From the Divisions of Internal Medicine and Cardiovascular Diseases
(D.H., D.R.H., D.M.R., A.L.), and Endocrinology and the Endocrine Research
Unit (R.A.R.), Mayo Clinic and Foundation, Rochester, Minn; and the Division
of Endocrinology and Department of Pediatrics, University of Pennsylvania,
Philadelphia, Pa (P.C.).
Correspondence to Amir Lerman, MD, Division of Cardiovascular Diseases, Mayo Clinic, 200 First St SW, Rochester, MN 55905. E-mail lerman.amir{at}mayo.edu
Independent of their metabolic and growth-promoting
properties, insulin and related peptides also have vasoactive
actions9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 ranging from
vasoconstriction9 13 16 24 to
vasodilation.9 10 11 12 14 15 17 18 19 20 21 22 23 24 Studies in animal
models and humans have reported conflicting results regarding the
vasoactive effects of insulin9 10 11 12 13 14 15 16 17 18 19 24 and
IGF-I,9 19 21 22 23 with disparate effects reported
for different vascular beds of the same species9
and even for different vessel types in the same organ (ie, afferent and
efferent arterioles of the kidney24).
The past several years have seen a major surge of interest in the
cardiovascular actions of insulin and
IGF-I.25 Specifically, the actions of the two
peptides, or the lack thereof, may have
pathophysiological consequences, such as abnormal
vascular tone. The vascular effects of insulin and IGF-I have been
extensively studied in peripheral vessels in
physiological and
pathophysiological states,25
but there are few data as yet regarding the direct actions of these 2
peptides on the coronary vasculature. In similarity to the
peripheral vasculature, however, insulin and IGF-I
potentially modulate coronary vascular tone, and an impairment
in their actions may result in abnormal vascular tone. This possibility
is underscored by the epidemiological evidence linking impaired insulin
action with abnormal coronary artery
tone26 27 28 29 and with increased morbidity
attributed to coronary artery disease.30
The present study was therefore designed to examine the hypothesis
that insulin and IGF-I are vasodilators of porcine epicardial arteries
in vitro and to elucidate possible mechanisms for their actions.
In vitro determination of vascular reactivity was performed as we
previously described.31 In brief, the hearts were
placed into cold modified Krebs-Ringer bicarbonate solution of the
following millimolar composition (control solution): 118.3 NaCl, 4.7
KCl, 2.5 CaCl2, 1.2 MgSo4,
1.2 KH2PO4, 25
NaHCO3, 0.026 calcium EDTA, and 11.1 glucose.
Segments 2 to 3 mm long of the left circumflex coronary
artery were dissected. When indicated, the endothelium
was mechanically removed from the vessels. Each vessel was connected to
an isometric force transducer (Grass Instruments) and suspended in an
organ chamber filled with 25 mL of control solution (37°C; pH 7.4)
and gassed with 94% O2 and 6%
CO2. Isometric tension was recorded
continuously. The arteries were allowed to stabilize at a resting
tension for 1 hour. Viability of the vessels was confirmed by a
contractile response to 20 mmol/L KCl at baseline, at 2 g, at
4 g, and at 6 g, each time after the potassium had been
washed out. At 6 g, all vessels were then exposed to
10-6 mol/L substance P (Sigma Chemical Co), an
endothelium-dependent vasodilator, to verify the
functional integrity of the vascular endothelium. All
chambers were then washed out using the control solution.
After an equilibration period of 30 minutes, the agents detailed in the
specific protocols below were added. In the vast majority of cases, the
vasorelaxing response reached a plateau after 3 to 4 minutes, at which
time the next concentration was added. In all cases, we waited until a
plateau had been reached before adding the next dose. Stock solutions
of each agent were prepared every day. Drugs were dissolved in
distilled water such that volumes of <0.2 mL were added to the organ
chambers. All concentrations are expressed as the concentration within
the bath solution.
Determination of Vasorelaxation Effects of Insulin and
IGF-I
Determination of the Role of Endothelium in
Vasorelaxation Effects of Insulin and IGF-I
NO Pathway
In addition, to examine whether the effects of L-NMMA on the
vasorelaxation response to insulin and IGF-I in vessels without
endothelium are related to the NO pathway, arteries
without endothelium were exposed to
10-4 mol/L L-NMMA and the precursor for NO
synthase, 10-3.5 mol/L L-arginine
hydrochloride (Sigma); after equilibration for 20 minutes, arteries
were contracted with ET-1 and relaxed with cumulative concentrations of
either insulin or IGF-I. The concentrations of L-NMMA and
L-arginine were derived from prior studies, which also
demonstrated that to reverse the effects of L-NMMA,
L-arginine should be given at concentrations 3 to 10 times
higher than the concentration of L-NMMA.34
Potassium Channels
cGMP
L-Type Calcium Channel Blocker
Data Analysis
Vasorelaxation Effects of Insulin and IGF-I
Role of Endothelium in Vasorelaxation Response to
Insulin and IGF-I
Role of NO in Vasorelaxation Response to Insulin and IGF-I
In vessels without endothelium, the incubation of
vessels with both L-arginine and L-NMMA before exposure to
cumulative concentrations of either insulin (n=6) or IGF-I (n=6) did
not reverse the attenuated vasorelaxation response observed with L-NMMA
alone (data not shown).
Role of cGMP in Vasorelaxation Response to Insulin and
IGF-I
Role of Potassium Channels in Vasorelaxation Response to Insulin
and IGF-I
Role of L-Type Calcium Channels in Vasorelaxation Response to
Insulin and IGF-I
Insulin and IGF-I are known to have vasoactive properties ranging from
vasoconstriction to vasodilation.9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 These
prior studies have demonstrated that insulin and IGF-I have similar
vasoactive actions and exert different vasoactive actions on various
vascular beds in the same species, and even in the same organ. Few data
are available regarding the vasoactive actions of insulin and IGF-I on
coronary arteries. A prior study demonstrated that incubation
of porcine coronary epicardial arteries with insulin in vivo
did not affect coronary artery tone, although insulin
potentiated the coronary vasoconstrictor response to
thromboxane
A2.16 In the present
study, when coronary epicardial arteries were first contracted
with ET-1, both peptides exerted a concentration-dependent
coronary vasorelaxation effect.
Endothelium and NO
In vitro incubation with insulin or IGF-I causes the release of NO from
endothelium-intact as well as
endothelium-denuded vessels, an effect attributed to
production of the inducible form of
NO.23 39 40 41 In these studies, the incubation
times were prolonged and were dependent on new protein synthesis. Of
interest, in studies in humans, infusion time rather than infusion rate
is the main determinant of the vasodilator response to
insulin.25 It is
postulated39 that the increased vascular smooth
muscle cell production of inducible NO caused by insulin and
IGF-I reduces
[Ca2+]i,41 42 43
thus resulting in vasorelaxation.
In our study, the coronary vasorelaxation effects of insulin
and IGF-I were similarly exerted in coronary arteries with and
without endothelium. In addition, L-NMMA, an NO
synthase inhibitor, attenuated the coronary
vasorelaxation effect of insulin and IGF-I in arteries with and without
endothelium. These findings may indicate that insulin
and IGF-I exerted their effects through the stimulation of the
inducible isoform of NO. However, several findings refute this
possibility. First, the effects of L-NMMA were not reversed by the
addition of L-arginine. In addition, the activation of
endogenous NO results in an increase in cGMP
production.44 Using ODQ, an agent that
potently blocks the production of cGMP by soluble guanylyl
cyclase,36 we did not detect an effect on the
response to insulin and IGF-I. Finally, in our experiments, the
duration of exposure to insulin or IGF-I was brief. Muniyappa et
al40 have shown that NO levels increase
significantly only after 4 hours of exposure to IGF-I (eg, in our
experiments the exposure to cumulative concentrations of insulin or
IGF-I lasted approximately 45 minutes). Thus, in our experimental
conditions the vasorelaxation responses of porcine epicardial arteries
in vitro to insulin or IGF-I were not dependent on the
endothelium or on the NO pathway. However, in different
conditions entailing prolonged exposure of vessels to insulin or IGF-I,
the vascular smooth muscle NO pathway may mediate, at least in part,
the vasoactive effects of insulin and
IGF-I.23 39 40 41
One may speculate as to the mechanism underlying the attenuation of the
effects of insulin and IGF-I by L-NMMA in our experiments. Arginine
analogues such as L-NMMA have recently been shown to have a direct
effect on ATP-sensitive potassium channels in feline and rat pial
arterioles.45 Low concentrations of L-NMMA
inhibited the vasodilation mediated by ATP-sensitive potassium channels
independently of an effect on NO and cGMP. Thus, in the present
study, L-NMMA may have had a direct effect on potassium channels that
was independent of NO activation.
Potassium Channels
TEA blocks both calcium-dependent and voltage-dependent potassium
channels.30 Higher concentrations of TEA are
required to inhibit voltage-dependent potassium channels than
calcium-dependent potassium channels.35 In our
study, we used high concentrations of TEA to inhibit the effect of
insulin and IGF-I. The lack of effect by charybdotoxin, an agent with
no effect on voltage-dependent potassium channels but a selective
effect on calcium-dependent potassium channels, supports the hypothesis
that the effects of TEA were exerted on the former.
Voltage-dependent potassium channels are found in porcine
coronary artery smooth muscle cells at a concentration of
approximately 5000 channels per cell.46 There are
several types of voltage-dependent potassium channels, differing in
their voltage dependence and their sensitivity to
inhibitors.35 Smooth muscle
depolarization can both activate and inactivate
these channels, and thus the current through the channels depends on
the balance between these opposing processes. Our study did not
determine the mechanism through which insulin and IGF-I
activate these channels. However, the IGF axis may
activate these channels by modulating the cation balance
intracellularly or across the cell membrane through receptor-mediated
and voltage-mediated mechanisms, including the inhibition of calcium L
channels.41 42 43 In our study, the
inhibitor of calcium L channels diltiazem had no effect on
the coronary vasorelaxation response of insulin and IGF-I.
Thus, the activation of voltage-dependent potassium channels may be the
result of a direct effect of both insulin and IGF-I on the channel, or
alternatively it may reflect the voltage changes resulting
predominantly from intracellular kinetics of cations rather than a
shift in cation balance across the cell membrane.
Similar Effect With Insulin and IGF-I
Implications Regarding Pathophysiological States
In conclusion, both insulin and IGF-I have coronary
vasorelaxation properties in vitro. The underlying mechanisms for the
coronary vasorelaxation effects of both peptides may involve
their interaction with vascular smooth muscle cell potassium channels.
These findings suggest that insulin and IGF-I play a role in the
regulation of coronary vasomotor tone and may have therapeutic
implications in pathophysiological states.
Received December 22, 1997;
first decision January 15, 1998;
accepted March 12, 1998.
© 1998 American Heart Association, Inc.
Scientific Contributions
Insulin and Insulin-like Growth Factor-I Cause Coronary Vasorelaxation In Vitro
Abstract
Top
Abstract
Introduction
Methods
Results
Discussion
References
Abstract—Insulin and insulin-like
growth factor-I (IGF-I) may play a role in the modulation of
coronary artery tone, yet there are few data regarding their
vasoactive effects on the coronary vascular bed. We evaluated
the vasorelaxation effects of insulin and IGF-I on porcine
coronary epicardial vessels in vitro and elucidated possible
mechanisms. Porcine epicardial arteries were contracted with
10-7 mol/L endothelin-1 and relaxed with cumulative
concentrations of either insulin or IGF-I (10-12 to
10-7 mol/L). The above experiments were repeated in
vessels without endothelium. Vessels were also
incubated with the nitric oxide synthase inhibitor
NG-monomethyl-L-arginine
(L-NMMA; 10-4 mol/L) with and without 10-3.5
mol/L L-arginine, the potassium channel blocker
tetraethylammonium (TEA; 10-2
mol/L), and the guanylyl cyclase inhibitor
1H-[1,2,4]oxadiazolo[4,3,-
]quinoxalin-1-one (ODQ;
10-5.5 mol/L); vessels were then contracted with
endothelin-1 and relaxed with insulin or IGF-I. Insulin and IGF-I were
also added after contraction with 60 mmol/L KCl. Insulin and IGF-I
caused a similar decrease in coronary epicardial tension after
contraction with endothelin-1 (relaxation of 28±4% [n=7] and
25±3% [n=8] with insulin and IGF-I, respectively;
P<0.0001 for both peptides). Removal of the
endothelium did not affect these responses. Incubation
with L-NMMA, but not ODQ, attenuated the vasorelaxation response to
insulin and IGF in vessels without endothelium.
L-Arginine did not reverse this effect of L-NMMA. KCl and
TEA attenuated the vasorelaxation effect of both insulin and IGF-I.
Thus, both insulin and IGF-I caused
non–endothelium-dependent coronary
vasorelaxation in vitro, probably through a mechanism involving the
activation of potassium channels. These findings suggest that insulin
and IGF-I participate in the regulation of coronary
vasomotor tone.
Key Words: insulin • growth factors • pigs • arteries • endothelium • potassium channels • nitric oxide
Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
The three peptide
hormones in the IGF family—insulin, IGF-I, and IGF-II—have
approximately 50% of their amino acids in
common.1 2 3 Whereas insulin is produced and
secreted by the pancreas as proinsulin, the liver is the main source of
circulating IGF-I levels.1 2 3 Unlike insulin,
however, IGF-I is also produced by various cell types, including
endothelial4 5 and vascular
smooth muscle cells,6 7 8 and is considered a
significant paracrine/autocrine factor.1 2 3
Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
Animals
The study procedures and handling of animals were reviewed and
approved by the Mayo Foundation Institutional Animal Care and Use
Committee. Juvenile domestic crossbred pigs were killed with an
intravenous overdose of pentobarbital sodium (30 mg/kg;
Sleepaway, Fort Dodge Laboratories). After death, the hearts were
harvested for in vitro analysis.
To determine the coronary epicardial vasodilator effects
of insulin and IGF-I, epicardial arteries were contracted with
10-7 mol/L ET-1 (Phoenix Pharmaceuticals); after
equilibration for 20 minutes, arteries were relaxed with cumulative
concentrations of either 10-12 to
10-7 mol/L insulin (Eli Lilly) or
10-12 to 10-7 mol/L IGF-I
(Sigma). ET-1 was chosen as the vasoconstrictor agent at this dose in
these experiments for 2 reasons. First, 10-7
mol/L ET-1 produces sustained contraction of porcine coronary
vessels. To support this observation, a control group was established
to verify that the loss of tension was due to insulin and IGF-I and not
due to the loss of the vasoconstrictor effect of ET-1: distilled water
was added to control vessels at the same time that insulin and IGF-I
were added in the experimental group. In the control group, no
significant vasorelaxation was evident. In preliminary experiments,
lower concentrations of ET-1 did not result in sustained contraction.
Second, prior studies have shown that ET-1 caused vasoconstriction of
porcine coronary arteries by increasing smooth muscle
calcium,32 33 but
physiological concentrations of insulin blunted
this rise in smooth muscle cell calcium.32 Thus,
we postulated that insulin and IGF-I would be effective in attenuating
ET-1–induced contraction.
To determine the role of the endothelium in the
coronary epicardial vasodilator effects of insulin and IGF-I,
the endothelium was removed and the above experiments
were repeated. Removal of the endothelium was verified
by the lack of relaxation to substance P after contraction with
potassium. At the end of all experiments, 10-3.5
mol/L papaverine (Sigma) was added to verify that the vessels
maintained vasodilating capacity.
Additional experiments were conducted to elucidate the
mechanisms involved in the vasorelaxation effects of insulin and IGF-I.
To determine the effect of the NO pathway, the NO synthase
inhibitor L-NMMA at 10-4 mol/L
(Sigma) was added 20 minutes before the addition of ET-1. Arteries with
and without endothelium were then exposed to cumulative
concentrations of either insulin or IGF-I.
To determine whether potassium channels mediate the vasoactive
effects of insulin and IGF-I, additional vessels with intact
endothelium were exposed to 60 mmol/L KCl for 20
minutes before exposure to cumulative concentrations of insulin and
IGF-I. Furthermore, to identify the potassium channel mediating the
vasorelaxation effect of insulin and IGF-I, arteries with intact
endothelium were exposed to the following potassium
channel inhibitors for 20 minutes before contraction with
ET-1 and the addition of insulin or IGF-I: the ATP-sensitive potassium
channel inhibitor glyburide (10-6
mol/L; Research Biochemical International), the calcium-dependent
potassium channel inhibitor charybdotoxin
(10-7 mol/L; Sigma), and the potassium channel
inhibitor TEA (10-2 mol/L; Sigma).
The reported concentration for half-block
Ki for glyburide is 20 to 200 nmol/L, for
TEA 200 µmol/L and 10 mmol/L (for calcium-dependent and
voltage-dependent potassium channels, respectively), and for
charybdotoxin 10 nmol/L.35
In additional experiments, vessels without
endothelium were exposed to
10-5.5 mol/L ODQ (Biomol Research Laboratories),
a potent inhibitor of soluble guanylyl cyclase, 20 minutes
before contraction with ET-1 and the addition of insulin or IGF-I. ODQ
at this dose inhibits the rise in cGMP induced by NO donors in vascular
smooth muscle.36 In addition, in preliminary
experiments, the incubation of porcine coronary arteries
without endothelium with 10-5.5
mol/L ODQ caused a shift to the right
(EC50=10-6 mol/L versus
EC50=10-8 mol/L) in the
relaxation response to cumulative concentrations
(10-10 to 10-5 mol/L) of
the NO donor diethylamine NONOate (Cayman Chemical).
To examine whether the effects of insulin and IGF-I are exerted
by inhibition of calcium influx through the L-type calcium channel,
vessels with intact endothelium were exposed to the
calcium channel blocker diltiazem (10-6 mol/L;
Sigma) 20 minutes before contraction with ET-1 and the addition of
insulin or IGF-I. In porcine coronary arterial
strips, diltiazem at this dose has been shown to inhibit increases in
intracellular calcium and tension development induced by cumulative
applications of extracellular calcium during potassium-induced
contraction.37
Results are presented as mean±SEM. The contraction
attained with ET-1 for each vessel at baseline was considered as
baseline (0% relaxation). Subsequent measurements of coronary
artery relaxation are expressed as a percent reduction in contraction
(the maximal relaxation attained with papaverine being 100%
relaxation). In all experiments, n refers to the number of vessels.
Experiments were performed in parallel in harvested vessels, to
preclude a situation whereby all vessels in 1 experiment were harvested
from only 1 animal (on average each experiment was conducted using
vessels from 3 to 4 animals). For statistical analysis, ANOVA
or repeated-measure ANOVA followed by Bonferroni's t test
was used. A two-tailed value of P
0.05 was considered
significant.
Results
Top
Abstract
Introduction
Methods
Results
Discussion
References
Vessel Integrity After Endothelium Removal
Substance P caused complete vasodilation after 20 mmol/L
KCl-induced contraction in endothelium-intact vessels
but did not cause any vasorelaxation in
endothelium-denuded vessels. The maximal response to
20 mmol/L KCl was similar for endothelium-intact
and endothelium-denuded vessels (8.8±0.6 versus
8.1±0.7 g for endothelium-intact and
endothelium-removed vessels, respectively;
P=0.23).
The mean contractile responses to ET-1 in the insulin and
IGF-I experiments in vessels with intact endothelium
were 8.5±1.5 and 10.8±1.8 g, respectively. Both insulin (Figure 1
, left) and IGF-I (Figure 1, right)
caused a significant decrease in coronary epicardial tension
after contraction with ET-1 (relaxation of 28±4% and 25±3% with
insulin and IGF-I, respectively; P<0.0001 for each
peptide). The vasorelaxation responses to both insulin and IGF-I were
significant at concentrations 
10-10 mol/L.
There was no significant difference in the vasorelaxation response
attained with both agents.
View larger version (9K):
[in a new window]
Figure 1. Vasorelaxation effect of cumulative concentrations
of insulin (left) and IGF-I (right) on epicardial coronary
artery tone of vessels with (E+) and without (E-)
endothelium after contraction with ET-1.
The mean contractile responses to ET-1 in the insulin and IGF-I
experiments in vessels without endothelium were
8.1±1.4 and 5.9±1.1 g, respectively. Removal of the
endothelium did not affect the vasorelaxation response
to insulin (Figure 1
, left; P=0.98 for comparison with
intact endothelium) and IGF-I (Figure 1, right;
P=0.95 for comparison with intact
endothelium).
The mean contractile responses to ET-1 after incubation with
L-NMMA in the insulin and IGF-I experiments in vessels with intact
endothelium were 11.8±1.3 and 9.2±1.1 g,
respectively. In vessels without endothelium, the mean
contractile responses were 7.0±1.1 and 10.9±1.2 g, respectively. The
incubation of vessels with L-NMMA attenuated the vasorelaxation
response to insulin in vessels with intact endothelium
(Figure 2, left; P=0.009 for
comparison with the response to insulin without L-NMMA in vessels with
intact endothelium), as well as in vessels without
endothelium (Figure 2, left; P=0.01 for
comparison with the response to insulin without L-NMMA in vessels
without endothelium). Similarly, the incubation of
vessels with L-NMMA attenuated the vasorelaxation response to IGF-I in
vessels with intact endothelium (Figure 2, right;
P=0.05 for comparison with the response to IGF-I without
L-NMMA in vessels with intact endothelium), as well as
in vessels without endothelium (Figure 2, right;
P=0.04 for comparison with the response to IGF-I without
L-NMMA in vessels without endothelium).
View larger version (9K):
[in a new window]
Figure 2. Vasorelaxation response to insulin (left) and
IGF-I (right) after incubation with L-NMMA for vessels with (E+) and
without (E-) endothelium.
The mean contractile responses to ET-1 after incubation with ODQ
in the insulin and IGF-I experiments in vessels without
endothelium were 8.9±0.9 and 10.2±1.4 g,
respectively. The incubation of vessels without
endothelium with ODQ did not affect the vasorelaxation
response to insulin (Figure 3, left;
P=0.93 for comparison with the response to insulin without
ODQ in vessels without endothelium) or the response to
IGF-I (Figure 3, right; P=0.70 for comparison with the
response to IGF-I without ODQ in vessels without
endothelium).
View larger version (10K):
[in a new window]
Figure 3. Vasorelaxation response of vessels without
endothelium (E-) to insulin (left) and IGF-I (right)
with and without incubation with ODQ.
The incubation of vessels with intact endothelium
with 60 mmol/L KCl completely abolished the vasorelaxation
response to insulin (Figure 4, left;
P<0.0001 for comparison with the response to insulin
without KCl) and IGF-I (Figure 4, right; P<0.0001 for
comparison with the response to IGF-I without KCl). There was no
statistically significant difference in the contraction to KCl in the
insulin and IGF-I experiments (2.0 and 3.1 g, respectively).
Neither glyburide (n=6) nor charybdotoxin (n=6) attenuated the
vasorelaxation response to insulin after contraction with ET-1 (data
not shown). Likewise, the ATP-sensitive potassium channel
inhibitor glyburide (n=6) and the calcium-dependent
potassium channel inhibitor charybdotoxin (n=7) did not
attenuate the vasorelaxation response to IGF-I after contraction with
ET-1 (data not shown). In contrast, the potassium channel
inhibitor TEA significantly attenuated the vasorelaxation
response to insulin (Figure 4, left; P=0.005 for comparison
with the response to insulin without TEA in vessels with intact
endothelium) and IGF-I (Figure 4, right;
P=0.03 for comparison with the response to IGF-I without TEA
in vessels with intact endothelium). The mean
contractile responses to ET-1 after incubation with TEA in the insulin
and IGF-I experiments in vessels with intact
endothelium were 9.7±0.7 and 9.9±1.1 g, respectively.
There was no difference in the contraction to ET-1 between vessels that
were and were not exposed to TEA (P=0.88).
View larger version (13K):
[in a new window]
Figure 4. Vasorelaxation response of vessels with
endothelium (E+) to insulin (left) and IGF-I (right)
with and without incubation with TEA and contraction with ET-1. The
responses to insulin and IGF-I are also shown after contraction with
KCl. *P<0.05 vs the response with either KCl or TEA.
The exposure of vessels with intact endothelium to
diltiazem before the contraction with ET-1 did not affect the
vasorelaxation response to insulin (n=6) or IGF-I (n=6) (data not
shown).
Discussion
Top
Abstract
Introduction
Methods
Results
Discussion
References
Main Findings
The principal finding of the present study was that both
insulin and IGF-I caused dose-dependent coronary vasorelaxation
of porcine epicardial arteries in vitro that were precontracted with
ET-1. Removal of the endothelium did not alter the
vasoactive actions of insulin and IGF-I, suggesting that their effects
were primarily exerted on the vascular smooth muscle and not through
the endothelium. The underlying mechanism for the
coronary vasorelaxation effect involved the activation of
potassium channels and was independent of cGMP production.
These findings support a role for insulin and IGF-I in the regulation
of coronary vascular tone.
Different mechanisms have been proposed to explain the vasoactive
actions of insulin and related peptides.25 37 38
In several studies, insulin- or IGF-I–induced vasoconstriction or
vasodilation was attenuated or abolished by the removal of the
endothelium9 or by inhibition of
the production of endogenous
NO.9 14 23 These findings have led to speculation
that NO derived from the endothelium mediates the
vasodilator effects of insulin and IGF-I. Others have failed to show an
effect of endothelium removal on the vasoactive effects
of insulin.18 38
In our study, potassium completely abolished the vasorelaxation
effect of insulin and IGF-I, indicating that the mechanism involved
potassium channels. Furthermore, the coronary vasorelaxation
effect of insulin and IGF-I were attenuated by pretreatment with high
doses of the potassium channel inhibitor TEA.
Charybdotoxin, a specific inhibitor of the
calcium-activated potassium channel, and glyburide, an
inhibitor of the ATP-sensitive potassium channel, did not
attenuate the response to insulin and IGF-I.
In our study, the concentrations of insulin and IGF-I that were
examined ranged from the physiological to the
pharmacological. A concentration-dependent vasorelaxation response was
evident at low concentrations (10-10 mol/L).
The type I IGF receptor binds IGF-I with high affinity and insulin with
low affinity, whereas the insulin receptor binds IGF-I with a much
lower affinity than insulin. Because of the vasorelaxation response
attained with low concentrations of insulin and IGF-I, the vasoactive
effects of each peptide may be attributed to its respective receptor. A
prior study9 suggested that the vasoactive
responses of insulin are mediated by the IGF-I receptor in the rat
mesenteric artery. It is possible that in different vascular beds or
species, or in vessels of different caliber, the role of the IGF-I
receptor is more prominent. Alternatively, IGF-I may also exert its
actions, at least in part, through receptor-independent
mechanisms.47 48 It is also possible that hybrid
IGF-I–insulin receptors mediate the effect of these
peptides.49 These types of receptors are indeed
common in the heart.50
In pathophysiological states such as
atherosclerosis, the NO pathway may be impaired,
resulting in impaired coronary vasomotor
tone.51 Given that the effects of insulin and
IGF-I were not dependent on the NO pathway or on the generation of
cGMP, the vasorelaxation effects of insulin and IGF-I may remain intact
in pathophysiological states involving an impaired
rise in cGMP in response to NO activation. Indeed, Najibi et
al52 have demonstrated that atherosclerotic
rabbit carotid arteries may vasodilate through the activation of
potassium channels even when the NO pathway is impaired. Manipulation
of the IGF pathway in pathophysiological states may
thus maintain coronary epicardial vasomotor tone.
Selected Abbreviations and Acronyms
ET
=
endothelin
IGF-I
=
insulin-like growth factor-I
L-NMMA
=
NG-monomethyl-L-arginine
NO
=
nitric oxide
ODQ
=
1H-[1,2,4]oxadiazolo[4,3,-
]quinoxalin-1-one
TEA
=
tetraethylammonium
Acknowledgments
This study was supported by the Mayo Foundation, Miami Heart
Research Institute, the Ruth and Bruce Rappaport Vascular Biology
Program, and the National Institutes of Health (Dr Rizza, grant
DK-29553).
References
Top
Abstract
Introduction
Methods
Results
Discussion
References
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 J. R. Burgoyne, M. Madhani, F. Cuello, R. L. Charles, J. P. Brennan, E. Schroder, D. D. Browning, and P. Eaton Cysteine Redox Sensor in PKGIa Enables Oxidant-Induced Activation Science, September 7, 2007; 317(5843): 1393 - 1397. [Abstract] [Full Text] [PDF]
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 O. Chan, K. Inouye, E. Akirav, E. Park, M. C. Riddell, M. Vranic, and S. G. Matthews Insulin Alone Increases Hypothalamo-Pituitary-Adrenal Activity, and Diabetes Lowers Peak Stress Responses Endocrinology, March 1, 2005; 146(3): 1382 - 1390. [Abstract] [Full Text] [PDF]
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 A. Tivesten, A. Barlind, K. Caidahl, N. Klintland, A. Cittadini, C. Ohlsson, and J. Isgaard Growth hormone-induced blood pressure decrease is associated with increased mRNA levels of the vascular smooth muscle KATP channel J. Endocrinol., October 1, 2004; 183(1): 195 - 202. [Abstract] [Full Text] [PDF]
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 V. Randriamboavonjy, J. Schrader, R. Busse, and I. Fleming Insulin Induces the Release of Vasodilator Compounds From Platelets by a Nitric Oxide-G Kinase-VAMP-3-dependent Pathway J. Exp. Med., February 2, 2004; 199(3): 347 - 356. [Abstract] [Full Text] [PDF]
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L. Sternik, S. Samee, H. V. Schaff, K. J. Zehr, L. O. Lerman, D. R. Holmes, J. Herrmann, and A. Lerman C-Reactive Protein Relaxes Human Vessels In Vitro Arterioscler. Thromb. Vasc. Biol., November 1, 2002; 22(11): 1865 - 1868. [Abstract] [Full Text] [PDF]
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 P. J.M Best, J. C Burnett Jr., S. H Wilson, D. R Holmes Jr., and A. Lerman Dendroaspis natriuretic peptide relaxes isolated human arteries and veins Cardiovasc Res, August 1, 2002; 55(2): 375 - 384. [Abstract] [Full Text] [PDF]
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 J. A. S. Barreto-Filho, M. R. S. Alcantara, R. Salvatori, M. A. Barreto, A. C. S. Sousa, V. Bastos, A. H. Souza, R. M. C. Pereira, P. E. Clayton, M. S. Gill, et al. Familial Isolated Growth Hormone Deficiency Is Associated with Increased Systolic Blood Pressure, Central Obesity, and Dyslipidemia J. Clin. Endocrinol. Metab., May 1, 2002; 87(5): 2018 - 2023. [Abstract] [Full Text] [PDF]
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 M. Kimura, A.-M. Jefferis, H. Watanabe, and J. Chin-Dusting Insulin Inhibits Acetylcholine Responses in Rat Isolated Mesenteric Arteries via a Non-Nitric Oxide Nonprostanoid Pathway Hypertension, January 1, 2002; 39(1): 35 - 40. [Abstract] [Full Text] [PDF]
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 J. M. STULAK, A. LERMAN, M. R. PORCEL, J. A. CACCITOLO, J. C. ROMERO, H. V. SCHAFF, C. NAPOLI, and L. O. LERMAN Renal Vascular Function in Hypercholesterolemia Is Preserved by Chronic Antioxidant Supplementation J. Am. Soc. Nephrol., September 1, 2001; 12(9): 1882 - 1891. [Abstract] [Full Text] [PDF]
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C. Vecchione, S. Colella, L. Fratta, M. T. Gentile, G. Selvetella, G. Frati, B. Trimarco, and G. Lembo Impaired Insulin-Like Growth Factor I Vasorelaxant Effects in Hypertension Hypertension, June 1, 2001; 37(6): 1480 - 1485. [Abstract] [Full Text] [PDF]
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G. Zhao, R. L. Sutliff, C. S. Weber, J. Wang, J. Lorenz, R. J. Paul, and J. A. Fagin Smooth Muscle-Targeted Overexpression of Insulin-Like Growth Factor I Results in Enhanced Vascular Contractility Endocrinology, February 1, 2001; 142(2): 623 - 632. [Abstract] [Full Text] [PDF]
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 B. Capaldo, V. Guardasole, F. Pardo, M. Matarazzo, F. Di Rella, F. Numis, B. Merola, S. Longobardi, and L. Sacca Abnormal Vascular Reactivity in Growth Hormone Deficiency Circulation, January 30, 2001; 103(4): 520 - 524. [Abstract] [Full Text] [PDF]
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 K. Aguan, J. A. Carvajal, L. P. Thompson, and C. P. Weiner Application of a functional genomics approach to identify differentially expressed genes in human myometrium during pregnancy and labour Mol. Hum. Reprod., December 1, 2000; 6(12): 1141 - 1145. [Abstract] [Full Text] [PDF]
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C. L. Oltman, N. L. Kane, D. D. Gutterman, R. S. Bar, and K. C. Dellsperger Mechanism of coronary vasodilation to insulin and insulin-like growth factor I is dependent on vessel size Am J Physiol Endocrinol Metab, July 1, 2000; 279(1): E176 - E181. [Abstract] [Full Text] [PDF]
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