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Am J Physiol Heart Circ Physiol 294: H954–H960, 2008.
First published December 14, 2007; doi:10.1152/ajpheart.00557.2007.
Metabolic response to an acute jump in cardiac workload: effects
on malonyl-CoA, mechanical efficiency, and fatty acid oxidation
Lufang Zhou,1 Hazel Huang,2 Celvie L. Yuan,3 Wendy Keung,4 Gary D. Lopaschuk,4
and William C. Stanley2,3,5
Departments of 1Biomedical Engineering, 2Physiology and Biophysics, and 3Nutrition, Case Western Reserve University,
Cleveland, Ohio; 4Department of Pediatrics, University of Alberta, Edmonton, Alberta, Canada; and 5Division of Cardiology,
Department of Medicine, University of Maryland, Baltimore, Maryland
Submitted 13 May 2007; accepted in final form 11 December 2007
acetyl-CoA carboxylase; AMP-activated protein kinase; exercise;
fatty acids; heart; mitochondria
ONE OF THE MAJOR DETERMINANTS of myocardial oxygen consumption (MV̇O2) at a given rate of left ventricular (LV) power
generation is mitochondrial substrate selection (23, 44). Under
normal resting conditions, fatty acid oxidation is the predominant source of energy for cardiac power generation (60 –
80%); however, studies in humans (39), dogs (30, 31), and pigs
(25) in vivo and in isolated perfused rat (3, 21) and mouse (20)
hearts show that, with high rates fatty acid oxidation, the
external power is reduced for a given MV̇O2 (3, 30, 39). In the
failing heart or during acute ischemia and/or reperfusion,
pharmacological treatment with agents that inhibit myocardial
Address for reprint requests and other correspondence: W. C. Stanley,
Division of Cardiology, Dept. of Medicine, Univ. of Maryland-Baltimore,
20 Penn St., HSF2, Rm. S022, Baltimore, MD 21201 (e-mail: wstanley
@medicine.umaryland.edu).
H954
fatty acid oxidation (5, 6) or directly activate carbohydrate
oxidation (2, 28, 41) increases LV function without affecting
MV̇O2 and, therefore, improves LV mechanical efficiency (defined as the ratio of external LV power to LV energy expenditure). However, the effect of inhibition of fatty acid oxidation
on cardiac function and MV̇O2 during high cardiac workloads
in the healthy heart is not known.
Under normal or ischemic conditions, fatty acid oxidation
strongly inhibits the mitochondrial enzyme pyruvate dehydrogenase, which inhibits oxidation of pyruvate and, thus, glucose
and lactate uptake and oxidation (35). On the other hand, it has
been shown that inhibition of fatty acid oxidation increases
pyruvate oxidation and glucose uptake and oxidation at rest
and during exercise (26, 41, 45). Fatty acid oxidation in the
heart is regulated at the level of the mitochondrial outer
membrane by the activity of carnitine palmitoyltransferase I
(CPT-I), which is inhibited by malonyl-CoA on the cytosolic
side of the enzyme (22, 44). Several studies have shown an
inverse relationship between myocardial malonyl-CoA content
and fatty acid oxidation (9, 24, 37, 40, 43) and, specifically,
that adrenergic stimulation corresponds with a reciprocal increase in fatty acid oxidation and decrease in malonyl-CoA
content (13–16, 24, 36). Malonyl-CoA is produced by acetylCoA carboxylase (ACC), which is inhibited when phosphorylated at serine 79 by AMP-activated protein kinase (AMPK),
whereas AMPK is activated by phosphorylation at threonine
172 (10). We previously found that the reduction in malonylCoA content when MV̇O2 was increased by adrenergic stimulation was not associated with reduced ACC activity or increased AMPK activity in pigs (15); however, tissue was
sampled 15–30 min after the initiation of stimulation. It is not
known whether there are changes in malonyl-CoA content and
activation of ACC and AMPK during the initial minutes of the
transition from a low to a high cardiac workload.
The present study evaluated the effect of inhibition of fatty
acid oxidation on myocardial LV function and mechanical
efficiency. We hypothesized that a switch in myocardial energy
substrate use from fatty acid to carbohydrates would increase
LV power without affecting oxygen consumption and, therefore, improve myocardial energy efficiency. The second aim of
the present study was to determine whether there is a decrease
in malonyl-CoA content with an abrupt short-term increase in
cardiac workload. We hypothesized that, with 5 min of increased cardiac workload, the myocardial content of malonylThe costs of publication of this article were defrayed in part by the payment
of page charges. The article must therefore be hereby marked “advertisement”
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0363-6135/08 $8.00 Copyright © 2008 the American Physiological Society
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Zhou L, Huang H, Yuan CL, Keung W, Lopaschuk GD, Stanley
WC. Metabolic response to an acute jump in cardiac workload: effects on
malonyl-CoA, mechanical efficiency, and fatty acid oxidation. Am J
Physiol Heart Circ Physiol 294: H954–H960, 2008. First published
December 14, 2007; doi:10.1152/ajpheart.00557.2007.—Inhibition of
myocardial fatty acid oxidation can improve left ventricular (LV)
mechanical efficiency by increasing LV power for a given rate of
myocardial energy expenditure. This phenomenon has not been assessed at high workloads in nonischemic myocardium; therefore, we
subjected in vivo pig hearts to a high workload for 5 min and assessed
whether blocking mitochondrial fatty acid oxidation with the carnitine
palmitoyltransferase-I inhibitor oxfenicine would improve LV mechanical efficiency. In addition, the cardiac content of malonyl-CoA
(an endogenous inhibitor of carnitine palmitoyltransferase-I) and
activity of acetyl-CoA carboxylase (which synthesizes malonyl-CoA)
were assessed. Increased workload was induced by aortic constriction
and dobutamine infusion, and LV efficiency was calculated from the
LV pressure-volume loop and LV energy expenditure. In untreated
pigs, the increase in LV power resulted in a 2.5-fold increase in fatty
acid oxidation and cardiac malonyl-CoA content but did not affect the
activation state of acetyl-CoA carboxylase. The activation state of the
acetyl-CoA carboxylase inhibitory kinase AMP-activated protein kinase decreased by 40% with increased cardiac workload. Pretreatment
with oxfenicine inhibited fatty acid oxidation by 75% and had no
effect on cardiac energy expenditure but significantly increased LV
power and LV efficiency (37 ⫾ 5% vs. 26 ⫾ 5%, P ⬍ 0.05) at high
workload. In conclusion, 1) myocardial fatty acid oxidation increases
with a short-term increase in cardiac workload, despite an increase in
malonyl-CoA concentration, and 2) inhibition of fatty acid oxidation
improves LV mechanical efficiency by increasing LV power without
affecting cardiac energy expenditure.
CARDIAC METABOLISM AT HIGH WORKLOAD
METHODS
Studies were performed in accordance with the National Institutes
of Health Guide for the Care and Use of Laboratory Animals (NIH
Publication No. 85-23, revised 1996) and with prior approval of the
Institutional Animal Care and Use Committee at Case Western Reserve University. Twenty-four domestic pigs of either sex (35.1 ⫾ 1.1
kg body wt) were entered into the study. Data from these studies on
the regulation of pyruvate dehydrogenase activity, NADH, glycogen
concentration, and basic hemodynamics have been reported separately (48).
Surgical preparation. The surgical preparation has been previously
described in detail (24, 38, 48). Briefly, overnight-fasted pigs were
sedated with tiletamine-zolazepam (Telazol, 6 mg/kg im), anesthetized
with isoflurane by mask (5%), ventilated with 100% O2, and maintained
on isoflurane (0.75–1.5%) and ketamine (4 mg䡠kg⫺1 䡠min⫺1 iv) to keep
PCO2 and pH in the normal range (⬎100 mmHg PO2, 35– 45 mmHg PCO2,
and pH 7.35–7.45). A femoral artery and vein were catheterized for blood
sampling and infusion, respectively, and the animals were treated with
heparin (200 U/kg bolus followed by 100 U䡠kg⫺1 䡠min⫺1 iv) to prevent
clotting and thrombus formation. The heart was exposed via a midline
sternotomy, and the left atrium was catheterized for infusion of
dobutamine and atropine. A vascular occluder was placed around the
ascending aorta and constricted during dobutamine treatment. The
cardiac anterior interventricular vein was catheterized for coronary
venous blood sampling, and a Doppler ultrasonic flowmeter was
placed around the proximal left anterior descending coronary artery
(LAD) to record blood flow continuously (Transonics). Four sonomicrometry crystals were placed at midmyocardial depth in the base,
apex, and septum of the lateral wall of the LV to continuously
measure LV volume with an online commercial system (Sonometrics). A high-fidelity pressure transducer (Millar Instruments) was
positioned in the LV, the signal was integrated with LV volume, and
the LV pressure-volume loop area was calculated for each beat.
Experimental protocol. Three groups of pigs were studied: 1) an
untreated group subjected to increased cardiac workload with dobutamine infusion (DOB, n ⫽ 8), 2) an oxfenicine-treated group that was
subjected to increased cardiac workload (DOB ⫹ OXF, n ⫽ 8), and
3) a control group (CON, n ⫽ 8) with sham instrumentation and
normal cardiac workload. At the beginning of the protocol, [9,103
H]oleate tracer was infused (40 ␮Ci/h iv) for the measurement of
fatty acid oxidation, and oxfenicine treatment was initiated in the
DOB ⫹ OXF group (30 mg/kg iv bolus oxfenicine followed by an
infusion at 30 mg 䡠 kg⫺1 䡠 min⫺1). After a 50-min equilibrium period,
animals in the DOB and DOB ⫹ OXF groups were subjected to 5 min
of increased cardiac work induced by constriction of the aortic cuff
sufficient to maintain the peak LV systolic pressure at ⬃190 mmHg,
during simultaneous infusion of dobutamine (100 ␮g/kg as a bolus
followed by 40 ␮mol 䡠 kg⫺1 䡠 min⫺1) and atropine (2 mg iv bolus) into
AJP-Heart Circ Physiol • VOL
the left atrium to increase heart rate and contractility. Arterial and
venous blood samples for measurement of blood glucose and lactate
concentrations were taken before (⫺5 and ⫺1 min) and during (20,
45, and 75 s and 2, 3, 4, and 5 min) dobutamine treatment, and
samples were drawn for plasma free fatty acid and 3H2O concentrations at ⫺5 and ⫺1 min and 3, 4, and 5 min of dobutamine infusion.
After 5 min of increased workload, a large (⬃3 g) punch biopsy was
rapidly excised from the LAD bed, immediately freeze clamped on
aluminum blocks precooled in liquid nitrogen, and stored at ⫺80°C
for later analysis (40). The CON group received the same infusion of
[9,10-3H]oleate, arterial and coronary venous blood samples were
drawn at 50 – 60 min, and a myocardial biopsy was immediately
obtained as described for the other two groups.
Analytic methods. Arterial and venous O2 saturation and hemoglobin were measured spectrophotometrically with a hemoximeter (AVOX System, San Antonio, TX), and pH, PCO2, and PO2 were
measured in a blood gas analyzer (Nova Biomedical, Waltham, MA).
Subsequent biochemical analysis was performed with the investigator
blinded to treatment. Blood samples were analyzed for concentrations
of glucose and lactate, and plasma was assayed for free fatty acids,
[3H]oleate, and 3H2O, as previously described (24, 38). Malonyl-CoA
and adenine nucleotide contents were assayed by high-pressure liquid
chromatography with UV detection, as previously described (40). All
tissue concentrations were expressed per gram wet weight of tissue.
The amounts of total and phosphorylated ACC and AMPK were
assessed by Western blot using specific antibodies for phosphorylated
(serine 79) ACC and phosphorylated (threonine 172) AMPK, as
previously described (24, 27). AMPK activity was measured on
myocardial homogenates, as previously described (11).
Calculations. Myocardial blood flow was measured from the ultrasonic flowmeter and normalized by dividing by the weight of the
heart being perfused by the LAD (34, 40). The net uptakes
(␮mol 䡠 kg⫺1 䡠 min⫺1) of glucose, lactate, free fatty acids, and oxygen
were calculated as arterial-venous difference ⫻ blood flow. The rate
of exogenous fatty acid oxidation (␮mol 䡠 g⫺1 䡠 min⫺1) was calculated
as [release of 3H2O (disintegrations per minute per milliliter) ⫻
myocardial blood flow] ⫼ arterial specific radioactivity of free fatty
acids (disintegrations per minute per micromole) (38).
Stroke volume was calculated as LV end-diastolic volume ⫺ LV
end-systolic volume and cardiac output as stroke volume ⫻ heart rate.
LV stroke work (J) was calculated as LV pressure (Pa) ⫻ volume and
LV power (W) as LV stroke work ⫻ heart rate (4). LV energy
expenditure was calculated from MV̇O2 with the assumption of 20.2
J/ml of O2 (46) and LV mechanical efficiency as LV power ⫼ LV
energy expenditure.
Statistical analysis. All hemodynamic variables; rates of free fatty
acid, glucose, and lactate uptake; rate of fatty acid oxidation; and
tissue metabolite concentrations were compared between resting conditions and increased cardiac work and between DOB and DOB ⫹
OXF groups using a one- or two-way ANOVA with Bonferroni’s post
hoc test for multiple comparisons, as appropriate. Significance was set
at P ⬍ 0.05, and values are means ⫾ SE.
RESULTS
As recently published separately from these experiments,
there was a significant increase in heart rate, peak LV pressure,
myocardial blood flow, and MV̇O2 in the DOB and DOB ⫹
OXF groups compared with the CON group, whereas there
were no differences between the DOB and DOB ⫹ OXF
groups (48). Cardiac output, stroke work, and LV energy
expenditure increased to a similar extent in the DOB and
DOB ⫹ OXF groups (Table 1). Oxfenicine significantly increased LV power compared with the DOB group (1.72 ⫾ 0.35
vs. 1.28 ⫾ 0.13 W at 45 s and 1.70 ⫾ 0.33 vs. 1.27 ⫾ 0.23 W
at 5 min; Fig. 1), despite no effect on LV energy expenditure
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CoA would decrease due to activation of AMPK and inhibition
of ACC, resulting in greater fatty acid oxidation, as previously
observed with longer-duration adrenergic stimulation (13–16,
24, 36). Studies were performed in an established open-chest
pig model, with animals subjected to an abrupt 5-min increase
in cardiac workload induced by simultaneous adrenergic stimulation, parasympathetic blockade, and aortic constriction (24,
38, 48). Oxfenicine, a CPT-I inhibitor, was used to inhibit fatty
acid oxidation at the level of transport into the mitochondria.
There are numerous approaches to inhibition of fatty acid
oxidation (29, 42). In the present study, oxfenicine was selected, because it is devoid of cardiovascular effects under
normal conditions (5, 7, 38, 48), has a rapid onset of action
(19), and consistently reduces fatty acid oxidation in the pig
heart under conditions of increased cardiac workload (24,
38, 48).
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Table 1. Hemodynamic responses to short-term increased cardiac power with or without pretreatment with oxfenicine
in pigs
DOB (n ⫽ 8)
Stroke work, J
Cardiac output, l/min
Peak ⫹dP/dt, mmHg/s
LV energy expenditure, W
DOB ⫹ OXF (n ⫽ 8)
CON (n ⫽ 8)
Baseline
45 s
5 min
Baseline
45 s
5 min
0.33⫾0.03
2.8⫾0.3
1,500⫾170
1.41⫾0.12
0.27⫾0.03
2.5⫾0.3
1,420⫾120
1.69⫾0.19
0.39⫾0.06*
3.7⫾0.5*
4,720⫾540*
4.04⫾0.39*
0.35⫾0.06
3.2⫾0.4
4,920⫾670*
4.81⫾0.40*
0.26⫾0.03
2.3⫾0.3
1,320⫾150
1.41⫾0.17
0.49⫾0.07*
3.8⫾0.8*
5,910⫾400*
3.96⫾0.42*
0.40⫾0.05
3.7⫾0.8
6,430⫾440*
5.21⫾0.54*
Values are means ⫾ SE. CON, control group with sham instrumentation and normal cardiac workload; DOB, untreated group subjected to increased cardiac
workload with dobutamine infusion; DOB ⫹ OXF, oxfenicine-treated group subjected to increased cardiac workload; LV, left ventricular; ⫹dP/dt, 1st derivative
of LV pressure with respect to time. *P ⬍ 0.05 vs. CON.
Fig. 1. Effect of oxfenicine (OXF) on myocardial left ventricular (LV)
external power and mechanical efficiency under normal conditions and at 45 s
and 5 min of increased cardiac workload. *P ⬍ 0.05 vs. baseline (Pre). †P ⬍
0.05 vs. untreated (dobutamine) at the same time.
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oxidation was also suppressed by oxfenicine at rest (14 ⫾ 9 vs.
49 ⫾ 14 nmol䡠g⫺1 䡠min⫺1, P ⬍ 0.05) and at a high cardiac
workload (Fig. 3). Lactate uptake was decreased in the DOB
and DOB ⫹ OXF groups in the 1st min of increased cardiac
energy expenditure (Fig. 2B), with 4 of 8 pigs in the DOB
Fig. 2. Net myocardial glucose, lactate, and fatty acid uptake as a function of
time for dobutamine (DOB) and DOB ⫹ OXF groups at rest and during
increased cardiac energy expenditure.
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(Table 1). As a result, a significant improvement in mechanical
efficiency was seen in the DOB ⫹ OXF group compared with
the DOB group (Fig. 1).
The arterial concentrations of lactate, glucose, and fatty
acids were unchanged over the course of the study, and values
were similar among the three experimental groups (data not
shown). Increased cardiac work significantly increased glucose
and fatty acid uptake in the DOB group (Fig. 2). Pharmacological inhibition of CPT-I suppressed fatty acid uptake in the
DOB ⫹ OXF group under resting and high cardiac work
conditions and further enhanced glucose and lactate uptakes
compared with the DOB group (Fig. 2). Myocardial fatty acid
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CARDIAC METABOLISM AT HIGH WORKLOAD
group and 2 of 8 pigs in the DOB ⫹ OXF group showing a net
release of lactate at 20 and/or 45 s.
The rate of free fatty acid oxidation was greater in the DOB
group than in the DOB ⫹ OXF group before (data not shown)
and during dobutamine treatment (Fig. 3). Malonyl-CoA concentration was greater in the DOB and DOB ⫹ OXF groups
than in the CON group (P ⬍ 0.05), with no significant
difference between these two groups (Fig. 3). Neither the
amount of total and phosphorylated ACC nor the ratio of
phosphorylated to total ACC was different among the groups
(Table 2). Although total AMPK was not different among
groups, phosphorylated AMPK and the ratio of phosphorylated
to total AMPK was significantly reduced in the DOB group
compared with the CON group, whereas the DOB ⫹ OXF
group was not different from the other two groups.
The cardiac content of AMP and ADP was not different
among the groups (Table 3), although ATP content was 20%
lower in the DOB and DOB ⫹ OXF groups than in the CON
group. Interestingly, using a less precise luciferase assay to
measure ATP and ADP content on these same samples, we
Table 2. Summary of Western blot assessment of total and
phosphorylated AMPK and ACC
DOB
DOB ⫹ OXF
247⫾9
154⫾3
237⫾21
88⫾17*
281⫾32
127⫾11
48⫾8
29⫾2
60⫾7
33⫾5
69⫾12
41⫾3
CON
AMPK
Total
Phosphorylated
ACC
Total
Phosphorylated
Phosphorylated-to-total
ACC ratio
0.70⫾0.13
0.58⫾0.10
DISCUSSION
The results of the present study show that, despite the
expected increase in myocardial fatty acid oxidation during an
acute work jump, there was a paradoxical increase in the tissue
content of malonyl-CoA, an established inhibitor of CPT-I and
cardiac fatty acid oxidation. In addition, the increase in malonyl-CoA was not due to activation of ACC but did correspond
to a decrease in AMPK activation. Oxfenicine had little effect
on malonyl-CoA content, despite a 75% decrease in fatty acid
oxidation, suggesting that the increase in malonyl-CoA was
largely independent of changes in fatty acid oxidation.
The second main finding of the present study is that inhibition of fatty acid oxidation with oxfenicine increases LV power
without increasing cardiac energy expenditure. This phenomenon has been observed under conditions of normal workload,
during demand-induced ischemia, and with postischemic reperfusion (41, 44). The present study extends this concept to conditions
of high workload similar to intense exercise in healthy people
and suggests the provocative idea that CPT-I inhibition might
improve exercise performance in short-term intense aerobic
athletic events. These observations are consistent with previous
studies showing that switching from fatty acid to carbohydrate
Table 3. Myocardial content of adenine nucleotides
0.86⫾0.31
Values are means ⫾ SE in arbitrary units. AMPK, AMP-activated protein
kinase; ACC, acetyl-CoA carboxylase. *P ⬍ 0.05.
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previously showed approximately twice the variability but,
again, no differences among the groups (48).
ATP
ADP
AMP
CON
DOB
DOB ⫹ OXF
4.64⫾0.11
1.08⫾0.02
0.159⫾0.004
3.64⫾0.15*
1.12⫾0.05
0.180⫾0.019
3.47⫾0.23*
1.13⫾0.03
0.181⫾0.011
Values are means ⫾ SE in ␮mol/g. *P ⬍ 0.004 vs. CON.
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Fig. 3. Myocardial malonyl-CoA content and rate of fatty acid oxidation (FAO) at the end of the protocol. *P ⬍ 0.05 vs. control (CON).
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response to high-dose dobutamine and increased aortic pressure in pigs. Although AMPK appears to play a central role in
the regulation of cardiac energy metabolism under many conditions (10), the results of the present in vivo study show that,
despite a significant decrease in phosphorylated AMPK and
AMPK activity (Fig. 4), glucose uptake, glycogenolysis, and
fatty acid oxidation are stimulated (Table 1, Figs. 1–3) (48).
Taken together, activation of AMPK is not an essential regulatory component of the metabolic response to a step increase
in cardiac workload.
Cardiac ATP content, as measured by high-pressure liquid
chromatography, was decreased by 20% under conditions of
high workload; however, AMP and ADP were not increased,
suggesting a net loss of adenine nucleotides during the 5 min
of increased cardiac workload. This is consistent with our
previous observation of an increased adenosine production and
efflux with dobutamine-induced work in pigs (17). In terms of
metabolic regulation, since there was also no increase in AMP
or phosphorylated AMPK, it appears that the modest fall in
ATP is not a major regulator of the energetic response to an
acute jump in workload.
Fig. 4. Myocardial phosphorylated AMP-activated protein kinase (P-AMPK)to-total AMPK ratio as measured by Western blot and myocardial AMPK
activity. *P ⬍ 0.05 vs. CON.
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utilization increases LV energy efficiency (2, 3, 20, 21, 25, 30,
32, 39). Compared with fatty acids, carbohydrates are more
oxygen efficient (i.e., for a given amount of ATP synthesis,
fatty acid oxidation requires 11% more oxygen consumption
than pyruvate), and high concentrations of fatty acids have also
been shown to uncouple oxidative phosphorylation and increase oxygen utilization in isolated mitochondria and cells
(44). Thus the improved efficiency with oxfenicine is likely
attributed to greater ATP synthesis per oxygen consumption
and/or more effective ATP use by the heart.
We previously observed that 15–30 min of intense adrenergic stimulation in pigs results in a reciprocal decrease in
malonyl-CoA content and an increase in fatty acid uptake and
oxidation (12, 26, 38), suggesting that less malonyl-CoA inhibition of CPT-I is a primary mechanism for the increase in
cardiac fatty acid oxidation observed with physiological
stresses such as an acute bout of exercise (15, 16, 24). In
contrast, the present investigation found that the rate of fatty
acid oxidation increased with dobutamine treatment, despite a
2.5-fold increase in tissue malonyl-CoA concentration (Fig. 3).
Since malonyl-CoA exists in cytosol and mitochondria (18, 22,
47), it is possible that the cytosolic malonyl-CoA decreased,
while mitochondrial malonyl-CoA increased. Malonyl-CoA
inhibits CPT-I on the cytosolic side of the enzyme (22, 44) and
is produced in the cytosol and mitochondrial matrix from
acetyl-CoA (44). The supply of acetyl-CoA is a major regulator of malonyl-CoA formation (36, 37). The increase in workload in the present experiment caused a 50% increase in the
acetyl-CoA concentration (48), presumably due to the rapid
stimulation of acetyl-CoA formation by pyruvate dehydrogenase, which may have triggered a selective increase of malonyl-CoA in the mitochondrial matrix. Thus the increased tissue
malonyl-CoA content may be due to a specific increase in
mitochondrial malonyl-CoA, as previously suggested (24). It is
impossible to prove this on the basis of current experimental
results, since malonyl-CoA was measured in whole tissue
without distinguishing cytosolic and mitochondrial compartments. Future studies should rapidly separate the cytosol and
mitochondria and measure these CoAs in these two compartments, although accurate measurements of cytosolic and mitochondrial malonyl-CoA have yet to be made because of technical difficulties with this approach. In any case, the results of
the present study clearly indicate that a fall in total tissue
malonyl-CoA content is not essential for the increase in cardiac
fatty acid oxidation in response to an acute increase in cardiac
workload.
Consistent with our previous studies in pigs (15, 24), there
was no increase in the activation of AMPK or ACC phosphorylation with increased cardiac workload. Studies in working
perfused rat hearts also found no increase of AMPK activity 1
or 15 min after a two- to threefold increase of cardiac power
(1). In addition, mice expressing a cardiac-specific dominantnegative AMPK␣2 subunit have normal ATP content and
glycogen depletion in response to acute exercise stress, stress
echocardiography, and have a normal maximal exercise capacity (33). On the other hand, 10 min of treadmill running in rats
approximately doubled AMPK activity and phosphorylated (at
threonine 172) AMPK and also doubled the amount of phosphorylated (at serine 79) ACC (8). The results of the present
study and our previous work in pigs (15, 24) consistently
suggest that AMPK is not activated nor is ACC inhibited in
CARDIAC METABOLISM AT HIGH WORKLOAD
ACKNOWLEDGMENTS
The authors thank Drs. Monika Duda, Isidore Okere, and Naveen Sharma
for assistance with the animal experiments.
GRANTS
This work was supported by National Institutes of Health Grants HL074237 and GM-66309 and by a grant from the Canadian Institutes of Health
Research. G. D. Lopaschuk is a Medical Scientist of the Alberta Heritage
Foundation for Medical Research.
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In conclusion, the results of the present study show that
inhibition of fatty acid oxidation improves LV mechanical
efficiency by increasing LV contractile power without affecting MV̇O2 during an acute bout of high workload. This finding
suggests a novel approach to improving LV mechanical efficiency at high cardiac workloads with drugs that optimize
myocardial energy metabolism and presents the possibility that
inhibition of CPT-I could potentially enhance performance in
athletic events that are limited by cardiac pump function. In
addition, we observed a paradoxical elevation of malonyl-CoA
concentration and fatty acid oxidation at high workloads.
Lastly, we observed a significant fall in AMPK activation
under high energy demand, which further illustrates that activation of AMPK is not an essential component of the metabolic
response to the increase in cardiac workload.
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40.
CARDIAC METABOLISM AT HIGH WORKLOAD