Selective Inhibition of Inducible Nitric Oxide Synthase by Agmatine

Abstract

ABSTRACT-Agmatine was about as potent as aminoguanidine to inhibit the activity of the inducible form of nitric oxide synthase (iNOS) in isolated rat aorta. Like aminoguanidine, agmatine was devoid of significant activity on the constitutive form of NOS. Agmatine inhibited the conversion of [3H]L-arginine in [3H]L-citrulline in partially purified iNOS from macrophages (IC50=262±39.9 pM). Thus, our data suggest that agmatine may act as endogenous inhibitor of iNOS.Keywords: Agmatine, Nitric oxide, Inducible enzyme L-Arginine is a nutritionally semiessential amino acid involved in a variety of physiological processes (1). Arginine plays a role in protein and creatine biosynthesis and, as an intermediate in the urea cycle, is converted to ornithine by arginase. However, other enzymes that catalyze the conversion of arginine have been identified. In this respect, nitric oxide synthase (NOS) produces the potent vasodilator nitric oxide (NO) (2, 3), and arginine decarboxylase converts arginine to agmatine (4). This latter enzyme, orginally observed in bacteria, has been recently found in mammalian brain (5). In addition, agmatine was identified as an endogenous clonidine-displacing substance (5). Thus, it was hypothesized that arginine may have an antihypertensive effect through NO and agmatine formation by NOS and arginine decarboxylase, respectively (6). However, Pinthong et al. (7) showed that despite recognition at a2-adrenoceptor binding sites, agmatine failed to produce functional a2-adrenoceptor activity not only in the isolated guinea pig ileum but also in other functional models of a2-adrenoceptors. In this respect, agmatine failed to mimic the effect of clonidine in isolated rat vas deferens or isolated porcine palmar lateral vein. In addition, in rat cerebral cortex, agmatine produced a concentrationdependent inhibition of 3H-clonidine, but failed, unlike UK-14304, to inhibit forskolin-stimulated cyclic AMP. Thus collectively, the results indicate that agmatine may not posses sufficient a2-adrenoceptor agonist properties to influence cardiovascular function as recently observed (8). Although a possible role of agmatine as a substrate for NOS has been ruled out (9), other guanidines such as aminoguanidine may act against NO formation (10).Thus, the aim of this study was to test the possible interaction of agmatine with NO synthesis. Thoracic aortae were excised from male Sprague Dawley rats (270-360g;Charles River, Paris, France). Rings, 2-mm-wide, were suspended (2 g tension) in organ baths containing 20 ml of Krebs-Henseleit solution (for composition see below) at 371C and gassed with 95% 02 /5% CO2. After a 1-hr rest period, contractile responses were measured using force-displacement transducers connected to a data collection system (IOS; Dei Leirre, Mitry Mory, France). Physiological solution was composed of: 118 mM NaCl, 4.7 mM KC1, 2.5 mM CaC12, 1.2 mM KH2PO4, 1.2 mM MgSO4, 25 mM NaHCO3 and 11 mM glucose.In a first series of experiments, rings possessing a functional endothelium were precontracted wit...

Agmatine Suppresses Nitric Oxide Production and Attenuates Hypoxic-Ischemic Brain Injury in Neonatal Rats

“…In the present study, HI rat pups showed elevations in NO metabolite concentrations in the brain during reperfusion 6 h after hypoxia, and agmatine treatment (100 mg/kg) eliminated this increase in NO metabolites. The results indicate that agmatine suppresses NO production after hypoxia in vivo as is known to occur in vitro (22)(23)(24)(25). Reduction of NO generation may therefore be one mechanism of neuroprotection by agmatine.…”

“…Agmatine also inhibits all isoforms of NOS (22)(23)(24)(25), with highest reported activity (Ki ϭ 29 M) as an irreversible inactivator of neuronal NOS (25). Treatments with agmatine are neuroprotective in vitro (21,22), as well as in gerbil ischemic brain injury, adult rat ischemic brain injury (26), and adult rat spinal cord injury (16,27,28). The effects of agmatine in neonatal HI brain injury have not previously been tested, nor has correlation been studied between the neuroprotective properties of agmatine and NO production in the brain.…”

Effect of agmatine on the blood system parameters of rats under the condition of experimental diabetes mellitus

“…Агматин в організмі діє як нейротрансмітер і нейромодулятор, є ендогенним лігандом для імідазольних рецепторів (І 1 /І 2 ) і, з меншою афінністю, для α 2 -адренорецепторів, N-метил-D-аспартат і серотоніно-вих рецепторів [15]. Як аналог L-аргініну, за рахунок наявності гуанідинової групи, цей поліамін також виконує роль конкурентного інгібітора NO-синтази [8]. Одним із біологічних ефектів агматину в організмі, що реалізуються рецептор-опосередко-ваним шляхом, є здатність впливати на метаболізм глюкози за умов її підвищеної концентрації [12].…”

Agmatine Suppresses Nitric Oxide Production and Attenuates Hypoxic-Ischemic Brain Injury in Neonatal Rats

“…Agmatine also inhibits all isoforms of NOS (22-25), with highest reported activity (Ki ϭ 29 M) as an irreversible inactivator of neuronal NOS (25). Treatments with agmatine are neuroprotective in vitro (21,22), as well as in gerbil ischemic brain injury, adult rat ischemic brain injury (26), and adult rat spinal cord injury (16,27,28). The effects of agmatine in neonatal HI brain injury have not previously been tested, nor has correlation been studied between the neuroprotective properties of agmatine and NO production in the brain.…”