CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 4, May 2012
AFRICA
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ROS formation via NADPH oxidase and xanthine oxidase.
60
For example, Gao
et al.
reported that TNF-
α
induces ED via
increased NADPH oxidase activity in coronary arterioles of mice
with type 2 diabetes.
61
In addition, TNF-
α
has been implicated
in the downregulation of eNOS expression (and therefore
decreased NO production) by accelerating eNOS mRNA
degradation.
34,60,62
According to Zhang
et al.
, ED observed in
myocardial ischaemia–reperfusion injury may be attributable to
increased TNF-
α
expression via the enhancement of xanthine
oxidase activity.
63
Other harmful stimuli-induced mechanisms associated with
the development of increased oxidative stress and decreased
eNOS activation/eNOS uncoupling include activation of
cholesterylester transport protein (CETP), downregulation of
lipoprotein lipase, downregulation of peroxisome-proliferator
activated receptor (PPAR), downregulation of protein kinase
A (PKA), activation of caveolin, activation of rho-kinase and
downregulation of sphingosine-1-phosphate.
64
Assessment of endothelial function
Direct mechanical endothelial function
measurements
Direct endothelial function measurement in humans has the
potential to become an important clinical tool in diagnosing
or predicting the development of cardiovascular disease in the
presence or absence of cardiovascular risk factors. Furthermore,
in recognition of evidence pointing towards a pathophysiological
link between ED and IHD, a number of human experiments have
been conducted in which clinical assessment of ED is explored
as a possible predictor or prognostic marker of cardiovascular
events.
65
An ideal method for the direct measurement of endothelial
function should be safe, cost-effective, non-invasive, repeatable,
reproducible and standardised between laboratories.
5,65
Current
methods of assessing endothelial function include flow-mediated
dilation (FMD), forearm plethysmography, finger-pulse
plethysmography, pulse curve analysis and quantitative coronary
angiography
65
(Table 2).
In a recent review article, FMD was recognised as a commonly
undertaken, non-invasive technique to assess endothelial
function.
66
In this article, a meta-analysis of 14 studies (more
than 8 300 subjects) showed that FMD was strongly predictive
of future cardiovascular events. Despite the evidence in favour of
the independent prognostic value of vascular/endothelial function
measurements, the authors conceded that more and larger human
studies should be undertaken to confirm this finding.
Biomarkers
Although many biomarkers of endothelial function have been
identified, only some may have potential clinical use. Therefore,
the measurement of endothelial biomarkers remains a tool
mainly utilised in the experimental animal and
in vitro
laboratory
setting.
Reduction of NO bioavailability
Reduction in endothelial-derivedNOproduction or bioavailability
represents a measurable parameter that is suggestive of the
development of ED. Laboratory-based studies most often make
use of indirect NO measurements (measurements of metabolic
products of NO) to confirm ED, such as nitrogen oxide levels
67
or
levels of stable degradation products, nitrite and nitrate.
68
Many
researchers also measure expression and/or activation of eNOS,
the main enzymatic source of NO in endothelial cells, to further
validate their findings.
69,70
In humans, changes in NO production can also be detected
in plasma by measurement of NO metabolites. Kiettisanpipop
et al.
reported a decrease in plasma NO metabolite levels in
patients with severe pulmonary hypertension compared to
those with moderate hypertension.
71
Oestrogen replacement
therapy has been shown to increase plasma NO levels while
decreasing ET-1 levels and thus improving endothelial function
in postmenopausal women.
72
Heiss
et al
. demonstrated that
plasma levels of NO derivatives (nitrosyl/nitroso species) were
decreased in patients with endothelial dysfunction.
73
It remains
to be seen, however, whether the measurement of plasma NO or
NO-derived metabolites will become a widely used clinical tool
with predictive properties.
ADMA
ADMA has emerged as a mediator, independent risk factor and,
from a clinical perspective, potentially promising marker of
ED.
29
As explained earlier, ADMA is endogenously synthesised
via methylation of arginine residues in the nuclear proteins
74
and competitively inhibits eNOS, resulting in decreased NO
production, which may induce eNOS uncoupling. Synthesis
TABLE 2. CLINICAL DETECTION TECHNIQUES OF ENDOTHELIAL FUNCTION
Method
Brief description
Forearm plethysmography
Involves intrabrachial infusion of endothelial-dependent vasodilators such as acetylcholine, meta-
choline, substance P and bradykinin, with subsequent measurement of changes in endothelial
function of forearm arterioles
Flow-dependent dilation of the brachial artery This method employs a high-resolution ultrasound to quantify flow-mediated dilation of the
brachial artery
Finger-pulse plethysmography (ENDO-PAT) A novel non-invasive technique that measures changes of the pulse-wave amplitude during reac-
tive hyperaemia. Low pulse-wave amplitudes are associated with compromised endothelial func-
tion and are therefore good predictors of cardiovascular disease
Pulse curve analysis
A non-invasive technique that relies on the measure of arterial stiffness to quantify endothelial
function
Quantitative coronary angiography following
intracoronary infusion of acetylcholine
An invasive approach of quantifying endothelial function, which involves intracoronary infusion
of the endothelium-dependent vasodilator, acetylcholine, and subsequent measurment of the vaso-
motor response
