Download Acipimox-Mediated Plasma Free Fatty Acid Depression per se

Vol. 61, No. 3
Printed in U.S.A.
0021-972x/96/$03.00/0
Journal
of Clinical
Endocrinology
and Metabolism
Copyright
0 1996 by The Endocrine
Society
Acipimox-Mediated
Plasma Free Fatty Acid Depression
per se Stimulates
Growth
Hormone
(GH) Secretion
in
Normal
Subjects and Potentiates
the Response to Other
GH-Releasing
Stimuli*
ROBERTO
PEINO,
FERNANDO
CLARsA V. ALVAREZ,
CARLOS
CORDIDO,
DIEGUEZ,
ANGELA
PERALVA,
FELIPE
F. CASANUEVA
AND
Hospital J. Canalejo (F.C.) and the Departments
of Physiology (C.V.A., C.D.) and Medicine
(A.P., R.P.,
F.F.C.), School of Medicine,
and Complejo Hospitalario
de Santiago,
Santiago de Compostela
University, Santiago de Compostela, Spain
ABSTRACT
Increases
in plasma free fatty acids (FFA) inhibit
the GH response
to a variety
of stimuli;
however,
the role of FFA depression
in GH
control
is far from understood.
In the present
work, FFA reduction
was obtained
by the administration
to normal
subjects
of acipimox,
a
lipid-lowering
drug devoid of side-effects.
Each subject tested underwent two paired tests. In one, acipimox
was administered
orally at a
dose of 250 mg at -270 min and at a dose of 250 mg at -60 min; in
the matched
test, placebo was given at similar
intervals.
To induce
GH release, four stimuli
acting through
different
mechanisms
were
used: pyridostigmine
(120 mg, orally) at -60 min, GHRH
(1 cLg/kg, iv)
at 0 min, GH-releasing
peptide
(GHRP-6;
His-D-Trp-Ala-Trp-D-PheLys-NH,;
1 pg/kg, iv) at 0 min, and finally,
GHRH
plus GHRP-6
at
the same doses at 0 min. GH secretion
was analyzed
as the area under
the secretory
curve (AUC; mean 2 SE, micrograms
per L/120 min).
Acipimox
pretreatment
aione (n = 6) induced
a reduction
in FFA
levels compared
with placebo treatment.
The FFA reduction
led to a
sustained
GH secretion
that increased
from 2.4 ? 1.8 pg/L at -120
min to 14.2 i 4.0 at 120 min. The GH AUC for placebo was 266 5 100,
and that for acipimox
was 1781 5 408 (P i 0.05). In the pyridostigmine-treated
group (n = 61, the acipimox-pyridostigmine
AUC (2046
-C 323) was higher (P < 0.05) than the placebo-pyridostigmine
AUC
(764 ? lOl), but was not different
from the AUC of acipimox
alone.
Previous
FFA reduction
nearly
doubled the GHRH-mediated
GH secretion
(n = 6; placebo-GHRH
AUC,
1817 ? 365; acipimox-GHRH
test, 3228 + 876; P < 0.05). A similar
enhancement
was observed
when the stimulus
employed
was GHRP-6
(n = 6; placebo-GHRP-6
AUC, 2034 ? 295; acipimox-GHRP-6,
4827 2 703; P < 0.05). Furthermore,
even the most potent
GH stimulus
known
to date, i.e.
GHRH
plus GHRP-6,
was enhanced
by the FFA suppression
(placeboGHRH-GHRP-6
AUC, 2034 + 277; acipimox-GHRH-GHRP-6,5809
?
758; P < 0.05). The enhancing
effect of lowering
FFA levels was
additive
regardless
of the stimulus
employed.
These results indicate
that 1) FFA reduction
per se stimulates
GH
secretion
with a delayed time of action; 2) FFA reduction
enhanced
in
an additive
manner
the GH secretion
elicited by such different
stimuli
as pyridostigmine,
GHRH,
and GHRP-6;
and 3) the observation
that
FFA reduction
enhanced
the response
to the most potent
GH stimulus, GHRH
plus GHRP-6,
suggests
that FFA suppression
acts by a
separate
mechanism.
FFA reduction
may have value in the clinical
setting for assessing
GH reserve.
(J Clin Endocrinol Metub 81: 909913, 1996)
G
L-DOPA, deep sleep, physical exercise, or administration of
GHRH or GHRP-6 (5-11). There is now compelling evidence
indicating that the FFA blockade of GH secretion is exerted
at the pituitary level, probably by direct inhibition of somatotroph function (11-13). More controversial is the role of
FFA depression in GH control. The original study showing
that FFA reduction by nicotinic acid administration stimulated GH release (14) was thought to be confounded by the
intense side-effects of the drug. Other evidence that FFA
depression was responsible for the secretion of GH came
from use of the experimental drug BM 11.189,an adenosine
derivative (15). The subsequent unavailability of the drug
precluded further studies. Recent studies with acipimox, a
nicotinic acid analog that blocks lipolysis and is devoid of
side-effects (16, 17), have provided the best tool for further
understanding the role of FFA depression in GH regulation.
In the last few years considerable attention has been devoted to the series of hexapeptides devised by Bowers and
co-workers (18, 191,of which GH-releasing peptide (His-DTrp-Ala-Trp-n-Phe-Lys-NH,, known asGHRP-6) is the most
representative. GHRP-6 releasesGH through specific recep-
H SECRETION is tightly linked to metabolic alterations
as well as to variations in the intake or availability of
lipids, amino acids, and carbohydrates (l-3). A classicfeedback relationship has been postulated between GH and the
metabolically active component of lipids, i.e. the nonesterified fatty acids, also called free fatty acids (FFA). GH has a
direct lipolytic effect on adipose tissue, leading to the release
of glycerol, FFA, and ketone bodies (4). Pharmacological
reduction of EEA is associatedwith GH release,whereas an
increase in FFA reduces both basal and stimulated GH secretion (3).
FFA elevations reduce or block GH secretion stimulated by
a variety of stimuli or conditions, such as FFA depression,
hypoglycemia, arginine infusion, protein meal, glucagon,
Received
May 18, 1995. Revision
received
July 18,
August
1, 1995.
Address
all correspondence
and requests for reprints
nueva,
M.D., Ph.D., P.O. Box 563, E-15780, Santiago
Spain.
*This work was supported
by grants from the Fondo
Sanitaria,
Spanish Ministry
of Health,
and a Research
Fundacion
Ramon Areces.
1995. Accepted
to: F. F. Casade Compostela,
de Investigation
Grant from the
909
910
PEINO
tors (20-22) and by combined pituitary and hypothalamic
stimulation (23, 24). This novel mechanism of action made
GHRP-6 a convenient drug to study GH regulation in man
(25-27).
This study examined the effect of acipimox-induced FFA
reduction on basal and stimulated GH secretion in normal
subjects, using GH stimuli that operate at the hypothalamic
(pyridostigmine), pituitary (GHRH), or combined (GHRP-6)
levels. The aim of this study was to further define the mechanism of action by which FFA reduction operates to enhance
GH secretion.
JCE & M . 1996
Vol81
. No 3
ET AL.
Placebo
Placebo
l
Aciprmox
5-s-e
0
l
Subjects
1-J
I-v
1,10’1-
o
Placebo
Aaplmox
and Methods
Eighteen normal male volunteers,
aged 19.6 i- 0.3 yr (range, 18-23 yr)
participated
in this study after providing
informed
consent. All of them
had normal life styles, were taking no medication,
and were within 10%
of their ideal body weight.
The study was approved
by the faculty
bioethical
committee.
Tests were started at 0800 h after an overnight
fast, with the subjects
recumbent.
An indwelling
catheter was placed in a forearm
vein and
kept permeable
with a slow infusion
of 150 mmol/L
NaCl. The first
blood sample was obtained
at -120 min, and additional
blood samples
were obtained
at appropriate
intervals
over the following
4 h of testing.
The paired tests were performed
in random
order 1 week apart, with
each subject serving as his own control.
Six of the volunteers
were studied four times on different
days. On
day 1, placebo was administered
at -270 and -60 min. On the second
day, they received acipimox
(Olbemox,
Farmitalia,
Barceloma,
Spain) at
a dose of 250 mg, orally, at -270 min and acipimox
at a dose of 250 mg,
orally,
at -60 min. On the third day, they received
pyridostigmine
(Mestinon
Roche, Madrid,
Spain) at a dose of 120 mg, orally, at -60 min.
Finally, on another day, they received 250 mg acipimox,
orally, at -270
min and the same dose at -60 min plus 120 mg pyridostigmine,
orally,
at -60 min.
A second group of six volunteers
was tested for 2-day periods twice.
On the first 2 days they were given GHRH
[GHRH-(l-29)NHz;
Geref,
Serono, Madrid,
Spain) at a dose of 1 pg/kg,
iv, at 0 min, preceded
by
either placebo or acipimox
pretreatment
(250 mg, orally, at -270 min
and 250 mg, orally, at -60 min). One month later, on each day, they were
given GHRP-6
(His-o-Trp-Ala-Trp-o-Fhe-Lys-NH,;
Peninsula
Laboratories, Madrid,
Spain), prepared
as previously
described
(25), at a dose
of 1 pg/kg,
iv, at 0 min, preceded
by either placebo or acipimox
pretreatment
(250 mg, orally, at -270 min and 250 mg, orally, at -60 min).
The third group
of six volunteers
underwent
two tests. On both
occasions they received
a combined
administration
of GHRH
(1 pg/kg,
iv) plus GHRP-6
(1 pg/kg,
iv) at 0 min preceded
by either placebo or,
on the second day, acipimox
pretreatment
(250 mg, orally, at -270 min
and 250 mg, orally, at -60 min).
Serum GH concentrations
were determined
using a time-resolved
fluoroimmunoassay
(Delfia,
Pharmacia,
Barcelona,
Spain) with a GH
sensitivity
of 0.02 pg/L and coefficients
of variation
of 6.3% (0.4 wg/L),
5.3% (10.2 pg/L),
and 4.2% (43.4 pg/L).
FFA levels were determined
by
a enzymatic
calorimetric
method
(NEFA-HA,
Wako, Zaragoza,
Spain).
All samples from a given subject were analyzed
in the same assay run.
Hormone
levels are presented
and analyzed
as absolute values or as the
mean GH peak. The areas under the secretory
curve (AUC)
were calculated by a trapezoidal
method
and compared
between
groups by the
Wilcoxon
rank test. The statistical level of significance
was set at P < 0.05.
Results
The administration of acipimox induced a FFA reduction
during the entire test (Fig. 1). The area under the curve (AUC)
after placebo pretreatment (107.3 + 15.2 mmol/L +120 min)
was significantly higher (P < 0.05) than that after acipimox
pretreatment (25.6 + 7.8). The acipimox-induced FFA reduction elicited a sustained increase in GH levels from 2.4 2 1.8
pg/L at -120 min to 14.2 2 4.0 pg/L at the end of the test
-120
-60
0
30
Minutes
60
90
120
FIG. 1. Mean + SE serum GH levels in six normal
subjects after the
administration
of placebo in two different
tests at 0 min. In one test,
subjects were pretreated
with acipimox
at a dose of 250 mg, orally, at
-270 min and at a dose of 250 mg, orally, at -60 min; in the matched
test, placebo was administered
at the same intervals.
The FFA reduction was associated
with higher
(P < 0.05) GH values than those
in placebo-pretreated
subjects
from -60 min and thereafter.
0
-120
Placebo
-60
0
30
Minutes
60
90
120
FIG. 2. Mean ? SE serum GH levels in six normal
subjects after the
administration
of pyridostigmine
(120 mg, orally)
at -60 min on 2
different
days. In one test, subjects were pretreated
with acipimox
at
a dose of 250 mg, orally, at -270 min and at a dose of 250 mg, orally,
at -60 min; in the matched
test, placebo was administered
at the
same intervals.
(120 min), with a mean GH peak of 15.6 -C3.6 pg/L. The GH
AUC was 266 -C 100 pg/L*120 min for placebo and 1781 t
408 pg/L.120 min for acipimox-treated subjects(P < 0.05;see
Fig. 6).
Administration of pyridostigmine (Fig. 2), a cholinergic
drug that is thought to affect GH release by suppressing
hypothalamic somatostatin, induced the expected moderate
increase in GH levels, with a mean GH peak of 9.9 + 1.2
pg/L. When FFA were reduced by previous acipimox administration, an enhancement of pyridostigmine-mediated
GH releasewas observed (mean GH peak, 18.3 -C1.7 pg/L).
FFA DEPRESSION
The effect of acipimox was simply additive, because the AUC
of acipimox-pyridostigmine
(2046 ? 323) was higher (P <
0.05) than that of placebo-pyridostigmine
(764 -C 101), but
was not statistically different from that of acipimox alone
(1781 k 408; see Fig. 6).
The pharmacological reduction in FFA also increased the
GH releaseelicited by GHRH administration (Fig. 3). GHRHstimulated GH secretion in placebo-pretreated subjects
(mean peak, 23.8 ? 4.8 pg/L) nearly doubled in acipimoxpretreated GHRH-stimulated subjects (mean peak, 54.5 +
14.3 pg/L). The action of FFA reduction was most evident
when comparing the AUCs (see Fig. 6; 1817 2 365 for placebo-GHRH and 3228 2 876 for acipimox-GHRH; P < 0.05).
A similar enhancement was observed when the stimulus
employed was GHRP-6. As shown in Fig. 4, GHRP-6 induced
a larger and more synchronized GH discharge than GHRH,
with a mean peak of 33.3 2 5.8 pg/L, and the previous FFA
drop due to acipimox pretreatment notably increased its
effectiveness (mean peak, 64.9 2 9.0 pg/L). The placeboGHRP-6 AUC (2034 -C295) was significantly lower (P < 0.05)
than that of acipimox-GHRP-6 (4827 2 703; see Fig. 6).
The combined administration of GHRH and GHRP-6 in
placebo-pretreated subjects induced the expected large GH
discharge (mean peak, 58.3 -+-5.5 pg/L; AUC, 3791 + 277;
Fig. 5). Even this potent stimulus was enhanced by FFA
reduction causedby acipimox pretreatment (mean peak, 82.6
?I 12.2 pg/L; AUC, 5809 2 758; P < 0.05).
When the AUCs for all conditions in this study were compared (Fig. 6), the FFA reduction was seento further increase
in an additive manner the responsesto all GH stimuli tested.
The FFA reduction peusewas a more potent GH releaser than
pyridostigmine and was as potent as the classicalGH secretagogues GHRH and GHRP-6.
One subject in one test experienced a mild facial flushing
80
1
1
1
GHRH
0 Placebo
Acipimox
AND GH RELEASE
911
I
80
70
I
GHRP-6
0
.
-120
Placebo
Aclpimox
-60
T
0
30
60
SO
120
FIG. 4. Mean
t- SE serum
GH levels
in six volunteers
after the
administration
of GHRP-6
(1 fig/kg, iv) at 0 min on 2 different
days.
On one occasion,
subjects
were pretreated
with acipimox
at a dose
of 250 mg, orally,
at -270
min and at a dose of 250 mg, orally,
at
-60 min; in the matched
test, placebo
was administered
at the
same intervals.
I
1
1
GHRH + GHRP- 6
100
90
-I
0 Placebo
l
Aciplmox
80 1
4
70-
:
g
60.
b
I
50-
l
-120
-60
0
30
Minutes
60
SO
120
FIG. 5. Mean t SE serum GH levels in six volunteers
after the combined administration
of GHRH
(1 pg/kg, iv) plus GHRP-6
(1 pg/kg, iv)
at 0 min on 2 different
days. In one test, FFA levels were lowered
by
pretreatment
with acipimox
at a dose of 250 mg, orally, at -270 min
and at a dose of 250 mg, orally,
at -60 min; in the matched
test,
placebo was administered
at the same intervals.
-120
-60
0
30
Minutes
FIG. 3. Mean
t SE serum
GH levels
in
administration
of GHRH
(1 pg/kg, iv) at
On one occasion,
subjects
were pretreated
of 250 mg, orally,
at -270
min and at a
-60 min; in the matched
test, placebo
same intervals.
60
SO
120
six volunteers
after the
0 min on 2 different
days.
with acipimox
at a dose
dose of 250 mg, orally,
at
was administered
at the
1.5 h after the first dose of acipimox. No side-effects were
reported in the other tests
Discussion
FFA are common metabolites carried by the blood stream,
and their levels oscillate widely during the day depending on
PEINO ET AL.
GHRH
GHRP
- 6
GHRH
GHR+P
- 6
FIG. 6. Mean t SE AUCs in five groups of six normal
subjects
stimulated with placebo, pyridostigmine,
GHRH,
GHRP-6,
or GHRH
plus
GHRP-6
on 2 different
days. In one test, the subjects were pretreated
with acipimox
(250 mg, orally, at -270 min and 250 mg, orally, at -60
min); in the matched
test, placebo was administered
at the same
intervals.
*, P < 0.05 us. the same stimulus
without
FFA reduction
(placebo pretreated).
+, P < 0.05, either higher or lower us. subjects
treated
with acipimox
alone.
the fed/fast state of the subject (28). These circulating compounds notably increase in some pathological states, and it
is not known at present whether the abnormally high levels
of FFA presented acutely in hypoxia and ketoacidosis or
chronically in obesity and pregnancy could account for some
of the morbid complications associated with such states.
After their increasein plasma and despite the buffering effect
of serum albumin, the amphiphilic FFA molecules rapidly
partition into the cell plasma membrane, influencing the
physicochemical state of lipid domains (29). After incorporation into the membrane, FFA perturb the bilayer structure
of the membrane in a manner similar to some anesthetics,
leading to alterations in membrane-cytoskeleton interactions
and altering the functioning of the integral proteins (30-32).
Considering that these integral membrane proteins are receptors, channels, or enzymatic systems implicated in transduction signals, it is easy to envision the degree of cell perturbation induced by an immoderate increasein plasmaFFA.
Consistent with this view, it has been reported that FFA are
able to alter such different cell functions as platelet aggregation, lymphocyte mitogenesis, or cell to cell substrate adhesion (33). We have also shown that FFA are capable of
reversibly blocking the early intracellular signals elicited by
epidermal growth factor in fibroblasts (34-36) and altering
the gene expression of some hypothalamic hormones (37).
Although the precise point of action of FFA is at present
unknown, their effects are relevant and widespread.
It hasbeen shown in vim that an increasein FFA blocks GH
secretion elicited by all known stimuli (31, and this nonselective blockade is exerted directly on somatotroph cells (ll13). In vitro, FFA block, in minutes and in a dose-related
manner, somatotroph function (11). Unfortunately, the effects on somatotroph function of acute FFA reduction have
been not studied due to the unsuitability or unavailability of
adequate lipid-lowering drugs. The use by Pontiroli and
co-workers (16, 17) of acipimox, a new inhibitor of lipolysis
JCE
& M l 1996
Vol81
. No 3
devoid of significant side-effects in humans, opened a new
way to perform such studies. In the present work, we took
advantage of acipimox to obtain a well tolerated reduction in
FFA levels, which lasted for hours, to further understand the
mechanism of action of those compounds in both basal and
stimulated GH secretion.
The first observation of the present work is that FFA reduction per sewas able to significantly stimulate GH secretion. In fact, FFA reduction-mediated GH releasewas greater
than 7 pg/L at 0 min and thereafter. FFA reduction was by
no means a weak GH stimulus, considering that both the
mean GH peak and the AUC were higher than those after
pyridostigmine treatment and as large as those induced by
either GHRH or GHRP-6, both administered at a saturating
dose. Perhaps the most interesting observation of the present
work was the long period needed for FFA reduction before
releasing GH and the long lasting effect, with GH levels still
rising at 120 min when the test was ended. This delayed effect
is peculiar to FFA reduction and different from those of other
stimuli such asGHRH and hypoglycemia, perhaps reflecting
an intrinsic characteristic of the action of FFA. Conversely, to
observe a blockade of GH secretion, FFA levels must be
elevated for a long period before there is an effect on the GH
response to stimuli (11).
Acipimox-mediated FFA reduction was able to enhance
the GH releaseelicited by four GH stimuli, each thought to
act by a different mechanism. For example, pyridostigmine,
an indirect cholinergic agonist, is widely accepted for releasing GH, operating at the hypothalamic level and through
the inhibition of somatostatin release (38). GHRH acts directly at the pituitary, stimulating somatotroph cells. The
main action of GHRP-6 is exerted at the hypothalamic level
through undetermined mediators (24), with an ancillary
stimulatory action at the somatotroph cell (20). Despite these
different mechanismsof action, FFA reduction enhanced the
GH secretory response in a consistent and additive manner.
This additive action, i.e. the GH released by acipimox plus
stimulus was the arithmetical sum of the GH released by
acipimox alone and the stimulus alone, suggeststhat FFA
reduction operates to release GH by mechanisms different
from reduction of somatostatin release, release of endogenous GHRH, or release of the endogenous ligand of the
GHRP-6 receptor. The observation that FFA reduction was
able to further increase the effectiveness of GHRH plus
GHRP-6, again in an additive manner, particularly emphasizes that the reduction in plasma FFA acts by a separate
mechanism. Although that mechanism is not known, based
on our previous work we postulated that a sustained plasma
FFA reduction leads to a parallel reduction in the amount of
FFA molecules residing in the plasma membrane, altering
the physicochemical state of receptors, channels, or any other
integral protein and making the somatotroph cell more sensitive to any stimulus. Data from cell biology studies support
this working hypothesis. Plasma-borne FFA molecules already residing in the cell plasma membrane are not covalently linked; on the contrary, they are included in the
bilayer as wedges (39). If the gradient of FFA from plasma
toward membrane is not maintained, FFA molecules quickly
disappear from the plasma membrane, altering its state (34).
FFA DEPRESSION
Considerable experimental work is needed to test this hypothesis.
In conclusion, acipimox-induced
FFA depression peu se
resulted in delayed GH release.Furthermore, FFA reduction
enhanced, in an additive manner, GH secretion elicited by
diverse stimuli, including pyridostigmine, GHRH, GHRP-6,
and GHRH plus GHRP-6. These results suggest that FFA
reduction actsthrough a mechanism separatefrom that of the
other stimuli. Considerable work is needed before it can be
ascertained whether this GH stimulus can be used to assess
the GH secretory reserve in the clinical setting.
Acknowledgment
The expert
knowledged.
technical
assistance
of Ms.
Mary
Lage
is gratefully
ac-
References
1. Dieguez
C, Page MD, Scanlon
MF. 1988 Growth
hormone
neuroregulation
and its alterations
in disease states. Clin Endocrinol
(0x0.
28:109-143.
2. Casanueva
FF. 1992 Physiology
of growth
hormone
secretion
and action.
Endocrinol
Metab Clin North Am. 21:483-517.
3. Dieguez
C, Casanueva
FF. 1995 Influence
of metabolic
substrates
and obesity
on growth
hormone
secretion.
Trends Endocrinol
Metab.
655-59.
4. Mary Lee Vance. 1993 Metabolic status and growth hormone secretion in man.
J Pediatr
Endocrinol.
6:3-4.
5. Quabbe
HJ, Bratzke
B, Siegers U, Elban K. 1972 Studies
on the relationship
between
plasma free fatty acids and growth
hormone
secretion
in man. J Clin
Invest. 51:2388-2398.
6. Blackard
WG, Boyden
CT, Hinson
TC, Nelson
NC. 1969 Effect of lipid and
ketone infusions
on insulin-induced
growth
hormone
elevations
in rhesus
monkeys.
Endocrinology.
85:1180-1185.
Blackard
WG, Hull EW, Lopez A. 1971 Effects of lipids
on growth
hormone
secretion
in humans.
J Clin Invest. 50:1439-1443.
Fineberg
SE, Horland
AA, Merimee
TJ. 1972 Free fatty acid concentrations
and growth
hormone
secretion
in man. Metabolism.
21:491-498.
Lucke C, Adelman
N, Glick
SM. 1972 The effect of elevated
free fatty acids
on the sleep-induced
human growth
hormone
peak. J Clin Endocrinol
Metab.
35:407-412.
10. Imaki T, Shibasaki
T, Shizume
K, et al. 1985 The effect of free fatty acids on
growth
hormone-releasing
hormone-mediated
GH secretion
in man. J Clin
Endocrinol
Metab.
60:290-294.
11 Casanueva
FF, Vil anueva
L, Dieguez
C, et al. 1987 Free fatty acids block
growth
hormone
(GH) releasing
hormone-stimulated
GH secretion
in man
directly
at the pituitary.
J Clin Endocrinol
Metab.
65:634-642.
12 Alvarez
CV, Mallo F, Burguera
B, Cacicedo
L, Dieguez
C, Casanueva
FF. 1991
Evidence
for a direct pituitary
inhibition
by free fatty acids of in viva growth
hormone
responses
to growth
hormone-releasing
factor in the rat. Neuroendocrinology.
53:185-189.
13. Renier
G, Abribat
T, Brazeau
P, Deslauriers
N, Gaudreau
P. 1990 Cellular
mechanism
of caprylic
acid-induced
growth
hormone
suppression.
Metabolism. 39:1108-1112.
14. Irie M, Sakuma
M, Tsushima
T, Shizume
K, Nakao K. 1967 Effect of nicotinic
acid administration
on plasma growth
hormone
concentrations.
Proc Sot Exp
Biol Med. 126:708-711.
15. Quabbe
HJ, Luyckx AS, L’Age M, Schwarz
C. 1983 Growth
hormone,
cortisol,
and glucagon
concentrations
during
plasma
free fatty acid depression:
different effects of nicotinic
acid and an adenosine
derivative
(BM 11.189). J Clin
Endocrinol
Metab. 57:410-414.
16. Pontiroli
AE, Lanzi R, Monti
LD, Pozza G. 1990 Effect of acipimox,
a lipid
lowering
drug on growth
hormone
(GH) response
to GH-releasing
hormone
in normal
subjects.
J Endocrinol
Invest. 13:539-542.
17. Pontiroli
AE, Lanzi R, Monti
LD, Sandoli
E, Pozza G. 1991 Growth
hormone
(GH) auto feedback
on GH response
to GH-releasing
hormone.
Role of free
fatty acids and somatostatin.
J Clin Endocrinol
Metab.
72492-495.
18. Bowers CY, Momany
FA, Chang D, Hong A, Chang K. 1980 Structure-activity
relationships
of a synthetic
pentapeptide
that specifically
releases GH in vitro.
Endocrinology.
106:663-667.
AND GH RELEASE
913
19. Momany
FA, Bowers
CY, Reynolds
GA, Hong A, Newlander
K. 1984 Conformational
energy
studies
and in vitro activity
data on active GH releasing
peptides.
Endocrinology.
114:1531-1536.
20. Bowers
CY, Sartor AO, Reynolds
GA, Badger
TM. 1991 On the actions of the
growth
hormone-releasing
hexapeptide,
GHRP-6.
Endocrinology.
128:20272035.
21. Bowers
CY, Momany
FA, Reynolds
GA, Hong A. 1984 On the in vitro and in
viva activity
of a new synthetic
hexapeptide
that acts on the pituitary
to
specifically
release growth
hormone.
Endocrinology.
1141537-1545.
22. Bowers
CY, Reynolds
GA, Durham
D, Barrera CM, Pezzoli
SS, Thorner
MO.
1990 Growth
hormone
(GH)-releasing
peptide
stimulates
GH release in normal
men and acts synergistic
with GH-releasing
hormone.
J Clin Endocrinol
Metab.
70:975-982.
23. Popovic
V, Damjanovic
S, Micic
D, Petakov
M, Dieguez
C, Casanueva
FF.
1994 Growth
hormone
(GH) secretion
in active acromegaly
after the combined
administration
of GH-releasing
hormone
and GH-releasing
peptide.
J Clin
Endocrinol
Metab.
79:456-460.
24. Popovic
V, Damjanovic
S, Micic D, Djurovic
M, Dieguez
C, Casanueva
FF.
1995 Blocked
growth
hormone-releasing
peptide
(GHRP-6).induced
GH secretion
and absence
of the synergic
action
of GHRP-6
plus GH-releasing
hormone
in patients
with hypothalamopituitary
disconnection:
evidence
that
GHRP-6
main action is exerted
at the hypothalamic
level. J Clin Endocrinol
Metab.
80:942-947.
25. Cordido
F, Peiialva
A, Dieguez
C, Casanueva
FF. 1993 Massive
growth
hormone (GH) discharge
in obese subjects
after the combined
administration
of
GH-releasing
hormone
and GHRP-6:
evidence
for a marked
somatotroph
secretory
capability
in obesity.
J Clin Endocrinol
Metab.
76:819-823.
26. Alster DK, Bowers CY, Jaffe CA, Ho PJ, Barkan AL. 1993 The growth hormone
(GH) response
to GH-releasing
peptide
(His-o
Trp-Ala-Trp-D
Phe-Lys-NH,),
GH-releasing
hormone,
and thyrotropin-releasing
hormone
in acromegaly.
J
Clin Endocrinol
Metab.
77842-845.
27. Leal-Cerro
A, Pumar A, Garcia
E, Dieguez
C, Casanueva
FF. 1994 Inhibition
of growth
hormone
release after the combined
administration
of GHRH
and
GHRP-6
in patients
with Cushing’s
syndrome.
Clin Endocrinol
(Oxf). 41:649654.
28. Hochachka
PW. 1986 Defense
strategies
against
hypoxia
and hypothermia.
Science.
231:234-241.
29. Renaud
G, Bouma
ME, Foliot
A, Infante
R. 1985 Free fatty-acid
uptake by
isolated
rat hepatocytes.
Arch Int Physiol
Biochim.
93:313-319.
30. Sanderman
H. 1978 Regulation
of membrane
enzymes
by lipids.
Biochim
Biophys
Acta. 515:208-237.
31. Pjura WJ, Kleinfeld
AM, Hoover
RL, Karnovsky
MJ. 1984 Partition
of fatty
acids and fluorescent
fatty acids into membranes.
Biochemistry.
23:2039-2043.
32. Klausner
RD, Kleinfeld
AM, Hoover
RL, Karnovsky
MJ. 1980 Lipid domains
in membranes.
Evidence
derived
from structural
perturbations
induced
by free
fatty acids and lifetime
heterogeneity
analysis.
J Biol Chem. 255:1286-1295.
33. Karnovsky
MJ. 1979 Lipid
domains
in biological
membranes.
Am J Pathol.
97:212-221.
34. Casabiel
X, Pandiela
A, Casanueva
FF. 1991 Regulation
of epidermalgrowth-factor-receptor
signal transduction
by cis-unsaturated
fatty acids. Biothem J. 278:679-687.
35. Casabiel
X, Zugaza
JL, Pombo
CM, Pandiela
A, Casanueva
FF. 1993 Oleic
acid blocks epidermal
growth factor-activated
early intracellular
signals without altering
the ensuing
mitogenic
response.
Exp Cell Res. 205:365-373.
36. Zugaza
JL, Casabiel
X, Bokser
L, Casanueva
FF. 1995 Oleic acid blocks
EGF-induced
Cazti release without
altering
cellular
metabolism
in fibroblast
EGFR T17. Biochem
Biophys
Res Commun.
207:105-110.
37. Sefiaris
RM, Lewis MD, Lago F, Dominguez
F, Scanlon
MF, Dieguez
C. 1993
Effects of free fatty acids on somatostatin
secretion,
content
and mRNA levels
in cortical
and hypothalamic
fetal rat neurones
in monolayer
culture.
J Mol
Endocrinol.
10:207-214.
38. Massara
F, Ghigo E, Molinati
P, et al. 1986 Potentiation
of cholinergic
tone by
pyridostigmine
bromide
reinstates
and potentiates
the growth
hormone
responsiveness
to intermittent
administration
of growth hormone
releasing
factor in man. Acta Endocrinol
(Copenh).
113:12-6.
39. Klausner
RD, Bhalla DK, Dragsten
P, Hoover
RL, Karnovsky
JK. 1980 Model
for capping
derived
from inhibition
of surface
receptor
capping
by free fatty
acids. Proc Nat1 Acad Sci USA. 77:437-441.
40. Widmaier
EP. 1992 Metabolic
feedback
in mamalian
endocrine
systems. Harm
Metab Res. 24:147-153.