697
Flow-Mediated Dilatation
of the Basilar Artery In Vivo
Kenichiro Fujii, Donald D. Heistad, and Frank M. Faraci
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Flow-mediated dilatation has been described mainly in peripheral conduit arteries. The goal of
this study was to examine mechanisms and functional implications of flow-mediated dilatation
in large cerebral arteries in vivo. Vessel diameter and velocity of blood flow through the basilar
artery were measured using a cranial window in 45 anesthetized rats. Mean blood flow velocity
through the basilar artery increased by 94±8% during unilateral common carotid artery
occlusion and 203±13% during bilateral occlusion. Diameter of the basilar artery increased by
10±1% during unilateral common carotid artery occlusion and 29±2% during bilateral
occlusion from control diameter of 275+±8 ,um. Vasodilatation appeared with a delay of 13±1
seconds after the onset of the increase in flow velocity. With systemic arterial pressure
maintained at baseline levels, pressure in the basilar artery (servonull) decreased initially
during carotid occlusion, and during dilatation of the basilar artery, pressure was restored
partially toward normal. Indomethacin (10 mg/kg i.v.), topical application of tetrodotoxin (10-6
M), N Gmonomethyl L-arginine (5 x 10-6 M), tetraethylammonium chloride (10-2 M), glibenclamide (10` M), SKF 525A (3 10` M), and ouabain (10-' M) had no effect on flow-mediated
dilatation. These findings indicate that 1) pronounced dilatation of the basilar artery occurs in
response to an increase in blood flow in vivo, 2) dilatation of large arteries attenuates
reductions in cerebral microvascular pressure during increases in blood flow, and 3) flowmediated dilatation of the basilar artery does not appear to depend solely on cyclooxygenase
activity, formation of nitric oxide, voltage-dependent or ATP-sensitive K' channels, activity of
cytochrome P-450-dependent monooxygenase, or sodium pump activity. (Circulation Research
1991;69:697-705)
X
In blood vessels, "flow-mediated dilatation" occurs in response to an increase in blood flow.
This response was first described in large peripheral arteries1 and is usually relatively small in
amplitude. In general, the response appears to involve an endothelium-dependent mechanism.2-11 A
similar mechanism has been described in cerebral
blood vessels in vitro, but the response to increases in
flow appears to be contraction or relaxation, depending on the initial level of basal tone.12
If blood flow through large cerebral arteries, which
have considerable resistance, increased and these
From the Departments of Internal Medicine and Pharmacology,
Veterans Administration Medical Center and Cardiovascular Center, University of Iowa College of Medicine, Iowa City, Iowa.
Presented in part at the 63rd Scientific Sessions of the American
Heart Association, Dallas, Tex., November 1990.
Supported by a Medical Investigatorship and research funds
from the Veterans Administration, by National Institutes of
Health grants HL-38901, HL-16066, NS-24621, HL-14388, and
HL-14230, and by a grant from the Iowa Affiliate of the American
Heart Association (1A90-G-5). F.M.F. is an Established Investigator of the American Heart Association.
Address for correspondence: Frank M. Faraci, PhD, Department of Internal Medicine, Cardiovascular Center, University of
Iowa College of Medicine, Iowa City, IA 52242.
Received January 9, 1991; accepted May 14, 1991.
large arteries failed to dilate, microvascular pressure
(distal perfusion pressure) would fall and, thereby,
impair tissue perfusion through a "steal" phenomenon. It has been hypothesized, but not demonstrated,
that flow-mediated dilatation of large arteries would
decrease resistance of large arteries and restore
microvascular pressure toward normal.'10"3 Thus,
flow-mediated dilatation of large arteries may be
important in preventing a decrease in perfusion
pressure.
The first goal of this study was to determine
whether flow-mediated dilatation occurs in large
cerebral arteries in vivo. Our hypothesis was that
flow-mediated dilatation may be of large magnitude
in the cerebral circulation, because resistance of
large arteries is high relative to other vascular beds.14
The second goal of this study was to test the hypothesis that flow-mediated dilatation may contribute to
the maintenance of microvascular pressure and, thus,
tissue perfusion during increases in blood flow.
Several mediators of flow-mediated dilatation have
been suggested in the peripheral circulation. In the
aorta and iliac arteries, a diffusible vasodilator such
as endothelium-derived relaxing factor may be in-
698
Circulation Research Vol 69, No 3 September 1991
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volved.1516 In the microcirculation of skeletal muscle,
prostaglandins may mediate the response.17 In addition, recent studies suggest a crucial role of K'
current within endothelium-dependent18 or Na+-dependent19 mechanisms in smooth muscle cells in
flow-mediated dilatation. The third goal of this study
was to examine potential chemical mediators and
ionic mechanisms that may mediate flow-mediated
dilatation in the cerebral circulation.
obtained. The Doppler shift is proportional to velocity of blood flow.
Pressure in the basilar artery was measured using
sharpened micropipettes (2-5-,um tip diameter)
filled with 0.8 M NaCI and coupled to a servonull
pressure-measuring device (model 4A, Instrumentation for Physiology & Medicine). The tip of the
micropipette was inserted into the lumen of the
basilar artery using a Leitz micromanipulator.
Materials and Methods
Animal Preparation
Experiments were performed on 45 male SpragueDawley rats (350-450 g) anesthetized with pentobarbital (50 mg/kg i.p.). The trachea was cannulated,
and the rats were mechanically ventilated with room
air and supplemental oxygen. Skeletal muscle paralysis was produced after surgery with gallamine triethiodide (5-10 mg/kg). Because the rats were paralyzed, we evaluated them approximately every 30
minutes for adequacy of anesthesia. When pressure
to 'a paw evoked a change in blood pressure or heart
rate, additional anesthesia was administered intravenously at a rate of -20-25 mg. kg`1 * hr-1.
Catheters were placed in both femoral arteries to
measure systemic arterial pressure and to obtain arterial blood samples. A femoral vein was cannulated for
infusion of drugs. Arterial blood gases were monitored
and maintained within normal limits throughout the
experiment: Pco2 was 39±1 (mean±SEM) mm Hg,
Po2 was 146± 9 mm Hg, and pH was 7.38 +0.01. Rectal
temperature was monitored and maintained at 37°C
with a heating pad. Both common carotid arteries
were exposed through a ventral midline incision in the
neck, separated from the vagosympathetic trunks, and
loosely encircled with sutures for later occlusion.
A craniotomy was prepared over the ventral brain
stem as described in detail previously,20-23 and a
portion of the dura mater was opened. The cranial
window was suffused with artificial cerebrospinal
fluid (CSF), warmed to 37°C, and bubbled continuously with a gas mixture of 5% C02-95% N2 to
produce normal levels of pH and Pco2. In CSF
sampled from the craniotomies, Pco2 was 37±1
mm Hg, Po2 was 91±3 mm Hg, and pH was
7.40±0.01. Diameter of blood vessels was measured
using a microscope equipped with a television camera coupled to a video monitor and image shearing
device (model 908, Instrumentation for Physiology &
Medicine, Inc., San Diego, Calif.). The images were
recorded on videotape for later analysis. With this
system, the standard deviation of 10 consecutive
measurements (every 10 seconds) of diameter of a
basilar artery with a diameter of -250 gm was <2.0
,um.
Velocity of blood flow through the basilar artery
was measured with a method described in detail
previously.24 Briefly, a pulsed Doppler crystal
(0.3x0.4 mm) was placed perpendicular to the basilar artery by using a micromanipulator. The crystal
was rotated until a maximum Doppler shift was
Experimental Protocol
Vessel diameter and velocity of blood flow through
the basilar artery were measured under control condition, during unilateral common carotid occlusion
(UCO), during bilateral occlusion (BCO), and after
recovery from occlusion. Changes in systemic arterial
pressure were kept to a minimum by withdrawal of
arterial blood. In eight rats, diameter of branches of
the basilar artery was also measured.
In six rats, pressure in the basilar artery was
measured. We were unable to measure microvascular
pressure with a pipette and velocity of blood flow
with a Doppler crystal simultaneously because of
limited space within the neck incision and cranial
window. Pressure in the basilar artery was also
measured during graded hemorrhagic hypotension to
examine the effect of pressure per se on vessel
diameter.
Dilatation of the basilar artery was reproducible
under control conditions (up to four to five times).
The response also was examined before and during
topical application of 10`6 M tetrodotoxin (TTX),
5 x 10`6 M N0-monomethyl L-arginine (L-NMMA),
10`2 M tetraethylammonium chloride (TEA), 10-5 M
glibenclamide, 3 xi0-5 M SKF 525A, or 10`5 M ouabain. SKF 525A was obtained from Smith Kline &
French Laboratories, Philadelphia, Pa. All other compounds were obtained from Sigma Chemical Co., St.
Louis, Mo. Glibenclamide was dissolved in dimethyl
sulfoxide and diluted with saline to the desired concentration. All other drugs were dissolved in saline.
The two vehicles had no effect on baseline diameter or
vasodilatation in response to carotid occlusion. Concentrations of the drugs were chosen based on previous experiments in vitro25-33 and our previous studies
in vivo.23,3435 Antagonists were superfused over the
craniotomy by adding them to the constantly flowing
artificial CSF. Responses were also examined before
and after intravenous administration of 10 mg/kg
indomethacin or replacement of 84% Na+ in artificial
CSF by Li+ (low Na+, with the remaining Na+ added
as NaHCO3 to maintain normal pH of CSF). Carotid
occlusion was started at least 10 minutes after application of each antagonist or change in composition of
CSF, except that BCO was performed only 6 minutes
after application of TTX. During superfusion of YTX,
the response to BCO but not UCO was examined to
reduce the time of exposure to ITX, because superfusion of 15TX for longer than 10 minutes produced
marked hypotension. It is possible that TTX penetrated into the ventrolateral medulla and acted on
Fujii et al Flow-Mediated Dilatation in Brain
699
TABLE 1. Effects of Interventions on Responses of the Basilar Artery to Carotid Occlusion
Baseline diameter
Change (%)
gm
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Tetrodotoxin (n=4)
Control
1,uM
Indomethacin (n=6)
Control
10 mg/kg
L-NMMA (n=5)
Control
5 ,.M
TEA (n=4)
Control
10 nM
Glibenclamide (n=5)
Control
101tM
SKF 525A (n=3)
Control
30,uM
Ouabain (n=4)
Control
248±16
Change in flow velocity
(%)
UCO
BCO
Change in diameter
(%)
UCO
BCO
...
295±83
...
-2±2
...
322+53
...
28±7
25±6
301±19
...
...
-2±2
158±16
147±53
215+42
240±68
22±11
15±4
27±5
27±5
249+20
...
...
-6+1*
151±53
138±38
295±63
284±80
18±3
17±3
36±6
38±6
252±19
.
-22+3*
120±7
144±29
235±19
276±59
12±1
12±1
34±5
31+4
267±11
...
...
6±2
57±10
87±19
188±20
226±17
4±1
3+1
22±4
18±3
272+3
...
1±1
103±30
90±23
237±8
232±15
7±1
9±3
26±5
29±8
...
...
...
249±11
99±13
204±20
15±4
35±4
10,uM
...
-17+4*
84±6
225±41
13±1
33±5
Low Na' CSF (n=5)
Control (157 mM)
281+14
...
45+13
212±47
5±2
26±6
25 mM
...
-6±4
67±14
3±2
191±37
26±7
Values are mean±SEM. UCO, unilateral common carotid occlusion; BCO, bilateral common carotid occlusion; n,
number of rats; L-NMMA, NG-monomethyl L-arginine; TEA, tetraethylammonium chloride; CSF, cerebrospinal fluid.
*p<0.05 vs. control.
local neurons to produce hypotension. One or two
interventions (antagonists or change in ionic composition) were tested in each rat.
Efficacy of Antagonists
Superfusion of TTX for longer than 10 minutes
produced marked hypotension that was probably due
to penetration of TTX into the ventrolateral medulla
and effects on local neurons that are involved in
autonomic control. This hypotensive response suggests efficacy of TTX. In addition, the concentration
of TTX (10`6 M) used in this study is sufficient to
abolish neurally mediated action potentials in vitro25
and to inhibit relaxation of cerebral arteries in response to transmural stimulation in vitro.33 We35
have shown previously that the dose of indomethacin
used in this study (10 mg/kg) completely blocked
dilatation of cerebral vessels to arachidonic acid in
vivo. L-NMMA (5x 10-6 M) inhibited steady-state
dilatation of the basilar artery in response to 10-5 M
acetylcholine (49+±9% versus 5+4%, n=5, p<0.05).
We34 have shown previously that L-arginine inhibits
constriction of the basilar artery in response to
L-NMMA and inhibits the effect of L-NMMA on the
dilator response to acetylcholine. TEA (10-2 M)
produced significant (22%) constriction of the basilar
artery in the present study (Table 1), and significantly
increased the frequency of spontaneous vasomotion
of the basilar artery in a previous study.23 Ouabain
(10-` M) produced significant vasoconstriction (17%)
in the present study (Table 1) and abolished spontaneous vasomotion of the basilar artery in a previous
study.23 These observations suggest that TEA and
oubain were efficacious. The concentration of glibenclamide that we used is sufficient to inhibit hyperpolarization of smooth muscle of cerebral vessels in
vitro.32 SKF 525A (10`7 M) significantly inhibits
endothelium-dependent relaxation in canine coronary arteries in vitro. We used 3 x 10-5 M SKF 525A,
because higher concentrations reportedly release nitric oxide and prostacyclin from endothelial cells.30
Statistical Analysis
All values are expressed as mean+SEM. One-way
analysis of variance for repeated observations within
each rat was used for comparison of stepwise response. When a significant F value was found, comparisons between mean values were made with Fisher's test for least significant difference. Paired t test
was used for comparison of the response before and
after interventions. A value ofp <0.05 was considered
to be significant.
Circulation Research Vol 69, No 3 September 1991
700
Velocity of
Blood Flow
(kHz)
1.
3r
Mean
Velocity
(kHz)
Bilateral
Carotid
Release of
Occlusion
Carotid
Occlusion
Occlusion
1.5 -
300
.
Diameter
2:0 L
(PLM )
.......... ....
.
.
.
- --
200-
Arterial
Pressure
(mmHg)
,1,
JAUMA
AWAIAUM
FIGURE 1. Recording of velocity of
blood flow through the basilar artery
(pulsatile and mean), measurement of
diameter of the basilar artery every 1-5
seconds, and recording of systemic arterial pressure (pulsatile and mean) under
control conditions and during carotid
occlusion. The break in the recording
during bilateral carotid occlusion is for 1
minute.
.11
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300 [
Mean
Arteral
Pressure 150 _
(mmHg)
blood flow through the basilar artery and diameter of
the basilar artery increased in a graded manner
during UCO and BCO.
Baseline diameter of the basilar artery and of three
progressively smaller groups of branches of the basilar artery is shown in Figure 3. Diameter of the
basilar artery in this group of eight rats increased by
-27% during BCO. In contrast, diameter of the
branches of the basilar artery did not change significantly during BCO (Figure 3).
Results
Responses to Carotid Occlusion
An example of the effect of carotid occlusion on
velocity of blood flow and diameter of the basilar
artery is shown in Figure 1. Velocity of blood flow
through the basilar artery began to increase with no
detectable time delay after UCO or BCO. Diameter
of the basilar artery began to increase after a delay of
13 +±1 seconds after the onset of the increase in flow
velocity. Increased velocity and diameter were sustained for the duration of the occlusion. After release
of carotid occlusion, velocity returned rapidly to
preocclusion levels, and diameter returned gradually
to preocclusion levels. Changes in systemic arterial
pressure were kept to a minimum by withdrawal and
reinfusion of arterial blood.
Average changes in aortic pressure, velocity of
blood flow, and vessel diameter during carotid occlusion are summarized in Figure 2. Systemic arterial
pressure was maintained near the control level of
127±4 mm Hg during carotid occlusion. Velocity of
A Velocity of
A Systemic
Arterial Pressure
(mmHg)
20
Pressure in the Basilar Artery
An example of the effect of carotid occlusion on
diameter of the basilar artery, pressure in the basilar
artery, and systemic arterial pressure is shown in
Figure 4. Pressure in the basilar artery fell immediately after UCO or BCO, while systemic arterial
pressure remained constant. Pressure in the basilar
artery returned toward normal after dilatation of the
basilar artery.
A
Blood Flow
Diameter
(%)
(%)
30
300
1
*t
0
15
150
*
FIGURE 2. Bar graphs showing changes in systemic arterialpressure, velocity ofbloodflow through
the basilar artery, and diameter of the basilar artery
during unilateral carotid occlusion (UCO) or bilateral occlusion (BCO) (n=45 rats). Increases in
arterial pressure during carotid occlusion were prevented by withdrawal of arterial blood. All values are
mean+±SEM. *p<0.05 vs. before occlusion;
tp<0.05 vs. UCO.
*20
0
0
uco
BCO
uco
BCO
_
UCO
__
BCO
Fujii et al Flow-Mediated Dilatation in Brain
Baseline Diameter (,im)
A
Diameter (%)
A
30
300
Basilar Arterial Pressure
(mmHg)
0
701
Diameter
A
(%)
30
20
10
150
-20 I_
15
*
o
*
0
-10
Branches
of
Basilar
Basilar
Artery
Basilar
Artery
Branches
of
Basilar
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FIGURE 3. Bar graphs showing baseline diameter of the
basilar artery and branches of the basilar artery (left panel)
and percent change in diameter of these vessels during bilateral
carotid occlusion (right panel) (n=8). Filled, hatched, and
dotted bars in each panel indicate three progressively smaller
groups of branches of the basilar artery. All values are
mean + SEM. *p < 0. 05 vs. before occlusion.
Changes in
pressure
and diameter of the basilar
artery during BCO and hemorrhagic hypotension are
shown in Figure 5. Pressure in the basilar artery
decreased by 8+2 mm Hg (n=6, p<0.05) under
steady-state conditions after BCO, while systemic
arterial pressure was maintained at control levels
(change= 1 1 mm Hg, p=NS). Similar or larger decreases in pressure in the basilar artery induced by
withdrawal of blood did not significantly alter diameter of the basilar artery (Figure 5), which suggests
that an autoregulatory response to the small reduction in basilar pressure during carotid occlusion
cannot explain the marked dilatation of the basilar
±
artery.
To examine the relation between dilatation of the
basilar artery and pressure in the basilar artery, we
analyzed changes in systemic arterial pressure and
pressure in the basilar artery (Figure 6). Systemic
arterial pressure was maintained near the control level
oI
-40
Bilateral
Carotid
Occlusion
Diameter
Effect of Antagonists
The effects of several interventions on baseline
diameter and changes in velocity of blood flow and
vessel diameter during carotid occlusion are shown in
200 L
+
+
300-
Unilateral
Bilateral
Basilar
Arterial
Pressure
~~~~~~~~~~~~~~~~~~~~........
30
seconds
250
Carotid
Occlusion
150
Carotid
Occlusion
REME
Hypotension
of 117±5 mm Hg during carotid occlusion. Pressure in
the basilar artery decreased by 9±1 mm Hg immediately after UCO (before the onset of dilatation of the
basilar artery) from a control level of 106±5 mm Hg.
Under steady-state conditions during UCO, the decrease in pressure in the basilar artery was only 5±2
mm Hg. Similarly, after BCO, the decrease in pressure
was larger (14±2 mm Hg) initially than during steadystate conditions (8±2 mm Hg). The results suggest
that the increase in pressure gradient from aorta to
basilar artery during carotid occlusion was attenuated
by dilatation of the basilar artery and large arteries
upstream from the basilar artery.
-
(gm)
,
FIGURE 5. Bar graphs showing changes in pressure in the
basilar artery and diameter of the basilar artery during bilateral
carotid occlusion and hemorrhagic hypotension. Filled and
hatched bars indicate two levels ofgraded reductions in basilar
artery pressure. All values are mean ±SEM (n=6-10).
*p< 0.05 vs. control.
A
300
,
Bilateral
Carotid
Occlusion
Hypotension
Sytemic Arterial Pressure
(mmHg)
in
0
A
Basilar Arterial Pressure
(mmHg)
0
ja
1
1
-L.
T
L.
early
SS
eady SS
-10
-
(mmHg)
300 rArterial
Pressure
(mmHg)
UCO
150
0 L-
FIGURE 4. Measurement of diameter of the basilar artery
2-3 seconds, and recordings of pressure in the basilar
artery and systemic arterial pressure under control conditions
and during carotid occlusion.
every
-10
BCO
-20'
eady SS
UCO
early SS
BCO
FIGURE 6. Bar graphs showing changes in systemic arterial
pressure (left panel) and pressure in the basilar artery (right
panel) immediately after (early) unilateral carotid occlusion
(UCO), during steady state (SS) after UCO, immediately after
bilateral occlusion (BCO), and during steady state after BCO
(n=6). All values are mean +SEM. *p<0.05 vs. before occlusion; tp<0. 05 vs. immediately after UCO or BCO.
702
Circulation Research Vol 69, No 3 September 1991
Table 1. Treatment with L-NMMA, TEA, and ouabain decreased baseline diameter. Dilatation in response to topical application of nitroglycerin, however, was not altered significantly by these
interventions. Dilatation in response to 10`6 M nitroglycerin was 37+7% under control conditions and
31+5% during application of 10 mM TEA (n=3,
p=NS), and 38+±8% under control conditions and
37+9% during application of 10-5 M ouabain (n=3,
p=NS). We have shown previously that dilatation in
response to nitroglycerin is not altered during application of L-NMMA.34 Other interventions summarized in Table 1 had no effect on baseline diameter of
the basilar artery.
The increase in velocity of blood flow and diameter
of the basilar artery during UCO or BCO was not
altered by any of the interventions shown in Table 1.
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Discussion
There are several major new findings in the present study. First, the basilar artery dilates during
carotid occlusion in response to an increase in blood
flow. The response is large relative to that observed
in other vascular beds. Second, cerebral microvascular pressure tends to be restored toward normal
during dilatation of the basilar artery. The finding
implies that flow-mediated dilatation of large arteries
attenuates the fall in pressure that occurs along large
cerebral arteries during increases in blood flow and,
thereby, preserves distal perfusion pressure. Third,
flow-mediated dilatation of the basilar artery does
not appear to depend solely on production of nitric
oxide, cyclooxygenase activity, activity of cytochrome
P-450-dependent monooxygenase, voltage-dependent or ATP-sensitive K' channels, or activity of
Na+,K+-ATPase.
Mechanism of Increase in Velocity of Blood Flow
During Carotid Occlusion
Pressure in the basilar artery fell during carotid
artery occlusion when systemic arterial pressure remained constant or was maintained at control levels.
Nevertheless, velocity of blood flow through the
basilar artery increased. The pressure difference
between upstream and downstream vessels is a determinant of flow velocity. Therefore, even when
upstream pressure decreases, velocity of blood flow
can increase if there is a larger reduction in downstream pressure. Reductions in downstream pressure
almost certainly occurred in the circle of Willis
during carotid occlusion, so that velocity of flow
increased despite a reduction in pressure in the
basilar artery. The mechanism of increase in velocity
of flow during carotid occlusion appears to be similar
to that observed in previous studies 335,6,1117,36,37 in
which increases in flow velocity were obtained by
distal vasodilatation induced pharmacologically,6,11,36
by an arteriovenous shunt,3,5,6'36 or by occlusion of
parallel arterioles or arteries.1737
Mechanism of Vasodilatation in Response to
Carotid Occlusion
There are several possible mechanisms that could
account for dilatation of the basilar artery in response to carotid occlusion. These include neurogenic, myogenic, metabolic (diffusion of a vasodilator
substance from an ischemic area), ascending dilatational (propagated dilatation within the vessel wall),
or flow-mediated mechanisms.
Studies of the femoral artery36 and large coronary
arteries4 suggest that neurogenic mechanisms, including reflex vasodilatation, do not play a major role
in dilatation during increases in flow. The studies
demonstrated that transsection of the artery distal to
the recording site, or adrenergic or ganglionic blockade, did not impair the dilatation. In the present
study, TTX, which selectively blocks sodium channels
and can thereby abolish neurally mediated responses,25 had no effect on dilatation of the basilar artery
during bilateral carotid occlusion. Adrenergic, cholinergic, and nonadrenergic, noncholinergic vasodilatator mechanisms all are inhibited by TTX.33 This
finding suggests that neural mechanisms that require
sodium channel activity for excitation have little, if
any, role in dilatation during carotid occlusion.
Pressure in the basilar artery decreased during carotid occlusion with systemic pressure constant. Similar
or larger decreases in pressure induced by hemorrhage
did not alter the diameter of the basilar artery, which
suggests that the vessel does not autoregulate very well
and that an autoregulatory (myogenic) response to the
small reduction in pressure in the basilar artery cannot
explain the pronounced dilatation of the basilar artery
during carotid occlusion.
Diameter of branches of the basilar artery did not
increase during carotid occlusion. These vessels probably are not exposed to an increase in blood flow. This
observation provides some evidence against a metabolic mechanism or a role for ascending dilatation of
the basilar artery. If the dilatation were due to diffusion of vasodilator substances from cerebrum, for
example, dilatation would be expected to occur in
small branches as well as the basilar artery.
It is not possible to distinguish definitely between
flow-mediated and propagated responses.38,39 In the
dog femoral artery, propagation of a signal along the
vessel wall probably does not account for vasodilatation, because dilatation was preserved after transsection of the artery distal to the recording site.36 In the
present study, absence of dilatation in branches
suggests that dilatation of the basilar artery is not due
to propagated dilatation within the wall. Based on
these observations, we conclude that dilatation of
basilar artery during carotid occlusion occurs primarily in response to an increase in blood flow.
There was always a time delay between the onset
of increase in velocity of blood flow and the onset of
increase in vessel diameter. A similar delay has been
Fujii et al Flow-Mediated Dilatation in Brain
observed in flow-mediated responses in other vascular beds in vivO510,13'36'37 and in vitro.40 The cause for
this delay is not clear but could reflect the time
required for the increase in flow to trigger the
production or release of mediators or for the mediators to produce detectable dilatation.
Endothelial cells appear to sense changes in shear
stress or blood flow,41'42 but the role of endothelium
in flow-mediated dilatation is controversial. Endothelium appears to play a crucial role in the response
to an increase in flow in dog femoral artery,3,5-8 dog
coronary arteries,2,4,9 and rat cremaster arterioles.10
In contrast, a major component of flow-mediated
relaxation still occurs after removal of endothelium
in the rabbit ear artery studied in vitro.40
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Mediators of Flow-Mediated Dilatation
Several mediators of flow-mediated dilatation have
been identified or proposed in the peripheral circulation. In the aorta and femoral arteries, a diffusible
vasodilator similar to endothelium-derived relaxing
factor may be involved.7"15"16 In rat cremaster microcirculation, inhibition of synthesis of nitric oxide (a
possible endothelium-derived relaxing factor) did not
inhibit the dilator response of arterioles during increases in blood flow.17 In the rabbit ear artery
studied in vitro, hemoglobin, which completely reversed acetylcholine-induced relaxation, had an inconsistent effect on relaxation during increases in
blood flow.40 In the present study, L-NMMA had no
effect on flow-mediated dilatation, which suggests
that this mechanism in the basilar artery does not
require production of nitric oxide.
Several studies suggest that flow-mediated dilatation is not attenuated by inhibitors of cyclooxygenase.34,7'1340 Arteriolar dilatation in the microcirculation of skeletal muscle, however, is inhibited by
indomethacin and meclofenamate.17 In this study,
indomethacin had no effect on flow-mediated dilatation, suggesting that flow-mediated dilatation of the
basilar artery is not dependent on cyclooxygenase
products. The response also was not blocked by SKF
525A, a cytochrome P-450-dependent monooxygenase inhibitor.27'29
Ionic Mechanisms
Recently, it has been proposed that activation of a
hyperpolarizing K' current within endothelium may
produce dilatation in response to an increase in
flow.18 A study16 in isolated rabbit iliac arteries also
suggests that activation of K' channels is the transducer and endothelium-derived relaxing factor is the
effector of flow-mediated dilatation. In our study,
TEA, which blocks voltage-dependent K' channels,26
and glibenclamide, which blocks ATP-sensitive K'
channels,31'32 did not affect flow-mediated dilatation
of the basilar artery. Many potassium channels have
been identified or proposed to exist,43 and we cannot
exclude an important role of other K' channels.
In resistance arteries in vitro, replacement of 85%
of total Na+ ions by Li+ abolished flow-mediated
703
dilatation, suggesting that sodium-dependent mechanisms may be important in mediating flow-mediated
dilatation.19 In our study, replacement of normal
CSF by low Na+ CSF did not affect the flow-mediated
dilatation of the basilar artery. We cannot, however,
exclude a role of sodium-dependent mechanisms,
because blood flowing through the basilar artery has
a normal concentration of Na+. Ouabain, which
inhibits activity of Na4,K+-ATPase,28 had no effect
on flow-mediated dilatation.
Functional Significance of Flow-Mediated Dilatation
Pressure in the basilar artery decreased after carotid occlusion, and this reduction in pressure was
restored toward normal as the basilar artery dilated. A
major determinant of pressure in the basilar artery is
the ratio of upstream resistance (from aorta to basilar
artery) to downstream resistance.14 During carotid
artery occlusion, resistance in downstream vessels
would be expected to decrease, in response to a
reduction in perfusion pressure, relative to resistance
of upstream vessels. Consequently, pressure in the
basilar artery would decrease. A decrease in pressure
during increases in blood flow also was observed in the
femoral artery during increases in blood flow produced by distal vasodilatation induced pharmacologically36 or by an arteriovenous shunt.3'36 In the present
study, dilatation of upstream vessels (presumably
flow-mediated dilatation) would decrease resistance
of large upstream vessels and restore pressure in the
basilar artery toward normal. Thus, restoration of
pressure in the basilar artery toward normal during
increases in blood flow suggests that there may be
dilatation of large arteries, such as the vertebral
artery, upstream from the basilar artery.
Flow-mediated dilatation appears to minimize
the drop in perfusion pressure that occurs along
large cerebral arteries during increases in blood
flow. This finding provides direct evidence to support the hypothesis"10"13 that flow-mediated dilatation helps to preserve microvascular pressure, or
distal perfusion pressure, and thereby protects
against a steal phenomenon.
The magnitude of the increase in diameter that
was observed in the basilar artery in response to
increases in blood flow is large relative to other
vascular beds. For example, the increase in vascular
diameter in response to a similar increase in blood
flow is 5-10% in large coronary arteries4,1113 and
2-10% in the femoral artery.3'6'8'36 In peripheral
blood vessels, the magnitude of flow-mediated dilatation appears to increase as the size of the artery or
arteriole decreases,3"17'37 which suggests that the response to increased flow is somehow dependent on
vessel tone or diameter. A change in diameter that is
relatively small in magnitude is magnified, in terms of
vascular resistance. The large magnitude of the response in the basilar artery suggests that flow-mediated dilatation is especially important in the cerebral
704
Circulation Research Vol 69, No 3 September 1991
circulation, where resistance of large arteries is high
relative to other vascular beds.14
In summary, prominent dilatation occurs in the basilar artery in response to an increase in blood flow in
vivo. Flow-mediated dilatation of the basilar artery
does not appear to be dependent on production of
nitric oxide, cyclooxygenase activity, cytochrome
P-450-dependent monooxygenase activity, Na+,K+ATPase activity, or several voltage-dependent or ATPsensitive K' channels. It remains possible that redundant mechanisms participate in this response and that
some combinations of these factors have additive or
synergistic effects that cannot be attenuated by a single
inhibitor. Thus, the cellular mechanisms and mediators
that account for this striking response in cerebral
vessels in vivo remain unidentified.
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KEY WORDS * basilar artery * blood flow * carotid occlusion
* microvascular pressure * endothelium-derived relaxing factor
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
Flow-mediated dilatation of the basilar artery in vivo.
K Fujii, D D Heistad and F M Faraci
Downloaded from http://circres.ahajournals.org/ by guest on June 17, 2017
Circ Res. 1991;69:697-705
doi: 10.1161/01.RES.69.3.697
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