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Department of Physiology and Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri 65211
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ABSTRACT |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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The coronary
vasculature located distal to a chronic occlusion
(collateral-dependent) has been shown to exhibit altered reactivity to
vasoactive agonists. Thus we evaluated effects of chronic coronary artery occlusion on vasomotor responsiveness of collateral-dependent arteries isolated from a canine model of Ameroid occlusion of the left
circumflex (LCX) coronary artery. We compared in vitro responses of
large (~1.3- to 1.4-mm-ID) and small (~0.6-mm-ID) LCX arteries
located distal to an occlusion with responses of similar-sized segments
of the unoccluded left anterior descending (LAD) coronary artery.
-Adrenergic receptor-mediated contractile responses to
norepinephrine
(10
9-104
M) and phenylephrine
(109-104
M) in the presence of propranolol were markedly enhanced in large LCX
arteries compared with LAD arteries (P < 0.001). Prazosin (1 µM), an
1-adrenergic receptor
antagonist, abolished contractile responses of LCX and LAD arteries to
norepinephrine. Inhibition of nitric oxide synthesis with
N
-nitro-L-arginine
methyl ester (100 µM) enhanced norepinephrine-induced contractions of
LAD arteries to a greater extent than contractions of LCX arteries. We
simultaneously measured myoplasmic free
Ca2+ (fura 2 fluorescence ratio)
and contractile responses in LCX and LAD arteries denuded of
endothelium; norepinephrine-induced increases in myoplasmic free
Ca2+ and contractile tension were
significantly enhanced in LCX arteries compared with LAD arteries. In
addition, large and small LCX arteries exhibited impaired relaxation in
response to adenosine
(108-103
M) compared with LAD arteries (P < 0.05). In contrast, relaxation in response to the 
-adrenergic
agonist isoproterenol
(109-104
M) and sodium nitroprusside
(1010-104
M) was not significantly different in LCX and LAD arteries. Thus collateral-dependent coronary arteries exhibit enhanced -adrenergic vasoconstriction and impaired vasorelaxation in response to adenosine. The enhanced -adrenergic contractile responsiveness involves at
least two mechanisms: 1)
enhanced 1-adrenergic
reactivity of smooth muscle and
2) decreased
-adrenergic-induced synthesis of nitric oxide by the endothelium.
coronary artery occlusion; norepinephrine; phenylephrine; adenosine; myoplasmic free calcium
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INTRODUCTION |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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AFTER OCCLUSION of a major coronary artery, myocardium previously supplied by the occluded artery receives blood flow via the collateral circulation. Collateral arteries develop sufficiently to maintain normal perfusion of the collateral-dependent myocardium during resting conditions. However, there is evidence that indicates that the vasomotor reactivity of collateral-dependent vasculature located distal to a chronic coronary occlusion is significantly altered (23, 29, 30). Specifically, Sellke and colleagues reported that receptor-mediated endothelium-dependent relaxation of collateral-dependent microvessels is impaired in dogs 3-6 mo after induction of gradual coronary occlusion (30) and in pigs after 4-7 wk (29). In contrast to microvessels, larger branches of chronically occluded coronary arteries were reported to exhibit normal endothelium-dependent relaxation (2, 30). Coronary microvessels isolated from collateral-dependent myocardium were also found to be hyperresponsive to the potent vasoconstrictor vasopressin (23, 30).
Previous studies of vascular reactivity within hearts subjected to
chronic coronary occlusion have primarily focused on responsiveness of
collateral arteries and endothelium-dependent relaxation of collateral-dependent microvessels (2, 17, 23, 30). The objective of the
current study was to evaluate vasomotor responsiveness of
collateral-dependent arteries to selected adrenergic agonists and
cyclic nucleotide-dependent vasodilators. We used a canine model of
chronic occlusion of the proximal left circumflex (LCX) coronary
artery. We compared vasomotor responses of large (~1.3- to 1.4-mm-ID)
and small (~0.6-mm-ID) conduit arteries isolated from normal
[left anterior descending (LAD)] and collateral-dependent (LCX) regions of the heart. Specifically, we evaluated effects of
chronic coronary occlusion and collateral perfusion on vasomotor responsiveness to the -adrenergic agonists norepinephrine
(1 and
2) and phenylephrine
(1) compared with responses
to K+ and prostaglandin
F2
(PGF2). In addition, we
examined the responsiveness of collateral-dependent LCX arteries to the cyclic nucleotide-dependent vasodilators adenosine, isoproterenol, and
nitroprusside. We identified significant alterations in vasomotor responsiveness of collateral-dependent arteries. A secondary objective of our study was to gain insight into the specific cellular mechanisms underlying the alterations in vascular reactivity in the
collateral-dependent myocardium. Specifically, we investigated the
potential involvement of alterations in synthesis of nitric oxide
and/or changes in myoplasmic free
Ca2+
(Cam) regulation in the altered
responsiveness of collateral-dependent arteries.
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MATERIALS AND METHODS |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Induction of Chronic Coronary Artery Occlusion
Adult mongrel male dogs (25-35 kg) were anesthetized with acepromazine maleate (0.8 mg/kg sc) and pentobarbital sodium (25 mg/kg iv) and ventilated mechanically. An Ameroid constrictor (2.75-4.0 mm ID; Research Instruments and Manufacturing, Corvallis, OR), chosen to fit snugly around the vessel, was placed around the proximal LCX using sterile techniques. 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 on 11 November 1984 and were approved by the Animal Care and Use Committee of the University of Missouri.Preparation of Coronary Artery Rings for Studies of Vasomotor Function
We studied vasomotor function of coronary arteries isolated from hearts 4 mo (123 ± 1 days) after implantation of the Ameroid occluder. Occlusion of the LCX artery was confirmed by visual inspection of the occluder on the day of each experiment. Complete occlusion of the LCX artery occurred in 68 of 75 dogs that were instrumented with an Ameroid occluder. We performed separate analyses on data collected from the subset of dogs in which sterile surgery was performed, but the Ameroid occluder did not produce occlusion.On the day of the experiment, dogs were anesthetized with pentobarbital sodium (40 mg/kg), and the hearts were removed rapidly and placed in cold Krebs bicarbonate buffer. Hearts were kept in aerated iced Krebs buffer during isolation of coronary vessels. We isolated proximal portions of the LAD (normal) and LCX (collateral-dependent: distal to the Ameroid occluder) coronary arteries. These arteries had average inner diameters of 1.3-1.4 mm and were termed large conduit arteries. In addition, we excised smaller size-matched epicardial branches of the LAD (normal) and LCX (collateral-dependent) arteries with average inner diameters of 0.6 mm, which we termed small conduit arteries.
Arteries were cleaned of myocardium and connective tissue, cut into rings with axial lengths of 3.5-4.0 mm, and measured with a Filar microscope eyepiece (Hitschfel Instruments, St. Louis, MO). Arterial rings were carefully mounted on two stainless steel wires (Rocky Mountain Orthodontics). One wire was attached to a force transducer (model FT03c, Grass Instrument, Quincy, MA), and the other was attached to a micrometer (Stoelting/Prior Microdrive, Wood Dale, IL). After they were mounted, arterial rings were lowered into 20-ml tissue baths (Harvard Apparatus, S. Natick, MA) containing Krebs bicarbonate buffer maintained at 37°C and gassed with 95% O2-5% CO2.
After 1 h of equilibration, coronary arteries were systematically stretched to the optimum of the length-active tension relationship (Lmax). Arteries were progressively stretched in increments equal to 10% of the initial vessel outer diameter. After each stretch, a contraction was elicited with 30 mM K+. Lmax was defined as the circumferential length at which the active tension produced was <5% greater than the tension produced at the previous length. We have found that passive and active length-tension relationships are qualitatively and quantitatively similar in LCX and LAD arteries. Lmax (expressed as percentage of outer diameter) and resting tension at Lmax (in grams) were not significantly different between large and small conduit LAD and LCX arteries (data not shown). All protocols were performed with arteries stretched to Lmax.
In Vitro Evaluation of Vasomotor Responses
Concentration-response relationships were determined by cumulative additions of small aliquots of concentrated stock solutions directly to the tissue bath. Drug concentrations were increased when the response to the previous concentration was maximal. For selected experiments, we performed paired experiments using two rings cut from each artery and evaluated responses in parallel in the absence and presence of 100 µM N-nitro-L-arginine
methyl ester (L-NAME). Arterial
rings were incubated with L-NAME
for at least 10 min before evaluation of vasomotor responses.
Endothelium-dependent relaxation was studied in all arteries;
relaxation responses to the Ca2+
ionophore A-23187 (10 µM) and bradykinin (1 µM) were not
significantly different in LCX and LAD vessels, indicating preservation
of endothelial integrity in collateral-dependent arteries.
Simultaneous Measurement of Fura 2 Fluorescence and Contractile Tension
Contractile tension and Cam were measured simultaneously in large conduit coronary rings using a specially designed myograph and microfluorometry instrumentation and methods described previously (6, 32). Arterial rings (1-mm axial length) were carefully denuded of endothelium by gentle rubbing of the luminal surface with suture. Because the adventitia of these arteries produces high autofluorescence, a portion (~0.5 mm2) of the adventitia was removed. This carefully cleaned portion of the arterial ring was positioned directly over the objective during the experiment so that fura 2 fluorescence from the smooth muscle could be measured directly without the high autofluorescence of the adventitia. Smooth muscle of the arterial rings was loaded with fura 2 by incubation of the arteries with 10 µM fura 2 acetoxymethyl ester (fura 2-AM) for 2 h at 37°C. The fura 2loading solution contained 0.5% cremophor el and 5% bovine serum albumin and was vortexed and sonicated for 1 min to increase solubilization of fura 2-AM. Arteries were rinsed for 30 min at 37°C to remove extracellular fura 2-AM.Coronary rings were mounted on two stainless steel wires: one was attached to a digital micrometer to permit control of circumferential length (stretch) of the vessel, and the other was attached to a force transducer (Kulite Semiconductor Products, Leonia, NJ) for measurement of force. After being mounted on the myograph, arteries were lowered into a superfusion chamber positioned on the heated stage of a microfluorometry system described previously (32, 33). Light from a xenon arc lamp was passed to arterial rings via a liquid light guide through a rotating wheel containing 340- and 380-nm interference filters. The fluorescence emission at 510 nm was reflected to a photomultiplier tube with a dichroic mirror. The fluorescence was analyzed with an analog fluorescence signal processor and an analog-to-digital converter. Fluorescence and tension values were sampled every 5 s.
Arteries were superfused with Krebs buffer bubbled with 95% O2-5% CO2 and heated to 37°C. Each ring was stretched to Lmax (optimal length), as determined by repeated exposures to 80 mM K+ at increasing vessel lengths, as described above. Autofluorescence of arterial rings was determined at the end of each experiment by quenching the Ca2+-sensitive fura 2 fluorescence via exposure of the arteries to 10 mM Mn2+ and 5 µM ionomycin (6, 33). Fluorescence ratio was calculated after subtraction of vessel autofluorescence. Data acquisition and transformations were performed using AxoBASIC software customized for multichannel data acquisition (32, 33).
Solutions and Drugs
The Krebs bicarbonate solution for all experiments contained (in mM) 131.5 NaCl, 5.0 KCl, 1.2 NaH2PO4, 1.2 MgCl2, 2.5 CaCl2, 25 NaHCO3, and 10.1 glucose (bubbled with 95% O2-5% CO2, pH 7.4). This solution also contained 25 µM EDTA. For studies of-adrenergic contractile
responsiveness, 3 µM propranolol was added to the solution. Solutions
with elevated K+ concentrations
used for depolarizing smooth muscle were produced by equimolar
replacement of NaCl with KCl. Drugs were obtained from Sigma Chemical
(St. Louis, MO) unless otherwise indicated. We purchased endothelin-1
from Peninsula Laboratories (Belmont, CA), ionomycin from Calbiochem
(La Jolla, CA), PGF2 from Upjohn (Kalamazoo, MI), and fura 2-AM from Molecular Probes (Eugene, OR).
Data Analyses
Contractile responses of coronary arteries were expressed as absolute values in grams of developed tension (force). The concentration of agonist causing 50% of the maximal response was designated EC50 and was calculated using nonlinear regression analysis of the concentration-response data for each artery. Measurements of Cam were expressed as the ratio of fura 2 fluorescence at 340- and 380-nm excitation wavelengths because of uncertainties regarding extrapolation of in vitro calibrations to in vivo measurements (33). Concentration-response curves were compared using two-way analysis of variance for repeated measures. Subsequently, differences between individual points were ascertained using Fisher's test for least significant difference. We determined that data from experiments in which we simultaneously measured fura 2 fluorescence and contractile tension had non-Gaussian characteristics that could lead to erroneous conclusions if normal theory-dependent analyses were used. Therefore, these data were analyzed using Wilcoxon signed-ranks test. For all analyses, P < 0.05 was considered significant. Values are means ± SE.| |
RESULTS |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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Artery Dimensions and Passive Characteristics
Dimensions of the arterial rings used in this study are presented in Table 1. Proximal large conduit arteries had mean inner diameters averaging 1.3-1.4 mm. Large conduit LCX arteries tended to be slightly smaller than corresponding LAD arteries. Inner diameters of small conduit arteries averaged ~0.6 mm. Length-active tension relationships were qualitatively similar in large and small conduit LAD and LCX arteries. Lmax and resting tension at Lmax were not significantly different between the two groups of arteries (data not shown).
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Contractile Responses
Small conduit arteries.
We evaluated contractile responses of small conduit coronary
arteries to K+,
PGF2, endothelin, and
norepinephrine. Contractions of small LCX and LAD arteries in response
to increasing concentrations of K+
(5-100 mM), PGF2
(108-104
M), and endothelin
(1010-108
M) were not significantly different (P > 0.05, data not shown). Norepinephrine did not produce significant
increases in contractile tension in small LCX or LAD arteries. We did
not observe any alteration in the contractile responsiveness of small
arteries located distal to a chronic coronary occlusion.
Large conduit arteries.
Contractile responses of large conduit arteries to
K+ and
PGF2 are presented in Fig.
1. Concentration-dependent contractile responses of large LAD and LCX arteries to depolarization with K+ and
PGF2 were not significantly
different. Contractile responses to endothelin (30 nM) were also not
significantly different between large LCX and LAD arteries (data not
shown).
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1- and
2-adrenergic agonist
norepinephrine were markedly enhanced compared with contractions of
similar-sized LAD arteries (P < 0.001; Fig.
2A).
Norepinephrine-induced contractions of LAD and LCX arteries were
abolished in the presence of the 1-adrenergic receptor
antagonist prazosin (1 µM). Contractions of large LCX arteries in
response to the selective
1-adrenergic receptor agonist
phenylephrine were also significantly larger than contractions of LAD
arteries (P < 0.001; Fig.
3). Importantly, norepinephrine
concentration-response curves of LCX arteries isolated from the subset
of animals in which the Ameroid did not produce coronary occlusion were
not significantly different from those of LAD arteries isolated from
the same dogs (Fig. 2B).
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-adrenergic vasoconstriction, we evaluated norepinephrine-induced contractions in the presence of the nitric oxide synthase inhibitor L-NAME (100 µM).
Concentration-dependent increases in contractile tension were
significantly enhanced in large LAD arteries pretreated with
L-NAME compared with paired LAD
arteries not treated with the inhibitor
(P < 0.001; Fig. 2 vs. Fig.
4). In contrast, pretreatment with
L-NAME did not significantly
alter the concentration-response relationship to norepinephrine in
large LCX arteries (P > 0.05). In
the presence of L-NAME,
contractile responses of LCX arteries to norepinephrine remained
significantly enhanced compared with contractions of LAD arteries
(P < 0.01; Fig. 4).
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was not
altered by the presence of
L-NAME; contractions of LCX and
LAD arteries remained not different after pretreatment with
L-NAME (data not shown). Thus a
disparity in the effects of inhibition of nitric oxide synthesis on
contractile responses of LCX and LAD arteries was observed only with
norepinephrine.
Cam Responses to Norepinephrine
In additional studies we simultaneously measured Cam and contractile responses to norepinephrine in large conduit LCX and LAD arteries to determine whether the enhanced-adrenergic contractile responsiveness of LCX
arteries involved concomitant alterations in
Cam regulation. Simultaneous
measurements of fura 2 fluorescence ratio and contractile tension were
performed in large arteries denuded of endothelium and, thus,
represented responses of the smooth muscle that were independent of
effects of vasoactive substances released from the endothelium. Figure
5 illustrates average time-dependent increases in Cam (ratio) and
contractile tension in response to norepinephrine (3 and 30 µM) in
large LCX and LAD arteries. Norepinephrine-induced increases in
Cam (30 µM) and contractile
tension (3 and 30 µM) were significantly enhanced in smooth muscle of
LCX arteries compared with LAD arteries
(P < 0.05).
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Relaxation Responses
Adenosine.
We evaluated concentration-dependent relaxation in response to
adenosine in large and small conduit arteries preconstricted with
PGF2 (30 µM) or
K+ (30 mM). Adenosine-induced
relaxation of large conduit LCX arteries was significantly impaired
compared with relaxation of LAD arteries (Fig.
6). Relaxation to adenosine was also
significantly attenuated in small conduit LCX arteries compared with
similar-sized LAD arteries, but only when these arteries were
preconstricted with PGF2 (data
not shown).
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. Maximal relaxation of
large LCX and LAD arteries from nonoccluded dogs averaged 65 ± 9 and 78 ± 7, respectively (P > 0.05). Maximal relaxation of
small LCX and LAD arteries averaged 85 ± 9 and 83 ± 2, respectively (P > 0.05).
Effects of inhibition of nitric oxide synthesis were evaluated on
adenosine-mediated relaxation of large conduit arteries preconstricted
with PGF2. Pretreatment with
L-NAME did not inhibit
adenosine-induced relaxation of LCX or LAD arteries. Adenosine relaxation of large conduit LCX arteries remained significantly impaired in the presence of
L-NAME
(P < 0.01, data not shown).
Isoproterenol.
Relaxation in response to the -adrenergic agonist isoproterenol
was evaluated in large and small conduit coronary arteries. In contrast
to relaxation responses to adenosine, isoproterenol-induced relaxation
of large and small conduit LCX arteries was not significantly different
from relaxation of similar-sized LAD arteries
(P > 0.05, data not shown).
Nitroprusside. Relaxation in response to sodium nitroprusside was evaluated in large and small conduit coronary arteries. Relaxation in response to nitroprusside was not significantly different between large (Fig. 7) and small (data not shown) LCX and LAD arteries.
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DISCUSSION |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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The current study documents important alterations in the vasomotor
responsiveness of coronary vasculature located distal to a chronic
occlusion. We determined that large epicardial arteries located distal
to the occlusion exhibit enhanced contractile responsiveness to the
-adrenergic agonists norepinephrine and phenylephrine in the
presence of -adrenergic receptor blockade. In contrast to enhanced
-adrenergic-mediated vasoconstriction, contractions of LCX arteries
in response to K+,
PGF2, and endothelin were not
altered. Another new finding is that relaxation of large and small
collateral-dependent arteries in response to adenosine is impaired.
Importantly, -adrenergic-mediated vasoconstriction and
adenosine-mediated vasorelaxation were not altered in LCX arteries
isolated from dogs in which the Ameroid did not produce coronary
occlusion. These findings indicate that these alterations do not
reflect regional differences between normal LCX and LAD coronary
arteries, nor do these changes appear to result from the surgical
procedures. Thus chronic coronary occlusion produces potentially
detrimental changes in vasomotor reactivity of the distal vasculature,
resulting in an imbalance between vasoconstriction (enhanced
-adrenergic vasoconstriction) and vasodilation (impaired
adenosine-mediated vasodilation). Pathophysiological consequences of
this imbalance may involve increased vasoconstrictor tone
and/or an increased propensity for coronary vasospasm,
potentially resulting in initiation or aggravation of myocardial
ischemia in the collateral-dependent region of hearts with chronic
coronary occlusion.
Mechanisms of Enhanced -Adrenergic Contractile
Responsiveness
-adrenergic receptors is opposed by
1-adrenergic receptor-induced
vasoconstriction (4, 10). Although activation of
2-adrenergic receptors produces
vasoconstriction of canine coronary arterioles (9, 35),
2-adrenergic receptors do not normally mediate adrenergic vasoconstriction of large canine arteries (4). Instead, 2-adrenergic
receptor stimulation has been demonstrated to produce
endothelium-dependent relaxation in large canine coronary arteries (1,
5). 2-Adrenergic-mediated
relaxation of canine coronary arteries is attenuated by inhibitors of
nitric oxide synthesis (5), indicating that
2-adrenergic receptors on the endothelium of canine coronary arteries are coupled to the
synthesis/release of nitric oxide.
To gain insight into which subtype of -adrenergic receptor is
involved in the altered contractile responsiveness of LCX arteries, we
evaluated effects of the
1-adrenergic receptor
antagonist prazosin on norepinephrine-induced contractions of LCX and
LAD arteries. Prazosin abolished norepinephrine-induced contractions of
collateral-dependent LCX and normal LAD arteries, suggesting a dominant
role of 1-receptor activation
in these responses. We also determined that LCX arteries exhibit
enhanced contractile responsiveness to the selective
1-adrenergic agonist
phenylephrine compared with LAD arteries. Thus our data indicate that
1-adrenergic vasoconstriction
is enhanced in large LCX arteries located distal to a chronic
occlusion.
We investigated the role of nitric oxide in modulating -adrenergic
receptor-mediated vasoconstriction by evaluating the effects of
L-NAME on contractile responses
of large LCX and LAD arteries to norepinephrine. In these experiments,
inhibition of nitric oxide synthesis with
L-NAME (100 µM) greatly
enhanced norepinephrine-induced contractions of LAD arteries
(P < 0.01) but had no significant effect on contractions of collateral-dependent LCX arteries. However, concentration-response curves for norepinephrine remained significantly different in LCX and LAD arteries treated with
L-NAME
(P < 0.01), which is consistent with
the significant role of enhanced
1-adrenergic vasoconstriction.
In contrast to effects of L-NAME
on norepinephrine contractions, pretreatment with
L-NAME did not alter the
relative responsiveness of LCX and LAD arteries to
K+ depolarization or
PGF2. Thus our data indicate
that decreased synthesis of nitric oxide is involved in the enhanced
contractile responsiveness of LCX arteries to norepinephrine and that
this impaired nitric oxide production is likely selective for
-adrenergic stimulation.
Results from experiments in which we simultaneously measured
Cam and contractile responses to
norepinephrine in coronary arteries denuded of endothelium indicate
that the enhanced -adrenergic contractile responsiveness also
involves changes in reactivity of smooth muscle of LCX arteries
independent of alterations in endothelial function. Norepinephrine
elicited minimal Cam and contractile responses in LAD arteries, consistent with the report of
Shogakiuchi et al. (31) that normal coronary artery smooth muscle cells
do not exhibit significant Cam
responses to -adrenergic receptor activation. However,
Cam and contractile responses to norepinephrine were enhanced in parallel in smooth muscle of LCX arteries located distal to the chronic coronary occlusion.
Cam and contractile force are not
necessarily directly related during agonist stimulation, even in normal
vascular smooth muscle. Under many circumstances, contractions can be
sustained with little or no increase in
Cam (25). However, our
simultaneous measurements of Cam
and contraction indicate that the augmented -adrenergic contractions
of LCX arteries do result, at least in part, from parallel enhancement
of -adrenergic-mediated increases of
Cam. Furthermore, our results
suggest that enhanced contractile and Cam responses observed in LCX
arteries are selective for -adrenergic receptor stimulation, since
enhanced responses were not observed to
K+,
PGF2, or endothelin (Fig. 1,
other data not shown). Therefore, our results indicate that
-adrenergic receptors and/or signaling mechanisms including
Cam regulation are selectively
upregulated in the smooth muscle of arteries located distal to coronary
occlusion.
Collectively, our data indicate that the enhanced -adrenergic
contractile responsiveness of large LCX arteries located distal to a
chronic coronary occlusion involves alterations in -adrenergic responsiveness of the endothelium and smooth muscle. Comparison of the
relative responsiveness to norepinephrine, an
1- and
2-adrenergic agonist, and to
phenylephrine, an 1-adrenergic
agonist, also supports the role of dual mechanisms underlying enhanced
-adrenergic vasoconstriction of LCX arteries. Maximal
norepinephrine-induced contractions of LCX arteries were more than
sevenfold larger than contractions of LAD arteries. In contrast,
maximal contractions of LCX arteries in response to phenylephrine were
only 2.5-fold larger than contractions of LAD arteries. Thus the
disparity between -adrenergic contractile responses of
collateral-dependent LCX and normal LAD arteries is greater during
combined stimulation of 1- and
2-adrenergic receptors than
during stimulation of only
1-receptors. Evaluation of the
vasodilator effects of
2-adrenergic stimulation on
preconstricted arteries will be required to confirm a specific
alteration in
2-adrenergic-mediated nitric
oxide release in collateral-dependent arteries. However, all our data
are consistent with the conclusion that enhanced
1-adrenergic vasoconstriction and impaired 2-adrenergic
stimulation of nitric oxide synthesis from the endothelium underlie the
enhanced -adrenergic contractile responsiveness of LCX arteries
located distal to a chronic coronary occlusion.
Mechanisms of Impaired Relaxation
The vasodilatory effects of adenosine are generally considered to be mediated by activation of adenylate cyclase in vascular smooth muscle via coupling of A2 adenosine receptors to guanine nucleotide-binding stimulatory (Gs) proteins (13, 19, 21). Similarly, relaxation in response to isoproterenol involves coupling of-adrenergic receptors to activation of adenylate cyclase via Gs proteins (19, 28). This
correlation in signaling pathways suggests that attenuated relaxation
of LCX arteries to adenosine does not likely result from impairment of
mechanisms common to the signal transduction of both agonists (i.e.,
Gs proteins, adenylate cyclase,
adenosine 3',5'-cyclic monophosphate-dependent protein kinase), given that -adrenergic relaxation was not altered in LCX
arteries. Alternatively, impaired adenosine-mediated responses of LCX
arteries may result from decreased number of adenosine receptors or
decreased efficiency of adenosine receptor coupling to signaling
events. Our results do not discriminate between these possibilities.
Relaxation of collateral-dependent LCX arteries to nitroprusside, a
guanosine 3',5'-cyclic monophosphate-dependent vasodilator,
was not significantly different from relaxation of normal LAD arteries.
Unaltered relaxation responses to nitroprusside and isoproterenol
indicate that the attenuated adenosine relaxation exhibited by LCX
arteries does not result from a general functional impairment of smooth
muscle.
Relation to Other Studies
This is the first report of altered responsiveness of large epicardial coronary arteries located distal to a chronic coronary occlusion. Potential mechanisms of altered vasomotor function of the collateral-dependent vasculature have been proposed previously by Sellke et al. (30). One possibility is that ischemia produced by coronary occlusion alters the responsiveness of arteries located in the collateral-dependent myocardial region. Indeed, ischemia and reperfusion have been shown to produce impairment of adenosine-mediated relaxation (12, 24). Other possible explanations of our findings may relate to the chronic decreases in distending pressure and/or reduced levels of fluid shear stress present in the collateral-dependent vasculature distal to occlusion. The pressure differential across the occlusion averages 90% during the time of Ameroid occlusion (27), gradually decreasing to 15-20% after significant development of the collateral circulation (30). In addition, as suggested by Sellke et al., the pulsatile nature of blood flow may be altered in the vasculature located distal to the occlusion. Recently, mechanical forces have been shown to be important regulators of gene expression in vascular cells. Interestingly, expression of-adrenergic receptors has been shown to be regulated by stretch in
coronary vascular smooth muscle cells (22). However, the effects of the
changes in hemodynamic forces resulting from coronary occlusion on
-adrenergic and adenosine receptor populations in the distal
vasculature remain to be elucidated.
Disruption of adrenergic innervation of the collateral-dependent vasculature by surgical dissection and/or myocardial ischemia could theoretically produce denervation supersensitivity in collateral-dependent arteries (14, 34). However, Roth and co-workers (26) previously determined that gradual Ameroid occlusion of the proximal LCX artery in dogs does not significantly alter the number of catecholamine-containing nerve terminals in the collateral-dependent vasculature or myocardium. Furthermore, we found that placement of the Ameroid occluder around the LCX artery without coronary occlusion did not produce alterations in responsiveness of distal segments of the LCX (Fig. 2). Finally, sympathetic denervation supersensitivity is characterized by an increase in sensitivity to adrenergic agonists (14, 34). However, EC50 values for phenylephrine in LCX and LAD arteries were not significantly different. Thus our results are inconsistent with the phenomenon of denervation supersensitivity.
Implications
Convincing evidence exists for a significant role of-adrenergic
coronary vasoconstriction in the initiation and aggravation of
myocardial ischemia. In experimental models of coronary stenosis or
during reductions in coronary perfusion pressure, adrenergic-mediated vasoconstriction detrimentally modulates metabolic coronary dilation (8, 18). -Adrenergic vasoconstriction distal to a coronary stenosis
is sufficient to limit oxygen supply enough to impair myocardial
function severely (8). A significant role of -adrenergic coronary
vasoconstriction in exercise-induced myocardial ischemia has been
demonstrated in humans. Indeed, sympathetic activation by exercise has
been shown to induce critical narrowing of stenotic coronary arteries
sufficient to produce ischemic myocardial dysfunction and angina
pectoris (7, 15). -Adrenergic receptor antagonists reduce
exercise-induced S-T segment depression (3, 11) and angina pectoris (3)
and increase exercise capacity (11, 16) in patients with chronic stable
angina. Kalsner and Richards (20) observed enhanced
norepinephrine-induced contractions in coronary arteries from patients
with coronary artery disease, suggesting that our findings in dogs may
reflect pathological sequelae to experimental gradual coronary
occlusion that are similar to those observed clinically in humans.
Furthermore, augmented -adrenergic vasoconstriction may have
increased significance in the setting of impaired adenosine-mediated
relaxation responses, producing a pathophysiological imbalance and,
potentially, ischemia of collateral-dependent myocardium.
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ACKNOWLEDGEMENTS |
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The authors greatly appreciate the expert technical contributions of M. L. Mattox and Q. Zhong.
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FOOTNOTES |
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These studies were supported by research funds from the American Heart Association and National Heart, Lung, and Blood Institute Grant HL-47812, Training Grant HL-07094, and Program Project Grant PO1-HL-52490. M. Sturek is the recipient of National Heart, Lung, and Blood Institute Research Career Development Award HL-02872. J. A. Rapps was supported by a predoctoral fellowship from the American Heart Association, Missouri Affiliate.
Present address of J. A. Rapps: Dept. of Physiology, Medical College of Wisconsin, 8701 Watertown Plank Rd., PO Box 26509, Milwaukee, WI 53226-0509.
Address for reprint requests: J. L. Parker, Dalton Cardiovascular Research Center, University of Missouri, Columbia, MO 65211.
Received 23 December 1996; accepted in final form 13 June 1997.
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REFERENCES |
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Top
Abstract
Introduction
Materials & Methods
Results
Discussion
References
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