CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 1, February 2012
14
AFRICA
statistical significance. To obtain the concentrations of SBE
producing 50% of the maximal stimulant effect (EC
50
), concen-
tration–response slopes were analysed using linear regression
analysis.
Results
Effect of SBE on rabbit isolated aortic ring strips
Incubation of aortic rings, harvested from normal rabbits, with
SBE (50–400 mg/ml) resulted in a significant (
p
<
0.01–0.001)
upward shift in baseline tone, with a mean positive baseline shift
of 0.56 ± 0.05 g (
n
=
6–8) in endothelium-intact, and 0.45 ± 0.03
g (
n
=
6–8) in endothelium-denuded aortic ring preparations
(Fig. 1A). Removal of the functional endothelium significantly
(
p
<
0.01) increased the SBE-induced contractile responses of
the preparations (Fig. 1B). The EC
50
of SBE’s contractile effects
on the endothelium-intact and endothelium-denuded aortic rings
were calculated to be 92
±
3 mg/ml and 93
±
5 mg/ml, respec-
tively.
The contractile effects of SBE were partially reversed by
washing out the SBE solutions and then allowing the tissue to
rest for five to 10 minutes (Fig. 1A). The contractile tension
of the endothelium-intact aortic rings in response to SBE was
increased by L-NAME (100
µ
M) (Fig. 1C), but not significantly
(
p
>
0.05), compared with SBE-only treatment.
Pre-incubation of the aortic rings with indomethacin (20
µ
M)
markedly inhibited or abolished the contractile effects of SBE
and this was characterised by a downward shift in baseline tone
(Fig. 1D). No visible change in baseline tone was observed with
SBE in Ca
2+
-free KHS. Pre-incubation of the tissues with vera-
pamil (1–3
µ
g/ml), however, partially inhibited the contractile
effects of SBE in both endothelium-intact and -denuded rabbit
aortic ring preparations (Fig. 1E).
Effect of SBE on rat isolated portal veins
The rat portal veins exhibited spontaneous rhythmic, myogenic
contractions with a mean amplitude of 0.89
±
0.11 g (
n
=
6–8).
SBE raised the baseline tone and subsequently caused marked (
p
<
0.05–0.01) concentration-dependent increases in the amplitude
of the contractions (Fig. 2A, B). The EC
50
concentration of SBE’s
contractile effect on the preparations was calculated to be 36
±
6 mg/ml (
n
=
6–8). The contractile effects of SBE were partially
reversed by washing out the SBE solutions and allowing the
tissue to rest for five to 10 minutes.
In Ca
2+
-free KHS, the portal vein preparations were devoid
of contractions. SBE sequentially added to the bath fluid caused
significant concentration-related increases (
p
<
0.05–0.001) in
the baseline tone, and induced powerful rhythmic contractions
(Fig. 2C). These contractile effects were completely reversed by
washing out the SBE (four to five times) and allowing the venous
tissue to rest for five to 10 minutes.
Pre-incubating the tissue with verapamil (1–3
µ
g/ml)
profoundly attenuated the contractile effects of SBE in a
concentration-dependent manner, and subsequently decreased
the contractile amplitudes of the portal vein preparations,
although an upward shift in baseline tone was still observed (Fig.
2D). Pre-incubation of the isolated venous tissue with prazosin
(1–3
µ
g/ml), on the other hand, did not affect the SBE-induced
contractions (Fig. 2E).
Effect of SBE on reserpinised rat isolated portal veins
Rat isolated portal vein preparations which were taken from
reserpine pre-treated animals (1 mg/kg subcutaneously) exhib-
ited spontaneous, rhythmic, myogenic contractions. SBE sequen-
tially added to the bath fluid caused significant concentration-
dependent increases (
p
<
0.05–0.001) in the baseline tone. The
contractile effects of SBE were partially reversed by washing it
out and allowing the tissue to rest for five to 10 minutes. However,
the contractile effects of SBE on the rat portal veins were not
modified by prior reserpinisation of the animals (Fig. 2F).
Discussion
The findings of this study show that SBE possessed contractile
effects on rabbit isolated aortic ring preparations. Graded concen-
trations of SBE (50–200 mg/ml) markedly (
p
<
0.01–0.001)
contracted freshly mounted, treatment-naïve, endothelium-intact
aortic ring preparations (Fig. 1B) in a concentration-dependent
manner, with an EC
50
value of 92
±
3 mg/ml.
These results contradict earlier observations of Ojewole,
13
in
which an aqueous stem bark extract of
S birrea
produced signifi-
cant concentration-dependent relaxation (
p
<
0.05–0.001) of rat
isolated endothelium-intact aortic ring preparations. This was
attributed to the formation and release of endothelium-derived
nitric oxide by the phytochemicals present in SBE. However, the
present study reports for the first time that SBE induced contrac-
tions in vascular smooth muscles.
The discrepancy in the findings of these two studies could be
due to the differences in the animal species and the morphologi-
cal parts of the plant used. The differences could also be due to
variability in the extracts, considering that not all of the bioactive
components are extractable in water.
In our study, addition of L-NAME to the bath fluid 10
minutes prior to treatment of the rabbit endothelium-intact
aortic ring preparations with SBE increased the SBE-induced
contractile effects (Fig. 1C), compared to SBE-only treat-
ment, but the differences were not statistically significant. The
increase in contractility could be attributed to the inhibitory
effect of L-NAME on nitric oxide (NO) synthesis. The presence
of endothelium-derived NO could have partially reduced the
SBE-induced contractile effects in the endothelium-intact aortic
ring preparations. NO is a potent vasodilator and vasorelaxant,
14,15
and inhibition of its synthesis would result in an increase in the
contractile effects of certain compounds; hence the increase
in SBE-induced vasoconstriction in tissues pre-incubated with
L-NAME.
On the other hand, pre-incubating aortic ring preparations
with indomethacin, a non-selective cyclo-oxygenase (COX)
inhibitor, resulted in a downward shift in baseline tone, suggest-
ing inhibition of the contractile effects of SBE (Fig. 1D). This
observation may indicate that in rabbit isolated aortic ring
preparations, the SBE-induced contractile responses may have
occurred either via the COX-1 or COX-2 pathways, which are
responsible for releasing vasoconstrictors such as prostaglandin
F
2
, prostaglandin E and endothelin-1.
16
Inhibition of the COX
pathway with indomethacin would therefore promote relaxation,
as with NO.
The magnitude of the contractile effects of SBE was slightly
less in the endothelium-denuded aortic rings than in the intact
rings (Fig. 1B). This could be due to the absence of powerful
