Antagonistic action of imidazolineoxyl N-oxides against endothelium-derived relaxing factor/.bul.NO (nitric oxide) through a radical reaction

Abstract

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.

Role of GABA receptors in nitric oxide inhibition of dorsolateral periaqueductal gray neurons

“…These data suggest that NO increases synaptic GABA release in the PAG, and that the site of action of NO is most likely to locate at presynaptic GABAergic terminals (Sulzer and Pothos, 2000). Furthermore, the effects of SNAP and L-arginine on mIPSCs of the dl-PAG were completely eliminated by the specific NO scavenger carboxy-PTIO (Akaike et al, 1993) and the specific nNOS inhibitor TRIM (Handy et al, 1995), respectively (Fig. 1&2).…”

β‐Carotene effectively scavenges toxic nitrogen oxides: nitrogen dioxide and peroxynitrous acid

“…It is known that NO specifically reduces PTIO into a different nitroxide radical, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl (PTI), and the conversion of the ESR signals of PTIO into those of PTI indicates generation of NO [15]. Thus, N0 2 (40 nmol) in air was introduced into chloroform containing 40 nmol PTIO and 7.2 nmol ß-carotene.…”

Central Infusion of L-Arginine or Superoxide Dismutase Does Not Alter Arterial Pressure in SHR.

“…Furthermore, intravenous or intraperitoneal injection and oral treatment of tempol reduces arterial pressure in SHR (15,25). Although the radical form of NO can react with tempol and, as a result, lose its physiological activity (26), tempol can restore NO-dependent relaxation in the presence of oxidant stress (27). From these results, we hypothesized that the central infusion of tempol could inhibit the degeneration of NO by superoxide, thereby preserving the inhibitory action of NO in the central nervous system, and, as a consequence, reducing blood pressure in SHR.…”