Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario N1G 2W1; Department of Medicine, McMaster University, Hamilton, Ontario, Canada L8N 3Z5; and Department of Clinical Chemistry, Huddinge University Hospital, S 141 86 Huddinge, Sweden
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
To investigate the regulation of intramuscular
fuel selection, we measured the malonyl-CoA (M-CoA) content in human
skeletal muscle at three exercise power outputs [35, 65, and 90%
maximal rate of O2 consumption
(O2 max)]. Four
males and four females cycled for 10 min at one power output on three
separate occasions with muscle biopsies sampled at rest and at 1 and 10 min. The respiratory exchange ratio was 0.84 ± 0.03, 0.92 ± 0.02, and >1.0 at 35, 65 and 90%
O2 max, respectively.
Muscle lactate content increased and phosphocreatine content decreased
as a function of power output. Pyruvate dehydrogenase
a activity increased from 0.40-0.64 mmol · kg wet
muscle
1 · min
1
at rest to 1.57 ± 0.28, 2.80 ± 0.41, and 3.28 ± 0.27 mmol · kg wet
muscle
1 · min
1
after 1 min of cycling at the three power outputs, respectively. Mean
resting M-CoA contents were similar at all power outputs (1.85-1.98 µmol/kg dry muscle). During exercise at 35%
O2 max, M-CoA decreased from rest at 1 min (1.85 ± 0.29 to 1.20 ± 0.12 µmol/kg dry muscle) but returned to rest level by 10 min (1.86 ± 0.25 µmol/kg dry muscle). M-CoA content did not decrease
during cycling at 65%
O2 max. At 90%
O2 max, M-CoA did not
increase despite significant acetyl-CoA accumulation (the substrate for M-CoA synthesis). The data suggest that a decrease in M-CoA content is
not required for the increase in free fatty acid uptake and oxidation
that occurs during exercise at 35 and 65%
O2 max. Furthermore,
M-CoA content does not increase during exercise at 90%
O2 max and does not
contribute to the lower rate of fat oxidation at this power
output.
fatty acid oxidation; acetyl-coenzyme A; acetyl-coenzyme A carboxylase; carnitine palmitoyltransferase I; high-performance liquid chromatography; malonyl-coenzyme A
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
MALONYL-COA (M-CoA) has recently been the subject of much interest due to its potential role in intramuscular fuel selection during exercise. Carnitine palmitoyltransferase (CPT) I, the key enzyme responsible for the transport of fatty acids into mitochondria for oxidation, is potently inhibited by M-CoA in vitro (3, 21, 27). It is currently believed that resting levels of M-CoA in rodent muscle are sufficiently high to inhibit excessive entry of free fatty acids (FFA) into the mitochondria at rest. During exercise, when the demand for energy from fat oxidation increases, rat skeletal muscle M-CoA content decreases during treadmill running and in response to electrical stimulation (9, 28, 35). This contraction-induced decrease in M-CoA has been postulated to relieve CPT-I inhibition and allow increased transport of FFA into the mitochondria for oxidation (35). To date, however, no direct measurements of fatty acid oxidation or any aspect of fat metabolism have been correlated with M-CoA contents in rodent muscle during exercise. Furthermore, research investigating M-CoA content in humans is extremely limited.
A recent investigation from our laboratory examining both
rodent and human skeletal muscle (22) reported a 52% reduction in
rodent red gastrocnemius M-CoA content in response to electrical stimulation at 0.7 Hz, which agrees with previous reports in rodent skeletal muscle (9). However, the M-CoA content in human vastus lateralis muscle remained constant during cycle exercise for 10 min at
40% maximal rate of O2
consumption (O2 max)
and for 60 min at 65%
O2 max. This occurred
despite significant increases in fat utilization from rest to exercise
and over time during exercise at 65%
O2 max (22). On the
basis of these results, we concluded that measured levels of
M-CoA in exercising muscle did not predict fatty acid oxidation rates
in human muscle during low- and moderate-intensity exercise. However,
we suggested that a more detailed time course of the human
skeletal muscle M-CoA response to exercise was required.
Saddik et al. (26) reported that heart muscle M-CoA content increased concomitantly with the carbohydrate (CHO)-derived acetyl-CoA level. It is well known that acetyl-CoA accumulates rapidly in human skeletal muscle at the onset of high-intensity exercise (6, 10). It is possible that the increase in acetyl-CoA stimulates the activity of acetyl-CoA carboxylase (ACC), the enzyme responsible for M-CoA production, leading to an increase in M-CoA content (rather than a decrease) at high power outputs. Previous studies in rodent skeletal muscle, however, have reported reductions in M-CoA with contraction, even when CHO availability was enhanced (11), but M-CoA measurements at different power outputs have not been conducted.
Due to the importance of exercise intensity in the determination of
intramuscular fuel selection (19, 30), the purpose of this study was to
examine the time course of M-CoA kinetics early in exercise in human
skeletal muscle at varying power outputs. It was hypothesized that
M-CoA would be unaffected at 1 and 10 min of exercise at 35 and 65%
O2 max and may increase
as a function of increasing muscle acetyl-CoA levels at 90%
O2 max.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Subjects
Four males and four females volunteered to participate in the study. The subjects were not well trained, but all were healthy and physically active (age, 22.1 ± 0.3 yr; weight, 77.7 ± 3.6 kg,Experimental Protocol
Before the experiment, all participants underwent a continuous incrementalThe experimental trials were performed on 2 days, separated by 2-3 wk. On each test day, subjects reported to the laboratory at the same time of day having eaten a meal high in CHO 2-4 h before the experiment. Daily food records were kept for 48 h preceding the initial trial, and subjects were instructed to replicate their consumption before the second trial. CHO intake represented 60.0 ± 4.3% of the total caloric intake of the pretrial diet, whereas fat and protein comprised 25.0 ± 2.4 and 15.0 ± 2.0%, respectively.
On one of the test days, exercise consisted of 10 min of cycling on an
electronically braked ergometer (Excalibur; Quinton Instrument,
Seattle, WA) at 35%
O2 max, followed by
at least 1 h of rest and then 10 min of cycling at 65%
O2 max. On the other
day, subjects cycled for 10 min at 90%
O2 max. Test days were randomly assigned. Before each exercise bout, one leg was prepared
for percutaneous needle biopsy of the vastus lateralis with three
incisions of the skin through to the deep fascia, under local
anesthesia (2% lidocaine without epinephrine) as described by
Bergstrom (2). Biopsy samples were obtained at rest after at least 30 min of complete rest on a bed and were immediately frozen in liquid
N2. Exercise biopsy samples were
obtained at 1 and 10 min at each power output. Rate of
O2 consumption
(
O2), CO2 output
(
CO2), and the respiratory
exchange ratio (RER) were determined using a metabolic cart (Quinton
Q-plex 1; Quinton Instrument). At 35 and 65%
O2 max, expired gases
were collected between 4 and 8 min of exercise, whereas at 90%
O2 max, gas was
collected between 2-4 and 7-9 min. The rate of fat oxidation
(g/min) was calculated from the
CO2 and
O2 using the RER (12).
Muscle Sampling and Analyses
A piece of frozen wet muscle (20-30 mg) was removed under liquid N2 for the determination of the activity of pyruvate dehydrogenase (PDH) in its active form (PDHa), as described previously (7, 24).The remainder of the muscle samples were freeze-dried, dissected free of blood and connective tissue, and powdered. A portion of dry muscle (~5 mg) was extracted in 0.5 M perchloric acid and 1 mM EDTA, neutralized to pH 7.0 with 2.2 M KHCO3, and analyzed for acetyl-CoA and acetylcarnitine (5) as well as phosphocreatine (PCr), creatine, ATP, glucose, and lactate (1, 16).
M-CoA was extracted from aliquots of dry muscle (~8-14 mg) with ice-cold 0.5 M PCA in the ratio of 10 µl/mg dry muscle as previously described (22). M-CoA esters were separated and quantified as described in previous reports (18, 26).
All muscle metabolites were normalized to the highest total creatine content for a given individual's biopsies to correct for nonmuscle contamination. This correction averaged 9.2 ± 1.2%.
Statistical Analysis
Data are expressed as means ± SE. A paired t-test was used to compare RER data from eight subjects at 35 and 65% ![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Respiratory Gas Exchange
During exercise at 35 and 65%Muscle Metabolites
PCr, ATP, glucose, and lactate. Muscle ATP remained unchanged from rest during exercise at 35, 65, and at 1 min at 90%
|
M-CoA. At rest, muscle M-CoA data were
similar between power outputs (35%, 1.85 ± 0.29; 65%, 1.85 ± 0.46; 90% O2 max, 1.98 ± 0.26 µmol/kg dry muscle; Fig. 1).
M-CoA was significantly reduced from rest at 1 min during exercise at
35%
O2 max but
returned to rest level by 10 min (Fig. 1,
P < 0.02). No differences in M-CoA
content occurred during exercise at 65 or 90%
O2 max.
|
Acetyl group accumulation. Muscle
acetyl-CoA remained constant during exercise at 35%
O2 max and was
elevated above rest at 10 min at 65%
O2 max
(P < 0.06) and at 1 and 10 min at
90%
O2 max (Fig.
2A,
P < 0.001). Acetylcarnitine
increased at 10 min during exercise at all three power outputs (Fig.
2B; 35%, P < 0.002; 65 and 90%,
P < 0.001).
|
PDH activity. Resting
PDHa activity was similar before
all exercise trials and increased above rest after 1 min of cycling as
a function of power output (Fig. 3,
P < 0.001).
PDHa activity did not change from
1 to 10 min at the lower two power outputs but increased at 90%
O2 max.
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The primary purpose of this investigation was to measure M-CoA in human
skeletal muscle biopsy samples at rest and during the onset of exercise
at varying power outputs. At rest, individual M-CoA contents ranged
from 0.78 to 3.41 µmol/kg dry muscle, but mean values were similar
across power outputs (Fig. 1). Generally, M-CoA did not change in
response to exercise at the lower two power outputs, although M-CoA
fell significantly at 1 min at 35% O2 max and then
returned to rest level by 10 min. M-CoA did not increase at the high
power output despite significant activation of PDH and accumulations of
acetyl-CoA and acetylcarnitine.
M-CoA and Intramuscular Fuel Selection
Observed decreases in M-CoA content during exercise have led to the suggestion that it is a regulator of fat-CHO interaction in rodent skeletal muscle at rest and during exercise (20, 28, 35). It has also been postulated to play the same role in contracting human skeletal muscle, although M-CoA was not measured in these investigations (17, 32). Two of the main regulatory enzymes responsible for substrate choice for oxidation are PDH and CPT-I, which control the entry of CHO and FFA into the mitochondria, respectively. M-CoA is a potent inhibitor of CPT-I in vitro (3, 27). Because the M-CoA content may be influenced by acetyl-CoA, which is an end product of the PDH reaction and a substrate for M-CoA formation, alterations in M-CoA content could provide a mechanism to control the balance between CHO and fat oxidation.It has long been established that exercise intensity is an important
factor responsible for determining the amount of fat or CHO utilization
during exercise (19, 30). A recent study (25) with well-trained, fasted
cyclists demonstrated that at low power outputs (25%
O2 max), 90%
of the energy expenditure was derived from fat. At 65%
O2 max, fat oxidation
fell to 50% of total energy expenditure, but absolute fat oxidation
rates were greater than at 25%
O2 max, since energy
demand also increased. Finally, at high power outputs (>85%
O2 max), both
the absolute and relative contribution of fat decreased (25). Less
information is available regarding intramuscular fuel selection in
untrained, fed subjects, although a greater reliance on CHO would be
expected at exercise intensities above 50%
O2 max (30).
During low to moderate submaximal exercise, when fat provides a
significant proportion of the fuel required for contraction, a
reduction in M-CoA may relieve inhibition at CPT-I and allow more FFA
transport into the mitochondria for oxidation. During high-intensity
exercise (i.e., 85%
O2 max or greater),
when the contribution from fat to total energy supply is reduced, high PDHa activity leads to CHO-derived
accumulations of acetyl-CoA and acetylcarnitine. If a portion of this
acetyl-CoA accumulation occurs in the cytoplasm and stimulates ACC to
produce M-CoA, then M-CoA-induced inhibition of CPT-I may contribute to
the reduced FFA oxidation during high-intensity exercise.
In the present study, the RER data suggested that exercise at 35 and
65% O2 max was
associated with a large energy contribution from fat oxidation. On the
basis of previous measurements with rodent skeletal muscle, it was
expected that M-CoA content would decrease at these workloads. However,
our previous work with M-CoA after 10 min of exercise suggested that a
decrease in M-CoA was not involved in the increase in FFA transport and
oxidation. In the present study, M-CoA fell significantly only at 1 min
at 35%
O2 max and
then returned to preexercise level by 10 min. At 65%
O2 max, M-CoA remained
unchanged throughout exercise. The calculated fat oxidation rates were
0.24 ± 0.04 g/min at 35% and 0.18 ± 0.03 g/min at 65%
O2 max despite similar
M-CoA contents. At 90%
O2 max, a
level at which fat contributed less energy, M-CoA also remained
constant.
The early decline in M-CoA at 35%
O2 max may be explained
by fiber-type recruitment. In rodent skeletal muscle, M-CoA content was
reported to vary among different fiber types and to correlate with
mitochondrial content of muscle fibers (36). In resting rats, M-CoA
concentration in the white region of the quadriceps (fast glycolytic)
was only 28% of the deep red quadriceps content (slow oxidative).
Furthermore, during treadmill running at 21 m/min up a 15% grade, the
most rapid decrease in rodent muscle M-CoA occurred in the red
quadriceps, whereas the slowest decrease occurred in white quadriceps
(36). Although regions of rat skeletal muscle are homogeneous with
respect to fiber type, human skeletal muscle is heterogeneous. Human
vastus lateralis muscle is composed of ~50% type I, 40% type IIa,
and 10% type IIb fibers (31). The M-CoA content within specific human
fiber types is not known, but the contents of many metabolites in human
type I and II fibers are similar (8, 13, 33). However, if there were
specific recruitment of mainly type I fibers during exercise at 35%
O2 max, a decrease in
the M-CoA content of ~67% in these fibers would be required to
account for the early ~33% decrease in total muscle M-CoA at this
power output.
Fiber-type recruitment cannot, however, explain the return of M-CoA to
resting levels by 10 min of exercise at 35%
O2 max. It is possible
that the early decline at 35% "primes" the fatty acid oxidation
machinery at the onset of exercise and that the mechanism responsible
for the early decline at 35%
O2 max is overridden by
other regulators over time and at higher power outputs. In addition,
there is the possibility that the M-CoA-induced inhibition of CPT-I is
maximal at rest, such that further increases are not necessary during
high-intensity exercise.
M-CoA Interaction With CPT-I
M-CoA content did not correlate with fat utilization during exercise at varying power outputs, but the possibility of M-CoA-induced inhibition of CPT-I and subsequent regulation of fatty acid oxidation cannot be dismissed. M-CoA-induced inhibition of CPT-I may be reduced independent of a change in M-CoA, as other factors could account for the reduction in inhibition. Certainly, pH is known to affect the sensitivity of CPT-I to inhibition by M-CoA. At pH 6.8, rodent muscle CPT-I has been shown to bind M-CoA more efficiently and to have a lower IC50 for M-CoA (concentration of M-CoA required for 50% inhibition of CPT-I activity), a higher Michaelis constant (Km) for carnitine, and a lower maximal velocity than at higher pH (21). If human muscle CPT-I is also sensitive to pH, this may help explain why M-CoA did not change at the higher power outputs, at a time when fatty acid utilization was reduced. Certainly the M-CoA-CPT-I interaction in skeletal muscle is complex, as in vitro experiments predict that CPT-I activity should be fully inhibited at resting and exercise M-CoA concentrations (3, 27), and this is not the case in vivo.At present, very little is known regarding the regulatory properties of the muscle isoform of CPT-I, particularly in humans. Both acetyl-CoA and free CoA were reported to antagonize the inhibitory effect of M-CoA on total CPT activity in human skeletal muscle homogenates by increasing the inhibitory constant (Ki) for M-CoA (39). It should be noted, however, that, despite increasing the Ki for M-CoA, addition of both acetyl-CoA and free CoA acted in a synergistic manner with M-CoA to increase (not decrease) inhibition of total CPT activity. Results from this study are difficult to interpret, however, since total CPT activity (i.e., CPT-I and CPT-II) was measured rather than CPT-I activity alone (39). Further in vitro experimentation is required to determine the human CPT-I response to physiological concentrations of various CoA esters. Specifically, inhibitor studies of human muscle CPT-I are needed to determine whether M-CoA binds directly at the catalytic site or at a separate regulatory site.
Regulation of Muscle ACC Activity
Tissue levels of M-CoA will be dependent upon the activity of ACC and the rate of M-CoA degradation. Tissue-specific isoforms of ACC have been identified, and human and rat skeletal muscle appear to contain a unique 272- to 275-kDa isoform (ACC-In vitro investigations have reported ACC- to have a higher
Km for acetyl-CoA
than ACC-
(4), and research in perfused working heart muscle has
suggested that muscle ACC-
may be substrate regulated (26). In the
present study, M-CoA did not increase above resting level in any
subject, at any time point or power output, despite concomitant
increases in acetyl-CoA and acetylcarnitine at 65 and 90%
O2 max. This result
suggests that M-CoA production in human skeletal muscle during
moderate- to high-intensity exercise is not regulated by acetyl-CoA
content.
Unfortunately, measurements of acetyl-CoA and acetylcarnitine in biopsy samples are made on total tissue content, making it difficult to determine what proportion of the measured acetyl groups are mitochondrial or cytosolic. Clearly, in order to affect ACC, some acetyl-CoA must be in the cytoplasm. It is likely that cytoplasmic acetyl-CoA content increases to some extent during exercise, as the large increases in acetylcarnitine are believed to be primarily cytosolic.
Recent studies of rodent soleus muscle incubated at rest with glucose
and insulin reported increased M-CoA formation modulated by changes in
the cytosolic concentration of citrate rather than by stable increases
in ACC- activity (29). In contrast, it was suggested that ACC-
activity was reduced after 5 and 30 min of treadmill exercise, despite
a significant increase in muscle citrate (37). In the present study,
citrate was not determined, but previous studies with similar protocols
have established that significant increases in citrate occur during
exercise in humans (10, 23). It is possible that
1) significant increases in muscle
citrate (and acetyl-CoA) offset a potential contraction-induced decrease in ACC-
activity, and 2)
ACC-
may be regulated by fuel supply at rest, but this control
mechanism is overridden during exercise. Saha et al. (28) suggested
that fuel supply and contraction-induced alterations in M-CoA were
regulated by different systems in isolated rodent muscle.
In summary, human skeletal muscle M-CoA content remained remarkably
constant during exercise at varying power outputs. Regardless of the
mechanism(s) regulating M-CoA formation and/or degradation, M-CoA content was not correlated with fatty acid oxidation
rate. Therefore, if M-CoA is involved in CPT-I regulation during
exercise at 35 and 65%
O2 max, it is not
primarily due to decreases in its concentration. Conversly, M-CoA
content did not increase during exercise at 90%
O2 max and therefore
did not contribute to the lower rate of fat oxidation at this power
output. In conclusion, other factors may be present during exercise
that interact with M-CoA and CPT-I and override its inhibitory effects
on CPT-I activity.
![]() |
ACKNOWLEDGEMENTS |
---|
We thank Drs. David Dyck and Jack Rosenfeld for invaluable technical assistance.
![]() |
FOOTNOTES |
---|
This work was supported by operating grants from the Natural Sciences and Engineering Research and Medical Research Councils of Canada and by a student grant from the Gatorade Sports Science Institute, Barrington, IL. G. J. F. Heigenhauser is a Career Investigator of the Heart and Stroke Foundation of Ontario.
Address for reprint requests: L. L. Spriet, Dept. of Human Biology and Nutritional Sciences, Univ. of Guelph, Guelph, Ontario, Canada N1G 2W1.
Received 29 September 1997; accepted in final form 18 February 1998.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Bergmeyer, H. U.
Methods of Enzymatic Analysis. New York: Academic, 1974.
2.
Bergstrom, J.
Percutaneous needle biopsy of skeletal muscle in physiological and clinical research.
Scand. J. Clin. Lab. Invest.
35:
609-616,
1975[Medline].
3.
Berthon, P. M., R. A. Howlett, G. J. F. Heigenhauser, and L. L. Spriet. Human skeletal
muscle carnitine palmitoyltransferase I activity determined in intact
isolated mitochondria. J. Appl.
Physiol. In press.
4.
Bianchi, A.,
J. L. Evans,
A. J. Iverson,
A. Norlund,
T. D. Watts,
and
L. A. Witters.
Identification of an isozymic form of acetyl-CoA carboxylase.
J. Biol. Chem.
265:
1502-1509,
1990
5.
Cederblad, G.,
J. I. Carlin,
D. Constantin-Teodosiu,
P. Harper,
and
E. Hultman.
Radioisotopic assays of CoASH and carnitine and their acetylated forms in human skeletal muscle.
Anal. Biochem.
185:
274-278,
1990[Medline].
6.
Constantin-Teodosiu, D.,
G. Cederblad,
and
E. Hultman.
Acetyl group accumulation and pyruvate dehydrogenase activity in human muscle during incremental exercise.
Acta Physiol. Scand.
143:
367-372,
1991[Medline].
7.
Constantin-Teodosiu, D.,
G. Cedeblad,
and
E. Hultman.
A sensitive radiosotopic assay of pyruvate dehydrogenase complex in human muscle tissue.
Anal. Biochem.
198:
347-351,
1991[Medline].
8.
Constantin-Teodosiu, D.,
S. Howell,
and
P. L. Greenhaff.
Carnitine metabolism in human muscle fiber types during submaximal dynamic exercise.
J. Appl. Physiol.
80:
1061-1064,
1996
9.
Duan, C.,
and
W. W. Winder.
Nerve stimulation decreases malonyl-CoA in skeletal muscle.
J. Appl. Physiol.
72:
901-904,
1992
10.
Dyck, D. J.,
C. T. Putman,
G. J. F. Heigenhauser,
E. Hultman,
and
L. L. Spriet.
Regulation of fat-carbohydrate interaction in skeletal muscle during intense aerobic cycling.
Am. J. Physiol.
265 (Endocrinol. Metab. 28):
E852-E859,
1993
11.
Elayan, I. M.,
and
W. W. Winder.
Effect of glucose infusion on muscle malonyl-CoA during exercise.
J. Appl. Physiol.
70:
1495-1499,
1991
12.
Frayn, K. N.
Calculation of substrate oxidation rates in vivo from gaseous exchange.
J. Appl. Physiol.
55:
628-634,
1983
13.
Greenhaff, P. L.,
J.-M. Ren,
K. Soderlund,
and
E. Hultman.
Energy metabolism in single human muscle fibers during contraction without and with epinephrine infusion.
Am. J. Physiol.
260 (Endocrinol. Metab. 23):
E713-E718,
1991
14.
Ha, J.,
J. K. Lee,
K. S. Kim,
L. A. Witters,
and
K. H. Kim.
Cloning of human acetyl-CoA carboxylase- and its unique features.
Proc. Natl. Acad. Sci. USA
93:
11466-11470,
1996
15.
Hardie, D. G.
Regulation of fatty acid synthesis via phosphorylation of acetyl-CoA carboxylase.
Prog. Lipid Res.
28:
117-146,
1989[Medline].
16.
Harris, R. C.,
E. Hultman,
and
L.-O. Nordesjo.
Glycogen, glycolytic intermediates and high-energy phosphates determined in biopsy samples of musculis quadriceps femoris of man at rest.
Scand. J. Clin. Lab. Invest.
33:
109-120,
1974[Medline].
17.
Jeukendrup, A. E.,
W. H. M. Saris,
F. Brouns,
D. Halliday,
and
A. J. M. Wagenmakers.
Effects of carbohydrate (CHO) and fat supplementation on CHO metabolism during prolonged exercise.
Metabolism
45:
915-921,
1996[Medline].
18.
King, M. T.,
P. D. Reiss,
and
N. W. Cornell.
Determination of short-chain coenzyme A compounds by reversed phase high-performance liquid chromatography.
Methods Enzymol.
166:
70-79,
1988[Medline].
19.
Krogh, A.,
and
J. Lindhard.
The relative value of fat and carbohydrate as sources of muscular energy.
Biochem. J.
14:
290-363,
1920.
20.
McGarry, J. D.,
and
N. F. Brown.
The mitochondrial carnitine palmitoyltransferase system. From concept to molecular analysis.
Eur. J. Biochem.
244:
1-14,
1997[Abstract].
21.
Mills, S. E.,
D. W. Foster,
and
J. D. McGarry.
Effects of pH on the interaction of substrates and malonyl-CoA with mitochondrial carnitine palmitoyltransferase I.
Biochem. J.
219:
601-608,
1984[Medline].
22.
Odland, L. M.,
G. J. F. Heigenhauser,
G. D. Lopaschuk,
and
L. L. Spriet.
Human skeletal muscle malonyl-CoA at rest and during prolonged submaximal exercise.
Am. J. Physiol.
270 (Endocrinol. Metab. 33):
E541-E544,
1996
23.
Putman, C. T.,
L. L. Spriet,
E. Hultman,
D. J. Dyck,
and
G. J. F. Heigenhauser.
Skeletal muscle pyruvate dehydrogenase activity during acetate infusion in humans.
Am. J. Physiol.
268 (Endocrinol. Metab. 31):
E1007-E1017,
1995
24.
Putman, C. T.,
L. L. Spriet,
E. Hultman,
M. I. Lindinger,
L. C. Lands,
R. S. McKelvie,
G. Cederblad,
N. L. Jones,
and
G. J. F. Heigenhauser.
Pyruvate dehydrogenase activity and acetyl group accumulation during exercise after different diets.
Am. J. Physiol.
265 (Endocrinol. Metab. 28):
E752-E760,
1993
25.
Romijn, J. A.,
E. F. Coyle,
L. S. Sidossis,
A. Gastaldelli,
J. Horowitz,
E. Endert,
and
R. R. Wolfe.
Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration.
Am. J. Physiol.
265 (Endocrinol. Metab. 28):
E380-E391,
1993
26.
Saddik, M.,
J. Gamble,
L. A. Witters,
and
G. D. Lopaschuk.
Acetyl-CoA carboxylase regulation of fatty acid oxidation in the heart.
J. Biol. Chem.
268:
25836-25845,
1993
27.
Saggerson, E. D.,
and
C. A. Carpenter.
Carnitine palmitoyltransferase and carnitine octanoyltransferase activities in liver, kidney cortex, adipocyte, lactating mammary gland, skeletal muscle and heart.
FEBS Lett.
129:
229-232,
1981[Medline].
28.
Saha, A. K.,
T. G. Kurowski,
and
N. B. Ruderman.
A malonyl-CoA fuel-sensing mechanism in muscle: effects of insulin, glucose, and denervation.
Am. J. Physiol.
269 (Endocrinol. Metab. 32):
E283-E289,
1995
29.
Saha, A. K.,
D. Vavvas,
T. G. Kurowski,
A. Apazidis,
L. A. Witters,
E. Shafrir,
and
N. B. Ruderman.
Malonyl-CoA regulation in skeletal muscle: its link to cell citrate and the glucose-fatty acid cycle.
Am. J. Physiol.
272 (Endocrinol. Metab. 35):
E641-E648,
1997
30.
Saltin, B.,
and
J. Karlsson.
Muscle glycogen utilization during work of different intensities.
In: Advances in Experimental Medicine and Biology, edited by B. Pernow,
and B. Saltin. New York: Plenum, 1971, vol. 11, p. 289-299.
31.
Schantz, P.,
E. Randall-Fox,
W. Hutchison,
A. Tyden,
and
P. O. Astrand.
Muscle fibre type distribution, muscle cross-sectional area and maximal voluntary strength in humans.
Acta Physiol. Scand.
117:
219-226,
1983[Medline].
32.
Sidossis, L. S.,
A. Gastaldelli,
S. Klein,
and
R. R. Wolfe.
Regulation of plasma fatty acid oxidation during low- and high-intensity exercise.
Am. J. Physiol.
272 (Endocrinol. Metab. 35):
E1065-E1070,
1997
33.
Tesch, P.,
and
J. Karlsson.
Effects of exhaustive, isometric training on lactate accumualtion in different muscle fiber types.
Int. J. Sports Med.
5:
89-91,
1984[Medline].
34.
Widmer, J.,
K. S. Fassihi,
S. C. Schlichter,
K. S. Wheeler,
B. E. Crute,
N. King,
N. Nutile-McMenemy,
W. W. Noll,
S. Daniel,
J. Ha,
K. H. Kim,
and
L. A. Witters.
Identification of a second human acetyl-CoA carboxylase gene.
Biochem. J.
316:
915-922,
1996[Medline].
35.
Winder, W. W.,
J. Arogyasami,
R. J. Barton,
I. M. Elayan,
and
P. R. Vehrs.
Muscle malonyl-CoA decreases during exercise.
J. Appl. Physiol.
67:
2230-2233,
1989
36.
Winder, W. W.,
J. Arogyasami,
I. M. Elayan,
and
D. Cartmill.
Time course of the exercise-induced decline in malonyl-CoA in different muscle types.
Am. J. Physiol.
259 (Endocrinol. Metab. 22):
E266-E271,
1990
37.
Winder, W. W.,
and
D. G. Hardie.
Inactivation of acetyl-CoA carboxylase and activation of AMP-activated protein kinase in muscle during exercise.
Am. J. Physiol.
270 (Endocrinol. Metab. 33):
E299-E304,
1996
38.
Witters, L. A.,
J. Widmer,
A. N. King,
K. Fassihi,
and
F. Kuhajda.
Identification of human acetyl-CoA carboxylase isozymes in tissue and in breast cancer cells.
Int. J. Biochem.
26:
589-594,
1994[Medline].
39.
Zierz, S.,
and
A. G. Engel.
Different sites of inhibition of carnitine palmitoyltransferase by malonyl-CoA, and by acetyl-CoA and CoA, in human skeletal muscle.
Biochem. J.
245:
205-209,
1987[Medline].