CARDIOVASCULAR JOURNAL OF AFRICA • Vol 22, No 1, January/February 2011
12
AFRICA
TEA (Fig. 6B and 6D).
These findings also indicate that the mechanisms accounting
for the hypo-responsiveness to agonist-induced contractions and
the relaxing effect of NAC partially involved the activation of
TEA-sensitive K
+
channels.
Discussion
This study provides new insights into the functional interaction
between endothelium-derived NO and endothelium-independent
nitric oxide donors. Our data demonstrated that in rings with
intact endothelium from rat aorta pre-exposed to GSNO, there
was no change in vascular reactivity following the administration
of the contractile agonist norepinephrine, but the low molecu-
lar weight thiol, NAC, exerted a relaxant effect. These results
provide evidence that NO stores can be formed in the presence
of intact endothelium. These data confirm and complete previous
studies from the existing literature in which it was reported that
among NO donors, some of them (S-nitrosating agents) induced
a long-lasting vasorelaxation, especially in endothelium-denuded
arteries.
9,15-17
One of the interesting results reported in this present study
is the hypo-responsiveness observed in vessels treated with the
NO synthase inhibitor L-NAME after GSNO exposure. Indeed,
these results suggest that in vessels with endothelial dysfunc-
tion in which a significant decrease in the production of NO is
observed, it is possible to maintain the vascular tone at normal
levels. Moreover, our results showed that treatment with GSNO
did not alter the relaxations induced by endogenous vasorelaxa-
tion agents such as acetylcholine.
Our results also indicate that the inhibitor of cGMP produc-
tion in smooth muscle, ODQ, enhanced norepinephrine contrac-
tion in control rings with intact endothelium and GSNO-treated
rings from rat aorta, but had no effect in the denuded control
rings. ODQ also abolished NAC-evoked relaxations in GSNO-
treated rings with and without endothelium, similar to that seen
in the controls. Since the attenuation of the contractile response
induced by GSNO and the relaxation effects of NAC were abol-
ished by ODQ, this hypo-reactivity and the associated relaxation
mechanisms involved the NO/cGMP pathway. These results
confirm previous studies that soluble guanylyl cyclase mediates
vasorelaxant activity associated with endogenous NO production
or exogenous sources such as nitric oxide donors.
15,18-20
Other observations support the idea that ODQ is less potent in
inhibiting relaxations by NO, therefore implicating a component
of NO-induced relaxation that is independent of sGC/cGMP.
21
The important endogenous production of NO could therefore
mask the effects of an additional supplementation with nitric
oxide donors under these conditions. Several data indicated that
in rat aorta the NO-synthase inhibitor, L-NAME, completely
abolished the relaxation to acetylcholine,
22,23
suggesting that in
these arteries, the activation of the NO/cGMP pathway appears
to be more relevant than the relaxing pathway.
It may also suggest that GSNO could desensitise the sGC,
as mentioned in the work of Sayed
et al
.
24,25
Indeed, exposure to
nitric oxide and to S-nitrosothiols caused S-nitrosylation of sGC,
which directly desensitised sGC to stimulation by nitric oxide.
They also reported that S-nitrosylation and desensitisation were
prevented by treatment with N-acetyl-cysteine (NAC), a precur-
sor of glutathione, clinically used to prevent development of
nitrate tolerance.
24
In another article, the same authors reasoned
that sGC, as the main or even the sole receptor activated by NO,
could be targeted by S-nitrosylation to induce its desensitisation,
thus constituting an exquisite process of sGC modulation by
negative feedback.
25
They tested the hypothesis that sGC desensitisation was
induced by S-nitrosylation and showed that (i) the NO-stimulated
activity of semipurified sGC was reduced by pre-treatment with
GSNO and correlated with its S-nitrosylation; (ii) NO-stimulated
activity was reduced by ~ 50% with 50
µ
M GSNO pre-treatment,
compared with GSH.
25
Our results also indicate that the unselective K
+
channel
blocker TEA caused a large increase in sensitivity of the rings
to NE without modifying the maximal response to the agonist
in rings denuded of endothelium. In a previous work, Terluk
et
al
. had already reported that the non-specific potassium channel
blockers TEA and charybdotoxin, a Ca
2+
-activated K
+
channel
blocker, inhibited the hypo-responsiveness to phenylephrine
induced by the NO donors.
15
By contrast, 4-aminopyridine, an inhibitor of voltage-gated
potassium channels, and glibenclamide, which specifically
blocks ATP-sensitive K
+
channels, had no effect. Furthermore,
opening of potassium channels, more specifically the calcium-
activated subtype, plays a predominant role in this NO-induced
hypo-responsiveness to phenylephrine in the rat aorta.
15
Finally, it
was also established that incubation of NO donors with rat aortic
rings induced substantial reduction in phenylephrine-induced
contractions. This effect was long lasting and involved calcium-
dependent potassium channels.
26
Some reports indicate that in small vessels such as the
mesenteric arteries, the contribution of voltage-dependent and
Ca
+
-activated large-conductance K
+
channels appears to be
more relevant than that of the NO/cGMP pathway.
27,28
Other
data also indicate that NO and its donors can directly stimulate
BK(Ca) activity in cells isolated from the rat mesenteric artery.
This ability of NO to directly open BK(Ca) channels could
play an important functional role in NO-induced relaxation of
the vascular smooth muscle cells in this low-resistance artery.
29
S-nitrosothiols can also activate SK(Ca) + IK(Ca) channels.
Since S-nitrosothiols decompose to NO, stored S-nitrosothiols
may mediate bradykinin-induced, EDHF-dependent relaxation.
30
Moreover, soluble guanylate cyclase activation and opening
of potassium channels play an important role in NO-induced
hypo-responsiveness to norepinephrine in the rat aorta.
Conclusion
The present study demonstrated that formation of releasable NO
stores remained effective in rings with a functional endothelium,
but endothelium-derived NO blunted the hypo-responsiveness
to GSNO-induced vasorelaxation. Not only can GSNO simulate
NO-induced hypo-responsiveness to contractile agonists without
modifying the basal tone in isolated, intact vessel, but this model
could also be particularly useful to prevent endothelial dysfunc-
tion when vascular failure could be imminent.
This work was partially supported by a grant from Fondation de France.
References
1. Taddei S, Virdis A, Ghiadoni L, Sudano I, Salvetti A. Endothelial
dysfunction in hypertension.
J Cardiovasc Pharmacol
2001;
38
(Suppl
2): S11–14.
