(Circulation. 2000;101:1960.)
© 2000 American Heart Association, Inc.
Clinical Investigation and Reports |
From the Division of Cardiology (J.-M.C., J.B.H., W.W.L., A.C.L., T.-J.W., P.-S.C.), Department of Medicine, Division of Cardiothoracic Surgery (L.C., T.A.D.), Department of Surgery, Cedars-Sinai Medical Center and University of California Los Angeles School of Medicine; Department of Pathology (M.C.F., I.P.S.), University of California Los Angeles School of Medicine; Department of Pathology (P.L.W.), Veterans Affairs Medical Center and University of California San Diego; and Division of Neurology (L.S.C.), Childrens Hospital Los Angeles and University of Southern California Keck School of Medicine.
Correspondence to Lan S. Chen, MD, Division of Neurology 82, Childrens Hospital Los Angeles, 4650 Sunset Blvd, Los Angeles, CA 90027.
| Abstract |
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Methods and ResultsWe studied 53 native hearts of transplant recipients, 5 hearts obtained at autopsy of patients who died of noncardiac causes, and 7 ventricular tissues that had been surgically resected from the origin of ventricular tachycardia. The history was reviewed to determine the presence (group 1A) or absence (group 1B) of spontaneous ventricular arrhythmias. Immunocytochemical staining for S100 protein, neurofilament protein, tyrosine hydroxylase, and protein gene product 9.5 was performed to study the distribution and the density of sympathetic nerves. The average left ventricular ejection fraction was 0.22±0.07. A total of 30 patients had documented ventricular arrhythmias, including ventricular tachycardia and sudden cardiac death. A regional increase in sympathetic nerves was observed around the diseased myocardium and blood vessels in all 30 hearts. The density of nerve fibers as determined morphometrically was significantly higher in group 1A patients (total nerve number 19.6±11.2/mm2, total nerve length 3.3±3.0 mm/mm2) than in group 1B patients (total nerve number 13.5±6.1/mm2, total nerve length 2.0±1.1 mm/mm2, P<0.05 and P<0.01, respectively).
ConclusionsThere is an association between a history of spontaneous ventricular arrhythmia and an increased density of sympathetic nerves in patients with severe heart failure. These findings suggest that abnormally increased postinjury sympathetic nerve density may be in part responsible for the occurrence of ventricular arrhythmia and sudden cardiac death in these patients.
Key Words: nervous system tachycardia death, sudden cardiomyopathy
| Introduction |
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| Methods |
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Group 1 (n=53) included ventricular tissues that were obtained from the native hearts of the transplant recipients. Paraffin-embedded tissue blocks were collected retrospectively (n=41) from January 1994 to June 1996, and fresh ventricular tissues were obtained prospectively (n=12) from December 1996 to February 1998. The fresh tissues were obtained at the time of transplant surgery and were processed immediately. Among group 1 patients, 25 had ischemic heart disease and 28 had idiopathic dilated cardiomyopathy. The group 1 patients were divided into 2 subgroups: patients with a history of VT or SCD (group 1A, n=30) and patients without a history of VT or SCD (group 1B, n=23). Patients with a history of VT or SCD were those who had documented spontaneous sustained VT, nonsustained VT (>3 beats), or SCD requiring either medical therapy or the implantation of an implantable cardioverter-defibrillator.
Group 2 (n=5) included ventricular tissues that were obtained from 5 patients who underwent an autopsy between December 1996 and February 1998 and who died of noncardiac causes and had no history of heart diseases or ventricular arrhythmia. Causes of death were breast cancer (n=1), ovarian cancer (n=1), hepatic failure (n=2), and motor vehicle accident (n=1). The tissues were harvested 8 to 16 hours after death.
Group 3 (n=7) included paraffin-embedded tissue blocks that were collected from 7 patients with coronary artery disease who underwent intraoperative computerized mapping and surgical ablation for sustained monomorphic VT between 1988 and 1991. Tissues excised from the origin of VT were used for the study.
Immunocytochemistry
Tissue Fixation
All ventricular tissues of the 41 retrospective
patients were formalin fixed for
48 hours, dehydrated with graded
alcohol, cleared in xylene, embedded in paraffin wax, and stored for
2 years before immunocytochemical studies. No frozen sections were
obtained.
For tissues that were collected prospectively, both frozen and paraffin-fixed sections were obtained. To prepare frozen tissue sections, transmural tissue slices of 2- to 3-mm thickness were cut, embedded in OCT compound (Tissue Tek; Sakura Finetechnical Co, Ltd), and snap-frozen with Cytocool II Tissue Freezing Aerosol (Stephens Scientific).
Immunostaining Procedures
Transmural sections of 5 µm were cut perpendicular to the
epicardium from either a frozen or a paraffin-embedded tissue block and
mounted onto slides. Before being processed for
immunostaining, paraffin sections were deparaffinized
and rehydrated. Frozen sections were prefixed with 10% buffered
formalin for 2 hours because S100 is a highly soluble protein. The
slides were then reacted with a 3% hydrogen peroxide/methanol solution
to inactivate endogenous peroxidase, followed
by a final 5-minute wash with PBS. For S100 and NF staining, paraffin
sections were treated with a trypsin solution for 10 minutes in 37°C
at pH 8.1. For TH staining in paraffin sections, slides were treated
with Target Unmasking Fluid (Signet Laboratories, Inc) for 10 minutes
at 90°C in a microwave oven and then washed with PBS after being
cooled down at room temperature. For PGP 9.5, slides were incubated in
Retrieve-All (Signet Laboratories, Inc) in a circulating water bath
maintained at 90°C. To reduce nonspecific staining, sections were
incubated with Serum-Free Protein Block (DAKO Corp) for 10 minutes.
Sections were incubated with primary antibodies for 1 hour and with
biotinylated secondary antibodies (DAKO Corp) and then with
ABComplex/HRP (DAKO) for 30 minutes each at room temperature. Sections
were thoroughly washed with PBS between each staining. The
immunoreactive products were visualized through incubation of
tissue sections for 2 to 3 minutes with DAKO Liquid DAB
Substrate-Chromogen System. The sections were then counterstained with
dilute hematoxylin, rinsed with an acid/alcohol solution for 3 seconds,
dehydrated through a graded alcohol series, and mounted.
For double immunostaining, sections were thoroughly washed with Tris buffer solution (because PBS inhibits the alkaline phosphatase reaction) after reaction with peroxidase and DAB. Tissue sections were incubated at room temperature with primary and secondary antibodies for 30 minutes each and then with a complex of avidin/biotin/alkaline phosphatase (ABComplex/AP; DAKO Corp) for 30 minutes, with sufficient washing with Tris buffer solution between steps. The sections were then developed with a fast red chromogen system to allow visualization of the nerves. The fast red chromogen system yielded a purple-red stain, whereas the DAB chromogen system yielded a yellow-brown stain. The sections were then lightly counterstained with hematoxylin, rinsed with an acid/alcohol solution, and mounted with aqueous mounting medium.
Sources of Antibodies
Primary antibodies used in this study were rabbit anti-cow S100
(recognizing S100A and S100B, 1:1000 dilution; DAKO Corp), monoclonal
mouse anti-human NF protein (1:300 dilution; DAKO Corp), monoclonal
mouse anti-rat TH (working concentration 0.2 µg/mL;
Boehringer Mannheim Biochemica), and rabbit anti-human receptor
trkA (1:100 dilution; Santa Cruz Biotechnology, Inc). PGP 9.5
antibodies (1:4000 dilution) were obtained from Chemicon International
Inc.
Negative and Positive Controls
For each protein marker, negative and positive control stains
were performed in the same staining session. Negative controls were
obtained by either replacing primary antibodies with rabbit anti-human
von Willebrand factor (factor VIIIrelated antigen, 1:500
dilution; DAKO Corp) or omitting primary antibodies. Positive controls
for S100, NF, and TH were obtained by immunostaining
the tissue sections from schwannoma, cerebellum, and sympathetic
ganglia, respectively.
Measurement of the Density of Nerve Fiber and Statistics
The density of S100-labeled nerve fibers was calculated through
computer-assisted image analysis with the MCID imaging
processing system (Imaging Research Inc). The procedure was performed
by an investigator (W.W.L.) who was blinded to the patients clinical
history.
For each section, 2 areas of myocardium were studied: (1) the periphery of necrotic or fibrotic myocardium and (2) the perivascular regions. If the periphery of injured myocardium was not easily identified or no myocardium injury existed, nerve fiber density was measured in the non-necrotic myocardium that contained most nerve structures. For each area, 6 fields that contained most nerve fiber structures were chosen visually to calculate the total number and the total length of nerve fiber structures. Under a x10 objective, 1 image was digitized into a 639x479-pixel array and equaled an area of 0.953 mm2.
The nerves were differentiated from the other structures by the use of the grain count program within the MCID. Briefly, all of the immunostained structures were highlighted with an arbitrarily set threshold. The chosen threshold included all nerve fibers that could be easily recognized by microscopy at 10x10 magnification. Shape criteria were used to discount structures based on the degree of roundness. A circle is 1°, and a line is 0°. In most circumstances, a cutoff of 0.4° excluded all of the structures other than nerve fibers (eg, myocytes, blood vessel, fibroblasts, or artifacts). Size criteria were used to discount any nonspecific staining of <1x10-8 mm2.
The nerve fiber density for each patient is expressed as the mean of the total number of nerve fibers per millimeters squared and the mean of the total length (mm) of nerve fibers per millimeters squared, which were calculated from all chosen fields. The data for each patient were calculated from 1 to 3 sections.
| Results |
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Double immunostaining of TH and S100 colocalized
the same structures (Figure 2
).
Immunostaining for S100, TH, and neurofilament protein
(NF) conducted on serial sections from frozen tissues also showed that
these 3 markers labeled the same structures (Figure 3
). These observations suggest that the
S100 immunoreactive nerve fiberlike structures contain not only
Schwann cells but also nerve axons and that the majority of these
fibers are likely sympathetic nerves.
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Regional Increase of Innervation in Injured Myocardium
No tyrosine kinase A (trkA) immunoreactivity was observed
except in the blood vessel walls. The lack of trkA immunoreactivity in
the cardiac tissue is consistent with the observation reported
by others.7 Large nerve trunks and small nerve fascicles
immunoreactive to S100 were consistently observed in all tissue
sections. Consistent immunostaining for NF and
TH was obtained only in frozen tissue sections (12 explant hearts and 5
autopsy hearts) and not in sections from paraffin-embedded tissue
blocks (41 explant hearts and 7 surgically resected tissues), which was
likely due to long formalin fixation and storage times.
The distribution of nerves was much less homogeneous in the
ventricles with myocardial injury (groups 1A and 1B) than in the
ventricles from normal hearts (group 2). The S100-immunopositive nerve
fibers were most abundant at the periphery of necrotic tissues (Figure 4A
) or in the perivascular regions
(Figure 4B
). In some sections, nerve fascicles were scattered in
a "swarm-like" pattern at the junction between necrotic and
surviving myocardium (Figure 4C
). Clusters of
S100-labeled small nerve twigs were also found in some injured areas
with fibrosis and revascularization (Figure 4D
). In contrast to the periphery of damaged
myocardium, no S100-reactive nerve fibers were seen in the
central zone of necrotic tissues or dense fibrotic tissues. TH-positive
nerve fibers were observed in the injured areas or around the
coronary arteries in both patients with ischemic
cardiomyopathy (Figure 5A
) and patients with idiopathic dilated
cardiomyopathy (Figure 5B
).
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In group 3, the ventricular tissues resected from the
origin of ventricular tachycardia (VT) showed
fibrosis intermingled with normal-appearing myocytes. Similar to those
seen in explanted hearts, S100-immunopositive nerve fibers were
abundant and easily identified in the tissue fragments from 6 patients
(Figure 6A
), and TH-positive nerves were
detected in 3 patients (Figure 6B
).
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Taking these observations together, it appears that ischemic or nonischemic myocardial injury results in inhomogeneous distribution of nerve fibers. A regional increase of sympathetic nerves was commonly observed in the injured myocardium. The tissues from arrhythmogenic sites had abundant sympathetic nerve fibers.
Relationship Between Density of S100-Immunoreactive Nerve Fibers in
the Ventricles and History of Ventricular Tachyarrhythmias
The density of nerve fibers
(Table
) was measured only in tissues
stained with S100 because the immunoreactivities to TH or NF were not
consistently observed in 41 retrospectively collected
paraffin-embedded tissues. There was no difference in the nerve
densities between patients with ischemic and
nonischemic cardiomyopathy. Therefore, the
53 patients who underwent heart transplantation were classified only
according to history of VT. All patients with VT were treated with
antiarrhythmic drugs, an implantable cardioverter-defibrillator, or
both. Group 1A had a higher nerve density than the control group (group
2). In contrast, the density of nerves did not differ significantly
between group 1B and group 2. This quantitative result demonstrates
that an increase in nerve fiber density in the myocardium
is associated with a history of VT in the patients with severe organic
heart disease.
| Discussion |
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Visualization of Nerves in Autopsied Heart
According to Hirsch et al,8 the first descriptions of
cardiac innervation were made in the mid-19th century by
Lugwig9 and Bidder10 in studies of the frog
heart. Since then, many investigators have studied cardiac nerves with
the use of conventional histological
techniques.8 Recent advancements in immunocytochemical
techniques have allowed investigators to study the sympathetic and
parasympathetic nerve distributions in autopsied
hearts.11 12 13 14 15 Chow et al16 studied the
effects of death on the immunofluorescence of
autonomic nerves that supply the human ventricular
myocardium. Percutaneous myocardial samples
were obtained at 5 to 11 days after death. The authors demonstrated
that depending on the type of antibodies used, immunohistochemical
techniques can be used on human hearts obtained within 6 days of death.
The results of these studies and the present study indicate that
visualization of nerves in the autopsied heart is now possible as a
routine procedure.
Myocardial Injury Is Associated With Inhomogeneous
Innervation With Regional Hyperinnervation, Most Likely of Sympathetic
Nerve Origin
Consistent with other reports, the present study
demonstrates that the distribution of nerve fibers in the ventricles is
altered after myocardial injury resulting from either ischemia
or cardiomyopathy. The necrotic
myocardium is associated with denervation,3
whereas the peripheries of myocardial scar and perivascular regions are
richly innervated.6 17 Regional
hyperinnervation is related with myocardial injury because nerve fiber
density was significantly higher in patients with organic heart disease
than in patients without heart disease. Animal models of myocardial
ischemia indicated that the increase in nerve fibers at the
borders of injured myocardium resulted from the
proliferative regeneration of Schwann cells and axons.18
Whether the increased nerve fibers are all sympathetic nerves cannot be
concluded in the present study because the parasympathetic nerves
were not stained. However, it is well known that
ventricular tissue is innervated mainly by
sympathetic nerves. Parasympathetic nerve fibers also innervate
the ventricles, but they are much less numerous.19 We have
shown that the majority of cardiac nerves that stained positive for
S100 are immunoreactive to TH and that colocalization of S100 and TH is
consistently observed. It is likely that a significant portion,
if not all, of regenerated nerves in the perinecrotic or perivascular
region are sympathetic nerves. This notion is supported by the
demonstration of sympathetic denervation and reinnervation in
myocardium injuries with the use of
131I-labeled
metaiodobenzylguanidine
scanning.20
The significant increase in nerve growth factor (NGF) activity had also been observed in the Schwann cells of the sympathetically denervated rat iris.21 In addition, it has been demonstrated that interleukin-1 released by macrophages plays a role in the augmented local production of NGF after peripheral nerve injury.22 Insults that damage myocardium can also injure nerves (denervation) and cause inflammation with macrophage infiltration. An induction of NGF synthesis in non-neural cells that results from myocardial injuries may provoke sympathetic nerve regeneration. We propose that the increased quantity of sympathetic nerves may be secondary to injury-related nerve sprouting. However, whether nerve sprouting actually was responsible for these observations could not be determined in this retrospective study.
S100-Immunoreactive Structures in Ventricles Are Mostly Sympathetic
Nerve Fibers
S100 protein has been used as a maker of cardiac nerve
fibers.6 17 18 S100 families consist of homodimers or
heterodimers of S100
and S100ß subunits.23 24 S100A
(
ß) is expressed predominantly in neurons, S100B (ßß) is
expressed predominantly in glial cells and neurons, and S100A0 (
)
is expressed predominantly in skeletal and heart muscle24
and in neurons.25 The anti-S100 antibody used in the
present study is polyclonal rabbit anti-cow S100 specific for S100A
and S100B, which strongly cross-reacts with human S100A and S100B but
does not cross-react with S100A0. Furthermore, the expression of S100
by Schwann cells depends on the axon (ie, the Schwann cellaxon
contact).24 Immunostaining of serial
sections and double immunostaining in the present
study show that S100-labeled structures are also labeled by NF, PGP
9.5, and TH. Small nerve fascicles immunoreactive to TH were easily
observed in frozen sections of myocardium. These data
suggest that most S100-immunopositive nerve fiberlike structures
demonstrated in the present study are sympathetic nerves.
An Increase in Nerve Fiber Density Is Associated With a History of
Ventricular Tachyarrhythmia
The second major finding of the present study was that an
increase in cardiac sympathetic innervation observed in our patients
was significantly associated with a history of ventricular
arrhythmia. It is well known that ventricular
arrhythmia and SCD occur in patients with poor
ventricular function of various causes.26
Although patients with ischemic heart disease may have
arrhythmias triggered by ischemia and anatomic
conduction block due to myocardial scarring, the mechanism of
arrhythmogenesis in patients with idiopathic
cardiomyopathy is unclear. The enhanced spatial
inhomogeneity of cardiac innervation may be associated with the
occurrence of ventricular arrhythmia in patients
with idiopathic cardiomyopathy. As sympathetic
nerve activation exerts significant effects on
electrophysiological properties such as
automaticity, refractoriness, and conduction velocity of myocardial
cells,27 28 29 an enhanced spatial inhomogeneity in cardiac
innervation might amplify the spatial inhomogeneity of these
electrophysiological properties and
therefore facilitate the initiation of ventricular
arrhythmia. In the present study, we observed that
abnormally increased nerve density not only occurred in all of our
patients with congestive heart failure but also was an independent
factor associated with the occurrence of ventricular
arrhythmia. It is therefore reasonable to propose that regional
hyperinnervation may play a significant role in arrhythmogenesis in
patients with chronic severe congestive heart failure.
Study Limitations
A major limitation was the retrospective nature of this work. Most
tissues were fixed in formalin and stored in paraffin-embedded tissue
blocks for 1 to 2 years before they were stained. This may have
accounted for the negative TH and NF stains in some tissues. However,
in freshly collected and fixed tissues, we have shown that S100, TH,
and PGP 9.5 stained the same nerves. Because S100 staining was positive
in all retrospectively collected tissue blocks, we were able to
quantify the nerves in each tissue without difficulty. This limitation
should not invalidate the conclusion of the study.
In summary, the present study demonstrated that a regional increase in sympathetic nerves in the ventricles was present in patients with severe organic heart diseases and that the density of nerve fibers is significantly associated with history of ventricular tachyarrhythmias. The data suggest that abnormal distribution of sympathetic nerves in the ventricle may contribute to the occurrence of VT in patients with severe organic heart disease.
| Acknowledgments |
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Received June 14, 1999; revision received November 2, 1999; accepted November 15, 1999.
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