Absence Of NO Research Articles

Assessment of the low-temperature EnviNOx® variant for catalytic N2O abatement over steam-activated FeZSM-5

Remarkable progress towards implementation of catalytic technology for N2O mitigation in nitric acid plants has been experienced in recent years. EnviNOx®, one of the commercial processes developed by Uhde, accomplishes the tail-gas abatement of N2O and NOx over a single reactor using iron-containing zeolite catalysts. Two variants of the process are available depending on the tail-gas temperature. This manuscript evaluates the low-temperature abatement process (<700K), where NOx and N2O are selectively reduced by NH3 and CH4, respectively. To this end, activity tests in mixtures with N2O, O2, NO, NH3, and CH4 at different temperatures and partial pressures of reactants were carried out over steam-activated FeZSM-5. Ammonia and nitric oxide strongly inhibit the selective catalytic reduction (SCR) of N2O by CH4, shifting the temperature for N2O conversion over 700K. Controlled ammonia dosing is required for the efficient operation of the low-temperature EnviNOx® variant over the particular iron zeolite used in this study. The optimal inlet NH3 concentration is determined by the NO concentration in the tail-gas and its temperature. In the absence of NO and NH3, the CH4-SCR of N2O over FeZSM-5 is effective above 623K. Still, this process cannot retrofit a number of existing plants with lower tail-gas temperatures.

NO2 Reduction with Nitromethane over Ag/Y: A Catalyst with High Activity over a Wide Temperature Range

Nitromethane (NM) is a very efficient reductant for converting NO2 to N2 over Ag/Y: Between 140 °C and 400 °C, the N2 yield is close to 100%. This high N2 yield results from the ability of Ag/Y to effectively catalyze the reaction between NM and NO2. This high catalytic activity of Ag/Y is minimally affected by surface bound CN−, NC−, or acetate, all of which are stable at temperatures below ∼300 °C. At T ≥ 400 °C, there is a reaction path that yields N2 from NM even in the absence of NO2. However even at 400 °C, under typical deNOx conditions, most N2 molecules are formed as a result of the reaction of NM and NO2.

Inactivation of inducible nitric oxide synthase protects intestinal pacemaker cells from postoperative damage

Abdominal surgery causes postoperative gastrointestinal dysmotility which can progress to paralytic ileus. Surgery causes inflammatory responses leading to loss of interstitial cells of Cajal (ICC), which generate intestinal pacemaker activity. Here, we demonstrate that a deficiency in or pharmacological inhibition of inducible nitric oxide synthase (iNOS) before surgery protects ICC from postoperative damage. Ileal segments from wild-type, iNOS and cyclooxygenase-2 (COX-2) knockout mice were resected and reconstructions were performed by end-to-end anastomoses. Wild-type animals were exposed to iNOS inhibitors before surgery, and electrical activity and ICC were examined 5 h after surgery. Intestinal surgery on wild-type mice caused a significant reduction in ICC and pacemaking at distances up to 5 cm from the anastomosis site. ICC networks and pacemaking were protected in iNOS(-/-) mice. In animals treated preoperatively with iNOS inhibitors, pacemaker activity was depressed only at the anastomosis site. COX-2 deficiency also muted postoperative disruption in pacemaker activity. Postoperative surgical damage consists of a local response and a more widespread response in which ICC and pacemaker activity are disrupted. Damage to ICC and pacemaking was greatly attenuated in the absence of NO derived from iNOS. Thus, management of iNOS expression or activity prior to intestinal surgery protects against postsurgical dysmotility and reduces the severity of postoperative ileus.

The effect of nitrate, Fe(III) and bicarbonate on the degradation of bisphenol A by simulated solar UV-irradiation

The photoinitiated degradation of bisphenol A (BPA, 520 μ mol / L ) was investigated using a solar simulator in the absence/presence of NO 3 - , Fe(III), and HCO 3 - . The concentrations of NO 3 - , Fe(III), and HCO 3 - were 0–160, 0–10, and 0 – 820 μ mol / L , respectively, and were chosen to simulate a natural aquatic environment. The experimental region was explored using a Box–Behnken design for three factors, extended to experimentally include all eight possible combinations of presence/absence of the factors studied. The results show that, after 7 h of irradiation, photolysis occurs only to a minimal degree (2%) in the absence of NO 3 - and HCO 3 - . Increasing the concentration of NO 3 - and HCO 3 - gives rise to up to 24% degradation after 7 h of irradiation. The concentration of Fe(III) was found to play no active role under the conditions studied. A simple linear model is given that very well describes the results obtained.

Isotopic Analysis of N and O in Nitrite and Nitrate by Sequential Selective Bacterial Reduction to N2O

Nitrite is an important intermediate species in the biogeochemical cycling of nitrogen, but its role in natural aquatic systems is poorly understood. Isotopic data can be used to study the sources and transformations of NO2- in the environment, but methods for independent isotopic analyses of NO2- in the presence of other N species are still new and evolving. This study demonstrates that isotopic analyses of N and O in NO2- can be done by treating whole freshwater or saltwater samples with the denitrifying bacterium Stenotrophomonas nitritireducens, which selectively reduces NO2- to N2O for isotope ratio mass spectrometry. When calibrated with solutions containing NO2- with known isotopic compositions determined independently, reproducible delta15N and delta18O values were obtained at both natural-abundance levels (+/-0.2-0.5 per thousand for delta15N and +/-0.4-1.0 per thousand for delta18O) and moderately enriched 15N tracer levels (+/-20-50 per thousand for delta15N near 5000 per thousand) for 5-20 nmol of NO2- (1-20 micromol/L in 1-5 mL aliquots). This method is highly selective for NO2- and was used for mixed samples containing both NO2- and NO3- with little or no measurable cross-contamination. In addition, mixed samples that were analyzed with S. nitritireducens were treated subsequently with Pseudomonas aureofaciens to reduce the NO3- in the absence of NO2-, providing isotopic analyses of NO2- and NO3- separately in the same aliquot. Sequential bacterial reduction methods like this one should be useful for a variety of isotopic studies aimed at understanding nitrogen cycling in aquatic environments. A test of these methods in an agricultural watershed in Indiana provides isotopic evidence for both nitrification and denitrification as sources of NO2- in a small stream.

Reduction of N2O by CO over Ceria-Modified Three-Way Pt−Rh Catalysts: Kinetic Aspects

This study reports an extensive kinetic investigation of the CO + N2O reaction which is a major reaction involved with automotive exhaust catalysts during the overall reduction of NO, particularly during the start of a cold engine. Steady-state rate measurements were performed between 217 and 250 °C on Pt−Rh/γ-Al2O3−CeO2 with partial pressures in the range (4.5−12.0) × 10-3 and (2.0−7.4) × 10-3 atm respectively for CO and N2O. Particular attention has been paid toward the influence of ceria on the catalytic performances of noble metals. A bifunctional mechanism is proposed involving the redox properties of ceria which correctly explains the beneficial effect of ceria on the reduction of N2O by CO in the absence of NO. However, subsequent comparisons with previous optimized kinetic and thermodynamic constants for the CO + NO reaction on the same catalyst suggest that the promotional effect of ceria is suppressed. When N2O and NO coexist in the gas phase, only noble metals are active near the light-off temp...

Effects of particle acidity and UV light on secondary organic aerosol formation from oxidation of aromatics in the absence of NO x

Recent laboratory observations have shown that particle acidity increases secondary organic aerosol (SOA) yields. However, these studies have mainly focused on biogenic precursors such as isoprene and terpenes. In this paper, the effects of particle acidity on the SOA yields from aromatic precursors under both dark and UV–visible light conditions were characterized through controlled chamber experiments. SOA was produced from oxidation of toluene and 1,3,5-trimethylbenzene (135-TMB) with OH radicals created by ozonolysis of 2-methyl-2-butene (MB). Particle acidity, described with proton concentrations, varied with inorganic seed aerosol composition and humidity (20–52%). Overall, in the presence of acidic seeds, greater increases in SOA yields were observed for the toluene system than the 135-TMB system. UV irradiation reduced SOA yields for both toluene and 135-TMB systems to different extents.

Prediction of tropospheric ozone concentrations by using the design system approach

Data on the concentrations of non-methane hydrocarbons (NMHC), nitrogen oxide (NO), nitrogen dioxide (NO2), carbon monoxide (CO), and meteorological parameters (air temperature and solar radiation) were used to predict the concentration of tropospheric ozone using the Design-Ease software. These data were collected on hourly basis over a 12-month period. Sampling of the data was conducted automatically. The effect of the NMHC, NO, NO2,CO, temperature and solar radiation variables in predicting ozone concentrations was examined under two scenarios: (i) when NO is included with the absence of NO2; and (ii) when NO2 is addressed with the absence of NO. The results of these two scenarios were validated against ozone actual data. The predicted concentration of ozone in the second scenario (i.e., when NO2 is addressed) was in better agreement with the real observations. In addition, the paper indicated that statistical models of hourly surface ozone concentrations require interactions and non-linear relationships between predictor variables in order to accurately capture the ozone behavior.

Atmospheric Chemistry of a Model Biodiesel Fuel, CH3C(O)O(CH2)2OC(O)CH3: Kinetics, Mechanisms, and Products of Cl Atom and OH Radical Initiated Oxidation in the Presence and Absence of NOx

Relative rate techniques were used to study the kinetics of the reactions of Cl atoms and OH radicals with ethylene glycol diacetate, CH3C(O)O(CH2)2OC(O)CH3, in 700 Torr of N2/O2 diluent at 296 K. The rate constants measured were k(Cl + CH3C(O)O(CH2)2OC(O)CH3) = (5.7 +/- 1.1) x 10(-12) and k(OH + CH3C(O)O(CH2)2OC(O)CH3) = (2.36 +/- 0.34) x 10(-12) cm3 molecule-1 s-1. Product studies of the Cl atom initiated oxidation of ethylene glycol diacetate in the absence of NO in 700 Torr of O2/N2 diluent at 296 K show the primary products to be CH3C(O)OC(O)CH2OC(O)CH3, CH3C(O)OC(O)H, and CH3C(O)OH. Product studies of the Cl atom initiated oxidation of ethylene glycol diacetate in the presence of NO in 700 Torr of O2/N2 diluent at 296 K show the primary products to be CH3C(O)OC(O)H and CH3C(O)OH. The CH3C(O)OCH2O* radical is formed during the Cl atom initiated oxidation of ethylene glycol diacetate, and two loss mechanisms were identified: reaction with O2 to give CH3C(O)OC(O)H and alpha-ester rearrangement to give CH3C(O)OH and HC(O) radicals. The reaction of CH3C(O)OCH2O2* with NO gives chemically activated CH3C(O)OCH2O* radicals which are more likely to undergo decomposition via the alpha-ester rearrangement than CH3C(O)OCH2O* radicals produced in the peroxy radical self-reaction.

Cysteine-674 of the Sarco/Endoplasmic Reticulum Calcium ATPase Is Required for the Inhibition of Cell Migration by Nitric Oxide

Nitric oxide inhibits smooth muscle cell migration after arterial injury, but the detailed mechanism is not fully understood. The sarco/endoplasmic reticulum calcium ATPase (SERCA) lowers cell Ca2+ by increasing intracellular Ca2+ uptake and inhibiting extracellular Ca2+ influx. Our previous studies showed that NO causes cyclic GMP-independent arterial relaxation by increasing SERCA activity by inducing reversible S-glutathiolation at cysteine-674. Because Ca2+ is an important second messenger for cell migration, we hypothesized that NO also inhibits cell migration through redox regulation of SERCA activity via cysteine-674. To test our hypothesis, overexpression of either wild type (WT) or mutant SERCA in which cysteine-674 was mutated to serine was accomplished by stable transfection of HEK 293 or adenoviral expression in rat aortic smooth muscle cells (VSMCs). In the cell models expressing mutant SERCA, biotinylated-iodoacetamide (BIAM) and biotinylated-glutathione labeling of SERCA was decreased, and NO failed to increase SERCA activity or decrease Ca2+ influx, thus validating that the expression of mutant SERCA prevents its redox-dependent activation. In the absence of NO, fetal bovine serum stimulated migration of both cell types expressing WT or C674S SERCA at similar rates. The NO donor S-nitrosopenicillamine inhibited migration of cells with WT SERCA, but had no effect on the migration of either HEK cells or VSMCs with C674S SERCA. The same result was obtained in VSMCs in which endogenous NO was produced by iNOS induced by interleukin (IL)-1beta. Blocking cyclic GMP did not prevent the inhibition of migration by NO. In cells overexpressing SERCA, the cyclic GMP-independent, redox regulation of SERCA cysteine-674 is required for the inhibition of cell migration by both exogenous and endogenously generated NO.

Rate constant and products for the reaction of Cl atom with n‐butyraldehyde

AbstractThe photooxidations of n‐butyraldehyde initiated by Cl atom were carried out at room temperature (298 ± 2K) and 1 atm pressure. The rate coefficient for the reactions of Cl atom with n‐butyraldehyde was determined as k = (2.04 ± 0.36) × 10−10 cm3 molecule−1 s−1 by using relative rate techniques. The photooxidation products of n‐butyraldehyde reaction with Cl atom were also studied by using both gas chromatography‐mass spectrometry (GC‐MS) and gas chromatography techniques. C2H5CHO, CH3CHO, CO and CO2 were the major products observed. In the absence of NO, the observed yields of C2H5CHO, CH3CHO, and CO were 60%, 3%, and 9%, respectively. However, when NO was introduced into the reaction chamber and the initial ratios of [NO]0/[n‐butyraldehyde]0 were between 1 and 8, the yield of C2H5CHO decreased to 33%, whereas that of CH3CHO and CO rose up to 21% and 25%, respectively. On the basis of mechanism data deduced in this study and the fraction molar yields, the approximate branching ratios for Cl atom attack at C(O)H, α‐, β‐, and γ‐positions in n‐butyraldehyde could be derived as ⩾42%, &lt;25%, 21%, and ⩽12%, respectively. © 2007 Wiley Periodicals, Inc. 39: 168–174, 2007

The atmospheric oxidation of diethyl ether: chemistry of the C2H5–O–CH(O˙)CH3 radical between 218 and 335 K

The products of the Cl atom initiated oxidation of diethyl ether (DEE) were investigated at atmospheric pressure over a range of temperatures (218-335 K) and O(2) partial pressures (50-700 Torr), both in the presence and absence of NO(x). The major products observed at 298 K and below were ethyl formate and ethyl acetate, which accounted for approximately equal to 60-80% of the reacted diethyl ether. In general, the yield of ethyl formate increased with increasing temperature, with decreasing O(2) partial pressure, and upon addition of NO to the reaction mixtures. The product yield data show that thermal decomposition reaction 3, CH(3)CH(2)-O-CH(O.)CH(3)--> CH(3)CH(2)-O-CH=O + CH(3), and reaction 6 with O(2), CH(3)CH(2)-O-CH(O.)CH(3) + O(2)--> CH(3)CH(2)-O-C(=O)CH(3) + HO(2) are competing fates of the CH(3)CH(2)-O-CH(O )CH(3) radical, with a best estimate of k3/k6 approximately equal to 6.9 x 10(24) exp(-3130/T). Thermal decomposition via C-H or C-O bond cleavage are at most minor contributors to the CH(3)CH(2)-O-CH(O.)CH(3) chemistry. The data also show that the CH(3)CH(2)-O-CH(O.)CH(3) radical is subject to a chemical activation effect. When produced from the exothermic reaction of the CH(3)CH(2)-O-CH(OO.)CH(3) radical with NO, prompt decomposition via both CH(3)- and probably H-elimination occur, with yields of about 40% and < or =15%, respectively. Finally, at temperatures slightly above ambient, evidence for a change in mechanism in the absence of NO(x), possibly due to chemistry involving the peroxy radical CH(3)CH(2)-O-CH(OO.)CH(3), is presented.

Coordinated Regulation of the Neisseria gonorrhoeae-truncated Denitrification Pathway by the Nitric Oxide-sensitive Repressor, NsrR, and Nitrite-insensitive NarQ-NarP

Neisseria gonorrhoeae survives anaerobically by reducing nitrite to nitrous oxide catalyzed by the nitrite and nitric oxide reductases, AniA and NorB. P(aniA) is activated by FNR (regulator of fumarate and nitrate reduction), the two-component regulatory system NarQ-NarP, and induced by nitrite; P(norB) is induced by NO independently of FNR by an uncharacterized mechanism. We report the results of microarray analysis, bioinformatic analysis, and chromatin immunoprecipitation, which revealed that only five genes with readily identified NarP-binding sites are differentially expressed in narP(+) and narP strains. These include three genes implicated in the truncated gonococcal denitrification pathway: aniA, norB, and narQ. We also report that (i) nitrite induces aniA transcription in a narP mutant; (ii) nitrite induction involves indirect inactivation by nitric oxide of a gonococcal repressor, NsrR, identified from a multigenome bioinformatic study; (iii) in an nsrR mutant, aniA, norB, and dnrN (encoding a putative reactive nitrogen species response protein) were expressed constitutively in the absence of nitrite, suggesting that NsrR is the only NO-sensing transcription factor in N. gonorrhoeae; and (iv) NO rather than nitrite is the ligand to which NsrR responds. When expressed in Escherichia coli, gonococcal NarQ and chimaeras of E. coli and gonococcal NarQ are ligand-insensitive and constitutively active: a "locked-on" phenotype. We conclude that genes involved in the truncated denitrification pathway of N. gonorrhoeae are key components of the small NarQP regulon, that NarP indirectly regulates P(norB) by stimulating NO production by AniA, and that NsrR plays a critical role in enabling gonococci to evade NO generated as a host defense mechanism.

Reaction paths of the formation and consumption of nitroorganic complex intermediates in the selective catalytic reduction of nitrogen oxides with propylene on zirconia-pillared clays according to in situ spectroscopic data

It was found that the adsorption and catalytic properties of nanosized ZrO2 particles as the pillar constituents of ZrO2-pillared clay and bulk ZrO2 are essentially different. The interaction of NO with the surface of bulk ZrO2 resulted in the formation of three types of nitrate complexes. Only two nitrate species were formed on ZrO2-pillared clay (the monodentate species was absent). Only an acetate complex was formed in the interaction of a mixture of propylene and oxygen with the surface of bulk ZrO2, whereas an isopropoxide complex was the main propylene activation species on ZrO2-pillared clay. On the surface of ZrO2-pillared clay, isopropoxide and nitrate intermediates formed a complex structurally similar to adsorbed dinitropropane. On the surface of bulk ZrO2, acetate and monodentate nitrate complexes formed a complex structurally similar to adsorbed nitromethane. The dinitropropane complex on ZrO2-pillared clay was consumed in reactions with surface nitrates. The decomposition reaction of a dinitropropane compound with the formation of acetate complexes and ammonia predominated on the surface containing no nitrate complexes in the absence of NO + O2 from a gas phase. The found differences in reactant activation species and their thermal stabilities explained differences in the activities of bulk ZrO2 and nanosized ZrO2 particles as pillars in pillared clay in the course of the selective catalytic reduction of nitrogen oxides with propylene in an excess of oxygen.

Understanding the activation mechanism induced by NOx on the performances of VOx/TiO2 based catalysts in the total oxidation of chlorinated VOCs

A previous investigation of the chlorobenzene combustion activity of VOx/TiO2, VOx–WOx/TiO2 and VOx–MoOx/TiO2 catalysts in the presence of NO pointed out the activation effect of NO. The suggested three-step mechanism based on catalytic performances data only was: (1) chlorobenzene is oxidized on the surface of the VOx phase (as described by Mars–van Krevelen), (2) NO gets oxidized to NO2, mainly on WOx and MoOx, and (3) the in situ produced NO2 assists O2 in the reoxidation of the VOx phase thus speeding up the oxidation step of the Mars–van Krevelen mechanism. The latter effect macroscopically corresponds to the observed increase of chlorobenzene conversion. This contribution aims at validating this hypothetical mechanism by pointing out the favourable occurrence of an oxidation of NO to NO2 on the WOx and MoOx phases and by pointing out the higher efficiency of NO2 than O2 to reoxidize the reduced VOx sites. In addition, the present contribution clearly demonstrates that, in the absence of NO, the chlorobenzene total oxidation occurred following the Mars–van Krevelen mechanism. Moreover, a thorough characterization of the oxidation state of the vanadium proving that the improvement of the catalyst activity brought by the simultaneous presence of NO and O2 is linked to the stronger reoxidation of the VOx active phase. Furthermore, plotting all the catalytic activity data versus the mean vanadium oxidation level clearly depicts, for the first time, the strong dependence between them. Under a mean vanadium oxidation level of 4.82 the catalyst is inactive while above 4.87 the activity is stabilized at a high level of conversion independent of the vanadium oxidation level.

Using Acetylene for Selective Catalytic Reduction of NO in Excess Oxygen

AbstractAcetylene as a reducing agent for selective catalytic reduction of NO (C2H2‐SCR) was investigated over a series of metal exchanged HY catalysts, in the reaction system of 0.16% NO, 0.08% C2H2, and 9.95% O2 (volume percent) in He. 75% of NO conversion to N2 with hydrocarbon efficiency about 1.5 was achieved over a Ce‐HY catalyst around 300 °C. The NO removal level was comparable with that of selective catalytic reduction of NOx by C3H6 reported in literatures, although only one third of the reducing agent in carbon moles was used in the C2H2‐SCR of NO. The protons in zeolite were crucial to the C2H2‐SCR of NO, and the performance of HY in the reaction was significantly promoted by cerium incorporation into the zeolite. NO2 was proposed to be the intermediate of NO reduction to N2, and the oxidation of NO to NO2 was rate‐determining step of the C2H2‐SCR of NO over Ce‐HY. The suggestion was well supported by the results of the NO oxidation with O2, and the C2H2 consumption under the conditions in the presence or absence of NO.

Role of nitrogen dioxide in the oxidation of diesel soot on promoted mixed catalysts with fluorite and perovskite structures

The oxidation of soot on catalysts with the perovskite and fluorite structures (including platinum-promoted catalysts) in the presence and in the absence of NO2 was studied using in situ IR spectroscopy and temperature-programmed techniques (TPR, TPD, and TPO). It was found that, as a rule, the temperature of the onset of soot oxidation considerably decreased upon the addition of NO2 to a flow of O2/N2, whereas the amount of oxygen consumed in soot oxidation considerably increased. To explain these facts, we hypothesized that the initiation of soot combustion in the presence of NO2 was related to the activation of the NO2 molecule through the formation (at a low temperature) and decomposition (at a high temperature) of nitrate structures on the catalyst. Superequilibrium amounts of NO2 resulted from the decomposition of nitrate complexes immediately on the catalyst for soot combustion. Based on a comparison between catalyst activities and data obtained by TPR and the TPD of oxygen, a conclusion was drawn that the presence of labile oxygen in the catalyst is a necessary but insufficient condition for the efficient occurrence of a soot oxidation reaction in the presence of NO2. The introduction of platinum as a constituent of the catalyst increased the amount of labile oxygen and, as a consequence, increased the amount of highly reactive nitrate complexes. As a result, this caused a decrease in the temperature of the onset of soot combustion.

Evidence for Involvement of Both IK Ca and SK Ca Channels in Hyperpolarizing Responses of the Rat Middle Cerebral Artery

Endothelium-derived hyperpolarizing factor responses in the rat middle cerebral artery are blocked by inhibiting IKCa channels alone, contrasting with peripheral vessels where block of both IKCa and SKCa is required. As the contribution of IKCa and SKCa to endothelium-dependent hyperpolarization differs in peripheral arteries, depending on the level of arterial constriction, we investigated the possibility that SKCa might contribute to equivalent hyperpolarization in cerebral arteries under certain conditions. Rat middle cerebral arteries (approximately 175 microm) were mounted in a wire myograph. The effect of KCa channel blockers on endothelium-dependent responses to the protease-activated receptor 2 agonist, SLIGRL (20 micromol/L), were then assessed as simultaneous changes in tension and membrane potential. These data were correlated with the distribution of arterial KCa channels revealed with immunohistochemistry. SLIGRL hyperpolarized and relaxed cerebral arteries undergoing variable levels of stretch-induced tone. The relaxation was unaffected by specific inhibitors of IKCa (TRAM-34, 1 micromol/L) or SKCa (apamin, 50 nmol/L) alone or in combination. In contrast, the associated smooth-muscle hyperpolarization was inhibited, but only with these blockers in combination. Blocking nitric oxide synthase (NOS) or guanylyl cyclase evoked smooth-muscle depolarization and constriction, with both hyperpolarization and relaxation to SLIGRL being abolished by TRAM-34 alone, whereas apamin had no effect. Immunolabeling showed SKCa and IKCa within the endothelium. In the absence of NO, IKCa underpins endothelium-dependent hyperpolarization and relaxation in cerebral arteries. However, when NOS is active SKCa contributes to hyperpolarization, whatever the extent of background contraction. These changes may have relevance in vascular disease states where NO release is compromised and when the levels of SKCa expression may be altered.

Tropospheric degradation of 2-hydroxy-2-methylpropanal, a photo-oxidation product of 2-methyl-3-buten-2-ol: Kinetic and mechanistic study of its photolysis and its reaction with OH radicals

This article presents the first study of the chemical reactivity of a photo-oxidation product of the 2-methyl-3-buten-2-ol (MBO) in the troposphere, the 2-hydroxy-2-methylpropanal (HMPr). Experiments were performed under controlled conditions for pressure (atmospheric pressure) and temperature (296±2 K) in the LISA indoor simulation chamber and in the EUPHORE outdoor simulation chamber to understand the transformations of HMPr in the atmosphere, using FTIR spectrometry. Reaction with OH radicals was studied in the presence and in the absence of NO x . All the kinetic studies were carried out by relative rate study using isoprene and/or n-butylether as the reference compound. The rate constants found for these reactions were J=(1.01±0.39)×10 −5 s −1 and k HMPr+OH=(1.52±0.36)×10 −11 molecule −1 cm 3 s −1. For the reaction with OH radicals, an important production of acetone was observed, with a yield of R acetone=1.06±0.06.

Clonidine induced endothelium-dependent tonic contraction in circular muscle of the rat hepatic portal vein

Clonidine, an alpha2-agonist, has been shown to be useful in the treatment of hepatic portal hypertension in cirrhosis. The mechanism has been attributed to a clonidine-induced decrease in sympathetic activity. While clonidine has been shown to stimulate the alpha2-adrenoceptors of blood vessels, there is limited knowledge of the effects of clonidine on the circular muscle of the hepatic portal vein which regulates its blood flow. To investigate clonidine-induced contraction of the circular muscle of the hepatic portal vein and to clarify the possible role of the endothelium in the contraction, we examined the effects of clonidine on the isometric contraction of endothelium-intact and -removed ring preparations of the rat hepatic portal vein. In endothelium-intact preparations, clonidine caused a concentration-dependent increase in the amplitude of contractions. Inhibition of NO synthesis with Nomega-nitro-L-arginine (L-NNA) elevated the resting tone, and increased the amplitude of the clonidine-induced contractions. Inhibition of cyclooxygenase by diclofenac did not change the amplitude of the clonidine-induced contractions observed both in the presence and absence of L-NNA. Application of a single concentration of clonidine induced a clear increase in amplitude of both twitch and tonic contractions. Twitch and tonic contractions induced by clonidine were inhibited by yohimbine. When the endothelium was damaged by sodium deoxycholate, tonic contractions induced by clonidine were completely suppressed, whereas the increase in twitch contractions was not influenced by chemical damage of the endothelium. Neither SKF-96365, a nonselective cation channel blocker, nor superoxide dismutase, a free radical scavenger, in the presence of catalase, changed the tonic contraction induced by clonidine. These results indicate that stimulation of alpha2-adrenoceptors enhanced twitch contractions and induced tonic contractions in the circular muscle of the rat hepatic portal vein, especially in the absence of NO. The latter, but not the former, occurs through an endothelium-dependent pathway.