From the Departments of Physiology (J.A.R., Q.Z., P.R.M., J.L.P.) and
Medicine (P.R.M.), Dalton Cardiovascular Research Center, University of
Missouri, Columbia. Dr Parker is now in the Department of Medical Physiology,
Texas A&M University, College Station.
Correspondence to Janet L. Parker, PhD, Department of Medical Physiology, Texas A&M University, College Station, TX 77843. E-mail jparker{at}medicine.tamu.edu
Results—Collateral arteries isolated after 2 months exhibited
markedly reduced endothelium-dependent relaxation in
response to acetylcholine (ACh; 10-10 to 10-4
mol/L) and bradykinin (BK; 10-11 to 10-6
mol/L) compared with relaxation of noncollateral coronary
arteries (P<0.01). In contrast,
endothelium-independent relaxation of collateral
arteries to nitroprusside was only slightly reduced compared with
relaxation of noncollateral arteries (P<0.05).
Endothelium-dependent relaxation of collateral arteries
isolated after 4 and 9 months was increased significantly, to the
extent that relaxation to ACh and BK was not significantly different
between collateral and noncollateral arteries at these periods.
Inhibition of nitric oxide synthesis with
NT-nitro-L-arginine methyl
ester (L-NAME; 100 µmol/L) markedly inhibited ACh-induced
relaxation in all noncollateral arteries and in collateral arteries
isolated after 9 months. However, neither L-NAME nor
indomethacin (5 µmol/L) alone inhibited
ACh-mediated relaxation of collateral arteries isolated after 4 months.
ACh-induced relaxation of these collateral arteries was only inhibited
when arteries were preconstricted with 30 mmol/L K+
and pretreated with L-NAME and indomethacin (ie, when
synthesis/effects of nitric oxide, prostaglandins, and
endothelium-derived hyperpolarizing factor were
inhibited).
Conclusions—Development of endothelium-dependent
relaxation in canine coronary collateral arteries is not
complete after 2 months. After 4 months,
endothelium-dependent relaxation of collateral arteries
is similar to relaxation of noncollateral arteries, but the relaxation
exhibits decreased dependence on synthesis of nitric oxide and
increased involvement of prostaglandins and
endothelium-derived hyperpolarizing factor(s). After 9
months of development, collateral arteries exhibit normal nitric
oxide–dependent relaxation, similar to noncollateral arteries.
Mature collateral arteries have been reported to exhibit functional
endothelium-dependent relaxation in response to
acetylcholine (ACh), bradykinin (BK), and substance
P3,6,7; however, the time course of development
of endothelial function in maturing collateral arteries
after coronary occlusion has not been studied. Direct
evaluation of endothelial function of collateral
arteries is difficult in vivo because of complicating influences
(myocardial metabolites, circulating hormones, and sympathetic nerves)
and the complex system of series and parallel resistances that
contribute to regulation of blood flow to collateral-dependent
myocardium. Therefore, we performed in vitro evaluations of
endothelium-dependent relaxation of collateral and
noncollateral coronary arteries 2, 4, and 9 months after
induction of coronary occlusion with an ameroid occluder. These
studies enabled us to compare endothelium-dependent
relaxation responses of collateral arteries directly with responses of
similar-size normal coronary arteries and collateral-dependent
arteries (located distal to the occlusion). We also investigated the
relative roles of nitric oxide, prostaglandins, and
endothelium-derived hyperpolarizing factor(s) (EDHF) as
mediators of relaxation of collateral arteries at progressive stages of
collateral development.
Preparation of Coronary Artery Rings
Coronary rings were mounted on two stainless steel wires (Rocky
Mountain Orthodontics) in individual tissue baths as described
previously.5 Arteries were bathed in Krebs
bicarbonate buffer containing (in mmol/L) NaCl 131.5, KCl 5.0,
NaH2PO4 1.2,
MgCl2 1.2, CaCl2 2.5,
NaHCO3, and glucose 10.1 (37°C, bubbled with
95% O2/5% CO2). This
solution also contained 3 µmol/L propranolol and
25 µmol/L EDTA.
Coronary arteries were allowed to stabilize for 1 hour before
the experiment. Then, each coronary ring was systematically
stretched to the optimum of its length-active tension relation by
increments equal to 10% of the initial vessel diameter. After each
stretch, a contraction was induced with 30 mmol/L
K+. Arteries were studied at their optimal
length, which was defined as the length at which the active tension
produced was <5% greater than the tension produced at the previous
length.
In Vitro Evaluation of Relaxation Responses
Data Analyses
Development of Endothelium-Dependent
Relaxation
Noncollateral LAD and LCx isolated 2 months after ameroid placement
relaxed 70% to 80% in response to ACh and BK (Figures 1
ACh- and BK-induced relaxation was not significantly different between
collateral-dependent LCx and normal LAD isolated 2 and 4 months after
ameroid placement. However, ACh-induced relaxation of
collateral-dependent LCx isolated after 9 months was reduced
significantly compared with relaxation of LAD isolated from the same
hearts. In contrast, relaxation in response to BK was not significantly
different between LCx and LAD isolated after 9 months.
Development of
Endothelium-Independent Relaxation
Role of Nitric Oxide
L-NAME markedly inhibited ACh-induced relaxation of all noncollateral
LAD and LCx (P<0.01, Figures 4
Bradykinin Relaxation
The effects of L-NAME on relaxation to BK were similar in collateral
arteries preconstricted with either K+ or
endothelin. L-NAME did not significantly decrease maximal BK-induced
relaxation of collateral arteries isolated after 4 months, which was in
contrast to marked inhibition of relaxation of noncollateral arteries
preconstricted with K+ (Figure 6A
Nitroprusside Relaxation
Role of Prostaglandins
Bradykinin Relaxation
Combined Effects of L-NAME and Indomethacin
Time Course of Development of
Endothelium-Dependent Relaxation
Findings from studies of endothelial function during
regrowth after vascular injury are consistent with our results.
For example, agonist-mediated endothelium-dependent
relaxation has been shown to be impaired in canine coronary
arteries with newly developed endothelial cells 5 weeks
after balloon injury.9 Thus, it is plausible that
receptor/signal transduction coupling mechanisms in proliferating
endothelial cells may require extended periods (>2
months) to develop in collateral arteries as well.
Mechanisms of Endothelium-Dependent Relaxation in
Developing Coronary Collateral Arteries
In contrast to our findings, Flynn and
colleagues7 reported that
NG-monomethyl-L-arginine
(L-NMMA) produced similar inhibition of ACh- and BK-mediated relaxation
of collateral and noncollateral coronary arteries. The
inconsistency in our results may be explained by the
differences in the experimental design or in the time allowed for
collateral development. Flynn et al studied collateral arteries
isolated over a broad, unspecified range of time ("a minimum of 12
weeks"), whereas we studied collateral arteries isolated at specific
time intervals after ameroid implantation.
Dulas and coworkers8 studied
endothelium-dependent relaxation of collateral arteries
isolated 6 months after coronary occlusion. In contrast to the
report of Flynn et al,7 Dulas and colleagues
found that L-NMMA produced less inhibition of ACh relaxation of
collateral arteries than of noncollateral coronary arteries,
indicating decreased dependence on nitric oxide synthesis. These
investigators concluded that the greater residual relaxation in
collateral arteries induced by ACh after nitric oxide synthase and
cyclooxygenase blockade could result from enhanced
EDHF production in collateral vessels. The results of Dulas et
al correlate well with our findings. Taken in concert, these results
indicate that endothelium-dependent relaxation of
maturing collateral arteries progressively develops increased
dependence on nitric oxide synthesis between 4 and 9 months after
coronary occlusion.
Implications and Conclusions
In contrast to risks posed by delayed development of
endothelium-dependent relaxation in collateral arteries
early after coronary occlusion, the mechanisms underlying
relaxation of collateral arteries studied after 4 months may enhance
the ability of the collateral circulation to provide adequate perfusion
of the collateral-dependent myocardium. In contrast to the
marked dependence of noncollateral canine coronary arteries on
nitric oxide synthesis, these collateral arteries exhibit an increased
capacity for production of multiple
endothelium-derived relaxing factors in response to
receptor-dependent agonists. This redundancy or reserve of
endothelium-derived mediators of relaxation may promote
and maintain adequate perfusion of the collateral-dependent
myocardium under diverse conditions and thereby prevent
myocardial ischemia and infarction.
Received March 23, 1998;
revision received May 12, 1998;
accepted May 20, 1998.
2.
Hautamaa PV, Dai X, Homans DC, Bache RJ. Vasomotor
activity of moderately well-developed canine coronary
collateral circulation. Am J Physiol. 1989;256:H890–H897.
3.
Angus JA, Ward JE, Smolich JJ, McPherson GA.
Reactivity of canine isolated epicardial collateral coronary
arteries. Circ Res. 1991;69:1340–1352.
4.
Peters KG, Marcus ML, Harrison DG. Vasopressin and the
mature coronary collateral circulation. Circulation. 1989;79:1324–1331.
5.
Rapps JA, Jones AW, Sturek M, Magliola L, Parker JL.
Mechanisms of altered contractile responses to vasopressin and
endothelin in canine coronary collateral arteries.
Circulation. 1997;95:231–239.
6.
Altman J, Dulas D, Pavek T, Laxson DD, Homans DC,
Bache RJ. Endothelial function in well-developed canine
coronary collateral vessels. Am J Physiol. 1993;264:H567–H572.
7.
Flynn NM, Kenny D, Pelc LR, Warltier DC, Bosnjak J,
Kampine JP. Endothelium-dependent vasodilation of
canine coronary collateral vessels. Am J
Physiol. 1991;30:H1797–H1801.
8.
Dulas D, Altman J, Hirata-Dulas C, Bache RJ.
Endothelium-dependent vasodilation in well-developed
coronary collateral vessels. J Cardiovasc
Pharmacol. 1997;28:488–493.
9.
Cox RH, Haas KS, Moisey DM, Tulenko TN. Effects of
endothelium regeneration on canine coronary
artery function. Am J Physiol. 1989;257:H1681–H1692.
10.
Rubanyi GM, Romero JC, Vanhoutte PM. Flow-induced
release of endothelium-derived relaxing factor.
Am J Physiol. 1986;250:H1145– H1149.
11.
Cox DA, Vita JA, Treasure CB, Fish RD, Alexander RW,
Ganz OP, Selwyn AP. Atherosclerosis impairs
flow-mediated dilation of coronary arteries in humans.
Circulation. 1989;80:458–465.
12.
Quyyumi AA, Dakak N, Andrews NP, Gilligan DM, Panza JA,
Cannon RO. Contribution of nitric oxide to metabolic
coronary vasodilation in the human heart.
Circulation. 1995;92:320–326.
13.
Radomski VW, Palmer RMJ, Moncada S. Characterization of
the L-arginine:nitric oxide pathway in human platelets.
Br J Pharmacol. 1990;101:325–328.[Medline]
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Togna G, Gandolfi C, Andreoni A, Fumagalli A,
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© 1998 American Heart Association, Inc.
Basic Science Reports
Development of Endothelium-Dependent Relaxation in Canine Coronary Collateral Arteries
Abstract
Top
Abstract
Introduction
Methods
Results
Discussion
References
Background—Little information
exists regarding development of vasomotor control mechanisms during
coronary collateral artery maturation. Therefore, we studied
endothelium-dependent relaxation of canine collateral
arteries isolated 2, 4, and 9 months after placement of an ameroid
occluder around the proximal left circumflex coronary artery.
Key Words: acetylcholine • bradykinin • nitric oxide • collateral circulation • coronary occlusion
Introduction
Top
Abstract
Introduction
Methods
Results
Discussion
References
Viability of myocardium located distal to an
occlusion or severe stenosis depends on blood flow through
coronary collateral arteries. Several
studies1 2 3 have shown that mature collateral
arteries exhibit vasomotor activity in response to a variety of
agonists and thus play an active role in regulating blood flow to the
dependent myocardium. Pharmacological reactivity of
collateral arteries differs from that of normal coronary
arteries. For example, collateral arteries are more responsive to
vasopressin than normal coronary
arteries.1 4 5 In contrast, collateral arteries
are less responsive than normal coronary arteries to other
vasoconstrictors, including endothelin, prostaglandin
F2
, and the thromboxane
A2 mimetic U46619.3 5
Methods
Top
Abstract
Introduction
Methods
Results
Discussion
References
Induction of Collateral Artery Growth
Adult mongrel male dogs (23 to 37 kg) were
anesthetized with acepromazine maleate (0.8 mg/kg SC) and
sodium pentobarbital (25 mg/kg IV) and ventilated mechanically. Using
sterile techniques, we placed an ameroid constrictor (2.75 to 4.0
mm inner diameter, Research Instruments and Manufacturing) around the
proximal left circumflex coronary artery (LCx). During
surgery and recovery, dogs received buprenorphine hydrochloride (0.3 mg
IV or IM) as needed for pain relief. Antibiotics were given immediately
before surgery (900 000 U penicillin IM) and for 5 days after surgery
(800 mg sulfamethoxazole and 160 mg trimethoprim). All experimental
procedures were in accordance with the "Position of the American
Heart Association on Research Animal Use," adopted November 11, 1984,
and approved by the Animal Care and Use Committee of the University of
Missouri.
We studied the function of coronary arteries
isolated from dogs 2, 4, and 9 months after implantation of the
ameroid. Dogs were anesthetized with sodium pentobarbital (40
mg/kg), and hearts were rapidly removed and placed in aerated cold
Krebs bicarbonate buffer. Collateral arteries were easily identified as
tortuous, epicardial vessels extending from branches of the left
anterior descending coronary artery (LAD) to branches of the
LCx. Midportions of collateral arteries were excised and cleaned of fat
and connective tissue. Size-matched branches from the LAD (normal) and
LCx (collateral-dependent) were isolated from the same hearts. Each
artery was cut into rings. Care was taken to avoid damage to the
intimal surface. Using a thin section cut from the end of each ring, we
measured vessel dimensions (outer diameter, inner diameter, and vessel
wall thickness) with a Filar microscope eyepiece (Hitschfel
Instruments, Inc).
Relaxation responses were studied with arteries preconstricted
with 30 to 40 mmol/L K+ or 30 nmol/L
endothelin. Some rings were incubated continuously with enzyme
inhibitors beginning at least 30 minutes before evaluation
of relaxation responses. Concentration-response relationships to
various agonists were determined by cumulative additions of small
aliquots (20 to 150 µL; total volume <5% of bath volume) of
concentrated stock solutions directly to the tissue bath. Unless
otherwise indicated, drugs were purchased from Sigma Chemical Co. We
purchased endothelin-1 from Peninsula Laboratories, Inc and BK from
Bachem.
Relaxation responses were expressed as percent decrease of the
precontraction. The concentration of vasodilator causing 50% of the
maximal relaxation response was designated as the
EC50 and was determined by nonlinear regression
analysis of the concentration-response data for each vessel.
Concentration-response curves were compared by two-way ANOVA for
repeated measures followed by Fisher's test for least significant
difference. EC50 values and maximal relaxation
values were analyzed by two-way ANOVA, and pairwise multiple
comparisons between groups were performed with the Student-Newman-Keuls
test. A P value <0.05 was considered significant. Data are
presented as mean±SEM, and n values reflect the
number of animals.
Results
Top
Abstract
Introduction
Methods
Results
Discussion
References
Vessel Dimensions
Dimensions of the collateral, LAD, and LCx studied 2, 4, and
9 months after ameroid placement are presented in Table 1
. All dimensions of
LAD and LCx arterial rings were not significantly
different. Outer diameters of collateral arteries isolated after all
three periods were not significantly different from outer diameters of
LAD and LCx. However, collateral arteries had thicker walls than
noncollateral arteries, resulting from greater thickness of the intimal
layer of collateral vessels. Inner diameters of all collateral arteries
were smaller than inner diameters of LAD and LCx isolated at the same
periods.
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Table 1. Dimensions of Canine Coronary Arterial
Rings
We evaluated endothelium-dependent
relaxation of collateral and noncollateral coronary arteries 2,
4, and 9 months (60±2, 120±2, and 279±2 days) after placement of an
ameroid occluder around the proximal LCx.
K+-induced precontractions of collateral arteries
isolated after all periods were significantly smaller than contractions
of noncollateral vessels, averaging 34%, 41%, and 47%, respectively,
of contractions of the LAD and LCx. Relaxation responses of all
arteries were expressed as a percentage of the stable
K+ precontraction. and 2).
In contrast, maximal relaxation in response to ACh and BK averaged only
35% and 25%, respectively, in collateral arteries isolated after 2
months (P<0.01 versus noncollateral arteries). Relaxation
of collateral arteries isolated after 4 and 9 months was increased
significantly compared with relaxation of collateral arteries isolated
after 2 months. Relaxation in response to ACh and BK was not
significantly different between collateral and noncollateral arteries
at these later periods.
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Figure 1. Endothelium-dependent relaxation
in response to ACh in canine collateral artery (COL), LAD, and LCx
isolated 2, 4, or 9 months after placement of an ameroid occluder.
Arteries were preconstricted with 30 to 40 mmol/L K+.
Values are expressed as the mean±SEM. The number of animals used for
each study is indicated in parentheses. Relaxation of COL isolated
after 2 months was attenuated significantly compared with relaxation of
LAD and LCx (P<0.01). In contrast, 4 and 9 months after
ameroid placement, ACh relaxation responses were not significantly
different between collateral and noncollateral arteries. Relaxation of
collateral-dependent LCx and normal LAD isolated after 2 and 4 months
was not different. However, relaxation of LCx isolated after 9 months
was reduced significantly compared with relaxation of LAD
(P<0.05).
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Figure 2. Endothelium-dependent relaxation
in response to BK in COL, LAD, and LCx isolated 2, 4, or 9 months after
placement of an ameroid occluder. Arteries were preconstricted with 30
to 40 mmol/L K+. Each value is the mean±SEM. COL
isolated after 2 months exhibited reduced relaxation in response to BK
(P<0.01; COL versus LAD and LCx). In contrast, 4 and 9
months after ameroid placement, concentration-dependent relaxation in
response to BK was not significantly different in collateral, LAD, and
LCx.
Maximal relaxation to the endothelium-independent
vasodilator averaged >80% in collateral and noncollateral arteries
isolated at all periods (Figure 3).
However, sodium nitroprusside (NP)-induced relaxation of collateral
arteries studied at all periods was slightly but significantly
attenuated compared with relaxation of noncollateral arteries.
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Figure 3. Endothelium-independent relaxation
in response to increasing concentrations of sodium NP in COL, LAD, and
LCx isolated 2, 4, or 9 months after ameroid implantation. NP-mediated
relaxation of COL isolated at all periods was reduced significantly
compared with the relaxation of LAD and LCx
(P<0.01).
Acetylcholine Relaxation
We determined the effect of inhibition of nitric oxide synthesis
with L-NAME (100 µmol/L) on
endothelium-dependent relaxation of collateral arteries
isolated after 4 and 9 months. In studies performed 4 months after
ameroid placement, arteries were preconstricted either with 30 to
40 mmol/L K+ (Figure 4A) or with a maximal concentration (30
nmol/L) of endothelin (Figure 4B). As observed with
K+, contractions produced by endothelin were
significantly smaller in collateral than in noncollateral
arteries; endothelin-induced precontractions averaged 2.5±0.7,
8.1±0.7, and 7.7±0.8 g in collateral, LAD, and LCx vessels,
respectively. Neither K+- nor endothelin-induced
precontractions were altered significantly by L-NAME pretreatment.
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Figure 4. Effects of L-NAME (100 µmol/L) on
relaxation in response to ACh in COL, LAD, and LCx isolated 4 months
after ameroid placement. Coronary rings were preconstricted
with either 30 to 40 mmol/L K+ (A) or 30 nmol/L
endothelin (B). Relaxation of the LAD and LCx was significantly
inhibited by L-NAME (P<0.01). In contrast, relaxation
of COL was not significantly altered by L-NAME
(P>0.05). Similar results were obtained in arteries
preconstricted with either K+ or
endothelin. 
and 5). L-NAME significantly increased the
EC50 for ACh in noncollateral arteries (Tables
2 and
3). In contrast,
pretreatment with L-NAME did not significantly alter ACh relaxation of
collateral arteries isolated after 4 months and preconstricted with
either high K+ (Figure 4A) or endothelin (Figure 4B). However, 9 months after ameroid placement, relaxation of
collateral arteries in response to ACh was markedly inhibited by L-NAME
(P<0.01, Figure 5).
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Figure 5. Effects of L-NAME on ACh-mediated relaxation of
COL, LAD, and LCx isolated 9 months after ameroid placement. L-NAME
significantly inhibited the relaxation of all three types of arteries
(P<0.01).
The relative role of nitric oxide in mediating relaxation of
noncollateral coronary arteries to BK appeared to depend on the
type of precontraction (K+ versus endothelin).
L-NAME markedly attenuated BK-mediated relaxation of noncollateral
arteries preconstricted with K+
(P<0.01, Figure 6A). However,
when noncollateral arteries were preconstricted with endothelin, L-NAME
produced only a slight decrease (
15%) in maximal BK-induced
relaxation of LAD and LCx (P<0.05; Figure 6B) and a small
increase in EC50 (P<0.05, Table 3).
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Figure 6. Effects of L-NAME on relaxation responses to BK of
COL, LAD, and LCx isolated 4 months after ameroid placement.
Coronary rings were preconstricted with either 30 to 40
mmol/L K+ (A) or 30 nmol/L endothelin (B). L-NAME markedly
inhibited the relaxation of LAD and LCx preconstricted with
K+ (P<0.01). In contrast, maximal
BK-induced relaxation of COL preconstricted with K+ was not
significantly inhibited by pretreatment with L-NAME. However, the BK
concentration-response relationship of COL was shifted to the right in
the presence of L-NAME (P<0.05). L-NAME significantly
inhibited the BK-mediated relaxation of all arteries preconstricted
with endothelin; however, the degree of inhibition was markedly less
than that observed when the arteries were preconstricted with
K+.
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Table 3. pEC50 Values for Relaxation on
Endothelin-Induced Precontraction at 4
Months ). However,
L-NAME significantly increased the EC50
(decreased pEC50) for BK in collateral arteries (Tables 2 and 3). BK-induced relaxation of collateral arteries isolated after 9
months was markedly inhibited by L-NAME, similar to the relaxation of
noncollateral arteries (P<0.05, Figure 7).
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Table 2. pEC50 Values for Relaxation on
K+-Induced Precontraction at 4
Months
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Figure 7. Effects of L-NAME on BK-mediated relaxation of
COL, LAD, and LCx isolated 9 months after ameroid placement. Arteries
were preconstricted with 30 to 40 mmol/L K+.
L-NAME significantly inhibited the relaxation of all three artery types
(P<0.05).
L-NAME did not significantly alter the relaxation of LAD or LCx to
NP (data not shown). L-NAME slightly enhanced the relaxation of
collateral arteries to higher concentrations of NP
(>10-7 mol/L; P<0.05), but did not
significantly alter the EC50 value for NP (Table 2).
Acetylcholine Relaxation
The cyclooxygenase inhibitor
indomethacin (5 µmol/L) did not significantly
alter ACh-induced relaxation of either collateral or noncollateral
arteries isolated after 4 months (Figure 8A).
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Figure 8. Effects of indomethacin (INDO;
5 µmol/L) on ACh (A) and BK (B) relaxation of COL, LAD, and LCx
isolated 4 months after ameroid placement. ACh-induced relaxation of
COL, LAD, and LCx was not significantly altered by pretreatment with
indomethacin. Relaxation responses of LAD and LCx in
response to BK were also not significantly different in the presence of
indomethacin. However, indomethacin
produced a significant rightward shift in the concentration-response
relationship for BK in COL (P<0.05). Maximal
BK-mediated relaxation of COL was not altered by
indomethacin.
BK-mediated relaxation of collateral arteries isolated after 4
months was slightly inhibited by indomethacin (Figure 8B). The EC50 for BK in collateral arteries was
increased in the presence of indomethacin from 32±26
to 138±113 nmol/L (P<0.05). Relaxation of noncollateral
arteries in response to BK was not altered by
indomethacin.
We evaluated the effects of combined inhibition of nitric oxide
synthase and cyclooxygenase on relaxation of
arteries preconstricted with either endothelin or high
K+ to investigate the role of EDHF as a mediator
of ACh relaxation of collateral arteries isolated after 4 months.
ACh-induced relaxation of LAD and LCx preconstricted with either
endothelin or K+ was markedly inhibited by
combined pretreatment with L-NAME and indomethacin
(P<0.01, Figures 9 and 10). In contrast, ACh-induced
relaxation of collateral arteries preconstricted with endothelin was
not altered by combined inhibition of nitric oxide synthase and
cyclooxygenase with these drugs
(P>0.05, Figure 9). However, when collateral arteries were
preconstricted with K+, ACh-mediated relaxation
was markedly inhibited in the presence of L-NAME and
indomethacin (P<0.01, Figure 10).
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Figure 9. Combined effects of L-NAME (100 µmol/L) and
indomethacin (5 µmol/L) on ACh-induced
relaxation of COL, LAD, and LCx preconstricted with 30 nmol/L
endothelin. Arteries were isolated 4 months after ameroid placement.
L-NAME and indomethacin significantly inhibited the
relaxation of LAD and LCx (P<0.01). In contrast, ACh
relaxation of COL was not significantly altered by L-NAME and
indomethacin.
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Figure 10. Effect of L-NAME alone and the combined effects
of L-NAME and indomethacin on ACh relaxation of COL,
LAD, and LCx preconstricted with 30 to 40 mmol/L K+.
Arteries were isolated 4 months after ameroid placement. L-NAME alone
did not significantly alter ACh relaxation of COL but produced marked
inhibition of relaxation of LAD and LCx (P<0.01).
Pretreatment with L-NAME and indomethacin significantly
inhibited ACh-induced relaxation of COL, LAD, and LCx preconstricted by
depolarization with K+ (P<0.01).
Discussion
Top
Abstract
Introduction
Methods
Results
Discussion
References
The present study is the first systematic evaluation of
the chronologic development of endothelium-dependent
relaxation in maturing coronary collateral arteries. Our
results indicate that development of agonist-mediated
endothelium-dependent relaxation is not complete in
collateral arteries 2 months after ameroid implantation. After 4 months
of development, endothelium-dependent relaxation of
collateral arteries in response to ACh and BK appears to be similar to
endothelium-dependent relaxation of noncollateral
coronary arteries; however, this relaxation exhibits decreased
dependence on synthesis of nitric oxide compared with relaxation of
noncollateral arteries. In contrast, 9 months after ameroid
implantation, endothelium-dependent relaxation of
collateral arteries appears to be mediated primarily by synthesis of
nitric oxide, similar to noncollateral arteries. These observations may
have important implications relative to the critical role of collateral
arteries in providing blood flow to collateral-dependent
myocardium.
This is the first study to evaluate
endothelial function of collateral arteries isolated as
early as 2 months (60 days) after ameroid placement. Previous studies
have demonstrated intact endothelium-dependent
responses to ACh and BK in well-developed coronary collateral
arteries.3 7 8 However, the present study has
provided a new finding, that collateral arteries in early stages of
development exhibit reduced relaxation in response to
endothelium-dependent vasodilator agonists. Potential
mechanisms responsible for the marked attenuation of agonist-mediated
endothelium-dependent relaxation in immature collateral
arteries could include a paucity of muscarinic and kininergic receptors
on endothelial cells, deficiencies of signal
transduction elements coupling endothelial receptors to
synthesis of mediators of relaxation, low intrinsic activity of nitric
oxide synthase, and/or dysfunctional guanylyl cyclase–cGMP cascade in
smooth muscle. Impairment of guanylyl cyclase–cGMP cascade in smooth
muscle of collateral arteries appears to be unlikely as a primary
mechanism of impairment because relaxation in response to the nitric
oxide donor NP was only mildly decreased in collateral arteries
isolated after 2 months compared with noncollateral arteries.
Our results indicate that endothelium-dependent
relaxation of collateral arteries isolated after 4 months is not
dependent on synthesis of nitric oxide. These findings are in distinct
contrast to those from noncollateral coronary arteries isolated
from the same hearts. Results of our studies suggest that all three
known endothelium-derived mediators (nitric oxide,
prostaglandins, and EDHF) are involved in ACh-induced
relaxation of collateral arteries isolated after 4 months. Inhibition
of the synthesis or action of any one or two of the known mediators of
endothelium-dependent relaxation did not significantly
attenuate ACh relaxation of collateral arteries. ACh relaxation of
collateral arteries isolated after 4 months was attenuated
significantly only when the synthesis and/or action of nitric oxide,
prostaglandins, and EDHF were inhibited
simultaneously. These results indicate that
prostaglandins and EDHF have important roles as mediators
of endothelium-dependent relaxation in collateral
arteries isolated after 4 months and that
endothelium-dependent relaxation of collateral arteries
at this period involves a redundancy or reserve of mediators of
relaxation. However, endothelium-dependent relaxation
of collateral arteries isolated after 9 months was primarily dependent
on synthesis of nitric oxide, which indicates that the roles of
prostaglandins and EDHF as redundant mediators of
endothelium-dependent relaxation diminish with time
during collateral development.
Our findings have potentially important implications relative to
understanding collateral artery function and regulation of perfusion of
collateral-dependent myocardium. Reduced
endothelium-dependent vasodilation in collateral
arteries early in development suggests the possibility of an increased
propensity for vasospasm of collateral arteries and myocardial
ischemia soon after occlusion of a major coronary
artery. Endothelium-derived relaxing factors have been
implicated as important mediators of flow-mediated vasodilation of
coronary vessels10 11 and
coronary metabolic
vasodilation.12
Endothelium-derived relaxing factors (nitric oxide,
prostacyclin) also exert protective antithrombotic
effects.13 14 Thus, the decreased capability to
produce endothelium-derived relaxing factors in
developing collateral arteries may result in reduced vasodilation and
increased thrombogenesis, potentially limiting blood flow to
collateral-dependent myocardium.
Acknowledgments
The authors greatly appreciate the technical contributions made
by M. L. Mattox and M. A. Tanner. These studies were
supported by funds from the American Heart Association and National
Institutes of Health grant HL-47812, training grant HL-07094, and
program project P01 HL-52490. Dr Rapps was supported by a
predoctoral fellowship from the American Heart Association,
Missouri Affiliate.
References
Top
Abstract
Introduction
Methods
Results
Discussion
References
1.
Harrison DG, Chilian WM, Marcus ML. Absence of
functioning -adrenergic receptors in mature canine
coronary collaterals. Circ Res. 1986;59:133–142.
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