Exercise, Muscle and Metabolism Unit, School of Health Sciences, Deakin University, Burwood, Victoria 3125, Australia
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ABSTRACT |
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This study examined the effect of
epinephrine on glucose disposal during moderate exercise when
glycogenolytic flux was limited by low preexercise skeletal muscle
glycogen availability. Six male subjects cycled for 40 min at 59 ± 1% peak pulmonary O2 uptake on two occasions, either
without (CON) or with (EPI) epinephrine infusion starting after
20 min of exercise. On the day before each experimental trial, subjects
completed fatiguing exercise and then maintained a low carbohydrate
diet to lower muscle glycogen. Muscle samples were obtained after 20 and 40 min of exercise, and glucose kinetics were measured using
[6,6-2H]glucose. Exercise increased plasma epinephrine
above resting concentrations in both trials, and plasma epinephrine was
higher (P < 0.05) during the final 20 min in EPI
compared with CON. Muscle glycogen levels were low after 20 min of
exercise (CON, 117 ± 25; EPI, 122 ± 20 mmol/kg dry matter),
and net muscle glycogen breakdown and muscle glucose 6-phosphate levels
during the subsequent 20 min of exercise were unaffected by epinephrine
infusion. Plasma glucose increased with epinephrine infusion (i.e.,
20-40 min), and this was due to a decrease in glucose disposal
(Rd) (40 min: CON, 33.8 ± 3; EPI, 20.9 ± 4.9 µmol · kg1 · min
1,
P < 0.05), because the exercise-induced rise in
glucose rate of appearance was similar in the trials. These results
show that glucose Rd during exercise is reduced by elevated
plasma epinephrine, even when muscle glycogen availability and
utilization are low. This suggests that the effect of epinephrine does
not appear to be mediated by increased glucose 6-phosphate, secondary
to enhanced muscle glycogenolysis, but may be linked to a direct effect
of epinephrine on sarcolemmal glucose transport.
glucose transport; exercise
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INTRODUCTION |
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ALTHOUGH PREVIOUS STUDIES have demonstrated a reduction in glucose disposal during exercise when plasma epinephrine levels are elevated (12, 14, 31), the mechanism or mechanisms underlying the decreased glucose disposal have yet to be fully elucidated. The most widely held view is that glucose disposal is reduced because of inhibition of glucose phosphorylation by elevated glucose 6-phosphate (G-6-P), secondary to greater flux through glycogenolysis. Indeed, we recently demonstrated that decreased glucose uptake during moderate-intensity cycle exercise with epinephrine infusion was associated with increased skeletal muscle glycogenolysis and G-6-P accumulation (31).
Another possibility is that epinephrine directly affects
GLUT4-mediated glucose transport across the sarcolemma,
although this is equivocal. GLUT4 is phosphorylated via -adrenergic
pathways (13), and this may inhibit GLUT4 transporter
activity, as demonstrated in rat adipocytes (16) and
adipocyte plasma membrane vesicles (24). Bonen et al.
(2) reported GLUT4 translocation to the sarcolemma but
reduced glucose transport, implying decreased GLUT4 intrinsic activity,
in rat skeletal muscle with epinephrine administration. Although
-adrenergic receptor stimulation results in phosphorylation of GLUT4
in rat skeletal muscle (19), no inhibitory effect of epinephrine on insulin (1, 19)- and contraction
(1)-stimulated 3-O-methyl-D-glucose
transport has been reported.
In the present study, we attempted to examine the effect of elevated plasma epinephrine on glucose disposal during exercise with reduced muscle glycogen availability. Under such conditions, exercise results in less muscle glycogenolysis (9) and G-6-P accumulation (29). Thus we hypothesized that if the epinephrine-induced reduction in glucose disposal was primarily mediated via inhibition of glucose phosphorylation by G-6-P, secondary to increased glycogenolysis, elevated epinephrine would have no, or a reduced, effect on glucose disposal in the glycogen-depleted state.
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METHODS |
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Subjects.
Six recreationally active males (23 ± 1 yr; 74 ± 3 kg)
volunteered to participate in this study, which was approved by the Deakin University Human Research Ethics Committee. All experimental procedures and possible risks were explained to subjects, verbally and
in writing, and subjects provided their written consent. Peak pulmonary
oxygen uptake (O2 peak) was determined
during an incremental cycling test to volitional exhaustion on an
electromagnetically braked cycle ergometer (Lode Instruments,
Groningen, The Netherlands) and averaged 60.5 ± 2.2 ml · kg
1 · min
1.
Experimental design.
At least 2 days after the determination of
O2 peak, subjects performed fatiguing
cycle exercise late in the afternoon. Exercise consisted of constant
load exercise at power outputs that varied between 60 and 80%
O2 peak, interspersed with a series of
three 1-min sprints at 100%
O2 peak with 2-min recovery periods. Subjects ceased cycle exercise after 2 h and began arm-cranking exercise, which consisted of four 5-min intervals at a power output of 35-50 W. The arm exercise was
employed to provide an alternate site for glucose disposal after the
completion of the fatiguing exercise to minimize glycogen synthesis in
leg skeletal muscle. After arm exercise, subjects recommenced cycle exercise, which consisted of 1-min efforts at 100%
O2 peak with 2-min recovery periods.
Fatigue was defined as the point when subjects were unable to maintain
their cadence >60 rpm. Upon completion of exercise, subjects were
provided with a standardized meal (4.5 MJ, 73% fat, 9% carbohydrate,
18% protein) and were instructed to abstain from caffeine, alcohol,
tobacco, and exercise.
Analytic techniques.
Plasma glucose (G) and lactate were measured using an automated
analyzer (EML105, Radiometer, Copenhagen, Denmark). Plasma FFA were
analyzed using an enzymatic, colorimetric method (Wako NEFA-C test kit,
Wako Chemicals, Richmond, VA). Plasma catecholamines were
determined using a single-isotope radioenzymatic method (TRK 995, Amersham, Buckinghamshire, UK), and plasma insulin (Phadeseph, Pharmacia & Upjohn, Uppsala, Sweden) was measured by radioimmunoassay. Plasma [2H]glucose enrichment (E) was measured as
previously described (31). The rates of glucose appearance
(glucose Ra) and disappearance (glucose Rd)
were calculated using a modified one-pool non-steady-state model
(30), with the assumption of a pool fraction of 0.65 and an estimate of the glucose space as 25% of body mass, to estimate the
volume of distribution (V)
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Statistical analysis. Results from the two trials were compared using a two-way ANOVA for repeated measures, with statistical significance defined as P < 0.05. Where appropriate, specific differences were located by the Newman-Keuls post hoc test. All data are presented as means ± SE (n = 6).
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RESULTS |
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Plasma epinephrine levels were similar between trials before and
throughout the initial 20 min of exercise. The infusion of epinephrine
at 20 min increased (P < 0.05) the plasma epinephrine concentration at 30 and 40 min compared with CON (Fig.
1). Plasma norepinephrine increased
(P < 0.05) during exercise in both trials, and
epinephrine infusion further enhanced (P < 0.05) this
response (see Table 2).
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During exercise, O2 increased
(P < 0.05) with time, and there was no difference
between trials (Table 1). Respiratory
exchange ratio was not different between trials, and estimated
carbohydrate oxidation was not different between trials during the
first 20 min of exercise. Epinephrine infusion increased
(P < 0.05) carbohydrate oxidation in EPI at 30 min;
however, no difference between trials was observed at 40 min (Table 1).
Heart rate was elevated (P < 0.05) in EPI at 30 and 40 min of exercise compared with CON (Table 1).
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Plasma lactate increased (P < 0.05) at the onset of
exercise in both trials, and the infusion of epinephrine resulted in
higher lactate levels during exercise (Table
2). Plasma FFA were elevated (P < 0.05) late in exercise but were not different
between trials (Table 2). Plasma insulin decreased
(P < 0.05) during exercise and was unaffected by
epinephrine infusion (Table 2).
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The combination of fatiguing exercise and a low carbohydrate diet on
the preceding day resulted in low muscle glycogen levels after the
initial 20 min of exercise that were not different between trials.
Although exercise from 20 to 40 min decreased (P < 0.05, time effect) muscle glycogen further, very little net
glycogenolysis occurred in each trial, and no difference between trials
was evident (Table 3). G-6-P
levels during exercise were not different between trials. Muscle
lactate, CP, creatine, and ATP were not different with either treatment
or time (Table 3).
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Plasma glucose was higher (P < 0.05) in EPI compared
with CON at 35 and 40 min of exercise (Fig.
2). Glucose Ra increased (P < 0.05, time effect) in both trials, but no
differences between trials were observed (Fig. 2). Thus the increased
plasma glucose was due to a lower glucose Rd in EPI.
Glucose Rd increased (P < 0.05) from rest
during both trials, and although glucose Rd continued to
rise in CON, the infusion of epinephrine decreased (P < 0.05) Rd in EPI after 30 and 40 min of exercise (Fig.
2).
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DISCUSSION |
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The present study has demonstrated that the decrease in glucose disposal observed with elevated epinephrine during moderate exercise is unlikely to be due to inhibition of glucose phosphorylation, secondary to enhanced muscle glycogenolysis and increased muscle G-6-P levels. Rather, our results suggest, albeit indirectly, that the effect of epinephrine may be mediated via effects on sarcolemmal glucose transport.
Previous studies have demonstrated an inhibitory effect of epinephrine
on glucose disposal during exercise. The infusion of epinephrine to
physiological levels decreased glucose uptake during exercise in
adrenalectomized, epinephrine-deficient humans (12) and in
men who underwent anesthesia of the celiac ganglion to block endogenous
epinephrine production (17). Furthermore, the present
investigation and a recent study from our laboratory (31) demonstrated a decrease in glucose Rd during moderate cycle
exercise when plasma epinephrine was elevated. In addition, in studies employing -adrenergic blockade, an increase in glucose uptake across
a range of exercise intensities has been observed (27, 28). Taken together, these findings support the hypothesis that epinephrine decreases glucose disposal during exercise.
The mechanism or mechanims underlying the decreased glucose Rd during exercise are possibly related to effects on sarcolemmal glucose transport, glucose phosphorylation, and/or glucose delivery. It has been suggested previously that the increase in glycogenolysis associated with elevated plasma epinephrine (7, 14, 25, 31) results in an accumulation of cytosolic G-6-P, which inhibits hexokinase (21) and glucose phosphorylation. Indeed, we have previously demonstrated a twofold increase in glycogen utilization, greater accumulation of G-6-P (6.5-8 mmol/kg dry matter), and reduced whole body glucose Rd with epinephrine infusion during moderate exercise (31). In the present study, the combination of fatiguing exercise and a low carbohydrate diet was successful in reducing muscle glycogen levels and flux through glycogenolysis and resulted in G-6-P levels during exercise that were only 25-30% of those we observed in our previous study (31), even with epinephrine infusion. Despite the marked differences in muscle G-6-P levels, elevation of plasma epinephrine to similar levels reduced glucose Rd to the same extent in both studies. Thus the inhibitory effect of epinephrine on glucose Rd appears to be independent of the muscle G-6-P level; this lends support to the possibility of a direct effect of epinephrine on sarcolemmal glucose transport.
It has been suggested previously that -adrenergic stimulation
inhibits glucose transport via a cAMP-dependent pathway
(18), and this was supported by the findings that
isoproterenol caused phosphorylation of GLUT4 (13) and
markedly decreased insulin-stimulated glucose uptake in rat adipocytes
(13, 16).
-Adrenergic stimulation was also shown to
phosphorylate GLUT4 in rat skeletal muscle (19); however,
no change in insulin (1, 3, 19)- or contraction (1)-stimulated glucose transport was observed with
epinephrine infusion in skeletal muscle. In contrast, Bonen et al.
(2) reported decreased GLUT4 intrinsic activity in a rat
skeletal muscle with epinephrine administration. Moreover, epinephrine reduced insulin-stimulated glucose transport in a dose-dependent manner
in rat hindlimb preparations, despite unaltered GLUT4 translocation (8).
Another possibility that cannot be discounted is a vasoconstrictor effect of increased epinephrine and norepinephrine (Table 2), which could potentially reduce glucose delivery to contracting skeletal muscle and limit glucose Rd. We have no data in the present study to support or refute such a mechanism; however, it should be noted that any reduction in skeletal muscle blood flow on glucose Rd may have been partly offset by the higher plasma glucose concentration after epinephrine infusion (Fig. 2).
The observation that glycogen utilization was not different between trials was somewhat unexpected, as epinephrine has been shown to increase muscle glycogen utilization during exercise (7, 14, 25, 26, 31). Furthermore, the magnitude of epinephrine-stimulated glycogen breakdown is thought to be independent of muscle glycogen concentration in rat epitrochlearis (15), which suggests that glycogenolysis should have been greater in EPI. Although we detected no significant change in net glycogen utilization between trials (19 ± 9 and 25 ± 8 mmol/kg dry matter for CON and EPI, respectively), we cannot rule out the possibility that greater muscle glycogenolysis may have occurred at the onset of epinephrine infusion, but this small change was not detected over the 20-min sampling period. The absence of an effect of epinephrine on glycogenolysis during exercise in the glycogen-depleted state may be due to the release of phosphorylase from the glycogen-protein-sarcoplasmic reticulum complex (6). The release of phosphorylase would cause an uncoupling of phosphorylase, phosphorylase kinase, and Ca2+, all of which are essential for the activation of the kinase (6). Although epinephrine stimulation results in phosphorylation and activation of the kinase, any effect of epinephrine would be inhibited by the uncoupling of phosphorylase from the kinase and Ca2+. Partial support for this hypothesis was reported in glycogen-depleted rats, where the ability of epinephrine to activate phosphorylase was blunted by exhaustive exercise (4). Although the underlying mechanism is unknown, our results suggest that, in contrast to situations in which preexercise glycogen concentration is normal (7, 31), epinephrine may not enhance glycogen breakdown when preexercise glycogen concentrations are low.
Finally, it is worth noting the effect of muscle glycogen on glucose Rd during exercise. In the present study, in which preexercise muscle glycogen levels were ~150 mmol/kg dry matter, glucose clearance over the first 20 min of exercise in CON averaged 113 ± 16 ml/kg. This was higher (P < 0.05) than the value of 71 ± 10 ml/kg for the corresponding trial in our previous study (31), when preexercise muscle glycogen was ~500 mmol/kg dry matter. Subject characteristics and absolute and relative exercise intensities were similar, whereas plasma glucose and insulin levels were lower (P < 0.05) in the present study. The higher glucose clearance with low muscle glycogen is consistent with previous results obtained in contracting, perfused rat muscle (5, 11) and is thought to be due to effects of muscle glycogen availability on both sarcolemmal glucose transport and intracellular glucose metabolism.
In summary, the results of the present study have shown that glucose uptake is reduced during exercise with elevated plasma epinephrine and that this was not associated with elevated G-6-P, secondary to enhanced muscle glycogenolysis. Rather, the inhibitory effect of epinephrine on glucose disposal during moderate exercise may be due to reduced sarcolemmal glucose transport; this warrants further investigation.
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ACKNOWLEDGEMENTS |
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We acknowledge the excellent medical assistance of Dr. Andrew Garnham, School of Health Sciences, Deakin University, and we thank Dr. Mark Febbraio, Department of Physiology, The University of Melbourne, for use of laboratory facilities in the plasma catecholamine analyses, and Dom Caridi, Department of Chemistry and Biology, Victoria University, for technical assistance in measuring plasma [2H]glucose enrichment.
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FOOTNOTES |
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This study was supported by the Australian Research Council.
Address for correspondence: M. Hargreaves, School of Health Sciences, Deakin Univ., Burwood, Victoria 3125, Australia (E-mail: mharg{at}deakin.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.
May 21, 2002;10.1152/ajpendo.00098.2002
Received 4 March 2002; accepted in final form 13 May 2002.
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