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The Beneficial Effects of LongChain, Polyunsaturated n-3
Fish Oil Fatty Acids on the
Cardiovascular System
Alexander Leaf, M. D.
Jackson Professor of Clinical Medicine,
Emeritus
Harvard Medical School, Boston 02114, USA
The structure of this lecture:
 Back-ground material
To 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.
 Author’s 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).
 Author’s equally humble contribution to the
accumulating evidence that n-3 fish oil fatty
acids prevent SCD in patients at high risk.
Background
The beginning of the interest in fish oil to
improve function of the heart occurred early in
the 1970’s 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.
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.
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.
Polyunsaturated fatty acids come in two classes:
The most prevalent in our diet are the n-6 (or
also called ω6) class. The parent compound
of this class is linoleic acid (LA), an 18-carbon
fatty acid with two unsaturated C=C 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 ω6. This LA
can be further desatuated and elongated to
form arachidonic acid (C20:4n-6, AA).
Arachidonic acid is the physiologically most
active member of the n-6 class of fatty acids.
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.
The effect of n-3 fish oil fatty acids in preventing
arrhythmias in a dog model of sudden cardiac
death:
To 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.
The Table 1 summarizes our
experiments on the dogs:
The prevention of fatal arrhythmias by the
emulsion of fish oil concentrate (P<0.005),
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.
Effects of n-3 PUFAs on cultured neonatal rat
cardiomyocytes:
To 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.
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.
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.
Figure 6. Effects of
EPA on activation
and inactivation of
human myocardial
Na+ channel
(- plus 1-subunits)
transiently
expressed in
HEK293t cells.
Our current hypothesis:
Our 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, voltagedependent 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.
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.
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.
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).
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.
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%.