Department of Kinesiology, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
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ABSTRACT |
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To investigate the
hypothesis that training-induced increases in muscle mitochondrial
potential are not obligatory to metabolic adaptations observed during
submaximal exercise, regardless of peak aerobic power
(O2 peak)
of the subjects, a short-term training study was utilized. Two groups
of untrained male subjects (n = 7/group), one with a high (HI) and the other with a low (LO)
O2 peak (means ± SE; 51.4 ± 0.90 vs. 41.0 ± 1.3 ml · kg
1 · min
1;P < 0.05), cycled for 2 h/day at 66-69% of
O2 peak for 6 days. Muscle tissue was extracted from vastus lateralis at 0, 3, and 30 min
of standardized cycle exercise before training (0 days) and after 3 and
6 days of training and analyzed for metabolic and enzymatic changes.
During exercise after 3 days of training in the combined HI + LO group,
higher (P < 0.05) concentrations (mmol/kg dry wt) of phosphocreatine (40.5 ± 3.4 vs. 52.2 ± 4.2) and lower (P < 0.05) concentrations
of Pi (61.5 ± 4.4 vs. 53.3 ± 4.4), inosine monophosphate (0.520 ± 0.19 vs. 0.151 ± 0.05), and lactate (37.9 ± 5.5 vs. 22.8 ± 4.8) were
observed. These changes were also accompanied by reduced levels of
calculated free ADP, AMP, and Pi.
All adaptations were fully expressed by 3 min of exercise and by 3 days
of training and were independent of initial
O2 peak levels.
Moreover, maximal activity of citrate synthase, a measure of
mitochondrial capacity, was only increased with 6 days of training
(5.71 ± 0.29 vs. 7.18 ± 0.37 mol · kg
protein
1 · h
1;
P < 0.05). These results demonstrate
that metabolic adaptations to prolonged exercise occur within the first
3 days of training and during the non-steady-state period. Moreover,
neither time course nor magnitude of metabolic adaptations appears to
depend on increases in mitochondrial potential or on initial aerobic power.
enzymatic, adaptation, cycling, oxidative phosphorylation
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INTRODUCTION |
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IN PREVIOUS STUDIES, we have been able to demonstrate that extensive adaptations, both cardiovascular and metabolic, occur within the first several days of prolonged, submaximal cycle training in untrained males (13). In skeletal muscle, as an example, a standardized protocol of moderate exercise elicits less of a reduction in cellular energy state and glycogen and less of an increase in lactate after the training (13). All of these adaptations are at least qualitatively similar to what has been reported for training extending over several weeks and months (22).
What is potentially unique about these short-term training studies is not that extensive adaptations occur but that they appear to occur in the absence of changes in mitochondrial enzyme potential (13). Increase in the potential for oxidative phosphorylation has long been accepted as fundamental, at least for the muscle metabolic adaptations that occur (11, 22).
However, recently, our findings have been challenged. Spina et al. (38)
have reported that a training protocol similar to one that we have
employed, namely 2 h of exercise per day at between 60 and 70% of peak
aerobic power
(O2 peak), resulted in
an increase in the maximal activities of a number of mitochondrial
enzymes within 7 days. The increase in mitochondrial enzyme potential was also accompanied by a decrease in respiratory exchange ratio and
blood lactate concentration during submaximal exercise
(38). Spina et al. have assumed that the muscle metabolic
adaptations, although not measured, would be consistent with what we
have reported for short-term training. In a subsequent
study, Chesley et al. (6), with a training program involving a similar
daily training stimulus and for a similar number of days, reported
adaptations in both mitochondrial enzyme potential and in the metabolic
response to submaximal exercise. The results of these two studies
support the hypothesis that metabolic adaptations are mechanistically linked to increases in the potential for oxidative phosphorylation.
Such a conclusion may be premature because neither study examined
time-dependent adaptations, namely the adaptations resulting during the
7-day period of training. Conceivably, the metabolic adaptations could
have preceded the increases in mitochondrial enzyme potential during
this period. Indeed, we have shown that changes in muscle metabolic
behavior during submaximal exercise may occur as early as 3-4 days
of training (14). In addition, it is not at all clear what the
importance of the initial aerobic power
(O2 peak) is, in the
time course of adaptations, even when training at the same percentage
of
O2 peak. Although a prerequisite for entry into our studies was that the subjects not be
regularly active, the range of
O2 peak has been
substantial (5, 14, 15, 17). In spite of the fact that the initial
O2 peak for
the males in the study by Spina et al. (38) cannot be determined
because of a failure to separate the males from the females, the
average
O2 peak was
substantially lower than that observed in several of our studies (5,
15, 17). It is possible that the subjects with the lower
O2 peak might
demonstrate both a larger and earlier onset adaptation in
O2 peak and
mitochondrial potential.
In this study, our objective was to examine the time-dependent
adaptations to short-term training as modified by initial
O2 peak. We have
hypothesized that the group with the lower
O2 peak would demonstrate the more rapid adaptation in the metabolic response to
exercise and that this adaptation would occur in the absence of
increases in mitochondrial enzymatic potential. In contrast, in the
group with the higher
O2 peak, both the
mitochondrial and metabolic adaptations would occur coincidentally and
not until later in the training.
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METHODS |
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Subjects. Male subjects were recruited
for the study and divided into two groups based on the
O2 peak that they
were able to attain during a progressive cycle task to fatigue.
Subjects in the low
O2 peak group (LO)
achieved a
O2 peak
(means ± SE in
ml · kg
1 · min
1)
of 41.0 ± 1.3, whereas subjects in the high
O2 peak group (HI)
achieved a
O2 peak of
51.4 ± 0.90. For the LO group (n = 7), mean age, height, and weight were 21.6 ± 1.0 yr, 176 ± 3.0 cm, and 72.3 ± 3.9 kg, respectively. Comparable values for the HI
group (n = 7) were 21.0 ± 0.63 yr,
175 ± 2.2 cm, and 71.6 ± 1.9 kg. All subjects were healthy (as
determined by questionnaire), and none were engaged in physical
exercise on a regular basis. Subdivision into HI and LO groups was
accomplished on the basis of testing a large group of subjects who
displayed widely varying
O2 peak values.
Written consent was obtained from all volunteers as required after
approval of the study by the Office of Human Research.
Experimental design. To investigate
the time-dependent adaptations to short-term training as modified by
pretraining O2 peak, subjects trained for 6 consecutive days. To determine the
training-induced effects, the responses to a standardized cycle
ergometer task were examined before the training (0 days), at 3 days,
and after 6 days of training. The standardized ergometer task was
performed for 30 min at power outputs designed to elicit the same
relative percentage of
O2 peak
based on the pretraining measures of
O2 peak. For the LO
group, the power output (W) was 127 ± 7.3 W and for the HI group
was 159 ± 4.5 W. These work levels elicited 69 and 66% of the
O2 peak in the LO and
HI group, respectively. Differences in relative work intensities
between groups were not significant (P > 0.05).
The training was conducted under supervision at the same absolute intensity as used in the cycle test throughout the period of training. On each training day, exercise was performed for a maximum of 2 h without interruption or until fatigue. Where fatigue occurred, the subjects were allowed to stop exercise and take a short break. This pattern continued until 2 h of exercise had been performed. In general, breaks were necessary during the first few days of training. Training was conducted in normal room conditions (24-26°C dry bulb temperature and 50-60% relative humidity), and water was provided ab libitum.
Before the exercise tests, the subjects were prepared for blood and
muscle sampling and cardiovascular measurements. For blood sampling, a
small Teflon catheter with a three-way stopcock was placed in a dorsal
vein of a prewarmed hand and kept warm throughout the exercise (and
blood sampling) with a heating pad. For each exercise test, three sites
on the vastus lateralis were prepared for biopsies (2). Biopsies were
performed before the exercise and at 3 and 30 min of the exercise. Two
tissue samples were extracted from each site at each time point. The
first sample, obtained as rapidly as possible, particularly in the case
of exercise, was immediately plunged into liquid
N2, stored at 80°C, and
subsequently analyzed for substrates and metabolite concentrations. The
tissue obtained before exercise from the second biopsy was treated the same way as the first and used for analyses of enzymatic activities. The order of sampling from the nine sampling sites was selected randomly and distributed over both legs.
The initial cycle test was conducted 1-2 days before training, the test at 3 days was performed ~24-30 h after the third training session, and the test at 6 days was performed ~24-36 h after training. Testing was conducted in the same environmental conditions used for the training. At least 4 h before the exercise tests, subjects consumed a liquid supplement consisting of one can of Ensure (1.045 kJ; 14.8% protein, 31.5% fat, and 53.7% carbohydrate; Ross Laboratories, Montreal, PQ, Canada). All subjects were requested to refrain from any other supplement, including coffee before the testing. During the experimental period, the subjects were requested to maintain their normal dietary practices.
Measurements of
O2 peak were obtained
at least 1 wk before the start of training and between 36 and 48 h
after the final submaximal exercise test.
Analytical procedures.
O2 peak was
determined as described previously with an electronic cycle (Quinton
870) and an open-current gas collection system (23). The values used
for peak were the highest values obtained over a 30-s period. The
submaximal tests were conducted with the same ergometer and collection
system. During the submaximal exercise, the gas collection measurements were performed over a 3- to 4-min period immediately before the biopsies, which were performed at 3 and 30 min.
For analyses of the muscle metabolic changes, the tissue was freeze-dried and analyzed with fluorometric techniques (18, 19). Each muscle sample was analyzed for the concentration of high-energy phosphate compounds [ATP and phosphocreatine (PCr)] and their metabolites [inorganic phosphate (Pi) and creatine (Cr)], glycogen, and a range of glycolytic intermediates. The adenine nucleotides (ATP, ADP, AMP) and inosine monophosphate (IMP) were measured with ion-pair reversed-phase high-performance liquid chromatography techniques (24). The free concentrations of ADP (ADPf) and AMP (AMPf) were estimated on the basis of the near-equilibrium nature of the Cr phosphokinase reaction and the adenylate kinase reaction, respectively, as previously described (15). Free Pi concentration during exercise was estimated as the difference between resting and exercise PCr concentrations plus the resting free Pi concentration. Resting concentration of free Pi was assumed to be 2.5 or 10.8 mmol/kg dry wt (3). The total Cr content (TCr), obtained for each individual by averaging the results from all nine biopsies, was used to adjust individual metabolite concentrations to minimize the effect of blood and connective tissue. As in previous studies, we have found that training has no systematic effect on TCr (14, 17). Muscle glucose, pyruvate, and lactate were not corrected for extracellular water content because of the uncertainty of water content in this space. It should also be emphasized that the calculation of ADPf and AMPf involves making a number of assumptions. These have been discussed in an earlier paper (16).
Maximal activities were determined on a number of enzymes designed to
represent glycolysis [phosphofructokinase (PFK)], glucose phosphorylation [hexokinase (HEX)], and the citric acid
cycle [citrate synthase (CS) and succinic dehydrogenase
(SDH)]. Enzymatic activities were performed from muscles hand
homogenized in a phosphate buffer (pH 7.4) containing 5 mM
-mercaptoethanol, 0.5 mM EDTA, and 0.02% BSA. Homogenates were
diluted in 20 mM imidazole buffer with 0.2% BSA. With the exception of
PFK and SDH, in which the measurements were performed on fresh
homogenates, all other enzymes were assayed with frozen homogenates
(20). The enzyme assays were performed at 24-25°C after the
procedures of Henriksson et al. (20) as modified in our laboratory
(16). Protein was determined with the Lowry technique as modified by
Schacterle and Pollock (36).
For both the metabolites and enzyme assays, samples were analyzed in duplicate. On a given analytical day, all samples for two subjects were analyzed, one from each group.
Statistical procedures. The data were
analyzed with both two-way and three-way ANOVA procedures for repeated
measures. Two-way ANOVA procedures were employed to examine the effect
of training on
O2 peak and related
parameters and enzymatic activities. Three-way ANOVA was used for the
variables measured during submaximal exercise. Training state (0, 3, and 6 days), initial
O2 peak levels (LO,
HI), and exercise time (0, 15, 30 min) represented the independent
variables. Where no group differences were found (HI vs. LO), two-way
ANOVA procedures were used on the combined HI-LO group. Where
significant differences were found, Newman-Keuls procedures were used
to locate differences between specific means. Significance was set at
the 0.05 level.
In general, where no group differences were found, only the results of the combined group have been presented. The combined group results are presented in graphic form.
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RESULTS |
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Maximal exercise. The
O2 peak obtained during
a progressive cycle exercise to fatigue was 41.0 ± 1.3 and 51.3 ± 0.90 ml · kg
1 · min
1
for the LO and HI groups, respectively (Table
1). Six days of training was without effect
in increasing
O2 peak
in either group.
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Submaximal exercise. No effect of
either exercise duration or training duration was found for
O2 (Table
2). For both the LO and HI groups,
steady-state
O2 was attained
by 15 min of exercise, regardless of the state of training. As
expected, the absolute
O2 was
persistently higher during exercise in the HI group compared with the
LO group. Before training, the LO and HI groups were exercising at a
O2 that represented 69 and
66% of
O2 peak,
respectively.
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Blood lactate was altered both by exercise and by training (Fig.
1). With exercise, lactate increased during
the first 15 min and then remained stable over the remaining 15 min of
exercise. Lactate at both 15 and 30 min of exercise was reduced with
training, an effect that was fully manifested within 3 days. No
differences in the time course of the response was observed between
groups.
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Of the high-energy phosphates, ATP and PCr, ATP was not affected by
exercise and training (Table 3). In
contrast, PCr was altered (Fig. 2). These
effects were independent of initial
O2 peak levels. Before
training, PCr was reduced ~44% by 3 min of exercise with no further
reductions noted during the remainder of the exercise. Training
attenuated the decrease in PCr, an effect that was observed at 3 min of
exercise and persisted until 30 min. Only 3 days of training were
needed for the adaptation in PCr to be fully expressed, regardless of
group. The concentrations of IMP were also altered by exercise and
training (Fig. 2). Before training, IMP was elevated at both 3 and 30 min of exercise. At both 3 and 6 days of training, no elevation in IMP
was observed with exercise. Only 3 days of training were needed to
elicit reductions in exercise IMP levels.
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As expected, changes in Pi and Cr,
two metabolites of high-energy phosphate metabolism, were also observed
(Fig. 3). By 3 min of exercise, the
increases in both Pi and Cr were
maximal. Reductions were observed in both metabolites at 3 and 30 min
of exercise, after 3 and 6 days of training. However, only in the case
of Pi did a further reduction
occur (3 min) with an additional 3 days of training.
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Before training, glucose 6-phosphate
(G-6-P) increased by 3 min of
exercise and then declined by 30 min, the value remaining higher than
at rest (Fig. 4). Training blunted the
increase that was observed at 3 min of exercise, an effect that was
apparent by 3 days of training and persisted with the additional 3 days of training. Muscle glucose concentration was altered by exercise but
not by training. With exercise, glucose increased at 3 min of exercise
and remained elevated throughout the exercise. As with
G-6-P, glucose 1-phosphate
(G-1-P) was elevated at both 3 and
30 min of exercise with the peak value observed early in exercise (Fig.
5). After 6 days of training, the increase
in G-1-P was blunted but only at 3 min
of exercise. Only exercise altered fructose 6-phosphate
(F-6-P) concentrations. With
exercise, F-6-P was elevated at both 3 and 30 min. No group differences in the response to training were
observed for any of the glycolytic metabolites examined.
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The increase in muscle lactate observed with exercise was fully
realized by 3 min (Fig. 6). Thereafter,
lactate remained elevated. Decreases in exercise lactate concentration
occurred during the first 3 days of training, with no further decreases
observed thereafter. The decreases were observed at both the 3- and
30-min time points examined. No changes were observed in pyruvate with
training. However, exercise did elevate pyruvate at 30 min. The
lactate-to-pyruvate ratio, a measure of the cytosolic redox potential,
increased maximally at 3 min of exercise before training and then
declined over the remaining period of exercise (Fig.
7). At both 3 and 6 days of training, the
lactate-to-pyruvate ratio was reduced both early and late in exercise.
Training beyond 3 days did not exaggerate the adaptation. No group
differences were observed in either the pyruvate or lactate response to
training.
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Changes in calculated ADPf,
AMPf, and free
Pi concentration were also found
(Fig. 8). For
ADPf and
AMPf, a main effect was found for
both exercise and training. For exercise, both
ADPf and
AMPf increased at 3 min and
continued to increase at 30 min. By 3 days of training, both
metabolites were reduced during exercise regardless of duration.
Training for an additional 3 days was without further effect. The
concentration of free Pi was also increased by exercise and decreased by training. In the case of this
metabolite, further increases were not observed beyond 3 min of
exercise regardless of training duration. Progressive reductions occurred in exercise free Pi
concentration at both 3 and 6 days of training. Time course adaptations
in ADPf,
AMPf, and free
Pi were independent of initial
O2 peak.
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Muscle glycogen levels were altered by exercise and training but not by
initial O2 peak levels.
For both the LO and HI groups, a progressive reduction in glycogen
levels was observed at 3 and 30 min (Fig.
9). Three days of training resulted in
overall elevations in glycogen levels at rest and at 3 and 30 min of
exercise, effects that persisted throughout the remainder of the
training. At 6 days of training, glycogen levels were higher than at 3 days both before exercise and at 3 min of exercise.
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Muscle enzymes. Changes in the maximal
activities of two enzymes were observed with training (Table
4). Of the two marker enzymes selected as a
measure of mitochondrial potential, SDH and CS, both were elevated and
only at 6 days of training. The enzyme of glucose phosphorylation, HEX,
although increased, failed to reach significance
(P = 0.11). The two training groups,
LO and HI, did not display any time-dependent differences in the enzymatic response. However, in general, the activity of both SDH and
CS was higher in the HI group. The enzyme used to represent glycolytic
potential, PFK, remained unchanged by training regardless of the group.
No effect of training was observed in protein concentration.
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DISCUSSION |
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This study has clearly demonstrated that a range of metabolic
adaptations, typical of the trained state, occurs in response to a
short period of daily, prolonged exercise. The adaptations are
extremely rapid, occurring within 3 days of the onset of training. An
additional 3 days of training failed to produce additional effects. The
initial metabolic adaptations occur in the absence of increases in
mitochondrial potential, as indicated by the maximal activities of two
representative mitochondrial enzymes. Moreover, the initial or
pretraining O2 peak
failed to alter both the time courses and magnitude of the metabolic
adaptations that were observed. On the basis of these results, we
conclude that mechanisms other than an increase in the potential for
oxidative phosphorylation must be involved in inducing the early
training responses in muscle metabolism.
We have attempted to provide some insight into the nature of the
mechanisms involved through our exercise and muscle sampling protocol.
A submaximal exercise protocol was selected, which is known to result
in a steady-state O2 by ~3
min (39). Muscle biopsies performed at 3 and 30 min of exercise allowed
us to determine the time at which the metabolic adaptations occurred.
As in a previous study (14), we have found that the metabolic changes to both exercise and training were fully expressed by 3 min of exercise. The increases in IMP (used as a measure of the change in
ATP), the decreases in PCr, and near stoichiometric increases in Cr and
Pi and increases in lactate
occurred during the non-steady-state period. It was also during the
non-steady-state period that the reductions in IMP, Cr,
Pi, and lactate and more preserved
PCr were observed after training. Sustaining the exercise for 30 min failed to result in further changes in either the high-energy phosphates or lactate. The most probable reason for the metabolic imbalance, observed during the non-steady-state period, relates to an
inability of oxidative phosphorylation to satisfy the ATP requirements
of the contracting muscle and the need to recruit the high-energy
phosphates (12). Short-term training appears to attenuate the
discrepancy that occurs between the ATP-synthesizing pathways and ATP
utilization processes during the early period.
We have proposed, based on previous observations of increases in
O2 kinetics during exercise
after short-term training (31), that increases in oxidative
phosphorylation provide additional ATP during the nonsteady state,
promoting less of a reduction in energy state. Theoretically, all of
the factors regulating oxidative phosphorylation could be implicated in
the increase in
O2
kinetics that was observed, namely
O2,
H+, ADP,
Pi, and oxidizable substrates.
Training-induced increases in O2 delivery to the working muscle is an inviting possibility (12), particularly because we have previously shown that increases in femoral artery blood flow kinetics occur relatively early in training (37). The depression that we have observed in the lactate-to-pyruvate ratio early in exercise after training is consistent with a reduction in the NADH-to-NAD+ ratio and alleviation of cellular hypoxia (35).
Increased availability of citric acid cycle intermediates mediated via increases in pyruvate or free fatty acid entry into the mitochondria also appears possible. Increased pyruvate entry would not be expected, however, because in previous work with a 5-day training model, we found reductions in carbohydrate oxidation (32). Increased fat oxidation could be involved given the results of Odland et al. (28), who found that during exercise of a comparable intensity with that used in this study, infusion of an Intralipid-heparin solution, which resulted in a substantial increase in blood free fatty acids, induced some of the adaptations observed with short-term training. With Intralipid-heparin, an improved cellular energy state, increased fat utilization, and glycogen sparing occurred (28). The 5-day training model results in a small but significant increase in total fat oxidation as a result of increases in intramuscular triglyceride oxidation (32). However, different mechanisms appear to be involved with a short-term training model because, unlike the Intralipid experiments, muscle lactate is depressed. In addition, the training model demonstrates that the metabolic adaptations occur during the nonsteady state and not later in exercise as observed with Intralipid (9).
As has been previously observed (13), reductions in both blood and muscle lactate concentration during exercise also accompany the adaptations in the phosphate energy system and also occur early in exercise. The association between the two events suggests a coupling between high-energy phosphate metabolism and glycogenolysis-glycolysis. It has been proposed that the more protected energy state may dampen the activation of phosphorylase and PFK as a result of reduced concentrations of one or more of the allosteric activators free Pi, ADPf, and AMPf (6, 22). Interestingly, we have found reductions in the calculated values of all three of these metabolites at 3 min of exercise after 3 days of training, similar to what was observed for lactate.
The changes in the glycolytic intermediates, G-1-P and G-6-P, both of which were observed to accumulate less after training, are also consistent with an enhanced inhibition of glycogenolysis and/or glucose phosphorylation (25). Because no training-induced increase in F-6-P was observed during exercise, PFK would not appear to be a principal control site in the apparent reduction in glycolytic flux that occurs. Reductions in PFK activity may have occurred, consistent with allosteric control by AMPf and free Pi concentration (25), both of which were reduced with training; however, the reduction would not appear to be rate limiting.
Glycogen depletion results provide additional support for
phosphorylase as being the regulatory site that is
altered with training. The training program resulted in higher glycogen
levels during exercise after training. However, the higher glycogen
levels can be explained, in part, by the higher training-induced
resting levels. When the initial levels are adjusted for, a reanalysis of the data indicates a reduced depletion rate after training. At 30 min, as an example, glycogen loss amounted to 175 mmol · glucosyl
U1 · kg
dry wt
1 before training and
112 and 113 mmol · glucosyl
U
1 · kg
dry wt
1 at 3 and 6 days of
training (P <
0.05). There
are clear indications that a substantial fraction of the reduced
depletion occurs early in exercise. We have previously found a reduced
rate of glycogen depletion with short-term training (5, 14, 15) as have others (6) who have also concluded that because phosphorylase (total
and phosphorylase a) was unaltered
early in exercise after training, that posttranslational control of
phosphorylase, mediated by reductions in free
Pi concentration and
AMPf, is responsible for the
blunted glycogenolysis.
It must be emphasized that at present it can only be speculated that a reduced glycolytic flux has occurred because only muscle lactate concentration was measured. Evidence exists based on lactate exchange across the working muscle (34) and stable isotope measurements with 13C lactate tracer (7, 33) that increases in lactate removal may be important. This possibility is further supported by the observation that a lactate transporter, monocarboxylate transporter 1, is also upregulated soon after the onset of training (4). Unfortunately, given the questionable assumptions inherent in both procedures, it is not possible to unequivocally argue for either adaptations in production or removal as the predominant mechanism (10).
The adaptations that we have observed in muscle metabolism during
moderate intensity exercise in response to short-term training clearly
occur during the early phase of the exercise. It is during this phase
that recruitment of high-energy phosphate metabolism occurs and
glycogenolysis and glycolysis are rapidly accelerated. When exercise is
sustained, no further alteration in phosphorylation potential occurs.
The metabolic adaptations expressed early in the exercise persist as
exercise is continued. During the steady-state period, at the exercise
intensity that we have used, which is below the lactate threshold,
oxidative phosphorylation appears to supply essentially all of the ATP
requirements of the contracting muscle (12). The results of this study
and others (6, 14, 15) indicate that in steady state, whole body
O2 and probably
O2 leg (12) remain unaltered.
Consequently, if muscle
O2 is
unaltered in the face of a training-induced decrease in
ADPf and free
Pi concentration, respiratory
control sensitivity must be enhanced (8). The enhancement would appear
to occur during the non-steady-state adjustment to moderate exercise.
The critical question is what mediates the enhanced sensitivity?
We have reported that the adaptive effects in muscle metabolism occur
in conjunction with increased
O2 kinetics (31) accompanied by an increased blood flow (
) (37) and
ostensibly oxygen delivery (
CaO2)
to the working muscle. However, for this hypothesis to be viable, it
must be demonstrated that events outside the muscle cell, rather than
events within the muscle cell, dictate the level of oxidative
phosphorylation during the non-steady-state adjustment period. Grassi
et al. (12) have examined muscle
O2 kinetics (
O2 leg) during moderate
exercise and concluded that there are two phases during the on
transition: an initial phase, phase I, which lasts for 10-15 s,
followed by a second phase, phase II, which extends for 1-2 min.
Phase I is characterized by a large increase in blood flow and only
modest increases in
O2 leg, suggesting that during this period constraints within the muscle limit
oxidative phosphorylation. During phase II, a monoexponential increase
in
O2 leg occurred with
approximately similar time courses observed for
CaO2 leg and
arteriovenous O2 content across the leg. Our
experimental design did not permit us to determine if the metabolic
adaptations were specific to phase I or phase II. In previous studies
with a short-term training model, we have been able to determine that
the increase in blood flow kinetics occurs with the first 1-2 min
(37), suggesting that a CaO2
limitation occurs during this period.
It has been shown that during phase II, a single reaction with
first-order kinetics controls
O2 muscle (27, 29). This single reaction is dependent on changes in the energy state. Previous work has shown that the decrease in PCr is also nonexponential (1, 26).
Based on this association, the more conserved PCr level that we have
observed during exercise with training would suggest a decrease in
O2 kinetics, not an increase,
as we have observed (31). For an increase in
O2 kinetics to have occurred in the face of a more protected high-energy phosphorylation potential, mitochondrial control sensitivity would have to be altered during the
non-steady-state period.
It has been hypothesized that the increase in respiratory control sensitivity occurs as a result of increases in mitochondrial or oxidative capacity (8, 22, 29). However, our results do not support this proposition. We have found that after 3 days of training, the metabolic adaptations that occurred were not accompanied by increases in the maximal activities of CS or SDH, the two enzymes selected to represent oxidative potential. After 6 days of training, oxidative potential increased but no further exaggeration occurred in the metabolic response. These results suggest that other factors are involved in the regulation of mitochondrial metabolism (21).
The changes that we have observed in the marker enzymes of oxidative
capacity with 6 days of training are similar to what has been recently
reported (6, 38) but contradict what we reported previously (15, 17,
34). Because, in previous studies, we have employed subjects with
widely different
O2 peak values, it was
hypothesized that both the enzymatic and metabolic adaptations may
occur with a different time frame. We have found no evidence that there
is a time-dependent effect of training in the present study, because
the group with the low
O2 peak and the group with the high
O2 peak
showed the same response at both 3 and 6 days of training.
Consequently, it is possible that our earlier studies failed to find an
increase in mitochondrial potential when it did in fact occur (15-17).
However, it should be emphasized that a recent study (34) has also
failed to find increases in CS activity, in the face of extensive
metabolic adaptations, after 7 days of training with a similar protocol
to the one we have employed (14, 15, 17).
In this regard, a potential limitation must be acknowledged. It is possible that the enzymes selected to represent mitochondrial potential in our studies (16) and others (6, 34, 38) may not have been appropriate. The enzymes have been selected because they exist in constant proportion with other mitochondrial enzymes (30). Controversy continues to exist as to what the rate-limiting enzyme is.
In summary, we have been able to demonstrate that metabolic adaptations
occur during moderate exercise within the first 3 days after the onset
of training. These adaptations, which are expressed during the
non-steady-state adjustment period, are characterized by a more
protected phosphorylation state (less recruitment of high-energy
phosphates) and an apparent reduction in glycogenolysis as indicated by
lower muscle lactate levels. The improved energy state with training is
unaccompanied by increases in mitochondrial potential and dissociated
from the initial aerobic power
(O2 peak of the
subjects). Although the mechanisms remain elusive, it is strongly
suspected that the metabolic adaptations are a consequence of a
reduction in cellular hypoxia during the onset of exercise, secondary
to improved blood flow and oxygen delivery to working muscles.
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ACKNOWLEDGEMENTS |
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We would like to thank the Natural Sciences and Engineering Research Council of Canada for financial support.
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FOOTNOTES |
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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. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: H. J. Green, Dept. of Kinesiology, Univ. of Waterloo, Waterloo, ON, Canada N2L 3G1.
Received 20 October 1998; accepted in final form 9 March 1999.
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