JACC: CARDIOVASCULAR IMAGING
VOL. 3, NO. 1, 2010
© 2010 BY THE AMERICAN COLLEGE OF CARDIOLOGY FOUNDATION
ISSN 1936-878X/10/$36.00
PUBLISHED BY ELSEVIER INC.
DOI:10.1016/j.jcmg.2009.11.002
EDITORIAL VIEWPOINT
Adrenergic Excess, hNET1 Down-Regulation, and
Compromised mIBG Uptake in Heart Failure
Poverty in the Presence of Plenty*
Nezam Haider, PHD,† Ragavendra R. Baliga, MD, MBA,‡ Y. Chandrashekhar, MD,§
Jagat Narula, MD, PHD†
Long Beach, California; Columbus, Ohio; and Minneapolis, Minnesota
An increased adrenergic drive contributes to augmented myocyte contractility and altered peripheral
vasoreactivity in heart failure (HF) and appears
intuitively compensatory (1). The increased adrenergic contents in the synaptic cleft eventually result
in desensitization of the post-synaptic myocardial
See page 64
adrenergic receptors (2), cardiac remodeling, worsening HF, poor exercise tolerance, increased susceptibility to arrhythmias, and sudden cardiac death
(3). Systemic norepinephrine (NE) levels correlate
with mortality (4,5), and the beneficial effects of
beta-receptor blockade in HF support the role of
the adrenergic system in the progression and pathophysiology of HF (6 –9).
In the sympathetic neuron, tyrosine is actively
delivered into the axoplasm, where it is converted to
dopamine by cytoplasmic enzymes. Dopamine is
then transported to intraneuronal vesicles where
NE is synthesized and stored (Fig. 1). The action
potential at the nerve terminal triggers the release of
NE in the synaptic cleft, where it activates the
post-synaptic beta1-adrenergic receptors on the sarcolemmal membrane of the cardiac myocyte; cleft
NE also activates the pre-synaptic ␣2a- and ␣2cadrenergic receptors to inhibit further NE release.
*Editorials published in JACC: Cardiovascular Imaging reflect the views of
the authors and do not necessarily represent the views of JACC: Cardiovascular Imaging or the American College of Cardiology.
From the †Memorial Heart and Vascular Institute, University of California Irvine, Long Beach, California; ‡Ross Heart Hospital, Ohio State
University, Columbus, Ohio; and the §Veterans Administration Hospital,
University of Minnesota, Minneapolis, Minnesota. H. William Strauss,
MD, served as Guest Editor for this paper.
Almost all NE is actively removed from the synaptic cleft by a transporter, human norepinephrine
transporter 1 (hNET1).
In patients with HF, there are increased quantities of NE in the synaptic cleft. A small proportion
of this NE spills into the bloodstream (increasing
plasma NE levels), and the remainder is transported
by hNET1 back into the axoplasm. However, there
is gradual down-regulation of the transporter and
increased levels of cleft NE, an excess that results in
further impairment of NE reuptake (10). This
concept has been best portrayed by radionuclide
metaiodobenzylguanidine (mIBG) imaging; mIBG
is an NE analog that demonstrates storage, transport, and reuptake characteristics similar to NE in
sympathetic neurons.
Reduced NE reuptake results in low mIBG
concentration in the neurons, and the accelerated
release kinetics lead to an enhanced mIBG washout
rate. In clinical imaging, myocardial mIBG labeled
with radioactive iodine uptake is commonly quantified in terms of the heart to mediastinal (H/M)
uptake ratio. The lowest H/M (or the highest
washout rate) in patients with HF is associated with
the worst prognosis; mIBG uptake has been proposed to be a superior predictor of outcomes than
left ventricular (LV) ejection fraction or B-type
natriuretic peptide. Beta-blockers have been shown
to improve myocardial retention of mIBG (11,12),
and the improved mIBG uptake correlates with the
morphologic characteristics of reverse remodeling.
In this issue of iJACC, Drakos et al. (13) employed mIBG imaging to substantiate the role of
the adrenergic system in both the pathogenesis and
recovery of patients with HF. A markedly reduced
mIBG uptake in end-stage HF was substantially
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Haider et al.
Editorial Viewpoint
Figure 1. NE Cascade From Synthesis to Vesicular Storage, Release Into
Synaptic Cleft, Reuptake, and Partial Inactivation
AC ⫽ adenyl cyclase; COMT ⫽ catechol-o-methyltransferase; DA ⫽ dopamine; DHPG ⫽ dihydroxyphenylglycol; DOPA ⫽ dihydroxyphalanine; MAO ⫽
monoamine oxidase; NE ⫽ norepinephrine; NMN ⫽ normetanephrine.
reversed by unloading of the ventricle after left
ventricular assist device (LVAD) implantation; the
H/M mIBG uptake ratio on delayed images and the
washout rate were both significantly improved.
These results suggest that LVAD restore NE reuptake mechanisms. These data are consistent with
our recent observations of the significant reduction
in hNET1 expression in myocardial samples obtained from patients with advanced HF and marked
improvment after ventricular unloading with
LVAD.
To determine the status of hNET1 expression in
end-stage HF, we analyzed 8 hearts explanted from
heart transplant recipients, 4 with idiopathic dilated
and 4 with ischemic cardiomyopathy; hNET1 expression in cardiomyopathic hearts was compared
with 8 unused donor (normal) hearts. We determined hNET1 protein levels by immunoblot analysis; hNET1 protein expression was significantly
reduced in HF hearts compared with control hearts
(p ⬍ 0.001) (Figs. 2A and 2B). We also analyzed
paired specimens from 5 additional patients before
and after LVAD placement. The hNET1 protein
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was virtually absent in 4 of 5 pre-LVAD cardiac
tissues, and the post-LVAD samples showed a
dramatic increase in the hNET1 proteins levels
(p ⬍ 0.05) (Figs. 3A and 3B). The molecular data
indicated the feasibility of restoration of hNET1
expression in assisted hearts.
Norepinephrine-induced endoplasmic reticulum
stress and an impaired post-translational glycosylation of NET can accelerate its degradation and may
result in lower hNET density in HF. To elucidate
this possibility, we analyzed the hNET glycosylation profile in normal and HF tissues. There was no
difference in NET1 glycosylation in normal and HF
hearts (Fig. 2C), suggesting that there was no
defect in surface expression of hNET on the sarcolemma. Almost all of the hNET protein was glycosylated in these heart samples; a barely detectable
unglycosylated fraction of hNET was present in the
supernatant.
It is expected that the superfluous NE in the
synaptic cleft will normally be efficiently reabsorbed by the hNET1. In fact, patients with ␣2c
polymorphism who exhibit a retarded ability for
the feedback control of NE release from the
neurons and release excessive amounts of NE in
the synaptic cleft demonstrate a higher delayed
mIBG H/M ratio. It is surmised that an increased
activity of hNET1 serves as a protective molecular adaptation to partially offset the deleterious
consequences of an incessant bombardment of
the myocyte with toxic amounts of NE. However,
such compensatory mechanisms are gradually
overwhelmed, and persistent NE excess leads to
beta-adrenoceptor desensitization and worsening
of HF. Data from transgenic models of exaggerated NE release support the importance of NE
reuptake mechanisms. Three months after aortic
banding, dramatically reduced survival was observed in alpha2A-knockout (52%) and alpha2Cknockout (47%) mice owing to HF with enhanced LV hypertrophy, myocardial fibrosis, and
elevated circulating catecholamines, compared
with wild-type and alpha2B-deficient animals
(86%).
Multiple experimental studies have demonstrated
that genetic alterations in the NE receptors or their
downstream signaling pathways may be associated
with cardiomyopathy, with the myocytes possibly
the innocent victims. A cardiomyocyte-specific
transgenic overexpression of G␣q results in maladaptive myocardial hypertrophy and is also associated with reproducible peripartum cardiomyopathy
and HF; inhibition of G␣q-induced signaling at-
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Haider et al.
Editorial Viewpoint
Figure 2. hNET1 in Cardiomyopathic and Normal Human Hearts
(A) Immunoblot analysis for the assessment of human norepinephrine transporter 1 (hNET1) expression demonstrates decreased levels in
cardiomyopathic hearts as compared with control human hearts (1 control and 2 cardiomyopathy sample is shown). Equal protein in
each well was confirmed by glyceraldehyde 3-phosphate dehydrogenase (GAPDH) protein level. (B) Densitometric analyses of hNET1
immunoblot (Fluor-S MultiImager; Bio-Rad, Hercules, California) are expressed as a bar graph and represent the intensity of the hNET1
bands’ mean ⫾ SEM (*p ⬍ 0.05) of 4 IDCM, 4 ISCM, and 8 control hearts. Values were corrected by the respective values of GAPDH for
each sample. (C) The hNET1 glycosylation analysis demonstrated that most of the hNET1 protein in cardiomyopathic and control human
hearts was glycosylated, which bound and precipitated with con-A Sepharose beads (pellet); barely detectable unglycosylated hNET1 not
bound to con-A Sepharose beads remained in the supernatant. GAPDH immunoblot confirmed equal protein in each well. AU ⫽ arbitrary unit; IDCM ⫽ idiopathic dilated cardiomyopathy; ISCM ⫽ ischemic cardiomyopathy.
tenuates mitogen-activated protein kinase activation and cardiac hypertrophy. Genetic polymorphism of the rate-limiting enzyme tyrosine
hydroxylase also induces structural changes in the
myocardium. The exact regulation of the enzymatic
cascade through pre-synaptic adrenergic and muscarinic receptors, hNET1 efficiency, and deficits of
intraneuronal monoamine oxidase have not been
elucidated. It is noteworthy that the reduction of
myocardial NE uptake and loss of neuronal NE in
HF is not due to the loss of structural integrity of
the cardiac sympathetic nerves, as exemplified by
the unaffected expression of the pan-neuronal protein gene product 9.5.
Evolutionarily, the sympathetic neurons, as
compared with the parasympathetic neurons, are
primitive. Unlike parasympathetic neurons,
which have evolved with cholinesterase protection, sympathetic neurons rely completely on the
neuronal reuptake mechanism to manage NE
excess and hence are easily overwhelmed. In
absence of a metabolic inhibitor of NE in the
synaptic cleft, the excess NE results in adrenoceptor modulation, myocyte apoptosis (14), and
myocellular loss, further worsening myocardial
function. It seems that we pay the price for the
lack of evolution in the sympathetic system.
Nerve growth factor overexpression in mice
facilitates NE reuptake and prevents onset of HF.
Injection of nerve growth factor into the stellate
ganglia of rats with HF resulted in improved NE
uptake, replenishment of cardiac NE stores, and
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Haider et al.
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Figure 3. hNET1 Before and After LVAD Placement in Human Hearts
(A) Immunoblot showed very low levels of hNET1 protein in pre-left ventricular assist device (LVAD) sample, which were markedly
increased in post-LVAD (1 representative sample is shown). Equal protein in each well was confirmed by GAPDH protein level. (B)
Densitometric analyses of hNET1 immunoblot from 5 paired pre- and post-LVAD samples showed restoration of hNET1 levels in
post-LVAD myocardial samples. Values were corrected by the respective values of GAPDH for each sample. Abbreviations as in
Figure 2.
improvement in LV function (15); this NE consolidation occurred without any change in the
number of sympathetic nerves. Similarly, overexpression of NET1 resulted in marked improvement of HF in a rabbit model (16). The NET1
gene transfer normalizes not only protein expression of the NET1 but also beta1-adrenergic
receptors and sarcoplasmic reticulum Ca2⫹⫹ATPase, thus replicating some of the molecular
effects that typically accompany pharmacologically managed HF. Another important finding
was the complete recovery of NE uptake by LV
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Editorial Viewpoint
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Key Words: norepinephrine y
beta adrenoceptor
desensitization y sympathetic
nervous system y neurohumoral
agonists y heart transplantations
y heart failure.
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