© 1995 American Heart Association, Inc.
Articles |
Effects of Action Potential Duration on Excitation-Contraction Coupling in Rat Ventricular Myocytes
Action Potential Voltage-Clamp Measurements
From the Departments of Medical Physiology and Medicine, The University of Calgary, Alberta, Canada.
Correspondence to W.R. Giles, 3330 Hospital Dr NW, Calgary, Alberta, Canada T2N 4N1.
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Abstract Although each of the fundamental processes involved in excitation-contraction coupling in mammalian heart has been identified, many quantitative details remain unclear. The initial goal of our experiments was to measure both the transmembrane Ca2+ current, which triggers contraction, and the Ca2+ extrusion due to Na+-Ca2+ exchange in a single ventricular myocyte. An action potential waveform was used as the command for the voltage-clamp circuit, and the membrane potential, membrane current, [Ca2+]i, and contraction (unloaded cell shortening) were monitored simultaneously. Ca2+-dependent membrane current during an action potential consists of two components: (1) Ca2+ influx through L-type Ca2+ channels (ICa-L) during the plateau of the action potential and (2) a slow inward tail current that develops during repolarization negative to
-25 mV and continues during diastole. This slow inward tail
current can be abolished completely by replacement of extracellular
Na+ with Li+, suggesting that it is due
to electrogenic Na+-Ca2+ exchange. In
agreement with this, the net charge movement corresponding to the
inward component of the Ca2+-dependent current
(ICa-L) was approximately twice that during the slow inward
tail current, a finding that is predicted by a scheme in which the
Ca2+ that enters during ICa is extruded
during diastole by a 3 Na+–1 Ca2+
electrogenic exchanger. Action potential duration is known to be a
significant inotropic variable, but the quantitative relation between
changes in Ca2+ current, action potential duration,
and developed tension has not been described in a single myocyte. We
used the action potential voltage-clamp technique on ventricular
myocytes loaded with indo 1 or rhod 2, both Ca2+
indicators, to study the relation between action potential duration,
ICa-L, and cell shortening (inotropic effect). A
rapid change from a "short" to a "long" action potential
command waveform resulted in an immediate decrease in peak
ICa-L and a marked slowing of its decline (inactivation).
Prolongation of the action potential also resulted in slowly developing
increases in the magnitude of Ca2+ transients
(145±2%) and unloaded cell shortening (4.0±0.4 to 7.6±0.4 µm).
The time-dependent nature of these effects suggests that a change in
Ca2+ content (loading) of the sarcoplasmic reticulum
is responsible. Measurement of [Ca2+]i
by use of rhod 2 showed that changes in the rate of rise of the
[Ca2+]i transient (which in rat
ventricle is due to the rate of Ca2+ release from
the sarcoplasmic reticulum) were closely correlated with changes in the
magnitude and the time course of ICa-L. These findings
demonstrate that Ca2+ release from the sarcoplasmic
reticulum can be modulated by the action potential waveform as a result
of changes in ICa-L.
Key Words: action potential duration • action potential clamp • excitation-contraction coupling • Ca2+ current • Na+-Ca2+ exchange • intracellular Ca2+
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Introduction |
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In mammalian myocardium, Ca2+ influx during the action potential (AP) triggers contraction by releasing Ca2+ from the sarcoplasmic reticulum (SR).1 2 3 4 The available data suggest that 80% to 90% of the Ca2+ that activates the myofilaments is from SR release, and the remaining 10% to 20% is derived from Ca2+ influx during the AP.3 4 5 6 7 The decline of [Ca2+]i and relaxation are due to (1) reuptake of Ca2+ into the SR and (2) Ca2+ extrusion by the Na+-Ca2+ exchanger.3 4 8 For [Ca2+]i homeostasis and steady state levels of contraction to be maintained during normal cardiac function, Ca2+ influx during successive cardiac cycles must be balanced by an equivalent Ca2+ efflux.9 10 However, direct experimental evidence describing the ionic mechanisms responsible for this Ca2+ balance is lacking. For example, the amount of Ca2+ that must be extruded during repolarization and diastole remains unknown due to a lack of quantitative data concerning the size and time course of (1) the Ca2+ influx during an AP waveform, (2) the changes in total [Ca2+] during contraction, and (3) the extent to which Ca2+ ions are bound by intracellular buffers. This information has been very difficult to obtain because Ca2+ influx during an AP is a complex function of the time and voltage dependence of L-type Ca2+ channels6 and their dependence on [Ca2+]i. In addition, the electrogenic Na+-Ca2+ exchange process is voltage dependent.9 10
Results obtained from bovine chromaffin cells,11 molluscan neurons,12 rat gonadotrophs,13 and both skeletal14 and smooth muscles15 have demonstrated that significant buffering of [Ca2+]i by endogenous Ca2+-binding ligands occurs during the time course of a single [Ca2+]i transient and that the on and off rates for Ca2+ binding to these ligands contribute to the shape of measured [Ca2+]i transients. Experimental results from intact16 17 18 19 and permeabilized20 21 22 cardiac cells have also suggested that during excitation-contraction coupling, a large fraction of total cytoplasmic Ca2+ is bound very rapidly to high-affinity Ca2+-binding ligands and/or buffers.
We used the AP voltage-clamp technique23 24 to make quantitative measurements of the Ca2+ influx during an AP and to study the interactions between Ca2+ fluxes due to (1) Ca2+ channels and (2) the Na+-Ca2+ exchanger. Our results show that during steady state stimulation with AP waveforms, an essential aspect of [Ca2+]i homeostasis is the balance of Ca2+ influx during the inward component of the Ca2+-dependent current (ICa-L) and Ca2+ efflux due to Na+-Ca2+ exchange.
In their classic paper, Wood et al25 studied the inotropic
effects of changes in AP shape in voltage-clamped ventricular muscle.
They and others26 27 28 29 concluded that the strength of
contraction was governed by the amount of Ca2+
released from intracellular storage sites and that this was positively
correlated with the height and/or duration of preceding APs. Recent
work on intact cardiac myocytes has shown that changes in
stimulation frequency,30 inhibition of repolarizing
K+ currents,31 32 and stimulation of
1-,33 34 ß1-, and
ß2-adrenoceptors35 can produce an increase
in AP duration and contraction in the majority of mammalian species,
including humans.31 36 37
Despite these well-established correlations between AP duration and contractility, the underlying mechanisms have not been studied in single myocytes from mammalian ventricle. Data obtained by use of photolabile Ca2+ channel antagonists38 or microinjections of Ca2+ on skinned cardiac cells39 have shown that altered Ca2+ uptake and release by the SR is importantly involved. In voltage-clamped cardiac myocytes, slowing the rate of repolarization40 or abruptly increasing the duration of depolarizing pulses6 40 41 can cause a time-dependent increase in contraction. This could be due to increased Ca2+ influx through voltage-dependent Ca2+ channels,38 40 41 decreased Ca2+ extrusion by the Na+-Ca2+ exchanger,40 41 42 or a combination of both. We used the AP voltage-clamp method to study the roles of the Ca2+ current and Na+-Ca2+ exchange in the modulation of cardiac contractility by AP duration. Of particular interest was the possibility that the trigger for release of Ca2+ from the SR in intact cardiac cells can be graded by the rate of change of free [Ca2+]i. Our results show that AP prolongation results in a substantial increase in net Ca2+ entry through L-type Ca2+ channels. This augmented Ca2+ influx through L-type Ca2+ channels enhances contractility by increasing SR Ca2+ release, and it is balanced during steady state stimulation by a corresponding increase in Ca2+ extrusion by the Na+-Ca2+ exchanger.
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Materials and Methods |
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Cell Isolation
The procedure used for isolation of adult rat ventricular myocytes has been described in detail previously.40 In brief, male Sprague-Dawley rats (250 to 300 g) were anesthetized with ether and killed by cervical dislocation. The heart was then removed and retrogradely perfused on a horizontal Langendorff apparatus for 4 to 6 minutes at 35°C with control Tyrode's solution containing collagenase (0.014 g/mL; Yakult Co Ltd) and pronase (0.014 g/mL; Sigma Chemical Co). The right ventricle was then separated from the rest of the heart and minced in 10 mL of a low-Ca2+ Tyrode's solution (50 µmol/L free Ca2+) containing 0.2 mg/mL each collagenase and pronase, 10 mg/mL fatty acid–free bovine serum albumin (BSA, Sigma). After the tissue segments were gently agitated for 8 to 10 minutes at 35°C, they were drawn off at 2-minute intervals and placed into 4-mL aliquots of enzyme-free Tyrode's solution containing 10 mg/mL BSA and 100 µmol/L Ca2+. The cells were stored in this enzyme-free solution at 22°C to 23°C until use.
Solutions and Chemicals
The control Tyrode's solution used for cell isolation contained
(mmol/L) NaCl 121, KCl 5, CaCl2 1, sodium acetate 2.8,
MgSO4 1, NaHCO3 24,
Na2PO4 1.1, and glucose 10. This solution was
gassed with 95% O2 and 5% CO2, pH 7.4.
The HEPES-buffered Tyrode's solution used in the electrophysiological
experiments contained (mmol/L): NaCl 140, KCl 5, sodium acetate 2.8,
MgSO4 1, CaCl2 1 or 2 (as indicated), HEPES 10,
glucose 10, CsCl 3, 4-aminopyridine (4-AP) 3, and tetrodotoxin 0.015.
pH was adjusted to 7.4 with NaOH and the solution was gassed with 100%
O2. Nominally Ca2+-free Tyrode's
solution was made by replacing CaCl2 with equimolar
MgCl2. In some experiments CdCl2 (100
µmol/L), ryanodine (1 to 5 µmol/L), thapsigargin (5 µmol/L), or
4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS; 150
µmol/L) was added to the solution. Ryanodine was obtained from
Calbiochem Corp. 4-AP, tetrodotoxin, thapsigargin, and DIDS were
obtained from Sigma Chemical Co. Stock solutions of DIDS, tetrodotoxin,
and ryanodine were made with distilled water. A stock solution of 4-AP
(300 mmol/L) was made with distilled water, and pH was adjusted
to 7.4 using 2N HCl. Thapsigargin was dissolved in 100% dimethyl
sulfoxide (DMSO). The final concentration of DMSO in the Tyrode's
solution was 
0.1%. The fluorescent Ca2+
indicators indo 1 (potassium salt) and rhod 2 (ammonium salt) were
obtained from Molecular Probes.
The pipette filling solution contained (mmol/L): cesium aspartate 120, CsCl 30, HEPES 5, MgCl2 1, and disodium ATP 5. pH was adjusted to 7.1 with CsOH. In some experiments, 100 µmol/L indo 1 or 300 µmol/L rhod 2 was added to the pipette solution.
Electrophysiological Methods
Membrane potential and currents were recorded at 22°C by use
of standard whole-cell voltage-clamp techniques. Pipettes used for
whole-cell voltage clamp had DC resistances of 1.5 to 2 M
, and the
series resistance after electronic compensation averaged 1.0±0.1 M
(mean±SEM; n=25). Membrane potentials were corrected by -10 mV to
compensate for liquid junction potentials between the external and
pipette solutions. Membrane potential and currents were low pass
filtered at 5 kHz and sampled at 1 kHz with a 12-bit analog-to-digital
convertor (DT2801A, Data Translation). Cell shortening and
Ca2+-indicator fluorescence signals (see
below) were filtered at 100 Hz and digitized simultaneously with the
membrane potential and current recordings. The digitized signals were
stored in a microcomputer for later analysis with a custom software
package and a commercial plotting program (SIGMAPLOT,
Jandel Scientific).
The capacitance of ventricular myocytes, measured from the integral of the current transient resulting from 5 mV depolarizing steps, averaged 99±8 pF (mean±SEM, n=10). The cell dimensions (lengthxwidthxdepth) were 110(±3)x21(±2)x7(±1) µm3, giving an estimated single-cell volume of 1.6x10-11 L.
Myocytes were voltage-clamped with either rectangular steps or AP-shaped waveforms. The AP waveforms were recorded in separate experiments from rat ventricular myocytes that were superfused with either control Tyrode's solution or control Tyrode's solution containing 3 mmol/L 4-AP.32 34 APs recorded in control solution and after addition of 4-AP had durations at 0 mV of 6 and 44 milliseconds and are designated AP-S and AP-L, respectively. When used as voltage-clamp command signals, these AP waveforms were fed to the D/A output of a DT2801A board in a separate microcomputer and filtered at 4 kHz prior to delivery to the command input of the patch-clamp amplifier.
Ca2+-dependent membrane currents were identified by subtracting recordings obtained in the absence and presence of CdCl2 (100 µmol/L) or after rapid replacement of extracellular Ca2+ by Mg2+. This was done with a multibarrelled local superfusion device40 that allowed solution changes to be made in <1 second.
In some experiments both "long" and "short" AP waveforms were used in the same myocyte. It was essential to separate changes in membrane currents that were due to differences in AP waveform from changes that were due to time-dependent rundown of the currents. In those experiments in which both AP waveforms were used, an additional rectangular depolarization was also applied to activate ICa-L. The time-dependent rundown of ICa-L was estimated from the scaling factor, which was required for normalization of the amplitude of the Ca2+ currents produced by the step depolarizations. For six different cells, the average scaling factor, calculated over a 10-minute interval between application of different AP waveforms, was 1.24±0.13 (mean±SEM). In most experiments, the time difference between use of AP-L and AP-S waveforms was <10 minutes.
Fluorescence Measurements
The apparatus for indo 1 or rhod 2 fluorescence measurements was
built around a Nikon Diaphot microscope. For experiments in which indo
1 was used, UV light (365±10 nm) from a 150-W xenon arc lamp was
carried to the epifluorescence port of the microscope by a liquid light
guide. The light was reflected by a long-pass dichroic mirror (with
50% transmission at 385 nm) and then passed to the cell through a x40
Fluo objective (numerical aperture, 1.3) for excitation of the indo 1
in the myocyte. Exposure of the cell to UV light was limited by an
electronic shutter (Uniblitz, Vincent Associates), which was opened at
selected times during the voltage-clamp protocols. The myocyte was
simultaneously transilluminated by means of the bright-field condenser
on the microscope with light filtered through a Schott long-pass glass
filter (type RG630) with 50% transmission at 630 nm. The bright-field
illumination and the fluorescence emitted from the myocyte were
collected by the objective and directed to the video camera port of the
microscope, where the light was split by a long-pass dichroic mirror
with 50% transmission at 550 nm. Light at wavelengths >550 nm was
used to measure unloaded cell shortening with a video edge-detection
device,43 and light at wavelengths <550 nm was directed
to a pair of photomultiplier tubes for measurement of indo 1
fluorescence. Interference filters positioned in front of the
photomultiplier tubes selected fluorescence emissions at wavelengths of
410±20 nm and 500±20 nm. Background fluorescence at both wavelengths
was subtracted after a gigaohm seal was made. After the patch was
ruptured, 3 to 5 minutes was allowed for indo 1 to diffuse into the
myocyte before recordings were begun. Signals from the photomultiplier
tubes were digitized, and changes in the ratio of fluorescence
intensity at 410 nm to that at 500 nm (F410/500) were taken
as a measure of changes in
[Ca2+]i.44 In experiments
in which rhod 245 was used to estimate changes in
[Ca2+]i, the dye was excited
with light at 545±10 nm, and fluorescence was measured with a single
photomultiplier with an interference filter at 580±20 nm. The
relatively low Ca2+ affinity of rhod 2
(Kd=1.3 µM) made it suitable for measuring the
onset of the intracellular Ca2+ transient with good
signal-to-noise ratio.46
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Results |
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Ca2+ Influx During the AP
Ca2+ currents recorded from a rat ventricular myocyte using step voltage-clamp commands applied from a holding potential of -60 mV are shown in Fig 1A
. The Tyrode's
and pipette-filling solutions blocked Na+ and
K+ currents. The net Ca2+-dependent
membrane currents were identified by rapidly exposing the myocyte to
Ca2+-free solution and then subtracting the
resulting current from the control record (see "Materials and
Methods"). As demonstrated previously in rat ventricular
myocytes,47 depolarizing steps elicit only L-type
Ca2+ currents (ICa-L), which activate
rapidly and then inactivate during the depolarization. Similar
Ca2+ currents were produced by steps from holding
potential of -80 mV in the presence of tetrodotoxin,48
suggesting that all of the Ca2+ current in rat
ventricular myocytes is carried by L-type Ca2+
channels. The current-voltage relation for peak Ca2+
current is shown in Fig 1B. As shown previously in rat ventricular
myocytes,47 peak current had a bell-shaped dependence on
membrane potential, with a threshold for activation close to -40 mV, a
maximum near 0 mV, and an apparent reversal potential between +60 and
+70 mV.
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An example of the Ca2+-dependent current produced by
an AP-shaped voltage-clamp command (AP-L; see "Materials and
Methods") is shown in Fig 2A. The current consisted
of two distinct components: ICa-L, which was
present during the upstroke and repolarization of the AP, and a
much smaller inward component, which activated slowly and then declined
to zero as the AP waveform repolarized to the diastolic potential.
Similar small, slowly declining inward currents were also observed
after repolarization of step depolarizations (Fig 1A
). Fig 2B is a plot
of the Ca2+-dependent current as a function of
membrane potential during the AP. The threshold for activation of
current during the upstroke of the AP was -40 mV, similar to that
for Ca2+ currents activated by step depolarizations
(Fig 1). The Ca2+ current peaked at +25 mV during
repolarization of the AP, then declined to near zero when the membrane
potential reached -25 mV. During further repolarization of the AP,
a small inward current slowly activated; it reached a peak value near
-70 mV, then eventually declined to zero as the AP repolarized to the
diastolic potential of -80 mV. The data shown in Fig 2 are
representative of results obtained in nine other myocytes.
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Ca2+ Balance During the Cardiac Cycle
Na+-Ca2+ exchange is an important
mechanism for maintaining intracellular Ca2+
homeostasis in cardiac myocytes,9 10 and previous work
with rat ventricular myocytes has shown that the electrogenic
Na+-Ca2+ exchanger produces an inward
membrane current when it extrudes Ca2+ from the cell
at potentials negative to its reversal potential.48 The
cardiac Na+-Ca2+ exchanger has an
obligatory requirement for Na+, and removal of
external Na+ blocks efflux of Ca2+ ions
and hence inward currents produced by exchanger
activity.49 Fig 3 shows the effect of
complete removal of external Na+ on
Ca2+-dependent currents during AP voltage clamp. In
this experiment, the control solution around the myocyte was exchanged
in 500 milliseconds for one in which Na+ was completely
replaced by Li+. As shown in Fig 3A, removal of
Na+ completely abolished the small, late component of
inward current but had little effect on the magnitude or time course of
the initial inward current component, ICa-L. The
accompanying records of unloaded cell shortening show that only very
small changes in contraction amplitude occurred after replacement of
Na+ by Li+, although the duration of the
contraction was slightly prolonged. This indicates that the increase in
[Ca2+]i during both contractions was
very similar, and therefore large differences in
[Ca2+]i cannot account for suppression
of the current. Replacement of Na+ by Li+ also
resulted in an outward shift in the holding current at the diastolic
potential (-80 mV), from -13±6 pA in control to +2±3 pA (mean±SEM)
after removal of Na+ (n=5).
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) and the first
action potential following complete substitution of Na+
with Li+ (
). Bottom, Record shows unloaded cell
shortening corresponding to membrane currents indicated above.
Vm indicates membrane voltage; Im,
membrane current; and PS, peak shortening. B, Record shows expanded
recordings from the same cell showing the effect of rapid removal of
Na+ on Ca2+-dependent membrane current.
Note that replacement of Na+ with Li+
completely suppressed the late component of inward current. C, Graph
shows effect of Na+ removal on late inward current. Data
were pooled from five myocytes.
The dependence of the late component of inward current on external
Na+ ions makes it probable that this current was due to
Na+-Ca2+ exchange; hence, it is denoted
below as Iex. It is unlikely that this current was produced
by a Ca2+-activated nonspecific cation conductance,
because in cardiac myocytes these channels are highly permeable to
Li+ as well as to Na+ ions.50 It
is also unlikely that it is generated by a
Ca2+-activated Cl- channel, which could
carry inward current at potentials negative to the reversal potential
for Cl- (-40 mV under these experimental conditions).
Although the presence of this type of Cl- channel has not
been reported in rat ventricular myocytes, a transient outward
Ca2+-activated Cl- current has been
demonstrated in rabbit ventricle51 and
atrium,52 where it was blocked by the stilbene derivative
DIDS. Application of 150 µmol/L DIDS to five rat ventricular myocytes
had no significant effect on Iex or the magnitude and time
course of unloaded cell shortening (data not shown).
The relation between the Ca2+-dependent current and
changes in the ratio of fluorescence at 410 nm to that at 500 nm (taken
as a measure of [Ca2+]i) during AP
voltage clamp in a cell loaded with indo 1 is shown in Fig 4A. The increase in
[Ca2+]i lagged behind
ICa-L, and peak
[Ca2+]i occurred 100 milliseconds
after the upstroke of the AP waveform.
[Ca2+]i then declined with a time
course that paralleled the decline of Iex. The close
correlation between the time course of the decay phase of the
[Ca2+]i transient and the decline of
Iex is shown more clearly in Fig 4B. The time-dependent
decreases of both [Ca2+]i and
Iex were well fitted by the following single-exponential
function: a+b · exp(-t/
), with time constants of 144 and 119
milliseconds, respectively. The average time constants for decay of
Iex and [Ca2+]i were
190±30 and 206±25 milliseconds (mean±SEM), respectively, in a total
of seven myocytes.
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For intracellular Ca2+ homeostasis to be maintained
during steady state conditions, Ca2+ influx during
the AP must be balanced by an equivalent Ca2+ efflux
during repolarization and diastole.9 10 If
Na+-Ca2+ exchange is the primary
mechanism by which Ca2+ ions are extruded from the
cell,3 48 then the net number of Ca2+
ions extruded during Iex should equal the number entering
during ICa-L, under conditions in which the
magnitude of [Ca2+]i transients and
cell shortening are constant. If the stoichiometry of exchange is 3
Na+:1 Ca2+,53 the
charge movement due to Ca2+ extrusion during the
time course of Iex is 2x
Iex and the charge
movement during ICa-L is ICa-L, if this
current is produced solely by Ca2+ ions. Fig 4B
shows how these integrals were estimated from the
Ca2+-dependent current. ICa-L and
Iex were not completely separated in many of the cells (eg,
Fig 3); hence, ICa-L was integrated from the upstroke of
the AP to the point at which the current reached its minimum value
during repolarization, and Iex was integrated from this
minimum to 1 second after the start of the AP. This estimation of
Ca2+ influx ignores the possible contribution of
Na+-Ca2+ exchange currents to
ICa-L. In particular, the
Na+-Ca2+ exchanger might reverse during
the plateau of the AP and generate an outward
current.48 54 55 56 The data in Fig 3 suggest that the
magnitude of such a current was probably small under the experimental
conditions used here. Removal of external Na+, which
would alter outward Na+-Ca2+ exchange
currents, had little effect on the magnitude or time course of
ICa-L. Moreover, our previous work48 showed
that outward Na+-Ca2+ exchange current
in rat ventricular myocytes was small and hence transported little
Ca2+ during depolarizations as short as an AP. This
integration procedure probably leads to underestimation of the integral
for Iex, because it is likely that ICa-L
and Iex overlap near the current minimum, obscuring the
initial part of Iex; this can be seen, for example, in Fig 3B. No attempt was made to correct the Iex integrals for
this error.
Fig 4B is a plot of 2x Iex versus
ICa-L; the slope of the least-squares regression line was
0.942. The data shown were pooled from experiments with seven myocytes,
and in some cells two AP waveforms of different duration were used (see
below). All of the cells from which these data were obtained had steady
state unloaded twitch contractions with amplitudes in the range of 4 to
8 µm (single-ended shortening). The results in Fig 4B strongly imply
that Na+-Ca2+ exchange removes almost
all of the Ca2+ that enters the cell during
ICa-L, at least under the conditions of these
experiments.
The majority of the Ca2+ that activates
contraction in rat ventricular myocytes is released from the
SR3 6 7 16 19 57 ; ie, Ca2+ that enters
the cell via L-type Ca2+ channels contributes only
minimally to initiation of contraction. The plant alkaloid ryanodine
depletes SR Ca2+ stores in cardiac
myocytes,58 59 and it has been used to demonstrate the
dependence of contraction on Ca2+ release from the
SR. Fig 5 illustrates the effect of ryanodine on
Ca2+-dependent membrane currents,
[Ca2+]i transients, and unloaded cell
shortening during AP voltage-clamp of a myocyte loaded with 100
µmol/L indo 1. After a 10-minute exposure to ryanodine
(10-6 mol/L), the [Ca2+]i
transient and cell shortening were completely inhibited.
ICa-L was not significantly changed by ryanodine, but the
Na+-Ca2+ exchange current
Iex was suppressed.
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Buffering of [Ca2+]i by
Intracellular Ligands
The data in Fig 5 show that treatment of a myocyte with ryanodine
results in block of both [Ca2+]i
transients and contraction, even though the magnitude of the
Ca2+ influx into the cell during ICa-L
was not significantly altered by the drug. The net charge movement
during ICa-L in the presence of ryanodine was 45
picocoulombs, corresponding to an influx of 0.23 fmol
Ca2+. Ultrastructural studies of rat ventricular
muscle have shown that 50% of the total cell volume is occupied by
myofibrils and intracellular organelles.60 Hence, for a
cell volume of 16 pL (see "Materials and Methods"), this
Ca2+ influx would result in an increase in
intracellular Ca2+ of 28 µmol/L. Because the
peak increase in free [Ca2+]i in rat
ventricular myocytes during contraction is on the order of 1 to 2
µmol/L,17 18 19 this implies that a large fraction of the
Ca2+ entering the cell through L-type
Ca2+ channels is inaccessible to the myofilaments;
it is buffered by binding to intracellular
ligands.3 4 8 17 18 Moreover, exogenous ligands (eg,
Ca2+-indicator dyes such as indo 1) can contribute
significantly to the buffering of Ca2+ in whole-cell
recording conditions.17 18 61 Therefore, it seemed
possible that indo 1 may have been a significant additional
intracellular Ca2+ buffer and hence prevented a rise
in free Ca2+ and the accompanying cell contraction.
Support for this interpretation of the results shown in Fig 5 was
obtained in a separate series of experiments in which a mixture of 250
µmol/L EGTA and 100 µmol/L CaCl2 was added to the
pipette solution to achieve a pCa of 7. In all three of the myoyctes
studied under these conditions, application of ryanodine
(5x10-6 mol/L) led to a complete suppression of unloaded
cell shortening (data not shown).
In the absence of exogenous intracellular
Ca2+-binding ligands, Ca2+
entering the cell through L-type channels during the AP did activate a
small contraction. Fig 6 shows an example of the effect
of inhibition of SR Ca2+ release and uptake on
Ca2+-dependent membrane current and unloaded cell
shortening. Application of ryanodine (5x10-6 mol/L)
reduced the amplitude of cell shortening to 9% of its control value
and greatly prolonged its duration, but unlike those cells that
contained indo 1 or the EGTA-Ca2+ mixture,
contractions were not completely abolished. The
Na+-Ca2+ exchange component of membrane
current was suppressed by ryanodine, but neither the magnitude nor the
time course of ICa-L was significantly altered. Subsequent
addition of thapsigargin (5x10-6 mol/L), a blocker of SR
Ca2+ uptake in cardiac myocytes,62 to
the ryanodine-containing solution resulted in a marked increase in cell
shortening to 17% of the peak control contraction. In nine
myocytes, peak shortening was decreased to 8±2% (mean±SEM) of the
control value after exposure to ryanodine, and it was decreased
to 20±4% of the control value when ryanodine was added in
combination with thapsigargin. These results are consistent with
previous suggestions that Ca2+ ions entering the
cell through sarcolemmal Ca2+ channels have access
to the myofilaments but that the SR removes 50% of these ions
before they can interact with the myofilaments.62 63
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Inotropic Effects of Changes in AP Duration
An example of the effect of changes in duration of the AP
voltage-clamp waveform on Ca2+-dependent membrane
current is shown in Fig 7. Changing the voltage-clamp AP
waveform from the short AP-S to the long AP-L (see "Materials and
Methods") resulted in a large decrease in peak ICa-L
(from 1.2 nA to 0.65 nA) but an increase in its time to peak (7.3 to
13.6 milliseconds) and the half-time for the current to decay back to
its diastolic level (6.1 to 20.0 milliseconds). The
Na+-Ca2+ exchange current,
Iex, was also altered by the change in AP waveform,
and its amplitude increased by 40% after the switch to the AP-S
waveform. The current changes shown in Fig 7 were qualitatively similar
in nine additional cells in which AP waveforms were switched from AP-S
to AP-L, or vice versa. These data are summarized in Table 1, which also shows that the net influx of
Ca2+ (ie, ICa-L) during the two
AP waveforms differed by a factor of close to 2, decreasing on average
from 33.4 pC for AP-L to 16.2 pC for AP-S. Using the estimated single
cell volume of 16 pL, and assuming that only 50% of this intracellular
volume is accessible to Ca2+ ions, we calculate that
the mean increase in total Ca2+ concentration would
be 10 and 21 µmol/L per AP-S and AP-L waveforms, respectively.
Table 1 shows that for both AP-S and AP-L, the net influx of
Ca2+ during ICa-L was approximately
balanced by the efflux of Ca2+ during
Iex.
|
|
Changes in AP duration also produced significant effects on
[Ca2+]i transients and unloaded cell
shortening. An example of these effects is shown in Fig 8; steady state Ca2+ currents,
[Ca2+]i transients, and unloaded cell
shortening were recorded simultaneously during stimulation of the
myocyte with both AP-S and AP-L waveforms. Switching of the AP waveform
from AP-S (Fig 8A) to AP-L (Fig 8B) resulted in a large increase in the
magnitude and duration of contraction as well as in the magnitude of
the [Ca2+]i transient. Data from five
myocytes in which AP-S and AP-L waveforms were applied to the same cell
are summarized in Table 2. On average, prolongation of
the AP waveform resulted in an increase in the amplitude and time to
peak of the [Ca2+]i transient and
unloaded cell shortening, as well as an increase in the 50% relaxation
time of the contraction. The time constants of decay of the
[Ca2+]i transient
(indo, Table 2) produced by each AP waveform were
not significantly different. Note that for both AP-S and AP-L the time
constants for decay of Iex and
[Ca2+]i transients were also very
similar (compare with data shown in Fig 4). Alteration of the AP
waveform had no significant effect on the diastolic indo 1 fluorescence
ratio (F410/500, 0.8±0.1), suggesting that there
was no change in resting [Ca2+]i under
these experimental conditions.
|
|
The results shown in Fig 8 and in Tables 1 and 2 were obtained under
steady state conditions, ie, after Ca2+-dependent
currents, [Ca2+]i transients, and
unloaded cell shortening were constant for each AP waveform. Fig 9 shows the time-dependent changes in
Ca2+ currents and unloaded cell shortening that
resulted from an abrupt change in AP duration. In the experiment shown
in Fig 9A the AP-S waveform was applied to a myocyte at a rate of 0.14
Hz until cell shortening reached a steady state. The
Ca2+-dependent current and contraction corresponding
to this steady state are denoted "B0." At the time of the next
"beat," the AP waveform was changed to AP-L; currents and
contractions produced by the first (B1) and seventh (B7) "beats"
are shown in Fig 9A. Note that the change in membrane current occurred
immediately after the switch from the AP-S to the AP-L waveform; ie,
the currents corresponding to B1 and B7 were identical. In contrast,
the switch from AP-S to AP-L resulted in a 20% increase in the first
contraction (B1). This initial increase in peak shortening was followed
by a considerably larger, time-dependent increase, and by B7 the
contraction had reached a new steady state amplitude that was 87%
larger than B0. These findings suggest that the majority of the
positive inotropic effect resulting from AP prolongation is caused by
increased SR Ca2+ loading and release. The increase
in SR Ca2+ loading is due to enhanced
Ca2+ entry during ICa-L and a delay in
Ca2+ extrusion by the
Na+-Ca2+ exchanger during the slower
repolarization.
|
Fig 9B shows the effect of an abrupt decrease in AP duration on
Ca2+-dependent current and unloaded cell shortening.
There was an immediate increase in peak ICa-L and a small
decrease (15%) in cell shortening, followed by a marked time-dependent
decrease (43%) in twitch amplitude.
The effects of a rapid switch from AP-S to AP-L on the magnitude and
time course of [Ca2+]i transients are
shown in Fig 10. The relatively low affinity
Ca2+ indicator rhod 245 46 was used in
these experiments because it allowed good signal-to-noise ratio
recordings to be made of the rising phase of the
Ca2+ transient. As was observed for cell-shortening
measurements (Fig 9), an abrupt increase in AP duration resulted in a
slight increase in the amplitude of the first rhod 2 fluorescence
transient following the switch in AP waveform (B1, Fig 10). Continued
stimulation with AP-L resulted in a substantial additional increase in
the amplitude of the [Ca2+]i
transient. As in the cell-shortening experiments, these changes in the
[Ca2+]i transient occurred in the
absence of significant changes in ICa-L. These data confirm
that the majority of the positive inotropic effect resulting from AP
prolongation is due to increased loading of the SR with
Ca2+.
|
F580), and rhod 2 dF/dt.
The recordings denoted B0 correspond to AP-S. B1 and B9 denote the
first and ninth beats following a switch to AP-L. Note that the rapid
switch to an AP-L waveform resulted in a significant decrease in dF/dt.
Continued stimulation with AP-L resulted in time-dependent increases in
the peak of the rhod 2 transient and dF/dt.
[Ca2+]o was 1 mmol/L and stimulation
frequency was 0.1 Hz. Vm indicates membrane voltage;
Im, membrane current.
Previous work on crayfish skeletal46 and mammalian
cardiac4 8 64 muscle has shown that the rate of
Ca2+ release from the SR can be estimated from the
rate of change of the intracellular Ca2+ transient.
Fig 10 also shows the rate of change of rhod 2 fluorescence, dF/dt.
Note that AP prolongation resulted in a decrease in the maximum value
of dF/dt, despite the increase in the amplitude of the transient. This
effect is obvious when the measurements of dF/dt for B0 and B1 are
compared. Even though the peak values of the rhod 2 transients were
nearly equal, maximum dF/dt for the prolonged AP was only 50% as
large as that for the shorter AP. Even after the transient had reached
its much larger, steady state amplitude (B9), its maximum dF/dt was
smaller than that for the shorter AP (B0). These data are consistent
with previous work showing that SR Ca2+ release can
be controlled by the rate of change of free [Ca2+]
near the SR Ca2+ release sites in guinea pig and rat
ventricle,19 39 64 65 66 as well as with recent mathematical
models of excitation-contraction coupling in mammalian
ventricle.19 67
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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In mammalian ventricle, excitation-contraction coupling involves at least four different processes3 68 69 : (1) Ca2+ influx, (2) Ca2+ release from the SR, (3) Ca2+ extrusion due to Na+-Ca2+ exchange, and (4) reuptake or sequestration of Ca2+ into the SR via an ATP-requiring Ca2+ pump. Each of these processes has been studied in detail in rat heart and in the myocardium of a variety of other mammalian species, and the AP voltage-clamp technique has been used to measure ICa-L23 and the relation between ICa-L and [Ca2+]i transients in guinea pig ventricle.24 We measured Ca2+ influx, Ca2+ extrusion, the [Ca2+]i transient, and cell shortening simultaneously in a mammalian ventricular myocyte and obtained new information concerning the ionic basis for the positive inotropic effect that results from an increase in AP duration in rat ventricle. Although our methods have limitations and complete answers to some of these questions may require additional technique development, our results provide a well-integrated and plausible scheme for excitation-contraction coupling in myocytes from rat ventricle. A summary of our major findings, together with comments on their significance and limitations, can be presented in relation to each of the steps in excitation-contraction coupling.
Ca2+ Influx
Our results show that the Ca2+ current
corresponding to a physiological stimulus (AP waveform) in rat
ventricle provides a net influx of Ca2+ that is
relatively large. This charge movement, if it were distributed
uniformly in 50% of the intracellular volume and if no
Ca2+ buffering took place, would increase
[Ca2+]i by 10 to 20 µmol/L. Under
our experimental conditions, virtually all of this
Ca2+ influx is due to current flow through L-type
Ca2+ channels (as opposed to reversal of the
Na+-Ca2+ exchanger), because superfusion
of cells with Cd2+ or verapamil blocked all of the inward
current.48 Results that we have previously
published48 and those from the present experiments
demonstrate that under our experimental conditions,
Ca2+ influx due to
Na+-Ca2+ exchange is very small and does
not result in either a significant increase in
[Ca2+]i or trigger
Ca2+-induced Ca2+ release. This
finding differs from the results of Levi et al.55 At
present, no clear explanation is apparent for this disparity.
Although it has been reported that both
Na+-Ca2+ exchange70 and SR
Ca2+ release71 are very temperature
sensitive, our experiments, which were done at both 23°C (as in the
present study) and at 32°C, yielded very similar
results.48
The possibility that T-type Ca2+ channels contributed significantly to the Ca2+ influx (as is the case in guinea pig ventricular myocytes) was ruled out because inward currents elicited from holding potentials of either -50 or -80 mV were very similar. Previously, Tytgat et al72 also failed to identify any significant T-type Ca2+ current in rat ventricular myocytes.
Because activation of ICa-L elicited very little cell
shortening and no [Ca2+]i transient
(in the presence of intracellular indo 1) when Ca2+
release from the SR was blocked (Figs 5 and 6), much of the
transmembrane Ca2+ influx must be very rapidly
buffered or its intracellular distribution must be restricted to small
microdomains just beneath the sarcolemma.73 74 75 76 77 Although
our experimental design and methodologies were not suitable for
evaluating these possibilities, previous electrophysiological
measurements,78 79 theoretical work,73 77 and
time-resolved measurements of [Ca2+]i
distribution80 that use, for example, confocal microscopy
have provided evidence for significant localized transient changes in
[Ca2+]i in a variety of cell types,
including rat ventricle.81 82 83
Ca2+-Induced Ca2+ Release
As has been demonstrated previously, enzymatically isolated rat
ventricular myocytes offer a valid model for studies of
excitation-contraction coupling,84 and provided that the
intracellular milieu is not strongly dialyzed and relatively low
concentrations of Ca2+-sensitive dyes are used, they
exhibit Ca2+-induced Ca2+
release. Previous results from both intact myocytes and skinned fibers
have demonstrated that in rat ventricle,
Ca2+-induced Ca2+ release gives
rise to a very large change in [Ca2+]i
consistent with some 95% to 99% of the
Ca2+, which is bound to the contractile
proteins, being from the SR.7 57 64 Our findings are
consistent with this, and they also show that the
Ca2+ influx through L-type Ca2+
channels is relatively large in rat ventricle, as it is in guinea pig
ventricle.85 This means that the safety factor for the
triggering mechanism for Ca2+-induced
Ca2+ release is large. Moreover, we showed that
changes in the Ca2+ waveform (eg, those due to
lengthening the AP duration) can significantly modulate both the rate
and the extent of Ca2+ release from the SR in rat
ventricle. Previous studies have suggested that this may be the
case,65 73 86 and a recent report87
demonstrates that the kinetics of Ca2+ release
channels reconstituted into black lipid bilayers can be modulated by
[Ca2+]i.
Ca2+ Extrusion/Sequestration in Rat
Ventricle
For [Ca2+]i homeostasis and
steady state contraction levels to be maintained, it is necessary that
Ca2+ influx during the AP be balanced (on average)
by an equivalent Ca2+ efflux during repolarization
and diastole.9 10 The present experiments demonstrate
that the AP voltage-clamp technique can be used to measure the flux of
Ca2+ due to voltage-dependent
Ca2+ channels and the
Na+-Ca2+ exchanger. If a stoichiometry
of 3 Na+:1 Ca2+ is assumed, the close
correlation between charge movement during ICa-L and that
during the slow inward current (Fig 4 and Table 1) demonstrates that
[Ca2+]i homeostasis in rat ventricular
myocytes is achieved by a balance of Ca2+ entry
through L-type Ca2+ channels and
Ca2+ extrusion by the
Na+-Ca2+ exchanger. Several observations
support this conclusion. First, in all experiments, Cs+ in
the pipette solution ensured that K+ currents were blocked.
Second, the slow inward current was suppressed completely after
replacement of extracellular Na+ with Li+ (Fig 3). Third, the Ca2+-dependent membrane current was
not changed significantly by the Cl- channel blocker DIDS,
which has been shown to block Ca2+-dependent
Cl- currents in mammalian ventricular and atrial
cells.51 52
The regression plot in Fig 4 provides evidence that almost all
(95%) of the Ca2+ that enters the cell during
the AP is extruded by the Na+-Ca2+
exchanger during repolarization and diastole. If the
Ca2+ influx due to ICa-L contributes
little to activation of the myofilaments (Fig 6), net
Ca2+ uptake by the SR during the declining phase of
the [Ca2+]i transient must be
approximately equal to the amount of Ca2+ released
during activation of contraction.3 88 Balke et
al18 have estimated that the maximal rate of
Ca2+ pumping by the SR Ca2+
ATPase in intact rat ventricular myocytes is 160
µmol/L · s-1, if
Km is 240 nmol/L. Similar values have
been reported by Wimsatt et al21 for SR uptake in
permeabilized rat ventricular myocytes (160
nmol · min-1 · mg protein-1;
Km=0.5 µmol/L). These results suggest
that SR Ca2+ pumping is the dominant process
determining the rate of decline of
[Ca2+]i in rat cardiac myocytes. Our
data are consistent with this, because complete removal of
extracellular Na+ prolonged the time to peak shortening but
had no effect on the rate of relaxation of contraction (Fig 3). The
approximately twofold increase in the extent of cell shortening after
thapsigargin treatment (Fig 6) suggests that the SR
Ca2+ binding/pumping is sufficiently fast to be able
to limit changes in [Ca2+]i due to
Ca2+ influx during the AP.3 4 18 63
Effects of AP Prolongation on Excitation-Contraction Coupling in
Rat Ventricle
Our results demonstrate that a marked prolongation of the AP
can result in a large, time-dependent positive inotropic effect. As
shown in Fig 7 and Table 1, AP prolongation in rat ventricular myocytes
resulted in a twofold reduction in peak ICa-L and a marked
slowing of its decline. The mechanisms controlling inactivation,
deactivation, and reactivation of Ca2+ channels are
complex,89 90 and involve an interaction between time- and
voltage-dependent gating as well as Ca2+-induced
inactivation. Compared with the AP-S waveform, the longer and more
depolarized plateau of the AP-L waveform results in a significant
reduction in the electrochemical driving force for
Ca2+ influx during the time that the
Ca2+channels are activated; this may be the major
factor responsible for the decrease in peak ICa-L in
response to the AP-L waveform in accounting for the differences in peak
ICa-L. The prolonged plateau is also in a range of membrane
potentials at which a portion of the Ca2+ channel
remains activated. In contrast, the much more rapid repolarization of
AP-S probably results in closure of the channels primarily by
deactivation, a kinetic process that is considerably faster than
inactivation.89 The net effect is that the
Ca2+ influx during AP-L is about twice that during
AP-S (Table 1). A second phenomenon that could contribute to the
decreased rate of decline of the Ca2+ current during
AP-L is that of channel reopening. This has been described in L-type
Ca2+ channels of guinea pig
ventricle,91 and it is also an important feature of
ICa-L kinetics in most recent mathematical models of the
cardiac AP.92 Additional experiments will be necessary to
determine the relative contribution of Ca2+ channel
inactivation, deactivation, or reopening to the observed
ICa-L kinetics during the AP-S and AP-L waveforms.
Perhaps the most important effect of AP prolongation in rat
ventricle is that the resulting alteration of transmembrane
Ca2+ influx significantly changes the rate and
extent of Ca2+ release from the SR, as measured by
changes in rhod 2 transients. Previous findings have suggested that
this may be the case,4 19 and the recent paper by Han et
al93 provides direct evidence for this. Data summarized in
Tables 1 and 2 show a strong positive correlation between the increase
in net charge movement during ICa-L and the increase in
[Ca2+]i transient and cell shortening
after AP prolongation. As expected from the results in Figs 5 and 6,
depletion of SR Ca2+ storage with ryanodine
completely blocked this phenomenon, demonstrating that the change in
Ca2+ transients and resulting cell shortening
depended on SR Ca2+ release. In addition, because
alteration of AP duration resulted in an immediate change in the
magnitude and time course of ICa-L but a much slower change
in contractions and Ca2+ transients, the majority of
the inotropic effect probably is due to alterations in loading of the
SR with Ca2+. Thus, the increase in
Ca2+ influx during the AP plateau results in a
positive inotropic effect only indirectly: ie, by changing
Ca2+ release and/or Ca2+
loading.4 8 64 65 Previous experimental results from
crayfish skeletal muscle fibers78 79 and
Ca2+ release channels from mammalian ventricle
reconstituted into black lipid bilayers87 have
demonstrated significant effects of both steady state
[Ca2+] and the rate of change of
[Ca2+] on channel open time and, by implication,
on SR Ca2+ release.
In summary, our results provide detailed information concerning the magnitude and nature of the interactions between three processes that are responsible for excitation-contraction coupling in rat ventricle. In rat ventricle it has been demonstrated previously38 65 that the L-type Ca2+ current acts as a trigger for the much more pronounced release of Ca2+ from the SR. Our results agree, and they also demonstrate that the Ca2+ current is relatively large. Its size and slow time course of inactivation give rise to a substantial flux that, by itself, could significantly change [Ca2+]i. However, this does not occur, leading to the conclusion that fast, relatively high capacity myoplasmic buffers bind most of the Ca2+ entering the cell through L-type Ca2+ channels.94 Although the identity of this buffer system remains unknown, the physicochemical properties and cytosolic distribution of calmodulin make this Ca2+-binding protein a very likely candidate.18 The presence of significant [Ca2+]i buffering and binding makes it necessary to be able to carry out spatially localized, very rapid measurements of the changes in [Ca2+]i to answer some important questions.80 81 82 83 Our global or spatially averaged measurements of [Ca2+]i therefore have significant limitations. Nevertheless, these results show that one important component underlying the positive inotropic effect of AP prolongation is based on the ability of the resulting change in transmembrance Ca2+ influx, ICa-L, to enhance the rate and the size of Ca2+-induced Ca2+ release from the SR.
Although our findings provide an integrated view of Ca2+ homeostasis in rat ventricular myocytes, extrapolation of these findings to other tissues and/or species should be made with caution. For example, it is well known that the microanatomic features and the size and time course of ICa-L in guinea pig ventricle differ very significantly from those of rat ventricle and that the rabbit model represents an intermediate situation.68 However, it is interesting that recent data suggest that the electrophysiological behavior of the rat ventricle (eg, the complement of K+ currents, frequency-induced changes in AP duration)32 is similar to that of myocytes from the epicardium of human left ventricle.74 95 96 It is possible, therefore, that some of the features of excitation-contraction coupling in rat ventricle are similar to those in the epicardium of human ventricle.
|
Acknowledgments |
|---|
This study was supported by the Medical Research Council of Canada, the Heart and Stroke Foundation of Canada, and the Alberta Heritage Foundation for Medical Research. Dr Giles is an Alberta Heritage Foundation Medical Scientist. Dr Bouchard was the recipient of a Medical Research Council Postdoctoral Fellowship. We thank S. Somers for help in the construction of the fluorescence setup and C. Collins for secretarial assistance.
|
Footnotes |
|---|
Previously published as preliminary results in abstract form (Biophys J. 1994;66:A91).
Received October 21, 1994; accepted January 23, 1995.
|
References |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
1. Fabiato A. Calcium-induced release of calcium from cardiac sarcoplasmic reticulum. Am J Physiol. 1983;245:C1-C14.
2.
Fabiato A. Time and calcium dependence of
activation and inactivation of calcium induced release of calcium from
the sarcoplasmic reticulum of a skinned canine cardiac Purkinje
cell. J Gen Physiol. 1985;85:247-289.
3.
Barry WH, Bridge JHB. Intracellular calcium
homeostasis in cardiac myocytes.
Circulation. 1993;87:1806-1815.
4. Wier WG. Dynamics of control of cytosolic calcium ion concentration. Annu Rev Physiol. 1990;52:467-485. [Medline] [Order article via Infotrieve]
5.
Negretti N, O'Neill SC, Eisner DA. The
relative contribution of different intracellular and sarcolemmal
systems to relaxation in rat ventricular myocytes.
Cardiovasc Res. 1993;27:1826-1830.
6.
Cleeman L, Morad M. Role of
Ca2+ channel in cardiac excitation-contraction
coupling in the rat: evidence from Ca2+ transients
and contraction. J Physiol (Lond). 1991;432:283-312.
7. Varro A, Negretti N, Hester SB, Eisner AD. An estimate of the calcium content of the sarcoplasmic reticulum in rat ventricular myocytes. Pflugers Arch. 1993;423:158-160. [Medline] [Order article via Infotrieve]
8. Wier WG. [Ca2+]i transients during excitation-contraction coupling of mammalian heart. In: Fozzard HA, Haber E, Jennings RB, Katz AM, Morgan HE, eds. The Heart and Cardiovascular System. New York, NY: Raven Press Publishers; 1992:1223-1248.
9. Noble D. Sodium-calcium exchange and its role in generating electric current. In: Nathan RD, ed. Cardiac Muscle: The Regulation of Excitation and Contraction. Orlando, Fla: Academic Press; 1986:171-198.
10. Hilgemann D. "Best estimates" of physiological Na/Ca exchange function: calcium conservation and the cardiac electrical cycle. In: Zipes DP, Jalife J. Cardiac Electrophysiology: From Cell to Bedside. Philadelphia, Pa: WB Saunders Co; 1990:51-61.
11.
Neher E, Augustine GJ. Calcium gradients and
buffers in bovine chromaffin cells. J Physiol
(Lond). 1992;450:273-301.
12.
Smith SJ, Zucker RS. Aequorin response
facilitation and intracellular calcium accumulation in molluscan
neurons. J Physiol (Lond). 1980;300:167-196.
13.
Tse A, Tse FW, Hille B. Calcium homeostasis in
identified rat gonodatrophs. J Physiol (Lond). 1994;477:511-525.
14.
Schneider MF, Simon BJ. Inactivation of calcium
release from the sarcoplasmic reticulum in frog skeletal
muscle. J Physiol (Lond). 1988;405:727-745.
15.
Stehno-Bittel L, Sturek M. Spontaneous
sarcoplasmic reticulum calcium release and extrusion from bovine, not
porcine, coronary artery smooth muscle. J Physiol
(Lond). 1992;451:49-78.
16.
Sipido K, Wier WG. Flux of
Ca2+ across the sarcoplasmic reticulum of guinea-pig
cardiac cells during excitation-contraction coupling.
J Physiol (Lond). 1991;435:605-630.
17. Berlin JR, Konishi M. Ca2+ transients in cardiac myocytes measured with high and low affinity Ca2+ indicators. Biophys J. 1993;65:1632-1647. [Medline] [Order article via Infotrieve]
18.
Balke CW, Egan TM, Wier WG. Processes that
remove calcium from the cytoplasm during excitation-contraction
coupling in intact rat ventricular cells. J Physiol
(Lond). 1994;474:447-462.
19.
Wier WG, Egan TM, Lopez-Lopez JR, Balke CW.
Local control of excitation-contraction coupling in rat heart
cells. J Physiol (Lond). 1994;474:463-471.
20.
Fabiato A. Myoplasmic free calcium concentration
reached during the twitch of an intact isolated cardiac cell and during
calcium-induced release of calcium from the sarcoplasmic reticulum of a
skinned cardiac cell from the adult rat or rabbit ventricle.
J Gen Physiol. 1981;78:457-497.
21.
Wimsatt DK, Hohl CM, Brierley GP, Aultschuld RA.
Calcium accumulation and release by the sarcoplasmic reticulum
of digitonin-lysed adult mammalian ventricular cardiomyocytes.
J Biol Chem. 1990;265:14849-14857.
22.
Hove-Madison L, Bers DM. Passive Ca buffering
and SR Ca uptake in permeabilized rabbit ventricular myocytes.
Am J Physiol. 1993;264:C677-C686.
23. Doerr T, Denger R, Doerr A, Trautwein W. Ionic currents contributing to the action potential in single ventricular myocytes of the guinea-pig studied with action potential clamp. Pflugers Arch. 1990;413:599-603.
24.
Arreola J, Dirksen RT, Shieh R-C, Williford DJ, Sheu
S-S. Ca2+ current and Ca2+
transients under action potential clamp in guinea-pig ventricular
myocytes. Am J Physiol. 1991;261:C393-C397.
25.
Wood EH, Heppner R, Weidmann S. Inotropic
effects of electric currents. Circ Res. 1969;24:409-445.
26. Morad M, Goldman Y. Excitation-contraction coupling in heart muscle: membrane control of development of tension. Prog Biophys Mol Biol. 1973;27:257-313.
27. Allen DG. On the relationship between action potential duration and tension in cat papillary muscle. Cardiovasc Res. 1977;11:210-218. [Medline] [Order article via Infotrieve]
28. Wohlfart B. Relationship between peak force, action potential duration and stimulus interval in rabbit myocardium. Acta Physiol Scand. 1979;106:395-409. [Medline] [Order article via Infotrieve]
29. Schouten VJA. The negative correlation between action potential duration and force of contraction during restitution in rat myocardium. J Mol Cell Cardiol. 1986;18:1033-1045. [Medline] [Order article via Infotrieve]
30. Boyett MR, Jewell B. Analysis of the effects of changes in rate and rhythm upon electrical activity in the heart. Prog Biophys Mol Biol. 1980;36:1-52. [Medline] [Order article via Infotrieve]
31. Shibata EF, Drury T, Refsum H, Aldrete V, Giles WR. Contributions of a transient outward current to repolarization in human atrium. Am J Physiol. 1989;258:H1773-H1781.
32.
Clark RB, Bouchard RA, Sanchez-Chapula E,
Salinas-Stephanon E, Giles WR. Heterogeneity of action potential
waveforms and repolarizing potassium currents in rat ventricle.
Cardiovasc Res. 1993;27:1795-1799.
33.
Fedida D, Shimoni Y, Giles WR. -Adrenergic
modulation of the transient outward current in rabbit atrial
myocytes. J Physiol (Lond). 1990;423:257-277.
34.
Fedida D, Bouchard RA. Mechanisms for the
positive inotropic effect of 1-adrenoceptor stimulation
in rat cardiac myocytes. Circ Res. 1992;71:673-688.
35.
Xiao R-P, Lakatta EG.
ß1-Adrenoceptor stimulation and
ß2-adrenoceptor stimulation differ in their effects on
contraction, cytosolic Ca2+, and
Ca2+ current in single rat ventricular
cells. Circ Res. 1992;73:286-300.
36. Gwathmey JK, Slawsky MT, Briggs GM, Morgan JP. Role of intracellular sodium in the regulation of intracellular calcium and contractility: effects of DPI 201-106 on excitation-contraction coupling in human ventricular myocardium. J Clin Invest. 1988;82:1592-1605.
37. Hoey A, Amos GJ, Ravens U. Comparison of the action potential prolonging and positive inotropic activity of DP1 201-106 and BDF 9148 in human ventricular myocardium. J Mol Cell Cardiol. 1994;26:985-994. [Medline] [Order article via Infotrieve]
38. Morad M, Cleeman L. Role of Ca2+ channel in development of tension in heart muscle. J Mol Cell Cardiol. 1987;19:257-313.
39.
Fabiato A. Simulated calcium current can both
cause calcium loading in and trigger calcium release from the
sarcoplasmic reticulum of a skinned canine cardiac Purkinje
cell. J Gen Physiol. 1985;85:291-320.
40.
Bouchard RA, Clark RB, Giles WR. Regulation of
unloaded cell shortening by sarcolemmal sodium-calcium exchange in
isolated rat ventricular myocytes. J Physiol (Lond). 1993;469:583-599.
41.
London B, Krueger JW. Contraction in
voltage-clamped internally perfused single heart cells. J
Gen Physiol. 1986;88:475-505.
42.
Bers DM, Lederer WJ, Berlin J. Intracellular Ca
transients in rat cardiac myocytes: role of Na-Ca exchange in
excitation-contraction coupling. Am J Physiol. 1990;258:C944-C954.
43. Steadman BW, Moore KB, Spitzer KW, Bridge JHB. A video system for measuring motion in contracting heart cells. IEEE Trans Biomed Eng. 1988;35:264-268. [Medline] [Order article via Infotrieve]
44.
Grykiewicz G, Poenie M, Tsien RY. A new
generation of Ca2+ indicators with greatly improved
properties. J Biol Chem. 1987;260:3440-3450.
45.
Minta A, Kao JPY, Tsien RY. Fluorescent
indicators for cytosolic calcium based on rhodamine and fluorescein
chromophores. J Biol Chem. 1989;264:8171-8178.
46.
Gyorke S, Palade P.
Ca2+-dependent negative control mechanism for
Ca2+-induced Ca2+ release in
crayfish muscle. J Physiol (Lond). 1994;476:315-322.
47.
Josephson IR, Sanchez-Chapula J, Brown AM. A
comparison of calcium currents in rat and guinea-pig single ventricular
cells. Circ Res. 1984;54:144-154.
48.
Bouchard RA, Clark RB, Giles WR. Role of
sodium-calcium exchange in activation of contraction in rat
ventricle. J Physiol (Lond). 1993;472:391-413.
49. Kimura J, Noma A, Irisawa H. Na-Ca exchange current in mammalian heart cells. Nature. 1987;319:596-597.
50.
Ehara T, Noma A, Ono K. Calcium-activated
non-specific cation channel in ventricular cells isolated from adult
guinea-pig hearts. J Physiol (Lond). 1988;403:117-133.
51.
Zygmunt AC, Gibbons WR. Calcium-activated
chloride current in rabbit ventricular myocytes.
Circ Res. 1991;68:424-437.
52.
Zygmunt AC, Gibbons WR. Properties of the
calcium-activated chloride current in heart. J Gen
Physiol. 1992;99:391-414.
53.
Eisner D, Lederer WJ. Na-Ca exchange:
stoichiometry and electrogenicity. Am J Physiol. 1985;248:C189-C202.
54. Noble D, Noble SJ, Berr CL, Earm YE, Ho WK, So IK. The role of sodium-calcium exchange during the cardiac action potential. Ann N Y Acad Sci. 1991;639:334-354. [Medline] [Order article via Infotrieve]
55.
Levi AJ, Brooksby P, Hancox JC. A role for
depolarisation induced calcium entry on the Na-Ca exchanger in
triggering intracellular calcium release and contraction in rat
ventricular myocytes. Cardiovasc Res. 1993;27:1677-1690.
56.
Le Guennec J-Y, Noble D. Effects of rapid
changes of external Na+ concentration at different moments
during the action potential in guinea-pig myocytes. J
Physiol (Lond). 1994;478:493-504.
57. Valdiomillos M, O'Neill SC, Smith GL, Eisner DA. Calcium-induced calcium release activates contraction in intact cardiac cells. Pflugers Arch. 1989;413:676-678. [Medline] [Order article via Infotrieve]
58.
Sutko J, Willerson J. Ryanodine alteration of
the contractile state of rat ventricular myocardium: comparison with
dog, cat and rabbit ventricular tissues. Circ
Res. 1980;46:332-343.
59.
Banajamali HS, Gao WD, MacIntosh BR, ter Keurs
HEDJ. Force–interval relations of twitches and cold
contractures in rat cardiac trabeculae: effect of ryanodine.
Circ Res. 1991;69:937-948.
60.
Page E. Quantitative ultrastructural
analysis in cardiac membrane physiology. Am J
Physiol. 1978;235:C147-C158.
61. Blatter LA, Wier WG. Intracellular diffusion, binding and compartmentalization of the fluorescent calcium indicators indo 1 and fura-2. Biophys J. 1990;58:1491-1499.[Medline] [Order article via Infotrieve]
62.
Negretti N, O'Neill SC, Eisner DA. The effects
of inhibitors of sarcoplasmic reticulum function on the systolic
Ca2+ transient in rat ventricular myocytes.
J Physiol (Lond). 1993;468:35-52.
63.
Janczewski AM, Lakatta EG. Buffering of calcium
influx by sarcoplasmic reticulum during the action potential in
guinea-pig ventricular myocytes. J Physiol (Lond). 1993;471:343-363.
64.
Beuckelmann DJ, Wier WG. Mechanism of release of
calcium from sarcoplasmic reticulum in guinea-pig cardiac
cells. J Physiol (Lond). 1988;405:233-255.
65.
Nabauer M, Callewaert G, Cleeman L, Morad M.
Regulation of calcium release is regulated by calcium, not
gating charge, in cardiac myocytes. Science. 1989;244:800-803.
66.
Wier WG, Cannell MB, Berlin JR, Marban E, Lederer WJ.
Cellular and subcellular heterogeneity of
[Ca2+]i in single heart cells revealed
by fura-2. Science. 1987;235:325-328.
67. Stern MD. Theory of excitation-contraction coupling in cardiac muscle. Biophys J. 1992;63:479-517.
68. Bers DM. Excitation-Contraction Coupling and Cardiac Contractile Force. Norwell, Mass: Kluwer Academic Publishers; 1991.
69. Gibbons WR, Zygmunt AC. Excitation-contraction coupling in heart. In: Fozzard HA, Haber E, Jennings RB, Katz AM, Morgan HE, eds. The Heart and Cardiovascular System. 2nd ed. New York, NY: Raven Press Publishers; 1992:1249-1279.
70.
Vornanen M, Shepherd N, Isenberg G.
Tension-voltage relations of single myocytes reflect Ca release
triggered by exchange at 35°C but not 23°C. Am J
Physiol. 1994;267:C623-C632.
71.
Sitsapesan R, Montgomery RA, MacLeod KT, Williams AJ.
Sheep cardiac sarcoplasmic reticulum calcium-release channels:
modification of conductance and gating by temperature. J
Physiol (Lond). 1991;434:469-488.
72. Tytgat J, Vereecke J, Carmeliet E. A combined study of sodium current and T-type calcium current in isolated cardiac cells. Pflugers Arch. 1990;417:142-148. [Medline] [Order article via Infotrieve]
73. Stern MD, Lakatta EF. Excitation-contraction coupling in the heart: the state of the question. FASEB J. 1992;6:2992-3015.
74.
Wendt-Gallitelli MF, Isenberg G. Total and free
myoplasmic calcium during a contraction cycle: x-ray microanalysis
in guinea-pig ventricular myocytes. J Physiol
(Lond). 1991;435:349-372.
75. Bers DM, Peskoff A. Diffusion around a cardiac calcium channel and the role of surface bound calcium. Biophys J. 1991;59:703-721. [Medline] [Order article via Infotrieve]
76. Imredy JP, Yue DT. Submicroscopic Ca2+ diffusion mediates inhibitory coupling between individual Ca2+ channels. Neuron. 1992;9:197-207. [Medline] [Order article via Infotrieve]
77. Nowycky MC, Pinter MJ. Time courses of calcium and calcium-bound buffers following calcium influx in a model cell. Biophys J. 1993;64:77-91. [Medline] [Order article via Infotrieve]
78.
Gyorke S, Palade P. Role of local
Ca2+ domains in activation of
Ca2+-induced Ca2+ release in
crayfish muscle fibers. Am J Physiol. 1993;264:C1505-C1512.
79.
Gyorke S, Palade P. Calcium-induced calcium
release in crayfish skeletal muscle. J Physiol
(Lond). 1992;457:195-210.
80. Escobar AL, Monck JR, Fernandez JM, Vergara JL. Localization of the site of Ca2+ release at the level of a single sarcomere in skeletal muscle fibres. Nature. 1994;367:739-741. [Medline] [Order article via Infotrieve]
81.
Lipp P, Niggli E. Modulation of
Ca2+ release in cultured neonatal rat cardiac
myocytes: insight from subcellular release patterns revealed by
confocal microscopy. Circ Res. 1994;74:979-990.
82.
Cheng H, Lederer WJ, Cannell MB. Calcium sparks:
elementary events underlying excitation-contraction coupling in heart
muscle. Science. 1993;262:740-744.
83. Cannell MB, Cheng H, Lederer WJ. Spatial non-uniformities in [Ca2+]i during excitation-contraction coupling in cardiac myocytes. Biophys J. 1994;67:1942-1956. [Medline] [Order article via Infotrieve]
84.
Capogrossi MC, Kort AA, Spurgeon HA, Lakatta ED.
Single adult rabbit and rat cardiac myocytes retain the
Ca2+- and species-dependent systolic and diastolic
contractile properties of intact muscle. J Gen
Physiol. 1986;88:589-613.
85.
McDonald TF, Pelzer D, Trautwein W. Does the
calcium current modulate the contraction of the accompanying beat?
Circ Res. 1981;49:576-583.
86.
Pozzan T, Rizzuto R, Volpe P, Meldolesi J.
Molecular and cellular physiology of intracellular calcium
stores. Physiol Rev. 1994;74:595-636.
87. Uehara A, Yasukochi M, Imanaga I. Calcium modulation of single SR potassium channel currents in heart muscle. J Mol Cell Cardiol. 1994;26:195-202. [Medline] [Order article via Infotrieve]
88.
Bassani JWM, Bassani RA, Bers DM. Relaxation in
rabbit and rat cardiac cells: species-dependent differences in cellular
mechanisms. J Physiol (Lond). 1994;476:279-293.
89. McDonald TF, Pelzer S, Trautwein W, Pelzer DJ. Regulation and modulation of calcium channels in cardiac, skeletal, and smooth muscle cells. Physiol Rev. 1994;74:366-461.
90. DeFelice LJ. Molecular and biophysical view of the Ca channel: a hypothesis regarding oligomeric structure, channel clustering, and macroscopic current. J Membr Biol. 1993;133:191-202. [Medline] [Order article via Infotrieve]
91.
Rose WC, Balke CW, Wier WG, Marban E.
Macroscopic and unitary properties of physiological ion flux
through L-type Ca2+ channels in guinea-pig heart
cells. J Physiol (Lond). 1992;456:267-284.
92.
Luo C-H, Rudy Y. A dynamic model of the cardiac
ventricular action potential, I: simulations of ionic currents and
concentration changes. Circ Res. 1994;74:1071-1098.
93.
Han S, Schiefer A, Isenberg G.
Ca2+ load of guinea-pig ventricular myocytes
determines efficacy of brief Ca2+ currents as
trigger for Ca2+ release. J Physiol
(Lond). 1994;480:411-421.
94. Berlin JR, Bassani JWM, Bers DM. Intrinsic cytosolic calcium buffering properties of single rat cardiac myocytes. Biophys J. 1994;67:1775-1787. [Medline] [Order article via Infotrieve]
95.
Wettmer E, Amos GJ, Posival H, Ravens U.
Transient outward current in human ventricular myocytes of
subepicardial and subendocardial origin. Circ
Res. 1994;75:473-482.
96. Antzelevitch C, Litovsky SH, Lukas A. Epicardium versus endocardium: electrophysiology and pharmacology. In: Zipes DP, Jalife J, eds. Cardiac Electrophysiology: From Cell to Bedside. Philadelphia, Pa: WB Saunders Co; 1990:386-396.
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|
|
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|
|
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|
|
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|
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|
|
|
|
|
|
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|
|
|
|
|
|
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|
|
|
|
|
|
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|
|
|
|
|
|
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|
|
|
|
|
|
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|
|
|
|
|
|
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|
|
|
|
|
|
L. Hove-Madsen, A. Llach, and L. Tort Quantification of Ca2+ uptake in the sarcoplasmic reticulum of trout ventricular myocytes Am J Physiol Regulatory Integrative Comp Physiol, December 1, 1998; 275(6): R2070 - R2080. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. Tavi, C. Han, and M. Weckstrom Mechanisms of Stretch-Induced Changes in [Ca2+]i in Rat Atrial Myocytes : Role of Increased Troponin C Affinity and Stretch-Activated Ion Channels Circ. Res., November 30, 1998; 83(11): 1165 - 1177. [Abstract] [Full Text] [PDF]
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X.-K. Liu, A. Katchman, M.-D. Drici, S. N. Ebert, I. Ducic, M. Morad, and R. L. Woosley Gender Difference in the Cycle Length-Dependent QT and Potassium Currents in Rabbits J. Pharmacol. Exp. Ther., May 1, 1998; 285(2): 672 - 679. [Abstract] [Full Text]
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A. D Wickenden, R. Kaprielian, Z. Kassiri, J. N Tsoporis, R. Tsushima, G. I Fishman, and P. H Backx The role of action potential prolongation and altered intracellular calcium handling in the pathogenesis of heart failure Cardiovasc Res, February 1, 1998; 37(2): 312 - 323. [Abstract] [Full Text] [PDF]
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A. Yao, H. Matsui, K. W. Spitzer, J. H. B. Bridge, and W. H. Barry Sarcoplasmic reticulum and Na+/Ca2+ exchanger function during early and late relaxation in ventricular myocytes Am J Physiol Heart Circ Physiol, December 1, 1997; 273(6): H2765 - H2773. [Abstract] [Full Text] [PDF]
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G.A. Langer and A. Peskoff Role of the Diadic Cleft in Myocardial Contractile Control Circulation, November 18, 1997; 96(10): 3761 - 3765. [Full Text]
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P. H. Reinhardt, C. A. Ward, W. R. Giles, and P. Kubes Emigrated Rat Neutrophils Adhere to Cardiac Myocytes via {alpha}4 Integrin Circ. Res., August 19, 1997; 81(2): 196 - 201. [Abstract] [Full Text]
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K. W. Linz and R. Meyer Modulation of L-type calcium current by internal potassium in guinea pig ventricular myocytes Cardiovasc Res, January 1, 1997; 33(1): 110 - 122. [Abstract] [Full Text] [PDF]
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C.J. Grantham and M.B. Cannell Ca2+ Influx During the Cardiac Action Potential in Guinea Pig Ventricular Myocytes Circ. Res., August 1, 1996; 79(2): 194 - 200. [Abstract] [Full Text]
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R. Sah, R. J. Ramirez, and P. H. Backx Modulation of Ca2+ Release in Cardiac Myocytes by Changes in Repolarization Rate: Role of Phase-1 Action Potential Repolarization in Excitation-Contraction Coupling Circ. Res., February 8, 2002; 90(2): 165 - 173. [Abstract] [Full Text] [PDF]
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R. Sah, G. Y. Oudit, T.-T. T. Nguyen, H. W. Lim, A. D. Wickenden, G. J. Wilson, J. D. Molkentin, and P. H. Backx Inhibition of Calcineurin and Sarcolemmal Ca2+ Influx Protects Cardiac Morphology and Ventricular Function in Kv4.2N Transgenic Mice Circulation, April 16, 2002; 105(15): 1850 - 1856. [Abstract] [Full Text] [PDF]
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