Ghrelin infusion in humans induces acute insulin resistance and lipolysis independent of growth hormone signaling.
ABSTRACT Ghrelin is a gut-derived peptide and an endogenous ligand for the growth hormone (GH) secretagogue receptor. Exogenous ghrelin stimulates the release of GH (potently) and adrenocorticotropic hormone (ACTH) (moderately). Ghrelin is also orexigenic, but its impact on substrate metabolism is controversial. We aimed to study direct effects of ghrelin on substrate metabolism and insulin sensitivity in human subjects.
Six healthy men underwent ghrelin (5 pmol . kg(-1) . min(-1)) and saline infusions in a double-blind, cross-over study to study GH signaling proteins in muscle. To circumvent effects of endogenous GH and ACTH, we performed a similar study in eight hypopituitary adults but replaced with GH and hydrocortisone. The methods included a hyperinsulinemic-euglycemic clamp, muscle biopsies, microdialysis, and indirect calorimetry.
In healthy subjects, ghrelin-induced GH secretion translated into acute GH receptor signaling in muscle. In the absence of GH and cortisol secretion, ghrelin acutely decreased peripheral, but not hepatic, insulin sensitivity together with stimulation of lipolysis. These effects occurred without detectable suppression of AMP-activated protein kinase phosphorylation (an alleged second messenger for ghrelin) in skeletal muscle.
Ghrelin infusion acutely induces lipolysis and insulin resistance independently of GH and cortisol. We hypothesize that the metabolic effects of ghrelin provide a means to partition glucose to glucose-dependent tissues during conditions of energy shortage.
- SourceAvailable from: Snezana Tepavcevic[Show abstract] [Hide abstract]
ABSTRACT: We investigated the effects of ghrelin on protein expression of the liver antioxidant enzymes superoxide dismutases (SODs), catalase (CAT), glutathione peroxidase (GPx), and glutathione reductase (GR), nuclear factor κB (NFκB) and inducible nitric oxide synthase (iNOS). Furthermore, we aimed to investigate whether extracellular regulated protein kinase (ERK1/2) and protein kinase B (Akt) are involved in ghrelin-regulated liver antioxidant enzymes and iNOS protein expression.Archives of Medical Science 08/2014; 10(4):806-816. · 1.89 Impact Factor
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ABSTRACT: Controlling meal related glucose excursions continue to be a therapeutic challenge in diabetes mellitus. Mechanistic reasons for this need to be understood better to develop appropriate therapies. To investigate delayed gastric emptying effects on postprandial glucose turnover, insulin sensitivity and beta cell responsivity and function, as a feasibility study prior to studying patients with type 1 diabetes, we used the triple tracer technique, C-peptide and oral minimal model approach in healthy subjects. A single dose of 30 μg of pramlintide administered at the start of a mixed meal was used to delay gastric emptying rates. With delayed gastric emptying rates, peak rate of meal glucose appearance was delayed and rate of endogenous glucose production (EGP) was lower. C-peptide and oral minimal models enabled the assessments of beta cell function, insulin sensitivity and beta cell responsivity simultaneously. Delayed gastric emptying induced by pramlintide improved total insulin sensitivity and decreased total beta cell responsivity. However, beta cell function as measured by total disposition index did not change. The improved whole body insulin sensitivity coupled with lower rate of appearance of EGP with delayed gastric emptying provides experimental proof of the importance of evaluating pramlintide in artificial endocrine pancreas approaches to reduce postprandial blood glucose variability in patients with type 1 diabetes.American journal of physiology. Endocrinology and metabolism. 07/2014;
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ABSTRACT: Ghrelin is a 28 amino acid peptide mainly derived from the oxyntic gland of the stomach. Both acylated (AG) and unacylated (UAG) forms of ghrelin are found in the circulation. Initially, AG was considered as the only bioactive form of ghrelin. However, recent advances indicate that both AG and UAG exert distinct and common effects in organisms. Soon after its discovery, ghrelin was shown to promote appetite and adiposity in animal and human models. In response to these anabolic effects, an impressive number of elements have suggested the influence of ghrelin on the regulation of metabolic functions and the development of obesity-related disorders. However, due to the complexity of its biochemical nature and the physiological processes it governs, some of the effects of ghrelin are still debated in the literature. Evidence suggests that ghrelin influences glucose homeostasis through the modulation of insulin secretion and insulin receptor signaling. On the other hand, insulin was also shown to influence circulating levels of ghrelin. Here, we review the relationship between ghrelin and insulin and we describe the impact of this interaction on the modulation of glucose homeostasis.World journal of diabetes. 06/2014; 5(3):328-341.
Ghrelin Infusion in Humans Induces Acute Insulin
Resistance and Lipolysis Independent of Growth
Esben Thyssen Vestergaard, Lars Christian Gormsen, Niels Jessen, Sten Lund, Troels Krarup Hansen,
Niels Moller, and Jens Otto Lunde Jorgensen
OBJECTIVE—Ghrelin is a gut-derived peptide and an endoge-
nous ligand for the growth hormone (GH) secretagogue receptor.
Exogenous ghrelin stimulates the release of GH (potently) and
adrenocorticotropic hormone (ACTH) (moderately). Ghrelin is
also orexigenic, but its impact on substrate metabolism is
controversial. We aimed to study direct effects of ghrelin on
substrate metabolism and insulin sensitivity in human subjects.
RESEARCH DESIGN AND METHODS—Six healthy men un-
derwent ghrelin (5 pmol ? kg?1? min?1) and saline infusions in a
double-blind, cross-over study to study GH signaling proteins in
muscle. To circumvent effects of endogenous GH and ACTH, we
performed a similar study in eight hypopituitary adults but
replaced with GH and hydrocortisone. The methods included a
hyperinsulinemic-euglycemic clamp, muscle biopsies, microdi-
alysis, and indirect calorimetry.
RESULTS—In healthy subjects, ghrelin-induced GH secretion
translated into acute GH receptor signaling in muscle. In the
absence of GH and cortisol secretion, ghrelin acutely decreased
peripheral, but not hepatic, insulin sensitivity together with
stimulation of lipolysis. These effects occurred without detect-
able suppression of AMP-activated protein kinase phosphoryla-
tion (an alleged second messenger for ghrelin) in skeletal
CONCLUSIONS—Ghrelin infusion acutely induces lipolysis and
insulin resistance independently of GH and cortisol. We hypoth-
esize that the metabolic effects of ghrelin provide a means to
partition glucose to glucose-dependent tissues during conditions
of energy shortage. Diabetes 57:3205–3210, 2008
observation that GHS-R is located in peripheral tissues
suggests that ghrelin may exert direct effects (4). The
effects of ghrelin on substrate in humans are uncertain, but
insulin resistance and stimulation of lipolysis have been
reported (5–7). However, it remains difficult to segregate
hrelin, an endogenous ligand for the growth
hormone (GH) secretagogue receptor (GHS-R),
stimulates GH and adrenocorticotropic hor-
mone (ACTH) secretion (1) in addition to hav-
direct effects from effects related to GH and cortisol, and
we have recently demonstrated that somatostatin infusion
fails to sufficiently suppress ghrelin-induced GH and cor-
tisol secretion (8). Hormonally replaced hypopituitary
patients constitute a means for studying putative GH- and
cortisol-independent effects of ghrelin in human subjects
We aimed to study potential direct effects of ghrelin
on substrate metabolism and insulin sensitivity in the
postabsorptive state. In one experiment in healthy
adults, we assessed whether ghrelin-induced GH release
translated into GH signaling in skeletal muscle, in the
event of which the importance of abrogating indirect
effects of ghrelin is obvious. Second, we studied the
effects of ghrelin exposure on whole-body and regional
substrate metabolism in the basal and insulin-stimulated
state in hypopituitary patients on stable replacement
with GH and hydrocortisone.
RESEARCH DESIGN AND METHODS
The studies were conducted in accordance with the Helsinki Declaration and
following the approval by the local ethics committee, the Danish Medicines
Agency, and the Good Clinical Practice (GCP) unit of Aarhus University
Hospital. Both protocols were registered (Clinicaltrials.gov identification
study 1: NCT00116025 and study 2: NCT00139945).
Preparation of synthetic ghrelin. Synthetic human acylated ghrelin
(NeoMPS, Strasbourg, France) was dissolved in isotonic saline and sterilized
by double passage through a 0.8/0.2-?m pore-size filter (Super Acrodisc;
Gelman Sciences, Ann Arbor, MI).
Study 1: subjects and study protocol. Six healthy men (aged 23 ? 1 years,
BMI 23.5 ? 0.4 kg/m2) were examined as previously described (6). They
received a constant infusion of saline or ghrelin (5 pmol ? kg?1? min?1)
starting at 0 min. At 90 min, a muscle biopsy was obtained from the lateral
vastus muscle with a Bergstro ¨m biopsy needle (Fig. 1).
Study 2: subjects and study protocol. Eight hypopituitary men (aged 53 ?
4 years, BMI 31.6 ? 1.0 kg/m2) on stable replacement therapy with GH and
hydrocortisone (for ?3 months) participated. None of the patients had
diabetes (A1C 5.7 ? 0.1% [range 4.9–6.0]) or any concomitant chronic disease.
Each patient was studied on two occasions with 5-h infusions of saline or
ghrelin (5 pmol ? kg?1? min?1) in a randomized double-blind, cross-over
design. Both study days commenced at 0800 h after an overnight fast (?9 h),
with the subjects remaining fasting.
One intravenous cannula was inserted in the antecubital region for
infusion, and one intravenous cannula was inserted in a heated dorsal hand
vein for sampling of arterialized blood. At t ? 0 min, saline or a primed-
continuous ghrelin infusion (5 pmol ? kg?1? min?1) was commenced. The
bolus dose was estimated from the elimination rate constant of ghrelin (k01)
(6) and infused over a 20-min interval to avoid an overshoot of steady-state
levels. Muscle biopsies were obtained at 120 min, as described above. A
hyperinsulinemic-euglycemic clamp (insulin 0.6 mU ? kg?1? min?1; Actrapid,
Novo Nordisk, Denmark) was performed from 120 to 300 min. Plasma glucose
was clamped at 5.0 mmol/l by adjusting the rate of infusion of 20% glucose
according to plasma glucose measurements every 10 min. Insulin sensitivity
was calculated from the glucose infusion rate (GIR) during the clamp. The
period from 0 to 120 min is referred to as the basal period and the period from
120 to 300 min as the clamp period. Blood samples were obtained as indicated
in Fig. 2.
From Medical Department M (Endocrinology and Diabetes), Aarhus Univer-
sity Hospital, Aarhus, Denmark.
Corresponding author: Esben Thyssen Vestergaard, firstname.lastname@example.org.
Received 8 January 2008 and accepted 26 August 2008.
Published ahead of print at http://diabetes.diabetesjournals.org on 5 Septem-
ber 2008. DOI: 10.2337/db08-0025. Clinical trial reg. nos. NCT00116025 and
© 2008 by the American Diabetes Association. Readers may use this article as
long as the work is properly cited, the use is educational and not for profit,
and the work is not altered. See http://creativecommons.org/licenses/by
-nc-nd/3.0/ for details.
The costs of publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked “advertisement” in accordance
with 18 U.S.C. Section 1734 solely to indicate this fact.
DIABETES, VOL. 57, DECEMBER 20083205
Tracers. A primed-continuous infusion of [3-3H]-glucose (bolus 12 ?Ci, 0.17
?Ci/min; NEN Life Science Products, Boston, MA) was initiated at t ? 0 min
and continued throughout. Glucose rate of appearance (Ra) was calculated at
10-min intervals from 90 to 120 min and 270 to 300 min using Steele’s
non–steady-state equations (9). During the clamp, endogenous glucose pro-
duction was calculated by subtracting the GIR from Ra. Oxidative rates of
glucose (GOX) and lipids were calculated from indirect calorimetry (Deltatrac;
Datex Instruments, Helsinki, Finland) after correction for protein oxidation,
which was estimated from the urinary excretion of urea. Nonoxidative
glucose disposal was calculated as whole-body glucose disposal (Rd) minus
the rate of GOX(10).
Microdialysis. Microdialysis was performed and analyzed as described
previously (8). Catheters (CMA 60, molecular cutoff of 20 kDa, membrane
length 30 mm; CMA, Stockholm, Sweden) were placed in the lateral vastus
muscle, in the subcutaneous adipose tissue, and in the femoral subcutaneous
adipose tissue. The abdominal and femoral adipose tissue blood flow was
estimated by Xe washout (11).
Blood samples and measurements. Plasma glucose was analyzed in dupli-
cate using the glucose oxidase method (Beckman Instruments, Palo Alto, CA).
Serum ghrelin (total levels) was measured in duplicate by an in-house assay
(12). Serum GH, cortisol, and insulin were analyzed with a double monoclonal
immunofluorometric assay (Delfia; Perkin Elmer, Wallac Oy, Turku, Finland).
Serum free fatty acids (FFAs) were determined using a commercial kit (Wako
Chemicals, Neuss, Germany). Plasma catecholamines were measured by
liquid chromatography (13). Plasma glucagon was measured by radioimmu-
noassay (14). Glucose, glycerol, lactate, and urea in the microdialysis dialysate
were measured in duplicate by an automated spectrophotometric kinetic
enzymatic analyzer (CMA 600; CMA).
Western blotting and phosphatidylinositol 3-kinase assay. Muscle biop-
sies were homogenized as previously described (15). Aliquots of protein were
resolved by SDS-PAGE, and proteins were electroblotted onto nitrocellulose
membranes. Immunoblotting was performed using primary antibodies as
follows: p signal transducers and activators of transcription (STAT5)a and -b,
STAT5, pSTAT3, STAT3, p extracellular signal–regulated kinase (ERK)1 and
-2, ERK-1 and -2, p AMP-activated protein kinase (AMPK)?, AMPK?-pan, p
acetyal-CoA carboxylase (ACC), pAkt, pAkt substrate, and Akt substrate 160
(AS160). Membranes were incubated with horseradish peroxidase–coupled
secondary antibodies, visualized by BioWest enhanced chemiluminescence
(UVP LabWorks, Upland, CA) and quantified by the UVP BioImaging System.
Densitometric measurements were adjusted to an internal control. Phospha-
tidylinositol 3-kinase (PI3K) activity was assessed, as previously described
Statistics. Results are expressed as means ? SE. Systemic levels of hor-
mones, metabolites, and GIR were analyzed by two-way ANOVA. The inter-
action between time and treatment (time ? treatment) was considered the
term of interest. The Bonferroni correction was used to account for multiple
comparisons when appropriate. Pairwise comparisons were carried out by
Student’s two-tailed paired t test when appropriate. P values ?0.05 were
considered significant. Statistical analysis was performed using SPSS version
14.0 for Windows.
Study 1. Ghrelin infusion stimulated endogenous GH
secretion, which peaked at t ? 60 min (1.1 ? 0.9 ?g/l
[saline] vs. 33.3 ? 8.0 ?g/l [ghrelin]; P ? 0.008). A
significant elevation in serum FFA levels was recorded
(0.4 ? 0.04 ?g/l [saline] vs. 1.0 ? 0.1 ?g/l [ghrelin]; P ?
0.003). The levels of serum cortisol (268 ? 24 nmol/l
[saline] vs. 400 ? 57 nmol/l [ghrelin]; P ? 0.06) and plasma
glucose (5.2 ? 0.1 mmol/l [saline] vs. 5.5 ? 0.1 mmol/l
[ghrelin]; P ? 0.16) were similar. Western blots performed
on skeletal muscle biopsies revealed distinct STAT5 phos-
phorylation in all six subjects 30 min after the endogenous
GH burst (Fig. 3).
Study 2. Pituitary surgery had been performed in all
cases, and GH deficiency was documented by GH stimu-
lation tests (insulin tolerance test [n ? 7] or arginine test
[n ? 1]; means ? SE peak GH 0.3 ? 0.1 ?g/l).
Hormones and metabolites. Hormones and metabolites
are shown in Fig. 2. Serum ghrelin concentrations were
similar at baseline on the 2 study days (0.51 ? 0.06 ?g/l
[saline] vs. 0.49 ? 0.06 ?g/l [ghrelin]; P ? 0.38) and
correlated inversely with BMI (saline r ? ?0.83, P ? 0.01;
ghrelin r ? ?0.73, P ? 0.04). Plasma levels of norepineph-
rine and epinephrine were comparable on both study days,
and plasma levels of glucagon were also similar (10.9 ? 0.9
pmol/l [saline] vs. 9.3 ? 0.5 pmol/l [ghrelin], P ? 0.10 at t ?
120 min; and 8.6 ? 0.5 pmol/l [saline] vs. 7.1 ? 0.8 pmol/l
[ghrelin], P ? 0.14 at t ? 300 min).
Resting energy expenditure and glucose and lipid
metabolism. Data on resting energy expenditure and
respiratory quotient (RQ) in study 2 are given in Table 1.
Energy expenditure, RQ, or lipid oxidation were not
significantly affected by ghrelin in the basal or in the clamp
period. The increase in RQ (RQclamp? RQbasal) during the
clamp, however, was larger in the saline study (0.07 ? 0.01
vs. 0.03 ? 0.01 [ghrelin], P ? 0.03).
FFAs. Ghrelin infusion induced an 80% increase in FFAs
to 0.62 ? 0.03 mmol/l at t ? 120 (P ? 0.05), followed by a
return to placebo levels during the clamp period (Fig. 2C).
Glucose. Ghrelin induced a rapid increase in plasma
glucose levels with a peak value of 6.1 ? 0.2 mmol/l at t ?
120 min (P ? 0.009) (Fig. 2E). During the clamp, glucose
levels gradually decreased toward postabsorptive levels
on the ghrelin day, resulting in comparable glucose levels
during the final 30 min of the clamp. The GIR was
significantly decreased during ghrelin administration (Fig.
4A) (P ? 0.01), and the corresponding M value was
reduced by ?60% (P ? 0.001). Ghrelin did not significantly
impact glucose metabolism in the basal state (Table 1 and
Fig. 4C) but reduced the rates of oxidative, nonoxidative,
and total glucose disposal during the clamp period.
Regional substrate metabolism (microdialysis). Inter-
stitial muscle glucose levels fluctuated in parallel with
those in the circulation (Fig. 2F). By contrast, interstitial
glucose in fat remained stable also during ghrelin infusion.
Ghrelin did not significantly influence the levels of inter-
stitial glycerol, lactate, or urea in either tissue (data not
GH, insulin, and AMPK signaling. Densitometric quan-
titative bar graphs and representative Western blots from
skeletal muscle biopsies are provided in Fig. 3. Adminis-
Glucose (adjustable, 20%)
Insulin (0.6 mU/kg/min)
0 60 120 180 240 300
[3-3H]-glucose (12 µCi + 0.17 µCi/min)
Basal periodClamp period
Saline or ghrelin (5 pmol/kg/min)
0 60 120 180 240 300 Time (minutes)
Saline or ghrelin (5 pmol/kg/min)
FIG. 1. Study protocol. Please refer to RESEARCH DESIGN AND METHODS for
METABOLIC EFFECTS OF GHRELIN
3206DIABETES, VOL. 57, DECEMBER 2008
tration of ghrelin translated into STAT5 phosphorylation in
the healthy subjects but not in the hypopituitary pa-
tients. Ghrelin exerted no effects on total protein levels,
and no effects were recorded with regard to AMPK,
ACC, STAT3, ERK-1 or -2, Akt, or AS160 phosphoryla-
tion in study 2. Insulin receptor substrate–associated
PI3K activity was also not modified by ghrelin infusion
(data not shown).
We document for the first time that ghrelin induces
peripheral insulin resistance and stimulates lipolysis in the
absence of GH and cortisol release. This investigation is
also the first to document that ghrelin-induced endoge-
nous GH release translates into Janus kinase/STAT signal-
ing in skeletal muscle.
In some human studies, ghrelin administration increases
plasma levels of glucose and FFAs (6–8,16) and reduces
glucose disposal (5,8), indicating insulin resistance. These
effects are, however, partly attributable to GH and/or
cortisol secretion (5–8). There are both clinical and in
vitro data to suggest that ghrelin directly suppresses
glucose-induced insulin secretion from the ?-cell (17),
whereas administration of a ghrelin antagonist does the
opposite (18). Moreover, ghrelin knockout mice display
enhanced glucose-induced insulin release from isolated
FIG. 2. Hormones and metabolites during saline and ghrelin administration in study 2. A: Serum levels of ghrelin increased in response to ghrelin
infusion to a plateau of 5.33 ? 0.45 ?g/l in the basal state and a higher plateau of 5.86 ? 0.50 ?g/l during the clamp period (mean ghrelin levels
basal period vs. clamp period P ? 0.001). B: Serum levels of GH. C: Serum levels of FFA. D: Serum levels of insulin. Serum insulin was similar
during both basal and clamp conditions. E: Plasma glucose levels. F: Interstitial skeletal muscle glucose levels. Printed P values refer to two-way
ANOVA significance levels. f, saline infusion; ?, ghrelin infusion. All data are presented as means ? SE.
E.T. VESTERGAARD AND ASSOCIATES
DIABETES, VOL. 57, DECEMBER 2008 3207
Saline GhrelinSaline GhrelinSaline Ghrelin
Percentage of control
Saline GhrelinSaline Ghrelin
Saline GhrelinSaline GhrelinSaline GhrelinSaline Ghrelin
Percentage of control
P = 0.98 P = 0.10 P = 0.95 P = 0.67 P = 0.79 P = 0.71 P = 0.10
STAT5 STAT3 ERK1 ERK2 AMPK Akt AS160
Percentage of control
P = 0.20 P = 0.51 P = 0.15 P = 0.94 P = 0.57 P = 0.87 P = 0.58 P = 0.43
pSTAT5 pSTAT3 pERK1 pERK2 pAMPK pACC pAkt PAS
Percentage of control
P = 0.004
P = 0.54
FIG. 3. A: Effects of ghrelin infusion on STAT5 phosphorylation and total STAT5 levels in skeletal muscle in healthy subjects (study 1). Values
are means ? SE. B: Representative Western blots and quantitative bar-graphs regarding JAK/STAT, MAPK, AMPK, and insulin signaling pathways
in skeletal muscle in hypopituitary patients (study 2) during ghrelin and saline infusion. PAS, pAkt substrate.
Metabolic parameters during saline and ghrelin infusion in hypopituitary men (study 2)
RQ (ratio O2/CO2)
Energy expenditure (kcal/24 h)
Lipid oxidation (mg ? kg?1? min?1)
Endogenous glucose production
(mg ? kg?1? min?1)
0.82 ? 0.02
1902 ? 58
0.74 ? 0.08
0.83 ? 0.01
1916 ? 61
0.67 ? 0.05
0.89 ? 0.01
1931 ? 49
0.34 ? 0.05
0.86 ? 0.01
1906 ? 66
0.48 ? 0.08
1.59 ? 0.181.63 ? 0.160.740.58 ? 0.190.57 ? 0.150.95
Data are means ? SE. Paired analysis of treatments.
METABOLIC EFFECTS OF GHRELIN
3208 DIABETES, VOL. 57, DECEMBER 2008
islets (18,19) and exhibit increased peripheral insulin
sensitivity (19). Ghrelin/GHS-R double knockout mice
show lower glucose levels after a glucose tolerance test
and a more rapid drop in plasma glucose levels after an
insulin tolerance test (20).
Our study in hypopituitary men demonstrates that ghrelin
in humans directly suppresses insulin-stimulated glucose
disposal and stimulates FFA release. The observations at the
whole-body level were corroborated by the demonstration of
increased concentrations of glucose in skeletal muscle inter-
stitial tissue. Previously, it has been demonstrated that in
humans glucose disposal rates during hyperinsulinemic
clamp conditions are predominantly determined by the rate
of glucose uptake into skeletal muscle (21). By contrast, we
observed no significant effect of ghrelin on hepatic insulin
sensitivity. AMPK appears to be a key messenger in ghrelin
signaling in several tissues (22–24), and AMPK is also a
recognized cellular energy sensor (25). In our study, we did
either AMPK or ACC in skeletal muscle. The biopsies,
assessment of aberrations in insulin signaling pathways. The
Glucose infusion rate (mg x kg-1 x min-1)
P < 0.01
Glucose infusion rate (mg x kg-1 x min )
P < 0.001
Glucose Rd (mg x kg-1 x min-1)
P = 0.012
* GOX clamp P = 0.009
NOGD clamp P = 0.033
Accumulated glucose infusion
(mg x kg-1)
P = 0.002
FIG. 4. A: Glucose metabolism during saline and ghrelin administration in study 2. Hyperinsulinemic clamp. Glucose infusion rates and M value during
saline and ghrelin administration. F, saline infusion; ?, ghrelin infusion. B: Accumulative glucose infusion dosage during saline and ghrelin
administration. The accumulative glucose dose was significantly decreased during the clamp period in the ghrelin study (P ? 0.002). C: Glucose
utilization during the terminal 30 min of basal and clamp periods in saline and ghrelin studies. Ghrelin did not significantly impact glucose metabolism in the
basal period. During the clamp ghrelin infusion reduced the rates of oxidative, nonoxidative, and total glucose disposal (P ? 0.009, P ? 0.03, and P ? 0.012,
respectively). All data are presented as means ? SE.
E.T. VESTERGAARD AND ASSOCIATES
DIABETES, VOL. 57, DECEMBER 2008 3209
molecular mechanisms by which ghrelin causes insulin
resistance in humans thus remain to be further ascer-
tained and should be studied in both basal and insulin-
Serum FFA levels increased in response to ghrelin,
which did not translate into increased lipid oxidation. It
should be noted that hypopituitary adults are moderately
obese, which is likely to influence any effect of ghrelin on
FFA turnover. More importantly, infusion of ghrelin does
not imitate the secretory pattern of endogenous ghrelin.
Furthermore, this study was powered to detect effects of
ghrelin on insulin sensitivity (8), and lack of significance
with regard to lipid metabolism may be due to a ? error.
In conclusion, we demonstrate for the first time in
humans that ghrelin directly induces lipolysis and resis-
tance to insulin-stimulated glucose disposal. We also dem-
onstrate that ghrelin-induced GH release translates into
GH signaling in skeletal muscle, emphasizing the signifi-
cance of accounting for GH when evaluating any effects of
ghrelin. The physiological significance of the direct meta-
bolic effects of endogenous ghrelin remains unclear, but
we propose that ghrelin in concert with GH partition
substrate metabolism during conditions of energy short-
age in such a way as to restrict glucose utilization to
insulin-independent tissues such as the brain.
This study was supported by an unrestricted grant from
Novo Nordisk as well as grants from the Novo Nordisk
Foundation, the A.P. Moller Foundation, the World Anti-
Doping Agency, and the FOOD Study Group/Ministry of
Food, Agriculture, and Fisheries and Ministry of Family
and Consumer Affairs. Microdialysis catheters were kindly
supplied by Roche.
The GCP unit of Aarhus University Hospital is acknowl-
edged for monitoring that GCP guidelines were followed.
S. Sorensen, M. Moller, E. Carstensen, and E. Hornemann
are acknowledged for excellent technical assistance.
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