Multiple cytokines are required to induce hepatocyte nitric oxide production and inhibit total protein synthesis

The role of xanthine oxidase in oxidative damage caused by cytokines in cultured mouse hepatocytes

In the present study we have examined the potential role of xanthine oxidase (XO) in the intracellular oxidative stress induced by combinations of recombinant murine TNF alpha (rMuTNF alpha) and murine interferon-gamma (IFN gamma) in cultured mouse hepatocytes. IFN gamma alone and the combination of rMuTNF alpha and IFN gamma increased XO activity after a 4 hr exposure period. rMuTNF alpha alone increased XO activity only after 24 hr. At the later time point, the increased XO activity was accounted for by decreased XDH activity. However, the apparent conversion of XDH to XO cannot account for the early effects of rMuTNF alpha on hepatocyte function, particularly the onset of an oxidative stress (as indicated by efflux of GSSG from the hepatocytes). This effect is observed after two hours, and it is temporally the earliest sign of alteration of cellular function caused by rMuTNF alpha. Increased XO activity was not observed until 4 hr after treatment with rMuTNF alpha/IFN gamma. In addition, inhibition of XO activity with allopurinol did not ameliorate GSSG efflux from hepatocytes treated with the cytokines. However, the ATP depletion caused by the combination of rMuTNF alpha and IFN gamma and the cytotoxicity observed with the combined cytokines in cells pre-treated with BCNU (to inhibit glutathione reductase) was inhibited by allopurinol. These results show that the onset of oxidative stress in cultured mouse hepatocytes is not due to conversion of XDH to XO. However, events which follow the efflux of GSSG, such as ATP depletion and cytotoxicity in cells with impaired anti-oxidant defenses, may be partially due to increased XO activity, especially in cells treated with both rMuTNF alpha and IFN tau.

Nicotinamide inhibits nitric oxide synthase mRNA induction in activated macrophages

Nitric oxide (NO) is a potent mediator involved in many biological functions including inflammation and non-specific immunity. Murine macrophages possess the prototype of high-output NO synthase which is not constitutively expressed but induced within a few hours by immunological stimuli. In this study, we explored the possibility of controlling the activity of the inducible NO synthase by interfering with the transduction signal which triggers its induction, in the RAW 264.7 macrophage cell line. We found that nicotinamide, an inhibitor of ADP-ribosylation, prevented NO synthase induction in RAW 264.7 cells after stimulation with interferon gamma (IFN-gamma) and lipopolysaccharide (LPS). Furthermore, the level of NO synthase mRNA was measured by Northern-blot analysis and we found that nicotinamide prevents expression of NO synthase mRNA in IFN-gamma- and LPS-stimulated cells. Nicotinamide was also found to inhibit other macrophage functions expressed in response to IFN-gamma, i.e. tumour necrosis factor secretion and the expression of the Ia antigen of the major histocompatibility complex. Analysis of the pattern of ADP-ribosylated proteins revealed that nicotinamide as well as cholera toxin prevented the ADP-ribosylation of a 107-117 kDa protein found constitutively ADP-ribosylated in stimulated and non-stimulated macrophage extracts. Together, our results indicate ADP-ribosylation as a crucial point of the signalling pathway which leads to NO synthase mRNA induction.

Targets for sepsis therapies: tumor necrosis factor versus interleukin-1

Clinical testing of therapies for sepsis that target tumor necrosis factor and interleukin-1 is currently in progress. It is now clear from early clinical results that patients display a heterogeneous response to anti-cytokine therapies that may be related to the stage or severity of disease. Experimental results show that either cytokine may contribute to the metabolic derangements that lead to organ dysfunction in severe sepsis. They also provide a physiological basis for understanding the benefits of anti-cytokine therapy in the most severely ill patients.

Interleukin-1 contributes to the induction of nitric oxide synthase by endotoxin in vivo

We investigated the role of interleukin-1 in the induction of a Ca(2+)-independent nitric oxide (NO) synthase by bacterial endotoxin in vivo. In anaesthetized rats, pretreatment with interleukin-1 receptor antagonist (interleukin-1ra; 16 mg kg-1 i.v., followed by an infusion of 2.4 mg kg-1 h-1) ameliorated the delayed hypotension and tachycardia in response to endotoxin (2 mg kg-1 i.v.). Endotoxaemia for 3 h induced a Ca(2+)-independent NO synthase activity in the lung and reduced the contractions to noradrenaline in the thoracic aorta ex vivo. Treatment with interleukin-1ra attenuated both the induction of NO synthase in the lung (by 46 +/- 5%) and the endotoxin-induced hyporeactivity to noradrenaline in the aorta. Thus, endogenous interleukin-1 contributes to the induction of NO synthase in response to endotoxin in vivo.

Arginine transport in human liver. Characterization and effects of nitric oxide synthase inhibitors

OBJECTIVE

Arginine transport was characterized and studied in human liver.

SUMMARY BACKGROUND DATA

Plasma arginine uptake may regulate hepatocyte intracellular availability and the subsequent biosynthesis of nitric oxide (NO), but little is known about arginine transport across the human hepatocyte plasma membrane.

METHODS

The authors characterized plasma membrane transport of 3[H]-L-arginine in hepatic plasma membrane vesicles (HPMVs) and in hepatocytes isolated and cultured from human liver biopsy specimens. They also studied the effects of the NO synthase inhibitors omega-nitro-L-arginine methyl ester (L-NAME) and N-methyl-arginine (NMA) on arginine transport in HPMVs and in cultured cells.

RESULTS

Arginine transport was saturable, Na(+)-independent, temperature and pH sensitive, and was inhibited by the naturally occurring amino acids lysine, homoarginine, and ornithine (System y+ substrates). Arginine transport by both vesicles and cultured hepatocytes was significantly attenuated by NO synthase inhibitors, suggesting that the arginine transporter and the NO synthase enzyme may share a structurally similar arginine binding site. Dixon plot analysis showed the blockade to occur by competitive, rather than noncompetitive, inhibition. In vivo treatment of rats with lipopolysaccharide (LPS) resulted in a twofold stimulation of saturable arginine transport in the liver. This LPS-induced hepatic arginine transport activity was also inhibited by L-NAME. These data indicate that arginine transport by human hepatocytes is mediated primarily by the Na(+)-independent transport System y+.

CONCLUSIONS

Besides inhibition of the NO synthase enzyme, the ability of arginine derivatives to block NO production may also be due to their ability to competitively inhibit arginine transport across the hepatocyte plasma membrane. The use of selective arginine derivatives that compete with arginine at the plasma membrane level may be a metabolic strategy that can be used to modulate the septic response.

Hepatocyte injury by activated neutrophils in vitro is mediated by proteases

OBJECTIVE

This study determined the mechanism used by neutrophils (PMNs) to induce hepatocellular injury.

SUMMARY BACKGROUND DATA

Neutrophils have been shown to be potent mediators of cell and tissue injury and have been hypothesized to contribute to the hepatic injury that occurs after trauma and infection. Oxygen radical scavengers protect the liver in vivo from inflammatory injury and it has been suggested that PMNs are the source of these toxic oxygen radicals. The specific mechanism used by PMNs to produce hepatocellular damage, however, has not been determined.

METHODS

Neutrophils were cultured in vitro with hepatocytes (HCs) and stimulated with phorbol 12-myristate 13-acetate (PMA) to induce HC injury in the presence of oxygen radical scavengers and protease inhibitors.

RESULTS

PMA induced a PMN-mediated HC injury that was dependent on the number of PMNs present and the concentration of PMA. Protease inhibitors reduced the extent of HC injury, while oxygen radical scavengers had no effect. Hydrogen peroxide, directly applied, was able to injure HCs, but only at concentrations greater than those that could be produced by PMA-stimulated PMNs.

CONCLUSIONS

PMNs are cytotoxic to cultured HCs, predominantly due to the release of proteolytic enzymes, while HCs appear relatively resistant to oxidative injury. Involvement of neutrophil toxic oxygen radicals in hepatic damage in vivo may require impairment of HC antioxidant defenses or may involve injury to nonparenchymal liver cells with secondary effects on HCs.

Macrophages produce nitric oxide at allograft sites

OBJECTIVE

The current study was designed to determine which cytokines produced during an alloimmune response stimulate macrophage nitric oxide (.N = O) production at allograft sites.

SUMMARY BACKGROUND DATA

Previous work has demonstrated that rat sponge matrix allograft infiltrating cells produce more .N = O on stimulation with alloantigen than syngeneic graft-infiltrating cells. Addition of NG-monomethyl-L-arginine (NMA), an inhibitor of .N = O synthesis, promotes allospecific cytolytic T-lymphocyte effector function.

METHODS

Polyurethane sponges were implanted subcutaneously in recipient Lewis rats and injected with 10 x 10(6) ACl splenocytes. On various days after grafting, graft-infiltrating cells were harvested for in vitro study. Adherent macrophages from the graft infiltrating cell population were obtained by a 2- to 3-hour incubation to plastic dishes with subsequent washing to remove nonadherent cells.

RESULTS

Stimulation of unseparated graft-infiltrating cell populations with lipopolysaccharide or interferon-tau resulted in enhanced .N = O synthesis by allograft infiltrating cells compared with syngeneic graft-infiltrating cells, early after grafting. Macrophages recovered from an allograft site spontaneously produce more .N = O than macrophages recovered from syngeneic grafts (p < 0.001). Significantly enhanced levels of .N = O were produced by allograft macrophages compared with syngeneic graft macrophages on stimulation with lipopolysaccharide or interferon-tau (p < or = 0.025).

CONCLUSIONS

Nitric oxide appears to be produced in response to the local cytokines secreted by an ongoing rejection reaction. Nitric oxide serves under these circumstances to modulate the alloimmune response.

The effect of interleukin 1 alpha on acetaminophen-induced hepatotoxicity

The protective effect of interleukin 1 alpha (IL-1 alpha) in mice with acetaminophen (AAP)-induced hepatitis was investigated. IL-1 alpha had a significant protective effect if given 2 or more hours (up to 24 hours) before AAP; it significantly reduced mortality of mice and decreased serum transaminase level. The maximal effect was obtained with the dose of 1000 U (166 ng/kg) IL-1 alpha. Pretreatment with IL-1 significantly increased the synthesis of prostaglandin E2 (PGE2) in samples of liver tissue from AAP-treated mice, but had no effect on the synthesis of leukotriene C4 (LTC4). Pretreatment with indomethacin (IMC) did not abrogate significantly the protective effect of IL-1. Thus, the hepatoprotective effect of IL-1 alpha can not be entirely explained by the stimulation of prostaglandin (PG) synthesis.

Molecular cloning and expression of inducible nitric oxide synthase from human hepatocytes

Nitric oxide is a short-lived biologic mediator for diverse cell types. Synthesis of an inducible nitric oxide synthase (NOS) in murine macrophages is stimulated by lipopolysaccharide (LPS) and interferon gamma. In human hepatocytes, NOS activity is induced by treatment with a combination of tumor necrosis factor, interleukin 1, interferon gamma, and LPS. We now report the molecular cloning and expression of an inducible human hepatocyte NOS (hep-NOS) cDNA. hep-NOS has 80% amino acid sequence homology to macrophage NOS (mac-NOS). Like other NOS isoforms, recognition sites for FMN, FAD, and NADPH are present, as well as a consensus calmodulin binding site. NOS activity in human 293 kidney cells transfected with hep-NOS cDNA is diminished by Ca2+ chelation and a calmodulin antagonist, reflecting a Ca2+ dependence not evident for mac-NOS. Northern blot analysis with hep-NOS cDNA reveals a 4.5-kb mRNA in both human hepatocytes and aortic smooth muscle cells following stimulation with LPS and cytokines. Human genomic Southern blots probed with human hep-NOS and human endothelial NOS cDNA clones display different genomic restriction enzyme fragments, suggesting distinct gene products for these NOS isoforms. hep-NOS appears to be an inducible form of NOS that is distinct from mac-NOS as well as brain and endothelial NOS isozymes.

Metabolic basis for management of the septic surgical patient

The physiologic events accompanying postoperative septic complications in surgical patients represent a coordinated response to bacterial invasion, which is aimed at maintaining the function of key organ systems. When sepsis is prolonged or overwhelming, physiologic dysfunction and multiorgan failure develop. This review outlines the pathophysiologic response to sepsis and correlates it with recent therapeutic advances in the metabolic management of the postoperative septic patient.

Malaria antigen and cytokine-induced production of reactive nitrogen intermediates by murine macrophages: no relevance to the development of experimental cerebral malaria

The in vitro production of reactive nitrogen intermediates (RNI) by murine macrophages was evaluated in response to heat-stable malaria antigen and cytokines. Malaria antigen, interferon-gamma (IFN-gamma) and tumour necrosis factor (TNF) induced RNI production in macrophages in a dose-dependent way. RNI production steadily increased over a 2-day period and was enhanced when the malaria antigen was co-incubated with IFN-gamma and/or TNF. RNI production induced by either IFN-gamma or malaria antigen or a combination of the two was suppressed by pentoxifylline in a dose-dependent manner. Pentoxifylline did not significantly influence TNF-induced RNI production. L-N-monomethyl arginine reduced malaria antigen, IFN-gamma and TNF-induced RNI production when these reagents were used in combination or alone. An anti-TNF monoclonal antibody (mAb) reduced IFN-gamma-induced RNI production, but did not significantly alter the malaria antigen-induced RNI synthesis by macrophages. The influence of inhibitors of nitric oxide synthase, L-N-monomethyl arginine and N omega-nitro-L-arginine, was studied in experimental cerebral malaria. They did not exert any significant effect on the development of cerebral malaria in Plasmodium berghei ANKA-infected CBA/J mice.

Cytokines, endotoxin, and glucocorticoids regulate the expression of inducible nitric oxide synthase in hepatocytes

Nitric oxide (NO.) is a short-lived mediator which can be induced in a variety of cell types and produces many physiologic and metabolic changes in target cells. The inducible or high-output NO. synthase (NOS) pathway was first characterized in macrophages activated by lipopolysaccharide (LPS) and interferon gamma (IFN-gamma). Hepatocytes also express an inducible NOS following exposure to the combination of endotoxin (LPS) and tumor necrosis factor (TNF), interleukin 1 (IL-1), and IFN-gamma. In this study, to identify which of these cytokines, if any, was acting to induce the gene expression for hepatocyte NOS, we measured the levels of rat hepatocyte NOS mRNA by Northern blot analysis after stimulation by various combinations of endotoxin and cytokines in vitro. We found the mRNA for hepatocyte NOS to be a single band at approximately 4.5 kilobases which was maximally up-regulated (approximately 70-fold) by the combination of TNF, IL-1, IFN-gamma, and LPS. Abundance of NOS mRNA peaked 6-8 hr after stimulation and then declined by 25% at 24 hr. Unstimulated hepatocytes in vitro showed only a trace mRNA band after prolonged autoradiographic exposure. As single agents, TNF and IL-1 were the most effective inducers of hepatocyte NOS mRNA. Combinations of two or three stimuli revealed strong synergy between TNF, IL-1, and IFN-gamma. The increased mRNA levels correlated with elevated nitrogen oxide release and cGMP levels in the culture supernatants. Dexamethasone and cycloheximide inhibited induction of mRNA for hepatocyte NOS in a dose-dependent fashion. The addition of NG-monomethyl-L-arginine had no effect on mRNA levels but effectively blocked NO. formation. The inducible hepatocyte NOS mRNA was also detected in rat hepatocytes following chronic hepatic inflammation triggered by Corynebacterium parvum injection in vivo. These data demonstrate that the inducible NOS is functional in rat hepatocytes both in vitro and in vivo and that this pathway is under complex control. Endotoxin and inflammatory cytokines act synergistically to up-regulate gene expression for hepatocyte NOS, whereas glucocorticoids down-regulate the mRNA.

Protective effects of inhibitors of nitric oxide synthase in immune complex-induced vasculitis

1. The ability of analogues of L-arginine (N-iminoethyl-L-ornithine (L-NIO), NG-monomethyl-L-arginine (L-NMMA), NG-nitro-L-arginine methyl ester (L-NAME) and NG-nitro-L-arginine (L-NNA)) to protect against inflammatory injury induced by activated neutrophils was investigated in rats following intradermal or intrapulmonary deposition of immune complexes. 2. The descending order of potency for protective effects of these analogues was: L-NIO > L-NMMA > L-NNA = L-NAME. The approximate IC50 value for L-NIO in the dermal vasculitis model was 65 microM. For all other compounds, the IC50 values were > 5 mM. 3. The protective effect of L-NIO in the skin was reversed in a dose-dependent manner by the presence of L-arginine, but not by D-arginine. L-Arginine also reversed the protective effects of L-NIO in immune complex-induced lung injury. 4. The protective effects of L-NIO were not associated with reductions in neutrophil accumulation, as measured by extraction from tissues of myeloperoxidase. 5. These data demonstrate that L-NIO has the most potent protective effects against immune complex-induced vascular injury induced by activated macrophages. Furthermore, they indicate that this injury is dependent upon the generation of nitric oxide.

Tumor necrosis factor alpha inhibits hepatocyte mitochondrial respiration

Although direct cytotoxicity is a well-established phenomenon of tumor necrosis factor alpha (TNF alpha)-induced tissue damage, the intracellular events leading to cell death are still poorly understood. To study the cytotoxic effects of TNF alpha on normal parenchymal cells, rat hepatocytes were purified and incubated with various concentrations of TNF alpha. Mitochondrial respiration, total protein synthesis, and enzyme release were measured to assess metabolic performance and cell integrity. Treatment with TNF alpha suppressed mitochondrial respiration in a concentration-dependent fashion, resulting in a reduction of the activity of complex I of the respiratory chain to 67.0 +/- 3.5% of that of untreated hepatocytes by 2000 U/mL TNF alpha. Under these conditions protein synthesis and the release of intracellular enzymes were significantly increased. Both hepatocellular enzyme release and inhibition of mitochondrial respiration appear to be associated with the generation of reactive oxygen intermediates by the hepatocyte itself, because oxygen radical scavengers prevented these adverse effects of TNF alpha. Inhibition of protein synthesis by cycloheximide as well as addition of cyclic adenosine monophosphate synergistically enhanced the suppression of mitochondrial respiration by TNF alpha, resulting in complex I activity of 6.9 +/- 1.6% and 24.9 +/- 2.9% of that of untreated cells. These data indicate that inhibition of mitochondrial respiration is one of the mechanisms by which TNF alpha induces tissue injury.

Cytokines and lipopolysaccharide induce nitric oxide synthase in cultured rat pulmonary artery smooth muscle

In the current study, we describe cytokine and Escherichia coli lipopolysaccharide (LPS) induction of nitric oxide (NO) synthase mRNA levels in cultured smooth muscle from rat pulmonary artery (RPASM). Exposure of RPASM to interleukin-1 beta, interferon-gamma, or LPS alone did not significantly affect NO synthesis, as determined by nitrite concentrations in media. Exposure to tumor necrosis factor-alpha caused a modest (2x) increase in nitrite production. In contrast, exposure to a combination of the above three cytokines and LPS caused a large increase in NO synthesis. Exposure of RPASM to this combination caused an increase in mRNA levels of NO synthase (as described by Northern blot analysis with 32P-cDNA probe to an inducible form of NO synthase present in murine macrophages) that was apparent as early as 4 h. Expression of the induced gene product after exposure to the cytokine and LPS mixture was evident by significant increases in nitrite production at 12 h. Production of nitrite was completely abolished in the presence of NG-monomethyl-L-arginine (NMA), and this inhibition was reversible by the addition of excess L-arginine. NO synthase mRNA levels were not affected by NMA. The nitrite production induced by the combination of cytokines and LPS was abolished by pretreating cells with cycloheximide. These data indicate that a combination of cytokines and LPS affect expression of the gene for the inducible form of NO synthase in cultured RPASM.

Suppression of IL-2-induced SAA gene expression in mice by the administration of an IL-1 receptor antagonist

The hepatic acute phase response induced by the administration of interleukin (IL)-2 is most likely mediated by secondary cytokines. In this investigation, we examined the role of endogenous IL-1 in the synthesis of the hepatic acute phase protein serum amyloid A (SAA) during IL-2 treatment. The injection of IL-2 induced SAA gene expression in the liver. The concurrent administration of an IL-1 receptor antagonist (IL-1RA) markedly reduced hepatic SAA mRNA levels and, to a lesser extent, SAA protein levels in the serum. Although IL-1 is an inducer of IL-6 production, the administration of the IL-1RA had no effect on circulating IL-6 levels in IL-2-treated mice. These findings suggest that the production of IL-1 is an important factor in the induction of SAA mRNA in mice undergoing immunotherapy with IL-2.

Differential effects of monoclonal antibodies to tumor necrosis factor alpha and gamma interferon on induction of hepatic nitric oxide synthase in experimental gram-negative sepsis

To investigate the stimuli required for the induction of nitric oxide synthase (NOS) in sepsis, we have analyzed the levels of this enzyme in the livers of mice infected with a 90% lethal dose of Escherichia coli in a model of gram-negative sepsis. Hepatic NOS levels are markedly induced in this model, with peak values occurring 12 to 22 h following infection. Treatment with TN3-19.12, a neutralizing monoclonal antibody to tumor necrosis factor alpha (TNF-alpha), resulted in complete protection from death in this model of sepsis but had no significant effect on the level of induction of hepatic NOS. Treatment with H22, a monoclonal antibody to gamma interferon (IFN-gamma), also gave significant protection against death and, in addition, did lead to a decrease in the level of induction of the hepatic NOS. Treatment of mice with pure TNF-alpha (0.2 microgram), IFN-gamma (2,000 U), or a combination of the two did not induce the hepatic NOS, but treatment with the combination led to significant mortality (probability of survival at 22 h, 0.32). Thus, the level of induction of NOS within the liver either in sepsis or by the coadministration of TNF-alpha and IFN-gamma does not correlate with death.

Microglial-produced nitric oxide and reactive nitrogen oxides mediate neuronal cell death

The role of inflammatory cytokines in the pathogenesis of neurological diseases is not well understood. The neurotoxic effects of cytokines could be mediated by immunostimulation of glial cells to produce toxic concentrations of nitric oxide (NO) and reactive nitrogen oxides. Cultured microglia and meningeal fibroblasts, but not Type 1 astrocytes, were induced by lipopolysaccharides and cytokines to synthesize NO and reactive nitrogen oxides from L-arginine. In co-cultures of immunostimulated microglia and cerebellar granule neurons, neurotoxicity was blocked by an inhibitor of NO synthase, NG-nitroarginine, and by oxyhemoglobin, which inactivates NO. Microglial-induced neurotoxicity was also partially attenuated by the N-methyl-D-aspartate (NMDA) receptor antagonists, MK-801 and 2-amino-5-phosphovalerate (APV). Superoxide dismutase, which stabilizes NO through inactivation of superoxide anion, augmented microglial-mediated neurotoxicity either alone or in combination with MK-801 or APV. Hence, immunostimulated microglia mediate neurotoxicity by NO, reactive nitrogen oxides, superoxide anion and NMDA-like substances. These findings suggest a novel role for microglial-produced NO and reactive nitrogen oxides as a neurotoxic agent in neurodegenerative disease states.

Stimulation of the nitric oxide synthase pathway in human hepatocytes by cytokines and endotoxin

Nitric oxide (NO) is a short-lived biologic mediator that is shown to be induced in various cell types and to cause many metabolic changes in target cells. Inhibition of tumor cell growth and antimicrobial activity has been attributed to the stimulation of the inducible type of the NO synthase (NOS). However, there is limited evidence for the existence of such inducible NOS in a human cell type. We show here the induction of NO biosynthesis in freshly isolated human hepatocytes (HC) after stimulation with interleukin 1, tumor necrosis factor (TNF), IFN-gamma, and endotoxin. Increased levels of nitrite (NO2-) and nitrate (NO3-) in culture supernatants were associated with NADPH-dependent NOS activity in the cell lysates. The production of NO2- and NO3- was inhibited by NG-monomethyl L-arginine and was associated with an increase in cyclic guanylate monophosphate release. The data presented here provide evidence for the existence of typical inducible NO biosynthesis in a human cell type.

Gut ischemia/reperfusion-induced liver dysfunction occurs despite sustained oxygen consumption

We have previously shown that gut ischemia/reperfusion (I/R) causes liver dysfunction in vivo (increased [I125]albumin leak, decreased mitochondrial redox potential). Our purpose was to investigate liver dysfunction due to gut I/R in an ex vivo model where oxygen delivery (DO2) could be controlled. Rats underwent laparotomy (sham) or 45 min of superior mesenteric artery (SMA) occlusion (I/R) and 6 hr later the gut and liver were isolated in situ. Pressures were monitored while recirculating blood was perfused via the hepatic artery (2.5 ml/min) for 90 min and the SMA (7.5 ml/min) for the first 30 min, then the portal vein (7.5 ml/min) for 60 min. Both gut and liver DO2 and VO2 (Fick method) were maintained throughout the study period in the gut I/R as well as sham groups. Despite maintenance of liver VO2, however, gut I/R resulted in a marked and persistent reduction in bile flow. In conclusion, dysfunctional bile production after gut I/R is not due to impaired VO2, but rather gut-liver signaling yet to be defined.

Association between synthesis and release of cGMP and nitric oxide biosynthesis by hepatocytes

Hepatocytes are known to synthesize nitric oxide (NO) from L-arginine via an inducible NO synthase. Studies were performed to determine the relationship between hepatocyte NO production and the stimulation of hepatocyte soluble guanylate cyclase. A combination of lipopolysaccharide (LPS), interferon-gamma, tumor necrosis factor, and interleukin-1 stimulates the biosynthesis of large quantities of nitrite and nitrate (NO2- + NO3-). Hepatocyte NO2- + NO3- production was associated with only small increases in intracellular guanosine 3',5'-cyclic monophosphate (cGMP) levels but much greater increases in extracellular cGMP release over an 18-h time period. This cGMP synthesis was dependent on the L-arginine concentration and was inhibited in a reversible manner by NG-monomethyl-L-arginine. The cytokines or LPS added alone induced small increases in nitrogen oxide production and concomitant minor elevations in cGMP release. Atrial natriuretic peptide also stimulated the release of cGMP by hepatocytes which appeared to be independent of the cytokine+LPS-induced cGMP release. The addition of probenecid reduced the cGMP release by 66%, while cell damage was excluded as a cause for the extracellular release. Addition of 3-isobutyl-1-methylxanthine, but not M&B 22948, increased hepatocyte intra- and extracellular cGMP levels after cytokine+LPS stimulation. Induction of nitrogen oxide synthesis by hepatocytes in vivo by injecting rats with killed Corynebacterium parvum resulted in increased cGMP levels in freshly isolated hepatocytes and increased cGMP release by the hepatocytes when placed in culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Tumor necrosis factor induced oxidative stress in isolated mouse hepatocytes

Tumor necrosis factor alpha (TNF alpha) is a macrophage-derived cytokine which participates in homeostatic tissue repair. It is also a potentially useful antitumor agent. Liver toxicity, however, limits TNF alpha's clinical utility and suggests that it may play a role in liver toxicity of various etiologies. To determine the direct effects of TNF alpha on hepatocytes, in the absence of infiltrating leukocytes and other inflammatory mediators, an isolated mouse hepatocyte model has been used in the present study. Hepatocytes exposed to recombinant human TNF alpha (1-10 micrograms/ml) exhibited intracellular GSH depletion and GSSG efflux during the first 2 hr of exposure, but no cytotoxicity was observed. However, TNF alpha was toxic to hepatocytes pretreated with 1,3-bis(chloroethyl)-1-nitrosourea to inhibit GSSG-reductase activity. Furthermore, these cells exhibited a greater efflux of GSSG upon exposure to TNF alpha. TNF alpha also caused a marked decrease in cellular ATP concentrations, which occurred after initiation of effects on the glutathione pool. These findings indicate that high concentrations of TNF alpha induce an oxidant stress in isolated hepatocytes. The antioxidants mannitol and benzoate, as well as the iron chelator deferoxamine, reduced the extent of TNF alpha-induced oxidant effects in hepatocytes, which indicates that the oxidant stress may involve hydroxyl radical generation. Hepatocytes treated with ruthenium red or fructose were less susceptible to TNF alpha-induced ATP depletion, which suggests that mitochondrial calcium cycling may be involved in disruption to energy metabolism.

Nitrogen oxide levels in patients after trauma and during sepsis

The mediators responsible for maintenance of the hyperdynamic state and the low systemic vascular resistance (SVR) observed in sepsis have not been elucidated. Nitric oxide (.N = O) is a mediator with numerous functions, including regulation of vascular tone and a role in macrophage-mediated cytostasis and microbiostasis. Thirty-nine critically ill trauma and septic patients were studied to determine the relationship between .N = O production and the hyperdynamic state. high plasma levels of NO2-/NO3- (the stable end products of .N = O) were observed in septic patients (p less than 0.02). Low SVR and high endotoxin levels were associated with high NO2-/NO3- values (p = 0.029, p = 0.002). Changes in .N = O levels may mediate the vasodilation seen in sepsis. Low NO2-/NO3- levels were observed in trauma patients (p less than 0.001) and remained low even in the presence of sepsis (p = 0.001).

Tissue injury caused by deposition of immune complexes is L-arginine dependent

Nitric oxide (NO.), a free radical that is generated from L-arginine by stimulated endothelial cells, neutrophils, activated macrophages, and other cell types, reacts with superoxide anion (O2.-) to form peroxynitrite, which itself may be tissue toxic or can then react further to form the highly reactive and toxic hydroxyl radical (HO.). Because vascular injury produced by tissue deposition of immune complexes is linked to formation of toxic products derived from activated neutrophils, we have assessed whether immune complex-induced injury of rat lung and dermal vasculature is arginine dependent. The arginine analogue, NG-monomethyl-L-arginine (N-MeArg), which blocks NO. formation, protects against immune complex-induced vascular injury in rats. The protective effects of N-MeArg are reversed by the presence of L-arginine but not D-arginine. Additionally, in the absence of N-MeArg, injury is enhanced by the presence of L-arginine but not by D-arginine. Protection by N-MeArg is not associated with diminished recruitment of polymorphonuclear leukocytes. Bronchoalveolar lavage fluids from animals undergoing immune complex deposition in lung contain the decomposition products of NO.--namely, nitrite and nitrate. In the presence of N-MeArg these products are greatly diminished. These data suggest that immune complex-induced injury of rat lung and skin is L-arginine dependent. These data also suggest that in vivo metabolic products of L-arginine, such as NO(.), are directly or indirectly linked to immune complex-induced tissue injury.

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

Although nitric oxide (.N = O) biosynthesis is inducible in rat hepatocytes (HC), the physiological significance of .N = O production by these cells is unknown. Short exposure of HC to authentic .N = O led to a concentration-dependent inhibition of mitochondrial aconitase, NADH-ubiquinone oxidoreductase, and succinate-ubiquinone oxidoreductase (complexes I and II of the mitochondrial electron transport chain). Most susceptible to .N = O inhibition was mitochondrial aconitase, in which a reduction in enzyme activity to 20.2 +/- 1.6% of control was observed. In contrast to mitochondrial aconitase, cytosolic aconitase activity was not inhibited by .N = O. After exposure to a maximal inhibitory concentration of .N = O, mitochondrial aconitase activity recovered completely within 6 h. Complex I did not fully recover within this incubation period. Endogenous .N = O biosynthesis was induced in HC by a specific combination of cytokines and lipopolysaccharide. After 18 h of incubation with these stimuli, a significant inhibition of mitochondrial aconitase activity to 70.8 +/- 2.4% of controls was detected. However, this was due only in part to the action of .N = O. A non- .N = O-dependent inhibition of mitochondrial function appeared to be mediated by tumor necrosis factor.