Effect of exogenous and endogenous nitric oxide on mitochondrial respiration of rat hepatocytes

Near-infrared spectroscopy in experimental pneumococcal meningitis in the rabbit: cerebral hemodynamics and metabolism

Near-infrared spectroscopy is a noninvasive technique which measures oxidized cytochrome aa3, oxygenated Hb, and deoxygenated Hb and calculates total Hb in tissue. This technique, in conjunction with measurement of cerebral blood flow, was used in rabbits with experimental bacterial meningitis to determine whether there was evidence for cerebral energy depletion and alterations in the cerebral vascular bed with infection. Rabbits with meningitis had a significant reduction in cerebral blood flow, cerebral oxidized cytochrome aa3 and a relative increase in the deoxygenated Hb fraction and a decrease in the oxygenated Hb fraction compared with uninfected controls. Total Hb was not significantly different between the two groups. These findings may help clarify the mechanism for some of the intracranial pathophysiologic abnormalities in meningitis.

Competition for tetrahydrobiopterin between phenylalanine hydroxylase and nitric oxide synthase in rat liver

Tetrahydrobiopterin (BH4) is an important cofactor for two hepatic enzymes, inducible nitric oxide synthase (iNOS) and phenylalanine hydroxylase (PAH), and competition for BH4 between the two enzymes might limit hepatic iNOS or PAH activity. To test this hypothesis, we determined whether conversion of phenylalanine to tyrosine was modified by changes in NO synthase activity, and conversely whether NO synthesis was limited by the rate of phenylalanine conversion to tyrosine in rat hepatocytes and perfused livers. NO production was decreased only slightly, when flux through PAH was maximized in isolated perfused livers, and in isolated hepatocytes only when BH4 synthesis was inhibited. Increases in NO synthesis did not reduce tyrosine formation from phenylalanine. Phenylalanine markedly increased biopterin synthesis, whereas arginine had no effect. Thus, basal BH4 synthesis appears to be adequate to support iNOS activity, whereas BH4 synthesis is increased to support PAH activity.

Role of endothelium-derived nitric oxide in the modulation of canine myocardial mitochondrial respiration in vitro. Implications for the development of heart failure

The mechanism responsible for the regulation of cardiac function by endogenous nitric oxide (NO) remains unclear. In this investigation, O2 consumption by freshly isolated myocardial muscle segments from the left ventricular free wall of canine hearts was quantified by a Clark-type O2 electrode at 37 degrees C. S-nitroso-N-acetylpenicillamine (SNAP, 9 +/- 3% to 50 +/- 8%), bradykinin (BK, 14 +/- 3% to 30 +/- 5%), or carbachol (CCh, 15 +/- 4% to 29 +/- 4%) significantly attenuated tissue O2 consumption at doses of 10(-7) to 10(-4) mol/L (mean +/- SE, P < .05). The effects of BK and CCh, but not SNAP, were blocked by 10(-4) mol/L NG-nitro-L-arginine, consistent with both BK and CCh stimulating NO biosynthesis and with SNAP decomposing to release NO, respectively. Similar doses of 8-Br-cGMP caused a respiratory inhibition, but to a lesser extent (9 +/- 2% to 14 +/- 6%). A mitochondrial uncoupler, 2,4-dinitrophenol (at 1 mmol/L), blocked the effects of 8-Br-cGMP, but not those of SNAP, BK, or CCh, suggesting that the major site of action of NO is on mitochondrial electron transport. Myocardial muscle from dogs with pacing-induced heart failure had a basal O2 consumption rate of 251 +/- 21 nmol.min-1.g-1, which was 54% higher than the rate seen in muscle from normal healthy canine hearts. The inhibitory effects of BK and CCh on O2 consumption were not observed in failing cardiac tissue, but SNAP showed an unaltered inhibitory effect. Therefore, our results indicate that NO released from microvascular endothelium by BK, stimulation of muscarinic receptors, and perhaps flow velocity may play an important physiological role in the control of cardiac mitochondrial respiration, and the loss of this regulatory function may contribute to the development of heart failure.

ARL 17477, a potent and selective neuronal NOS inhibitor decreases infarct volume after transient middle cerebral artery occlusion in rats

We tested the effects of administration of a selective neuronal nitric oxide synthase (nNOS) inhibitor, ARL 17477, on ischemic cell damage and regional cerebral blood flow (rCBF), in rats subjected to transient (2 h) middle cerebral artery (MCA) occlusion and 166 h of reperfusion (n = 48) and in rats without MCA occlusion (n = 25), respectively. Animals were administered ARL 17477 (i.v.): 10 mg/kg; 1 mg/kg; 3mg/kg; N-nitro-L-arginine (L-NA) 10 mg/kg L-NA 1 mg/kg; and Vehicle. Administration of ARL 17477 1 mg/kg, 3 mg/kg and 10 mg/kg reduced ischemic infarct volume by 53 (p < 0.05), 23, and 6.5%, respectively. L-NA 1 mg/kg and 10 mg/kg increased infarct volume by 2 and 15%, respectively (p > 0.05). Administration of ARL 17477 (10 mg/kg) significantly (p < 0.05) decreased rCBF by 27 +/- 5.3 and 24 +/- 14.08% and cortical NOS activity by 86 +/- 14.9 and 91 +/- 8.9% at 10 min or 3 h, respectively, and did not alter mean arterial blood pressure (MABP). L-NA (10 mg/kg) significantly reduced rCBF by 23 +/- 9.8% and NOS activity by 81 +/- 7% and significantly (p < 0.05) increased MABP. Treatment with 3 mg/kg and 1 mg/kg ARL 17477 reduced rCBF by only 2.4 +/- 4.5 and 0%, respectively, even when NOS activity was reduced by 63 +/- 13.4 and 45 +/- 15.7% at 3 h, respectively, (p < 0.05). The data demonstrate that ARL 17477 inhibits nNOS in the rat brain and causes a dose-dependent reduction in infarct volume after transient MCA occlusion.

Modification of macrophage glyceraldehyde-3-phosphate dehydrogenase in response to nitric oxide

A potential cytotoxic, self-destructive role of endogenously generated and exogenously supplied nitric oxide (NO) was studied in two mouse monocytic macrophage cell lines (RAW 264.7 and J774.1). Our attention centered on NO-mediated glyceraldehyde-3-phosphate dehydrogenase (GAPDH) modification and inhibition of the Krebs cycle enzyme, aconitase, related to macrophage cell death. NO formed by an active inducible nitric oxide synthase significantly decreased cell viability in the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) cytotoxicity assay. Similarly, cell viability was inversely and dose-dependently correlated to increasing concentrations of the NO-releasing compound, sodium nitroprusside. Biochemically, we noticed a correlation between endogenously derived or exogenously generated NO and inhibition of GAPDH as well as aconitase enzyme activity. The involvement of NO was further substantiated by the use of NG-monomethyl-L-arginine. Associated with decreased GAPDH enzyme activity, 32P-NAD(+)-dependent modification of the enzyme in the cytosol of pretreated cells was hindered. This reflects intracellular protein modification as a result of NO signalling. Using sodium nitroprusside we achieved GAPDH translocation from the cytosol to the plasma membrane or the nucleus of treated cells. However, despite GAPDH modification, lactate production was not rate limiting during NO intoxication. Furthermore, blocking the iron-sulfur-containing enzyme, aconitase, is insufficient to produce macrophage cell death. Although RAW 264.7 and J774.1 cells show substantial variation in their sensitivity towards NO it can be concluded that NO-mediated macrophage cell death is not linked to energy depletion. For GAPDH, NO-mediated protein modification may be related to functions of the enzyme, other than its glycolytic role.

Effects of 7-nitroindazole, NG-nitro-L-arginine, and D-CPPene on harmaline-induced postural tremor, N-methyl-D-aspartate-induced seizures, and lisuride-induced rotations in rats with nigral 6-hydroxydopamine lesions

The present behavioral study was undertaken to investigate whether neuronal nitric oxide (NO) synthase mediates the abnormal consequences of increased NMDA receptor-mediated synaptic transmission in models of postural tremor, Parkinson's disease and epilepsy. We used 7-nitroindazole, a selective inhibitor of neuronal NO synthase, and NG-nitro-L-arginine (L-NAME), an unspecific NO synthase inhibitor, and compared their action with that of the competitive NMDA receptor antagonist 3-[(R)-2-carboxypiperazin-4-yl]-prop-2-enyl-1-phosphonic acid (D-CPPene). In both mice and rats, 7-nitroindazole, L-NAME and D-CPPene dose dependently reversed the harmaline-induced increase of cerebellar cyclic guanosine-5'-monophosphate (cGMP) levels. For subsequent behavioral experiments we used doses of 7-nitroindazole, L-NAME and D-CPPene which were equipotent in preventing harmaline-induced cGMP increase. Harmaline-induced tremor in mice and rats was suppressed by D-CPPene, but not by 7-nitroindazole or by L-NAME. This effect of D-CPPene was not due to unspecific suppression of motor activity, since D-CPPene did not affect locomotor activity at doses which reduced tremor. D-CPPene, but not 7-nitroindazole and L-NAME potentiated the antiparkinsonian action of the dopamine agonist lisuride in rats with unilateral 6-hydroxydopamine lesions of the substantia nigra. D-CPPene antagonized seizures induced by intracerebroventricular injection of NMDA in mice. In contrast, 7-nitroindazole and L-NAME had only a tendency to prevent seizures and to delay the latency to onset of seizures. We conclude from these results that neuronal NO synthase does not serve as a major mediator of increased NMDA receptor-mediated synaptic transmission in animal models of Parkinson's disease, postural tremor and epilepsy. The novel observation that D-CPPene suppresses harmaline-induced tremor leads us to suggest that NMDA receptor antagonists should be considered as novel therapeutics for postural tremor.

Regulation of energy metabolism by interleukin-1beta, but not by interleukin-6, is mediated by nitric oxide in primary cultured rat hepatocytes

The effects of inflammatory cytokines (interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha) on energy metabolism were studied in primary cultured rat hepatocytes. Adenine nucleotide (ATP, ADP, and AMP) content, lactate production, the ketone body ratio (acetoacetate/beta-hydroxybutyrate) reflecting the liver mitochondrial redox state (NAD+/NADH), and nitric oxide formation were measured. Insulin increased ATP content in hepatocytes and had a maximal effect after 8-12 h of culture. Both interleukin-1beta and interleukin-6, but not tumor necrosis factor-alpha, significantly inhibited the ATP increase time- and dose-dependently. Interleukin-1beta and interleukin-6 also stimulated lactate production. During the same period, interleukin-1beta but not interleukin-6 decreased the ketone body ratio. Furthermore, interleukin-1beta markedly stimulated nitric oxide formation in hepatocytes, and this increase was blocked by NG-monomethyl-L-arginine (a nitric oxide synthase inhibitor) and by interleukin-1 receptor antagonist. NG-monomethyl-L-arginine reversed inhibition of the ATP increase, decrease in the ketone body ratio, and increase in lactate production, which were induced by interleukin-1beta. Interleukin-1 receptor antagonist completely abolished all of the effects induced by interleukin-1beta. These results demonstrated that interleukin-1beta and interleukin-6 affect the insulin-induced energy metabolism in rat hepatocytes by different mechanisms. Specifically, interleukin-1beta inhibits ATP synthesis by causing the mitochondrial dysfunction, a process which may be mediated by nitric oxide.

Involvement of nitric oxide in the deregulation of cytosolic calcium in cerebellar neurons during combined glucose-oxygen deprivation

Nitric oxide (NO) has been proposed as a neuronal messenger molecule in hypoxic/ischemic cell injury (Nowicki et al., 1991; Trifiletti, 1992). We conducted studies in a model of combined glucose-oxygen deprivation using cultured rat cerebellar granule cells. Experiments were designed to test the hypothesis that sustained elevation of cytosolic calcium ([Ca2+]i) and NO generation act in concert to trigger neuronal injury after anoxic insult. A hypoxic state was achieved by perfusing the cells with medium pre-equilibrated with argon gas. [Ca2+]i was monitored using digital-imaging fluorescence microscopy in cells loaded with fura-2 AM. Under short-term hypoxic conditions, cells displayed a progressive and sustained, moderate increase of [Ca2+]i, which returned to near basal levels on restoration of O2-containing medium. Prolonged hypoxic conditions (> 60 min) caused irreversible elevation of [Ca2+]i followed by disruption of cell membrane integrity, as indicated by severe swelling, loss of regular cell shape and processes, leakage of dye fura-2, and propidium iodide uptake ("point of no return"). Pretreatment with NG-nitro-L-arginine methyl ester (L-NAME, 100 microM), a specific NO synthase inhibitor, markedly delayed the onset of intensity of the rise of [Ca2+]i. The hypoxia-induced elevation of [Ca2+]i was also greatly attenuated if L-NAME (100 microM) was added to the argon-perfused medium before the cells demonstrated signs of irreversible injury. Prolonged or repeated hypoxic conditions, however, caused a rapid and intense increase of [Ca2+]i, which could not be blocked by inhibition of NO synthase (NOS). In addition, reoxygenation after the "point of no return," as characterized above, greatly potentiated [Ca2+]i overload and facilitated the process of cell injury. The potentiation and facilitation of cell damage, as demonstrated by rapid massive increase of [Ca2+]i and subsequent cell death, was not blocked by NOS inhibitor, L-NAME.

Role of nitric oxide and superoxide anion in leukotoxin-, 9,10-epoxy-12-octadecenoate-induced mitochondrial dysfunction

The present study was carried out to explore the involvement of nitric oxide (NO) and superoxide anion (O2.-) in Leukotoxin (Lx)-induced suppression of mitochondrial respiration. Glutamate- and succinate-dependent oxygen consumption and cytochrome c oxidase activity were assayed. Lx-induced mitochondrial damage was significantly attenuated by the pretreatment of lung with 4 x 10(-4) M NG-monomethyl-L-arginine (L-NMMA) or 500 units/ml superoxide dismutase (SOD) in ex vivo. However, L-NMMA plus SOD pretreatment showed no additive effect on the recovery of mitochondrial functions. The same assay was performed after the exposure of intact mitochondria to NO containing solution (1.25 x 10(-5) M) or 0.1 mM KO2/18-Crown-6 solution, which generated O2.-(6.4 x 10(-5) M). NO, but not O2.-, significantly inhibited the respiration of isolated mitochondria in vitro. Thus, there were great discrepancies in the involvement of NO and O2.- between ex vivo and in vitro system. Together with the previous reports, these facts suggested that the mechanisms by which NO and O2.- probably from vascular constituent cells inhibit mitochondrial respiration function of isolated perfused rat lung may not be simply due to their direct reactions with mitochondrial electron transport chain components, but may rely on the formation of peroxynitrite, and/or peroxynitrite-derived oxidants.

Nitrone-based free radical traps as neuroprotective agents in cerebral ischaemia and other pathologies

Nitrone-based spin trapping compounds have been shown to protect experimental animals from pathology associated with ischaemia/reperfusion injury, endotoxaemia, natural and accelerated aging, certain xenobiotics, and physical trauma. Moreover, these compounds have an intriguing nootropic action. Nitrones affect pathophysiological correlates in both the central nervous system and peripheral organ systems. These compounds have been shown to affect cellular oxidation state and oxidatively sensitive enzyme systems, but the precise mode of nitrone action has not been elucidated. Recent discoveries regarding the ability of nitrones to suppress gene transcriptional events associated with pathophysiological states, particularly the elaboration of NF kappa B-regulated cytokines and inducible nitric oxide synthase, argue that nitrones may act at a proximal level to oxidatively sensitive signal amplification systems.

Characterization of nitric oxide dependent changes in carbohydrate hepatic metabolism during septic shock

The role of nitric oxide in the alterations of liver carbohydrate metabolism during septic shock has been studied in fed and starved animals injected with bacterial lipopolysaccharide (LPS). One h after LPS injection an hyperglycemic peak was observed followed by hypoglycemia when the plasma nitric oxide concentration increased. However, in animals pharmacologically treated with nitric oxide donors only hypoglycemia was observed. In isolated hepatocytes from LPS treated rats an impairment of the gluconeogenic flux was observed accompanied by a decrease in the mRNA levels of the glucose transporter GLUT-2 and phosphoenolpyruvate carboxykinase, at the time that increased the mRNA levels of the inducible form of nitric oxide synthase. These results suggest that part of the effects observed in response to LPS challenge are due to early signaling molecules (cytokines and other factors molecules) whereas other effects can be attributed to nitric oxide synthesis which in turn has specific effects on hepatic metabolism.

Nitric oxide. An important signaling mechanism between vascular endothelium and parenchymal cells in the regulation of oxygen consumption

BACKGROUND

Nitric oxide (NO) is known to be an inhibitor of mitochondrial function. However, the physiological significance of endothelium-derived NO in the control of tissue respiration is not established.

METHODS AND RESULTS

Tissue O2 consumption by skeletal muscle slices of the triceps brachii of normal dogs was measured with a Clark-type O2 electrode/tissue bath system at 37 degrees C. S-Nitroso-N-acetylpenicillamine (SNAP), carbachol (CCh), or bradykinin (BK) decreased tissue O2 consumption by 12 +/- 3% to 55 +/- 8%, 15 +/- 6% to 36 +/- 11%, or 21 +/- 5% to 42 +/- 4% at doses of 10(-7) to 10(-4) mol/L, respectively. The effects of both CCh and BK but not SNAP were eliminated by nitro-L-arginine (NLA, 10(-4) mol/L), consistent with SNAP decomposing to release NO and both CCh and BK stimulating endogenous NO production from L-arginine. Oxygen consumption was also decreased by 8-bromo-cGMP. The mitochondrial uncoupler dinitrophenol blocked the effects of 8-bromo-cGMP but only slightly altered those of SNAP, indicating that the major site of action of NO is the mitochondria. In normal, chronically instrumented, resting conscious dogs, blockade of NO synthase by NLA increased mean arterial pressure by 28 +/- 2.5 mm Hg and hind limb vascular resistance by 114 +/- 12% and decreased blood flow by 39 +/- 3%. Most important, NLA also increased O2 uptake by 55 +/- 9% in hind limb skeletal muscle (P < .05), associated with decreases in PO2 and O2 saturation and an increase in reduced hemoglobin in hind limb venous blood.

CONCLUSIONS

Our results indicate that NO release from vascular endothelial cells appears to play an important physiological role in the regulation of tissue mitochondrial respiration in skeletal muscle and perhaps other organ systems.

Inhibition of biotransformation by nitric oxide (NO) overproduction and toxic consequences

Hepatic nitric oxide (NO) biosynthesis is induced by local or systemic inflammation. The highly reactive NO radical binds to prosthetic iron groups such as heme or iron-sulfur clusters leading to either activation or inhibition of enzymes such as guanylate cyclase, cyclooxygenase and aconitase. It has been known for years that NO also binds to the heme moiety of cytochrome P450s (CYP) with high affinity. However, it was demonstrated recently that binding of NO to CYPs also inhibits their enzymatic activity. This is true for exogenously applied as well as for endogenously synthesized NO. Suppression of CYP-dependent metabolism, which is a major problem of inflammatory liver diseases, can be significantly reversed by inhibition of NO synthesis in vivo under experimental conditions. We investigated whether these findings are applicable as a novel therapeutic principle in severe inflammatory liver dysfunction.

Tumor necrosis factor-alpha and nitric oxide production in endotoxin-primed rats administered carbon tetrachloride

Tumor necrosis factor-alpha (TNF alpha) is elevated in the sera of rats administered non-lethal doses of carbon tetrachloride (CCl4) followed by endotoxin. Elevated TNF alpha levels are correlated with the increased release of hepatic enzymes indicating hepatic damage. Under these conditions, nitric oxide (NO) was also produced in the liver as evidenced by the formation of nitrosyl complexes which were measured by electron paramagnetic resonance (EPR) spectroscopy. Decreased nitrosyl complex formation occurred in livers following treatment with either an inhibitor or macrophage activation (gadolinium trichloride; GdCl3), an inhibitor of cytokine responses (dexamethasone) or a NO synthase inhibitor (NG-monomethyl-L-arginine; 1-NMA), GdCl3 or dexamethasone treatment decreased, while 1-NMA treatment increased, TNF alpha serum level. Taken together, these data suggest that TNF alpha and NO are induced following CCl4 and LPS exposure and may be important regulators in the hepatotoxicity of this liver injury model.

Nitrogen oxide-induced autoprotection in isolated rat hepatocytes

Pretreatment of rat hepatocytes with low-dose nitrogen oxide (addition of SNAP in vitro or induction of nitric oxide synthase in vitro or in vivo) imparts resistance to killing and decrease in aconitase and mitochondrial electron transfer from a second exposure to a higher dose of SNAP. Induction of this resistance is prevented by cycloheximide, indicating upregulation of protective protein(s). Ferritin levels are increased as are non-heme iron-NO EPR signals. Tin-protoporphyrin (SnPP) prevents protection, suggesting involvement of hsp32 (heme oxygenase) and/or guanylyl cyclase (GC). Cross-resistance to H2O2 killing is also observed, which is also prevented by cycloheximide and SnPP. Thus, hepatocytes possess inducible protective mechanisms against nitrogen oxide and reactive oxygen toxicity.

Involvement of a transforming-growth-factor-beta-like molecule in tumor-cell-derived inhibition of nitric-oxide synthesis in cerebral endothelial cells

Nitric oxide (NO) has been shown to exert cytotoxic effects on tumor cells. We have reported that EC219 cells, a rat-brain-microvessel-derived endothelial cell line, produced NO through cytokine-inducible NO synthase (iNOS), the induction of which was significantly decreased by (a) soluble factor(s) secreted by DHD/PROb, an invasive sub-clone of a rat colon-carcinoma cell line. In this study, the DHD/PROb cell-derived NO-inhibitory factor was characterized. Northern-blot analysis demonstrated that the induction of iNOS mRNA in cytokine-activated EC219 cells was decreased by PROb-cell-conditioned medium. When DHD/PROb cell supernatant was fractionated by affinity chromatography using Con A-Sepharose or heparin-Sepharose, the NO-inhibitory activity was found only in Con A-unbound or heparin-unbound fractions, respectively, indicating that the PROb-derived inhibitory factor was likely to be a non-glycosylated and non-heparin-binding molecule. Pre-incubation of DHD/PROb-cell supernatant with anti-TGF-beta neutralizing antibody completely blocked the DHD/PROb-derived inhibition of NO production by EC219 cells. Addition of exogenous TGF-beta 1 dose-dependently inhibited NO release by EC219 cells. The presence of active TGF-beta in the DHD/PROb cell supernatant was demonstrated using a growth-inhibition assay. Moreover, heat treatment of medium conditioned by the less invasive DHD/REGb cells, which constitutively secreted very low levels of active TGF-beta, increased both TGF-beta activity and the ability to inhibit NO production in EC219 cells. Thus, DHD/PROb colon-carcinoma cells inhibited NO production in EC219 cells by secreting a factor identical or very similar to TGF-beta.

Nitric-oxide-mediated activation of iron-regulatory protein controls hepatic iron metabolism during acute inflammation

The molecular regulation of intracellular iron metabolism has been studied in the livers of rats undergoing an acute inflammatory reaction following turpentine injection. Treatment induced an increase in the steady-state level of the transferrin receptor (TfR) mRNA, peaking 18 h after treatment and returning to control levels 24 h after treatment, with no change in TfR gene transcription. RNA band-shift assays documented an activation of the cytoplasmic RNA-binding protein called the iron-regulatory protein (IRP), in parallel with a rise in the amount of TfR transcripts. A 2-3-fold increase in the amount of H and L ferritin subunit mRNAs was found 12-18 h after turpentine treatment. Surprisingly, higher accumulation of ferritin mRNAs did not result in appreciable differences in the liver ferritin content. This might be due to the concomitant rise of IRP activity, which is known to prevent ferritin mRNA translation. The absence of significant changes in the total iron and ferritin contents prompted us to investigate the role of nitric oxide (NO), an inflammatory mediator which is also known to modulate the activity of IRP. Northern-blot analysis showed a marked enhancement in the expression of the inducible form of nitric oxide synthase mRNA in turpentine-treated rats. Furthermore, the activation of IRP and the increase of the TfR mRNA content that occur in turpentine-treated rats were abolished by treatment with N5-nitro-L-arginine, a specific nitric oxide synthase inhibitor. The present data suggest that NO-mediated activation of IRP regulates alterations of hepatic iron homeostasis that occur in acute inflammation.

Nitric oxide: an overview

Nitric oxide (NO), a paracrine-acting gas enzymatically synthesized from L-arginine, is a unique biologic mediator that has been implicated in a myriad of physiologic and pathophysiologic states. It is an important regulator of vascular tone and may be the mediator of the hemodynamic changes involved in sepsis and cirrhosis. In addition, there is increasing evidence that NO is involved in coagulation, immune function, inhibitory innervation of the gastrointestinal tract, protection of gastrointestinal mucosa, and the hepatotoxicity of cirrhosis. It has already been speculated that NO may represent a point of control or intervention in a number of disease states. The purpose of this paper is to provide the surgeon with a broad overview of the scientific and clinical aspects of this important molecule.

Oxidative metabolism in sepsis and sepsis syndrome

The high mortality associated with sepsis syndrome and multiple organ dysfunction syndrome has persisted despite extraordinary research efforts in the laboratory and the intensive care unit. These syndromes produce systemic tissue damage that is likely to result from widespread inflammation and subsequent endothelial injury. This article reviews the oxidative metabolic effects and responses to sepsis syndrome at several levels: the oxygen transport system, the cell, and the mitochondrion. Specifically, aerobic metabolism of carbon substrates and oxygen is altered in sepsis. As a result of systemic inflammation and nonmetabolic oxygen use, oxidative stress may occur both outside and inside the cell. The consequences of these oxidative processes during sepsis may be ongoing cell damage mediated by reactive oxygen and nitrogen oxide species that culminates in multisystem organ failure.

Sulfhydryl involvement in nitric oxide sequestration and nitric oxide induced guanylyl cyclase activation in vascular smooth muscle

In the present study, the role of vascular smooth muscle sulhydryl groups was investigated with respect to sequestration of nitric oxide (NO) and activation of soluble guanylyl cyclase by NO. Vascular smooth muscle 100,000 x g supernatant (soluble) fraction was prepared in phosphate buffer, using the medial layer of bovine pulmonary artery. The soluble fraction was incubated with 100 pmol NO for 5 min in a sealed flask at 37 degree C under anerobic conditions in the presence or absence of the sulfhydryl reagent, N-ethylmaleimide (NEM, 5 mM). NO sequestration by the soluble fraction was measured as an indicator of NO binding. Total thiol content was measured in the soluble fraction with and without exposure to NEM. Guanylyl cyclase activity was measured in the soluble fraction with and without exposure to NO and a combination of NO and NEM. NEM decreased total thiol content in the soluble fraction from 103.59 nmol/mL to undetectable levels, and decreased guanylyl cyclase activity to below basal levels. The percentage of NO sequestered by the soluble fraction was inhibited by NEM by approximately 25% from a control value of 26.52 +/- 9.39 to 18.72 +/- 8.52, n = 13, p < 0.05. The data indicate that sulfhydryl groups are essential for guanylyl cyclase activation by NO, and are also involved in the sequestration of NO by the vascular smooth muscle soluble fraction.

Endotoxemia causes ileal mucosal acidosis in the absence of mucosal hypoxia in a normodynamic porcine model of septic shock

OBJECTIVE

To evaluate the hypothesis that splanchnic ischemia and mucosal hypoxia are responsible for lipopolysaccharide-induced intramucosal acidosis in pigs.

DESIGN

Prospective, randomized, unblinded study.

SETTING

Surgical research laboratory at a large, university-affiliated medical center.

SUBJECTS

Anesthetized, mechanically ventilated swine.

INTERVENTIONS

Pigs were infused with lactated Ringer's solution (12 mL/kg/hr) and, starting at 30 mins, 25-mL boluses of dextran-70 (maximum 15 mL/kg/hr) to maintain cardiac output at 90% to 110% of the baseline value for each pig. Ileal mucosal hydrogen ion concentration was measured tonometrically. A segment of distal ileum was exteriorized, opened, and placed on a platform to permit measurement of mucosal PO2, using an array of Clark-type microelectrodes and a computerized data acquisition and analysis system. Mucosal perfusion was measured using laser-Doppler flowmetry. The control group (n = 4) received no further interventions. Pigs in the lipopolysaccharide group (n = 6) were infused with 150 micrograms/kg of Escherichia coli lipopolysaccharide over 60 mins. To assess the effect of mucosal acidosis on mucosal PO2 in nonendotoxemic animals, intramucosal hydrogen ion concentration, mucosal PO2, and mucosal perfusion were measured in pigs rendered hypercarbic through deliberate hypoventilation (hypercarbia group; n = 4).

MEASUREMENTS AND MAIN RESULTS

Infusion of lipopolysaccharide resulted in a significant increase in intramucosal hydrogen ion concentration. However, in the lipopolysaccharide group, mucosal perfusion did not change significantly and mucosal PO2 increased significantly. In the hypercarbia group, hypercarbia was associated with significant increases in both intramucosal hydrogen ion concentration and mucosal PO2.

CONCLUSIONS

Mucosal hypoxia is not responsible for lipopolysaccharide-induced mucosal acidosis in this normodynamic pig model of septic shock. A rightward shift of the oxyhemoglobin dissociation curve (the Bohr effect) can explain the increase in mucosal oxygenation observed in endotoxemic pigs.

Hepatic nitric oxide production following acute endotoxemia in rats is mediated by increased inducible nitric oxide synthase gene expression

In the present studies, we analyzed the effects of acute endotoxemia on hepatocyte nitric oxide production and functional activity. Treatment of rats with 5 mg/kg of lipopolysaccharide (LPS), which induces acute endotoxemia, caused an increase in nitric oxide production in the liver, as measured by electron paramagnetic spin trapping, which was evident within 6 hours. This was associated with expression of inducible nitric oxide synthase (iNOS) messenger (m) RNA in hepatocytes and in sinusoidal cells throughout the liver lobule. Acute endotoxemia also caused alterations in hepatic structure, including hypertrophy, vacuolization, and chromosomal emargination, however these changes were not apparent for 24 to 48 hours. Hepatocytes isolated from endotoxemic rats released increased amounts of nitric oxide, measured by nitrite production, in response to interferon gamma (gamma-IFN) alone or in combination with LPS, tumor necrosis factor alpha, macrophage-colony stimulating factor, granulocyte/macrophage-colony stimulating factor, or hepatocyte growth factor. These results show that hepatocytes are sensitized by acute endotoxemia to respond to inflammatory mediators and growth factors. Increased nitrite production by hepatocytes was due to increased expression of iNOS mRNA and protein and was correlated with the time following induction of acute endotoxemia. Thus, cells isolated 48 hours after induction of acute endotoxemia released significantly more nitrite than cells recovered after 6 hours, a response that was not due to alterations in hepatocyte viability. Hepatocytes isolated from endotoxemic rats also exhibited a marked increase in proliferative capacity when compared with cells from control rats. Nitric oxide production by hepatocytes in vitro was associated with inhibition of cell growth and protein synthesis, which was reversed by the nitric oxide synthase inhibitor, NG-monomethyl-l-arginine (L-NMMA). Agarose gel electrophoresis showed extensive cytoplasmic DNA fragmentation in hepatocytes treated with LPS and gamma-IFN, a characteristic of apoptosis, which was also reversed by L-NMMA. These results, together with our findings that treatment of rats with an inhibitor of nitric oxide synthase partially reversed the structural alterations in the liver associated with acute endotoxemia suggest that nitric oxide may contribute to the pathophysiologic response to this bacterially derived toxin.

Does nitric oxide play a role in liver function?

Nitric oxide (NO) is becoming increasingly recognised as a signalling molecule in many organs, although its role in the liver remains to be fully elucidated. There is no doubt that liver cells can produce NO in response to a variety of stimuli including Corynebacterium parvum-infection, lipopolysaccharide (LPS) and a variety of cytokines. Within the liver, NO modulates some fundamental intracellular functions such as protein synthesis, mitochondrial electron transport and components of the citric acid cycle. Intercellular roles for NO in the liver may include drug metabolism and blood storage. Also, NO acts to protect the liver from immunological damage in models of hepatic inflammation. Understanding the role of NO in the liver may provide insight into the functioning of this organ in health and disease.

Effect of recombinant human tumor necrosis factor-alpha on cerebral oxygen uptake, cerebrospinal fluid lactate, and cerebral blood flow in the rabbit: role of nitric oxide

Among the important pathophysiologic alterations in the brain in bacterial meningitis are abnormalities of cerebral circulation and metabolism; however, the precise mechanisms by which these disturbances occur are not completely delineated. It has been recently recognized that cytokines are produced by tissues in the central nervous system in meningitis and play a critical role in the host inflammatory response. Because these mediators are involved in circulatory and metabolic disturbances in other tissues in sepsis, we investigated the role of tumor necrosis factor-alpha in the central nervous system in a rabbit model. We found that injection of recombinant human TNF into the cisterna magna in the rabbit led to an acute reduction in cerebral oxygen uptake and a more prolonged reduction in cerebral blood flow. This was accompanied by an increase in intracranial pressure and an increase in cerebrospinal fluid lactate. Reduction in oxygen uptake and increases in intracranial pressure and CSF lactate were blocked by pretreatment with L-NAME, an inhibitor of nitric oxide synthase. Reduction in cerebral blood flow was not affected by L-NAME treatment and was due to increased cerebrovascular resistance and reduced oxygen demand. These results suggest that TNF may be a critical mediator of changes in cerebral circulation and metabolism and that some of these changes occur via the nitric oxide pathway.

Nitric oxide induces necrotic but not apoptotic cell death in oligodendrocytes

We have investigated the mechanism of nitric oxide-induced damage in glial cells. Genomic DNA isolated from astrocytes and microglia, treated for 18 h with varying concentrations of a nitric oxide donor, was analysed by electrophoresis. No DNA damage was evident. Oligodendrocytes, treated with 2 mM nitric oxide for 3-48 h, showed single stranded breaks at 48 h but no laddering of nucleosomic fragments of DNA. When analysed by electron microscopy, ultrastructural changes in oligodendrocytes treated with 1 mM nitric oxide for 24 h showed intact nuclei but alterations in membranes and organelles characteristic of necrosis, including disrupted mitochondria with dissolution of their christae. Astrocytes, a glial cell type that we have previously shown to be much less sensitive to nitric oxide-induced damage, did not show ultrastructural changes. DNA analysis by flow cytometry of glial cells treated with nitric oxide supported the apparent necrotic-type death in oligodendrocytes. Double staining of oligodendrocytes, using Hoechst 33342 and propidium iodide for the simultaneous assessment of both apoptotic and necrotic cells, demonstrated that, while the proportion of dead cells increased with time and increasing concentrations of nitric oxide, the death was due to necrosis and not apoptosis. In this study, we demonstrate that direct exposure to soluble nitric oxide, produced in vitro from a nitric oxide donor chemical, ultimately kills oligodendrocytes by necrosis. Microglia and astrocytes maintain DNA and organelle integrity when exposed to exogenous nitric oxide.

Nitric oxide synthase inhibition and extracellular glutamate concentration after cerebral ischemia/reperfusion

BACKGROUND AND PURPOSE

Transient cerebral ischemia in rats results in selective loss of neuronal viability, eg, hippocampal CA1 neurons. The neurochemical variables responsible for this selective vulnerability to ischemia/reperfusion (IR) appear to involve excitatory amino acids. In brain IR, excitatory amino acid toxicity may be modulated by endogenous nitric oxide (NO.) gas. To investigate NO. in global brain IR, we measured the effects of NO. synthase (NOS) inhibition on interstitial excitatory amino acids in rats. Changes in postischemic cerebral blood flow and blood-brain barrier function also were evaluated.

METHODS

Forebrain ischemia was produced by systemic hypotension and occlusion of both carotid arteries for 15 minutes. Blood flow was restored for 60 minutes by unclamping the carotids and reinfusing with blood. A microdialysis probe was placed into the cortex and hippocampus using a stereotaxic device. Interstitial glutamate concentration was measured during IR with high-performance liquid chromatography. A competitive NOS inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME), was given intraperitoneally 30 minutes before ischemia in doses of 1, 4, and 20 mg/kg. Changes in cerebral blood flow and blood-brain barrier during IR were determined using laser-Doppler flowmetry and microdialysis with sodium fluorescein.

RESULTS

Glutamate in the dialysate during IR increased transiently 10-fold and returned to baseline levels by 30 minutes of reperfusion. Animals treated with L-NAME 30 minutes before ischemia also showed increases in glutamate concentration during ischemia, but glutamate remained elevated during reperfusion. The increase in glutamate concentration during reperfusion caused by L-NAME was prevented by L-arginine. The administration of L-arginine and L-NAME together decreased extracellular glutamate concentration during ischemia. Cerebral blood flow decreased to about 5% of baseline values during ischemia but increased approximately fourfold relative to control values on reperfusion. The hyperemic responses after ischemia were not different between IR groups treated with or without L-NAME. Brain ischemia increased the permeability of the blood-brain barrier to fluorescein; however, this change was attenuated by L-NAME administration at 20 mg/kg.

CONCLUSIONS

NOS inhibition did not attenuate extracellular glutamate accumulation during ischemia and increased its concentration on reperfusion. The elevated glutamate concentration after IR in L-NAME-treated rats did not appear to be due to either a decrease in cerebral blood flow response after ischemia or increases in local blood-brain barrier permeability. For the most part, the blood-brain barrier was spared in the immediate postischemic period by L-NAME treatment. These data suggest that NO. production may oppose synaptic excitatory amino acid accumulation and presumably excitotoxicity during IR.

Bioenergetic and oxidative stress in neurodegenerative diseases

Aging is a major risk factor for several common neurodegenerative diseases, including Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), and Huntington's disease (HD). Recent studies have implicated mitochondrial dysfunction and oxidative stress in the aging process and also in the pathogenesis of neurodegenerative diseases. In brain and other tissues, aging is associated with progressive impairment of mitochondrial function and increased oxidative damage. In PD, several studies have demonstrated decreased complex I activity, increased oxidative damage, and altered activities of antioxidant defense systems. Some cases of familial ALS are associated with mutations in the gene for Cu, Zn superoxide dismutase (Cu, Zn SOD) and decreased Cu, Zn SOD activity, while in sporadic ALS oxidative damage may be increased. Defects in energy metabolism and increased cortical lactate levels have been detected in HD patients. Studies of AD patients have identified decreased complex IV activity, and some patients with AD and PD have mitochondrial DNA mutations. The age-related onset and progressive course of these neurodegenerative diseases may be due to a cycling process between impaired energy metabolism and oxidative stress.

Inhibition of brain macrophage/microglial respiratory chain enzyme activity in experimental autoimmune encephalomyelitis of the Lewis rat

We measured the activities of five respiratory chain enzymes in brain macrophages/microglial cells from Lewis rats with experimental autoimmune encephalomyelitis (EAE) and found a significant reduction of the activity of nicotinamide adenine dinucleotide-dehydrogenase (NADH-DH), succinate cytochrome c reductase (SCCR) and succinate dehydrogenase (SDH) when compared with age-matched healthy control animals. The inhibition of NADH-DH (complex I) was specific for EAE, while we also found a reduction of SCCR and SDH activities (complex II) in newborn rats and adjuvant-immunised rats. Activities of NADH cytochrome c reductase (NCCR) and cytochrome c oxidase (COX) were not significantly changed. These observations demonstrate an impairment of brain macrophage/microglial respiratory chain function in central nervous system inflammation.

Nitric oxide and intracellular heme

Figure 2 depicts a working hypothesis for these results. Activation of .NO synthesis results in nitrogen oxide-induced loss of protein-bound heme from CYP proteins, which remain relatively intact. This heme liberation results in a decrease in heme synthesis (decreased ALAS) and an increase in heme degradation (increased HO). In addition, .NO synthesis results in direct inhibition of ferrochelatase, which further contributes to inhibition of heme synthesis. There also appears to be a mechanism to repair or resynthesize CYP after .NO synthesis is inhibited. Finally, a result of this effect may be protection against cellular injury, since increased HO is an important response against cellular injury from a variety of insults.

The herbal medicine sho-saiko-to induces nitric oxide synthase in rat hepatocytes

We have examined the effects of the herbal medicine sho-saiko-to (SST) on nitric oxide (NO) biosynthesis in rat hepatocytes by measuring the stable end-product nitrite and the mRNA of inducible NO synthase (iNOS). Interferon-gamma (IFN) by itself failed to induce NO synthesis (IFN: 1-1,000 u/ml). SST also did not elicit NO synthesis at concentrations up to 300 micrograms/ml when administered alone, but dose-dependently induced NO production in the presence of IFN. Whereas SST or IFN induced barely detectable levels of iNOS mRNA when administered alone, a combination of SST and IFN markedly induced iNOS mRNA in the cells. SST also modestly increased NO synthesis caused by interleukin-1 or bacterial lipopolysaccharide as a single agent, or in combination with IFN. On the other hand, SST had no effects on the NO synthesis produced by iNOS which were already induced. Thus, we found that SST stimulates cultured hepatocytes to produce NO by inducing iNOS gene expression under appropriate conditions. The capability of SST to induce NO biosynthesis might be related to the therapeutic efficacy of SST on the liver diseases.

Lack of role for nitric oxide (NO) in the selective destabilization of endothelial NO synthase mRNA by tumor necrosis factor-alpha

The constitutive expression of endothelial nitric oxide (NO) synthase (cNOS) is essential for the physiological regulation of vascular tone and structure. The mechanism of downregulation of steady state cNOS mRNA in human umbilical vein endothelial cells exposed to tumor necrosis factor-alpha (TNF-alpha) was investigated by using Northern blot analysis of total cellular RNA. TNF-alpha produced a dose- and time-dependent decrease in cNOS mRNA expression that was near maximal at 10 U/mL and 6 hours of exposure, respectively. In contrast, steady state expression of endothelin-1 and plasminogen activator inhibitor-1 (PAI-1) mRNA was upregulated by TNF-alpha. The pharmacological generation of NO using sodium nitroprusside (10 mumol/L) and S-nitroso-acetylpenicillamine (100 to 400 mumol/L) had no effect on cNOS mRNA levels, and TNF-alpha-induced downregulation of cNOS was not prevented by coincubation with the inhibitors of NO synthesis N omega-nitro-L-arginine methyl ester (1 mmol/L) and NG-monomethyl L-arginine (10 mmol/L). Under control conditions, cNOS and PAI-1 mRNA were stable after treatment with actinomycin D for periods greater than 24 hours, whereas endothelin-1 message was rapidly degraded (half-life, < 1 hour). Pretreatment with TNF-alpha (30 U/mL) selectively reduced that half-life of cNOS mRNA to less than 12 hours without altering the stability of PAI-1 message. TNF-alpha-induced destabilization of cNOS mRNA could be partially prevented by coincubation with cycloheximide (1 mumol/L) but was not reproduced by addition of sodium nitroprusside.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of mitochondrial beta-oxidation as a mechanism of hepatotoxicity

Severe and prolonged impairment of mitochondrial beta-oxidation leads to microvesicular steatosis, and, in severe forms, to liver failure, coma and death. Impairment of mitochondrial beta-oxidation may be either genetic or acquired, and different causes may add their effects to inhibit beta-oxidation severely and trigger the syndrome. Drugs and some endogenous compounds can sequester coenzyme A and/or inhibit mitochondrial beta-oxidation enzymes (aspirin, valproic acid, tetracyclines, several 2-arylpropionate anti-inflammatory drugs, amineptine and tianeptine); they may inhibit both mitochondrial beta-oxidation and oxidative phosphorylation (endogenous bile acids, amiodarone, perhexiline and diethylaminoethoxyhexestrol), or they may impair mitochondrial DNA transcription (interferon-alpha), or decrease mitochondrial DNA replication (dideoxynucleoside analogues), while other compounds (ethanol, female sex hormones) act through a combination of different mechanisms. Any investigational molecule should be screened for such effects.

Regulation and function of inducible nitric oxide synthase during sepsis and acute inflammation

During sepsis and inflammation profound changes in physiological function are induced by a variety of mediators, including endotoxin, various cytokines, and NO. Many of these mediators, in addition to their other functions, induce the synthesis of NO through the induction of iNOS within a variety of cell types. The regulation of iNOS expression is quite complex. Of interest is the fact that the functions of NO during sepsis range from modulating perfusion to mediating cytotoxicity. In addition, it is unique that many tissues not characterized as being involved in immune function express iNOS in a manner similar to that of tissues involved in immune function. The role of NO during episodes of acute inflammation appears to be a protective one; however, there are examples of chronic localized inflammation in both animal and human models which suggest that chronic iNOS expression may be detrimental. Further investigations into the regulation and function of NO in both the acute and chronic settings are necessary in order to fully understand this small yet unique molecule.

Nanomolar concentrations of nitric oxide reversibly inhibit synaptosomal respiration by competing with oxygen at cytochrome oxidase

Nitric oxide (NO) reversibly inhibited oxygen consumption of brain synaptosomes. Inhibition was reversible, occurred at the level of cytochrome oxidase, and was apparently competitive with oxygen, with half-inhibition by 270 nM NO at oxygen concentrations around 145 microM and by 60 nM at around 30 microM O2. Isolated cytochrome oxidase was inhibited by similar levels of NO. These levels of NO are within the measured physiological and pathological range for a number of tissues and conditions, suggesting that NO inhibition of cytochrome oxidase and the competition with oxygen may occur in vivo.

Quantitation of nitrotyrosine levels in lung sections of patients and animals with acute lung injury

Activated alveolar macrophages and epithelial type II cells release both nitric oxide and superoxide which react at near diffusion-limited rate (6.7 x 10(9) M-1s-1) to form peroxynitrite, a potent oxidant capable of damaging the alveolar epithelium and pulmonary surfactant. Peroxynitrite, but not nitric oxide or superoxide, readily nitrates phenolic rings including tyrosine. We quantified the presence of nitrotyrosine in the lungs of patients with the adult respiratory distress syndrome (ARDS) and in the lungs of rats exposed to hyperoxia (100% O2 for 60 h) using quantitative immunofluorescence. Fresh frozen or paraffin-embedded lung sections were incubated with a polyclonal antibody to nitrotyrosine, followed by goat anti-rabbit IgG coupled to rhodamine. Sections from patients with ARDS (n = 5), or from rats exposed to hyperoxia (n = 4), exhibited a twofold increase of specific binding over controls. This binding was blocked by the addition of an excess amount of nitrotyrosine and was absent when the nitrotyrosine antibody was replaced with nonimmune IgG. In additional experiments we demonstrated nitrotyrosine formation in rat lung sections incubated in vitro with peroxynitrite, but not nitric oxide or reactive oxygen species. These data suggest that toxic levels of peroxynitrite may be formed in the lungs of patients with acute lung injury.

Role of nitric oxide in the regulation of oxygen consumption in conscious dogs

The role of nitric oxide (NO) in the regulation of O2 consumption was studied in chronically instrumented conscious dogs. A specific NO synthesis inhibitor, nitro-L-arginine (NLA, 30 mg/kg i.v.), significantly increased mean arterial pressure from 100 +/- 4 to 134 +/- 5 mm Hg (mean +/- SEM) and total peripheral resistance by 157 +/- 16% and reduced cardiac output by 47 +/- 3% and heart rate by 34 +/- 6% after 120 minutes. Changes in arterial blood gases were not observed. There were significant changes in PO2 (-14 +/- 2 mm Hg), O2 saturation (-21 +/- 2%), the percentage of hemoglobin as oxyhemoglobin (-21 +/- 2%), and O2 content (-3.0 +/- 0.9 vol%) and a significant increase in percent reduced hemoglobin (21 +/- 1%) in mixed venous blood, associated with an increase in O2 extraction (5.1 +/- 0.2 vol%) (all P < .01). O2 consumption was increased from 124 +/- 6 to 155 +/- 9 mL/min (P < .05). Methoxamine, titrated to have hemodynamic effects similar to those of NLA (eg, mean arterial pressure increased from 97 +/- 4 to 131 +/- 5 mm Hg), had much smaller effects on venous blood gases, hemoglobin, and O2 extraction (2.3 +/- 0.7 vol%) and no significant effect on O2 consumption. NLA also caused an increase in O2 consumption of 37 +/- 8% (P < .01) in quietly resting conscious dogs that had undergone pretreatment with hexamethonium and atropine, but no significant change in O2 consumption in dogs anesthetized with barbiturate.(ABSTRACT TRUNCATED AT 250 WORDS)

Duration of expression of inducible nitric oxide synthase in glial cells

Lipopolysaccharide (LPS) or a combination of interleukin (IL)-1 beta and interferon (IFN)-gamma cause transcriptional induction of a calcium-independent nitric oxide synthase (NOS) in astrocytes and C6 glioma cells. LPS induction of NOS in C6 cells was evidenced by a small amount of nitrite accumulation as compared with cells exposed to IL-1 beta/IFN-gamma, but the maximal NOS activity achieved (as revealed by cGMP formation) was the same. The NOS activity induced by LPS in C6 cells was maximal at 4 to 8 hr and then rapidly decreased, while NOS activity induced by IL-1 beta/IFN-gamma slowly decreased after 4 hr. In addition, the effects of re-presenting IL-1 beta/IFN-gamma to both astrocytes and C6 cells after maximal induction of activity of the inducible form of NOS were studied. The re-addition of cytokines prolonged both NOS mRNA expression and also enzyme activity, suggesting effects at either the transcriptional (further induction) or translational level (mRNA stability). These results imply that the time course of NO production by induced astrocytes depends both upon the nature of the inducing stimulus and the frequency of the cells' exposure to it.

Endogenous nitric oxide attenuates ethanol-induced perturbation of hepatic circulation in the isolated perfused rat liver

The purpose of this study was to clarify the role of endogenous nitric oxide in ethanol-induced perturbation of microcirculation and hepatic injury in perfused rat liver. Infusion of ethanol into the portal vein at 25 and 100 mmol/L increased portal pressure, which is an indicator of hepatic vasoconstriction, in a concentration-dependent fashion. Portal pressure started to rise immediately after ethanol load, then decreased gradually and remained at higher than basal levels throughout the period of ethanol infusion. Release of lactate dehydrogenase into the effluent perfusate began to increase after 30 min of ethanol infusion and continued to increase during the 60-min period of ethanol infusion. The lactate dehydrogenase level in the effluent perfusate at 60 min was dependent on the ethanol concentration (0 mmol/L, 8 +/- 3 IU/L; 25 mmol/L, 16 +/- 2 IU/L; 100 mmol/L, 52 +/- 6 IU/L). Simultaneous infusion of NG-monomethyl-L-arginine, a nitric oxide synthesis inhibitor, enhanced significantly the ethanol-induced increase in portal pressure by 100% to 400% and increased lactate dehydrogenase release by 40% to 80%. The effect of NG-monomethyl-L-arginine on the ethanol-induced increase in portal pressure was completely reversed by the co-infusion of an excess dose of L-arginine. Change in portal pressure averaged over 60 min of ethanol infusion correlated with levels of lactate dehydrogenase release 60 min after the initiation of ethanol infusion (r = 0.77, p < 0.01). In conclusion, inhibition of the action of endogenous nitric oxide was associated with an increase in hepatic vasoconstriction and hepatocellular damage.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversibility of interleukin-1 beta-induced islet destruction and dysfunction by the inhibition of nitric oxide synthase

We have examined the reversibility of NO-mediated islet dysfunction and destruction induced by interleukin-1 beta (IL-1 beta). Previous studies have shown that pretreatment of islets for 18 h with IL-1 beta results in an inhibition of glucose-stimulated insulin secretion that requires 4 days incubation in the absence of IL-1 beta to restore islet secretory function. In this study we use a sequential experimental design in which islets are first exposed to IL-1 beta for 18 h, and then treated with the NO synthase inhibitor NG-monomethyl-L-arginine (NMMA). Insulin secretion is inhibited by 98% after the 18 h incubation with IL-1 beta, and this inhibition is reversed in a time-dependent fashion by NMMA, with complete recovery of insulin secretion observed 8 h after the inhibition of NO synthase. Inhibition of NO synthase also restores IL-1 beta-induced inhibition of mitochondrial aconitase activity in a time-dependent fashion that mimics the recovery of glucose-stimulated insulin secretion by islets. Ferrous iron and the reducing agents cysteine and thiosulphate accelerate the rate of recovery of insulin secretion, and ferrous iron and thiosulphate stimulate the recovery of islet aconitase activity, suggesting that iron-sulphurcentre reconstitution may be involved in the recovery process. The recovery process also appears to require mRNA transcription, because the transcriptional inhibitor actinomycin D prevents the recovery of insulin secretion by islets after the inhibition of NO synthase. Although IL-1 beta induces the co-expression of NO synthase and cyclo-oxygenase by islets, cyclo-oxygenase is not involved in the recovery of glucose-stimulated insulin secretion. Inhibition of NO synthase also prevents IL-1 beta-induced islet destruction, which otherwise occurs during a 96 h continuous exposure to this cytokine. The destructive effects of IL-1 beta on islet viability are prevented if NMMA is added to islet cultures during the first 24 h of exposure to IL-1 beta, but islet destruction is not prevented if NMMA is added after the first 48 h exposure to IL-1 beta. These results show that IL-1 beta-induced islet dysfunction is reversed by the inhibition of NO synthase, that recovery of insulin secretion is stimulated by iron and reducing agents, and that the recovery process appears to require mRNA transcription. We also show that it is possible to rescue islets from the destructive effects of IL-1 beta if NO synthase is inhibited early after its induction.