Influence of bilirubin and other antioxidants on nitrergic relaxation in the pig gastric fundus

Bilirubin acts as a multipotent guardian of cardiovascular integrity: more than just a radical idea

Bilirubin, a potentially toxic catabolite of heme and indicator of hepatobiliary insufficiency, exhibits potent cardiac and vascular protective properties. Individuals with Gilbert's syndrome (GS) may experience hyperbilirubinemia in response to stressors including reduced hepatic bilirubin excretion/increased red blood cell breakdown, with individuals usually informed by their clinician that their condition is of little consequence. However, GS appears to protect from all-cause mortality, with progressively elevated total bilirubin associated with protection from ischemic heart and chronic obstructive pulmonary diseases. Bilirubin may protect against these diseases and associated mortality by reducing circulating cholesterol, oxidative lipid/protein modifications, and blood pressure. In addition, bilirubin inhibits platelet activation and protects the heart from ischemia-reperfusion injury. These effects attenuate multiple stages of the atherosclerotic process in addition to protecting the heart during resultant ischemic stress, likely underpinning the profound reduction in cardiovascular mortality in hyperbilirubinemic GS. This review outlines our current knowledge of and uses for bilirubin in clinical medicine and summarizes recent progress in revealing the physiological importance of this poorly understood molecule. We believe that this review will be of significant interest to clinicians, medical researchers, and individuals who have GS.

Nitric Oxide Scavenging by Hydroxocobalamin May Account for Its Hemodynamic Profile

BACKGROUND

Antidotal doses of hydroxocobalamin are associated with transient increases in blood pressure in some animals and humans. These studies in anesthetized rabbits were undertaken to explore the possible mechanisms underlying the hemodynamic effects of hydroxocobalamin by investigating 1) possible hemodynamic effects of cyanocobalamin, which is formed on a molar-to-molar basis when hydroxocobalamin binds cyanide, and 2) the interference of hydroxocobalamin with the endothelial nitric oxide system.

METHODS

Study 1 investigated the hemodynamic effects of cyanocobalamin. This study included two treatment arms: 1) cyanocobalamin (75 mg/kg, IV) followed by saline (n = 7) and 2) saline followed by cyanocobalamin (n = 7). Study 2 assessed the hemodynamic effects of hydroxocobalamin (75 mg/kg, IV) in the presence and absence of the nitric oxide synthase inhibitor L-Nomega-nitro-L-arginine methyl ester (L-NAME; 30 mg/kg, IV). Nitric oxide synthase inhibition itself increases blood pressure. Thus, as part of Study 2, the hemodynamic effects of hydroxocobalamin were also investigated in the presence of an equipressor dose of angiotensin II (ANGII; 0.05 microg/kg/min, IV) in order to determine whether elevated blood pressure per se could interfere with hydroxocobalamin's hemodynamic effects. This study included six treatment arms (designated as first treatment + second treatment): saline + saline (n = 5), L-NAME + saline (n = 7), saline + hydroxocobalamin (n = 7), L-NAME + hydroxocobalamin (n = 7), ANGII + hydroxocobalamin (n = 7), and ANGII + saline (n = 7).

RESULTS

In Study 1, the effects of cyanocobalamin on hemodynamic parameters were indistinguishable from those of saline. In Study 2, hydroxocobalamin infusion was associated with moderate hemodynamic effects, including an increase in systemic vascular resistance, an increase in blood pressure, and a decrease in cardiac output. Administration of L-NAME abolished the effects of hydroxocobalamin on all hemodynamic parameters. ANGII at a dose producing a pressor response comparable to that of L-NAME did not influence the hydroxocobalamin-associated hemodynamic changes.

CONCLUSION

These studies in anesthetized rabbits demonstrate that the moderate pressor effect of hydroxocobalamin is not related to the formation of cyanocobalamin but is very likely related to the scavenging of nitric oxide by hydroxocobalamin.

Investigation of a possible interaction between the heme oxygenase/biliverdin reductase and nitric oxide synthase pathway in murine gastric fundus and jejunum

This study investigated the possible interaction between the heme oxygenase (HO)/biliverdin reductase (BVR) and nitric oxide synthase (NOS) pathway in murine gastric fundus and jejunum, since previous studies have shown that both HO-2 and BVR are expressed in interstitial cells of Cajal (ICCs) and co-localized with neuronal NOS in a large proportion of myenteric neurons along the gastrointestinal tract. Neither HO inhibition by chromium mesoporphyrin (CrMP) nor co-incubation with CO or biliverdin/bilirubin affected nitrergic neurotransmission - i.e. relaxations induced by non-adrenergic non-cholinergic (NANC) nerve stimulation or exogenous NO - under normal physiological conditions. However, biliverdin/bilirubin reversed the inhibitory effect of the superoxide generator LY83583 on exogenous NO-induced relaxations in both tissues. When gastric fundus muscle strips were depleted of the endogenous antioxidant Cu/Zn superoxide dismutase (SOD) by the Cu-chelator DETCA, electrically induced NANC relaxations were also affected by LY82583; however, biliverdin/bilirubin could not substitute for the loss of Cu/Zn SOD when this specific antioxidant enzyme was depleted. In jejunal muscle strips, the combination DETCA plus LY83583 nearly abolished contractile phasic activity and, hence, did not allow studying nitrergic relaxation in these experimental conditions. In conclusion, this study does not establish a role for HO/CO in inhibitory NANC neurotransmission in murine gastric fundus and jejunum under normal physiological conditions. However, the antioxidants biliverdin/bilirubin might play an important role in the protection of the nitrergic neurotransmitter against oxidative stress.

Role of superoxide dismutase enzymes and ascorbate in protection of nitrergic relaxation against superoxide anions in mouse duodenum

AIM

The aim of this study was to investigate whether superoxide dismutase (SOD) enzymes and ascorbate play a role in the protection of the nitrergic relaxation against superoxide anion inhibition in the mouse duodenum.

METHODS

The effects of exogenous SOD, N,No-bis(salicylidene) ethylenediamine chloride (EUK-8; a synthetic cell-permeable mimetic of the manganese SOD [Mn-SOD] and ascorbate on relaxant responses induced by nitrergic nerve stimulation), exogenous nitric oxide (NO), and nitroglycerin were investigated in isolated mouse duodenum tissues.

RESULTS

Diethyldithiocarbamate (DETCA) inhibited the relaxation to exogenous NO and nitroglycerin, but not relaxation to electrical field stimulation (EFS). SOD and ascorbate partially prevented the inhibitory effect of DETCA on relaxation to NO, abut not to nitroglycerin. The DETCA-induced inhibition on nitroglycerin was prevented by EUK-8. Hemoglobin, 2- (4-carboxyphenyl)-4,4,5,5-tetramethylimidazolinel-oxyl-3-oxide, and hydroxocobalamin inhibited the relaxation to NO, but not to EFS and nitroglycerin in the presence of DETCA. Pyrogallol and hydroquinone inhibited the relaxation to NO, but not to EFS and nitroglycerin. This inhibition was prevented by exogenous SOD and ascorbate, but was not prevented by EUK-8. Pyrogallol and hydroquinone did not inhibit the EFS-induced relaxation in the presence of DETCA. Duroquinone and 6-anilino-5.8-quinolinedione inhibited the relaxation to EFS, NO, and nitroglycerin, and this inhibition was prevented by EUK-8.

CONCLUSION

These results suggest that the nitrergic neurotransmission in the mouse duodenum is protected by endogenous tissue antioxidants against superoxide anions, and Mn SOD, in addition to copper/zinc SOD, can protect NO from attack from superoxide anion generators intracellularly. Also, the possibility that the endogenous neurotransmitter may not be the free NO but a NO-containing or NO-generating molecule in the mouse duodenum remains open.

The location of photodegradable nitric oxide store in the mouse stomach fundus

The aim of this study was to investigate the location of photodegradable nitric oxide (NO) store using a pharmacological approach in mouse gastric fundus. The ultraviolet light irradiation (UV; 360 nm, 60 s), electrical field stimulation (EFS; 4 Hz, 25 V, 1 ms, 15s-train), exogenous nitric oxide (NO; 10 microM), nitroglycerin (100 microM) and isoproterenol (5 nM) induced relaxation in mouse gastric fundus preparations in the absence or presence of an intact mucosa. The NO scavenger, haemoglobin (20 microM), significantly inhibited the relaxation of intact and denuded mucosa stomach fundus to UV light irradiation, EFS and NO, but not to nitroglycerin and isoproterenol. The superoxide anion generator, pyrogallol (50 microM), inhibited relaxation of intact and denuded mucosa stomach fundus induced by UV light irradiation, EFS, NO, but not to nitroglycerin and isoproterenol. The inhibition observed with pyrogallol was prevented by exogenous Cu/Zn superoxide dismutase (SOD; 100 U/ml), a membrane impermeable antioxidant. The Cu/Zn SOD inhibitor, diethyldithiocarbamic acid (DETCA; 8 mM), inhibited the relaxation of intact and denuded mucosa stomach fundus to UV light irradiation, EFS, NO and nitroglycerin but not those to isoproterenol. Exogenous SOD (100 U/ml) partially prevented the inhibitory effect of DETCA on relaxation to UV light irradiation, EFS, NO but not to nitroglycerin. DETCA-induced inhibition of the nitroglycerin-induced relaxation was partially prevented by the cell-permeable polyethylene-glycol-superoxide dismutase (100 U/ml). These results indicate that photodegradable NO store is, at least in part, unlikely to be within smooth muscle cells, and furthermore, that UV light-induced relaxation is not dependent on gastric mucosal layer.

Role of heat shock proteins (molecular chaperones) in intestinal mucosal protection

Most studies into the pathogenesis of inflammatory bowel diseases (IBD) have primarily focused on the cytotoxic agents and processes involved in producing mucosal injury, including the immune system. However, less consideration has been given to the inherent mechanisms of cytoprotection and cellular repair in the intestinal mucosa. This review will focus on intestinal mucosal protection against cytotoxic agents and cellular stress mainly from the viewpoint of expression and function of heat shock proteins, in their role of "molecular chaperones," as internal cytoprotectants. Elucidation of such stress-responses in the intestinal mucosa may provide a better understanding of the mechanisms of cytoprotection and cellular repair, and present new strategies for IBD therapy.

In vitro analysis of the effect of hyperbilirubinemia on rabbit ureter and bladder

Spontaneous resolution of intrauterine pelvic dilatations after birth is an expected outcome. In nonobstructive pelvic dilatations, changes in ureteral and bladder physiology may also play a part. We aimed to demonstrate the effect of increased concentrations of bilirubin on ureteral and bladder muscles in vitro. Normal and pathologic concentrations of bilirubin (3.5x10(-7)-10(-5)M and 10(-4)-4x10(-4)M, respectively) caused no change in the basal ureter tension (343.9+/-29.4 mg). Normal concentrations of bilirubin caused no difference in basal bladder tension (430.2+/-70.2 mg), but pathologic concentrations caused a decrease of 303.8+/-52.9 mg. Normal and pathologic amounts of bilirubin were cumulatively applied to rabbit ureteral and bladder tissues both after reaching basal tension and when contracted with KCl (80 mM and 120 mM KCl for ureter and bladder, respectively). The cumulative addition of normal bilirubin concentrations to the ureteral tissues precontracted with KCl produced 86.4+/-7.2% relaxation, while the addition of pathologic bilirubin concentrations produced a relaxation of 133.9+/-17.4%, which was significantly higher (p=0.04). Similarly, the addition of normal concentrations of bilirubin to the bladder tissues precontracted with KCl produced a maximal relaxation of 35.3+/-2.2%, while pathologic concentrations produced a maximal relaxation of 53.5+/-3.5%, which was significantly higher (0.001). Consequently, high concentrations of bilirubin caused a mild relaxation in basal ureteral and bladder tensions, while pathologically increased concentrations led to significant relaxation in both types of precontracted tissues. We suggest that high bilirubin levels may partly but not directly contribute to the spontaneous recovery of hydronephrosis because of the relaxation effect on bladder while probably causing susceptibility to urinary tract infections because of relaxation of both ureteral and bladder tissues.

Gastrointestinal Function Regulation by Nitrergic Efferent Nerves

Gastrointestinal (GI) smooth muscle responses to stimulation of the nonadrenergic noncholinergic inhibitory nerves have been suggested to be mediated by polypeptides, ATP, or another unidentified neurotransmitter. The discovery of nitric-oxide (NO) synthase inhibitors greatly contributed to our understanding of mechanisms involved in these responses, leading to the novel hypothesis that NO, an inorganic, gaseous molecule, acts as an inhibitory neurotransmitter. The nerves whose transmitter function depends on the NO release are called "nitrergic", and such nerves are recognized to play major roles in the control of smooth muscle tone and motility and of fluid secretion in the GI tract. Endothelium-derived relaxing factor, discovered by Furchgott and Zawadzki, has been identified to be NO that is biosynthesized from l-arginine by the constitutive NO synthase in endothelial cells and neurons. NO as a mediator or transmitter activates soluble guanylyl cyclase and produces cyclic GMP in smooth muscle cells, resulting in relaxation of the vasculature. On the other hand, NO-induced GI smooth muscle relaxation is mediated, not only by cyclic GMP directly or indirectly via hyperpolarization, but also by cyclic GMP-independent mechanisms. Numerous cotransmitters and cross talk of autonomic efferent nerves make the neural control of GI functions complicated. However, the findingsrelated to the nitrergic innervation may provide us a new way of understanding GI tract physiology and pathophysiology and might result in the development of new therapies of GI diseases. This review article covers the discovery of nitrergic nerves, their functional roles, and pathological implications in the GI tract.

AZD3582 increases heme oxygenase-1 expression and antioxidant activity in vascular endothelial and gastric mucosal cells

AZD3582 [4-(nitrooxy)-butyl-(2S)-2-(6-methoxy-2-naphthyl)-propanoate] is a COX-inhibiting nitric oxide donator (CINOD). Incubation of human endothelial cells (derived from umbilical cord) with AZD3582 (10-100muM) led to increased expression of heme oxygenase (HO)-1 mRNA and protein. Heme oxygenase-1 (HO-1) is a crucial mediator of antioxidant and tissue-protective actions. In contrast, naproxen (a non-selective NSAID) and rofecoxib (a selective inhibitor of COX-2), did not affect HO-1 expression. Pre-treating endothelial cells with AZD3582 at concentrations that were effective at inducing HO-1 also reduced NADPH-dependent production of oxygen radicals. Antioxidant activity in the endothelial cells persisted after AZD3582 had been washed out from the incubation medium. When added exogenously to the cells at low micromolar concentrations, the HO-1 metabolite, bilirubin, virtually abolished NADPH-dependent oxidative stress. AZD3582-induced blockade of free-radical formation was reversed in the presence of the HO-1 inhibitor, tin protoporphyrin-IX (SnPP). Similar results were obtained in human gastric mucosal cells (KATO-III). Our results demonstrate that HO-1 is a novel target of AZD3582.

The role of carbon monoxide in the gastrointestinal tract

Carbon monoxide (CO) is a biologically active product of haem metabolism that contributes to the normal physiology of the gastrointestinal tract. In this article, we review recent data showing that CO is an integral regulator of gastrointestinal motility and an important factor in the response to gastrointestinal injury. CO is generated by haem oxygenase-2 (HO-2), which is constitutively expressed in many inhibitory neurones of the vertebrate enteric nervous system. The membrane potential gradients along and across the muscle layers of the gastrointestinal tract require the generation of CO by haem oxygenase-2. The presence of CO is also necessary for normal inhibitory neurotransmission in circular smooth muscle and appears to permit nitric oxide-mediated inhibitory neurotransmission. Genetic deletion of the haem oxygenase-2 gene in mice slows gut transit. The other major CO synthetic enzyme, haem oxygenase-1 (HO-1) is induced under conditions of stress or injury. Recent studies have demonstrated that up-regulation of haem oxygenase-1 protects the gut from several types of gastrointestinal injury, suggesting that CO or induction of HO-1 may find therapeutic use in gastrointestinal diseases and injuries. Furthermore, it is anticipated that the understanding of CO-mediated signalling in the gastrointestinal tract will inform studies in other tissues that express haem oxygenases.

Influence of polyethylene-glycol-superoxide dismutase and combined depletion and repletion of antioxidants on nitrergic relaxation in the pig gastric fundus

In circular smooth muscle strips of porcine gastric fundus, polyethylene-glycol-superoxide dismutase, a membrane-permeable analogue of endogenous copper/zinc (Cu/Zn) superoxide dismutase, reversed the inhibitory effect of the superoxide anion generator 6-anilino-5,8-quinolinedione (LY83583) on electrically induced nitrergic relaxations of fundic tissues which are depleted of the endogenous antioxidant Cu/Zn superoxide dismutase by diethyldithiocarbamate, to the same extent as exogenously added Cu/Zn superoxide dismutase. Addition of a second antioxidant together with Cu/Zn superoxide dismutase does not result in a higher degree of reversal of the inhibitory effect of LY83583. Depletion of either tissue glutathione or tissue catalase in combination with diethyldithiocarbamate does not increase the inhibitory action of LY83583 or the nitric oxide (NO)-scavenger hydroxocobalamin upon nitrergic relaxations (electrically induced or by exogenous NO) when compared to their action in the presence of diethyldithiocarbamate alone. In conclusion, these results demonstrate that endogenous Cu/Zn superoxide dismutase is the essential antioxidant responsible for safeguarding peripheral nitrergic neurotransmission, whereby extracellular protection of endogenous NO is most important.

Interaction of bilirubin and biliverdin with reactive nitrogen species

Bilirubin (BR) and biliverdin (BV), two metabolites produced during haem degradation by haem oxygenase, possess strong antioxidant activities toward peroxyl radical, hydroxyl radical and hydrogen peroxide. Considering the importance attributed to nitric oxide (NO) and its congeners in the control of physiological and pathophysiological processes, we examined the interaction of BR and BV with NO and NO-related species in vitro. Exposure of BR and BV to agents that release NO or nitroxyl resulted in a concentration- and time-dependent loss of BR and BV, as assessed by high performance liquid chromatography. Peroxynitrite, a strong oxidant derived from the reaction of NO with superoxide anion, also showed high reactivity toward BR and BV. The extent of BR and BV consumption largely depended on the NO species being analysed and on the half-lives of the pharmacological compounds considered. Of major importance, BR and BV decomposition occurred also in the presence of pure NO under anaerobic conditions, confirming the ability of bile pigments to scavenge the gaseous free radical. Increasing concentrations of thiols prevented BR consumption by nitroxyl, indicating that bile pigments and thiol groups can compete and/or synergise the cellular defence against NO-related species. In view of the high inducibility of haem oxygenase-1 by NO-releasing agents in different cell types, the present findings highlight novel anti-nitrosative characteristics of BR and BV suggesting a potential function for bile pigments against the damaging effects of uncontrolled NO production.

Investigation of the potential modulatory effect of biliverdin, carbon monoxide and bilirubin on nitrergic neurotransmission in the pig gastric fundus

In porcine gastric fundus, we have investigated the colocalization of the bile pigment biosynthetic enzymes heme oxygenase-2 and biliverdin reductase with neuronal nitric oxide synthase (nNOS), the effect of carbon monoxide (CO) on fundic circular smooth muscle and the possible modulatory effect of the bile pigments biliverdin and bilirubin on CO-mediated relaxations and on nitrergic relaxation. Heme oxygenase-2 and biliverdin reductase immunoreactivity was present in all nNOS containing myenteric neurons. CO induced a concentration-dependent relaxation of fundic circular smooth muscle strips, which was completely blocked by the specific guanylate cyclase inhibitor 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (ODQ). 3-(5'-hydroxymethyl-2'-furyl)-1-benzylindazole (YC-1), biliverdin and bilirubin strongly enhanced the amplitude of the CO-induced relaxation. Tin protoporphyrin had no effect on electrically induced nitrergic relaxation, but spectrophotometric analysis learned that incubation of porcine gastric fundus circular muscle strips with tin protoporphyrin did not influence heme oxygenase activity. In conclusion, our data suggest that nitrergic neurons in the pig gastric fundus are able to produce biliverdin and bilirubin, and that these agents potentiate the relaxant effect of CO, which is formed concomitantly with biliverdin by heme oxygenase-2.

Heme oxygenase modulates oxidant-signaled airway smooth muscle contractility: role of bilirubin

Reactive oxygen species (ROS) increase the contractile response of airway smooth muscle (ASM). Heme oxygenase (HO) catabolizes heme to the powerful antioxidant bilirubin. Because HO is expressed in the airways, we investigated its effects on ASM contractility and ROS production in guinea pig trachea. HO expression was higher in the epithelium than in tracheal smooth muscle. Incubation of tracheal rings (TR) with the HO inhibitor tin protoporphyrin (SnPP IX) or the HO substrate hemin increased and decreased, respectively, ASM contractile response to carbamylcholine. The effect of hemin was reversed by SnPP and mimicked by the antioxidants superoxide dismutase (SOD) and catalase. Hemin significantly reduced the effect of carbamylcholine in rings treated with the guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), compared with ODQ-treated rings without hemin incubation, suggesting that the CO-guanosine 3',5'-cyclic monophosphate pathway was not involved in the control of tracheal reactivity. SnPP and hemin increased and decreased ROS production by TR by 18 and 38%, respectively. Bilirubin (100 pM) significantly decreased TR contractility and ROS production. Hemin, bilirubin, and SOD/catalase decreased phosphorylation of the contractile protein myosin light chain, whereas SnPP significantly augmented it. These data suggest that modulation of the redox status by HO and, moreover, by bilirubin modulates ASM contractility by modulating levels of phosphorylated myosin light chain.

The nonadrenergic noncholinergic relaxation of anococcygeus muscles of bile duct-ligated rats

Previous studies have shown the naloxone-induced withdrawal syndrome and the development of tolerance in the tissues of cholestatic animals. Increased neuronal nitric oxide synthase (nNOS) expression is reported to exist in morphine-tolerant animals. This, together with evidence for nitric oxide (NO) overproduction in cholestasis, suggested the possibility of an alteration of nonadrenergic noncholinergic (NANC) relaxation of anococcygeus muscles of cholestatic rats. To study this, we used three main groups of animals: unoperated, sham-operated and bile duct-ligated. Electrical field stimulation, in the presence of atropine and guanethidine, caused NANC relaxation in the anococcygeus muscle which was enhanced in bile duct-ligated animals. N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS blocker, caused a dose-dependent inhibition of the NANC relaxation. The IC(50)'s of L-NAME in 7-day (7.30+/-0.87 microM), 14-day (6.98+/-0.70 microM) and 21-day (8.25+/-1.40 microM) bile duct-ligated groups were significantly different from those of unoperated (1.69+/-0.30 microM) and sham-operated groups (1.90+/-0.27 microM). L-NAME (100 microM) completely inhibited the NANC relaxation response, suggesting that NANC relaxation in the rat anococcygeus muscle is mediated mainly via NO. The contraction response of the intact muscle to phenylephrine, an alpha(1)-adrenoceptor agonist, and the relaxation response of the phenylephrine-contracted muscle to sodium nitroprusside, an NO donor, were not different in unoperated, sham-operated and 7-day bile duct-ligated groups. These results showed that the smooth muscle component of NANC relaxation is not altered in anococcygeus muscles of bile duct-ligated rats. It can thus be concluded that the NANC relaxation in the anococcygeus of cholestatic rats is more resistant to a NOS blocker, providing evidence for increased nitrergic neurotransmission in the anococcygeus muscles of cholestatic rats.

Immunohistochemical localization of the antioxidant enzymes biliverdin reductase and heme oxygenase-2 in human and pig gastric fundus

The intrinsic antioxidant capacities of the bile pigments biliverdin and bilirubin are increasingly recognized since both heme degradation products can exert beneficial cytoprotective effects due to their scavenging of oxygen free radicals and interaction with antioxidant vitamins. Several studies have been published on the localization of the carbon monoxide producing enzyme heme oxygenase-2 (HO-2), which concomitantly generates biliverdin; histochemical data on the distribution of biliverdin reductase (BVR), converting biliverdin to bilirubin, are still very scarce in large mammals including humans. The present study revealed by means of immunohistochemistry the presence of BVR and HO-2 in mucosal epithelial cells and in the endothelium of intramural vessels of both human and porcine gastric fundus. In addition, co-labeling with the specific neural marker protein-gene product 9.5 (PGP 9.5) demonstrated that both BVR and HO-2 were present in all intrinsic nerve cell bodies of both submucous and myenteric plexuses, while double labeling with c-Kit antibody confirmed their presence in intramuscular interstitial cells of Cajal (ICC). Our results substantiate the hypothesis that BVR, through the production of the potent antioxidant bilirubin, might be an essential component of normal physiologic gastrointestinal defense in man and pig.

Influence of antioxidant depletion on nitrergic relaxation in the pig gastric fundus

1. The hypothesis that endogenous tissue antioxidants might explain the inability of the superoxide generators 6-anilino-5,8-quinolinedione (LY83583) and hydroquinone (HQ) and of the NO-scavengers hydroxocobalamin (HC) and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) to affect nitrergic neurotransmission in the porcine gastric fundus was tested by selective pharmacological depletion of respectively Cu/Zn superoxide dismutase (Cu/Zn SOD) and reduced glutathione (GSH) in circular smooth muscle preparations. 2. Diethyldithiocarbamate (DETCA; 3x10(-3) M), which almost completely abolished tissue Cu/Zn SOD activity, had no effect per se on nitrergic relaxations induced by either electrical field stimulation (EFS; 4 Hz, 10 s) or exogenous nitric oxide (NO; 10(-5) M). In these DETCA-treated tissues however, electrically-induced nitrergic relaxations became sensitive to inhibition by LY83583 (10(-5) M) or HC (10(-4) M), but not by HQ (10(-4) M) or c-PTIO (10(-4) M); only for the combination of DETCA plus LY83583, this inhibition was partially reversed by exogenous Cu/Zn SOD (1000 u ml(-1)). 3. Immunohistochemical analysis of porcine gastric fundus revealed a 100% colocalization of Cu/Zn SOD and neuronal nitric oxide synthase (nNOS) in the intrinsic neurons of the myenteric plexus. 4. Buthionine sulphoximine (BSO; 10(-3) M) in the absence or presence of LY83583 (10(-5) M) or HC (10(-4) M) did not alter nitrergic relaxations, although it reduced per se the tissue GSH content to 62% of control. 5. Pharmacological depletion studies, corroborated by immunohistochemical data, thus suggest a role for Cu/Zn SOD but not for GSH in nitrergic neurotransmission in the porcine gastric fundus.

Pre- and postjunctional protective effect of neocuproine on the nitrergic neurotransmitter in the mouse gastric fundus

1. Electrical field stimulation (EFS) of non-adrenergic non-cholinergic nerves of the mouse gastric fundus induced frequency-dependent transient relaxations which were mimicked by nitric oxide (NO), added as acidified NaNO(2). The NO donors S-nitrosocysteine, S-nitrosoglutathione, SIN-1 and hydroxylamine induced sustained concentration-dependent relaxations. The NO synthase blocker L-nitro arginine (L-NOARG; 300 microM) abolished the relaxations to EFS without affecting the relaxations to NO. 2. The copper(I) chelator neocuproine (10 microM) enhanced the relaxations to EFS and NO but inhibited those to S-nitrosocysteine and S-nitrosoglutathione. Neocuproine potentiated the relaxations to SIN-1, which releases NO extracellularly, without affecting the relaxations to hydroxylamine, which releases NO intracellularly. 3. The potentiating effect of neocuproine on the relaxations to EFS was more pronounced after inhibition of catalase with 3-amino-1,2,4-triazole (1 mM) but not after inhibition of Cu/Zn superoxide dismutase (SOD) with diethyl dithiocarbamic acid (DETCA, 1 mM). The potentiating effect of neocuproine on relaxations to NO was not altered by 3-amino-1,2,4-triazole or DETCA treatment. 4. The relaxations to EFS were significantly inhibited by the oxidants hydrogen peroxide (70 microM) and duroquinone (10 microM) but only after inhibition of catalase with 3-amino-1,2,4-triazole or after inhibition of Cu/ZnSOD with DETCA respectively. 5. Our results suggest that neocuproine can act as an antioxidant in the mouse gastric fundus and that both catalase and Cu/ZnSOD protect the nitrergic neurotransmitter from oxidative breakdown. Since inhibition of catalase but not inhibition of Cu/ZnSOD potentiated the effect of neocuproine on relaxations to EFS without affecting the relaxations to NO, catalase may protect the nitrergic neurotransmitter mainly at a prejunctional site whereas Cu/ZnSOD protects at a postjunctional site.

Interaction of hypoxanthine/xanthine oxidase with nitrergic relaxation in the porcine gastric fundus

The influence of hypoxanthine (HX)/xanthine oxidase (XO) on short-term [electrical field stimulation (EFS; 4 Hz) for 10 s and 3 min; bolus of exogenous NO (10(-5) M)] and long-term [EFS (4 Hz) and continuous NO-infusion for 20 min] nitrergic relaxations was investigated in circular muscle strips of the pig gastric fundus. HX (3x10(-4) M) / XO (64 mu ml(-1)) did not affect EFS for 10 s and 3 min; the short-lasting relaxation in response to a bolus of exogenous NO (10(-5) M) was changed into a biphasic relaxation with a small and short first phase followed by a larger and prolonged second phase. Cu/Zn superoxide dismutase (Cu/Zn SOD; 1000 u ml(-1)) and uricase (100 mu ml(-1)) respectively enhanced the amplitude of the first phase and diminished the amplitude of the second phase. Ascorbate (5x10(-4) M) and bilirubin (2x10(-4) M) prevented the prolonged component. Exposure to HX/XO during long-term EFS elicited a complete, stable reversal of relaxation starting after a delay. During continuous NO-infusion, HX/XO induced an immediate, complete but transient reversal. The antioxidants bilirubin, ascorbate, alpha-tocopherol, urate, glutathione and Cu/Zn SOD, the hydrogen peroxide degrading enzyme catalase, the hydroxyl radical scavengers dimethylsulphoxide and mannitol, and the cofactor flavin adenine dinucleotide did not influence the reversal induced by HX/XO during either EFS or NO-infusion. The cell-permeable manganese SOD mimetic EUK-8 modified the stable reversal during long-term EFS into a transient one. The results suggest that a nitrated uric acid derivative is responsible for the prolonged second phase in the relaxation to a bolus of exogenous NO in the presence of HX/XO. The exact underlying mechanism of the reversal induced by HX/XO during sustained relaxation remains unclear.