| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Cardiovascular Division, The Lankenau Hospital and Medical Research Center, Wynnewood 19096; and Department of Medicine, Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania 19107
 |
ABSTRACT |
|---|
Top
Abstract
Introduction
Methods
Results
Discussion
References
|
|---|
Recent studies indicate that regression of left ventricular hypertrophy (LVH) normalizes the in situ electrophysiological abnormalities of the left ventricle. This study was designed to determine whether regression of LVH also normalizes the abnormalities of individual membrane currents. LVH was induced in rabbits by renal artery banding. Single ventricular myocytes from rabbits with LVH at 3 mo after renal artery banding demonstrated increased cell membrane capacitance, prolonged action potential duration, decreased inward rectifier K+ current density, and increased transient outward K+ current density compared with myocytes from age-matched controls. Additional rabbits were randomized at 3 mo after banding to treatment with either vehicle or captopril for an additional 3 mo. Myocytes from LVH rabbits treated with vehicle showed persistent membrane current abnormalities. However, myocytes isolated from LVH rabbits treated with captopril had normal cell membrane capacitance, action potential duration, and membrane current densities. Captopril had no direct effect on membrane currents of either control or LVH myocytes. These data support the hypothesis that the action potential prolongation and membrane current abnormalities of LVH are reversed by regression. Normalization of membrane currents probably explains the reduced vulnerability to ventricular arrhythmia observed in this LVH model after treatment with captopril.
renovascular hypertension; action potential; inward rectifier potassium current; transient outward potassium current; L-type calcium current
| |
INTRODUCTION |
|---|
|
Top
Abstract
Introduction
Methods
Results
Discussion
References
|
|---|
LEFT VENTRICULAR HYPERTROPHY (LVH) is associated with an increased risk of sudden cardiac death that is thought to be due to malignant ventricular arrhythmia. The increased vulnerability to ventricular arrhythmia appears to be the result of action potential prolongation and altered repolarization (1, 6). The changes in action potential duration and repolarization are not uniform (10) and are associated with an increased vulnerability of inducible arrhythmia (12, 13, 24). Cellular studies indicate that prolongation of the action potential in response to LVH appears to be the result of a summation of changes in the transient outward K+ current (Ito), delayed and inward rectifier K+ currents (IK and IK1, respectively), and the L-type Ca2+ current (ICa,L) (6).
Recent evidence in animal models of LVH suggests that regression of LVH is associated with normalization of ventricular electrophysiology (17, 20). These studies used either a constricting aortic band in cats (17) or renovascular hypertension in rabbits (20) to produce LVH. Regression of LVH was observed after removal of the aortic band or treatment with captopril and was associated with normal in situ ventricular electrophysiology in both animal models (17, 20).
The present study was designed to determine whether regression of LVH produced by angiotensin-converting enzyme (ACE) inhibition would also reverse the ion channel abnormalities associated with LVH. ACE inhibition was chosen as a means of producing regression of LVH because ACE inhibitors (ACEI) have no significant in situ electrophysiological effect on normal or hypertrophied hearts (19, 20, 26), have been shown to reliably produce regression of hypertrophy in this animal model (20), and are available clinically for the treatment of hypertension.
| |
METHODS |
|---|
|
Top
Abstract
Introduction
Methods
Results
Discussion
References
|
|---|
This study was carried out in conformance with the guidelines published by the American Heart Association, and the protocol was approved by the Lankenau Medical Research Center Animal Use Committee.
Experimental Groups
Adult New Zealand White rabbits (1.8-2.2 kg) underwent unilateral nephrectomy and placement of a constricting clip on the contralateral renal artery to produce LVH using techniques reported previously (20). Rabbits banded in this manner uniformly develop LVH within 3 mo (20). Ventricular myocytes were isolated from rabbits 3 mo after banding (LVH 3-mo group) and from-age matched controls (control 3-mo group) for cellular electrophysiological study.Additional rabbits underwent unilateral nephrectomy with contralateral
renal artery banding to produce LVH. Three months after banding, these
rabbits were treated for an additional 3 mo with either captopril at a
dose of 5 mg · kg
1 · day1
(regress group) or vehicle (LVH 6-mo group). Age-matched control rabbits received vehicle added to their diet for 3 mo (control 6-mo
group). The last oral administration of captopril was given ~24 h
before death.
Myocyte Isolation
Single ventricular myocytes were isolated enzymatically using perfusion techniques reported previously (20). At the end of perfusion, noncardiac tissue attached to the heart was trimmed and the weight subtracted from the total to obtain the net heart weight. To minimize the influence of transmural variability on action potential duration (APD) or membrane currents, we studied myocytes from the midlayer of the myocardial tissue only. The posterior wall of the left ventricle was cut off, and the epicardial and endocardial tissues were carefully trimmed. The central one-third of the tissue was minced in high K+, low Cl solution containing (in
mmol/l) 80 potassium glutamate, 20 K2HPO4, 20 KCl, 5 MgCl2, 0.5 K2EGTA, 2 Na2ATP, 5 Na-pyruvic acid, 5 creatine, 20 taurine, 10 glycine, 10 glucose, and 5 HEPES with the
addition of 0.05 mg/ml DNase I, and myocytes were dispersed by
agitation. The resulting cell suspension was filtered through 290-µm
nylon mesh. Ten minutes after dispersion, cells were transferred to a
HEPES-buffered 1 mmol/l Ca2+
Tyrode solution containing (in mmol/l) 137 NaCl, 5 KCl, 1 MgCl2, 1 CaCl2, 10 glucose, and 10 HEPES,
with pH adjusted to 7.4 with NaOH, and stored at 10°C.
Cellular Electrophysiological Study
Action potentials were recorded at 37 ± 0.1°C with the use of the conventional microelectrode technique in the bridge mode of Axoclamp-2A (Axon Instruments, Foster City, CA). Microelectrodes were fabricated with a vertical pipette puller (model 730, David Kopf Instruments, Tujunga, CA) and had resistances of 30-50 M
when
filled with 3 mol/l KCl. Cells were placed in a water-jacketed plastic
chamber that was continuously perfused with 2 mmol/l
Ca2+ Tyrode solution. After
microelectrode penetration, the cell was continuously stimulated by a
2-ms suprathreshold pulse repeated at 2 Hz. Action potentials were
recorded using pCLAMP software (version 5.5.1, Axon Instruments) at a
sampling rate of 2 kHz.
Whole cell membrane currents were recorded at a room temperature of
23-24°C with the use of the whole cell patch-clamp technique in the voltage-clamp mode of Axopatch-1C (Axon Instruments).
Axopatch-1C was interfaced with a personal computer through a TL-1 DMA
interface (Axon Instruments). pCLAMP software was used for data
acquisition and analysis. Room temperature was used because the large
amplitudes of IK1
and ICa,L at
physiological temperatures precluded an adequate voltage clamp.
Ito current was
also measured at room temperature for consistency, although some
Ito experiments
were repeated at 37°C. Patch pipettes were pulled from borosilicate
glass tubing on a horizontal puller P-80/PC (Sutter Instruments,
Novato, CA) and had resistances of 0.5-1.5 M when filled with
pipette solutions. Cell capacitance and series resistance were
determined from the current transient induced by a hyperpolarization
voltage stepping from 
40 to 50 mV when ionic currents
were either inactivated or blocked. Series resistances were usually no
larger than 3 M and were compensated electronically to the maximal
extent before oscillation occurred.
IK1.
When IK1 was
recorded, cells were held at 40 mV to inactivate
Na+ current.
ICa,L was blocked
by Cd2+ in the bath solution (in
mmol/l: 140 NaCl, 5 KCl, 1 CaCl2,
1 MgCl2, 10 glucose, 10 HEPES, and
0.3 CdCl2, with pH adjusted to 7.4 with NaOH). The pipette solution contained (in mmol/l) 120 potassium
aspartate, 20 KCl, 10 HEPES, 5 K2EGTA, 1 MgCl2, and 6 MgATP, with pH
adjusted to 7.3 with KOH.
IK1 was recorded
at membrane potentials between 100 and 30 mV to avoid
activating voltage-dependent K+
currents. Test pulses with a duration of 450 ms were applied in 2-s
intervals. The amplitude of
IK1 was measured
at the end of the test pulses.
ICa,L.
To isolate ICa,L,
Na+ currents were inactivated by
holding the cell at 40 mV.
K+ currents were blocked by
replacing K+ with
Cs+ in both bath and pipette
solutions and including tetraethylammonium (TEA) chloride in the
pipette solution. The bath solution contained (in mmol/l) 140 NaCl, 5 CsCl, 1 MgCl2, 2 CaCl2, 10 glucose, and 10 HEPES,
with pH adjusted to 7.4 with NaOH. The pipette solution contained (in
mmol/l) 151 CsOH, 10 L-aspartic
acid, 20 taurine, 20 TEA chloride, 5 glucose, and 10 EGTA, with pH
adjusted to 7.5 with
H3PO4.
MgATP (5 mmol/l) and GTP (sodium salt; 0.4 mmol/l) were added to the
pipette solution immediately before use, and the final pH was ~7.3.
40 mV to a
test potential of 30 to 60 mV in 10-mV increments. The pulses
were given in an 8-s interval. For
ICa,L recordings,
currents were sampled at 2.5 kHz. The
Ca2+ conductance at each potential
was determined from the equation g = I/(Em Erev),
where g is the membrane
Ca2+ conductance,
I is the magnitude of
ICa,L at a given
membrane potential
(Em), and
Erev is the
apparent reversal potential determined from the current-voltage
relationship of
ICa,L. The
voltage dependency of the peak conductance was determined by dividing
the peak conductance at each potential by the maximal peak conductance
(g/gmax)
and plotting each value against the test potential. The voltage
dependency of inactivation was determined by holding the cell for 2 s
at potentials ranging from 40 to 10 mV, after which a 180-ms
test pulse to 15 mV was given to fully activate the current. The amount of inactivation at each holding potential was calculated by dividing the magnitude of the current recorded from each holding potential by
the largest current obtained at a 35 mV holding potential (I/Imax)
and plotting that value against the holding potential.
Ito.
When Ito was
studied, currents through Na+
channels were eliminated by substituting
N-methyl-D-glucamine
for Na+.
Ca2+ currents were blocked by 0.3 mmol/l Cd2+ in the bath solution,
and Ca2+-activated
K+ current was minimized by
buffering the pipette solution with 5 mmol/l EGTA.
IK1 was blocked
by 0.1 mmol/l BaCl2. The bath
solution contained (in mmol/l) 140 N-methyl-D-glucamine
chloride, 5 KCl, 1 MgCl2, 1 CaCl2, 10 glucose, 10 HEPES, 0.3 CdCl2, and 0.1 BaCl2, with pH adjusted to 7.4 with
N-methyl-D-glucamine.
The same pipette solution used for
IK1 recording was
used for Ito
recording. Junction voltage of ~10 mV arising from the use of
potassium aspartate was subtracted to correct for the results of both
IK1 and
Ito. The
voltage-clamp protocol used for recording the current-voltage relationship of
Ito consisted of
180-ms pulses stepping from a holding potential of 90 mV to a
test potential of 30 to 40 mV in 10-mV increments. Ten seconds
were allowed for the recovery of
Ito between
pulses.
90 mV.
Ito tail currents
were measured on return to a constant membrane potential of 30
mV from a number of test potentials. The test potentials were from 20 to 30 mV in 10-mV increments. The duration of test potentials decreased from 10.5 to 9.0 ms in a 0.3-ms decrement, because
Ito reached
maximal activation sooner at more depolarized test potentials. Tail
current increased as the step potential became more depolarized. Activation of Ito
was measured as the ratio
I/Imax,
in which I is the tail current after a
short step to various depolarizing potentials and
Imax is the
maximal tail current after a step to 30 mV. Voltage-dependent
inactivation was measured with a 270-ms test pulse to 30 mV from a
series of holding potentials ranging from 80 to 10 mV.
Ito measured at
various holding potentials were normalized to the maximal
Ito obtained at a
holding potential of 80 mV. The recovery of
Ito from
inactivation was studied using a double-pulse protocol. Two
depolarizing pulses (from 90 to 20 mV, 180 ms) with a varying
interpulse interval were applied every 60 s. The magnitude of
Ito elicited by
the second pulse was expressed as a percentage of the
Ito during the
first pulse and plotted against the interpulse duration.
Acute effects of captopril on membrane currents. To determine whether captopril had any direct effect on membrane currents in this animal model, the current-voltage relationship of each of the membrane currents was examined before and after superfusion of the myocytes with 10 µM captopril. Myocytes from both control 3-mo and LVH 3-mo groups were studied using the same experimental techniques described above.
Data Analysis
Results are expressed as means ± SE. Statistical analysis was performed using GraphPad Prism 2.0 (GraphPad Software). A Student's t-test was used for comparisons between the LVH 3-mo and control 3-mo groups. Other comparisons were made using one-way ANOVA. Statistical difference was determined as P < 0.05.| |
RESULTS |
|---|
|
Top
Abstract
Introduction
Methods
Results
Discussion
References
|
|---|
Regression of LVH with Captopril
LVH developed 3 mo after renal artery banding and contralateral nephrectomy in this animal model. All rabbits in the LVH 3-mo group developed hypertrophy, manifested as a 23% increase in the heart weight-to-body weight ratio (HW/BW) and a 35% increase in the cell membrane capacitance (Cm) (Table 1). HW/BW was not significantly different between LVH 3-mo and LVH 6-mo groups, indicating that no additional hypertrophy developed beyond that manifested at 3 mo. Treatment with captopril beginning 3 mo after renal artery banding and continuing for an additional 3 mo caused regression of hypertrophy as evidenced by normalization of HW/BW and Cm (regress group, Table 1).
|
Effect of Regression on APD
APD measured at 60 (APD60) and 90% repolarization (APD90) was prolonged in myocytes isolated from rabbits in the LVH 3-mo group, as has been reported in this (20) and other animal models. Action potential prolongation persisted without further increase at 6 mo after renal artery banding (LVH 6-mo group, Table 1). APD measured at both 60 and 90% repolarization was normal in captopril-treated rabbits [regress group, P = not significant (NS) vs. control 6-mo group; Table 1 and Fig. 1; see Ref 20]. Resting membrane potential and APD at 30% repolarization (APD30) were not affected by either the development of LVH or treatment with captopril (control 3-mo group vs. LVH 3-mo group, P = NS; control 6-mo group vs. LVH 6-mo group vs. regress group, P = NS; Table 1).
|
Effect of Regression on IK1 Density
Representative IK1 traces from a control 3-mo myocyte (169 pF) and an LVH 3-mo myocyte (247 pF) are shown in Fig. 2A. IK1 amplitude was measured at the end of the test pulse and normalized for Cm to derive current density. IK1 density-voltage relationships for control 3-mo and LVH 3-mo rabbits are shown in Fig. 2B. LVH caused a significant decrease in IK1 density over a broad range of membrane potentials, particularly in the membrane potential range encountered during phase III repolarization of the action potential, from40 to 70 mV. At 100 mV, inward
IK1 density decreased from 14.8 ± 0.4 pA/pF (n = 25) in control 3-mo group to 11.2 ± 0.5 pA/pF
(n = 25) in LVH 3-mo group, a 24%
reduction, whereas at 60 mV (the membrane potential at which
outward IK1 peaked), IK1
density decreased from 2.4 ± 0.1 pA/pF
(n = 25) to 1.6 ± 0.1 pA/pF (n = 25), a 33% reduction.
IK1 density
remained decreased in LVH 6-mo myocytes over the same range of
potentials (Fig. 2C), with a peak
outward IK1
density of 1.5 ± 0.1 pA/pF (n = 23) at 60 mV. Myocytes obtained from the regress group showed normalization of
IK1 density over
the entire range of test pulses, with values not significantly
different from those from the control 6-mo group (Fig.
2C). The outward
IK1 density at
60 mV was 2.2 ± 0.2 pA/pF
(n = 20) and 2.3 ± 0.1 pA/pF (n = 18) for the regress and
control 6-mo groups, respectively. Reduction of
IK1 density in
LVH rabbits was associated with a decrease in the rate of
phase III repolarization of action
potential
(dVm/dt),
and restoration of
IK1 density in
captopril-treated rabbits was associated with the normalization of
dVm/dt
(Table 1).
|
Effect of Regression on ICa,L
Representative ICa,L traces from control 3-mo and LVH 3-mo myocytes are shown in Fig. 3A. Larger peak currents were recorded in LVH myocytes, but, when normalized for Cm (an electrophysiological correlate of cell size), LVH did not significantly alter ICa,L at 3 mo after banding (Fig. 3B). At 6 mo after renal artery banding, however, a small but statistically significant reduction of ICa,L density was observed at potentials between20 and +30 mV (LVH 6-mo group vs.
control 6-mo group, Fig. 3C). At 10 mV, ICa,L density decreased from 19.0 ± 0.8 pA/pF (n = 32) in the control 6-mo group to 16.4 ± 0.7 pA/pF
(n = 28) in the LVH 6-mo group, a 14%
reduction. Myocytes obtained from the regress group showed
normalization of
ICa,L density,
with values not significantly different from those of age-matched
controls over the entire range of test pulses (Fig.
3C).
ICa,L density at
10 mV was 19.1 ± 0.7 pA/pF (n = 21) for the regress group. Steady-state voltage-dependent inactivation and activation of
ICa,L were not
different among myocytes isolated from control, LVH, and regress groups
(Fig. 4 and Table
2).
|
|
|
Effect of Regression on Ito
A rapidly inactivating current component (Ito) and a current component that persisted at the end of depolarization pulses (Isus) were recorded (Fig. 5A). Isus was measured at the end of the test pulses, and the difference between the peak current and Isus was taken as Ito. Both Ito and Isus were normalized by the corresponding Cm and plotted against test potentials (Fig. 5B). Isus density was unchanged by the development of LVH (LVH 3-mo group vs. control 3-mo group, Fig. 5B). Hypertrophied myocytes, however, had a significant increase in Ito density compared with controls. At 40 mV, Ito density increased from 3.6 ± 0.3 pA/pF (n = 27) in control 3-mo group to 4.8 ± 0.3 pA/pF (n = 23) in LVH 3-mo group, a 33% increase. Voltage-dependent inactivation and activation of Ito were unchanged between these two groups. Data were fit by the Boltzmann equation. The fitting parameters, V0.5 (potential at which conductance is half-maximal) and k (slope factor describing curve) of inactivation were33.7 and 6.7 mV, respectively, for control
3-mo group and 33.7 and 6.5 mV, respectively, for LVH 3-mo
group; V0.5 and
k of activation were 8.5 and
9.3 mV, respectively, for control 3-mo group and 8.2 and
8.6 mV, respectively, for LVH 3-mo group. Time-dependent
recovery of Ito from inactivation was similar between control and LVH rabbits (Fig.
6A).
|
|
The increase in Ito density persisted at 6 mo after banding, whereas Isus remained unchanged (LVH 6-mo group vs. control 6-mo group, Fig. 5C). Regression of LVH with captopril normalized Ito density over the entire range of test potentials and had no effect on Isus (regress group vs. LVH 6-mo group), with the current density of Ito being no different than that in control myocytes (regress group vs. control 6-mo group, Fig. 5C).
The Ito density-voltage relationships shown in Fig. 5 were obtained at a stimulus frequency of 0.1 Hz. Because of its characteristically slow recovery from inactivation (Fig. 6A), the contribution of Ito to repolarization is likely to be small at physiological heart rates. At the pacing rate of 2 Hz, at which we recorded action potentials, the amplitude of Ito was significantly smaller than that recorded at 0.1 Hz. Figure 6B (inset) shows outward K+ current traces generated by the 1st, 2nd, and 20th stimuli at a stimulus frequency of 2 Hz. Ito amplitude was plotted as a function of the number of stimuli. Ito amplitude at the 20th stimulus was only 16.9 ± 2.4% (n = 8) of that at the 1st stimulus in control rabbits. When repeated at physiological temperatures, Ito continued to demonstrate slow recovery from inactivation. At 37°C, Ito amplitude at the 20th stimulus was ~33% of that at the 1st stimulus.
Effect of Captopril on Membrane Currents
To determine whether any of the changes in membrane current densities observed in the regress group were due to a direct electrophysiological effect of captopril, we examined the current-voltage relationship of IK1, ICa,L, and Ito before and after superfusion with 10 µM captopril in myocytes from control 3-mo and LVH 3-mo rabbits. As shown in Fig. 7, we found that bath application of captopril (10 µM) had no significant effect on IK1, ICa,L, and Ito of control myocytes. Similar results were also obtained from the hypertrophied myocytes (data not shown).
|
| |
DISCUSSION |
|---|
|
Top
Abstract
Introduction
Methods
Results
Discussion
References
|
|---|
This study demonstrates normalization of APD, cell membrane capacitance, and membrane current abnormalities of ventricular myocytes after regression of LVH with chronic captopril treatment. Each of the individual membrane current abnormalities we examined (IK1, ICa,L, and Ito) was shown to be completely reversible. Furthermore, normalization of these membrane current abnormalities was associated with normalization of APD, which probably explains the reduced vulnerability to ventricular arrhythmia observed after regression of LVH in this rabbit model (20). This is the first report to demonstrate reversibility of membrane current abnormalities due to LVH. This study extends our previous work and indicates that regression normalizes the abnormal electrophysiology of LVH at the membrane current level as well as in vivo (17, 20). This is also one of the first reports to evaluate cellular and membrane current abnormalities with the use of an animal model whose electrophysiology has been characterized by in vivo studies. Other studies have examined the membrane currents responsible for prolongation of APD, but they have not done so in animals shown to have increased vulnerability to arrhythmia.
Normalization of the electrophysiological abnormalities produced by LVH may have significant impact if our findings can be reproduced in clinical studies. LVH is associated with an increased risk of sudden death thought to result from malignant ventricular arrhythmia. The in vivo electrophysiological abnormalities of LVH have already been shown to be normalized by regression. The current study demonstrates normalization of in vitro abnormalities and provides a mechanism for explaining the in vivo findings reported earlier (20). Taken together, these studies provide the basis for hypothesizing a clinical benefit to be associated with regression of LVH.
A variety of animal models have been used to study the electrophysiology of LVH. Our results describe membrane current abnormalities in this animal model and indicate that a reduction in IK1 density is responsible for prolongation of the action potential. Prolongation of the action potential can be produced by increased inward currents, reduction of outward currents, or both. ICa,L was unchanged at 3 mo after renal artery banding, a time when APD was found to be prolonged. At 6 mo after banding, ICa,L was actually somewhat reduced. This decrease in inward current would be expected to shorten APD, but it appeared to have no significant effect, because both APD60 and APD90 of LVH 6-mo myocytes were not different from APD60 and APD90 of LVH 3-mo myocytes. The lack of effect was probably due to the small magnitude of reduction in ICa,L.
We studied the predominant outward currents present in rabbit ventricular myocytes and found an increase in Ito density and a decrease in IK1 density. The increase in Ito density would be expected to accelerate phase I repolarization of the action potential and shorten APD30. However, the slow recovery of Ito from inactivation at physiological pacing cycle lengths results in a smaller contribution of Ito to phase I repolarization at the cycle length at which we recorded action potentials (2 Hz). This probably explains why APD30 was unchanged in both the LVH 3-mo and LVH 6-mo groups.
Consistent with other reports (2, 3), we found a decrease in outward
current in the voltage range corresponding to phase III of the action potential in hypertrophied myocytes.
This decrease was mainly due to a reduction in
IK1 density.
IK1 has been
shown to provide outward current during the final phase of action
potential repolarization (7), and close correlation between
IK1 and the rate
of phase III repolarization of the
action potential has been demonstrated in both rabbit and guinea pig
ventricular cells (8, 25). We found that the reduction of
IK1 density in
LVH rabbits was accompanied by the reduced rate of
phase III repolarization of action
potential and prolonged APD.
IK1 density was
normalized by regression of LVH, which is consistent with the normal
phase III repolarization rate in
myocytes isolated from captopril-treated rabbits. Resting membrane
potential was not significantly different between LVH and control
animals despite the reduction in
IK1. We
hypothesize that this occurred because the reversal potential was
unchanged and because the density of
IK1 measured near
the resting membrane potential (80 mV) was not significantly
different between these groups (Fig. 2). We did not study the delayed
rectifier K+ current because of
its small size in rabbit ventricular myocytes even at 37°C (5, 23).
Although the study of other currents may also be important to fully
characterize this animal model, the major finding remains that each of
the membrane current abnormalities due to LVH that have been evaluated
to date are normalized by regression of hypertrophy.
Many studies of hypertrophied myocytes or myocytes from failing hearts have observed a reduction in Ito density (2, 4, 9, 15, 21, 22, 28, 30). However, Li and Keung (14), Ten Eick et al. (27), and the present study have reported an increase in Ito density in hypertrophied ventricular myocytes. The discrepant findings are probably due to different methods for inducing ventricular hypertrophy, different animal species, and the marked heterogeneity with which Ito is distributed in the ventricle. We intentionally used myocytes isolated only from the mid one-third of the posterior wall of the left ventricle to avoid introducing transmural heterogeneity into our evaluation. Our conclusions regarding the reversibility of membrane current abnormalities with regression of LVH can therefore only be applied to myocytes from this region. Studies of how LVH alters the membrane currents in other layers and from other regions of the left ventricle in this model are ongoing. Preliminary data from these experiments (18) suggest that LVH does not alter the transmural gradient of APD in rabbits. However, until the final results from these experiments are available, consideration of the methodological differences between our study and the work of others should be borne in mind when evaluating our results.
ICa,L density was unchanged at 3 mo after banding but was reduced somewhat at 6 mo after banding. This finding demonstrates another methodological concern to be addressed in the study of the electrophysiology of LVH, the duration of hypertrophy. We chose to study isolated myocytes at 3 mo after banding because in vivo vulnerability to arrhythmia and increased dispersion of refractoriness are present at this time (20).
The electrical abnormalities observed in the rabbit renovascular model of LVH closely parallel those recently reported in rats with LVH that developed as part of ventricular remodeling after myocardial infarction (MI) (16). Both models show increased dispersion of refractoriness/repolarization and increased APD. These similarities raise the possibility that the abnormal electrophysiology of post-MI hypertrophy may also be reversible. If so, some of the benefit observed with the use of ACEI post-MI may be due to prevention or regression of hypertrophy in the noninfarcted tissue. Postinfarction hypertrophy can be prevented by ACEI (29). We speculate that the lower incidence of sudden death observed in one post-MI trial of an ACEI may be due in part to prevention or regression of hypertrophy. A lower incidence of ventricular fibrillation, but not monomorphic ventricular tachycardia, was observed in post-MI patients treated with the ACEI trandolapril (11), as would be expected if ACEI prevented or reversed repolarization abnormalities.
In summary, we observed increased cell membrane capacitance, prolonged APD, increased Ito density, and decreased IK1 density in myocytes isolated from the midlayer of the left ventricle of rabbits with induced renovascular hypertension. These abnormalities were reversed by the chronic treatment of LVH with captopril. We did not study each of many different membrane current abnormalities that have been described in the setting of LVH (1, 2) because our main goal was to determine whether any of the membrane current abnormalities of LVH are reversible. Therefore, important questions remain as to whether the other membrane currents altered by LVH are reversible and whether normalization is observed in other regions of the myocardium such as the epicardium and endocardium.
| |
ACKNOWLEDGEMENTS |
|---|
The authors thank Rose Marie Wells and Nicole Ewing for assistance in the preparation of this manuscript.
| |
FOOTNOTES |
|---|
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: S. J. Rials, Lankenau Medical Office Bldg. East, 100 Lancaster Ave, Ste. 556, Wynnewood, PA 19096.
Received 9 February 1998; accepted in final form 23 June 1998.
| |
REFERENCES |
|---|
|
Top
Abstract
Introduction
Methods
Results
Discussion
References
|
|---|
1.
Aronson, R. S.
Mechanisms of arrhythmias in ventricular hypertrophy.
J. Cardiovasc. Electrophysiol.
2:
249-261,
1991.
2.
Beuckelmann, D. J.,
M. Nabauer,
and
E. Erdmann.
Alterations of K+ currents in isolated human ventricular myocytes from patients with terminal heart failure.
Circ. Res.
73:
379-385,
1993
3.
Brooksby, P.,
A. J. Levi,
and
J. V. Jones.
The electrophysiological characteristics of hypertrophied ventricular myocytes from the spontaneously hypertensive rat.
J. Hypertens.
11:
611-622,
1993[Medline].
4.
Cerbai, E.,
M. Barbieri,
Q. Li,
and
A. Mugelli.
Ionic basis of action potential prolongation of hypertrophied cardiac myocytes isolated from hypertensive rats of different ages.
Cardiovasc. Res.
28:
1180-1187,
1994
5.
Clay, J. R.,
A. Ogbaghebriel,
T. Paquette,
B. I. Sasyniuk,
and
A. Shrier.
A quantitative description of the E-4031-sensitive repolarization current in rabbit ventricular myocytes.
Biophys. J.
69:
1830-1837,
1995[Medline].
6.
Hart, G.
Cellular electrophysiology in cardiac hypertrophy and failure.
Cardiovasc. Res.
28:
933-946,
1994
7.
Ibarra, J.,
G. E. Morley,
and
M. Delmar.
Dynamics of the inward rectifier K+ current during the action potential of guinea pig ventricular myocytes.
Biophys. J.
60:
1534-1539,
1991[Medline].
8.
Jurkiewicz, N. K.,
J. Wang,
B. Fermini,
M. C. Sanguinetti,
and
J. J. Salata.
Mechanism of action potential prolongation by RP 58866 and its active enantiomer, tertikalant. Block of the rapidly activating delayed rectifier K+ current, IKr.
Circulation
94:
2938-2946,
1996
9.
Kaab, S.,
B. Nuss,
N. Chiamvimonvat,
B. O'Rourke,
P. H. Pak,
D. A. Kass,
E. Marban,
and
G. F. Tomaselli.
Ionic mechanism of action potential prolongation in ventricular myocytes from dogs with pacing-induced heart failure.
Circ. Res.
78:
262-273,
1996
10.
Keung, E. C.,
and
R. S. Aronson.
Non-uniform electrophysiological properties and electrotonic interaction in hypertrophied rat myocardium.
Circ. Res.
49:
150-158,
1981
11.
Kober, L.,
C. T. Pederson,
J. E. Carlsen,
H. Bagger,
P. Eliasen,
K. Lyngborg,
J. Videbaek,
D. S. Cole,
L. Auclert,
N. C. Pauly,
E. Aliot,
S. Persson,
and
A. J. Camm.
A clinical trial of the angiotensin-converting-enzyme inhibitor trandolapril in patients with left ventricular dysfunction after myocardial infarction.
N. Engl. J. Med.
333:
1670-1676,
1995
12.
Kowey, P. R.,
T. D. Friehling,
J. Sewter,
Y. Wu,
A. Sokil,
J. Paul,
and
J. Nocella.
Electrophysiological effects of left ventricular hypertrophy. Effect of calcium and potassium channel blockade.
Circulation
83:
2067-2075,
1991
13.
Kowey, P. R.,
R. O'Brien,
Y. Wu,
J. Sewter,
A. Sokil,
J. Nocella,
and
S. J. Rials.
Effect of gallopamil on electrophysiologic abnormalities and ventricular hypertrophy in the feline heart.
Am. Heart J.
124:
898-905,
1992[Medline].
14.
Li, Q.,
and
E. C. Keung.
Effects of myocardial hypertrophy on transient outward current.
Am. J. Physiol.
266 (Heart Circ. Physiol. 35):
H1738-H1745,
1994
15.
Potreau, D.,
J. P. Gomez,
and
N. Fares.
Depressed transient outward current in single hypertrophied cardiomyocytes isolated from the right ventricle of ferret heart.
Cardiovasc. Res.
30:
440-448,
1995[Medline].
16.
Qin, D.,
Z. Zhang,
E. B. Caref,
M. Boutjdir,
P. Jain,
and
N. El-Sherif.
Cellular and ionic basis of arrhythmias in postinfarction remodeled ventricular myocardium.
Circ. Res.
79:
461-473,
1996
17.
Rials, S. J.,
Y. Wu,
N. Ford,
F. J. Pauletto,
S. V. Abramson,
A. M. Rubin,
R. A. Marinchak,
and
P. R. Kowey.
Effect of left ventricular hypertrophy and its regression on ventricular electrophysiology and vulnerability to inducible arrhythmia in the feline heart.
Circulation
91:
426-430,
1995
18.
Rials, S. J.,
Y. Wu,
T. Liu,
X. Xu,
D. B. Bharucha,
R. A. Marinchak,
and
P. R. Kowey.
Left ventricular hypertrophy increases vulnerability to bradycardia and d-sotalol induced early afterdepolarizations (Abstract).
J. Am. Coll. Cardiol.
31:
252A,
1998.
19.
Rials, S. J.,
Y. Wu,
F. J. Pauletto,
S. V. Abramson,
R. A. Marinchak,
and
P. R. Kowey.
Effect of an intravenous angiotensin-converting enzyme inhibitor on the electrophysiologic features of normal and hypertrophied feline ventricles.
Am. Heart J.
132:
989-994,
1996[Medline].
20.
Rials, S. J.,
Y. Wu,
X. Xu,
R. A. Filart,
R. A. Marinchak,
and
P. R. Kowey.
Regression of left ventricular hypertrophy with captopril restores normal ventricular action potential duration, dispersion of refractoriness, and vulnerability to inducible ventricular fibrillation.
Circulation
96:
1330-1336,
1997
21.
Rozanski, G. J.,
Z. Xu,
R. T. Whitney,
H. Murakami,
and
I. H. Zucker.
Electrophysiology of rabbit ventricular myocytes following sustained rapid ventricular pacing.
J. Mol. Cell. Cardiol.
29:
721-732,
1997[Medline].
22.
Rozanski, G. J.,
Z. Xu,
K. Zhang,
and
K. P. Patel.
Altered K+ current of ventricular myocytes in rats with chronic myocardial infarction.
Am. J. Physiol.
274 (Heart Circ. Physiol. 43):
H259-H265,
1998
23.
Salata, J. J.,
N. K. Jurkiewicz,
B. Jow,
K. Folander,
P. J. Guinosso, Jr.,
B. Raynor,
R. Swanson,
and
B. Fermini.
IK of rabbit ventricle is composed of two currents: evidence for IKs.
Am. J. Physiol.
271 (Heart Circ. Physiol. 40):
H2477-H2489,
1996
24.
Shechter, J. A.,
K. M. O'Conner,
T. D. Friehling,
R. Mark,
C. Uboh,
and
P. R. Kowey.
Electrophysiologic effects of left ventricular hypertrophy in the intact cat.
Am. J. Hypertens.
2:
81-85,
1989[Medline].
25.
Shimoni, Y.,
R. B. Clark,
and
W. R. Giles.
Role of an inwardly rectifying potassium current in rabbit ventricular action potential.
J. Physiol. (Lond.)
448:
709-727,
1992
26.
Stark, G.,
U. Stark,
S. Nagl,
W. Klein,
E. Pilger,
and
H. A. Tritthart.
Acute effects of the ACE inhibitor lisinopril on cardiac electrophysiological parameters of isolated guinea pig hearts.
Clin. Cardiol.
14:
579-582,
1991[Medline].
27.
Ten Eick, R. E.,
K. Zhang,
R. D. Harvey,
and
A. L. Bassett.
Enhanced functional expression of transient outward current in hypertrophied feline myocytes.
Cardiovasc. Drugs Ther.
7:
611-619,
1993.
28.
Tomita, F.,
A. L. Bassett,
R. J. Myerburg,
and
S. Kimura.
Diminished transient outward currents in rat hypertrophied ventricular myocytes.
Circ. Res.
75:
296-303,
1994
29.
Vaughan, D. E.,
and
M. A. Pfeffer.
Angiotensin converting enzyme inhibitors and cardiovascular remodelling.
Cardiovasc. Res.
28:
159-165,
1994
30.
Xu, X.,
and
P. M. Best.
Decreased transient outward K+ current in ventricular myocytes from acromegalic rats.
Am. J. Physiol.
260 (Heart Circ. Physiol. 29):
H935-H942,
1991
This article has been cited by other articles:
|
|
 |
|
  E. Kaplinsky Do we understand why regression of left ventricular hypertrophy is beneficial? Cardiovasc Res, December 1, 2003; 60(3): 463 - 464. [Full Text] [PDF]
|
|
|
|
 |
|
 C. Chouabe, J. Amsellem, L. Espinosa, P. Ribaux, S. Blaineau, P. Megas, and R. Bonvallet Reversibility of electrophysiological changes induced by chronic high-altitude hypoxia in adult rat heart Am J Physiol Heart Circ Physiol, April 1, 2002; 282(4): H1452 - H1460. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
G.-X. Yan, S. J. Rials, Y. Wu, T. Liu, X. Xu, R. A. Marinchak, and P. R. Kowey Ventricular hypertrophy amplifies transmural repolarization dispersion and induces early afterdepolarization Am J Physiol Heart Circ Physiol, November 1, 2001; 281(5): H1968 - H1975. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 K. N. Jew, M. C. Olsson, E. A. Mokelke, B. M. Palmer, and R. L. Moore Endurance training alters outward K+ current characteristics in rat cardiocytes J Appl Physiol, April 1, 2001; 90(4): 1327 - 1333. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 X. Xu, S. J. Rials, Y. Wu, J. J. Salata, T. Liu, D. B. Bharucha, R. A. Marinchak, and P. R. Kowey Left Ventricular Hypertrophy Decreases Slowly but Not Rapidly Activating Delayed Rectifier Potassium Currents of Epicardial and Endocardial Myocytes in Rabbits Circulation, March 20, 2001; 103(11): 1585 - 1590. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Visit Other APS Journals Online |
,
n = 12; 
,
n = 13) and activation (
,
n = 17; 
,
n = 16) of
ICa,L from
control 3-mo group and LVH 3-mo group.
I/Imax,
current recorded from each holding potential divided by largest current
obtained at holding potential. B:
comparison of voltage dependency of steady-state inactivation and
activation of
ICa,L from
control 6-mo group (
,
n = 8, and 
,
n = 9). Data were fit by the Boltzmann
equation:
I/Imax
or
g/gmax = {1 + exp[(V 
