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CARDIOVASCULAR JOURNAL OF AFRICA • Volume 30, No 1, January/February 2019
36
AFRICA
Means and proportions were compared using independent
t
-tests and chi-squared tests, respectively. We performed single,
partial and forward stepwise multiple regression analyses to
investigate associations between relevant cardiovascular
variables and renin, aldosterone and ARR, as well as between
aldosterone and noradrenaline:creatinine ratio. In partial
regression analyses we adjusted for age, body mass index (BMI)
and gender. Covariates included in the models were age, waist-
to-hip ratio, gender, GGT, cotinine, urinary Na
+
:K
+
ratio, total
cholesterol:HDL-C ratio, HbA
1c
, TNF-
α
, eGFR and TPR.
Results
Table 1 compares the black and white groups in which the
frequency of low renin level was higher in the black group
compared to whites (80.3% vs 58.7%,
p
<
0.001).The ethnic
groups had similar mean ages (
p
=
0.35) and gender distribution
(
p
=
0.78). Twenty-four-hour, day- and night-time systolic BP
(SBP) and diastolic BP (DBP) as well as HR were higher in
the black group (all
p
<
0.001), while percentage dipping in
SBP, DBP or HR was lower in the black group compared to
the white group (all
p
≤ 0.042). Aldosterone (
p
=
0.015) and
noradrenaline:creatinine ratio (
p
=
0.044) were higher in the white
group compared to the black group, while blacks had lower
renin level (
p
<
0.001) and a higher ARR (
p
=
0.007) compared
to whites.
We performed Pearson and partial correlations (adjusting for
age, gender and BMI) to investigate the associations of surrogate
measures of sympathetic activity (noradrenaline:creatinine
ratio, 24-hour HR and night-time dipping in HR) with renin,
aldosterone and ARR (Fig. 1, Tables 2, 3). In blacks, before
and after full adjustment in multiple regression analysis,
24-hour HR associated positively with renin (
β
=
0.20,
p
=
0.024), while night-time dipping in HR associated negatively
with aldosterone (
β
=
–0.18,
p
=
0.024) and ARR (
β
=
–0.20,
p
=
0.011) (Table 4).
Percentage dipping in SBP was positively associated with
aldosterone (
β =
0.23,
p
=
0.008) and ARR (
β =
0.18,
p
=
0.038),
while dipping in DBP was positively associated with aldosterone
level (
β =
0.24,
p
=
0.007) (Table 4). In whites, the renin level was
positively associated with dipping in DBP (
β
=
0.16,
p
=
0.033)
(Table 4).
Table 5 indicates the following associations in the black group:
24-hour SBP (
β =
–0.22,
p
=
0.006) and 24-hour DBP (
β =
–0.20,
p
=
0.009) associated negatively with renin level. There was also
a borderline significant association of noradrenaline:creatinine
ratio with aldosterone (
β =
0.19,
p
=
0.051) (Table 5). In the white
group, 24-hour SBP (
β =
–0.15,
p
=
0.021) and 24-hour DBP (
β =
–0.16,
p
=
0.019) associated negatively with renin level (Table 5).
Discussion
The main finding of this study is that in the black group, which
consisted mainly of participants with low renin levels, 24-hour
HR and its dipping as surrogate measures of sympathetic
nervous system activity related independently and adversely
to renin, aldosterone and ARR. Furthermore, there was a
tendency towards a positive association between aldosterone
and noradrenaline levels (
p
=
0.051). Previous observations in the
SABPA cohort indicated a blunted baroreceptor sensitivity and
depressed HR variability, supporting the possibility of higher
sympathetic activity in this population group.
25,26
A novel finding of this study was that despite higher aldosterone
levels in whites compared to blacks, aldosterone and its ratio
to renin were associated with less dipping of night-time HR in
blacks only. A higher HR is associated with total mortality rate,
and night-time HR predicts cardiovascular mortality rate in the
general population.
27
In blacks, a more pronounced sympathetic
drive, as shown by an exaggerated cardiovascular reactivity to
Table 1. Comparison between black and white groups
Variable
s
Blacks
(
n
=
127)
Whites
(
n
=
179)
p
-value
Age (years)
43.0 ± 7.33
44.0 ± 10.8 0.35
Women,
n
(%)
64 (50.4)
93 (52.0)
0.78
Hypertensive,
n
(%)
77 (60.6)
66 (36.9)
<
0.001
Anthropometric measurements
Body mass index (kg/m
2
)
30.2 ± 7.26
27.5 ± 6.01
0.001
Waist-to-hip ratio
0.87 ± 0.11
0.87 ± 0.10 0.51
Resting cardiovascular measurements
a
Cardiac output (l/min)
6.72 ± 1.88
6.44 ± 1.94 0.21
Heart rate (bpm)
67.9 ± 10.3
66.3 ± 10.7 0.17
Stroke volume (ml)
100 ± 27.2
97.9 ± 24.5 0.43
Total peripheral resistance
(mmHg/ml/s)
1.03 ± 0.42
1.03 ± 0.53 0.93
Systolic BP
24-hour (mmHg)
132 ± 15.4
124 ± 11.9
<
0.001
Daytime (mmHg)
137 ± 15.4
129 ± 11.8
<
0.001
Night-time (mmHg)
123 ± 16.1
113 ± 13.8
<
0.001
Diastolic BP
24-hour (mmHg)
82.5 ± 10.5
73.8 ± 9.94
<
0.001
Daytime (mmHg)
87.7 ± 11.4
81.4 ± 8.70
<
0.001
Night-time (mmHg)
73.4 ± 11.7
66.4 ± 9.01
<
0.001
Heart rate
24-hour (bpm)
79.3 ± 9.70
73.7 ± 10.2
<
0.001
Daytime (bpm)
84.2 ± 10.4
78.7 ± 10.7
<
0.001
Night-time (bpm)
71.0 ± 12.0
64.9± 10.3
<
0.001
% Dipping
Systolic BP (mmHg)
10.1 ± 6.34
12.2 ± 6.33
0.003
Diastolic BP (mmHg)
15.9 ± 11.9
18.2 ± 7.90
0.042
Heart rate (bpm)
14.6 ± 8.78
17.2 ± 9.25
0.015
Biochemical measurements
Aldosterone (pg/ml)
43.7 (10.5; 170) 55.0 (21.8; 219)
0.015
Renin (pg/ml)
3.47 (0.95; 9.33) 5.75 (2.24; 12.3)
<
0.001
Low renin status,
n
(%)
102 (80.3)
105 (58.7)
<
0.001
Aldosterone-to-renin ratio
12.5 (2.95; 67.6) 9.55 (2.88; 33.1)
0.007
Glycosylated haemoglobin (%)
6.03 (5.25; 8.71) 5.50 (5.01; 6.31)
<
0.001
Total cholesterol:HDL-C
4.07 (2.29; 7.41) 4.68 (2.82; 7.94)
0.0002
C-reactive protein (mg/l)
3.80 (0.28; 26.3) 2.04 (0.99; 8.91)
<
0.001
Cotinine (ng/ml)
2.75 (1.00; 148) 1.86 (1.00; 209)
0.043
Gamma-glutamyltransferase (U/l) 43.6 (20.4; 138) 18.6 (7.08; 74.1)
<
0.001
Tumour necrosis factor-
α
(IU/ml)
3.00 ± 2.13
1.90 ± 2.00
<
0.001
Estimated glomerular filtration
rate (ml/min/1.73 m
2
)
114 ± 27.8
94.5 ± 17.1
<
0.001
Urinary Na
+
:K
+
ratio
5.47 ± 4.11
4.09 ± 2.73
0.0004
Urinary noradrenaline:creatinine
ratio
1.45 (0.95; 2.51) 1.55 (0.85; 2.69)
0.044
Lifestyle factors
Self-reported smoking,
n
(%)
22 (17.3)
26 (14.6)
0.52
Self-reported alcohol use,
n
(%)
29 (22.8)
91 (51.1)
<
0.001
a
Obtained from the Finometer device. Values are arithmetic mean ± standard
deviation; geometric mean (5th and 95th percentile interval) for logarithmically
transformed variables. BP, blood pressure; HDL-C, high-density lipoprotein
cholesterol. Bold text indicates
p
<
0.05.