CARDIOVASCULAR JOURNAL OF AFRICA • Vol 21, No 4, July/August 2010
184
AFRICA
It has also been shown that peripheral vascular adrenergic
hyperactivity contributes to human hypertension
8
and is char-
acterised by (1) down-regulation of
α
-adrenergic receptors, (2)
an impairment in the neuronal uptake of nor-epinephrine from
sympathetic nerve terminals, and (3) an altered functional inter-
action at the level of the vascular wall between nor-epinephrine,
epinephrine, and other humoral (such as angiotensin II), meta-
bolic (including insulin and leptin), or endothelium-derived
substances, e.g. asymmetrical dimethylarginine (ADMA).
8
Altogether, the above proposed mechanisms coupled to low-
renin hypertension,
6
salt sensitivity
1
and increased vascular
responses in Africans
2,6,7,9
and African-Americans may contribute
to an increase in peripheral vasoconstrictor tone, as well as an
abnormal vasomotor responsiveness to adrenergic stimuli in
hypertension.
Overall, subjects maintain their characteristic response tenden-
cies (central or vascular) across mental tasks but it was also
shown that when the task was long enough, certain individuals
shifted from cardiac to vascular responders. This could support
the findings of Malan
et al
.
7,9
where
β
-adrenergic responses on
exposure to acute mental stress were elicited in high ACAfricans
in a rural environment opposed to
α
-adrenergic responses in
high AC Africans in an urban environment. Therefore, we might
propose that if a hyperkinetic–hyperactive peripheral adrenergic
sympathetic drive is elicited during acute/chronic stress, coupled
to dissociation between a behavioural and a physiological defen-
sive high AC
β
-adrenergic coping style, it could imply uncontrol-
lable stress.
2
A resultant high vascular resistance pattern emerg-
es, augmenting possible inward eutrophic vascular remodelling
and inflammation, a hallmark of hypertension.
5
Implementing Fig. 1, these data suggest a physiological
adaptation process of black Africans associated with an urban
environment or chronic psychosocial stress. Moving from a
traditional rural African setting (a collectivistic cultural context
with support networks) to an urban area (an individualistic
cultural environment without social support) is likely to exacer-
bate stress. Over time, the combined effects of stress, enhanced
vascular reactivity, associated with
α
-adrenergic stimulation and
synergistic effects on cortisol may further impact on depres-
sion/stress via the hypothalamus–pituitary–adrenal cortex axis
(submitted to
Biol Psych
2010) and predispose these individuals
to greater cardiovascular risk. Clearly, our understanding of these
interactions is critical for the development of recommendations
for early prevention of hypertension and the metabolic syndrome
in urbanised Africans.
Future studies are, however, required to establish if stress
responses to behaviourally induced stressors could prospectively
predict hard clinical CVD endpoints, such as myocardial infarc-
tion and cardiovascular death. Laboratory-induced responses
are relatively stable within an individual and consistent across
time, although the responses might not always be reflective of
everyday life. Therefore, future work is required that examines
not only a battery of laboratory measures but also the associa-
tions of ambulatory responses to real-life stress in relation to risk
of CVD.
The first well-controlled psycho-physiological study inAfrica,
the SABPA I study (Sympathetic activity and Ambulatory Blood
Pressure study on Africans) was conducted in 2008/2009. It
is an example of such a study where genetic polymorphisms,
adrenergic, pro-inflammatory, prothrombic, blood pressure,
heart rate variability (HRV), ischaemic events, oxidative stress,
and stress hormone responses to acute mental stress were evalu-
ated in urban teachers from South Africa. In addition, the study
will incorporate ambulatory measures during the normal work-
ing day. Preliminary findings from SABPA I indicated that the
prevalence of HIV/AIDS in the black SABPA teachers was only
9%, opposed to the overwhelmingly increased hypertension
prevalence rates in the black teachers [males (79%), females
(45%)]. This implies that CVD risk attributable to other risk
factors apart from the inflammatory status contributed to their
overall health profile, and risk factors should be broadened
without concentrating on only the HIV/AIDS factor. Data from
the SABPA I and SABPA II follow-up (2011/2012) studies
will therefore provide an estimate of the health/disease burden
attributable to established and emerging risk factors for CVD,
metabolic syndrome, psychological distress and progression of
subclinical atherosclerosis.
Future studies should examine whether non-pharmacological
interventions that reduce SNS hyper-responsiveness, such as
physical activity and/or active relaxation breathing techniques
could lessen sympathetic drive and increase vagal outflow.
LEONÉ MALAN, NT MALAN, A DU PLESSIS
Hypertension in Africa Research Team (HART), School for
Physiology, Nutrition and Consumer Sciences, North-West
University, Potchefstroom Campus, Potchefstroom, South
Africa
MP WISSING, JC POTGIETER
Psychosocial Behavioural Sciences, North-West University,
Potchefstroom Campus, Potchefstroom, South Africa
YK SEEDAT
The Renal Hypertension Unit, Nelson Mandela School of
Medicine, University of Kwa-Zulu Natal, Durban, South Africa
Fig. 1. Implementing the general adaptation syndrome
10
to interpret the dissociation between behavioural and
physiological AC cardio-neuro-metabolic responses in
urban black Africans. HT
=
hypertension; AC
=
active
coping; IFG
=
impaired fasting glucose.
1
ALARM REACTION
Urban
environment
2
RESISTANCE STAGE
↑
vascular responses,
↑
HT,
↑
glucose,
↑
stress
(
↑
Prolactin,
↓
Testosterone,
↑
Cortisol:Testosterone,
↑
Cortisol:Prolactin)
3
EXHAUSTION STAGE
Dissociation between
behavioural and
physiological AC cardio-
neuro-metabolic responses:
↑
lifestyle diseases (HT,
IFG, sensitized defensive
pathway, loss of control,
chronic stress)
