691
Altered Performance of Rat Cardiac Muscle
Follows Changes in Mechanical Stress
during Relaxation
ALLEN W. WIEGNER AND OSCAR H. L. BING
SUMMARY We compared two sets of loading conditions for their effect on the mechanical performance of
isolated rat trabecular muscle: (1) loading which approximated that of the intact ventricle; (2) nonphysiological
afterloaded isotonic contractions. Improved performance, manifested by increased shortening and rate of shortening
at constant load, was seen when lengthening occurred at light loads, as in the intact heart. In contrast, lengthening
occurring at the same load against which shortening had taken place was followed by diminished performance.
Viscous elements in series with the muscle could not account for this phenomenon. The results suggest the presence
of an autoregulatory response of myocardium to mechanical stress during relaxation.
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During a normal cardiac cycle, the aortic valve closes
following ventricular ejection, and intraventricular pressure declines. Subsequent ventricular filling occurs at low
pressure, as ventricular wall fibers are extended to their
precontraction length. In an isolated muscle preparation
which is loaded to simulate this cycle of events, isometric
relaxation precedes isotonic relaxation, that is, in a "physiologically sequenced contraction" (PSC), force is permitted to decline prior to muscle lengthening.1
Afterloaded isotonic contractions (AIC), apparently a
holdover from studies of skeletal muscle,2 represent the
lifting of a load and the subsequent reextension of the
muscle by the same load during relaxation. With AICs,
the isotonic relaxation phase precedes isometric relaxation; i.e., lengthening occurs before the decline of active
force. Thus, AICs simulate the ventricular relaxation
dynamics imposed by an incompetent semilunar valve,
where ejected blood is allowed to return to the ventricle.
This nonphysiological relaxation sequence has been used
to date in many studies of the innate properties of cardiac
muscle.
In the present study we have compared these two
modes of contraction in isolated muscle preparations, to
evaluate the effects of load during relaxation on overall
muscle performance.
Methods
Trabeculae carneae were dissected from the left ventricles of freshly decapitated rats and mounted vertically in
a Plexiglas chamber containing Krebs-Henseleit solution3
From the Department of Electrical Engineering and Computer Science,
Massachusetts Institute of Technology, Cambridge, Massachusetts, and
Thorndike Memorial Laboratory, Harvard Medical School, Beth Israel
Hospital, Boston, Massachusetts.
Supported by Grant 1147 of the Greater Boston Chapter of the
Massachusetts Heart Association, National Institutes of Health Grants
HL 18338, HL 00072, and Training Grant CM 01555. Dr. Wiegner is
the recipient of a fellowship from the Johnson & Johnson Associated
Industries Fund. Dr. Bing is the recipient of a National Institutes of
Health Career Development Award.
Address for reprints: O. H. L. Bing, M.D., Cardiovascular Unit, Beth
Israel Hospital, 330 Brookline Avenue, Boston, Massachusetts 02215
Received February 2, 1977; accepted for publication April 19, 1977.
at 28°C, bubbled with a mixture of 95% O2 and 5% CO2.
An electronic servosystem1'4 was used to control the
force and/or length of the muscle preparation. Length
was controlled by means of a low inertia DC motor
(General Scanning model G100PD) with integral displacement transducer, while a semiconductor strain gauge
transducer (DSC Incorporated model DSC-3) measured
force. Servosystem feedback was adjusted to obtain step
response times of 1 msec for a length step and 5 msec for
a force step. System frequency response to a sinusoidal
length command (0.1 mm peak to peak) was found to be
flat within 1 dB from 0 to 500 Hz. Electrical compliance
of the system was less than 1 //.m/g. The preparations
were stimulated at a rate of 12/min by parallel platinum
electrodes delivering 5-msec pulses at voltages 10% above
threshold. The upper end of the muscle was held by a
spring clip and connected by a length of 30-gauge stainless
steel tubing to the motor lever arm above the chamber.
The lower end of the muscle was held by a spring clip
directly attached to the immersed force transducer.
The physiologically sequenced contractions that occurred with this system consist of the following four
phases: (1) isometric contraction: following stimulation,
the muscle is held at constant length while contractile
force increases; (2) isotonic shortening: when active force
equals the afterload, force is held constant at that level
and muscle shortening occurs; (3) isometric relaxation:
the point of maximum shortening is detected and the
muscle is held at its shortest attained length while force is
allowed to decline to the preload level; (4) isotonic
lengthening: with force held at preload level, the muscle
returns to its original length. To perform afterloaded
isotonic contractions, phases 3 and 4 are interchanged,
such that lengthening occurs before the decline of active
force.
Following a 30-minute period of isotonic contractions
at a preload of approximately 0.5 g/mm2, the muscles
were stretched to the apices of their length-tension curves
(Ln,ax) and allowed to equilibrate for 30 minutes while
contracting with an afterload equal to 25% of peak
isometric active force. Fifteen-minute periods of AICs
CIRCULATION RESEARCH
692
RELAXATION
A. Isometric - tsotonic
VOL. 41, No. 5, NOVEMBER 1977
SEQUENCE
8.
Isotonic - Isometric
o Afterlood Increased
• Relaxation Sequence Changed (PSC to AIC)
4
o
o
—
o
o
g wt
t
o
T
0
-
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FIGURE 1 Oscilloscope traces showing muscle length in millimeters (top) and muscle force in grams-weight (bottom). A, Trace 1
is a record of the first physiologically sequenced contraction (PSC)
following a 15-minute period of afierloaded isotonic contractions
(AIC). Trace 2 is a PSC recorded 15 minutes later which shows
increased shortening and more rapid relaxation. B, Trace 3,
recorded immediately after trace 2, is the first of a subsequent 15minute period of AICs. In trace 4, 15 minutes later, shortening
has decreased and relaxation slowed. This cycle of events was
repealable.
were subsequently alternated with equal periods of PSCs.
Afterload and resting length were held constant; only the
relaxation sequence changed. The first and last contractions of each 15-minute period were recorded with a
Tektronix model D15 storage oscilloscope and model C5
camera (Fig. 1). The entire cycle as shown in Figure 1
was performed twice with four muscles, three times with
one muscle, and once with one muscle. Repeated cycles
with a given muscle preparation were averaged to obtain
the values for that muscle used in statistical analysis.
Statistical significance was evaluated by Student's paired
Mest.
Results
Table 1 summarizes the observed changes. After 15
minutes of PSCs, there was no change in the rate of force
development (+dF/dt) during the isometric phase of the
contraction. However, an increase (8%) in the maximum
velocity of muscle shortening, combined with negligible
change in the time to peak shortening, resulted in increased shortening (8%). Relaxation parameters showed
the largest changes: isotonic lengthening proceeded 26%
faster after 15 minutes of PSCs.
It is evident from Figure 1 that the force-time product
(FTP), or muscle force integrated over the course of a
o
-2 —
~"
o
o
•
|
•
-4
—
-6
o
•
-
-8
1
*
1
20
1 1
40
1 *l
60
1
1
1
80
1
100
120
INCREASE IN. FORCE-TIME PRODUCT
FIGURE 2 The effect of a 15-minute increase in force-time
product (FTP) on muscle shortening. Upon changing from PSCs
to AICs, the increase in the FTP is followed by a decrease in the
amount of shortening after 15 minutes (0). When the FTP is
increased by simply increasing the afterload (O), after the initial
step decrease in shortening, no consistent changes in shortening
appear during the subsequent 15-minute period. The change in
shortening (mean ± SEM,) for each group is indicated.
twitch, of an AIC exceeds that of a PSC at the same
afterload. One might suggest that viscous elements, either
in series with the contractile element (CE), or at the
damaged ends of the muscle where it is attached to the
experimental apparatus, would be stretched by the increased FTP, resulting in a shorter CE. In this case the
present effect could be a result of the CE operating at a
shorter length during AICs. However, two arguments
suggest that this is not the case.
Following the onset of a period of AICs, a small
decrease in preload is seen at constant resting length.
This implies the existence of a small series viscous element
which is stretched by the increased FTP. However, this
fall in preload occurs within the first several twitches
following the onset of AICs, while decreased shortening
has a time constant of approximately 5 minutes, an order
of magnitude greater. Thus, these early series viscous
effects do not appear to be temporally correlated with the
TABLE 1 Effect of Relaxation Sequence on Muscle Contraction Parameters
PSC
AIC
Max. shortening velocity (muscle lengths/sec)
Total shortening (% of Lmax)
Time to peak shortening (msec)
Duration of isometric relaxationt (msec)
Max. lengthening velocity! (muscle lengths/sec)
1 1
1.04 at
10.1 it
190 at
95 it
1.92 at
0.03
0.4
2
6
0.09
1.12
10.9
189
78
2.42
±
±
±
±
±
0.04
0.5
2
4
0.18
<0.01
<0.01
NS*
<0.01
<0.01
Fifteen-minute periods of AICs were alternated with 15-minute periods of PSCs. The first and last
contractions of each period were recorded. The AIC column lists parameter values after a 15-minute
period of AICs, while the PSC column reflects a 15-minute period of equilibration with PSCs. Data
are from six muscles, mean ± SEM.
* NS = not significant.
t Since isometric relaxation follows lengthening in AICs, relaxation parameters are determined
from the PSCs which bracket a 15-minute period of AICs or PSCs.
MECHANICAL STRESS DURING MYOCARDIAL RELAXATION/W/eg/jtv and Bing
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later changes. In an additional experiment, the afterload
was altered at the onset of a period of AICs or PSCs to
test the effect of FTP on twitch parameters. Figure 2
shows that changing the relaxation sequence from PSC to
AIC increased the FTP and consistently decreased
shortening by 4-5% in this series of muscles. Changes in
FTP alone resulted in random variations in shortening
with a mean near zero. This implies that changes in FTP
alone are not responsible for the effect described here.
Improved performance associated with properly sequenced relaxation slightly resembles the effect of catecholamine administration, in that total shortening, velocity of shortening, and especially rate of relaxation are
enhanced. However, time to peak shortening is not reduced and the effect is not abolished by /3-adrenergic
blockade with 10~4 M propranolol, even though contractile
force is decreased approximately 50% at that concentration . If this effect were mediated by changes in intracellular Ca2+, one might expect to find contractile changes
which parallel the effect of an increased concentration of
calcium in the bath. On the contrary, we find that increasing the bath calcium strongly increases +dF/dt and maximum shortening velocity, with little change in the isometric relaxation rate.
Discussion
We have found that altered muscle performance follows
a change in mechanical stress during relaxation. Since this
is an intrinsic alteration of myocardial performance at
constant fiber length, it may represent a form of homeometric autoregulation. This term was introduced by Sarnoff et al.,5 who suggested two examples: the apparent
positive inotropic effects resulting from increased heart
rate (the Bowditch effect, or "Treppe") and increased
afterload (the Anrep effect). Subsequent research has
indicated that Treppe may be a consequence of intracellular ionic shifts with alteration in contraction rate.6 The
Anrep effect has been related to coronary vessel flow
redistribution following changes in aortic impedence and,
as such, it would not be a true example of homeometric
autoregulation.7 Although the present studies do not
elucidate the basic mechanism responsible for the inotropic effect reported here, it may be of interest to
consider the possible effect of stress during relaxation on
the machinery of contraction.
693
In the sliding filament theory of muscle contraction,
force is proportional to the number of attached "crossbridges," the sites of interaction of the actin and myosin
filaments.8 In an AIC, lengthening takes place while the
number of attached cross-bridges is large, whereas in a
PSC, external lengthening occurs when fewer crossbridges remain attached. One might speculate that gradual
conformational changes in the actomyosin complexes of
the thick and thin filaments may result from forced
dislocation of the cross-bridges by the afterload during an
AIC and lead to altered muscle performance.
Previous studies have shown that quick stretches and
quick releases during the rising phase of the twitch alter
the intensity and time course of a given contraction9-10
and that a change in performance seems to accompany
large changes in afterload."-l2 The present study uses no
stretches, releases, or changes in afterload, yet demonstrates that changes only in the load during relaxation can
reversibly alter both contraction and relaxation dynamics
of future twitches.
References
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isolated heart muscle. Biochem Biophys Res Commun 56: 947-951,
1974
2. Jewell BR, Wilkie DR: The mechanical properties of relaxing muscle.
J Physiol 152: 30-47, 1960
3. Krebs HA, Henseleit K: Untersuchungen fiber die Harnstoffbildung
im Tierkorper. Hoppe Seylers Z Physiol Chem 210: 33-66, 1932
4. Wiegner AW: A contraction sequence controller for isolated cardiac
muscle experiments. Progress Report, Research Laboratory of Electronics, No. 115. Cambridge, Massachusetts Institute of Technology,
1975, pp 312-313
5. Sarnoff SJ, Mitchell JH, Gilmore JP, Remensnyder JP: Homeometric
autoregulation in the heart. Circ Res 8: 1077-1091, 1960
6. Langer GA: Ion fluxes in cardiac contraction and excitation and their
relation to myocardial contractility. Physiol Rev 48: 708-757, 1968
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Heart. Ciba Foundation Symposium 2A. ASP, Amsterdam, 1974, pp
257-271
8. Huxley AF, Simmons RM: Proposed mechanism of force generation
in striated muscle. Nature 233: 533-538, 1971
9. Kaufmann RL, Bayer RM, Harnasch C: Autoregulation of contractility in the myocardial cell. Pfluegers Arch 332: 96-116, 1972
10. Bozler E: Mechanical control of the time-course of contraction of
frog heart. J Gen Physiol 65: 329-344, 1975
11. Parmley WW, Brutsaert DL, Sonnenblick EH: Effects of altered
loading on contractile events in isolated cat papillary muscle. Circ
Res 24:521-532,1969
12. Donald TC, Peterson DM, Walker AA, Hefner, LL: Afterloadinduced homeometric autoregulation in isolated cardiac muscle. Am
J Physiol 231 (Part 2): 545-550, 1976
Altered performance of rat cardiac muscle follows changes in mechanical stress during
relaxation.
A W Wiegner and O H Bing
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Circ Res. 1977;41:691-693
doi: 10.1161/01.RES.41.5.691
Circulation Research is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1977 American Heart Association, Inc. All rights reserved.
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