Active and Passive Immunization Against Angiotensin II in the
Rat and Rabbit
EVIDENCE FOR A NORMAL REGULATION OF THE RENIN-ANGIOTENSIN SYSTEM
By Peter Oster, Harald Bauknecht, and Eberhard Hackenthal
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ABSTRACT
Active or passive immunization has been used repeatedly as a tool in studies on the role
of the renin-angiotensin system in the control of blood pressure and kidney function. The
results are not consistent among each other, and they are also at variance with other
studies using different approaches. To evaluate the possible causes of these discrepancies,
the biological characteristics of angiotensin antibodies were studied in rats. Following the
intravenous injection of angiotensin II antibodies (purified by affinity chromatography),
the plasma concentration of the antibodies declined in a two-exponential curve with half
times of 11 hours and 7 days, which probably reflect distribution in the extracellular space
and elimination, respectively. Plasma angiotensin II levels rose from preinjection levels of
100 pg/ml to 12,000 pg/ml within 10 minutes and then declined concomitantly with the
decline in antibody concentration. We calculated that only a small fraction of the
circulating antibody was occupied by angiotensin II. Plasma renin concentrations were
initially elevated both in controls and in antibody-injected rats, but they returned to the
control level after 30 minutes and remained at that level throughout the rest of the
experiment (10 days). This fact indicates that feedback mechanisms which control renin
secretion, such as free plasma angiotensin II concentrations, are in the normal range. We
therefore concluded that the renin-angiotensin system in antibody-injected rats was
regulated at a normal level. Similar conditions seemed to exist in rabbits actively
immunized against angiotensin II; these animals exhibited high concentrations of total
immunoreactive angiotensin II (up to 200,000 pg/ml) and a small increase in plasma
aldosterone and corticosterone concentrations.
• One of the most frequently used principles in
studies on the biological role of an endogenous
humoral or local transmitter is to prevent its effects
at the presumed site of action by either inhibiting
its synthesis or preventing its access to the effector
site. This principle has also been applied to studies
on the role of the renin-angiotensin system in the
control of arterial blood pressure, aldosterone
secretion, and kidney function. The synthesis of
angiotensin II, the principal effector of the system,
can be blocked by inhibitors of renin (e.g., pepstatin) or converting enzyme (e.g., SQ 20,881). To
prevent the interaction of angiotensin II with tissue
receptors, antibodies against angiotensin II and,
more recently, competitive antagonists of angiotensin II (e.g., l-Sar-8-Ala-angiotensin II) have been
used. A critical review on the suitability of the
various agents has recently been published by
Davis and his colleagues (1).
From the Department of Pharmacology, University of Heidelberg, Heidelberg, Germany.
This work was supported by the Deutsche Forschungsgemeinschaft within the SFB 90 Cardiovasculares System.
A preliminary account of this work has been published
previously in abstract form (Clin Sci Med 48:27s-30s, 1975).
Received February 19, 1975. Accepted for publication August
12, 1975.
Circulation Research, Vol. 37, November 1975
For the purpose of such studies, antibodies
have the primary advantage of a prolonged persistence in the circulation compared with the short
half-life of a few minutes or less of the low-molecular weight antagonists. However, studies with active immunization against angiotensin II (2-8)
have failed to demonstrate any participation of the
renin-angiotensin system in blood pressure control
and the pathogenesis of renal hypertension, although in the latter condition such a participation
has been convincingly demonstrated with other
inhibitors (reviewed by Davis et al. [1]).
Passive immunization, i.e., injection of angiotensin II antiserum, appears to be a more promising
tool (9-17). In the two-kidney model of renovascular hypertension (one renal artery clamped, the
contralateral kidney left untouched), the injection
of angiotensin antiserum reduces arterial blood
pressure (10, 13-15). Reports on a blood pressure
effect of antiserum in other forms and stages of
renal hypertension and in normotensive animals
are less consistent. Moreover, in most studies, the
reduction in blood pressure following antiserum
injection is only of short duration, i.e., a few
minutes, despite continued blockade of the pressure effect of injected angiotensin (10, 12, 14, 15), a
phenomenon that is usually taken as evidence for
607
608
OSTER, BAUKNECHT. HACKENTHAL
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adequate angiotensin-blocking concentrations of
antibody in the circulation.
However, some methodological aspects of these
studies have not been taken into consideration,
which may explain some of these inconsistencies
and others to be discussed later. Crude undiluted
antiserum, which has been used in most of these
studies, not only contains heterologous proteins,
renin, and renin substrate but also high concentrations of angiotensin II (up to 200 ng/ml). The
affinity of the antibody for angiotensin II has not
been determined in most of these studies. It may be
assumed that low-avidity antibodies cannot effectively compete with angiotensin receptors for angiotensin II. The biological characteristics of the
injected antibody such as distribution, biological
half-life in the circulation, and loading with angiotensin II and the reaction of the renin-angiotensin
system are not known, although these data are
essential for the interpretation of results. To provide a more defined basis for such studies, we
investigated some of these biological characteristics of purified angiotensin II antibodies.
Methods
ANTIBODIES
The purification by affinity chromatography and the
characterization of antibodies against 5-Ile-angiotensin
II have been described previously (18). The preparation
used in this study had a titer of 1:200,000; its protein
content was 2.3 mg/ml and its apparent affinity constant
was 7 x 1010 liters/mole. A binding capacity of 11.44
/ig/ml (10.94 nmoles/ml) was calculated from doublereciprocal plots of radioimmunoassay standard curves.
The cross-reaction with angiotensin I w a s 0.2%. The
antibody preparation was free of renin, renin substrate,
and angiotensin I and II.
EXPERIMENTAL PROTOCOL
Seven male Sprague-Dawley rats with a body weight of
275 ± 17 (SD) g received 0.2 ml of the antibody preparation in a total volume of 0.4 ml of saline injected into the
saphenous vein under light ether anesthesia. Four control rats received 0.4 ml of saline. Blood samples (0.5 ml)
were collected into tubes containing 25 ^liters of 125 mM
Na2EDTA and 25 mM o-phenanthrolin. To keep stimulation of renin secretion by closely spaced blood samplings
as low as possible, the rats were divided into four groups
(two antibody-injected rats and one control rat each).
Blood was taken at 7 minutes, 90 minutes, and 10 hours
from group 1, at 30 minutes, 90 minutes, and 10 hours
from group 2, at 7 minutes and 4 hours from group 3, and
at 30 minutes and 4 hours from group 4 (consisting of one
antibody-injected rat and one control rat only). Subsequently, blood was obtained from all of the rats at 1, 2, 3,
4, 6, 8, and 10 days after antibody injection.
Until 6 hours following antibody injection, the rats
were kept in the laboratory; subsequently, they were
transferred to the animal unit, where they were housed at
a constant temperature of 24°C with a relative humidity
of 55% and lights on from 6 AM to 6 PM. The rats had free
access to food (Ssniff-pellet diet) and tap water. Body
weight and food and water consumption were recorded
daily.
PLASMA ANGIOTENSIN II ASSAY
Following extraction of rat or rabbit plasma with a
cation exchange resin according to the procedure described by Dusterdick and McElwee (19), the angiotensin eluted from the resin was estimated by radioimmunoassay. Details have been described previously (20).
Complete dissociation of angiotensin II bound to antibody by the extraction procedure was ascertained by
control experiments, in which angiotensin II, when it was
added to antibody dilutions in buffer or antibody-containing plasma, was recovered by the extraction procedure to the same extent (85-90%) as it was from normal
plasma. The radioactivity of 125I-labeled angiotensin II
added to antibody-containing plasma was completely
removed from the plasma by the extraction.
PLASMA ANTIBODY CONCENTRATION
Serial dilutions of plasma were incubated at 0°C with
10 pg of monoiodinated '"I-labeled angiotensin II in a
total volume of 200 /iliters of 0.1M Tris-acetate solution, pH 7.4, containing 0.2% human serum albumin.
After 20 hours of equilibration, bound and free angiotensin were separated by plasma-coated charcoal (20).
Following centrifugation, the charcoal pellet was
counted in a well-type gamma scintillation counter.
The titer is defined as the dilution of the antibody-containing preparation at which exactly 5 pg of radioiodinated angiotensin II is bound by the antibody. If this
amount of antibody is taken as an arbitrary unit, the
concentration in a given sample can be expressed as the
reciprocal of the titer in arbitrary antibody units per
0.2 ml (the volume of equilibration). To eliminate possible interferences by angiotensin I or II, the plasma
samples were extracted with a cation exchange resin
prior to titer estimation, as described earlier.
PLASMA RENIN CONCENTRATION
Renin concentration was estimated by incubating 50
/xliters of the plasma sample with 200 ^liters (400 pmoles)
of a rat renin substrate preparation (21) containing 2 mM
dimercaptopropanol, 4 mM 8-hydroxyquinolin, and 10
mM Na2EDTA in 0.1M TES-NaOH buffer, pH 7.2, at
37°C. At 0, 60, and 120 minutes 50 ^liters of the reaction
mixture was removed, adjusted to pH 6.0 with 200 jiliters
of 0.1M ammonium acetate, and mixed with 0.1 ml of a
suspension of a cation exchange resin (Biorad AG 50 W x
2, 100-200 mesh, H + form). This mixture was transferred
to a small column containing an additional 0.1 ml of the
resin suspension. The column was washed with 5 ml of
0.1M ammonium acetate, pH 6.0, and angiotensin I was
eluted with 2.0 ml of 10% methanolic ammonia. The
eluate was dried at 30 °C in a stream of filtered air, and
the residue was dissolved in 400 ^liters of 0.1M Tris-acetate solution, pH 7.4. Subsequently, angiotensin I was
estimated by radioimmunoassay. Under these conditions, the recovery of angiotensin I from the column,
when it was added in buffer solution or to plasma
incubation mixtures, was 86 ± 5% (SD). Exact pH
adjustment of the incubation mixture prior to column
separation is essential for the dissociation of angiotensin
Circulation Research, Vol. 37, November 1975
All ANTIBODIES AND RENIN-ANGIOTENSIN SYSTEM
I from the antibody, which, in spite of its low cross-reactivity with angiotensin I (0.2%), partially prevents the
binding of angiotensin I to the exchange resin when the
incubation mixture is passed through the column at
more acid or more alkaline conditions. Under the conditions described, the antibody present in the plasma
samples did not interfere with the generation of angiotensin I during incubation or with the recovery of
angiotensin from the column. This fact was ascertained
by incubating normal rat plasma with or without the
addition of appropriate amounts of angiotensin II antibody and by adding the same amount of antibody to
identical incubation mixtures after the incubation.
There was no difference in angiotensin I generation
between the three groups.
RABBIT EXPERIMENTS
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Nine New Zealand white rabbits were immunized
against 5-Ile-angiotensin II coupled to porcine gamma
globulin by the carbodiimide method of Greenwood et al.
(22). The coupling product was emulsified with complete
Freund's adjuvant and injected in 0.2-ml volumes (corresponding to 0.1 mg of angiotensin II) intramuscularly
and subcutaneously fortnightly for 3-6 months. Shamimmunization was performed with the same emulsion
without angiotensin II.
ESTIMATION OF ALDOSTERONE. CORTICOSTERONE. SODIUM. AND
OSMOLALITY IN RABBIT PLASMA
Plasma aldosterone concentrations were estimated by
radioimmunoassay using a modification of the method of
Mayes et al. (23), and corticosterone was determined by
the method of Vecsei (24). Serum sodium was estimated
by flame photometry (Zeiss FA 2), and serum osmolality
was determined by measurement of freezing point
depression (Knauer osmometer).
Results
ANTIBODY KINETICS
The decrease in antibody concentration in the
circulation following the intravenous injection of
angiotensin II antibody is shown in Figure 1. A
more rapid decline occurred during the first day
followed by a slow but steady decrease in the
subsequent days. This curve can be described as
the sum of at least two exponential functions,
which are obtained by extrapolation of the terminal slope of the curve, as shown in Figure 1. The
half-times for these two functions are 11 hours and
7 days, respectively.
609
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1:2OOO
1>1OOO
1500
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1125
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i
2
3
4
8
8
10 <fc"»s
FIGURE 1
Plasma concentrations of angiotensin II antibodies injected
intravenously into seven rats. The two exponential functions
were obtained by extrapolating the terminal slope and by
plotting the differences from this slope to the experimental
points, respectively. Due to the alternating blood sampling
protocol {see Methods), the data (means ± SE) were obtained
from four antibody-injected rats at 7 minutes, 90 minutes, and
10 hours, from three antibody-injected rats at 30 minutes and 4
hours, and from two control rats each at 7, 30, and 90 minutes
and 4 and 10 hours. Subsequently, each point represents data
from all seven antibody-injected rats or four control rats,
respectively. This protocol also applies to the data given in
Figures 2 and 3. trt = half-time.
points were obtained from separate groups. In
addition, the rats were transferred from the laboratory to the animal units between the fourth and
tenth hours. Control rats also exhibited a small
initial rise in plasma angiotensin II concentrations,
which returned to stable levels after 30 minutes.
PLASMA RENIN CONCENTRATIONS
Plasma concentrations of renin, as shown in
Figure 3, were initially elevated in both the antibody-injected and the control group, with no significant difference between the two groups. This
short-lasting elevation probably reflects stimula-
PLASMA CONCENTRATIONS OF ANGIOTENSIN II
As shown in Figure 2, a dramatic increase in
plasma angiotensin concentration from control levels of about 100 pg/ml to 12,000 pg/ml occurred
within 30 minutes following antibody injection.
During the following 24 hours, the angiotensin
concentration fell to 2,000 pg/ml; it then decreased
slowly in the subsequent days. The high value at 10
hours as compared with that at 4 hours is probably
due to the experimental protocol, since these two
Circulation Research, Vol. 37, November 1975
10 days
FIGURE 2
Plasma concentration of immunoreactive angiotensin II in seven
rats injected with antibodies against angiotensin II (open
circles) compared with that in four control rats (solid circles).
Values represent means ± SE. See the legend to Figure 1 for
an explanation of the sampling protocol.
OSTER. BAUKNECHT, HACKENTHAL
610
TABLE 1
Antibody Titers and Concentrations of Immunoreactiue
Angiotensin II in Plasma of Rabbits Immunized against
Angiotensin II
Rabbit
no.
T
W
8
4h
Plasma antibody
titer
Plasma angiotensin
II concentration
(pg/ml)
1:640,000
1:180,000
1:90,000
1:83,000
1:39,000
1:22,000
1:13,000
1:11,800
1:5,600
113,000
93,000
207,000
22,400
6,600
20,500
32,300
23,600
440
10 days
FIGURE 3
Plasma renin concentration in seven rats injected with angiotensin II antibodies (open circles) compared with that in four
control rats (solid circles). Values represent means ± SE. See the
legend to Figure 1 for an explanation of the sampling protocol.
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tion of renin secretion by the experimental procedure (preparation of the saphenous vein and two
anesthesia periods within 30 minutes) and explains
the initial rise in the angiotensin II concentration
in the control rats. The low angiotensin II concentration at 4 hours is also reflected in the lower renin
concentration at this time. During the rest of the
first day and the subsequent days of the experiment, there was no difference in renin concentration between antibody-injected rats and controls.
Hematocrit values were determined at days 2, 7,
and 10 of the experiment; there was no difference
between the two groups. There were also no differences in body weight gain and food and water
intake (Fig. 4) between the two groups.
ACTIVE IMMUNIZATION
All immunized rabbits had developed antibodies
after 3-6 months; their titers ranged from 1:5,600
to 1:640,000. Plasma angiotensin II concentration
was markedly elevated in immunized rabbits compared with that in controls, ranging from 440 to
207,000 pg/ml plasma (Table 1). The three rabbits
with the highest angiotensin II concentrations had
the highest antibody concentrations, and the lowest antibody titer was associated with the lowest
angiotensin II concentration. Plasma angiotensin
II concentration in sham-immunized rabbits
ranged from 50 to 190 pg/ml. The mean plasma aldosterone concentration in six immunized rabbits
was 29.4 ± 3 (SE) ng/100 ml, which is elevated
above the normal mean value for sham-immunized
rabbits, 18.4 ± 2.9 ng/100 ml (Table 2). Changes
in plasma corticosterone levels paralleled those
in aldosterone. Serum sodium and serum osmolality were similar in both groups.
Discussion
,4
»
-5
-4
-3
-2
8
-1
9
10 daya
FIGURE 4
Body weight, water intake, and food intake in seven rats
injected with angiotensin II antibodies (open circles) and four
control rats (solid circles) before and during the experimental
period. Values are means ± SE.
Following the injection of angiotensin antibodies, the plasma concentration of the antibodies
declined in a curve that can be described as the
sum of at least two exponential functions. This
change is in good agreement with the kinetics of
gamma-G-immunoglobulins in general (25). Halflives reported for the slow component, which reflects catabolism of the globulin, vary between 5.3
and 7.2 days in the rat. The fast component (or
components) represents distribution in the extracellular space, with half-times ranging from a few
hours to 1 day (for tabulation of data see ref. 25). It
can therefore be assumed that angiotensin antibodies injected intravenously distribute in the
extracellular volume with a half-time of 11 hours
and are catabolized with a half-life of 7.0 days. For
Circulation Research, Vol. 37, November 1975
611
All ANTIBODIES AND RENIN-ANGIOTENSIN SYSTEM
TABLE 2
Plasma Concentrations of Angiotensin II, Corticosterone, Aldosterone, and Sodium and Plasma
Osmolality in Rabbits Actiuely Immunized against Angiotensin IIand in Sham-Immunized Rabbits
Angiotensin II
(pg/ml)
Immunized
rabbits
Controls
P
Corticosterone Aldosterone
(ng/100 ml)
(ng/lOOml)
Sodium
(mEq /liter)
Osmolality
(mosmoles/kg)
57751 ± 22685
4.1 ±0.5
29.4 ± 3
143 ± 0.6
308.7 ± 5.7
82.6 ± 20.8
0.001
2.4 ±0.4
0.02
18.4 ± 3
0.02
142.1 ± 1.2
298.8 ± 4.9
NS
NS
All values are means ± SE.
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quantitative considerations, an arbitrary antibody
unit was used (see Methods). Accordingly, each rat
received 200,000 antibody units. The plasma concentration after 7 minutes was about 4,000
units/0.2 ml (Fig. 1); thus, the volume of distribution was 10 ml, which is in fair proximity of the
expected plasma volume (11.7 ml) in these rats
(26). The total distribution volume in the body
following equilibration can be derived from the
intercept of the slow terminal slope of the decay
curve with the ordinate. It was calculated to be
about 26 ml, which means that the antibody did
not distribute evenly in the total extracellular
space (which in these rats was approximately 65 ml
[27]) and, furthermore, that under equilibrium
conditions 60% of the antibody present in the body
was distributed in the interstitial space and only
40% was present in the circulation.
The only evidence for the presence of angiotensin
antibodies in the interstitial space has been provided by Eide et al. (8), who found similar concentrations in renal lymph and plasma. This observation was made, however, in actively immunized
rabbits.
Since angiotensin antibodies injected into the
circulation are expected to bind circulating angiotensin, thereby preventing it from being metabolized, it is not surprising to find elevated angiotensin concentrations (Fig. 2). The quantitative pattern, however, is remarkable.
Only 7 minutes after antibody injection, the
angiotensin II concentration had increased a hundredfold over control values; there was an additional small increase until 30 minutes, and thereafter angiotensin II concentrations declined rapidly
during the first day and much more slowly during
the subsequent days. It is obvious that the angiotensin II concentration curve closely resembles the
antibody decay curve. Part of the immunoreactive
material probably represents fragments of angiotensin II. Since the known fragments have a
similar or smaller affinity to the antibody than
Circulation Research, Vol. 37, November 1975
does angiotensin II (20), it may be assumed that
the relative contribution of fragments to the total
immunoreactive material referred to as angiotensin
II is not larger than it is in normal plasma.
Despite high angiotensin II concentrations, the
antibody was far from being saturated. The antibody concentration at 7 minutes was about 21,500
arbitrary units/ml (for definition see Methods)
with a total binding capacity of 245,000 pg of
angiotensin II. Thus, only 4.1% of the binding sites
of the antibody were occupied, provided that the
value for the binding capacity, which had been
measured in vitro, is also valid under in vivo
conditions. The same calculation for the subsequent experimental periods from 30 minutes
through 10 days showed that the total angiotensin
II concentrations corresponded to 4.7, 3.2, 1.5, 3.3,
1.7, 2.6, 2.9, 2.2, 3.3, 3.1, and 3.0% of the total
binding capacity of the respective antibody concentrations. Thus, there was a remarkable constancy of the fractional antibody occupation
throughout the whole experiment. It seems reasonable to conclude that this constancy reflects a
steady state of angiotensin turnover, which is
reached within a few minutes following the injection of antibody.
This conclusion is supported by the renin concentration values shown in Figure 3. Except for a
short initial rise in renin concentration, which was
probably due to the experimental stress at the
beginning of the experiment since it also occurred
in the controls, renin concentrations were in the
normal range throughout the entire experiment.
These results are not only consistent with the
concept of a steady state, but, moreover, they
suggest that this steady state is close to, if not
identical with, that in untreated rats. This suggestion is based on the well-documented negative
feedback control of renin release by angiotensin.
Elevations of blood angiotensin II concentration
within the physiological range inhibit the secretion
of renin (28, 29), whereas a competitive antagonist
612
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of angiotensin II (l-Sar-8-Ala-angiotensin II) produces a severalfold increase in plasma renin concentration (30, 31). Therefore, any deviation from
normal free concentrations of angiotensin II would
tend to stimulate or depress renin secretion, which
is apparently not the case in the antibody-injected
rats. It may thus be concluded that normal renin
concentrations in these rats reflected normal free
angiotensin II concentrations, despite enormous
concentrations of total angiotensin II.
This conclusion raises the question of what
normal free concentrations of angiotensin are.
Total angiotensin II concentrations, ranging from
52 to 142 pg/ml (20), do not necessarily represent
free angiotensin II. There are reports on angiotensin II binding proteins in rat and human plasma
(32-35). It is conceivable that these proteins have
sufficient capacity and affinity for angiotensin II to
reduce free concentrations of angiotensin II to very
low levels. This speculative view is supported by
the present study. From the apparent association
constant and the binding capacity of the antibody,
it can be calculated that free angiotensin II concentrations in antibody-injected rats are below 1
pg/ml. For reasons outlined earlier, it may be
assumed that similar concentrations of free angiotensin II exist in normal rats, i.e., probably only a
small fraction of the total plasma angiotensin II is
biologically active. It is tempting to speculate
further that the binding factors may vary in
concentration and affinity, thus permitting
changes in free angiotensin II concentrations without variations in total angiotensin II concentration.
In actively immunized rabbits, a situation similar to that in antibody-injected rats seems to exist.
Renin concentrations reported by Oates and Stokes
(36) are elevated only early in the course of
immunization but are in the normal range after 3
months of immunization, despite continuous
booster injections and an increase in the antibody
titers. Normal plasma renin levels in immunized
rabbits have also been reported by Johnston et al.
(3). The very high total angiotensin II concentrations represent only 1-2% of the total binding
capacity of the antibody. Free concentrations of
angiotensin II are expected to be in the range of 0.5
to 5 pg/ml in immunized rabbits, if the calculation
made for rat plasma is applied to rabbit plasma.
Walker et al. (37) have reported an average of 4
pg/ml of free angiotensin II in angiotensin Il-immunized rabbits. Thus, even in actively immunized rabbits, a steady state with probably normal
free concentrations exists. It is conceivable that the
renin-angiotensin system is regulated at a normal
OSTER, BAUKNECHT. HACKENTHAL
level in these rabbits and can respond to stimuli as
well as it does in the nonimmunized rabbit. This
situation would explain the lack of protection
against the development of renal hypertension in
such animals (2-7). A slight increase in plasma
aldosterone and corticosterone in immunized rabbits (Table 2) is in accordance with elevated rather
than decreased angiotensin II levels.
In several studies on the pathogenesis of renal
hypertension, the injection of angiotensin II antiserum into animals with experimental renal hypertension failed to reduce the elevated blood pressure
or a decrease was observed only for a short period of
a few minutes (10, 12-15). These results have been
interpreted as evidence against the participation of
the renin-angiotensin system in the development of
hypertension in the animal model studied. A different explanation has been offered by Thurston and
Swales (16), who have proposed that angiotensin is
formed and acts at vascular sites not accessible to
the large antibody molecule. This explanation is
based on the observation that in the same animal
model (two-kidney renovascular hypertension) the
angiotensin II antagonist, l-Sar-8-Ala-angiotensin
II, effectively reduces the elevated blood pressure.
In view of the results obtained in the present
study, we favor an alternative interpretation. If
crude antiserum is infused or injected, the angiotensin II present in the serum is already in equilibrium with the antibody. The injection may therefore have no effect at all on the levels of endogenous
free angiotensin. But even if angiotensin levels are
influenced, this disturbance will be of very short
duration: a steady state is probably restored within
a few minutes, i.e., the animal is regulating its
renin-angiotensin system at a normal level. Consequently, no long-term blood pressure decrease is to
be expected even under conditions in which the
renin-angiotensin system significantly contributes
to the maintenance of elevated blood pressure.
However, under conditions in which angiotensin
concentrations are changing rapidly, in particular,
when angiotensin is injected, the antibody in the
circulation will function as a "buffer." The large
free binding capacity will neutralize the high
concentration of angiotensin, and the antibody will
subsequently release "excess" angiotensin more
slowly at sites with lower free concentrations,
where it is rapidly destroyed by angiotensinases.
This mechanism explains the observation that in
antibody-injected animals no blood pressure response can be observed on injection of angiotensin
(10, 12, 14, 15). The degree of unresponsiveness will
Jbe critically dependent on the concentration of the
Circulation Research, Vol. 37, November 1975
All ANTIBODIES AND RENIN-ANGIOTENSIN SYSTEM
613
injection solution, the speed of the injection, the
affinity of the antibody, and the circulating conditions. Thus, lack of a blood pressure response
cannot be taken as evidence for complete blockade
of all responses to angiotensin. This fact is illustrated by the observation that in rabbits immunized against angiotensin II the injection of angiotensin II results in increased aldosterone secretion
without any blood pressure response (38).
Angiotensin vascular receptors: Their avidity in relationship to sodium balance, the autonomic nervous system,
and hypertension. J Clin Invest 51:58-67, 1972
16. THURSTON H, SWALES JD: Action of angiotensin antagonists
and antiserum upon the pressor response to renin: Further evidence for the local generation of angiotensin II.
Clin Sci Mol Med 46:273-276, 1974
Acknowledgment
18. BAUKNECHT H, OSTER P, HACKENTHAL E: Purification of
The authors thank Dr. P. Vecsei for the estimation of plasma
aldosterone and corticosterone and gratefully acknowledge the
expert technical assistance of Ms. Ulla Rausch and Mr. Helmut
Dick.
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P Oster, H Bauknecht and E Hackenthal
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Circ Res. 1975;37:607-614
doi: 10.1161/01.RES.37.5.607
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