Download Does the 2-Agonist Clenbuterol Help to Maintain

Does the ␤2-Agonist Clenbuterol Help to Maintain
Myocardial Potential to Recover During
Mechanical Unloading?
Hiroshi Tsuneyoshi, MD; Wnimunk Oriyanhan, MD; Hideo Kanemitsu, MD;
Reiko Shiina, BS; Takeshi Nishina, MD, PhD; Satoshi Matsuoka, MD, PhD;
Tadashi Ikeda, MD, PhD; Masashi Komeda, MD, PhD
Downloaded from http://circ.ahajournals.org/ by guest on June 16, 2017
Objective—Chronic mechanical unloading induces left ventricular (LV) atrophy, which may impair functional recovery
during support with an LV-assist device. Clenbuterol, a ␤2-adrenergic receptor (AR) agonist, is known to induce
myocardial hypertrophy and might prevent LV atrophy during LV unloading. Furthermore, ␤2-AR stimulation is
reported to improve Ca2⫹ handling and contribute to antiapoptosis. However, there is little information on the effects
of clenbuterol during LV unloading.
Methods and Results—We investigated LV atrophy and function after LV unloading produced by heterotopic heart
transplantation in isogenic rats. After transplantation, rats were randomized to 1o f 2 groups (n⫽10 each). The
clenbuterol group received 2 mg䡠kg⫺1䡠d⫺1 of the drug for 2 weeks; the control group received normal saline. The weight
of unloaded control hearts was 48% less than that of host hearts after 2 weeks of unloading. Clenbuterol significantly
increased the weight of the host hearts but did not prevent unloading-induced LV atrophy. Papillary muscles were
isolated and stimulated, and there was no difference in developed tension between the 2 groups. However, the
inotropic response to the ␤-AR agonist isoproterenol significantly improved in the clenbuterol group. The mRNA
expression of myocardial sarco(endo)plasmic reticulum Ca2⫹-ATPase 2a (SERCA2a) and fetal gene shift (myosin
heavy chain [MHC] mRNA isozyme) was also significantly improved by clenbuterol treatment. There was no
difference in ␤1-AR mRNA expression between the 2 groups. In contrast, ␤2-AR mRNA was significantly
decreased in the clenbuterol-treated, unloaded heart. This indicates that clenbuterol may downregulate ␤2-ARs. In
the evaluation of apoptosis, mRNA expression of caspase-3, which is the central pathway for apoptosis, tended to
be better in the clenbuterol group.
Conclusions—During complete LV unloading, clenbuterol did not prevent myocardial atrophy but improved gene
expression (SERCA2a, ␤-MHC) and ␤-adrenergic responsiveness and potentially prevented myocardial apoptosis.
However, chronic administration of clenbuterol may be associated with downregulation of ␤2-ARs. (Circulation. 2005;
112[suppl I]:I-51–I-56.)
Key Words: cardiomyopathy 䡲 heart-assist device 䡲 heart failure 䡲 inotropic agents 䡲 receptors, adrenergic, beta
R
ecent clinical observations in some patients with endstage heart failure have demonstrated that a left ventricular–assist device (LVAD) can improve cardiac function and
be used as a “bridge to recovery” without heart transplantation.1,2 However, its success rate as a bridge to recovery is
low,3 and it is difficult to predict which patients will show
functional improvement. In addition, the mechanisms of
functional recovery, if any, are unknown. Chronic mechanical
unloading induces cardiac atrophy, and this may impair the
recovery of function.4 Recently, to prevent cardiac atrophy
and improve myocardial function during mechanical unloading, administration of a ␤2-adrenergic receptor (AR) agonist,
clenbuterol, was advocated in combination with mechanical
unloading.5 Clenbuterol is known to induce muscle hypertro-
phy in animal hearts without adverse effects,6 similar to
physiologic hypertrophy. Furthermore, ␤2-AR stimulation is
reported to improve Ca2⫹ handling and reduce apoptosis.7,8
However, antiatrophic effects or other beneficial effects of
clenbuterol during mechanical unloading are not fully understood. Thus, we tested the hypothesis that clenbuterol induces
beneficial changes in heart size and biophysiologic characteristics of the unloaded heart by using a rat heterotopic heart
transplantation model. The heterotopically transplanted hearts
were mechanically unloaded in a manner similar to an LVAD
and underwent cardiac atrophy. We evaluated histologic
changes, gene expression, and contractile function of isolated
papillary muscles in both host and transplanted (ie, unloaded)
hearts.
From the Department of Cardiovascular Surgery, Graduate School of Medicine, Kyoto University, Kyoto, Japan.
Correspondence to Masashi Komeda, Department of Cardiovascular Surgery, Graduate School of Medicine, Kyoto University, 4 Kawahara-cho,
Shogoin, Sakyo-ku, Kyoto, Japan, 606-8507. E-mail [email protected]
© 2005 American Heart Association, Inc.
Circulation is available at http://www.circulationaha.org
DOI: 10.1161/CIRCULATIONAHA.104.525097
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August 30, 2005
TABLE 1.
Effect of Clenbuterol on Heart Size
Control
Clenbuterol
Host Heart
Unloaded Heart
Host Heart
Unloaded Heart
LV weight, g
0.60⫾0.04
0.31⫾0.04*
0.71⫾0.08†
0.32⫾0.06*
Myocyte diameter, ␮m
25.3⫾1.6
21.5⫾2.1*
26.5⫾1.9
21.7⫾2.5*
Abbreviations are as defined in text.
*P⬍0.05 vs host heart; †P⬍0.05 vs control group.
Methods
Experimental Design
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All experimental procedures were conducted according to Kyoto
University’s guidelines for animal care. Inbred male Lewis rats
(weighing 150 to 200g) were used in this study. Heterotopic heart
transplantation was performed as previously described.9 In brief, the
ascending aorta of the donor heart was anastomosed end-to-side on
the recipient’s abdominal aorta, and the pulmonary artery of the
donor was anastomosed end-to-side on the recipient’s inferior vena
cava. Thus, the donor heart continued to beat because of perfusion
through the coronary arteries, but the LV was unloaded. Transplanted hearts spontaneously resumed beating within a few seconds
of reperfusion. The total procedure time was ⬍40 minutes. After
heterotopic transplantation, surgical mortality was ⬍5%. The transplanted rats were randomly divided into 2 groups (n⫽10 each). The
control group consisted of rats that were injected with subcutaneous
saline (0.5 mL) once daily for 2 weeks. The treatment group was
injected with subcutaneous clenbuterol (MP Biomedicals, Inc) once
daily at a dose of 2 mg/kg body weight for 2 weeks.
Papillary Muscle Function
Papillary muscle function was examined at experimental end points.
The animals were anesthetized with pentobarbital sodium (100
mg/kg) via intraperitoneal injection and heparinized (500 U) via
intravenous infusion. After the chest or abdomen was opened, the
heart was rapidly removed. The heart was placed into normal
Tyrode’s solution containing (in mmol/L) Na⫹ 142, K⫹ 5.6, Mg2⫹ 1,
Cl⫺ 154.6, HEPES 5, and glucose 11. The entire heart, right
ventricle, and LV were weighed. The posterior papillary muscle was
ligated, dissected free, and mounted in a tissue bath containing
Krebs-Henseleit solution (in mmol/L) Na⫹ 152, K⫹ 3.6, Mg2⫹ 0.6,
Ca2⫹ 2.5, Cl⫺ 135, HCO3⫺ 25, H2PO4⫺ 1.3, SO42⫺ 0.6, glucose 5.6,
and 2,3-butanedione monoxime 30, pH 7.4. The bath was maintained
at a constant temperature of 37°C and bubbled with 95% O2 and 5%
CO2. The papillary muscle was stimulated at 1 Hz with impulses of
5-ms duration and current ⬇20% above threshold. The muscle was
stretched to the length at which maximum tension development
occurred. After stabilization, isometric tension was recorded digitally
at the maximum tension position, and papillary muscle diameter was
measured with a calibrated eyepiece. Developed tension was normalized to cross-sectional area.
After baseline measurements were completed, developed tension
was recorded during exposure to the ␤-adrenergic agonist isoproterenol. Isoproterenol HCl was dissolved in distilled water and added to
the bath to produce cumulative concentrations of 10⫺8 and 10⫺7
mol/L. Developed tension was measured when the response was
maximal (5 to 10 minutes after each addition of isoproterenol).
After removal of the papillary muscle from the heart, the LV
myocardium was transversely sliced into 2-mm-diameter sections at
the base of the papillary muscles and fixed in 10% buffered formalin.
After slices were taken, the remaining LV myocardium was frozen at
⫺80°C until analyzed.
Pathologic Studies
Transverse sections of LV myocardium were stained with
hematoxylin-eosin to evaluate myocyte size. Mean cardiomyocyte
diameter was calculated by measurement of 50 cells in the myocardium under microscopy (magnification ⫻400). Cells were accepted
for measurement if they met the following criteria: (1) cross sections
of cardiomyocytes were present; (2) cardiomyocytes had a visible
nucleus; and (3) their cellular membranes were intact. Other transverse sections were used for in situ detection of apoptosis by terminal
dUTP nick end-labeling (TUNEL) assay. The TUNEL assay was
performed in accordance with the manufacturer’s protocol (Takara
Biomedicals). TUNEL-positive cells were counted, in a blinded
manner (400 to 500 cells), in 30 randomly chosen fields for each
experiment.
Analysis of mRNA Expression
Total mRNA was prepared from the frozen LV pieces with TRIzol
(Life Technologies Inc) reagent, reverse-transcribed, and amplified
with an ABI Prism 7700 sequence detector (Applied Biosystems).
Polymerase chain reaction (PCR) conditions were 40 cycles of
denaturing at 94°C for 20 seconds and primer annealing/extension at
62°C for 60 seconds. The nucleotide sequences of PCR primers and
TaqMan probes were as follows: ␣-myosin heavy chain (MHC)
forward primer, 5⬘-TGACAAATCTGCCTACCTCATGG-3⬘ and reverse primer, 5⬘-TGACATACTCGTTACCCACCTTCA-3⬘; TaqMan probe 5⬘-CTGCTCAAGGGTCTGTGTGTCACCCTCG-3⬘;
␤-MHC forward primer, 5⬘-CAAGTCAGCCTACCTCATGGG-3⬘
and reverse primer, 5⬘-TGACTCGAGGGTGGCACAA-3⬘; TaqMan
probe 5⬘-CTGAACTCGGCTGACCTGCTCAAGG-3⬘; caspase-3
forward primer, 5⬘-AATTCAAGGGACGGGTCATG-3⬘ and reverse
primer, 5⬘-GCTTGTGCGCGTACAGTTTC-3⬘; and TaqMan probe
5⬘-TTCATCCAGTCACTTTGCGCCATG-3⬘. The PCR sequences
of sarco(endo)plasmic reticulum Ca2⫹-ATPase2a (SERCA2a), ␤1AR, and ␤2-AR were previously reported in our studies.9,10 The
TaqMan rodent glyceraldehyde 3-phosphate dehydrogenase
(GAPDH) control reagent was used to detect rat GAPDH as the
internal standard. Expression levels of the target gene were normalized to the GAPDH level in each sample.
Statistical Analysis
All data are expressed as the mean⫾SEM. Statistical differences of
paired data between host and unloaded hearts were evaluated by a
paired t test, and an unpaired t test was used for comparing unpaired
data. Two-way ANOVA with repeated measures was used to assess
the inotropic response to isoproterenol. Statistical analyses were
performed with Statview for Windows, version 5.0 (SAS Institute
Inc). A value of P⬍0.05 was considered statistically significant.
Results
Effects of Clenbuterol on Heart Size
As shown in Table 1, there was an 18% increase in LV weight
(from 0.60⫾0.04 to 0.71⫾0.08 g) in the clenbuterol-treated,
host hearts compared with the control host hearts. In contrast,
the size of the unloaded hearts in control animals decreased
by 48%, from 0.60⫾0.04 to 0.31⫾0.04 g, and myocyte
diameter decreased from 25.3⫾1.6 to 21.5⫾2.1 ␮m after 2
weeks of unloading; these changes were not affected by
clenbuterol treatment.
Myocardial Expression of mRNAs Encoding
SERCA2a, ␣-MHC, and ␤-MHC
As shown in Figure 1, mRNA expression of SERCA2a
significantly decreased in the unloaded heart compared with
Tsuneyoshi et al
Effect of Clenbuterol During Mechanical Unloading
Figure 1. Expression of SERCA2a mRNA. All values are
mean⫾SEM *P⬍0.05 vs host hearts in the same treatment
group; †P⬍0.05 vs control (host or unloaded hearts). Abbreviations are as defined in text.
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the host heart in the control group. However, this impaired
expression of SERCA2a improved in the clenbuterol-treated
animals. The isoforms of ␣- and ␤-MHC mRNAs are shown
in Figure 2. There was no significant difference in ␣-MHC
mRNA between the 2 groups. In the unloaded heart, ␤-MHC
mRNA expression significantly increased compared with the
host heart. By contrast, clenbuterol treatment significantly
reduced ␤-MHC mRNA expression induced by mechanical
unloading. Therefore, clenbuterol can prevent the adverse
changes in SERCA2a gene expression and the fetal gene shift
of MHC isoforms.
Evaluation of Myocardial Apoptosis
To evaluate the degree of myocardial apoptosis, we evaluated
TUNEL staining and the expression of caspase-3 mRNA
(Figure 3). The unloaded heart showed significant increases
in caspase-3 mRNA expression and TUNEL-positive cells. In
the clenbuterol-treated group, caspase-3 expression of the
unloaded heart showed a nonsignificant decrease compared
with the control group (P⫽0.08). TUNEL-positive cells were
significantly increased in unloaded hearts in the control
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Figure 3. Expression of caspase-3 mRNA (left). Caspase-3
mRNA expression in clenbuterol-treated, unloaded hearts
showed a nonsignificant decrease compared with untreated,
unloaded hearts (P⫽0.08 vs control-group). Average number of
TUNEL-positive myocytes in groups is shown (right). All values
are mean⫾SEM *P⬍0.05 vs host heart in the same treatment
group. Abbreviations are as defined in text.
group. However, there was no significant difference in the
clenbuterol-treated group.
Papillary Muscle Function
As shown in Table 2, papillary muscle cross-sectional area in
the unloaded heart was significantly smaller than in the host
heart. Although papillary muscles in unloaded hearts showed
atrophic changes, developed tension was maintained even
after mechanical unloading. There was no significant difference in developed tension between the clenbuterol- and the
saline-treated groups. The inotropic response to isoproterenol
is shown in Figure 4. Although isoproterenol produced a
dose-related increase in developed tension in the normal
heart, the unloaded heart revealed a diminished inotropic
response. Furthermore, administration of clenbuterol improved the inotropic response to isoproterenol in the unloaded
heart.
Expression of ␤-AR mRNA
To assess the effect of inotropic response in the clenbuteroltreated group, we measured the expression of ␤1- and ␤2-AR
mRNAs. There was no significant difference in ␤1-AR
mRNA among the 4 groups, as shown in Figure 5. However,
clenbuterol reduced the expression of ␤2-AR mRNA in the
unloaded heart compared with the host heart.
Discussion
Figure 2. Expression of ␣-MHC and ␤-MHC mRNA isoforms. All
values are mean⫾SEM *P⬍0.05 vs host hearts in the same
treatment group; †P⬍0.05 vs control among the host or
unloaded heart group. Abbreviations are as defined in text.
In the clinical setting, excessive or prolonged ventricular
unloading with an LVAD causes disuse cardiac atrophy.11
These atrophic changes are thought to impede the functional
recovery associated with an LVAD.5 To prevent atrophy
during mechanical unloading, clenbuterol was administered
in a recent study.5 Clenbuterol is a selective ␤2-adrenergic
agonist known to produce muscle hypertrophy in the rat
heart.5 The hypertrophied muscle shows normal histology and
normal gene expression. In contrast, isoproterenol, a ␤1- and
␤2 -adrenergic agonist, produces cardiac hypertrophy with
myocyte necrosis, fibrosis, and increased expression of fetal
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August 30, 2005
TABLE 2.
Papillary Muscle Contractions
Control
Papillary muscle area, mm2
Clenbuterol
Host Heart
Unloaded Heart
Host Heart
Unloaded Heart
0.39⫾0.1
0.27⫾0.06*
0.41⫾0.09
0.29⫾0.06*
Developed tension, g
0.28⫾0.09
0.22⫾0.06
0.27⫾0.12
0.23⫾0.1
Developed tension/area, g/mm2
0.70⫾0.1
0.75⫾0.40
0.67⫾0.35
0.68⫾0.47
Abbreviations are as defined in text.
*P⬍0.05 vs host heart.
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genes.12 In the present study, the dose of clenbuterol given by
subcutaneous injection was selected on the basis of a prior
study,6 which showed that clenbuterol induced hypertrophy
of the normal rat heart. Although clenbuterol induced hypertrophy, as shown by an 18% increase in weight of the host
heart, clenbuterol treatment did not alter the weight of the
unloaded heart. If clenbuterol influences heart weight during
mechanical unloading, then the response should occur similarly in both the hemodynamically unloaded heart and the
working host heart. These results suggest that clenbuterol
does not prevent cardiac atrophy during mechanical unloading in this model. Atrophy of the heart is strongly related to
hemodynamic loading. Experimental studies13 have reported
that heart size and growth are determined in large part by the
volume loading status of the heart. Moreover, atrophy is a
complex and highly regulated phenomenon involving the
interplay of multiple signaling pathways and the simultaneous activation of protein synthesis and protein degradation.
Although the mechanism of atrophy was unclear, this study
revealed that stimulation of ␤2-ARs could not prevent cardiac
atrophy during mechanical unloading.
Administration of clenbuterol did not affect LV size in the
unloaded heart. However, the results showed other beneficial
effects on molecular markers. The mRNA expression of
␤-MHC was significantly lower and SERCA2a higher in the
clenbuterol-treated, unloaded heart compared with the control, unloaded heart. In small animals, where the predominant
MHC isoform is the ␣-type, a switch to the ␤-MHC isoform
is often seen in the failing and hypertrophic heart.14
SERCA2a activity was also decreased in these hearts. Down-
Figure 4. Inotropic response to isoproterenol (expressed as the
increase in developed tension) as a percentage of baseline. All
values are mean⫾SEM *P⬍0.05 vs host hearts in the same
treatment group; †P⬍0.05 vs control in unloaded hearts. Abbreviations are as defined in text.
regulation of SERCA2a is thought to impair intracellular
Ca2⫹ handling,15 which plays an important role in functional
recovery with an LVAD.16 In this heterotopic transplantation
model, the LV cavity was completely unloaded, and these
unloaded normal hearts underwent significant alterations in
SERCA2a and MHC isozymes. Prior studies14,17 reported an
increase in ␤-MHC in heterotopically transplanted hearts that
included a switch from the “adult” to the “fetal” isoform.
Despite significant atrophy of the unloaded heart, the molecular changes in SERCA2a and MHC isoforms were similar to
those observed in the hypertrophied heart.17 Thus, opposite
hemodynamic changes in unloaded and hypertrophic hearts
induce a similar pattern of gene response. In the normal heart,
clenbuterol treatment induced cardiac hypertrophy without
alterations in SERCA2a and ␣,␤-MHC.6 This means that
clenbuterol induces a physiologic hypertrophy in the normal
heart. However, it is unknown whether clenbuterol induces
molecular alterations in the mechanically unloaded heart.
This study is the first report to resolve these issues. There are
some studies that showed evidence of beneficial effects of
␤2-adrenergic agonists in only failing and hypertrophied
hearts. Wong et al7 reported that clenbuterol increased myocardial SERCA2a levels in a rat model of hypertrophy
induced by pressure overload. Ahmet et al8 reported that
␤2-adrenergic agonists could prevent LV remodeling and
improve LV function in rat hearts with dilated cardiomyopathy. In the present study, clenbuterol-induced hypertrophy
was not observed in the unloaded hearts; however, clenbuterol can prevent the adverse changes in SERCA2a gene
Figure 5. Expression of ␤1- and ␤2-AR mRNAs. All values are
mean⫾SEM *P⬍0.05 vs host hearts in the same treatment
group; †P⬍0.05 vs control host hearts or unloaded hearts.
Abbreviations are as defined in text.
Tsuneyoshi et al
Effect of Clenbuterol During Mechanical Unloading
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expression and MHC isoform shifts not only in the hypertrophied heart but also in the unloaded heart. These results
suggest that administration of clenbuterol can improve molecular changes in both hypertrophied and unloaded hearts.
Myocardial apoptosis, which is 1 of the factors that can
deteriorate LV function, was evaluated in the unloaded heart.
Schena et al18 reported that ventricular unloading increased
myocardial apoptosis in the heterotopic heart transplantation
model, as indicated by TUNEL staining and caspase-3
activity. TUNEL staining is commonly used for detection of
apoptosis by labeling the DNA-free 3 ⬘⬘⬘-OH ends of DNA
fragments. Caspase-3, which is 1 of the interleukinconverting enzyme families of cysteine protease, is known to
mediate apoptosis in cardiomyocytes. The process of apoptosis has many pathways, but caspase-3 is thought to be central
in the apoptotic pathway.19 Therefore, caspase-3 activity can
be used as a molecular marker of myocardial apoptosis. The
present study showed significantly increased caspase-3
mRNA and TUNEL-positive cells in the heterotopically
transplanted heart. In addition, caspase-3 expression in the
unloaded heart tended to decrease in the clenbuterol group
compared with the control group. However, there was no
significant difference in TUNEL-positive cells in the clenbuterol group. The results of the present study suggest that
clenbuterol may have blocked the apoptosis pathway via
caspase-3 during mechanical unloading. Prior studies8 support these findings. ␤2-AR stimulation is reported to protect
cardiac myocytes from apoptosis, whereas ␤1-AR stimulation
is proapoptotic. In fact, Ahmet et al8 reported that administration of a ␤2-AR–selective agonist improved LV function
and attenuated apoptosis in a rat ischemic model. They
suspected that the effect of the ␤2-AR agonist to reduce
apoptosis might be mediated through ␤2-AR–inhibitory
G(Gi)-protein coupling. Gi opposes the action of Gs, which
appears to be central to ␤1-AR–stimulated apoptosis. Moreover, a recent study20 demonstrated that a ␤2-AR agonist
inhibited caspase-3 activity via direct inhibition of p38
mitogen-activated protein kinase. Activation of the ␤2-AR
pathway might play an important role in the prevention of
apoptosis during mechanical unloading.
The contractile function of papillary muscles was maintained in the unloaded heart. However, ␤-adrenergic responsiveness deteriorated in the unloaded heart compared with the
host heart. Clenbuterol did not change contractile function in
either the host or unloaded heart but improved ␤-adrenergic
responsiveness in the unloaded heart. The decrease in
␤-adrenergic responsiveness is due mainly to a decrease in
␤-AR density and impairment of the ␤-adrenergic signaling
pathway, including Gi protein, ␤-adrenergic kinase, and
intracellular Ca2⫹ homeostasis.21,22 In this study, we measured
␤-AR density, as indicated by the expression of ␤1- and
␤2-AR mRNAs. The ␤1-AR was not altered in either the host
heart or the unloaded heart. In contrast, clenbuterol induced
downregulation of ␤2-ARs in the unloaded heart. Isoproterenol is a nonselective ␤-adrenergic agonist, which stimulates
both ␤1- and ␤2-ARs, and mainly affects myocardial contractility through ␤1-AR stimulation. These findings suggest that
the improvement of ␤-adrenergic responsiveness in the
clenbuterol-treated, unloaded heart may not have been caused
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by an increase in ␤1-AR density. Although the present study
does not clarify the specific mechanism by which clenbuterol
improves ␤-adrenergic responsiveness, these beneficial effects may be related to the improvement in intracellular
homeostasis. The molecular data on SERCA2a, fetal gene
shifts of MHC, and apoptosis support this mechanism.
Downregulation of ␤2-ARs is an important issue, because
␤2-AR signaling may play a significant role in functional
recovery during mechanical unloading. ␤2-AR signaling is
valuable for maintaining heart function and distinctly differs
from that of ␤1-AR. However, ␤2-AR is easier to downregulate than ␤1-AR.21 In this study, ␤2-AR was downregulated in
the clenbuterol-treated, unloaded heart. Previous studies have
also shown that chronic administration of clenbuterol decreased total ␤-AR density, mostly ␤2-AR.23 Downregulation
of ␤2-ARs may deteriorate heart function. Recently, experimental studies have reported that cardiac ␤2-AR overexpression increases contractile function in chronic heart failure
models.24 Tevaearai and Koch24 delivered the ␤2-AR transgene during heterotopic heart transplantation, and this resulted in higher levels of ␤2-AR expression in the heart. In the
present study, ␤2-AR stimulation by clenbuterol demonstrated
beneficial effects during mechanical unloading but was accompanied by downregulation of ␤2-ARs. Before application
of these results to the clinical setting, future studies are
needed to focus on the optimal dose of clenbuterol during
mechanical unloading to minimize receptor downregulation
of ␤2-ARs.
The current study has some limitations. First, clenbuterol
was systemically given to animals. This drug may influence
the hemodynamics of the host heart and contribute to the
beneficial effects observed in the unloaded heart. In our
preliminary studies, administration of clenbuterol induced
cardiac hypertrophy but did not change hemodynamics such
as blood pressure. Petrou et al25 also reported that clenbuterol
did not alter hemodynamics in an ovine model. Thus, the
hemodynamic influences of clenbuterol on the host heart
should be minimal. Second, the LV cavity of the heterotopically transplanted heart was completely unloaded, and this
may mimic the full unloading that occurs during support with
an LVAD in the clinical setting. Although partial unloading
of the LV cavity may be desirable, this rat model is convenient and can provide much information about the functional
recovery associated with mechanical unloading.
Conclusions
During mechanical unloading by heterotopic heart transplantation, clenbuterol did not prevent myocardial atrophy but
improved gene expression (SERCA, ␤ MHC) and
␤-adrenergic responsiveness and potentially prevented myocardial apoptosis. These effects of clenbuterol may contribute
to the recovery of LV function after mechanical unloading.
However, chronic administration of clenbuterol may be
associated with downregulation of ␤2-ARs. Future studies are
needed to focus on the optimal dose and duration of clenbuterol therapy in mechanical unloading.
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Does the β2-Agonist Clenbuterol Help to Maintain Myocardial Potential to Recover
During Mechanical Unloading?
Hiroshi Tsuneyoshi, Wnimunk Oriyanhan, Hideo Kanemitsu, Reiko Shiina, Takeshi Nishina,
Satoshi Matsuoka, Tadashi Ikeda and Masashi Komeda
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Circulation. 2005;112:I-51-I-56
doi: 10.1161/CIRCULATIONAHA.104.525097
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