Vol. 276, Issue 4, H1305-H1312, April 1999
Effect of NO on transmural distribution of blood flow in
hypertrophied left ventricle during exercise
Dirk J.
Duncker1,2,
Jay H.
Traverse1,
Yutaka
Ishibashi1, and
Robert J.
Bache1
1 Cardiology Division,
Department of Medicine, University of Minnesota Medical School,
Minneapolis, Minnesota 55455; and
2 Laboratory for Experimental
Cardiology, Thoraxcenter, Erasmus University Rotterdam, 3000 DR
Rotterdam, The Netherlands
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ABSTRACT |
When exercise in the presence of a coronary
artery stenosis results in subendocardial ischemia,
administration of a nitric oxide (NO) donor increases subendocardial
blood flow, whereas NO synthesis blockade worsens subendocardial
hypoperfusion. Because left ventricular hypertrophy (LVH) is also
associated with subendocardial hypoperfusion during exercise, this
study tested the hypothesis that alterations of NO availability can
similarly influence subendocardial blood flow in the hypertrophied
heart. Studies were performed in seven dogs in which ascending aortic
banding resulted in an 80% increase in LV weight. Myocardial blood
flow was measured with microspheres during treadmill exercise that
increased heart rates to 216 ± 8 beats/min. During control
exercise, mean myocardial blood flow in animals with LVH was similar to
that in historic controls, but the ratio of subendocardial to
subepicardial blood flow was lower in animals with hypertrophy (0.88 ± 0.07) than in controls (1.36 ± 0.08;
P < 0.05). Blockade of NO synthesis with NG-nitro-L-arginine
(L-NNA; 1.5 mg/kg ic) caused no
change in heart rate or LV systolic pressure during exercise.
Furthermore, L-NNA did not
worsen subendocardial hypoperfusion during exercise. Intracoronary infusion of nitroglycerin (0.4 µg · kg
1 · min1)
did not significantly alter either mean blood flow or the transmural distribution of perfusion during exercise in the hypertrophied hearts.
Thus, unlike the subendocardial underperfusion that occurs when a
stenosis limits coronary blood flow, alterations of NO availability did
not alter subendocardial hypoperfusion in the hypertrophied hearts.
myocardium; nitroglycerin; NG-nitro-L-arginine; subendocardium
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INTRODUCTION |
IN THE NORMAL resting awake animal subendocardial blood
flow is greater than subepicardial flow, reflecting greater systolic wall stress and oxygen requirements in the deeper myocardial layers. This gradient of blood flow favoring the subendocardium is also maintained during treadmill exercise (12, 13). In
contrast, in the hypertrophied heart subendocardial blood flow
undergoes a subnormal increase during exercise, resulting in a
subendocardial-to-subepicardial blood flow ratio that is significantly
less than unity (2, 3, 6, 21, 22). A similar redistribution of blood
flow away from the subendocardium is observed when exercise is
performed in the presence of a coronary artery stenosis (12, 14, 24, 26). When blood flow during exercise is limited by a coronary stenosis,
alterations of nitric oxide (NO) availability can modulate the
transmural distribution of perfusion. Thus, when a coronary stenosis
resulted in hypoperfusion during exercise that was most severe in the
subendocardium, administration of the NO synthase inhibitor
NG-nitro-L-arginine
(L-NNA) aggravated the
hypoperfusion (12, 24). Worsening of the hypoperfusion occurred with no
change in distal coronary pressure, indicating that interruption of NO production decreased resistance vessel dilation in the ischemic myocardium. In contrast, when an NO donor was administered with no
change in pressure distal to the coronary stenosis, blood flow was
selectively increased in the deeper myocardial layers with no change in
flow to the subepicardium (14, 26). This finding suggested that NO
acted to selectively dilate the penetrating arteries that deliver blood
to the subendocardium. Fujii et al. (18) have reported that the
pressure drop across the penetrating arteries is increased in
hypertrophied hearts, suggesting that these vessels could have a
greater influence on the transmural distribution of blood flow in
hypertrophied than in normal hearts. Consequently, this study was
performed to test the hypothesis that blocking NO synthesis would
worsen subendocardial hypoperfsion and that supplying exogenous NO
would improve subendocardial blood flow in the hypertrophied heart
during exercise. Studies were performed in chronically instrumented
dogs trained to run on a treadmill.
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METHODS |
Studies were performed in seven mongrel dogs with left ventricular
hypertrophy (LVH). All experiments were performed in accordance with
the "Guiding Principles in the Care and Use of Laboratory Animals" as approved by the Council of the American Physiological Society and with prior approval of the Animal Care Committee of the
University of Minnesota.
Production of Supravalvular Aortic Stenosis
At 6-9 wk of age, dogs were sedated with acepromazine (0.4 mg/kg
im), anesthetized with thiamylal sodium (25-30 mg/kg iv), intubated, and ventilated with oxygen-enriched room air. A thoracotomy was performed through the third right intercostal space. The ascending aorta was dissected from the surrounding tissue ~1.5 cm above the
aortic valve and encircled with a polyethylene band 2.5 mm in width.
While LV and distal aortic pressures were measured simultaneously, the
band was tightened until a 20- to 25-mmHg peak systolic pressure gradient was achieved across the aortic constriction. The chest was
then closed, air was evacuated from the thorax, and the animals were
allowed to recover. Thereafter, animals were maintained in enclosed
runs on a standard laboratory diet until 10-14 mo of age, at which
time they were returned to the laboratory for study.
Surgical Instrumentation
After dogs were sedated with acepromazine (0.5 mg/kg im), they were
anesthetized with pentobarbital sodium (30-35 mg/kg iv), intubated, and ventilated with a mixture of oxygen (30%) and room air
(70%). Respiratory rate and tidal volume were set to keep arterial
blood gases within physiological limits. A left thoracotomy was
performed through the fifth intercostal space, and the heart was
suspended in a pericardial cradle. A polyvinyl chloride catheter (OD
3.0 mm) filled with heparinized saline was inserted into the left
internal thoracic artery and advanced into the ascending aorta. Similar
catheters were introduced into the right and left atria through the
atrial appendages and into the left ventricle at the apical dimple. A
solid-state micromanometer (model P5; Konigsberg Instruments, Pasadena,
CA) was also introduced into the left ventricle at the apex.
Approximately 1.5 cm of the proximal left anterior descending coronary
artery (LAD) was dissected free, and a Doppler flow velocity probe
(Craig Hartley, Houston, TX) was positioned around the artery.
Immediately distal to the flow probe, a hydraulic occluder was placed
around the vessel. A silicone catheter (ID 0.3 mm) bonded to a larger
silicone catheter (ID 1.6 mm) was introduced into the coronary artery
immediately distal to the hydraulic occluder. The pericardium was then
loosely closed, and the catheters and electrical leads were tunneled
subcutaneously to exit at the base of the neck. The chest was closed in
layers and the pneumothorax evacuated. Catheters were flushed daily
with heparinized saline.
Hemodynamic Measurements
Studies were performed with the animals in the awake state 2-3 wk
after surgery. Phasic and mean aortic pressure and coronary perfusion
pressure were measured with Gould P23 XL pressure transducers positioned at midchest level. LV pressure was measured with the micromanometer that was calibrated from the fluid-filled LV catheter. Rate of rise of LV pressure (LV
dP/dt) was obtained via electronic differentiation of the LV pressure signal. Mean coronary pressure was
measured with the silicone catheter in the LAD. Phasic and mean
coronary blood flow were measured with a Doppler flowmeter system
(Craig Hartley). Data were recorded on an eight-channel paper recorder
(Coulbourne Instruments, Lehigh Valley, PA).
Myocardial Blood Flow
To measure the transmural distribution of myocardial blood flow, we
injected 3 × 106
microspheres [15 µm in diameter and labeled with either
141Ce,
51Cr,
85Sr,
95Nb, or
46Sc (NEN, Boston, MA)] into
the left atrium. Before injection, microspheres were agitated for 15 min in an ultrasonic bath. An arterial blood reference sample was
withdrawn at a constant rate of 15 ml/min starting 10 s before
injection of microspheres and continuing for 90 s.
At the end of the experiment, the area perfused by the LAD was
identified by injection of 5 ml of Evans blue dye into the coronary
artery catheter. Immediately thereafter, the dogs were killed with an
overdose of intravenous pentobarbital sodium. The hearts
were excised, weighed, and fixed in 10% buffered Formalin. The atria,
aorta, right ventricular free wall, and large epicardial blood vessels
were dissected from the left ventricle and discarded. Duplicate
samples were obtained from the center of the blue-stained region of the
left ventricle perfused by the LAD and were divided into four layers of
equal thickness from epicardium to endocardium and placed in vials for
counting. Each myocardial sample weighed at least 0.5 g so that the
minimum number of microspheres was >400 per sample. Myocardial and
blood reference samples were counted in a gamma spectrometer (Packard
Instruments, Downers Grove, IL). The counts per minute were corrected
for background activity and for overlapping activity between isotopes.
Blood flow per gram of myocardium
(
m) was
computed as: m = r × Cm/Cr,
where r is the
rate of withdrawal of the reference blood sample (in ml/min),
Cm is the counts per minute per
gram of the myocardial specimen, and
Cr is the counts per minute of the
reference blood sample.
Experimental Protocols
L-NNA.
The effects of L-NNA on the
coronary blood flow responses to exercise in the hypertrophied left
ventricle were studied in seven dogs. After the dogs had been standing
quietly on the treadmill for 10 min, baseline measurements were made of
LV, aortic, and coronary pressures, maximum LV
dP/dt (LV
dP/dtmax), and
coronary blood flow velocity. The dogs then underwent a three-stage
exercise protocol (4.8 km/h at 0% grade, 6.4 km/h at 5% grade, and
6.4 km/h at 15% grade). Each stage was 3 min in duration;
hemodynamic measurements were obtained during the last 30 s of each
stage. At the highest level of exercise, radioactive microspheres were also injected for determination of the transmural distribution of
myocardial blood flow. After 90 min of rest, animals received an
intracoronary infusion of L-NNA
(1.5 mg/kg) delivered over 15 min into the coronary artery catheter.
The efficacy of this dose was previously shown in normal dogs (12, 24)
and was confirmed in two dogs with LVH under resting conditions in the present study (Fig. 1). Five minutes after
the infusion was completed, the exercise protocol was repeated.
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Fig. 1.
Increases in coronary blood flow produced by intracoronary infusions of
acetylcholine (A) and sodium
nitroprusside (B) under control
conditions (open symbols) and after intracoronary administration of 1.5 mg/kg NG-nitro-L-arginine
(L-NNA; solid symbols) in 2 resting dogs with left ventricular hypertrophy (LVH). Data are means ± SE.
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Nitroglycerin.
On a different day the seven dogs underwent an identical exercise
protocol under control conditions. After 90 min of rest, animals
received a continuous infusion of nitroglycerine (0.4 µg · kg1 · min1)
into the coronary artery. Five minutes after the infusion was started,
the exercise protocol was repeated. Hemodynamic measurements were
obtained and microspheres injected after steady-state exercise conditions had been achieved.
Data Analysis
Heart rate, LV peak systolic and end-diastolic pressures (measured at
onset of positive LV dP/dt), LV
dP/dtmax, mean
aortic and mean coronary pressures, and the mean coronary Doppler shift were measured from the strip-chart recordings. Coronary blood flow was
computed using the following equation: = 2.5 × 
f × d2, where
is coronary blood flow (in ml/min),
f is the Doppler shift (in kHz),
and d is the internal diameter of the
coronary artery (in mm) within the flow probe (27). The factor 2.5 is a
constant derived from the speed of sound in tissue
(c = 1.5 × 105 cm/s), the ultrasonic
frequency of the sound beam emitted
(f0 = 10 MHz),
the cosine of the angle at which the sound beam is emitted (45°),
and unit conversion factors (c × 
/4 × 3)/(2f0 × cos 45°). Because in chronically instrumented animals
the flow probe adheres to the coronary artery, the internal diameter of the flow probe is equal to the external diameter of the artery. To
obtain the internal diameter of the coronary artery, we subtracted the
arterial wall thickness, which in our experience is 20% of the
external diameter of the coronary artery. In this way, any error in the
computation of the coronary internal diameter would affect control and
intervention conditions equally.
Comparisons of systemic and coronary hemodynamic data were performed
using two-way (exercise level and treatment) analysis of variance with
replications. Comparison of myocardial blood flow data was performed
using two-way (myocardial layer and treatment) analysis of variance
with replications. When a significant effect was observed, individual
comparisons were made using the Wilcoxon signed-rank test and a
modified Bonferroni correction (23). Statistical significance was
accepted at P < 0.05 (two tailed). All data are presented as means ± SE.
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RESULTS |
Anatomic Data
The mean LV weight of animals with supravalvular aortic stenosis was
186 ± 13 g, whereas mean body weight was 23.0 ± 0.9 kg. The
LV-to-body weight ratio was 8.0 ± 0.3 g/kg, which represents an
~80% increase in relative LV mass compared with 4.1-4.8 g/kg in
historic control dogs (5, 15).
Systemic Hemodynamics
Treadmill exercise caused an increase in heart rate from 113 ± 3 beats/min at rest to a maximum of 216 ± 8 beats/min during the
heaviest stage of exercise (P < 0.01), an increase in LV systolic pressure from 206 ± 12 to 294 ± 15 mmHg (P < 0.01), an
increase in LV
dP/dtmax from
2,580 ± 220 to 5,540 ± 770 mmHg/s
(P < 0.01), and an increase in LV
end-diastolic pressure from 13 ± 2 to 35 ± 2 mmHg
(P < 0.01), but had no significant
effect on mean aortic pressure (88 ± 3 mmHg; Table
1). Neither intracoronary
L-NNA (Table 1) nor
nitroglycerin (Table 2) had a significant
effect on any of the systemic hemodynamic variables either at rest or during exercise.
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Table 1.
Systemic hemodynamics in seven dogs with LVH during graded levels of
treadmill exercise under control conditions and after intracoronary
administration of the NO synthase inhibitor L-NNA
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Table 2.
Systemic hemodynamics in seven dogs with LVH during graded levels of
treadmill exercise under control conditions and after intracoronary
administration of nitroglycerin
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Coronary Hemodynamics
Exercise increased coronary artery pressure from 115 ± 3 mmHg at
rest to a maximum of 161 ± 8 mmHg during the heaviest level of
exercise and increased coronary blood flow from 57 ± 4 to 109 ± 15 ml/min (both P < 0.01; Table 1).
Neither L-NNA (Table 1) nor
nitroglycerin (Table 2) had a significant effect on coronary artery
pressure or coronary blood flow either at rest or during exercise.
Furthermore, when coronary blood flow was plotted as a function of the
product of heart rate times LV systolic pressure (an estimate of
myocardial oxygen demand), neither
L-NNA nor nitroglycerine had a
significant effect on the relation between the rate-pressure product
and coronary blood flow (Fig. 2).
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Fig. 2.
Relation between myocardial oxygen demand estimated as product of heart
rate and left ventricular (LV) systolic pressure [rate-pressure
product (RPP) × 103] and coronary
blood flow in 7 dogs with LVH. Relations are shown under control
conditions (open symbols) and in presence of
L-NNA (1.5 mg/kg ic;
A) or nitroglycerin (0.4 µg · kg1 · min1
ic; B) (solid symbols). Neither
L-NNA nor nitroglycerin had a
significant effect on the relation. Data are means ± SE.
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Regional Myocardial Blood Flow
Myocardial blood flow rates measured with microspheres are shown in
Tables 3 and 4, whereas the transmural
distribution of blood flow is shown in Fig.
3. Subendocardial-to-subepicardial blood
flow ratios during exercise were 0.88 ± 0.07 in the anterior LV
wall perfused by the LAD and 0.92 ± 0.07 in the posterior control region. These values were not significantly different from unity. These
ratios in the hearts with LVH were less than those in normal exercising
dogs with heart rates of ~200 beats/min in which we observed that the
transmural distribution of myocardial blood flow significantly favored
the subendocardium with a subendocardial-to-subepicardial blood flow
ratio of 1.36 ± 0.08 (3). During exercise, neither L-NNA (Table 3 and Fig. 3) nor
nitroglycerin (Table 4 and Fig. 3) produced
a change in blood flow to any of the four myocardial layers. Neither
L-NNA nor nitroglycerin caused a
significant change in the subendocardial-to-supepicardial blood flow
ratio.
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Table 3.
Myocardial blood flow in seven dogs with LVH during treadmill
exercise under control conditions and after intracoronary
administration of L-NNA
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Fig. 3.
Myocardial blood flow in 4 layers across LV wall from subepicardium
(Epi) to subendocardium (Endo) of 7 exercising dogs with LVH.
Myocardial blood flow is shown during control exercise (open symbols)
and during exercise in presence of
L-NNA (1.5 mg/kg ic;
A) or nitroglycerin (0.4 µg · kg1 · min1
ic; B) (solid symbols). Neither
L-NNA nor nitroglycerin had a
significant effect on blood flow in any myocardial layer. Data are
means ± SE.
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Table 4.
Myocardial blood flow in seven dogs with LVH during treadmill
exercise under control conditions and after intracoronary
administration of nitroglycerin
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DISCUSSION |
In agreement with previous reports, subendocardial blood flow relative
to subepicardial flow was significantly less in hypertrophied than in
normal left ventricles during equivalent levels of treadmill exercise,
resulting in a lower subendocardial-to-subepicardial blood flow ratio
in the hypertrophied hearts (2, 3, 6, 16, 21, 22). When a coronary
artery stenosis produced a similar pattern of relative subendocardial
underperfusion during exercise in normal hearts, blockade of NO
synthesis caused worsening of subendocardial hypoperfusion (12, 24),
whereas administration of nitrovasodilators (including nitroglycerin)
improved subendocardial blood flow (14, 26). In contrast to those
findings, when pressure-overload LVH resulted in relative
subendocardial hypoperfusion during exercise, neither inhibition of NO
production nor administration of nitroglycerin significantly altered
subendocardial blood flow in the present study. The implications of
these findings will be discussed in detail.
Role of NO in Normal Heart
Metabolic vasodilation in response to increasing myocardial oxygen
needs is mediated principally by opening of ATP-sensitive potassium
channels in coronary arterioles <100 µm in diameter (25, 31). The
endothelial NO pathway represents an endogenous mechanism by which
vasomotor tone in coronary vessels that are not under direct metabolic
control can be adjusted in response to changes in myocardial metabolic
needs (29, 30). The increase in blood flow produced by metabolic
vasodilation of the arterioles causes increased shear on the
endothelium of the small arteries. Because these small coronary
arteries (diameters 100-400 µm) account for as much as 40% of
total coronary resistance (11), NO-dependent vasodilation of this
segment of the coronary vascular bed is likely to be important during
the high metabolic demand associated with exercise. Estimates of NO
production in the heart from measurements of the arteriovenous
difference in the sum of nitrite plus nitrate have demonstrated that
exercise is associated with increased coronary NO production (8).
However, in normal dogs blockade of NO synthesis with
L-NNA did not impair the ability
to increase coronary blood flow during treadmill exercise (1).
Interestingly, in that study blockade of NO synthesis caused modest but
significant increases of coronary blood flow that were associated with
significantly increased myocardial oxygen consumption (1). This effect
may have been, in part, the result of an increased rate-pressure
product secondary to the increased arterial pressure caused by
L-NNA or may result from the
ability of NO to modulate myocardial oxygen consumption by direct
effects on mitochondrial respiration (8). Coronary venous oxygen
content was slightly but significantly decreased during exercise after
blockade of NO synthesis (1). However, the relation between myocardial
oxygen consumption and coronary venous oxygen tension was not
significantly altered during exercise (1, 25), indicating that
endogenous NO is not mandatory for the coronary vasodilation that
occurs during normal exercise. Similarly, inhibition of NO production
had no effect on the transmural distribution of blood flow in normally
perfused myocardium during exercise (12).
Role of Endogenous NO in Ischemic Heart
The contribution of endogenous NO to coronary vasodilation during
exercise is greater in hypoperfused than in normal myocardium. In
chronically instrumented dogs an arterial stenosis that decreased distal LAD coronary pressure to 55 mmHg during treadmill exercise caused a decrease in mean blood flow measured with microspheres to 1.09 ± 0.13 ml · min1 · g1
in the region perfused by the stenotic LAD compared with 2.57 ± 0.50 ml · min1 · g1
in a remote, normally perfused region (12). When NO synthesis was
blocked with L-NNA, with no
change in distal coronary pressure, mean blood flow in the LAD region
during exercise decreased further to 0.68 ± 0.11 ml · min1 · g1.
Although blockade of NO synthesis decreased blood flow in all transmural myocardial layers, the relative reduction of flow was most
prominent in the subendocardium (12). Studies of isolated coronary
microvessels have demonstrated that NO production occurs in both small
arteries and arterioles (28, 29). Thus the decrease in flow in response
to L-NNA could have been the
result of vasoconstriction either in the small arteries that are not
under metabolic control but that contribute significantly to total
coronary resistance (>100 µm in diameter) (28, 29) or in arterioles
that are under metabolic control (<100 µm in diameter) (28).
Tschudi et al. (35) demonstrated that isolated porcine small coronary
arteries (~300 µm in diameter) responded to
NG-monomethyl-L-arginine with
vasoconstriction, indicating tonic release of NO even in the absence of
flow. Kuo et al. (33) provided evidence that basal release of NO also
occurs in isolated canine coronary arterioles (40-80 µm in
diameter), but only in the presence of flow. In the in vivo canine
heart, blockade of NO production caused constriction of arterioles
(<100 µm in diameter), whereas results in arterial vessels >100
µm in diameter were equivocal (28, 32); nevertheless, vessels of both
sizes dilated in response to acetylcholine (32). The differences
between studies could be related to differences in species or
experimental conditions, but it is clear that NO production can occur
in coronary arterial microvessels of all sizes.
Response to Exogenous NO in Normal and Ischemic Heart
Selke et al. (34) demonstrated important differences in the response of
isolated coronary arterial segments to nitroglycerin depending on
vessel size. They found that nitroglycerin was a potent dilator of both
large epicardial conduit arteries and small coronary arteries (>190
µm in diameter) but had little effect on arterioles. The importance
of this finding is that the small arteries that are not under metabolic
control contribute up to 40% of total coronary resistance (11). During
high work states that result in metabolic vasodilation of the
arterioles, vasoconstriction of the small arteries could have the
potential to decrease myocardial blood flow. In this situation
nitrovasodilators, which act principally at the level of the large and
small arteries with little effect on the arterioles, could increase
blood flow in regions where metabolic arteriolar vasodilation had
already occurred. We previously observed that when exercise in the
presence of a coronary stenosis resulted in hypoperfusion, the
administration of nitrovasodilators such as nitroglycerin or the NO
donors ITF-296 and ITF-1129 caused an increase in blood flow in the
hypoperfused region (14, 26). Because the increase in coronary flow
occurred with no change in distal coronary pressure, the findings imply
that NO caused vasodilation of resistance vessels within the ischemic
region. Interestingly, the effects of the NO donor were selective for the ischemic region, because these agents caused no alteration of
coronary blood flow or its transmural distribution in the normally perfused region (14, 26).
An interesting effect of the nitrovasodilators in the presence of a
coronary stenosis was a preferential increase in blood flow to the
deeper myocardial layers of the hypoperfused region, resulting in an
increase of the subendocardial-to-subepicardial blood flow ratio.
Selective augmentation of blood flow to the subendocardium in an
ischemic region suggests that these agents exert an effect on the
penetrating arteries. The penetrating arteries that conduct blood from
the epicardial arteries to the subendocardial microvasculature range
from 100 to 500 µm in diameter (9, 17), the size range at which NO
has its greatest effect. Chilian (10) reported that in the normal heart
intravascular pressure in arterial segments 100 µm in diameter is
considerably lower in the subendocardium than in the subepicardium,
indicating that a significant pressure drop occurs across the
penetrating arteries. Furthermore, Fujii et al. (18) demonstrated that
this pressure gradient was increased in dogs with LVH secondary to
renovascular hypertension. Those measurements were obtained in isolated
perfused hearts that were arrested in diastole so that the greater
pressure loss across the penetrating arteries could not be attributed
to differences in extravascular forces between the normal and
hypertrophied ventricles.
NO in Hypertrophied Heart
Neither supplying exogenous NO with intracoronary nitroglycerin nor
blocking endogenous synthesis of NO with
L-NNA had a significant effect
on myocardial blood flow during exercise. The lack of effect of
L-NNA and nitroglycerin on
myocardial blood flow was not due to insufficient dosing of these
compounds. Thus the intracoronary dose of 1.5 mg/kg of
L-NNA used in the present study
has previously been demonstrated to result in effective inhibition of
acetylcholine-induced NO-mediated coronary vasodilation in normal dogs
(14, 24, 25), and this was confirmed in two dogs with LVH in the
present study. Moreover, in normal dogs, this dose of
L-NNA aggravated myocardial
hypoperfusion during exercise in the presence of a coronary artery
stenosis (12, 24). Similarly, the dose of 0.4 µg · kg1 · min1
of intracoronary nitroglycerin used in the present study (resulting in
a total dose of ~5 µg/kg) is more than adequate to produce a
maximum effect, as based on previous reports that intravenous doses as
low as 0.3-15 µg/kg produced marked large coronary artery vasodilation (4, 7, 36). In addition, we observed that intravenous
nitroglycerin administered in a dose of 2 µg · kg1 · min1
(which was without any systemic hemodynamic effect) increased subendocardial blood flow distal to a coronary artery stenosis during
exercise (26).
The hypothesis that the lack of response to inhibition of NO production
or the lack of response to exogenous NO was not the result of decreased
sensitivity to NO in the hypertrophied heart is suggested by the
coronary blood flow responses to nitroprusside and acetylcholine. In
normal dogs, nitroprusside in an intracoronary dose of 3 µg · kg1 · min1
resulted in increases in coronary blood flow of 30-70 ml/min from
a baseline of 45-50 ml/min (25). Similarly, in the LVH dogs in the
present study, this dose of nitroprusside produced an increase in
coronary flow of 60 ± 18 ml/min from a baseline of 57 ± 4 ml/min. Furthermore, the responses to acetylcholine appeared normal in
the LVH hearts; acetylcholine in an intracoronary dose of 0.5 µg · kg1 · min1
caused a 73 ± 10 ml/min increase in coronary flow, whereas in normal dog hearts we previously observed that this dose of
acetylcholine caused increases in coronary flow of 60-70 ml/min
(25). These findings suggest that the coronary resistance vessels in
the hypertrophied hearts respond normally to exogenous NO
(nitroprusside) and that NO production via receptor-mediated
stimulation of NO synthase (acetylcholine) is intact in the resistance
vessels of the LVH heart. Therefore, the findings of the present study
cannot be ascribed to reduced sensitivity to NO in the microcirculation of the hypertrophied heart.
In normal hearts, blood flow to the subendocardium is maintained
greater than that to the subepicardium, reflecting greater systolic
wall stress and oxygen requirements in the deeper myocardial layers.
This preferential delivery of blood to the subendocardium is also
maintained during exercise in normal hearts. In contrast, in the
hypertrophied hearts in the present study, the preferential delivery of
blood flow to the subendocardium was lost, with a subendocardial-to-subepicardial blood flow ratio not significantly different from unity. We have recently shown that loss of the normal
preferential blood flow to the subendocardium in the hypertrophied hearts is not the result of exhaustion of vasodilator reserve in this
region (13), suggesting that the smaller increase of subendocardial
blood flow during exercise does not imply subendocardial ischemia. Hittinger et al. (22) reported that, unlike exercise in normal dogs, which resulted in an increase of systolic wall thickening in both the subendocardium and the subepicardium, exercise in animals with pressure-overload hypertrophy resulted in increased subepicardial systolic wall thickening but decreased subendocardial thickening. In that study subendocardial wall thickening decreased within the first 3 s after the onset of exercise, at a time when LV
pressure and wall stresses had not yet increased (21). This suggests
that the decreased subendocardial-to-subepicardial blood flow ratio in
the hypertrophied hearts during exercise may have been the result of
relatively lower metabolic demands in this region rather than
insufficient perfusion. However, it is possible that heavier levels of
exercise than were used in the present study could result in exhaustion
of vasodilator reserve in the subendocardium. This is supported by data
reported by Hittinger et al. (22) showing that, after a period of heavy
exercise (heart rates of ~250 beats/min), subendocardial wall
thickening remained below the preexercise level for 
1 h, suggesting
postischemic subendocardial stunning. Furthermore, there is ample
clinical evidence that the pressure-overload hypertrophied heart is
susceptible to subendocardial ischemia (3, 19, 20).
In conclusion, the present study confirms previous reports that the
subendocardial-to-subepicardial blood flow ratio is abnormally decreased during exercise in animals with LVH. However, whereas exercise produced subendocardial underperfusion in the presence of a
coronary artery stenosis, the blockade of NO production did not worsen
subendocardial hypoperfusion and a supply of exogenous NO did not
improve subendocardial blood flow in the hypertrophied hearts. Thus
alterations of NO production do not influence the relative
subendocardial underperfusion that occurs in the hypertrophied heart
during exercise.
| |
ACKNOWLEDGEMENTS |
This study was supported by National Heart, Lung, and Blood
Institute Grants HL-21872 and HL-20598. D. J. Duncker's research was
made possible in part by a research fellowship awarded by the Royal
Netherlands Academy of Arts and Sciences.
| |
FOOTNOTES |
The costs of publication of this
article were defrayed in part by the
payment of page charges. The article
must therefore be hereby marked
"advertisement"
in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: R. J. Bache,
Div. of Cardiology, Dept. of Medicine, Univ. of Minnesota Medical
School, Box 508 UMHC, 420 Delaware St. SE, Minneapolis, MN 55455 (E-mail: bache001{at}maroon.tc.umn.edu).
Received 18 August 1998; accepted in final form 5 January 1999.
| |
REFERENCES |
1.
Altman, J. D.,
J. Kinn,
D. J. Duncker,
and
R. J. Bache.
Effect of inhibition of nitric oxide formation on coronary blood flow during exercise.
Cardiovasc. Res.
28:
119-124,
1994[Abstract/Free Full Text].
2.
Bache, R. J.
Effects of hypertrophy on the coronary circulation.
Prog. Cardiovasc. Dis.
31:
403-440,
1988.
3.
Bache, R. J.,
D. Alyono,
X. Dai,
T. R. Vrobel,
D. G. Parrish,
and
D. C. Homans.
Myocardial blood flow during exercise in left ventricular hypertrophy produced by aortic banding and perinephritic hypertension.
Circulation
76:
835-842,
1987[Abstract/Free Full Text].
4.
Bache, R. J.,
R. M. Ball,
F. R. Cobb,
J. C. Rembert,
and
J. C. Greenfield.
Effects of nitroglycerin on transmural myocardial blood flow in the unanaesthetized dog.
J. Clin. Invest.
55:
1219-1228,
1975.
5.
Bache, R. J.,
and
X. Z. Dai.
Myocardial oxygen consumption during exercise in the presence of left ventricular hypertrophy secondary to supravalvular aortic stenosis.
J. Am. Coll. Cardiol.
15:
1157-1164,
1990[Abstract].
6.
Bache, R. J.,
T. R. Vrobel,
W. S. Ring,
R. W. Emery,
and
R. W. Andersen.
Regional myocardial blood flow during exercise in dogs with chronic left ventricular hypertrophy.
Circ. Res.
48:
76-87,
1981[Free Full Text].
7.
Berdeaux, A.,
B. Ghaleh,
J. L. Dubois-Rande,
B. Vigue,
C. Drieu La Rochelle,
L. Hittinger,
and
J. F. Giudicelli.
Role of vascular endothelium in exercise-induced vasodilation of large epicardial coronary arteries in conscious dogs.
Circulation
89:
2799-2808,
1994[Abstract/Free Full Text].
8.
Bernstein, R. D.,
F. Y. Ochoa,
X. Xu,
P. Forfia,
W. Shen,
C. I. Thompson,
and
T. H. Hintze.
Function and production of nitric oxide in the coronary circulation of the conscious dog during exercise.
Circ. Res.
79:
840-848,
1996[Abstract/Free Full Text].
9.
Boucek, R. J.,
A. R. Morales,
R. Romanelli,
and
M. P. Judkins.
Coronary artery branch arteries and collateral circulation: the vascular supply to specialized areas of the heart.
In: Coronary Artery Disease: Pathologic and Clinical Assessment. Baltimore, MD: Williams and Wilkins, 1983, p. 108-130.
10.
Chilian, W. M.
Microvascular pressures and resistances in the left ventricular subepicardium and subendocardium.
Circ. Res.
69:
561-570,
1991[Abstract/Free Full Text].
11.
Chilian, W. M.,
S. M. Layne,
C. Klausner,
C. L. Eastham,
and
M. L. Marcus.
Redistribution of coronary microvascular resistance produced by dipyridamole.
Am. J. Physiol.
256 (Heart Circ. Physiol. 25):
H383-H390,
1989[Abstract/Free Full Text].
12.
Duncker, D. J.,
and
R. J. Bache.
Nitric oxide contributes to coronary vasodilation distal to a coronary artery stenosis that results in myocardial hypoperfusion during exercise.
Circ. Res.
73:
629-640,
1994[Abstract/Free Full Text].
13.
Duncker, D. J.,
Y. Ishibashi,
and
R. J. Bache.
Effect of treadmill exercise on the transmural distribution of blood flow in the hypertrophied left ventricle.
Am. J. Physiol.
275 (Heart Circ. Physiol. 44):
H1274-H1282,
1998[Abstract/Free Full Text].
14.
Duncker, D. J.,
J. Mizrahi,
and
R. J. Bache.
Nitrovasodilators ITF 296 and isosorbide dinitrate exert anti-ischemic activity by dilating coronary penetrating arteries.
J. Cardiovasc. Pharmacol.
25:
823-832,
1995[Medline].
15.
Duncker, D. J.,
J. Zhang,
and
R. J. Bache.
Coronary pressure-flow relation in left ventricular hypertrophy. Importance of changes in back pressure versus changes in minimum resistance.
Circ. Res.
72:
579-587,
1993[Abstract/Free Full Text].
16.
Duncker, D. J.,
J. Zhang,
M. Crampton,
and
R. J. Bache.
1-Adrenergic activity restricts subendocardial blood flow in the hypertrophied left ventricle of exercising dogs.
Basic Res. Cardiol.
90:
73-83,
1995[Medline].
17.
Estes, E. H.,
M. L. Entman,
H. B. Dixon,
and
D. B. Hackel.
The vascular supply of the left ventricular wall: anatomic observations, plus a hypothesis regarding acute events in coronary artery disease.
Am. Heart J.
71:
58-67,
1966[Medline].
18.
Fujii, M.,
D. W. Nuno,
K. G. Lamping,
K. C. Dellsperger,
C. L. Eastham,
and
D. G. Harrison.
Effect of hypertension and hypertrophy on coronary microvascular pressure.
Circ. Res.
71:
120-126,
1992[Abstract/Free Full Text].
19.
Goodwin, J. R.
Hypertrophic diseases of the myocardium.
Prog. Cardiovasc. Dis.
16:
199-238,
1973[Medline].
20.
Harris, C. N.,
W. S. Aronow,
D. P. Parker,
and
M. A. Kaplan.
Treadmill stress tests in left ventricular hypertrophy.
Chest
63:
353-357,
1973[Abstract/Free Full Text].
21.
Hittinger, L.,
R. P. Shannon,
S. Kohin,
W. T. Manders,
P. Kelly,
and
S. F. Vatner.
Exercise-induced subendocardial dysfunction in dogs with left ventricular hypertrophy.
Circ. Res.
66:
329-343,
1990[Abstract/Free Full Text].
22.
Hittinger, L.,
Y. T. Shen,
T. A. Patrick,
N. Hasebe,
K. Komamura,
T. Ihara,
W. T. Manders,
and
S. F. Vatner.
Mechanisms of subendocardial dysfunction in response to exercise in dogs with severe left ventricular hypertrophy.
Circ. Res.
71:
423-434,
1992[Abstract/Free Full Text].
23.
Hochberg, Y.
A sharper Bonferroni procedure for multiple tests of significance.
Biometrika
74:
800-802,
1988.
24.
Ishibashi, Y.,
D. J. Duncker,
and
R. J. Bache.
Endogenous nitric oxide masks 2-adrenergic coronary vasoconstriction during exercise in the ischemic heart.
Circ. Res.
80:
196-207,
1997[Abstract/Free Full Text].
25.
Ishibashi, Y.,
D. J. Duncker,
J. Zhang,
and
R. J. Bache.
ATP-sensitive K+ channels, adenosine, and nitric oxide-mediated mechanisms account for coronary vasodilation during exercise.
Circ. Res.
82:
346-359,
1998[Abstract/Free Full Text].
26.
Ishibashi, Y.,
J. Mizrahi,
D. J. Duncker,
and
R. J. Bache.
The nitric oxide donor ITF 1129 augments blood flow during exercise-induced myocardial ischemia.
J. Cardiovasc. Pharmacol.
30:
374-382,
1997[Medline].
27.
Ishida, T.,
R. M. Lewis,
C. J. Hartley,
M. L. Entman,
and
J. B. Field.
Comparison of hepatic extraction of insulin and glucagon in conscious and anesthetized dogs.
Endocrinology
112:
1098-1109,
1983[Abstract/Free Full Text].
28.
Jones, C. J.,
D. V. DeFily,
J. L. Patterson,
and
W. M. Chilian.
Endothelium-dependent relaxation competes with 1- and 2-adrenergic constriction in the canine epicardial coronary microcirculation.
Circulation
87:
1264-1274,
1993[Abstract/Free Full Text].
29.
Jones, C. J.,
L. Kuo,
M. J. Davis,
D. V. DeFilly,
and
W. M. Chilian.
Role of nitric oxide in the coronary microvascular responses to adenosine and increased metabolic demand.
Circulation
91:
1807-1813,
1995[Abstract/Free Full Text].
30.
Jones, C. J. H.,
L. Kuo,
M. J. Davis,
and
W. M. Chilian.
Regulation of coronary blood flow: coordination of heterogeneous control mechanisms in vascular microdomains.
Cardiovasc. Res.
29:
585-596,
1995[Medline].
31.
Komaru, T.,
K. G. Lamping,
C. L. Eastham,
and
K. C. Dellsperger.
Role of ATP-sensitive potassium channels in coronary microvascular autoregulatory responses.
Circ. Res.
69:
1146-1151,
1991[Abstract/Free Full Text].
32.
Komaru, T.,
K. G. Lamping,
C. L. Eastham,
D. G. Harrison,
M. L. Marcus,
and
K. C. Dellsperger.
Effect of an arginine analogue on acetylcholine-induced dilation of isolated coronary arterioles.
Am. J. Physiol.
259 (Heart Circ. Physiol. 28):
H1063-H1070,
1990[Abstract/Free Full Text].
33.
Kuo, L.,
W. M. Chilian,
and
M. J. Davis.
Interaction of pressure and flow-induced responses in porcine coronary resistance vessels.
Am. J. Physiol.
261 (Heart Circ. Physiol. 30):
H1706-H1715,
1991[Abstract/Free Full Text].
34.
Selke, F. W.,
P. R. Myers,
J. N. Bates,
and
D. G. Harrison.
Influence of vessel size on the sensitivity of porcine microvessel to nitroglycerin.
Am. J. Physiol.
258 (Heart Circ. Physiol. 27):
H515-H520,
1990[Abstract/Free Full Text].
35.
Tschudi, M.,
V. Richard,
F. R. Bühler,
and
T. F Lüscher.
Importance of endothelium-derived nitric oxide in porcine coronary resistance arteries.
Am. J. Physiol.
260 (Heart Circ. Physiol. 29):
H13-H20,
1991[Abstract/Free Full Text].
36.
Vatner, S. F.,
M. Pagani,
W. T. Manders,
and
A. D. Pasipoularides.
Alpha adrenergic constriction and nitroglycerin vasodilation of large coronary arteries in the conscious dog.
J. Clin. Invest.
65:
5-14,
1980.
Am J Physiol Heart Circ Physiol 276(4):H1305-H1312
0002-9513/99 $5.00
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