Background and purpose: Mitochondrial aldehyde dehydrogenase (ALDH-2) has been shown to provide a pathway for bioactivation of organic nitrates and to be prone to desensitization in response to highly potent, but not to less potent, nitrates. We therefore sought to support the hypothesis that bioactivation by ALDH-2 critically depends on the number of nitrate groups within the nitrovasodilator. Experimental approach: Nitrates with one (PEMN), two (PEDN; GDN), three (PETriN; glyceryl trinitrate, GTN) and four (pentaerithrityl tetranitrate, PETN) nitrate groups were investigated. Vasodilatory potency was measured in isometric tension studies using isolated aortic segments of wild type (WT) and ALDH-2 À/À mice. Activity of the cGMP-dependent kinase-I (reflected by levels of phosphorylated VAsodilator Stimulated Phosphoprotein, P-VASP) was quantified by Western blot analysis, mitochondrial dehydrogenase activity by HPLC. Following incubation of isolated mitochondria with PETN, PETriNchromophore and PEDN, metabolites were quantified using chemiluminescence nitrogen detection and mass spectrometry. Key results: Compared to WT, vasorelaxation in response to PETN, PETriN and GTN was attenuated about 10fold in ALDH-2 À/À mice, identical to WT vessels preincubated with inhibitors of ALDH-2. Reduced vasodilator potency correlated with reduced P-VASP formation and diminished biotransformation of the tetranitrate-and trinitrate-compounds. None of these findings were observed for PEDN, GDN and PEMN. Conclusions and implications: Our results support the crucial role of ALDH-2 in bioactivating highly reactive nitrates like GTN, PETN and PETriN. ALDH-2-mediated relaxation by organic nitrates therefore depends mainly on the number of nitrate groups. Less potent nitrates like PEDN, GDN and PEMN are apparently biotransformed by other pathways.

Background and purpose:The chronic use of organic nitrates is limited by serious side effects including oxidative stress, nitrate tolerance and/or endothelial dysfunction. The side effects and potency of nitroglycerine depend on mitochondrial aldehyde dehydrogenase (ALDH-2). We sought to determine whether this concept can be extended to a new class of organic nitrates with amino moieties (aminoalkyl nitrates). Experimental approach: Vasodilator potency of the organic nitrates, in vitro tolerance and in vivo tolerance (after continuous infusion for 3 days) were assessed in wild-type and ALDH-2 knockout mice by isometric tension studies. Mitochondrial oxidative stress was analysed by L-012-dependent chemiluminescence and protein tyrosine nitration. Key results: Aminoethyl nitrate (AEN) showed an almost similar potency to glyceryl trinitrate (GTN), even though it is only a mononitrate. AEN-dependent vasodilatation was mediated by cGMP and nitric oxide. In contrast to triethanolamine trinitrate (TEAN) and GTN, AEN bioactivation did not depend on ALDH-2 and caused no in vitro tolerance. In vivo treatment with TEAN and GTN, but not with AEN, induced cross-tolerance to acetylcholine (ACh)-dependent and GTN-dependent relaxation. Although all nitrates tested induced tolerance to themselves, only TEAN and GTN significantly increased mitochondrial oxidative stress in vitro and in vivo. Conclusions and implications:The present results demonstrate that not all high potency nitrates are bioactivated by ALDH-2 and that high potency of a given nitrate is not necessarily associated with induction of oxidative stress or nitrate tolerance. Obviously, there are distinct pathways for bioactivation of organic nitrates, which for AEN may involve xanthine oxidoreductase rather than P450 enzymes. (2009) 8-amino-5-chloro-7-phenylpyrido[3,4-d]pyridazine-1,4-(2H,3H)dione sodium salt; L-NAME, N w -nitro-L-arginine methyl ester hydrochloride; NO, nitric oxide; NS2028, 4H-8-bromo-1,2,4-oxadiazolo[3,4-d]benz [b][1,4]oxazin-1-one; PBS, phosphate buffered saline; PETN, pentaerithrityl tetranitrate; PETriN, pentaerithrityl trinitrate; PTIO, 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl 3-oxide; ROS, reactive oxygen species; TEAN, triethanolamine trinitrate British Journal of Pharmacology

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Neither therapeutic dosage of nitrovasodilators nor the development of tolerance correlates with nitrate groups in these molecules. Clinically, low dosages of glyceryl trinitrate (GTN) develop tolerance, but 100-fold higher dosages of pentaerythrityl tetranitrate (PETN) do not. Vasorelaxation was studied on prostaglandian F2alpha (PGF2alpha)-precontracted porcine pulmonary arteries in organ bath procedure. In vitro tolerance was induced by incubating the arteries with different nitrate concentrations and thereafter concentration-response curves were repeated. Furthermore, 14 mg/kg PETN were daily administered to rats by gavage; PETN and metabolites were measured in feces and blood. In vitro, the vasodilator potencies increased from mononitrates to tetranitrates (pD2: 4.14 to 8.18); PETN was the most potent vasodilator. In vitro tolerance was found with PETN and trinitrates but not with dinitrates and mononitrates. Thus, in vitro tolerance correlated with the in vitro potency of nitrates but not with the vasodilator potency of NO donors in general, because S-nitroso-N-aectyl-D-penicillamine and N-phenylpiperazin-NONOate were more potent than GTN but did not induce tolerance. After feeding of rats with PETN, pentaerythrityl dinitrate (PEdiN) and mononitrate (PEmonoN) but neither PETN nor PEtriN (both detected in feces) were found in the blood. The missing systemic bioavailability of PETN and PEtriN may explain the discrepancy between in vitro and in vivo findings. We conclude that the partially denitrated metabolites PEdiN and PEmonoN contribute to the moderate and tolerance-devoid clinical activity of PETN.

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