703
From Isolated Vessels
to the Catheterization Laboratory
Studies of Endothelial Function in the Coronary
Circulation of Humans
David G. Harrison
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In 1980, Furchgott and Zawadskil first showed
that an intact endothelium must be present for
acetylcholine to relax isolated vessels. This
was subsequently found to be true for many other
vasoactive substances.2-6 As a result of work from
several laboratories, this response has been shown
to occur because the endothelium releases a shortlived nonprostanoid compound or family of compounds collectively termed the "endotheliumSee p 458 and p 466
derived relaxing factor" (EDRF).7,8 These substances relax vascular smooth muscle by activating
guanylate cyclase9,10 and by producing transient
smooth muscle hyperpolarization.11 The role of
EDRF in regulating vascular tone was first implicated in large vessels but has now been extended to
the microcirculation.12,13
Identity of Endothelium-Derived Relaxing Factor
The chemical identity of EDRF has been the
subject of intense investigations. Many of EDRF's
biologic activities are shared by nitric oxide.14
These include the ability to activate guanylate
cyclase, inhibition of platelet aggregation, short
biologic half-life, and inactivation by hemoglobin.
In 1987, Palmer et al15 reported that nitric oxide is
synthesized and released by cultured endothelial
cells and that EDRF may be nitric oxide. The
evidence for nitric oxide being the sole EDRF is
far from conclusive, however. The amount of
nitric oxide released from cultured endothelial
cells is insufficient to account for all of the vasodilator activity of EDRF simultaneously released
The opinions expressed in this editorial comment are not
necessarily those of the editors or of the American Heart
Association.
From the Cardiovascular Center and Veterans Administration
Medical Center, University of Iowa College of Medicine, Iowa
City, Iowa.
Dr. Harrison is an Established Investigator of the American
Heart Association.
Address for correspondence: David G. Harrison, MD, E317A
General Hospital, University of Iowa, Iowa City, Iowa 52242.
Received June 2, 1989; revision accepted June 5, 1989.
from the same source.16 Moreover, recent studies
have shown that EDRF may not be authentic nitric
oxide but a nitrosyl compound (such as a nitrosothiol) that is substantially more potent than
nitric oxide.17 Further, nitric oxide cannot account
for some of EDRF's effects such as hyperpolarization of vascular smooth muscle. Hopefully,
future research will provide further insight into the
chemical identity of these substances.
Abnormalities of Endothelium-Dependent Vascular
Relaxation in Atherosclerosis
Shortly after the original observation by Furchgott
and Zawadski, several groups began to examine
endothelium-dependent vascular relaxation in vessels removed from experimental animals with dietinduced atherosclerosis. The rationale underlying
these studies was that many of the substances that
elicit the release of EDRF (serotonin, norepinephrine, acetylcholine, vasopressin, and others) also
exert a direct vasoconstrictor effect on vascular
smooth muscle. The release of EDRF normally
modulates the constrictor action of these compounds. If the endothelium were dysfunctional (and
thus did not release EDRF) in the setting of atherosclerosis, these neurohumoral agents could conceivably produce exaggerated vascular constriction
because of their unopposed effect on the smooth
muscle. The unopposed vasoconstrictor actions of
these agents may account for abnormal vasomotor
phenomenon encountered in individuals with coronary atherosclerosis. In 1986 and 1987, several
groups reported that endothelium-dependent relaxations of vessels removed from animals with dietinduced atherosclerosis were markedly impaired,
whereas responses to agents that directly relax
smooth muscle were either normal or only minimally impaired.18-22Subsequent studies have shown
that this defect of vascular function in atherosclerosis is not due to decreased responsiveness of
vascular smooth muscle to EDRF but related to
either decreased release of EDRF from the endothelium or the release of a defective EDRF.
704
Circulation Vol 80, No 3, September 1989
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Recently, this defect in endothelial function was
shown not to be limited to larger vessels but that it
also extends to endothelial function in the microcirculation. In vivo studies of the rabbit cremaster
muscle have shown that short-term cholesterol feeding markedly impairs acetylcholine-induced vasodilation of vessels as small as 25-gm diameter.23
Preliminary in vitro studies in our laboratory on
coronary resistance vessels from monkeys with
diet-induced atherosclerosis suggest that endothelium-dependent responses are strikingly abnormal
in these vessels, whereas responses to endotheliumindependent vasodilators are unchanged. These findings show that endothelial dysfunction can occur in
vessels that do not develop overt atherosclerosis
and indicate that abnormal vasomotor control in
atherosclerosis can involve microvessels as well as
large vessels.
Close on the heels of the demonstration that
endothelium-dependent vascular relaxation is abnormal in atherosclerotic animals, several groups began
to report similar abnormalities of endotheliumdependent vascular relaxation in atherosclerotic
human coronary arteries. Two groups have studied
human coronary arteries removed from hearts of
transplant recipients in vitro.24,25 In these experiments, endothelium-dependent relaxations were
depressed in rings of vessels with atherosclerosis
compared with relaxations in nonatherosclerotic
vessels. Subsequently, several groups have examined the vasodilatation produced by acetylcholine
(presumed to be dependent on an intact endothelium) in the cardiac catheterization laboratory.26-29
In general, most of these studies have shown that
vessels with even minimal atherosclerosis do not
dilate to acetylcholine, whereas normal vessels do.
Release of Endothelium-Derived Relaxing Factor by
Mechanical Stimuli
In addition to the large number of neurohumoral
agents that can stimulate the release of EDRF,
mechanical stimuli such as changes in shear stress
or pulsatile flow also modulate EDRF release.30,31
Teleologically, this would appear important in maintaining vascular shear stress at a constant level
during periods of increased blood flow by providing
a mechanism for local dilatation of large vessels in
response to changes in blood flow that are mediated
by downstream vessels. Two recent studies have
shown that flow-mediated vasodilatation occurs in
the brachial artery of humans.32,33
In this issue of Circulation, Cox et a134 and
Drexler et a135 show that this important regulatory
function of the endothelium exists in the human
coronary circulation. Furthermore, unequivocal evidence is provided that flow-induced, endotheliumdependent vasodilatation is abnormal in humans
with coronary atherosclerosis.35 These studies were
made possible first by initial observations in animals
and second by the application of important new
technology to the study of the coronary circulation
in humans. Studies such as these would be impossible without a reasonably accurate method of measuring blood flow velocity and quantitative
approaches to the measurement of arterial dimensions from the coronary angiogram. These observations represent an extension from experimental
studies in animals (in vivo and in vitro) to clinical
practice. Efforts such as these are important because
occasional findings in experimental animals have
not been applicable to clinical situations.
The implications of abnormal large vessel dilatation in response to increased flow in atherosclerosis
are several. In addition to the effect of exposing the
vessel wall to increased shear stress, the potential
exists for an increase in pressure losses during high
flow states across the epicardial coronary artery.
Under normal conditions, there is minimal pressure
gradient (<5 mm Hg) from the coronary ostium to
the most distal areas of the large epicardial coronary arteries. Under conditions of increased flow
(such as may exist during pharmacologic- or exerciseinduced vasodilatation), this gradient may substantially increase and exceed 20 mm Hg. This may, in
a small but important way, affect the driving pressure for perfusion in the coronary microcirculation.
In addition, the pressure gradient that develops in
the proximal coronary arteries during pharmacologic vasodilatation may decrease the pressure at
the origin of coronary collateral arteries, effectively
reducing coronary collateral blood flow. This phenomenon, which has been termed the "collateral
steal phenomenon," would be exacerbated when
the proximal coronary arteries could not dilate in
response to increased vascular shear.
Future Directions
Numerous questions remain to be answered
regarding endothelial regulation of vascular smooth
muscle and how this regulation is altered by several
disease states including atherosclerosis. Several
questions remain to be answered regarding the
biochemical identity of EDRF and how its biosynthesis is altered by disease. There are a myriad of
other functions of the endothelium that may be
abnormal in atherosclerosis.
The advent of new technology has substantially
enhanced the capability of obtaining accurate information regarding coronary vasoactive reactivity in
the cardiac catheterization laboratory so that many
additional studies will likely be forthcoming. One
inhibitor of EDRF, methylene blue, has recently
been used in humans, unmasking the constrictor
effect of acetylcholine.36 Such an approach, used
cautiously, may provide an additional tool to examine dual opposing vascular effects of various neurohumoral substances. Abnormal endotheliumdependent vascular relaxation can be reversed after
successful dietary treatment of atherosclerosis in
experimental animals.37 Whether or not this can
occur in humans has yet to be established. Newer
therapies for correction of hypercholesterolemia
Harrison Endothelial Function
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should allow such studies to be forthcoming in the
near future. For example, Shimokawa and Vanhoutte have shown that dietary supplementation
with fish oil can enhance endothelium-dependent
vascular relaxation38 and in part correct abnormal
responses observed in experimental atherosclerosis.39 Because endothelium-dependent relaxations are abnormal in the coronary resistance
circulation of cholesterol-fed animals, such an abnormality probably exists in humans with atherosclerosis. Studies of blood flow with the Doppler catheter should allow the study of resistance vessel
responses in normal and hypercholesterolemic subjects. Atherosclerosis develops in the setting of a
wide range of plasma cholesterol levels, including
low plasma cholesterols. It will be interesting to
determine whether or not the plasma cholesterol
level has any direct bearing on abnormalities of
endothelial function in humans. Last, other than
atherosclerosis, a number of pathologic conditions
including ischemia with reperfusion,4041 hypertension,42 and diabetes43'44 are associated with abnormal endothelium-dependent vascular relaxation in
experimental animals. Whether or not these commonly encountered disorders alter coronary vascular reactivity in humans is unclear. Studies in patients
with these diseases similar to those presented in this
issue of Circulation will undoubtedly be forthcoming and should provide important additional information regarding abnormalities of the coronary circulation in other disease states.
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From isolated vessels to the catheterization laboratory. Studies of endothelial function in
the coronary circulation of humans.
D G Harrison
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Circulation. 1989;80:703-706
doi: 10.1161/01.CIR.80.3.703
Circulation is published by the American Heart Association, 7272 Greenville Avenue, Dallas, TX 75231
Copyright © 1989 American Heart Association, Inc. All rights reserved.
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