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Vol. 292, Issue 2, 737-742, February 2000
Department of Pharmaceutics, School of Pharmacy, University at
Buffalo, State University of New York, Buffalo, New York.
Nitric oxide (NO) donors are believed to exert their vasodilatory
action through the activation of soluble guanylate cyclase (sGC), the
heme site of which can be specifically inhibited by 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one
(ODQ). We examined the vascular relaxation of the rat aorta mediated by
eight different NO donors in the presence of ODQ (0.1, 1, or 10 µM),
and demonstrated that these NO donors displayed different sensitivities
toward ODQ inhibition (ANOVA, P < .05). Among the
NO donors studied, S-nitrosothiols such as
S-nitroso-N-acetylpenicillamine (SNAP) and S-nitrosoglutathione exhibited partial resistance
toward ODQ inhibition at 0.1 µM ODQ, whereas nitroglycerin (NTG)
showed nearly complete inhibition at this concentration of ODQ. Three
NO donors representing increasing sensitivity toward ODQ inhibition,
SNAP < sodium nitroprusside (SNP) < NTG, were chosen for
additional mechanistic studies. ODQ (1 µM) inhibition of vascular
relaxation by SNAP and SNP, but not that by NTG, was partially reversed
by a sulfhydryl donor, N-acetylpenicillamine (100 µM),
and by a phosphodiesterase inhibitor, zaprinast (10 µM), specific for
cGMP. Our results strongly indicate that the vascular relaxation
mechanism(s) of NO donors is not identical for each. In the rat
aorta, NTG appeared to exhibit its vasodilatory effect exclusively
through activation of the heme site of sGC. On the other hand, in the
intact vascular tissue, SNAP and SNP could bring about vasodilation
through a secondary pathway. These results are consistent with the view
that SNAP and SNP, but not NTG, can induce vascular relaxation
additionally through the activation of the sulfhydryl site of
sGC.
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