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Circulation: Arrhythmia and Electrophysiology is published by the American Heart Association. 7272
2008;1;103-109; originally published online January 1,
Circ Arrhythm Electrophysiol
Bayles, Flora Socratous, Alex Agrotis, Garry Jennings, Gavin Lambert and Gautam
Elisabeth Lambert, Nina Eikelis, Murray Esler, Tye Dawood, Markus Schlaich, Richard
in Patients With Postural Tachycardia SyndromeCLINICAL PERSPECTIVE
Altered Sympathetic Nervous Reactivity and Norepinephrine Transporter Expression
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Altered Sympathetic Nervous Reactivity and Norepinephrine
Transporter Expression in Patients With Postural
Elisabeth Lambert, PhD; Nina Eikelis, PhD; Murray Esler, MD, PhD; Tye Dawood, PhD;
Markus Schlaich, MD, PhD; Richard Bayles, BSc; Flora Socratous, BSc; Alex Agrotis, PhD;
Garry Jennings, MD, PhD; Gavin Lambert, PhD; Gautam Vaddadi, MD
Background—Clinical observations in patients with postural tachycardia syndrome (POTS) suggest abnormal sympathetic
nervous system activity and a dysfunction of the norepinephrine (NE) transporter (NET).
Methods and Results—We examined sympathetic nervous system responses to head-up tilt by combining NE plasma
kinetics measurements and muscle sympathetic nerve activity recordings and by quantifying NET protein content in
peripheral sympathetic nerves in patients with POTS compared with that in controls. POTS patients had an elevated
heart rate during supine rest (81?2 bpm versus 66?2 bpm in healthy subjects [HS], P?0.01). Head-up tilt to 40°
induced a greater rise in heart rate in patients with POTS (?24?4 bpm versus ?13?2 bpm in HS, P?0.001). During
rest in the supine position, muscle sympathetic nerve activity, arterial NE concentration, and whole-body NE spillover
to plasma were similar in both groups. Muscle sympathetic nerve activity response to head-up tilt was greater in the
POTS group (?29?3 bursts/min in patients with POTS and ?13?2 bursts/min in HS, P?0.001), but the NE spillover
rise was similar in both groups (51% in the POTS subjects and 50% in the HS). Western blot analysis of NET protein
extracted from forearm vein biopsies in patients with POTS and HS demonstrated a decrease in the expression of NET
protein in patients with POTS.
Conclusion—Patients with POTS exhibit a decrease in NET protein in their peripheral sympathetic nerves. Paradoxically,
whole-body NE spillover to plasma during rest in the supine position and in response to head-up tilt is not altered despite
excessive nerve firing rate in response to the head-up tilt. (Circ Arrhythmia Electrophysiol. 2008;1:103-109.)
Key Words: tachycardia ? norepinephrine ? nervous system, sympathetic
heart rate (HR) on standing that is typically not accompanied
by a decrease in blood pressure (BP). Common symptoms
include fatigue, palpitations, exercise intolerance, and light-
headedness. Overall, patients with POTS experience clear
limitations across multiple domains of quality of life, includ-
ing physical and social functioning.1
he postural tachycardia syndrome (POTS) is a form of
orthostatic intolerance characterized by a dramatic rise in
Clinical Perspective see p 109
Numerous mechanisms have been invoked in the patho-
genesis of POTS, such as hypovolemia,2inadequate vasocon-
striction, peripheral denervation,3and excessive venous pool-
ing.4,5Moreover, it has been suggested that the functional
distribution of central sympathetic tone to the heart and
vasculature is abnormal in POTS.6The examination of the
sympathetic nervous system, estimated by measuring direct
muscle sympathetic nervous activity (MSNA), has yielded
equivocal results, with various reports showing high,6low,7
or normal8sympathetic activity in subjects with POTS. High
levels of circulating norepinephrine (NE) have been reported
in a number of studies,6,9,10but this is perhaps due to
decreased NE clearance11rather than increased NE release.
An alternative explanation for increased NE levels may be
an impairment of the clearance of the NE from the synaptic
cleft by the NE transporter (NET), the presynaptic transmem-
brane pump responsible for neuronal reuptake of NE. This
hypothesis is supported by a previous finding of a function-
ally significant genetic mutation in the NET in a family
kindred with POTS.12Furthermore, selective NET blockade
in healthy subjects induces a HR response to head-up tilt
similar to that of POTS patients.13The present study explores
the sympathetic nervous system response to progressive
head-up tilt in subjects with POTS compared with that of
healthy subjects, examines the NET protein content in pe-
ripheral sympathetic nerves assessed by subcutaneous vein
Received November 4, 2007; accepted April 11, 2008.
From the Human Neurotransmitter Laboratory (E.L., N.E., M.E., T.D., M.S., R.B., F.S., G.L., G.V.) and Cell Biology Laboratory (A.A.), Baker Heart
Research Institute (G.J.), Melbourne, Victoria, Australia.
Correspondence to Elisabeth Lambert, PhD, Baker Heart Research Institute, PO Box 6492, St Kilda Road Central, Melbourne, Victoria 8008, Australia.
© 2008 American Heart Association, Inc.
Circ Arrhythmia Electrophysiol is available at http://circep.ahajournals.orgDOI: 10.1161/CIRCEP.107.750471
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biopsy, and explores the possibility that a defect in NET
underpins the POTS phenotype.
The research protocol conformed to the relevant guidelines of the
National Health and Medical Research Council of Australia and was
approved by the Alfred Hospital Ethics Review Committee. A total
of 18 healthy subjects (10 men, 8 women, 27?3 years old) and 14
POTS patients (4 men, 10 women, 31?3 years old) participated in
the study after giving their written informed consent. The subjects
with POTS underwent an exhaustive medical evaluation to exclude
any other relevant medical condition. All patients with POTS shared
the common clinical characteristics central to the diagnosis, such as
recurrent episodes of presyncope while standing, freedom from
postural hypotension (decrease in systolic BP while standing in the
clinic ?20 mm Hg), and the presence of posture-related tachycardia
(mean HR increase on standing recorded in the clinic?41?3 bpm).
The most common symptoms of the POTS patients included fatigue
(n?10), palpitations (n?9), cognitive impairment (n?8), syncope
(n?7), and chest pain (n?4). All patients with POTS were unmedi-
cated. They were either newly diagnosed and never treated or had
stopped any medication for at least 7 days (7 days for ?-adrenergic–
blocking drugs, 21 days for fludrocortisone). The reference popula-
tion had none of the clinical characteristics listed earlier, and their
mean HR increase on standing recorded in the clinic was 11?2 bpm.
Participants were placed in the supine position on a tilt table and
were instrumented for plasma NE kinetics measurements and micro-
neurography recordings. Tilt testing was performed in 12 subjects
with POTS (3 men and 9 women, 27?1 years old) and 15 healthy
controls (7 men and 8 women, 24?2 years old). All subjects were
tested in the morning, after a light breakfast. Caffeine and alcohol
intake was excluded from 7 PM on the evening before the study. The
radial artery was cannulated percutaneously (3F, 5 cm, Cook) to
enable arterial BP monitoring and blood sampling for catecholamine
measurement. Subjects were instrumented with an intravenous can-
nula in an antecubital vein. A lead III ECG was recorded, and
respiration measurements were determined by using a transducer based
on a piezoelectric device (ADI Instruments, Castle Hill, NSW, Austra-
lia). After instrumentation, subjects were allowed to rest for at least 30
minutes. BP, ECG, respiration rate, and MSNA were measured for 10
minutes, and blood samples were collected from the subjects at rest.
Subjects were tilted to angles of 20°, 30°, and 40° for 10 minutes at each
angle. Blood samples were taken at the end of each tilt angle, and all
other parameters were collected continuously.
Multiunit sympathetic nerve firing rates in postganglionic fibers
distributed to the skeletal muscle vasculature were recorded by using
clinical microneurography as previously described.14The common
peroneal nerve was located by palpation and electrical stimulation by
using a surface probe. A tungsten microelectrode (FHC, Bowdoin-
ham, Maine) was inserted percutaneously and adjusted until satis-
factory spontaneous MSNA was observed in accordance with pre-
viously described criteria.14MSNA was expressed as burst
frequency (bursts per minute) and burst incidence (bursts per 100
heart beats). BP, ECG, and MSNA were digitized with a sampling
frequency of 1000 Hz (PowerLab recording system, model ML785/
8SP, ADI Instruments).
Cardiac Baroreflex Sensitivity
Baroreflex sensitivity was assessed by the sequence method,15using
BaroCor software (AtCor Medical, West Ryde, NSW, Australia). This
procedure identifies “spontaneous” sequences of 3 or more consecutive
beats in which systolic BP progressively rises (by at least 1 mm Hg) and
cardiac interval lengthens, or systolic BP progressively decreases (by at
1 beat. For each sequence, the linear correlation coefficient between
cardiac interval and systolic BP was computed, and the sequence was
validated when r?0.80. The slope between cardiac interval and systolic
BP was calculated for each validated sequence, and an average slope
was calculated for each recording.
Assessment of Spontaneous Arterial Baroreflex
Control of MSNA
Over a 3- to 5-minute period, diastolic pressures of individual heart
beats were grouped in intervals of 2 mm Hg, and for each interval,
the percentage of diastoles associated with a sympathetic burst was
plotted against the mean of the pressure interval. Muscle sympathetic
bursts were advanced by 1.3 second to compensate for baroreflex
delay.16The reflex gain was defined as the slope of the regression
line17and was assessed for each subject in the supine position.
The NE appearance, or spillover, rate to plasma was determined by
using the principle of isotope dilution during an intravenous infusion
of a tracer dose of tritiated NE. Participants were infused with a
tracer infusion of3H-labeled NE via the peripheral venous cannula at
0.6 to 0.8 ?Ci/min, after a priming bolus of 12 ?Ci. Arterial blood
sampling for measurement of endogenous and radiolabeled NE was
done after a minimum of 30 minutes’ infusion time to ensure that
steady-state plasma conditions had been reached and that plasma
NE-specific activity could be determined.18Blood was collected into
chilled tubes containing reduced glutathione and EGTA. Plasma was
separated by refrigerated centrifugation (4°C at 3000g) and stored at
?80°C until assayed. The plasma concentrations of NE and its
intraneuronal metabolite, 3,4-dihydroxyphenylglycol (DHPG), were
measured in all patients with POTS and 11 of the healthy subjects.
The total-body NE spillover to plasma and the total-body clearance
rate of NE were determined according to the following formulas:
Total Spillover Rate ? [3H]NE Infusion Rate (dpm/min)/
Plasma NE Specific Radioactivity (dpm/pmol)
Total Body Clearance ? [3H]NE Infusion Rate (dpm/min)/
Arterial [3H]NE Concentration (dpm/mL)
Plasma Renin Activity Determination
Blood samples were drawn in prechilled tubes and centrifuged at
2000g at 4°C. Plasma was stored at ?80°C until analysis. Plasma
renin activity (PRA, expressed as nanograms per milliliter per hour)
was determined by incubating the plasma samples at 37°C for 90
minutes and then by measuring the amount of angiotensin I gener-
ated with a commercial radioimmunoassay kit (Ren-CT2; CIS Bio
Noradrenaline Transporter Expression From Vein
Biopsies of Healthy Subjects and POTS Patients
A small vein biopsy was performed in 6 patients with POTS (3 men,
3 women) and 3 healthy controls (2 men, 1 women). Four of the
subjects with POTS but none of the 3 healthy subjects had completed
the tilt test. A skin incision was performed on the dorsum of the
forearm under local anesthesia to identify a vein with a diameter of
approximately 1 mm. One centimeter of the vein was removed after
ligation at both ends with absorbable suture material. One to 3 skin
sutures were used to provide adequate closure of the skin. After
removal, the vein was frozen in liquid nitrogen.
The tissue samples were homogenized in RIPA buffer, containing
50 mmol/L Tris-HCl (pH 7.5), 100 mmol/L NaCl, 2 mmol/L EDTA
(pH 8.0), 0.1% SDS, 0.5% NA deoxychloate, 1% Triton X-100, and
protease inhibitors (leupeptin, PMSF, aprotinin, and pepstatin).
Western blots were prepared by using lysates from vein biopsies.
Proteins were separated on 7.5% SDS-acrylamide gel and transferred
onto polyvinylidene fluoride (PVDF) membranes. Immunodetection
was performed by using anti-NET antibody (NET17–1; MAb Tech-
nologies, Inc). Immunoreactive bands were visualized by using a
104 Circ Arrhythmia Electrophysiol
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chemiluminescence system (NEN Life Science). Two specific bands
at 80 and 50 kDa were detected. ?-Tubulin (Sigma) was used as a
marker protein to assess protein loading of the gel. Quantification of
band density was performed on scanned images, with NIH Image
(version 1.62) software used to perform densitometry analyses.
Noradrenaline Transporter Expression From Vein
and Heart Biopsies of Deceased Donors
Left ventricles and forearm veins used in this study were obtained
during autopsy at the Victorian Institute of Forensic Medicine.
Informed consent of donor’s next of kin, arranged by the Donor
Tissue Bank of the Victorian Institute of Forensic Medicine, was
acquired before the autopsy. The Alfred Hospital Ethics Review
Committee approved tissue retrieval performed at the Victorian
Institute of Forensic Medicine.
The tissue samples were obtained from donors with no known
history of heart disease. Donors were also screened with regard to
their known medical history, particularly to exclude diabetes and
other endocrine disorders. For all tissues acquired at autopsy, time
elapsed since death did not exceed 72 hours. All tissues were
snap-frozen in liquid nitrogen on site and kept at ?80°C until
processed. The source of obtained tissue included 2 men (ages 40
and 67 years; body mass index 49 and 20 kg/m2; causes of death,
motor accident and ruptured ventricle, respectively) and 1 woman
(age 34 years; body mass index 36 kg/m2; cause of death, multiple
The tissues were homogenized in a similar fashion to vein
biopsies. GAPDH (sc-32233; Santa Cruz Biotechnology, Inc) was
used as a housekeeping protein.
Data were analyzed with 2-way ANOVA for repeated measures.
Pairwise multiple-comparison procedures were used when appropri-
ate. Results are reported as mean?SEM. Values of P?0.05 were
considered statistically significant differences.
The authors had full access to and take full responsibility for the
integrity of the data. All authors have read and agree to the
manuscript as written.
Western blot analysis revealed the presence of an 80-kDa band
in the positive control, a human neuroblastoma cell line, and in
the 3 controls. This band was less visible in 5 of the 6 patients
with POTS. Quantification by using ?-tubulin indicated a
significant reduction in NET protein content in the POTS group
(P?0.0017; Figure 1). The NET expression in the heart and
veins extracted from the deceased donors indicated the presence
of NET in both regions. There was a 2.8-fold increase in NET
expression in the heart compared to that in the forearm vein
(1.28, 3.86, and 3.30, respectively; Figure 1).
Baseline Sympathetic Parameters
Baseline averages for the 2 groups of subjects who underwent
the tilt test are summarized in the Table. There was no
difference in either systolic or diastolic BP during rest in the
supine position. Average HR at rest was significantly higher
in the POTS groups (P?0.001). There was no difference
between the 2 groups in baseline MSNA, regardless of
whether expressed as burst incidence or burst frequency.
Likewise, plasma NE, plasma DHPG concentration, whole-
body NE spillover, and PRA did not differ between the 2
groups. Plasma NE clearance tended to be lower in POTS
patients, but this did not reach significance (P?0.09). The
ratio of DHPG to NE plasma concentration was lower in the
POTS group (P?0.04). Patients with POTS, when compared
with the healthy subjects, displayed reduced cardiovagal
baroreflex function (P?0.001). The sympathetic baroreflex
gain was not significantly different between the 2 groups.
The tilt test had to be aborted in 1 healthy subject who
developed some symptoms of orthostatic intolerance at the
40° angle and in 3 patients with POTS (2 during the 40° and
1 during the 30° angle).
Progressive head-up tilt increased HR in healthy subjects (?7
bpm at 30°, P?0.005; ?13 bpm at 40°, P?0.001) and in POTS
patients (?8 bpm at 20°, P?0.05; ?15 bpm at 30°, P?0.001;
?24 bpm at 40°, P?0.001). As expected, the HR response was
more marked in the POTS patients at each angle (P?0.001;
Figures 2 and 3). Systolic BP, diastolic BP, and respiration rate
remained unaltered during the tilt test in both groups. Muscle
sympathetic nerve recording was done during rest in the supine
position in all patients and healthy subjects. The recording site
was maintained up to the highest angle reached in all but 1
of tilt 40° and at 20°). MSNA, as expressed in burst incidence,
increased during tilting in healthy subjects (?12 bursts per 100
heartbeats at a tilt of 40°; P?0.001) and in POTS patients (?12
bursts per 100 heartbeats at a tilt of 20°, P?0.001; ?19 bursts
per 100 heartbeats at a tilt of 30°, P?0.001; and ?24 bursts per
100 heartbeats at a tilt of 40°, P?0.001). The MSNA response
to head-up tilt was greater in the POTS patients than in healthy
Figure 1. Top, Western blot indicating abundance of sympa-
thetic nerve NET protein in forearm vein biopsies from healthy
volunteers and patients with POTS. NET protein was in 80- and
50-kDa forms, which is typical. Loading conditions were identi-
cal, with ?-tubulin as the loading marker. Protein extracted from
a human neuroblastoma cell line, SK-N-BE,2was used as a
positive control. *P?0.05, POTS vs controls. Bottom, Western
blot indicating NET expression in hearts and forearm veins from
3 human tissue donors. NET was expressed at higher rates in
the heart samples of all 3 donors. GAPDH was used as a
housekeeping protein. For the heart samples, 2 ?g of total pro-
tein was loaded in wells, whereas 20 ?g of total protein was
loaded in wells of the corresponding forearm vein samples.
Lambert et al Sympathetic Function in POTS
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subjects (30°, P?0.05; and 40°, P?0.01). Similarly, when
MSNA was expressed as burst frequency, differences be-
tween patients and controls were also evident after tilt
(Figures 2 and 3).
Plasma NE concentrations, whole-body NE spillover to
plasma, and NE clearance from plasma were measured in 9
patients with POTS and in 11 healthy subjects at all tilt angles
and in 1 POTS patient up to 30° only. Plasma NE concen-
tration increased during head-up tilt similarly in healthy
subjects and in POTS patients (Figure 4, healthy subjects
?115 pg/mL at 30°, P?0.001, and ?182 pg/mL at 40°,
P?0.001; POTS patients ?101 pg/mL at 30°, P?0.001, and
?184 pg/mL at 40°, P?0.001). NE clearance decreased to
the same extent in both groups (to 1.94?0.13 L/min in
healthy subjects at 40°, P?0.001, and to 1.53?0.12 L/min in
POTS subjects, P?0.022). NE spillover to plasma increased
similarly in both groups (healthy subjects ?206 ng/min at 40°,
P?0.001; POTS patients ?216 ng/min at 30°, P?0.002, and
?182 ng/min at 40°, P?0.016). Plasma DHPG concentrations
remained unaffected by the tilt test in both groups. The ratio of
plasma DHPG to plasma NE decreased equally in both groups
(healthy subjects to 4.34?0.38 at 30°, P?0.001, and to
P?0.003, and to 3.01?0.37 at 40°, P?0.001).
PRA increased similarly during tilting in the 2 groups of
subjects (from 0.88?0.36 to 1.75?0.48 ng/(mL ? h) at tilt
40°, P?0.05 in healthy subjects and from 0.80?0.17 to
1.73?0.45 ng/(mL ? h), P?0.01 in the POTS patients). Car-
diac baroreflex function progressively decreased during the
tilt test in both groups, but the values remained higher in the
healthy subjects until 30° (12.9?0.7 for the healthy group
and 5.4?0.8 ms/mm Hg for the POTS patients, P?0.002).
Underwent the Tilt Test
Baseline Average Values of the Participants Who
Body mass index, kg/m2
Systolic BP, mm Hg
Diastolic BP, mm Hg
MSNA, bursts per 100
Plasma NE, pg/mL
Plasma NE spillover, ng/min
Plasma NE clearance, L/min
Plasma DHPG, pg/mL
Plasma renin activity,
ng/(mL · h)
Cardiac baroreflex gain,
Sympathetic baroreflex gain,
burst incidence, mm Hg
Figure 2. HR, BP, and MSNA responses at supine rest and
head-up tilt in 1 healthy subject (top) and in a subject with
Figure 3. HR, systolic BP, diastolic BP, and MSNA responses at
supine rest and head-up tilt in the control subjects (F) and in
subjects with POTS (E). *P?0.05, **P?0.01, ***P?0.001, POTS
Figure 4. Plasma NE concentrations, NE spillover to plasma, NE
clearance, and plasma DHPG concentrations at supine rest and
in response to head-up tilt in control subjects (F) and in sub-
jects with POTS (E).
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Two striking new observations about the pathophysiology of
POTS emerge from this study. First, NET protein content is
decreased in peripheral sympathetic nerves of POTS patients.
The abnormal ratio between plasma concentration of dihy-
droxyphenylglycol and NE, found in the present study and
previously described,9,12provides some evidence of impaired
NE uptake. Second, the sympathetic nervous system response
to head-up tilt is disturbed in patients with POTS, who
displayed a preferential and enhanced sympathetic activation
in the outflow to the skeletal muscle vasculature during
head-up tilt, not evident in the total-body NE spillover
response, indicative of overall sympathetic activation that
was not different from that of normal subjects.
We investigated NET protein content in POTS patients,
given previous strong evidence that NET dysfunction may be
implicated in this syndrome. Shannon et al12showed the
existence of a specific genetic mutation in exon 9 of the NET,
which resulted in a 98% loss of function in 1 patient with
POTS. Second, reboxetine, a highly selective NE uptake
inhibitor, induces a phenotype that resembles POTS, increas-
ing HR in the supine position and inducing a dramatic
increase in HR in the upright position.13The genetic mutation
identified by Shannon et al12was not found in any of our
patients (data not presented) and has indeed not been ob-
served in any other patients with POTS.19The origin of
decreased NET protein in peripheral sympathetic nerves in
our POTS patients remains unknown. Decreased NET abun-
dance is unlikely to result from regulation of NE itself, not
only because POTS patients did not display higher plasma
NE concentrations, but also because experimental data sup-
port the idea that NE does not play a role in the regulation of
NET.20,21Whether decreased NET is a cause or a conse-
quence of POTS remains to be unequivocally demonstrated.
Given NET impairment in peripheral sympathetic nerves in
POTS patients, one would expect that this would be associ-
ated with higher NE spillover to plasma in the supine position
and in response to head-up tilt. At a given level of sympa-
thetic nerve firing, impairment of NE neuronal reuptake
would be expected to lead to higher levels of transmitter
overflow to plasma. Paradoxically, NE spillover to plasma
was normal in the supine position in patients with POTS, and
the increase with head-up tilt was of similar magnitude to that
observed in control subjects.
A number of reports have drawn attention to the presence
of high sympathetic nervous activity in the supine position in
subjects with POTS because plasma NE concentrations have
commonly been found to be elevated.6,9,12However, more
accurate estimation of sympathetic nervous activity by using
total-body NE spillover, as used in the present study, has
demonstrated normal sympathetic activity in POTS pa-
tients.10,11,22Elevated plasma NE concentrations may arise
from a reduction in plasma NE clearance11,22rather than the
activation of the sympathetic nervous system, given that NE
clearance from plasma is dependent on NE neuronal uptake.
We found plasma NE clearance to be marginally but not
significantly decreased in POTS patients. The decrease in
NET abundance we describe in POTS, together with a
decrease in the ratio of plasma concentration of DHPG and
NE, would be expected to be paralleled by an increase in
plasma NE concentration and NE spillover to plasma. With-
out such an increase in plasma NE, our cohort of patients with
POTS clearly are not “hyperadrenergic.” Whether there
occurs a specific activation of the cardiac sympathetic out-
flow remains to be determined. The reason for a lack of a
direct effect of NET dysfunction on plasma NE concentration
and NE spillover is unclear. Vincent et al23demonstrated that
NET inhibition by duloxetine caused a dose-dependent in-
crease in the levels of plasma venous NE both in the supine
and the upright positions. However, selective NET inhibition
by reboxetine did not alter NE plasma concentration in the
supine and upright positions in another study.24The mismatch
between NET abundance and NE spillover to plasma in
patients with POTS may reflect profound changes in NE
content within adrenergic nerves or abnormalities in NE
release, although this remains to be demonstrated. Indeed,
Jacob et al22found that sympathetic challenge induced by the
cold pressor test, sodium nitroprusside infusion, and tyramine
infusion increased NE spillover in the arms to a similar extent
in the POTS and control groups, although the increases in the
legs were smaller in the patients with POTS than in the
normal subjects. Another study reported that NE spillover to
plasma failed to increase in patients with POTS in response to
standing or to a tyramine injection.11
In agreement with normal NE spillover to plasma, direct
sympathetic nerve recording to the skeletal muscle confirmed
normal sympathetic firing activity in patients with POTS
during rest in the supine position. Muenter Swift et al8also
found normal MSNA; however, discordant MSNA results
have previously been observed in patients with POTS.6,7
Our finding of an excessive increase in MSNA during
head-up tilt in patients with POTS is in agreement with a
previous report.8Increased sympathetic nerve activity to the
skeletal muscle was also observed in patients with POTS in
response to a hypotensive challenge induced by sodium
nitroprusside.7The fact that MSNA increased disproportion-
ately to the rise in plasma NE concentration and NE spillover
during head-up tilt suggests the existence of a mismatch
between the nerve traffic and NE release. There have been
some suggestions that patients with POTS may have distal
sympathetic denervation,22,25whereby reduction in the num-
ber of nerves could perhaps increase the neural activity in the
remaining ones. Alternatively, there could be a functional
defect of postganglionic sympathetic neurons. For example,
patients with POTS tend to have a greater chronotropic
response to isoproterenol than do normal volunteers, suggest-
ing adrenoreceptor hypersensitivity,9and similarly, they are
hypersensitive to the pressor effect of phenylephrine.11
Our finding that patients with POTS do not display overall
increased sympathetic nervous activity as a whole but greater
response to postural stimulation, together with a mismatch
between nerve traffic and NE release, suggests the presence of
abnormal neuronal function that goes beyond NET dysfunction.
It is important to note that it is not known whether the
reduced NET protein that we document in the peripheral
sympathetic nerves is paralleled by such a reduction in the
sympathetic nerves in the legs and in the heart. Tissue
samples obtained from 3 deceased subjects enable us to
Lambert et al Sympathetic Function in POTS
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demonstrate that both peripheral veins and heart tissue were
rich in NET, therefore supporting to a degree the use of
peripheral veins in the assessment of NET abundance. The
heart is more dependent on the activity of the NET to clear the
NE from synaptic clefts than is the case in the vascular beds,26
and therefore, a reduction in NET function in the heart could
underlie a greater HR rise during sympathetic activation in
patients with POTS. Goldstein et al10documented higher
cardiac NE spillover to plasma in subjects with POTS than in
controls and noted that increments in HR during yohimbine
infusion correlated significantly positively with that in car-
diac NE spillover. However, cardiac extraction of3H-labeled
NE, taken as an index of the activity of the NET, was not
reduced, which suggests that high cardiac NE spillover was
more likely due to increased cardiac sympathetic firing and
high NE release rather than decreased NE reuptake. Our
results favor the idea that NET impairment is also present in
cardiac sympathetic nerves in patients with POTS. If NET
impairment is present in most sympathetic nerves in patients
with POTS, it is probable that it would affect the regional
sympathetic system differentially. It is important to recognize
that POTS is a heterogeneous condition, with patients pres-
enting physiological and clinical differences. Hence, our
results of reduction in NET protein cannot necessarily be
generalized to all patients with POTS. Because of the rela-
tively small number of patients in whom vein biopsies were
obtained, we are unable at this point in time to quantitatively
link the defect in NET to their clinical characteristics.
Other mechanisms could contribute to the abnormal cardio-
vascular response to the upright posture in patients with POTS.
The marked impairment of cardiac baroreflex sensitivity seen in
patients with POTS when compared with healthy control sub-
jects, described by others,8suggests that vagal impairment also
contributes to the excessive tachycardia seen in patients with
POTS. Impairment of the renin-angiotensin system may induce
with previous observations.28However, PRA in patients with
POTS seems inappropriately low, given the degree of hypovo-
lemia that they exhibit.28Another important mechanism that
could induce excessive sympathetic response in patients with
POTS is the increase in venous pooling in the lower extremi-
ties.29Such an increase in venous pooling would require aug-
mented sympathetic activation to the lower extremities to main-
tain arterial pressure, which may perhaps translate into increased
sympathetic nerve firing in the skeletal muscle, as we described.
Stewart et al5demonstrated that venous pooling in patients with
POTS occurs as a result of blunted arterial vasoconstriction
rather than a defect in venous compliance.
In summary, we have documented that patients with POTS
exhibit decreased NET protein in their peripheral sympathetic
nerves. However, during rest and head-up tilt, whole-body
NE spillover to plasma was not accentuated. Further investi-
gations should aim at delineating the molecular mechanism
underlying the defect of NET expression and should specif-
ically target the heart to establish whether a cardiac defect in
NET is responsible for the magnification of the cardiac
sympathetic response and tachycardia elicited during upright
The authors thank the staff of the Donor Tissue Bank of the Victorian
Institute of Forensic Medicine for invaluable assistance in acquiring
human tissue for research.
Sources of Funding
This study was supported by a National Health and Medical
Research Council of Australia (NHMRC) program grant (No.
225108). E. Lambert and M. Schlaich are supported by NHMRC
Career Development Awards, and M. Esler and G. Lambert are
supported by NHMRC Research Fellowships.
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The simple act of standing can be a challenge for some people, being accompanied by weakness, dizziness, or fainting. The
postural orthostatic tachycardia syndrome (POTS) is one specific disorder causing problems of this type. The defining
characteristic of POTS is an excessive rise (increases by 30 bpm or more or heart rate in excess of 120 bpm) in heart rate
upon standing. Often the rise in heart rate is accompanied by presyncope or fainting in the absence of postural hypotension.
Although the underlying pathophysiology of POTS remains unclear, there is evidence to suggest that elevated
norepinephrine spillover from the heart correlates with heart rate in POTS patients. Reuptake of norepinephrine into
sympathetic nerves after its release terminates the neural signal. A fault in transmitter inactivation augments the effects of
sympathetic nerve traffic. In the heart, approximately 80% to 90% of released norepinephrine is recaptured into
sympathetic nerves, making the heart more sensitive than all other organs to impairments of transmitter reuptake. Indeed,
an abnormality in neuronal norepinephrine reuptake could sensitize the heart to sympathetic activation and its
consequences. In this study, we found that patients with POTS exhibit decreased norepinephrine transporter protein in their
peripheral sympathetic nerves. Future studies should concentrate on the molecular mechanism underlying the defect in
norepinephrine transporter and establish whether it is responsible for the excessive cardiac sympathetic response and
tachycardia elicited during the upright posture.
Lambert et al Sympathetic Function in POTS
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