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Summary of Dissertation
The importance of dietary cholesterol in influencing the
physiology and pathology of the cardiovascular system has been
emphasized in a number of studies both on animals and
humans. A high cholesterol diet results in increased
cholesterol levels, not only in the plasma, but in the
myocardium as well. Cholesterol, a component of the lipid
bilayer of the myocardial sarcolemma, has been proposed to
play a prominent role in regulating myocardial
sarcolemmal function. The cell membrane with high cholesterol
content alters its physical properties such as phase transition,
fluidity homogeneity and permeability and, subsequently,
modifies the functions of certain membrane-bound enzymes and
receptors.Thus, the pathogenic mechanism of dietary
cholesterol on cardiac function can be attributed to the
development of atherosclerosis,leading to the occlusion of
essential coronary arteries feeding the heart muscle, and to
alterations in structural lipid compositions, resulting in
changes in enzymatic activities and receptor functions of
the myocardial sarcolemma. In fact, these two mechanisms can
operate simultaneously in cases of long-standing
hypercholesterolemia, which which is a relatively common
pathological condition in industrialized countries and
Taiwan as well.
For above reasons, we designed a series of studies not only to
explore the effects of high dietary cholesterol on plasma
cholesterol level and lipid compositions of the myocardial
sarcolemma but also to examine the effects of high dietary
cholesterol on certain membrane-bound enzymatic activities
(Na+-K+-ATPase, Mg2+-ATPase, 5''-nucleotidase and adenylate
cyclase) and receptor functions(ouabain receptor and β-
adrenergic receptor) of the myocar -dial sarcolemma.Furthermore,
we proceeded with another study investigate the effects of
ischemia onthe activities of Na+-K+-ATPase, adenylate cyclase
and properties of ouabain andβ-adrenergic receptors of the
myocardial sarcolemma in normal andcholesterol-fed rabbits.
In the first part of this study, male New Zealand white rabbits
were fed with standard chow (control group) or standard chow
supplementedwith 10% coconut oil and 0.25% cholesterol
(group A); 0.5% cholesterol (group B); or 1.0% cholesterol
(group C). The plasma levels of cholesterol were meas ured
every two weeks. After 8 weeks of feeding, the rabbits were
sacrificed;a myocardial sarcolemma fraction was then
prepared from the left ventricular myocardium. The
myocardial sarcolemma was analyzed for lipid mpositions by
thin-layer chromatography with flame ionization detection
method. At the end of the study, the plasma cholesterol levels
were found to be significantly increased in the cholesterol-
fed group, as compared with those of the control group. In
addition, the severity of hypercholesterolemia in the
cholesterol- fed group was significantly correlated with dietary
cholesterol level. In terms of lipid compositions of
myocardial sarcolemma, the cholesterol content , but not the
phospholipid content, wasignificantly increased in the
cholesterol-fed groups, thereby, resulting in an increased
cholesterol/ phospholipid ratio in the cholesterol-fed
groups. However, the cholesterol content, as well as the
cholesterol/phospholipid ratio, of the myocardial sarcolemma
did not significantly differ among cholesterol-fed groups.
In the second study,male New Zealand white rabbits were fed with
standard chow (control group) or standard chow supplemented with
0.5% cholesterol and 10% coconut oil (cholesterol-fed group).
Assay of activities of Na+-K+- ATPase, Mg2+-ATPase,
5''-nucleotidase, adenylate cyclase and the binding studies
of either [3H]ouabain or [125I]iodocyanopindolol ([125I]ICYP)
were performed in the myocardial sarcolemma fromthe control
and cholesterol-fed rabbits. A decrease in Na+-K+-ATPase
activity and an increase in adenylate cyclase activity were
found in the cholesterol-fed group. These changes were
selective since Mg2+-ATPase and 5''-nucleotidase activities
remained unchanged In the [3H]ouabain receptor studies, a
decrease in the number of maximum binding sites for [3
H]ouabain was found in the cholesterol-fed group. The
binding affinity for [3H]ouabain was not significantly different
between the control and cholesterol-fed rabbits. In the β-
adrenergic receptor studies, a decrease in the number of
maximum bindinsites and an increase of binding affinity, for
β-adrenergic receptor were found in the cholesterol-fed
rabbits.
In the further study, with the same animal preparation, the
rabbits underwent a thoracotomy and myocardial ischemia
was induced by occlusion of the coronary artery. Myocardial
samples from the ischemic and non-ischemic regions of the left
ventricle of control and cholesterol-fed rabbits were taken
for study. Assay of activities of Na+-K+-ATPase, Mg2+-ATPase,
adenylate cyclase and [3H]ouabain as well as [125I]ICYP binding
studies were then performed. The results indicated that
ischemia caused a reduction not only in Na+-K+-ATPase activity
but also in [3H]ouabain binding sites of both control and
cholesterol-fed rabbits. Either hypercholesterolemia or
ischemia alone led to a reduction of myocardial Na+-K+-ATPase
activity and [3H]ouabain binding sites. In addition, combining
ischemia and hypercholesterolemia produced an additive
effect, with a further decrease in both myocardial Na+-K+-
ATPase activity and [3H]ouabain binding sites. Neither the
activity of Mg2+-ATPase nor the binding affinity for [3
H]ouabain was significantly affected by hypercholesterolemia
and ischemia. Moreover, ischemia caused a reduction in [125
I]ICYP binding sites in both control and cholesterol-fed
rabbits. The extent of ischemia-induced percent decrease of
[125I]ICYP binding sites was similar in the control and
cholesterol-fed rabbits. The binding affinity for [125I]ICYP
was not affected by ischemia. Ishcemia also significantly
decreased not only the basal adenylate cyclase activity but
also adenylate cyclase activities stimulated by ther
isoproterenol or forskolin in both control and
cholesterol-fed rabbits. However, the ischemia-induced
percent decrease of adenylate cyclase activity was
significantly greater in the cholesterol-fed rabbits as compared
with that of the control. These results indicated that
hypercholesterolemia might exaggerate certain aspects of
functional deterioration arising during myocardial
ischemia.
In conclusion, a high cholesterol diet induced
hypercholesterolemia, and the severity of hypercholesterolemia
closelycorrespondedto dietary cholesterol content. A
high cholesterol diet also induced an increase of
cholesterol conontent of myocardial sarcolemma did not
significantly differ among three study groups fed with 0.25%,
0.5% or 1.0% cholesterol. A high myocardial sarcolemmal
cholesterol content led to changes in functns of certain
membrane-bound enzymes and receptors,which might play critical
roles for the myocardial dysfunction associated with long-
standing hypercholeste- rolemia. Although earlier studies have
demonstrated many pathophysiological changes arise during
myocardial ischemia, a majority of them were performed under
conditions of physiological normo-cholesterolemic condition.
Results in this study confirmed that hypercholesterolemia
could exaggerate certain aspects of functional deterioration
arising during myocardial ischemia. These findings implied
that the pathophysiological changes related to
hpercholesterolemia deserved further investigations.
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