, three incisions were made in the skin and fascia overlying the vastus lateralis muscle under local anesthetic. A catheter (20 gauge Terumo, Tokyo, Japan) was inserted into the forearm vein to allow multiple blood samples to be obtained. Subjects inserted a rectal thermistor probe (Monatherm Mallinckrodt Medical, St. Louis, MO) 10 cm beyond the anal sphincter, and a monitor (PE3000, Polar, Kempele, Finland) was fitted to each subject to allow for heart rate (HR) measures via telemetry. Twenty minutes before exercise, muscle temperature (Tmu) was recorded with the use of a needle thermistor probe (model YSI 525, Yellow Springs Instruments, Yellow Springs, OH) inserted to a depth of 4 cm into a biopsy incision site. A basal blood sample was obtained and HR and rectal temperature (Tre) were also recorded. At this point, subjects entered the chamber set to the predetermined environmental condition and remained seated for 20 min. The protocol was designed this way to mimic environmental heat stress conditions where subjects would commence exercise with slightly higher Tmu and/or Tre. After 20 min of passive exposure to each environmental condition, a muscle biopsy was obtained and rapidly frozen in liquid nitrogen, a blood sample was obtained, and Tre, Tmu, and HR were measured. Subjects then commenced cycling at the predetermined power output for 5 min. At 1 min of exercise, subjects ceased cycling, a muscle biopsy was obtained, and Tmu and Tre were measured. Subjects recommenced cycling within 45 s of stopping for the additional 4 min, at which time another muscle biopsy and blood sample were obtained and Tre, Tmu, and HR were measured. The time from the cessation of exercise to freezing of the biopsy was <20 s for both the 1- and 5-min sample. Subjects were not permitted to eat or consume any fluid until the cessation of data collection.

Blood and muscle analyses. Plasma epinephrine was determined with a single-isotope (3H) radioenzymatic assay system (Amersham kit TRK 995, Little Chalfont, UK) as previously described (14). A small piece of frozen muscle (10-20 mg) was removed under liquid N2 for the determination of PDH transformation state (PDHa), as described by Constantin-Teodosiu et al. (7) and modified by Putman et al. (30). The remainder of the muscle biopsy sample was freeze-dried; dissected of all visible blood, connective tissue, and fat; and powdered for subsequent analysis.

One aliquot (3-4 mg) of powdered freeze-dried muscle was extracted for determination of the percentage of Phos in the more active a form via a spectrophotometric assay (6). Briefly, the powdered freeze-dried muscle was homogenized at -25°C in 200 µl of buffer containing 100 mM Tris, 60% glycerol, 50 mM KF, and 10 mM EDTA (pH 7.0). Homogenates were then diluted with 800 µl of the above buffer without glycerol and homogenized further at 0°C. The homogenate was either added to a reagent in the presence (for total a + b activity) or absence (a form) of AMP, and the activities were measured following the production of glucose 1-phosphate spectrophotometrically at 30°C. The maximal velocity (Vmax) of total (a + b; Vmax tot) and a (Vmax a) forms as well as the percentage of Phos in the a form, which is represented as a mole fraction of total Phos (a + b), was calculated from the measured activities described by Chasiotis et al. (4). Phos a was not measured at rest, because an accurate resting Phos a value is obtainable only if the biopsy is held from liquid N2 freezing for >30 s (5), requiring a separate biopsy. Resting Phos a has been measured previously and is ~10% (5).

A second aliquot (1 mg) of freeze-dried muscle was extracted with 250 µl of 2 M HCl, incubated at 100°C for 2 h, and then neutralized with 750 µl of 0.667 M NaOH for subsequent determination of glycogen via an enzymatic analyses with fluorometric detection (29). A third aliquot (6 mg) of freeze-dried muscle was extracted with 750 µl of 0.5 M perchloric acid-1 mM EDTA and neutralized with 150 µl of 2.1 M KHCO3. This extract was used for determination of lactate, ATP, creatine, PCr, glucose 6-phosphate (G-6-P), and pyruvate by enzymatic spectrophotometric and fluorometric assays (1, 20). Acetyl-CoA was measured by the radiometric measure as previously described (3).

Calculations. Free ADP and free AMP concentrations were calculated by assuming equilibrium of the creatine kinase and adenylate kinase reactions (10). Free ADP was calculated using the measured ATP, PCr, and creatine content (10), and H+ concentration was estimated from the muscle lactate content according to the regression equation of Sahlin et al. (32). Free AMP was calculated from the ATP concentration and the estimated free ADP. Free Pi was calculated by adding the assumed resting free Pi of 10.8 mmol/kg dry muscle (8) to the values of change in PCr minus change in G-6-P between rest and each exercise time point. All metabolites and the activities of Phos and PDH were normalized to the highest total creatine measurement from the six samples obtained for each subject.

Statistical analysis. All data from Heat and Con were compared with the use of a two-way analysis of variance (time × treatment) with repeated measures. For significant (P < 0.05) effects, pairwise comparisons were made using a Student-Newman-Keuls post hoc analysis. All analyses were performed with Statistica (Stat Soft, Berkley, CA).


    RESULTS
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Physiological responses. Resting Tmu was similar when comparing Heat with Con, averaging 34.2 ± 0.3 and 34.4 ± 0.2°C, respectively. However, 20 min of passive exposure to heat increased (P < 0.05) Tmu in Heat compared with Con, and this difference was maintained throughout exercise such that values at 5 min of exercise were 37.5 ± 0.1 vs. 36.9 ± 0.1°C for Heat and Con, respectively (P < 0.05). Tre was not different at rest, preexercise, or at 1 and 5 min of exercise between the two trials. HR was not different at rest, preexercise, and 5 min of exercise (Table 1).

                              
View this table:
[in this window]
[in a new window]
  		Table 1.   Tmu, Tre, and HR at rest, preexercise, and at 1 and 5 min of cycling at 70% VO2max in control condition (20°C) or heat (40°C)

Regulatory enzymes. Phos Vmax tot was similar between trials at both 1 and 5 min of exercise, but there was a main time effect (P < 0.05) for Vmax a when comparing 5 min with 1 min (Table 2). There was no difference at 1 or 5 min of exercise when comparing Heat with Con for either Vmax tot or Vmax a. The resulting percentage of Phos in the more active a form was not different at 1 or 5 min of when comparing Heat with Con (Fig. 1). PDHa activity was similar between Heat and Con preexercise, averaging 0.87 ± 0.06 and 0.76 ± 0.12 mmol · kg wet muscle-1 · min-1, respectively (Fig. 1). There was no difference in PDHa activity between trials during exercise. PDHa activity was higher (P < 0.05) at 1 min of exercise compared with rest. Of note, at 5 min of exercise, PDHa activation was higher (P < 0.05) when compared with 1 min of exercise and with preexercise, indicating that the activity of this enzyme was not fully activated at this power output after 1 min of exercise.

                              
View this table:
[in this window]
[in a new window]
  		Table 2.   Skeletal muscle phosphorylase activities at 1 and 5 min of exercise at 70% VO2max in control condition (20°C) or heat (40°C)



View larger version (16K):
[in this window]
[in a new window]
  		Fig. 1.   Top: phosphorylase transformation, in % active (a) form, at 1 min () and 5 min () of cycling. Bottom: pyruvate dehydrogenase (PDH) activation in mmol · kg · wet muscle (ww) -1 · min-1 preexercise and at 1 and 5 min of cycling at 70% maximal O2 uptake at 20°C (; control) or 40°C (). Values are means ± SE; n = 6 subjects for phosphorylase transformation, and n = 5 subjects for PDH activation. # Main time effect, 5 min compared with 1 min, P < 0.05. *Difference from preexercise values, P < 0.05. dagger  Difference from 1 min, P < 0.05.

Muscle metabolites. Preexercise glycogen concentrations were not different when comparing Heat with Con, averaging 336 ± 43 and 325 ± 38 mmol/kg dry muscle, respectively (Table 3). At 5 min of exercise, glycogen concentrations had decreased (P < 0.05) in both trials, but there were no differences in the concentration of this metabolite when comparing trials. Preexercise concentrations of G-6-P, lactate, creatine, PCr, pyruvate, and ATP were all similar (Table 3). G-6-P, lactate, and creatine were all higher (P < 0.05) and PCr was lower (P < 0.05) at 1 and 5 min compared with preexercise concentrations. There were no differences when comparing Heat with Con for any of these metabolites. Pyruvate and ATP at 1 and 5 min were not different compared with preexercise concentrations, and there was no difference between trials. Preexercise acetyl-CoA concentrations were similar when comparing Heat with Con, and, although there was no significant increase in acetyl-CoA concentration at 1 min of exercise, concentrations of this metabolite were higher (P < 0.05) at 5 min of exercise compared with rest. No differences in acetyl-CoA were observed during exercise when comparing Heat with Con.

                              
View this table:
[in this window]
[in a new window]
  		Table 3.   Muscle metabolite concentrations preexercise and at 1 and 5 min of cycling at 70% VO2max in control condition (20°C) or heat (40°C)

Allosteric activators. Preexercise concentrations of free ADP, free AMP, and free Pi were similar between Heat and Con (Table 3). Concentrations of free ADP, free AMP, and free Pi were higher (P < 0.05) at 1 and 5 min of exercise compared with preexercise, but there was no difference in the concentration of any of these metabolites when comparing trials.

Epinephrine. Basal plasma epinephrine concentrations were similar when comparing Heat with Con, and, although concentrations of this hormone increased (P < 0.05) with 5 min of exercise, values remained similar when comparing trials (Fig. 2).


View larger version (13K):
[in this window]
[in a new window]
  		Fig. 2.   Plasma epinephrine concentration during 20 min of passive sitting and 5 min of cycling at 70% maximal O2 uptake in an environmental chamber at 20°C () or 40°C (). Values are means ± SE; n = 6 subjects. *Increase at 5 min compared with 0 and -20 min, P < 0.05.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

The results from this study demonstrated that, despite resulting in an increased Tmu, mild heat stress did not increase the plasma epinephrine response or the activation of Phos or PDHa during 5 min of exercise. Consequently, no significant differences in glycogen utilization or muscle lactate accumulation were observed during exercise.

In the present study, no differences in CHO metabolism was observed when comparing Heat with Con. An obvious difference between the results from the present study and those reported previously (14, 15, 17, 18, 27) is the exercise duration. Previous studies that have observed an effect of heat stress on CHO metabolism during exercise have sampled muscle before and after exercise lasting from 40 to 135 min. It is also important to note that, in a previous study where the level of heat stress imposed during the hot trial was similar to that imposed in the present study, no difference in the rate of muscle glycogenolysis was observed (38). In the present study, our rationale for examining muscle metabolism at the onset of exercise was based on the previous findings of Howlett et al. (22). In this study, both Phos and PDHa activity were fully activated by 1 min of exercise and remained elevated after 10 min of exercise at 65% VO2 max. On the basis of these data, we predicted that the effect of heat stress on these regulatory enzymes must occur within minutes of the onset of exercise. In contrast with the findings of Howlett et al., in the present study, PDHa activity was higher when comparing 5 min with 1 min. It is possible, therefore, that had exercise progressed further than 5 min and the effect of heat stress was exacerbated, we may have seen differences in PDHa and CHO oxidation when comparing Heat with Con. This suggestion is purely speculative because the previous studies that have observed differences in CHO metabolism when comparing exercise in a cool environment with that in a hot environment had not examined the effect of heat stress on CHO metabolism and enzyme activities early in exercise.

Of note, the subjects in the present study, although having participated in some form of regular weekly aerobic exercise, were not as highly endurance trained as those subjects recruited in previous studies. This difference may be important when one examines the regulation of Phos and glycogen use. Although Ca2+ is responsible for the initial activation of Phos to its a form, setting a potential upper limit for glycogenolytic flux (4, 31), posttransformation regulation of activation once in a form occurs via the presence of positive allosteric modulators such as AMP (22, 28, 30). It has been demonstrated that free AMP is reduced with endurance training (5), and it is therefore possible that, in the present study, exercise at 70% VO2 max was of such an intensity that it resulted in marked elevations in positive allosteric modulators, rendering the environmental condition unimportant. Indeed, the calculated concentrations of the allosteric activators of Phos were higher in this, compared with our laboratory's previous, study in endurance-trained men (15).

Although our laboratory previously observed a marked increase in CHO oxidation when comparing exercise in the heat with that in a cooler environment (18), we did not observe any difference in PDHa in the present study. It is important to note, however, that all previous studies that have observed differences in CHO oxidation (14, 15, 18) or contracting leg RQ (17) during exercise and heat stress have also observed marked differences in circulating epinephrine. Interestingly, when comparing dehydration-induced heat stress with a euhydrated trial, Gonzalez-Alonso et al. (17) observed differences in leg RQ only late in exercise in the dehydration trial (after 120 min), when plasma epinephrine concentrations were also different. In contrast, early during exercise (within 120 min), when the degree of heat stress was not sufficient to result in differences in the sympathoadrenal response, no differences in CHO oxidation were observed when comparing trials. In the present study, 5 min of exercise did not result in differences in epinephrine, glycogen use, or PDHa activity. Taken together, these data suggest that PDHa may be acutely sensitive to alterations in epinephrine. In support of this, our laboratory recently demonstrated that epinephrine infusion increases PDHa activity and glycogenolysis after 1 and 20 min of exercise at ~60% VO2 max (36). Whether the stimulus for the increase in PDH activity is mediated by a direct effect of epinephrine on PDH kinase or PDH phosphatase is not known. Studies in the isolated rat heart demonstrate that epinephrine increases PDH activity (21, 25) and intramitochondrial Ca2+ concentration (9). In addition, beta 2-adrenergic stimulation of mouse skeletal muscle increases tetanic myoplasmic Ca2+ concentration by enhancing sarcoplasmic Ca2+ release (2). Taken together, these data suggest that epinephrine may increase PDH activation via a Ca2+-stimulated activation of PDH phosphatase. We suggest, therefore, that, during exercise in the heat, the activation of PDHa is only augmented when the increase in epinephrine concentrations is exacerbated, ultimately leading to differences in CHO oxidation. In the present study, this did not occur.

Our laboratory has previously shown that manipulating Tmu in the absence of any other physiological changes increases glycogen use after 20 min of exercise at 70% VO2 max (34). Our laboratory has also shown that elevated temperature decreased sarcoplasmic reticulum Ca2+ reuptake in vitro (35), which may result in an increase in cytosolic Ca2+, which can activate Phos (33). In the present study, despite a higher Tmu at 0 min, due to the passive exposure, and at 1 min and 5 min of exercise in Heat compared with Con, neither Phos nor glycogenolysis was different. It is important to note, however, that in the present study the difference in Tmu at 5 min was 0.6°C, whereas, in our laboratory's previous experiment where Tmu was manipulated, the difference at the onset of exercise was 6.9°C (34). This indicates that the magnitude of difference in Tmu in the present study was not sufficient to result in changes in CHO metabolism.

In summary, our data demonstrate that, whereas heat stress resulted in a small difference in contracting Tmu during 5 min of exercise, it was not sufficient to result in major physiological changes such as an altered sympathoadrenal response. As a consequence, the activity of the key regulatory enzymes for CHO metabolism or glycogen use were not different at these times. We hypothesize, therefore, that as exercise in the heat progresses and differences in circulating epinephrine become greater, the activity of CHO regulatory enzymes is augmented, leading to differences in CHO use. Further research examining PDH and Phos activity progressively throughout prolonged exercise in the heat is required to test this hypothesis.


    ACKNOWLEDGEMENTS

The authors acknowledge the technical assistance of Drs. Rod Snow and Damien Angus.


    FOOTNOTES

This study was supported by The Australian Research Council.

Address for reprint requests and other correspondence: M. Febbraio, Dept. of Physiology, The Univ. of Melbourne, Parkville, Victoria 3010, Australia (E-mail: m.febbraio{at}physiology.unimelb.edu.au).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Received 23 April 2001; accepted in final form 5 July 2001.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

1.   Bergmeyer, HU. Methods of Enzymatic Analysis. New York: Academic, 1974.

2.   Cairns, SP, Westerblad H, and Allen DG. Changes of tension and [Ca2+]i during beta -adrenoceptor activation of single, intact fibres from mouse skeletal muscle. Pflügers Arch 425: 150-155, 1993[ISI][Medline].

3.   Cederblad, G, Carlin JI, Constatin-Teodosiu D, Harper P, and Hultman E. Radioisotopic assays of CoASH and carnitine and their acetylated forms in human skeletal muscle. Anal Biochem 195: 274-278, 1990.

4.   Chasiotis, D, Hultman E, and Sahlin K. Acidotic depression of cyclic AMP accumulation and phosphorylase b to a transformation in skeletal muscle of man. J Physiol (Lond) 335: 197-204, 1982[Abstract/Free Full Text].

5.   Chesley, A, Heigenhauser GJF, and Spriet LL. Regulation of muscle glycogen phosphorylase activity following short-term endurance training. Am J Physiol Endocrinol Metab 270: E328-E333, 1996[Abstract/Free Full Text].

6.   Chesley, D, Hultman E, and Spriet LL. Effects of epinephrine infusion on muscle glycogenolysis during intense aerobic exercise. Am J Physiol Endocrinol Metab 268: E127-E134, 1995[Abstract/Free Full Text].

7.   Constantin-Teodosiu, D, Cederblad G, and Hultman E. A sensitive radioisotopic assay of pyruvate dehydrogenase complex in human muscle tissue. Anal Biochem 198: 347-351, 1991[ISI][Medline].

8.   Constantin-Teodosiu, D, Simpson EJ, and Greenhaff PL. Effects of pyruvate, dichloroacetate and adrenaline infusion on pyruvate dehydrogenase complex activation and tricarboxylic acid cycle intermediates in human skeletal muscle (Abstract). J Physiol (Lond) 506P: 102P, 1998.

9.   Crompton, M, Kessar P, and Al-Nasser I. Adrenergic mediated activation of the cardiac mitochondrial Ca2+ uniporter and its control of intramitochondrial Ca2+ in vivo. Biochem J 216: 333-342, 1983[ISI][Medline].

10.   Dudley, GA, Tullson PC, and Terjung RL. Influence of mitochondrial content on the sensitivity of respiratory control. J Biol Chem 262: 9109-9114, 1987[Abstract/Free Full Text].

11.   Febbraio, MA. Alterations in energy metabolism during exercise and heat stress. Sports Med 31: 47-59, 2001[ISI][Medline].

12.   Febbraio, MA, Carey MF, Snow RJ, Stathis CG, and Hargreaves M. Influence of elevated muscle temperature on metabolism during intense, dynamic exercise. Am J Physiol Regulatory Integrative Comp Physiol 271: R1251-R1255, 1996[Abstract/Free Full Text].

13.   Febbraio, MA, Lambert DL, Starkie RL, Proietto J, and Hargreaves M. Effect of epinephrine on muscle glycogenolysis during exercise in trained men. J Appl Physiol 84: 465-470, 1998[Abstract/Free Full Text].

14.   Febbraio, MA, Snow RJ, Hargreaves M, Stathis CG, Martin IK, and Carey MF. Muscle metabolism during exercise and heat stress in trained men: effect of acclimation. J Appl Physiol 76: 589-597, 1994[Abstract/Free Full Text].

15.   Febbraio, MA, Snow RJ, Stathis CG, Hargreaves M, and Carey MF. Effect of heat stress on muscle energy metabolism during exercise. J Appl Physiol 77: 2827-2831, 1994[Abstract/Free Full Text].

16.   Fink, WJ, Costill DL, and Van Handel PJ. Leg muscle metabolism during exercise in the heat and cold. Eur J Appl Physiol 34: 183-190, 1975.

17.   Gonzalez-Alonso, J, Calbert JAL, and Nielsen B. Metabolic and thermodynamic responses to dehydration-induced reductions in muscle blood flow in exercising humans. J Physiol (Lond) 520: 577-589, 1999[Abstract/Free Full Text].

18.   Hargreaves, M, Angus D, Howlett K, Marmy-Conus N, and Febbraio M. Effect of heat stress on glucose kinetics during exercise. J Appl Physiol 81: 1594-1597, 1996[Abstract/Free Full Text].

19.   Hargreaves, M, Dillo P, Angus D, and Febbraio M. Effect of fluid ingestion on muscle metabolism during prolonged exercise. J Appl Physiol 80: 363-366, 1996[Abstract/Free Full Text].

20.   Harris, RC, Hultman E, and Norsdesjö LO. Glycogen, glycolytic intermediates and high energy phosphates determined in muscle biopsy samples of musculus quadriceps femoris of man at rest. Scand J Clin Lab Invest 33: 109-119, 1974[ISI][Medline].

21.   Hiraoka, T, DeBuysere M, and Olson MS. Studies of the effects of beta-adrenergic agonists on the regulation of pyruvate dehydrogenase in the perfused rat heart. J Biol Chem 255: 7604-7609, 1980[Free Full Text].

22.   Howlett, RA, Parolin ML, Dyck DJ, Hultman E, Jones NL, Heigenhauser GJF, and Spriet LL. Regulation of skeletal muscle glycogen phosphorylase and PDH at varying exercise outputs. Am J Physiol Regulatory Integrative Comp Physiol 275: R418-R425, 1998[Abstract/Free Full Text].

23.   Kjær, M, Howlett K, Langfort J, Zimmerman-Belsing T, Lorentsen J, Bulow J, Ihlemann J, Feldt-Rasmussen U, and Galbo H. Adrenaline and glycogenolysis in skeletal muscle during exercise: a study in adrenalectomised humans. J Physiol (Lond) 528: 371-378, 2000[Abstract/Free Full Text].

24.   Kozlowski, S, Brezinska Z, Kruk B, Kaciuba-Uscilko H, Greenleaf JE, and Nazar K. Exercise hyperthermia as a factor limiting physical performance: temperature effect on muscle metabolism. J Appl Physiol 59: 766-773, 1985[Abstract/Free Full Text].

25.   McCormack, JG, and Denton RM. The activation of pyruvate dehydrogenase in the perfused rat heart by adrenaline and other inotropic agents. Biochem J 194: 639-643, 1981[ISI][Medline].

26.   Nielsen, B, Savard G, Richter EA, Hargreaves M, and Saltin B. Muscle blood flow and muscle metabolism during exercise and heat stress. J Appl Physiol 69: 1040-1046, 1990[Abstract/Free Full Text].

27.   Parkin, JM, Carey MF, Zhao S, and Febbraio MA. Effect of ambient temperature on human skeletal muscle metabolism during fatiguing submaximal exercise. J Appl Physiol 86: 902-908, 1999[Abstract/Free Full Text].

28.   Parolin, ML, Chesley A, Matsos MP, Spriet LL, Jones NL, and Heigenhauser GJF Regulation of skeletal muscle glycogen phosphorylase and PDH during maximal intermittent exercise. Am J Physiol Endocrinol Metab 277: E890-E900, 1999[Abstract/Free Full Text].

29.   Passonneau, JV, and Lauderdale VR. A comparison of three methods of glycogen measurement in tissues. Anal Biochem 60: 405-412, 1974[ISI][Medline].

30.   Putman, CT, Spriet LL, Hultman E, Lindinger MI, Lands LC, McKelvie RS, Cederblad G, Jones NL, and Heigenhauser GJF Pyruvate dehydrogenase activity and acetyl group accumulation during exercise after different diets. Am J Physiol Endocrinol Metab 265: E752-E760, 1993[Abstract/Free Full Text].

31.   Richter, EA, Ruderman NB, Gavras H, Belur ER, and Galbo H. Muscle glycogenolysis during exercise: dual control by epinephrine and contractions. Am J Physiol Endocrinol Metab 242: E25-E32, 1982[Abstract/Free Full Text].

32.   Sahlin, K, Harris RC, Nylind B, and Hultman E. Lactate content and pH in muscle samples obtained after dynamic exercise. Pflügers Arch 367: 143-149, 1976[ISI][Medline].

33.   Spriet, LL, and Howlett RA. Metabolic control of energy production during physical activity. In: Perspectives in Exercise Science and Sports Medicine. The Metabolic Bases of Performance in Sport and Exercise. Carmel, IN: Cooper, 1999, vol. 12, p. 1-52.

34.   Starkie, RL, Hargreaves M, Lambert DL, Proietto J, and Febbraio MA. Effect of temperature on muscle metabolism during submaximal exercise in humans. Exp Physiol 84: 775-784, 1999[Abstract].

35.   Warmington, SA, Hargreaves M, and Williams DA. A method for measuring sarcoplasmic reticulum calcium uptake in the skeletal muscle using Fura-2. Cell Calcium 20: 73-82, 1996[ISI][Medline].

36.   Watt, MJ, Howlett KF, Febbraio MA, Spriet LL, and Hargreaves M. Adrenaline increases skeletal muscle glycogenolysis, PDH activation and carbohydrate oxidation during moderate intensity exercise in humans. J Physiol (Lond) 534: 269-278, 2001[Abstract/Free Full Text].

37.   Wendling, PS, Peters SJ, Heigenhauser GJF, and Spriet LL. Epinephrine infusion does not enhance net muscle glycogenolysis during prolonged aerobic exercise. Can J Appl Physiol 21: 271-284, 1996[Medline].

38.   Yaspelkis, BB, III, Scroop GC, Wilmore KM, and Ivy JL. Carbohydrate metabolism during exercise in hot and thermoneutral environments. Int J Sports Med 14: 13-19, 1993[ISI][Medline].

39.   Young, AJ, Sawka MN, Levine L, Burgoon PW, Latzka WL, Gonzalez RR, and Pandolf KB. Metabolic and thermal adaptations from endurance training in hot and cold water. J Appl Physiol 78: 793-801, 1995[Abstract/Free Full Text].

40.   Young, AJ, Sawka MN, Levine L, Cadarette BS, and Pandolf KB. Skeletal muscle metabolism during exercise is influenced by heat acclimation. J Appl Physiol 59: 1929-1935, 1985[Abstract/Free Full Text].

J APPL PHYSIOL 91(5):2282-2288
8750-7587/01 $5.00 Copyright © 2001 the American Physiological Society



This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF) Free
Right arrow Submit a response
Right arrow Alert me when this article is cited
Right arrow Alert me when eLetters are posted
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Saunders, P. U.
Right arrow Articles by Febbraio, M. A.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Saunders, P. U.
Right arrow Articles by Febbraio, M. A.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
Visit Other APS Journals Online