(Circulation. 1999;100:II-328.)
© 1999 American Heart Association, Inc.
Myocardial Protection and Vascular Biology |
Serotonin-Induced Coronary Contraction Increases After Blood Cardioplegia-Reperfusion
Role of COX-2 Expression
From the Division of Cardiothoracic Surgery, Department of Surgery (C.M., Jiany Li, F.W.S.), and Cardiovascular Division (Jian Li, M.S.), Department of Medicine of Beth Israel-Deaconess Medical Center and Harvard Medical School, Boston, Mass.
Correspondence to Frank W. Sellke, MD, Division of Cardiothoracic Surgery, Beth Israel-Deaconess Medical Center, East Campus, Dana 905, 330 Brookline Ave, Boston, MA 02215. E-mail fsellke{at}caregroup.harvard.edu
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Abstract |
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Background—Coronary contraction has been implicated in causing suboptimal myocardial function after coronary bypass surgery. Addition of blood to cardioplegic solutions has been shown to improve endothelial function after cardioplegia. In this study, the effects of blood cardioplegia and brief reperfusion on vascular reactivity in patients with coronary artery disease and the expression (mRNA and protein) of enzymes involved in vasomotor regulation were examined.
Methods and Results—The atrial appendages of patients undergoing coronary artery surgery were harvested before cardiopulmonary bypass (control, n=8) and after bypass from a nonischemic tissue atrial segment exposed to cold, hyperkalemic blood cardioplegia (mean, 60 minutes) and a brief period (10 minutes) of reperfusion (CP-Rep, n=8). Responses of atrial arterioles were studied in vitro with video-microscopy. Reverse-transcriptase polymerase chain reaction and Western blotting were used to examine the expressions and protein content, respectively, of enzymes involved in vasomotor regulation. Serotonin caused a minimal dilation under baseline conditions but after CP-Rep elicited a potent contractile response that was inhibited in the presence of the selective inducible cyclooxygenase (COX-2) inhibitor NS398. Substance P caused an endothelium-dependent relaxation of atrial arterioles through release of nitric oxide, and ADP caused relaxation mediated through release of prostaglandins. After CP-Rep, relaxation to substance P was impaired, whereas endothelium-independent relaxation to nitroprusside and response to ADP were unchanged. Expression and protein level of COX-2 were significantly increased after CP-Rep. In contrast, expression of inducible (nitric oxide synthase-2) or constitutive endothelial (nitric oxide synthase-3) nitric oxide synthase, prostacyclin synthase, and constitutive cyclooxygenase (COX-1) were not altered after CP-Rep.
Conclusions—CP-Rep increases serotonin-induced contraction of human microvessels caused by the release of products of COX-2 and the impaired release of nitric oxide. These findings have implications regarding altered coronary microvascular regulation and the cause of coronary spasm after cardiac surgery.
Key Words: coronary disease • heart diseases • endothelium • nitric oxide • surgery • microcirculation
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Introduction |
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Cardioplegia is widely and successfully used for the protection of the myocardium against ischemic injury during cardiac surgery.1 2 Cardiac dysfunction after cardiopulmonary bypass and ischemic arrest may be related to reduced myocardial perfusion secondary to altered vasomotor regulation and coronary artery spasm.3 Although the effects of cardioplegic solutions on endothelium-dependent relaxation,4 ß-adrenergic regulation,5 and myogenic vasomotor regulation6 are well known, these experiments have been performed in normal animals without coronary artery disease. Patients undergoing CABG surgery suffer from atherosclerosis and in general have underlying endothelial dysfunction. Therefore, cardioplegia-induced endothelial dysfunction may be a moot point in these patients who already possess dysfunctional endothelium. No studies have examined the effects of cardioplegia and reperfusion (CP-Rep) on vascular reactivity in patients with underlying endothelial dysfunction caused by atherosclerotic vascular disease. The alteration in vasomotor pathways after cardioplegia and reperfusion in patients may be due to changes in expression or activities of either nitric oxide synthase (NOS) or cyclooxygenase (COX).
This study was designed to examine the effect of blood cardioplegia and brief reperfusion on vascular responses of human atrial microvessels to serotonin (5-HT) and other vasoactive substances and to correlate these responses to possible alterations in gene expressions and protein levels of constitutive endothelial NOS-3 and inducible NOS-2 in the human heart, as well as the constitutive cyclooxygenase (COX-1), inducible cyclooxygenase (COX-2), and prostacyclin synthase.
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Methods |
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The atrial appendages were harvested from patients undergoing CABG. Aspirin was discontinued
24 hours before surgery, but other
medications were continued up to the time of surgery. Double
cannulation of 3–0 polypropylene sutures was placed in the atrial
appendage. After administration of heparin, a single 2-stage atrial
cannula was placed and secured with the superior suture. The lower
suture was not secured so that this portion of atrium would be exposed
to cardioplegic solution and would be reperfused after release of the
aortic cross clamp. A piece of right atrial appendage was harvested
above the superior cannulation suture after heparinization but before
initiation of cardiopulmonary bypass (control group). Another
piece of atrium was harvested after cross clamp removal and termination
of cardiopulmonary bypass before protamine administration. An
initial 800 to 1000 mL of cold-blood (0°C to 4°C) hyperkalemic
(25 mmol/L K+) cardioplegic solution was
delivered antegrade into the aortic root. This was followed at 8- to
15-minute intervals with 250 to 300 mL of cold, low-KCl (12 mmol/L
K+) cardioplegic solution. The cardioplegic
solution consisted of a 4:1 mixture of oxygenated blood
with a hyperkalemic crystalloid solution, resulting in 25 or 12
mmol/L K+. The compositions of the high- and
low-potassium crystalloid solutions were 25 and 12 mmol/L KCl,
respectively, together with 5 g/L mannitol, 20 mL/L citrate phosphate
dextrose solution, 4 mmol/L trihydroxy methylamino methane in 5%
dextrose, and 0.2235% saline solution. A cold, moist gauze pad was
placed on the heart for topical cooling. Patients were systemically
cooled to 30°C to 32°C. Rewarming was initiated before the mammary
distal anastomosis was begun. Mean cardioplegia time was 58±7 minutes,
and the mean time from cross clamp removal until second tissue harvest
was 10±3 minutes. Tissue in the control and CP-Rep groups was
immediately frozen in liquid nitrogen after harvest in preparation for
molecular biology studies or was placed in cold (5°C to 10°C)
Krebs’ buffer solution of the following composition (in mmol/L):
118.3 NaCl, 4.7 KCl, 2.5 CaCl2, 1.2
MgSO4, 1.2
NaH2PO4, 25
NaHCO3, and 11.1 glucose for vascular
reactivity studies. The study was approved by the clinical research committee of Beth Israel Deaconess Medical Center.
In Vitro Atrial Microvascular Studies
Atrial microvessels (70- to 180-µm ID) were dissected with a
10x to 60x dissecting microscope (Olympus Optical). Microvessels were
placed in a microvessel chamber, cannulated with dual glass
micropipettes measuring 40 to 80 µm in diameter, and secured
with 10–0 nylon monofilament suture (Ethicon). Oxygenated
(95% O2/5% CO2) Krebs’
solution warmed to 37°C was continuously circulated through the organ
chamber. The vessels were pressurized to 40 mm Hg in a no-flow
state with a burette manometer filled with Krebs’ buffer solution.
With an inverted microscope (40x to 200x, Olympus CK2, Olympus
Optical) connected to a video camera, the vessel image was
projected onto a black and white television monitor. An electronic
dimension analyzer (Living System Instrumentation) was used to
measure internal lumen diameter. Measurements were recorded
(Graphtec). Vessels were allowed to bathe in the organ chamber for 30
minutes before an intervention.
Microvessel Study Protocols
Relaxation responses of microvessels were examined after
development of spontaneous tone with or without supplemental
precontraction with the thromboxane
A2 analog U46619. Baseline diameter was defined
as the ID within minutes of cannulation and placement in the bath when
the diameter tended to be at a maximum and spontaneous contraction has
not yet occurred. At the completion of an experiment, papaverine
(10-4 mol/L) was applied to confirm that the
initial diameter reading was similar to the maximally dilated diameter.
If the spontaneous contraction was <30% of the initial baseline
diameter, incremental concentrations of U46619
(10-8 to 10-6 mol/L) were
applied so that the final precontraction was 30% to 60% of the
initial baseline diameter. Vascular responses to serotonin
(5-HT, 10-9 to 10-4
mol/L), ADP (10-9 to 10-4
mol/L), substance P (10-15 to
10-6 mol/L), and sodium nitroprusside (SNP,
10-9 to 10-4 mol/L) were
examined. Selected experiments were performed in the presence of
10-4 mol/L
NG-nitro-L-arginine
(LNNA), 10-6 mol/L indomethacin,
or 10-6 mol/L selective COX-2
inhibitor NS-398. Blocking drugs were applied for 20
minutes before a dose-response intervention was performed. All drugs
were applied extraluminally. Measurements were made and recorded 2
to 3 minutes after drug administration, when the response had
stabilized. Once substance P was applied to a vessel, the vessel was
discarded to avoid tachyphylaxis. One to 4 interventions were performed
on each vessel. The order of drug administration was random. Vessels
were washed 3 times with Krebs’ buffer solution and allowed to
equilibrate in drug-free Krebs’ buffer solution for 15 to 30 minutes
between interventions.
Expression of NOS-3 and NOS-2 mRNA
For NOS-3 and NOS-2 mRNA studies, the semiquantitative
reverse-transcriptase (RT) polymerase chain reaction (PCR) was
performed because the signal intensities for NOS-3 and NOS-2 were not
sufficient for quantitative analysis by Northern hybridization.
Primers were designed on the basis of the published NOS-37
and NOS-2 sequences.8 The primers of the sense
5'-CAGTGTCCAAC-ATGCTGCTGGAAATTG-3' corresponding to bases 1050
through 1076 and the antisense 5'-TAAAGGTCTTCTTCCTGGTGATG-CC-3'
corresponding to bases 1511 through 1535 were used to amplify a 486-bp
fragment of NOS-3. For NOS-2, the primer of sense
5'-GCCTCGCTCTGGAAAGA-3' corresponding to bases 1425 through 1441 and
the antisense 5'-TCCATGCAGACAA-CCTT-3' corresponding to bases 1908
through 1924 were used to amplify a 500-bp fragment of NOS-2.
An equal amount of total RNA was used for RT-PCR. For quantification, GAPDH was amplified from the same amount of RNA to correct for variation of different samples. The PCR products were loaded in 1% agarose gel and then scanned and quantified with Image-Quant software (Molecular Dynamics).
Expression of COX-1, COX-2, and Prostacyclin Synthase mRNA
Primers were designed on the basis of the published
COX-1,9 COX-2,10 and prostacyclin synthase
sequences.11 For COX-1, the primer of the sense
5'-TCTTTGCACAACACTTCACC-3' corresponding to bases 601 through 620 and
the antisense 5'-GTACTCATTGAAGGGCTGCA-3' corresponding to bases 1381
through 1400 were used to amplify a 799-bp fragment of COX-1. For
COX-2, the primer of the sense 5'-TAAACTGCGCCTTTTCAAGG-3'
corresponding to bases 781 through 800 and the antisense
5'-GTGATACTTTCTGTACTGCG-3' corresponding to bases 1381 through 1400
were used to amplify a 619-bp fragment of COX-2. For prostacyclin
synthase, the primer of the sense 5'-AGTGTCAAAAGTCGCCTGTG-3'
corresponding to bases 721 through 740 and the antisense
5'-TTCTTCTCTGATCCGTCAGG-3' corresponding to bases 1261 through 1280
were used to amplify a 559-bp fragment of prostacyclin synthase.
As above, an equal amount of total RNA was used for semiquantitative RT-PCR. For quantification, GAPDH was amplified from the same amount of RNA to correct for variation of different samples. The PCR products were loaded in 1% agarose gel and then scanned and quantified with Image-Quant software (Molecular Dynamics).
Expression of NOS-2, NOS-3, COX-1, and COX-2 Proteins
Total proteins from atrial tissues were obtained by
homogenizing in a lysis buffer containing 1% NP-40,
0.5% sodium deoxycholate, and 0.1% SDS and centrifuging at
12 000g for 10 minutes at 4°C. Protein concentration of
the supernatant was measured by spectrophotometry at 595 nm (DU640,
Beckman) of an aliquot developed for 10 minutes in protein assay dye
reagent (Bio-Rad). Total protein (40 µg/lane) was fractionated on
10% SDS-PAGE transferred to a polyvinylidene difluoride
membrane (Immobilon-P, Millipore). Equal protein loading and transfer
efficiency were visualized by Ponceau red staining. The membrane was
incubated with 5% nonfat dry milk powder and 0.05% Tween-20 in PBS
for 12 hours at 4°C to block nonspecific absorption and then was
immunoblotted with the monoclonal mouse
anti-endothelial NOS antibody (Transduction
Laboratories) at 1:2500 (vol/vol) dilution or the monoclonal mouse
anti-inducible NOS antibody (Transduction Laboratories) at 1:500
dilution (vol/vol) for 2 hours for NOS-3 and NOS-2 Western, or with the
polyclonal goat anti–COX-1 (Santa Cruz) at 1:1000 dilution (vol/vol)
or with the polyclonal goat anti–COX-2 at 1:500 dilution (vol/vol)
antibody (Santa Cruz). After washing with PBS, the membrane was
incubated for 1 hour in 5% milk powder PBS containing 1:3000 diluted
goat anti-mouse IgG conjugated to horseradish peroxidase (Vector
Laboratories) or anti-goat IgG conjugated to horseradish peroxidase
(Santa Cruz Biotechnology). Peroxidase activity was visualized with an
enhanced chemiluminescence substrate system (Amersham). Densitometry of
digitized images of immunoprobed membranes (ScanJet 4c, Hewlett
Packard) was performed by use of Image-Quant software (Molecular
Dynamics).
Drugs
Substance P and NS-398 were obtained from RBI. LNNA, U46619,
ADP, SNP, indomethacin, and 5-HT were obtained from
Sigma Chemical Co. ADP, 5-HT, LNNA, and SNP were dissolved in ultrapure
distilled water. U46619 was dissolved in ethanol to make a 10-mmol/L
stock solution. Indomethacin was dissolved in minimal
ethanol to make a 20-mmol/L stock solution. All stock solutions were
stored at -20°C. Dilutions were prepared daily.
Data Analysis
The relaxation responses were expressed as percent relaxation of
the spontaneous and/or U46619-induced vascular contraction (mean±SEM)
of the microvessels. Because all vascular responses could not be
performed before and after cardioplegia, paired comparisons could not
be performed. Comparisons of dose-response curves were performed by
2-way ANOVA with repeated measures or 1-way ANOVA, followed by
Scheffé’s multiple range test when indicated. Student’s
t test was used to compare changes in gene and protein
expressions. P<0.05 was considered to indicate
significance.
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Results |
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Patient Population
We studied responses of atrial microvessels from 20 patients undergoing nonemergent CABG. The average age was 68±5 years. Fifteen patients were male; 8 continued to smoke until just before surgery; and 6 had non–insulin-dependent diabetes mellitus. Mean total cholesterol concentration was 206±24 mg/dL; mean LDL was 140±20 mg/dL; and mean HDL was 34±5 mg/dL. Triglycerides were 190±30 mg/dL. Mean ejection fraction was 49±5%.
Vessel Characteristics
Atrial microvessel ID ranged from 71 to 180 µm, averaging
106±4 µm in the control group and 123±13 µm in the
CP-Rep group. Precontraction after spontaneous constriction and/or
after application of the thromboxane
A2 analog was 59±3% in the control group and
64±6% in the CP-Rep group. Similar amounts of U46619 were required to
produce adequate precontraction in both groups.
In Vitro Response to 5-HT
5-HT, a platelet-derived substance that produces both
receptor-mediated endothelium-dependent relaxation and
direct vascular smooth muscle contraction, had a minimal net effect on
control microvessels. In contrast, 5-HT elicited a significant
contraction after CP-Rep. This contraction was significantly reduced in
the presence of the COX inhibitor
indomethacin and was completely inhibited in the
presence of the selective COX-2 inhibitor NS-398 (Figure 1A
). This implies that a significant portion
of the enhanced contractile response observed after cardioplegia is due
to the release of prostanoid contracting substances.
Indomethacin prevented the contractile response to 5-HT
and induced a significant relaxation of control microvessels. This
suggests that 5-HT causes the release of contractile
prostaglandins even under nonischemic conditions in
atherosclerotic human microvessels. LNNA produced a significant
contraction response to 5-HT in control microvessels, suggesting that
part of the response to 5-HT is due to the stimulated release of nitric
oxide (Figure 1B).
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P<0.05 vs CP-Rep (2-way ANOVA for repeated
measures).
In Vitro Response to ADP and Substance P
The responses of microvessels to ADP before and after cardioplegia
were similar. The responses of microvessels to ADP were not altered in
the presence of LNNA but were significantly reduced in the presence of
indomethacin (Figure 2A).
Thus, in contrast with most other species, ADP does not elicit the
release of endothelium-derived nitric oxide in human
atrial microvessels. This response was not affected by CP-Rep. The
response of microvessels to substance P was markedly reduced in the
presence of LNNA. The endothelium-dependent relaxation
response to substance P was significantly reduced after CP-Rep compared
with the response of microvessels from the control group (Figure 2B).
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In Vitro Response to SNP
The relaxations of atrial microvessels to SNP, which operates
through an endothelium-independent cGMP-mediated
pathway, were similar in both groups, suggesting no alteration of the
ability of the smooth muscle to relax through the cGMP pathway after
CP-Rep (Figure 3).
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Gene Expression of NOS-3, NOS-2, COX-1, COX-2, and
Prostacyclin Synthase
To examine whether the endothelium dysfunction
observed after cardioplegia is due to an altered expression of 1 of the
isoforms of NOS or to enzymes responsible for synthesis of
prostaglandins, the expressions of NOS-3, NOS-2, COX-1,
COX-2, and prostacyclin synthase were analyzed by
semiquantitative RT-PCR. Gene expressions of NOS-3 and NOS-2, as well
as the expressions of COX-1 and prostacyclin synthase, were not altered
after cardioplegia. In contrast, COX-2 expression was significantly
increased (2.5-fold, P<0.05) after CP-Rep (Figure 4).
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Protein Expression of NOS-3, NOS-2, COX-1, and COX-2
NOS-3 was slightly but significantly reduced after CP-Rep,
possibly because of a posttranscriptional mechanism. NOS-2 was not
affected by CP-Rep (Figure 5). COX-1 protein
level was significantly reduced and COX-2 was significantly increased
after CP-Rep (Figure 6).
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Discussion |
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The major finding of this study is that blood cardioplegia and brief reperfusion cause an enhanced contractile response to 5-HT of human atrial microvessels that is completely prevented by COX-2 inhibition. This enhanced microvascular response to 5-HT is due to an increased production and release of contractile prostaglandin substances, likely due to increased expression of the inducible COX isoform. As opposed to the inducible isoform of NOS, COX-2 seems to be regulated by agonist stimulation, such as to 5-HT. Prostaglandins are formed by the action of isoforms of COX in a 2-step conversion of arachidonic acid. First, the enzyme converts arachidonic acid to a cyclic endoperoxide (PGG2) by the action of COX. This is followed by cleavage of the peroxide to yield to an endoperoxide (PGH2). These unstable intermediate products of arachidonic acid metabolism are rapidly converted to the prostaglandins (prostaglandins E2, F2, I2, and thromboxane A2) by specific isomerase enzymes.12 Prostacyclin synthase transforms PGH2 into what is the most characteristic prostanoid formed by resting endothelial cells, prostacyclin.. Prostacyclin is a potent platelet antiaggregatory agent and has antiadhesive and smooth muscle relaxing properties. PGH2 also undergoes spontaneous or enzymatic transformation to prostaglandins F2
, E2, and
D2. Two isoforms of COX exist. The constitutive
isoform (COX-1) is present in such tissues as the heart, gut, and
kidney, in which prostaglandin production plays a
cytoprotective role in maintaining normal
physiological process.13 Both COX-1
and COX-2 use the same endogenous substrate,
arachidonic acid, and form the same product by the
same catalytic mechanism; their major difference lies in their
pathological functions. In inflammatory processes, the inducible
isoform of cyclooxygenase (COX-2) is expressed in
many cells, including fibroblasts and macrophages, and accounts
for the release of large quantities of proinflammatory
prostaglandins at the site of inflammation.13
Which prostaglandin products (constrictor versus
vasodilator) predominate depends mostly on the relative activity of the
2 isoforms of COX but also on the secondary pathways that yield the
different prostanoids. When COX activity is overexpressed, prostacyclin
synthase activity could be the limiting step in the biosynthesis of
prostaglandin I2; other prostanoids
may be synthesized instead. In addition, the oxidative state and other
conditions in the tissue could influence which
prostaglandins are synthesized. Indomethacin may not have completely prevented the enhanced microvascular contraction to 5-HT after cardioplegia because indomethacin is roughly 60 times more potent at inhibiting COX-1 than COX-2 in intact cells.14 The inducing factor leading to increased expression of COX-2 was not examined in the present study, but it is most likely related to myocardial ischemia or hypoxia or the exposure of vessels to inflammatory cytokines during cardiopulmonary bypass and cardioplegic arrest. Endothelial injury, platelet accumulation and aggregation, and release of serotonin and other cell-derived mediators could promote COX-2 expression. It has been shown that COX-2 can be induced by the platelet product serotonin in rat mesangial cells.15
Cardiac operations incorporating cardiopulmonary bypass cause a
systemic inflammatory response, which can lead to organ injury and
postoperative morbidity. Causative factors include surgical trauma,
contact of blood with the extracorporeal circuit, and lung reperfusion
injury on discontinuing bypass. The perioperative
response to such procedures includes activation of the complement,
coagulation, fibrinolytic, and kallikrein cascades; activation of
neutrophils with degranulation and protease enzyme release; oxygen
radical production; and synthesis of various cytokines
from mononuclear cells (including tumor necrosis factor,
interleukin-1,16 17 and interleukin-6). Interleukin-1ß,
tumor necrosis factor-, and other cytokines are important
components of inflammation and the immune response. Adherence to
foreign surfaces activates monocyte production of
interleukin-1.18 Increased production of
interleukin-1 has been transiently found after cardiopulmonary
bypass, maximal at 24 hours coinciding with a peak in body
temperature.19 By contrast, plasma interleukin-1 was not
detected in 2 other reports,20 21 perhaps reflecting a
more important role as a paracrine mediator.22
Interleukin-1 has been shown to induce expression of COX-2 mRNA as
early as 15 minutes after exposure.23
NOS-2 in atrial tissue was significantly expressed even before cardiopulmonary bypass. Other studies have demonstrated NOS-2 mRNA and protein expression in human arteries with transplant coronary artery disease24 or in myocardium of patients with heart failure caused by either dilated cardiomyopathy or ischemic heart disease.25 No induction of NOS-2 after CP-Rep could be observed in our study, probably because the time that the hearts were subjected to cardioplegia was not long enough for observation of such induction. Synthesis of nitric oxide by NOS-2 is typically delayed 4 to 6 hours in response to inflammatory stimuli such as cytokines.26 This hypothesis remains to be determined because it has been shown27 that nitric oxide amplifies interleukin-1–induced COX- 2 expression. Interleukin-1 also induces NOS-2 expression in various cell types28 29 and may contribute to the inflammatory state observed after cardiac surgery. Interestingly, the NOS-3 protein level was decreased after CP-Rep, possibly because of alterations in mRNA or protein half-life. This may contribute to the endothelial dysfunction observed after cardiac surgery.
In conclusion, patients with atherosclerotic coronary arteries and baseline endothelial dysfunction may develop coronary vasoconstriction in response to 5-HT after CP-Rep. The release of 5-HT associated with platelet activation and the increased expression of COX-2 after CP-Rep may potentially contribute to myocardial ischemia after cardiac surgery.
Clinical Implications
Coronary artery contraction or spasm has been estimated to
occur in 
2.5% of patients after cardiac surgery.30 In
addition, ECG changes detected by Holter monitoring occur in 1% to 3%
of patients in the postoperative period.31 32 Furthermore,
a reduction in cardiac performance has been described in young
patients undergoing repair of transposition of the great vessels, a
condition not associated with coronary obstruction of altered
endothelial function.33 It is possible
that altered myocardial perfusion, in addition to myocardial edema and
inflammation, is 1 cause of this decrease in myocardial function. 5-HT
is a platelet-derived vasoactive agent that is released on
platelet activation during myocardial ischemia or
cardiopulmonary bypass or in response to other stimuli. 5-HT is
a prototypical agent to examine vascular responsiveness because it both
affects vascular smooth muscle and causes relaxation through the
release of endothelium-dependent substances. It has
been shown that serotonin is released into the
coronary circulation during angioplasty and that this
vasoactive substance may contribute to the occurrence of
vasoconstriction distal to the dilated site.34
Arterial 5-HT concentrations of
0.5x10-8 mol/L have been reported in patients
without coronary artery disease,35 and in a canine
model of coronary thrombosis, coronary sinus levels
>2x10-6 mol/L were found.36 It is
likely that local levels of 5-HT are increased even more in the
coronary microcirculation and other regions in which
platelet activation occurs during myocardial ischemia.
A previous study in pigs demonstrated that cold-blood cardioplegia preserves endothelium-dependent relaxation better than a purely crystalloid cardioplegic solution.37 In this previous study, indomethacin inhibited the increased contractile response to 5-HT, as was the case in the present study. However, in the present study, microvessels were examined from human subjects with underlying endothelial dysfunction as a consequence of hypercholesterolemia and other risk factors. The short- and long-term patency of saphenous vein bypass grafts in the arterial circulation could be increased if aspirin (cyclooxygenase inhibitor) is administered before the postoperative period. It is possible that this is due not only to reduced platelet activity but also to improved perfusion of myocardium distal to the site of a vein to coronary anastomosis. Finally, because only atrial microvascular responses were examined, it remains to be determined whether responses of the ventricular microcirculation and other vascular territories are affected in the same manner.
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Acknowledgments |
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This work was supported by National Institutes of Health grants RO1-HL-46716, RO1-HL-53793, and P50-HL-56993.
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