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RESEARCH ARTICLE
SA JOURNAL OF DIABETES & VASCULAR DISEASE
66
VOLUME 13 NUMBER 2 • DECEMBER 2016
Discussion
Currently, the world is suffering from a silent epidemic starting
with obesity and culminating in type 2 diabetes.
3
Two of the most
debilitating complications of obesity, especially centrally located
obesity, responsible for the high morbidity and mortality associated
with such patients are hypertension and heart disease.
19,20
In view of
the need for effective medication to supplement lifestyle changes
to control these disease states, utilisation of plant-based therapies
are currently strongly advocated.
7,21
Such therapies offer potentially
cost-effective management but need scientific validation of their
effects.
Previous studies from our laboratory demonstrated that the dried
and ground pods of the
P glandulosa
tree have a potential benefit
in the management of both type 1 and type 2 diabetes.8 In view of
the insulin-sensitising effects on isolated cardiomyocytes from rats
treated with
P glandulosa
, we aimed to determine whether this
product has any cardioprotective or anti-hypertensive effects.
In this study, we used three different animal models. The first was
a model of pre-diabetes (DIO), as also indicated by the biometric
data presented in Table 1. These animals were fed an obesity-
inducing diet containing only 16% fat.
11,13
DIO animals become
insulin resistant but not diabetic, as the blood glucose levels never
rose above ~ 6.5 mmol/l. This was however significantly higher
than the levels found in the control, chow-fed animals. In order to
keep the blood glucose levels low, the animals presented with high
plasma insulin concentrations.
Although
P glandulosa
treatment did not significantly alter these
parameters, the clinically important two-hour blood glucose values
after a glucose tolerance test were significantly higher in the DIO
animals and were effectively lowered by the treatment (Fig. 1). This
underscores the slight effect on blood glucose handling previously
reported.
8
Determination of infarct size in ex vivo perfused rat hearts as a
measure of myocardial damage incurred by ischaemia followed by
reperfusion, is taken as the gold standard to prove cardioprotection.
15
We previously showed that hearts from the DIO rats developed
larger infarct sizes when subjected to regional ischaemia followed
by reperfusion.
10
After eight weeks of treatment of DIO rats or CIRKO mice with
P glandulosa
, it was clearly demonstrated that there was an infarct-
sparing effect elicited by ingestion of this plant material (Figs 2, 3).
As the CIRKO mice do not possess a myocardial insulin receptor, the
protection found in these animals confirmed the results obtained
in the rat model and underscores that protection does not occur
via the insulin-secretory effects of
P glandulosa
, as previously
reported.
8
One of the best-described and researched mechanisms of
protection of the heart against ischaemia–reperfusion injury and
infarction is activation of the PI-3K, PKB/Akt pathway, normally
activated by various extracellular substances.
22-24
Activation of this
pathway has several anti-apoptotic effects, leading to limitation of
the development of an infarct after ischaemia.
In addition, activation of PKB/Akt is a pre-requisite for glucose
uptake by the heart.
25
Myocardial glucose is taken up via the two
transporters Glut 1 and Glut 4. An improved ability to import and
utilise glucose is cardioprotective when the heart is subjected to the
absence of oxygen, as induced by ischaemia. The heart then uses
the energy generated by glycolysis to protect itself.
Measurement of the expression of both Glut 1 and Glut 4 showed
no differences between hearts from control and DIO rats. However,
the lower ratio of phosphorylated to total protein of PKB/Akt found
in hearts from the DIO animals may have been detrimental during
an ischaemic incident. In addition, there was lower expression of
the p85 subunit of PI-3K documented in these hearts, which may
have exacerbated this effect.
Both of these detrimental changes were improved by
P glandulosa
treatment. The changes documented in the phosphatase PTEN will
further the positive effects found in both PI-3K and PKB/Akt as the
lower expression and elevated phosphorylation of this enzyme will
elevate the activity of PKB/Akt when the latter is stimulated.
18
PTEN
normally inactivates PKB/Akt.
17
These changes may play a central
role in the protection that
P glandulosa
treatment confers on the
heart.
The second rat model was aimed at specifically inducing the
development of hypertension. A modification of a high-fat diet
was used (HFD).
12
These animals, in contrast to the DIO animals,
developed severe hypertension within a four-week period, as
shown in Fig. 6A and B. Not only was
P glandulosa
treatment
able to prevent the development of hypertension when given in
conjunction with the high-fat diet, but it normalised elevated blood
pressure within two weeks.
The hormonal effects associated with a high-fat diet in rats,
namely elevated vasopressin as well as activation of the renin–
angiotensin system, leading to elevated aldosterone levels may
both be involved in the development of hypertension in these
animals.
26-28
Vasopressin, the anti-diuretic hormone leads to water
retention and therefore the development of high blood pressure. In
addition, it is associated with vasoconstriction.
28
Similar effects can
be expected from elevated sympathetic activity, leading to elevated
aldosterone levels. Measuring the 24-hour urine output of the HFD
animals underscored this, as the HFD animals had a significantly
lower urinary output than the controls.
According to Lee and Blaufox,
29
a volume of 16–17 ml urine
can be expected from normal animals in the weight range of our
experimental rats (control 258.49 ± 15.03 vs HFD 327 ± 12.90 g,
p
< 0.05,
n
= 14 per group) while a high-fat diet will result in
concentration of this volume, indicating water retention. It can also
be speculated that, in parallel with the latter effect, there will be
vasoconstriction, contributing to the observed hypertension.
The treatment with
P glandulosa
was able to alleviate this,
thereby adding to the myocardial protection observed. To highlight
this argument, in the present study both the ACE inhibitor and
Fig. 7.
Rats on the high-fat diet were individually placed in metabolic cages for
the collection of urine over a 24-hour period. Data were collected at 12 weeks
after the diet was started. **
p
< 0.01, ***
p
< 0.001,
n
= 9 per group.