(Hypertension. 1997;30:1465-1470.)
© 1997 American Heart Association, Inc.
Articles |
From the Department of Pharmacology, Vacomed, IFRMP 23, Rouen University Medical School and Rouen Hospital, Rouen, France (R.J., V.R., A.B., C.T.); the Division of Clinical Pharmacology, Basel University Hospital, Basel (W.E.H., L.L.), and Cardiology, Cardiovascular Research, Zurich University Hospital, Zurich, Switzerland (T.F.L.).
| Abstract |
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Key Words: arteries endothelium-derived factors endothelium elasticity stress
| Introduction |
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| Methods |
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Instrumentation
Measurements were performed while subjects were supine in a
quiet, air-conditioned room maintained at a constant temperature
(22°C to 24°C). Under local anesthesia (lidocaine 1%),
an 18-gauge catheter was inserted into the left brachial artery for
continuous measurement of arterial pressure (Statham
P23Pb)10 and infusion of L-NMMA.8,9 Radial
artery internal diameter and wall thickness were continuously measured
using a high-precision A-mode echo-tracking device with an axial
resolution of 150 µm and a theoretical resolution of less than
1 µm on artery wall displacements (NIUS 02,
Asulab).11,12 Briefly, a 10-MHz focused transducer was
positioned over the radial artery. The probe was set perpendicularly to
the artery without any skin contact using a stereotaxic arm
with micrometric screws, while proper positioning was adjusted using a
stereo Doppler mode. After switching to A-mode, the echoes
reflected by the interfaces between blood and artery wall on one side
and between artery wall and surrounding tissue on the other side could
be identified for both the anterior and posterior walls and visualized
on the screen display. After selection by the operator of the desired
interfaces, the echoes reflected were tagged by electronic trackers,
allowing continuous recording of the artery internal diameter
and wall thickness.3 Radial artery internal cross-sectional
area was then calculated from the measurement of internal diameter.
Furthermore, radial artery blood flow velocity was continuously
recorded using an 8 MHz Doppler probe (Doptek 2002, Deltex).
Radial artery flow was calculated from the measurements of velocity and
internal cross-sectional area. The method has been validated with
target measurements and comparisons between in vitro and in vivo
measurements.12 The reproducibility of the method in humans
assessed by calculating the coefficient of variation between
consecutive measurements was 2±1%, 3±1%, 7±2%, respectively, for
arterial internal diameter, wall thickness, and blood
flow.38
From the measured variables, arterial pressure (p),
internal diameter (d), and wall thickness (h), the cross-section
(S)pressure curve was fitted using an arctangent model as proposed by
Tardy et al11:
S=
[
/2+tang-1([p-ß]/
)],
S=
d2/4, where
, ß, and
are the three optimal
fit parameters of the model.
To eliminate the hysteresis loop in the diameter-pressure graph due to the distance between the sites of measurements of arterial diameter and pressure, we used a previously reported method that is based on the evaluation of the phase lag between the diameter and pressure signals from sectional pulse-wave velocity calculated at each level of pressure.11
By taking the derivate of the cross-section-pressure function, the
cross-sectional compliance (C) was expressed as
C=(
/
)/[1+([p-ß]/
)2].
Assuming isotropic and incompressible material, the incremental elastic
modulus (Ei) for a thick-walled artery was expressed as a function of
pressure13 as Ei=3/8[d(d+2h)2/
h(d+h)]
p/
d.
With the same assumptions, the midwall stress (s) was calculated
as3
=2p(rire/r)2/(re2-ri2),
where re and ri, are the external and internal
radii, respectively, and r the radius at midwall
(r=(re+ri)/2).
Protocol Design
Subjects were allowed to rest for at least 30 minutes after
instrumentation. Then heart rate, arterial pressure, radial
artery flow, diameter, and wall thickness were recorded for 10
minutes during saline infusion at a constant rate of 0.8 L ·
min-1 10-3. After control measurements
(physiological saline), two doses of L-NMMA (1 and
4 µmol L-1 min-1, Clinalfa) were
infused for 5 minutes at the same infusion rate as
physiological saline in the 11 volunteers. In 7
subjects, after control measurements (physiological
saline), two doses of acetylcholine (3 and 30 nmol L-1
min-1, Ciba Vision AG) were infused for 3 minutes
(infusion rate of 0.8 L min-1 10-3) 40
minutes before L-NMMA infusion. All parameters were
continuously recorded from baseline to the end of the infusion
period of L-NMMA and acetylcholine. From this continuous
recording mean values were calculated from 15 consecutive
cardiac cycles during the last minute of each infusion period. As
previously reported, when an oscillatory pattern was noted, we extended
the calculations to 30 to 40 consecutive cardiac cycles in order to
cover the half period of oscillation and to obtain stable
mean values.9 At each time, parameters were
calculated both at operational pressure (mean arterial
pressure) and to avoid the passive effects of changes in systemic
arterial pressure, at a constant pressure of 80
mm Hg. This level of pressure was chosen because it was reached at
each time point of the experiment in all subjects.
Statistical Analysis
Results are expressed as mean±SEM. The analysis of the
effect of L-NMMA and acetylcholine on the different
parameters investigated was done by repeated measures
ANOVA, followed by Tukey's test when applicable. The relations
obtained after L-NMMA and acetylcholine at the maximal doses were
compared with those obtained at baseline using an ANCOVA with midwall
stress as covariate and subjects and periods as factors. The
analysis was repeated adding arterial flow as
covariate to consider the effect of associated change in blood flow.
For the latter analysis, reported values were adjusted least
square mean±SEM. A value of P<.05 was considered
statistically significant.
| Results |
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Forearm infusion of L-NMMA or acetylcholine did not modify
arterial pressure or heart rate (Table 2
). In addition, there was no significant
change in the cross-sectional area of the arterial wall
during the three periods (from 4.77±0.44 to 4.77±0.44
m2 · 10-6 during the control period,
NS; from 4.78±0.37 to 4.79±0.42 m2 ·
10-6 after 4 µmol · L-1
· min-1 L-NMMA, NS; from 4.93±0.28 to 4.82±0.28
m2 · 10-6 after 30 nmol ·
L-1 · min-1 acetylcholine, NS).
|
L-NMMA decreased radial blood flow (Table 2
) but did not affect radial
artery internal diameter, wall thickness, or midwall stress (Fig 1
). After inhibition of NO synthesis, the
incremental elastic modulus of the arterial wall decreased
(P<.05) and compliance increased (P<.05, Fig 1
).
|
Acetylcholine increased radial blood flow (Table 2
) and internal
diameter (P<.05), and decreased radial artery wall
thickness (P<.05, Fig 2
).
Those changes in arterial geometry obtained without any
change in arterial pressure were associated with an
increase in midwall stress (P<.05). Despite this increase
in midwall stress, acetycholine decreased incremental elastic modulus
of the arterial wall (P<.05) and increased mean
arterial compliance (P<.05, Fig 2
).
|
Effects of L-NMMA on Diameter, Modulus, and ComplianceMidwall
Stress Curves
To assess the effect of L-NMMA at identical levels of wall-loading
conditions, arterial diameter and elastic modulus were
assessed as a function of midwall stress. Fig 3
shows the diameterand elastic
modulusmidwall stress curves at baseline and during L-NMMA infusion.
The diameter increased with wall stress. After L-NMMA the
diametermidwall stress curve was not shifted significantly.
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The elastic modulus increased with midwall stress. After L-NMMA infusion the modulusmidwall stress curve was shifted downward (P<.01). Thus, NO synthesis inhibition was associated with a decrease in arterial wall stiffness at every level of midwall stress.
Fig 4
shows the compliancemidwall
stress curves at baseline and during L-NMMA infusion. Compliance
decreased in a curvilinear manner when wall stress increased. During NO
synthesis inhibition, compliance increased at every level of stress
(P<.01).
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The decreased arterial wall stiffness and increased compliance that occurred in the absence of any change in arterial pressure or geometry suggest that inhibition of NO synthesis was associated with a paradoxical isometric smooth muscle relaxation.
Effects of Acetylcholine on Diameter, Modulus, and
ComplianceMidwall Stress Curves
Fig 5
(panels a and b) shows the
diameterand elastic modulusmidwall stress curves at baseline and
during acetylcholine infusion. After acetylcholine there was an
increase in diameter at each level of midwall stress
(P<.01) and a downward shift of the modulusmidwall stress
curve toward the lower values of modulus, reflecting the decrease in
arterial wall stiffness at every level of midwall stress
(P<.01).
|
Fig 5c
shows the compliancemidwall stress curves at baseline and
during acetylcholine infusion. After acetylcholine, there was an upward
shift of the compliancemidwall stress curve toward higher values of
compliance (P<.01) reflecting, at the level of the artery
chamber, the effect of the increase in artery diameter and the decrease
in artery wall stiffness at every level of stress.
Effects of Arterial Blood Flow on Mechanical Changes
Induced by L-NMMA and Acetylcholine
When the decrease in arterial flow during L-NMMA
was taken as a covariate, the decrease in the elastic incremental
modulus of the arterial wall (1.58±0.05 versus 1.44±0.05
kPa · 103; P<.025) and the increase in
arterial compliance (3.99±0.07 versus 4.48±0.07
m2 · kPa · 10-8;
P<.0001) were still significant. These results demonstrate
that the isometric relaxation observed could not be fully explained by
the decrease in arterial flow.
In addition, when the increase in arterial flow during acetycholine was taken as a covariate, the decrease in the elastic incremental modulus of the arterial wall (1.14±0.02 versus 0.57±0.02 kPa · 103; P<.0001), the increase in radial artery internal diameter (2.92±0.00 versus 3.10±0.00 m · 10-3; P<.0001) and the increase in arterial compliance (2.74±0.21 versus 9.21±0.21 m2 · kPa · 10-8; P<.0001) were still significant. These results demonstrate that the decrease in arterial tone and thus the vasodilatation observed after acetylcholine could not be fully explained by the increase in arterial flow but could be due in part to a direct flow-independent effect.
| Discussion |
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Using the high resolution echo-tracking method, it was possible to obtain continuous and stable measurements of internal diameter and wall thickness. This was illustrated by the stable value of the calculated cross-sectional area of the arterial wall during control and infusion periods. Moreover, the high reproducibility of the measurements obtained with this method was previously demonstrated in our laboratory3,8,9 and was shown to be similar to that reported by others.14
In addition, the Doppler system enabled us to obtain simultaneous and continuous recordings of arterial blood flow at the same site at which mechanical parameters were measured. Thus, it was possible for the first time in humans to study the effects of NO synthesis inhibition on the mechanical properties of the arterial wall of the radial artery and to consider these effects in relation to the simultaneous changes in arterial flow. This method enables us to assess the direct effects of inhibition or stimulation of NO synthesis on the mechanical properties of the conduit arteries independently from those induced by the associated changes in flow.
In agreement with previous results obtained with plethysmography6,15 or with Doppler technique,9 the administration of L-NMMA decreased radial blood flow, demonstrating the existence of a basal release of NO in the forearm vascular bed. Moreover, this decrease in arterial flow after L-NMMA demonstrates that the doses we used were able to significantly affect the release of NO at least at the level of resistance arteries. However, at the level of conduit arteries, L-NMMA, administered at a dose that abolished the radial artery response to acetylcholine9 as well as the flow-dependent vasodilatation of the radial artery,8 did not significantly decrease radial diameter. Such a lack of effect on conduit artery is in agreement with previous results obtained in humans at the level of the pulmonary artery16 and the proximal site of coronary arteries17and stresses the fact that conduit artery can exhibit reactivity to vasoactive substances differently than resistance arteries.
In the present study, the relaxant effect of acetylcholine was directly assessed from the calculated decrease in wall stiffness, ie, the incremental elastic modulus of the arterial wall. This decrease in stiffness occurred despite the opposite effect of the increase in midwall stress, secondary to the increase in diameter at constant pressure.18 Moreover, since compliance is proportional to arterial diameter and inversely proportional to wall stiffness, the increase in arterial compliance after the administration of acetylcholine may be explained both by the increase in arterial diameter and by the decrease in arterial wall stiffness related to the decrease in arterial tone. This is in agreement with results previously reported with acetylcholine19 or with exogenous NO donors,20 which increase both arterial diameter and distensibility.
It must be noted that the NO-mediated relaxing effect of acetylcholine in vivo may occur through two distinct mechanisms, ie, direct effects and indirect flow-mediated vasorelaxation. However, the fact that the decrease in wall stiffness and the increase in compliance remained significant even when the changes in blood flow were taken into account suggests that at least part of the relaxing effect is flow-independent, confirming in humans the data previously obtained in animal models.21
The most surprising finding of our study was the demonstration that acute inhibition of NO synthesis was associated with a dose-dependent decrease in arterial wall stiffness of the radial artery, as evidenced by the decrease in incremental elastic modulus of the arterial wall. This occurred in the absence of changes in mean midwall stress, ie, in constant wall loading conditions, and was confirmed at each level of stress by the downward shift of the elastic modulusmidwall stress curve. In addition, this decrease in stiffness was observed without a change in mean arterial diameter and wall thickness, and thus represents a paradoxical isometric smooth muscle relaxation of the arterial wall after inhibition of NO synthesis. This decrease in wall stiffness induced an isometric increase in arterial compliance. A similar increase in arterial compliance was noted after endothelium removal at the level of the carotid artery in the rat.22 This change was explained by the increase in arterial diameter after endothelium removal.23 However, our results cannot be fully superposed with these previously described data, since endothelium removal not only suppresses NO-dependent tone but also other pathways involved in the regulation of vascular tone, and may markedly affect the balance between vasoconstrictor and vasodilator tone. In addition, our results on compliance cannot be explained by changes in arterial diameter since diameter did not change after L-NMMA. Our results could also suggest that the arterial diameter or tone of large arteries in humans may not be directly dependent on NO synthesis or that the basal release of NO may be absent or minimal at this level. However, the fact that the vasodilatory response to nitroprusside was increased in this vascular bed after L-NMMA9 suggests the presence of a basal release of NO, since potentiation of the response to exogenous NO after L-NMMA has been previously considered to be an index of basal NO release.2426 Finally, our results are consistent with recent in vitro experiments, which demonstrated a rightward shift of the stress-strain relationship of the isolated rat coronary artery after incubation with L-NAME, reflecting a paradoxical decrease in the arterial wall stiffness after NO-synthase inhibition. These effects were partially reversed by the administration of L-arginine and were associated with a downward shift of the incremental elastic modulusstress curve.27
Given the marked role of NO as an inhibitor of smooth muscle contraction, it would be expected that inhibition of NO release should be associated with an increase in the effects of vasoconstrictor influences (such as angiotensin II, catecholamines, or endothelin) and thus with an increased arterial isometric tone. One possibile explanation of these paradoxical effects would be that L-NMMA acts as a partial agonist on NO synthase. This is unlikely given the marked decrease in blood flow observed after L-NMMA. One more likely explanation is that the paradoxical decrease in arterial tone observed in our experiments is the consequence of compensating vasodilatory mechanisms occurring after the inhibition of NO synthesis. Such compensatory mechanisms could also explain why L-NMMA did not affect arterial diameter in our experiments or in previously published studies.16,17 Indeed, in the coronary circulation of dogs, chronic treatment with L-NMMA is associated with an increase in the basal release of vasodilator prostanoids. However, there was no evidence to support the existence of such a mechanism after acute NO-synthesis inhibition.28 In contrast, in isolated perfused rabbit carotid and porcine epicardial coronary arteries, acute administration of NO attenuates the release of EDHF.29 Therefore, these results suggest that the decrease in NO resulting from an acute NO-synthesis inhibition may be associated with a compensating increase in the release of EDHF. However, the role of EDHF cannot be easily assessed in humans in vivo, and thus whether compensatory release of EDHF may explain the decrease in wall stiffness after L-NMMA is not known and cannot be answered from the present study.
Finally, we evaluated the potential role of the modifications of flow in the L-NMMAinduced changes in the mechanical properties of the artery wall. Since we previously demonstrated that flow-dependent dilatation was converted to a vasoconstriction after the administration of L-NMMA8 (possibly mediated by an endothelium-dependent vasoconstrictor substance), it is possible that the observed relaxation is the consequence of the decrease in this flow-dependent vasoconstriction, secondary to the decrease in flow. However, the analysis of the effect of L-NMMA repeated with both midwall stress and arterial flow as cofactors demonstrated that inhibition of NO synthesis decreases isometric smooth muscle tone independently of the changes in flow. This lack of role of flow is also supported by the decrease in arterial wall stiffness observed after L-NAME in isolated, pressurized arteries in the absence of flow.27
Conclusion
The present investigation demonstrates that NO is involved in
the regulation of the mechanical properties of a peripheral
conduit artery and suggests that the inhibition of NO synthesis is
associated at this level with the development of compensating
vasodilator mechanisms. The effectiveness of this compensating
mechanism in pathological states in which NO availability is decreased
remains to be assessed.
| Selected Abbreviations and Acronyms |
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| Acknowledgments |
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| Footnotes |
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Received May 9, 1997; first decision June 12, 1997; accepted July 7, 1997.
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F. A Dinenno, H. Tanaka, K. D Monahan, C. M Clevenger, I. Eskurza, C. A DeSouza, and D. R Seals Regular endurance exercise induces expansive arterial remodelling in the trained limbs of healthy men J. Physiol., July 1, 2001; 534(1): 287 - 295. [Abstract] [Full Text] [PDF] |
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D. J. Blackman, J. A. Morris-Thurgood, J. J. Atherton, G. R. Ellis, R. A. Anderson, J. R. Cockcroft, and M. P. Frenneaux Endothelium-Derived Nitric Oxide Contributes to the Regulation of Venous Tone in Humans Circulation, January 18, 2000; 101(2): 165 - 170. [Abstract] [Full Text] [PDF] |
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J. Lambert, M. van den Berg, M. Steyn, J. A. Rauwerda, A. J.M. Donker, and C. D.A. Stehouwer Familial hyperhomocysteinaemia and endothelium-dependent vasodilatation and arterial distensibility of large arteries Cardiovasc Res, June 1, 1999; 42(3): 743 - 751. [Abstract] [Full Text] [PDF] |
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C.A Hamilton, R Williams, V Pathi, G Berg, K McArthur, A.R McPhaden, J.L Reid, and A.F Dominiczak Pharmacological characterisation of endothelium-dependent relaxation in human radial artery: comparison with internal thoracic artery Cardiovasc Res, April 1, 1999; 42(1): 214 - 223. [Abstract] [Full Text] [PDF] |
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