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"Bells and whistles" scientists
call them, the intricate devices by which cells get their work done,
and it's not hard to see why. Consider the cellular events that lead
up to a heart beat: First there's the electrical impulse along the
cell's membrane; then channels open in the cell membrane allowing
sodium to flow into the cell; then more channels open and calcium
enters and binds to a tiny structure near the membrane; then much
more calcium explodes out of that structure into the cell's inner
fluid and combines with another protein; then that protein changes
shape to allow two other proteins to come together; then the joined
proteins move in such a way as to shorten the cell, pulling the ends
of the cell inward-this is the actual contraction-and then the whole
process reverses itself as the heart relaxes. This is what happens
in a heart muscle cell (myocyte) every time the heart beats.
Over the last two decades,
scientists have delved into this wealth of bells and whistles and
come up with some intriguing finds. They have discovered age-related
changes in myocytes that help explain changes in the heart as a
whole; begun to see how these changes could interact with disease
processes; and found clues to how exercise affects the biochemistry
of cells. The result is a string of new insights and novel hypotheses
about aging and disease.
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Contraction
and Relaxation Prolonged
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Take for example, the bells
and whistles that regulate how long it takes a myocyte to contract
and relax. Since the 1950s, scientists have known that calcium plays
a major role in the contraction and relaxation of muscle, including
heart muscle. Calcium entering the myocyte's inner fluid or cytosol
binds with other contractile proteins to bring about contraction;
calcium leaving the cytosol allows the cell to relax.
When scientists learned,
in the early 1970s, that muscle from older hearts takes longer to
relax than muscle from younger hearts, they speculated that it could
have something to do with calcium. A way to test this hypothesis
came to hand about a decade later via a protein called aequorin.
Aequorin binds to calcium and gives off a blue light, showing how
much calcium is in a cell at any one time. When Edward Lakatta and
Clive Orchard at NIA injected aequorin into myocytes within heart
muscle in laboratory dishes, the blue light showed that the calcium
levels, after a contraction, fell more slowly in older myocytes.
Or putting it in biologists' terms, the calcium transient
was longer. Here was the cellular mechanism that explained why hearts
relax more slowly with age.
Here also is a good example
of how discoveries in the realm of cellular biology have begun to
throw light on the relationships between aging and disease. The
longer calcium transient, for one thing, appears to be a way that
the heart adjusts to age, or more specifically to the stiffer arteries
that accompany aging. "It makes sense from an engineering standpoint
to have a longer contraction if you're pumping blood into stiffer
vessels," points out Lakatta. "The down-side," he adds, "is that
when you alter the kinetics of calcium cycling in the cell, various
stresses can more easily throw the calcium out of balance."
And when calcium cycling
gets out of balance, levels of calcium may oscillate and chaos can
ensue. Individual cells may fire off rapidly and independently,
resulting in arrhythmias-variations from the normal heart
beat rhythm-and fibrillation, which is a very rapid twitching
of individual muscle fibers. Fibrillation in the left ventricle
leads quickly to acute heart failure and to death if not treated.
Heart failure rises exponentially
with advancing age, and studies of calcium oscillations in the myocytes
suggest possible reasons. Older hearts are more prone to spontaneous
calcium oscillations than younger hearts and it takes fewer oscillations
in the older heart to bring about fibrillation. As a result, some
scientists speculate that age-related changes in calcium cycling-changes
that initially help the heart adapt to stiffer arteries and larger
loads-eventually make it more vulnerable to calcium-dependent arrhythmias
and heart failure.
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The Calcium
Pump
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The next question, of course,
was why: Why is the calcium transient longer in older cells? Perhaps,
bio-chemists speculated, it is because the calcium stays in the inner
fluid longer.
Each myocyte has a structure,
or organelle, that stores calcium. Called the sarcoplasmic
reticulum, it releases calcium into the inner fluid before contraction
and removes calcium from the inner fluid after contraction. Could
the calcium be spending longer in the inner fluid because the sarcoplasmic
reticulum was not removing it as quickly in older cells?
With technology that became
available in the late 1970s, researchers were able to answer this
question. Jeffrey Froehlich and his colleagues at NIA isolated the
sarcoplasmic reticulum from the rest of the cell, placed it in a
test tube, and then added calcium. The result confirmed their hypothesis:
The sarcoplasmic reticulum took up the calcium more slowly in the
samples from older animals than in those from younger animals.
Subsequent studies confirmed
that the sarcoplasmic reticulum-or more exactly, a protein on this
organelle-removes calcium from the inner fluid more slowly in older
hearts. Researchers have found that older cells have lower amounts
of this particular protein, often called the calcium pump protein
because it removes the calcium in a series of repeated movements.
So the sarcoplasmic reticulum removes calcium from the inner fluid
more slowly in older hearts because there are fewer calcium pump
proteins to do the work.
Once scientists had learned
about the pump protein, the next question was about that protein's
gene. Proteins make up a huge category that includes enzymes, growth
factors, hormones-almost all the substances that are responsible
for the day-to-day functioning of living organisms. Proteins are
produced by genes in the nucleus of every cell. Each protein has
its own gene. Cells translate gene codes into proteins through a
complex, multistep process called gene expression. Any alteration
in this process can lead to changes in the end product, the protein.
In the case of the pump protein,
the gene that produces it is only about half as active in older
hearts as in younger hearts, according to recent studies. On the
more hopeful side, exercise conditioning may alter the picture.
(see Exercise and the Aging Myocyte, below).
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Researchers
are beginning to find out why exercise is so good for the heart by
looking at how if affects heart muscle cells or myocytes. At NIA,
scientists have found that laboratory animals that exercise regularly
have hearts that work more like the hearts of younger animals.
Now NIA grantee
Charlotte Tate is exploring the molecules and genes in myocytes
to find out why exercise makes a difference.
In an early
experiment, Tate and her colleagues at the Baylor College of Medicine
compared three groups of rats. One older group was put on an endurance
training program, running on a treadmill five days a week. One older
group remained sedentary, and the third, a middle-age group, was
also sedentary.
One of the
key differences among the groups turned out to be how quickly calcium
was removed from the cell's inner fluid after a contraction. In
the older exercising group, calcium made its exit faster than in
the sedentary older group. The rate of removal in the older exercising
group was close to that in the middle-age group. The scientists
concluded that at least one reason exercise improves heart performance
is that it somehow allows cells to remove calcium from the inner
fluid at a faster rate.
The next
step is to delve even deeper into this mechanism: Why does calcium
exit from the inner fluid faster after endurance training? Tate's
laboratory, now at the University of Houston, is looking for clues
in the pump protein that transports calcium out of the inner fluid.
Endurance training may alter the way this pump protein is manufactured
and thus may help explain the impact of regular exercise.
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A Longer
Action Potential
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In addition to the calcium
transient, two other clusters of events in myocytes seem to be affected
by age. One is the action potential. This is a transient alteration
in the amounts of positive and negative charges on either side of
the myocyte membrane. The action potential triggers the opening of
sodium and then calcium channels in the membrane.
The action potential is prolonged
in older hearts and may contribute to the longer calcium transient,
although the connection, if it exists, is still not clear. Researchers
studying changes in the action potential think an age-related switching
of genes is a possibility, but the mechanisms and the effects are
uncertain.
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Contractile
Proteins
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The other mechanisms that change
with age involve the contractile proteins-actin, myosin, troponin,
and others-that interact to shorten, or contract, the myocyte. These
contractile proteins pass through a series of steps, triggered by
calcium, which bring actin and myosin together into "crossbridges."
The crossbridges use energy released during the transaction to shorten
the cell. With age, one part of the crossbridge alters-the part called
the myosin heavy chain.
The myosin heavy chain can be produced in two slightly different
forms, one dubbed alpha, the other beta. In experimental animals,
the alpha myosin heavy chain decreases with age, while the beta
increases. The same seems to be true in the human atrium. When the
proportion of alpha myosin heavy chain is reduced in isolated cells,
the contraction speed is slower.
Changes in the myosin heavy
chain have been traced back to the genes involved-alpha is expressed
less with age, beta more-but why the genes should switch activity
remains a mystery. "It's one of the cutting edge questions," says
Lakatta. "There's no doubt that some genes change their activity
with aging. We just don't know why or how yet." In general, though,
the expression of genes in the aging heart tends to revert back
to patterns of gene expression seen in the fetus.
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Scientists are
beginning to understand what happens in the cells and molecules of
arteries during atherosclerosis-the build-up of fatty deposits on
the inside of arteries. Atherosclerosis significantly increases the
risk of heart disease and stroke.
Blood vessels
consist of several kinds of cells. Lining the inner surface are
endothelial cells which are attached to a basement membrane. Moving
toward the outer surface, a layer of tissue called the intima comes
next, followed by the inner elastic membrane. Next comes a layer
of vascular smooth muscle cells between layers of elastin and finally
an outer layer of connective tissue.
In the normal process of aging, some vascular smooth muscle cells
migrate to the intima, which grows larger. In atherosclerosis, some
of these cells in the intima invade the basement membrane and form
clumps of tissue that narrow the passageway inside the artery (the
lumen). The process seems to be triggered, at least partly, by the
build-up of fatty streaks with cholesterol along the artery walls.
This much
was known by 1992 when Rebecca Pauly, Michael Crow, and others at
NIA set out to learn what happened in the vascular smooth muscle
cells during atherosclerosis. Studying the cells in laboratory cultures,
they found that these cells are able to invade the basement membrane
because they secrete enzymes that attack the protein in the membrane.
They also found that specific antibodies and other substances could
keep the enzymes from attacking the membrane.
The next
step is to find out what makes the smooth muscle cells secrete the
damaging enzymes and invade the basement membrane. Once the scientists
have identified the steps that lead to this event, it may be possible
to design strategies to prevent atherosclerosis.
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Enlargement
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Myocytes undergo another interesting
alteration with age: They get larger. Individual myocytes do not divide
like other cells in the body. However, they do increase in size by
20 to 40 percent during adult life. Others die and are replaced by
collagen. This enlargement of the remaining myocytes seems to be the
principal mechanism for the thickening of the heart walls - the hypertrophy
- that occurs during normal aging.
Much evidence suggests that
myocyte enlargement and the consequent thickening of the heart walls
are ways that the heart adjusts to increased loads, especially from
the growing stiffness of the arteries. Extra loads may also be imposed
by disease.
One reason cardiovascular
researchers are so intrigued by myocyte enlargement is because of
its possible links to disease. While enlargement seems to occur
in response to aging and stiffening arteries, it is exaggerated
by disease, such as coronary artery disease and high blood pressure.
Enlargement occurs with high blood pressure regardless of age.
While myocyte enlargement
seems to be one way the heart adapts to increased loads, there is
also evidence that at the oldest ages, it no longer adapts as much.
Older animals have less enlargement in response to heart overloads
than younger animals. This failure or slowing of the adaptive response
may explain why 80-year-olds are much more likely to experience
heart failure following a heart attack than 60-year-olds.
In fact, according to one
school of thought, heart failure rises with age for the same reason.
That is, the reason older people have a higher risk of heart failure
is not just because their hearts have had longer to deteriorate.
It is because of specific, age-associated changes in healthy hearts
and blood vessels that hinder the heart's ability to adapt to disease
states. In this view, normal age-associated changes are risk factors
for heart disease in older persons.
Selected
Readings
Froehlich JP, Lakatta EG,
Beard
E, Spurgeon HA, Weisfeldt ML, and Gerstenblith G. Studies of sarcoplasmic
reticulum function and contraction duration in young adult and aged
rat myocardium. Journal of Molecular and Cellular Cardiology 10:427-438,
1978.
Lakatta EG, Gerstenblith
G, Angell CS, Shock NW, and Weisfeldt MI. Prolonged contraction
duration in aged myocardium. Journal of Clinical Investigation 55:61-68,
1975.
O'Neill L, Holbrook NJ, Fargnoli
J, and Lakatta EG. Progressive changes from young adult age to senescence
in mRNA for rat cardiac myosin heavy chain genes. Cardioscience
2:1-5, 1991.
Orchard CH and Lakatta EG.
Intracellular calcium transients and developed tensions in rat heart
muscle. A mechanism for the negative interval-strength relationship.
Journal of General Physiology 86:637-651, 1985.
Pauly RR, Passaniti A, Crow
M, Kinsella JL, Papadopoulos N, Monticone R, Lakatta EG, and Martin
GR. Experimental models which mimic the differentiation and dedifferentiation
of vascular cells. Circulation 86 (Supplement III): III68-73, 1992.
Spurgeon HA, Steinbach MF,
and Lakatta EG. Chronic exercise prevents characteristic age-related
changes in rat cardiac contraction. American Journal of Physiology
244 (Heart Circulation Physiology 132): H513-518, 1983.
Tate CA, Taffet GE, Hudson
ED, Blaylock SL, McBride RP, and Michael LH. Enhanced calcium uptake
of cardiac sarcoplasmic reticulum in exercise-trained old rats.
American Journal of Physiology 258 (Heart Circulation Physiology
27): H431-435, 1990.
Wei JY, Spurgeon HA, and
Lakatta EG. Excitation-contraction in rat myocardium: alterations
with adult aging. American Journal of Physiology 246 (Heart Circulation
Physiology 15): H784-791, 1989.
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