Comparison of P2 Receptor Subtypes Producing Dilation in
Rat Intracerebral Arterioles
Tetsuyoshi Horiuchi, MD; Hans H. Dietrich, PhD; Kazuhiro Hongo, MD; Ralph G. Dacey, Jr, MD
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Background and Purpose—P2 receptors are important regulators of cerebrovascular tone. However, there is functional
heterogeneity of P2Y receptors along the vascular tree, and the functionality of P2Y receptors in small arterioles has not
been studied in detail. We investigated the effects of activating P2Y1 and P2Y2 receptors and their underlying dilator
mechanisms in rat intracerebral arterioles.
Methods—We used computer-aided videomicroscopy to measure diameter responses from isolated and pressurized rat
penetrating arterioles (39.9⫾1.2 ␮m) to the natural P2 receptor agonist ATP in addition to ADP-␤-S (P2Y1-selective) and
ATP-␥-S (P2Y2-selective) and inhibitors of signaling pathways.
Results—Extraluminal application of ATP-␥-S and ADP-␤-S initiated a biphasic response (initial constriction followed by
the secondary dilation) similar to ATP-induced responses. Pyridoxal phosphate-6-azophenyl-2⬘,4⬘-disulphonic acid
(0.1 mmol/L; a P2Y1 receptor antagonist) blocked ADP-␤-S– but not ATP-␥-S–induced dilation and affected the
ATP-mediated dilation at low concentrations. N␻-Monomethyl-L-arginine partially inhibited the dilation of ATP and
ADP-␤-S but not ATP-␥-S. High K⫹ saline suppressed the dilation of all agonists. Indomethacin had no effect.
Conclusions—Both P2Y1 and P2Y2 receptors are functionally present in cerebral arterioles. ATP stimulates P2Y1 receptors at
low concentrations, while high concentrations of ATP activate P2Y2 in addition to P2Y1 receptors. Nitric oxide is involved
in P2Y1 but not P2Y2 receptor activation. Potassium channels play an important role in the regulation of P2Y
receptor–mediated dilation. (Stroke. 2003;34:1473-1478.)
Key Words: adenosine 䡲 cerebral circulation 䡲 nitric oxide 䡲 potassium channels 䡲 receptors, purinergic P2
T
he importance of purinergic regulation has been recognized in the cerebral circulation. ATP is one of the
purines and the natural agonist to control blood flow. Purines
released from the parenchyma may be important in regulating
microvascular blood flow.1 Both vasoconstriction and vasodilation can be produced by ATP in the cerebral vasculature.2,3 These vasomotor responses depend on distribution of
P2 receptor subtypes. Thus, species, location, and size of
vessels greatly influence the vasomotor response to ATP.3
It has been demonstrated that both endothelial P2Y1 and P2Y2
receptors are present and their stimulation dilates rat middle
cerebral artery.4 – 6 In contrast, we2 and others4 speculated that
P2Y1 receptor may not be present or its function may be silent
in the cerebral microcirculation under physiological conditions because 2-methylthio-ATP (2-MeSATP; a P2Y1-receptor
agonist) did not dilate the cerebral arteriole effectively.
However, P2Y1 and P2Y2 receptors have been detected in
primary cultures of rat brain capillary endothelium.1,7 In rat
middle cerebral artery, P2Y1 releases only nitric oxide (NO),
while P2Y2 stimulation liberates both NO and endotheliumderived hyperpolarizing factor (EDHF).5 It is of interest that
the involvement of NO in response to ATP-induced dilation
decreases along the cerebrovascular tree, whereas EDHF
seems to be the major contributor to ATP-induced dilation in
smaller vessels.4
It is not known whether P2Y1 receptors are functionally
present in penetrating arterioles, which are important regulators of cerebral microvascular blood flow. Furthermore, the
mechanism of P2Y1 receptor–induced dilation has not been
studied. Therefore, in rat isolated cerebral arterioles, we
evaluated the functional P2Y receptor distribution using ADP␤-S (a selective P2Y1 receptor agonist) and ATP-␥-S (a
selective P2Y2 receptor agonist) on arteriolar diameter. We
compared the results with the natural agonist ATP and also
investigated the relaxing factor(s) released by these agonists.
Materials and Methods
The Animal Studies Committee at Washington University approved
the experimental protocol for this study. Fifty-six male SpragueDawley rats (weight, 350 to 450 g; Harlan, Indianapolis, Ind) were
anesthetized with pentobarbital sodium (65 mg/kg IP) and decapitated. The brain was immediately removed and stored in cold
physiological salt solution (PSS). The technique for isolation and
cannulation of the intracerebral arteriole has been showed in previous studies.8 –10 In short, the cerebral penetrating arterioles were
Received September 12, 2002; final revision received December 17, 2002; accepted December 18, 2002.
From the Department of Neurosurgery, Washington University School of Medicine, St Louis, Mo (T.H., H.H.D., R.G.D.), and Department of
Neurosurgery, Shinshu University School of Medicine, Matsumoto, Japan (T.H., K.H.).
Correspondence to Hans H. Dietrich, PhD, Department of Neurosurgery, Washington University School of Medicine, Box 8057, 660 S Euclid Ave,
St Louis, MO 63110. E-mail [email protected]
© 2003 American Heart Association, Inc.
Stroke is available at http://www.strokeaha.org
DOI: 10.1161/01.STR.0000071527.10129.65
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Passive Diameter, Tone Diameter, and pH Response in Isolated Rat Intracerebral Arterioles
Experimental Group
ATP
ATP-␥-S
ADP-␤-S
n
19
17
20
Passive diameter Dmax, ␮m
Tone diameter Dtone (%Tone), ␮m
56.4⫾2.3
60.0⫾2.0
59.8⫾2.0
37.7⫾2.3* (33.7%)
41.5⫾1.8* (31%)
40.6⫾1.8* (32.2%)
pH 6.8 diameter, ␮m
46.9⫾3.0†
53.0⫾2.5†
52.3⫾1.9†
pH 7.65 diameter, ␮m
28.2⫾1.7†
32.1⫾2.0†
29.9⫾1.4†
The values are mean⫾SEM, and n is the number of observations. The vessels developed spontaneous tone by
constricting more than 30% (%Tone in parentheses) from the maximum passive diameter Dmax to the tone diameter
Dtone (* denotes P⬍0.05 compared with passive diameter) and responded to pH challenge († denotes P⬍0.05
compared with tone diameter). There were no statistically significant differences in passive, tone or pH diameters
between experimental groups.
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obtained from the proximal portion of middle cerebral artery. An
arteriole without branches was transferred to a temperaturecontrolled organ bath and cannulated with glass pipettes. The
luminal pressure was maintained at 60 mm Hg, and no luminal
perfusion was given in all experiments. The vessel was magnified
with an inverted microscope (Diaphot, Nikon) equipped with a video
camera and monitor. The internal diameter of the vessel was
measured with a computerized diameter tracking system (Diamtrak,
Montech Pty Ltd; 320⫻200 pixels; 0.5 ␮m per pixel), and the
measurements were stored digitally (WinDaq, DataQ Instruments)
for further evaluation.
After the diameter was measured at 60 mm Hg (maximum
diameter [Dmax]), the organ bath temperature was brought to 37.5°C
with circulating PSS (pH 7.3) at a rate of 0.5 mL/min. After an
equilibration period, the arterioles developed spontaneous tone
(Dtone), constricting by at least 20%. Subsequently the vessel viability
was confirmed with diameter responses to acidosis (pH 6.8) and
alkalosis (pH 7.65).
Only 1 arteriole was used from each brain. Vasomotor responses
to ATP, ATP-␥-S (selective P2Y2 receptor agonist),3 and ADP-␤-S
(selective P2Y1 receptor agonist)3 were determined by changing the
bath solution in a cumulative manner. Initially, we generated a
concentration-response curve for each agonist as control response.
Then we reapplied the agonist in the presence of the inhibitor. We
applied only 1 agonist on each vessel and generated concentrationresponse curves maximally 3 times per vessel. In preliminary
experiments, we found no difference among 3 concentrationresponse curves obtained in the same vessel.
We investigated effects of 0.1 mmol/L pyridoxal phosphate-6azophenyl-2⬘,4⬘-disulphonic acid (PPADS) on ATP-, ATP-␥-S–, and
ADP-␤-S–induced responses. We2 have previously shown that
0.1 mmol/L PPADS is an antagonist for P2X1 and P2Y1 but not P2Y2
receptors in this preparation.
The effects of 10 ␮mol/L N␻-monomethyl-L-arginine (L-NMMA;
a NO synthase inhibitor)9 or 10 ␮mol/L indomethacin (cyclooxygenase inhibitor) on ATP-, ATP-␥-S–, and ADP-␤-S–mediated
vessel responses were examined in response to the 3 agonists. We
also used the high-K⫹ saline (30 and 80 mmol/L) to negate any
effects of K⫹ channels.11 Isotonic high-K⫹ PSS was made by
substituting NaCl with an equimolar amount of KCl.
The composition of PSS was as follows (in mmol/L): 144 NaCl, 3
KCl, 2.5 CaCl2, 1.4 MgSO4, 2.0 pyruvate, 5.0 glucose, 0.02 EDTA,
2.0 3-(N-morpholino) propanesulfonic acid (MOPS), 1.21 NaH2PO4.
ATP, ATP-␥-S, ADP-␤-S, L-NMMA, indomethacin, and PPADS
were obtained from Sigma Chemical.
All data are presented as mean⫾SEM; n represents the number of
vessels.
Vessel tone is calculated as follows: %Tone⫽[(Dmax⫺Dtone)/
Dmax]⫻100.
For concentration-response curves, the results are presented as
percentages of the maximal dilation of the vessel, calculated as
follows: %Maximum Dilation⫽[(Dagonist⫺Dbase)/(Dmax⫺Dbase)]⫻100,
where Dmax is the maximum diameter of the vessel at 60 mm Hg
before the development of spontaneous tone, Dtone is the spontaneous
tone, Dbase is the baseline diameter of the vessel before the stimulation with agonist, and Dagonist is the diameter of the vessel after
agonist stimulation. Dmax is identical to the diameter produced by
papaverine12 and calcium-free buffer.13 With this method, the maximal dilation is represented as 100%, and baseline diameter is 0%.
Dilation from baseline is presented as positive and constriction as
negative relative values.
Comparisons were made with the use of paired Student’s t test or
ANOVA with a post hoc Bonferroni test, as appropriate. The
acceptable level of significant was defined as P⬍0.05.
Results
All vessels (n⫽56) developed spontaneous tone and responded to pH challenge (Table). There were no significant
differences in passive diameter, spontaneous tone, dilation to
acidosis, and constriction to alkalosis between vessel groups
(ATP, ATP-␥-S, or ADP-␤-S) (Table).
Previous studies in our laboratory showed that extraluminal ATP caused a biphasic response consisting of initial
constriction followed by secondary dilation.2,10 Extraluminal
ATP-␥-S and ADP-␤-S also produced dilation after constriction, as seen with ATP (Figure 1). We previously demonstrated that the initial constriction is caused by smooth muscle
cell P2X1 receptors.2 We also showed that this constriction did
not affect the subsequent dilation.2 The following results,
therefore, focus on the secondary dilation induced by the
agonists. Group data for concentration responses of ATP,
ATP-␥-S, and ADP-␤-S are shown in Figure 1. Dilator
responses to ATP-␥-S and ADP-␤-S (10 and 100 ␮mol/L)
were significantly more potent than those to ATP. ATP,
ATP- ␥ -S, and ADP- ␤ -S at 100 ␮ mol/L produced
60.1⫾4.3%, 90.2⫾2.5%, and 90.2⫾2.5% of maximal dilation, respectively. Furthermore, ATP showed a right-shifted
EC50 value of 9.69 ␮mol/L compared with ATP-␥-S
(EC50⫽6.40 ␮mol/L) and ADP-␤-S (EC50⫽3.72 ␮mol/L).
We compared the control dose-response curves for ATP,
ATP-␥-S, and ADP-␤-S in Figure 1 with the respective
curves in the subsequent protocols and found them to be not
statistically different (ANOVA).
Figure 2 illustrates the effects of 0.1 mmol/L PPADS (a
P2X1 plus P2Y1 receptor antagonist at this concentration2) on
ATP-, ATP-␥-S–, and ADP-␤-S–induced dilation. PPADS
had no effect on the baseline vessel diameter. PPADS
significantly inhibited dilation to ADP-␤-S but not to ATP␥-S. Interestingly, ATP-mediated dilations at 10 nmol/L to 1
␮mol/L were inhibited by PPADS, whereas PPADS did not
affect 10 ␮mol/L ATP– and 0.1 mmol/L ATP–induced
Horiuchi et al
P2 Receptor in Cerebral Arterioles
1475
acin affected neither the control diameter nor dilations to the
3 agonists.
Figure 5 demonstrates the effects of 30 and 80 mmol/L
KCl on ATP-, ATP-␥-S–, and ADP-␤-S–induced dilation.
The high-K⫹ saline (30 and 80 mmol/L) decreased the
baseline diameter of the vessels (41.6⫾2.5 ␮m [control
diameter]; 35.0⫾2.3 ␮m [30 mmol/L]; 24.1⫾2.1 ␮m
[80 mmol/L]) and attenuated the dilation of ATP, ADP-␤-S,
and ATP-␥-S in a dose-dependent manner. Papaverine (10
␮mol/L) fully dilated 3 vessels in the presence of high-K⫹
saline, indicating that high-K⫹ saline did not cause vascular
paralysis.
Discussion
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Figure 1. Concentration-response curves comparing vasomotor
responses induced by ATP (n⫽19), ATP-␥-S (P2Y2-receptor agonist; n⫽17), and ADP-␤-S (P2Y1-receptor agonist; n⫽20). All agonists cause a biphasic response consisting of initial transient
constriction followed by secondary dilation. *P⬍0.05, significantly different compared with ATP-induced responses.
dilation. The initial constrictions of all agonists were abolished by 0.1 mmol/L PPADS (data not shown).2 In preliminary experiments, 3 ␮mol/L PPADS inhibited the agonistinduced initial constriction. However, 3 ␮mol/L PPADS did
not affect the secondary dilation.
Figure 3 shows the effects of 10 ␮mol/L L-NMMA on
ATP-, ATP-␥-S–, and ADP-␤-S–induced dilation. The vessels constricted significantly to L-NMMA from 42.2⫾2.0 to
32.5⫾1.7 ␮m. L-NMMA attenuated the dilation of ATP and
ADP-␤-S but not ATP-␥-S. In preliminary experiments (2
vessels), 100 ␮mol/L L-NMMA did not produce further
inhibition of the dilation.
Figure 4 shows the effects of 10 ␮mol/L indomethacin on
ATP-, ATP-␥-S–, and ADP-␤-S–induced dilation. Indometh-
The purpose of the present study was to clarify the functional
contribution of P2Y1 receptors in particular to purinergic
stimulation with the use of specific agonists/antagonists and
to elucidate the relaxing factors released to specific P2Y
receptor stimulation. We found that ADP-␤-S and ATP-␥-S
potently dilate rat intracerebral arterioles, indicating P2Y1 and
P2Y2 receptor presence, respectively. With the use of the
specific P2Y1 inhibitor PPADS, low concentrations of ATP
stimulated P2Y1 receptors, whereas P2Y2 receptors were additionally activated by higher concentrations of ATP. NO
partially contributed to P2Y1 receptor– but not P2Y2 receptor–
produced dilation. Cyclooxygenase products were not involved. Potassium channels may play a major role in dilation
to both P2Y1 and P2Y2 receptor stimulation.
Distribution of P2 Receptors in Cerebral Arterioles
Extraluminal application of ATP, ADP-␤-S, and ATP-␥-S
resulted in a biphasic response consisting of the initial
constriction followed by the secondary dilation. The initial
constriction was inhibited by PPADS (3 ␮mol/L and
0.1 mmol/L). PPADS is a P2 receptor antagonist,3 and its
effects depend on concentration, species, and vessel location.
In our preparation, 3 ␮mol/L PPADS inhibits P2X1 receptor
but not P2Y receptor.2 A higher concentration of 0.1 mmol/L
PPADS, however, blocks P2X1 plus P2Y1 receptors but not P2Y2
receptors.2 These results indicate that both ADP-␤-S and
ATP-␥-S activate P2X1 receptors to induce transient arteriolar
constriction. We2 reported that biphasic vasomotor responses
Figure 2. Effects of 0.1 mmol/L PPADS on
vasomotor responses induced by ATP (n⫽5),
ATP-␥-S (n⫽4), and ADP-␤-S (n⫽5). *P⬍0.05,
significantly different compared with the percentage of dilation in presence of PPADS. Note
that ATP-mediated dilations at low concentration are inhibited by PPADS.
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Figure 3. Effects of 10 ␮mol/L L-NMMA on
vasomotor responses induced by ATP (n⫽5),
ATP-␥-S (n⫽4), and ADP-␤-S (n⫽6). *P⬍0.05,
significantly different compared with percentage
of dilation in presence of L-NMMA.
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after extraluminal application of agonists not only resulted in
the activation of smooth muscle cells (to cause transient
constriction) but also activated the endothelium to cause
vessel dilation. In cerebral macro- and micro-circulation, P2Y2
receptor subtypes function as an important regulator of
cerebral blood flow.2,4 – 6 However, on the basis of previous
studies,2,4 P2Y1 receptor subtypes seem to be less important in
the regulation of the cerebral microcirculation because
2-MeSATP as a P2Y1 agonist produced no vasodilation in
third-order branches of the middle cerebral artery and large
penetrating arterioles of the rat.4 These findings suggested
that functional P2Y1 receptors may be absent in the cerebral
microcirculation. Recently, our study,2 which also used
2-MeSATP, confirmed their results. However, we2 also
speculated that 2-MeSATP is not a potent P2Y1 receptor
agonist in the cerebral arteriole because 2-MeSATP can also
stimulate P2X1 receptors in our preparation.2 However, we
excluded that the P2X1 stimulation has an inhibitory/subtractive effect on ATP-induced secondary dilation.2 Another
explanation for the apparent lack of potency of 2-MeSATP is
based on differences in smooth muscle sensitivity to the
relaxing factors along the vessel trees, ie, NO.4
In the present study ATP-␥-S in addition to ADP-␤-S
dilated the cerebral arterioles of the rat in a dose-dependent
manner. PPADS (0.1 mmol/L) can clearly distinguish P2Y1
from P2Y2 receptor subtypes in our preparation and inhibited
ADP-␤-S–induced dilation. Thus, P2Y1 receptors are also
present in the arteriole and can regulate the vascular tone in
the cerebral microcirculation. Furthermore, the water-soluble
purinergic agonists we used reached endothelial receptors on
the abluminal side to cause endothelium-dependent dilation.
Our results are consistent with studies in which P2Y agonists
are used intraluminally.4 – 6 However, if endothelial P2y1 and
P2y2 receptors are heterogeneously distributed on luminal
versus abluminal surfaces, it is possible that the effects of the
receptor specific agonists we used may differ when applied
intraluminally.
ADP-␤-S and ATP-␥-S strongly dilated the vessels compared with ATP. It is a possibility that ATP was metabolized
by ectonucleotidases to decrease its efficacy. However, we
found that ADP as a dephosphorylated ectonucleotidase
product dilated penetrating arterioles in a manner similar to
that of ATP.14 Thus, ATP degradation by ectonucleotidases to
cause the reduced dilation seems less likely.
Our results show that high concentrations of ATP (10 to
100 ␮mol/L) activate P2Y2 in addition to P2Y1 receptors,
resulting in arteriolar dilation. By contrast, low concentrations of ATP (10 nmol/L to 1 ␮mol/L) mainly stimulated P2Y1
receptors. These results indicate that ATP acts as both a P2Y1
and P2Y2 receptor agonist, and its function depends on the
Figure 4. Effects of 10 ␮mol/L indomethacin on
vasomotor responses induced by ATP (n⫽4),
ATP-␥-S (n⫽4), and ADP-␤-S (n⫽4).
Horiuchi et al
P2 Receptor in Cerebral Arterioles
1477
Figure 5. Effects of 30 and 80 mmol/L KCl on
vasomotor responses induced by ATP (n⫽5),
ATP-␥-S (n⫽5), and ADP-␤-S (n⫽5). *P⬍0.05,
significant difference vs control; †P⬍0.05, significant difference vs 30 mmol/L.
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agonist concentration. Sipos et al7 discussed the possible
relevance of multiple purinergic receptors on the brain
endothelium and concluded that in addition to liberating
various dilators such as NO and EDHF, multiple receptors
could also contribute to “cross-talk” between second messenger systems.
Mechanisms of P2Y1 and P2Y2 Purinoceptor–
Induced Vasodilation
It is well known that the activation of endothelial P2Y1 and P2Y2
receptors in the cerebral circulation dilates the vessel via
different mechanisms.4,5,15 The stimulation of endothelial P2
receptors dilates vessels through releasing NO, prostanoid,
and/or EDHF in the vasculature.3 However, these relaxing
factors may vary from species, size, and location of vessels,
even though the P2Y receptor subtype is the same.3 Prostacyclin seems to be a less important factor in response to
endothelial P2Y receptor stimulation in cerebral circulation.3,4
These findings are consistent with the present study because
inhibition of cyclooxygenase in the intracerebral arteriole had
no effect in any agonist-induced dilations. Thus, a cyclooxygenase metabolite such as prostacyclin was not involved in
P2Y-induced dilation. However, we cannot exclude a possible
contribution of cyclooxygenase metabolites to P2Y receptor–
induced dilation after NO inhibition because we did not
examine the combined effects of L-NMMA and
indomethacin.
Our laboratory reported that ATP hyperpolarized the
smooth muscle cell, resulting in dilation of the cerebral
arteriole,16 and oxyhemoglobin (a scavenger of NO) partially
inhibited ATP-induced dilation.14 We also reported that an
intact endothelium is necessary for the ATP-induced dilation.2 These findings indicate that endothelial NO production
and a hyperpolarization factor caused the dilation of the
cerebral arteriole to ATP. In the rat middle cerebral artery,4,15
P2Y1 receptor activation produces vasodilation exclusively
through NO release, whereas P2Y2 receptor activation results
in vasodilation via NO and EDHF. Interestingly, EDHF
rather than NO prominently contributes to dilation of P2Y
receptor stimulation in the cerebral microcirculation compared with the macrocirculation.4
In the present study NO was partially involved in both
ATP- and ADP-␤-S–induced (P2Y1 receptor–induced) dilations. This contribution was observed at low but not high
concentration of ADP-␤-S. By contrast, ATP-␥-S (P2Y2 receptor agonist) dilated the arteriole via a NO-independent pathway. High-K⫹ saline strongly diminished dilation in response
to both P2Y1 and P2Y2 receptor stimulation, suggesting that
potassium channels contribute to the vessel dilation. Potassium channels are one of the main contributors to the
EDHF-induced dilation.17,18 It is possible that NO dilates the
vessel via potassium channel stimulation. However, our
previous studies12,19 demonstrated that NO/potassium channel
interactions did not seem to be important in the cerebral
arteriole. Thus, NO and potassium channel activation may act
as the mediator of P2Y1 receptor independently. On the basis
of this finding, the relaxing factors released in response to P2Y
receptor stimulation depended on receptor subtypes and
agonist concentrations in rat cerebral arterioles.
Intracellular Mechanisms and Mediators Released
to Purinergic Stimulation Along the
Cerebrovascular Tree
P2Y1 and P2Y2 receptors are G protein coupled.3 However, the
subsequent signal transduction pathway linked to P2Y receptors varies between the cell types even though receptor
subtypes on the cells may be identical.3 Nevertheless, an
increase in endothelial intracellular Ca2⫹ concentration
[(Ca2⫹)in] is seen as an essential intracellular response.3,5 In rat
middle cerebral artery, Marrelli5 reported that P2Y2 receptor
stimulation resulted in a significantly greater increase in
(Ca2⫹)in than P2Y1 receptor stimulation. P2Y1 stimulation produced a purely NO-dependent dilation, while P2Y2 stimulation
caused the release of both NO and EDHF. These results
suggest that (Ca2⫹)in thresholds regulate NO- and EDHFdependent dilation. Our results, however, indicate that in
cerebral arterioles P2Y1 stimulation releases both NO and an
indomethacin-independent factor, while P2Y2 stimulation was
independent of NO. Thus, our results differ from those seen
in middle cerebral arteries but agree with studies in pial
arterioles, in which P2Y1 stimulation depended partly (approximately one half) on NO.20 However, if P2Y2 stimulation
results in a higher calcium increase than P2Y1, we would also
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June 2003
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expect a release of NO, which is calcium dependent. Since we
did not find NO involvement after P2Y2 stimulation, a possible
explanation is that P2Y2 receptors are directly coupled to
phospholipase A2, which could release EDHF.21 Further
studies are needed to elucidate the intracellular coupling of
P2Y1 and P2Y2 receptors in rat penetrating arterioles.
In summary, the results of the present study provide
functional evidence that both P2Y1 and P2Y2 receptors are
present and mediate dilation of rat cerebral arterioles via
different mechanisms. The P2Y1 receptor dilates via both NO
and potassium channels (probably EDHF), whereas the P2Y2
receptor dilates the cerebral arteriole via potassium channel
activation but not NO. Cyclooxygenase products are not
involved. At low concentrations, the natural agonist ATP
stimulates predominantly P2Y1 receptors with subsequent NO
release. Higher ATP concentrations stimulate P2Y2 in addition
to P2Y1 receptors, releasing a NO- and cyclooxygenaseindependent but potassium channel– dependent factor(s). Our
results confirm the functional P2Y receptor heterogeneity
along the cerebrovascular tree, where stimulation of similar
P2Y receptors results in the release of different dilators.
Acknowledgments
This study was supported by National Institutes of Health grants
HL57540 and NS30555.
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Tetsuyoshi Horiuchi, Hans H. Dietrich, Kazuhiro Hongo and Ralph G. Dacey, Jr
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Stroke. 2003;34:1473-1478; originally published online May 1, 2003;
doi: 10.1161/01.STR.0000071527.10129.65
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