Download Effects of aging on the work output and efficiency of rat papillary

Cardiovascular Research 48 (2000) 111–119
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Effects of aging on the work output and efficiency of rat papillary muscle
Helen Kiriazis, Colin L. Gibbs*
Department of Physiology, Monash University, Clayton, Victoria 3168 Australia
Received 6 December 1999; accepted 29 May 2000
Abstract
Objectives: This study aimed to investigate the effect of aging on the work output and efficiency of rat papillary muscle. Methods:
The mechanical and energetic properties of left ventricular papillary muscle preparations isolated from 6-, 15-, and 27- to 32-month-old
Sprague–Dawley rats were measured in myothermic experiments at 278C at a stimulus frequency of 0.167 Hz. Results: We found that the
basal metabolism measured in quiescent papillary muscles was significantly reduced in the 27- to 32-month-old group (4.9 mW g 21
compared to 7.7 and 7.0 mW g 21 in the 6- and 15-month groups). In isotonic experiments, the work output (at a range of afterloads) was
significantly depressed for the 27- to 32-month group being only 52% of the work output of the 6-month group. This outcome was due to
a decrease in both the extent of muscle shortening only, 66% of 6- and 15-month data, and in the maximum force developed. The reduced
work was accompanied by a parallel decrease in energy consumption (enthalpy) and hence, the net mechanical efficiency (work / active
enthalpy3100%) was not altered. A force–length– area (FLA) analysis was applied to the isotonic data and an energy: FLA regression
line was obtained for each preparation. We found that there were no significant differences in either the intercept or slope of the energy:
FLA relation with age. Contractile efficiency (3963%) in the 27- to 32-month group was not significantly different to that found in the
6-month (4364%) or 15-month (4063% group). Conclusion: There are no changes in the mechanical performance or efficiency of
cardiac muscle from young (6-month-old) or adult (15-month-old) rats but in the aged and senescent rats (27–32-month-old) there is a
pronounced decline in stress development and shortening ability leading to a fall in work output. Mechanical and contractile efficiency
however remain unchanged in old age and the data resembles that obtained in pressure overload hypertrophy.  2000 Elsevier Science
B.V. All rights reserved.
Keywords: Aging; Energy metabolism; Hypertrophy
This article is referred to in the Editorial by N.
Westerhof ( pages 4 – 7) in this issue.
1. Introduction
Biochemical, mechanical and electrophysiological properties of the myocardium are altered in aging in a similar
fashion to that observed as a consequence of pressure
overload hypertrophy (see Ref. [1] for review). Although
there are numerous studies in the literature on the mechanical and energetic effects of cardiac overload, there are
few which have investigated the energetic consequences of
aging in the working myocardium.
*Corresponding author. Tel.: 161-3-9905-2513; fax: 161-3-99055583.
E-mail address: [email protected] (C.L. Gibbs).
It is well known that with aging there is a clear decline
in cardiac function such that in old age or senescence there
is a progressive prolongation of contractile duration and
time to peak tension and a diminished ability to generate
force [1–5]. There are also changes in cellular Ca 12
kinetics [6,7] but the data in respect of systolic [Ca 21 ]i are
not consistent [8], cross-bridge turnover rate is decreased
[9,10]. The age-associated changes in calcium cycling have
been clearly documented by several groups [4,6,7,11,12]
and it has been shown that there are age-dependent
changes in the calcium transient such that under b-adrenergic-stimulated senescent hearts have a decreased ability
to increase the [Ca 21 ]i transient [7].
It has been established that with aging and with pressure
overload hypertrophy there are alterations in ventricular
myosin heavy chain expression [10,13,14] and this leads to
Time for primary review 29 days.
0008-6363 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved.
PII: S0008-6363( 00 )00144-9
112
H. Kiriazis, C.L. Gibbs / Cardiovascular Research 48 (2000) 111 – 119
an increase in the economy with which isometric force is
maintained. There has however been no isotonic energetic
investigation of work output per beat and mechanical
efficiency in senescence. In the present investigation we
have measured the work output in isotonic contractions
and have analysed our data using both a conventional
definition of mechanical efficiency and have made a force–
length–area (FLA) analysis of the same data to calculate
contractile efficiency [15]. The FLA analysis not only
allows contractile efficiency to be measured but also
provides an independent estimate of the activation or
tension-independent heat associated with Ca 21 -ATPase of
the SR [16,17]. In order to allow estimates of gross
mechanical efficiency (work / active 1 basal enthalpy3
100%) to be made, which can then be compared with in
vivo mechanical efficiency estimates, we also measured
the basal metabolism or resting heat production of papillary muscles from the different age groups.
2. Methods
2.1. Dissection of rat left ventricular muscle
preparations
Three age groups were studied. Young adult (5.7- to
6.5-month-old, n56; 6-month group), mature adult (14.3to 15.7-month-old, n56; 15-month group) and senescent
(26.6- to 32.2-month-old, n56; 27- to 32-month group)
male Sprague–Dawley rats were studied. All rats were
killed by cervical dislocation, and their hearts rapidly
removed and placed into warm (|348C) physiological
solution until the heart vented the blood from its chambers.
The dissection then took place in Krebs solution containing
30 mM BDM at room temperature. Krebs–Henseleit
solution composed of (in mM) 118 NaCl, 4.75 KC1, 1.18
MgSO 4 , 1.18 KH 2 PO 4 , 24.8 NaHCO 3 , 2.5 CaC1 2 , 11.1
glucose and insulin (10 IU / l) at room temperature. This
solution was aerated with 95% oxygen–5% carbon dioxide
and had a pH of 7.4. The investigation conforms with
principles outlined in the Declaration of Helsinki and was
approved by the Monash University Animal Ethics Committee. BDM protects the myocardium against dissection
damage [18] and viable, (cross-sectional area |0.5 mm 2 )
rat myocardium preparations have been obtained in the
presence of BDM by splitting (cutting) left ventricular
papillary muscles longitudinally [18,19]. The same dissection method was employed in the present study as it
allowed myocardial preparations with a relatively uniform
cross-sectional area and of a size suitable for isolated
muscle experiments, to be obtained even from large (old)
rats. The right ventricular free wall was removed, and the
interventricular septum was bisected and pinned open.
After allowing the heart to be exposed to BDM for |10–
15 min, one of the left ventricular papillary muscles was
split longitudinally, separating a strip of muscle around
one-third to one-half of the papillary muscle. Ties (braided
noncapillary 5-0 silk thread) were placed around both ends
of the split muscle strip and the free ends of the ties were
then hooked onto a small C-shaped spring, which kept the
muscle close to its resting length upon isolation from the
heart. The preparation was ready to be mounted onto the
thermopile and immersed in normal Krebs–Henseleit
solution (i.e., without BDM). Only one muscle preparation
was obtained from each heart. Mean muscle mass, length,
and cross-sectional areas, respectively, were (mean6S.E.):
3.260.3 mg, 6.260.4 mm, 0.5360.05 mm 2 for the 6month group; 2.260.3 mg, 6.460.4 mm, 0.3460.04 mm 2
for the 15-month group; 3.160.5 mg, 5.960.4 mm,
0.5260.07 mm 2 for the 27- to 32-month group.
2.2. The muscle–thermopile system
Details on heat measurements and the experimental
set-up have been described in earlier papers [13,18]. The
muscles were mounted on a wire-wound electroplated
thermopile, enabling measurements of heat production.
One muscle tie was clamped to the frame of the thermopile
and the other tie was attached, via a tungsten connecting
wire, to the lever system of an ergometer (model 300 H,
Cambridge Technology) which measured force and
changes in the length of the preparation. The total compliance of the ergometer, tungsten wire, and silk ties was
1.6310 23 m N 21 . Force, length and heat, could be measured simultaneously. The preparations were stimulated
through platinum electrodes. Stimulation per se (7.0–8.0 V
d.c. pulses of 0.2–0.5 ms duration, at 0.167 Hz), did not
produce any measurable heat (i.e., stimulus heat was
negligible).
The muscle–thermopile system was enclosed in a glass
chamber containing 45–50 ml of aerated (95% oxygen–
5% carbon dioxide) Krebs–Henseleit solution (no BDM).
As heat produced by the muscle is quickly lost to the
surrounding solution, heat measurements were made only
when the solution was drained out of the muscle–thermopile chamber into an adjacent glass chamber. This
considerably reduces the rate of heat loss. For any one
muscle, the time-course of heat loss, with the solution
drained away from the muscle, was exponential, and
reproducible over the course of the experiment: the average heat rate loss of all muscles in the four main groups
was 23.961.5% s 21 (n518). Heat signals recorded from
contracting muscles were electronically corrected for heat
loss. Both chambers were immersed in a thermostatically
controlled water bath maintained at 278C.
2.3. Experimental protocol
Initially, preparations were continuously stimulated to
contract isotonically at 0.167 Hz, while in solution, under a
light preload (0.5–1.0 g) for a |1-h period of equilibration,
H. Kiriazis, C.L. Gibbs / Cardiovascular Research 48 (2000) 111 – 119
after which muscle length was set to optimum (Lmax ) for
isometric force production.
2.4. Basal metabolism
The resting heat rate measurements (basal metabolism)
were made with the muscle quiescent (i.e., unstimulated),
by draining the solution out of the muscle–thermopile
chamber. The output of the muscle–thermopile system
when immersed in solution was zero. Upon draining the
solution, the heat signal rose, and within a few minutes,
reached a steady-state from which the magnitude of the
basal metabolism was determined. Resting heat rate measurements were made at the beginning of each experimental run. The recording runs which were made out of
solution in a moist aerated environment were separated by
equilibration periods, back in solution of |30-min duration. For each preparation, resting heat rate data were
plotted against time post-cardiac excision. As the time at
which basal metabolism was measured varied between
muscles, values were interpolated at specific time-points
(150, 180, 240 and 300 min) from graphs of the experimental data, allowing comparison between groups.
Active muscle heat production, recorded when stimulated,
was measured above the basal metabolism baseline. During experimental runs, muscles were stimulated by trains
of 15 stimuli at 0.167 Hz and the total heat production
(i.e., initial plus recovery heat) was measured over a 2- to
3-min period. Up to six such runs were made in any
recording period. There was little sign of deterioration in
the muscles’ contractile performance over the recording
period but in both the isotonic and isometric experiments
mirror image load (isotonic) or length (isometric) runs
were usually made in the next recording session and the
data from two identical runs was averaged.
113
relaxation work is done by the load as it stretches the
muscle back to Lmax , and the energy that appeared as
mechanical work is returned to the muscle as heat (due to a
slow relaxation rate, negligible energy is lost into the
afterload stop). From the data obtained, enthalpy, external
work (load3distance shortened; normalised for muscle
mass), mechanical efficiency (work / enthalpy3100%), and
muscle shortening (as a percentage of Lmax ) were calculated. These parameters were plotted against the total load,
which was normalised by expressing it as a fraction of Po .
As the loads used were not usually an exact fraction of Po ,
values of enthalpy, work etc., were interpolated at 0.1,
0.2, . . . ,1.0 Po from graphs of the experimental data.
2.6. Force–length area analysis
As part of the current study, a force–length–area (FLA)
analysis was performed as previously detailed [19]: (i) For
each preparation length–tension curves were obtained (see
Fig. 1). An active length–tension (load) curve was established for each preparation using the isotonic data, by
plotting the average distance the muscle shortened at each
load. A passive length–tension curve was established using
data obtained by progressively shortening the muscle until
passive force was negligible, see above. (ii) The FLA was
calculated for each load. FLA is determined by the sum of
two energy terms, external work and potential energy,
except for the isometric case where it is described by a
2.5. Isotonic experiments
For each preparation a number of isotonic runs were
performed. Additional afterloads were progressively added
to the fixed preload, such that the total load (preload and
afterload) was increased in steps of |0.5–1.0 g until it was
equal to Po (the load at which contractions became
isometric). Thereafter the afterload was progressively
decreased to zero. An afterload stop prevented the muscle
from being stretched at increased afterloads; therefore, the
initial muscle length remained at Lmax . To obtain measurements at very light loads, occasionally loads lighter than
the preload were used. Mechanical and heat measurements
were made at each set load. As a train of contractions was
used, values for muscle shortening were averaged for each
set. In addition, the total heat production (normalised for
muscle mass) in a train of contractions was divided by 15
to obtain the average heat production per contraction. With
the isotonic contractions, the measured heat production is
actually the enthalpy (heat plus work), because during
Fig. 1. Active and passive length–tension curves for one preparation. The
active length–force (load) relation (upper curve) was established using
isotonic data and plotting the shortening that took place under the various
afterloads. The passive length–tension relation (lower curve) was established from isometric studies and plotting passive force values obtained at
lengths less than Lmax (the length at which maximum force was
generated). At Lmax (origin, X), Po denotes the load at which contractions
became isometric (i.e., no shortening). Similar graphs were established
for each preparation and used to calculate the force–length area (FLA).
See Section 2.6 for details.
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H. Kiriazis, C.L. Gibbs / Cardiovascular Research 48 (2000) 111 – 119
single term, that for potential energy [15]. In Fig. 1 an
example is shown for a load of 19.6 mN, the work term is
represented by the area enclosed by ABCDA and the
potential energy is determined from the adjacent area,
DCZD; therefore, the area enclosed by ABCZDA, is the
FLA for the load. When the load is equal to or greater than
Po , the area enclosed by the path ABYCZDA, represents
the potential energy (work is zero in the isometric case),
and also gives the FLA. (iii) the FLA (work plus potential
energy) data were normalised for muscle mass, and plotted
against the total energy (enthalpy) data for the individual
loads. A linear regression line was fitted to the total
energy: FLA data obtained for each preparation.
2.7. Isometric experiments and calibrations
At certain times during the day, isometric experimental
runs were made. The data was needed for the FLA analysis
and allowed checks to be made that there was no deterioration of the preparations with time. At the end of each
experiment the muscle–thermopile system was calibrated
as described previously [14,17], and the muscle mass and
length at (Lmax ) were measured.
2.8. Statistics
Results are expressed as mean6S.E. Statistical analysis
of the data was performed using SPSS (Chicago, IL, USA).
Tests for homogeneity of variances (Bartlett-Box, Cochrans) were carried out, and revealed that some data
required square root or logarithmic (log 10 ) transformation
prior to statistical testing. One-way, or two-way repeatedmeasures analyses of variance (ANOVA) were performed
on the raw data, or transformed data (when appropriate), to
test for differences between the groups. Statistical significance was indicated by P,0.05. Where the ANOVA
revealed a significant difference between groups, the least
significant difference multiple range test [20] was used to
identify differences between mean values.
3. Results
Fig. 2. Resting heat rate at 150, 180, 240 and 300 min post cardiac
excision for 6-, 150- and, 27- to 32-month groups. There was a significant
effect of time on the magnitude of the resting heat rate. The mean resting
heat rate was lower for the 27- to 32-month group compared to the
6-month group (see Section 3.1 for details).
ever, a significant decrease in the resting heat rate with
time (P,0.001) for all age levels. Analysis performed
using the first time-point (150 min; one-way ANOVA), did
show a statistically significant decrease in the resting heat
rate for the 27- to 32-month group in comparison to the
6-month group (7.6560.41, 7.0461.12 and 4.9860.38
mW g 21 for the 6-, 15- and 27- to 32-month groups
respectively; P50.047). To investigate whether the magnitude of the drop in basal metabolism with increasing
time post-cardiac excision was affected by age, the resting
heat rate data at 180, 240 and 300 min were expressed as a
fraction of the 150 min data. The effect of time was
significant (P,0.001) but there was no significant difference between the groups (P50.252) implying that the
proportional decrease in resting heat rate with time was
similar for the three groups.
3.1. Resting heat rate
3.2. Isometric and isotonic mechanical data
Many measures of resting heat rate or basal metabolism
were made in each quiescent muscle over the course of an
experiment (5 h). To allow comparison at specific times
e.g., 150, 180, 240 and 300 min post cardiac excision,
values were interpolated from heat rate versus time plots
made for each preparation. Fig. 2 shows the mean data for
the 6-, 15- and, 27- to 32-month groups. The resting heat
rate data collected for the 15-month group had a large
variance compared to the other two groups and, analysis of
variance performed using the 6-, 15-, and 27- to 32-month
groups and incorporating the four time-points (i.e., twoway repeated-measures ANOVA), revealed no significant
differences between groups (P50.063). There was, how-
Developed stress at Lmax , Po , was significantly decreased
for the 27- to 32-month group in comparison to preparations obtained from the 6- and 15-month animals
(52.765.3, 64.764.7, and 31.461.2 mN mm 22 for the 6-,
15- and 27- to 32-month groups, respectively; P,0.001).
Mean muscle shortening data (expressed as a percentage
of Lmax ) against load, are presented in Fig. 3. There was a
significant attenuation in the degree of muscle shortening
for the 27- to 32-month group in comparison to the 6- and
15-month groups (P50.005). Data obtained for the 27- to
32-month-old animals were scaled down by a factor of
0.66 in comparison to the 6- and 15-month-old animals.
H. Kiriazis, C.L. Gibbs / Cardiovascular Research 48 (2000) 111 – 119
115
Fig. 3. Muscle shortening against load. Shortening has been expressed as
a percentage of Lmax , and the load has been expressed as a fraction of Po .
The extent of muscle shortening for the 27- to 32-month group is quite
depressed, in comparison to the 6- and 15-month groups. The difference
was statistically significant.
3.3. Work enthalpy and mechanical efficiency
As described in the Methods, the muscles were made to
shorten against a range of afterloads. Mean enthalpy
(heat1work), work and mechanical efficiency data plotted
against (normalised) load, are shown in Fig. 4. There was a
significant decrease in both enthalpy and work for the 27to 32-month group in comparison with the 6- and 15month groups (P,0.001 for both parameters). Relative to
the 6-month group data, enthalpy was decreased by 52%
and work by 55% for the 27- to 32-month group. The
mechanical efficiency profile was not significantly different
between groups (P50.396). Although mechanical efficiency appears to be lower for the 27- to 32-month group
than the 6- and 15-month groups at the light loads (0.1, 0.2
Po ), this difference was not significant even when analysis
of the data was restricted to these loads alone (P50.082).
It should be noted that the work term calculated in the
results described above, excludes the work done by the
stretched parallel elastic component work (PECW), see
also Fig. 1. (The PECW is determined from the area under
the passive length–tension curve used to calculate the
FLA), whereas the area XABCDX represents the work
(load3shortening). It should also be pointed out that
external work is commonly calculated as load3muscle
shortening, as this is simple to determine, and when
passive tension is low, the PECW is relatively small
compared to the work term.
As there was a marked decrease in muscle shortening
for the 27- and 32-month group in comparison to the adult
Fig. 4. Enthalpy (heat plus work), work (load3distance shortened) and
mechanical efficiency (work / enthalpy3100%) per contraction against
load (expressed as a fraction of Po . There was no significant differences in
enthalpy and work for the 6- and 15-month groups. Enthalpy and work
were significantly attenuated for the 27- to 32-month group compared to
the adult (6-, 15-month) groups. Mechanical efficiency was similar for the
three groups.
animal groups (Fig. 3), this implied that the PECW at each
load represented a relatively greater portion of the work
(load3shortening) for this group. In order to take into
account this difference, the work-PECW data were also
analysed, see Fig. 5. There was no significant difference
between the 6- and 15-month groups, whereas, workPECW was significantly attenuated for the 27- to 32-month
group (P,0.001): mean work-PECW values were 60%
lower than those of the 6-month group. In comparison to
the work data shown in Fig. 4 (i.e., work calculated as:
load3distance shortened), the work-PECW data were, on
average, decreased by 1763%, 1863% and 2663% for
the 6-, 15- and 27- to 32-month groups, respectively;
H. Kiriazis, C.L. Gibbs / Cardiovascular Research 48 (2000) 111 – 119
116
represents the contractile efficiency [15]. There were no
significant differences in intercept (P50.105), or slope
(P50.765) and, hence, no difference in contractile efficiency (P50.658), between the groups. The intercept on
the total energy axis provides an estimate of the activation
heat, see Discussion.
4. Discussion
Fig. 5. Mean work excluding the parallel elastic component work (i.e.,
work-PECW), versus fractional load for the different age groups. In
contrast, the work shown in Fig. 4 includes the energy stored in the
parallel elastic component (due to the passive force exerted by the
preload). There was a significant depression in work-PECW for the
preparations obtained from the 27- to 32-month-old animals in comparison to 60 and 15-month-old animals.
however, there was no significant difference between the
groups (P50.084). As ‘work-PECW’ values were less than
‘work’ values, mechanical efficiency data evaluated using
work-PECW were also proportionally lower in comparison
to the mechanical efficiency data shown in Fig. 4. Statistical analysis on these mechanical efficiency data (obtained
for 0.1, 0.2, . . . ,0.9Po ) reached the same conclusion stated
above namely, there were no significant differences between the different age groups.
3.4. FLA analysis and contractile efficiency
The isotonic data were also subjected to a different form
of analysis. The FLA for each load was calculated as
explained in the Methods and plotted against the total
energy (enthalpy) obtained for each load level. The data
were fitted with a linear regression line details are given in
Table 1. The reciprocal of the slope of the line (3100%)
Table 1
Details on mean total energy: FLA (force–length area) regression lines,
and contractile efficiency a
Group
Intercept
(mJ g 21 )
Slope
r
Contractile
efficiency
%
6-month
15-month
27- to 32-month
4.7960.93
3.8861.05
2.0860.44
2.4460.24
2.5960.19
2.6860.24
0.95560.011
0.96160.010
0.95360.020
4364
4063
3963
a
Values are means6S.E. Regression lines are shown in Fig. 5.
Contractile efficiency was calculated as the reciprocal of the slope3
100%. r, Correlation coefficient.
As mentioned in the Introduction there are very clear
effects of aging on several of the cellular ATPases that
have been shown to underwrite rat cardiac expenditure
[16,21]. There are changes in both cellular Ca 21 kinetics
and in cross-bridge turnover rate and these changes must
and do affect cardiac function and energetics [13–15]. It
has been known for some time that when myosin heavy
chain alters, that the economy and even the mechanical
efficiency of a contraction can be altered [10,13,14,22].
Since there are significant age-induced alterations in
ventricular myosin heavy chain expression [9,10], one
would predict aging would alter the economy of a contraction. However, it is much less clear from the literature
whether mechanical efficiency alters with aging or pressure
overload hypertrophy. Likewise the clear changes in
cellular calcium handling [1,6,12] that have a large effect
on contractile duration have not always produced clear
changes in activation heat magnitude.
This paper attempts to look at age-dependent changes in
cellular mechanics and energetics in isotonically contracting papillary muscles from rat hearts. It is important to
note that the mechanical and energetic characteristics of
cardiac preparations obtained from the 6- and 15-month
animal (adult) groups were similar, and this implies that
the intrinsic properties of the Sprague–Dawley rat myocardium (assessed at 278C, 1 / 6 Hz stimulation and 2.5 mM
calcium) did not undergo changes over this age range.
Hence, it is unlikely that any differences between the
measured parameters in the senescent (27- to 32-monthold) and adult animals can be attributed to developmental
changes.
4.1. Isotonic and isometric mechanical performance in
senescence
In comparison to the 6- and 15-month groups, papillary
muscle preparations dissected from 27- to 32-month
animals had a markedly reduced work (or work-PECW)
profile against load. This outcome was due to both a
decrease in muscle shortening as a function of load and to
reduced stress generation (i.e., heavy loads could not be
lifted). A decrease in isotonic shortening has been observed previously in papillary muscles obtained from 24month-old Fischer rats, in comparison to 6- and 12-monthold animals [5] and is also a prominent feature of longterm pressure overload hypertrophy [14]. Interestingly, this
H. Kiriazis, C.L. Gibbs / Cardiovascular Research 48 (2000) 111 – 119
phenomenon does not seem to persist at the cellular level
as the relative extent of shortening in myocytes (isolated
from Wistar rats) during a twitch is not affected by
advancing age [23]. It may be that age-related changes in
the extracellular matrix of the myocardium (e.g., accumulation of collagen and fibrosis) impedes normal shortening
of the myocytes in the whole heart or in papillary muscle
preparations [24]. The fall in peak stress development in
the senescent group is quite marked and the changes in the
extracellular matrix may contribute. This decline in performance is also very similar to that seen in long-term
pressure overload where the mechanical and biochemical
consequences have been well documented [13,14].
Elzinga and Westerhof [25] established pump function
curves for contracting feline hearts and showed that there
was a curvilinear relationship between mean intraventricular pressure and flow when measured over the entire
cardiac cycle and that changes in contractility or cardiac
end-diastolic volume could shift the relationship. Our
aging data, which show a decrease in stress generation and
shortening would be explicable in terms of a decrease in
contractility or an ‘apparent’ decrease in end-diastolic
volume. The second explanation seems unlikely unless
there has been an over-compensation of cell lengthening
with too many sarcomeres in series and hence at less
favourable points on their ascending length–tension curve.
4.2. Energetic consequences of senescence
4.2.1. Basal metabolism
The magnitude of the resting heat rate was reduced for
the 27- to 32-month group in comparison to the 6-month
group (see Fig. 2). The reasons underlying the timedependent decrease in the magnitude of the resting heat
rate post cardiac excision, though well documented in both
saline perfused hearts and in myothermic investigations are
unknown [26,27]; the phenomenon does not occur under
some conditions [28] but has been seen in myothermic
experiments on rat, rabbit and cat hearts and a timedependent decline is even evident in saponin skinned rat
cardiac trabeculae [21] where it was attributed to loss of
membrane-bound ATPases.
It is important to realise that the basal metabolism
accounts for 20–30% of the total cardiac metabolism and
to assess why the basal metabolism of the senescent heart
is depressed. There is recent experimental evidence in
saponin skinned rat trabeculae that after the initial timedependent decline, some 40% of the remaining basal
ATPase is non-membrane bound and may reflect unregulated myosin ATPase activity, about 14% can be linked to
the Na 1 –K 1 pump and about 8% reflects basal SR Ca 21 ATPase activity. The remaining 38% is abolished by
Triton treatment and hence is thought to be membrane
bound possibly associated with T-tubules and microsomes.
Alternatively the attenuation of basal metabolism with
age may relate to morphometric changes observed in the
117
left ventricle for this rat strain. Anversa et al. [25] have
reported (i) that there is a loss of myocytes and replacement fibrosis (hence, reducing the basal metabolism per g
of tissue) and, (ii) that existing myocytes are hypertrophied. Less energy (ATP) is required to maintain ionic
homeostasis in enlarged (hypertrophied) myocytes as there
is a lower surface area to volume ratio so that the enlarged
cells would require less Na 1 / K 1 -ATPase activity per unit
surface area, resulting in a reduced energetic cost.
It is important to establish the magnitude of any changes
in basal metabolism since in most in vivo measurements of
gross mechanical efficiency the basal metabolic rate is in
the denominator of the efficiency calculation and a fall in
the basal metabolism term could make the senescent heart
look ‘efficient’ in spite of a possible decline in net
efficiency.
4.2.2. Active metabolism
When a muscle contracts, additional energy is liberated
above the basal metabolism. This active metabolism is
made up of two components; an activation one related to
the energetic cost of EC coupling and another related to
cross-bridge activity [17,29]. In the present investigation
activation heat was measured in isotonic experiments as
the intercept on the energy:FLA curve. The activation
energy term predominantly involves ATP usage by the
calcium ATPases of the sarcoplasmic reticulum and sarcolemma and the cost of sodium pump activity [16,21].
There was no statistically significant change in the activation heat but we have been able to show [30] that when the
senescent heart is challenged pharmacologically it can
produce only a modest increase in activation heat compared with the several fold increments seen in the young
and adult groups and this result is very much in line with
expectations from calcium transient data [7].
4.3. Efficiency
There are several different efficiency definitions in use
and the interpretative difficulties that result have been
discussed in a recent review [31] where a comparison was
made of mechanical and contractile efficiencies. Mechanical efficiency defined as work / total enthalpy must by
definition be load-dependent i.e., it must be zero at zero
load or at loads $Po . Mechanical efficiency was unaltered
between adulthood (6-, 15-month-old) and senescence (27to 32-month-old). The substantial decrease in work output,
observed for the 27- to 32-month group in comparison to
the adult groups, was offset by a decrease in enthalpy
leaving net mechanical efficiency unaltered.
In isometric contractions, no external work is done and
conventionally the energy cost of force production is
assessed by measuring economy defined as the energy
liberated for unit developed force or for unit force–time
integral. We and others have found that the isometric
economy is unchanged across the young and adult age
118
H. Kiriazis, C.L. Gibbs / Cardiovascular Research 48 (2000) 111 – 119
groups but increases in the senescent hearts. This result
would seem to be at odds with the isotonic results but it
has been pointed out that economy and efficiency are
different entities with efficiency being dimensionless and
economy being proportional to 1 / velocity and hence to the
rate at which crossbridges are broken [32]. With age, the
rate constant ‘g’ of the Huxley model would be predicted
to decrease due to the shift to the slow myosin isoform and
one would expect economy to increase since proportionally
more cross-bridges will achieve a full power stroke before
being detached at the lower velocity of sarcomere shortening.
In the present investigation, we also measured contractile efficiency. The contractile efficiency, derived from the
FLA analysis, represents the proportion of energy used that
can be converted to mechanical energy (work and potential
energy); this value is reasonably constant regardless of the
loading conditions [15]. The strength of the FLA model
rests on its ability to accurately predict the oxygen
consumption per beat (it is clearly better than other
mechanical indices) and ‘contractile efficiency’ does measure the total mechanical energy rather than only external
work. The major problem for the model relates to its
unsatisfactory molecular explanation. There is however
considerable modeling work being done to address this
problem. In the present investigation, the contractile
efficiency was found to be around 40%, the same value as
for various other animals species (see Table 1 in [15]) and
aging did not alter contractile efficiency and this is in
agreement with the studies of Suga and colleagues who
have measured contractile efficiency in puppies and adult
dogs [15,33].
The poor in vitro isotonic mechanical performance noted
for the senescent papillary muscles in the present investigation, might lead one to expect that in vivo the
cardiac index (cardiac output per body surface area) would
be very depressed. Much earlier experiments by Spann and
colleagues [34] upon pressure-overloaded hypertrophic cat
hearts showed that in vivo there was only a 20% depression of the feline cardiac index but when papillary muscles
were isolated from the same hearts they had a greatly
decreased ability to generate isometric stress (only 50% of
normal). Presumably this means that in vivo the heart
works at greater end-diastolic volumes and with its contractile performance stimulated by higher levels of sympathetic outflow, and can maintain an adequate cardiac index
in spite of greatly reduced intrinsic contractility.
In a recent review it was concluded that the aged heart
seems to resemble a pressure-overloaded heart [1]. The
cardiac energetic profile of the senescent rat group in the
present study is in line with many findings in the isolated
perfused whole heart literature, reviewed by Lakatta [1],
and for the most part our data resembles the results we and
others have obtained in various models of pressure overload.
Acknowledgements
This work was performed with the support of the
National Health and Medical Research Council of Australia.
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