Clinical Science (1984) 67, 383-387
383
The basal levels of active and inactive plasma renin
concentrationin infancy and childhood
T. FISELIER, F. D E R K X * , L. MONNENS, P. VAN MUNSTER, P. PEER
A N D M. SCHALEKAMP*
Department of Paediatrics and Department of Statistical Consultation, University of Nijmegen, Nijmegen, and
*Department of Internal Medicine I, Erasmus University, Rotterdam, The Netherlands
(Received 14 December 1983; accepted 28 March 1984)
summary
1. Basal plasma renin activity, active and inactive plasma renin concentration were measured
in 89 healthy recumbent children aged between 1
week and 16 years.
2. A significant (P< 0.001) age-related decrease for active (r = -0.60), inactive (r =-0.59)
and total renin concentration (r = - 0.66) was
observed.
3. After correction for the influence of age,
active renin concentration correlated with plasma
renin activity (r = 0.81), but not with inactive
renin concentration (r = 0.18).
4. The proportions of active and inactive
renin were not related to age, and the overall
percentage of inactive renin was 79%.
Key words: child, prorenin, renin.
Introduction
Inactive renin is a large molecular weight form of
renin without intrinsic activity. It can be converted in vitro into an active form of renin [ 1,2].
Inactive renin, often called ‘prorenin’, is present in
plasma of adults in much higher concentrations
than active renin and accounts for between 70%
and 95% of the total circulating renin [3]. In
contrast to adults, there is almost no information
on inactive renin in childhood [4-61.
Correspondence: T. J . W. Fiselier, Department
of Paediatrics, University of Nijmegen, Geert
Grooteplein Zuid 20, 6500 HB Nijmegen, The
Netherlands.
The aim of this paper is to present basal levels
of active and inactive renin concentrations in
infants and children between 1 week and 16 years
of age. Trypsin was used for activation [7].
Methods
Basal levels of plasma renin activity and active
and inactive plasma renin concentrations were
measured in 89 healthy infants and children, 54
boys and 35 girls, aged between 1 week and 16
years. All children were hospitalized and recovering from minor diseases or minor operations. They
had normal blood pressure and showed no
evidence of renal, cardiovascular and endocrine
diseases, or disorders of acid-base and electrolyte
metabolism. They were without medication. The
children were arbitrarily divided into the following age groups: 1 week to 3 months (n = 1l),
3-6 (n = lo), 6-9 (n = 9) and 9-12 months
(TI = 8), 1-4 (n = 18), 4-8 (n = 15), 8-12 (n =
10) and 12-16 years (n = 8). Venous blood
samples were taken at 08.30 to 09.00 hours.
Children had taken nothing by mouth and had
been in a controlled supine posture since midnight. Infants were not fed later than 3 h before
sample collection and remained at least 3 h in a
supine posture. The venepuncture was performed
as quickly as possible with a minimum of discomfort. All children were on an unrestricted
hospital diet. The venepuncture was only performed when this puncture was required for
routine examinations.
Blood was collected into ice-cooled tubes
containing 10 mg of potassium EDTA. Plasma was
384
T. Fiselier et al.
immediately separated in a refrigerated centrifuge,
quickly frozen and kept stored at -20°C until
time of assay.
Plasma renin activity was measured with a
commercial kit (Clinical Assays). Briefly, the
sample was incubated at pH 6.0 using the maleate
buffer, in the presence of phenylmethylsulfonyl
fluoride, for 9 0 m i n at 37°C. The angiotensin I
(ANG I) generated was measured by radioimmunoassay. The results are expressed in ng of ANG I
h-' ml-'. The sensitivity of the plasma renin
activity (PRA) assay is 0.15 ng of ANG I h-' m1-l.
The within run precision represented by the coefficient of variation for PRA at level < 10 ng of
ANG 1 h-' ml-' is 5% and at level > 10 ng of ANG
I h-' ml-' it is 4%. The between run coefficient of
variation at these levels is 8% and 10%respectively.
The assay of active and inactive plasma renin
concentrations has been reported [7]. Inactive
renin was measured as the difference between the
renin concentration after activation of the test
sample (total renin concentration) and the concentration before activation (active renin concentration). Inactive renin was activated by
Sepharose-bound trypsin in a final concentration
of 0.25 mg of trypsin/ml for 24 h at 4OC. Under
these conditions maximal activation of inactive
renin was obtained. Plasma renin concentration
was measured after incubation of the plasma
sample with sheep renin substrate in a final concentration of 2.5 nmol of IleS-ANG I equivalents/
ml, which corresponds to about ten times K,, in
the presence of 8-hydroxyquinoline, phenylmethylsulfonyl fluoride and aprotinin, for 3 h at pH 7.5
and 37°C. Renin concentration is expressed as
p-units of the WHO human kidney renin standard,
lot MRC 68/356, per ml.
Interassay variability for the assay of active and
inactive renin concentration was evaluated bv
weekly measurements of naturally occurring active
and inactive renin in a normal plasma pool (stored
at -20°C) during a 9 month period. The mean
value of active renin was 27 p-units/ml(36 assays)
with a standard deviation of 3 p-unitslml (coefficient of variation 11%). The mean value of
total renin was 254p-units/ml with a standard
deviation of 26 p-unitslml (coefficient of variation
10%). The mean value of inactive renin was 227
p-units/ml with a standard deviation of 2 4 punits/
ml (coefficient of variation 11%).
Statistical analysis
All statistical methods 181 were undertaken on
logarithmically transformed -data because of the
skew distribution of the raw data within the age
groups. The mean and the one standard deviation
range after transformation were transformed back
to the model median and the one standard deviation range of the raw data.
In calculating the Pearson's coefficient of
correlation and the partial correlation eliminating
the effect of age, logarithmic transformation of
age was also performed to obtain a better linear
relation.
When plasma values were not different between
contiguous age groups according to Duncan's test,
these groups were joined.
With one-way analysis of variance an investigation was made of whether age groups were
different from each other. If there was a significant difference the simultaneous method of
Scheff6 indicated which of the age groups differed
from each other.
All tests were performed at a significance level
of 0.05.
Results
Since the Duncan analysis made no distinction
between some age groups, a new subdivision was
made: 1 week to 3 months, 3-12 months, 1-4,
4-8 and 8-1 6 years.
The age-related decrease for circulating active
and inactive renin and plasma renin activity is
shown in Figs. 1 and 2 and in Table 1. Correlation
with age is significant (P<O.OOl) for active
(r = - 0.60), inactive (r = - 0.59) and total
plasma
renin
concentration
(r = - 0.66).
Differences between age groups are indicated in
Table 1.
180
.s
140
5
120
5
8
100
"5
'g .@
'
80
0 6o
d
$
9
40
2o
1 3 6 9122 4
6
(months)
Age
n = 11 n = 27 n = 18 n = 1s
FIG. 1. Circulating levels of active renin in
different age groups. Model median and the one
standard deviation range are indicated.
385
Active and inactive renin in children
P = 0.09). The proportion of renin in the active or
lo ol- I i I
in the inactive form is not related to age ( r =
-0.09 and r = 0.07 respectively) and no significant differences between the various age groups
are found (Table 1). For the whole group of
infants and children 19%of renin was in the active
form, and 79% in the inactive form. The 95%
confidence intervals were 17-20% and 76-8 1%
respectively.
.
Discussion
The higher activity of the renin-angiotensinaldosterone system in infancy and childhood in
comparison with adult values is well known
I
19-13]. Our data not only confirm the age-related
1 3 6 9 12 2 4 6 8 10 12 14 16 decrease in ulasma renin-activitv and aciive renin
(months)
Age
(years)
Concentration in childhood, b i t also show conn=ll n=27 n = l 8 n = 1 5
n=l8
sistently higher levels of plasma inactive renin
than of active renin. In addition, circulating
FIG. 2. Circulating levels of inactive renin in
levels are decreasing with maturation.
different age groups. Model median and the one
When the renin-angiotensin system is stimustandard deviation range are indicated.
lated, such as during chronic sodium depletion,
chronic diuretic therapy and chronic converting
enzyme blockade, prorenin as well as active renin
Values for plasma renin activity are strongly
increase, but prorenin levels seem t o increase
correlated with active renin levels (r = 0.88)
proportionally less than active renin [2]. The
(Fig. 3) also after correction for the influence of
percentage active renin of total circulating renin
age (r = 0.81). The correlation is not different
found in infants and children is in the same range
between infants (r = 0.86) and children (r = 0.83).
as that reported in adults [3]. However, the
Active plasma renin concentration is related to
measured percentages reported in the literature
inactive renin concentration (r = 0.47, P < 0.001),
are widely different [7]. When our values are
compared with those reported in adults and
but this correlation is no longer significant after
measured by exactly the same methods [7],
correction for the influence of age (r = 0.18,
____
TABLE 1. Model medians and observed ranges in parentheses for. plasma renin activity (PRA), active
(APRC), inactive (IPRC) and total (TPRC) plasma renin concentration, and proportion o f active renin in
different age groups
Age group
n
PRA
(nr! of ANG I h-' m l - 9
APRC
-x 100%
APRC
Olunitslml)
IPRC
(r-unitslml)
TPRC
(r-unitslml)
110.3*
(66-172)
521.5*
(281-941)
637.3*
(347-1058)
17
(11-26)
322.8 1
(164-830)
19
(743)
22
(9-14)
TPRC
1 week to 3 months
11
3-12 months
27
61.8(20-144)
248.2
(78-64
1-4 years
18
49.7
(21-130)
169.7
4-8 years
15
(0.9-9.2)
38.0
(10-93)
161.8
(91-296)
205.4
(110-389)
19
(5-39)
2.05
(0.9-7.6)
32.1(11-97)
172.6
(105-297)
208.8(131-345)
15
(8-42)
23
(14-43)
196
(1 38-312)
8-16 years
18
Adults$
(Derkx er al. [7])
17
8.77*
(4.0-14.6)
+
+I
7
10.9
(4.3-17.5)
* Significantly higher than the other age groups; significant difference between age groups; $ adults not included in
statistical evaluation.
T. Fiselier et al.
386
0
0
0
0
0
0
0
0
0
'
.O
0
.
0
.
0
0
o o
.
Q
0 0
0 '
.o
. .
-
I
0
I
I
20
I
I
40
l
I
I
I
I
I
I
I
I
I
60
80
100
120
140
Active plasma renin concentration
I
I
160
I
I
180
(p-units/ml)
FIG. 3. Relation between plasma renin activity and plasma concentration of active
renin in children aged between 1 week and 16 years. ( 0 ) Less than 1 year old; ( 0 ) more
than 1 year old. Conversion of traditional to SI units: 1297 ng of ANG I = 1 nmol
of ANG I.
active renin is a higher percentage of total renin
in infancy and childhood. This is expected in a
stimulated system. The percentage of active renin
in infancy, however, is the same as in older
children (Table 1). An age-dependent difference in
the proportion of inactive renin converted to
active renin intracellularly (renal kallikrein?)
before release into the circulation may explain this
discrepancy.
High levels of inactive renin are reported in
newborns ([4], cryoactivation) and in 1-15 year
old children ([6], trypsin activation). In the latter
group, orthostasis for 30 min resulted in an
increase in active renin and in a decrease in
inactive renin, while total renin did not change.
These results are in conflict with the results reported in adults after such a short stimulus. The
same authors [6] found no inactive renin at all in
infants under 1 year of age when active renin was
high. Not enough information about the activation
methods used (cryoactivation or trypsin activation) in the three published abstracts [4-61
about prorenin in childhood is provided to allow
comment on the results. The plasma concentration
of active renin declincs progressively with age, and
the mean levels in 8-16 year old children are still
higher than those reported in adults and measured
with the same methods [7]. This decline is in
agreement with the reported age-related levels of
circulating angiotensin I and I1 [13]. However, the
levels of inactive renin reach values in adults at a
younger age. The mean values and observed ranges
for plasma inactive renin in children older than
1 year are similar to the values reported in adults
[7]. If inactive renin is the cellular intermediate of
renin biosynthesis, the high plasma level of
inative renin at young age may reflect a higher
rate of biosynthesis.
Plasma renin activity is a function of the concentrations o f both renin and renin substrate [14].
The reaction between renin and its substrate is
dependent on substrate concentration [I 51. Both
renin and renin substrate concentration are higher
at a young age compared with adult values [12,
161. The relationship between plasma renin
activity and active plasma renin concentration
reflects the rate of conversion between active
renin and endogenous substrate. As this relationship is not different between infants under 1 year
of age and 1-16 year old children (Fig. I), basal
Active and inactive renin in children
plasma renin activity seems not significantly
influenced by the reported age-related decrease in
renin substrate concentration. This is supported
by our earlier findings that the addition of 66.6
and 133.2 p-units of renin (WHO standard preparation of human renin, lot MRC 68/356) to 1.0 ml
of plasma of infants (n = 11) and adults (n = 20)
resulted in a similar generation of angiotensin I. In
infants the mean increase in PRA per p-unit of
renin added was 0.095 ng of ANG I h-* ml-' f
0.017 (SD) and 0.094f 0.015 after the addition of
66.6 and 133.2 p-units of renin respectively. In
adults these values were 0:091 fO.012 and 0.091 f
0.012 respectively. Therefore, normal substrate
concentrations in infants and adults do not significantly influence the generation rate of angiotensin
I. From the data of Skinner et al. [15] it can be
seen that small changes in renin substrate concentration hardly influence the initial reaction
velocity because of the shape of the substrate
concentration-angiotensin I generation rate curve.
The presented data about circulating active and
inactive renin concentration are required to
evaluate the renin-angiotensin system in infancy
and childhood in a more complete way.
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