Chemical Composition of the Rabbit Aorta during
Development
By D. Ian McCloskey and Edward G. Cleary
ABSTRACT
Changes in both the relative and the absolute amounts of collagen and elastin in
segments of the aortic wall of New Zealand white rabbits (1-130 weeks old) were
determined by chemical analysis. As in other mammals, elastin was the major component
of the wall of the thoracic aorta although the proportion of collagen increased along the
abdominal aorta and into the more distant arteries. Data on the absolute amounts of
collagen and elastin per aorta showed that collagen and elastin deposition was most rapid
during the early postnatal weeks. Although this deposition slowed in later weeks, it
continued in both segments of the aorta throughout the period of this study. The
proportion of the wall mass attributable to collagen and to elastin increased very rapidly
during the first postnatal weeks and then, after 20 weeks, declined. The results of the
present study indicate that there is a significant increase in some unidentified nonscleroprotein component within the aortic wall of older rabbits.
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KEY WORDS
collagen
elastin
• Harkness et al. (1) have observed that the
scleroprotein composition of the canine aortic wall
varies at different anatomical sites; elastin is
the major component in the thoracic aorta and
collagen is the predominant component in the
abdominal aorta and its major peripheral
branches. They (1) have also found this pattern in
the aortic walls of young puppies of unspecified
age. Several subsequent studies (2-6) have shown
that this pattern of scleroprotein distribution
within the walls of the major arteries is common to
many mammalian species and some birds. However, these studies have revealed a number of
differences between species. To what extent this
pattern of scleroprotein distribution is genetically
determined or to what extent the developing arterial system differentiates in response to locally
acting hemodynamic and physical stresses is unknown. Only one previous study—an investigation
of the rat aorta—has detailed the changes with age
in scleroprotein composition of different arterial
segments (7). The present study describes the
From the Department of Physiology, University of Sydney,
Sydney. New South Wales, Australia, and the Department of
Pathology, University of Adelaide, Adelaide, South Australia.
This study was supported by grants from the Life Assurance
Medical Research Fund of Australia and New Zealand and from
the National Heart Foundation of Australia.
Dr. McCloskey s present address is School of Physiology and
Pharmacology, University of New South Wales, Kensington,
2033, Australia.
A brief account of these results was published previously in
abstract form (Aust J Sci 25:111, 1962).
Received April 23, 1973. Accepted for publication March 13.
1974.
828
nonscleroproteins
arteries
media
changes in both the relative and the absolute
amounts of collagen and elastin in segments of the
wall of the aorta and its major branches in healthy
rabbits from postnatal development to maturity.
Methods
One hundred and twenty-one New Zealand white
rabbits (1-130 weeks old) of either sex were studied. The
rabbits were fed standard rabbit food pellets, fresh
carrots, and tap water.
DETERMINATION OF BLOOD PRESSURE
Mean arterial blood pressure was determined on
restrained unanesthetized rabbits under quiet conditions
by percutaneous puncture of the femoral artery as
described by Goldblatt (8).
PREPARATION OF VESSELS
The rabbits were killed by cervical fracture. The
anterior chest wall was removed, and the abdominal
cavity was opened to expose the aorta and its main
branches. These vessels were excised as a unit, and loose
adventitial tissue was removed by blunt dissection. The
vessels were opened longitudinally, washed free of blood
with physiologic saline, and blotted gently between filter
papers. The arterial branch orifices were excised from the
aorta, which was then divided at the level of the upper
border of the crura of the diaphragm into thoracic and
abdominal portions. All of the thoracic aorta distal to the
aortic valvular ring was used; the lower limit of the
abdominal aorta was the aortic bifurcation. The two
portions were weighed separately.
CHEMICAL ANALYSES
For chemical analyses, the thoracic and abdominal
aortas were cut into segments. Transection of the thoracic aorta immediately below the insertion of the ductus
arteriosus separated the thoracic descending aorta from
the ascending aorta and the aortic arch. The thoracic
Circulation Research, Vol. XXXIV. June 1974
829
AGE CHANGES IN RABBIT AORTA
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descending aorta was then divided into three segments of
equal length, which will be referred to as the upper.
the middle, and the lower thoracic segments. The
abdominal aorta was also divided into three segments of
equal length. The common carotid arteries, as defined
anatomically, were used whole. The common and external iliac arteries were taken together as the vessels
between the aortic bifurcation and the inguinal ligament
and will be referred to as the iliac arteries. The axial
artery distal to the inguinal ligament as far as a point
two-thirds of the distance down the thigh was taken as
the femoral artery.
Definition of these vessels was readily reproducible at
all ages studied. For each age group, comparable samples
were pooled from several rabbits; the number of rabbits
used varied from two to ten depending on the mass of
tissue available. Samples were pooled so that enough
tissue was available for gravimetric quantification of the
elastin residues. To measure the residues reliably, samples weighing more than 0.1 g (fresh weight) were
needed. In most instances, samples of fresh tissue
weighing between 0.2 g and 0.5 g were available for each
estimation. Estimations were made on vascular segments from 19 groups of rabbits.
DETERMINATION OF COLLAGEN AND ELASTIN
Each tissue sample was minced finely with scissors in
a tared weighing bottle and weighed (wet weight). It was
then dried to constant weight in a vacuum over phosphorus pentoxide (dry weight). The dried samples were
repeatedly extracted with ether to remove fat and then
dried again to constant weight (dry fat-free weight). The
dry fat-free weight was used as the most reproducible
reference standard for purposes of comparison.
Collagen was extracted from the segments as gelatin
by autoclaving in distilled water for 6 hours at 30 psi.
After hydrolysis in 6N hydrochloric acid, the collagen
contents of the extracts were determined by colorimetric
assay of hydroxyproline. A modification of the procedure
of Neuman and Logan (9) was used. This procedure has
been described in detail elsewhere (procedure 1, Jackson
and Cleary [10]). A hydroxyproline content of 14.4% for
mammalian collagens was assumed for the collagen
calculations. Elastin was determined gravimetrically as
the dry residue remaining after extraction with hot alkali
(method C, Jackson and Cleary [10]). For each arterial
sample, results were calculated as a percent of the dry
fat-free weight.
Weighed samples of dried elastin were hydrolyzed in
6N hydrochloric acid in sealed tubes by autoclaving for 4
hours at 40 psi. The hydroxyproline contents of the
hydrolysates were determined colorimetrically as previously described.
To establish the reliability of the colorimetric assay for
hydroxyproline with these tissues and to exclude the
possibility of interference with color development by
some component in the aortic gelatin and the elastin
hydrolysates from different segments and the same
segments at various ages, recovery experiments were
carried out after known amounts of pure hydroxyproline
had been added to the hydrolysates.
HISTOLOGY
Vessels were removed from 1-, 20-, and 58-week-old
rabbits. After preparation for chemical analysis, segCirculation Research. Vol. XXXIV. June 1974
ments were sectioned for histological examination. The
amount of adventitia remaining on each segment was
checked, and histological appearance was compared with
chemical data. Sections were stained with hematoxylin
and phloxine. with aldehyde fuchsin, hematoxylin. and
light green, and with aldehyde fuchsin. hematoxylin. and
picro-indigo-carmine combinations (5).
Results
BLOOD PRESSURE
Mean arterial blood pressure rose steadily7 from
an initial value of 30 mm Hg in 1-week-old rabbits.
Adult values (90-100 mm Hg) were attained by the
age of 10-12 weeks. Thereafter, blood pressure
showed only minor variations about this value up
to 130 weeks of age. These findings are very similar
to those reported by Bauer (11) and Dawes et al.
(12).
REPRODUCIBILITY OF CHEMICAL ANALYSES
Duplicate hydroxyproline determinations
agreed to within ± 1.5%, and recovery of hydroxyproline added to hydrolysates of both collagen and
elastin from different vascular segments and from
the same segment from different age groups was
within the range of 100 ± 2%. The reproducibility
of collagen determinations on tissues from any one
vascular segment was better than ± 2%. The
autoclave procedure extracted more than 98% of
the collagen hydroxyproline from all segments from
newborn, young, and old rabbits.
For elastin determinations, the size of the tissue
sample affected the reproducibility. For those samples with a wet weight of 100-500 mg, elastin
content was reproducible to within ± 2%. For the
samples containing only small amounts of elastin,
e.g., abdominal aorta segments from young rabbits, replicate elastin analyses fell within a wider
range of ± 5%.
PATTERNS OF SCLEROPROTEIN DISTRIBUTIONS
The typical distribution of collagen and elastin
along the aorta and its main branches in New
Zealand white rabbits is represented in Figure 1.
The values for the ascending aorta-aortic arch
segments have been omitted from the figure in the
interest of clarity. The elastin content of these
segments was identical to that in the remainder of
the thoracic aorta, but the collagen content was
slightly less than that in the thoracic descending
aortic segments. The proportion of the dried defatted vessel wall mass composed of both elastin and
collagen was relatively constant along the aorta but
declined in the iliac and more distant "muscular"
arteries.
The collagen and elastin contents relative to
830
McCLOSKEY, CLEARY
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ABDOMINAL
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FIGURE 1
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Distribution of collagen (squares) and elastin (circles) along the
aorta and its main branches in 33-week-old rabbits. The total
scleroprotein (diamonds) content of individual segments is also
shown. Values are expressed as percent of the dry fat-free weight
(D.F.F.WT.) of the segments which were taken from the
portions of the vessel indicated on the diagram of the aorta.
60
80
AGE (weeks)
100
120
60
80
AGE (weeks)
100
120
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B
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total scleroprotein content changed considerably
along the arterial system. Elastin was predominant
throughout the thoracic aorta. In the proximal half
of the abdominal aorta, the elastin content decreased and the collagen content increased. These
changes continued into the distal aorta and peripheral branches, in which collagen was the predominant component. The carotid arteries also showed a
preponderance of collagen.
CHANGES IN SCLEROPROTEIN CONTENT WITH AGE
Although the pattern of distribution of collagen
and elastin shown in Figure 1 was found in the major arteries of rabbits at all ages examined, there
were variations during development in the contents
of collagen and elastin in both thoracic and abdominal aortic segments. These changes are shown in
Figure 2A and B.
Weeks 1-20.—For both the thoracic and the
abdominal segments of the aorta during weeks
1-20, a progressive increase occurred in the proportion of the wall mass composed of collagen. Expressed algebraically, the collagen content of the
thoracic aorta increased in this period according to
the equation
Aortic collagen (thoracic)
= 0.20 x age (weeks) + 15.5. (1)
The increase was significant (P < 0.001). The
increase in the collagen content of the abdominal
aorta is described by the equation
20
L0
1U0
FIGURE 2
A: Changes with age in the collagen contents of the thoracic
(solid squares) and abdominal (open squares) segments of the
rabbit aorta. B: Changes with age in the elastin contents of the
thoracic (solid circles) and abdominal (open circles) segments of
the rabbit aorta. Results are expressed as a percent of the dry
fat-free weight (D.F.F.WT.) of the whole of the segment
analyzed. Regression lines have been drawn from the computed
data for the changes in these components before and after 20
weeks of age.
Aortic collagen (abdominal)
= 0.29 x age (weeks) + 24.4. ^ '
Again, the increase was significant {P < 0.001).
The changes in elastin content of both the
thoracic and abdominal segments of the aorta
during weeks 1-20 are more difficult to describe
than are the changes in collagen content. In both
segments, a sharp rise in the elastin content
occurred in the first 4 weeks. Although the elastin
content of the thoracic aorta attained its highest
value by the fourth week, the elastin content of the
abdominal aorta appeared to continue to increase
until a maximum was reached at about the twentieth week (the same time the collagen maximum
occurred). This latter increase can be described by
the equation
Circulation Researth, Vol. XXXIV, June 1974
831
AGE CHANGES IN RABBIT AORTA
Aortic elastin (abdominal)
= 0.05 x age (weeks) +32.1. (3)
The increase was significant (P< 0.01).
After 20 Weeks.—Figure 2 shows that the collagen and elastin contents of both thoracic and
abdominal aortic segments declined after 20 weeks
and that the rates of these decreases were greater
after 66 weeks of age. Linear regressions were fitted
to the data for collagen content; the equations
describing the changes after 20 weeks are as follows.
Aortic collagen (abdominal)
(4)
= -0.05 x age (weeks) + 32.74.
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The decrease was significant (0.01 < P < 0.025).
Aortic collagen (thoracic)
<§\
= -0.029 x age (weeks) + 20.41.
The decrease was significant (P< 0.001).
Linear regressions were also fitted to the data for
elastin content. Because the onset of the decline in
the percent of elastin present was not clear-cut, the
regression equations were calculated for the data
obtained after 18 weeks. Inclusion of the additional
values between 18 and 20 weeks will tend to reduce
rather than increase the extent and the significance
of the decline in elastin values. Nevertheless, a
highly significant decrease in the elastin proportions in both the abdominal and the thoracic aortic
segments occurred. These changes are described by
the following equations.
Aortic elastin (abdominal)
= -0.079 x age (weeks) + 43.62.
(6)
weeks, total scleroprotein content increased rapidly
in both the thoracic and the abdominal segments of
the aorta. The increase in the abdominal segment
was of greater magnitude and occurred over a
longer period of time than that in the thoracic
segment of the aorta. Following this initial postnatal phase, scleroprotein content remained relatively stable in both segments for some months,
after which it declined in both segments.
NONSCLEROPROTEIN CONTENT AND AGE
The changes in total scleroprotein content can
be looked at in another way. The proportion of the
wall mass not composed of collagen and elastin can
be considered as the nonscleroprotein content.
Obviously, a rise in sclefoprotein content means a
commensurate fall in nonscleroprotein content and
vice versa. Figure 3A and B graphically shows the
changes in the proportion of the aortic wall mass
composed of materials other than collagen and
elastin. (These graphs show the inverse of the
changes in contents of collagen plus elastin.) The
changes in the postnatal phase were prominent.
The nonscleroprotein content of both segments of
the aorta increased after 60 weeks of age. Because
few data points were available after 60 weeks, regression lines were fitted to the data after 20 weeks
of age. Both aortic segments showed a significant
increase (P < 0.005 for each) in nonscleroprotein
contents after the twentieth week. The rate of
increase in the proportion of nonscleroproteins was
even greater after the first year. The increases
might be greater in the abdominal segment than
they are in the thoracic segment.
ABSOLUTE AMOUNTS OF ELASTIN AND COLLAGEN
The changes with age in the absolute amounts
of elastin and collagen are shown separately in
The decrease was significant (P < 0.001).
Aortic elastin (thoracic)
= -0.044 x age (weeks) +51.95.
A
(7)
Again, the decrease was significant (P < 0.001).
The possibility that the elastin content of the
thoracic aorta decreased after the fourth week was
also considered. However, little change in the slope
of the regression equation resulted; the decrease in
elastin content remained highly significant (P <
0.001).
TOTAL SCLEROPROTEIN CONTENT AND AGE
As a result of the changes in collagen and elastin
contents, changes in the proportion of the aortic
wall mass composed of scleroproteins (elastin plus
collagen) occurred with age. In the early postnatal
Circulation Research. Vol. XXXIV. June 1974
r°
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t
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t
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a
JO
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FIGURE 3
Changes with age in the proportion of the wall of rabbit aorta
segments composed of nonscleroprotein material (material other
than collagen and elastin). A: Changes in the thoracic segment
of the aorta. B: Changes in the abdominal segment of the aorta.
Results are expressed as a percent of the dry fat-free weight
(D.F.F.WT.) of the whole segments analyzed.
832
McCLOSKEY. CLEARY
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FIGURE 4
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A: Changes with age in the absolute amounts of collagen
(squares) and elastin (circles) in the thoracic portion of the
rabbit aorta. B: Changes with age in the absolute amounts of
collagen (squares) and elastin (circles) in the abdominal portion
of the rabbit aorta. Results are expressed as milligrams of tissue
obtained from the whole of the segment analyzed. The curves
have been-fitted visually to approximate lines of best fit for the
data.
Figure 4A and B. In the thoracic aorta, elastin and
collagen accumulated at relatively constant rates
with a ratio of approximately 3:1 throughout the
first 20 weeks of life. Thereafter, the rate of increase
in both components slowed considerably. However,
the deposition of both components in this portion
of the aorta continued throughout the duration of
this study. In the period after 20 weeks of age, the
increases in elastin and collagen had a ratio of
about 1.9:1. In the abdominal aorta, the initial
phase of rapid elastin and collagen accumulation at
a ratio of approximately 4:3 appeared to be of
shorter duration, and the absolute mass of both
components leveled off after 8-12 weeks. Again, in
this segment of the vessel, the deposition of both
components continued at a slow rate throughout
the period of the study; the ratio of the increase in
elastin to that in collagen was approximately 1.4:1.
HYDROXYPROLINE CONTENT OF ELASTIN
The mean value for the hydroxyproline content
of dried elastin samples was 1.67 ± 0.09% (SE) (10
estimations).
HISTOLOGY
Examination of histological sections from different levels of the vessels prepared for chemical
analyses revealed that the removal of adventitial
tissue was uniformly complete throughout the
thoracic aorta. In the abdominal aorta, a thin rind
of residual adventitia remained. This rind was
composed mainly of collagen-staining material
with a few fibroblasts and occasional elastic-staining fibers. There was no obvious change with age in
the proportion of aortic adventitial residue.
Throughout the aorta, the intima consisted of a
single layer of endothelial cells and a small amount
of subendothelial connective tissue. Therefore, the
main mass of the vessel wall, as analyzed, consisted
of the media.
A 20% reduction in the number of medial elastic
lamellas between the thoracic descending aorta
and the lower half of the abdominal aorta was
observed. Concurrently, the elastic lamellas became thinner and the smooth muscle elements in
the lower abdominal portion appeared to increase.
No pathological changes were observed in either
the media or the intima of these vessels.
The amount of adventitial tissue remaining on
the more peripheral muscular arteries was relatively greater and represented between 10 and 15%
of the wall thickness.
Discussion
This study indicated that the pattern of scleroprotein distribution in wall segments along the
rabbit aorta and its major branches is similar to
that reported for other mammals. The histological
examination revealed that the changes in the
distribution of scleroproteins along the vessels
occur in aortic segments that consist almost entirely of tunica media.
The importance of the scleroproteins in the
aortic wall relates to their influence on the physical
properties of the vessel. Elastin is readily extensible and rubberlike, but collagen is much stiffer and
inextensible. The reduction in elastin content and
the increase in collagen content of the aortic wall as
it becomes more distant from the heart is associated with an increase in stiffness of the more
peripheral portions of the arterial tree (13, 14). This
nonuniformity of chemical composition and physical properties is in turn associated with changes in
the contour of the arterial pressure pulse and with
an increase in the rate of transmission of these
pulse waves as they pass into the more peripheral
arteries (15).
The data from the present study on New Zealand
white rabbits showed that, despite the low arterial
blood pressure and the relative immaturity of the
cardiovascular system at birth and in the early
postnatal weeks in this species, the adult pattern of
scleroprotein distribution is already established by
the end of the first postnatal week (when our first
observations were made). Additional observations
on newborn rabbits of another breed (Cleary,
unpublished data) have shown that the pattern of
scleroprotein distribution at birth is very similar to
that of the 1-week-old rabbits used in the present
study.
Circulation Research. Vol. XXXIV. June 1974
AGE CHANGES IN RABBIT AORTA
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Although the actual pattern of scleroprotein
distribution along the aorta does not change, extensive changes in the relative amounts of the individual components do occur during the portion of the
life-span considered in this study. In the early
postnatal weeks the proportions of both collagen
and elastin in the two aortic segments increase
(Fig. 2). These increases in relative contents of the
scleroproteins correspond to the phase of rapid
deposition of collagen and elastin in the aortic wall
segments during the first 15-20 weeks of life (Fig.
4). After this initial phase, collagen and elastin
continue to accumulate at a reduced rate, at least
throughout the remainder of the period of this
study. The ratios of the rate of accumulation of
elastin to that of collagen are different in the two
aortic segments, but for each segment very little
change in this ratio occurs with further increase in
age.
Our observation of a significant continuing decrease in the proportion of the wall mass attributable to collagen after 20 weeks of age is surprising
because of the well-known "sclerosis," or increase
in collagen, reported to occur in the aortic media of
older animals of many species (16-18). Saxton (19)
has shown that aging in rabbits is associated with
progressive replacement of aortic smooth muscle
cells by collagen; our own histological studies on
these vessels appear to be consistent with this view.
However, it is obvious from our chemical data
that this explanation is not sufficient. On a wholeorgan basis, there is chemical evidence of continuing collagen and elastin deposition with increasing
age; yet, simultaneously, collagen and elastin concentrations in both aortic segments decrease. Because the chemical findings are in opposition to the
usually accepted interpretations, we have meticulously ruled out possible sources of error in our
chemical assays.
Erroneous collagen values could result from
contamination of the vascular gelatins by some
substance(s) which interferes with color development in the hydroxyproline assay. The accumulation with age of a substance which suppresses color
development or the decrease in a substance which
enhances it could produce an apparent but false
fall in collagen content. Both of these possibilities
were excluded in this study by the demonstration
that recovery of pure hydroxyproline added to
vascular gelatins from young, mature, and old
rabbits was complete and within the range of 100 ±
2%. Furthermore, an apparent but false decline
with age in collagen content could result from
incomplete extraction of gelatin from the older
Circulation Research, Vol. XXXIV. June 1974
833
vessels. We found that more than 98% of the total
hydroxyproline (other than that in the alkaliinsoluble elastin) was present in the gelatins from
young, mature, and old rabbit aortas extracted
using the autoclave procedure. There was no systematic difference in the proportion of unextracted
hydroxyproline with age; thus, this source of error
was also excluded.
Finally, it should be noted that the collagen
results were obtained by assuming that the hydroxyproline content of vascular collagens does not
vary with age. Although to our knowledge there is
no direct evidence for this assumption in rabbit
aorta, there is considerable evidence from other
tissues and from other species to support such an
assumption. Hydroxyproline content does not vary
with age in bone collagen in fowls (20), in tail tendon
and skin collagens in rats (21), and in lung and skin
collagens in humans (22). Some workers (23) have
reported an increase with age in the hydroxyproline
content of collagens from lung and tail tendon in
rats. If these latter findings are valid and applicable to rabbit aorta collagens, they would indicate a
decrease with age in collagen content even greater
than that proposed in the present study. We know
of no report indicating that the hydroxyproline
content of collagen from any one soft tissue in a
single species decreases with age. Thus, the decrease in collagen content observed by chemical
assay in the aging rabbit aorta appears to be a real
one, and some alternative explanation will have to
be found for the histological observations.
Cliff (24) has attempted to reconcile biochemical
analyses showing a 10% increment in collagen
content in rat thoracic aortic segments between
the ages of 4 months and 3 years (5) with the very
obvious sclerosis seen histologically and electron
microscopically in the aged rat aorta. He found
that the distribution of collagen in the rat aortic
media alters with advancing age so that, instead of
being arranged in large well-localized bundles, it is
spread diffusely and randomly throughout the
medial elastic network. Cliff (24) has suggested
that this change in distribution could account for
an apparent increase in collagen in histological
sections.
Although the continuing increase with age in the
absolute amount of collagen in the aortic wall
might have been predicted (probably for inaccurate reasons) from histological examination, neither histological appearance nor previous reports
suggested the continuing and even greater increase
with age in aortic elastin. Saxton (19) has reported
an increase in the number of fine elastic fibrils
834
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between the elastic membranes of the aorta in old
rabbits and a concurrent reduction in the thickness
of the elastic lamellas themselves. Similar observations have been made by Cliff (24) in rats up to 36
months of age.
Furthermore, the conclusion that elastin is biologically inert and has a small rate of turnover in
adult and aging animals (25-28) is widely accepted. However, the experiments from which
these conclusions were derived measured the incorporation and the specific activity of 14C-glycine in
insoluble aortic elastins in adult rats and adult
rabbits. Since radioactive glycine was readily incorporated into these insoluble elastins, there must
have been a continuing elastin synthesis at the
time. The failure to detect a decrease in the specific
activity of glycine in aortic elastins was probably a
function of the short time interval over which the
observations were made, i.e., 9 days in the rats and
28 days in the rabbits.
These same criticisms cannot be applied to the
findings of Walford et al. (27). These workers also
studied the changes in the specific activity of
14
C-glycine in rat aortic elastin, but the radioactive
glycine was incorporated during the rapid growth
phase of young rats and the elastin specific activity
was measured in rats killed at repeated intervals
up to 930 days. The specific activity of the elastin
fell rapidly during continued growth until the age
of 120 days, after which it remained constant. This
otherwise excellent experiment is subject to the
criticism that the method used to purify the
insoluble elastin does not apply uniformly at all
ages. Hass (29), who was largely responsible for
reviving interest in the method used by Walford et
al. (27), showed that a longer extraction time
(36-72 hours) was necessary to remove collagen
from human vascular elastic tissue in older vessels.
This same stipulation might apply to rat vessels
and until this possibility is investigated, the results
of Walford et al. (27) must also be accepted with
reservations.
Results from two reports by Wolinsky (30, 31)
show an increase in elastic tissue in the aortas of
normal rats between the ages of 28 weeks and 18
months. Unfortunately, no observations were made
on rats of intermediate ages; thus, it is impossible
to say definitely that elastin synthesis was still
occurring at 18 months of age and to comment on
the changes in older rats.
Our own results clearly indicate continuing elastin accumulation with increasing age in the aortic
wall of the rabbit, even though relatively harsh
methods of purification were used.
McCLOSKEY. CLEARY
Our observations on the changes in collagen and
elastin in the aortic wall of the rabbit highlight the
fact that examination of the changes in the content
of collagen and elastin relative to the whole wall
mass not only can give an incomplete understanding of the changes in collagen and elastin metabolism but also can actually lead to an erroneous
interpretation. This fact can be clearly seen by
examining the changes in the rabbit aorta after 20
weeks of age. The results for the changes in the
absolute amounts of collagen and elastin per aorta
show that both components continue to accumulate throughout the period of the study, although
the percent of both relative to wall mass is declining. Wolinsky (30) has already drawn attention to
this important observation in his study of the
effects of hypertension on rat aorta. Whenever
possible, quantification of absolute amounts of
aortic components in identical vascular segments
should be performed if metabolic changes are to be
interpreted.
The demonstration of continuing accumulation
of collagen and elastin in the aorta in the face of
reductions in collagen and elastin content expressed as a percent of the dry fat-free weight
indicate that some other nonscleroprotein component is accumulating in the aortic wall at a rate
faster than the combined rates of elastin and
collagen deposition. The electron microscopic data
of Cliff (24) for thoracic aortas of aging breeder rats
show an accumulation of granular, tubular, and
vesicular debris lying free within the extracellular
space. This material appears to be derived from
degeneration and necrosis of medial cells. Although
the female breeder rat has an increased susceptibility to aortic disease, similar changes might occur in
the aging rabbit aorta. Such changes could account
for some of the increase in the nonscleroprotein
content with advancing age. Our own data do not
permit speculation about the nature of this debris;
however, they do enable us to exclude the possibility that the material is derived from collagen, since
we have demonstrated that hydroxyproline does
not accumulate in the residue after collagen extraction.
Our attempts to analyze the relative importance
of genetic and hemodynamic influences in the
determination of the scleroprotein distribution pattern in the relatively immature, low-pressure system of the young rabbit have been inconclusive.
The presence at birth of the adult pattern of
scleroprotein distribution along the aortic wall
appears to favor genetic determination. Even so,
we have demonstrated that the accumulation of
Circulation Research. Vol. XXXIV. June 1974
835
AGE CHANGES IN RABBIT AORTA
both elastin and collagen in the aortic wall reaches
its highest rate at 1-3 weeks of age and begins to
taper off after 10 weeks. This phase of most rapid
scleroprotein deposition predates by 2-3 weeks the
period of most rapid increase in total body weight
in this breed of rabbits (32) and corresponds very
closely to the period of increasing blood pressure.
Additional information relevant to the question of
hemodynamic influences can probably be obtained
from experiments designed to alter the hemodynamic stresses in the rabbit aorta during the period
of rapid vascular growth.
Acknowledgment
The authors are grateful to Professor M. G. Taylor for
encouragement and advice during the progress of this study.
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Chemical Composition of the Rabbit Aorta during Development
D. IAN MCCLOSKEY and EDWARD G. CLEARY
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Circ Res. 1974;34:828-835
doi: 10.1161/01.RES.34.6.828
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