Calcium-Activated Potassium Channels and NO Regulate
Human Peripheral Conduit Artery Mechanics
Jeremy Bellien, Robinson Joannides, Michele Iacob, Philippe Arnaud, Christian Thuillez
Downloaded from http://hyper.ahajournals.org/ by guest on June 17, 2017
Abstract—The role of NO in the regulation of the mechanical properties of conduit arteries is controversial in humans, and
the involvement of an endothelium-derived hyperpolarizing factor (EDHF), acting through calcium-activated potassium
(KCa) channels, has never been investigated at this level in vivo. We assessed in healthy volunteers, after oral
administration of aspirin (500 mg), the effect of local infusion of NG-monomethyl-L-arginine (L-NMMA; 8 ␮mol/min
for 8 minutes), an NO synthase inhibitor, tetraethylammonium chloride (TEA; 9 ␮mol/min for 8 minutes), a KCa
channels inhibitor, and the combination of both on radial artery internal diameter, wall thickness (echo tracking), blood
flow (Doppler), and pressure. The incremental elastic modulus and compliance were fitted as functions of midwall
stress. L-NMMA decreased modulus and increased compliance at high levels of midwall stress (all P⬍0.05) without
affecting radial diameter. TEA reduced radial diameter from 2.68⫾0.07 to 2.50⫾0.08 10⫺3 m, increased the modulus,
and decreased the compliance at all levels of stress (all P⬍0.05). Combination of both inhibitors synergistically
enhanced the increase in modulus, the decrease in diameter (from 2.71⫾0.10 to 2.42⫾0.09 10⫺3 m), and compliance
compared with TEA alone (all P⬍0.05). These results confirm that inhibition of NO synthesis is associated with a
paradoxical isometric smooth muscle relaxation of the radial artery. They demonstrate the involvement of KCa channels
in the regulation of the mechanical properties of peripheral conduit arteries, supporting a role for EDHF at this level in
vivo. Moreover, the synergistic effect of L-NMMA and TEA shows that KCa channels compensate for the loss of NO
synthesis to maintain peripheral conduit artery diameter and mechanics. (Hypertension. 2005;46:210-216.)
Key Words: arteries 䡲 elasticity 䡲 compliance 䡲 nitric oxide 䡲 potassium channels 䡲 endothelium-derived factors
V
ascular endothelium regulates smooth muscle tone by
the release of vasorelaxant and vasoconstricting factors.1,2 Smooth muscle cell relaxation induces a decrease in
wall stiffness through a reduction in isometric tone. The
presence of a basal release of NO resulting in a permanent
NO-dependent vasodilatory influence has been largely demonstrated at the arteriolar level.1–3 However, at the level of
conduit arteries, although the contribution of NO to agonists
and flow-mediated endothelium-dependent vasodilatation has
been demonstrated, local administration of NO synthase
inhibitors such as NG-monomethyl-L-arginine (L-NMMA) do
not constantly induce a decrease in diameter or an increase in
the measured indicators of arterial stiffness, thus questioning
the role of a basal release of NO in the regulation of conduit
artery geometry and mechanics.2–10 In addition, although NO
synthase inhibition was associated to an increased sensitivity
to exogenous NO as expected from the suppression of an
endogenous NO release,3,11 we previously observed a surprising decrease in arterial stiffness without change in arterial
pressure and diameter after L-NMMA, suggesting the development of compensatory vasorelaxant mechanisms.7 In this
way, prostacyclin (PGI2) derived from endothelial cyclooxygenase appears to play no role in this regulation in physiological condition as after acute NO synthase inhibition.2,4
Whether an increase in vascular calcium-activated potassium
(KCa) channel activity after NO synthesis inhibition could
contribute to the maintenance of basal conduit artery diameter
and mechanics has never been investigated in vivo in humans.
Indeed, endothelium-derived hyperpolarizing factor (EDHF)
is described as the non-NO and non-PGI2 factor leading to
smooth muscle cell hyperpolarization and relaxation by the
opening of vascular KCa channels.12,13 In humans, the recent
demonstration of the contribution of KCa channels in the
regulation of forearm arteriolar resistance strongly suggests a
role for EDHF in vivo.14,15 Moreover, the involvement of KCa
channels and EDHF in the control of conduit artery mechanics and diameter is consistent with previous animal and ex
vivo studies.16 –19 Thus, the present study was designed to
assess in vivo, at the level of the radial artery, the effect of the
acute vascular KCa channel blockade in the regulation of basal
conduit artery diameter and mechanical properties before and
after inhibition of NO synthesis.
Methods
Subjects
Nine healthy male volunteers (mean⫾SEM 23⫾1 years) were
examined on 3 separate days, with a 3- to 4-week washout period
Received February 8, 2005; first decision February 24, 2005; revision accepted April 1, 2005.
From the Departments of Pharmacology (J.B., R.J., M.I., C.T.) and Pharmacy (P.A.), Rouen University Hospital, France.
Correspondence to R. Joannides, Département de Pharmacologie, INSERM U644, IFRMP 23, CHU de Rouen, 76031 Rouen Cedex, France. E-mail
[email protected]
© 2005 American Heart Association, Inc.
Hypertension is available at http://www.hypertensionaha.org
DOI: 10.1161/01.HYP.0000165685.83620.31
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Bellien et al
NO, KCa Channels, and Conduit Artery Mechanics
211
TABLE 1. Radial Artery Parameters Obtained at Operational Pressure Before and After
Infusion of L-NMMA and TEA and Their Combination
Arterial Parameters
⫺3
Internal diameter (10
m)
Time
L-NMMA
TEA
Base
2.699⫾0.099
2.678⫾0.071
2.707⫾0.101
⫺0.5⫾0.9
⫺6.8⫾0.7*†
⫺10.8⫾0.5*†‡
0.356⫾0.015
0.338⫾0.013
0.341⫾0.012
Change (%)
Wall thickness (10⫺3 m)
Base
Change (%)
Radius-to-wall thickness ratio
Base
Change (%)
Cross-sectional area (10⫺6 m2)
Midwall stress (kPa)
Elastic modulus (103 kPa)
⫺8
Compliance (10
2
m /kPa)
Base
⫺1.5⫾1.2
3.798⫾0.038
⫺0.8⫾2.0
3.710⫾0.206
5.9⫾2.1
3.988⫾0.123
12.3⫾2.3*†
L-NMMA⫹TEA
11.4⫾1.5*†‡
3.975⫾0.121
20.0⫾1.2*†‡
3.645⫾0.201
3.747⫾0.254
0.8⫾1.3
0.6⫾0.4
Change (%)
⫺1.3⫾1.0
Base
48.2⫾2.0
51.5⫾2.3
47.8⫾3.3
Change (%)
3.8⫾2.0
⫺10.9⫾3.3*†
⫺18.0⫾1.9*†‡
1.22⫾0.24
1.06⫾0.22
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Base
1.15⫾0.25
Change (%)
⫺9.0⫾2.4*
Base
5.04⫾0.19
5.16⫾0.81
5.12⫾0.80
Change (%)
5.5⫾3.5
⫺21.4⫾4.8*†
⫺44.5⫾5.8*†‡
⫺1.8⫾4.2
22.0⫾5.1*‡
Values are mean⫾SEM.
*P⬍0.05 vs corresponding control value; †P⬍0.05 vs L-NMMA; ‡P⬍0.05 vs TEA.
between each. The forearm volume was measured by using the water
displacement method to adjust the doses of the inhibitors to be
infused. All the procedures followed were in accordance with our
institutional guidelines. The protocol was approved by the Consultative Committee for the Protection of Persons Engaged in Biomedical Research of Haute-Normandie, and all participants gave informed written consent.
Instrumentation
Systemic blood pressure and heart rate were measured on the
dominant arm by mean of a brachial cuff oscillometric device
(Dinamap). Radial internal diameter, wall thickness, blood flow, and
arterial pressure were continuously measured in the nondominant
arm using a high-precision echo-tracking device (NIUS 02) coupled
to a Doppler system (Doptek) and a finger photoplethysmograph
(Finapres) as described previously.2,3,7,20 From arterial blood samples, total blood viscosity was measured using a cone-plate viscometer (Ex100 constant temperature bath).20,21 From the individual
values of radial artery internal diameter (d), blood flow (Q), and total
blood viscosity (␮), the mean arterial wall shear stress (␶) was
calculated as ␶⫽[(4 ␮Q)/(␲r3)] and r⫽d/2).20
Mechanics of the Radial Artery
From the measured variables arterial pressure (p), internal diameter
(d), and wall thickness (h), the cross-section (S)–pressure curve was
fitted as S⫽␣[␲/2⫹tang⫺1([p⫺␤]/␥)], S⫽␲d2/4, where ␣, ␤, and ␥
are the 3 optimal fit parameters of the model.22 The cross-sectional
compliance (C) was expressed as C⫽(␣/␥)/[1⫹([p⫺␤]/␥)2] and the
incremental elastic modulus (Ei) as Ei⫽3/8[d(d⫹2h)2/h(d⫹h)]⭸p/
⭸d.23 The midwall stress (␴) was calculated as ␴⫽2p(ri 䡠 re)2/(re2⫺ri2),
where re and ri are the external and internal radii, respectively, and
r the radius at midwall (r⫽(re⫹ri)/2).7,20 Finally, from the individual
values, the diameter–midwall stress, the modulus–midwall stress,
and the compliance–midwall stress curves were constructed to assess
the effect of the inhibitors at identical levels of wall-loading
conditions.
Study Protocol
Subjects were supine in a quiet, air-conditioned room (22°C to
24°C). A 27-gauge needle was inserted under local anesthesia (1%
lidocaine) into the brachial artery of the nondominant arm, and saline
was infused (1 mL/min). After instrumentation, oral aspirin (UPSA
500 mg; laboratoire UPSA) was given to block vascular cyclooxy-
genase activity.15,24 After 30 minutes of resting, arterial parameters
were recorded at baseline for 5 minutes. Then, sodium nitroprusside
(SNP; 20 nmol/L per minute) was infused for 3 minutes to assess the
NO-mediated endothelium-independent dilatation. Thirty minutes
after completion of SNP infusion and return to basal values of radial
artery blood flow and diameter, for 8 minutes, subjects received, in
random order, either the NO synthase inhibitor L-NMMA (8 ␮mol/L
per minute; Clinalfa) or the vascular KCa channel inhibitor tetraethylammonium chloride (TEA; 9 ␮mol/L per minute; Clinalfa), or
their combination. At the end of the inhibitor infusion, the same
protocol of SNP administration was repeated. From this continuous
recording, mean values were calculated from 15 consecutive cardiac
cycles during the last minute of each infusion period. At each time,
all parameters were calculated at operational pressure (mean arterial
pressure). In absence of change in systemic blood pressure during the
different experimental procedures, only digital arterial pressure used
for calculations is shown.
Statistical Analysis
Results are expressed as mean⫾SEM. The effects of the inhibitors
on the mean values were compared using ANOVA with subjects,
periods, and inhibitors as factors, followed by a modified Student t
test when applicable. This analysis was repeated with the mean wall
shear stress or the variation of the radial artery flow as covariate. The
relationships obtained after the inhibitors were compared with those
obtained at baseline using an ANCOVA with midwall stress as
covariate and subjects and periods as factors. A value of P⬍0.05 was
considered statistically significant.
Results
Mean Hemodynamic and Mechanical Parameters
At baseline before infusion of L-NMMA and TEA, and the
combination of L-NMMA and TEA, there was no significant
difference between the mean values of the geometric and
mechanical parameters of the radial artery (Table 1) and
between the mean values of arterial pressure and heart rate
(Table 2).
None of the inhibitors affected arterial pressure or heart
rate (Table 2). In addition, cross-sectional area was not modified
by L-NMMA or TEA, or their combination (Table 1).
212
Hypertension
July 2005
TABLE 2. Arterial Hemodynamic Parameters Measured Before
and After Infusion of L-NMMA and TEA and
Their Combination
Parameters
Inhibitors
Before
After
Systolic arterial pressure (mm Hg)
L-NMMA
112⫾3
112⫾2
Diastolic arterial pressure (mm Hg)
Mean arterial pressure (mm Hg)
Pulse pressure (mm Hg)
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Heart rate (bpm)
TEA
112⫾5
114⫾6
L-NMMA⫹TEA
116⫾7
118⫾8
L-NMMA
62⫾2
64⫾2
TEA
61⫾3
61⫾3
L-NMMA⫹TEA
62⫾4
61⫾3
L-NMMA
79⫾1
83⫾2
TEA
79⫾3
80⫾4
L-NMMA⫹TEA
80⫾5
80⫾4
L-NMMA
50⫾4
48⫾3
TEA
51⫾4
52⫾5
L-NMMA⫹TEA
54⫾5
57⫾5
L-NMMA
64⫾3
63⫾3
TEA
59⫾2
62⫾2
L-NMMA⫹TEA
60⫾2
64⫾2
Values are mean⫾SEM.
After L-NMMA, internal diameter, wall thickness, radiusto-wall thickness ratio, and midwall stress were not modified
(all NS). Radial artery flow decreased from 11.7⫾2.6 to
9.2⫾1.8 10⫺3 L/min (P⬍0.05), and the mean wall shear stress
decreased from 4.3⫾0.6 to 3.5⫾0.5 10⫺1 Pa (P⬍0.05). After
inhibition of NO synthesis, the pulse diameter to pulse
pressure ratio increased nonsignificantly by 9⫾5%, from
8.0⫾1.1 10⫺2 m/Pa. The incremental elastic modulus of the
arterial wall decreased (P⬍0.05), and the radial artery compliance increased, however, not significantly.
After TEA, internal diameter decreased (P⬍0.05), and wall
thickness increased, however, not significantly. The radiusto-wall thickness ratio decreased (P⬍0.05), explaining, in
absence of change in arterial pressure, the decrease in radial
artery midwall stress (P⬍0.05). Radial artery flow decreased
from 10.9⫾2.3 to 8.5⫾1.8 10⫺3 L/min (P⬍0.05), but as a
result of the concomitant decrease in radial artery diameter
and flow, the mean wall shear stress was not significantly
modified by TEA (from 4.6⫾0.8 to 4.4⫾0.8 10⫺1 Pa). After
the blockade of KCa channels, the pulse diameter-to-pulse
pressure ratio decreased by ⫺18⫾5%, from 8.9⫾1.7 10⫺2
m/Pa (P⬍0.05). The incremental elastic modulus of the
arterial wall was not modified (NS), but the radial artery
compliance decreased (P⬍0.05).
After the combination of L-NMMA and TEA, internal
diameter decreased, and wall thickness increased (both
P⬍0.05). The radius-to-wall thickness ratio decreased
(P⬍0.05), explaining, in absence of change in arterial pressure, the decrease in radial artery midwall stress (P⬍0.05).
Radial artery flow decreased from 10.5⫾2.4 to 7.7⫾0.8 10⫺3
L/min (P⬍0.05), but as a result of the concomitant decrease
in radial artery diameter and flow, the mean wall shear stress
was not significantly modified by the combination (from
4.0⫾0.6 to 4.0⫾0.7 10⫺1 Pa). After NO synthesis inhibition
combined to KCa blockade, the pulse diameter-to-pulse pressure ratio decreased by ⫺34⫾5%, from 8.5⫾1.1 10⫺2 m/Pa
(P⬍0.05). The incremental elastic modulus of the arterial
wall increased, and the radial artery compliance decreased
(both P⬍0.05). The effect of the combination of L-NMMA
and TEA on all these parameters was more pronounced than
those of TEA alone (all P⬍0.05).
The effects of L-NMMA and TEA and their combination
on the geometric and mechanical parameters of the radial
artery remained significant even after mean wall shear stress
or changes in radial artery flow were included as covariates
into analysis (all P⬍0.05).
Internal Diameter–, Elastic Modulus–, and
Arterial Compliance–Midwall Stress Curves
Figure 1 shows the effect of L-NMMA on the internal
diameter–, elastic modulus–, and arterial compliance–midwall stress curves. Before and after L-NMMA, the radial
artery diameter and the elastic modulus of the arterial wall
increased, and the radial artery compliance decreased with the
midwall stress (both P⬍0.001). After L-NMMA, there was no
significant change in the diameter–midwall stress curve. The
modulus–midwall stress curve was shifted downward
(P⬍0.001), reflecting a decrease in arterial wall stiffness. The
compliance–midwall stress curve was shifted upward
(P⬍0.001). These effects were more marked at high levels of
midwall stress. The decrease in arterial wall stiffness and the
increase in compliance, which occurred in the absence of any
change in arterial pressure or geometry, suggest that inhibition of NO synthesis was associated with an isometric smooth
muscle relaxation.
Figure 2 shows the effects of TEA and of the combination
of L-NMMA and TEA on the internal diameter–, elastic
modulus–, and arterial compliance–midwall stress curves.
The radial artery diameter and the elastic modulus of the
arterial wall increased, and the radial artery compliance
decreased with the midwall stress in all cases (all P⬍0.001).
Figure 1. Radial artery diameter (left)–,
incremental elastic modulus (middle)–,
and compliance–midwall (right) stress
curves obtained before (E) and after
(F) infusion of L-NMMA. After L-NMMA,
in absence of significant shift of the
diameter–stress curve (NS), there was
a downward shift of the incremental
modulus–stress curve at high values of
stress (P⬍0.001) that indicates the
decrease in wall stiffness and an
upward shift of the compliance–stress
curve (P⬍0.001) that reflects at the
level of the arterial chamber the decrease in wall stiffness obtained after NO synthase inhibition. P⬍0.001 for stress effect.
Bellien et al
NO, KCa Channels, and Conduit Artery Mechanics
213
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Figure 2. Radial artery diameter (left)–,
incremental elastic modulus (middle)–,
and compliance–midwall (right) stress
curves obtained before (E) and after (F)
infusion of TEA (A) and before (E) and
after (F) infusion of the combination of
L-NMMA and TEA (B). TEA induced a
downward shift of the diameter–stress
curve (P⬍0.001) and an upward shift of
the incremental modulus–stress curve
(P⬍0.001), indicating that the vasoconstriction of the radial artery observed
after TEA was associated with a simultaneous increase in wall stiffness at each
level of midwall stress. Thus, the downward shift of the compliance–stress
curve (P⬍0.001) after KCa channel blockade is explained by the decrease in
diameter and the increase in wall stiffness. After the combination of L-NMMA
and TEA, the curves were shifted in the
same direction as after TEA alone (all
P⬍0.001) but with a larger magnitude (all
P⬍0.01 vs TEA alone), thus indicating a
synergistic effect of NO synthase inhibition and KCa channel blockade. P⬍0.001
for stress effect.
After TEA, there was a downward shift in the diameter–
midwall stress curve and an upward shit in the modulus–
midwall stress curve (both P⬍0.001). Thus, KCa channel
blockade was associated with a smooth muscle contraction
that explains the decrease in arterial diameter and the increase
in arterial wall stiffness at each level of stress. As a result, the
compliance–midwall stress curve was shifted downward at
each level of stress (P⬍0.001).
After the combination of L-NMMA and TEA, the internal
diameter–, elastic modulus–, and arterial compliance–midwall stress curves were shifted in the same way as after TEA
alone (all P⬍0.001). However, the shift of these curves was
more pronounced after the combination of L-NMMA and
TEA than after TEA alone (all P⬍0.01). Thus, when combined with KCa channel blockade, the NO synthesis inhibition
was associated to a larger smooth muscle contraction than
after TEA alone.
Radial Artery Endothelium-Independent Dilatation
Figure 3 shows the effects of SNP on the internal diameter–,
elastic modulus–, and arterial compliance–midwall stress
curves in control conditions. After SNP, there was an upward
shift of the diameter–midwall stress curve and a downward
shift in the modulus–midwall stress curve (both P⬍0.001).
As a result, the compliance–midwall stress curve was shifted
upward at each level of stress (P⬍0.001).
SNP induced an increase in radial artery diameter before
(2.724⫾0.072 to 3.293⫾0.109 10⫺3 m; P⬍0.05) and after
⫺3
L-NMMA (from 2.691⫾0.095 to 3.360⫾0.128 10
m;
P⬍0.05), before (2.694⫾0.084 to 3.274⫾0.117 10⫺3 m;
P⬍0.05) and after TEA (2.502⫾0.078 to 3.143⫾0.116 10⫺3
m; P⬍0.05), and before (from 2.694⫾0.084 to 3.274⫾0.117
10⫺3 m; P⬍0.05) and after their combination (2.415⫾0.093
to 3.207⫾0.134 10⫺3 m; P⬍0.05). The increase in radial
artery diameter was enhanced by L-NMMA alone and in
combination with TEA (both P⬍0.05), whereas TEA alone
did not significantly modify this response.
Discussion
Our results obtained from humans demonstrate that KCa
channels are involved in regulation of the mechanical properties of peripheral conduit arteries, supporting a role for
EDHF at this level in vivo. In addition, the synergistic effect
of L-NMMA and TEA on radial artery constriction and
arterial wall stiffening in absence of such effects after
administration of L-NMMA alone shows that KCa channels
compensate for the loss of NO synthesis to maintain peripheral conduit artery diameter and mechanics.
Figure 3. Radial artery diameter (left)–,
incremental elastic modulus (middle)–,
and compliance–midwall (right) stress
curves obtained before (E) and after
(F) infusion of SNP in control conditions. SNP induced an upward shift of
the diameter–stress curve (P⬍0.001)
and a downward shift of the incremental modulus–stress curve (P⬍0.001),
indicating that the vasodilatation
observed after administration of exogenous NO is associated to a decrease in
wall stiffness. Thus, the upward shift of
the compliance–stress curve (P⬍0.001) after SNP is explained by the increase in diameter and the decrease in wall stiffness.
214
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July 2005
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The present study was performed in healthy subjects at the
level of the radial artery to assess in vivo the role of NO and
vascular KCa channels and their potential interaction in the
regulation of basal conduit artery diameter and mechanics.
All experiments were performed after administration of
aspirin to inhibit vascular cyclooxygenase and to exclude a
role for PGI2 in our results.15,24 The inhibition of endothelial
NO synthesis was obtained with L-NMMA, infused at a dose
of 8 ␮mol/min, known to abolish the radial artery dilatation to
high gradual doses of acetylcholine without affecting hemodynamics.3 In addition, we used the nonspecific inhibitor of
vascular KCa channels, TEA, as a pharmacological tool for the
exploration of EDHF activity.14,15 Indeed, although the biological nature of EDHF remains to be discussed, its endothelium-dependent hyperpolarizing mechanism appears mainly
related to the opening of vascular KCa channels leading to
smooth muscle cell relaxation.12,13 Thus, TEA was infused at
a dose of 9 ␮mol/min to obtain, for a basal radial artery flow
of 10 mL/min, a local concentration of 1 mmol/L higher than
those shown to selectively block a single KCa channel without
affecting the function of other potassium channels.15,25 As
expected, L-NMMA reduced radial artery flow, confirming
the role of NO in the regulation of the basal arteriolar tone in
humans.1–3,5–7,14,15 In addition, as more recently reported by
use of plethysmography in healthy subjects,15 TEA reduced
radial artery flow in our study, also demonstrating a role of
KCa channels in the control of basal arteriolar tone.
Concerning the conduit arteries, the local administration of
L-NMMA did not significantly modify blood pressure nor the
radial diameter but decreased the arterial wall stiffness, thus
demonstrating, in accordance with our previous report,7
although less marked, paradoxical isometric smooth muscle
cell relaxation after NO synthase inhibition. An absence of
decrease in the diameter or alteration in the mechanical
properties of conduit arteries was reported in vivo despite
local administration of doses of L-NMMA up to 60 ␮mol/min
in absence of significant change in blood pressure.2– 8,26
Conversely, other studies demonstrated a significant effect
of local NO synthesis inhibition on arterial diameter and
mechanics.9,10,27–30 These results could be explained by
the heterogeneity of the vascular tree5,27,28 and by the increase in blood pressure after systemic administration of
L-NMMA.29 –31 In addition, the invasive methodological approaches used in some studies could have provided more
sensitivity, but they also could have potentiated the vascular
effect of NO synthesis inhibition by majoring the local
decrease in flow9 or by interfering with KCa channel activity.10,32 In this context, L-NMMA induced in our subjects an
enhanced dilatation to SNP compared with baseline conditions. This hypersensitivity of the smooth muscle cells to a
nitrovasodilator after NO synthase inhibition has been observed previously in vitro and in vivo and reflects the
suppression of an endogenous NO release in the arterial
wall.3,11 Thus, this hypersensitivity and the absence of decrease in resting diameter despite the concomitant decrease in
flow after L-NMMA once again suggest more the presence of
compensatory mechanisms occurring after inhibition of NO
synthesis to maintain this diameter than the absence of NO
basal release.3 Thus, according to the magnitude or the ability
to develop such mechanisms, the effect of L-NMMA on
diameter and mechanics could vary, explaining also the
apparent divergences observed. Indeed, although the impact
of arterial pressure on this mechanism needs to be evaluated,
the higher blood pressure level observed in the present study,
although within the normal range, could have decreased the
magnitude of the effect noted after L-NMMA compared with
our previous experiment.7
In this context, TEA induced a significant decrease in
radial artery diameter and compliance obtained at operational
pressure and at each level of stress, demonstrating for the first
time in vivo that vascular KCa channels are involved in the
regulation of basal peripheral conduit artery mechanics in
humans. At operational pressure, the elastic modulus was not
modified. This was the consequence of the simultaneous
decrease in midwall stress attributable to the lower radius-towall thickness ratio after vasoconstriction. When evaluated at
stable wall-loading conditions by use of the elastic modulus–
midwall stress curve, TEA was associated to an increase in
modulus at each level of stress, thus confirming the wall
stiffness increase and the withdrawing of a relaxant influence
after KCa channel blockade. The effect of TEA on radial artery
geometry and stiffness cannot be related to a flow-dependent
mechanism because radial artery mean wall shear stress (ie,
the physiological flow stimulus) was not significantly affected by TEA. In addition, as described previously in human
resistance arteries,14,15 TEA did not modify radial artery
dilatation to SNP, arguing against a decrease in smooth
muscle cell ability to relax after KCa channel blockade.
Previous animal and ex vivo experiments have shown that
vascular KCa channels are involved in the endotheliumdependent dilatation of conduit arteries.16 –19,33,34 However,
their role in the regulation of basal conduit artery diameter
and tone has been less investigated. Nevertheless, it was
demonstrated that KCa channels contribute to the control of
the resting membrane potential of human gastroepiploic
arteries.18 In addition, Popp et al16 showed that the activation
of muscular KCa channels was mainly dependent of the basal
release of an EDHF in porcine coronary arteries and participates in the regulation of arterial compliance. Thus, our
results obtained at the level of the radial artery suggest that
the activation of vascular KCa channels involved in the
regulation of resting diameter and mechanics results from the
basal release of an EDHF in vivo.
Finally, we observed that the combined administration of
L-NMMA and TEA induced a greater decrease in radial artery
diameter and compliance than TEA alone. In addition, this
combination induced a significant increase in the operational
elastic modulus despite the larger decrease in midwall stress
than after TEA alone, thus supporting the dramatic increase in
wall stiffness at each level of stress reflected by the upward
shift of the modulus–midwall stress curve. As already discussed for TEA alone, this effect could not be related to a
flow-dependent mechanism or to a decrease in vascular
smooth muscle cell reactivity. In this context, KCa channel
blockade unmasks the vasoconstrictor effect of L-NMMA on
the radial artery, thus giving evidence of the role of NO in the
regulation of resting diameter and mechanics at the level of a
peripheral conduit artery in humans.9 Moreover, this syner-
Bellien et al
gistic effect of the combined administration of L-NMMA and
TEA demonstrates that vascular KCa channels compensate for
the loss of endothelial NO synthesis to maintain the basal
radial diameter and mechanics, explaining why L-NMMA,
when administrated alone, was ineffective to induce radial
artery constriction and, in contrast, decreased the isometric
wall artery muscular tone. These results are consistent with
previous studies showing that an increase in NO availability
decreases EDHF-mediated conduit artery dilatation in animals35 and that at the opposite, chronically impaired NOdependent dilatation present in pathology is associated to an
upregulation of EDHF, acting as a back-up mechanism to
preserve endothelium-dependent dilatation of resistance and
conduit arteries.19,36 – 41
Perspectives
NO, KCa Channels, and Conduit Artery Mechanics
9.
10.
11.
12.
13.
14.
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Our in vivo study gives evidence for the first time that an
alternative pathway acting through the stimulation of KCa
channels is activated at baseline and upregulated during NO
deficiency at the level of human peripheral conduit arteries to
maintain resting diameter and mechanical properties in physiological state. These results strongly support the hypothesis
of a basal release of an EDHF at this level. Whether this
mechanism is still effective in aging humans and in pathophysiological conditions needs further investigation. This
could be of importance in the regulation of cardiovascular
coupling and arterial conductance at rest and during exercise,
and therefore its alteration could contribute to the pathophysiology of many cardiovascular diseases.42– 44 Furthermore, the
presence of such mechanisms at the level of epicardial
coronary arteries or proximal conduit arteries more frequently
concerned by atherosclerosis45 could participate in the prevention of atherosclerosis in addition to NO.46
15.
16.
17.
18.
19.
20.
Acknowledgments
21.
This study was supported by a grant from the Societe Francaise
d’Hypertension Arterielle and was performed with the help of the
CIC-INSERM-Rouen University Hospital.
22.
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Calcium-Activated Potassium Channels and NO Regulate Human Peripheral Conduit
Artery Mechanics
Jeremy Bellien, Robinson Joannides, Michele Iacob, Philippe Arnaud and Christian Thuillez
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Hypertension. 2005;46:210-216; originally published online May 2, 2005;
doi: 10.1161/01.HYP.0000165685.83620.31
Hypertension is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
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