CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 1, February 2012
AFRICA
15
vasoconstrictors such as endothelins, prostanoids and prosta-
glandins, which are normally expressed in intact, functional
endothelium.
The ability of verapamil, an L-type calcium channel blocker,
to partially inhibit the SBE-induced contraction of the rabbit
aortic ring preparations may indicate involvement of calcium
ions in the contractile responses (Fig. 1E). This suggests that
SBE may have caused membrane depolarisation, thus facilitating
mobilisation of Ca
2+
from the extracellular fluid into the muscle
cells via the L-type voltage-gated calcium channels.
Alternatively, SBE could have promoted the accumulation of
intracellular Ca
2+
by inhibition of the Na
+
/Ca
2+
exchangers or the
Na
+
/K
+
channels, which remove calcium from the cells. Ca
2+
-free
treatment of the rabbit aortic ring preparations, however, had
no visible effect when SBE was added to the bath fluid. This
suggests the absence of SBE-induced intracellular Ca
2+
release
in the rabbit preparations, perhaps due to diminished influx of
extracellular calcium.
The findings of the present study indicated that SBE also
caused contractile effects on rat isolated portal veins. Graded
concentrations of SBE raised the basal tone, and subsequently
caused marked concentration-dependent increases in the ampli-
tudes of the myogenic contractions of the venous tissue, with an
EC
50
value of 36
±
6 mg/ml (Fig. 2B, C). SBE-induced contrac-
tions were not modified by pre-incubation of the tissues with
prazosin, suggesting that the contractile responses of the venous
Fig. 1. Effects of SBE on rabbit isolated aortic ring
preparations. A: SBE (200 mg/ml) was sequentially added
to the bath fluid and then washed out four to five
times. B: Contractile response to graded doses of SBE
on endothelium-intact and endothelium-denuded rabbit
isolated aortic ring preparations, respectively (
n
=
6–8 for
each data point). **
p
<
0.01; ***
p
<
0.001 for endothelium-
denuded rings versus control.
φφ
p
<
0.01;
φφφ
p
<
0.001
for endothelium-intact rings versus control (zero). C:
Effect of L-NAME (100
µ
M) on SBE-induced (100 mg/ml)
contraction of rabbit isolated endothelium-intact aortic
ring preparations (
n
=
6–8 for each data point). D: Effects
of endomethacin (20
µ
M) on SBE-induced (200 mg/ml)
contractions on endothelium-intact rabbit isolated aortic
ring preparations. E: Effects of verapamil (2
µ
g/ml) on
SBE-induced (200 mg/ml) contractions on endothelium-
intact and -denuded rabbit isolated aortic ring prepara-
tions, respectively.
A
B
C
D
E
0.6
0.4
0.2
0.0
Contractile tension (g)
SBE concentration (mg/ml)
***
endothelium-denuded
aortic rings
endothelium-intact
aortic rings
0
50
100
150
200
250
**
0.5
0.4
0.3
0.2
0.1
0.0
Contractile tension (g)
SBE
L-NAME + SBE
Verapamil (2
µ
g/ml)
