The Beneficial Effects of LongChain, Polyunsaturated n3 Fish Oil Fatty Acids on the Cardiovascular S

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The Beneficial Effects of Long-Chain,
Polyunsaturated n-3 Fish Oil Fatty Acids on the
Cardiovascular SystemAlexander Leaf, M.
D.Jackson Professor of Clinical Medicine,
EmeritusHarvard Medical School, Boston 02114,
USA
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The structure of this lecture? Back-ground
materialTo inform you of the beginning interest
in fish and fish oil on heart disease.? The
current status of the evidence that ingesting
fish or fish oils benefit the cardiovascular
systems of animals and humans.? Authors humble
contributions to our understanding of the
mechanisms by which the n-3 fish oil fatty acids
prevent fatal ventricular arrhythmias (Sudden
Cardiac Death, SCD).? Authors equally humble
contribution to the accumulating evidence that
n-3 fish oil fatty acids prevent SCD in patients
at high risk.
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BackgroundThe beginning of the interest in fish
oil to improve function of the heart occurred
early in the 1970s when two Danish physicians,
Bang and Dyerberg, who were aware that the
mortality among Greenland Eskimos from heart
disease was only about one-tenth that in Denmark
and the USA. This was despite the knowledge that
the total energy provided from dietary fats was
about 38 in all three populations. Dyerberg and
Bang then surmised this striking difference in
cardiac mortality might be due to the different
fats in the diet of the Greenland Inuits and that
of Danes and US citizens.
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Although there were earlier suggestions that
polyunsaturated fatty acids might be
antiarrhythmic, it was the definitive findings of
two Australians McLennan and Charnock, who first
demonstrated the antiarrhythmic action of the
fish oil fatty acids. Their basic experiment was
simple and clear. They fed rats diets for 3- or
4-months in which they could control the major
fat component. At the end of the dietary period,
they ligated the coronary arteries of the rats
and counted the number of animals that died of
sustained ventricular fibrillation (VF). In one
publication, McLennan reported that slightly more
than 40 of animals fed a diet with saturated fat
providing 12 of energy calories died of
sustained VF.
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Polyunsaturated fatty acids are essential fatty
acids for two reasons. First, they must be
obtained from the diet, we cannot synthesize them
in our bodies as we can for saturated and
monounsaturated fatty acids. Second, they are
absolutely essential for optimal development,
growth and function of the brain, heart and
probably other systems.
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Polyunsaturated fatty acids come in two classes
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The most prevalent in our diet are the n-6 (or
also called ?6) class. The parent compound of
this class is linoleic acid (LA), an 18-carbon
fatty acid with two unsaturated CC bonds. Since
the first of these unsaturated bonds encountered
when one counts from the methyl end of the fatty
acid chain toward the carboxylic end of the fatty
is the 6th carbon atom, hence the appellation n-6
or ?6. This LA can be further desatuated and
elongated to form arachidonic acid (C204n-6,
AA). Arachidonic acid is the physiologically
most active member of the n-6 class of fatty
acids.
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The n-3 fatty acids are derived from vertebrate
animals in the oceans, the source of n-3 fish oil
fatty acids. By contrast the n-6 class of
polyunsaturared fatty acids derives from plant
seed oils, such as corn oil, sunflower seed, and
soy bean oils, the common table and cooking oils,
which are much too abundant in our diets. As
stated both classes of fatty acids are essential,
but for some of their cyclooxegenase,
lipoxyngenase and epoxygenase derivatives have
actions in our bodies, which oppose each other in
important ways. These will be discussed later in
this review.
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The effect of n-3 fish oil fatty acids in
preventing arrhythmias in a dog model of sudden
cardiac deathTo see if we could confirm the
surprising findings of McLennan, we turned to a
highly reliable dog model of sudden cardiac
death. Working with George E. Billman, who
prepared the dogs surgically by ligating the left
descending coronary artery, producing a large
anterior wall infarction and in the same
operation placing an hydraulic cuff around the
right circumflex artery so that it could be
compressed at will. The dogs were then trained to
run on a treadmill during the month allowed for
them to recover from the surgery and the MI.
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The Table 1 summarizes our experiments on the
dogs
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The prevention of fatal arrhythmias by the
emulsion of fish oil concentrate (Plt0.005),
confirms the studies of McLennan and associates.
They used feeding experiments, which were
criticized because of possible confounding
factors occurring in long term feeding studies in
animals. We infused the fatty acids just before
an ischemic stress in our prepared dogs. We
believed that if the n-3 fatty acid infusion were
promptly associated with an effect in the
protocol we used, we could then feel confident
the effect resulted from what had just been
infused.
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Effects of n-3 PUFAs on cultured neonatal rat
cardiomyocytesTo learn the biochemical or
physiologic effects of these n-3 fatty acids,
which explain their antiarrhythmic action, the
effects of the n-3 PUFAs on cultured neonatal
cardiomyocytes were studied. One can quickly
remove the hearts from several one to two day old
rat pups and separate the individual myocytes
enzymatically. The myocytes are then plated on
microscope covers-slips and grown in an
appropriate culture medium.
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At this point we had found that the arrhythmias
induced in the isolated neonatal rat
cardiomyocytes could in every instance be
prevented by the prior addition of the EPA or DHA
to the superfusate bathing the cells. Adding the
EPA or DHA after an arrhythmia was induced, would
stop the arrhythmia. It was apparent that the n-3
PUFA were affecting the excitability/automaticity
of the cardiomyocytes, so the effects of the n-3
PUFAs on the electrophysiololgy of the myocytes
were examined.
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Heart, brain and muscle are excitable tissues and
their function is to generate electrical currents
to signal their actions in the body. This they do
by activating and then inactivating ion channels
in their plasma membranes to allow specific ions
to move through their plasma membranes, thus
creating ionic currents. In heart cells these
ionic currents create action potentials by the
sequential opening and closing of fast
voltage-dependent sodium currents into the
cardiomyocytes see Fig. 5.
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Figure 6. Effects of EPA on activation and
inactivation of human myocardial Na channel
(?- plus ?1-subunits)
transiently expressed in HEK293t cells.
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Our current hypothesisOur current hypothesis
regarding the mechanism of action of the n-3
polyunsaturated fatty acids to prevent fatal
arrhythmias is based on their actions to inhibit
the fast, voltage-dependent sodium current and
the L-type calcium currents. With a myocardial
infarction there occurs a gradient of
depolarization of cardiomyocytes. In the central
core of the ischemic zone cells rapidly
depolarize and die. The depolarization results
from deficiency of ATP in the ischemic cells
causing a dysfunctional Na,K-ATPase and the rise
of interstitial K concentrations in the ischemic
zone.
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Thus any further small depolarizing stimulus (for
example, current of injury) may elicit an action
potential, which if it occurs at a vulnerable
moment during the cardiac electrical cycle, may
initiate an arrhythmia. With non-homogeneous
rates of conduction of the action potential in
the ischemic tissue reentry arrhythmias are
likely. In the presence of the n-3
PUFAs, however, a
voltage-dependent shift of the steady state
inactivation curve to more hyperpolarized
potentials occurs. The consequence of this
hyperpolarizing shift is that sodium channel
availability is decreased, and the potential
necessary to return these Na channels in
partially depolarized myocytes to a closed but
activatable state is physiologically
unobtainable.
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Figure 7. Effects of EPA on resting and
inactivated hH1? Na channels. Current tracings
were evoked by voltage steps from 150 mV to 30
mV (A) and from 70 to 30 mV (B) in the absence
and presence of 5 ?M EPA. Each value represents
6 15 cells. Normalized current was calculated
as INa(( (EPA)/INa(( (control) from the same
corresponding cell.
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The L-type Ca2 current, ICa,L Not all fatal
cardiac arrhythmias are caused by dysfunction of
the Na channel. Many serious arrhythmias can be
triggered by excessive cytosolic free Ca2
fluctuations. In clinical practice these may be
seen in patients with extensive bone metastases,
hyperparathyroidism, immobilization of
extremities (which have in common hypercalcemia)
and cardiac glycoside toxicity due to the
inhibition of the Na-K-ATPase depolarizing the
heart cell and allowing increase in cytosolic
free calcium concentrations via the Na/Ca
exchanger (the high intracellular Na moving out
of the cell in exchange for Ca2 increasing in
the heart cell).
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It is of interest to compare the actions of the
n-3 fatty acids with that of available
phamaceutical drugs both of which inhibit the Na
and Ca2 ion channels, such as the Class 1
antiarrhythmic Na channel blockers or the L-type
Ca2 channel blockers. There are several striking
differences a) The n-3 fatty acids have been
part of the human diet for hundreds of thousands
of years, during the time our genes were being
adapted to our environment, including the diet of
our hunter-gatherer forebears and they are safe.
b) By contrast the available
antiarrhythmic pharmaceutical drugs are all
potentially toxic.
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Although at present we think that inhibitory
effects of the PUFAs on INa and ICaL seem the
major effects accounting for their antiarrhythmic
actions, we are not unmindful that they affect
other sarcolemmal ion currents as well. By whole
cell voltage clamp measurements they have been
reported to also inhibit K currents - the
transient outward current, Ito, and the delayed
rectifier current, IK, but not the inward
rectifying current, IK1. However, these effects
on the important repolarizing K currents would
have the effect to prolong the action potential
duration, a potentially proarrhythmic effect
whereas the PUFAs, significantly shorten the
action potential duration by some 20.