(Stroke. 2001;32:516.)
© 2001 American Heart Association, Inc.
Original Contributions |
Altered Expression of P2 Receptor mRNAs in the Basilar Artery in a Rat Double Hemorrhage Model
From the Departments of Neurosurgery and Microbiology (E.B.), University of Mississippi Medical Center, Jackson.
Correspondence to John H. Zhang, MD, PhD, Department of Neurosurgery, University of Mississippi Medical Center, 2500 N State St, Jackson, MS 39216. E-mail jzhang{at}neurosurgery.umsmed.edu
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Background and Purpose—Extracellular ATP might induce cerebral vasospasm after subarachnoid hemorrhage through P2 receptor. To investigate the roles of P2 receptor subtypes in vasospasm, we examined the changes in mRNA expression of P2 receptor subtypes in basilar arteries from double cisternal blood injection rat models.
Methods—One hundred male Sprague-Dawley rats, each weighing 350 to 400 g, were divided into 2 groups of 50. In the first group (n=50), the autologous arterial blood (0.2 to 0.3 mL) was injected into the cisterna magna on days 0 and 2. The rats were killed on day 3, 5, or 7 (n=10 in each group). In the sham group (n=10), the rats were injected with saline (0.3 mL) instead of blood. Ten rats were killed without blood or saline injection and served as control. The basilar arteries from rats in each group were used for reverse transcription and polymerase chain reaction. In another group of 50 rats, the same experiment was conducted, and the basilar arteries were collected for transmission electron microscopic study.
Results—In the subarachnoid hemorrhage groups, transmission electron microscopy showed the reduction in vessel perimeter on days 5 and 7 to be approximately 30% to 40%. The P2X1 mRNA level was significantly decreased on day 3 and recovered on days 5 and 7. The P2Y1 mRNA level was transiently increased on day 5, and the P2Y2 mRNA level was elevated from day 5 to day 7 (P<0.05).
Conclusions—The differential expression of the P2 receptors indicates that P2X1 subtype might not play an important role in vasospasm. The upregulation of P2Y1 and P2Y2 receptors might enable ATP to produce contraction at low levels of concentration.
Key Words: adenosine triphosphate • muscle, smooth • phenotype • receptors, purinergic P2 • rats
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Cerebral vasospasm is a leading cause and a frequent complication of the morbidity and mortality of subarachnoid hemorrhage (SAH). Cerebral vasospasm is characterized by a delayed, prolonged constriction, occurring mainly in vascular smooth muscle cells,1 2 and by cell proliferation within the arterial wall.3 The cause of vasospasm might be vasoactive substances (such as oxyhemoglobin, purine, and pyrimidine nucleotides) released into the subarachnoid space by the dissolution of the resultant blood clot4 5 or by vasoactive agents released from the vessel wall (such as endothelin). These spasmogens might produce gene expression changes that lead not only to a prolonged contraction but also to cell differentiation, cell proliferation, and cell death.3 6
The P2 receptor (P2 nucleotide receptor) was also referred to as P2 purinergic receptor (purinoceptor) previously. A large number of P2 receptor subtypes can be divided into 2 major families: the ligand-gated ion channel P2X receptors and the G protein–coupled P2Y receptors. More than 7 P2X and 8 P2Y subtypes were identified. P2 receptors play a central role in the functions of extracellular nucleotides in peripheral and central neuronal tissues, in the regulation of lung surfactant secretion, and in the regulation of the cardiovascular system. P2 receptors have been identified in cerebral arteries. Among the many identified P2 receptor subtypes, P2X1, P2Y1, and P2Y2 are the major functional populations expressed in vascular tissues.7 The P2X1 receptor exists mainly in smooth muscle cells. Activating P2X1 induces Ca2+ influx and cerebral arterial contraction.8 P2Y1 and P2Y2 are G protein–coupled receptors. Activating P2Y receptors leads to increased intracellular Ca2+, cerebral arterial contraction,5 9 10 11 12 and vasospasm in animals.5 13 The P2Y1 and P2Y2 subtypes, in particular among the P2 receptors, are also involved in mitogenesis via the mitogen-activated protein kinase pathway.8
Since ATP and P2 receptors are believed to be involved in vasospasm, we examined the expression of P2X1, P2Y1, and P2Y2, the most frequently expressed and studied P2 receptors in vascular tissue, in the basilar artery in a rat double hemorrhage model.
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Materials and Methods |
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Rat Double Hemorrhage Model
The protocol for this study complies with the Guide for the Care and Use of Laboratory Animals published by the National Institute of Laboratory Animal Resources (Commission on Life Sciences, National Research Council) and approved by the Institutional Animal Care and Use Committee at the University of Mississippi Medical Center. Thirty Sprague-Dawley male rats (each weighing 350 to 400 g) were anesthetized with an intraperitoneal injection of ketamine (100 mg/kg) and xylazine (10 mg/kg) and were allowed to breathe spontaneously in a supine position. The inguinal region was shaved and prepared in a sterile manner. After we exposed the left femoral artery and prepared to obtain arterial blood by cannulation, a midline incision on the dorsal surface of the neck was made from the cranial vertex to the lower cervical spine. The suboccipital and nuchal muscles were divided bilaterally to expose the atlanto-occipital membrane. The rat was then turned over to a supine position again, the blood was drawn, and the rat was placed again in the prone position. With the rat in a prone, head-down position, a 27-gauge needle was inserted into the cisterna magna via an atlanto-occipital membrane puncture. Correct positioning of the needle was determined by successful aspiration of cerebrospinal fluid. After a nonheparinized syringe was used to aspirate 0.3 mL of cerebrospinal fluid, 0.25 to 0.3 mL of autologous arterial blood was withdrawn from the femoral artery and then carefully injected into the cisterna magna over 3 minutes. After injection, the puncture site was immediately sealed with ethyl cyanoacrylate glue. The muscles were sutured layer to layer, and the skin incision was closed. The rats were placed prone in a head-down position at a 30° angle for 20 minutes to ensure rostroventral blood distribution around the basal intracranial arteries. They were kept warm by a heating blanket and monitored closely until they recovered. The day of the first injection was set at day 0. On day 2, the rats received a second injection following the same procedure, but blood was withdrawn from the right femoral artery. One rat from day 3 group died immediately after the second blood injection and was excluded from this study. On days 3, 5, and 7, the rats scheduled for death were euthanatized with an overdose of anesthesia and then decapitated. The basilar arteries were immediately removed under surgical microscope with minimal mechanical manipulation. The vessels were snap-frozen in liquid nitrogen and kept at -80°C until further analysis. On days 0 and 2, these investigators substituted a sterile 0.9% NaCl solution for blood and administered a double injection to a sham group of 10 rats. The sham rats were killed on day 7. Basilar arteries were collected from a control group of 10 rats that underwent no surgical procedure.
Transmission Electron Microscopy and
Imaging Analysis
A separate study of 50 rats using this same double
hemorrhage model was also conducted. Two rats died immediately
after the second blood injection and were excluded from this study.
Rats were euthanatized and killed via left ventricular
perfusion of 2% glutaraldehyde-phosphate buffer at
physiological blood pressure (100 mm Hg).
Basilar arteries were removed and postfixed with 2%
glutaraldehyde over a period of 1 week. Ultrathin cross
sections of the basilar arteries were stained with uranyl acetate and
examined with transmission electron microscopy (TEM). Morphometric
determination for lumen perimeter was determined by using a Kodak
digital camera and a DigiVision Pro image analysis system (both
attached to the LEO 906 TEM). The perimeters of the basilar artery were
calculated by imaging analysis. The transverse sections of
basilar artery were scanned by a computer and analyzed as a
digital image. The perimeter of the vessels was measured by tracing the
entire luminal surface of the intima, and the perimeter of the vessels
was calculated. The values from each group were expressed as a
percentage of the lumen perimeter of control rat basilar
artery.
Reverse Transcription and Polymerase Chain
Reaction
Total RNA was isolated from rat basilar arteries with
the use of RNAzol B. Total RNA was reverse-transcribed to cDNA for use
in polymerase chain reaction (PCR). Amplification was performed with
the thermal cycler Power Block II (ERICOMP). The thermal cycle
profile consisted of denaturation for 1 minute at 92°C, annealing of
primers for 1 minute at 56°C, extension for 30 seconds at 72°C, and
a final extension step at 72°C for 7 minutes. Reaction conditions and
cycle numbers were optimized for each receptor subtype. A relative
quantification of the cDNA for each receptor subtype was performed in
the logarithmic phase of amplification to obtain a linear relationship
between the cycle number and product amplification. An
amplification of both the receptor template and an internal control,
glyceraldehyde 3-phosphate dehydrogenase (GAPDH), was
run in parallel. The PCR primers and their expected product size,
as previously
reported,7 14 were
as follows: X1 forward (fwd) primer, 5'-AGAGGCACTACTACAAGCAGAA-3'; X1
reverse (rev) primer, 5'-GGTAAGGCTGTGGGAAAGA-3' (product size, 434
bp); Y1 fwd primer, 5'-CTGCCTGAGTTGGAAAGA-3'; Y1 rev primer,
5'-TCCCAGTGCCAGAGTAGA-3' (663 bp); Y2 fwd primer,
5'-ACCCGCACCCTCTATTACT-3'; Y2 rev primer, 5'-CTTAGATACGATTCCCCAACT-3'
(538 bp); GAPDH fwd primer, 5'-ACCACAGTCCATGCCATCAC-3'; GAPDH rev
primer, 5'-TCCACCACCCTGTTGCTGTA-3' (452 bp).
We used a semiquantitative PCR rather than a quantitative PCR. We used the housekeeping gene GAPDH as an internal standard. The aliquots of the reverse transcription (RT) products were used with the same amount of cDNA in PCR with primers for GAPDH and with primers for P2 subtypes in each sample. Since GAPDH is a housekeeping gene, the intensity of the resulting GAPDH-PCR product should be the same as if we had used an identical amount of cDNA from control and experimental samples. Thus, the difference between ratios for specific gene/GAPDH is due to the change of the specific gene. Consequently, we can estimate the specific gene mRNA change through the change of this ratio. PCR products were electrophoresed in an ethidium bromide–containing 2% agarose gel in Tris-acetate/EDTA buffer. The gels were analyzed with the use of Gel Doc 1000 and Quantity One software (Bio-Rad). We measured the volume of bands of GAPDH and specific genes in each sample. To correct for any variation in RNA content or cDNA synthesis between samples, each sample was normalized according to its GAPDH content. The ratios of the receptor PCR product/GAPDH product were expressed as a percent increase from those of the control. Because the number of samples of rat basilar arteries was small, 3 to 4 samples from each group were pooled for 1 experiment. Three experiments (from 10 different samples) were averaged for calculation. The linear exponential phases for P2 subtypes and GAPDH PCR were 25 to 36 cycles. Thus, we used 32 cycles for P2X1, P2Y1, and P2Y2 and 28 cycles for GAPDH.
Chemicals
RNAzol B was purchased from TEL-TEST, Inc.
Superscript II RNase H-RT, oligo(dT)12–18
primer, specific primer pairs, and 
X174RF
DNA/HaeIII fragments
(marker) were obtained from GIBCO BRL. Amplitaq DNA polymerase, PCR
buffer, and dNTPs were obtained from
Perkin-Elmer.
Data Analysis
The data were calculated as a ratio of the band
volume of target to that of the internal standard and were expressed as
mean±SEM. The statistical analysis was performed with ANOVA
followed by Fisher’s protected least significant difference test.
Differences were considered to be significant at
P<0.05.
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Results |
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Rat Double Hemorrhage Model
All animals were drowsy on days 1 and 3 after cisternal blood injection but resumed normal behavioral patterns and feeding habits the next day. Gross macroscopic observation/inspection of basilar arteries before removal from the brains revealed blood clots on the basal surface of the brains from day 3 and 5 samples. The clots were still present but noticeably smaller on samples taken on day 7. Arterial constriction was also observed.
TEM and Imaging Analysis
Histological results showed corrugation
of the internal elastic lamina and contraction of smooth muscle cells
in basilar arteries. Smooth muscle contraction and corrugation of the
internal elastic lamina were most severe on days 5 and 7
(Figure 1A
and 1B). Imaging analysis demonstrated an
overall reduction of the diameter up to 40% in samples collected on
days 5 to 7
(Figure 1C).
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Expression of P2
Receptors
The expression of P2X1 receptors
decreased significantly
(P<0.05) on day 3 and
increased to the normal range on days 5 through 7 in samples from SAH
rats. The expression of P2Y1 receptors increased
significantly (P<0.05) on day
5 but decreased to the normal range on day 7. The expression of
P2Y2 receptors increased on day 5 and remained
above normal (P<0.05) in
samples taken on day 7. These results are summarized in
Figures 2 and 3. In all RT-PCRs of these mRNAs, the
results from the sham operation group on day 7 were measured at the
same level as the control group.
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The main observations in this study are as follows: (1) double blood injection produced mild to moderate vasospasm on days 5 to 7; (2) mRNA expression of P2X1 receptors in the basilar arteries from the rats assigned to the SAH injection groups was downregulated on day 3 and recovered on days 5 through 7; (3) mRNA expression of P2Y1 receptors was upregulated on day 5 and returned to normal on day 7; and (4) mRNA expression of P2Y2 receptors was upregulated on day 5 and remained above normal on day 7.
Extracellular ATP and Vasospasm
The source of the extracellular nucleotides
is a cellular component of subarachnoid blood clots.
Millimolar-level concentrations of ATP and ADP are included in
erythrocytes, and a high level of UTP is also in
platelets.15 Some
experimental data have supported the role of extracellular ATP in
vasospasm. ATP induces Ca2+
elevation9 and
contraction12 of cerebral
artery. Even though intraluminal application of ATP produced
relaxation, extraluminal application of ATP, a situation that more or
less resembles SAH, produced vasoconstriction in cerebral
arteries.16 The removal of
ATP by incubating erythrocyte lysate with apyrase (an enzyme that
breaks down ATP and ADP into AMP) abolished the action of erythrocyte
lysate to elevate
Ca2+.5
Furthermore, ATP produced vasospasm in rat femoral arteries and in a
monkey model of cerebral
vasospasm.5 13
Contradictory evidence regarding the role of ATP in cerebral vasospasm
also exists. First, an ATP-induced contraction is less than that of
erythrocyte lysate.12
Second, the ATP level in the bloody cerebrospinal fluid from a canine
double hemorrhage model is measured at the nanomolar level,
which is considered to produce no contraction (Yin, MD, PhD,
unpublished data, 2000). However, these contradictory data do
not rule out possible molecular events (except contractions) involving
smooth muscle cells in which a previously high ATP level is required to
induce delayed and prolonged vasocontraction.
Differential Expression of P2 Receptors and
Vasospasm
P2 receptors play an important
role in regulating cerebral vascular tone. Even though >15
P2 receptor subtypes have been discovered, the
most frequently reported P2 receptors in
vascular tissue are the P2X1,
P2Y1, and P2Y2 subtypes.
The P2X1 receptor is a ligand-gated cation
channel that exists mainly in smooth muscle cells and mediates
vasoconstriction.17 18
The P2Y1 and P2Y2 (or
P2U) subtypes are G protein–coupled
receptors.19 Activation of
P2Y1 receptors, which exist mainly in
endothelial cells, leads to vasodilatation.
P2Y1 receptors are also found less frequently in
smooth muscle cells. The P2Y2 receptor is found
in endothelial and smooth muscle cells and is
responsible for vasodilatation or constriction, respectively. According
to You et
al,16 20 the
relaxant effect of P2Y1 and
P2Y2 receptors is endothelium
dependent and is related to the generation of nitric oxide or
prostacyclin and endothelium-dependent hyperpolarizing
factors.
The extracellular nucleotides released from subarachnoid clots are supposed to stimulate all P2 receptor subtypes in cerebral arteries. Although the pathological roles of P2 receptors have been implicated in the development of post-SAH cerebral vasospasm, the differential role of each subtype has not been clearly understood. P2 receptors are involved in contraction or relaxation of cerebral arteries as well as in other cellular functions, such as proliferation and mitogenesis.8 Differential expression of P2 receptor subtypes occurs when smooth muscle undergoes phenotypic changes.7
The P2X1 Subtype
In contractile smooth muscle cells,
,ß-methylene-ATP, a potent agonist of P2X1
receptors, induces a transient increase in intracellular
Ca2+,21
indicating the expression of P2X1 receptors.
Similarly, a small and sustained P2X1
receptor–mediated intracellular Ca2+
elevation was observed in primary cultures of rat aorta smooth muscle
cells.22 However, this
P2X1 receptor–mediated response did not occur
in subcultured rat smooth muscle
cells,7 23 24 25
indicating P2X1 receptor downregulation and
phenotypic change. Although P2X1 contributes to
ATP-induced contraction in rat vascular smooth muscle cells, our data
suggest that contractile P2X1 might not be
involved in chronic vasospasm: P2X1 mRNA
expression was transiently downregulated on day 3 after double
hemorrhage. Postsynaptic P2X1
downregulation in the arterial wall might also impair the
fine neural regulation of cerebrovascular smooth muscle tone. Because
the data of mRNA expression were sampled only on days 3, 5, and 7, this
study does not rule out a possible role for P2X1
in "acute vasospasm," which might occur immediately after the blood
injection.
The P2Y1 Subtype
P2Y1 exists mainly in
endothelial cells and contributes to
endothelium-dependent
relaxation.11 16 26
A potent agonist of the P2Y1 receptor (2
MeS-ATP) induces Ca2+ release in cultured
rat smooth muscle cells but not in freshly isolated
cells,23 which indicates an
upregulation of P2Y1 receptors in cultured
smooth muscle cells. Because the P2Y1 receptor
is involved in mitogenic effect, the upward regulation of
P2Y1 receptors in culture has also been observed
to contribute to an increased progression of cell cycles in smooth
muscle cells.7 Even though
P2Y1 expression in contractile smooth muscle
cells was detected, its role was described in cell mitogenesis, and its
role in smooth muscle contraction remains to be determined. In this
study the mRNA expression of P2Y1 transiently
upregulated and peaked around day 5. This result might be interpreted
as an increase in P2Y1 mRNA expression in
endothelial cells, as an increase in
P2Y1 expression in smooth muscle cells, or as
both. Because the nature of multiple layers of smooth muscle cells
contrasts with the nature of a thin layer of
endothelial cells, we speculate that the altered mRNA
P2Y1 expression might reflect an enhanced
expression in smooth muscle cells. Endothelial cells
were not removed in this study because it is difficult to remove the
endothelium from rat basilar arteries, and the
procedure followed in this study required the samples to be frozen
immediately after euthanasia to avoid possible decay or contamination
of the RNA.
The P2Y2 Subtype
The mRNA expression of P2Y2 was
upregulated, and the time course of its upregulation was
consistent with the time course of cerebral vasospasm. This
P2Y2 upregulation might have led to a
contractile response (even to a low concentration of extracellular ATP)
and might have contributed to cerebral vasospasm. According to some
investigators, P2Y2 also contributes to
mitogenesis7 22 by
cooperating with other growth factors, such as serum or
platelet-derived growth factor. After balloon injury in the
endothelium-damaged intimal lesions, the
neointimal subpopulation of smooth muscle cells on luminal
surface demonstrates a P2Y2 subtype
overexpression.27
Extracellular ATP also regulates this
P2Y2-specific upregulation process through the
mitogen-activated protein kinase
pathway28 and plays an
important role in atherosclerotic intimal hyperplasia. Thus,
P2Y2 mRNA upregulation might be involved in
spastic arterial mitogenesis. In a recent article outlining
the P2 receptor subtype changes that occurred
during the switching of smooth muscle phenotypes,
P2X1 and P2Y1 were
expressed in freshly isolated rat aortas. If rat aortic smooth muscle
cells were cultured, however, the P2X1 subtype
disappeared, and P2Y1 and
P2Y2 were
overexpressed.7 However, in
this rat model the endothelial cells did not detach
markedly, nor did subintimal cells proliferate. It is speculated that
the upregulation of P2Y2 receptor in this study
might be related to the long-term contractile response of the basilar
artery instead of tissue proliferation.
The cause of the upregulation of P2Y2 receptor in smooth muscle cells undergoing cerebral vasospasm is unclear. The most likely cause is blood clot and its lysate (such as erythrocyte lysate, hemoglobin, ATP, and their degenerative products). Perhaps these spasmogens directly or indirectly (by releasing other factors such as endothelin or growth factors and together with these factors) stimulate smooth muscle cells. In addition, hemoglobin and other factors cause cytotoxicity and possibly trigger inflammatory response and generate cytokines. Platelet or leukocyte infiltration might lead to the production of 5-hydroxytryptamine, UTP, thromboxanes, and cytokines. In turn, the conditions fostered by these growth factors might stimulate the basilar artery, resulting in changes of P2Y2 receptors.
Rat Double Hemorrhage Model of
SAH
Developing a reliable model for rat SAH that closely
parallels the pathogenesis of vasospasm in humans has proven
difficult.29 30 31 32 33 34 35 36
Single hemorrhage rat models do not closely resemble the
biphasic phenomena characteristic of SAH-induced vasospasm in
large-animal models. Additionally, most single injection models have
been conducted in acute studies—ie, those investigating only the
initial acute constriction—while excluding the characteristic delayed
spasm resulting in cerebral ischemia and the concomitant
morbidity and mortality.2
Other problems with previous models include the indirect evaluation of
vasospasm (via angiogram or cerebral blood flow determination) or
direct evaluation by gross observation. The illustrations of the small
rat cerebral arteries proved to be difficult in these studies.
Investigating the molecular changes associated with SAH-induced
vasospasm requires a reliable rat model because most documented genes
are from either rats or mice. A double hemorrhage model in rats
has created vasospasm that resembles angiographically the time course
of vasospasm in large-animal
models.37 Although massive
single injections (0.5 mL) of blood have also been found to generate a
similar time course for constriction, we found this method to be
undesirable because it led to increased mortality (ie, high incidence
of respiratory arrest due to extremely high inspiratory
capacity, as noted in unpublished observations [R.C. Carpenter,
BS, et al, unpublished data, 1999]).
According to our results, the profile of the cerebral vasospasm produced in this rat double hemorrhage model of SAH falls in the mild to moderate range, and we observed few severe vasospasms. The morphological changes as observed by TEM are consistent with those observed in most animal models (such as corrugation of the elastic lamina, which indicates vasoconstriction). Other features that have been described in humans (such as endothelial damage, smooth muscle migration, proliferation, necrosis, and apoptosis) were not observed to any appreciable extent. Therefore, this rat double hemorrhage model is more or less an experimental albeit atypical SAH model designed to parallel cerebral vasospasm observed in humans or in larger-animal models. Because this model did not successfully produce severe cerebral vasospasm, the molecular changes in P2 receptor expression can only be analyzed within the context of an SAH model.
By examining the mRNA expression of P2 receptors, this study demonstrates an upregulation of P2Y receptors that might enhance a contractile response to extracellular ATP even at lower levels. The P2X1 receptor might not be involved in chronic vasospasm, even though its role in acute vasospasm cannot be excluded. For several reasons, we recommend additional studies using larger animals (such as canines or primates) as models. First, these models replicate human vasospasm more closely (eg, severe contraction, endothelial damage). Second, separating the endothelial cells from the relatively larger cerebral arteries of canines or primates (as opposed to those of rodents) might prove useful (removal of endothelial cells is a crucial step in identifying the sources of different P2 receptor mRNAs). Third, the protein expression of different P2 receptors and the functional activities of these receptors during cerebral vasospasm need further examination.
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Acknowledgments |
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This study was partially supported by a Grant-in-Aid to Dr Zhang from the American Heart Association, Southeast Affiliation.
Received June 28, 2000; revision received September 22, 2000; accepted November 2, 2000.
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Editorial Comment
Department of Neurosurgery, University of California–Davis, Sacramento, California
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferencesIntroduction References |
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The authors describe an interesting piece of vascular physiology, applied to subarachnoid hemorrhage in the rat. They are careful to point out the differences in (especially) morphology between vasospastic arteries in humans and in rats, but on balance this model appears to be at least as valid as the widely used dog double-hemorrhage model. This is useful, not only because rats are cheaper, easier to handle, and have less "pet appeal," but also because most of the genes known to play a role in vascular physiology have not been described in dogs, cats, or monkeys, but rather in rats and mice. Obviously, having less "pet appeal" does not absolve us from treating rats as humanely as possible, but the authors have clearly adhered to the guidelines described in Materials and Methods.
One of the vexing problems in this type of work is that practically every agonist and every receptor modulate both vasodilation and vasoconstriction, depending not only on the various contributions of the agonist and receptor but also on the doses of the former. This, then, makes this work more suitable for learning about vascular physiology than about vasospasm. Nevertheless, the switch in phenotype rather than change in sensitivity to agonists with the upregulated expression P2Y2 mRNA is an interesting clue in understanding the phenomenon of "vasospasm."
The authors note that "Converting a phenotype to synthetic smooth muscle during a prolonged contraction (such as cerebral vasospasm) might reduce smooth muscle sensitivity to contractile or more likely relaxant stimulation, and lead to resistance to vasodilators," The reverse may be true as well (see above), and this possibly explains some of the effects of prophylactic balloon dilation through its endothelium-modulating effects, in which vessels were found to become unresponsive to vasodilator and vasoconstricting agents after SAH!R1
Received June 28, 2000; revision received September 22, 2000; accepted November 2, 2000.
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1. Megyesi JF, Findlay JM, Vollrath B, Cook DA, Chen MH. In vivo angioplasty prevents the development of vasospasm in canine carotid arteries: pharmacological and morphological analyses. Stroke. 1997;28:1216–1224.
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