© 1996 American Heart Association, Inc.
Articles |
Adenosine-Induced Vasoconstriction In Vivo
Role of the Mast Cell and A3 Adenosine Receptor
From the Department of Molecular Physiology and Biological Physics, University of Virginia School of Medicine, Charlottesville.
Correspondence to Dr Brian R. Duling, Department of Molecular Physiology and Biological Physics, Box 449, University of Virginia School of Medicine, Charlottesville, VA 22908. E-mail brd@dayhoff.med.virginia.edu.
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Abstract |
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Abstract Adenosine, a vasodilator metabolite, is often produced in tissues where the demand for oxygen exceeds the supply. We have recently demonstrated in isolated cannulated arterioles that adenosine and its metabolite, inosine, can also cause vasoconstriction by stimulation of mast cells. Secondary release of histamine and thromboxane is responsible for the inosine-induced constriction in vivo. In the present study, we explored the vasomotor effects of adenosine in vivo and investigated the role of the A3 adenosine receptor in mediating vasoconstriction. In vivo, local application of adenosine (10-6 to 10-4 mol/L) to arterioles consistently caused dose-dependent vasodilation. A fraction of arterioles, however, exhibited a biphasic response, with constriction following dilation. This, too, was dose dependent; 37% of arterioles constricted by 12.7±4.3% of the initial diameter in response to 10-4 mol/L adenosine. In the presence of 8-(p-sulfophenyl)theophylline (8-SPT), an antagonist of A1 and A2 adenosine receptors, dilation in response to the same dose of adenosine was reduced, and constriction was enhanced; 85% of the tested arterioles constricted by -44.3±6.0% of the initial diameter. The A3 adenosine receptor has been shown to facilitate mediator release from mast cells, and its role was also examined. N6-(3-Iodo-4-aminobenzyl)adenosine (I-ABA), an agonist of A1 and A3 adenosine receptors, produced dose-dependent vasoconstriction. 1,3-Dipropyl-8-(4-acrylate)phenylxanthine (BW-A1433), an antagonist of A1, A2, and A3 receptors, significantly reduced the vasoconstrictor response to adenosine, which was unmasked during treatment with 8-SPT. In addition, both adenosine and I-ABA stimulated mast cell uptake of ruthenium red, indicating degranulation. The I-ABA–induced constriction was abolished by combined histamine and thromboxane receptor antagonists. We conclude that adenosine can cause vasoconstriction in vivo, which is often masked by A2 receptor–mediated vasodilation. Mast cells are stimulated in the course of the response, and the A3 adenosine receptor is involved in mediating constriction.
Key Words: ischemia • hamsters • arterioles • microcirculation • allergy
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Introduction |
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TopAbstractIntroductionMaterials and Methods Results Discussion References |
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Adenosine is a by-product of ATP metabolism that is released from cells into the surrounding tissue when oxygen demand exceeds supply.1 Adenosine is believed to be an important component in the metabolic regulation of blood flow in the coronary and cerebral circulations by acting at A2 adenosine receptors on vascular smooth muscle to cause vasodilation.2 3 Under normal conditions, cardiac interstitial levels of adenosine are on the order of 0.2 µmol/L, but during hypoxia or ischemia, interstitial concentrations may rise threefold.4
Previous studies in this laboratory have demonstrated that in addition to direct A2 receptor–mediated vasodilation, adenosine causes vasoconstriction of isolated perfused arterioles from the hamster cheek pouch.5 During cumulative dose-response curves, the adenosine-induced constriction occurred at a concentration of 10-6 mol/L, with higher concentrations causing vasodilation. Furthermore, the constriction was tachyphylactic and originated at focal sites along the vessel. Inosine, a metabolite of adenosine, also caused vasoconstriction, apparently by a similar mechanism. The response to adenosine and inosine was found to be due to stimulation of periarteriolar mast cells and the subsequent release of a vasoconstrictor. Further studies in vivo revealed that the arteriolar constriction resulting from mast cell activation induced by inosine was the result of histamine and thromboxane release.6
Mast cells have been found in most organs of the body, including the heart, brain, lungs, and kidneys.7 8 9 Their distribution around blood vessels10 and their ability to secrete numerous mediators suggests that they are well positioned to exert effects on blood flow as well as on venous permeability and leukocyte recruitment. Nucleoside-induced mast cell degranulation may also play a role in a variety of pathophysiological situations. As noted above, adenosine and inosine are produced in ischemic tissues, and stimulation of mast cells may contribute to the pathology of ischemic injury. In addition, adenosine has been shown to play a role in allergy and asthma,9 11 which are known to involve mast cells.
The mechanism by which adenosine elicits mast cell degranulation and arteriolar constriction has not been completely explored. Doyle et al5 invoked a role for nucleoside uptake and metabolism in mediating the constriction. In addition, it was suggested that a non–A1/A2 adenosine receptor (insensitive to blockade by 100 µmol/L 8-(p-sulfophenyl)theophylline [8-SPT]) could be involved, but this possibility has not been examined. A rat A3 adenosine receptor has recently been cloned and characterized. This receptor has been shown to be present on mast cells and to facilitate release of allergic mediators.12 We hypothesized that A3 adenosine receptor occupation contributed to mast cell stimulation and arteriolar constriction. In the present study, we characterize the vasomotor response to adenosine in vivo and explore the role of adenosine receptors, including the A3 receptor, in mediating the constrictor response.
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Materials and Methods |
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In Vivo Microscopy of the Hamster Cheek Pouch
All protocols were approved by the University of Virginia Animal Care and Use Committee. Hamster cheek pouches were everted and prepared for in vivo microscopy as previously described.13 Briefly, male Golden hamsters (average weight,
130 g) were
anesthetized with pentobarbital sodium (Nembutal, 70 mg/kg IP),
and a tracheal cannula was inserted. The left femoral vein was
cannulated for infusion of anesthetic in normal saline (10 mg/mL) and
for fluid replacement (0.4 mL/h). Animal temperature was maintained at
37°C using convective heating. The cheek pouch was exteriorized on a
Plexiglas pedestal and cleaned of adherent connective tissue. Cheek
pouches were superfused with a bicarbonate-buffered saline solution
at 37°C that contained (mmol/L) NaCl 131.9, KCl 4.7,
CaCl2 2.0, MgSO4 1.2, and NaHCO3
20.0. The superfusion solution was gassed with 5%
CO2/95% N2. After 30 minutes of
equilibration, microvessels were observed using a Nikon Optiphot
microscope equipped with a Leitz x50 objective (numerical aperture,
0.60). Images were projected to a Dage-MTI video camera with a
Newvicon tube, displayed on a video monitor, and recorded using a
Panasonic Omnivision II videocassette recorder (model NV-8950).
Source of Drugs
1,3-Dipropyl-8-(4-acrylate)phenylxanthine
(BW-A1433) and
N6-(3-iodo-4-aminobenzyl)adenosine
(I-ABA) were gifts from Burroughs Wellcome Co. SQ29,548 was obtained
from Cayman Chemicals. 8-SPT was obtained from Research Biochemicals
Inc, and ruthenium red was obtained from Polyscience Inc. All other
products were obtained from Sigma Chemical Co.
Drug Application
Test agents (adenosine, I-ABA) were applied
to discrete
sites along an arteriole by using glass micropipettes. We refer to this
as local application to distinguish it from application of a drug or
other substance in the superfusion solution (global application). Drugs
to be applied locally were diluted from stock solutions into
MOPS-buffered saline (mmol/L: NaCl 131.9, KCl 4.7, CaCl2
2.0, MgSO4 1.2, and MOPS 10.0, pH 7.4) and loaded into
glass micropipettes (tip diameter, 5 µm). Pipettes were placed 25
µm from the arteriolar wall, and drugs were ejected with a
Picospritzer (Picospritzer II, General Valve Corp) using 100% nitrogen
at a pressure of 20 psi. The pulse duration was adjusted roughly in
proportion to vessel diameter (range, 10 to 70 milliseconds).
Ruthenium red, 8-SPT, SQ29,548, diphenhydramine, and BW-A1433 were applied globally by inclusion in the superfusion solution. BW-A1433 was prepared as a 100 mmol/L stock solution in 500 mmol/L NaOH and was diluted 1:40 in 0.9% NaCl. The drug was delivered at a flow rate of 0.05 mL/min, through a side port, into the superfusate (flowing at 5 mL/min) using a syringe pump (model 355, Sage Instruments) to yield a final concentration on the cheek pouch of 25 µmol/L.
Ruthenium Red
We used mast cell uptake of ruthenium red as a
marker of
degranulation.14 15 Cheek pouches were superfused
with
0.001% ruthenium red in bicarbonate-buffered saline for at least
20 minutes before any stimulus. Arterioles were stimulated locally with
adenosine or I-ABA, and dye uptake into mast cells near the
point of drug application was monitored by videotaping images at
1-minute intervals after the stimulus. Images were obtained by
averaging 16 frames from the videotape at the following time points:
immediately before stimulation, during the vasomotor response, and at
the 6-minute time point (Image 1, Universal Imaging). Images were
subsequently processed and printed using Adobe Photoshop (Adobe Systems
Inc) and Designer (Micrografx).
Analysis of Diameter Changes
Changes in arteriolar diameter
were measured from the videotape
using a video caliper (Colorado Video). Resting diameter of arterioles
ranged from 8 to 70 µm (mean, 29 µm). Local application of
adenosine to arterioles in the hamster cheek pouch results in
consistent dilation and occasional constriction (see Fig 1
and
"Results"). To thoroughly examine the constrictor component of
the response to adenosine, we developed the following protocol
to assess diameter changes. Adenosine or I-ABA was applied
locally to cheek pouch arterioles, and two parameters were
measured. First, we noted the percentage of tested vessels that
constricted, ie, the "incidence" of constriction [(number
of vessels that constricted/number of vessels tested)x100].
Second, we determined the magnitude of the diameter change, both the
initial dilation and the subsequent constriction, measured as the
difference between the initial vessel diameter and the peak change and
expressed as a percentage of initial diameter [(change in luminal
diameter in micrometers/initial diameter in
micrometers)x100]. Given the biphasic nature of some
vasomotor responses, dilator and constrictor responses were considered
separately, and arterioles that did not respond were given a value of
0% constriction or dilation. Furthermore, because of the tachyphylaxis
in the constrictor response,5 6 single sites along
arterioles were tested only once.
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Statistical Analysis
Statistical analyses were carried out
using Statgraphics
statistics software (Jandel Scientific). The fractional incidence of
constriction was determined, and control groups were compared with the
treated groups using a z test. The magnitude of the response
was analyzed using a Mann-Whitney rank-sum test. In all
comparisons and tests, P<.05 was considered significant.
Data are expressed as mean±SEM.
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Results |
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Vasomotor Response to Local Application of Adenosine In Vivo
We first determined the vasomotor response to local application of adenosine in vivo. Three traces typifying the diameter changes following adenosine application are shown in Fig 1
. Trace A
shows a simple monotonic increase in diameter
typical of the arteriolar response to adenosine in most control
arterioles. Trace B depicts a second type of response seen less
commonly, in which a biphasic response consisting of vasodilation
followed by constriction was observed.
To quantify the constrictor
component of the vasomotor response to
adenosine, we applied adenosine
(10-6 to
10-4 mol/L) to a number of arterioles and
determined the percentage that constricted (Fig 2A).
There was a dose-dependent increase in the incidence of
constriction ranging from 20% at a dose of
10-6 mol/L to 37% at a dose of
10-4 mol/L. In addition, we determined
the peak magnitude of both the dilation and the constriction (Fig
2B).
As expected, adenosine application produced dose-dependent
vasodilation, but it also resulted in dose-dependent
vasoconstriction. Using 10-6 mol/L
adenosine, we observed 1.3±0.6% constriction, whereas
10-4 mol/L adenosine produced a
12.7±4.3% decrease in diameter.
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Role of the A3 Adenosine Receptor
Studies
With 8-SPT
We hypothesized that the direct vasodilator effects of
adenosine were competing with the vasoconstrictor response.
Since previous studies indicated that neither A1 nor
A2 adenosine receptors were involved in the
adenosine-induced constriction but were responsible for
vasodilation,5 we used an
A1/A2 receptor antagonist,
8-SPT, to block vasodilation and tested the response to
adenosine (10-4 mol/L) in the
manner described above. As shown in Fig 2, in the presence of
8-SPT
(10-4 mol/L), both the incidence and
magnitude of constriction increase markedly. The incidence of
constriction increased from 37% to 85%, and the magnitude of
constriction increased from 12.7±4.3% to 44.3±6.0%. Vasodilation
was reduced from 33.7±3.0% to 21.0±3.8% (P<.05),
presumably because of antagonism of A2 adenosine
receptors on vascular smooth muscle. A typical trace of the diameter
change in response to adenosine stimulation during treatment
with 8-SPT is shown in Fig 1C.
Studies With
I-ABA
The preceding data suggested that A1 and
A2 adenosine receptors were not involved in
mediating the constriction and that, in fact, blocking
A1/A2 receptors increased the
constriction. Therefore, we decided to explore the role of the recently
characterized A3 adenosine receptor, which has been
shown to be present on mast cells and to facilitate release of
allergic mediators.12 We first examined the vasomotor
response to varying concentrations of an
A1/A3 receptor agonist, I-ABA (Fig 3).
Derivatives of adenosine substituted at the
N6 position with iodobenzyl, such as I-ABA, are potent
agonists of A1 and A3 adenosine
receptors.16 17 Failure by 100 µmol/L 8-SPT to
block the
constrictor response indicated that the constriction was not mediated
by A1 receptors. Therefore, I-ABA was used to stimulate the
A3 adenosine receptor. Local application of I-ABA
produced dose-dependent vasoconstriction; both the incidence (Fig
3A) and magnitude (Fig 3B) of constriction
increased over a range of
I-ABA concentrations (0.1 to 30 µmol/L). At each concentration of
I-ABA, we also noticed a slight consistent vasodilation (8%
of initial diameter at each dose, not shown), which may be due to
slight activity of the I-ABA at an adenosine A2
receptor.
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Studies With BW-A1433
We next examined
the response to adenosine in the presence
of an adenosine receptor antagonist, BW-A1433.
Although the rat A3 receptor has been called xanthine
resistant, high concentrations of certain xanthines will block
the receptor; BW-A1433 has been reported to bind to rat A3
receptors with a Ki value of 15
µmol/L.18 In the present study, BW-A1433 (25
µmol/L) was superfused over the cheek pouch, as described, and the
vasomotor response to adenosine and I-ABA was measured. In
order to unmask the adenosine-induced constriction, the
following experiments were performed during superfusion of 8-SPT
(10-4 mol/L). To test the efficacy of the
antagonist, we examined the response to I-ABA (5 µmol/L)
and found that 25 µmol/L BW-A1433 reduced the constriction from
63.6±6.5% to 37.0±9.4%. Additionally, the
adenosine-induced constriction was significantly reduced by
BW-A1433 (Fig 4). The incidence of constriction was
reduced from 85% to 51% (Fig 4A), and the magnitude went from
-41.9±5.6% to -21.5±4.5% (Fig 4B).
The dilation
remained unchanged. The constrictor response to adenosine did
not change during superfusion with the vehicle for BW-A1433 (data not
shown).
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Role of the Mast Cells
A role for mast cells in
nucleoside-induced constriction has
been established both in vitro5 and in vivo.6
We decided to address the issue of mast cell involvement in the
response to adenosine in vivo and to further examine the role
of A3 adenosine receptors in mediating this
response by using ruthenium red as a marker of mast cell
degranulation14 15 ; results are shown in Fig
5. Ruthenium red (0.001%) was superfused for at least
20 minutes before any stimulus. Arterioles were treated with
adenosine (10-4 mol/L) or I-ABA
(60 µmol/L), and mast cell uptake of ruthenium red was assessed. Fig
5 (panels A through C) shows data obtained during treatment
with
adenosine. Fig 5A shows the arteriole immediately before
adenosine stimulation. In Fig 5B, adenosine was applied
via micropipette, and the arteriole constricted, as shown. Finally,
mast cell staining with ruthenium red increased, which indicates mast
cell activation (Fig 5C). In the group of arterioles tested for
ruthenium red uptake, adenosine caused both constriction and
dilation, as described above. In all cases, however, after
adenosine application, mast cells stained with ruthenium red
(n=5). Panels D through F of Fig 5 were obtained during
treatment with
the A1/A3 adenosine agonist
I-ABA. Fig 5D shows an arteriole immediately before I-ABA
stimulation.
I-ABA was applied via micropipette, the arteriole constricted (Fig
5E),
and mast cell staining with ruthenium red was observed (Fig 5F,
n=5).
In Fig 5, filled arrows track examples of mast cells that
degranulate
and stain with ruthenium red; open arrows track cells that do not stain
or degranulate.
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To further examine the role for mast cells in eliciting
constriction, the mediators responsible for
I-ABA–induced constriction were examined. We have previously
shown that mast cells are involved in inosine-induced
constriction and that combined histamine and
thromboxane antagonists abolished this
constriction.6 We tested the response to I-ABA (3
µmol/L) in the absence and presence of combined histamine (20
µmol/L diphenhydramine) and thromboxane (10 µmol/L
SQ29,548) blockade, as described previously.6 As shown in
Fig 6, the incidence of constriction was reduced from
91% to 19% (P<.001 versus control), and the magnitude was
reduced from 50.4±8% to 1.5±0.9% (P<.0001 versus
control).
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Discussion |
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TopAbstractIntroductionMaterials and Methods Results Discussion References |
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In the present study, we report that adenosine often causes vasoconstriction of hamster cheek pouch arterioles in vivo by a response that involves mast cell activation. It appears that this constriction is frequently masked by direct vasodilator effects on vascular smooth muscle. Furthermore, BW-A1433, a compound capable of blocking A3 receptors, attenuates the adenosine-induced constriction. I-ABA, an A3 agonist, mimics the constriction, which is mediated by histamine and thromboxane, and causes mast cell degranulation, supporting the hypothesis that the A3 adenosine receptor is involved in mediating this response.
In a previous report, we described and characterized a constriction elicited by adenosine in isolated perfused hamster cheek pouch arterioles.5 The constriction was found to be due to stimulation of periarteriolar mast cells. The first objective of the present study was to determine whether this effect was present in vivo as well. Our initial studies showed that constriction did occur, but not in every arteriole. Further examination of the response in a large number of arterioles showed that although not all arterioles constricted, there was a dose-dependent increase in both the percentage of arterioles that constricted and the degree of constriction.
In studies using adenosine with and without 8-SPT, we found
that direct A2 adenosine receptor–mediated
dilation was competing with constriction (Fig 2). Thus, in
vivo,
adenosine appears to initiate multiple conflicting vasomotor
signals that are integrated in the vascular smooth muscle. Under normal
circumstances, dilation predominates, but when dilation is prevented, a
constriction is unmasked. Previous studies indicate that the
constrictor response to adenosine and inosine exhibits
tachyphylaxis.5 6 Given such tachyphylaxis and the
predominant adenosine-induced vasodilation, it would be
expected that constriction would not be evident in most experimental
situations using adenosine.
In addition, these data implicate a mechanism that does not involve A1/A2 adenosine receptors. Of the adenosine receptors that have been cloned, rat A3 receptors can be distinguished from the other subtypes by their low affinity for xanthine antagonists such as 8-SPT. 8-SPT competes for radioligand binding to rat A1 and A2a receptors with Ki values of <1 µmol/L.18 In fact, the inability of 8-SPT to block adenosine-mediated responses has been used as evidence that A3 receptors mediate hypotension in the angiotensin II–supported circulation of the pithed rat.19 Failure of 100 µmol/L 8-SPT to block the constrictor response is consistent with the hypothesis that the response is mediated by A3 receptors exhibiting pharmacological properties similar to those of the rat A3 receptor.
Additional pharmacological tools were used to examine the role of the
A3 adenosine receptor in mediating constriction in
vivo. We found that the A1/A3 receptor
agonist I-ABA elicited both constriction and mast cell degranulation,
as demonstrated by ruthenium red staining and constrictor blockade by
histamine and thromboxane receptor antagonists.
Constriction was observed even in the presence of 8-SPT, suggesting
that I-ABA was acting at an A3 adenosine receptor.
Mast cells are the primary tissue storage site for histamine;
therefore, implication of histamine in mediating constriction provides
a link between A3 adenosine receptor activation and
mast cell degranulation. Furthermore, during treatment with the
nonselective adenosine antagonist BW-A1433, the
adenosine-induced constriction was attenuated. We observed
50% reduction in adenosine-induced constriction using
25 µmol/L BW-A1433. Given the Ki value of 15
µmol/L for the rat A3 receptor,18 these data
further support a role for the A3 adenosine
receptor in mediating constriction.
Together, the data are highly suggestive of a role for the A3 adenosine receptor in mediating mast cell degranulation and arteriolar constriction. However, there are limitations of pharmacological experiments. In the rat, there are few compounds that are highly selective for the A3 adenosine receptor.18 Although the data do not support involvement of known adenosine extracellular receptors, it is possible that the effects observed in the present study are the result of an unidentified adenosine receptor. More definitive proof for the involvement of the A3 receptor will require more selective reagents that are not available at the present time.
Adenosine has been shown by others to have a variety of effects on mast cells in vitro. It is capable of either potentiating or inhibiting mediator release from mast cells, but potentiation or inhibition seems to depend on a number of variables, including time of application and cell type.9 20 21 22 23 24 Both intracellular actions and membrane receptor sites have been proposed to account for the potentiating effects of adenosine on mast cell mediator release,9 23 and A2 adenosine receptors have been proposed to inhibit mast cell degranulation.9 24 It is possible that part of the effect of antagonizing A1/A2 receptors and unmasking constriction was due to blocking an inhibitory signal to mast cells. The A3 adenosine receptor has been shown to facilitate release of mediators in rat basophilic leukemia cells (RBL-3H3), a cultured mast cell–like cell line.12
A common observation of in vitro studies is that adenosine alone does not stimulate mast cells; rather, it enhances mast cell secretion in response to another stimulus, such as calcium ionophore or antibody cross-linking of IgE. In the present study, we observed mast cell stimulation in vivo after treatment with adenosine or I-ABA alone. The present data do not establish whether this was an effect of adenosine on mast cells alone or a secondary effect due to stimulation of other cell types within the preparation. However, given that mast cells do appear to be involved in the constrictor response5 and that in the hamster cheek pouch, mast cells have a preferential distribution along arterioles of all sizes,25 the most simple hypothesis is that mast cells alone are sufficient to respond to adenosine and release mediators that cause vasoconstriction. If the mast cell is the only cell type necessary to respond to adenosine and initiate constriction, then the present data suggest that there is an additional signal in vivo to allow mast cell stimulation by adenosine. Such a signal might be constitutively present in the preparation or might be generated as a result of adenosine application.
A wide array of substances and conditions are stimulatory to mast cells. Mast cells in vivo are exposed to various cytokines, as well as nerves and fibroblasts, and are embedded in a network of extracellular matrix, any of which might enhance releasability in response to adenosine.26 27 28 In addition, changes associated with surgical preparation of the cheek pouch, including physical trauma, as well as induction of slight inflammation could also influence the response of mast cells to adenosine.29 Any of these stimuli might be present at a subthreshold level that is not sufficient to stimulate mast cells, but in the presence of adenosine, mast cell activation occurs. Another possibility is that application of adenosine could generate a stimulatory signal and also activate the A3 adenosine receptor, which might result in enhanced release of mast cell mediators. The second hypothesis suggests that adenosine actually generates two signals that are stimulatory for mast cells, and this hypothesis is attractive for the following reasons. First, previous studies indicate that uptake and metabolism of adenosine are necessary to cause vasoconstriction in isolated arterioles,5 and uptake of adenosine has been proposed as a mechanism by which adenosine facilitates mast cell mediator release.23 30 Second, inosine, a metabolite of adenosine, elicits a similar response to adenosine both in vivo and in vitro.5 6
Metabolism of adenosine and inosine can result in generation of superoxide radicals by the enzyme xanthine oxidase.31 Superoxide and hydrogen peroxide, which is produced during dismutation of superoxide, stimulate mast cells in vivo and in vitro.32 33 It is possible that A3 adenosine receptor occupation facilitates mast cell degranulation induced by oxygen reactive species and that both of these signals could come from adenosine. Such a hypothesis consolidates the data suggesting that uptake and metabolism of adenosine are required to cause constriction5 with the data suggesting a role for the A3 adenosine receptor. Further experiments will be required to more fully understand which factors contribute to stimulating mast cell degranulation in vivo.
The data reported in the present study indicate that dilation is the predominant response to adenosine. However, we have observed that there are conditions under which constriction can occur and that the occurrence of constriction increases with adenosine concentration. During situations of hypoxia, interstitial concentrations of both adenosine and inosine rise, increasing the likelihood of vasoconstriction, since both have been shown to cause constriction in vivo.6 A vasoconstriction would further reduce blood flow to the hypoxic tissue, and such a response would be more damaging to already ischemic tissues. Perhaps more significantly, we established that even when arterioles respond to adenosine by dilating, mast cell stimulation still occurs; therefore, numerous mediators are certainly released.34 These mediators would be expected to initiate a cascade of events, including recruitment of leukocytes, ultimately contributing to an inflammatory response.34
Adenosine stimulation of mast cells in vivo has several interesting physiological and pathophysiological implications. Adenosine is present in ischemic tissue, and our data suggest that because mast cells may be stimulated by adenosine in vivo, they could serve as a tissue sensor of decreased oxygen. The mediators released by mast cells can exert long-term effects on the tissue microenvironment. In such a scenario, mast cell activation could contribute to the pathology seen during ischemia/reperfusion injury in the heart. In this phenomenon, reperfusion after ischemia leads to neutrophil accumulation and inflammatory tissue damage.31 It is possible that mast cells play a role in the initiation of this response. In a more positive role, mast cells have been shown to participate in angiogenesis.35 The ability to sense the level of tissue oxygen via stimulation by adenosine may be important to the mast cell contribution during angiogenesis.
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Acknowledgments |
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This study was supported by National Institutes of Health grants HL-12792 and HL-07284.
Received June 14, 1995; accepted January 19, 1996.
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TopAbstractIntroductionMaterials and Methods Results Discussion References |
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