Antagonistic action of imidazolineoxyl N-oxides against endothelium-derived relaxing factor/.NO through a radical reaction.
A labile inorganic free radical, nitric oxide (.NO), is produced by nitric oxide synthase from the substrate L-arginine in various cells and tissues. It acts as an endothelium-derived relaxing factor (EDRF) or as a neurotransmitter in vivo. We investigated the reactivity of stable radical compounds, imidazolineoxyl N-oxides such as 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide (PTIO), carboxy-PTIO, and carboxymethoxy-PTIO against .NO/EDRF in both chemical and biological systems. By using electron spin resonance (ESR) spectroscopy, imidazolineoxyl N-oxides were found to react with .NO in a stoichiometric manner (PTIO/.NO = 1.0) in a neutral solution (sodium phosphate buffer, pH 7.4) with rate constants of approximately 10(4) M-1 s-1, resulting in the generation of NO2-/NO3- and imidazolineoxyls such as 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl (PTI), carboxy-PTI, or carboxymethoxy-PTI. Furthermore, the effects of imidazolineoxyl N-oxides on acetylcholine- or ATP-induced relaxation of the smooth muscle of rabbit aorta were tested. The vasorelaxations were inhibited by all three imidazolineoxyl N-oxides markedly. The inhibitory effects of carboxy-PTIO was almost 2-fold stronger than those of .NO synthesis inhibitors, N omega-nitro-L-arginine and N omega-monomethyl-L-arginine. Generation of EDRF/.NO was identified by reacting the PTIO in aortic strips and quantitating the reaction product with ESR spectroscopy. Thus, it was clarified that imidazolineoxyl N-oxide antagonize EDRF/.NO via a unique radical-radical reaction with .NO.
Akaike
T
T
Department of Microbiology, Kumamoto University School of Medicine, Japan.
Yoshida
M
M
Miyamoto
Y
Y
Sato
K
K
Kohno
M
M
Sasamoto
K
K
Miyazaki
K
K
Ueda
S
S
Maeda
H
H
D000109:Acetylcholine / Q000494:pharmacology; D000818:Animals; D001011:Aorta / Q000187:drug effects*; D001565:Benzoates / Q000138:chemical synthesis / Q000494:pharmacology; D003497:Cyclic N-Oxides / Q000138:chemical synthesis / Q000378:metabolism / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005260:Female; D005609:Free Radicals; D007093:Imidazoles / Q000138:chemical synthesis / Q000737:chemistry / Q000378:metabolism / Q000494:pharmacology; D009126:Muscle Relaxation / Q000187:drug effects*; D009131:Muscle, Smooth, Vascular / Q000187:drug effects*; D009569:Nitric Oxide / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D011817:Rabbits
D001565:Benzoates; D003497:Cyclic N-Oxides; D005609:Free Radicals; D007093:Imidazoles; C079393:1,3-dihydroxy-4,4,5,5-tetramethyl-2-(4-carboxyphenyl)tetrahydroimidazole; C079391:2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide; C079392:2-phenyl 4,4,5,5-tetramethylimidazoline-1-oxyl; D009569:Nitric Oxide; C029899:imidazole; D000109:Acetylcholine