From the Department of Pathophysiology, Center of Internal Medicine
(R.S., G.H.) and the Department of Nuclear Medicine, Center of Radiology
(C.K., H.C., A.B.), University Essen, School of Medicine, FRG.
Correspondence to Gerd Heusch, MD, FESC, FACC, Department of Pathophysiology, Center of Internal Medicine, University Essen, School of Medicine, Hufelandstraße 55, 45122 Essen, FRG.
Methods and Results—In 12 anesthetized pigs,
[1-11C]acetate was injected as a bolus into the
cannulated left anterior descending coronary artery during
normoperfusion, inotropic stimulation, and early (5 to 45 minutes) and
prolonged ischemia (60 to 90 minutes). Regional myocardial
oxygen consumption (M
Conclusions—Kmono correlates to
M
Modeling of myocardial [1-11C]acetate clearance
assumes constant sizes of all labeled pools. However, during prolonged
hypoxia or ischemia, the myocardial aspartate and
glutamate concentrations decrease
substantially,8 24 26 thereby potentially
affecting the calculation of the rate constant of the
monoexponential fit of the
time–[1-11C]acetate curve. Particularly during
prolonged myocardial ischemia, such as in hibernating
myocardium, the determination of myocardial oxidative
metabolism may be important.27 28
Therefore, in the present study, the relationship between
myocardial [1-11C]acetate clearance and oxygen
consumption was assessed during normoperfusion, inotropic stimulation,
and early and more prolonged myocardial ischemia by injection
of [1-11C]acetate into the LAD of
anesthetized swine using an established model of short-term
myocardial hibernation29 30 and taking changes in
aspartate and glutamate concentrations during prolonged
ischemia into consideration.
Experimental Model
Regional Myocardial Dimensions
Regional Myocardial Blood Flow
Regional Myocardial Oxygen and Lactate Consumption
Myocardial Aspartate and Glutamate Concentrations
Radiochemistry
Coronary Venous 11CO2
Morphology
Experimental Protocol
Group 1 (n=6)
Group 2 (n=6)
Data Acquisition
Data Analysis
Similarly, kmono was corrected for the decrease
in the peak count rate within the region of interest. The peak count
rate during early and more prolonged ischemia was expressed as
a fraction of the peak count rate under control conditions and then
multiplied with kmono:
kmonocorr=kmonox(countsROI)ischemia÷(countsROI)control.
Statistics
Coronary Venous Blood Activity
Regional Myocardial Blood Flow, Metabolism, and
[1-11C]Acetate Kinetics
Relationships Between kmono and
M
Correction for the decreases in peak count rate based on all data
points obtained throughout the experiment
(y=1.11x10-3xx+2.96x10-2,
r=.91) only slightly increased the y axis
intercept of the relationship between myocardial oxygen consumption and
kmono compared with the uncorrected relationship
(Fig 4A
Critique of Methods
With systemic administration of the tracer, its delivery will be flow
dependent, vary between the region of interest and the reference
region, and therefore require normalization to myocardial blood flow.
In the present study, however, labeled acetate was injected only
into the cannulated LAD, and such normalization to myocardial blood
flow was not necessary.
The variation in the measured 11C activity within
the region of interest could represent the
physiological variation of either regional
myocardial blood flow or metabolism. With myocardial tissue
with an average weight <100 mg, substantial variations in regional
blood flow have indeed been demonstrated.37 Thus,
the variation of activity from pixel to pixel might reflect the
differences in blood flow distribution, because the pixel volume
averaged (2.4x3.4x3 mm3), which, when a
density of 1 is assumed, results in an average sample weight of only 25
mg.
The time constants k1 and
k2 from a two-compartment model, as first
introduced by Armbrecht et al,4 were not
calculated. Because of the limited acquisition period, especially
during ischemia, a reasonable determination of
k2 in the two-compartment model was not possible
because of the slow kinetics of the second component. It therefore
remains unclear whether the limitations outlined for the
monoexponential model with respect to altered amino
acid pool sizes are also valid for other compartment models. Because
the exchange rate between the TCA cycle and the glutamate pool is known
to be rapid,8 19 22 23 it is unlikely that the
second, slow component adequately reflects such rapid exchange. The
second, slow component of the two-compartment model might, however,
reflect the slow conversion of labeled glutamate to glutamine or the
further metabolism of glutamine that has already been
formed.4 In the present study, lactate
consumption increased during the first degree of inotropic stimulation
but once again decreased toward control values when inotropic
stimulation was further increased. Using 14C-labeled
glucose, Massie et al38 demonstrated an increased
lactate uptake but also lactate production during
dobutamine stimulation in the anesthetized pig in
situ, resulting in an almost unchanged net lactate uptake. Similar
findings were also obtained in anesthetized
dogs.39 The increased lactate production
during inotropic stimulation indicates some myocardium with
an oxygen supply-demand imbalance.40 Thus, during
inotropic stimulation, areas with increased oxygen demand and increased
oxygen supply coexist with areas with increased oxygen demand but
insufficient oxygen supply.
Myocardial alanine, which can be produced from either aspartate or
glutamate during ischemia,24 26 was not
measured in the present study. The increase in the alanine
concentration by transamination of glutamate would not trap activity
within the myocardium because only
NH2 is transferred to pyruvate. The myocardial
aspartate concentration decreased by 23%. Under the assumptions that
aspartate declined continuously during ischemia and that
aspartate was completely converted into alanine,
a maximal error of (30 minutes/90 minutesx23% total decline in
aspartate)= 8% could have occurred during the 30-minute measuring
interval. Therefore, by use of the decrease in the glutamate and
aspartate concentrations for correction, kmono
could be overcorrected by a maximum of 8%.
Amino Acid Metabolism and kmono
During ischemia, anaerobic glycolysis is the major
energy source, and lactate accumulates. In the presence of high
concentrations of tissue lactate, the conversion of pyruvate to lactate
is retarded owing to the mass action effect of lactate accumulation
with subsequent augmentation of glutamate-pyruvate
transamination.44 This might result in a net
reduction in lactate because alanine is produced from pyruvate; the
predominant amino acid for transamination of pyruvate is
glutamate.45 Indeed, also in the present
study, lactate production was attenuated during prolonged
ischemia, and the glutamate concentration significantly
decreased. Glutamate is further involved in the clearance of ammonia
from the heart. During anoxia and ischemia, ammonia is produced
within the myocardium by breakdown of adenosine to
inosine.26 46 Alanine, which is formed through
transamination of pyruvate (thereby accepting ammonia), acts as a
nontoxic carrier of ammonia.47 Therefore, despite
an increased glutamate uptake,44 48 the
myocardial glutamate concentration is significantly decreased during
ischemia.24 26
The decrease in amino acid concentrations during prolonged
ischemia—at an equivalent reduction of regional myocardial
oxygen consumption during early and late ischemia—was
associated with a relative increase in kmono back
toward control values. Correction of kmono was
performed under the assumption that at an unchanged regional myocardial
oxygen consumption, the flux rate through the TCA cycle also remained
unchanged. Understandably, then, correction for the decrease in the
aspartate and glutamate concentrations restored the close correlation
between regional myocardial oxygen consumption and
kmono observed during normoperfusion, inotropic
stimulation, and early ischemia (Fig 3B
Clinical Implications
While in clinical practice the correction of
kmono might be less important for the assessment
of myocardial viability, such correction offers the potential to better
identify the underlying mechanism of contractile dysfunction. It is
currently unclear whether hibernation as observed in the clinical
setting is a manifestation of and adaptation to a persistent reduction
in blood flow50 or the result of repetitive
stunning.51 The correct measurement of myocardial
oxygen consumption is a prerequisite to quantitatively correlate the
observed reduction in function and oxygen demand to that in oxygen
supply, and the matching between demand and supply is a hallmark of the
classic concept of hibernation.52 53 Whether or
not in a clinical setting with a single-spot PET measurement, comparing
the region of interest to a reference region, the correction proposed
by the present study using serial PET measurements in the region of
interest will be useful remains to be determined.
Received August 27, 1997;
revision received October 8, 1997;
accepted October 21, 1997.
© 1998 American Heart Association, Inc.
Basic Science Reports
Positron Emission Tomography Analysis of [1-11C]Acetate Kinetics in Short-term Hibernating Myocardium
Abstract
Top
Abstract
Introduction
Methods
Results
Discussion
References
Background—Modeling of the
time-[1-11C]acetate activity curve assumes a constant
concentration of labeled tricarboxylic acid cycle intermediates and
associated metabolites, such as glutamate and aspartate, which may,
however, decrease in short-term hibernating myocardium. 
O2, microliters per
minute per gram) was measured, and the absence of necrosis was verified
by triphenyl tetrazolium chloride staining. Inotropic stimulation
increased MO2 from 52.5±7.4 to
195.4±36.2 (mean±SD) and the rate constant (kmono,
minutes-1) of [1-11C]acetate clearance from
0.094±0.018 to 0.322±0.076. During early ischemia,
MO2 and kmono were decreased
to 24.3±8.5 and 0.061±0.011, respectively. Kmono closely
correlated to MO2 during normoperfusion,
inotropic stimulation, and early ischemia. In short-term
hibernating myocardium, however, at an unchanged
MO2, kmono increased toward
control values (0.080±0.014). Myocardial glutamate and aspartate
concentrations (biopsies) decreased to 47±26% and 77±18%; the peak
count rate decreased to 66±22% of its respective control value. After
correction for the decreases in glutamate and aspartate or in peak
count rate, kmono was again decreased (0.050±0.016 or
0.052±0.014, respectively), and a close relationship to
MO2 was restored. O2 in short-term hibernating
myocardium when the decreases in aspartate and glutamate or
in peak count rate are considered.
Key Words: tomography • ischemia, myocardial • hibernation, myocardial • metabolism
Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
The clearance of
[1-11C]acetate from the myocardium
has been used to assess myocardial oxidative metabolism by
PET in both animals1 2 3 4 5 6 7 8 and
humans.7 9 10 11 12 13 14 15 16 17 18 At increased cardiac workload and
myocardial oxygen consumption, the rate constant of the
monoexponential fit of the time
[1-11C]acetate curve is
increased,3 7 and it is decreased during early
acute ischemia, when myocardial oxygen consumption is
reduced.1 4 8
[1-11C]acetate is activated within the
cell and translocated into the mitochondria; more specifically, it is
incorporated into different TCA cycle intermediates and equilibrates
very rapidly with aspartate and glutamate.19 In
isolated rat hearts, the sum of the myocardial aspartate and glutamate
concentrations is 8 to 10 times higher than the sum of the TCA cycle
metabolites,8 19 20 and the exchange rate between
-ketoglutarate and glutamate is equal to21 or
higher than8 19 22 23 the TCA cycle rate itself.
Also in the anesthetized dog, the sum of the myocardial
aspartate and glutamate concentrations is 20 times higher than the
concentration of succinate,24 the predominant TCA
cycle intermediate.8 20 Two minutes after
[1-14C]acetate injection in isolated rat
hearts, >60% of the total activity injected is found in labeled
glutamate,8 whereas labeled TCA cycle metabolites
contribute only 8% to total tissue activity.8
With continuous [1-13C]acetate infusion in
conscious rabbits, 95% of total [11C] activity
within the myocardium is related to labeled
glutamate.25
Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
The experimental protocols used in this study were approved by
the bioethical committee of the district of Düsseldorf, and they
adhere to the guiding principles of the American Physiological
Society.
The experimental model has been described extensively
elsewhere29 30 ; in brief, in 12
enflurane-anesthetized Göttinger miniswine (20 to 40 kg)
of either sex, both common carotid arteries were cannulated for
measurement of arterial pressure and to supply blood to an
extracorporeal circuit. A left lateral thoracotomy was performed, and a
micromanometer was placed in the left ventricle
through the apex. Ultrasonic dimension gauges were implanted in the LV
myocardium to measure the thickness of the anterior and
posterior (control) walls. The proximal LAD was cannulated and perfused
from an extracorporeal circuit. Before coronary cannulation,
the pigs were anticoagulated with sodium heparin. The system included a
roller pump, windkessel, and side ports—distal from the
windkessel—for the injection of radiolabeled microspheres and
[1-11C]acetate and for the measurement of
coronary arterial pressure. The large epicardial
vein that paralleled the LAD was cannulated at the same site as the
coronary artery. Rectal temperature was monitored, and body
temperature was kept between 37°C and 38°C with heating pads. Heart
rate was controlled by left atrial pacing.
End diastole was defined as the point when the first
derivative of LV pressure (LV dP/dt) started its rapid upstroke after
crossing the zero line. Global LV end systole was defined as the time
point of peak negative LV dP/dt and regional end systole as the point
of maximal wall thickness within 20 ms before peak negative LV
dP/dt.31 Anterior systolic wall
thickening and regional myocardial work index are
reported.30
Radiolabeled microspheres were injected into the
coronary perfusion circuit to determine regional myocardial
blood flow. Blood flow to the tissue at the site of the ultrasonic
crystals is reported, and this piece of tissue was divided into
transmural thirds of approximately equal thickness. Transmural blood
flows in different pieces (average weight, 1.08±0.51 g) within and
along different heart slices revealed a coefficient of variation of
2.6±1.7%.
Oxygen content was measured with anaerobically
sampled blood drawn simultaneously from the
coronary vein and an artery
(Cavitron/LexO2-Con-k, Dr B.G. Schlag, Bergisch
Gladbach). Lactate was measured in simultaneously drawn
coronary venous and arterial blood samples by use
of enzymatic dehydrogenation and subsequent photometry of NADH.
Myocardial oxygen (MO2) and
lactate consumption were calculated by multiplying the
arterial-coronary venous difference by the
transmural blood flow at the crystal site.
Transmural myocardial biopsies (
10 mg each) were taken in six
swine of group 1 under control conditions and at the end of the
90-minute ischemic period (see below) with a modified dental
drill. Samples requiring more than 1 to 2 seconds for acquisition were
not used for this analysis. Samples were
homogenized in 0.33 mol/L perchloric acid
(Mikro-Dismembrator, B. Braun Melsungen), and the pH of the supernatant
was adjusted to 7.2 with 1.1 mol/L
K3PO4.
Aspartate was determined by transamination with -ketoglutarate and
glutamic-oxalacetic transaminase, followed by hydrogenation of the
formed oxalacetate with NADH and malic dehydrogenase. The decrease in
NADH extinction was measured photometrically at 340 nm (model 8452,
Hewlett-Packard Co).32 Glutamate was
determined by the glutamic dehydrogenase reaction. In this reaction,
NADH is formed stoichiometrically when the equilibrium is shifted by
the addition of hydrazine sulfate at pH
9.33 The NADH formation was measured at 340
nm.
[1-11C]acetate was prepared via
carboxylation of methylmagnesium bromide.34
The radiochemical yield, defined as the ratio of 11C activity determined in
[1-11C]acetate to the 11C activity at the beginning of the
acetate synthesis, averaged 68±7%. The specific activity was in the
range of 10 to 20 GBq ·
µmol-1. Radiochemical purity was
determined by high-performance liquid
chromatography to be >97.5% by use of a LiChrospher
100 RP-18 column (250x4 mm, 10 µm; Merck) with 0.0003N
H2SO4 as eluent
(1.0 mL · min-1). For each
intracoronary injection, activities of about 1 to 3 mCi
1-[11C]acetate were administered within
1 minute.
The vein that paralleled the LAD was cannulated and drained
into an unpressurized reservoir to avoid recirculation of labeled
substance. If [1-11C]acetate is completely
metabolized to 11CO2 within
the TCA cycle, its clearance from the myocardium reflects
myocardial oxygen consumption. The coronary venous blood was
sampled during and at 2, 3, 5, 7, 10, 15, 20, 25, 30, 35, and 40
minutes after the injection of [1-11C]acetate.
The total activity of the coronary venous blood was counted,
and thereafter 11CO2 was
eliminated from the coronary venous blood by acidification (pH
1.0) and bubbling with N2 for 5 minutes. The
remaining activity within the coronary venous blood was once
again assessed, corrected for the activity decay over time, and related
to total activity.
At the end of each study, the heart was removed and sectioned
from base to apex into five transverse slices in a plane parallel to
the AV groove. The tissue slices were immersed in a 0.09 mol/L sodium
phosphate buffer, pH 7.4, containing 1.0% triphenyl tetrazolium
chloride and 8% dextran to verify the absence of infarcted
tissue.
Two groups of swine were studied. Under control conditions, the
perfusion pump was adjusted so that the minimum coronary
arterial pressure was not <70 mm Hg to avoid any
initial hypoperfusion. Therefore, mean coronary
arterial pressure exceeded peak LV pressure. In both groups
of swine, each observation period began with the
simultaneous withdrawal of a pair of arterial
and coronary venous blood samples. During the blood sampling,
microspheres and [1-11C]acetate were
injected into the LAD perfusion system. Systemic
hemodynamic and regional myocardial dimension data were
recorded throughout the microspheres injection.
After control measurements, epinephrine was infused
intravenously (0.5 to 1.0 µg ·
kg-1 · min-1) to
increase heart rate by 30 bpm. During inotropic stimulation, the
perfusion pump was set to maintain mean coronary
arterial pressure equal to LV peak pressure to avoid any
hypoperfusion. The dose of epinephrine was then maintained, and
regional myocardial blood flow, function, and metabolism
were measured while [1-11C]acetate was once
again injected intracoronarily. Thereafter, heart rate was further
increased by 30 bpm with a further increased dose of
intravenous epinephrine (1.0 to 4.0 µg ·
kg-1 · min-1) and
additional left atrial pacing, and measurements were once again
repeated. Thereafter, the infusion of epinephrine and left
atrial pacing were stopped, and restoration of systemic
hemodynamics and regional myocardial function was
ensured. The perfusion pump was then adjusted to reduce
coronary inflow by 50%, a level previously shown to be
compatible with the development of short-term hibernating
myocardium over 90 minutes.29 30
Flow, function, and myocardial oxygen consumption were measured at 5
minutes of hypoperfusion, whereas the PET data for early
ischemia were acquired between 5 and 45 minutes of
hypoperfusion. Flow, function, and myocardial oxygen consumption were
once more measured at 90 minutes of hypoperfusion, and biopsies for the
determination of glutamate and aspartate concentration were taken,
whereas the PET data for prolonged ischemia were acquired
between 60 and 90 minutes of hypoperfusion.
To ensure that prior inotropic stimulation had no impact on the
results obtained during ischemia, in a second group of swine,
blood flow to the LAD was reduced to 50% immediately after control
measurements. Measurements were taken during early and prolonged
ischemia, as in group 1. After 90 minutes of ischemia,
the myocardium was reperfused for 2 hours.
PETs were acquired with an ECAT 953/15 scanner (Siemens/CTI)
with an axial field of view of 54 mm. Plane separation was
3.4 mm. The data were reconstructed with a Hanning filter and a
frequency cutoff of 0.5 to a 128 matrix. The emission data were
corrected for radioactive decay and for attenuation with a transmission
scan. A dynamic acquisition protocol with 30 frames of 1 second, 10
frames of 10 seconds, 8 frames of 60 seconds, and 6 frames of 300
seconds was used. The emission data were evaluated with a
monoexponential model. Transaxial scans were
analyzed. The axial and in-plane spatial resolution of the
images was about 9 mm. Because labeled acetate was injected into
the cannulated LAD, the perfusion territory was clearly delineated from
the left circumflex and right coronary artery perfusion
territory. Within the LAD perfusion territory, to avoid influences of
mixed perfusion territory (border zone), only the central area with a
distance of 2 to 3 mm from the left and right borders was used for
analysis. Therefore, the size of the region of interest ranged
from 17 to 28 mm in diameter, depending on heart size (different
swine) and the intervention used (control versus inotropic stimulation
versus ischemia). During ischemia, however, the size of
the region of interest remained unchanged within a given animal.
Because recirculation of labeled substance was avoided and acetate
taken up by the myocardium was metabolized to
CO2, the peak-to-noise ratio exceeded 50:1 during
normoperfusion and 30:1 during ischemia. The
parameters a and kmono of the
monoexponential model y(t) =a ·
e(-kmono · t) were determined pixelwise
by linear least-squares fitting. The linear fit was performed over the
time interval when the washout of tracer was dominated by
11CO2 (>95% of baseline)
and the logarithmic plot of the data was obviously linear. The
kmono values for a given region of interest were
averaged. Values for kmono thus are mean values
of six to eight heart slices, depending on the size of the perfusion
territory. The coefficient of variation of measured activity within the
region of interest throughout the slices averaged 11±3%, 10±4%,
12±4%, 20±10%, and 24±11% during control conditions, the two
degrees of inotropic stimulation, and early and late ischemia,
respectively. Within the same region of interest, the peak count rate
was determined during control conditions, inotropic stimulation, and
early and prolonged ischemia. To account for the different
amounts of injected activity, the true count rates are expressed per
millicurie of injected activity.
Hemodynamic data were recorded on an
eight-channel recorder and stored directly to the hard disk of an
AT-type computer. Hemodynamic and dimension
parameters were digitized and recorded over the time
period of microspheres injection with CORDAT II
software.35 The calculation of all
hemodynamic parameters was done on a
beat-to-beat basis, and data were then averaged over at least 33
cardiac cycles. kmono was corrected for the
observed decreases in the amino acid pool sizes with the following
assumptions: If acetate is metabolized completely to
CO2, the flux rate through the TCA cycle closely
correlates to myocardial oxygen consumption.6 The
flux rate through the TCA cycle is given by the product of the TCA
cycle intermediates and metabolites in equilibrium with the TCA cycle
times kmono. Therefore, as assumed in most
instances, at constant pool sizes of TCA cycle intermediates and
metabolites such as aspartate and glutamate,
kmono indeed correlates to myocardial oxygen
consumption. The total concentration of aspartate and glutamate is 8 to
10 times higher than the sum of the TCA cycle metabolites in isolated
rat hearts8 19 20 and in the
myocardium of anesthetized
dogs.24 Therefore, the flux rate can be
simplified as the sum of the aspartate and glutamate concentrations
times kmono. Alterations in the aspartate
and glutamate concentrations, at constant regional myocardial oxygen
consumption and thus flux rate through the TCA cycle, will then
inversely affect kmono. In conclusion, given the
observed changes in the glutamate and aspartate concentrations,
kmono was corrected by multiplying
kmono with the fraction of the glutamate (Glu)
and aspartate (Asp) concentration relative to their control values:
kmonocorr=kmonox[(Asp)ischemia+(Glu)ischemia]÷[(Asp)control+(Glu)control].
Statistical analysis was performed by use of SYSTAT
software. Because comparison of systemic hemodynamics,
regional myocardial blood flow, function, and metabolism
during normoperfusion and ischemia revealed no statistical
significant differences between groups 1 and 2 by use of a two-way
ANOVA, data at these time points were pooled and compared by use of a
one-way ANOVA for repeated measures. When significant differences were
detected, individual mean values were compared by use of
least-significant-difference post hoc tests. All data are reported as
mean±SD, and a value of P<.05 was accepted as indicating a
significant difference in mean values. Linear regression
analyses between regional myocardial oxygen consumption in the
LV area at risk and kmono and corrected
kmono were performed.
Results
Top
Abstract
Introduction
Methods
Results
Discussion
References
Systemic Hemodynamics and Regional Myocardial
Dimensions
With infusion of epinephrine, heart rate
(P<.05), LV end-diastolic pressure
(P=NS), peak pressure (P<.05), LV
dP/dtmax (P<.05), LV
dP/dtmin (P<.05), and
coronary blood flow (P<.05) increased (Table 1
). End-diastolic wall
thickness of the anterior myocardium (P=NS)
increased, whereas systolic wall thickening, in the presence of
increased heart rate and LV peak pressure, remained unaltered. The
anterior myocardial work index (P<.05) increased. Further
increases in the epinephrine dose in combination with left
atrial pacing resulted in a further increase in LV peak pressure
(P=NS), LV dP/dtmax and LV
dP/dtmin (both P<.05) and
coronary blood flow (P=NS). The
end-diastolic wall thickness of the anterior
myocardium tended to increase (P=NS), and
systolic wall thickening tended to decrease (P=NS);
the anterior myocardial work index remained unchanged. With
ischemia, the LV end-diastolic pressure was
increased (P=NS), and LV peak pressure (P=NS), LV
dP/dtmax (P=NS), and LV
dP/dtmin (P<.05) were reduced. In the
presence of significantly reduced coronary blood flow and
coronary arterial pressure (both
P<.05), end-diastolic wall thickness decreased
(P=NS), and both systolic wall thickening and the
work index of the anterior myocardium were reduced by
70% (both P<.05). Prolongation of ischemia
resulted in no further changes compared with early ischemia,
except that systolic wall thickening and the work index of the
anterior myocardium tended to be further depressed.
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Table 1. Systemic Hemodynamics and Regional
Myocardial Dimensions During Control Conditions, Inotropic Stimulation,
and Low-Flow Ischemia
During normoperfusion, >95% of total activity was related to
11CO2 at 5 minutes and
longer after the [1-11C]acetate injection (Fig 1A). During inotropic stimulation, >95%
of total activity was related to
11CO2 already at 3 minutes
after the [1-11C]acetate injection, whereas it
took almost 7 minutes during early and more prolonged myocardial
ischemia (Fig 1B) until >95% of total activity was related to
11CO2.
View larger version (26K):
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Figure 1. Original data from one animal on total and
11CO2-related activity in coronary
venous blood. During control conditions, >95% of total activity was
related to 11CO2 at 5 minutes and longer after
the [1-11C]acetate injection (A). During acute and
prolonged myocardial ischemia (B), it took almost 7 minutes
until >95% of total activity was related to
11CO2.
Infusion of low-dose epinephrine increased transmural
myocardial blood flow (P<.05) and oxygen consumption of the
anterior myocardium to twofold (P<.05) (Table 2). Myocardial lactate consumption
increased (P<.05). The rate constant of the
monoexponential fit of the time
[1-11C]acetate activity curve
(kmono) also increased to twofold
(P<.05). Further increases in the epinephrine dose
in combination with left atrial pacing resulted in an additional
increase in transmural myocardial blood flow to about fourfold the
baseline level (P<.05 versus preceding value) and a similar
increase in regional myocardial oxygen consumption (P<.05
versus preceding value). Myocardial lactate consumption returned toward
control values. The time constant kmono increased
further, however, only to 3.5-fold the baseline level
(P<.05 versus preceding value). With ischemia,
transmural myocardial blood flow and regional myocardial oxygen
consumption were reduced by 60% (both P<.05), and
lactate consumption under control conditions was reversed to net
lactate production (P<.05). The time constant
kmono was significantly reduced. With prolonged
ischemia, transmural myocardial blood flow and regional
myocardial oxygen consumption were not changed, whereas lactate
production was attenuated. In the absence of changes in
regional myocardial oxygen consumption, kmono
returned toward baseline values. Myocardial glutamate and aspartate
concentrations averaged 2.19±0.54 and 1.35±0.17 µmol ·
g-1 wet weight under control conditions,
respectively, and were reduced to 47±26% and 77±18% of their
control levels (both P<.05 versus control conditions) after
90 minutes of ischemia. The true count rates averaged
35 044±6540 cps/mCi under control conditions, 30 329±7658 cps/mCi
during inotropic stimulation, 27 077±8335 cps/mCi during early
ischemia, and 22 121±6358 cps/mCi during late
ischemia. In the six swine in which decreases in both the peak
count rate and glutamate and aspartate concentrations were measured, a
close correlation between these two variables existed (percent
change in peak count rate=0.99 times the percent change in the
glutamate and aspartate concentrations-1.76, r=.84).
Correction of kmono for the attenuation in peak
count rate during early ischemia decreased
kmono from 0.061±0.011 to 0.046±0.021
min-1. After correction for the decreases in
glutamate and aspartate or in peak count rate during prolonged
ischemia, kmono was again decreased from
0.080±0.014 to 0.050±0.016 or 0.052±0.014
minutes-1, respectively.
View this table:
[in a new window]
Table 2. Regional Myocardial Blood Flow,
Metabolism, and TCA Cycle Kinetics During Control
Conditions, Inotropic Stimulation, and Low-Flow Ischemia O2
A close correlation between kmono and
MO2 was observed when data
obtained during control conditions, inotropic stimulation, and early
ischemia were combined (Fig 2).
With prolonged ischemia, kmono was
increased, so that in short-term hibernating myocardium,
all data points relating kmono to regional
myocardial oxygen consumption were located above the regression line
determined during normoperfusion, inotropic stimulation, and early
ischemia (Fig 3A). Correction for
the significant decrease in labeled pool sizes of aspartate and
glutamate in group 1 restored a close relationship between
kmono and regional myocardial oxygen
consumption also in short-term hibernating myocardium (Fig 3B).
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Figure 2. A close correlation between kmono and
regional myocardial oxygen consumption existed during normoperfusion,
inotropic stimulation, and early myocardial ischemia.
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[in a new window]
Figure 3. Values for kmono in short-term
hibernating myocardium were located above the regression
line obtained during normoperfusion, inotropic stimulation, and early
acute myocardial ischemia (A). Correction for the significant
decrease in labeled pool sizes of aspartate and glutamate restored a
close relationship between kmono and
M O2 also in short-term hibernating
myocardium (B).). However, correction for the
decreases in peak count rate during prolonged ischemia once
more restored a close relationship between kmono
and regional myocardial oxygen consumption in short-term hibernating
myocardium (Fig 4B).
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[in a new window]
Figure 4. Correction for the decrease in peak count rate
early during ischemia slightly decreased the slope of the
relationship between myocardial oxygen consumption and
kmono determined during normoperfusion, inotropic
stimulation, and early ischemia (A). Correction for the
decrease in peak count rate during late ischemia once more
restored a close relationship between kmono and regional
myocardial oxygen consumption in short-term hibernating
myocardium (B).
Discussion
Top
Abstract
Introduction
Methods
Results
Discussion
References
In short-term hibernating myocardium, the
monoexponential rate constant per se does not correctly
reflect regional myocardial oxygen consumption. The lack of decrease in
the rate constant, in the presence of a significantly reduced regional
myocardial oxygen consumption, is related to alterations in the amino
acid pool sizes; correction for such a decrease in the pool sizes of
glutamate and aspartate or in peak count rate within the region of
interest restores a close relationship between the rate constant and
regional myocardial oxygen consumption.
In the present study, an established model of short-term
hibernation was used.29 30 36 A limitation of all
controlled experimental studies, including the present one, on
myocardial hibernation, however, is the limited observation period.
Under aerobic conditions, the oxidative metabolism of
fatty acids, lactate, and glucose provides most of the cardiac energy
requirements. Glutamate participates in the transport of electrons from
the cytosol into the mitochondria during oxidation of glucose or
lactate (malate-aspartate shuttle).41 This
process is strictly dependent on a high concentration of glutamate
because of the low affinity of the mitochondrial
carrier.42 High rates of ß-oxidation delay and,
vice versa, predominant use of glucose accelerate the incorporation of
the 13C label into glutamate in isolated rabbit
hearts.43 The somewhat smaller increase in
kmono than in regional myocardial oxygen
consumption during maximal inotropic stimulation (3.5-fold versus
4-fold) observed in the present study could relate to such altered
substrate use. 
). Similarly, correction
of kmono for the decrease in the peak count rate
within the region of interest also restored the close correlation
between regional myocardial oxygen consumption and
kmono (Fig 4B). These findings indicate that the
peak count rate and the decay of activity from the
myocardium after [1-11C]acetate
injection are closely related to the myocardial glutamate and aspartate
concentrations.
In patients with coronary artery disease, dynamic
PET imaging with [1-11C]acetate facilitates the
accurate identification of dysfunctional but still viable
myocardium, which is capable of recovering
systolic function after coronary
revascularization.11 13 14 49
[1-11C]acetate even exhibits positive
and negative predictive values superior to
fluorine-18-gluorodeoxyglucose.49
Selected Abbreviations and Acronyms
LAD
=
left anterior descending coronary artery
LV
=
left ventricular/left ventricle
PET
=
positron emission tomography
TCA
=
tricarboxylic acid
Acknowledgments
This work was supported by grants from the German Research
Foundation (He 1320/8–1 and Schu 843/3–1). We are grateful to Prof Dr
M. Schwaiger for his helpful suggestions.
References
Top
Abstract
Introduction
Methods
Results
Discussion
References
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