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,
-methylene-ATP in cat
pulmonary, mesenteric, and hindquarter vascular beds
Department of Pharmacology, Tulane University School of Medicine, New Orleans, Louisiana 70112
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Responses to the P2X-purinoceptor
agonist 
,
-methylene-ATP (,-MeATP) were investigated in the
pulmonary, hindquarter, and mesenteric vascular beds in the
cat. Under constant-flow conditions, injections of
,-MeATP caused dose-related increases in perfusion pressure in
the pulmonary and hindquarter beds and a biphasic response in the
mesenteric circulation. In the pulmonary vascular bed, the order of
potency was ,-MeATP > U-46619 > angiotensin II,
whereas, in the hindquarters, the order of potency was angiotensin II > U-46619 > ,-MeATP. The order of potency was
similar in the hindquarter and mesenteric beds when the pressor
component of the response to ,-MeATP was compared with responses
to angiotensin II and U-46619. The P2X-receptor antagonist
pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid attenuated the
pressor response to ,-MeATP in the hindquarter circulation and
the pressor component in the mesenteric vascular bed. Pressor responses
to ,-MeATP were not altered by cyclooxygenase, -adrenergic, or
angiotensin AT1 antagonists. These data show that
,-MeATP has potent pressor activity in the pulmonary circulation, where it was 100-fold more potent than angiotensin II. In contrast, ,-MeATP had modest pressor activity in the systemic bed, where it
was 1,000-fold less potent than angiotensin II. These data suggest that
responses to ,-MeATP are dependent on the vascular bed studied
and may be dependent on the density of P2X receptors in the vascular bed.
vasoconstrictor; pulmonary and peripheral vascular bed; pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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THE CARDIOVASCULAR EFFECTS of ATP were first reported by Drury and Szent-Gyorgyi in 1929 (12), and, since that early study, a great deal of information has been accumulated about responses to extracellular adenine nucleotides. In 1978, Burnstock (5) proposed that ATP acts on a distinct class of receptors called P2 receptors, whereas adenosine acts on P1 receptors. Subsequently, based on the rank order of activity of agonists on vascular and nonvascular smooth muscle, the P2 receptor was subdivided into P2X- and P2Y-receptor subfamilies (14, 25). P2X receptors are thought to be ligand-gated cation channels permeable to Na+, K+, and Ca2+ (13-18, 23, 25, 31, 33). There are seven cloned P2X receptors (1, 13, 23, 25). P2X1 receptors are located on vascular smooth muscle, and activation of P2X receptors most often causes vasoconstriction (15, 29). P2Y receptors are G-coupled receptors located on the endothelium and on vascular smooth muscle cells (4, 13, 25). Activation of endothelial P2Y receptors has been shown to lead to vascular smooth muscle relaxation (7, 16). The distribution of P2X and P2Y receptors may determine the properties of the response to a P2-receptor agonist in different regional vascular beds.
,-Methylene ATP (,-MeATP) is a degradation-resistant ATP
analog that has potent vasoconstrictor activity in the pulmonary vascular bed (10, 21, 22, 27). Pressor responses to
,-MeATP in the pulmonary vascular bed are inhibited by
pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), and
radioligand studies show that P2X receptors are localized in large and
small pulmonary arteries and veins (19, 22, 26). Responses
to ,-MeATP have been studied in rat tail arteries; femoral,
cerebral, and renal arteries; the perfused rat mesenteric vascular bed;
and saphenous veins (3, 9, 24, 28, 30, 34). P2X-receptor
antagonists have been developed, and PPADS has been shown to be an
antagonist at the endogenous P2X1 receptor and on
recombinant P2X1, P2X3, and P2X5 receptors (19, 24, 26, 35).
Although PPADS has been shown to selectively antagonize responses to
,-MeATP in the feline pulmonary vascular bed, little if anything
is known about responses to ,-MeATP and the effects of PPADS in
the systemic vascular bed in the cat. The purpose of the present study
was to compare responses to ,-MeATP in the pulmonary, mesenteric,
and hindquarter vascular beds and to investigate the effects of PPADS
on systemic vascular responses to the P2X agonist.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Ninety adult cats of either sex weighing 2.4-5.4 kg were sedated with ketamine hydrochloride (10-15 mg/kg im) and were anesthetized with pentobarbital sodium (30 mg/kg iv). Supplemental doses of pentobarbital were given, as needed, to maintain a uniform level of anesthesia. The trachea was cannulated, and the animals either breathed spontaneously or were ventilated with a Harvard model 607 ventilator at a volume of 40-60 ml at 15-22 breaths/min. An external jugular vein was catheterized for the intravenous (iv) administration of drugs, and a carotid artery was catheterized for the measurement of systemic arterial pressure.
Mesenteric preparation. For constant-flow perfusion of the mesenteric vascular bed, the superior mesenteric artery was approached through a midline abdominal incision and carefully cleared of surrounding tissue. The mesenteric vascular bed was denervated by ligating and cutting the perivascular nerves to the small intestine as they course along the superior mesenteric artery. After the administration of heparin sodium (1,000 U/kg), the femoral artery was cannulated and connected to the inlet side of the perfusion circuit. The outlet side of the perfusion circuit was connected to a catheter inserted into the superior mesenteric artery. Blood flow to the small intestine was maintained constant with a Sigmamotor model T-8 perfusion pump. Superior mesenteric arterial perfusion pressure was measured by way of a lateral tap in the perfusion circuit located between the pump and the outlet side of the perfusion circuit. Superior mesenteric arterial perfusion pressure and systemic arterial pressure were measured with Statham P23 pressure transducers and were recorded on a Grass model 7 polygraph. Mean pressures were derived by electronic averaging, and the perfusion rate was set so that superior mesenteric arterial perfusion pressure approximated systemic arterial pressure and was not changed during the experiment. The flow rate was determined by timed collection and ranged from 26-36 ml/min. The agonists used during these experiments were injected in small volumes (30 and 100 µl) and, in random fashion, directly into the superior mesenteric artery perfusion circuit, distal to the pump, as described previously (8).
Hindquarter preparation. For perfusion of the hindquarter vascular bed, a 3- to 4-cm segment of distal aorta was exposed through a ventral midline incision and was cleared of surrounding connective tissue by blunt dissection. After administration of heparin sodium (1,000 U/kg iv), the abdominal aorta was ligated, and catheters were inserted into the aorta proximal and distal to the ligature. Branches of the aorta distal to the origin of the external iliac arteries were ligated to restrict blood flow to the hindquarters. Blood was withdrawn from the proximal catheter and pumped at a constant flow rate with a Sigmamotor model T-8 pump into the distal aortic catheter. Hindquarter perfusion pressure and systemic arterial pressure were measured with Statham P23 transducers and were recorded on a Grass model 7 polygraph. Mean pressures were derived by electronic averaging, and the flow rate was set so that hindquarter perfusion pressure approximated systemic arterial pressure and was not changed during the experiment. The flow rate was determined by timed collection and ranged from 24 to 30 ml/min. Agonists were injected in small volumes and in a random sequence directly into the hindquarter perfusion circuit distal to the pump. The hindquarter vascular bed was denervated by ligating and cutting of the lumbar sympathetic chain ganglia between L3 and L4. These procedures have been described previously (2).
Pulmonary preparation. For studies in the pulmonary vascular bed, the animals were positioned in the supine position on a fluoroscopic table. The trachea was intubated with a cuffed pediatric endotracheal tube, and the animals spontaneously breathed room air enriched with 100% O2. Systemic arterial pressure was measured from a catheter inserted into the aorta from a femoral artery, and iv injections were made into a catheter positioned in the inferior vena cava from a femoral vein. For perfusion of the left lower lung lobe, a triple-lumen, 28-cm, 6-Fr balloon perfusion catheter was passed, under fluoroscopic guidance, from an external jugular vein through the pulmonary artery to the left lower lung lobe. After the animals had been heparinized (1,000 U/kg iv), the lobar artery was vascularly isolated by distension of the balloon cuff on the perfusion catheter. The lobe was perfused with a perfusion pump (model 1210, Harvard Instruments) by way of the catheter lumen beyond the cuff with blood withdrawn from a femoral artery. The perfusion rate was adjusted so that lobar arterial perfusion pressure approximated mean pressure in the main pulmonary artery and was not changed thereafter. The flow rate ranged from 28 to 45 ml/min. Left atrial pressure was measured with a radiopaque 6-Fr double-lumen catheter passed transseptally into the left atrium. Mean vascular pressures, measured with Spectromed DTX Plus transducers, zeroed at right atrial level and were recorded on a Grass model 7 polygraph. These procedures have been described previously (10, 11).
In the first set of experiments, responses to local injections of,-MeATP were compared in the pulmonary, mesenteric, and hindquarter vascular beds under constant-flow conditions. Dose-response curves for ,-MeATP, the thromboxane mimic U-46619, and
angiotensin II were compared in the three vascular beds, and doses of
the agonists were compared on a nanomole basis to take molecular weight into account. In these experiments, doses of the agonists were randomized, and sufficient time was permitted between agonist injections to ensure that tachyphylaxis did not occur. Control dose-response curves for ,-MeATP, angiotensin II, and U-46619 were similar or identical, indicating that tachyphylaxis did not occur
in these experiments.
In the second set of experiments, the role of P2X-receptor activation
in mediating responses to ,-MeATP was investigated in the
mesenteric and hindquarter vascular beds. Responses to ,-MeATP
were compared before and after administration of the P2X-receptor
antagonist PPADS (15 mg/kg iv). Administration of PPADS did not
significantly change systemic arterial, mesenteric, and hindquarter
perfusion pressure.
In the third set of experiments, the role of adrenergic receptors in
mediating responses to ,-MeATP was investigated in the mesenteric
and hindquarter vascular beds. Responses to ,-MeATP were
compared before and after administration of the -adrenergic-receptor antagonist phentolamine (1 mg/kg iv), and in some experiments a
dose-response curve for ,-MeATP was obtained before and after administration of phentolamine. Phentolamine decreased mesenteric perfusion pressure from 132 ± 7 to 101 ± 8 mmHg and
systemic arterial pressure from 123 ± 7 to 103 ± 6 mmHg and
decreased hindquarters perfusion pressure from 117 ± 11 to
92 ± 8 mmHg and systemic arterial pressure from 133 ± 15 to 107 ± 17 mmHg.
In the fourth set of experiments, the role of AT1-receptor
activation in mediating responses to ,-MeATP was investigated in
the mesenteric and hindquarter vascular beds. Responses to ,-MeATP were compared before and after administration of the angiotensin AT1-receptor antagonists losartan (1 mg/kg iv)
in the mesenteric vascular bed and candesartan (1 mg/kg iv) in the hindquarter vascular bed. The extent of AT1-receptor
blockade was similar with losartan and candesartan, because responses
to angiotensin II were reduced to a similar extent in both vascular beds, and, in additional experiments, losartan did not change the
response to ,-MeATP in the hindquarter vascular bed. In some
experiments, dose-response curves for ,-MeATP were compared before and after administration of the AT1-receptor
antagonists. Mesenteric and hindquarter perfusion pressures were not
altered by losartan or candesartan.
In the fifth set of experiments, the role of cyclooxygenase product
formation in modulating responses to ,-MeATP was investigated in
the mesenteric and hindquarter vascular beds. Responses to ,-MeATP were compared before and after administration of the cyclooxygenase inhibitor sodium meclofenamate (2.5 mg/kg iv), and in
some experiments dose-response curves for ,-MeATP were obtained
before and after administration of the cyclooxygenase inhibitor. Sodium
meclofenamate did not alter mesenteric or hindquarter perfusion pressure.
Drugs.
,-MeATP, sodium meclofenamate, phentolamine, and angiotensin II
(Sigma Chemical, St. Louis, MO) were dissolved in 0.9% NaCl solution.
PPADS tetrasodium (Research Biochemicals International, Natick, MA) and
losartan (Dupont-Merck, Wilmington, DE) were dissolved in 0.9% NaCl
solution. U-46619 (Upjohn, Kalamazoo, MI) was dissolved in 100%
ethanol at a concentration of 10 mg/ml and was diluted in 0.9% NaCl.
BAY K 8644 (Miles, New Haven, CT) was dissolved in a 1:4 solution of
cremophor EL and Tris and Tris · HCl (50 mM, pH 7.4). The
resulting suspension was warmed, and polyethylene glycol and Tris (pH
7.4) were added to make a stock solution that was stored (0°C) in a
brown bottle. Candesartan (Astra Zeneca) was dissolved in 1 N NaOH.
Working solutions of all agonists were prepared on a frequent basis,
stored in brown stoppered bottles, and kept on crushed ice during the
experiment. Vehicles for the drugs used in these studies did not
significantly alter baseline pressures or responses to the vasoactive
agents. All agonists were injected directly into the perfusion circuit
in small volumes in a random sequence, and sufficient time was
permitted between injections for perfusion pressure to return to
baseline values.
Statistical analysis. Responses were measured in absolute units (mmHg) and presented as means ± SE. The data were analyzed by using a one-way analysis of variance and Scheffé's F-test with a Bonferroni/Dunn correction factor or paired t-test. A P value of <0.05 was used as the criterion for statistical significance.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Responses to ,-MeATP in the pulmonary, hindquarter, and
mesenteric vascular beds.
Local injections of ,-MeATP into the lobar and hindquarter
perfusion circuits in the cat caused dose-related increases in perfusion pressure (Fig. 1, A
and B). In contrast, injections of ,-MeATP into the
mesenteric perfusion circuit resulted in a biphasic response
characterized by an initial increase in perfusion pressure followed by
a secondary smaller decrease in perfusion pressure (Fig.
1C). The time course of the changes in perfusion pressure in
response to ,-MeATP in the pulmonary, hindquarter, and mesenteric
vascular beds is compared in Fig. 2. The
increase in lobar arterial perfusion pressure in response to the 10-ng dose of ,-MeATP was rapid in onset, and pressure slowly returned toward the baseline value over a 350-s period (Fig. 2A). The
increase in hindquarter perfusion pressure in response to the 30-µg
dose of ,-MeATP was slow in onset and reached a peak ~50 s
after injection. Perfusion pressure returned to the baseline value
~250 s after injection (Fig. 2B). The increase in
mesenteric perfusion pressure in response to the 30-µg dose of
,-MeATP was rapid in onset and reached a peak in ~15 s.
Perfusion pressure then rapidly returned toward the baseline and
decreased below baseline value (Fig. 2C). The
secondary depressor response was slower in onset and decay (Fig.
2C). The response to the 30-µg dose of ,-MeATP in
the mesenteric vascular bed lasted ~270 s (Fig. 2C).
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,-MeATP were compared with responses to the
thromboxane A2 mimic U-46619 and to angiotensin II in the
pulmonary, hindquarter, and mesenteric vascular beds of the cat, and
these data are shown in Fig. 3. In the
pulmonary vascular bed, the order of potency was ,-MeATP > U-46619 > angiotensin II. The dose-response curve for
,-MeATP was ~0.5 log units to the left of the dose-response curve for U-46619 and ~1-1.5 log units to the left of the
dose-response curve for angiotensin II (Fig. 3A). In the
hindquarter vascular bed, the order of potency was angiotensin II > U-46619 > ,-MeATP. The dose-response curve for
,-MeATP was ~2 log units to the right of the curve for U-46619
and 3 log units to the right of the curve for angiotensin II (Fig.
3B). In the mesenteric vascular bed, the order of potency
was angiotensin II > U-46619 > ,-MeATP. The pressor
component of the dose-response curve for ,-MeATP was ~2 log
units to the right of the curve for U-46619 and ~3 log units to the
right of the curve for angiotensin II (Fig. 3C).
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Effect of PPADS.
The effect of the P2X-receptor antagonist PPADS on the response to
,-MeATP has been previously studied in the pulmonary vascular bed
of the cat by using a similar procedure, and, in that study, PPADS (15 mg/kg iv) was found to attenuate the pressor response to the
P2X-receptor agonist in a selective manner (22). In the
present study, the effects of PPADS in a dose of 15 mg/kg iv on
responses to ,-MeATP were investigated in the mesenteric and
hindquarter vascular beds, and these data are summarized in Figs.
4 and 5.
After administration of PPADS, the vasoconstrictor component of the
response to ,-MeATP was decreased significantly, and the
dose-response curve for the purinergic agonist was shifted to the right
in a parallel manner in the mesenteric vascular bed (Fig. 4). After
administration of PPADS, the magnitude of the secondary vasodilator
component of the response to the 30- and 100-µg doses of
,-MeATP was increased significantly in the mesenteric vascular
bed (Fig. 4). The duration of action of PPADS was short, and responses
to ,-MeATP returned toward control value 60 min after
administration of the antagonist (data not shown). The selectivity of
the inhibitory effects of PPADS on responses to ,-MeATP was assessed, and, in a dose of 15 mg/kg iv, the purinergic antagonist did
not alter vasoconstrictor responses to norepinephrine, BAY K 8644, and
U-46619 in the mesenteric vascular bed (Fig. 4).
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,-MeATP were significantly reduced, and the dose-response curve for the purinergic agonist was
shifted to the right in a parallel manner (Fig. 5). In a manner similar to that observed in the mesenteric vascular bed, the duration of the
inhibitory action of PPADS was short, and responses to ,-MeATP
returned toward control value 60 min after administration of the
antagonist (data not shown). PPADS did not alter responses to
norepinephrine, BAY K 8644, or U-46619 in the hindquarter vascular bed
(Fig. 5).
Role of AT1 and -adrenergic receptors and the
cyclooxygenase system.
The role of angiotensin AT1 and -adrenergic receptors in
mediating responses to ,-MeATP was investigated in the mesenteric and hindquarter vascular beds, and these data are summarized in Figs.
6 and 7.
Responses to ,-MeATP were not altered by the AT1-receptor antagonist losartan in the mesenteric vascular
bed or candesartan in the hindquarter vascular bed in a dose of the AT1 antagonists (1 mg/kg iv) that significantly inhibited
responses to angiotensin II (Fig. 6). The AT1-receptor
antagonists did not alter responses to U-46619 in both regional
vascular beds (data not shown).
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-adrenergic receptors in mediating responses to
,-MeATP was investigated in experiments with phentolamine (1 mg/kg iv) in the mesenteric and hindquarter vascular beds, and these
data are summarized in Fig. 7. After administration of phentolamine, responses to ,-MeATP were not altered at a time when
vasoconstrictor responses to norepinephrine were decreased
significantly (Fig. 7). Responses to U-46619 were not changed after
treatment with phentolamine (data not shown).
The role of cyclooxygenase product formation in modulating responses to
,-MeATP was investigated in the mesenteric and hindquarter vascular beds, and these data are summarized in Fig.
8. After administration of sodium
meclofenamate (2.5 mg/kg iv), responses to ,-MeATP were not
altered in the hindquarter and mesenteric vascular beds of the cat
(Fig. 8). The vasodilator response to the prostaglandin precursor
arachidonic acid was decreased significantly after administration
of the cyclooxygenase inhibitor (Fig. 8). Vasodilator responses
to acetylcholine were not changed in the mesenteric or hindquarter
vascular bed after administration of the cyclooxygenase inhibitor
(data not shown).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Results of the present study show that the selective P2X-receptor
agonist ,-MeATP increases perfusion pressure in the pulmonary and
hindquarter vascular beds and produces biphasic changes in perfusion
pressure in the mesenteric vascular bed. Inasmuch as blood flow was
maintained constant, the changes in perfusion pressure directly reflect
changes in vascular resistance in the three vascular beds studied.
Although vasoconstriction was observed in the pulmonary and hindquarter
vascular beds and a biphasic response was observed in the mesenteric
circulation, the response to ,-MeATP also differed with regard to
relative pressor activity compared with the responses to the
thromboxane A2 mimic U-46619 and to angiotensin II in the
pulmonary and systemic vascular beds. In the mesenteric vascular bed,
analysis of the dose-response curve for the pressor component of the
response to ,-MeATP showed that the purinergic agonist was
1,000-fold less potent than angiotensin II and 100-fold less potent
than U-46619. In terms of relative vasoconstrictor activity and in the
hindquarter vascular bed, ,-MeATP was 1,000-fold less potent than
angiotensin II and 100-fold less potent than U-46619. In the pulmonary
vascular bed, ,-MeATP was the most potent agonist and was
100-fold more potent than angiotensin II and 30-fold more potent than
U-46619 in increasing pulmonary vascular resistance. The results of
these comparative studies show marked differences in vasoconstrictor
activity in the regional vascular response to the P2X agonist
,-MeATP compared with responses to angiotensin II and the
thromboxane A2 mimic U-46619. The present study
demonstrates that ,-MeATP is one of the most potent
vasoconstrictor agents in the pulmonary vascular bed while having
modest vasoconstrictor activity in the systemic vascular bed. The
reason for the marked difference in relative pressor activity is
unknown but may reflect differences in the density of P2X receptors in
each bed. The comparison of P2X-receptor density using radioligand
techniques to quantify receptors in small arteries in three vascular
beds would provide additional mechanistic information in the present
study. The potent pressor response in the pulmonary circulation
compared with U-46619 and angiotensin II suggests a potential role for
P2X-receptor activation in pathophysiological conditions, such as
chronic hypoxia, acute respiratory distress syndrome, and pulmonary
hypertension. The role of P2X-receptor activation in pathophysiological
conditions may be important, and information can be obtained by
examining the effects of PPADS in experimental models of conditions,
such as the acute respiratory distress syndrome.
The purinergic receptor subtype and mechanism by which ,-MeATP
alters vascular resistance were investigated in the mesenteric and
hindquarter vascular beds. The vasoconstrictor response to ,-MeATP in the hindquarter circulation and the vasoconstrictor component of the biphasic mesenteric response were attenuated by PPADS,
a P2X-receptor antagonist (19, 25, 26, 35). The
dose-response curves for ,-MeATP were shifted to the right in a
parallel manner in both beds, indicating that the antagonism was
competitive. PPADS has been shown to block P2X1,
P2X2, P2X3, P2X5, and
P2Y1 receptors; however, the P2X1 receptor is
the major P2X-receptor subtype on vascular smooth muscle (23,
25). P2X1 receptors usually mediate
vasoconstriction, and the selectivity of the inhibitory effect of PPADS
on the response to ,-MeATP was assessed by investigating the
actions of the P2X-receptor antagonist on responses to agonists that
change vascular resistance by various mechanisms (25, 29,
32). The results of these studies show that PPADS did not alter
vasoconstrictor responses to norepinephrine, BAY K 8644, or U-46619 in
the mesenteric and hindquarter vascular beds. The results of these
studies suggest that increases in mesenteric and hindquarter vascular
resistance in response to ,-MeATP are mediated by the activation
of P2X1 receptors. These data are consistent with a
previous report showing that ,-MeATP induces vasoconstriction in
the cat intestine by activating P2X receptors (31). It has
also been reported that ,-MeATP causes dose-dependent
vasoconstriction by activation of P2X1 receptors in the
isolated perfused pulmonary vascular bed of the rat, and these
responses were blocked by PPADS (27). Additionally, the
distribution of P2X receptors in the cat lung has been studied by using
autoradiographic techniques (22). The results of
these studies show that P2X receptors are expressed in high density in
large and small pulmonary arteries, as well as in pulmonary veins
(22). In that study, with the use of a procedure similar
to the technique used in the present study, pulmonary vasoconstrictor
responses to ,-MeATP in the cat were attenuated in a selective
manner by PPADS in a dose of 15 mg/kg iv and were not altered by
cyclooxygenase inhibitors, -adrenergic-receptor blocking agents, or
histamine and serotonin blocking agents (22). Responses to
,-MeATP have been studied in careful detail in the pulmonary
vascular bed in the cat and, in a manner similar to that observed in
the hindquarter and mesenteric vascular beds, appear to be mediated in
a direct manner (10, 21, 22).
The role of cyclooxygenase product release and activation of
-adrenergic or angiotensin AT1 receptors in mediating or
modulating responses to ,-MeATP was investigated in the
mesenteric and hindquarter vascular beds. Responses to ,-MeATP
were not altered by the cyclooxygenase inhibitor sodium meclofenamate
in a dose that attenuated the response to the prostaglandin precursor
arachidonic acid or by phentolamine in a dose that reduced the pressor
response to norepinephrine. These results suggest that responses to
,-MeATP are not mediated by or modulated by the release of
cyclooxygenase products or the activation of -adrenergic receptors
in the two vascular beds. The observation that mesenteric and
hindquarter responses to ,-MeATP were not altered by losartan and
candesartan in a dose that attenuated responses to angiotensin II
suggests that activation of AT1 receptors is also not
involved in mediating responses to the purinergic agonist. Previous
studies have shown the involvement of these mechanisms in mediating or
in modulating responses to purinergic agonists in other organ systems
(4, 6, 19, 20, 24). The reason for the difference in the mechanism of action to ,-MeATP in these studies is unclear but may involve differences in the regional vascular bed or species studied.
In summary, results of the present study show that responses to the
P2X-receptor agonist ,-MeATP differ in the three vascular beds
studied in the cat. In the pulmonary vascular bed, ,-MeATP had
potent vasoconstrictor activity, whereas angiotensin II had modest
pressor activity. In the systemic vascular bed, the P2X-receptor agonist had modest pressor activity. In the mesenteric circulation, ,-MeATP produces a biphasic response with an initial
vasoconstrictor component and a secondary vasodilator component. In the
systemic vascular bed, ,-MeATP was far less potent than
angiotensin II or the thromboxane A2 mimic U-46619 in
increasing vascular resistance. Pressor responses to ,-MeATP are
antagonized in a selective manner by the P2X-receptor antagonist PPADS
in the mesenteric and hindquarter vascular bed. Responses to
,-MeATP were not altered by meclofenamate, phentolamine, or
AT1-receptor blocking agents, suggesting that the release
of cyclooxygenase products and the activation of -adrenergic or
AT1 receptors are not involved in mediating or modulating
responses in the mesenteric and hindquarter circulation. The results of
the present studies in the systemic vascular bed and previous studies
in the pulmonary vascular bed of the cat are consistent with the
hypothesis that ,-MeATP induces vasoconstriction by directly
activating a PPADS-sensitive P2X receptor and that the P2X-receptor
density or distribution may differ in different regional vascular beds.
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ACKNOWLEDGEMENTS |
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The authors thank Janice Ignarro for editorial assistance.
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FOOTNOTES |
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This study was supported in part by National Institutes of Health Grants HL-62000 and DK-44628, a grant from the American Heart Association Southeast Affiliate, and American Heart Association Grant 95001370. E. W. Inscho is an Established Investigator of the American Heart Association.
Present address of E. W. Inscho: Department of Physiology, Medical College of Georgia, Augusta, GA 30912.
Address for reprint requests and other correspondence: P. J. Kadowitz, Dept. of Pharmacology SL83, Tulane Univ. School of Medicine, 1430 Tulane Ave., New Orleans, LA 70112 (E-mail: pkadowi{at}tulane.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/japplphysiol.00262.2002
Received 27 March 2002; accepted in final form 12 June 2002.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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