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Clinical Science
(1981) 61,75-83
75
Acute effects
of
captopril
on
blood pressure and circulating
hormone levels in salt-replete and depleted normal subjects
and essential hypertensive patients
J.
A. MILLAR, B.
P.
McGRATH,
P.
G. MATTHEWS
AND
C. I. JOHNSTON
Monash University, Department
of
Medicine, Prince Henry's Hospital, Melbourne, Victoria, Australia
(Received
7
Julyll2 December
1980;
accepted
2
February
1981)
Summary
1.
The acute effects of
a
single oral dose of
captopril on blood pressure, pulse rate and
circulating levels of angiotensin
I
(ANG I),
angiotensin I1 (ANG II), renin, bradykinin and
catecholamines were studied in three groups:
eight normal subjects, six salt-depleted normal
subjects and
16
patients with essential hyper-
tension.
2.
Captopril treatment did not cause any
significant fall in supine blood pressure in
salt-replete normal subjects or patients with
untreated essential hypertension but was as-
sociated with
a
fall in mean blood pressure from
85 2
to
75
f
2
mmHg in salt-depleted normal
subjects and from
131
f
7
to
117
f
5
mmHg in
patients with essential hypertension treated with
diuretics. There was no change in pulse rate in
any group.
3.
Hormonal responses
to
captopril were
qualitatively similar in the three groups and
consisted of significant falls in ANG
I1
with
corresponding increases in ANG
I
and plasma
renin concentration. The changes in plasma renin
concentration and ANG
I
were greater in
salt-depleted normal subjects (mean values
at
90
min were
1140%
and
990%
of basal levels
respectively) than in salt-replete normal subjects
(410%, 190%)
and were blunted in patients with
essential hypertension
(140%, 120%).
Blood
bradykinin, noradrenaline and adrenaline con-
centrations did not change after captopril in any
group.
Correspondence: Professor C.
I.
Johnston, Monash
University, Department
of
Medicine, Prince Henry's
Hospital, Melbourne, Victoria, Australia.
4.
The parallel fall in blood pressure and ANG
I1
levels in salt-depleted normal subjects is
consistent with maintenance of blood pressure by
increased levels of ANG
I1
in sodium depletion.
5.
The failure of captopril
to
reduce acutely
blood pressure in patients with essential hyper-
tension despite suppression of plasma ANG
I1
and without change in circulating bradykinin
confirms that the renin-angiotensin system does
not play
a
primary role in essential hypertension.
Key words: angiotensin, angiotensin-converting
enzyme, bradykinin, sodium depletion.
Abbreviations: ANG
I,
ANG
11;
angiotensin
I
and I1 respectively.
Introduction
Converting enzyme (peptidyldipeptide hydrolase;
dipeptidyl carboxypeptidase; kinase
11;
EC
3.4.15.1)
utilizes both bradykinin and angio-
tensin
I
(ANG
I)
as substrates. Activity of this
enzyme therefore promotes both the breakdown
of bradykinin and the formation of angiotensin
I1
(ANG 1I) (Erdos,
1975).
Captopril
1-(~-3-
mercapto-2-methyl-3-oxopropyl)-~-proline
(SQ
14225:
Squibb Inc.) is
a
specific orally active
converting enzyme inhibitor (Ondetti, Rubin
&
Cushman,
1977)
which blocks the pressor and
smooth muscle contractile effects of exogenous
ANG
I
and potentiates the vasodepressor actions
of exogenous bradykinin
in vitro
and
in vivo
(Rubin, Antonaccio
&
Horovitz,
1978).
Captopril lowers blood pressure in various
forms af experimental hypertension (Bengis,
Coleman, Young
&
McCaa,
1978;
Freeman,
Davis, Watkins, Stephens
&
De Forrest,
1979).
The acute hypotensive action appears
to
be partly
0143-5221/81/070075-09$01.50/1
0
1981
The Biochemical Society and the
Medical
Research Society
76
J.
A.
Mi
'Ilar et al.
mediated by inhibition of the renin-angiotensin
system. However, it is also an effective hypo-
tensive agent in patients with essential hyper-
tension particularly when combined with a
diuretic (Case, Atlas, Laragh, Sealey, Sullivan
&
McKistry, 1978; Gavras, Brunner, Turini,
Kershaw, Tifft, Cuttelod, Gavras, Vukovich
&
McKinstry, 1978; Johnston, Millar, McGrath
&
Matthews, 1979). The precise mechanism of the
hypotensive action of this agent
is
at present
unclear. In chronic studies it has been shown to
reduce circulating ANG
11,
increase plasma renin
and ANG I, without
a
change in plasma
bradykinin (Atkinson
&
Robertson, 1979;
Johnston
et al.,
1979). However, in these studies
the fall in blood pressure with captopril treatment
did not correlate with pretreatment renin
or
ANG
I1
levels, suggesting that its chronic action on
blood pressure may not be mediated entirely by
inhibition of the renin-angiotensin system.
Evidence for angiotensin-converting enzyme
inhibition in man is based solely on the ability of
the drug to inhibit the pressor effects of ANG
I.
There is little information on the hormonal
changes in man
in vivo
associated with captopril.
Studies have been performed with
SQ
20881,
a
nonapeptide converting enzyme inhibitor (Wil-
liams
&
Hollenberg, 1977; Hulthen
&
Hokfelt,
1978; Niarchos, Pickering, Case, Sullivan
&
Laragh, 1979), but the biochemical effects of
SQ
20881 and captopril may not be identical
(Okuno, Kojdo, Konishi, Saruta
&
Kato, 1979).
Also many of these studies were performed
during negative sodium balance, which activates
the renin-angiotensin system and increases
urinary excretion of kallikrein.
In the present study we report the acute effects
of
a
single oral dose of captopril on blood
pressure in sodium-replete and -depleted normal
subjects and in 16 patients with essential hyper-
tension. Plasma renin concentration and blood
levels of ANG
I
and ANG
I1
and bradykinin
were measured before, and for
2-4
h after,
captopril and the changes in these hormones were
examined in relation to concurrent alterations in
blood pressure.
To
assess the role of the
autonomic nervous system and kidney in the
cardiovascular response to captopril, circulating
levels of noradrenaline and adrenaline and ex-
cretion of sodium and potassium were also
measured.
Materials and methods
Experimental protocol and subjects
The experimental procedure, to which all
subjects gave written consent, was approved by
the Ethics Advisory Committee of Prince
Henry's Hospital and conformed to the ethical
guidelines
of
the National Health and Medical
Research Council of Australia.
Three groups of subjects were studied.
Group
1.
This consisted of eight normal male
subjects aged 2046 years. During the 48 h
before the study they consumed
a
normal diet,
unrestricted in sodium content. They were taking
no other drugs, including alcohol
or
tobacco.
Group
2.
This consisted of six normal subjects
(five males, one female) salt-depleted by the
ingestion of
a
low-sodium diet containing 20
mmol of sodium and
60
mmol of potassium/day
for
a
period of
4
days and of chlorothiazide
(0.5
g) on the mornings of the first 2 days of the
low-sodium diet. Urine samples
(24
h) were
collected during the 4 day period for assessment
of sodium balance, which was calculated as the
cumulative difference between dietary intake and
urinary output of sodium. The study was per-
formed on the morning of day
5
in this group.
Group
3
(Table
I).
This was a group of 16
patients with essential hypertension (1 1 males,
five females) aged 28-69 years (mean 46 years).
All the patients were classified as WHO grade 1
or
2 and had normal intravenous pyelograms.
Ten patients were untreated, three patients had
been receiving long-term diuretic therapy alone
and three patients were receiving combined drug
TABLE
1.
Clinical details and
drug
therapy
of
patients
with essential hypertension
Patient Sex Age Creatinine Basal Treatment
no.
(years)
(mmol/l)
blood
pressure
(mmHg)
I
M
46 0.12 172/120 Nil
2
F
36
0.05
164/104 Nil
3
M
28
0.05
146/100 Nil
4 F 32 0.06 l22/98 Nil
5
M
66 0.06 160/104 Nil
6
M
38 0.10 132/96 Nil
7
M
36 0.09 160/89 Nil
8
F
42
0.07
158/101 Nil
9
F
49 0.09 155/91 Nil
10
M
54 0.09 164/106 Nil
I1
M
60 0.06 154/100 Hydrochlorothiazide
12
M
30 0.14 132/94 Frusemide
13
M
65
0.15
160/110 Frusemide
14
M
68
0.13 214/118 Alprenolol,
chlorothiazide'
15
M
46 0.22 195/115 Lahetalol, amiloride.
hydrochlorothiazide'
16
F
54 0.09 202/112 Propranolol,
6-methyldopa,
prazocin,
cyclopenthiazide'
*
Treatment withdrawn 24 h before the test.
Acute hormonal effects
of
captopril
77
therapy. In the latter group therapy was dis-
continued
24
h before the study.
All studies in normal subjects commenced at
09.30-10.00 hours. Patients with essential hyper-
tension were all studied after at least 24 h as
a
hospital in-patient. Each subject was lying recum-
bent in
a
quiet room from the start of the study
for
a
total of
105
min. Captopril
(50
mg to
normal subjects ?nd
25
or
50
mg to hypertensive
patients) was given orally after
a
period of 30 min
supine rest. Blood samples (33 ml) were taken via
an indwelling antecubital venous cannula on two
occasions before and at
30,
60, 90 and 120 min
after administration of the drug. In the normal
subjects urine was collected for the
2
h period
before and for 2 h after captopril.
The rest period was sufficient to achieve
a
steady state with regard to renin, ANG
I
and
ANG
I1
levels and both adrenaline and nor-
adrenaline. Blood pressure (measured auto-
matically with an Arteriosonde, Roche Products
or
a
Dynamap, Applied Medical Research) and
pulse rate were recorded at
5
min intervals
throughout.
Laboratory methods
At each sampling time, 33 ml of blood was
removed as follows: 3 ml was taken, immediately
after discarding dead-space blood, into chilled
5
ml syringes, containing
25
pl of converting
enzyme inhibitor
[O.
5
mol/l, 1,lO-phenanthroline
in
25%
(v/v) ethanol], for both bradykinin and
ANG I assays. A further
20
ml of blood for the
assay of ANG
I1
was quickly taken without
venous stasis and added to
a
chilled tube
containing
2
ml of dimercaptopropanol
(0.1
mol/l) and disodium
ethylenediaminetetra-acetate
(0.1
mol/l) in potassium phosphate (0.6 mol/l,
pH 7.4). Lastly, 10 ml of blood was removed and
dispensed equally into two tubes containing
lithium heparin for assay of .dlasma renin
concentration and plasma catecholamines.
The tube for catecholamine determination
contained
2.5
mg of reduced glutathione. Total
blood volume removed was 198 ml, less than that
required to stimulate renin release or increase
plasma ANG
11.
Measurements
Plasma renin concentration.
This was
measured by
a
method described previously
(Millar, Leckie, Morton, Jordan
&
Tree, 1980).
In this method endogenous ANG
I
is normally
undetectable. Because circulating ANG
I
was
increased
as
a
result of converting enzyme
inhibition, blank values were measured and
subtracted in
all
samples. Plasma renin con-
centration is reported as micro-international units
of renin per millilitre. The normal range for
plasma renin concentration in this laboratory is
10-38 p-i.u./ml. A concentration of 20 p-i.u./ml
gives
a
reaction velocity of
2.5
ng of ANG
I
generated h-' ml-' of plasma
at
37OC.
Blood bradykinin.
This was assayed by
radioimmunoassay as described by Mashford
&
Roberts (1972). The supernatant from the etha-
nol precipitation was remoyed by centrifugation,
and the precipitate was washed with 70% (v/v)
ethanol. The combined solutions were washed
with diethyl ether and evaporated to dryness at
4OOC. The solid residue was dissolved in
0.5
ml
of Tris/HCl
(0.1
mol/l, pH 7.4) containing
1,lO-phenanthroline (10 mmol/l) and
0.25%
human serum albumin. Radioimmunoassay was
performed on duplicate 100 pl samples of this
solution with rabbit anti-bradykinin serum and
'251-labelled [Tyr81bradykinin as trace. This trace
has full cross-reaction with the antibody. The
assay as described gives 101
&
1.4%
(n
=
13)
recovery of bradykinin standard
(5
ng), added to
the sampling syringe, and breakdown of added
bradykinin is prevented for at least
120
min
(Roberts, 1971). Validation of the assay and
values for inter- and intra-assay variation have
been reported elsewhere (Johnston, Millar, Cas-
ley, McGrath
&
Matthews, 1980).
Blood angiotensin
I.
This was assayed by
radioimmunoassay by the method of Johnston,
Mendelsohn
&
Casley (1969) in duplicate
50
pl
samples from the ethanolic extract, with specific
rabbit anti-ANG
I
serum which cross-reacted
with ANG
I1
by
0.02%.
Plasma angiotensin
II.
This was measured by
radioimmunoassay after extraction from plasma
on fuller's earth (Boyd, Landon
&
Peart, 1969).
Samples were cooled
in
ice and processed
immediately after venesection. Values for ANG
I1
were corrected for cross-reactivity of antibody
(0.5%)
with ANG
I
(Morton, Casals-Stenzel,
Lever, Millar
&
Riegger, 1979) when the con-
current concentration of ANG I exceeded 300
pg/ml of blood, equivalent to approximately
3-0
pg of ANG II/ml of plasma.
Plasma catecholamines.
These were assayed
by the radioenzymatic technique of Hortnagl,
Benedict, Grahame-Smith
&
McGrath (1977).
Plasma and urine electrolytes.
These were
measured by flame photometry.
Statistical methods
Significance
of
differences in mean values were
estimated by Student's two-tailed t-test. Cor-
78
J.
A.
Millar et
al.
relation coefficients were calculated by the
method of least-squares. Values are quoted and
shown graphically as means
L-
SEM.
Mean blood
pressure was computed as the sum
of
diastolic
blood pressure plus one-third of the pulse
pressure.
Results
Group
I
(salt-repleted normal subjects)
No
adverse effects were encountered. Supine
blood pressure did not decrease and there was no
change in pulse rate for 120 min after captopril
treatment.
The hormonal changes were similar in all
subjects, with
a
decrease in
ANG
I1
(P
<
0.05)
and corresponding increases in renin concen-
tration and
ANG
I
(P
<
0.001), but no change in
plasma bradykinin levels (Fig. 1). The peak
increase
for
plasma renin concentration and
ANG
I
occurred at 90 min after taking the drug.
The ratio of maximum (90 min)
to
basal values
for
renin and
ANG
I
were 4-1 and 1.9
respectively. There was a significant direct
correlation between renin and
ANG
I
(r
=
0.70,
P
<
0.001,
n
=
48).
Urine flow rate increased significantly in the
period after captopril treatment (85
k
18-143
L-
19 ml/h;
P
<
0.05,
n
=
8) as did the excretion
rate of both sodium (7.2 +'1.1-15.3
L-
1.4
mmol/h;
P
<
0.001) and potassium (2-9
k
0.3-5.4
k
0.5
mmol/h;
P
<
0.005).
However, in
five normal male subjects who collected urine
under similar conditions, but who did not take
captopril, the corresponding values were 94-18
1
ml/h, 4.2-12.2 and 3.1-4.8 mmol/h.
As
shown in Table 2 there were no significant
changes in plasma concentrations
of
adrenaline
or
noradrenaline.
Four additional normal subjects received 250
mg of captopril orally, and followed the same
I
Sodium-replete Sodium-depleted
85
-
I
75
900--500
I
360--
200
I
7
I
36
20
g0!504"b:
I
I
.-
2
.a
8
Captopril
(50
mgl
Captopril
(50
mg)
f=
0.5
L
0
30
60
90
Time
120
(min)
0
30
60
90
120
35
1.0
05
FIG.
1.
Changes in supine mean blood pressure and circulating levels
of
ANG
11,
ANG
I,
plasma
renin concentration and
bradykinin
in
salt-replete
(n
=
8)
and
salt-depleted
(n
=
6)
normal
subjects after administration of captopril
(50
mg orally),
as
indicated
by
the arrows. Results are
means
SEM.
Acute hormonal effects
of
captopril
79
study protocol outlined above. In these subjects
the hormonal responses were of
a
similar mag-
nitude to those seen in the group who received
50
mg (Table 3), suggesting that complete angio-
tensin-converting enzyme blockade is achieved in
normal man by
50
mg of captopril orally.
Group
2
(salt-depleted normal subjects)
Mean cumulative sodium deficit over 4 days of
low dietary salt intake combined with 2 days
treatment with chlorothiazide in this group was
204
f
50
mmol. Two subjects experienced
lightheadedness on standing after completion of
the study, but no other adverse effects were
noted. When .the basal hormone levels were
compared in this group of salt-depleted subjects
with those in group 1, significant increases were
observed in plasma renin concentration (132
f
34 vs 23
f
4 p-i.u./ml,
P
<
O.OOl),
ANG
I
(62
&
14 vs 30
f
3 pg/ml,
P
<
0.001)
and
ANG
I1 levels (38
f
8
vs 28
rl:
10 pg/ml,
P
<
0.001).
Basal blood bradykinin levels were similar in
TABLE
2.
Plasma levels of adrenaline and noradrenaline
(pmolhl)
during control,
60
and
120
rnin after administra-
tion
of
captopril
in
salt-replete (group
I)
and salt-depleted
(group 2) normal subjects, and in patients with essential
hypertension (group
3)
All
subjects were resting
supine
in
a
quiet room. Blood
samples were taken via
an
indwelling cannula.
n,
Number
of
subjects.
Group Plasma concn. (pmol/ml)
Before After captopril
captopril
(control)
60
rnin
120
min
1
(n
=
8)
Adrenaline
Noradrenaline
Adrenaline
Noradrenaline
Adrenaline
Noradrenaline
2
(n
=
6)
3
(n
=
8)
0.21
f
0.03 0.28
-1
0.04 0.26
f
0.05
0.84
f
0.20 0.98
f
0.32 1.87
+_
0.56
0.23
f
0.10
0.45
f
0.24 0.16 k 0.07
0.75
f
0.21 0.83
-1
0.17 0.66
f
0.14
0.14
+_
0.04 0.09
f
0.02 0.12
-1
0.02
0.95
f
0.28
0.70
f
0.11 0.92
-1
0.15
salt-depleted (940
f
70 pg/ml) and salt-repleted
normal subjects (9 10
f
70 pg/ml).
Mean blood pressure decreased from
85
If:
2
mmHg before captopril to 75
f
2 mmHg
(P
<
0.01)
at 60 min, but there was no change in pulse
rate.
As
in group 1 subjects after captopril
treatment there was a progressive decrease in
plasma
ANG
I1 with reciprocal increases in both
renin and
ANG
I
with again no change in
bradykinin (Fig. 1). Renin concentration and
ANG
I
levels were maximum at 90 min, but the
increases in both were much greater than in
group 1. The ratios of 90 rnin to basal values
were 11.4 and 9.9 respectively. There was
a
significant correlation between corresponding
values for renin and
ANG
I
(r
=
0.83,
n
=
36,
P
<
0.001)
(Fig. 2).
.
In this group of subjects
ANG
I1
levels
decreased significantly in parallel with mean
blood pressure changes after captopril treatment
(Fig.
1).
There was no increase
in
urine
flow
rate
or excretion of sodium or potassium in this group
and catecholamine levels did not change sig-
nificantly (Table 2).
Group
3
(patients with essential hypertension)
In the group of
10
previously untreated
patients oral captopril treatment was associated
with
a
small but not significant fall in supine
mean blood pressure of
5
mmHg.
A,
significant
fall of 14 mmHg
(P
<
0.01)
in blood mean
pressure, maximal at
90
min, was observed in the
group of
six
patients treated previously with
diuretics (Fig. 3). The average percentage falls
in
mean blood pressure at 90 rnin in previously
untreated and treated patients were 4.0
f
2.6
and
10
&
2.1% respectively.
The hormone responses in the two groups of
hypertensive patients were not significantly dif-
ferent and the results for the combined group are
shown
in
Fig.
4.
The changes observed in the
patients were qualitatively similar to those seen in
both normal groups, but the increases in renin
TABLE
3.
Plasma renin concentration and circulating levels of angiotensin
I.
angiotensin
II
and bradykinin in four normal
salt-replete subjects given captopril(250
mg
orally)
Values are means
5
S.E.M.
~~
Concn. in plasma
or
blood
Before captopril After captopril
60
rnin
90
min
120
min
41
f
12 59t
10
66f22 69-134
Control
1
Control
2 30
rnin
17
f
2
Plasma renin concn. (p-idml
of
plasma)
17
2
4 445
10
29-15
ANG
I
(pg/ml of blood)
23
f
5
19 f
3
22
t
2 34
f
3
ANG
I1
(pg/ml of blood)
27
f
5
23
It:
7 11f5 llf3
10
f
3
9f4
Bradykinin (pg/ml of blood)
820
f
60 800
f
60 770
f
100 810
-1
70 820
f
90 870
f
60
80
J.
A.
Millar
et
al.
0
100
200
300
400
ANG
I
(pg/ml)
FIG.
2.
Correlation between plasma renin concentration @-i.u./ml
of
plasma) and
ANG
I
(pg/ml
of blood) in six salt-depleted normal subjects given captopril
(50
mg orally). The equation for the
curve shown, obtained by the least-squares method, is
y
=
88
+
4.95
x
(r
=
0.83,
P
<
0.001,
n
=
36).
Captapril
125
mgl
i
-+++
'.
T
--i
20
I I
I
0
60
120
Time (min)
FIG.
3.
Change in mean blood pressure after capto-
pril
(25
mg
orally) in
10
patients with essential
hypertension who had received no prior therapy
(0)
compared with the fall in six patients who had received
diuretic therapy
(0).
and ANG
I
were less marked, the ratios
of
90
min to basal values for renin and ANG
I
being
1
8
and
1.3
respectively. A significant fall in the
blood ANG
I1
level was observed by
30
min
and
was maintained for
a
further period
of
90
min;
however, by 240 min the level had returned
towards normal.
Mean bradykinin levels were similar in both
groups
of
hypertensive patients. For the whole
group mean pretreatment bradykinin was 820
k
120 pg/ml. After captopril the levels were
700
f
T
..
$0
li0
2io
Time
(min)
FIG.
4.
Circulating levels of
ANG
I1
(pg/ml of
plasma), renin (,u-i.u./ml
of
plasma),
ANG
I
(pg/ml
of
blood) and bradykinin
(ng/ml)
in
16 salt-replete
patients with essential hypertension given captopril
(25
mg
orally), as indicated by the
arrow.
80,
760
f
80,
690
?
60
and
700
?
80
pg/ml at
30,
60, 90
and 120 min respectively (Fig.
4).
There was
no
correlation between resting plasma
renin concentration or ANG
I1
and the fall in
Acute hormonal effects
of
captopril
81
blood pressure with captopril in these supine
hypertensive patients.
No significant changes were observed in
plasma concentrations of adrenaline or nor-
adrenaline (Table
2).
Discussion
Captopril
(SQ
14225)
was introduced as
a
specific inhibitor of angiotensin-converting en-
zyme whose pharmacological effects
in
uiuo
were considered to be
a
result of decreased
formation of ANG
I1
(Ondetti
et al.,
1977;
Horovitz,
1979).
Some studies do support
a
unique angiotensin-mediated mechanism of ac-
tion. For example, captopril prevents the rise in
blood pressure in two-kidney, one-clip Goldblatt
hypertension, but merely delays hypertension in
the one-kidney, one-clip model (Koletsky, Par-
licko
&
Revera-Velez,
1971;
Freeman, Davis,
Williams, De Forrest, Seymour
&
Rowe,
1979),
and does not reduce blood pressure in deoxy-
corticosterone acetate (DOCAtsalt hypertension
in the rat (Douglas, Lanford
&
McCaa,
1979).
Conversely, there is evidence that the hypo-
tensive action of captopril is not mediated solely
by inhibition of the renin-angiotensin system. In
patients with essential hypertension there is no
relationship between the fall in blood pressure
with long-term treatment, and pre-existing renin
levels (Gavras
et al.,
1978;
Johnston
et al.,
1979).
This finding has been confirmed in the present
acute study. Swarz, Williams, Hollenberg, Moore
&
Dluhy
(1979)
have shown that infusion of
ANG I1 into hypertensive patients given the
nonapeptide-converting enzyme inhibitor
(SQ
20
88
1)
restores blood pressure to pretreatment
levels, but blood angiotensin was then greater
than that before treatment with the inhibitor.
Again, in salt-depleted rats, converting enzyme
inhibition produced
a
greater fall in blood
pressure than angiotensin receptor antagonism
with [Sarl,AlaalANG
I1
(Thurston
&
Swales,
1978).
This study also demonstrated that when
the renin-angiotensin system was blocked by
a
continuous infusion of saralasin captopril still
lowered the blood pressure. However, the evi-
dence in these last two studies is indirect and the
results may be explained by changes in the
sensitivity of the vascular tree to ANG
I1
during
treatment with captopril (Clappison, Millar,
Casley, Anderson
&
Johnston,
1980)
and the
agonist properties of saralasin respectively.
Lastly, captopril has been shown to lower the
blood pressure in nephrectomized animals and
man
(Man
in't Veld, Schicht, Derkx, de Bruyn
&
Schalekamp,
1980).
There is good evidence that blood pressure in
salt depletion is maintained by stimulation of the
renin-angiotensin system and raised levels of
ANG I1 (Sancho, Re, Burton, Barger
&
Haber,
1976).
Hence, our finding that captopril de-
creased blood pressure in salt-depleted normo-
tensive and hypertensive subjects
is
consistent
with
a
single mechanism of action via decreased
generation
of
ANG
11.
In
contrast to the present
study, Niarchos
et al.
(1979)
found that mean
blood pressure in salt-replete normal subjects in
the sitting position fell by
10
mmHg,
15
and
30
min after administration of
SQ 20
881.
These
opposing results may be an indication of subtle
differences in the effects of captopril and
SQ
20
881
or
the influence of posture on the
renin-angiotensin system. Conversely the failure
to observe
a
fall in recumbent blood pressure in
patients with essential hypertension suggests that
the renin-angiotensin system does not play a
significant primary role in this condition.
It is interesting that ANG
I1
levels fell in
all
groups of subjects to the same level, but only in
the salt-depleted subjects was any fall in blood
pressure observed. Similarly in patients with
essential hypertension the hormonal effects were
seen within
30
min without
a
fall in blood
pressure, but long-term blood pressure does fall in
Datients with essential hypertension without
a
rurther fall
in
plasma ANG
11.
This suggests that
although the acute hypotensive effect of angio-
tensin-converting enzyme inhibition may be due
to angiotensin inhibition the chronic long-term
hypotensive effect is mediated by other
mechanisms.
The response of the renin-angiotensin system
to converting enzyme inhibition was markedly
enhanced in the salt-depleted state (Fig.
1).
This
may indicate that the negative-feedback effect of
ANG
I1
on renin release
is
of relatively greater
magnitude in sodium depletion and acts to
attenuate the renin response to negative salt
balance. However, the blood pressure decrease
after captopril in the salt-depleted group may also
have contributed to the hyper-reninaemia. The
difference in the hormonal responses to capto-
pril in different states of salt balance emphasizes
the need to define salt intake when measuring
either the renin response to captopril
as
a
test of
renovascular hypertension as suggested by Case
&
Laragh
(1979)
or the blood pressure response
as
a
measure of dependency
of
hypertension on
ANGII.
In contrast to the pronounced changes
in
renin
and ANG
I
levels in normal subjects (Millar
&
Johnston,
1979)
the changes in circulating levels
of these hormones in patients with essential
hypertension were much less marked. This may
6
82
J.
A.
Millar et al.
be yet another example of blunted renin re-
sponsiveness in essential hypertension.
Changes in bradykinin have also been postu-
lated
as
contributing
to
the hypotensive effect of
captopril. Captopril does potentiate bradykinin
in
vitro
and
in vivo
(Murphy, Waldron
&
Goldberg,
1978) and blood bradykinin is cleared across the
lungs by degradation by converting enzyme
(Erdos, 1975). McCaa, Hall
&
McCaa (1978)
found increased bradykinin levels in dog plasma
during long-term captopril. However, the dose of
captopril was high
(20
mg/kg) and bradykinin was
measured by bioassay. We have also found raised
immunoreactive blood bradykinin levels in rats
given
a
similar dose
of
captopril
(30
mg/kg), but
not
at
doses similar to those used in the present
study (Matthews
&
Johnston, 1979). Williams
&
Hollenberg (1977) reported
a
transient increase
in immunoreactive bradykinin levels in salt-
depleted hypertensive patients given
SQ
20
881,
but no increases were found in normal subjects.
A more recent report from these authors shows
no significant increase in bradykinin in patients
with hypertension and chronic renal failure given
SQ
20
881 (Hollenberg, Swartz, Passan
&
Williams, 1979). In contrast to the above findings
and
to
our own results, Hulthen
&
Hokfelt (1978)
reported
a
brief decrease in blood bradykinin in
salt-replete hypertensive subjects given
SQ
20
881. Again, it is difficult
to
exclude dif-
ferences in the effects of
SQ
20
881 and capto-
pril as an explanation for these diverse findings.
However, the results of the present study suggest
that captopril,
at
doses which are clinically
effective in decreasing blood pressure, does not
increase circulating endogenous bradykinin in
spite
of
evidence of marked converting enzyme
inhibition as shown by sequential increases in
ANG I and renin and a decrease in ANG I1
levels.
Blood levels
of
hormones do not necessarily
reflect levels at effector sites in target tissues.
Indeed, dissociation of blood and tissue renin
levels has been described (Swales, 1979). We
have measured an increase in urinary bradykinin
excretion in dogs after captopril without changes
in blood levels (Clappison
et
al.,
1980). Hence,
captopril may lead
to
local increases
in
intrarenal
bradykinin which influences renal haemo-
dynamics and thus blood pressure.
The present study has directly confirmed that
captopril is an effective inhibitor of converting
enzyme
in vivo
in man and that major hormonal
changes affecting the circulating components
of
the renin-angiotensin system follow its
administration. The acute hypotensive effect of
captopril is probably mediated by inhibition
of
ANG I1 formation and not by any increase in
circulating bradykinin.
Acknowledgments
J.A.M. and P.G.M. are Senior Research
Officers of the National Health and Medical
Research Council (NH and MRC) of Australia.
The studies were supported by grants-in-aid from
the National Heart Foundation and the Life
Insurance Medical Research Fund. We thank
S.
Lamont, M. Hammat and
D.
Casley for expert
technical assistance. Captopril was supplied by
Dr
A. Jenkins, Squibb Pharmaceuticals.
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