1,721,111 research outputs found
Biotransformation to nitric oxide of organic nitrates in comparison to other nitrovasodilators
No full textNitrovasodilators are prodrugs which, although chemically heterogenous, exert their pharmacodynamic action via a common pathway, i.e. the release of nitric oxide (NO). The NO, which results from metabolism of nitrovasodilators in vascular and non-vascular cells, stimulates the cytosolic enzyme guanylyl cyclase leading to an increase in the concentration of intracellular cyclic guanosine monophosphate (cGMP). In general, the rate of NO generation from the individual compounds correlates well with the extent of cGMP increase and their potency to relax vascular tissue. The amounts of NO generated are sufficient to inhibit platelet aggregation and to induce disaggregation. Nitrovasodilators thus mimic the action of endothelium-derived relaxing factor (EDRF). After more than a century of empiric use, the application of nitrovasodilators today may be regarded as causal therapy, since these drugs act by substituting an endogenous factor, the production or release of which is impaired under pathophysiological circumstances associated with endothelial dysfunction. Marked differences exist between individual compound classes with regard to bioactivation mechanisms, cofactor requirements, and the extent and nature of the concomittant formation of metabolites other than NO. This review describes the discovery of the mode of action of nitrovasodilators and our current understanding of the pathways involved in their bioactivation and biodegradation with special emphasis on the enzymatic and non-enzymatic metabolism of organic nitrates. In addition, the in-vivo metabolism of NO is reviewed briefl
[Nitrogen monoxide (NO)- the active principle of organic nitrates]
No full textAlthough organic nitrates have been used in the treatment of patients with angina pectoris for more than 100 years, their mechanism of action was only disclosed during the last years. In the seventies it became already clear that nitrates act via the intracellular messenger cyclic 3,5'-guanosine-monophosphate (cGMP). Later on, S-nitrosothiols were suggested as possible intermediary messengers arising during metabolism of the nitrates. In parallel with the discovery of the endothelium-derived relaxing factor (EDRF) and its biochemical identification as nitric oxide (NO), it became clear that organic nitrates act via the release of NO in the vascular wall and thus by using metabolic pathways identical to those of endogenous EDRF. The target-enzyme for nitrates or for the NO released by them, respectively, thus is the soluble guanylylcyclase. The rate of enzymic stimulation induced by a given nitrate correlates closely with the rate of measured NO production from the nitrate molecule. The highest NO production was detected with nitroglycerin, followed by the group of dinitrates and mononitrates. In the vessel wall both endothelial cells as well as vascular smooth muscle cells can transform nitrates into NO. This might explain, why the antiaggregatory effect of nitrates is more pronounced in the presence of these cells or in vivo than it is in the absence of vascular cells in vitro. In spite of some differences in metabolism, nitrates are thus closely related by their end-product to the endothelium-derived relaxing factor and represent, therefore, an adequate substitution for NO missing in diseased blood vessels
Bioassay discrimination between nitric oxide (NO) and nitroxyl (NO-) using L-cysteine
No full textNitroxyl (NO-) is the one-electron reduction product of nitric oxide (NO.). Recently, NO- generating compounds were shown to possess potent vasorelaxant activity and this was attributed to the ready conversion of NO- to NO.. Because of its metastable character, direct chemical detection of NO- or its conjugated acid, HNO, has not been accomplished yet. In order to gain further insight into the cellular mode of action of NO- generating compounds we aimed at finding a means to discriminate NO- from NO. by bioassay. Using isolated rat aortic rings in organ baths, we here show that high concentrations of L-cysteine cause complete inhibition of the vasorelaxant response to NO- (generated from Angeli's salt and sodium nitroxyl) whereas responses to authentic NO. and S-nitrosocysteine are largely enhanced. Preliminary results indicate that the inhibition by L-cysteine of NO- activity may be mediated in part by enzymatic and non-enzymatic mechanisms. Whether or not NO- generating compounds will have promising therapeutic potential as a new class of NO.- donors will not least depend on their interference with enzymatic routes susceptible to inhibition by NO-
Human endothelial cells bioactivate organic nitrates to nitric oxide: implications for the reinforcement of endothelial defence mechanisms
No full textAlthough in therapeutic use for more than a century, the mode of cellular action of organic nitrates remains incompletely understood. Despite ample experimental evidence from animal studies to show that nitrates are metabolized to NO in the vascular smooth muscle, direct demonstration of such an activity in human vascular cells is still lacking. Moreover, the role of the endothelium in modulating the pharmacodynamic action of nitrates is far from clear. We therefore aimed to investigate whether or not human endothelial cells are capable of bioactivating these drugs to NO and whether the amounts generated are sufficient to elicit any biological effects. Using cultured human umbilical vein endothelial cells (HUVECs) as an established model system a combination of three different methods was used to address this issue: (1) quantification of NO formation upon endothelial nitrate metabolism using the oxyhaemoglobin technique; (2) evaluation of the second messenger response using radioimmunoassay for cGMP; and (3) assessment of mechanism and extent of potentiation of the anti-aggregatory effect of nitrates in the presence of endothelial cells as a relevant bioassay. We now show that superfusion of cultured human endothelial cells on microcarrier beads with either glyceryl trinitrate (GTN) or isosorbide dinitrate (ISDN; both at 0.1-100 mumol L-1) results in a concentration-dependent formation of NO. NO generation from isosorbide 5-mononitrate (IS-5-N) was below the detection limit. The amounts of NO produced (maximally 2.97 +/- 0.98 pmoles NO min-1 x mg protein with 100 mumol L-1 GTN; n = 8) were similar to those elicited upon challenge of the cells with 100 nM bradykinin. NO formation from either organic nitrate was accompanied, in a concentration-dependent and methylene blue-inhibitable manner, by stimulation of endothelial soluble guanylyl cyclase with consequent increases in the intracellular level of cGMP (maximally 32-fold over basal levels with ISDN), a significant portion of which was released into the extracellular space. Upon continuous 30 min superfusion or repeated application of high concentrations of GTN (100 mumol L-1) nitrate bioactivation to NO was subject to partial tachyphylaxis. Co-incubation of washed human platelets with HUVECs potentiated the anti-aggregatory action of nitrates in a cell number dependent and oxyhaemoglobin-sensitive manner and this effect, too, was accompanied by increases in intraplatelet cGMP levels. The potentiating effect was largely inhibited after blockade of sulfhydryl groups by pre-incubation of HUVECs with N-ethylmaleimide and completely abrogated after pretreatment of cells with the tissue fixative glutaraldehyde. These results demonstrate that human endothelial cells are capable of bioactivating organic nitrates to NO by an enzymatic, apparently thiol-sensitive pathway, in quantities sufficient to influence endothelial and platelet function. Besides the well known vasorelaxant action of organic nitrates, which is mainly due to their metabolism in the smooth muscle compartment, these drugs may therefore be endowed with a hitherto underestimated potential to directly influence endothelial functions via the NO/cGMP pathway. Through specific bioactivation in the endothelium itself organic nitrates can thus mimic and reinforce protective functions normally served by a functional endothelium such as the modulation of blood cell/vessel wall interactions and inhibition of cell proliferation
Vasodilator effects of PGE1 in the coronary and systemic circulation of the rat are mediated by ATP-sensitive potassium (K+) channels
Full text linkThis study was undertaken to investigate the possible involvement of K+ channels in PGE1-mediated vasodilatation. The increase in coronary flow elicited by PGE1 in isolated working rat hearts was attenuated by phentolamine and glibenclamide, inhibitors of ATP-regulated K+ channels, whereas apamin and charybdotoxin, inhibitors of calcium-activated K+ channels, were ineffective. In the anaesthetized rat, the duration of the hypotensive action of PGE1 was markedly attenuated by glibenclamide. It is concluded that the vasodilatory action of PGE1 in the coronary and systemic circulation of the rat is, at least in part, mediated via an opening of ATP-sensitive K+ channels
Oxidative release of nitric oxide accounts for guanylyl cyclase stimulating, vasodilator and anti-platelet activity of Piloty's acid: a comparison with Angeli's salt
No full textThe decomposition of benzenesulphohydroxamic acid (Piloty's acid; PA) and some of its derivatives has been reported to yield nitroxyl ions (NO-), a species with potent vasodilator properties. In a previous study we demonstrated that the oxidative breakdown of PA results in the formation of nitric oxide (NO) and suggested that NO rather than NO- may account for its vasorelaxant properties. Using isolated aortic rings in organ baths, we now show that high concentrations of cysteine potentiate the vasorelaxant response to PA, whereas responses to Angeli's salt (AS), a known generator of NO-, were almost completely inhibited. These different behaviours of PA and AS are mirrored by their distinct chemistries. By using HPLC it was shown that, at physiological pH and in the absence of oxidizing conditions, PA is a relatively stable compound. Direct chemical determination of NO, stimulation of soluble guanylyl cyclase, and measurement of platelet aggregation under various experimental conditions confirmed the requirement for oxidation to release NO from PA, and quite weak oxidants were found to be sufficient to promote this reaction. In contrast, at pH 7.4 AS decomposed rapidly to yield nitrite (NO2-) and NO-, bu did not produce NO on reaction with dioxygen (O2) or hydrogen peroxide (H2O2). Thus sulphohydroxamic acids are a new class of thiol-independent NO-donors that generate NO rather than NO- under physiological conditions
Dietary nitrate increases arginine availability and protects mitochondrial complex I and energetics in the hypoxic rat heart
Full text linkHypoxic exposure is associated with impaired cardiac energetics in humans and altered mitochondrial function, with suppressed complex I-supported respiration, in rat heart. This response might limit reactive oxygen species generation, but at the cost of impaired electron transport chain (ETC) activity. Dietary nitrate supplementation improves mitochondrial efficiency and can promote tissue oxygenation by enhancing blood flow. We therefore hypothesised that ETC dysfunction, impaired energetics and oxidative damage in the hearts of rats exposed to chronic hypoxia could be alleviated by sustained administration of a moderate dose of dietary nitrate. Male Wistar rats (n = 40) were given water supplemented with 0.7 mmol l(-1) NaCl (as control) or 0.7 mmol l(-1) NaNO3, elevating plasma nitrate levels by 80%, and were exposed to 13% O2 (hypoxia) or normoxia (n = 10 per group) for 14 days. Respiration rates, ETC protein levels, mitochondrial density, ATP content and protein carbonylation were measured in cardiac muscle. Complex I respiration rates and protein levels were 33% lower in hypoxic/NaCl rats compared with normoxic/NaCl controls. Protein carbonylation was 65% higher in hearts of hypoxic rats compared with controls, indicating increased oxidative stress, whilst ATP levels were 62% lower. Respiration rates, complex I protein and activity, protein carbonylation and ATP levels were all fully protected in the hearts of nitrate-supplemented hypoxic rats. Both in normoxia and hypoxia, dietary nitrate suppressed cardiac arginase expression and activity and markedly elevated cardiac l-arginine concentrations, unmasking a novel mechanism of action by which nitrate enhances tissue NO bioavailability. Dietary nitrate therefore alleviates metabolic abnormalities in the hypoxic heart, improving myocardial energetics
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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