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337
Effect of Age on the Development of Cardiac
Hypertrophy Produced by Aortic
Constriction in the Rat
Shogen Isoyama, Jeanne Y. Wei, Seigo Izumo, Peter Fort, Frederick J. Schoen,
and William Grossman
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To test the hypothesis that the capacity to develop left ventricular (LV) hypertrophy might diminish
with advancing age, we examined the hiypertrophic response to ascending aortic constriction in 3
groups of adult Fischer 344 rats (9 months, 18 months, and 22 months of age). Aortic constriction
was created so that aortic cross-sectional areas would be the same for the 3 groups of rats. Four weeks
after imposition of aortic constriction, there was no significant difference in peak LV pressure,
peak-to-peak and mean systolic pressure gradients between left ventricle and aorta, cardiac output,
LV minute work, or cross-sectional area of the aortic constrictions in the 3 groups. In 9-month-old
aortic-constricted rats, LV dry wt (LVDW)/body wt, LVDW/tibial length, and myocyte width
increased by 23% (p<0.01), 14% (p<0.01), and 27% (p<0.01), respectively, compared with
sham-operated rats. In contrast, in 18-month-old and 22-month-old aortic-constricted rats, LVDVV/
body wt and LVDW/tibial length were unchanged compared with sham-operated controls, and
increases in myocyte width were only modest 4 weeks following constriction. RNA concentration in
the myocardium 5 days after constriction increased by 21 % (p<0.001) in 9-month-old rats but showed
no significant rise in 18-month-old rats. These results suggest that advancing age is associated with
a diminished capacity to develop myocardial hypertrophy in response to acute pressure overload and
that a reduced ability to synthesize protein may be one of the major contributing factors to a diminished
capacity for hypertrophy in advanced age. (Circulation Research 1987;61:337-345)
C
ardiac hypertrophy may be regarded as an
adaptive physiologic response to increased
functional demands on the heart. 12 However,
when the functional demands exceed adaptive limits,
heart failure ensues. It is well appreciated that when
subjected to similar stress, heart failure occurs more
frequently in old patients compared with younger
cohorts. 3 This higher incidence of heart failure in old
patients might be associated with a diminished capacity
for hypertrophy in response to a given stress. 45
In animal studies, it has been suggested that the
hypertrophic response in the adult state may be
different from that in the growing phase. M Several
investigators have examined the effect of age on the
capacity for development of cardiac hypertrophy in
adult animals; however, the observations in these
studies are controversial. McCafferty and Edington9
reported that the extent of increase in heart weight in
From the Charles A. Dana Research Institute, the HarvardThorndike Laboratory, and the Department of Medicine (Cardiovascular and Gerontology Divisions), Beth Israel Hospital; the
Department of Pathology, Brigham and Women's Hospital; and the
Departments of Medicine and Pathology, Harvard Medical School,
Boston, Mass.
Supported in part by National Heart, Lung, and Blood Institute
grants HL-28939 (W.G.) and HL-29295 (J.Y.W.), Veterans Administration grant (J.Y.W.), and a Research Fellowship (S.I.) from
the American Heart Association, Massachusetts Affiliate.
Address for correspondence: Jeanne Y. Wei, MD, PhD, Beth
Israel Hospital, 330 Brookline Avenue, Boston, MA 02215.
Received August 11, 1986; accepted April 10, 1987.
response to exercise was greater in young adult rats than
in old rats. In contrast, Florini et al10 reported that
there was no age difference in the extent of myocardial hypertrophy produced by thyroid hormone administration in the mouse. In a /report by Zitnik and
Roth," there was no age difference in the extent or
time course of hypertrophic response after administration of thyroid hormone in the rat. These controversies might be attributable in part to the different
types of stimulus and/or different mechanisms of myocardial hypertrophy.
To test the hypothesis that the capacity for hypertrophic response of the myocardium might diminish
with age, ascending aortic constriction was employed
to increase left ventricular pressure load, and the extent
of myocardial hypertrophy under the same degree of
aortic constriction in young adult and old rats was
examined since increased left ventricular pressure is a
common trigger of heart failure in elderly patients.
Materials and Methods
We used 3 age groups of male Fisher 344 rats (Harlan
Sprague-Dawley Inc., Indianapolis, Ind.) under contract with the National Institute of Aging (first group,
9 months old; second group, 18 months old; third
group, 22 months old). The males of this colony show
a 50% mortality at 24 months of ;age. Before surgery,
all rats were housed for at least 3 days in the Animal
Quarters of the Harvard-Thorndike Laboratory, Beth
Israel Hospital, at 23 ± 1° C on a 12-hour light/dark
338
cycle and were fed Purina Rat Chow and tap water ad
libitum.
To compare the degree of hypertrophic response
among the 3 age groups of rats, we attempted to create
the same degree of aortic constriction rather than a
given level of left ventricular pressure as a stimulus to
the left ventricle for the following reasons: 1) left
ventricular pressure may depend on age, 2) left ventricular pressure may depend on the hypertrophic
response, and 3) severity of aortic constriction (crosssectional area of the constricted aorta) would be a more
stable, independent, and comparable parameter if the
body size estimated by body weight or tibial length
were similar between young and old rats.
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Operative Procedure for Ascending Aortic
Constriction
Each rat was anesthetized with methohexital sodium
(60 mg/kg i.p.), and endotracheal intubation was
performed with direct visualization. Under controlled
ventilation (Harvard Rodent Ventilator, model 683,
South Natick, Mass.) with room air, the left thorax was
opened at the third intercostal space to expose the
ascending aorta. The ascending aorta was dissected
free from the pulmonary artery and surrounding
tissues, and a surgical thread (1 -0 silk) was drawn under
the ascending aorta. A 16-gauge needle (1.6 mm o.d.)
was placed alongside the ascending aorta, and the
ascending aorta and needle were tied tightly together
with the thread. Then, the needle was removed rapidly,
leaving the ascending aorta constricted to a diameter of
1.6 mm. The thorax was closed immediately with a silk
suture while pulmonary inflation was maintained with
positive end-expiratory pressure (approximately 10 cm
H2O). After opening the chest, the aortic constriction
could be performed in 3-5 minutes so that approximately 20 minutes after injection of methohexital
sodium, the chests were closed, and the rats were
weaned from artificial ventilation. In age-matched
control rats, sham-operations were performed as described without inducing aortic constriction. Mortalities of 9-month-old, 18-month-old, and 22-month-old
rats with aortic constriction were 15, 23, and 40%,
respectively; 82% of these rats died within 7 days of
surgical procedures. Rats were maintained on standard
rat chow and water ad libitum for 4 weeks.
In pilot studies, aortic constriction was created using
15-, 16-, and 17-gauge needles in 9-month-old rats,
and gradients were measured between peak left ventricular and peak aortic pressures. The results revealed
that operative mortality was prohibitively high (over
50% within 3 days) in the rats constricted with a
17-gauge needle (pressure gradients of approximately
45 mm Hg). Pressure gradients of approximately 15
and 30 mm Hg were obtained using 15- and 16-gauge
needles, respectively. Also, in pilot studies, the time
course of development of left ventricular hypertrophy
in 9-month-old rats was examined using 16-gauge
needles. The rats were killed at 1, 2, 4, and 8 weeks
after aortic constriction, and the degree of hypertrophy
was estimated as cited below. The hearts of rats killed
Circulation Research
Vol 61, No 3, September 1987
at 1 week after aortic constriction attained 80% of the
level of left ventricular hypertrophy observed in those
of rats killed at 4 or 8 weeks after aortic constriction.
Therefore, in the present study, 16-gauge needles were
used to create aortic constriction of the same crosssectional area among the 3 age groups and rats were
killed at 4 weeks after operation to compare the extent
of hypertrophy among those groups.
Hemodynamic Measurements and Calculations
Body weights were measured before (initial body
weight) and 4 weeks after the surgery (final body
weight). At 4 weeks after the surgery, each rat was
anesthetized with urethane (1.1 g/kg i.p.). Urethane
was chosen as the anesthetic for hemodynamic measurements because it does not depress myocardial
function and has a longer duration of action.12 After
tracheal intubation was performed as described above,
the rat was placed on a slightly warmed electric heating
pad (39±1° C). Under controlled ventilation with
room air, the neck region was carefully opened, and the
right carotid artery was approached by dividing the
sternocleidomastoid, digastric, and omohyoid muscles
close to the trachea and retracting the sternocleidomastoid muscle.13 A polyethylene cannula (Intramedic
polyethylene tubing, PE-50, Becton Dickinson Co.,
Mountain View, Calif.) was inserted into the right
carotid artery, and aortic pressure was monitored using
a Statham P23 physiologic transducer. The left thorax
was opened at the third or fourth intercostal space. The
pericardium was carefully opened, and the heart was
exposed. The ascending aorta was gently dissected free
from the surrounding tissues, and an electromagnetic
flow probe (2.5-tnm diameter, BL 6025-H55, Biotronex Laboratory, Kensington, Md.) was positioned
around the ascending aorta at the distal portion of the
thread (used for constriction) to measure ascending
aortic flow, which represents cardiac output excluding
coronary flow. Particular care was taken so that the
electromagnetic flow probe did not change the aortic
pressure level or pulse contour. Zero level of aortic flow
was defined as the beginning of the upstroke of the
tracings.
Next, the left ventricular cavity was approached
from the left ventricular base with a 19-gauge needle,
through which the left ventricular pressure was measured using a strain gauge physiologic transducer
(model MS20-BA26AAS, Electromedics Inc., Englewood, Colo.). Zero pressure reference was taken at the
midlevel of the heart. Recordings of phasic and mean
pressures and flow were continuously displayed on a
multichannel recorder (recorder 260, Gould Inc.,
Instruments Division, Cleveland, Ohio). Left ventricular end-diastolic pressure was measured by high
amplification of the left ventricular pressure signal.
Cardiac output was calculated as heart rate times stroke
output (measured by the aortic flow probe) and was
divided by final body weight to give ml/min/kg. Left
ventricular minute work was calculated as heart rate
times left ventricular stroke work.l4 The cross-sectional
area of the constricted ascending aorta was calculated
Isoyama et al Effects of Age on Cardiac Hypertrophic Response
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using Gorlin's formula.15 After the hemodynamic
measurements were performed, each rat was killed,
and its heart was removed, stripped of fat and appendages, and divided into a right ventricular free wall
portion and a left ventricular-septal portion. After
determining the total left ventricular wet weight, the left
ventricle was cut into 2 portions. The free wall portion
was fixed with 10% neutral buffered formalin and
stored in this solution until histologic examination. The
other portion involving the ventricular septum was
reweighed and dried to constant weight at 70° C (48-72
hours). The dry wt/wet wt ratio of the second portion
was obtained, and total left ventricular dry weight was
calculated from the total left ventricular wet weight and
the dry wt/wet wt ratio of the second portion. One leg
was removed above the knee joint, and the length of
the tibia from the condyles to the tip of the medial
malleolus was measured using a method16 modified
from Yin et al.' 7
Histologic Examination
Transmural myocardial sections were cut in a transverse plane perpendicular to the apex-to-base axis.
Sections were processed conventionally for histologic
examination (dehydrated in graded alcohols and embedded in paraffin), but in 6-^tm sections, and stained
with hematoxylin-eosin. For each section, a mean
myocyte width was determined by measurement of
transnuclear widths of random, longitudinally oriented
myocytes in the circular midwal 1 muscle bundles'8 with
a calibrated microscope eyepiece reticle (10 cells for
each sample) on random fields at a magnification of
400 X . Degree of fibrosis was semiquantitatively
described for each specimen as follows: score 0, no
fibrotic lesion; score 1, minimal or focal fibrotic
lesions; score 2, focal fibrotic lesions and subendocardial scars; score 3, multifocal fibrotic lesions or
large subendocardial scars; score 4, generalized scarring. All measurements and histologic observations
were made by a single observer. All histologic data
were collected without knowledge of other data.
RNA Measurement in the Myocardium
To determine the protein-synthesizing capacity of the
myocardium,19 RNA content of myocardial tissue was
determined using 2 additional groups: 9-month-old rats
(n= 10) and 18-month-old rats (n = 10). The rats were
operated on as described above, creating aortic constriction or performing a sham operation, and killed 5
days after operation. Each sample of left ventricle was
saved in liquid nitrogen until biochemical analysis. The
individual samples were homogenized in 4 mol guanidine thiocyanate and extracted at 65° C with a
phenol/chloroform/isoamyl alcohol mixture.2021 After
centrifugation, the aqueous phase was saved, and the
organic phase was reextracted. The combined acqueous
phase was completely deproteinized by extraction
twice with phenol/chloroform/isoamyl alcohol followed by extraction twice with chloroform/isoamyl
alcohol. After ethanol precipitation and centrifugation,
the sample was dissolved in 6 mol guanidine hydro-
339
chloride and reprecipitated by a half volume of ethanol
to remove DNA and it-RNA.20 The RNA pellet was
dissolved in sterile water, and its concentration was
determined by measuring the absorbance at 260 nm. In
all cases, the 260/280 nm absorbance ratio was greater
than 2.
Statistical Analysis
Variables measured are expressed as mean±SEM.
The statistical significance of differences in mean
values from 2 groups of sham-operated and the
aortic-constricted rats were assessed by the unpaired
Student's t test. The Bonferroni correction was applied
for multiple comparisons to reduce the possibility of
chance significance.
Results
Changes in Heart Weight and Histologic Findings
All rats weighed approximately 340 g at the start of
the experiment. Body weights differed little over the
9-22-month-old age range. Final body weights increased slightly 4 weeks after initial surgery in both
sham-operated and aortic-constricted 9-month-old rats
but did not in either the 18- or 22-month-old rats. Tibial
length tended to be slightly increased with age
(4.38 ±0.03 cm in 9-month-old rats, 4.43 ±0.01 cm in
18-month-old rats, 4.45 ±0.02 cm in 22-month-old
rats, p<0.05) but did not show a significant difference
between sham-operated and aortic-constricted rats.
Figure 1 shows the left ventricular (LV) dry weight,
LV dry wt/final body wt, and LV dry wt/tibial length.
In sham-operated rats, LV dry weight increased and
was 7% higher in 22-month-old rats than in 9-monthold rats ( p < 0 . 1 ) . After aortic constriction, LV dry
weight in 9-month-old rats increased by 14% compared
with age-matched sham-operated rats (192 ± 7 vs.
1 6 8 ± 4 m g , p < 0 . 0 1 ) . In contrast, LV dry weight in the
18- and 22-month-old rats with aortic constriction did
not increase compared with sham-operated controls
(173 ± 5 vs. 1 7 1 ± 6 m g , NS.in 18-month-old rats and
1 8 0 ± 5 v s . 1 8 3 ± 7 m g , N S , i n 22-month-old rats with
sham-operation and aortic constriction, respectively).
Since changes in body size occur with aging and
surgery, it is important to examine changes in left
ventricular mass relative to parameters of body size.
Interestingly, the ratio of LV dry weight to final body
weight increased with age in sham-operated rats
(0.473 ±0.013 mg/g in 9-month-old rats, 0.492±
0.006 mg/g in 18-month-old rats, and 0.539±0.014
mg/g in 22-month-old rats, p<0.0\). However, the
ratio of LV dry weight to final body weight in
9-month-old rats increased by 23% in the aorticconstricted group (0.581 ± 0.031 mg/g) compared with
age-matched sham-operated rats (p<0.01), while in
18- and 22-month-old rats, there was no difference
between aortic-constricted and sham-operated rats.
Since there is evidence that the ratio of LV dry weight
to tibial length allows a more appropriate adjustment
for body size,' 617 ' 22 " this ratio was calculated for all
groups. Once again, there was evidence of increased
left ventricular mass in aortic-constricted 9-month-old
340
Circulation Research
p<0.01
|
Vol 61, No 3, September 1987
|Sham
NS
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(rc9)(7)
9 M
18 M 22 M
9 M
18 M 22 M
9 M
18 M 22 M
FIGURE 1. Left ventricular (LV) dry weights, LV dry wtlfinalbody wt, and LV dry wtltibial length in 9-month-old (9M), 18-month-old
(18M), and22-month-old (22M) rats. Sham, sham-operatedrats: AoC', aortic-constricted rats: NS, not significant: Bars, mean±SEM.
rats compared with age-matched sham-operated rats
(43.8± 1.6 vs. 38.5 ± 0 . 7 mg/cm,/?<0.01) but not in
18- and 22-month-old rats.
Right ventricular (RV) dry weight (31 ± 1 and 32 ± 1
mg in 9-month-old rats, 3 4 ± 2 and 31 ± 1 mg in
18-month-old rats, and 3 4 ± 1 and 3 4 ± 3 mg in
22-month-old rats with sham-operation and aortic
constriction, respectively), and the ratios of RV dry
wt/final body wt and RV dry wt/tibial length did not
show significant differences among the age groups or
between aortic-constricted and sham-operated rats.
Figure 2 shows representative photomicrographs of
myocardium from hearts of the 3 age groups. Myocyte
width increased with age as seen in Panels a, c, and e.
In the heart of a 9-month-old rat, aortic constriction
increased myocyte width (Panel b). However, the
increase was modest in the heart of 18- and 22-monthold rats (Panels d and f). The mean values of myocyte
width and fibrosis' score both increased significantly
with age as summarized in Table 1. Aortic constriction
increased the myocyte width by 27% in 9-month-old
rats, by 0% in 18-month-old rats, and by 12% in
22-month-pld rats.
Hemodynamic Changes
Hemodynamic data for each of the 3 age groups are
summarized in Table 2. There was no significant
difference in peak systolic left ventricular pressure,
peak aortic pressure, or mean aortic pressure among
the 3 age groups of rats with aortic constriction. Left
ventricular end-diastolic pressure increased slightly
with age and was slightly higher in the aorticconstricted rats than in the sham-operated rats in each
age group; however, these changes were not statistically significant. No age group showed a significant
difference in cardiac output between sham-operated
and aortic-constricted rats (Table 2). Heart rate tended
to decrease with age in sham-operated rats (p<0.01 in
22-month-old rats). However, there was no difference
between aortic-constricted and sham-operated rats.
Peak-to-peak pressure difference and mean systolic
pressure difference between left ventricle and aorta
were similar among the 3 age groups of aorticconstricted rats as seen in Figure 3. Additionally, there
was no significant difference in cross-sectional area of
the constricted ascending aorta or in left ventricular
minute work among the 3 age groups of rats with aortic
constriction (Figure -3).
RNA Concentration in the Myocardium
As summarized in Table 3, there was no significant
difference in RNA concentration in the myocardium
between the 2 age groups of sham-operated rats. RNA
concentration in aortic-constricted 9-month-old rats
increased by 2 1 % compared with age-matched shamoperated rats and was significantly higher than in
18-month-old rats with aortic constriction. In contrast,
RNA in 18-month-old rats with aortic constriction
showed only a 4% increase compared with shamoperated rats. These results correlated with changes in
left ventricular dry weight and myocyte width in 9- and
18-month-old rats.
Discussion
Many studies have been reported concerning the
myocardial response to stress in neonatal animals.6"8 It
Isoyama et al
Effects of Age on Cardiac Hypertrophic Response
341
m
«•
»•
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*
»l
«.
- -
-
n
-/
; .
t
FlGURI- 2. Histologic sections obtained from hearts of 9-month-old (Panels a and b), 18-month-old (Panels c and d), and'22month-old (Panels d and 0 rots with sham-operation1 and aortic-constriction, respectively. Stained with hematoxylin and eosin,
magnification 375 x .
342
Circulation Research
Histologic Analysis in the 3 Age Groups of Rats
18 months
9 months
AoC
Sham
AoC
Sham
7
n
5
7
9
Myocyte width (ju,m)
20.4±0.8 25.9±0.9*
29 .2 + 0 ,6§ 29.2 ±1 .4
1.7±0 .5
0.9 + 0.9
Fibrosis score
1.6±0 .5*
0.6±0.7
Vol 61, No 3, September 1987
Table 1,.
22 months
Sham
AoC
9
9
32. 4 ± 1 . 5 t §
2. l ± 0 . 2
29 .0±l .2§
2 .4±0. • 3 §
Sham, sham-operated rats; AoC, aortic-constricted rats; mean ± SEM.
*p<0.01, tp<0.05, statistical significance of differences between mean values in sham-operated and aorticconstricted rats in each age group. t p < 0 . 0 5 , §p<0.01, statistical significance of differences between mean values in 9month-old rats and in different age rats in sham-operated or aortic-constricted rats.
Downloaded from http://circres.ahajournals.org/ by guest on June 14, 2017
is generally accepted that a volume or pressure overload
in neonates results in significant cardiac enlargement
accompanied by mitotic division of both myocardial
and nonmyocardial cells (hyperplasia), while compensatory cardiac enlargement in adult animals is achieved
by growth of preexisting myocardial cells (hypertrophy) and nonmyocardial cell mitotic division.
However, there is little information concerning
adaptation of the adult heart to increased loading with
advancing age. The extent as well as time course of
cardiac hypertrophy induced by thyroid hormone in
rats of advanced age (22-24 months) was reported to
be similar to that in young adult rats." In another
report,10 there was a longer lag period before hypertrophy in mice of advanced age (26 months). McCafferty and Edington9 examined the effect of age on
Cardiac hypertrophy during chronic running exercise,
and reported that exercise increased heart weight in rats
3-15-months old. However, the extent of increase in
heart weight in response to exercise was greater in
young rats than in old rats. Furthermore, initiation of
exercise at an older age (approximately 20 months)
decreased the heart weight compared with age-matched
controls.
In our study, left ventricular hypertrophy was substantially less in rats of advanced age (18 to 22 months)
than in young rats in response to a similar grade of
increased afterload for 4 weeks. This diminished
hypertrophic response was also found in morphometric
measurement of individual myocytes. In fact, a sigTable 2.
nificant increase in left ventricular weight was not
observed, and an increase in myocyte width was
modest in the old rats following aortic constriction.
This diminished hypertrophic response in rats of
advanced age was coincident with their higher mortality after aortic constriction compared with younger
rats. Left ventricular end-diastolic pressure was
slightly higher in old rats (22 months) than in young rats
(9 months) at 4 weeks after aortic constriction, but there
was no hemodynamic or myocardial tissue evidence of
heart failure.
In this study, the degree of hypertrophy at 4 weeks
after operation was estimated, and the time course of
development of myocardial hypertrophy in old rats was
not examined. The choice of 4 weeks was made because
our pilot studies showed no difference in hypertrophic
response between 9-month-old rats killed at 4 or 8
weeks after aortic constriction. It seems unlikely that
the diminished myocardial hypertrophic response in
old animals was caused by a longer time lag to develop
hypertrophy for the following reasons. First, the longer
time lag, which was observed by Florini et al10 in old
mice treated with thyroid hormone, was only 9 days,
and the extent of hypertrophy in young (8 months old)
and old (26 months old) mice was almost equal at 9 days
after initiation of the treatment. Second, the hypertrophic response in young (9 months old) rats reached near
maximal level after only 1 week of aortic constriction
in our pilot study and in the report of Fanburg and
Posner.24 This time course of developing hypertrophy
Hemodynamic Data in the 3 Age Groups of Rats
9 months
Sham
n
Peak systolic LVP
End-diastolic LVP (mm Hg)
Peak systolic AoP
Mean AoP (mm Hg)
CO (ml/min)
CI (ml/min/kg)
Heart rate (beats/min)
AoC
7
5
91±3
4.6±0.8
89 + 3
69±4
80 + 9
223±21
371 ±17
127 + 13*
5.6±0.9
94±9
75 ±7
63 ±5
182±16
361 ±8
18 months
Sham
AoC
8
95 ±9
5.1 + 1.0
94+10
79+11
76 + 7
212±16
345 + 9
22 months
AoC
Sham
7
8
8
124±11*
6.0±0.6
98±10
77 + 7
73 ±5
208 ±17
370±14
88 + 9
5.8±0.4
89 ±10
70±9
59 + 7$
174±21
316±12§
121±lOt
6.4 + 0.5
95 ±7
81+6
57±7
177 + 24
339 ±24
Sham, sham-operated rats; AoC, aortic-constricted rats; LVP, left ventricular pressure; AoP, aortic pressure; CO,
cardiac output; CI, cardiac index; mean ± SE.
*/><0.05, t p < 0 . 0 1 , statistical significance of differences between mean values in sham-operated and aorticconstricted rats in each age group. tp<0.05, §p<0.01, statistical significance of differences between mean values in 9month-old rats and different age rats in sham-operated or aortic-constricted rats.
Isoyama et al
Effects of Age on Cardiac Hypertrophic Response
Peak-to-Peak AP
(mmHg)
40 r
30 --
I
Mean Systolic AP
(mmHg)
40
X
30
T X
T
T
20
20 •
•
10
10
0
9 M
18 M
22 M
9 M
Cross-Sectional
(cm2)
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0.010
0.005
•
T
8000i -
X
T
22 M
LV Minute Work
(mmHg ml/min)
0.020 i -
0.015
18 M
X
_T_
6000
4000 -
-
2000
•
0
•
0
9 M
18 M
22 M
9M
18 M
22 M
FIGURE 3. Pressure gradient (AP) between the left ventricle
(LV) and aorta, cross-sectional area of the constricted aorta,
and LV minute work in 9-month-old(9M), I8-month-old(I8M),
and 22-month-old (22M) rats with aortic constriction. Bars,
mean±SEM.
was quite similar to that in mice treated with thyroid
hormone in the report of Florini et al.10
In the report of Lipana and Fanburg,25 aortic constriction in hypophysectomized rats did not result in
cardiac hypertrophy estimated by increases in weight;
however, when the degree of constriction was increased, cardiac hypertrophy resulted. In the present
study, relatively moderate degrees of aortic constriction estimated by peak left ventricular pressure or by
pressure gradient between the left ventricle and aorta
were employed because mortality following aortic
constriction was high in old rats (22 months). Given a
more intense stimulus, it is possible that an increase in
Table 3. RNA Concentration in the Myocardium of the 2 Age
Groups of Rats
18 months
9 months
Sham
n
AoC
5
963±22
5
1,I68±31
I—p<0.001
1
/xg/g wet wt
AoC
Sham
5
980+10
5
1,021 ± 1 6
'
NS
p<0.005
1
NS
Sham, sham-operated rats; AoC, aortic-constricted rats; NS, not
significant, mean ± SEM.
343
left ventricular weight might be observed even in rats
of advanced age (assuming the animals survive).
In sham-operated rats, left ventricular weight and
myocyte width increased with age. In addition, histologic examination revealed that myocardial tissues
obtained from older rats showed more fibrosis. Similar
findings associated with senescence per se have been
reported by McCafferty and Edington,9 Li et al,16 Yin
et al,17-22 Wei et al,23 and Anversa et al.26 If the extent
of hypertrophy occurring with senescence alone has
reached a maximum level, it might be possible to obtain
further hypertrophy by pressure overload in advanced
age. However, in the reports of Florini et al10 and Zitnik
and Roth," stimulation by thyroid hormone produced
further myocardial hypertrophy, even in rats of advanced age. Therefore, in our study, the degree of
hypertrophy produced by pressure overload in rats of
advanced age should not have been limited by existing
hypertrophy associated with senescence, although it
seems likely that the hypertrophy caused by senescence
alone might change the level of sensitivity to stimulation for myocytes to develop further hypertrophy.
McCafferty and Ed ington9 examined the effect of age
on cardiac hypertrophy produced by chronic running
exercise and suggested that there might be a threshold
age (approximately 15 months old) beyond which the
initiation of a training program no longer affects heart
muscle growth. Although aortic constriction rather
than exercise was employed as the stimulus to cardiac
hypertrophy, our observations are consistent with their
findings.
Johnson et al27 and Meerson et al28 have reported that
in myocardium of old rats or old humans, RNA
concentration decreased, and the rate of RNA and
protein synthesis as well as degradation were diminished. In our study, young adult rats showed a
significant increase in myocardial RNA concentration
following aortic constriction, but old rats did not. Since
the majority (>97%) of total cellular RNA in our
preparation is ribosomal RNA, a failure to increase
ribosomal RNA in old rats indicates that the capacity
to synthesize protein is diminished. This diminished
capacity for protein synthesis appears to be one of the
contributing factors to the diminished hypertrophic
response in rats of advanced age.
The different findings in the extent of myocardial
hypertrophy in animals of advanced age produced by
thyroid hormone administration10" or, in our study,
aortic constriction may reflect differences in the
mechanisms by which these stimuli provoke myocardial hypertrophy.2A5IU9:'0 Many factors within the
myocardial cell7-8-291" modulate the process linking
increased circulatory demand to myocardial hypertrophy and/or the process of protein synthesis and
degradation. Thyroid hormone has been shown to
regulate cardiovascular responsiveness to catecholamines through stimulation of /3-adrenoceptors in
vivo" 32 and in vitro.33 Increases in /3-adrenoceptor
density" and protein synthetic rates10 after thyroid
hormone treatment may be reduced in the senescent
compared with the young adult heart. It has been
344
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demonstrated that aging has no effect on the extent of
thyroid-induced cardiac hypertrophy in mice and
rats. 10 " It is possible that thyroid hormone may
differentially modulate cardiovascular adrenergic responsiveness and protein synthesis in young adult and
senescent animals under similar conditions of pressure
overload.
Stimulation of a-adrenoceptors may modulate the
hypertrophic response of cardiac muscle at the cellular
level. In intact animals,3435 subhypertensive doses of
norepinephrine caused myocardial hypertrophy,35 Furthermore, estimated by cell volume, surface area, and
protein content,36-37 norepinephrine plus /3-receptor
blockade produced hypertrophy in cultured myocytes,
which suggests that stimulation of a-receptors may
trigger hypertrophy in myocardial cells. It is possible
that the circulating catecholamine or myocardial catecholamine levels were higher after aortic constriction
in young rats than in old rats and/or that the number or
affinity of a-adrenoceptors in old rats was diminished
compared with young rats38 in our study.
The role of baroreceptor function should also be
considered in interpreting the results of our study. In
our aortic-constriction preparation, the arterial baroreceptors were relatively unloaded, while the systemic
hypertension of renal or subdiaphragmatic origin,
these receptors would be subjected to pressure loading
comparable in degree to that experienced by the
cardiopulmonary receptors located in the left
ventricle.39 Baroreceptor interactions with protein synthetic mechanisms may be important in mediating the
age effect on the myocardial hypertrophic response.
Further studies are needed to determine whether these
factors may contribute to the diminished capacity for
myocardial hypertrophic response in rats of advanced
age.
Acknowledgments
The authors are grateful to Drs. Janice and Marc
Pfeffer for helpful advice and Helen Shing and Sara
Murray for technical assistance (histology).
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KEY WORDS
senescence
left ventricular hypertrophy
afterload • RNA
adult aging
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Effect of age on the development of cardiac hypertrophy produced by aortic constriction in
the rat.
S Isoyama, J Y Wei, S Izumo, P Fort, F J Schoen and W Grossman
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Circ Res. 1987;61:337-345
doi: 10.1161/01.RES.61.3.337
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