CARDIOVASCULAR JOURNAL OF AFRICA • Vol 22, No 1, January/February 2011
AFRICA
11
was similar to the controls (Fig. 5B, D). These findings indicate
that the mechanisms accounting for the hypo-responsiveness to
agonist-induced contractions and also the relaxing effect of NAC
involved activation of the soluble guanylyl cyclase.
Inhibition of the TEA-sensitive K
+
channels
Aorta segments pre-exposed to GSNO were incubated with K
+
channel blockers 30 min before the administration of the cumu-
lative dose of norepinephrine. When the contractile response
reached a plateau, cumulative concentration-effect curves for
NAC were created. Our results show that TEA (10 mM, 30
min) had no affect on the GSNO-induced loss in vessel reactiv-
ity but increased the sensitivity of control and GSNO-exposed
rings without endothelium (Fig. 6A, Table 1). Similar findings
were observed with control and GSNO-exposed rings with
intact endothelium (Fig. 6C, Table 1). The effects of NAC were
partially inhibited in GSNO-treated rings pre-incubated with
TABLE 1. CONTRACTILE EFFECT OF NOREPINEPHRINE EXPRESSED IN GRAMS OF DEVELOPED TENSION (E-MAX) AND
EC
50
VALUES, DETERMINED BY LOG-LOGIT REGRESSION INAORTAS PRE-EXPOSED TO GSNOAND INHIBITORS
Control
GSNO
ODQ
GSNO/ODQ
TEA
GSNO/TEA
Without endothelium
E-max (g)
2.46
±
0.082
1.93
±
0.074*
2.74
±
0.16
ns
2.66
±
0.045
ns
2.12
±
0.162
ns
1.76
±
0.123*
EC
50
(nM)
5.31
±
1.392
30.55
±
4.963*** 3.74
±
0.915*
4.30
±
1.464
ns
0.59
±
0.063** 6.77
±
0.154
ns
With endothelium
E-max (g)
2.04
±
0.184
2.07
±
0.123
ns
2.62
±
0.178
ns
2.51
±
0.094
ns
2.42
±
0.093
ns
2.39
±
0.179
ns
EC
50
(nM)
15.07
±
1.077
46.64
±
2.035** 2.48
±
0.086** 4.04
±
0.041** 5.20
±
0.144** 14.10
±
0.131
ns
Data are means
±
SEM of five experiments; ns: not significant; *
p
<
0.05; **
p
<
0.01 ***
p
<
0.001 in comparison with respective controls.
Fig. 6. The GSNO-induced hypo-reactivity and associated relaxation mechanisms partially involved the activation
of TEA-sensitive K
+
channels. (A) and (C) established that the unselective K
+
channel blocker TEA (10 mM, 30 min)
caused a large increase in sensitivity to NE of both controls (not exposed) and endothelium-denuded or intact rings
pre-exposed to GSNO. (B) and (D) indicate that the effects of NAC were partially inhibited by TEA on GSNO-treated
denuded or intact rings. Results are means
±
SEM of five experiments; ns: not significant; *
p
<
0.05; **
p
<
0.01 in
comparison to respective controls.
A
–9.5 –9.0 –8.5 –8.0 –7.5 –7.0 –6.5
log [NE], M
2.5
2.0
1.5
1.0
0.5
0.0
Contraction (g)
Without endothelium
C
D
–9.5 –9.0 –8.5 –8.0 –7.5 –7.0 –6.5
log [NE], M
0
20
40
60
80
–4
–3
–2
Relaxation (%)
log [NAC], M
2.5
2.0
1.5
1.0
0.5
0.0
Contraction (g)
With endothelium
CTL
TEA
GSNO
GSNO/TEA
ns
ns
B 0
20
40
60
80
–4
–3
–2
Relaxation (%)
log [NAC], M
Control
GSNO
ns
