Aldosterone-induced microvascular dysfunction as a cause of salt-sensitivity in obesity?

Currently, the incidence of obesity and obesity-related disorders is reaching
epidemic proportions, which entails an increasing burden for health care
systems. The association of obesity with other risk factors for type 2 diabetes
mellitus and cardiovascular disease, such as insulin resistance and
hypertension, is often referred to as the metabolic syndrome. During recent
years, salt-sensitivity of blood pressure has emerged as an additional
cardiovascular risk factor that is related to obesity and other key components
of the metabolic syndrome. The underlying pathophysiological mechanisms of
these interrelationships are complex and incompletely elucidated. A more
detailed insight, however, is essential to identify and treat persons at risk
for cardiovascular disease at an early stage. Recently, microvascular
dysfunction has been proposed as a link between insulin resistance and
hypertension in obese individuals. In addition, impairment of microvascular
function was found to be associated with salt-sensitivity of blood pressure.
Loss of NO-mediated vasodilation is an important feature of microvascular
dysfunction and may contribute to the pathogenesis of salt-sensitivity.
Furthermore, an impaired insulin-mediated microvascular NO production has been
suggested to underlie the reduction in insulin-stimulated glucose disposal that
is characteristic of insulin-resistant states. Increased aldosterone levels, as
observed in obese individuals, might be a cause of microvascular
dysfunction-induced salt-sensitivity and insulin resistance. Aldosterone not
only gives rise to sodium-retention in the distal tubule of the kidney, but was
also found to impair endothelial function and thus lower NO-availability. In
addition, elevated aldosterone levels are associated with both hypertension and
insulin resistance, which is illustrated in patients with primary
aldosteronism, but also in the general population. Whether and how
microvascular dysfunction is involved in the pathogenesis of salt-sensitivity,
has never been directly investigated in humans or more specifically, obese
individuals. The role of aldosterone in the pathogenesis of microvascular
dysfunction-induced salt-sensitivity has never been established as well.

To investigate whether increased aldosterone levels in obese individuals lead
to impairment of microvascular function through reduction of NO-availability,
and to establish the role of microvasculaire dysfunction in the pathogenesis of
salt-sensitive hypertension.

This is a randomized, double blind, placebo-controlled cross-over study.

After recruitment, participants will be either started on a low salt diet of 50
mmol NaCl/24h or a high-salt diet of 250 mmol NaCl/24h during one week.
Thereafter, measurements of microvascular function will be performed. This is
followed by a 2-week wash-out period of ad-libitum salt intake.
Subsequently, participants who initially used a low salt diet will now be
placed on a high salt diet and vice versa, for another week, where after
measurements of microvascular function will be repeated.
The sequence of variation in salt intake for each individual participant will
be determined by means of block randomization.

Participants will visit the study center 3 times: once for a screening visit
and at two occasions for measurements of microvascular function before and
during a hyperinsulinaemic, euglycaemic clamp test.
For screening purposes, 9 mL blood will be drawn to determine electrolytes,
renal function, lipid profile and glucose. After recruitment, participants
will be randomized to a period of 7 days of either low (50 mmol NaCl/24h) or
high (250 mmol NaCl/24h) salt intake. This might induce a slight decrease or
increase in blood pressure, which will most probably not be noticed by the
participants and carries no health risks.
Possible side effects of the high salt diet are mild gastro-intestinal
complaints, although not reported by all studies applying this intervention.
Since we use slow-release capsules, we expect that these side effects will be
reduced to a minimum or do not occur at all. Prior to the initial measurements,
antihypertensive therapy (if participants are receiving any) will be
temporarily ceased. This is not expected to cause health concerns.
At the end of the week of either low or high sodium ingestion, 24h urine will
be collected to verify compliance to the diet, and 24h blood pressure
measurements are performed.
This is followed by a first study day, during which microvascular function will
be assessed in the fasting state, before and during a hyperinsulinaemic,
euglycaemic clamp test for determination of insulin sensitivity.
Participants will be requested to abstain from alcohol 12 hours prior to the
study days, and not to perform strenuous exercise 48 hours beforehand.
Measurements of microvascular function include contrast-enhanced ultrasound of
skeletal muscle (CEUS), skin capillary microscopy, skin vasomotion analysis,
registration of heating-induced microvascular dilatation and determination of
endothelial glycocalyx thickness, and are merely noninvasive. The contrast
agent administered during the CEUS procedure has been proven to be a safe
imaging modality in previous investigations. The most prevailing side effect of
the hyperinsulinaemic euglycaemic clamp test is hypoglycaemia. A total amount
of ~ 140 mL blood is drawn for determination of electrolytes, renal function,
lipid profile, insulin, markers of endothelial activation and inflammation,
RAAS components, and glucose values during the hyperinsulinaemic, euglycaemic
clamp. This amount carries no risk for the participants.
The first study day is followed by a 2-week washout period of ad-libitum
salt-intake. Thereafter, participants who initially used a low salt diet will
now be placed on a high salt diet and vice versa, for another week. At the end
of this week, 24h urine collection and 24h blood pressure measurements will be
iterated, for verification of compliance to the diet and salt-sensitivity of
blood pressure.
Subsequently, all procedures performed during the first study day will be
repeated, to establish the effect of variation in sodium intake on
microvascular function and insulin sensitivity.
Participants will obtain no specific health benefits. The periods of low and
high sodium intake demand a strict compliance, but since the diet is adapted to
the individual caloric intake of the participants, and due to the relative
short-time interventions and the intervening wash-out period of ad-libitum salt
intake, the burden for the participants is somewhat reduced. Nevertheless, the
amount of time invested is considerable and a large amount of measurements will
be performed (determination of microvascular function, laboratory
investigations, 24h blood pressure measurements).This is justified, however,
since we might elucidate mechanisms leading to obesity-related complications,
which possess a major health risk. Participants will also gain insight into
their individual risk profile for the development of type 2 diabetes and
cardiovascular disease. All subjects will receive a compensation of ¤ 250,-
after completion of the study.