Hepatic basolateral plasma high-affinity Ca2+-ATPase is inhibited by nitric oxide and peroxynitrite anion

N-Acetylcysteine-induced vasodilatation is modulated by KATP channels, Na+/K+-ATPase activity and intracellular calcium concentration: An in vitro study

BACKGROUND

In this study, we aimed to investigate the role of ATP-sensitive potassium (K_ATP) channel, Na⁺/K⁺-ATPase activity, and intracellular calcium levels on the vasodilatory effect of N-acetylcysteine (NAC) in thoracic aorta by using electrophysiological and molecular techniques.

METHODS

Rat thoracic aorta ring preparations and cultured thoracic aorta cells were divided into four groups as control, 2mM NAC, 5mM NAC, and 10mM NAC. Thoracic aorta rings were isolated from rats for measurements of relaxation responses and Na⁺/K⁺-ATPase activity. In the cultured thoracic aorta cells, we measured the currents of K_ATP channel, the concentration of intracellular calcium and mRNA expression level of K_ATP channel subunits (KCNJ8, KCNJ11, ABCC8 and ABCC9).

RESULTS

The relaxation rate significantly increased in all NAC groups compared to control. Similarly, Na⁺/K⁺- ATPase activity also significantly decreased in NAC groups. Outward K_ATP channel current significantly increased in all NAC groups compared to the control group. Intracellular calcium concentration decreased significantly in all groups with compared control. mRNA expression level of ABCC8 subunit significantly increased in all NAC groups compared to the control group. Pearson correlation analysis showed that relaxation rate was significantly associated with K_ATP current, intracellular calcium concentration, Na⁺/K⁺-ATPase activity and mRNA expression level of ABCC8 subunit.

CONCLUSION

Our findings suggest that NAC relaxes vascular smooth muscle cells through a direct effect on K_ATP channels, by increasing outward K+ flux, partly by increasing mRNA expression of K_ATP subunit ABCC8, by decreasing in intracellular calcium and by decreasing in Na⁺/K⁺-ATPase activity.

Nitric oxide and peroxynitrite in health and disease

The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

Oxidative stress in pneumococcal meningitis: A future target for adjunctive therapy?

Despite antibiotic therapy and supportive intensive care, the morbidity and mortality of pneumococcal meningitis remain unacceptably high. During the last years, reactive oxygen (ROS) and nitrogen species (RNS), and peroxynitrite, were found to be produced in large amounts during pneumococcal meningitis. Although most likely intended to fight the invasive pathogens, they seem to lead to substantial collateral damage instead. This is because ROS and RNS can exert a vast variety of toxic actions, e.g., through lipid peroxidation, DNA strand breakage followed by PARP activation and subsequent cellular energy depletion, production of inflammatory cytokines, and activation of matrix metalloproteinases. Animal models of pneumococcal meningitis have shown that these interactions contribute to massive meningeal inflammation, disruption of the blood-brain barrier, alterations of the cerebral autoregulation, neuronal cell death, and cochlear destruction. Thus, the production of ROS and RNS seems at least in part to be responsible for the poor outcome of patients with pneumococcal meningitis. In consequence, reactive oxygen and nitrogen species such as peroxynitrite have been investigated as potential targets for adjunctive therapy in pneumococcal meningitis. Among the multiple agents tested, one promising drug is N-acetyl-l-cysteine (NAC), which significantly reduced cerebral and cochlear complications in animal models of experimental pneumococcal meningitis.

Nitric oxide inhibits mitochondrial monoamine oxidase activity and decreases outer mitochondria membrane fluidity

The recent finding that mitochondria contain a nitric oxide (NO) synthase suggests that this compound is involved in the regulation of various mitochondrial functions. Monoamine oxidase (MAO) is embedded in the outer mitochondrial membrane. NO modulates membrane fluidity. Thus, the aim of the present work was to study the effect of NO on mitochondrial MAO activity and membrane fluidity. An outer mitochondrial membrane fraction (OMMF) was obtained from rat liver. OMMF was incubated with various concentrations of S-nitroso-N-acetylpenicillamine (SNAP), a NO donor. MAO activity and fluidity were measured by a spectrophotometric assay and by the polarization of fluorescence technique, respectively. It was found that small concentrations of SNAP (0.4-40 microM) were capable of inhibiting MAO activity but unable to decrease fluidity significantly. In contrast, larger amounts of SNAP (40-300 microM) effectively decreased membrane fluidity, but were not able to further decrease MAO activity. This information suggests that mitochondrial MAO and membrane fluidity possess different sensitivity to the effect of NO. Unfortunately, the mechanism by which NO inhibits MAO remains unknown at present. However, it seems likely that the effect of NO on MAO activity is by a direct interaction of the compound or a metabolite to the protein.

Insights into the mechanism of erythrocyte Na+/K+-ATPase inhibition by nitric oxide and peroxynitrite anion

Evidence shows that Na(+)/K(+)-ATPase from kidney, brain and liver is inhibited by nitric oxide (NO) and peroxynitrite anion (ONO(2) (-)), but the mechanism is unknown. The aim of the present work was to study the inhibitory effect of NO and ONO(2) (-) on erythrocyte Na(+)/K(+)-ATPase. Erythrocyte membranes were isolated from male Wistar rats by hypotonic washing. The membranes (free from haemoglobin) were incubated for Na(+)/K(+)-ATPase activity measurement at various concentrations of ATP in the presence or absence of 400 microM SNAP (an NO donor) or 100 microM SIN-1 (an ONO(2)(-) donor). At these concentrations, SNAP and SIN-1 released about the same amount (100 microM) of NO or ONO(2)(-), respectively, as monitored by measuring NO(2)(-) + NO(3)(-). Both SNAP and SIN-1 decreased V(max) by ca. 75% but they did not decrease the apparent affinity of the Na(+)/K(+)-ATPase for the substrate (a decrease of K(m) was even observed after SNAP treatment). The pattern of this inhibition is compatible either with oxidation of SH groups directly involved in ATP binding but in a way that is not surmountable by increasing the substrate concentration ("non-competitive") or with oxidation of SH groups located outside the active site of the enzyme but important for the activity of the enzyme.

Infrared spectra of cis- and trans-peroxynitrite anion, OONO-, in solid argon

The peroxynitrite anion, of vast importance in biochemistry, is formed in vivo from the reaction of NO and O(-2). Laser ablation of 10 different metal targets with concurrent 7 K codeposition of NO/Ar and O(2)/Ar mixtures gives new metal-independent infrared bands at 1458.3 and 806.1 cm(-)(1), and at 1433.3 and 983.2 cm(-1), in addition to known O(-4) and (NO)(-2) absorptions. The new bands are not observed with CCl(4) added to capture electrons or in O(2) and NO experiments without laser ablation to produce electrons, which identifies new product anions. Based on (15)NO and (18)O(2) isotopic shifts, splitting patterns in mixed isotopic experiments, and comparison with DFT isotopic frequency calculations, the former absorptions are assigned to cis-OONO-, and the latter pair to trans-OONO-, which are isolated from metal cations trapped elsewhere in the matrix. The cis- and trans-peroxynitrite anion isomers are probably formed via the ion-molecule reaction between O(-2)and NO: the O(-2) anion, made by the capture of ablated electrons, is attested by the observation of O(-4). cis- and trans-OONO- are reversibly photoisomerized by visible and near-UV radiation. Collisional stabilization of the OONO- ion-molecule dimer complex during formation of the solid argon matrix appears to be crucial.