From the Institute of Physiology and Pathophysiology,
Johannes-Gutenberg-Universität, Mainz, Germany.
Correspondence to Dr Cor de Wit, Institute of Physiology and Pathophysiology, Johannes-Gutenberg-Universität Mainz, Duesbergweg 6, D-55099 Mainz, Germany. E-mail dewit{at}mailvms.zdv.uni-mainz.de
A well-adjusted endothelial release of NO can
principally exert such a control function. Several studies on isolated
vessels,5 6 7 as well as on isolated
organs,8 have demonstrated that a shear
stress–dependent augmentation of endothelial NO
release can effectively oppose pressure-induced constrictions. At least
in certain sections in the microvascular tree, an increase in pressure
with simultaneous myogenic response should elevate WSR and
eventually wall shear stress. In fact, recent analyses of WSR
in skeletal muscle microcirculation have demonstrated that it tended to
increase with augmentation in blood
pressure.9
A myogenic constriction occurs irrespective of the size or generation
in isolated arterial vessels,10 11
whereas in vivo usually only the smallest arterioles react to changes
in transmural pressure by vasoconstriction.12
This different behavior suggests the existence of opposing mechanisms
acting in vivo with different efficacies along the vascular tree. The
exact localization of these potentially counterbalancing mechanisms
would be an important prerequisite to understand the processes leading
to hypertension after impairment of endothelial
function. It is of particular interest to study this question in
skeletal muscle arterioles, because skeletal muscle vasculature plays
an important role in the control of total peripheral
resistance.
The aim of this study was to investigate the extent of the contribution
that myogenic responses make to the vasoconstriction induced by
infusion of NE or Ang II in the hamster cremaster in vivo. The second
aim of our study was to identify the major site of myogenic
constrictions within certain vascular segments. Third, we examined
whether the endothelium, through the release of NO,
could counterbalance these constrictions and how its action was
distributed throughout the vascular tree.
Experimental Setup
Experimental Protocol
In a second set of experiments in different animals, arteriolar
diameters and RBC velocities on stepwise (20 mm Hg) increases in
inflow pressure were measured. The inflow pressure to the cremaster (as
measured in the femoral artery) was initially adjusted to 60
mm Hg by partial aortic occlusion and then raised to 80, 100, and
120 mm Hg by stepwise deflation of the occluder under continuous
infusion of NE. Each pressure level was maintained for 1 minute. Before
the next arteriole was investigated, the infusion of NE was stopped,
and a recovery period of 15 minutes was allowed. Three to four
arterioles were examined in each animal consecutively. The same
arterioles were then investigated according to the same protocol 30
minutes after the NOS inhibitor L-NNA (30 µmol/L)
was added continuously to the superfusion buffer. This concentration
has been shown to be sufficient to block basal and ACh-induced NO
release.13 In some animals, Ang II (0.47
nmol/min IV) was used instead of NE to increase blood pressure. The
AT1 receptor blocker S4509 (0.5
µmol/L) was applied locally in these animals instead of the ARB. The
maximum diameters of the vessels were determined at the end of each
experiment by superfusion of the preparation with a combination of the
vasodilators adenosine (100 µmol/L), SNP (1
µmol/L), and papaverine (300 µmol/L). Finally, blood was
withdrawn from the carotid artery for blood gas analysis.
Because we infused vasoconstrictors to increase pressure, receptor
blockers had to be used to differentiate between the direct constrictor
effect and effects of the elevated arterial pressure. To
test the efficacy of ARBs, arteriolar responses on bolus injections of
NE (0.1 nmol, 10 µL in volume) into the femoral artery (from which
the cremaster arterioles emerge) were investigated in three animals.
The dilution of the blood due to the injected bolus was clearly visible
in all vessels investigated. The bolus injections were performed before
and during the local superfusion of ARB. Prior experiments have
revealed that the injection of 10 µL of saline alone did not change
the diameters of the cremaster arterioles significantly (not shown).
Additionally, in each experiment investigating pressure effects, the
efficacy of the receptor blockade was tested by local superfusion of NE
(0.1 µmol/L) or Ang II (10 nmol/L) before and during the
addition of ARB or S4509.
Solutions and Drugs
Statistics and Calculations
Superfusion of the arterioles with NE (0.1 µmol/L) induced an
arteriolar constriction by 33.9±2.0%, which was virtually abolished
after addition of prazosin, yohimbine (0.1 µmol/L each), and
propranolol (1 µmol/L, ARB) to the
superfusate. In addition, these blockers led to a slight
increase of arteriolar diameters (by 4.2±1.6%, P<.05,
Table 1
Arteriolar Responses on Increases in Pressure
In a separate series (four animals, local ARB), arterial
pressure was raised to a similar level (124±6 mm Hg) by
intravenous infusion of NE. In these animals, when the
increase in pressure in the hindquarter was blocked by partial aortic
occlusion, no diameter changes occurred (Fig 1
To study whether pressure-induced constrictions could be elicited
over the whole pressure range investigated here, we increased the
inflow pressure stepwise in another series. After systemic blood
pressure had reached a plateau of 137±2 mm Hg by
continuous infusion of NE (0.5 nmol/min IV), the preinflated occluder
was stepwise deflated to observe three subsequent stepwise increases in
inflow pressure (20 mm Hg each) starting at a baseline level of
60 mm Hg. In contrast to the large and rapid increase in pressure
described above, these pressure increments did not change the diameter
of LAs significantly. SAs, however, constricted on each increase in
pressure (Table 2
Effects of NOS Inhibition
Stepwise Increases in Pressure During Ang II Infusion
Influence of Vascular Tone on Myogenic Responses
Because we were interested in myogenic reactivity in the high normal
and hypertensive blood pressure range, we infused vasoconstrictors. The
intravenous infusion of NE induced an increase in blood
pressure and a constriction in the cremaster arterioles. The latter was
not attenuated by the blockade of adrenergic receptors. However, a
similar constriction on NE, which was applied only locally and
therefore accompanied by only small changes in arterial
pressure, was virtually abolished. This finding held true regardless of
whether the locally applied NE acted from the adventitial side (ie,
superfused) or from the luminal side of the arteriole (ie, injected
into the cremaster circulation). This demonstrates an effective
receptor blockade and thus indicates that intravenously
infused NE elicited pronounced vasoconstriction in the cremaster
arterioles by mechanisms that are independent of these receptors. In
fact, in the presence of receptor blockade, the
simultaneous increase in pressure elicited the
vasoconstriction. By blocking the increase in pressure in the
hindquarter, this constriction was completely abolished. This occurred
at presumably identical plasma concentrations of NE and at virtually
the same increases in pressure in the upper half of the body. Thus,
mechanisms other than the increase in pressure are unlikely to be
involved in the constriction. Moreover, vasoconstriction during
increases in inflow pressure was completely reversed when the inflow
pressure was suddenly brought back to the control level, despite the
ongoing vasoconstrictor infusion. Blood pressure in the upper half of
the body changed by no more than 6 mm Hg, and HR did not decrease
after the inflation of the occluder. It is therefore rather unlikely
that baroreceptor-mediated reflexes were involved in the vasodilation
on pressure reduction. Similarly, other studies have demonstrated that
intravenous infusion of phenylephrine elicited
constrictions in rat mesenteric arteries only if the inflow pressure
into this vascular bed increased at the same
time.3 Therefore, the observed vasoconstriction
(in the presence of the respective receptor blockade) is
consistent with a pressure-induced myogenic response of the
cremaster vessels. If such a myogenic constriction occurs in a similar
magnitude in other vascular beds, as suggested by other
workers,2 4 this constriction should considerably
amplify the direct effect of a circulating vasoconstrictor on the
peripheral vascular resistance. This is not to say that
myogenic constriction is the sole mechanism and action of
vasoconstrictors in the periphery. To induce myogenic constrictions, an
initial increase in pressure has to occur. This increase could have
been elicited by a NE-induced increase in cardiac output and/or,
especially in the case of Ang II, by direct pharmacological
constriction in some organs other than the cremaster. However, our
study shows that the myogenic component was, at least in the cremaster,
of a considerable magnitude. This myogenic constriction occurred
predominantly in SAs, whereas LAs responded only weakly and only when
the increase in pressure was rapid and large. It has to be kept in mind
that additional, ie, non–pressure-related, mechanisms might have
contributed to the observed responses in these vessels. In particular,
metabolic signals have been implicated in this regulation.
SAs have been shown to be more sensitive to metabolic
stimuli.17 18 Thus, it cannot be ruled out that
the exhibited constriction in the SAs has emerged partially because of
these metabolic regulatory mechanisms, because flow
increased with pressure. However, the capability of LAs to constrict in
response to metabolic stimuli seems to be
weak.17 18 Therefore, at least in large vessels,
the observed responses are likely to be myogenic.
In contrast to our in vivo experiments, LAs and even small arteries
reveal significant myogenic responses to small changes in pressure when
studied in vitro.6 10 11 19 20 However, after
inhibition of NOS, we found significant vasoconstrictions in LAs on
graded increases in pressure in vivo as well, and thus, the release of
NO represents part of the mechanism that controls myogenic
responsiveness of these vessels. This NO release was not due to a
receptor-dependent endothelial stimulation by NE or Ang
II as previously reported,21 because these
receptors were blocked. Inhibition of NOS led to an augmentation of
vascular tone, and merely an enhanced vascular tone might increase
myogenic responsiveness. Therefore, responses in LAs with similar tone
under control conditions and after inhibition of NOS were compared.
These data demonstrated that arterioles with a comparable degree of
activation constricted significantly in response to an increase in
pressure only in the presence of L-NNA (Fig 5
Several in vitro studies have demonstrated an influence of NO on
myogenic constriction only when the vessels were perfused and are
therefore under conditions of shear
stress.5 6 7 22 As shown here, and in a recently
published study,9 the WSR, and thus the wall
shear stress, increases roughly linearly with elevations in blood
pressure, and elevations in wall shear stress have been shown to
release NO.16 Consistent with shear
stress–dependent augmentation of NO release, our data show the
strongest effect of L-NNA at the highest WSR (Figs 3
Despite its very marked effect on LAs, L-NNA failed to enhance the
apparent myogenic responses in SAs. The reasons for this are not clear.
One plausible explanation is that L-NNA treatment and the subsequent
myogenic constriction of LAs caused an increased drop in pressure along
these LAs. Thus, further downstream, the increase in transmural
pressure would be attenuated. The apparent constancy of the myogenic
response in the SAs would reflect the net result of two compensatory
effects. These effects are an enhanced constriction in response to
increases in pressure (as observed in LAs) and, concomitantly, a
reduced increase in transmural pressure. Furthermore,
metabolic counterregulation may contribute to an
attenuation of the constrictions, because these SAs are controlled by
metabolic signals.17 18
The most important functional implication of this study is that NO
inhibits the myogenic response of LAs. These vessels are influenced
differently from small ones, because they are less tightly controlled
by metabolic signals. This finding is partially due to the
different distribution of
Received July 9, 1997;
first decision August 6, 1997;
accepted October 16, 1997.
© 1998 American Heart Association, Inc.
Scientific Contributions
Nitric Oxide Opposes Myogenic Pressure Responses Predominantly in Large Arterioles In Vivo
Abstract
Top
Abstract
Introduction
Methods
Results
Discussion
References
Abstract—A myogenic vasoconstriction
may amplify the effects of circulating vasoconstrictors. In cremaster
arterioles, the contribution of a myogenic component to the
constriction on intravenous infusion of
norepinephrine (NE) or angiotensin II (Ang II)
was studied. Second, the role of endothelium-derived
nitric oxide (NO) in the control of these myogenic constrictions and
its site of action in the resistance vascular bed was investigated. In
30 anesthetized (pentobarbital) hamsters, the cremaster was
prepared for intravital microscopy, and a pneumatic vessel occluder was
placed around the aorta to vary blood pressure in the hindquarter of
the animal. Intravenous infusion of NE (0.5 nmol/min)
increased the systemic blood pressure by 52±2 mm Hg.
Simultaneously, constrictions of up to 33±6% were
observed in the small arterioles (SAs; maximal inner diameter, 36 to
65 µm). The constrictions were not significantly altered by a
local adrenergic blockade but were abolished when the pressure
elevation in the cremaster arterioles was blocked by partial occlusion
of the abdominal aorta. Diameters in large arterioles (LAs; maximal
inner diameter, 65 to 127 µm), however, did not change
significantly on NE infusion. Similar responses in the arterioles were
observed when the local pressure was increased stepwise from 60 to
120 mm Hg by partial opening of the aortic occluder. However,
after treatment of the cremaster tissue with the inhibitor
of the NO synthase,
NG-nitro-L-arginine (L-NNA,
30 µmol/L), a significant pressure-induced constriction of up to
16±3% occurred in LAs, whereas the magnitude of the constriction in
SAs remained unchanged. L-NNA also abolished the increases in blood
flow that were observed with increments in pressure in control animals.
Similar results were obtained when Ang II was used to increase blood
pressure. We conclude that a myogenic constriction of SAs contributes
markedly to the overall response of cremaster arterioles to circulating
vasoconstrictors. NO effectively opposes the myogenic response in LAs,
thus preventing myogenic constrictions in a vascular region where
constriction cannot be fully controlled by metabolic
dilation. If this attenuating effect of NO on myogenic constriction
also takes place in other organs, it might be a decisive mechanism in
controlling changes of total peripheral vascular resistance
elicited by vasoconstrictors.
Key Words: endothelium • angiotensin II • norepinephrine • arterioles • blood pressure • microscopy
Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
The Bayliss effect, a
myogenic vasoconstriction in response to an increase in transmural
pressure and vice versa, is thought to be an important mechanism in
keeping capillary filtration pressure constant and in contributing to
the autoregulation of blood flow.1 It has also
been demonstrated that part of the peripheral
vasoconstriction in response to infusions of vasoconstrictors can be
attributed to the resulting increase in pressure rather than to a
direct pharmacological effect of the
compound.2 3 4 This finding suggests that the
myogenic vasoconstriction could represent a mechanism that
enhances increases in blood pressure by further increasing
peripheral resistance. This potential positive feedback
mechanism might lead to instability in the intact circulation and
therefore requires control through opposing mechanisms.
Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
Preparation
Male golden Syrian hamsters (80 to 150 g body weight) were
anesthetized by intraperitoneal injection
of pentobarbital (75 mg/kg), followed by continuous administration of
the anesthetic (5 to 10 mg · kg-1
· h-1) through a jugular vein catheter
throughout the experiment. The animals were artificially ventilated
(7025 Rodent Ventilator, Hugo Sachs Elektronik) to achieve
physiological arterial
O2 and CO2 partial
pressures. Arterial blood samples taken at the end of the
experiment revealed a pH of 7.39±0.02, and arterial
PO2 and
PCO2 were 123±15 and 37±2
mm Hg, respectively. The abdominal aorta was exposed distal to the
renal arteries through an abdominal midline incision. A pneumatic
occluder (Jones) connected to a syringe was placed around the aorta.
The left carotid and femoral arteries were cannulated to measure
pressure proximal and distal to the vascular occluder by means of
pressure transducers (Statham). The pressures were monitored by means
of a pen recorder and stored on computer disk for later
analysis. The right cremaster muscle was prepared for
intravital microscopy as described
previously.13 The care of the animals and
the conduct of the experiments were in strict accordance with the rules
of the German animal protection laws.
The muscle was superfused with warmed (34°C,
physiological for cremaster tissue)
bicarbonate-buffered salt solution at a rate of 8 mL/min. The
superfusion fluid had a pH of 7.35±0.01, a
PO2 of 33.0±1.6 mm Hg, and a
PCO2 of 38.8±0.7 mm Hg as
measured in samples taken at the edge of the cremaster. Three to five
arterioles of different sizes and vascular generation were studied in
one single animal. The arterioles were located in adjacent microscopic
fields and could be examined sequentially within 2 minutes by means of
a microscope (Metallux, Leitz) equipped with a video camera. Images at
a final magnification of x718 were stored on videotape. Arteriolar
inner diameters were measured off-line from digitized images (MVP-AT,
Matrox) with a laboratory computer program. On infusion of
vasoconstrictors, one arteriole was monitored continuously and the
other vessels were studied subsequently under the same protocol. In
addition, centerline RBC velocity was measured continuously by means of
a modified dual-slit cross-correlation method, which was described in
detail elsewhere.14
After the surgical preparation, the hamsters were allowed to
recover for 30 minutes before control diameters were taken. Vascular
vasodilator reactivity was tested by consecutive local superfusions (1
to 3 minutes) of the endothelium-independent NO-donor
SNP (1 µmol/L) or the endothelial stimulator ACh
(10 µmol/L). Arteriolar diameters were measured before and
during the application of the respective vasoactive agent. Thereafter,
the diameter and RBC velocity were monitored continuously in one
arteriole before and during the intravenous infusion of NE
(0.5 nmol/min) for 8 to 10 minutes. The recovery period before the next
arteriole was studied under the same conditions was at least 15
minutes. The same protocol was thereafter repeated in the presence of
the ARB prazosin, yohimbine (0.1 µmol/L each), and
propranolol (1 µmol/L), which were added to the
superfusion continuously. In a subset of experiments (four animals),
the increase in pressure on intravenous NE in the vessels
supplying the cremaster was blocked (as monitored in the femoral
artery) by partial occlusion of the abdominal aorta. A rapid deflation
of the vascular occluder after 3 minutes allowed us to investigate the
arteriolar responsiveness on a sudden rise in the inflow pressure.
The salt buffer used for superfusion was of the following
composition (in mmol/L): Na+ 143,
K+ 6, Ca2+ 2.5,
Mg2+ 1.2, Cl-
128, HCO3- 25,
SO42- 1.2, and
H2PO4-
1.2. ACh, SNP, adenosine, prazosin, and angiotensin
were obtained from Sigma Chemical Co, yohimbine and
propranolol from ICN Biochemicals, and L-NNA from Serva.
S4509 and NE were generous gifts from Hoechst (Frankfurt, Germany).
Propranolol (10 mmol/L) and yohimbine (1 mmol/L)
were dissolved in 1 mmol/L ascorbic acid, and prazosine (1
mmol/L) in 3:1 ethanol/ascorbic acid (1 mmol/L). These stock
solutions were stored at -20°C until use and further diluted in the
superfusion buffer. SNP (10 mmol/L) was dissolved in 1 mmol/L
sodium acetate on the day of the experiment. For all other solutions
and further dilutions, freshly prepared superfusion buffer was used.
All locally applied drugs (concentrated 100-fold over the final
concentration) were added to the superfusion fluid by means of a roller
pump at 1/100th of the total superfusion rate (0.08 mL/min) to obtain
the final concentrations indicated above.
Vascular tone is expressed as quotient of the vessel's resting
diameter divided by its maximal diameter. Changes of the arteriolar
inner diameter on treatment were calculated as a percentage of the
respective control diameter: %
change=[(DTr-DCo)/DCo]x100,
where DTr represents the diameter
after treatment and DCo the control
diameter before treatment. The mean RBC velocity
(VM) was calculated from the measured
centerline velocity divided by an empirical correction factor
(1.6).15 Arteriolar blood flow, Q, was calculated from the
formula
Q=VMxD2x
/4
and is expressed as nanoliters per second. WSR was calculated according
to the formula WSR=8xVM/D. Comparisons
within groups were performed with paired t tests, and
those between groups with ANOVA. For multiple comparisons, the
probability values were corrected according to Bonferroni. Differences
were considered significant at a corrected error probability of
P<.05.
Results
Top
Abstract
Introduction
Methods
Results
Discussion
References
Responses on Local Application of Vasoactive Substances
A total of 114 arterioles with maximal luminal diameters between
32 and 127 µm (66.4±1.8 µm) were studied in 30 animals.
The vessels exhibited varying degrees of spontaneous tone, ie, the
quotient of resting to maximal diameter ranged from 0.38 to 0.96
(0.73±0.01). This quotient was significantly lower in SAs (0.68±0.02,
n=57) than in LAs (0.77±0.02, n=57), indicating a higher basal tone in
small vessels. SAs belonged to A3 and A4 and LAs belonged to A1 and A2
generation of the arteriolar tree.16 Local
superfusion of SNP (1 µmol/L) or ACh (10 µmol/L) dilated
the arterioles by 30.6±3.4% and 42.8±2.9% (Table 1
).
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Table 1. Reactivity of Arterioles to Local Application of
Vasomotor Stimuli 
). To test the efficacy of the ARB to inhibit effects of
circulating NE, a bolus of NE was locally applied from the luminal side
of the arteriole. This bolus injection of NE (0.1 nmol) raised
arterial pressure only from 85±2 to 89±2 mm Hg
(P<.05). Nevertheless, it induced a vasoconstriction by
10.7±2.3% in LAs (n=8) and by 30.1±6.2% in SAs (n=7) within 1
minute. After addition of ARB to the superfusion, the arteriolar
constrictions on injected NE were completely abolished (LAs,
-1.3±0.8%; SAs, -2.0±3.2%, Table 1). Similarly, the constriction
induced by addition of Ang II (10 nmol/L) to the superfusion fluid
(-18.6±2.9%) was completely abolished in the presence of the
AT1 receptor blocker S4509 (0.5 µmol/L),
which induced a small increase of arteriolar diameter (by 7.0±1.9%,
P<.05, Table 1).
Intravenous infusion of NE (0.5 nmol/min) increased
the arterial blood pressure within 50±3 seconds from 75±2
to 125±2 mm Hg, which was maintained until the infusion of NE
was stopped. The HR decreased slightly from 333±5 to 321±7 bpm
(P<.05) during this infusion. The cremaster arterioles
exhibited different behavior on the intravenous infusion of
NE: The SAs constricted significantly (by -23.8±9.3%), and LAs had
no significant change in diameter. Local blood flow remained unchanged
in SAs and tended to increase in LAs (data not shown), whereas the WSR
increased in both groups (Fig 1). Local
ARB affected neither the NE-induced increase in blood pressure nor the
absolute (Fig 1) or relative diameter changes in SAs (-29.1±8.1
versus -32.7±5.7%, respectively) or LAs (-12.2±8.9 versus
-8.8±7.1%, respectively) on intravenous infusion of
NE.
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Figure 1. Contribution of adrenergic receptor stimulation
and pressure to the arteriolar responses on intravenous
infusion of NE. Bars depict, from top to bottom, inflow pressure (as
measured in the A. femoralis [A. fem.]), microvascular diameter, and
WSR before (open bars) and during (steady state, hatched bars) NE
infusion in LAs (left) and SAs (right). LAs only tended to constrict on
NE infusion in control animals (Con, seven animals), whereas SAs
constricted significantly. WSR increased in both groups. These
responses were not altered in the presence of local ARB but were
abolished when, in addition, the increase in inflow pressure (ARB-P 
,
seven animals) was blocked. *P<.05 vs before NE. n
indicates number of arterioles.). However, the sudden
release of the aortic occlusion, which increased the inflow pressure
rapidly by 49±4 mm Hg, led to a significant constriction in
both vessel types (LAs, -10.5±3.5%; SAs, -34.6±5.2%). The
reinflation of the aortic occluder brought the inflow pressure back to
the initial level and dilated the arterioles again back to control
diameter despite the ongoing intravenous application of NE
(Fig 2). Opening or closing the aortic
occluder changed systemic blood pressure by no more than 6
mm Hg.
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Figure 2. Systemic pressure changes and arteriolar responses
during changes of pressure in the lower half of the body. Changes of
arterial pressure in the upper (as measured in the A.
carotis [A. car.]) and lower half of the body (A. femoralis [A.
fem.]) as well as arteriolar diameter during intravenous
infusion of NE. Deflation or inflation (D, I) of the aortic occluder
led to only minor changes of the pressure in the upper half of the
body, whereas the cremaster inflow pressure changed substantially. A
weak but significant constriction was observed in LAs (left, n=9),
whereas SAs (right, n=7) constricted more vigorously. When the inflow
pressure was brought back to the initial pressure level, the vessels
reattained their control diameter despite the ongoing
intravenous infusion of NE. Note that LAs, in contrast to
SAs, were initially distended. Data were obtained in four different
animals, and all experiments were done in the presence of local
ARB. ). Together with these
pressure increments, RBC velocity, WSR, and local blood flow increased
in LAs as well as in SAs (Fig 3). The
reduction in inflow pressure back to baseline led to a dilation by
which the arterioles reattained their control diameter despite the
continuous infusion of NE (Table 2, Fig 3). Systemic blood pressure
increased by 3.4±0.8 mm Hg and HR from 409±5 to 415±5 bpm
after inflation of the occluder.
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Table 2. Vascular Diameters and RBC Velocities at Increasing
Inflow Pressures During NE-Induced Blood Pressure Rise in LAs and SAs
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Figure 3. Effect of NO inhibition on pressure-induced
changes of microvascular parameters during infusion of NE.
Small stepwise increases in inflow pressure (as measured in the A.
femoralis [A.fem.]) during NE infusion induced myogenic constrictions
in SAs (right, n=23), but not in LAs (left, n=14) under control
conditions (open symbols). L-NNA (solid symbols) unmasked a
pressure-dependent vasoconstriction in LAs, whereas it did not modify
it in SAs. Arteriolar blood flow increased only under control
conditions with pressure; nevertheless, the WSR increased under both
conditions. Data were obtained in six animals under local superfusion
with ARB. *P<.05 vs previous pressure level. Control
values, see Table 2 .
Addition of L-NNA to the superfusate (30 µmol/L)
decreased arteriolar diameters by 9.1±1.3% (P<.05) and
also significantly reduced ACh (10 µmol/L)–induced dilations
(from 44.0±3.6% to 29.6±2.2%) but not SNP-induced dilations
(27.4±4.0% versus 25.4±5.4%). In contrast to control conditions,
LAs constricted significantly now on each increase in inflow pressure,
whereas the constrictions of SAs were not different from control
animals. The local blood flow no longer increased on pressure
elevations, whereas augmentations of RBC velocity and WSR were still
found. Again, the reduction in inflow pressure back to baseline brought
the arteriolar diameters back to their respective control values
despite continuous NE infusion (Table 2, Fig 3), whereas systemic blood
pressure increased (by 5.3±0.9 mm Hg), and HR remained unchanged
(380±4 versus 387±4 bpm).
In different experiments (seven animals), Ang II instead of
NE was infused intravenously to increase
arterial pressure in the local presence of the
AT1 receptor blocker S4509 (0.5 µmol/L).
The blood pressure increased from 70.2±1.4 to 162±2 mm Hg.
Thus, four increments in inflow pressure of 20 mm Hg each were
investigated. The stepwise increases in pressure elicited virtually the
same diameter constrictions in SAs (n=12) as were found with similar
increments in pressure under NE. Only at the last increment, which was
not obtained in the NE group, did the vessels not constrict further.
Again, there were no significant constrictions in LAs (n=20) except for
the pressure change from 80 to 100 mm Hg. The constriction was
much weaker here than in small vessels (P<.05). At most
pressure increments, RBC velocity and WSR increased in LAs and SAs,
whereas local blood flow increased only in LAs. These changes were
completely reversible if the inflow pressure was returned to 60
mm Hg, despite the continued Ang II infusion (Table 3, Fig 4).
On the inflation of the occluder, small increases in systemic blood
pressure (by 3.3±1.0 mm Hg) but no changes in HR (389±5 bpm)
were observed. In the presence of L-NNA, each pressure elevation led to
significant constrictions in LAs, and local blood flow no longer
increased, as observed in control animals. The vasoconstrictions in SAs
were unaltered compared with control animals, as found in the NE group
(Table 3, Fig 4).
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Table 3. Vascular Diameters and RBC Velocities at Increasing
Inflow Pressures During Ang II–Induced Blood Pressure Rise in LAs and
SAs
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Figure 4. Effect of NO inhibition on pressure-dependent
changes of microvascular parameters during infusion of Ang
II. During stepwise increases in pressure under infusion of Ang II,
there was, in contrast to the NE infusion, a weak vasoconstriction of
LAs (left) already under control conditions (open symbols). L-NNA
(solid symbols) enhanced the vasoconstriction in these LAs but not in
SAs. Data were obtained in seven animals: 19 LAs and 13 SAs. All
experiments were done in the presence of AT1
receptor blockade (S4509, 0.5 µmol/L, local superfusion).
*P<.05 vs previous pressure level. Control values, see
Table 3 . A.fem. indicates A. femoralis.
Since the graded pressure increments during
intravenous infusion of NE or Ang II elicited virtually the
same arteriolar constrictions, these groups were analyzed
together to deduce the role of initial vascular tone on the myogenic
responsiveness. The arterioles were grouped according to their tone
(quotient of actual and maximal diameter) at the baseline pressure of
60 mm Hg, ie, just before the increase in pressure. Under control
conditions, significant vasoconstrictions on increases in pressure (40
or 60 mm Hg applied in 20 mm Hg increments) were found only
in arterioles with high tone (quotient from 0.55 to 0.70) but not in
those with moderate (quotient from 0.70 to 0.85) or low (quotient from
0.85 to 1.00) tone. In contrast, after L-NNA, significant
vasoconstrictions were observed in all groups, ie, also in those large
vessels that exhibited moderate or low tone after NOS inhibition (Fig 5). SAs behaved similarly before and
after L-NNA (data not shown).
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Figure 5. Enhanced myogenic constrictions after inhibition
of NOS in arterioles with similar initial tone. Diameter changes
(percentage change of the diameter at 60 mm Hg) after two (top)
or three (bottom) increments in inflow pressure (20 mm Hg each)
of LAs at different initial vascular tone (ie, tone at the baseline
pressure). Arterioles with moderate or low tone did not constrict
significantly on these increases in pressure under control conditions
(open bars). In contrast, in the presence of L-NNA (30 µmol/L,
hatched bars) a significant constriction in arterioles with similar
tone was observed. Digits within bars indicate number of arterioles.
Data include pressure elevations by NE and Ang II infusion in the local
presence of the respective blockers. *P<.05 vs
respective control diameter.
Discussion
Top
Abstract
Introduction
Methods
Results
Discussion
References
The experiments demonstrate that a pressure-induced
vasoconstriction contributes markedly to the arteriolar response on
intravenous infusion of a vasoconstrictor in the cremaster
vascular bed. They also show that this response occurs predominantly in
SAs, whereas LAs respond to such pressure elevations only weakly. The
major finding of this study is that myogenic constriction is extended
to LAs after inhibition of NOS. This finding suggests that one of the
basic mechanisms by which NO reduces blood pressure and enhances organ
blood flow is the blockade of a pressure-induced constriction in
LAs. ). Thus, the effect of
inhibition of NOS cannot be attributed solely to the augmentation of
vascular tone, but rather, NO can be implicated in playing a specific
role in preventing myogenic responses. Likewise, studies in isolated
organs have demonstrated that the same increase in basal vascular tone
led to different myogenic responses when endothelin or NOS
inhibitors were used.8 and 4). Therefore,
the shear stress–induced NO release may counteract the
pressure-induced vasoconstriction in these LAs. In accordance with this
view, myogenic constrictions in the isolated rabbit ear were unmasked
only after inhibition of NOS.23 Furthermore, a
recent study in the perfused rat kidney demonstrated that
vasoconstrictions in afferent arterioles induced by increases in
pressure coincided with a prolonged increase in
endothelial intracellular free
calcium,24 a signal that activates the
endothelial NOS.25 
-receptor subtypes along vessels of
different generations.17 18 Therefore, any
impairment of endothelial NO release will extend the
myogenic reactivity to the LAs, thereby enhancing the overall myogenic
responsiveness of a vascular bed. Consistent with these
results, it has previously been demonstrated that (myogenic)
autoregulation is enhanced after inhibition of NOS in isolated perfused
hearts.8 Furthermore, the adaptive increase in
blood flow is impaired during reactive and active hyperemia,
conditions that are associated with (re-)increases in blood
pressure.26 27 28 29 It is also tempting to speculate
that the amplification of the vasoconstrictor effects on
peripheral resistance2 3 4 described
above will be further enhanced after inhibition of NOS. It remains to
be established whether, under conditions of impaired NO release, a
shift of myogenic responsiveness to LAs contributes to the elevated
peripheral resistance observed in many hypertensive
patients. It would be particularly interesting to examine these
myogenic effects on peripheral resistance in hypertensive
patients, who respond with a decrease in blood pressure on infusion of
the NOS substrate L-arginine.30
Selected Abbreviations and Acronyms
ACh
=
acetylcholine
Ang II
=
angiotensin II
ARB
=
adrenergic receptor blockade/blocker
AT1
=
angiotensin II type 1 (receptor)
HR
=
heart rate
LA
=
large arteriole
L-NNA
=
NG-nitro-L-arginine
NE
=
norepinephrine
NO
=
nitric oxide
NOS
=
nitric oxide synthase
RBC
=
red blood cell
SA
=
small arteriole
SNP
=
sodium nitroprusside
WSR
=
wall shear rate
Acknowledgments
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG Po 307/1–3). The authors are grateful to Sarah Neuhaus for her
help in preparing the manuscript.
References
Top
Abstract
Introduction
Methods
Results
Discussion
References
1- and 2-adrenoceptor constriction to
metabolic inhibition during rat skeletal muscle
contraction. Circ Res. 1991;69:174–184.
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