(Circulation. 1997;96:1313-1319.)
© 1997 American Heart Association, Inc.
Articles |
Failure to Maintain a Low ADP Concentration Impairs Diastolic Function in Hypertrophied Rat Hearts
From the NMR Laboratory for Physiological Chemistry, Cardiovascular Division, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School (R.T., L.N., J.S.I.), and the Cardiovascular Division, Department of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School (B.H.L.), Boston, Mass.
Correspondence to Rong Tian, MD, PhD, NMR Laboratory for Physiological Chemistry, Brigham and Women's Hospital, 221 Longwood Ave, Room 247, Boston, MA 02115. E-mail rong{at}bustoff.bwh.harvard.edu
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Abstract |
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Background Mechanisms in addition to diastolic calcium overload may contribute to diastolic dysfunction in hypertrophied hearts. In this study, we tested the hypothesis that failure to maintain a low ADP concentration in hypertrophied hearts contributes to diastolic dysfunction by inhibiting the rate of cross-bridge cycling.
Methods and Results By perfusing isolated rat hearts with pyruvate and 2-deoxyglucose (2DG), we were able to perturb [ADP] with minimal changes in [ATP] and [inorganic phosphate] or the contribution of glycolytic ATP to ATP synthesis. The effects of 2DG were compared in aortic-banded (LVH, n=5) and sham-operated (control, n=5) rat hearts. 31P NMR spectroscopy was used to measure the concentrations of phosphorus-containing compounds. We found a threefold increase of left ventricular end-diastolic pressure (LVEDP) in LVH during 2DG perfusion, and this increase was concomitant with a threefold increase in intracellular free [ADP]. The [ADP] in the control hearts was maintained <40 µmol/L, and no change in LVEDP was observed. A linear relationship between increases in [ADP] and LVEDP was found (r2=.66, P=.001). Furthermore, the capacity of the creatine kinase reaction, a major mechanism for maintaining a low [ADP], was decreased in LVH (P=.0001).
Conclusions Increased [ADP] contributes to diastolic dysfunction in LVH, possibly due to slowed cross-bridge cycling. Decreased capacity of the creatine kinase reaction to rephosphorylate ADP is a likely contributing mechanism to the failure to maintain a low [ADP] in LVH.
Key Words: hypertrophy • diastole • adenosine • creatine kinase
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Introduction |
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Hypertrophied hearts exhibit prolonged diastolic relaxation and greater diastolic dysfunction than normal adult hearts during energy deprivation.1 2 3 It has been suggested that the susceptibility to impaired relaxation is due to diastolic calcium overload in these hearts.1 2 3 4 However, a recent study showed a greater impairment of relaxation during energy deprivation in myocytes isolated from hypertrophied hearts than in controls, even though the diastolic calcium level increased to a similar extent for both groups.5 These results show that it is very likely that mechanisms in addition to diastolic calcium overload also contribute to the impairment of relaxation in hypertrophied myocardium.
The rate of actomyosin cross-bridge cycling, directly responsible for
force generation and relaxation, is controlled by the concentrations of
substrates and products of the actomyosin ATPase reaction: ATP,
ADP, and Pi. The release of ADP has been shown to be a
rate-limiting step for cross-bridge dissociation in cardiac and
skeletal muscle fibers.6 7 Thus, an increase in free
[ADP] should inhibit the rate of ADP release from the actomyosin
complex and thereby reduce the rate of cross-bridge cycling. If this
were to occur, myofibrillar relaxation would be impaired. Under
physiological conditions, several mechanisms ensure
that the free [ADP] is maintained at a low level in myocytes. Among
them, the CK reaction plays a prime role. This is because the rate of
ATP turnover via this reaction is at least an order of magnitude higher
than that of oxidative phosphorylation, glycolysis, or
the adenylate kinase reaction.8 By rapidly
transferring a phosphoryl group from PCr to ADP via the CK reaction
(PCr+ADP+H+ 
ATP+Cr), the free [ADP] is normally
maintained in a low range despite the fluctuations in the rate of ATP
synthesis and utilization. Previous studies showed that the activity of
CK and the total content of Cr in severely hypertrophied and failing
hearts are significantly reduced, indicating a decreased capacity of
this reaction.9 10 11 This decrease may compromise the
ability of hypertrophied myocardium to maintain a low
intracellular free [ADP], especially under stress conditions.
This study was designed to test the hypothesis that failure to maintain
a low [ADP] may render the hypertrophied heart more susceptible to
diastolic dysfunction. Fig 1
illustrates our
strategy to perturb [ADP] with minimal changes in [ATP] and a low
[Pi] in an isolated perfused rat heart preparation. We
first perfused the hearts with a buffer containing pyruvate and glucose
(Fig 1, left) and then switched to another buffer in which glucose was
substituted with 2DG. Because 2DG is not an appropriate substrate for
the glycolysis, it accumulates as 2DG-P after entering the cell (Fig 1, right). Phosphorylation of 2DG requires energy from ATP
hydrolysis, and accumulation of 2DG-P traps free intracellular
Pi. As a result, the intracellular [ATP] and
[Pi] should decrease and free [ADP] should increase.
Since [ADP] is in the micromolar range and [ATP] is at the
millimolar level, a significant increase of [ADP] can occur with only
a slight decrease of [ATP]. By supply of a large amount of pyruvate
in the perfusate (5 mmol/L), ATP synthesis via oxidative
phosphorylation in these hearts was sustained. Under
these conditions, ATP synthesis via glycolysis was trivial (<1% of
total ATP production).12 Therefore, perturbing
glycolysis by supplying 2DG to these hearts did not substantially
decrease ATP synthesis, and normal [ATP] was maintained. Using this
strategy, we found that an increase in free [ADP] is closely related
to an increase in LVEDP in isolated rat hearts, and the greater
increase of [ADP] in hypertrophied hearts than in the control hearts
may be one mechanism responsible for the enhanced susceptibility to
developing diastolic dysfunction in these hearts.
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Methods |
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Animal Model of LVH
Weanling male Wistar rats (body weight, 75 to 100 g) were purchased from Charles River Breeding Laboratories (Wilmington, Del). LVH was induced by banding the ascending aorta as previously described.5 Briefly, the animal was anesthetized with intraperitoneal pentobarbital, the chest was opened, a stainless steel clip was placed on the ascending aorta, and the chest wall was quickly closed. The sham-operated animal was subjected to the same operation procedure but without the clip. Animals were fed normal rat chow and water ad libitum and were used 39 weeks after banding.
Isolated Perfused Heart Preparation and Experimental
Protocol
Rats were anesthetized by 50 mg sodium pentobarbital IP.
The chest was opened, and the heart was rapidly excised and arrested in
ice-cold buffer. The heart was immediately attached to a perfusion
apparatus, and retrograde perfusion via the aorta was
carried out at a constant coronary flow at 37°C. The flow was
adjusted to achieve mean perfusion pressures of 80 and 110 mm Hg
for control and LVH hearts, respectively. These perfusion pressures
were chosen in recognition of the difference between the in vivo
coronary perfusion pressures of the LVH and the control hearts.
Prior experience showed that this approach would achieve comparable
myocardial flow rates per gram of LV weight for the two
groups.3 13 Immediately after the perfusion began, the
root of the pulmonary artery was cut open to allow for the
right ventricular outflow. The flow of the thebesian veins
was drained by a thin polyethylene tube pierced through the LV apex. A
water-filled latex balloon was inserted into the left ventricle through
an incision in the left atrium and was connected to a pressure
transducer (Viggo-Spectramed P23XL) for continuous recording of
LV pressure. LVEDP was set to 10 mm Hg by adjustment of the
volume of the balloon. Thereafter, balloon volume was held constant
throughout the protocol. All hearts were paced at 4.5 Hz by a Grass
stimulator (Grass Instrument Co). The heart was surrounded by its own
perfusate in a glass NMR tube, and the perfusate level
was kept just above the left atrium by continuous suction through a
polyethylene tube.
All hearts were first perfused with insulin-free Krebs-Henseleit buffer of the following composition (mmol/L): NaCl 118.0, KCl 3.5, CaCl2 1.75, MgSO4 1.2, KH2PO4 1.2, EDTA 0.5, NaHCO3 25, pyruvate 5.0, and glucose 5.0 (equilibrated with 95% O2/5% CO2, pH 7.4) and stabilized for 15 to 20 minutes. After a baseline measurement, hearts were switched to a 2DG (Sigma Chemical Co) buffer in which glucose was substituted with 5 mmol/L of 2DG. All hearts were perfused with 2DG buffer for an additional 20 minutes. At the end of each experiment, the heart was weighed, frozen, and stored at -80°C for biochemical assays.
31P NMR Spectroscopy
Intracellular concentrations of PCr, ATP, Pi, and
2DG-P were measured by 31P NMR spectroscopy. Spectra were
obtained at 161.94 MHz on a GE-400 Omega spectrometer. The heart was
placed in a 20-mm NMR sample tube inserted into a broad-band probe
situated in an 89-mm-bore, 9.4-T superconducting magnet. Spectra were
collected without proton decoupling at a pulse width of 27 µs, 60°
pulse angle, recycle time of 2.3 seconds, and sweep width of 6000 Hz.
Spectra were analyzed with 20-Hz exponential multiplication and
zero- and first-order phase corrections. Resonance peaks were fitted to
a lorentzian function, and the areas under the peaks were calculated by
the commercially available program NMR1 (NMRi). By comparison of the
peak areas of fully relaxed (recycle time, 10 seconds) with those of
partially saturated (recycle time, 2.3 seconds) spectra, the correction
factors for saturation were calculated for [ß-P]ATP (1.0), PCr
(1.2), Pi (1.15), and 2DG-P (1.35). The baseline NMR
spectrum was collected after the stabilization period by averaging
signals from 520 free induction decays. During the 20-minute perfusion
with 2DG, NMR measurements were made every 2 minutes by averaging of
signals from 52 free induction decays.
Biochemical Assays
Ventricular tissue (5 to 10 mg) was thawed and
homogenized for 10 seconds at 4°C in potassium phosphate
buffer containing 1 mmol/L EDTA and 1 mmol/L
ß-mercaptoethanol, pH 7.4 (final concentration of 5 mg tissue/mL).
Aliquots were removed for assays of protein by the method of Lowry et
al14 with BSA as the standard and assays of total Cr
content by a fluorometric assay.15 Triton X-100 was then
added to the homogenate at a final concentration of 0.1%
for analysis of the total CK activity.16 The CK
activities were measured at 30°C and are expressed as international
units (IU=µmol/min) per milligram of cardiac protein. All reagents
used were at least analytic grade and were obtained from Sigma Chemical
Co.
Data Analysis
The ATP content of the isolated perfused rat heart at the end of
the stabilization period has been determined to be 31.1±1.8 nmol/mg
protein.17 By use of a value of 0.155 mg protein/mg
blotted wet tissue and the reported value of 0.48 µL intracellular
water/mg blotted wet tissue,18 [ATP] was calculated to
be 10.0 mmol/L. Therefore, the [ß-P]ATP peak area of the NMR
spectrum obtained at baseline was normalized by the heart weight and
was set to 10.0 mmol/L for the control group. The [PCr],
[Pi], and [2DG-P] for both control and LVH hearts and
[ATP] for LVH hearts were calculated from the ratios of their peak
areas per gram heart weight to that of [ß-P]ATP area per gram heart
weight for the controls. pHi was determined by comparison
of the chemical shift of Pi and PCr in each spectrum with
values from a standard curve.
Cytosolic free [ADP] was calculated from the equilibrium constant of the CK reaction19 and values obtained by NMR spectroscopy and biochemical assay: [ADP]=([ATP][free Cr])/([PCr][H+]Keq), where Keq=1.66x109 (mol/L)-1 at pH 7.0 and free [Mg2+]=1.0 mmol/L.
The product of myocardial total Cr concentration and total CK activity (Vmax) was used to estimate the maximal CK reaction velocity in vivo, which represented the capacity of energy reserve via this reaction20 : Capacity of the CK reaction=Vmax[total Cr].
Statistical Analysis
All results were expressed as mean±SEM. Measurements made
before and during 2DG perfusion were compared by repeated-measures
ANOVA for each group. Comparisons between the two groups at the time
points of interest were performed by unpaired t test. All
the statistical analyses were performed with Statview
(Brainpower Inc), and a value of P<.05 was considered
significant.
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Results |
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Five controls and five LVH rats were studied. The body weight was comparable in the LVH and the control groups (670±34 versus 693±33 g, P=.64), whereas the heart weight was markedly increased in the LVH group (2.96±0.22 versus 2.00±0.04 g, P=.002). The heart-weight-to-body-weight ratio was 52% higher in the LVH group (4.44±0.34 versus 2.92±0.14 mg/g, P=.003).
Table 1 shows the baseline LV function of both groups.
When the LVEDP was set to 10 mm Hg, the LV volume was not
different between the two groups. The LVDP was 54% higher in the LVH
group. To estimate the load per unit of myocardium, we
assumed a spherical LV for both groups. Since the LV volume was similar
(Table 1), the LV radius was comparable for the two groups.
By Laplace's law, the LV wall stress was inversely proportional to the
wall thickness, which was directly related to LV mass. Therefore,
LVDP was normalized by ventricular weight (LVDP/g) and was
used as an approximation of wall stress under our
experimental conditions.21 LVDP/g was not different
between LVH and control hearts.
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High-Energy Phosphate Content and the CK System
Table 2 shows the baseline concentrations of ATP,
PCr, Pi, and ADP measured by 31P NMR
spectroscopy and the activity of CK and the total Cr content measured
in tissue homogenates for LVH and control hearts. In LVH
hearts, [PCr] was lower while [Pi] and [ADP] were
higher than in controls. [ATP] and pHi were not
significantly different in the two groups (Table 2) and did not change
throughout the protocol. Total content of Cr but not the activity of CK
(Vmax) was decreased in LVH hearts. The capacity of the CK
system, estimated by the product of total Cr content and the
Vmax of the enzyme, was substantially reduced (by
43%).
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Effects of 2-DG on High-Energy Phosphate Metabolism
The accumulation of 2DG-P in hearts of both groups is shown in Fig 2. A significantly higher accumulation was observed in
the LVH group (P=.001). Thus, comparisons of the LV function
between the two groups were made both at the end of the protocol (after
20 minutes of 2DG perfusion) and at the time points when a similar
accumulation of 2DG-P was achieved (5.5±0.9 and 6.7±0.8 mmol/L,
P=.306) for both control and LVH groups (indicated by the
arrows).
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Fig 3 shows the changes in concentrations of ATP,
Pi, and ADP during the 20-minute perfusion with 2DG. There
was a small but significant decrease in [ATP] in both control
(-19%, P=.001) and LVH (-22%, P=.001) hearts,
and no difference between the two groups was found during the 20-minute
period. The [Pi] was kept low by 2DG perfusion and was
not significantly different in the two groups (0.85±0.15 versus
1.40±0.54 mmol/L, P=.364). Free [ADP] increased in
both groups by the end of the 20-minute 2DG perfusion. It increased
moderately in the controls (from 11 to 20 µmol/L) but
substantially in the LVH group and was >100 µmol/L by 20
minutes of 2DG perfusion. At the time points when accumulations of
2DG-P in LVH and control hearts were similar, [ADP] was significantly
higher in the LVH group than in the controls (indicated by the arrows
in Fig 3, P=.017).
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Effect of 2DG on LV Function
Fig 4 shows the LVSP/g and LVEDP in control and LVH
hearts during 20-minute 2DG perfusion. The LVSP/g was not different
between the two groups either at the baseline (P=.650) or
during 2DG perfusion (P=.215). Twenty minutes of 2DG
perfusion did not cause any change in LVEDP in the controls
(P=.886), whereas it caused a threefold increase in LVEDP in
LVH hearts (P=.001). Since it has been postulated that
glycolytic intermediates might play a role in inhibiting relaxation,
hearts were also compared at times of equal accumulation of 2DG-P. An
increased LVEDP was observed in LVH hearts compared with the controls
(arrows in Fig 4, P=.005). Thus, an increase in LVEDP did
not correlate well with the increase in 2DG-P.
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Relationship of [ADP] and LVEDP
In Fig 5, LVEDP before and during 20 minutes
of 2DG perfusion was plotted against the [ADP] obtained at the same
time point for each individual heart. By use of all data points of all
hearts, a linear relationship was obtained between the increase of
LVEDP and [ADP] (LVEDP=0.23[ADP]+5.76,
r2=.66, P=.001). Note that
[ADP] was maintained at a level <40 µmol/L in all control
hearts, whereas [ADP] in LVH hearts increased up to a concentration
of 150 µmol/L. There was no relationship between LVEDP and
LVDP/g, [ATP], or [Pi].
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Discussion |
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The contribution of decreased high-energy phosphate content to diastolic dysfunction has been well documented during ischemia and hypoxia.2 22 These studies, however, were not able to distinguish the effects of ATP depletion, ADP accumulation, or Pi accumulation on myocardial relaxation. Using a moderate dose of 2DG to perturb intracellular energy metabolism in isolated perfused rat hearts, we have successfully increased intracellular free [ADP] while maintaining a low [Pi] and relatively high [ATP]. This occurs because phosphorylation of 2DG requires hydrolysis of ATP to ADP and Pi, and accumulation of 2DG-P traps free intracellular Pi. Because the ATP synthesis via oxidative phosphorylation in these hearts was sustained by the presence of pyruvate and the amount of 2DG entered into the cell is relatively low in the absence of insulin in the perfusate, the [ATP] decreased only slightly. Importantly, [ATP] decreased to a similar extent in both control and hypertrophied hearts during 2DG perfusion. Thus, this strategy gives us a unique opportunity to test the hypothesis that an increase in intracellular free [ADP] alone is sufficient to cause impairment of myocardial relaxation in LVH.
Relationship Between [ADP] and Diastolic Function
Our results show a close relationship between the increase of
intracellular free [ADP] and the increase of LVEDP in isolated
isovolumic rat hearts (r2=.66,
P=.001). Previous studies using skinned skeletal muscle
fibers have shown that the dissociation of ADP from the actomyosin
complex is a rate-limiting step in cross-bridge cycling.6
The velocity of actin filaments sliding on cardiac myosin was markedly
reduced by high [ADP].7 This decrease was found to be
more pronounced for cardiac myosin than for skeletal muscle
myosin.7 This observation suggests that the intact heart
would have a high susceptibility for diastolic dysfunction
when [ADP] increases. We have recently shown that an increase in
intracellular free [ADP] was closely correlated with the elevated
LVEDP in well-oxygenated normal hearts with intact
glycolysis and unchanged [Pi].23 Taken
together, these results strongly indicate that the contribution of an
increased free [ADP] to the impairment of myocardial relaxation,
which has been suggested by studies using isolated contractile proteins
or skinned muscle fibers, also applies to the intact rhythmically
beating heart.
Furthermore, recent studies on smooth muscle fibers indicate that the effect of increased [ADP] on the conformation of myosin differs from that of ATP depletion.24 Thus, it is likely that independent mechanisms are responsible for the impairment of relaxation of muscle fibers caused by increased free [ADP] versus depletion of [ATP]. This supports the previous and current observations that diastolic dysfunction in intact hearts can be caused by marked increase in free [ADP] even at a high [ATP].23 We suggest that diastolic dysfunction caused by an increase of free [ADP] may be mechanistically distinct from the myocardial contracture observed in the rigor state due to decreased [ATP].
[ADP] in Hypertrophied Hearts
Previously, the increased susceptibility of hypertrophied hearts
to development of diastolic dysfunction during
metabolic stress has been attributed to increased
intracellular calcium concentration.1 2 3 The results
presented here show that increasing the free [ADP] also
impairs diastolic function. Our results suggest that
hypertrophied hearts have a reduced ability to maintain intracellular
free [ADP] in a low range, which renders these hearts more
susceptible than normal hearts to developing diastolic
dysfunction. This finding in the intact heart is consistent
with prior observations that exposure to 2DG caused markedly slowed
relaxation and incomplete relengthening in isolated hypertrophied
myocytes from aortic-banded rats compared with control rats despite
comparable mild elevation of diastolic calcium
levels.5
Role of the CK System
In the well-oxygenated heart, the intracellular free
[ADP] is rigorously maintained in the low micromolar range. The
precise value is determined in part by the carbon substrate used to
synthesize ATP. Free [ADP] is always many orders of magnitude lower
than intracellular [ATP] (10 mmol/L). Among other mechanisms,
the CK reaction plays a critical role in maintaining this very high
ATP-to-ADP ratio by rapidly transferring a phosphoryl group between PCr
and ATP. This is due to the high abundance of the enzyme and to the
rapid reaction velocity (an order of magnitude higher than oxidative
phosphorylation).8 In the present
study, we found that the capacity for the CK reaction, estimated as the
product of Vmax and total [Cr], was substantially
lower in hypertrophied hearts (decreased by 43%). This decreased
capacity of the CK reaction may contribute to the impaired ability of
hypertrophied hearts to maintain the [ADP] as low as that of the
controls. Results presented here show that free [ADP] was
higher in LVH hearts than in controls both during baseline conditions
and during 2DG perfusion even when accumulation of 2DG-P was similar in
the two groups. This observation is consistent with our
previous findings that acutely inhibiting the CK activity in hearts
with normal Cr content resulted in increases in free [ADP] and
LVEDP.23
Clinical Implications
Using 2DG perfusion as a tool to manipulate ADP levels, we found
that the capacity of the CK system for maintaining a low [ADP] was
decreased in hypertrophied hearts. This decrease may contribute to the
development of diastolic dysfunction in these hearts.
Because of the ubiquitous nature of hypertrophy and
diastolic dysfunction in the evolution of heart failure,
our findings have potential clinical significance. Changes in the CK
system can be found during the development of heart failure due to many
different kinds of cardiac pathological
conditions.11 20 25 Previous studies in hypertrophied and
failing human myocardium have shown changes in the CK
system25 26 27 and in LV diastolic
function28 29 similar to those reported here. Recent
studies using animal models of cardiac hypertrophy and
failure have shown that changes in the CK system of hypertrophied and
failing hearts occurred much earlier than any significant change in
[ATP].11 30 31 Furthermore, we recently showed that
decreased energy reserve via the CK system could be found in an early
stage of cardiac dysfunction when baseline function was normal but
contractile reserve was reduced.20 The findings of these
prior studies and the present experiments are consistent
with the clinical observation that impaired diastolic
function may occur early in cardiac hypertrophy, sometimes
much earlier than the development of systolic
dysfunction.28 29 Thus, a reduced capacity to maintain a
low [ADP] may be an early characteristic of hypertrophied and failing
myocardium, which contributes to further worsening of
cardiac function.
Limitations of the Study
Since the rate of 2DG-P accumulation was faster in hypertrophied
hearts than in controls, more 2DG-P accumulated in hypertrophied hearts
relative to the controls at the same time point. Therefore, we compared
the two groups at two time points: at the end of the 20 minutes of 2DG
perfusion and at a time when accumulation of 2DG-P was similar. It
would be ideal to match the rate of 2DG-P accumulation in the two
groups. However, it is difficult to increase the rate in control hearts
without using insulin, which by itself may introduce other confounding
factors, such as changes in enzyme activities and metabolic
fluxes. In addition, calculating free [ADP] by the CK reaction
equilibrium expression provides an average free intracellular [ADP].
It does not provide any information regarding free [ADP] in any
specific intracellular compartment. Nevertheless, it is likely that
intracellular free [ADP] for the entire functioning organ determined
in this way is related to the [ADP] near the myofibril.
It has been suggested that glycolytic ATP plays a special role in diastolic relaxation by supporting the sarcoplasmic reticulum Ca2+-ATPase activity.32 Using 2DG perfusion as a strategy to increase free [ADP], we also perturbed glycolysis in these hearts. There is a possibility that loss of glycolytic ATP may be responsible for the diastolic dysfunction in the LVH hearts. Our study was designed to minimize this possibility. In hearts supplied with glucose as a sole substrate, glycolytic ATP contributes 3% to 4% of total ATP production, and this contribution is enhanced when high glucose concentration is used and insulin is included in the perfusate. With 5 mmol/L pyruvate supplied to the heart, the ATP synthesis via glycolysis was kept minimal even in the presence of 5 mmol/L glucose (<1% of total ATP production).12 Thus, hearts studied here are relatively independent of the confounding factor of glycolytic ATP. In support of this conclusion, perturbing glycolysis in the control hearts did not cause diastolic dysfunction. Our data do not allow us to rule out the possibility that hypertrophied hearts may be more dependent on glycolytic ATP for cross-bridge cycling and that a very small amount of glycolytic ATP (<100 µmol/L) is critical for diastolic relaxation in LVH hearts.
In summary, we found a close relationship between an increase in intracellular free [ADP] and impairment of diastolic relaxation in hearts with cardiac hypertrophy. Decreased energy reserve via the CK reaction may be an important mechanism that contributes to the impaired capacity to maintain a low free [ADP] in hypertrophied hearts.
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Selected Abbreviations and Acronyms |
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Acknowledgments |
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This study was supported by National Institutes of Health grants HL-44431 (to Dr Lorell), HL-49574 (to Dr Judith K. Gwathmey and Dr Ingwall), HL-52350 (to Dr Ingwall), and HL-52864 (to Dr Lorell). Dr Luigino Nascimben was supported by a Merck Sharp & Dohme Italia S.p.a. fellowship award. We would like to thank Drs Liang Zhao and Judith K. Gwathmey for helpful discussions of this study and Dr Ellen O. Weinberg for assistance with the animal model. The expert surgical assistance of Soeun Ngoy in the preparation of the aortic-banded rats and the skillful technical assistance of Ilana Reis are greatly appreciated.
Received December 2, 1996; revision received February 20, 1997; accepted February 28, 1997.
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