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Cardiology Unit, College of Medicine, The University of Vermont, Burlington, Vermont 05401
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ABSTRACT |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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We tested the feasibility of an isolated,
balloon-in-ventricle, isovolumically contracting, crystalloid-perfused
mouse heart preparation (n = 10) for studies of cardiac mechanoenergetics using the end-systolic
pressure-volume relation (ESPVR) and myocardial oxygen consumption
(
O2)-pressure-volume area
(PVA) framework employed in larger species. The intraventricular
balloon method was shown to be accurate for measurement of left
ventricular volume, especially at relatively higher volumes. The ESPVR
demonstrated contractility-dependent curvilinearity. Average slope of
the ESPVR was 1,299 ± 369 (SD)
mmHg · g · ml
1,
with a volume intercept of 0.018 ± 0.006 ml. The
O2-PVA relation was well
fitted by a straight line, with average slope and
O2 intercept of 3.57 ± 1.31 × 105 ml
O2 · mmHg1 · ml1
and 0.92 ± 0.21 × 103 ml
O2 · beat1 · g1,
respectively. Decreasing perfusate
Ca2+ concentration resulted in a
decrease in the slope of the ESPVR, a decrease in the
O2 intercept of the
O2-PVA relation, but no
significant change in its slope. Hearts from hypothyroid
(n = 8) mice demonstrated similar
mechanoenergetic changes. We conclude that delineation of the ESPVR and
the O2-PVA relation is
feasible in the mouse heart. Our method should allow an assessment of
cardiac mechanoenergetics as sophisticated as that previously possible only in larger hearts.
myocardial oxygen consumption; pressure-volume area; ventricular volume
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INTRODUCTION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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GENETIC ENGINEERING METHODS have great promise for
elucidating the normal and pathological consequences of gene expression in the heart. Although a variety of animal species have been used in
genetic engineering experiments, the mouse has been employed most
extensively. Despite the considerable advantage of the mouse in terms
of expense and reproductive rate, analysis of function in mouse heart
is a challenge because of its small size. Despite the size limitation,
remarkable progress has been made using several methods including
radiolabeled microspheres combined with indicator dilution techniques
(1), classic isolated work-performing hearts (2, 5, 7, 9, 23, 26), left
ventricular pressure measurement in vivo (17, 24) and in situ (22),
contrast angiography (27), and echocardiography (8, 16, 17, 19, 31).
However, there are no published methods using a balloon-in-ventricle preparation with the capacity to measure and vary volume. This method
offers important advantages in that it is possible to make functional
measurements with controlled loading conditions and independent control
of coronary perfusion pressure. With the use of this type of
preparation, two pivotal approaches to ventricular mechanics and
energetics have been established, quantitative assessment of
ventricular contractility through analysis of the end-systolic pressure-volume relation (ESPVR) and quantitative assessment of total
mechanical energy generated by the ventricle in terms of pressure-volume area (PVA). Studies in which PVA has been correlated with measured oxygen consumption
(O2) under a variety of
hemodynamic loads and contractile states have provided new
understanding of the determinants of myocardial energy utilization
(28). The latter studies were performed originally in the canine heart
and then extended to other species, including human (20), rabbit (10),
and rat (33). The purpose of the present study was to test the
feasibility of an isolated, balloon-in-ventricle, isovolumically contracting, crystalloid-perfused mouse heart preparation for studies
of cardiac mechanoenergetics using the same framework employed in
larger species.
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METHODS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Isolated heart preparation.
All experimental procedures were approved by the Institutional Animal
Care and Use Committee of the University of Vermont. We studied 18 male
adult CD-1 mice weighing 21-41 g [mean 32.1 ± 5.4 (SD)
g]. Hypothyroidism was induced in eight of the mice by adding 0.8 mg/ml propiothiouracil (PTU) to the drinking water for 3 wk. Each mouse
was anesthetized with a 1:5 diluted mixture of ketamine hydrochloride
(100 mg/kg ip) and xylazine (5 mg/kg ip) and heparinized (5,000 IU/kg
ip). A tracheotomy was performed, and the mouse was ventilated with a
Harvard respirator. The chest was opened at the midline of the sternum.
Both pulmonary hili and the superior vena cava were ligated
simultaneously. The heart was removed from the chest and immediately
submerged in oxygenated, 30 mM 2,3-butanedione monoxime (BDM)-Ringer
solution (25) at room temperature. Under a microscope, the severed end
of the aorta was cannulated and perfused via a 20-gauge cannula with
warmed perfusate (35-37°C, composition provided below). The
right atrium (RA) was widely opened, and a flexible tube was inserted
through the RA into the right ventricle (RV). The RA and inferior vena cava were ligated, after which the left atrium was opened widely. A
collapsed, thin, high-density polyethylene balloon mounted on a
22-gauge polyethylene catheter (see below) was placed in the left
ventricle (LV) through the mitral orifice. Before each experiment, the
pressure-volume relationship of the balloon was measured. The balloon
was used only if the pressure was zero up to an intraballoon volume
0.03 ml. A 2.5-Fr micromanometer catheter (model SPR-524, Millar
Instruments, Houston, TX) was introduced just above the mitral orifice
via a side port. A 100-µl graduated syringe (model 80601, Hamilton,
Reno, NV) was connected to another port. Pacing electrodes connected to
an electronic stimulator (model S9, Grass Instruments, Quincy, MA) were
attached to the LV apex. The RV was kept collapsed by continuous
hydrostatic drainage to minimize RV
O2. The heart was placed in
a chamber with a heating jacket, and its temperature was maintained at
35-37°C. Coronary flow was measured by timed collections of
the RV drainage into a graduated cylinder. Coronary arteriovenous
O2 content difference was measured continuously with a dual-channel platinum
O2 electrode system (model 203B,
Instech, Plymouth Meeting, PA). Sodium dithionate, a compound which
extracts O2 from solution, was
used to zero the electrode at the start of each experiment. The gain of
the electrode system was calibrated using the perfusate solution, which
was saturated with 100% O2. The
reported value of oxygen solubility in this solution (2.3 vol/100 ml)
(5, 33) was used to calculate O2
content.
Preparation of perfusate. Perfusate was composed of (in mM) 108.0 NaCl, 4.0 KCl, 1.4 KH2PO4, 25.0 NaHCO3, 11.0 dextrose, 10.0 sodium pyruvate, and 2.5 CaCl2 (all from Sigma Chemical, St. Louis, MO). The solution was equilibrated with 95% O2-5% CO2 and warmed to 37°C. pH was adjusted to 7.35-7.45 by changing CO2. The perfusate was transported to the perfusion tubing by a variable-flow pump (Masterflex, Cole-Parmer, Chicago, IL). Coronary perfusion pressure was controlled by a pressurized arterial reservoir connected to a pressure regulator and compressed air. The temperature of the perfusate was maintained at 35-37°C with water jackets around the container and the pressurized arterial reservoir by constant-temperature circulators. Perfusate was not recirculated. To investigate acute changes in contractility, we also prepared a low Ca2+ perfusate, differing only with respect to the Ca2+ concentration ([Ca2+]), which was 1.5 mM.
Experimental protocol.
The hearts were stimulated electrically at 240 beats/min by LV pacing
and, in eight mice (no. 1-8 in
Tables 2 and 3), were initially perfused randomly with either the
normal (n = 5) or low
(n = 3)
Ca2+ perfusate. We waited 30 min
for initial stabilization and BDM washout. The protocol (see below) for
determining the ESPVR and the relationship between
O2 and PVA was then
executed. We then switched to the other
[Ca2+] perfusate and
waited 10 min before executing the protocol once again. In two control
and the eight PTU mice, the measurements were done only with normal
Ca2+ perfusate (2.5 mM).
Data analysis.
LV pressure, coronary perfusion pressure, and arteriovenous
O2 content difference were
recorded on a pen recorder and stored on a hard disk at 5-ms sampling
intervals for off-line analysis with a personal computer (Gateway 2000, North Sioux City, SD). We compared the effect of sampling intervals of
5 ms and of 0.5 ms on measurement of the first derivative of pressure
with respect to time (dP/dt) in the
same heart (n = 3). Maximal and
minimal dP/dt measured with sampling
intervals of 5 ms averaged 85% of those measured with 0.5-ms
intervals, but correlation between the two was highly linear
(r = 0.999). Thus our sampling
interval may underestimate the absolute value of
dP/dt but accurately reflects relative
changes. O2 per minute was
calculated as the product of coronary flow (ml/min) and arteriovenous
O2 content difference (vol%) and
was divided by heart rate to yield total
O2 per beat (in ml
O2/beat).
O2 was normalized per gram
LV weight to give O2 in
milliliters O2 per beat per gram.
LV volume was determined as the sum of the volume of water within the
LV balloon and the volume of the balloon walls and connector within the
LV. LV developed pressure was defined as the difference between peak
and minimum LV pressure during one cardiac cycle. End diastole was
defined as the time when LV positive
dP/dt increased to 10% of its peak value.
ESPVR.
Contractile state was quantified as the slope of the ESPVR,
Emax (in
mmHg · g · ml1).
End-systolic pressure was determined as peak isovolumic pressure. Both
linear and nonlinear regression analyses were performed (4) (E'max denotes the
slope for a nonlinear ESPVR). LV pressure,
dP/dt, coronary flow, and coronary
perfusion pressure were also measured or calculated at a reference LV
volume of 0.04 ml.
O2-PVA relation.
Total energy liberated by the LV was quantified as PVA (26). PVA is the
area circumscribed by the ESPVR, the end-diastolic pressure-volume
relation, and the systolic pressure-volume trajectory. For each
contractile state, O2 was
plotted as a function of PVA, and a linear regression analysis
(O2 = a × PVA + b) was performed. According to the
PVA concept, the slope of this equation a represents the oxygen cost of PVA,
and the intercept b is
O2 at zero PVA, i.e.,
unloaded O2. The reciprocal
of the slope (1/a) is contractile
efficiency for conversion of
O2 to PVA. In addition to a
linear fit, we also fitted
O2-PVA data points to a
second-order polynominal to test for curvilinearity. The RV fraction of
O2 at zero PVA was
subtracted for each experimental condition by assuming that the amount
of O2 consumed by the RV is
proportional to its weight
[O2 × LV
weight/(LV + RV weight)].
LV balloon. We employed a balloon constructed of high-density polyethylene, obtained from a commonly available plastic shopping bag. A small piece of this material was stretched with the round tip of a small centrifuge tube and gently mounted on a 22-gauge polyethylene intravenous catheter (SURFLO, Terumo, Japan). The length of the balloon was ~5 mm. The volume of the balloon wall plus the tip of the tubing within the balloon was determined (range 0.018-0.03 ml) by water replacement after loading a known volume of fluid within the balloon. Airtightness of the system was evaluated by measuring the intraballoon volume change after overinflating the balloon to ~150 mmHg of intraballoon pressure over a 2-h period. After 2 h, we could withdraw exactly the same amount of water as was instilled, indicating the balloon is airtight.
To test the accuracy of LV volume measurement using this balloon, we measured the residual volume between the balloon and the LV inner surface using similar methods to those described for canine (27) and rabbit hearts (11). Briefly, four hearts were fixed in 10% Formalin. The aorta, pulmonary artery, atria, and RV free wall were trimmed off, and the LV was filled with water. The balloon was inserted into the LV and slowly inflated with water to obtain intraventricular pressure of 5, 15, 40, and 90 mmHg. Then the balloon was deflated and slowly removed from the ventricle. The surface of the balloon and the LV endocardial surface were wiped carefully to absorb all residual water with pieces of blotting paper. These were then weighed with a precision balance with a resolution of 0.1 mg (0.1 µl). Measurements were repeated four times at each level of intraventricular pressure. Residual volume was also expressed as percent of total LV volume, which was determined as the sum of the volume of water within the LV balloon, the volume of the balloon walls and connector within the LV, and the residual volume. Because the 2.5-Fr pressure transducer cannot pass through a 22-gauge catheter, the sensing tip of the pressure transducer was positioned ~1 cm from the balloon. Frequency response characteristics of pressure measurement with our balloon system were evaluated using previously described methods (13). Briefly, the entire system with a volume-loaded balloon (<3 mmHg of intraballoon pressure) was placed within a 60-ml plastic syringe via a rubber stopper. The tip of the syringe was connected to Y-connector, one port of which was sealed by a thin rubber membrane. The pressure within the syringe was increased by adding air from another port and, after a steady period, suddenly reduced by cutting the rubber membrane. During this dynamic pressure change, intraballoon pressure was measured, and the damping coefficient (which is 0.64 with optimal damping) and undamped natural frequency were calculated. Our balloon system has an almost optimal damping coefficient of 0.619 and a high natural frequency (65.3 Hz), indicating that it has a frequency response that is flat up to 57.5 Hz (65.3 × 0.88) and applicable for measurement of pressures up to 5.8 Hz of heart rate (assuming that the physiologically significant signal is up to the 10th harmonic of the original wave). We can measure dP/dt accurately up to 2.9 Hz of heart rate, since measurement of the first 20 harmonics of the LV pressure is required for dP/dt measurement (13).Statistics.
Data are reported as means ± SD. Student's
t-test for paired data was used to
compare normal and low Ca2+
perfusate. Student's t-test for
unpaired data was used to compare normal and hypothyroid mice. The
least-squares method was used for estimation of the ESPVR and the
O2-PVA relation. A value of
P < 0.05 was accepted as the level
of significance.
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RESULTS |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Average LV and RV weights were 0.136 ± 0.030 and 0.034 ± 0.010 g, respectively.
Accuracy of volume measurement. The average volume of the residual space was 2.3 ± 0.7 µl (4.5 ± 1.8% of total volume) at a pressure of 90 mmHg, 2.8 ± 0.9 µl (8.5 ± 2.3%) at a pressure of 40 mmHg, 3.2 ± 1.0 µl (19.5 ± 8.2%) at a pressure of 15 mmHg, and 6.5 ± 1.8 µl (35.5 ± 11.6%) at a pressure of 5 mmHg.
Baseline mechanoenergetics. Representative LV pressure tracings (mouse 3) at various volumes are shown in Fig. 1. LV pressure, maximum and minimum dP/dt, coronary flow, and estimated coronary perfusion pressure measured at a LV volume of 0.04 ml are summarized in Table 1.
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O2-PVA relation from the
same heart as in Fig. 2A is shown in
Fig. 2B. PVA was calculated based on a
curvilinear ESPVR. O2-PVA
relations of all hearts are summarized in Table
3. Average contractile efficiency and
O2 intercept were 0.21 ± 0.08 and 0.92 ± 0.24 × 103 ml
O2 · beat1 · g1,
respectively.
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Influence of contractile state on ESPVR and
O2-PVA relationship.
The results from another heart (mouse
2) in which contractility was varied by changing the
perfusate [Ca2+] are
shown in Fig.
3A. The
decrease in [Ca2+]
from 2.5 to 1.5 mM decreased the slope of the ESPVR. With lower [Ca2+], curvilinearity
of ESPVR was no longer apparent, and R
values for nonlinear and linear ESPVRs were similar (mean 0.99 ± 0.01 vs. 0.97 ± 0.04), indicating contractility-dependent
curvilinearity in mouse hearts.
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O2-PVA relations of the same
heart as in Fig. 3A are shown in Fig.
3B. In this single case, at 2.5 mM
Ca2+, the
O2-PVA relation was found
statistically to be fit better by a second-order polynomial than a
linear function (R = 0.99 vs. 0.87).
E'max, developed
pressure, contractile efficiency, and
O2 intercept for the group
(mice 1-8) at each perfusate
[Ca2+] are summarized
in Fig. 4. The decrease in
[Ca2+] reduced
E'max and developed
pressure. It also decreased the
O2 intercept of the
O2-PVA relation without changing its slope.
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Influence of hypothyroidism on ESPVR and
O2-PVA relationship.
Figure 5 shows the effect of hypothyroidism
on E'max, developed
pressure, contractile efficiency, and
O2 intercept. Compared with
the control group (mice 1-10),
both E'max and
developed pressure were reduced in hypothyroid (PTU) animals. The
decreased contractility in these animals was accompanied by a decrease
in the O2 intercept of the
O2-PVA relation without a
change in slope.
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DISCUSSION |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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In this study, we have demonstrated for the first time the feasibility
of assessing the ESPVR and
O2-PVA relation in mouse hearts. The ESPVR shows contractility-dependent curvilinearity, and the
O2-PVA relation is well
fitted by a straight line. The response of these relations to changes
in perfusate [Ca2+]
was similar to that observed in hearts from larger species (29). The
results in hypothyroid animals demonstrate that our method is useful in
comparing mechanoenergetics between normal and dysfunctional hearts.
Comparison with previous studies. Previous studies of ventricular function in the in situ mouse heart have employed radiolabeled microspheres combined with indicator dilution techniques to measure cardiac output and stroke volume (1), quantitative angiography (27), and echocardiography (8, 16, 17, 19, 31). Several authors (2, 5, 7, 9, 23, 26) have published reports using a classic, isolated, perfused, work-performing heart. There are two reports of isolated, buffer-perfused, isovolumically contracting balloon-in-ventricle preparations. Galinanes and Hearse (6) reported an LV developed pressure of 52 ± 4 mmHg with a crystalloid perfusate containing 1.36 mM Ca2+. Brooks and Apstein (3) reported an LV developed pressure of 111 ± 4 mmHg in hearts perfused with Krebs-Henseleit solution containing 2.2 mM Ca2+. In neither of these studies was LV volume measured or varied. Our developed pressures of 73 ± 17 mmHg with 2.5 mM Ca2+ and 42 ± 14 mmHg with 1.5 mM Ca2+ are low compared with these reports. However, because LV volume was not reported, it is difficult to compare our results directly with these.
Previously, many investigators have employed LV dP/dt as an index of systolic and diastolic function in the mouse heart. In the isolated, working, in vivo heart, maximal LV dP/dt is reported to be 2,800-7,300 mmHg/s (2, 5, 7, 9, 14, 17, 23, 24, 26). Using a microcatheterization technique, Lorenz and Robbins (22) reported maximum and minimum dP/dt of 7,830 ± 670 and
8,614 ± 763 mmHg/s, respectively, in
intact, closed chest, anesthetized mice with native heart rates.
Compared with these reports, our dP/dt
values are apparently low. As mentioned in
METHODS, our sampling frequency of 5 ms modestly underestimates dP/dt. In
addition, the frequency response of our balloon system is such that
accurate dP/dt measurement is possible
only at the relatively low heart rate (for the mouse) employed in this
study. As a function of the force-frequency relation, the heart rate we
employed would be expected to result in lower
dP/dt values than those present at
native rates. Thus our method may be most useful in evaluating a
relative change during an intervention.
Studies of coronary flow in the mouse heart are also limited. In the
isolated, working heart preparation, coronary flow is reported to be
2.8-5 ml/min (1, 2, 5, 22, 25). Brooks and Apstein (3) reported a
coronary flow of 3.1 ± 0.2 ml/min in their isolated, isovolumically
contracting preparation perfused with Krebs-Henseleit buffer. The
coronary flow of 2.9 ± 0.5 ml/min in our preparation is in good
agreement with these previous reports.
There is only one report of
O2 in mouse heart. Chu et al.
(5) measured a O2 of 0.14 ml
O2 · min1 · g1
(heart weight) in their isolated, working heart preparation perfused with buffer containing 2.5 mM Ca2+
at 37°C. Because they did not measure LV volume and the contraction mode was different, it is difficult to compare our results with their
data. Nonetheless, our O2
value of 0.22 ml
O2 · min1 · g1
(LV weight) is fairly similar.
Using echocardiography, several investigators have reported mouse LV
dimensions of 2-4 mm (16, 17, 19, 30). In the present study, the
length of the long axis of our balloon was typically 5 mm. We used an
LV volume of 0.04 ml as a reference volume. If we assume the balloon is
ellipsoidal and has a long-axis radius of 5/2 = 2.5 mm, then the
short-axis dimension of the balloon is then 2 × 2 = 4 mm
(4/3
r2 × 2.5 × 101 = 4 × 102,
r = 2 × 101). Thus our volume
range seems to be physiological compared with in vivo echocardiographic
measurements.
LV volume measurement.
LV volume measurements by intraventricular balloon have been widely
used for evaluation of LV function. This method requires no geometric
assumptions. However, there is a potential methodological limitation
with all balloon methods, namely, the assumption that the balloon fills
all of the space within the ventricle. Previous studies have reported
excellent accuracy of volume measurements by the balloon method but
also indicate that errors increase at low diastolic pressures, when
fitting of the balloon may be impaired (11, 29). In addition, because
of the small size of the mouse heart, we could not cut the chordae
tendineae, which may increase the space between the balloon and LV wall
(29). The volume error (underestimation) of this method in
Formalin-fixed mouse hearts with intact chordae tendineae was <10%
at intraballoon pressures >40 mmHg but increased up to 35% at
intraballoon pressures <5 mmHg. However, the effects of this error on
the ESPVR (14% underestimation of
Emax and 16%
underestimation of V0, based on
the average ESPVR) and O2-PVA
relation (4% overestimation of the slope and 0.2% overestimation of
the O2 intercept,
based on the average O2-PVA relation) are relatively small. Thus, although one must be cautious in
measuring absolute LV volume at low pressure ranges, the method should
be acceptable for evaluation of mechanoenergetics.
ESPVR and O2-PVA
relationship.
The major technical advantages of our preparation are the use of a
balloon that allows measurement and variation of LV volume combined
with measurement of coronary flow and
O2, in conjunction with
independent control of coronary perfusion. These features are required
for measurement of the ESPVR and the
O2-PVA relation, approaches
which have become standards for estimation of ventricular contractility
and assessment of mechanoenergetics.
1
with perfusate containing 2.5 mM
Ca2+, which is quite similar to
values in blood- perfused rabbit hearts (1,360 ± 570 mmHg · g · ml1)
(10). The average
E'max of 876 ± 340 mmHg · g · ml1
with perfusate containing 1.5 mM
Ca2+ is similar to values
obtained with the same
[Ca2+] in
crystalloid-perfused rat hearts (645 ± 226 mmHg · g · ml1)
(33).
The curvilinearity of the ESPVR may be obscured when regression
analysis is applied to data within a relatively limited volume range.
In this situation, there is relatively little change in Emax when altered
contractility changes the ESPVR from convex to linear, or vice versa.
These phenomena were discussed extensively by Burkhoff et al. (4). In
the present study, we could reduce LV volume to a level at which LV
pressure was <20 mmHg and varied LV volume widely enough to evaluate
a curvilinear ESPVR. This also means that we can directly measure
O2 at nearly zero PVA rather
than employing considerable extrapolation to the
O2 axis intercept to estimate
this important energetic variable.
In contrast, O2-PVA relations
were well-fitted by straight lines, and in only 1 of 18 instances was
the fit better with a curvilinear function. Average slope and
O2 intercept with 2.5 mM Ca2+ were 3.57 ± 1.31 × 105 ml
O2 · mmHg1 · ml1
and 0.92 ± 0.21 × 103 ml
O2 · beat1 · g1,
respectively. Thus contractile efficiency, the reciprocal of the slope
of the relation, was 0.21 ± 0.07. This value is low compared with
reported values in canine (0.35-0.45) (28), rabbit (0.40 ± 0.04) (10), rat (0.53 ± 0.11) (33), and human (0.41 ± 0.06)
(20) hearts, but similar to some reports in canine heart (18, 34).
Contractile efficiency has been reported to be relatively insensitive
to acute changes in contractility and loading conditions. However,
changes in composition of the contractile proteins may alter efficiency
in thyrotoxic rabbit (12) and failing hearts (21, 34). Thus the lower
contractile efficiency in our preparation could be due to species
differences in contractile proteins. The fact that the hearts were
paced at less than one-half the usual intrinsic rate of the mouse may
have also influenced efficiency.
The value of the O2 intercept
of the mouse heart (0.92 ± 0.21 × 103 ml
O2 · beat1 · g1)
is higher than that reported in other species, such as canine (0.2- 0.3 × 103 ml
O2 · beat1 · g1)
(29), rabbit (0.33 ± 0.10 × 103 ml
O2 · beat1 · g1)
(10), and rat (0.38 ± 0.09 × 103 ml
O2 · beat1 · g1)
(33). One contributor to the high
O2 intercept may be the relatively low heart rate employed in this study. The
O2 intercept of the
O2-PVA relation reflects
mainly O2 for basal
metabolism and excitation-contraction coupling. Because basal
O2 is relatively insensitive
to changes in heart rate, basal
O2 per beat increases when
heart rate is lowered (15). This feature may be particularly exaggerated in our studies because the difference between the paced
heart rate we employed and the native rate of the mouse is larger than
in other species in which the
O2-PVA relation has been
quantified.
To test the feasibility of our method to detect differences between
normal and dysfunctional hearts, we also studied hearts from
hypothyroid mice. Using a work-performing preparation, Ng et al. (26)
and Grupp et al. (14) reported decreased LV systolic pressure,
dP/dt, and cardiac output in
hypothyroid mice. Lorenz and Robbins (22) also reported a decreased LV
systolic pressure and dP/dt in
hypothyroid, closed chest, anesthetized mice using in situ pressure
measurement. Our ESPVR data are consistent with these results. In
addition, the O2-PVA relation
in these animals revealed a decreased
O2 intercept without a change
in slope. These results are similar to our previous observations in
hypothyroid rabbits (12).
Limitations. We used non-blood-containing crystalloid perfusate in this study. The decreased O2-carrying capacity of buffer perfusate is compensated by increased coronary flow. However, the increased flow may contribute to increased diastolic stiffness (32). Recent reports indicate that buffer-perfused mouse hearts demonstrate increased diastolic pressure at heart rates >300 beats/min (3). The latter could be due to increases in coronary flow and/or ischemia at higher heart rates. In addition to considerations mentioned earlier, these results suggest that our preparation may be most useful at lower heart rates.
Conclusion.
Our results demonstrate that delineation of the ESPVR and the
O2-PVA relation is feasible
in the mouse heart and that our methods are accurate enough to detect
predictable acute and chronic changes in mechanoenergetics. This
approach should allow an assessment of cardiac mechanoenergetics as
sophisticated as that previously possible only in larger hearts.
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ACKNOWLEDGEMENTS |
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This research was supported by National Heart, Lung, and Blood Institute Grant HL-52087.
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FOOTNOTES |
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Address for reprint requests: M. M. LeWinter, Cardiology Unit, Fletcher Allen Health Care/MCHV campus, 111 Colchester Ave., Burlington, VT 05401.
Received 31 March 1997; accepted in final form 10 September 1997.
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REFERENCES |
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Top
Abstract
Introduction
Methods
Results
Discussion
References
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Burkhoff, D.,
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) and end-diastolic (
) pressure-volume relation. Solid line
indicates nonlinear regression of end-systolic pressure-volume
relation. Pressure-volume relation of balloon itself is also indicated
(+). B: relation between myocardial
oxygen consumption and pressure-volume area, with linear regression
line.
) to 1.5 mM (
) decreased slope of ESPVR.
B: influence of contractile state on
myocardial oxygen consumption-pressure-volume area relationship.
