Membrane Currents and Tension in Cat Ventricular Muscle
Treated with Cardiac Glycosides
By Terence F. McDonald, Hermann Nawrath, and Wolfgang Trautwein
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ABSTRACT
The effect of cardiac glycosides on membrane currents and tension in cat ventricular
muscle was studied using the single sucrose gap voltage clamp method. Complete
tension-voltage and current-voltage relations were obtained in5 five preparations before
and during treatment with dihydro-ouabain (DHO, 1.7 x 10" M). After 1-2 minutes of
DHO, the developed tension was 15% greater than control, but there was no change in
either the slow inward (calcium) current (I<-a) or the level of the outward current flowing at
the end of a 300-msec depolarization (Iout)- After 6-8 minutes of DHO, there was a 60%
increase in developed tension, a noticeable increase in resting tension, a 20% decrease in
I, a, and a smaller increase in IoUt. It seems possible that the reduction of
ICa was due to a
reduced driving force. In preparations treated with ouabain (5 x 10"7M, 3-5 minutes),
developed tension was 45-150% greater than control with no change in ICa or Iout between
-45 and +15 mv. We conclude that the inotropic action of these cardiac glycosides is not
mediated by an increase in I (a .
• During the last 5 years, a growing body of
evidence has accumulated suggesting that the slow
inward (calcium) current (I la ) is a vital link in the
excitation-contraction coupling of mammalian
ventricular muscle (1-4). This view has been further strengthened by the demonstration of a tight
coupling between the calcium-carrying system and
the tension in cat ventricular muscle (5). Thus,
changes in ICa must definitely be considered as a
mechanism by which drugs exert positive or negative inotropic effects.
The two classes of inotropic agents most widely
studied in cardiac muscle are the catecholamines
and the cardiac glycosides. Epinephrine increases
ICa in Purkinje fibers (6) and frog atrial trabeculae
(7). Similarly, norepinephrine increases I ( a in
mammalian ventricular muscle; moreover, it has
been concluded that this increase in calcium influx
is of primary importance in the positive inotropic
effect of the drug (8). A central question regarding
the mechanisms by which the cardiac glycosides
exert their inotropic effect on the myocardium is
whether they increase I t a . To explain the actions of
cardiac glycosides on the cardiac action potentials
of different species, Katz (9) has proposed an
hypothesis that involves an increase in ICa. Two
From the II. Physiologisches Institut, Universitat des Saarlandes, 6650 Homburg/Saar, West Germany.
This investigation was supported by the Deutsche Forschungsgemeinschaft SFB 38, Membranforschung.
Dr. McDonald is the recipient of s Canadian Heart Foundation Fellowship.
Dr. Nawrath's permanent address is Pharmakologisches
Institut, Universitat Mainz, 6500 Mainz, West Germany.
Received February 10, 1975. Accepted for publication August
28, 1975.
674
abstracts have appeared dealing with changes in
calcium-mediated action potentials during treatment of mammalian papillary muscles with cardiac glycosides (10, 11). Both of these reports noted
decreases in the rate of rise, the overshoot, and the
duration of these action potentials during inotropy.
The characteristics of the calcium-mediated action
potentials (10-13) are thought to be dependent on
the amplitude and the time course of ICa. But it
should not be forgotten that such responses can
only serve as a guide, since changes in outward
current can also significantly affect the shape of
these action potentials. We are only aware of two
published voltage clamp studies on glycosidetreated cardiac muscle.1 In frog ventricular muscle,
Morad and Greenspan (14) have reported that
acetylstrophanthidin (0.5-5 x 10"6M) reduces ICa
and increases outward current. However, their
study is open to criticism, because the currents
recorded in response to depolarizing steps were
inordinately large and the records suggesting a
decrease in ICa and an increase in outward current
were obtained at a time when the muscle was
depolarized and in partial contracture. Vassort (15)
has reported that ouabain increases the mechanical
response in frog atrial muscle with no change in ICa,
but no records were presented in his article.
Pharmacologic studies with the sucrose gap
method of voltage clamping are hindered by
changes that can occur in the core resistance of the
muscle in the gap. The core resistance often
1
The review article by Lee and Klaus (24) attributes an
unpublished study to H. Schobe, a reference which has been
further recited (40). The original reference appears to have been
an error (K. S. Lee and W. Klaus, personal communication).
Circulation Research, Vol. 37, November 1975
CARDIAC GLYCOSIDES. CALCIUM CURRENT. AND TENSION
increases with time, leading to an increase in the
flow of current through the extracellular shunt
resistance (16). Although the current wave form
does not appear to be changed by such increases in
shunt current, the recorded current magnitude will
reflect more the shunt current amplitude than the
true membrane current. For this reason, it is
advantageous to minimize the time between control and experimental voltage clamps. With this
requirement in mind, we studied the two cardiac
glycosides dihydro-ouabain and ouabain, both of
which have a rapid onset of action.
675
-300 msec-
-5mV
hP
-55 mV
OUT
Methods
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Hearts from adult cats of either sex were removed
under ether narcosis and placed in warm oxygenated
Tyrode's solution. Trabeculae or papillary muscles 0.20.7 mm in diameter and 3-5 mm in length were dissected
from the right ventricle and mounted in a bath having
three compartments. The middle compartment (sucrose)
was 1.5-2.3 mm in width and was separated from the
right (test) and left (KC1) compartments by rubber
sheets. Muscles were drawn from the test compartment
to the KC1 compartment through holes of appropriate
diameter burned through the two rubber sheets.
The test compartment was perfused with Tyrode's
solution of the following millimolar composition: NaCl
140.0, KC1 3.0, MgCl2 1.0, CaCl2 1.8, NaHCO3 12.0,
NaH2PO4 0.4, and glucose 5.0. The middle compartment
was perfused with sucrose solution containing 304 HIM
sucrose (enzymatic grade) and 0.01 mM CaCl2 dissolved
in deionized water. The KC1 compartment was perfused
with a solution identical to that of the test compartment
except that the NaCl was replaced with KC1. Tyrode's
solutions were equilibrated with 95% O2-5% CO2 and
sucrose solutions with 100% O2. The temperature of the
solutions was 35°C. A complete description of the voltage
clamp apparatus and the method of tension recording
has been published previously (16).
MEASUREMENT OF PARAMETERS
Figure 1 is a drawing of a typical record. The traces
from top to bottom represent voltage, current, and
tension. A 300-msec depolarization from -55 mv (holding potential) to - 5 mv results in a current wave form
which is thought to reflect the following sequence of
events. A rapid upward capacitive spike is followed by a
fast inward (negative deflection) sodium current. Due to
inactivation at this low holding potential, the sodium
current is only a small fraction of that which could be
elicited from a holding potential of —80 mv. Following
the rapid decay of the sodium current, the activation of
the slow inward current (I, a ) leads to a second negative
current deflection. I ca is gradually inactivated, and this
phenomenon, in combination with a time-dependent
increase in outward current, accounts for the shape of
the wave form following the second inward peak. In this
study, the amplitude of I t a was taken as the difference
between the maximum inward current (second peak) and
the current level at 300 msec. Analysis of the current
records accompanying long depolarizations and meaCirculation Research, Vol. 37, November 1975
FIGURE 1
Drawing of a typical voltage clamp and tension record showing
the parameters measured. The traces from top to bottom are:
(1) the voltage record (V) showing a 50-mv depolarization for
300 msec to -5 mv from the holding potential (hp) of -55 mv;
(2) the current record (I) obtained in response to the voltage
step, showing the fast inward sodium current (not measured)
(INa) and the way in which the slow inward calcium current
(I la ) and the outward current (I<,m) were estimated, and (3)
the tension record (T) in which the twitch amplitude was measured.
surements of conductances suggest that the time constant of ICa inactivation (T{) is about 85 msec at 0 mv; the
time-dependent outward current is activated with a time
constant of about 350 msec (McDonald and Trautwein,
unpublished observation). Therefore, I l a is 97% inactivated after 300 msec when the outward current is about
55% activated. If the current record consisted of an I, a of
amplitude 100 on which was superimposed a timedependent outward current of amplitude 20, then I (a
measured as in Figure 1 would be about 10% too large.
From the foregoing discussion, it can be seen that the
error in measuring ICa at 300 msec depends on the time
constants of the two currents involved and on their
relative amplitudes. Measurements of these parameters
at different potentials (McDonald and Trautwein, unpublished observation) lead us to conclude that ICa,
estimated as shown in Figure 1, is 5-10% too large at
potentials below 0 mv and 10-20% too large at potentials
above 0 mv.
The outward current reported in this paper (Iout) refers
to the difference between the current at the holding
potential and that flowing after 300 msec at the new
potential. As such, this measurement includes an instantaneous, voltage-dependent, time-independent current
as well as a component which increases with time (see
ref. 17 for outward current analysis in Purkinje fibers).
Since the latter component has a rather long activation
time constant, the measurement at 300 msec does not
estimate the maximum outward current at a given
potential. Nevertheless, any large changes in outward
current brought about by treatment with cardiac glycosides is detected.
676
MCDONALD, NAWRATH. TRAUTWEIN
DRUGS
CONTROL
DIHYDRO-OUABAIN
Dihydro-ouabain (DHO) and ouabain2 were dissolved
in distilled water and added to the Tyrode's solution as
required.
Results
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The calcium necessary for contraction may be
released from intracellular stores that can be rapidly filled or depleted. When these stores are overfilled, there may be a reduction in ICa amplitude
due to a reduction in the calcium equilibrium potential (E Ca ). If cardiac glycosides act to fill these
stores, one would expect a reduction in ICa. However, it is important to know whether this effect
occurs pursuant to an earlier increase in the magnitude of lea or whether another mechanism must
be sought. For this reason, we thought that it was
important to compare control voltage clamp records
with those obtained on the first sign of glycosideinduced inotropy as well as with those obtained
following longer glycoside exposure.
TIME-DEPENDENT CHANGES IM MEMBRANE CURRENTS AND
TENSION DURING TREATMENT WITH DHO
Figure 2 shows voltage clamp records obtained
before and 1 minute after treatment with DHO (1.7
x 10"5M). The holding potential was -53 mv and
the membrane was clamped for 300 msec to potentials between -33 and +27 mv at a rate of 12/min.
The drug increased tension by about 15% with
negligible effects on the amplitudes of ICa and loutLonger exposure to DHO produced a greater
inotropic action and also, within 10 minutes, an
increase in the resting tension. Figure 3 compares
voltage clamp records before and after 10 minutes
of treatment with DHO (1.7 x 10" 5 M). The holding potential was -55 mv, and 300-msec depolarizing steps were imposed to potentials between -15
and +25 mv at a rate of 12/min. The current and
tension records have been photographically superimposed with the lower trace in each case being the
control record. In this preparation, there was a
marked increase in resting tension on which was
superimposed a developed tension whose peak amplitude was nearly twice that of control. The current records indicate that ICa was reduced by about
40% and that the amplitude of Iout at 300 msec was
slightly greater than control.
The mean results of five complete experiments
with DHO are presented in Figure 4. The same
protocol was used in each experiment. From a
holding potential of -55 mv, the membrane was
depolarized for 300 msec to potentials between
2
Generous gifts from Boehringer Mannheim GmbH, Mannheim, Germany.
50 msec
FIGURE 2
Records of 300-msec depolarization before and after 1 minute
of treatment with dihydro-ouabain {DHO) (1.7 x 10~SM). Each
record shows, from top to bottom, the uoltage, the current, and
the tension. The potential was displaced from the holding potential of -53 mv to the different levels (Vc) shown between
the control and DHO sections of the figure. Note that DHO
increased tension by about 15% with no detectable changes in
current.
-45 and +25 mv. Following the control clamp run
(solid circles), records were taken 1-2 minutes
(open circles) and 6-8 minutes (open triangles)
after the addition of DHO (1.7 x K)- 5 M). Treatment with DHO for 1-2 minutes increased the
maximum tension by nearly 20%, but no changes
were noted in the amplitude of ICa or I out . After 6-8
minutes with DHO, muscles produced about 60%
more tension than control. Although not shown in
the figure, the increase in resting tension at this
time was 10-20% of the maximum control twitch
tension. Maximum I t a was reduced by about 20%,
and there was perhaps a 10% increase in Iout.
These changes in current-voltage relations can
account for the changes in action potential configuration seen with DHO. Typical records obtained
during a voltage clamp experiment are shown in
Figure 5. After 1 minute of DHO (1.7 x K)- 5 M), the
plateau phase of the action potential was unchanged, and the action potential duration was
increased by a few percent. The resting tension was
Circulation Research, Vol. 37, November 1975
677
CARDIAC GLYCOSIDES. CALCIUM CURRENT. AND TENSION
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It is also important to know whether the drug
changed the time course of ICa, thereby allowing a
smaller or a greater transfer of charge across the
membrane. As discussed in Methods, the slow
decline of current following the second inward peak
consists of both the inactivation of ICa and the
activation of an outward current. Although this
situation makes it impossible to get the correct ICa
inactivation time constant from a 300-msec current
record, a plot of the current at -10 mv provides a
reasonable estimate because ICa at this potential is
large relative to IoUt and its time course is about five
times faster than that of lout (McDonald and
Trautwein, unpublished observation). Semilogarithmic plots of the current in the control and the
3-minute ouabain records are also presented in
Figure 6, and the values are well fitted in both
cases by lines of identical slope (T = 85 msec).
W0mg[
50msec
FIGURE 3
Records of 300-msec depolarizations before and after 10 minutes of treatment with DHO (1.7 x 10~5M). The membrane
potential was clamped from a holding potential of —55 mu to
the potentials denoted in the upper right corner of each record.
The control (c) and DHO (d) traces have been superimposed to
demonstrate that DHO reduced ICa, slightly increased /„„„ and
increased both resting and developed tension.
unchanged, and the developed tension was nearly
25% greater than control. After 11 minutes of DHO,
there were significant changes in action potential
configuration. The resting potential was reduced
by a few millivolts as was the overshoot, and the
duration at 25% repolarization was only 60% that
of control. The resting tension was increased by 50
mg, and developed tension was approximately
twice the control value. These changes are in
agreement with earlier studies on DHO; Reiter (18)
has reported a small initial increase followed by a
decrease in the action potential plateau duration of
guinea pig papillary muscle treated with 10" 5 M
DHO.
EFFECT OF OUABAIN ON MEMBRANE CURRENTS AND TENSION
The other cardiac glycoside investigated in this
study was ouabain. Successful experiments were
obtained in three preparations. In one experiment,
the muscle was clamped from -50 to -10 mv for
300 msec before and during a 3-minute exposure to
ouabain (5 x 10"7M). The pulsing rate was 12/min.
Figure 6 shows clamp records and tension before
and after 3 minutes with ouabain. There was no
increase in resting tension, developed tension increased by 45%, and there were no detectable
changes in ICa or IoUt.
Circulation Research, Vol. 37, November 1975
-2C
-V
0
«
MEMBRANE POTENTIAL ImV)
.20
FIGURE 4
Summary of five voltage clamp experiments with dihydroouabain (DHO) (1.7 x /0~ 5 M). The symbols represent the
mean values of Iout, lCa, and developed tension before (solid
circles) and after 1-2 minutes (open circles) or 6-8 minutes
(triangles) with the drug. The abscissa refers to the membrane
potential during a 300-msec depolarization.
678
MCDONALD, NAWRATH, TRAUTWEIN
CONTROL
DHO 1mm
DHO 11 mm
t O msec
FIGURE S
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Action potentials {top traces) and tension {bottom traces) recorded during a voltage clamp experiment
with dihydro-ouabain (DHO) (1.7 x 10~hu).
Records from three preparations treated with DHO
gave similar results.
Another preparation was clamped to potentials
between —45 and +15 mv for 300 msec every 5
seconds before and after 5 minutes of treatment
with ouabain (5 x 10 7 M). The results are depicted
in Figure 7. Although there was no change in Ica
over the entire voltage range, developed tensions
were more than twice the control values. At this
time, the increase in resting tension was approximately 5 mg and Iout was marginally smaller than
control.
CONTROL
DEPENDENCE OF TENSION ON l C a BEFORE AND AFTER
TREATMENT W I T H CARDIAC GLYCOSIDES
The charge carried by I ( a during a typical
voltage clamp step does not appear to be great
enough to allow a role for I, a in directly activating
the contractile machinery (3). For this and other
reasons, ICa is thought to influence contractile
80 r
OUABAIN
BO msec
i-10
20 mV
5.0
2.5
M.0• - • CONTROL
O—O OUABAIN
SO
t»
BO
200
250
msec
50
WO
150
200
250
msec
FIGURE 6
Records of a 300-msec depolarization from -50 mu to -10 mv before and after 3 minutes of exposure to ouabain (5 x 10~7M).
Note that tension increased after ouabain but that no detectable changes in ICa or /„„, occurred. A semilogarithmic plot of
current decay below each record indicates that the time course
of ICa was unaffected by ouabain. The lines fitting the experimental values have identical slopes (T = 85 msec).
FIGURE 7
Comparison of tension-voltage and ICa-uoltage relations before
and after 5 minutes of exposure to ouabain (5 x 70~'M). Measurements are from 300-msec clamps to potentials between -45
and +15 mv with the holding potential at —55 mv. The increase
in resting tension was less than 5 mg.
Circulation Research, Vol. 37, November 1975
679
CARDIAC GLYCOSIDES. CALCIUM CURRENT. AND TENSION
(Al SINGLE EXPT.
(B) NORMALIZED MEANS ln=7)
1.0
1(15 •
z
us
0.1
Q05
0.3
0.5
10
5.0
005
01
'Co ° 5
1.0
(fraction of maximum)
FIGURE 8
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Relation between developed tension and ICa amplitude. The
data are from 300-msec depolarizing steps to potentials between -35 and +25 mv and are plotted in double-log fashion.
A: Data from a single experiment before and after 6 minutes
of DHO (1.7 x 10~SM). B: Maximum amplitude of lCa was
given a value 1 and the tension at that potential was also assigned a value 1. Other amplitudes during a control run or a
glycoside run were normalized accordingly. Values are means
± SE (N = 7 for control, N = 12 for DHO and ouabain runs).
All lines fitting the data points have a slope of 1. The arrows in
A emphasize values that deviate from a slope of 1, i.e., values
that indicate an increase in tension despite a decrease in ICa at
+25 mv.
strength by triggering the release of activated
calcium from intracellular stores (3-5). The mechanism by which this release occurs is not known,
but what is known is that there is a tight coupling
between the amplitude of I r a and the accompanying twitch strength (5).
This fact suggests an amplification factor or
gain between the magnitude of ICa and the quantity
of calcium released from the intracellular stores.
DHO and ouabain increased contractile force but
not ICa- The foregoing suggests that the glycosides
increase the quantity of calcium available for
release, change the relation between ICa and release
in such a way that smaller increments in ICa result
in proportionally larger releases of activator calcium, or both.
By considering ICa as the stimulus and tension as
the response, dose-response curves can be constructed from the data. Figure 8A shows a doublelog plot of tension versus ICa amplitude before and
after 6 minutes of treatment with DHO. The lines
drawn through the experimental values have slopes
of 1 and the fit suggests a linear relation, which
remains unchanged with DHO treatment. Figure
8B summarizes the results of seven experiments,
five with DHO and two with ouabain. Data were
normalized by assigning a value of 1 to the maximum amplitude of ICa during a voltage clamp run
and a value of 1 to the tension at that potential. All
Circulation Research, Vol. 37, November 1975
other values were then expressed as fractions of
these maximums. There is no significant difference
between control and glycoside data, and the mean
values are well described by a line with a slope of 1.
Although the data appear to indicate a perfectly
linear relation between tension and ICa, it must be
considered that tension continued to increase
slightly despite a decrease in ICa between +15 and
+ 25 mv (arrows on Fig. 8A) and that the amplitude
of ICa was probably overestimated by 5-20% within
the range of - 5 to +25 mv (see Methods).
Despite these limitations, the data indicate that
glycosides produce a simple shift to the left in the
relation between tension and ICa. In the frog heart,
ouabain has been reported to shift the (KC1)
depolarization-tension curve by about 30 mv toward the resting potential (19). We have no information on whether such a shift occurs in the
mammalian ventricle, but it seems certain that the
threshold for activation of ICa (and tension) was not
shifted to any large degree by ouabain (Fig. 7).
Discussion
Short exposures to DHO (1.7 x 1 0 5 M for 1-2
minutes) or ouabain (5 x 10" 7M for 3-5 minutes)
increased the contractile force of cat ventricular
muscle without changing ICa or the level of I out at
the end of a 300-msec depolarizing clamp. Muscles
treated with DHO for 6-10 minutes had an increased resting tension, a reduced ICa, and a slightly
increased IoUt. The reduction in I ( a and the smaller
increase in Iout appear to explain the reduced
plateau amplitude and duration of the action
potential observed after longer exposures to DHO.
There are many possible explanations for the
reduction in ICa after 6-10 minutes of exposure to
DHO, including (1) a shift in the negative direction
of the steady-state inactivation variable ( f j , or a
shift in the positive direction of the steady-state
activation variable (dx) (see refs. 4 and 5 for
descriptions of these calcium conductance variables), (2) a decrease in the maximum calcium
conductance (gLa), and (3) a reduction in the
calcium equilibrium potential (E Ca ). Any one or all
of these mechanisms could play a role. However, in
view of the coincidence between the increase in
resting tension and the decrease in ICa, the third
explanation, a reduction in E Ca , appears to be the
strongest candidate.
The results of this study point up a clear
difference between the mechanism of action of
these cardiac glycosides and that of the catecholamines. In Purkinje fibers (6) and atrial trabeculae
(7), epinephrine increases the amplitude of I t a . In
680
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ventricular muscle, the norepinephrine-induced increase in lea has been shown to be due to an
increase in the maximum calcium conductance
(g( a) (8). These voltage clamp data are consistent
with 45Ca flux studies demonstrating that catecholamines increase the influx of 45Ca in contracting guinea pig atriums (20, 21) and rat atriums
(22).
Muscles treated with cardiac glycosides for
longer periods of time had an increased resting
tension and a reduced ICa amplitude, suggesting an
accumulation of intracellular free calcium. These
facts are in agreement with the finding that toxic
doses (contracture) of cardiac glycosides increase
myocardial calcium content (23, 24). Prior to
changes in resting tension but at a time of positive
inotropy, there were no changes in the amplitude
or the time course of I l a . This situation indicates an
increase in the availability of activator calcium in
keeping with 45Ca flux data showing that cardiac
glycosides increase the exchangeable myocardial
calcium fraction (24, 25). What remains contentious is how this increase in the exchangeable
fraction comes about. Some investigators feel that
an increase in calcium influx is responsible, but
others hold that there is a redistribution of intracellular calcium. Langer and Serena (26) have measured an increase in 46Ca influx and a gain in total
muscle calcium of 10"4 moles/kg wet weight. They
feel that this gain of intracellular calcium is related
to an increase in intracellular sodium due to
inhibition of the sodium pump. It has been calculated (27) that a 5 mM increase in intracellular
sodium will increase muscle calcium by 10~5M
through the Na-Ca exchange mechanism (28).
That the inotropic effect of cardiac glycosides has
been shown by many (see ref. 24) to be unaccompanied by changes in intracellular sodium does not
completely negate this hypothesis, because a
change of 3 mM in total muscle sodium can be
difficult to distinguish from control values. Bailey
and Sures (29) have also reported an increased
calcium influx and a net gain in muscle calcium.
However, their net gain is associated with a compartment different from that which has been postulated to support contraction (30). Somewhat analogous is Nayler's finding (31) that ouabain increases
the amount of superficial Ca2+ available for displacement by La 3+ .
A different view is that the increased fraction of
exchangeable calcium during treatment with cardiac glycosides is brought about by a redistribution
of intracellular calcium, possibly by mobilization
of otherwise tightly bound calcium (24). This mode
of action requires neither an increased calcium
MCDONALD, NAWRATH. TRAUTWEIN
influx nor an increased myocardial calcium content. Results supporting this concept have been
reported in the literature (see ref. 24). In fact, there
are many reports suggesting that therapeutic concentrations of cardiac glycosides significantly reduce the myocardial calcium content (23, 32).
Considering that the total calcium content is in the
millimolar range, that the inotropic effect may
require only about 10"5M calcium, and that 45Ca
flux data are notoriously difficult to interpret, it is
little wonder that a consensus has not yet been
reached.
Finally, it should be noted that inotropically
active doses of ouabain have no effect on the
contracture tension of canine trabecular muscle
having highly permeable (EDTA) cell membranes
(33). Nor does 10"6M strophanthin increase either
cyclic contractions or tonic contraction in skinned
rat and rabbit ventricular cells (34). These results
argue against, but do not exclude in untreated
intact cells, a mechanism of intracellular redistribution of calcium.
The effect of cardiac glycosides on Iout was of
interest in another context. Both DHO and ouabain markedly reduce Iout over the entire voltage
range in sheep Purkinje fibers (35). Together with
further evidence involving changes in temperature
(36) and changes in external sodium concentration
(37), these findings indicate that a significant
fraction of Iout is generated by an electrogenic
sodium pump. Within 1 minute of treating Purkinje fibers with DHO (2 x 10" 5 M), the inhibition
of the electrogenic outward current can shift the
resting potential from -85 to -55 mv (35). Similar
shifts have been seen after 10 minutes with 10"6M
ouabain. Large reductions in I out were not seen in
the present work, and this finding is consistent
with other evidence which suggests that under
normal conditions such a large contribution of an
electrogenic sodium current does not exist in ventricular fibers. The decline in the resting potential
of ventricular muscle treated with moderately high
doses of cardiac glycosides occurs in a slow progressive manner (38-40) which is more consistent with
a gradual loss of internal potassium and a reduction in Ek than it is with the inhibition of an
electrogenic current component. Even 10"4M ouabain only reduces the resting potential of guinea
pig ventricular muscle by 5-10 mv over a 20minute period (41), a decline which can be accounted for by the loss of internal potassium (42).
Acknowledgment
We thank Mr. H. Ehrler for skillful electronics support and
Ms. R. Quint for excellent technical assistance.
Circulation Research, Vol. 37, November 1975
CARDIAC GLYCOSIDES. CALCIUM CURRENT. AND TENSION
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207:211-229, 1970
2. OCHI R, TRAUTWEIN W: Dependence of cardiac contraction
on depolarization and slow inward current. Pfluegers
Arch 323:187-203, 1971
3. NEW W, TRAUTWEIN W: Ionic nature of slow inward current
and its relation to contraction. Pfluegers Arch 334:24-38,
1972
relation between membrane potential and tension in frog
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T F McDonald, H Nawrath and W Trautwein
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Circ Res. 1975;37:674-682
doi: 10.1161/01.RES.37.5.674
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