(Circulation. 1996;93:120-128.)
© 1996 American Heart Association, Inc.
Articles |
High Frequency–Induced Upregulation of Human Cardiac Calcium Currents
From the Centre de Recherches de Biochimie Macromoléculaire, INSERM U 249 (C.P., S.L., J.N., S.R.), and the Service de Chirurgie Thoracique et Cardio-vasculaire, Hôpital Arnaud de Villeneuve (B.A., J.S.), Montpellier, France.
Correspondence to Sylvain Richard, PhD, Centre de Recherches de Biochimie Macromoléculaire, CNRS, UPR 9008, INSERM U 249, Route de Mende, BP 5051, 34033 Montpellier Cedex, France.
 |
Abstract |
|---|
 Top Abstract IntroductionMethods Results Discussion References |
|---|
Background In mammalian heart cells, Ca2+ influx through voltage-gated L-type Ca2+ channels can be upregulated by high rates of stimulation. We have investigated this important adaptive regulation in human cardiomyocytes.
Methods and Results Using the whole-cell
patch-clamp technique, we found a high frequency–induced
upregulation (HFIUR) of the
dihydropyridine-sensitive L-type
Ca2+ current (ICa) in human
cardiomyocytes. ICa was potentiated in a graded
manner with increasing rates of stimulation between 0.3 and 5 Hz. Both
moderate increase of ICa peak amplitude and marked slowing
of current decay contributed to large increases of Ca2+
influx (up to 80%). The maximal potentiation of ICa was
reached rapidly after the change in the rate of stimulation (no more
than a few seconds). ß-Adrenergic stimulation of the cells by
isoproterenol (1 µmol/L), which is well known to induce a slow (
1
minute) cAMP-mediated potentiation of ICa, could
enhance (when present) or promote (when absent) the HFIUR of
ICa. As a consequence, the increasing effect of
isoproterenol on Ca2+ influx through Ca2+
channels was dependent on the rate of stimulation. HFIUR of
ICa was altered in patients with ejection fraction lower
than 40% and in patients pretreated with Ca2+
antagonists or ß-blockers.
Conclusions Upregulation of Ca2+ entry through voltage-gated Ca2+ channels by high rates of beating may be involved in the frequency-dependent regulation of contractility (Bowditch "staircase") of the human heart. This process, which is highly sensitive to ß-adrenergic stimulation, may be crucial in adaptation to exercise and stress.
Key Words: calcium channels • contractility • heart rate • electrophysiology • inotropic agents
|
Introduction |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
Transsarcolemmal influx of Ca2+ into cardiac cells via voltage-gated L-type Ca2+ channels (ICa) plays a major role in the development and control of both heart contractility and pacemaking activity.1 2 3 One interesting adaptive property of these channels, both in frogs and in various mammalian species, is that they can be upregulated after repetitive depolarizations3 4 5 6 7 8 9 but only when applied from negative HPs (lower than –50 mV).10 11 12 Although voltage is the primary effector, modulation of Ca2+ channel activity by a variety of neurotransmitters, hormones, drugs, and intracellular second messengers is also fundamental.1 2 3 For example, ß-adrenergic regulation potentiates cardiac ICa, which results in a shift of the action potential plateau toward a more positive level, a subsequent increase of intracellular free Ca2+, and thereby increased contractility of cardiac cells. The intracellular second messenger cAMP is responsible for most of this stimulation.13 14 15 This regulation is a major physiological way of regulating the beating heart because sympathetic fibers terminate in all parts of the heart and, via the transmitters epinephrine and norepinephrine, induce chronotropic effects on the sinus node and concomitant increases in contractility. In the present study, we show that human cardiac L-type ICa can be upregulated by sudden increases in the rate of cell stimulation. This regulation is enhanced by ß-adrenergic stimulation.
|
Methods |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
Surgery
Most of the present study was conducted on myocytes obtained from the right atrium. Small fragments of the right atrial appendage (0.5 to 1 cm2) were obtained during open heart surgery (before cardiopulmonary bypass) from 33 patients aged 26 to 79 years (Table
), in accordance with institutional
guidelines for human subject research. As shown in the Table,
the
clinical diagnosis was aortic or mitral disease (stenosis or
insufficiency) or coronary artery disease. Most patients
previously had received medication comprising Ca2+-channel
antagonists, ß-adrenergic blockers, and/or ACEI. All
drugs were stopped 24 to 48 hours before surgery. During
anesthesia, all patients received a benzodiazepine,
pancuronium, morphine, pentobarbital, and antibiotics. The present
study also included myocytes obtained from other heart cavities. A
fragment of left atrium was obtained from a patient with mitral
insufficiency and who was treated with ACEI. Fragments of both left and
right ventricles were obtained from 5 patients (aged 38 to 49 years)
undergoing heart transplantation. All of these patients had
ischemic cardiopathies. They were diagnosed as having
end-stage heart failure (New York Heart Association class IV). They
had a severe alteration of left ventricular function and a
low EF (range, 13% to 27%). Therapeutic treatment included only ACEI
(no Ca2+-channel antagonists or
ß-adrenergic blockers). After explantation of the heart, small
tissue samples of the four cavities of the failing heart were removed
and plunged into Tyrode's solution at room temperature within 15
minutes after cardiectomy.
|
Cell Isolation
Fragments were immediately plunged into
Tyrode's solution at
room temperature, and cell dissociation was realized 1 to 2 hours after
removal for the right atria and 6 to 10 hours after removal for the
other cavities. The enzymatic (0.5 mg/mL protease, type 14 [Sigma];
0.6 mg/mL collagenase, type A, Clostridium
histolyticum, and 0.2 mg/mL elastase [Boehringer
Mannheim]) isolation procedure was derived from a procedure detailed
previously.16 Briefly, the Tyrode's solution used for
dissociation and transportation contained (in mmol/L): NaCl 136, KCl
10.8, MgCl2 1.1, dextrose 22, HEPES 25, glutamate 10,
CaCl2 (10 µmol/L), penicillin (60 µg/mL), and
streptomycin (100 µg/mL) and 0.002% phenol red indicator (pH
adjusted to 7.4 with NaOH). After the enzymatic step, dissociation was
achieved by mechanical agitation. Cells were stored in the following
solution (in mmol/L): choline chloride 119,
KH2PO4 1.5, MgCl2 1.7, HEPES 25,
glucose 5.8, succinate 5, pyruvic acid 10, creatine 5, BSA (1 mg/mL),
penicillin (60 µg/mL), and 0.002% phenol red indicator (pH adjusted
to 7.4 with tetraethyl-ammonium hydroxide). Only cells with clear
cross striation and without significant granulation were selected for
experiments (yield, approximately 60%).
Electrophysiological
Recordings
The waveforms of voltage-activated inward
ICa were measured 2 to 10 hours after cell dispersion by
use of the whole-cell patch-clamp technique.17
ICa was recorded no earlier than 5 minutes after the
whole-cell recording configuration was obtained. Conditions
were optimized to eliminate contaminating voltage-gated inward Na
(INa) and outward K (Iout)
currents.10 12 Bath solutions contained (mmol/L):
tetraethyl-ammonium chloride 130, CaCl2 2,
MgCl2 1.1, 4-amino-pyridine 4, HEPES 25, dextrose 22,
and phenol red (17.7 mg/L), adjusted to pH 7.4 with
tetraethyl-ammonium hydroxide, 290 to 310 mOsm. Recording
pipettes contained (in mmol/L): CsCl 130, EGTA 10, HEPES 25, (Mg) ATP
3, and (Mg) GTP 0.4, adjusted to pH 7.4 with CsOH, 290 to 310 mOsm.
Junction potentials between the intrapipette solution and the reference
electrode were canceled before obtaining tight seals. Because a major
limitation of voltage-clamp methods results from the presence of
large series resistance, experiments were performed with use of large
low-resistance pipettes (resistance, 1.5 to 2 M
when filled with
recording solutions). After seal formation (resistance range, 1
to 20 G) and membrane disruption, series resistance (estimated from
the decay of the capacitive transients) was typically 2 to 3 times the
pipette resistance (<6 M) and was electronically compensated by
>80%. Since currents measured at room temperature (20 to 22°C) were
<2 nA in all cells, voltage errors resulting from residual,
uncompensated series resistance (
1.2 M) were <3 mV and could not
introduce major errors. The voltage-clamp circuit was provided by a
Biologic (model RK-300) patch-clamp amplifier. All experimental
parameters, such as HPs, test potentials, and rate of
stimulation, were controlled with an IBM PC connected through a Tecmar
Labmaster analog interface (Axon Instruments) to the
electrophysiological equipment. Data
acquisition and analyses were performed by use of
PCLAMP software (Axon Instruments). Sampling frequencies
ranged from 5 to 10 kHz, and signals were filtered at 3 to 5 kHz before
digitization and storage.
L-Type ICa was routinely recorded
at a test pulse of
–10 mV delivered from a HP of -80 mV. No
low-threshold–activated T-type ICa was
evidenced in atrial or ventricular
myocytes.16 17 18 Although a
tetrodotoxin-sensitive
low-threshold ICa flowing through Na+
channels was recorded at times, this current had no significant
contribution because it was observed only from very negative HPs (95%
inactivated at -80 mV)19 and was small
at a test pulse of -10 mV. Use-dependent facilitation of
ICa was examined by use of trains of depolarizing
stimulations at various rates (see Figure legends). Rest
periods
between trains of stimulations were 
10 seconds. ICa
recorded at the first stimulation of a particular train was defined
as controls because there was no change of ICa waveforms
for low rates of stimulation (<0.3 Hz). In all cells included in the
Table
, the HFIUR of ICa was assessed routinely by
increasing the rate of stimulation from 0.1 Hz to 1 Hz (unless
otherwise noted). Nearly maximal effect was obtained within a few
stimulations (generally at the fourth stimulation, as shown in the
figures). The basal effect of ISO was examined at low rates of
stimulation.
|
Solutions
ISO (Sigma) was prepared as a concentrated stock
solution (10
mmol/L) in H2O that was diluted at the desired working
concentrations in the test solution. Control and test solutions were
applied to the exterior of each cell tested by use of a
multiple-capillary perfusion system (200-µm inner capillary
tubing; flow rate, 0.5 mL/min) placed in the vicinity of the cell
(<0.5 mm). Each capillary was fed by a reservoir 50 cm above the bath.
Rapid (at most, seconds) and complete solution changes can be made by
switching from the opening of one capillary to the next.
Current Measurements
Ca2+ entry was
quantified by integrating
ICa (pAxms) during the test pulse rather than by measuring
peak current. Indeed, the major change in ICa waveform
after an increase in the rate of stimulation occurred mainly in
ICa decay. The increase of peak ICa is only a
consequence of the slowing of current decay.12 Similar
analysis was performed to determine the effect of ISO. Results
are expressed as percent increase of ICa. The Table
includes only cells for which all three tests (ie, HFIUR of
ICa at the basal level, effect of ISO on
ICa, and HFIUR of ICa in the presence of
ISO) have been performed.
Statistical Analyses
Data were averaged, and values
(mean±SD) for individual hearts
are given in the Table. Comparisons between groups of patients
were
performed. Distributions of the quantitative variables, although
close to normal, were not described by gaussian curves (statistical
method of Shapiro-Wilk). Comparisons of the averaged
parameters (HFIUR of ICa at the basal level,
effect of ISO, and HFIUR of ICa in the presence of ISO; see
Table) as a function of drug treatment by Ca2+
antagonists or ß-adrenergic blockers and as a
function of EF were performed by use of Mann-Whitney
nonparametric tests. The group of patients given ACEI
and the untreated group were pooled together because there were no
significant differences between these two groups for the three
parameters examined. The influence of drug treatment and EF
on the variance of the three parameters was studied by use
of two-way ANOVA. Because of the limited number of observations,
interaction among factors was not studied. The above statistical
analysis was performed by use of SAS version
6.08.20 The effect of ISO on HFIUR of ICa was
determined by use of a binomial test based on the sign of the
difference between paired samples.
|
Results |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
HFIUR of ICa in Human Atrial Cells
During depolarization from a HP of -80 mV, ICa through L-type Ca2+ channels typically began to activate at
-30 mV. ICa had a maximum peak
amplitude between -10 and 0 mV and appeared to reverse positive
to +60 mV (data not shown; for details, see References 16 and 18).
ICa was routinely recorded at a test pulse of -10
mV, which evokes maximal current amplitude. When the rate of activation
of ICa was increased routinely from 0.1 Hz to 1 Hz, two
types of responses were observed. In one fraction of the cells, the
high rate of stimulation changed the waveform of ICa (Fig
1A). This was reversible when switching back and forth
between the two rates. In the other fraction of cells, there was no
alteration (Fig 1B). Interestingly, both effects were
consistent in nearly all cells originating from the same
patients. The discrepancy therefore concerned variations between
patients rather than variability among cells from the same patient (see
Table). In other words, patients (rather than cells) could be
divided
into two categories: (1) patients who showed an increase of
Ca2+ entry induced by high rates of stimulation and (2)
patients who lacked this response.
HFIUR of ICa consisted of
both a moderate increase of peak
current amplitude (range, 0% to 40%) and a slowing of current
inactivation. Both effects contributed to increased Ca2+
entry during depolarization (up to 80% in some cells). This increase
reflected a genuine change in Ca2+ channel activity since
the recording conditions used limit the contribution of other
ionic currents, such as voltage-activated or
Ca2+-activated K+
currents.2 10 12 21 The
increase occurred at all
depolarizations that activated an inward current (data not
shown), and the change in current waveform was graded with the rate of
stimulation (Fig 1C). Steady state was reached rapidly (within
seconds). The final waveforms of ICa stabilized between 2
to 10 stimulations regardless of the rate used. In the absence of
spontaneous rundown of Ca2+ channel activity,
ICa waveforms could remain stable for minutes if
voltage-dependent inactivation of Ca2+ channels was
prevented by use of short depolarizations. As previously demonstrated
in animal cardiomyocytes, there was no HFIUR when
ICa was evoked from HPs more positive than -50 mV. In
contrast, repetitive activation induced a decrease of ICa
because of the voltage-dependent inactivation of Ca2+
channels (data not shown). In other words, the rates of recovery of
ICa are voltage dependent and decrease with depolarized
HPs.10 11 12 As a result, an increase in
the rate of
stimulation from HPs -50 mV leads to use-dependent
inactivation (ie, a decrease in Ca2+ channel availability
for opening,12 which is expected to result in a reduced
force of contraction).
ß-Adrenergic Increase of ICa in Human Atrial
Cells
Extracellular application of the ß-adrenergic agonist ISO at
its maximally effective concentration (1 µmol/L) increased
ICa (Fig 2A) for all myocytes from all
patients. However, the effects varied among patients (Table).
There was
a delay of several seconds between application of ISO and onset of the
potentiation. This delay was due in part to the design of the perfusion
system plus the time required between activation of the
ß-receptor, activation of the G protein and intracellular
production of cAMP.13 14 15 It was
followed by a slow
increase of ICa peak amplitude, which reached a plateau
within 30 to 40 seconds (Fig 2B). This time course is thought
to
reflect phosphorylation of the Ca2+
channel by the activated catalytic subunit of protein kinase A,
which constitutes the major pathway by which this stimulation
occurs.1 2 3 13 14 15
As we demonstrated previously in animal
cells,22 ISO induced a marked leftward shift (10 mV) of
both the threshold and maximal activation of ICa (Fig
2C).
This shift leads to greater enhancement of ICa by ISO for
weak depolarizations. ß-Adrenergic stimulation therefore is expected
to enhance Ca2+ entry during the early depolarizing phase
of the action potential.
|
Enhancement of HFIUR of ICa by ISO
We have
investigated HFIUR of ICa in the presence of
ISO. Two sets of results were obtained. In most atrial myocytes in
which HFIUR of ICa was observed at the basal level, HFIUR
was enhanced by ISO (Fig 3A). As shown in the
Table,
this was true for 14 of 17 patients and was highly significant
(P<.006; see "Methods"). In terms of absolute
Ca2+ entry, the frequency-dependent potentiation of
ICa could lead to a large increase compared with that
produced by ISO at a low rate of stimulation (Fig 3A). In many
myocytes
(patients) lacking HFIUR of ICa, HFIUR was promoted
by ISO (Fig 3B; see Table for patients).
|
In contrast to the effects of ISO, high rates of stimulation did not
alter the voltage-dependent activation of Ca2+ channels
(Fig 3C). There was no change in the threshold of activation.
However,
the leftward shifts induced by ISO were conserved (Fig 3C).
Therefore,
the promotion of Ca2+ entry at relatively negative
potentials can be dramatically enhanced by high rates of stimulation.
The major implication of this regulation is that promotion of
Ca2+ entry during the early depolarizing phase of the
action potential by ß-adrenergic stimulation is expected to be
enhanced further by positive chronotropic effects such as those induced
by this stimulation on the beating heart. These effects could
contribute synergistically to both optimized excitability and faster
activation of the contraction of cardiac cells.
Rate-Dependent Modulation of the Effect of ISO on
ICa
The results discussed above suggest that rate of
stimulation is an
important modulator of the effect of ß-adrenergic stimulation on
Ca2+ channel activity, which is clearly illustrated in Fig
4. For the myocyte presented in Fig 4, high
rates of stimulation had no effect on ICa waveform under
control conditions (data not shown). However, HFIUR of ICa
was promoted after continuous ß-adrenergic stimulation by ISO.
Fig 4 clearly shows that the overall effect of ISO on current
waveform
depended crucially on the rate of stimulation, ie, on the
diastolic interval between two stimulations. The
potentiation of ICa, and thereby of Ca2+
entry, by ISO was dramatically enhanced by high rates of stimulation
(up to 75% at 5 Hz). Interestingly, the fine, graded modulation of
ICa occurred over a range of frequencies corresponding to
heart rates frequently encountered in human physiopathology.
|
Variability Among Patients
As indicated above, regulation of
Ca2+ channels by
high rates of activation and by ß-adrenergic stimulation varied
among patients (Table). Bearing in mind the variability of the
results,
the question arose as to whether the differences were significant, ie,
actually due to particular factors. A statistical analysis of
the entire population (33 patients) suggested that two major factors
influenced the HFIUR of ICa, the effect of ISO, and
HFIUR in the presence of ISO. The influence of EF was significant for
all three parameters examined. Significance levels were
P<.006 for basal HFIUR, P<.02 for effect of
ISO, and P<.002 for HFIUR in the presence of ISO. In
addition, the influence of drug treatment (Ca2+
antagonists or ß-adrenergic blockers) on the same
parameters was also significant: P<.0001,
P<.005, and P<.008, respectively.
Data given in
Fig 5 represent patients who were
subdivided into four groups on the basis of the statistical
analysis: untreated and treated patients with low or high EF.
In the untreated group, all patients (15 of 15) with an EF >40% had
Ca2+ channels capable of being upregulated by high rates of
stimulation, whereas the lack of this regulation was observed among
patients with an EF <40% (Table and Fig 5A).
It should be noted that
in this subgroup, ICa also tended to be less responsive to
ISO (Fig 5A), which failed to promote HFIUR (Fig
5C). In contrast, the
percentage increase of ICa by high rates of stimulation was
significantly higher after ISO application for 12 of 15 patients
(P<.02; see "Methods") in the high EF subgroup. Fig
5
also illustrates the effects of drug treatment in the high EF subgroup
of patients in whom HFIUR of ICa was abolished (Fig
5A). In
addition, there was a trend for a diminution of the effect of ISO (Fig
5B). Nevertheless, ISO promoted (or restored) HFIUR of
ICa
in most cases (8 of 11 patients) in which it was not detected under
control conditions (Fig 5C and Table).
|
HFIUR of ICa in Left Atrium and
Ventricles
Because most of the present study was undertaken on right
atrial cells, we tested whether the HFIUR of ICa could be
evidenced in myocytes originating from other parts of the human heart.
All three myocytes isolated from the left atrium of one patient (EF of
70%; treated with ACEI) exhibited HFIUR of ICa at the
basal level as illustrated in Fig 6A. HFIUR of
ICa was enhanced after ISO application (Fig 6A).
Ventricular myocytes also were isolated from left and right
ventricles obtained from five patients (see "Methods") with
end-stage heart failure (EF <27%). Only two patients exhibited
HFIUR of ICa at the basal level (one of two cells from the
left ventricle in one case, two of five cells from the right ventricle
in the other; Fig 6B and 6C). However, most
ventricular
cells isolated from these patients lacked HFIUR of ICa
(3±6% increase; n=11). In addition, ICa in these
cells
seemed to be less responsive to ISO (93±6% increase; n=11)
compared
with cells isolated from the right atrium of patients with EF >40%.
Although ISO could promote HFIUR, in association with a large increase
in ICa in some cells (Fig 6B and 6C),
the effect, on
average, was also smaller (22±26%; n=11). Taken together, these
results suggest that, although uncommonly observed, apparently because
of heart failure, HFIUR of Ca2+ channel activity also may
be a feature of ventricular cells.
|
|
Discussion |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
To our knowledge, the present study provides the first investigation of a rate-dependent regulation of L-type ICa in human cardiomyocytes. Our results suggest that regulation of Ca2+ channel activity by high rates of activation is a major adaptive mechanism of the healthy human heart. HFIUR of ICa may account, at least in part, for the frequency-dependent regulation of heart contractility.23 In addition to abnormal intracellular [Ca2+]i handling,24 25 26 an alteration of transsarcolemmal Ca2+ signaling via voltage-gated Ca2+ channels may be involved in heart failure. Our results also emphasize the novel idea that in addition to increasing the frequency of activation of ICa, the chronotropic effect of ß-adrenergic agents modulates Ca2+ entry via a change in ICa decay that may contribute thereby to an increase in contraction of the beating heart (Fig 7
).
|
HFIUR of ICa in Human Cardiomyocytes
Frequency-dependent regulation of Ca2+ channel
activity has been previously reported in canine,8 guinea
pig,9 11 and
rat7 10 12 21
ventricular myocytes. HFIUR of ICa, described
herein for the first time with human cardiomyocytes,
resembles that described in mammals. HFIUR of ICa has been
detected in myocytes from both right and left atria and ventricles,
suggesting that it is a general property of human cardiac
Ca2+ channels, ie, irrespective of their anatomic origin,
despite some variability among patients. This variability is related to
the degree of left ventricular function (EF <40%) or to
drug pretreatment by Ca2+ antagonists or
ß-blockers that could alter both HFIUR and modulation by ISO. The
basic mechanistic properties of HFIUR of ICa in human
cardiomyocytes described in the present study are
reminiscent of those described in detail in mammalian cells. For
example, in humans (the present study) as well as in other mammals,
HFIUR of cardiac ICa consists of both a moderate increase
in peak current amplitude and a slowing of current decay and occurs
exclusively from negative
HPs.6 8 10 11 12
The increase of
peak ICa is only a consequence of the slowing of current
decay, which could be accounted for by voltage-dependent and
time-dependent changes in the gating of Ca2+
channels.10 11 12 21 A
detailed analysis of
Ca2+ channel gating mechanisms in mammalian
cardiomyocytes has shown that HFIUR reflects a time-,
voltage-, and Ca2+-dependent overshoot in the reactivation
of ICa.8 11 12
Physiological Relevance
The frequency-dependent control of
transmembrane
Ca2+ entry via voltage-gated Ca2+ channels
provides human cardiac cells with a highly sophisticated short-term
system for regulation of intracellular Ca2+ homeostasis.
This regulation is rapid, accurate, and flexible enough to ensure a
fine, graded modulation of Ca2+ entry over a
physiopathological range of heart rates. There is little doubt that
this flexibility could play an important role in the control of cell
excitability, Ca2+ entry, and, thereby, contractile force.
In many animal species, it has been shown that increasing the heart
rate induces a positive inotropic effect, known as the
force-frequency effect or Bowditch "staircase"23
both in isolated cardiac muscle,27 in intact hearts of
anesthetized animals under controlled
hemodynamic conditions,28 and in conscious
animals.29 30 Our results suggest that HFIUR of
transmembrane Ca2+ channels mediates the
force-frequency regulation of myocardium
contractility in humans, at least in part. However, it
may also involve other aspects of heart physiology, such as regulation
of neurosecretion. In this regard, it is interesting to note that
increasing the frequency of atrial contraction directly increases
atrial natriuretic peptide secretion via a
Ca2+-dependent process31 and that this is
enhanced during atrial
tachycardias.32 33 34
Regulation by ß-Adrenergic Stimulation
A well-known
effect of ß-adrenergic stimulation on
cardiac myocytes is a large increase in ICa peak amplitude
and thereby in contraction. This primary effect is related to increased
activity of the pool of channels already capable of being
activated and recruitment of new
channels.1 2 3
Another effect, rarely taken into account but suggested by results of
the present study, concerns enhancement of HFIUR of ICa
that is expected from the positive chronotropism of ß-adrenergic
stimulation on the beating heart. This indirect modulation is related
to changes in the gating properties rather than to an increase in the
number of channels capable of being
activated.10 12 It generates a different signal,
ie, a more sustained entry of Ca2+, due to the
slowing of ICa decay at high rates of stimulation (Fig
7).
This regulation may involve cAMP-dependent
phosphorylation of Ca2+ channels, as has
been demonstrated previously in rat ventricular
cells.21 It is likely that the two routes defined here
account synergistically for the final inotropic effect of
ß-adrenergic agents on contraction of the beating
myocardium (Fig 7). Consistent with this
hypothesis, it has been reported recently35 that
ß-adrenergic stimulation enhances
force-frequency–induced contractile responses in the normal
heart of conscious animals.
Alteration by Heart Failure and Drug Treatment
Several
studies have demonstrated that after an increase in the
rate of stimulation,36 37 38 the force of
contraction
increases in normal hearts but decreases in failing hearts. Stimulation
by ISO preserves the positive force frequency in the nonfailing
myocardium but only partly reverses the negative
force-frequency relationship in the failing human
heart.39 In failing hearts in which the basal cAMP content
of cardiomyocytes is decreased because ß-adrenergic
receptors are downregulated,40 41 the force-frequency
relationship is
altered.36 37 38 39 All of
these observations
are consistent with the present findings of an alteration
of both HFIUR and ß-adrenergic stimulation of ICa.
This could be accounted for, at least in part, by a reduction in the
overall number of functional Ca2+ channels.42
However, the downregulation of ß-adrenergic receptors, which is a
characteristic of failing hearts,41 may also explain the
decreased response to ß-adrenergic stimulation and, as a
consequence, the alteration of HFIUR due to decreased cAMP-dependent
phosphorylation of Ca2+ channels. Indeed,
ß-adrenergic stimulation could not induce HFIUR of
ICa in hearts with reduced left ventricular
function, which contrasted with the promotion of HFIUR in treated
patients with an EF >40%.
HFIUR of ICa was altered in patients pretreated with Ca2+ antagonists and ß-adrenergic blockers (ie, Vaughan-Williams class II and IV antiarrhythmic agents). Since drug administration was stopped 24 to 48 hours before surgery, chronic treatment apparently induces a depression of HFIUR that persists long after removal of drugs. These observations are not new.16 18 At the moment, we have no precise interpretation for this. We can only speculate that (1) some drug (or active metabolite) is still present in cell membranes, or (2) there are long-term modifications induced by these drugs. Treatment with antagonists results in loss of the corresponding receptor (downregulation) and/or loss of its associated response (desensitization).18
In conclusion, HFIUR of Ca2+ channels allows a sudden adjustment of intracellular Ca2+ loading as stimulation frequency rises in the healthy myocardium. We anticipate that this regulation is crucial in the adaptation of the beating heart to stress and exercise. For example, force-frequency effects in the conscious animal are markedly enhanced by exercise43 in relation to reflex norepinephrine release and circulating catecholamines. This regulation is altered by drug treatment but also possibly by the disease itself.37 44
|
Selected Abbreviations and Acronyms |
|---|
|
|
Acknowledgments |
|---|
This work was supported by grants of the Association Recherche et Partage (to S.R.), Fondation Pour la Recherche Medicale, and the Association Française contre les Myopathies. We thank J.M. Davy, T. Barman, B. Crozatier, and C.L. Wolfe for helpful comments on the manuscript and M.C. Picot (Departement d'Information Medicale, CHU de Montpellier) for her assistance with statistical analysis.
|
Footnotes |
|---|
C.P. and S.M. are to be considered joint first authors.
Received March 14, 1995; revision received August 16, 1995; accepted August 20, 1995.
|
References |
|---|
|
TopAbstractIntroductionMethods Results Discussion References |
|---|
1. Hartzell HC. Regulation of cardiac ion channels by catecholamines, acetylcholine and second messenger system. Prog Biophys Mol Biol. 1988;52:165-247. [Medline] [Order article via Infotrieve]
2.
McDonald TF, Pelzer S, Trautwein W, Pelzer D.
Regulation and modulation of calcium channels in cardiac, skeletal, and
smooth muscle cells. Physiol Rev. 1994;74:365-507.
3. Tsien RW, Bean BP, Hess P, Lansman JB, Nilius B, Nowycky MC. Mechanisms of calcium channel modulation by ß-adrenergic agents and dihydropyridine calcium agonists. J Mol Cell Cardiol. 1986;18:691-710. [Medline] [Order article via Infotrieve]
4.
Noble S, Shimoni Y. The calcium and frequency
dependence of the slow inward current `staircase' in frog
atrium. J Physiol (Lond). 1981;310:57-75.
5. Schouten VJA, Morad M. Regulation of Ca2+ current in frog ventricular myocytes by the holding potential, c-AMP and frequency. Pflugers Arch. 1989;415:1-11. [Medline] [Order article via Infotrieve]
6.
Lee KS. Potentiation of the calcium-channel
currents of internally perfused mammalian heart cells by repetitive
depolarization. Proc Natl Acad Sci U S A. 1987;84:3941-3945.
7.
Fedida D, Noble D, Spindler AJ.
Use-dependent reduction and facilitation of Ca2+
current in guinea-pig myocytes. J Physiol
(Lond). 1988;405:439-460.
8.
Tseng GN. Calcium current restitution in
mammalian ventricular myocytes is modulated by
intracellular calcium. Circ Res. 1988;63:468-482.
9.
Zygmundt AC, Maylie J.
Stimulation-dependent facilitation of the high threshold calcium
current in guinea-pig ventricular myocytes.
J Physiol (Lond). 1990;428:653-671.
10.
Richard S, Tiaho F, Charnet P, Nargeot J, Nerbonne
JM. Two pathways for Ca2+ channel gating
differentially modulated by physiological
stimuli. Am J Physiol. 1990;258:H1872-H1881.
11. Peineau N, Garnier D, Argibay JA. Rate dependence of action potential duration and calcium current in isolated guinea-pig cardiocytes. Exp Physiol. 1992;77:615-625. [Abstract]
12.
Richard S, Charnet P, Nerbonne JM.
Voltage-and time-dependent interconversion between distinct
gating pathways of the high threshold cardiac calcium channel.
J Physiol (Lond). 1993;462:197-228.
13.
Nargeot J, Lester H, Nerbonne JM, Engels J. Time
course of the increase in the myocardial slow inward current after a
photochemically generated concentration jump of intracellular
cAMP. Proc Natl Acad Sci U S A. 1983;80:2395-2399.
14. Nerbonne JM, Richard S, Nargeot J, Lester HA. New photoactivable cyclic nucleotides produce intracellular jumps in cyclic AMP and cyclic GMP concentrations. Nature. 1984;310:74-76. [Medline] [Order article via Infotrieve]
15. Charnet P, Richard S, Gurney AM, Ouadid H, Tiaho F, Nargeot J. Modulation of Ca currents in isolated frog atrial cells studied with photosensitive probes: regulation by cAMP and Ca2+—a common pathway? J Mol Cell Cardiol. 1991;23:343-356. [Medline] [Order article via Infotrieve]
16. Ouadid H, Séguin J, Richard S, Chaptal PA, Nargeot J. Properties and modulation of Ca channels in adult human atrial cells. J Mol Cell Cardiol. 1991;23:41-54. [Medline] [Order article via Infotrieve]
17. Hamill OP, Marty A, Neher E, Sakmann B, Sigworth FJ. Improved patch-clamp techniques for high resolution current recording from cells and cell-free membrane patches. Pflugers Arch. 1981;391:85-100.[Medline] [Order article via Infotrieve]
18.
Le Grand BE, Hatem S, Deroubaix E, Couetil JP,
Coraboeuf E. Calcium current depression in isolated human atrial
myocytes after cessation of chronic treatment with calcium
antagonists. Circ Res. 1992;69:292-300.
19. Lemaire S, Piot C, Seguin J, Nargeot J, Richard S. Tetrodotoxin-sensitive Ca2+ and Ba2+ currents in human atrial cells. Receptors Channels. 1995;3:71-81. [Medline] [Order article via Infotrieve]
20. SAS/STAT User's Guide, Version 6. 4th ed. Cary, NC: SAS Institute, Inc; 1989;2:846.
21.
Tiaho F, Piot C, Nargeot J, Richard S.
Regulation of the frequency-dependent facilitation of L-type
calcium currents in rat ventricular cells.
J Physiol (Lond). 1994;477:237-252.
22. Tiaho F, Nargeot J, Richard S. Voltage-dependent regulation of L-type cardiac Ca2+ channels by isoproterenol. Pflugers Arch. 1991;419:596-602.[Medline] [Order article via Infotrieve]
23. Bowditch HP. Uber die Eigenthûmlichkeiten der Reizbarkeit, welche die Muskelfarsen des Herzens zeigen. Arb Physiol Anst Leipzig. 1871;6:139-176.
24.
Gwathmey JK, Copelas L, MacKinnon R, Schoen FJ, Feldman
MD, Grossman W, Morgan JP. Abnormal intracellular calcium
handling in myocardium from patients with end-stage
heart failure. Circ Res. 1987;61:70-76.
25. Gwathmey JK, Slawsky MT, Hajjar RJ, Briggs GM, Morgan JP. Role of intracellular calcium handling in force-interval relationship of human ventricular myocardium. J Clin Invest. 1990;85:1599-1613.
26.
Beuckelmann DJ, Näbauer M, Erdmann E.
Intracellular calcium handling in isolated ventricular
myocytes from patients with terminal heart failure.
Circulation. 1992;85:1046-1055.
27.
Koch-Weser J, Stinis JT. The influence of the
interval between beats on myocardial
contractility. Pharmacol Rev. 1963;15:601-652.
28.
Covell JW, Ross J Jr, Taylor R, Sonnenblick EH,
Braunwald E. Effects of increasing frequency of contraction on
the force velocity relation of the left ventricle.
Cardiovasc Res. 1967;1:2-8.
29.
Arentzen CE, Rankin JS, Anderson PAW, Feezor MD,
Anderson RW. Force-frequency characteristics of the left
ventricle in the conscious dog. Circ Res. 1978;42:64-71.
30. Freeman GL, Colston JT. Role of ventricular coupling in cardiac response to increased contractility in closed-chest dogs. J Clin Invest. 1990;86:1278-1284.
31. Schiebinger RJ, Li Y, Cragoe EJ. Calcium dependency of frequency-stimulated atrial natriuretic peptide secretion. Hypertension. 1994;23(part 1):710-716.
32. Schriffrin EL, Gutkowska J, Kuchel O, Cantin M, Genest J. Plasma concentration of atrial natriuretic factor in a patient with paroxysmal atrial tachycardia. N Engl J Med. 1985;312:1196-1197. [Medline] [Order article via Infotrieve]
33. Nicklas JM, DiCarlo LA, Koller PT, Morady F, Diltz EA, Shenker Y, Grekin RJ. Plasma levels of immunoreactive atrial natriuretic factor increase during supraventricular tachycardia. Am Heart J. 1986;112:923-928. [Medline] [Order article via Infotrieve]
34. Kojima S, Akabane S, Ohe T, Tsuchihashi K, Yamamoto K, Kuramochi M, Shimomura K, Ito K, Omae T. Plasma atrial natriuretic polypeptide and polyurea during paroxysmal tachycardia in Wolff-Parkinson-White syndrome patients. Nephron. 1986;44:249-252. [Medline] [Order article via Infotrieve]
35.
Kambayashi M, Miura T, Oh BH, Rockman HA, Murata K,
Roos J. Enhancement of the force-frequency effect on
myocardial contractility by adrenergic stimulation in
conscious dogs. Circulation. 1992;86:572-580.
36. Feldman MD, Gwathmey JK, Phillips P, Schoen F, Morgan JP. Reversal of the force-frequency relationship in working myocardium from patients with endstage heart failure. J Appl Cardiol. 1988;3:273-283.
37.
Mulieri LA, Hasenfuss G, Leavitt B, Allen PD, Alpert
NR. Altered myocardial force-frequency relation in human
heart. Circulation. 1992;85:1743-1750.
38. Bôhm M, La Rosee K, Schmidt U, Schulz C, Schwinger RHG, Erdmann E. Force-frequency relationship and inotropic stimulation in the nonfailing and failing human myocardium: implication for the medical treatment of heart failure. Clin Invest. 1992;70:421-425. [Medline] [Order article via Infotrieve]
39.
Schwinger RHG, Bohm M, Muller-Ehmensen J, Uhlman R,
Schmidt U, Stablein A, Uberfuhr P, Kreuzer E, Reichart B, Eissner HJ,
Erdman E. Effect of inotropic stimulation of the negative
force-frequency relationship in the failing human heart.
Circulation. 1993;88:2267-2276.
40. Bristow MR, Ginsburg R, Minobe W, Cubiciotti RS, Sageman V, Lurie K, Billingham ME, Harrison DC, Stinson EB. Decreased catecholamine sensitivity and beta-adrenergic-receptor density in failing human hearts. N Engl J Med. 1982;307:205-211. [Abstract]
41.
Muntz KH, Zhao M, Miller JC. Downregulation of
myocardial ß-adrenergic receptors: receptor subtype
selectivity. Circ Res. 1994;74:369-375.
42. Ouadid H, Albat B, Nargeot J. Calcium currents in diseased human cardiac cells. J Cardiovasc Pharmacol. 1995;25:282-291.[Medline] [Order article via Infotrieve]
43.
Miura T, Miyazaki S, Guth SD, Kambayashi M, Roos
J. Influence of the force-frequency relation on left
ventricular function during exercise in conscious
dogs. Circulation. 1992;86:563-571.
44. Pieske B, Hasenfuss G, Holubarsch R, Shwinger R, Bôhm M, Just H. Alteration of the force-frequency relationship in the failing human heart depends on the underlying cardiac disease. Suppl Basic Res Cardiol. 1992;87:213-221.
This article has been cited by other articles:
 |
|
 |
|
  V. Bito, F. R. Heinzel, L. Biesmans, G. Antoons, and K. R. Sipido Crosstalk between L-type Ca2+ channels and the sarcoplasmic reticulum: alterations during cardiac remodelling Cardiovasc Res, January 15, 2008; 77(2): 315 - 324. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Altamirano and D. M. Bers Effect of intracellular Ca2+ and action potential duration on L-type Ca2+ channel inactivation and recovery from inactivation in rabbit cardiac myocytes Am J Physiol Heart Circ Physiol, July 1, 2007; 293(1): H563 - H573. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 A. Grosu, T. Bombardini, M. Senni, V. Duino, M. Gori, and E. Picano End-systolic pressure/volume relationship during dobutamine stress echo: a prognostically useful non-invasive index of left ventricular contractility Eur. Heart J., November 2, 2005; 26(22): 2404 - 2412. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. A. Mulieri, M. D. Tischler, B. J. Martin, B. J. Leavitt, F. P. Ittleman, N. R. Alpert, and M. M. LeWinter Regional differences in the force-frequency relation of human left ventricular myocardium in mitral regurgitation: implications for ventricular shape Am J Physiol Heart Circ Physiol, May 1, 2005; 288(5): H2185 - H2191. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 T. Bombardini, M. Agrusta, N. Natsvlishvili, F. Solimene, R. Pap, F. Coltorti, A. Varga, G. Mottola, and E. Picano Noninvasive assessment of left ventricular contractility by pacemaker stress echocardiography Eur J Heart Fail, March 2, 2005; 7(2): 173 - 181. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. Brixius, H. Reuter, W. Bloch, and R. H.G. Schwinger Altered hetero- and homeometric autoregulation in the terminally failing human heart Eur J Heart Fail, January 1, 2005; 7(1): 29 - 35. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 G. F. Tomaselli and D. P. Zipes What Causes Sudden Death in Heart Failure? Circ. Res., October 15, 2004; 95(8): 754 - 763. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
F. Brette, L. Salle, and C. H. Orchard Differential Modulation of L-type Ca2+ Current by SR Ca2+ Release at the T-Tubules and Surface Membrane of Rat Ventricular Myocytes Circ. Res., July 9, 2004; 95(1): e1 - e7. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J Guo and H J Duff Inactivation of ICa-L is the major determinant of use-dependent facilitation in rat cardiomyocytes J. Physiol., March 15, 2003; 547(3): 797 - 805. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
J. Fauconnier, S. Bedut, J.-Y. Le Guennec, D. Babuty, and S. Richard Ca2+ current-mediated regulation of action potential by pacing rate in rat ventricular myocytes Cardiovasc Res, March 1, 2003; 57(3): 670 - 680. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 R. B. Abraham, M. Matza, S. Marmor, V. Rudick, I. Frolkis, I. Shapira, and A. A. Weinbroum Electromechanical impairment of human auricle and rat myocardial strip subjected to exogenous oxidative stress Eur. J. Cardiothorac. Surg., January 1, 2003; 23(1): 66 - 73. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
M. Petretta, M. L.E. Vicario, L. Spinelli, A. Ferro, A. Cuocolo, M. Condorelli, and D. Bonaduce Combined effect of the force-frequency and length-tension mechanisms on left ventricular function in patients with dilated cardiomyopathy Eur J Heart Fail, December 1, 2002; 4(6): 727 - 735. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. Herr, N. Smyth, S. Ullrich, F. Yun, P. Sasse, J. Hescheler, B. Fleischmann, K. Lasek, K. Brixius, R. H. G. Schwinger, et al. Loss of Annexin A7 Leads to Alterations in Frequency-Induced Shortening of Isolated Murine Cardiomyocytes Mol. Cell. Biol., July 1, 2001; 21(13): 4119 - 4128. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. J. F. Tucci, N. Murad, C. L. Rossi, R. J. Nogueira, and O. Santana Jr. Heart rate modulates the slow enhancement of contraction due to sudden left ventricular dilation Am J Physiol Heart Circ Physiol, May 1, 2001; 280(5): H2136 - H2143. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 F. Lombardi, A. Colombo, B. Basilico, R. Ravaglia, M. Garbin, D. Vergani, P. M. Battezzati, and C. Fiorentini Heart rate variability and early recurrence of atrial fibrillation after electrical cardioversion J. Am. Coll. Cardiol., January 1, 2001; 37(1): 157 - 162. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. S. Maier, P. Barckhausen, J. Weisser, I. Aleksic, M. Baryalei, and B. Pieske Ca2+ handling in isolated human atrial myocardium Am J Physiol Heart Circ Physiol, September 1, 2000; 279(3): H952 - H958. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. Barrere-Lemaire, C. Piot, F. Leclercq, J. Nargeot, and S. Richard Facilitation of L-type calcium currents by diastolic depolarization in cardiac cells: impairment in heart failure Cardiovasc Res, August 1, 2000; 47(2): 336 - 349. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. D. Prabhu and G. L. Freeman Altered LV inotropic reserve and mechanoenergetics early in the development of heart failure Am J Physiol Heart Circ Physiol, March 1, 2000; 278(3): H698 - H705. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
L. S Maier, C. Schwan, W. Schillinger, K. Minami, U. Schutt, and B. Pieske Gingerol, isoproterenol and ouabain normalize impaired post-rest behavior but not force-frequency relation in failing human myocardium Cardiovasc Res, March 1, 2000; 45(4): 913 - 924. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
U Ravens and D Dobrev Regulation of sarcoplasmic reticulum Ca2+-ATPase and phospholamban in the failing and nonfailing heart Cardiovasc Res, January 1, 2000; 45(1): 245 - 252. [Full Text] [PDF]
|
|
|
|
|
|
S. E. Bates and A. M. Gurney Use-dependent facilitation and depression of L-type Ca2+ current in guinea-pig ventricular myocytes: modulation by Ca2+ and isoprenaline Cardiovasc Res, November 1, 1999; 44(2): 381 - 389. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 E. Carmeliet Cardiac Ionic Currents and Acute Ischemia: From Channels to Arrhythmias Physiol Rev, July 1, 1999; 79(3): 917 - 1017. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. F. Tomaselli and E. Marban Electrophysiological remodeling in hypertrophy and heart failure Cardiovasc Res, May 1, 1999; 42(2): 270 - 283. [Full Text] [PDF]
|
|
|
|
|
|
B. O'Rourke, D. A. Kass, G. F. Tomaselli, S. Kaab, R. Tunin, and E. Marban Mechanisms of Altered Excitation-Contraction Coupling in Canine Tachycardia-Induced Heart Failure, I : Experimental Studies Circ. Res., March 19, 1999; 84(5): 562 - 570. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G.-R. Li, B. Yang, J. Feng, R. F. Bosch, M. Carrier, and S. Nattel Transmembrane ICa contributes to rate-dependent changes of action potentials in human ventricular myocytes Am J Physiol Heart Circ Physiol, January 1, 1999; 276(1): H98 - H106. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
S. D. Prabhu Ryanodine and the left ventricular force-interval and relaxation-interval relations in closed-chest dogs: insights on calcium handling Cardiovasc Res, December 1, 1998; 40(3): 483 - 491. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 F. Schroder, R. Handrock, D. J. Beuckelmann, S. Hirt, R. Hullin, L. Priebe, R. H. G. Schwinger, J. Weil, and S. Herzig Increased Availability and Open Probability of Single L-Type Calcium Channels From Failing Compared With Nonfailing Human Ventricle Circulation, September 8, 1998; 98(10): 969 - 976. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
C Mittmann, T Eschenhagen, and H Scholz Cellular and molecular aspects of contractile dysfunction in heart failure Cardiovasc Res, August 1, 1998; 39(2): 267 - 275. [Full Text] [PDF]
|
|
|
|
 |
|
 K. S. Lee and E. W. Lee Ionic Mechanism of Ibutilide in Human Atrium: Evidence for a Drug-Induced Na+ Current Through a Nifedipine Inhibited Inward Channel J. Pharmacol. Exp. Ther., July 1, 1998; 286(1): 9 - 22. [Abstract] [Full Text]
|
|
|
|
|
|
G. Hasenfuss Animal models of human cardiovascular disease, heart failure and hypertrophy Cardiovasc Res, July 1, 1998; 39(1): 60 - 76. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G. Hasenfuss Alterations of calcium-regulatory proteins in heart failure Cardiovasc Res, February 1, 1998; 37(2): 279 - 289. [Full Text] [PDF]
|
|
|
|
|
|
S. Richard, F. Leclercq, S. Lemaire, C. Piot, and J. Nargeot Ca2+ currents in compensated hypertrophy and heart failure Cardiovasc Res, February 1, 1998; 37(2): 300 - 311. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
P. P de Tombe Altered contractile function in heart failure Cardiovasc Res, February 1, 1998; 37(2): 367 - 380. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
K. R Sipido, T. Stankovicova, W. Flameng, J. Vanhaecke, and F. Verdonck Frequency dependence of Ca2+ release from the sarcoplasmic reticulum in human ventricular myocytes from end-stage heart failure Cardiovasc Res, February 1, 1998; 37(2): 478 - 488. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
R. G. Tieleman, C. D. J. De Langen, I. C. Van Gelder, P. J. de Kam, J. Grandjean, K. J. Bel, M. C. E. F. Wijffels, M. A. Allessie, and H. J. G. M. Crijns Verapamil Reduces Tachycardia-Induced Electrical Remodeling of the Atria Circulation, April 1, 1997; 95(7): 1945 - 1953. [Abstract] [Full Text]
|
|
|
|
|
|
C. Dumitrescu, P. Narayan, I. R. Efimov, Y. Cheng, M. J. Radin, S. A. McCune, and R. A. Altschuld Mechanical alternans and restitution in failing SHHF rat left ventricles Am J Physiol Heart Circ Physiol, April 1, 2002; 282(4): H1320 - H1326. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||