Ghrelin Modulates Sympathetic Nervous System Activity
and Stress Response in Lean and Overweight Men
Elisabeth Lambert, Gavin Lambert, Carolina Ika-Sari, Tye Dawood, Katie Lee, Reena Chopra,
Nora Straznicky, Nina Eikelis, Sara Drew, Alan Tilbrook, John Dixon, Murray Esler, Markus P. Schlaich
Downloaded from http://hyper.ahajournals.org/ by guest on June 17, 2017
Abstract—Ghrelin is a growth hormone–releasing peptide secreted by the stomach with potent effects on appetite.
Experimental and clinical studies indicate that ghrelin also influences cardiovascular regulation and metabolic function
and mediates behavioral responses to stress. We investigated the effects of ghrelin on blood pressure (BP), sympathetic
nervous system activity, and mental stress responses in lean (n⫽13) and overweight or obese (n⫽13) individuals.
Subjects received an intravenous infusion of human ghrelin (5 pmol/kg per minute for 1 hour) and saline in a randomized
fashion. Ghrelin decreased systolic (⫺6 and ⫺11 mm Hg) and diastolic BP (⫺8 mm Hg for both), increased muscle
sympathetic nervous system activity (18⫾2 to 28⫾3 bursts per min, P⬍0.05 and from 21⫾2 to 32⫾3 bursts per min,
P⬍0.001) in lean and overweight or obese subjects, respectively, without a significant change in heart rate, calf blood
flow, or vascular resistance. Ghrelin induced a rise in plasma glucose concentration in lean individuals (P⬍0.05) and
increased cortisol levels in both groups (P⬍0.05). Stress induced a significant change in mean BP (⫹22 and ⫹27 mm Hg),
heart rate (⫹36 and ⫹29 bpm), and muscle sympathetic nervous system activity (⫹6.1⫾1.6 and ⫹6.8⫾2.7 bursts per min)
during saline infusion in lean and overweight or obese subjects, respectively. During ghrelin infusion, the changes in BP and
muscle sympathetic nerve activity in response to stress were significantly reduced in both groups (P⬍0.05). In conclusion,
ghrelin exerts unique effects in that it reduces BP and increases muscle sympathetic nervous system activity and blunts
cardiovascular responses to mental stress. These responses may represent a combination of peripheral (baroreflex-mediated)
and central effects of ghrelin. (Hypertension. 2011;57:00-00.)
Key Words: ghrelin 䡲 obesity 䡲 stress 䡲 blood pressure 䡲 sympathetic nervous system
S
creased cardiovascular risk7 and possibly with metabolic
dysfunction.8 Experimental9 and clinical10 studies have
shown that acute psychological stress induced a rise in
plasma ghrelin concentration. Consequently, it has been
suggested that ghrelin may be a mediator of both behaviors
linked to food intake and body weight, as well as those
associated with psychological stress11; however, few data are
available to support such a hypothesis. Activation of ghrelin
signaling pathways in response to chronic stress has been
suggested as a potential homeostatic adaptation that helps an
individual to cope with stress.9 How ghrelin may influence
the cardiovascular system and stress responses is not known,
but there is some evidence to suggest that ghrelin may act
centrally to decrease sympathetic nervous system (SNS)
activity and blood pressure (BP),12 as well as the behavioral
responses to stress.5 To date, the effects of ghrelin administration on SNS activity and stress reactivity have not been
investigated in humans. We hypothesized that acute administration of ghrelin would inhibit sympathetic activity and
ince its discovery in 1999, ghrelin has attracted much
attention, because this growth hormone (GH)–releasing
peptide modulates multiple biological functions, including
food intake, glucose, and lipid metabolism, as well as
influencing cardiovascular function. Ghrelin’s ability to beneficially influence the cardiovascular system has been described in several clinical studies. Of particular interest are
results indicating that ghrelin has the ability to improve
cardiac function in chronic heart failure1 and endothelial
function in patients with the metabolic syndrome.2 Against
this background, the fact that circulating plasma levels of
ghrelin are decreased in obesity3 suggests that low ghrelin
levels might play a role in the etiology of obesity and the
associated cardiovascular risk that accompanies obesity. Indeed, ghrelin levels have been shown to be lower in obese
individuals3 and to rise after weight loss.4
Emerging data from animal experiments indicate that
ghrelin may also be linked to stress reactivity and to anxiety
and depression,5,6 conditions known to be linked with in-
Received February 7, 2011; first decision February 21, 2011; revision accepted March 25, 2011.
From the Laboratories of Human Neurotransmitters and Neurovascular Hypertension (E.L., G.L., C.I.-S., T.D., K.L., R.C., N.S., N.E., J.D., M.E.) and
Kidney Disease (M.P.S.), Baker IDI Heart & Diabetes Institute, Faculty of Medicine, Nursing and Health Sciences (G.L., M.E., M.P.S.), and Departments
of Physiology (E.L., S.D., A.T.) and General Practice (J.D.), Monash University, Melbourne, Victoria, Australia.
The funding organizations played no role in the design and conduct of the study; collection, management, analysis, or interpretation of the data; or
preparation, review, or approval of the articles.
Correspondence to Elisabeth Lambert, Human Neurotransmitters Laboratory, Baker IDI Heart & Diabetes Institute, PO Box 6492, St Kilda Road
Central, Melbourne, Victoria 8008, Australia. E-mail [email protected]
© 2011 American Heart Association, Inc.
Hypertension is available at http://hyper.ahajournals.org
DOI: 10.1161/HYPERTENSIONAHA.111.171025
1
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Hypertension
June 2011
alter the cardiovascular response to stress and that the
responses may differ between lean and overweight subjects.
Methods
over the 5-minute duration of the test. The MSNA was expressed as
burst incidence (bursts per 100 heartbeats).
CBF and Vascular Resistance
Twenty-six healthy male volunteers were recruited through 2 major
universities in the Melbourne metropolitan area. Thirteen lean (body
mass index: ⬍25 kg/m2) and 13 overweight (OW) or obese (Ob)
subjects (body mass index: ⬎25 kg/m2) participated in the study.
Subjects were aged between 19 and 25 years. They were nonsmokers
and not on any medications. None of the participants had a history of
metabolic, cardiovascular, or cerebrovascular disease. Physical examination and ECG were all normal. Subjects attended at 8:00 AM,
having fasted for 12 hours. The study protocol was approved by the
Alfred Hospital Ethics Committee, and all of the subjects gave
written informed consent before participating in the study.
Calf arterial blood flow was measured in the left leg using automated
venous occlusion plethysmography equipment (D.E. Hokansen,
Bellevue, WA). During measurements, an arterial occlusion cuff was
attached around the ankle to exclude the circulation of the foot, and
a venous occlusion cuff was placed around the thigh. Ankle cuffs
were inflated ⱖ60 seconds before measurements of flow to allow
CBF to stabilize. Strain gauges were attached around the thickest
part of the calf. Throughout the experiment, the leg was supported 10
cm above heart level to empty the venous system. CBF (in mL 䡠 100
mL⫺1 䡠 min⫺1) was calculated as the average of 12 measurements
before infusion and at times 12 to 15, 30 to 35, and 50 to 55 minutes.
Calf vascular resistance, expressed in arbitrary units, was calculated
as mean BP/CBF.
Study Protocol
Biochemical Analysis
Subjects
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Participants were studied in the supine position over one morning.
They were instrumented for muscle sympathetic nerve activity
(MSNA), heart rate (HR), BP recordings, and calf blood flow (CBF)
measurements. A venous catheter was placed in an antecubital vein
in each arm. One was used for drug infusion and the other for venous
blood sampling. All of the subjects underwent an identical testing
session. After a recovery period of 15 to 20 minutes, they received
either acylated ghrelin (Bachem) or saline for a period of 1 hour in
randomized order separated by a recovery period of ⱖ1 hour. The
recovery period was based on previous data showing that the half-life
of plasma ghrelin is very short, because it is degraded/eliminated
with a half-life of 10 minutes.13 Ghrelin was given at a dose of 5
pmol/kg per minute. Infusion rates were 20 mL/h for both ghrelin
and saline. The dose of ghrelin was chosen because it has been
shown previously to induce a 2- to 3-fold increase in plasma ghrelin
concentration, which reached steady state within 45 to 60 minutes.14,15 At 55 minutes postdrug infusion, mental stress (forced
mental arithmetic)16 was performed for a period of 5 minutes. At the
end of each stress test, subject-perceived stress levels were evaluated
using a 5-point scale of 0 (not stressful), 1 (somewhat stressful), 2
(stressful), 3 (very stressful), and 4 (very very stressful), as described
by Callister et al.17 Blood samples were taken for measurements of
glucose, insulin, ghrelin, cortisol, and adrenaline. A first blood
sample (10 mL) was taken just before the infusion of each drug, and
subsequent blood samples (10 mL) were taken at 55 minutes after
infusion (before stress) and at 60 minutes after infusion (after stress).
The participants were not made aware of the order of saline or
ghrelin administration.
Anxiety levels were examined immediately before the commencement of the investigation. The Spielberger State-Trait Anxiety
Inventories were used to assess the anxiety proneness (trait anxiety)
and the situational anxiety (state anxiety) of each subject.18
MSNA, BP, and HR Recording
A tungsten microelectrode (FHC, Bowdoinham, ME) was inserted
directly into the right peroneal nerve at the fibular head. A subcutaneous reference electrode was positioned 2 to 3 cm away from the
recording site. The nerve signal was amplified (⫻50 000), filtered
(bandpass, 700 to 2000 Hz), and integrated. Beat-to-beat BP was
measured to the digit using a Finometer device, and brachial BP was
assessed every 5 minutes (Dinamap). Brachial BP was used to assess
the effect of the drugs over time, and the Finometer was used to
determine changes in arterial BP occurring during mental stress. HR
was derived from continuous lead III ECG recordings.
Finometer BP, ECG, and MSNA were digitized with a sampling
frequency of 1000 Hz (PowerLab recording system, model ML
785/8SP, ADI Instruments, Bella Vista, New South Wales, Australia). MSNA was analyzed over a period of 3 to 5 minute before drug
infusion and at 10 to 15 minutes, 30 to 35 minutes, and 50 to 55
minutes after infusion. During mental stress, MSNA was averaged
Plasma concentration of adrenaline was measured by highperformance liquid chromatography with electrochemical detection.19 Insulin was measured by radioimmunoassay (Linco Research
Inc). The plasma concentration of cortisol was assayed using an
extracted radioimmunoassay, which was developed for fetal sheep
plasma20 using hydrocortisone (H4001, Sigma) as standard. The
mean recovery of [1,2,6,7 3H]cortisol (Perkin Elmer) from human
plasma using a dichloromethane extraction procedure was 82.68%.
The assay sensitivity was 0.33 ng/mL. The concentration of total
ghrelin in plasma was measured using a Ghrelin ELISA kit (Phoenix
Pharmaceuticals, Inc, Burlingame, CA) following the manufacturer’s
instructions.
Statistics
For each group of subjects the effects of the infusions (saline and
ghrelin) on brachial BP, HR, MSNA, and CBF and biochemical
measurements were analyzed using a 2-way repeated-measures
ANOVA (time ⫻ drug). The responses to stress for Finometer BP,
HR, MSNA, ghrelin, cortisol, and adrenaline were first compared in
each group of subjects using a 2-way repeated-measures ANOVA for
infusion (saline versus ghrelin) and stress (prestress versus stress).
Then, to compare possible differences in stress response between
lean and OW or Ob subjects, absolute changes (⌬) in cardiovascular
parameters were tested using a 2-way repeated-measures ANOVA
for the group comparison (lean versus OW or Ob) and infusion effect
(saline versus ghrelin). All of the post hoc comparisons were performed
by the Tukey test. The effects of saline and ghrelin on plasma cortisol,
adrenaline, and total ghrelin during stress were assessed for all of the
subjects combined by using a 1-way repeated ANOVA. Means were
considered significantly different when P⬍0.05.
Results
Complete sets of data were obtained in 11 lean subjects and
11 OW/Ob subjects. For 3 participants, the study was stopped
prematurely because of technical difficulties, and for 1
subject the study was stopped because the participant felt
unwell while receiving the ghrelin infusion (sensation of
flushes). The ghrelin infusion was overall well tolerated but
was associated with marked perspiration in 6 subjects. Table
1 presents the characteristics of the participants. OW/Ob
individuals had higher plasma levels of low-density lipoprotein cholesterol, triglycerides, and human C-reactive protein
and also presented with higher trait anxiety.
Effects of Saline and Ghrelin on BP, HR, MSNA,
CBF, and Calf Vascular Resistance
Figure 1 represents the BP and HR responses to both saline
and ghrelin infusions in lean and OW/Ob individuals. In both
Lambert et al
Table 1.
Baseline Characteristics of the Participants
Subjects
Lean
OW/Ob
11
11
Age, y
21.5⫾0.5
20.8⫾0.6
BMI, kg/m2
21.3⫾0.6
29.3⫾0.6‡
Waist circumference, cm
71.7⫾5.1
92.2⫾1.7‡
Hip circumference, cm
82.0⫾5.3
101.4⫾2.9‡
Cholesterol, g/L
3.95⫾0.18
4.47⫾0.21
HDL cholesterol, mmol/L
1.24⫾0.05
1.1⫾0.05
LDL cholesterol, mmol/L
2.30.141
2.79⫾0.18*
0.85⫾0.09
1.26⫾0.14*
N
Triglycerides, mmol/L
Cortisol, ng/ml
177⫾16
178⫾10
h-CRP, mg/L
0.78⫾0.12
1.95⫾0.35†
State anxiety
28⫾2
31⫾2
Trait anxiety
32⫾2
39⫾2‡
Ghrelin and Sympathetic Nervous System
groups, saline had no effect on BP, whereas ghrelin induced
a gradual reduction in BP. In lean subjects, ghrelin induced a
slight but not significant decrease in systolic BP compared
with that observed during saline infusion. Diastolic BP (DBP)
was significantly decreased during ghrelin compared with
saline infusion (ANOVA drug effect P⫽0.008), with the
changes significant between saline and ghrelin from 20
minutes postinfusion. At 55 minutes after ghrelin infusion,
DBP was 61⫾2 mm Hg compared with 67⫾2 mm Hg before
infusion (P⬍0.05).
Lean
Systolic and Diastolic BP (mmHg)
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In OW/Ob subjects, systolic BP was significantly decreased
during ghrelin compared with the saline infusion (ANOVA drug
effect P⫽0.031), with the changes significant from 25 minutes
after infusion. At 55 minutes after ghrelin infusion, systolic BP
was 112⫾1 mm Hg compared with 123⫾3 mm Hg before
infusion (P⬍0.05). DBP also decreased significantly during
ghrelin compared with saline infusion (ANOVA drug effect
P⫽0⬍0.001), with the changes significant between the 2 drugs
from 20 minutes after infusion. At 55 minutes after ghrelin
infusion, DBP was 60⫾1 mm Hg compared with 68⫾2 mm Hg
before infusion (P⬍0.05). Ghrelin or saline infusion did not
affect the HR in either of the subject groups.
Figure 2 shows the effects of saline and ghrelin infusions in
lean and OW/Ob individuals on MSNA expressed in bursts
per minute. In lean participants, MSNA increased from 18⫾2
to 28⫾3 bursts per min (55 minutes; P⬍0.001), whereas
MSNA remained stable during saline infusion (19⫾2 to
17⫾2 bursts per minute; P value not significant). The
changes in MSNA were significantly different between saline
and ghrelin (ANOVA, drug effect P⫽0.012), and the difference in infusions became significant from 12 minutes after
infusion. In OW/Ob participants, MSNA increased from
21⫾2 to 32⫾3 bursts per minute (55 minutes, P⬍0.001).
Saline did not influence MSNA (22⫾2 to 20⫾2 bursts per
minute, P value not significant). The changes in MSNA were
significantly different between saline and ghrelin (ANOVA,
drug effect P⫽0.002), and the difference became significant
from 30 minutes after infusion.
CBF recordings were performed in 7 subjects in each
group. As presented in Figure 3, in both lean and OW/Ob
individuals, CBF and calf vascular resistance remained stable
during the infusion of either saline or ghrelin.
OW indicates overweight; Ob, obese; BMI, body mass index; HDL, highdensity lipoprotein; LDL, low-density lipoprotein; h-CRP, human C-reactive
protein.
*P⬍0.05.
†P⬍0.01.
‡P⬍0.001.
Heart rate (bpm)
3
OW/Ob
Saline
130
Ghrelin
130
120
120
110
110
100
100
90
90
80
80
70
*
60
70
50
80
80
70
70
60
60
0
10
20
30
40
Time (min)
50
60
*
60
50
50
*
50
0
10
20
30
40
50
60
Time (min)
Figure 1. Systolic and diastolic blood pressures (BP) and heart rate (HR) during the infusion of saline and ghrelin in lean (left) and overweight/obese (OW/Ob; right) individuals. *P⬍0.05, ANOVA drug effect.
4
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June 2011
Lean
40
MSNA (bursts per min)
OW/Ob
Saline
Ghrelin
40
35
35
30
25
*
30
*
25
20
20
15
15
10
0
10
20
30
40
50
60
10
0
Time (min)
10
20
30
40
50
60
Time (min)
Figure 2. Muscle sympathetic nerve activity (MSNA) expressed in bursts per minute during the infusion of saline and ghrelin in lean
(left) and overweight/obese (OW/Ob; right) individuals. *P⬍0.05, ANOVA drug effect.
Effects of Stress on BP, HR, and MSNA
Given the fall in BP elicited by ghrelin, we examined in 6
additional subjects the MSNA response to BP reduction elicited
by the vasodilator anginine (sublingual, 600 ␮g). In this group
following anginine, systolic BP and DBP decreased by 7 and
6 mm Hg, respectively, within 15 to 20 minutes. This was
associated with a rise in HR of 4 bpm and an increase in MSNA
by 13 bursts per minute (P⬍0.05 for all parameters).
Figure 4 presents the mean BP, HR, and MSNA changes
observed under mental stress during saline and ghrelin
infusion for both lean and OW/Ob subjects. During saline
infusion, mental stress induced a significant elevation in
mean BP and HR and MSNA (P⬍0.05 versus prestress for
all). During ghrelin infusion, the maximum BP and MSNA
rises observed during mental stress were significantly altered
compared with that observed during the saline infusion in
both groups (ANOVA, drug effect P⬍0.001). The HR responses
to stress observed under ghrelin infusion were similar to those
during saline infusion.
Effects of Saline and Ghrelin on Plasma Ghrelin,
Glucose, Insulin, Cortisol, and Adrenaline
Ghrelin infusion induced a rise in total plasma ghrelin in both
groups. Ghrelin induced glucose to rise significantly in lean
subjects (Tables 2 and 3). The same trend was observed in
OW/Ob but it did not reach significance (P⫽0.11). Plasma
cortisol and adrenaline concentrations were increased during
ghrelin infusion (P⬍0.001) when all of the subjects were
combined (Table 3). Saline was without any effect on any of
these parameters.
CBF (ml/100ml/min)
Lean
CVR (units)
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Effects of Anginine on BP, HR, and MSNA
6
5
5
4
4
3
3
2
2
1
1
0
0
80
80
60
60
40
40
20
20
0
0
10
20
Given that plasma levels of adrenaline, cortisol, and total
ghrelin were similar between lean and OW/Ob subjects at rest
and before stress, changes in these parameters during stress
OW/Ob
Saline
Ghrelin
6
Effects of Saline and Ghrelin on Plasma Cortisol,
Adrenaline, and Total Ghrelin During Stress
30
Time (min)
40
50
60
0
Figure 3. Calf blood flow (CBF) expressed in
mL 䡠 100 mL⫺1 䡠 min⫺1 and calf vascular resistance
(CVR) during the infusion of saline and ghrelin in
lean (left) and overweight/obese (OW/Ob; right)
individuals.
0
10
20
30
40
Time (min)
50
60
Lambert et al
Table 2.
Ghrelin and Sympathetic Nervous System
5
Effects of Saline and Ghrelin Infusion on Plasma Total Ghrelin, Glucose, and Insulin Concentrations
Lean
OW/Ob
Before
Saline Infusion
Saline
Before
Ghrelin Infusion
Ghrelin
Before
Saline Infusion
Saline
Before
Ghrelin Infusion
Ghrelin
Total ghrelin, nmol/L
4.34⫾026
3.81⫾0.24
3.65⫾0.22
8.27⫾0.89†
4.12⫾0.4
3.82⫾0.3
4.49⫾0.7
11.89⫾2.2†
Glucose, mmol/L
4.45⫾0.12
4.51⫾0.12
NA
4.72⫾0.10†
4.75⫾0.1*
4.80⫾0.16
NA
5.01⫾0.13
Insulin, mU/L
13.4⫾1.9
9.5⫾1.9
14.1⫾3.1
12.8⫾1.95
21.1⫾1.9*
20.9⫾1.9
19.8⫾2.1
17.5⫾2.0
Drug
Note that plasma sample for glucose determination before infusion was only performed before the first preinfusion. OW indicates overweight; Ob, obese; NA, not
applicable.
*P⬍0.05 overweight vs lean.
†P⬍0.05 after vs before infusion.
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were assessed for all of the subjects combined (1-way
repeated ANOVA; Table 3). Plasma adrenaline rose during
the stress test during saline infusion (P⬍0.01), and this rise
was still apparent during ghrelin infusion (P⬍0.05). Plasma
cortisol and total ghrelin concentrations were not affected by
stress.
After mental arithmetic, stress perception was reported
between “somewhat stressful” and “stressful.” There was no
difference in the stress perception between the group of
subjects during saline infusion (lean: 2.63⫾0.45; OW/Ob:
3.18⫾0.58), and stress perception was not affected by ghrelin
(lean: 2.88⫾0.48; OW/Ob: 3.41⫾0.55).
Discussion
This study is the first to document the effects of ghrelin
administration in humans on resting SNS activity and cardiovascular response to mental stress. Our results indicate that
ghrelin did the following: (1) reduced BP; (2) increased
sympathetic nerve activity; and (3) blunted the sympathetic
and BP responses to mental stress without any effect on HR.
The responses to ghrelin do not differ between lean and
OW/Ob individuals.
In line with previous studies in humans,13,21 we report that
ghrelin induces a slight drop in BP. This decrease in BP has
been suggested previously to involve either a direct peripheral arterial vasodilation22 or a direct central inhibition of the
SNS.12 Contrary to expectation, our data clearly indicate that
MSNA is increased after peripheral ghrelin administration.
Whether sympathetic activation is triggered by arterial baroTable 3. Effects of Mental Stress on Plasma, Adrenaline
Cortisol, and Ghrelin Concentrations in All of the Subjects
Combined During Either Saline or Ghrelin Infusion
Saline Infusion
Ghrelin Infusion
Before
Stress
After
Stress
Before
Stress
After
Stress
Adrenaline,
pmol/L
11.6⫾1.7
30.6⫾6.9*
59.1⫾9.7†
78.5⫾10.0*‡
Cortisol,
ng/mL
161⫾16
159⫾16
210⫾8†
221⫾8‡
Ghrelin,
nmol/L
3.87⫾0.20
4.04⫾0.18
10.06⫾1.17†
Drug
*P⬍0.05 after vs before stress.
†P⬍0.05 ghrelin vs saline, before stress.
‡P⬍0.05 ghrelin vs saline, after stress.
9.20⫾1.05‡
reflex unloading is unsure. The drop in BP was not accompanied by any change in HR or significant changes in CBF or
calf vascular resistance, suggesting the absence of significant
vasodilation. Although ghrelin has been shown previously to
induce significant vasorelaxation in vitro,23 in patients, much
higher doses of ghrelin than the one used in the present study
are required to observe vasodilation.22 Likewise, it was found
that when administered at doses closer to the one used in the
present study, ghrelin exerts no vasodilation in the radial
artery24 and is without any effect on forearm blood flow.2
Nevertheless, anginine, which was associated with a similar
BP reduction to that observed after ghrelin, elicited a similar
degree of sympathetic activation. Given that vasodilation was
only investigated in the calf, we cannot rule out the possibility
that dilation occurred in other vascular beds and that baroreflex mechanisms may have contributed to the sympathoexcitation. Other alternatives include deactivation of lowpressure receptors caused by a possible decrease in central
venous pressure through GH receptor action25 or a direct
central effect. This last alternative is based on the knowledge
that ghrelin and ghrelin receptors have been identified in
many brain areas26 and that peripherally injected ghrelin can
activate neurons within the hypothalamus and may, in turn,
modulate resting BP and SNS activity in our subjects.
The present study is the first to demonstrate that ghrelin
blunts both BP and sympathetic activation associated with
mental stress in both lean and OW/Ob individuals. The fact
that baseline BP was decreased and MSNA augmented is
unlikely to affect the stress response, because isosorbide
dinitrate, a vasodilator that induced similar changes in these
parameters, has been shown not to alter sympathetic and BP
responses to mental stress.27 Ghrelin may possibly affect
stress response through a central effect. Peripheral or intraperitoneal administration of ghrelin induces c-fos expression
in the paraventricular nucleus of rats,28 an area involved in a
neural network mediating the autonomic, neuroendocrine,
and skeletal-motor responses of fear and anxiety,29 which
may be a prime area for mediating the effects of ghrelin on
the cardiovascular response to stress. Moreover, it is possible
that ghrelin acts to reduce the BP response to stress by a
peripheral mechanism, vasodilation is known to occur during
mental stress,30 and circulating adrenaline has been shown to
contribute to forearm vasodilation during mental stress.31
Because ghrelin induced a rise in circulating adrenaline
levels, this may boost the vasodilatory effect during stress
and, hence, restrain the BP rise.
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June 2011
25
20
15
Ghrelin
*
*
10
5
0
12
Change in MSNA
(bursts/min)
30
Change in HR
(bpm)
Change in mean BP
(mmHg)
Saline
50
35
40
30
20
10
0
Lean
OW/Ob
8
4
0
-4
-8
Lean
OW/Ob
*
Lean
*
OW/Ob
Figure 4. Maximum changes in mean blood pressure (BP), heart rate (HR), and muscle sympathetic nerve activity (MSNA) during mental stress test in lean and overweight/obese (OW/Ob) individuals during saline (open bars) and ghrelin (hatched bars) infusion. *P⬍0.05,
ANOVA drug effect.
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All of the cardiovascular effects of ghrelin observed during
rest and stress were very similar between the lean and OW/Ob
individuals, suggesting that ghrelin responses are unlikely to
be affected by weight. In agreement with Espelund et al,32
plasma ghrelin concentrations were identical between lean
and OW/Ob subjects, which is in contrast with previous
studies.3 However, the body mass index of our OW/Ob cohort
was lower than in this study (29 compared with 38 kg/m2),
which may explain the discrepancy.
Interestingly, although we observed that, during ghrelin
administration, BP and sympathetic response to stress were
blunted, perception of stress was not different. A previous
study provided evidence that MSNA is not primarily governed by the perception of stress,33 suggesting that perceived
stress does not modulate the autonomic responses to stress
and that there is a dissociation of MSNA and BP during
mental stress. The fact that ghrelin influences the cardiovascular responses to stress without affecting the stress perception indicates that ghrelin may act differently to modulate the
complex cardiovascular and emotional responses induced by
psychological stress. The effects of ghrelin administration on
cardiovascular response to stress have, to our knowledge,
never been studied in either animals or humans. Only anxietylike behaviors have been examined in rodents, and these have
yielded conflicting results with ghrelin inducing either anxiogenic5,34 or antianxiogenic effects.9
Overweight subjects presented with increased trait anxiety
but similar stress perception and cardiovascular responses to
the mental stress test compared with the lean individuals.
Some experimental and clinical studies have shown a link
between ghrelin response to stress and anxiety state; in rats,
acute stress induced a rise in plasma ghrelin, with the rise
being higher in animals who displayed more anxiety in
behavioral tests than other rat strains.6 Similarly, in humans,
plasma ghrelin increased during psychological stress, and this
rise was associated with a rise in cortisol but not in BP.10 In
the present study, although mental stress induced a rise in
plasma adrenaline concentrations, we found no effects on
plasma ghrelin or cortisol concentrations. The reason for this
lack of effect is not known but may involve differences in the
type of stress, its intensity, and length used compared with the
study from Rouach et al.10 As described previously, plasma
cortisol and adrenaline increased during ghrelin administration,35 indicating that ghrelin is able to modulate the
hypothalamic-pituitary-adrenal axis stress pathway. The in-
crease in cortisol and adrenaline was suggested previously to
be because of ghrelin’s adrenocorticotropic hormone–releasing property after activation of neuropeptide Y neurons in the
paraventricular nucleus.36 Adrenaline-releasing effect of
ghrelin may also involve the direct effect on the adrenal,
because GH-binding sites are present.37
In agreement with others, we found that ghrelin administration induced a significant increase in plasma glucose levels
in lean subjects. The change observed in our study is
comparable to that seen in the study from Vestergaard et al38
at a similar time point with the same dose. This hyperglycemic effect has been shown to be GH and cortisol independent.38 The mechanism of glucose release, in the present
study, is also not related, at least in the first hour, to any
decrease in insulin levels.35 However, it has been reported
that glucose is increased via a reduction in basal- and
insulin-stimulated disposal rate,38 and ghrelin was shown
recently to suppress glucose-stimulated insulin secretion39;
hence, insulin resistance is a likely candidate for the hyperglycemic effect of ghrelin. The findings of this study offer a
new hypothesis into ghrelin’s hyperglycemic actions: SNS
activation in a fasting state accelerates hepatic gluconeogenesis and in a fed state promotes glycogenolysis.40 Our
observation of an increase in MSNA could, therefore, be
another mechanism by which glucose is released into the
circulation during ghrelin infusion.
Limitations
Possible limitations of our study include the use of 1 short-term
dose of ghrelin and 1 type of stress, as well as reliance on
measurement of sympathetic activity in the muscle vascular bed.
The dose of ghrelin was chosen to elicit minimal BP drop.
Nagaya et al13 reported that a single injection of ghrelin at a dose
10 times higher than ours elicited an increase in plasma ghrelin
by ⬇61-fold and a drop in BP by 12 mm Hg. The dose that we
used, however, was more in the physiological range, because it
only doubled the plasma ghrelin concentration and moderately
decreased BP by 7 mm Hg. Ghrelin was constantly infused for a
limited period of 1 hour and may have triggered some acute
responses that may not have occurred if the peptide was infused
over a longer period of time. Effects of longer-term administration of the peptide would need to be addressed, because chronic
effects may result in different cardiovascular effects. Mental
stress resulted in a marked increase in BP and HR but a modest
rise in MSNA. Muscle sympathetic response to mental stress has
Lambert et al
been reported previously to present large individual differences33
and preferentially activates cardiac sympathetic activity.16 However, it consistently increases BP and HR, and Callister et al17
demonstrated that other types of stress, such as the Stroop color
word test, induced qualitatively similar responses.
11.
12.
13.
Perspectives
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Given the potential orexigenic and adipogenic14 properties of
ghrelin, treatment with an antagonist would seem logical in
obesity; however, the numerous actions of ghrelin need to be
clarified before it can be envisaged as to whether ghrelin
antagonists could be used for treating obesity and related
metabolic disorders. The findings of the present study provide
the first direct evidence in humans that ghrelin is able to
modulate resting sympathetic activity and the cardiovascular
response to mental stress. We41and others42 have previously
demonstrated abnormal MSNA in obesity and metabolic
disorders, as well as the deleterious impact of sympathetic
activation on cardiovascular health.41,42 Unraveling the mechanisms by which ghrelin affects both metabolic and cardiovascular functions may pose a challenge.
14.
15.
16.
17.
18.
19.
20.
Sources of Funding
E.L., M.P.S., M.E., J.D., and G.L. are supported by fellowships from
the National Health and Research Council of Australia.
Disclosures
M.E. serves on scientific advisory boards of Abbott (formerly Solvay)
Pharmaceuticals and ARDIAN Inc. M.P.S. serves on scientific advisory
boards for Abbott Pharmaceuticals and Novartis Pharmaceuticals. The
laboratories of M.E., M.P.S., E.L., and G.L. currently receive research
funding from ARDIAN Inc, Abbott Pharmaceuticals, Allergan, and
Scientific Intake.
21.
22.
23.
24.
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Ghrelin Modulates Sympathetic Nervous System Activity and Stress Response in Lean and
Overweight Men
Elisabeth Lambert, Gavin Lambert, Carolina Ika-Sari, Tye Dawood, Katie Lee, Reena Chopra,
Nora Straznicky, Nina Eikelis, Sara Drew, Alan Tilbrook, John Dixon, Murray Esler and
Markus P. Schlaich
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Hypertension. published online April 18, 2011;
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