SARCOMERE VELOCITY IN SINGLE HEART CELLS/Tarr et al.
growth in the fetal lamb. Circ Res 26: 289-299
Rudolph AM, Heymann MA (1973) Control of the foetal circulation. In Proceedings of the Sir Joseph Barcroft Centenary
Symposium: Foetal and Neonatal Physiology, edited by RS
Comline, KW Cross, GS Dawes, PW Nathanielsz. Cambridge,
Cambridge University Press, pp 89-111
Severs WB, Daniel-Severs AE (1973) Effects of angiotensin on
the central nervous system. Pharmacol Rev 25: 415-449
Smith FG Jr, Lupu AN, Barajas L, Bauer R, Bashore RA (1974)
The renin-angiotensin system in the fetal lamb. Pediatr Res
8: 611-620
Tripp ME, Heymann MA, Rudolph AM (1978) Hemodynamic
effects of prostaglandin Ei on lambs in utero. Adv Prostaglan-
189
din Thromboxane Res 4: 221-229
Tulenko TN (1979) Re"ional sensitivity to vasoactive polypeptides in the human umbilicoplacental vasculature. Am J Obstet Gynecol 135: 629-636
Tulenko TN, Millard RW (1977) Evidence for a physiological
role for fetal angiotensin in the regulation of the umbilicoplacental vasculature. Ann Rech Vet 8: 484-485
Vapaavouri EK, Shinebourne EA, Williams RL, Heymann MA.
Rudolph AM (1973) Development of cardiovascular responses
to autonomic blockade in intact fetal and neonatal lambs. Biol
Neonate 22: 177-188
Winer BJ (1971) Statistical Principles in Experimental Design,
ed 2. New York, McGraw-Hill
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
Characteristics of Sarcomere Shortening in
Single Frog Atrial Cardiac Cells during
Lightly Loaded Contractions
MERRILL TARR, JOHN W. TRANK, AND PAUL LEIFFER
SUMMARY We studied sarcomere performance in single isolated intact cardiac cells using techniques
that allow direct measurement of sarcomere length and force. This investigation dealt primarily with
sarcomere performance during twitch contractions under lightly loaded conditions. In such contractions, there was a significant portion of the contraction in which sarcomere shortening occurred at
constant velocity over a significant range of sarcomere lengths. The constant velocity phase of
shortening was followed by a phase of shortening in which sarcomere velocity decreased markedly.
Both the velocity and extent of sarcomere shortening depended on the stimulus parameters used to
excite the cell. With threshold stimulation, sarcomere velocities during the constant velocity phase of
shortening ranged from 1 to 5.5 /im/sec in different cells and significant slowing of sarcomere shortening
began at sarcomere lengths of 1.8-2.0 /im. In contrast, when cells were stimulated with a long duration
stimulus (200 msec) of large current strength, sarcomere velocities during the constant velocity phase
ranged from 6 to 12 /tm/sec, and significant slowing did not occur until a sarcomere length of about 1.6
lira was reached. The threshold stimulus strength-stimulus duration relationship was determined on
the single cell, and it was found to be of the type expected for a cell having an intact excitable membrane
capable of generating an action potential when depolarized to a fixed voltage threshold. The data
presented in this paper give direct evidence that the lightly loaded cardiac sarcomere has a velocity of
shortening which depends on the level of contractile activation but is independent of sarcomere length
at sarcomere lengths greater than about 1.6 pm. Circ Res 48: 189-200, 1981
THERE have been few direct determinations of the
relationships between sarcomere length and developed force in cardiac muscle. Elegant experiments
have been performed to determine these relationships in single cardiac cells in which the cell membrane has been removed, and these experiments on
skinned cardiac cells have given information about
the relationships between sarcomere length and
From the Department of Physiology, University of Kansas Medical
Center, College of Health Sciences and Hospital, Kansas City, Kansas.
Supported by U.S. Public Health Service Grant HL 18943 and a
Kansas Heart Association Fellowship to Paul Leiffer.
Address for reprints: Dr. Merrill Tarr, Department of Physiology,
University of Kansas Medical Center, College of Health Sciences and
Hospital, Kansas City, Kansas 66103.
Original manuscript received October 26, 1979; accepted for publication September 9, 1980.
developed force at various levels of contractile activation (Fabiato and Fabiato, 1976,1978). However
it is difficult to make an extrapolation from these
data to predict quantitatively sarcomere performance during a twitch contraction in the intact tissue.
Attempts to measure sarcomere performance directly during twitch contractions in intact tissue
have been limited due to the general difficulty of
directly measuring sarcomere lengths in intact living cardiac tissue. Recently, the laser diffraction
technique has been applied to very thin bundles of
intact cardiac tissue and it as been possible with
this technique to measure directly the performance
of a large group of sarcomeres during twitch contractions (Nassar et al., 1974; Krueger and Pollack,
1975). However, interpretation of data derived from
190
CIRCULATION RESEARCH
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
the intact tissue suffers from uncertainty about the
distribution of forces within the tissue (Manring et
al., 1977).
Ideally, one would like to determine sarcomere
performance in a very simple cardiac preparation
in which the sarcomere length can be measured
directly and in which the distribution of forces is
relatively well defined. Single cardiac cells perhaps
can be used for this purpose and attempts at such
investigations presently are being made by ourselves and other investigators (Tarr et al., 1979;
Brady et al., 1979).
The present paper presents results of an investigation of sarcomere shortening in single frog atrial
cells during twitch contractions in which the sarcomeres within the cell developed relatively small
forces (i.e., lightly loaded contractions). Sarcomere
performance at light loads is of interest for several
reasons. First, there are at present few data available in which the time course of sarcomere shortening in cardiac muscle has been measured directly
under these conditions, and the data available relate
only to rat ventricular tissue (Pollack and Krueger,
1976; Krueger and Wittenberg, 1978). The majority
of data, on a variety of cardiac tissues, relates
velocity of tissue shortening to tissue length (Brutsaert et al., 1971; Henderson and Brutsaert, 1974).
The interpretation of these data with regards to the
sarcomere velocity-sarcomere length relationship
suffers from inhomogeneities in sarcomere lengths
which exist in the tissue, as well as from the lack of
any definitive relationship between changes in overall tissue length and changes in sarcomere length
within the intact tissue (Winegrad, 1974; Gay and
Johnson, 1967; Nassar et al., 1974; Pollack and
Hunstman, 1974; Krueger and Pollack, 1975; Julian
and Sollins, 1975). Second, the sarcomere velocity
approaches the so-called unloaded or maximum
velocity (Vmax) as the external load approaches zero
provided internal loads are negligible. There has
been considerable controversy and interest as to
whether or not Vmax depends on sarcomere length
(Sonnenblick, 1962; Pollack, 1970). Third, there
appears to be considerable difference between the
length dependence of shortening velocity in cardiac
muscle compared to that in skeletal muscle. Studies
that have been done in skeletal muscle demonstrate
that the lightly loaded skeletal muscle sarcomere is
capable of shortening at constant and high velocity
down to sarcomere lengths as short as 1.6-1.7 fim
(Gordon et al., 1966; Costantin and Taylor, 1973).
In contrast, the data presently available on sarcomere or tissue shortening in intact cardiac tissue
indicate that the velocity of shortening decreases
markedly at sarcomere lengths below 1.9-2.0 /urn
(Brutsaert et al., 1971; Henderson and Brutsaert,
1974; Pollack and Krueger, 1976).
The results presented in this paper demonstrate
that during lightly loaded twitch contractions the
sarcomeres in the single cardiac cell shorten at
constant velocity over a significant range of sarco-
VOL. 48, No. 2, FEBRUARY 1981
mere lengths. Furthermore, the data demonstrate
that under some conditions the cardiac sarcomere
shortens at constant and high velocity down to
sarcomere lengths as short as 1.6 /nm before any
significant slowing occurs. Both the velocity and
extent of sarcomere shortening were dependent on
the stimulus parameters used to excite the cell, a
finding which indicates that the level of contractile
activation in the single cardiac cell can be altered
by the stimulus conditions. Data are also presented
that demonstrate that the isolated cardiac cell has
a stimulus strength-stimulus duration relationship
similar to the type expected for a cell having an
intact excitable membrane capable of generating an
action potential dependent on the fast inward sodium current.
Methods
Preparation of Isolated Frog Atrial Cells
Isolated single frog atrial cardiac cells generally
were prepared by trypsin-collagenase dispersion of
intact frog (Rana catesbeiana) atrial tissue, as described previously (Tarr and Trank, 1976). However, recently we have modified the cell isolation
procedure in the following ways. First, trypsin (0.5
mg/ml) alone is adequate to disperse the cells from
the tissue. Second, the cells are harvested without
centrifugation merely by removing the suspension
of dispersed cells from the remaining undigested
tissue by means of a pipette. A few drops of the
suspended cells then are placed directly into a culture dish containing 2.5 ml of Ringer's solution
having the following composition: NaCl =111 mM,
KC1 = 5.4 mM, CaCl2 = 1.8 mM, 10 mM
Tris(Hydroxymethyl)aminomethane, and glucose
= 4 mM. The pH of the Ringer solution was adjusted
to 7.3 at 25°C by adding 12.4 N HC1.
Preparation and Calibration of Cantilever
Force Beams
The methods for preparation and calibration of
the cantilevered glass force beams have been presented previously (Tarr et al., 1979).
Determination of Sarcomere Length and
Developed Force
A Reichert Bio vert 111 microscope was used to
view the cell, the sarcomere pattern within the cell,
and the position of the force beam to which the cell
was attached (see below). To obtain relatively high
magnification, a Zeiss 40 x long-working distance
(1.5 mm) objective having a numerical aperature of
0.6 was used; this objective is equivalent to a 62.5
X objective when placed on the Biovert microscope,
since the Biovert has a 250-mm tube length rather
than the conventional 160-mm tube length. The
microscope condenser also had a numerical aperature of 0.6 and it had a working distance of about
20 mm. A beam splitter was placed in the optical
path to allow simultaneous viewing by two TV
SARCOMERE VELOCITY IN SINGLE HEART CELLS/Tarr et al.
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
cameras. Each TV camera was focused on the real
image produced by the microscope objective; lenses
of different focal lengths were used to provide different magnifications. The total length resolution
was limited by the microscope objective to approximately 0.5 /im.
To measure the force developed by the cell during a twitch contraction, the cell was attached to a
poly -L -lysine- coated cantilevered force beam by
simply touching the force beam to the cell surface
near one end of the cell; the end of the cell was not
wrapped around the beam as was the case in our
previous work (Tarr et al., 1979). The other end of
the cell then was sucked into the end of a fluidfilled pipette under controlled vacuum. The suction
pipette was filled with Ringer's solution containing
bovine albumin (1 mg/ml). The albumin reduced
the tendency for the cell to stick to the inside
surface of the pipette and thereby reduced the drag
forces on the cell when the cell moved through the
pipette opening during a contraction. This fluidfilled pipette also served as a stimulating electrode.
Figure 1 gives an example of a typical TV image
of the preparation obtained from the TV microscope system. The upper half of the TV image
displays the edge of the force beam to which the
cell was attached and the sarcomere pattern as
viewed with one TV camera. The lower half of the
TV image displays the force beam, the cell, and the
suction pipette as viewed by the other TV camera.
An adjustable electronic reticle composed of vertical lines was added to the composite video to provide a direct and continuous dimensional reference
for the TV display; the reticle line spacing was
calibrated by comparison with the spacing of a stage
FIGURE 1 Monitor display obtained from the closed
circuit TV-microscope system. The upper part of the TV
screen shows the sarcomere pattern and force beam at
the magnification used for the determination of the
sarcomere length and developed force; the force beam is
barely visible at the far left edge of the picture. The lower
part of the TV screen shows the force beam, cell and
stimulus pipette at low magnification. The vertical reticle spacings are 5 fim (upper) and 22 fim (lower).
191
micrometer displayed through the TV microscope
system.
A permanent retrievable store of the TV display
was recorded on a video tape recorder (Sony AV
3600). Synchronized stroboscopic illumination was
used to "freeze" the motion of the sarcomeres and
force beam at time intervals of 16.67 msec. Each
TV frame was identified uniquely by a six-digit realtime clock and a two-digit frame count. Electrical
stimulation of the cell was provided by passing
constant current between the suction pipette (anode) and a Ag/AgCl electrode (cathode) placed in
the bathing medium. The stimulus was applied at
the rate of 0.2 Hz, and the application of the stimulus to the cell was identified by means of a video
display stimulus marker which generated a single
vertical line at the left margin of the TV display.
The onset and termination of this mark was coincident with the onset and termination of the stimulus. Both the stimulus marker and the digital
frame identification were recorded on the video
tape, along with the image of the preparation, and
these allowed precise analysis of the timing of the
cells mechanical events with respect to the electrical stimulus.
Analysis of the video-taped data was done using
the stop frame capability (pause mode) of the video
tape recorder in combination with a double TV
cursor (Tarr et al., 1979). For sarcomere length
determinations, the length occupied by 10 sarcomeres (upper half of TV display) was measured
with the TV cursors and an average sarcomere
length was calculated. The total length resolution
of the microscope optics (0.5 fim) limits the precision of an average sarcomere dimension determined
from a group of 10 sarcomeres to about 0.05 /j.m (0.5
/mi/10). Applying the 0.5-jum length resolution to
the measurement of the displacement of the force
beam gives a force resolution on the order of 2.5 nN
for a force beam having a compliance of 0.2 /im/nN,
and 1.0 nN for a force beam having a compliance of
0.5 jum/nN. The position of the force beam prior to
sucking the cell into the suction pipette was taken
as the zero force position. The force in the cell at
any given time during a twitch contraction (i.e.,
developed force) was determined from the displacement of the force beam relative to its position prior
to the initiation of the contraction. The total force
in the cell was the sum of the resting force plus
developed force.
In some experiments the performance of a cell
segment containing many sarcomeres (e.g., 30-60)
was investigated by measuring the distance between the point of cell attachment to the force
beam and a prominent marker (e.g., cell nucleus)
on the cell surface. The average sarcomere length
within the cell segment at any time during the
contraction was computed as the ratio of the segment length to the number of sarcomeres in the
segment. The number of sarcomeres in the cell
segment was assumed to be equal to the ratio of the
VOL. 48, No. 2, FEBRUARY 1981
CIRCULATION RESEARCH
192
resting cell segment length to the resting average
sarcomere length computed from a group of 10
sarcomeres.
In some experiments, a second noncompliant
poly-L-lysine glass beam was attached to the cell
surface by simply touching the beam to the cell at
a point close to the opening of the stimulus pipette.
The cell then was stimulated and allowed to develop
force and shorten in an auxotonic fashion. The
forces devloped by the cell during auxotonic twitch
contractions then were compared to those developed by the same cell during the lightly loaded
twitch contraction under identical conditions of
stimulus strength and stimulus duration. In this
manner it was possible to determine if the forces
developed by the cell during the lightly loaded
contraction were relatively small compared to the
force-generating capability of the cells.
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
Results
The Lightly Loaded Condition
The lightly loaded condition was accomplished
by attaching one end of the cell to a flexible cantilevered force beam and drawing the other end of
the cell through the opening of a fluid-filled pipette
with controlled suction. Under these conditions the
initial force on the cell was the resting force required
to set the initial sarcomere length, and the forces
developed by the cell during contraction were those
forces developed by the cell as the unattached end
of the cell moved through the opening of the suction
pipette during the contraction. Since the unattached end of the cell was relatively free to move
during the contraction, the forces developed by the
cell during these contractions were relatively small.
To determine whether these forces were indeed
small with respect to the force-generating capability
of the cell, the forces developed by a given cell
during auxotonic twitch contractions were compared to those developed during lightly loaded
twitch contractions under identical conditions of
stimulus strength and stimulus duration. It was not
possible to determine auxotonic forces on all cells
that were investigated under lightly loaded conditions. In some cases the cell either did not survive
the attachment of the second beam, or it developed
sufficient force during the auxotonic contraction to
pull free of its attachment to one of the beams. It
was possible, however, to obtain partial data on
nine different cells under a variety of conditions of
initial sarcomere length and stimulus conditions.
The data obtained from these cells are summarized
in Table 1. It is apparent that the peak total force
developed by the cells during the lightly loaded
contractions (PFL) was variable and increased as
the initial sarcomere length increased. The peak
total force developed by the cell during auxotonic
twitch contractions (PFA) also was variable and
depended on the stimulus parameters used to excite
the cell. The ratio PFLrPFA was variable and
ranged from a low of 0.03 to a high of 0.27. However,
when the initial sarcomere length was 2.6 jum or less
(i.e., when the resting force on the cell was small)
and the cell was stimulated with a long duration
stimulus (100-200 msec) of large amplitude (0.7 /XA
or greater), the ratio of PFL:PFA was always less
than 0.10. Thus, under these conditions it seems
reasonable to conclude that the forces developed by
the cell under the so-called lightly loaded conditions
were probably less than 10% of the force-generating
capabilities of the sarcomeres. The so-called lightly
loaded contractions represent conditions where the
external load on the cell during the contraction was
approximately that of the preload required to set
the initial sarcomere length.
1 Resting Forces and Peak Total Forces during Lightly Loaded and
Auxotonic Twitch Contractions
TABLE
I
D
SL
RF
Experiment no.
(MA)
(msec)
((im)
(nNl
1 3-1-79
0.2
0.7
1.0
0.7
0.7
0.4
1.0
0.7
0.7
1.0
1.0
0.9
0.9
100
100
100
100
20
50
100
10
100
10
200
10
200
10
100
10
100
2.5
2.5
2.5
2.7
2.6
2.6
2.6
2.5
2.5
2.2
2.2
2.8
2.8
2.8
2.8
2.4
2.4
0.9
0.9
0.9
21.6
13.3
6.9
6.9
3.1
3.1
2.5
2.5
7.0
2 4-16-79
2 4-17-79
1 4-24-79
1 5-21-79
2 5-22-79
1 5-31-79
2 6-1-79
4 6-1-79
0.8
0.8
0.8
0.8
PFL
(nN|
PFA
(n,\)
PFL:PFA
4.4
4.6
55.9
75.5
98.0
127.1
120.5
73.9
186.4
78.6
94.1
106.4
160.0
132.0
196.4
70.9
129.4
116.4
176.4
0.08
0.06
0.06
0.25
0.17
0.14
0.05
0.11
0.09
0.03
0.04
0.14
0.08
0.27
0.17
0.05
0.04
7.0
11.1
11.1
5.6
32.2
20.2
10.2
9.4
8.8
8.3
3.4
6.4
18.6
16.2
19.4
21.6
2.2
2.2
6.4
6.7
I = stimulus current; D = stimulus duration; SL = initial sarcomere length; RF = resting force; PFL = peak total
force (resting + developed) during lightly loaded contraction; PFA = peak total force during auxotonic contraction..
SARCOMERE VELOCITY IN SINGLE HEART CELLS/Tarr et al.
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
Characteristics of the Lightly Loaded
Contraction
Typical examples of the time course of sarcomere
shortening and the forces developed (change in
force from resting force) during lightly loaded
twitch contractions in one cell (Experiment no. 4 61-79, Table 1) are shown in Figure 2. Contractions
elicited by constant current stimulation (0.8 /iA) at
two different stimulus durations (10 and 100 msec)
are shown. This figure demonstrates the following
consistent findings concerning such lightly loaded
contractions. First, there is a considerable portion
of the contraction during which the sarcomeres
shorten at constant velocity (within the measurement error of ±0.05 fim). Second, the velocity of
sarcomere shortening during the constant velocity
phase depends markedly on the stimulus parameters used to excite the cell. In this example, the
constant velocity for the 10-msec stimulus duration
was 3.1 fim/sec compared to a velocity 9.7 jitm/sec
for the 100-msec stimulus duration. Third, the constant velocity phase of shortening is followed by a
phase of shortening in which sarcomere velocity
decreases markedly. As will be demonstrated in
other examples, the sarcomere length at which this
decrease in velocity occurs also depends on the
stimulus parameters used to excite the cell. Fourth,
the force developed by the cell during the sarcomere
shortening is not constant. The initial onset of
sarcomere shortening is associated with a rapid rise
2.6
o 10 msec
• 100 msec
122
18
500
250
UJ 4
O
a.
o
0
250
500
TIME (msec)
2 Sarcomere length vs. time (upper) and developed force vs. time (lower) during lightly loaded
twitch contractions elicited by constant current (0.8 fiA)
stimulation at two different stimulus durations (10 msec
and 100 msec). Zero time is the onset of the stimulus.
FIGURE
193
in developed force and the constant velocity phase
generally is associated with a decline in or only a
slight change in developed force. In the example
shown in Figure 2, the peak change in developed
force is about 4.4 nN (100-msec stimulus) and the
resting force in this cell was 2.2 nN (Table 1). Thus,
the total maximum force developed by this cell
during this contraction was 6.6 nN, assuming the
sarcomeres supported the total external load (resting force + developed force) as the sarcomeres
shortened. (At sarcomere lengths below 2.2 ^m, the
assumption that the sarcomere supports the total
external load is valid, since the resting sarcomere
lengths in these single cells at zero external load
were generally in the range of 2.1-2.4 jiim.) In comparison, the total force developed by this cell auxotonically (PFA) when stimulated with a 100-msec
duration stimulus was 176.4 nN. Thus, the ratio of
PFL:PFA in this cell was 0.04, and it seems reasonable to conclude that the total force supported by
the sarcomeres under the so-called lightly loaded
contraction was indeed small in relation to the
force-generating capabilities of the sarcomeres.
The Effects of Stimulus Parameters
The data presented in Figure 2 demonstrate that
the velocity of sarcomere shortening can depend on
the stimulus parameters. A detailed investigation of
the effects of the magnitude of stimulus current and
stimulus duration on the time course of sarcomere
shortening under lightly loaded conditions was done
on five different cells. Figure 3 gives a typical example of the effects of sequentially applied stimulus
currents of increasing magnitude (stimulus duration
of 200 msec) on the sarcomere kinetics and force
development during lightly loaded twitch contractions. In this example, several effects of stimulus
current are apparent. First, the delay between stimulation and the onset of shortening decreased as
the magnitude of the stimulus current increased.
Second, the velocity of sarcomere shortening during
the constant velocity phase of shortening increased
with an increase in stimulus current. Third, the
sarcomere length at which a subsequent slowing of
sarcomere velocity occurs (arrows) depended on the
magnitude of the stimulus current. Fourth, the total
extent of sarcomere shortening depended on the
magnitude of stimulus current. The effects of stimulus strength on sarcomere shortening also depended on stimulus duration. At short stimulus
durations (10-20 msec), the velocity and extent of
sarcomere shortening were not affected by the stimulus current strength. Similarly, at low current
strengths, the velocity and extent of sarcomere
shortening were not dependent on stimulus duration.
The effects of stimulus current strength and stimulus duration on sarcomere velocity during the constant velocity phase of shortening are summarized
in Figure 4. With a 10-msec stimulus duration, the
CIRCULATION RESEARCH
194
2.8r
pomps
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
500
VOL. 48, No. 2, FEBRUARY 1981
made on only 10 sarcomeres were representative of
the properties of a significant number of sarcomeres
within the cell, the properties of an increased number of sarcomeres were investigated in a few cells
where a prominent landmark on the cell surface
allowed an accurate measurement of a cell segment
length between the landmark and the point of attachment of the cell to the force beam (see Methods). The average sarcomere length at any time
during the contraction was computed from the ratio
of the segment length to the number of sarcomeres
in the segment. A typical example obtained from
one cell is shown in Figure 5. In this case the cell
segment length between the force beam and the
landmark on the cell surface contained 31 sarcomeres with an average sarcomere length of 2.59 /im.
The results of sarcomere performance computed
from the cell segment measurements were similar
to those obtained on only 10 sarcomeres. In the
example shown in Figure 5, sarcomere shortening
had a constant velocity phase, and at large stimulus
strengths significant slowing of sarcomere velocity
occurred only at sarcomere lengths below about 1.6
jum.
250
TIME (msec)
500
FIGURE 3 Sarcomere length and developed force vs.
time during lightly loaded twitch contractions elicited
by three different, stimulus current strengths at a constant stimulus duration of 200 msec. Resting force = 9
/IN.
velocity of sarcomere shortening for any given cell
was not dependent on stimulus strength, provided
the stimulus current was above threshold strength:
however, a great deal of cell-to-cell variability occurred in the sarcomere velocity ranging from a low
of about 1 fim/sec to a high of about 5.5 jum/sec.
With a stimulus duration of 100 msec, the velocity
of shortening was more dependent on stimulus
strength than with a 10-msec stimulus. For example,
in one cell, the velocity of sarcomere shortening
increased almost 5-fold (2.0-9.8 /im/sec). In contrast, in another cell the velocity increased about
two-fold from 1.4 to 3.2 fim/sec. With a 200-msec
stimulus duration, the velocity of shortening became very dependent on stimulus strength, and all
cells showed an increased velocity of shortening
with an increase in stimulus strength.
Sarcomere Kinetics from Segment Length
Measurements
To confirm that the observations with regard to
the velocity and extent of sarcomere shortening
The Slow Velocity Phase of Sarcomere
Shortening
In Figure 3 it is apparent that a phase of decreased sarcomere velocity follows the constant velocity phase of sarcomere shortening. The sarcomere length at which the slow velocity phase begins
also depends on stimulus strength. For example, in
Figure 3 at a current strength of 0.2 pA, the slow
velocity phase of sarcomere shortening begins at a
sarcomere length of 1.97 /j.m and at a total force of
about 14 nN (resting force = 9 nN). In contrast, at
a current strength of 0.9 /xA, the slow velocity phase
begins at a sarcomere length of 1.54, also at a total
10 msec
100 msec
200 msec
10
E
Q
9
._
6L
8 2
CURRENT(pamps)
4 Summary of the effects of stimulus current
strength and stimulus duration on the velocity of sarcomere shortening. Data from five different cells are
presented.
FIGURE
SARCOMERE VELOCITY IN SINGLE HEART
2.8r
imps
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
500
CELLS/Tarretal.
195
pette, since we found that the level of contractile
activation elicited by long duration stimulus (100200 msec) could be altered by changing the length
of cell in the stimulus pipette.
Figure 6 demonstrates the sarcomere performance for three different stimulus strengths when the
initial sarcomere length was 3.04 /tm (resting force
= 45 nN). The data presented in this figure were
obtained from the same cell, as were the data presented in Figure 3; the data in Figure 6 and 3 are to
be compared. In Figure 6, it is apparent the constant
velocity phase of sarcomere shortening occurs over
a large range of sarcomere length. For example, for
the case of a stimulus current of 0.9 JUA, the constant
velocity phase spans a sarcomere length range of
about 2.9 /mi down to about 1.8 ftm. It is also
apparent by comparing Figure 6 to Figure 3 that
the onset of the slow velocity phase occurs at an
increased sarcomere length and at an increased
total load when the contraction begins from an
initial sarcomere length of 3.04 fim. For example,
for a stimulus of 0.9 juA the slow velocity phase
3.2
r
pomps
250
TIME (msec)
500
FIGURE 5 Sarcomere length and developed force vs.
time as computed from cell segment length determinations. Data obtained from three contractions elicited by
stimulation with a 200-msec duration stimulus at three
different stimulus strengths. Resting force = 38 NN.
force of about 14 nN. The dependence of the onset
of the slow velocity phase on stimulus parameters
suggests that the onset of the slow velocity phase
depends more on the time course of contractile
activation than on sarcomere length per se. In other
words, the slow velocity phase begins when the
level of contractile activation decreases to the point
at which the total force supported by the sarcomeres is no longer small with respect to the forcegenerating capability of the sarcomeres. To test this
hypothesis, the sarcomere performance was compared for given stimulus strengths and durations
when the contractions were initiated from different
initial sarcomere lengths and thus had different
external loads. (An increase in sarcomere length
increased the preload on the cell, thereby increasing
the total force supported by the sarcomeres at
sarcomere lengths less than about 2.2 /um.) The
sarcomere length was increased by decreasing the
pressure in the suction pipette and thereby stretching the cell. In performing these experiments, it was
essential to keep the length of cell in the stimulus
pipette constant by repositioning the suction pi-
12
250
500
for
3CE
O
U
10
O
2 so
500
TIME (msec)
FIGURE 6 Sarcomere length and developed force vs.
time during lightly loaded contractions beginning from
a long initial sarcomere length. Stimulus duration = 200
msec. Resting force = 45 AIN.
VOL. 48, No. 2, FEBRUARY 1981
CIRCULATION RESEARCH
196
begins at a sarcomere length of 1.8 /*m (total force
= 60 nN) for the contraction beginning from the
3.04-jum length compared to 1.5 jum (total force =
14 nN) for the contraction beginning from the 2.61/xm length. For a stimulus of 0.2 /xA, the slow velocity phase begins at a length of 2.12 /im (total force
= 70 nN) for the contraction initiated from the 3.04jum length compared to 1.97 /im (total force = 14
nN) for the contraction initiated from the 2.61-jum
length. These results would be consistent with the
hypothesis that the primary determinant of the
onset of the slow velocity phase of sarcomere
shortening at sarcomere lengths greater than about
1.6 jum is the level of contractile activation relative
to the total force supported by the sarcomeres,
rather than sarcomere length per se.
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
The Threshold Stimulus Strength-Duration
Relationship
In light of the finding that the velocity of sarcomere shortening depended on stimulus strength and
duration, we were curious as to whether or not the
isolated single cardiac cell had a threshold strengthduration relationship similar to what would be expected for an excitable cardiac tissue having an
intact membrane. In other words, does the stimulus
excite an action potential in these cells? To answer
this question, the relationship between the magnitude of stimulus current and stimulus duration
which produced a contractile response having a
sharp threshold was determined on 10 single cardiac
cells. There was very little variability in the
strength-duration relationships of these single cells,
and the results obtained from five of the cells are
shown in Figure 7. The figure plots the ratio of
6 -
I/I
Rh
|508
LiJ
Of 0 6
|
04
0
20
40
60
80
100
STIMULUS DURATION (msec)
FIGURE 8 Effect of tetrodotoxin (TTX) on the threshold
strength-duration relationship of one cell. Stimulus current is plotted as a function of stimulus duration. TTX
was added to the bathing fluid to produce a final TTX
concentration as indicated.
stimulus current (I) to rheohase current (IRh) as a
function of the stimulus duration. The solid line
describes the Lapicque-Hill equation I/Iith = [1 —
exp (—t/r)]"1 having a time constant (T) of 15 msec.
The Lapicque-Hill equation (Lapicque, 1907; Hill,
1936) has been found to describe reasonably the
strength-duration relationship of a number of excitable tissues.
To determine whether the excitatory process
elicited by the stimulus depended on the fast inward
sodium current responsible for phase 0 of the cardiac action potential, the effect of tetrodotoxin on
the stimulus strength-duration relationship was investigated. Figure 8 gives typical results obtained
from one cell. Tetrodotoxin caused a rapid shift in
the strength-duration relationship such that it was
no longer possible to excite the cell with short
duration stimuli (10 msec or less). Also, the threshold stimulus current was increased at long duration
stimuli by tetrodotoxin.
Discussion
-16
30
60
STIMULUS DURATION (msec)
7 Ratio of threshold stimulus current (I) to
rheobase current (Im,) as a function of stimulus duration.
Data from five representative cells are presented. The
solid line describes the Lapicque-Hill equation (see text).
FIGURE
O Control
A TTX(80nM)
o TTX(l60nM)
At present, much of our knowledge concerning
the effect of sarcomere length on the velocity and
extent of sarcomere shortening at light or near zero
external loads has been derived from studies on
intact tissue in which the velocity of tissue shortening has been related to tissue length. For example,
Brutsaert et al. (1971) found in cat papillary muscle
that the velocity of muscle shortening either at
constant external preload or under so-called zero
load clamp conditions decreased markedly at tissue
lengths less than 88% of Lmax (Lma, is the tissue
length at maximum isometric tension). Similar results have also been reported for frog cardiac muscle
(Henderson and Brutsaert, 1974). These studies
along with studies in tetanized cat papillary muscle
(Ford and Forman, 1974), in which the tissue length
SARCOMERE VELOCITY IN SINGLE HEART CELLS/Tarr etal.
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
dependence of the force-velocity relationship of cardiac muscle was determined under conditions of
relatively constant activation, suggest that the unloaded sarcomere velocity (i.e., Vmax)in cardiac muscle is highly length dependent at sarcomere lengths
below about 1.9 jum. The interpretation of these
results with regard to the sarcomere velocity-sarcomere length relationship is uncertain, however,
since the inference of sarcomere length from overall
tissue length is uncertain. For example, Winegrad
(1974) has shown that connective tissue within the
intact tissue renders the relationship between overall muscle length and sarcomere length unpredictable, and many investigators have found significant
disproportionate changes in overall tissue length
relative to changes in sarcomere length within the
tissue (Gay and Johnson, 1967; Nassar et al., 1974;
Pollack and Hunstman, 1974; Kruegar and Pollack,
1975; Julian and Sollins, 1975). Furthermore, in the
intact tissue, significant nonuniformities of sarcomere length occur, and it is therefore difficult to
define the dependence of various parameters of
contractile performance on sarcomere length from
tissue length measurements alone.
We know of only two studies in cardiac muscle in
which the time course of sarcomere shortening at
light or near zero external loads has been measured
directly, and these studies have been limited to rat
ventricular tissue. Pollack and Krueger (1976) determined the time course of sarcomere shortening
in thin bundles of rat papillary muscle by using the
laser diffraction technique to measure sarcomere
length. They found that the velocity of sarcomere
shortening under very lightly loaded conditions decreased markedly at sarcomere lengths below 2.0
(im. For example, the velocity at a sarcomere length
of 1.8 jiim was only about 30% of that at a sarcomere
length of 2.2 jum. Recently, Krueger and Wittenberg
(1978) used the laser diffraction technique to measure sarcomere kinetics during twitch contractions
in isolated single rat ventricular cells. In a preliminary report they stated that in unattached cells
(i.e., external load of zero) the resting sarcomere
length ranged from 1.9 to 2.0 jum and that during
the twitch contraction the initial shortening velocity was nearly the maximal velocity. However, the
relationship between sarcomere velocity and sarcomere length was not given. Thus, the data presently available, in which direct measurments of
sarcomere kinetics during lightly loaded contractions have been made, appear to support the conclusions inferred from tissue velocity-tissue length
measurements that the unloaded sarcomere velocity is highly length dependent at sarcomere lengths
below 1.9-2.0 jum.
Our results on sarcomere shortening in single
frog atrial cells during lightly loaded contractions
show many similarities to those obtained from cardiac preparations where the external load was
clamped very close to zero. First, the velocities of
197
shortening are very similar. Henderson and Brutsaert (1974) found the peak velocity in zero load
clamped frog ventricular tissue to be in the range of
1-2 initial muscle lengths/sec. If we assume the
initial sarcomere length in the intact tissue was 2.3
jam, then an unloaded velocity of shortening of 1-2
initial muscle lengths/sec would give a sarcomere
velocity of 2.3-4.6 /nm/sec. These values are in the
range of sarcomere velocities which we obtained on
the single cell with short duration stimulation (Fig
4). The sarcomere velocities of 10-12 /im/sec reported by Pollack and Krueger (1976) and Krueger
and Wittenberg (1978) in rat ventricular tissue are
similar to the sarcomere velocities of 6-12 jum/sec
that we obtained when the single cell was stimulated with long duration stimuli of large current
strength. Second, similar to other cardiac preparations, significant sarcomere slowing occurred at a
sarcomere length of 1.8-2.0 /xm when the single cell
was stimulated with a threshold stimulus. Thus,
our results concerning sarcomere performance in
the intact single cardiac cell are in some respects
similar to those deduced from the performance of
intact cardiac tissue during lightly loaded contractions. However, there is one difference. The data
presented in this paper demonstrate for the first
time that, under some conditions, the cardiac sarcomere can shorten at constant and high velocity
down to a sarcomere length of about 1.6 ^m before
any significant slowing occurs.
The finding of a constant velocity of sarcomere
shortening under conditions in which the external
forces were relatively small tempts one to conclude
that Vmax is independent of sarcomere length and
time over the sarcomere length range in which the
constant velocity occurred. However, this conclusion might be erroneous. It is possible that timeand length-dependent variations in Vmax, isometric
force (Po), and internal loads could interact in such
a manner as to produce a constant velocity of
shortening over a significant range of sarcomere
lengths. The constant velocity phase cannot be
explained entirely by the changes in external load
that occurred during the lightly loaded contractions, since constant velocities of shortening were
obtained under conditions of nearly constant external loads (Fig. 2) and under conditions of increasing
load (Figs. 3 and 6), as well as under conditions of
decreasing load (Fig. 5). Unfortunately, it is beyond
the state of the art to obtain the information required to give a definitive answer as to how the
length-independence of the lightly loaded sarcomere velocity actually occurred, since this would
require information as to the length and time dependency of P o and internal loads. We must emphasize that the finding of a constant sarcomere
velocity over a significant range of sarcomere
lengths would be compatible with either of two
hypotheses: (1) a time- and length-independent
Vmax; and (2) a time- and length-dependent Vmax.
198
CIRCULATION RESEARCH
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
The effect of stimulus parameters on the velocity
and extent of sarcomere shortening that we found
in the single cardiac cell are similar to what Costantin and Taylor (1973) found in voltage-clamped
segments of skeletal muscle fibers in which the level
of contractile activation was controlled by the membrane potential imposed on the segment. These
investigators found that both the extent of sarcomere shortening and the velocity of sarcomere
shortening at a given sarcomere length could be
graded by varying the magnitude of the depolarizing
step applied to the cell membrane. For example,
when a 100-msec depolarizing step of 32 mV was
applied to the membrane, sarcomere shortening
occurred from an initial length of 2.3 jum down to
about 1.9 /urn, and a considerable slowing of sarcomere velocity began at a sarcomere length of about
2.1 jtim. In contrast, when the membrane was depolarized by 55 mV for 100 msec, sarcomere
shortening occurred with constant and high velocity
from an initial length of 2.1 jum down to about 1.6
(im. Shortening then continued with a markedly
decreased velocity on down to a sarcomere length
of about 1.3 /im. These results are similar to those
we obtained when the single cardiac cell was stimulated with 100- to 200-msec current pulses of various amplitudes (Fig. 3).
The finding that the extent of sarcomere shortening during lightly loaded contractions depends on
stimulus parameters indicates that the level of contractile activation in the single cardiac cell can be
altered by the stimulus parameters used to excite
the cell. For example, in Figure 3, the sarcomeres
shortened to about 1.8 /urn when the cell was stimulated with 200-msec current pulse of 0.2 JXA compared to a shortening to 1.4 jum when the cell was
stimulated with a 200-msec pulse of 0.9 juA. The
forces developed by the sarcomeres were similar in
both cases, and, therefore, the data demonstrate
that the sarcomere was able to develop the same
force at a shorter sarcomere length for the 0.9 /xA
case than for the 0.2 juA case. Similarly, the peak
forces developed during auxotonic twitch contractions depended on the stimulus parameters used to
excite the cell (Table 1). Thus, the ability of the
sarcomeres in the single cardiac cell to develop
force (i.e., P,,) can be altered by the stimulus parameters used to excite the cell.
The dependence of the velocity of sarcomere
shortening during lightly loaded contractions on
stimulus parameters strongly suggests, although it
does not prove, that Vmax in the single intact cardiac
cell depends on the level of contractile activation
(i.e., the intracellular calcium level). A dependence
of Vmax on contractile activation is to be expected
in light of the following observations. It has been
demonstrated that Vmax of glycerinated skeletal
muscle fibers depends on the level of intracellar
calcium (Julian, 1971; Wise et al., 1971). Also, Fabiato and Fabiato (1978) recently have computed
Vmax from tension and sarcomere oscillations in
VOL. 48, No. 2, FEBRUARY 1981
partially activated single-skinned dog cardiac cells
and found that Vmax was highly sensitive to pCa. It
seems reasonable to conclude that the level of intracellular calcium during the twitch contraction in
the single intact cardiac cell can be altered by the
stimulus parameters used to excite the cell. The
stimulus-dependent changes in intracellular calcium produce changes in Vmax and P,, which affect
both the velocity and extent of sarcomere shortening.
Exactly how the stimulus parameters alter the
intracellular calcium concentration is not clear at
present. In this regard it is important to note that
the data indicate that the single cell has an intact
membrane capable of producing an action potential
when electrically stimulated. This conclusion is supported by the following observations. The contractile response behaved in an all-or-none manner to
a threshold stimulus, in that there appeared to be
a very sharp stimulus threshold for contractile activation. The velocity and extent of sarcomere
shortening were not dependent on the stimulus
duration, provided a threshold current strength was
used to excite the cell. Also, the velocity and extent
of sarcomere shortening were not dependent on the
stimulus current magnitude, provided the stimulus
was of short duration (<50 msec). Identical contractile responses would be expected by stimulating
either with suprathreshold currents of short duration or with threshold currents of long duration if
the cell produced an all-or-none action potential in
response to these stimuli, since the magnitude and
time course of contractile activation would be the
same in both cases.
The conclusion that these single cardiac cells
produce all-or-none action potentials is supported
further by the nature of the threshold stimulus
strength-stimulus duration relationship obtained
from these single cells. First, the strength-duration
relationship of these isolated cells was found to be
reasonably described by the Lapicque-Hill equation, an equation that adequately describes the
strength-duration relationship of a variety of excitable tissues under a variety of stimulus conditions.
This relationship given by the equation I/LRH = [1
- exp (—t/r)]"1 describes the strength-duration
curve expected from a membrane having an equivalent circuit composed of a resistance (R) and capacitance (C) in parallel which produces an action
potential whenever the membrane is brought to a
fixed voltage threshold. In this equation, T = RC
and T equals the membrane time constant when the
membrane is polarized uniformly (Noble and Stern,
1966; Fozzard and Schoenberg, 1972). In our experiments it is certain that the cell was not polarized
uniformly, since the stimulus current entered the
cell in the region of the cell in the stimulus pipette
and exited the cell in the region between the stimulus pipette and the force beam. Nevertheless, the
value of T obtained from the strength-duration relationship for the single cell was on the order of 15
SARCOMERE VELOCITY IN SINGLE HEART CELLS/Ta/r et al.
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
msec; a value equal to the membrane time constant
of intact frog atrial tissue (Tarr and Sperelakis,
1964; Tarr and Trank, 1971). Second, the strengthduration relationship that we obtained on the single
frog atrial cell is remarkably similar to the strengthduration relationship reported by Goto et al. (1971)
for bundles of intact frog ventricular tissue. These
investigators found that the strength-duration relationship of the fast inward sodium current had a
chronaxie of 9.8 msec, a value remarkably close to
a chronaxie of about 10 msec which would be estimated from the strength-duration relationship of
the single cell (Fig. 7). Third, tetrodotoxin in a
concentration known to block the fast sodium current in intact frog atrial tissue (Tarr, 1971) produced
a shift in the strength-duration relationship of the
single cell (Fig. 8), as would be expected if the
excitatory response of the single cardiac cell depended on the fast inward sodium current. These
observations suggest that the single frog atrial cells
used in the present investigation had intact and
functional excitable membranes which produced an
action potential dependent on the fast inward sodium current when the membrane was depolarized
to the threshold potential.
Although the data strongly suggest that the single cell produced an action potential in response to
the stimulus current, we cannot give a definitive
answer as to how suprathreshold long-duration
stimuli altered the level of intracellular calcium
during the twitch contraction, since the relationships between applied current, action potential configuration, and excitation-contraction coupling are
unknown. One possibility is that the stimulus-dependent contractile responses were mediated via
the effects which the applied depolarizing current
had on the plateau phase of the action potential. It
is well established that depolarizing currents applied during the plateau of the cardiac action potential enhance contractile activation (Goto and McC.
Brooks, 1970). Another possibility is that the stimulus current not only excited an action potential
but also directly influenced the release of calcium
from storage sites either on the cell membrane or
from the sarcoplasmic reticulum within the cell.
There also have been several reports that high
current stimuli and long duration stimuli can elicit
catecholamine release in intact tissue, thereby producing augmentation of the contractile response
(Whalen, 1958; Furchgott et al., 1959; Brady et al.,
1960). However, these catecholamine-induced inotropies developed after a considerable delay and
increased with continued stimulation. In the single
cell, the change in contractile activation with large
stimulus currents was immediate; a change in stimulus current altered the contractile response on the
first stimulus. Furthermore, preliminary experiments have demonstrated that graded contractile
responses to long duration stimuli still exist in the
presence of DL-propranolol (10~ to 10~ g/ml).
Thus, the stimulus-dependent effects seen in the
199
single cell would not appear to be catecholamineinduced inotropies.
The data presented in this paper demonstrate
several important findings regarding the properties
of the isolated single cardiac cell. First, the isolated
cell has a stimulus strength-stimulus duration relationship of the type expected for a cardiac cell
having an intact excitable membrane capable of
generating an action potential. Second, under
lightly loaded conditions the sarcomeres shorten at
constant and high velocity over a large range of
sarcomere length, and this high velocity of shortening can occur down to sarcomere lengths as short
as 1.6 jum. Third, the level of contractile activation
of the single cell can be altered by the stimulus
parameters used to excite the cell; the level of
contractile activation affects both the velocity and
extent of sarcomere shortening during lightly
loaded contractions.
References
Brady AJ, Abbot BC, Mommaerts WFH (1960) Inotropic effects
of trains of impulses applied during the contraction of cardiacmuscle. J Gen Physiol 44: 415-432
Brady AJ, Tan ST, Ricchiuti NV (1979) Contractile force measured in unskinned isolated adult rat heart fibers. Nature 282:
728-729
Brutsaert DL, Claes VA, Sonnenblick EH (1971) Velocity of
shortening of unloaded heart muscle and the length-tension
relation. Circ Res 29: 63-75
Costantin LL, Taylor SR (1973) Graded activation in frog muscle
fibers. J Gen Physiol 61: 424-443
Fabiato A, Fabiato F (1976) Dependence of calcium release,
tension generation and restoring forces on sarcomere length
in skinned cardiac cells. Eur J Cardiol 4 (suppl): 13-27
Fabiato A, Fabiato F (1978) Myofilament-generated tension
oscillations during partial calcium activation and activation
dependence of the sarcomere length-tension relation of
skinned cardiac cells. J Gen Physiol 72: 667-699
Ford LE, Forman R (1974) Tetanized cardiac muscle. Ciba
Found Symp (new series) 24: 137-150
Fozzard HA, Shoenberg M (1972) Strength-duration curves in
cardiac Purkinje fibres: Effects of liminal length and charge
distribution. J Physiol (Lond) 226: 593-618
Furchgott RF, de Gubareff T, Grossman A (1959) Release of
autonomic mediators in cardiac tissue by suprathreshold stimulation. Science 129: 328-329
Gay WA, Johnson EA (1967) An anatomical evaluation of the
myocardial length-tension diagram. Circ Res 21: 33-43
Gordon AM, Huxley AF, Julian FJ (1966) The variation in
isometric tension with sarcomere length in vertebrate muscle
fibres. J Physiol (Lond) 184: 170-192
Goto M, McC. Brooks C (1970) Positive and negative inotropic
action of polarizing current on the frog ventricle. Am J Physiol
218: 1038-1045
Goto M, Kimoto Y, Kato Y (1971) A study on the excitationcontraction coupling of the bullfrog ventricle with voltage
clamp technique. Jap J Physiol 21: 159-173
Henderson AH, Brutsaert DL (1974) Force-velocity-length relationship in heart muscle: Lack of time-independence during
twitch contractions of frog ventricle strips with caffeine. Pfluegers Arch 348: 59-64
Hill AV (1936) Excitation and accommodation in nerve. Proc R
Soc Lond [Biol] 119: 305-355
Julian FJ (1971) The effect of calcium on the force-velocity
relation of briefly glycerinated frog muscle fibres. J Physiol
(Lond) 218: 117-145
Julian FJ, Sollins MR (1975) Sarcomere length-tension relations
in living rat papillary muscle. Circ Res 37: 299-308
Krueger JW, Pollack GH (1975) Myocardial sarcomere dynamics
200
CIRCULATION RESEARCH
during isometric contraction. J Physiol (Lond) 251: 627-643
Krueger JW, Wittenberg BA (1978) Dynamics of myofilament
sliding in single intact cardiac muscle cells (abstr). Circulation
58: (suppl II): 33
Lapicque L (1907) Recherches quantitative sur l'excitation electriquer des nerfs traitee comme une polarisation. J Physiol
(Paris) 9: 620-635
Manring A, Nassar R, Johnson EA (1977) Light diffraction of
cardiac muscle: An analysis of sarcomere shortening and muscle tension. J Mol Cell Cardiol 9: 441-459
Nassar R, Manring A, -Johnson EA (1974) Light diffraction of
cardiac muscle: Sarcomere motion during contraction. Ciba
Found Symp (new series) 24: 57-82
Noble D, Stern RB (1966) The threshold conditions for initiation
of action potentials by excitable cells. J Physiol (Lond) 187:
129-162
Pollack GH (1970) Maximum velocity as an index of contractility. Circ Res 26: 111-127
Pollack GH, Hunstman LL (1974) Sarcomere length-active force
relations in living mammalian cardiac muscle. Am J Physiol
227: 383-389
Pollack GH, Krueger JW (1976) Sarcomere dynamics in intact
VOL. 48, No. 2, FEBRUARY
1981
cardiac muscle. Eur J Cardiol 4 (suppl): 53-65
Sonnenblick EH (1962) Force velocity relations in mammalian
heart muscle. Am J Physiol 202: 931-939
Tarr M (1971) Two inward currents in frog atrial muscle. J Gen
Physiol 58: 523-543
Tarr M, Sperelakis N (1964) Weak electrotonic interaction between contiguous cardiac cells. Am J Physiol 207: 691-700
Tarr M, Trank J (1971) Equivalent circuit of frog atrial tissue as
determined by voltage clamp-unclamp experiments. J Gen
Physiol 58: 511-522
Tarr M, Trank JW (1976) Preparation of isolated single cardiac
cells from adult frog atrial tissue. Experientia 32: 338-339
Tarr M, Trank JW, Leiffer P, Shepherd N (1979) Sarcomere
length-resting tension relation in single frog atrial cardiac cells.
Circ Res 45: 554-559
Whalen WJ (1958) Apparent exception to the "all or none" law
in cardiac muscle. Science 127: 468-469
Winegrad S (1974) Resting sarcomere length-tension relation in
living frog heart. J Gen Physiol 64: 343-355
Wise RM, Rondinone JF, Briggs FN (1971) Effect of calcium on
force-velocity characteristics of glvcerinated skeletal muscle.
Am J Physiol 221: 973-979
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
Evidence That the Velocity of Sarcomere
Shortening in Single Frog Atrial Cardiac
Cells is Load Dependent
MERRILL TARR, JOHN W. TRANK, PAUL LEIFFER, AND NEAL SHEPHERD
SUMMARY Recent experiments using laser diffraction techniques to determine the time course and
extent of sarcomere shortening in thin bundles of cardiac tissue have given results which suggest that
the velocity of sarcomere shortening in cardiac muscle is independent of the developed force (Nassar
et a]., 1974; Krueger and Pollack, 1975). However, the anatomical complexity of the intact tissue
precludes a definitive interpretation of the data, since the exact relationship between the force being
borne by the total tissue to the force being borne by any observed group of sarcomeres is uncertain.
The single frog atrial cell provides a simple cardiac preparation in which the relationship between
sarcomere velocity and sarcomere force is well defined, since these cells are only 1-2 myofibrils wide.
The purpose of the present investigation was to determine if sarcomere velocity in the single frog atrial
cell is dependent on force by measuring the time course of sarcomere shortening in single cells under
conditions in which the cell developed markedly different forces. The results presented in this paper
give direct evidence that the velocity of sarcomere shortening in the single cardiac cell depends on the
force being developed by the sarcomeres. Thus, cardiac sarcomeres have a type of force-velocity
relationship, although the exact nature of this relationship could not be determined in these experiments. Circ Res 48: 200-206, 1981
IN preliminary experiments on single frog atrial
cells we found that the rate of force development
during auxotonic twitch contractions was relatively
constant for a significant portion of the rising phase
From the Department of Physiology, University of Kansas Medical
Center, College of Health Sciences and Hospital, Kansas City, Kansas,
and Department of Physiology, Duke University Medical Center. Durham, North Carolina.
Supported by U.S. Public Health Service Grant HL 18943. Kansas
Heart Association Fellowship to Paul Leiffer, and a North Carolina Heart
Association Grant-in-Aid to Dr. Neal Shepherd.
Address for reprints: Dr. Merrill Tarr, Department of Physiology,
University of Kansas Medical Center, College of Health Sciences and
Hospital, Kansas City, Kansas 66103.
Received January 4, 1980; accepted for publication August 7, 1980.
of force development. Since the rate of force development in these experiments was directly proportional to the average sarcomere shortening velocity
within the cell, the data indicated that the velocity
of sarcomere shortening remained relatively constant even though force was increasing dramatically. Other investigators have found similar results
in intact cardiac tissue (Nassar et al., 1974; Krueger
and Pollack, 1975). However, the interpretation of
the data derived from intact tissue is complicated
by uncertainties with regard to the distribution of
forces within the tissue (see Manring et al., 1977);
i. e., the relationship between the force being borne
Characteristics of sarcomere shortening in single frog atrial cardiac cells during lightly
loaded contractions.
M Tarr, J W Trank and P Leiffer
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
Circ Res. 1981;48:189-200
doi: 10.1161/01.RES.48.2.189
Circulation Research is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1981 American Heart Association, Inc. All rights reserved.
Print ISSN: 0009-7330. Online ISSN: 1524-4571
The online version of this article, along with updated information and services, is located on the
World Wide Web at:
http://circres.ahajournals.org/content/48/2/189
Permissions: Requests for permissions to reproduce figures, tables, or portions of articles originally published in
Circulation Research can be obtained via RightsLink, a service of the Copyright Clearance Center, not the
Editorial Office. Once the online version of the published article for which permission is being requested is
located, click Request Permissions in the middle column of the Web page under Services. Further information
about this process is available in the Permissions and Rights Question and Answer document.
Reprints: Information about reprints can be found online at:
http://www.lww.com/reprints
Subscriptions: Information about subscribing to Circulation Research is online at:
http://circres.ahajournals.org//subscriptions/