375
Abnormal Ion and Water Composition of Veins and
Normotensive Arteries in Coarctation Hypertension
in Rats
MOTILAL B. PAMNANI, M.D.,
P H . D . , AND HENRY W. OVERBECK, M.D.,
PH.D.
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SUMMARY We examined the water, sodium, and potassium
composition of the thoracic aorta, abdominal aorta (plus iliac
arteries), and veins (vena cava and portal vein) from rats with aortic
coarctation. The aortas of 10 rats (group A) were coarcted above the
renal arteries to produce hypertension. Control groups consisted of
10 rats sham-coarcted above and 10 rats coarcted below the renal
arteries. In group A rats heart weights and carotid artery pressures
were elevated over controls (P < 0.01), whereas there were no
significant differences in femoral arterial pressures. In group A rats
both the hypertensive thoracic aorta and the normotensive abdominal
aorta contained about 20% more water per unit of wet weight, and
about 35% and 60% more sodium and potassium, respectively, per
unit of dry weight than did the corresponding portions of aorta from
control rats (P < 0.01). In group A rats water (P < 0.01), sodium
(P < 0.02), and potassium (P < 0.05) contents of veins also were
increased. There were no significant correlations between level of
carotid arterial pressure and magnitude of changes in arterial and
venous composition, nor were there significant differences between
the magnitude of changes in the normotensive and hypertensive
portions of the aorta. These results indicate that in rats abnormalities in vascular wall salt and water content are not necessarily a
direct effect of the elevated pressure in hypertension.
IT IS WELL KNOWN that the ionic and water composition of arterial walls is abnormal in many forms of
hypertension, including human essential hypertension.1
What is not known with certainty is whether these abnormalities are merely the result of the elevated intravascular
pressures, as suggested by Hollander et al., 2 or whether, in
contrast, they may reflect changes in vascular electrolyte
metabolism underlying the pathogenesis of hypertension, as
suggested by Tobian.' To investigate these alternatives,
Hollander et al.2 coarcted the aortas of dogs and found the
ionic and water composition of the arterial wall to be normal
in normotensive portions of the vascular tree downstream
from the coarctation. In contrast, in the hypertensive
portions of the vascular tree upstream to the coarctation
there were typical abnormalities in ionic and water content.
These findings were confirmed by Villamil and Matloff.3
Thus it appears that in the dog with this form of hypertension such changes in vascular composition are mainly the
result of elevated intravascular pressures.
These studies in dogs have not been confirmed for other
species and the conclusions may not be applicable to man.
Furthermore, if the veins show similar abnormalities, this
hardly could be attributed to elevated intravascular pressures. Thus, in the present study we produced coarctation
hypertension in rats, using techniques developed by NollaPanades, 4 and studied the ionic and water composition of
hypertensive and normotensive portions of the vascular bed,
including veins. The results indicate that in rats with this
form of hypertension the abnormal ionic and water composition of the vascular wall is not merely a direct effect of the
high blood pressure.
From the Departments of Physiology and Medicine, Michigan Stale
University, East Lansing, Michigan.
Supported by U.S. Public Health Service Training Grant HL-05873 (Dr.
Pamnani) and Research Grant HL-16665 from the National Heart and Lung
Institute, and by research grants from the Michigan Heart Association.
A preliminary report of this research appeared as an abstract in Federation Proceedings 33: 358, 1974.
Address for repririts: Dr. M.B. Pamnani, Assistant Professor of Physiology, College of Human Medicine, Giltner Hall, Michigan State University,
East Lansing, Michigan 48824.
Received June 10, 1975; accepted for publication January 20, 1976.
Methods
Normotensive male Wistar rats (obtained from the Otago, New Zealand, colony5) approximately 2 months old and
weighing 150-180 g were randomly divided into three
groups. To create coarctation hypertension in 10 rats (group
A), we placed a silver clip, 0.813 mm in diameter, around the
abdominal aorta and iliac arteries downstream to the renal
arteries. In 10 rats (group B) a clip (diameter = 0.927 mm)
too large to constrict the aorta was similarly placed. In a
final group of 10 rats (group C), in an attempt to create
hypotension in the vessels of the hindquarters, we placed a
clip 0.800 mm in diameter on the aorta downstream to the
origin of the renal arteries. Systolic blood pressure in the
hindquarters of all rats was measured weekly by the tail
plethysmographic method* under light ether anesthesia.
One month after surgery carotid and femoral arterial
pressures were measured directly under light ether anesthesia. The rats were then killed by rapid exsanguination and
the blood was used for determination of serum sodium and
potassium by flame photometry, serum calcium and magnesium by atomic absorption spectrophotometry, and creatinine by the Autoanalyzer. The descending thoracic aorta,
abdominal aorta and iliac arteries downstream to the renal
arteries (or to the clip in group C), and the inferior vena cava
and portal vein were excised rapidly for analysis of vascular
wall ionic and water composition. Placed on a glass plate, all
specimens were rapidly cleaned of adventitia and opened
longitudinally. The tissues were blotted once with filter
paper to remove blood and surface fluid, and then weighed
to the nearest 0.01 mg. The tissue was then oven-dried at
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376
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100°C for 24 hours, and, when again cooled to room
temperature, reweighed.
The difference between the two weights was considered
water content and was expressed as milliliters per kilogram
of wet weight. For measurement of ionic content, 2 ml of 0.1
N nitric acid were added to each tissue and the tissue was
allowed to digest for 14 days. Digested specimens were again
placed in an oven at IOO°C for 48 hours to evaporate the
nitric acid. Tissue contents of sodium and potassium were
then measured by flame photometry (Beckman). Ion contents were expressed as milliequivalents per kilogram of
tissue, dry weight. Student's f-test' was used to compare
pressures and composition of the vessels among the groups.
Student's Mest for paired replicates was used within groups
to compare composition of thoracic and abdominal aortae.
Linear correlation coefficients were calculated to test for
associations between mean carotid arterial pressure and
vascular composition. The null hypothesis was rejected at
P Z 0.05.
Results
All rats remained healthy and showed no evidence of
cardiac or renal insufficiency or malignant hypertension at
the time of study. At this time body weights and serum
sodium, potassium, calcium, magnesium, and creatinine
concentrations did not differ significantly among the groups
(Table I). In contrast, heart weight (Table 1) was significantly increased in the rats with coarctation hypertension
(group A). Mean carotid arterial pressure also was significantly increased in group A rats as compared to both control
groups (Fig. 1); carotid pressures were similar in the two
control groups. Mean femoral arterial pressures in the three
groups did not differ significantly. The pressure gradient
between carotid and femoral arteries (Fig. I) was therefore
greater (P < 0.001) in rats with coarctation hypertension
(group A) than in either control group. This gradient also
was greater (P < 0.05) in the rats clipped below the renal
arteries (group C) than in the sham-clipped rats (group B).
After clipping, tail blood pressures measured weekly remained unchanged in the groups with the exception that 3
weeks after clipping there was a significant but very small
rise (about 4 mm Hg) in the rats with coarctation hypertension. This rise had disappeared by the 4th week.
Average weights of tissue obtained from thoracic aorta,
abdominal aorta (plus iliac arteries), and veins was 32 mg,
13 mg, and 25 mg, respectively. There were significant
"GROUP: B A C
CAROTID
ARTERY
VOL.
38, No. 5, MAY 1976
A
FEMORAL
GRADIENT BETWEEN
ARTERY CAROTID AND FEMORAL
ARTERIES
FIGURE 1 Mean (± SEM) carotid and femoral arterial pressures.
Pressures were directly measured in the femoral and carotid arteries
of rats from group A (solid bars), B (clear bars), and C (stippled
bars) at the time vascular tissue was obtained for analysis. Also
indicated are the mean pressure gradients between carotid and
femoral arteries of the groups. P values are represented for
comparison of each control group with the hypertensive group
(group A).
differences in vascular composition among the groups, as
shown in Figure 2. In the hypertensive rats of group A both
the hypertensive and the normotensive portions of the aorta
contained significantly more water, sodium, and potassium
per unit of tissue weight than did the corresponding portions
of aorta in the control rats. In these group A rats, compared
to the sham-clipped group (group B), water content increased by 22%, sodium content increased by 36%, and
potassium content increased by 60% in the hypertensive
portion of the aorta. Increases in the normotensive portion
of the aorta (19%, 36%, and 62%, respectively) did not
significantly differ from those in the hypertensive portion.
Additionally, in hypertensive rats of group A water and
ionic content was increased in the walls of veins, as
compared to veins in the control groups. For group A rats
there was no significant correlation between the level of
carotid blood pressure and the magnitude of changes in
arterial and venous composition. Comparison of vascular
composition between the two control groups revealed no
significant differences.
The values for vascular wall water content we report are
significantly lower than those reported previously.7 We
believe this difference can be attributed to a systematic error
TABLE 1 Mean Values ± SEM for Weights and Serum Electrolytes
Measurements
Body wt (g)
Heart wt (g/100 g body wt)
Serum [Na + ] (mEq/liter)
Serum [K+] (mEq/liter)
Serum [Cas+] (mEq/liter)
Serum [Mg1+] (mEq/liter)
Serum creatinine (mg/100 ml)
Group A
262.2
0.48
143.6
4.6
4.9
2.23
0.8
± 6.6
± 0.02
± 2.9
± 0.4
± 0.1
±0.13
± 0.1
Group B
264.6 ± 7.0
0.34 ±0.01*
144.6 ± 1.2
4.5
5.1
2.17
0.8
± 0.2
±0.1
± 0.07
± 0.1
Group C
280.8
0.35
145.2
4.5
5.0
2.26
0.9
±
±
±
±
±
±
±
4.9
0.01*
1.7
0.3
0.1
0.20
0.1
Values were obtained from 10 rats in each group except for serum creatinine. Tor which n = 8 in group A. 6 in
group B, and 8 in group C.
* P < 0.01 for comparison with group A rats.
VASCULAR WALL IN HYPERTENSION/Pamnaniand Overbeck
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GROUP: B A
C
ABDOMINAL
AORTA
B A
C
THORACIC
AORTA
B
A
C
VEINS
GROUP: B
A
C
ABDOMINAL
AORTA
B
A C
THORACIC
AORTA
B
A C
VEINS
% 200
I
GROUP:
ABDOMINAL
AORTA
FIGURE 2 Mean (± SEM) values for content of water (top), sodium
(middle), and potassium (bottom) in abdominal aorta, thoracic
aorta, and venous tissue of rats from groups A, B, and C. The P
values are represented for comparison of each control group with
the hypertensive group (group A).
introduced by tissue drying during our dissection procedures
which were carried out at room temperature. This systematic error does not affect our conclusions, because we used
identical dissection techniques for all rats in the study.
Discussion
In this study hypertension was produced only in the rats
whose aortas had been constricted above the renal arteries
(group A), confirming the findings of Nolla-Panades< and
supporting his conclusion that renal factors play an essential
role in the development of this form of hypertension in rats.
The results of our present study additionally clearly
indicate that abnormalities in vascular wall salt and water
377
content are not necessarily the effects of elevated intravascular pressures in coarctation hypertension in rats. The
changes we observed in the composition of both the
normotensive abdominal aorta downstream to the coarctation and the veins cannot be attributed to the direct effects of
increased intravascular pressure. Furthermore, changes in
composition of the hypertensive thoracic aortas upstream to
the coarctation were no greater than those in the normotensive abdominal aortas of the same rats. Finally, there were
no significant correlations between level of carotid arterial
pressure and magnitude of changes in vascular composition.
Our results for rats, therefore, do not agree with those of
Hollander et al.* and Villamil and Matloff,* who found, in
dogs with a similar degree and duration of aortic coarctation, that changes in aortic sodium and water composition
were confined to the hypertensive portions upstream to the
coarctation. Other than species differences, there were
technical differences between these previous studies and
ours: (I) Our coarctation was of the abdominal aorta,
whereas the other groups produced midthoracic coarctation;
(2) we sampled the entire aorta, whereas the other groups
took only representative samples; and (3) control animals in
the other series apparently had not undergone sham surgery,
whereas ours had. Thus the differences in results may be
explained either on the basis of species differences or on the
basis of such technical differences.
Regarding species differences, even in the hypertensive
thoracic aorta the changes we observed in rats differed
qualitatively and quantitatively from those found by the
other two groups in studies on dogs. For example, increases
in potassium content were the most impressive change we
observed in coarcted rats. Similiar changes in potassium
were reported by Phelan and Wong' in two-kidney Goldblatt hypertensive rats of the same strain, and by other
investigators also studying rat aorta in one- and two-kidney
Goldblatt hypertension.7 In contrast, in coarcted dogs,
Hollander et al. J found no significant changes in aortic
potassium content and Villamil and Matloff3 reported
significant decreases in potassium content of both hypertensive and normotensive portions of the aorta (which they
attributed to tissue injury). It is of interest that in dogs with
bilateral perinephritic hypertension, aortic potassium content has been reported to be unchanged."
Regarding technical differences, it is possible that the
degree of renal involvement in the hypertension may be
different in abdominal and thoracic coarctation. Although
there was no evidence for renal dysfunction in our rats, the
normal serum electrolyte and creatinine concentrations
certainly do not rule out occult renal abnormalities at the
time the vascular tissue was sampled. Thus it is possible that
our rats had elevated levels of renin, angiotensin, and
aldosterone. Chronic angiotensin infusions elevate arterial
sodium contents in dogs,' therefore it is possible that the
increased vascular sodium we found in our rats may have
been related to an action of angiotensin. On the other hand,
such angiotensin infusions have not been found to elevate
vascular wall water content.' Other renal factors not
involving the renin-angiotensin-aldosterone system may, of
course, also have played a role in the vascular changes we
observed.
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Whether or not renal factors were involved, we found no
good evidence that the vascular changes we observed were
causally related to elevated intravascular pressures. Thus, in
this regard rats and dogs appear to differ. It should be noted
especially that the composition of vessels from normotensive
vascular beds of humans with hypertension has not yet been
studied. Thus it is certainly too early to conclude on the
basis of available data that changes in vascular wall ionic
and water composition in humans with hypertension are the
result only of elevated intravascular pressure.
In hypertension abnormal water and ionic contents reportedly occur in many other tissues in addition to arteries,
including myocardium, 10 - " skeletal muscle, 10 ' 12 " 14 submaxillary gland, 8 brain, 10 ' 15 liver, gut, spleen, and skin.10 In the
only previous study of veins, normal wall composition was
found in spontaneously hypertensive rats. 16 We believe ours
is the first report that venous wall composition is abnormal
in hypertension.
This abnormal venous composition is noteworthy because
recently there has been increased interest in veins in
hypertension; elevated venous return to the heart due to
decreased venous compliance may help to account for the
increased cardiac output in early stages of hypertension.17
We have previously reported that femoral 18 and
mesenteric 19 venous compliances are decreased in perinephritic hypertensive dogs. We suggested that abnormal venous
wall ionic and water composition may underlie this decreased venous compliance. 19 It is also possible that abnormalities in ionic composition of veins may be related to the
enhanced responses of veins to norepinephrine and nerve
stimulation observed in coarctation hypertension.20 Furthermore, if abnormalities in ionic and water composition in
hypertension reflect underlying defects in ion metabolism, as
suggested by Tobian, 1 our present data would suggest that
such underlying defects may be present not only in arterial
but also in venous tissue.
Acknowledgments
We appreciate the excellent technical assistance of Donald L. Anderson
and Lynn Dietrich, and thank Dr. Francis J. Haddy for his valuable help in
the preparation of this manuscript.
VOL.
38, No. 5, MAY 1976
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M B Pamnani and H W Overbeck
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Circ Res. 1976;38:375-378
doi: 10.1161/01.RES.38.5.375
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