Carbon monoxide: an endogenous modulator of the nitric oxide-cyclic GMP signaling system

Inhibitors of nitric oxide synthase block carbon monoxide-induced increases in cGMP in retina

Previous studies indicate that the gaseous messengers carbon monoxide (CO) and nitric oxide (NO) can interact to cause robust increases in intracellular cGMP levels in the retina. The purpose of the present study was to investigate the biochemical basis of the interactions between NO and CO for these increases. Turtle retinas were incubated in vitro with CO to stimulate cGMP production in the presence or absence of the nitric oxide synthase inhibitors N-omega-nitro-L-arginine methyl ester and S-methyl-thiocitrulline. Cyclic GMP immunocytochemistry was then used to evaluate the changes in cGMP levels in response to these stimuli. The results indicated that CO itself stimulated increases in cGMP in bipolar and amacrine cells, and that the increases were completely blocked by SMTC and L-NAME. We postulate that the increases of cGMP in response to CO might be mediated, at least partly, by CO displacing and releasing NO from its intracellular storage pool(s).

Heme oxygenase-1-related carbon monoxide production and ventricular fibrillation in isolated ischemic/reperfused mouse myocardium

Heme oxygenase-1 (HO-1)-dependent carbon monoxide (CO) production related to reperfusion-induced ventricular fibrillation (VF) was studied in HO-1 wild-type (+/+), heterozygous (+/-), and homozygous (-/-) isolated ischemic/reperfused mouse heart. In HO-1 homozygous myocardium, under aerobic conditions, HO-1 enzyme activity, HO-1 mRNA, and protein expression were not detected in comparison with aerobically perfused wild-type and heterozygous myocardium. In wild-type, HO-1 hetero- and homozygous hearts subjected to 20 min ischemia followed by 2 h of reperfusion, the expression of HO-1 mRNA, protein, and HO-1 enzyme activity was detected in various degrees. A reduction in the expression of HO-1 mRNA, protein, and enzyme activity in fibrillated wild-type and heterozygous myocardium was observed. In reperfused/nonfibrillated wild-type and heterozygous hearts, a reduction in HO-1 mRNA, protein expression, and HO-1 enzyme activity was not observed, indicating that changes in HO-1 mRNA, protein, and enzyme activity could be related to the development of VF. These changes were reflected in the HO-1-related endogenous CO production measured by gas chromatography. In HO-1 knockout ischemic/reperfused myocardium, all hearts showed VF, and no detection in HO-1 mRNA, protein, and enzyme activity was observed. Thus, interventions that are able to increase endogenous CO may prevent the development of VF.

Analysis of pulmonary heme oxygenase-1 and nitric oxide synthase alterations in experimental hepatopulmonary syndrome

BACKGROUND & AIMS

Cirrhosis and portal hypertension due to chronic common bile duct ligation reproduce the features of human hepatopulmonary syndrome, whereas portal hypertension alone due to partial portal vein ligation does not. Nitric oxide contributes to experimental hepatopulmonary syndrome, but the nitric oxide synthase forms involved remain controversial. Recently, increased pulmonary heme oxygenase-1 expression and carbon monoxide production have also been found after common bile duct ligation. Our aim was to explore the role of the heme oxygenase-1/carbon monoxide pathway in the pathogenesis of experimental hepatopulmonary syndrome.

METHODS

Pulmonary heme oxygenase-1 expression and distribution were assessed in sham; 3-week partial portal vein ligation; and 1-, 2-, 3-, 4-, and 5-week common bile duct ligation animals by Northern, Western and immunohistochemical analysis relative to endothelial and inducible nitric oxide synthase levels and to hepatopulmonary syndrome development. In vivo heme oxygenase enzyme inhibition with tin protoporphyrin IX in common bile duct ligation animals was used to define effects on intrapulmonary vasodilatation and arterial blood gases.

RESULTS

Heme oxygenase-1 expression in pulmonary intravascular monocytes/macrophages and arterial carboxyhemoglobin levels increased progressively from 3 to 5 weeks after common bile duct ligation relative to controls (5-week protein levels were 15.94 +/- 1.75-fold those of sham animals; P < 0.001). Inducible nitric oxide synthase increased transiently in pulmonary intravascular monocytes/macrophages in 3-week common bile duct ligation animals, whereas pulmonary microvascular endothelial nitric oxide synthase increases began at 2 weeks and correlated with the onset of hepatopulmonary syndrome. Tin protoporphyrin treatment normalized carboxyhemoglobin and improved arterial blood gases and intrapulmonary vasodilatation, reflecting partial reversal of hepatopulmonary syndrome.

CONCLUSIONS

The heme oxygenase-1/carbon monoxide system is an important contributor to the progression of experimental hepatopulmonary syndrome in addition to alterations in the endothelial nitric oxide synthase/nitric oxide pathway.

Carbon monoxide neurotransmission activated by CK2 phosphorylation of heme oxygenase-2

Carbon monoxide (CO) is a putative gaseous neurotransmitter that lacks vesicular storage and must be synthesized rapidly following neuronal depolarization. We show that the biosynthetic enzyme for CO, heme oxygenase-2 (HO2), is activated during neuronal stimulation by phosphorylation by CK2 (formerly casein kinase 2). Phorbol ester treatment of hippocampal cultures results in the phosphorylation and activation of HO2 by CK2, implicating protein kinase C (PKC) in CK2 stimulation. Odorant treatment of olfactory receptor neurons augments HO2 phosphorylation and activity as well as cyclic guanosine monophosphate (cGMP) levels, with all of these effects selectively blocked by CK2 inhibitors. Likewise, CO-mediated nonadrenergic, noncholinergic (NANC) relaxation of the internal anal sphincter requires CK2 activity. Our findings provide a molecular mechanism for the rapid neuronal activation of CO biosynthesis, as required for a gaseous neurotransmitter.

Selective protection of renal tubular epithelial cells by heme oxygenase (HO)-1 during stress-induced injury

BACKGROUND

The renal pathology of human heme oxygenase (HO)-1 deficiency is characterized by advanced tubulointerstitial injury, whereas the glomerular structures are affected little. These facts suggest that the renal tubuli are dependent on intrinsic HO-1 production for their survival under oxidative stresses.

METHODS

We compared the patterns of HO-1 expression by primary cultured human mesangial cells (HMCs) and renal proximal tubular epithelial cells (HRPTECs) in vitro. Furthermore, the cytoprotective roles of HO-1 induced in these cells were evaluated by stress-induced cytotoxicity assays. HO-1 expressions in HRPTECs and HMCs were evaluated by immunoblotting, and by reverse transcriptase (RT) and/or real time polymerase chain reaction (PCR).

RESULTS

In HRPTECs, both HO-1 mRNA expression and protein production peaked at around 12 h and persisted until 24 h after hemin stimulation. In contrast, HO-1 mRNA expression and protein production by HMCs peaked at 4 h and 6 h respectively, and the levels declined rapidly, being undetectable at 24 h. The peak level of HO-1 expression was significantly higher in HRPTECs than in HMCs. Oxidative stress-induced cell injury in HRPTECs was significantly reduced when HO-1 production had been induced prior to the culture. In contrast, HO-1 induction had little cytoprotective effect on HMCs. Tin protoporphyrin (SnPP), an inhibitor of HO function, significantly reversed the cytoprotection by HO-1.

CONCLUSION

These data suggest that HRPTECs are more susceptible to oxidative stress and are significantly more dependent on HO-1 for protection against noxious stimuli than HMCs. Collectively, these results indicate that HO-1 is an important protective factor for kidney tissue, in particular, renal tubular epithelial cells.

Inhibition of heme oxygenase decreases sodium and fluid absorption in the loop of Henle

We previously demonstrated that carbon monoxide (CO) stimulates the apical 70-pS K+ channel in the thick ascending limb (TAL) of the rat kidney (Liu HJ, Mount DB, Nasjletti A, and Wang WH. J Clin Invest 103: 963-970, 1999). Because the apical K+ channel plays a key role in K+ recycling, we tested the hypothesis that heme oxygenase (HO)-dependent metabolites of heme may affect Na+ transport in the TAL. We used in vivo microperfusion to study the effect of chromium mesoporphyrin (CrMP), an inhibitor of HO, on fluid absorption (Jv) and Na+ absorption (JNa) in the loop of Henle and renal clearance methods to examine the effect of CrMP on renal sodium excretion. Microperfusion experiments demonstrated that addition of CrMP to the loop of Henle decreased Jv by 13% and JNa by 20% in animals on normal rat chow and caused a decrease in Jv (39%) and JNa (40%) in rats on a high-K+ (HK) diet. The effect of CrMP is the result of inhibition of HO because addition of MgPP, an analog of CrMP that does not inhibit HO, had no effect on Jv. Western blot analysis showed that HO-2 is expressed in the kidney and that the level of HO-2 was significantly elevated in animals on a HK diet. Renal clearance studies demonstrated that the infusion of CrMP increased the excretion of urinary Na+ (ENa) and volume (UV) without changes in glomerular filtration rate. The effect of CrMP on ENa and UV was larger in HK rats than those kept on normal chow. We conclude that HK intake increases HO-2 expression in the kidney and that HO-dependent metabolites of heme, presumably CO, play a significant role in the regulation of Na+ transport in the loop of Henle.

Effect of carbon monoxide on dopamine and glutamate uptake and cGMP levels in rat brain

After the recognition of nitric oxide (NO) as a messenger molecule in the nervous system, carbon monoxide (CO) has received attention with similar properties. The present study aims to elucidate the effects of CO on synaptosomal dopamine ((3)H-DA) and glutamate ((3)H-Glu) uptake and on cGMP levels; possible interaction between NO and CO systems was also evaluated. Our results provide evidence for the inhibition of DA and Glu uptake by CO in a time-, dose-, and temperature-dependent manner in rat striatum and hippocampus, respectively; the inhibition observed was sexually dimorphic with more pronounced effects in females. Basal cGMP levels were higher in female rats than males in the striatum and exogenous CO increased striatal cGMP levels only in males; no effect of CO was observed in the hippocampus. In vivo nitric oxide synthase (NOS) inhibition increased DA and Glu uptake; however, CO was still effective in inhibiting uptake following NOS inhibiton. Taken together, these findings suggest a role for CO in trans-synaptic regulation through modulation of DA and Glu transporters and of cGMP levels; the effect on cGMP levels is independent of NOS activity and appears to be sexually dimorphic and region specific.

Nitric oxide synthesis influences the renal vascular response to heme oxygenase inhibition

We studied the effects of the heme oxygenase (HO) inhibitor stannous mesoporphyrin (SnMP; 40 micromol/kg i.v.) on renal hemodynamics in anesthetized rats with and without 48-h pretreatment with N(G)-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide (NO) synthesis. SnMP decreased renal blood flow (RBF) and increased renal vascular resistance (RVR) in both groups. The SnMP-induced reduction of RBF in L-NAME-pretreated rats was more prominent than in rats without pretreatment (43 +/- 7 vs. 13 +/- 3%) as was the SnMP-induced elevation of RVR (87 +/- 31 vs. 14 +/- 5%). The renal vasoconstrictor effect of SnMP is linked, in part, to amplification of prevailing neurohormonal constrictor mechanisms, since in L-NAME-pretreated rats it was prevented by concurrent administration of prazosin or losartan. However, SnMP (15 micromol/l) also elicits vasoconstriction in isolated, pressurized renal interlobular arteries and the response is more intense in vessels obtained from L-NAME-pretreated rats than from rats without pretreatment. These data indicate that the status of NO synthesis conditions the vascular response to HO inhibition in the rat kidney.

Heme oxygenase-1 and heme oxygenase-2 have distinct roles in the proliferation and survival of olfactory receptor neurons mediated by cGMP and bilirubin, respectively

Heme oxygenase (HO) is implicated in protection against oxidative stress, proliferation and apoptosis in many cell types, including neurons. We utilized olfactory receptor neurons (ORNs) as a model to define the roles of HO-1 and HO-2 in neuronal development and survival, and to determine the mediators of these effects. The olfactory system is a useful model as ORNs display neurogenesis post-natally and do not contain nitric oxide synthase (NOS) activity, which could confound results. HO isoforms were expressed in ORNs during embryogenesis and post-natally. Mice null for either HO-1 or HO-2 displayed decreased proliferation of neuronal precursors. However, apoptosis was increased only in HO-2 null mice. Cyclic GMP immunostaining was reduced in ORNs in both genotypes, providing direct evidence that HO mediates cGMP production in vivo. Bilirubin immunostaining was reduced only in HO-2 null mice. These roles for HO-1 and HO-2 were confirmed using detergent ablation of the epithelium to observe increased neurogenesis of ORNs after target disruption in HO null mice. Primary cultures of ORNs revealed that proliferative and survival effects of HO were mediated through cGMP and bilirubin, respectively. These results support a role for HO, the CO-cGMP signaling system and bilirubin in neurodevelopment and in response to injury.

NOVELNEURALMODULATORS

The discovery that nitric oxide (NO) is produced by neurons and regulates synaptic activity has challenged the definition of a neurotransmitter. NO is not stored in synaptic vesicles and does not act at conventional receptors on the surface of adjacent neurons. The toxic gases carbon monoxide (CO) and hydrogen sulfide (H2S) are also produced by neurons and modulate synaptic activity. D-serine synthesis and release by astrocytes as an endogenous ligand for the "glycine" site of N-methyl D-aspartate (NMDA) receptors defy the concept that a neurotransmitter must be synthesized by neurons. We review the properties of these "atypical" neural modulators.

Visualization of gaseous monoxide reception by soluble guanylate cyclase in the rat retina

Immunohistochemistry using novel monoclonal antibodies (mAbs) allowed us to uncover tissue activities of soluble guanylate cyclase (sGC) fine tuned by NO and CO. Upon NO and CO applications in vitro, purified sGC increased the affinity to mAb3221 by 100- and 10-fold, respectively, but not to mAb28131. Immunohistochemistry for gas-generating enzymes revealed that NO occurred in amacrine, bipolar, and Müller's glia cells (MGCs), whereas CO was derived mostly from heme oxygenase (HO)-2 in MGCs. Basal sGC immunoreactivities in vivo to mAb3221 but not to mAb28131 were enhanced by injecting L-arginine and attenuated by blocking NO synthases, suggesting the ability of the former mAb to sense NO. Comparison of mAb-assisted immunohistochemistry suggested that sGC activities were enhanced by zinc protoporphyrin-IX, an HO inhibitor, and repressed completely by blocking NO. However, suggested roles of CO played in situ varied among different retinal layers. In inner plexiform and inner nuclear layers located in the proximity of the cellular NO sources, CO serves as a simple inhibitor of local sGC, while playing roles in housekeeping sGC activation in external limiting membrane standing far from them. These results suggest that CO generated in MGCs is a diffusible gas mediator regulating sGC in both autocrine and paracrine manners.

Effects of nitrogen monoxide and carbon monoxide on molecular and cellular iron metabolism: mirror-image effector molecules that target iron

Many effector functions of nitrogen monoxide (NO) and carbon monoxide (CO) are mediated through their high-affinity for iron (Fe). In this review, the roles of NO and CO are examined in terms of their effects on the molecular and cellular mechanisms involved in Fe metabolism. Both NO and CO avidly form complexes with a plethora of Fe-containing molecules. The generation of NO and CO is mediated by the nitric oxide synthase and haem oxygenase (HO) families of enzymes respectively. The effects of NO on Fe metabolism have been well characterized, whereas knowledge of the effects of CO remains within its infancy. In terms of the role of NO in Fe metabolism, one of the best characterized interactions includes its effect on the iron regulatory proteins. These molecules are mRNA-binding proteins that control the expression of the transferrin receptor 1 and ferritin, molecules that are involved in Fe uptake and storage respectively. Apart from this, activated macrophages impart their cytotoxic activity by generating NO, which results in marked Fe mobilization from tumour-cell targets. This deprives the cell of the Fe that is required for DNA synthesis and energy production. Considering that HO degrades haem, resulting in the release of CO, Fe(II) and biliverdin, it is suggested that a CO-Fe complex will form. This may account for the rapid Fe mobilization observed from macrophages after haemoglobin catabolism. Intriguingly, overexpression of HO results in cellular Fe mobilization, suggesting that CO has a similar effect to NO on Fe trafficking. Preliminary evidence suggests that, like NO, CO plays important roles in Fe metabolism.

Carbon monoxide inhibition of apoptosis during ischemia-reperfusion lung injury is dependent on the p38 mitogen-activated protein kinase pathway and involves caspase 3

Carbon monoxide (CO), a reaction product of the cytoprotective gene heme oxygenase, has been shown to be protective against organ injury in a variety of models. One potential mechanism whereby CO affords cytoprotection is through its anti-apoptotic properties. Our studies show that low level, exogenous CO attenuates anoxia-reoxygenation (A-R)-induced lung endothelial cell apoptosis. Exposure of primary rat pulmonary artery endothelial cells to minimal levels of CO inhibits apoptosis and enhances phospho-p38 mitogen-activated protein kinase (MAPK) activation in A-R. Transfection of p38alpha dominant negative mutant or inhibition of p38 MAPK activity with SB203580 ablates the anti-apoptotic effects of CO in A-R. CO, through p38 MAPK, indirectly modulates caspase 3. Furthermore, we correlate our in vitro apoptosis model with an in vivo model of A-R by showing that CO can attenuate I-R injury of the lung. Taken together, our data are the first to demonstrate in models of A-R that the anti-apoptotic effects of CO are via modulation of p38 MAPK and caspase 3.

Carbon monoxide as an attenuator of vasoconstriction in piglet cerebral arterioles

Carbon monoxide (CO) is an endogenous dilator in the newborn cerebral circulation. The present study addressed the hypothesis that endogenous CO attenuates pial arteriolar vasoconstriction caused by hypocapnia, platelet activating factor, and elevated blood pressure. Experiments used anesthetized piglets with implanted, closed cranial windows. Topical application of a metal porphyrin inhibitor of heme oxygenase was used to inhibit production of CO. Chromium mesopophyrin increased vasoconstriction in response to hypocapnia. The constrictor response to a topical stimulus, platelet activating factor, was also increased by application of chromium mesoporphyrin. Inhibition of heme oxygenase did not constrict pial arterioles in normotensive newborn pigs (mean arterial pressure of about 70 mmHg), but did constrict pial arterioles of piglets with experimentally induced increases in arterial pressure (mean arterial pressure greater than 90 mmHg). In fact, pial arterioles of normotensive piglets transiently dilated to chromium mesoporphyrin, whereas those of hypertensive piglets progressively constricted during 10 min of chromium mesoporphyrin treatment. Therefore, inhibition of heme oxygenase augments cerebral vasoconstriction in response to several very different constrictor stimuli. These data suggest endogenous CO attenuates vasoconstrictor responses in the newborn cerebral circulation.

Regulation of IL-1beta -induced GM-CSF production in human airway smooth muscle cells by carbon monoxide

Asthma, a chronic inflammatory disease of the airways, involves the increased expression of inflammatory mediators, including granulocyte-monocyte colony-stimulating factor (GM-CSF). Heme oxygenase-1 (HO-1), a stress-response protein, confers protection against oxidative stress. We hypothesized that carbon monoxide (CO), a byproduct of HO-1-dependent heme catabolism, regulates GM-CSF synthesis in human airway smooth muscle cells (HASMC). IL-1beta treatment induced a time-dependent induction of GM-CSF in HASMC. Furthermore, IL-1beta stimulated the major MAPK pathways, including ERK1/ERK2, JNK, and p38 MAPK. Exposure of HASMC to CO at low concentration (250 ppm) markedly inhibited IL-1beta-induced GM-CSF synthesis (>90%) compared with air-treated controls. CO treatment inhibited IL-1beta-induced ERK1/2 activation but did not inhibit JNK and p38 MAPK. Furthermore, CO increased cGMP levels in HASMC. Inhibition of guanylate cyclase by IH-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-1 (ODQ) abolished the inhibitory effects of CO on GM-CSF synthesis and ERK1/2 activation. Collectively, these data demonstrate that the inhibitory effect of CO on GM-CSF synthesis depends on ERK1/2 MAPK and guanylate cyclase/cGMP-dependent pathways.

Heme oxygenase-1 and carbon monoxide in pulmonary medicine

Heme oxygenase-1 (HO-1), an inducible stress protein, confers cytoprotection against oxidative stress in vitro and in vivo. In addition to its physiological role in heme degradation, HO-1 may influence a number of cellular processes, including growth, inflammation, and apoptosis. By virtue of anti-inflammatory effects, HO-1 limits tissue damage in response to proinflammatory stimuli and prevents allograft rejection after transplantation. The transcriptional upregulation of HO-1 responds to many agents, such as hypoxia, bacterial lipopolysaccharide, and reactive oxygen/nitrogen species. HO-1 and its constitutively expressed isozyme, heme oxygenase-2, catalyze the rate-limiting step in the conversion of heme to its metabolites, bilirubin IXalpha, ferrous iron, and carbon monoxide (CO). The mechanisms by which HO-1 provides protection most likely involve its enzymatic reaction products. Remarkably, administration of CO at low concentrations can substitute for HO-1 with respect to anti-inflammatory and anti-apoptotic effects, suggesting a role for CO as a key mediator of HO-1 function. Chronic, low-level, exogenous exposure to CO from cigarette smoking contributes to the importance of CO in pulmonary medicine. The implications of the HO-1/CO system in pulmonary diseases will be discussed in this review, with an emphasis on inflammatory states.

Carbon monoxide and the nervous system

Carbon monoxide (CO) is a colorless, tasteless, odorless, and non-irritating gas formed when carbon in fuel is not burned completely. It enters the bloodstream through the lungs and attaches to hemoglobin (Hb), the body's oxygen carrier, forming carboxyhemoglobin (COHb) and thereby reducing oxygen (O(2)) delivery to the body's organs and tissues. High COHb concentrations are poisonous. Central nervous system (CNS) effects in individuals suffering acute CO poisoning cover a wide range, depending on severity of exposure: headache, dizziness, weakness, nausea, vomiting, disorientation, confusion, collapse, and coma. At lower concentrations, CNS effects include reduction in visual perception, manual dexterity, learning, driving performance, and attention level. Earlier work is frequently cited to justify the statement that CO exposure sufficient to produce COHb levels of ca. 5% would be sufficient to produce visual sensitivity reduction and various neurobehavioral performance deficits. In a recent literature re-evaluation, however, the best estimate was that [COHb] would have to rise to 15-20% before a 10% reduction in any behavioral or visual measurement could be observed. This conclusion was based on (1) critical review of the literature on behavioral and sensory effects, (2) review and interpretation of the physiological effects of COHb on the CNS, (3) extrapolation from the effects of hypoxic hypoxia to the effects of CO hypoxia, and (4) extrapolation from rat behavioral effects of CO to humans. Also covered in this review article are effects of chronic CO exposure, the discovery of neuroglobin, a summary of the relatively new role for endogenous CO in neurotransmission and vascular homeostasis, groups which might be especially sensitive to CO, and recommendations on further research. The interested reader is directed to other published reviews of the literature on CO and historically seminal references that form our understanding of this ubiquitous gas.

Carbon monoxide inhibits human airway smooth muscle cell proliferation via mitogen-activated protein kinase pathway

The gaseous molecule carbon monoxide (CO) is elevated in the breath of individuals with asthma. The physiologic function of CO in asthma is poorly understood. Here we demonstrate that CO (250 ppm) markedly inhibits human airway smooth muscle cell (HASMC) proliferation, arresting cells at the G0/G1 phase. This CO-induced cell growth arrest of HASMC was associated with upregulation of p21 and downregulation of cyclin D1 expression. It is generally believed that the signaling pathway by which CO affects biologic processes is primarily mediated via the guanylyl cyclase/3',5'-Guanylate cyclic monophosphate (cGMP) pathway. To examine whether guanylyl cyclase/cGMP was involved in CO-induced growth arrest of HASMC, Rp-8-Br-cGMP, a selective inhibitor of cGMP-dependent protein kinase and ODQ, a selective inhibitor of soluble guanylate cyclase, were administered to HASMC in the presence of CO. Interestingly, CO-induced cell growth arrest was not reversed by these inhibitors. We next examined whether the extracellular signal-regulated kinase (ERK) 1/ERK2 mitogen-activated protein kinase (MAPK) signaling pathway may regulate the antiproliferative effect of CO. We first showed time-dependent activation of the various MAPKs in HASMC in response to serum, including phosphorylated ERK1/ERK2, p38, and JNK and then demonstrated that CO exerted negligible effect on activated p38 and JNK; however, ERK activation was significantly attenuated in the presence of CO. These data suggest that CO can inhibit HASMC proliferation via the ERK1/ERK2 MAPK pathway, independent of a guanylyl cyclase/cGMP independent pathway. CO may act as an important mediator of remodeling of human airways in asthma via its ability to regulate cell growth of airway smooth muscle cells.

Induction of heme oxygenase 1 by nitrosative stress. A role for nitroxyl anion

Nitric oxide and S-nitrosothiols modulate a variety of important physiological activities. In vascular cells, agents that release NO and donate nitrosonium cation (NO(+)), such as S-nitrosoglutathione, are potent inducers of the antioxidant protein heme oxygenase 1 (HO-1) (Foresti, R., Clark, J. E., Green, C. J., and Motterlini, R. (1997) J. Biol. Chem. 272, 18411-18417; Motterlini, R., Foresti, R., Bassi, R., Calabrese, V., Clark, J. E., and Green, C. J. (2000) J. Biol. Chem. 275, 13613-13620). Here, we report that Angeli's salt (AS) (0.25-2 mm), a compound that releases nitroxyl anion (NO(-)) at physiological pH, induces HO-1 mRNA and protein expression in a concentration- and time-dependent manner, resulting in increased heme oxygenase activity in rat H9c2 cells. A time course analysis revealed that NO(-)-mediated HO-1 expression is transient and gradually disappears within 24 h, in accordance with the short half-life of AS at 37 degrees C (t(12) = 2.3 min). Interestingly, multiple additions of AS at lower concentrations (50 or 100 microm) over a period of time still promoted a significant increase in heme oxygenase activity. Experiments performed using a NO scavenger and the NO electrode confirmed that NO(-), not NO, is the species involved in HO-1 induction by AS; however, the effect on heme oxygenase activity can be amplified by accelerating the rate of NO(-) oxidation. N-Acetylcysteine almost completely abolished AS-mediated induction of HO-1, whereas a glutathione synthesis inhibitor (buthionine sulfoximine) significantly decreased heme oxygenase activation by AS, indicating that sulfydryl groups are crucial targets in the regulation of HO-1 expression by NO(-). We conclude that NO(-), in analogy with other reactive nitrogen species, is a potent inducer of heme oxygenase activity and HO-1 protein expression. These findings indicate that heme oxygenase can act both as a sensor to and target of redox-based mechanisms involving NO and extend our knowledge on the biological function of HO-1 in response to nitrosative stress.

Heme oxygenase-1: redox regulation and role in the hepatic response to oxidative stress

Heme oxygenase (HO) catalyzes the oxidative cleavage of the alpha-mesocarbon of Fe-protoporphyrin-IX yielding equimolar amounts of biliverdin-IXalpha, free divalent iron, and carbon monoxide (CO). Among the three isoenzymes cloned to date, only HO-1 can be induced by a variety of seemingly disparate stimuli, most of which are linked by their ability to provoke oxidative stress. Although constitutive expression of HO-1 in the liver is restricted to Kupffer cells, the gene is inducible in nonparenchymal as well as in parenchymal liver cells. HO-1 induction potentially confers protection against oxidative stress in a variety of experimental models, such as liver ischemia/reperfusion secondary to transplantation or hemorrhage/resuscitation. Induction of HO-1 may protect the cell against oxidative injury by (a) controlling intracellular levels of "free" heme (a prooxidant), (b) producing biliverdin (an antioxidant), (c) improving nutritive perfusion via CO release, and (d) fostering the synthesis of the Fe-binding protein ferritin. Although protective effects of up-regulation of the HO pathway--presumably through production of bile pigments and CO--have been reported for a variety of cells and tissues, including the liver, evidence suggests that the protective action might be restricted to a rather narrow threshold of overexpression. High levels of HO-1 may even sensitize the cell to oxidative stress, e.g., through release of reactive iron. Transcriptional activation of the HO-1 gene is an integral part of the cellular response to oxidative stress, but its induction seems to be neither exclusively cytoprotective nor exclusively cytotoxic.

Oxidative stress induced by phenylketonuria in the rat: Prevention by melatonin, vitamin E, and vitamin C

Phenylketonuria (PKU) is an autosomal recessive disorder caused by a deficiency of the phenylalanine hydroxylation system and is characterized by a block in the conversion of phenylalanine (PHE) to tyrosine. We examined the effects of maternal hyperphenylalaninemia on the morphological and biochemical development of pup rat brain and cerebellum. In our model of PKU we evaluated a number of markers of oxidative stress such as Ehrlich adducts formation, lipid peroxidation, as well as the levels of reduced and oxidized glutathione, and the activities of the enzymes glutathione peroxidase and glutathione reductase. We also studied the expression of heme-oxigenase-1 and mitogen-activated protein kinase 1/2 (MAPK 1/2) as additional markers of oxidative stress. We demonstrate that PKU strongly increased most of the oxidative stress markers studied and induced significant morphological damage. We also showed that daily administration of melatonin (20 mg/kg BW), vitamin E (30 mg/kg BW), and vitamin C (30 mg/kg BW) until delivery prevented the oxidative biomolecular damage in the rat brain and cerebellum. Although no significant differences were observed among the antioxidants studied, it should be noted that the doses of melatonin were less than those for vitamins E and C. We conclude that PKU induces a clear state of oxidative stress that is somehow involved in the brain and body damage occurring in this inborn error. Moreover, melatonin and other antioxidants are capable of preventing completely the damage induced by PKU.

Heme oxygenase-1: the "emerging molecule" has arrived

Organisms on our planet have evolved in an oxidizing environment that is intrinsically inimical to life, and cells have been forced to devise means of protecting themselves. One of the defenses used most widely in nature is the enzyme heme oxygenase-1 (HO-1). This enzyme performs the seemingly lackluster function of catabolizing heme to generate bilirubin, carbon monoxide, and free iron. Remarkably, however, the activity of this enzyme results in profound changes in cells' abilities to protect themselves against oxidative injury. HO-1 has been shown to have anti-inflammatory, antiapoptotic, and antiproliferative effects, and it is now known to have salutary effects in diseases as diverse as atherosclerosis and sepsis. The mechanism by which HO-1 confers its protective effect is as yet poorly understood, but this area of invetsigation is active and rapidly evolving. This review highlights current information on the function of HO-1 and its relevance to specific pulmonary and cardiovascular diseases.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Role of carbon monoxide in glutamate receptor-induced dilation of newborn pig pial arterioles

The hypothesis that glutamate dilates pial arterioles of newborn pigs through the production of carbon monoxide (CO) was addressed. Anesthesized newborn pigs were equipped with cranial windows to measure pial arteriolar responses to stimuli. Heme oxygenase (HO) inhibitors added topically inhibited dilation to glutamate and to specific glutamate receptor agonists. The initial dilation to glutamate (10(-5) M) was 22% from baseline without an inhibitor and decreased to 9% with the HO inhibitor chromium mesoporphyrin (CrMP). Inhibition of dilation upon HO inhibition was similar when specific glutamate receptor agonists were employed. RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid caused 24% dilation from the baseline without an inhibitor, and the dilation was decreased to 1% with tin protoporphyrin (SnPP). (RS)-2-amino-3-(3-hydroxy-5-t-butylisoxazol-4-yl)propionic acid (kainate receptors) caused dilation of 18% from baseline without an inhibitor, but only 2% when tin mesoporphyrin was applied. 1-Aminocyclopropanecarboxylic acid (N-methyl-D-aspartate receptors) dilated pial arterioles 33% from baseline in control, but only to 2% in the presence of SnPP. Neither copper mesoporphyrin, which does not inhibit HO, nor light-inactivated CrMP affected the dilations. Furthermore, cerebral microvessels removed from the brain produced CO (stable isotope dilution gas chromatography-mass spectrometry), and this production was dose dependently increased by glutamate and inhibited by metal porphyrin HO inhibitors. These data suggest that dilation of newborn pig pial arterioles to glutamate and specific glutamate receptor agonists involves vascular production of CO. Additional cerebral sources of CO also could be stimulated by glutamate and contribute to the dilation.

G proteins and olfactory signal transduction

The olfactory system sits at the interface of the environment and the nervous system and is responsible for correctly coding sensory information from thousands of odorous stimuli. Many theories existed regarding the signal transduction mechanism that mediates this difficult task. The discovery that odorant transduction utilizes a unique variation (a novel family of G protein-coupled receptors) based upon a very common theme (the G protein-coupled adenylyl cyclase cascade) to accomplish its vital task emphasized the power and versatility of this motif. We now must understand the downstream consequences of this cascade that regulates multiple second messengers and perhaps even gene transcription in response to the initial interaction of ligand with G protein-coupled receptor.

Cerebrovascular effects of carbon monoxide

This review examines the influence of endogenous and exogenous carbon monoxide (CO) on the cerebral circulation. Although CO generated from neuronal heme oxygenase can modulate neurotransmission, evidence supporting its role in cerebral vasodilation is limited. In newborn piglets, heme oxygenase is enriched in microvessels and contributes to hypoxic vasodilation. Low CO concentrations dilate piglet arterioles by opening calcium-activated potassium channels. With inhalation of CO and formation of carboxyhemoglobin, cerebral vasodilation can be greater than that occurring with hypoxic hypoxia at equivalent reductions of arterial oxygen content. This additional vasodilation is probably attributable to additional release of hypoxic vasodilators secondary to increased oxyhemoglobin affinity, although direct effects of CO on cerebral arterioles may also occur. When CO exposure is prolonged, cerebral endothelium undergoes oxidant stress as evident by nitrotyrosine formation. As CO levels increase, modest decreases in oxygen consumption are detectable, which may reflect CO or nitric oxide interactions with cytochrome oxidase in regions with very low oxygen availability. If subsequent CO concentration increases sufficiently to depress cardiac function and limit cerebral perfusion, cerebral oxygen consumption becomes further reduced, and oxidant stress becomes amplified by leukocyte sequestration and xanthine oxidase activity with consequent lipid peroxidation. Specific regions of the brain, such as central white matter, globus pallidus, and hippocampus, are selectively vulnerable to CO toxicity, but whether the mechanisms involved in selective injury differ from other forms of hypoxia-ischemia needs to be clarified.

Measurement of endogenous carbon monoxide formation in biological systems

Endogenous carbon monoxide (CO) formation has been measured in different biological systems using a variety of analytical procedures. The methods include gas chromatography-reduction gas detection, gas chromatography-mass spectroscopic detection, laser sensor-infrared absorption, UV-visible spectrophotometric measurement of CO-hemoglobin or CO-myoglobin complex, and formation of (14)CO from (14)C-heme formed following [2-(14)C]glycine administration. CO formation ranged from a low of 0.029 nmol/mg of protein/h in chorionic villi of term human placenta to a high of 0.28 nmol/mg of protein/h in rat olfactory receptor neurons in culture and rat liver perfusate.

Modulation of endothelial cell apoptosis by heme oxygenase-1-derived carbon monoxide

It is well established that expression of heme oxygenase-1 (HO-1) acts in a cytoprotective manner in a variety of cell types, including in endothelial cells (EC). We have recently shown that HO-1 expression protects EC from undergoing apoptosis. We have also shown that the antiapoptotic effect of HO-1 is mediated through heme catabolism into the gas carbon monoxide (CO). In this review, we discuss the possible molecular mechanisms by which HO-1-derived CO suppresses EC apoptosis. We will review data suggesting that the antiapoptotic effect of CO acts through the activation of the p38 mitogen-activated protein kinase signal transduction pathway and requires the activation of the transcription factor nuclear factor-kappa B (NF-kappa B), as well as the expression of a subset of NF-kappa B-dependent antiapoptotic genes.

Cross talk between carbon monoxide and nitric oxide

Carbon monoxide and nitric oxide are two endogenously produced gases that can act as second messenger molecules. Heme oxygenase and nitric oxide synthase are the enzyme systems responsible for generating carbon monoxide and nitric oxide, respectively. Both carbon monoxide and nitric oxide share similar properties, such as the ability to activate soluble guanylate cyclase to increase cyclic GMP. It is becoming increasingly clear that these two gases do not always work independently, but rather can modulate each other's activity. Although much is known about the heme oxygenase/carbon monoxide and nitric oxide synthase/nitric oxide pathways, how these two important systems interact is less well understood. This review attempts to define the current known relationship between carbon monoxide and nitric oxide as it relates to their production and physiological function.

Nitric-oxide-synthase-mediated cyclic guanosine monophosphate production in neonatal rat cerebellar prisms is resistant to calcineurin inhibition

Although the macrolide immunosuppressant tacrolimus (FK506) is neuroprotective in animal models of focal and global cerebral ischaemia, the mechanism of this action is not known. FK506 inhibits the protein phosphatase calcineurin, whose substrates can include nitric oxide synthase (NOS), and the neuroprotective effect of FK506 has been attributed to inhibition of NOS activity. We have examined nitric oxide-mediated cyclic guanosine monophosphate (cGMP) accumulation in neonatal rat cerebellar prisms. As expected, N-methyl-D-aspartate (NMDA) induced a rapid, concentration dependent accumulation of cGMP that was inhibited by the NMDA receptor antagonist dizocilpine (MK801) and the NOS inhibitor L-nitro-arginine methyl ester. Phosphoserine immunopositivity following NMDA exposure was increased in the presence of FK506, confirming inhibition of calcineurin. However, FK506 had no effect on NMDA-stimulated cGMP accumulation. These findings suggest that the neuroprotective effect of FK506 may be mediated by mechanisms other than increased NOS phosphorylation.

Carbon monoxide-releasing molecules: characterization of biochemical and vascular activities

Carbon monoxide (CO) is generated in living organisms during the degradation of heme by the enzyme heme oxygenase, which exists in constitutive (HO-2 and HO-3) and inducible (HO-1) isoforms. Carbon monoxide gas is known to dilate blood vessels in a manner similar to nitric oxide and has been recently shown to possess antiinflammatory and antiapoptotic properties. We report that a series of transition metal carbonyls, termed here carbon monoxide-releasing molecules (CO-RMs), liberate CO to elicit direct biological activities. Specifically, spectrophotometric and NMR analysis revealed that dimanganese decacarbonyl and tricarbonyldichlororuthenium (II) dimer release CO in a concentration-dependent manner. Moreover, CO-RMs caused sustained vasodilation in precontracted rat aortic rings, attenuated coronary vasoconstriction in hearts ex vivo, and significantly reduced acute hypertension in vivo. These vascular effects were mimicked by induction of HO-1 after treatment of animals with hemin, which increases endogenously generated CO. Thus, we have identified a novel class of compounds that are useful as prototypes for studying the bioactivity of CO. In the long term, transition metal carbonyls could be utilized for the therapeutic delivery of CO to alleviate vascular- and immuno-related dysfunctions. The full text of this article is available at http://www.circresaha.org.

Lead neurotoxicity: heme oxygenase and nitric oxide synthase activities in developing rat brain

Chronic lead (Pb) exposure has been shown to result in cognitive deficits in children. Diffusible nitric oxide (NO) is a biological messenger known to regulate long-term potentiation (LTP) in the developing nervous system. Carbon monoxide (CO), as in the case of NO was shown to be a messenger molecule in the brain. Our previous studies indicated significant changes in neuronal nitric oxide synthase (nNOS) in the developing brain following Pb-exposure [Chetty et al. (2001) Int. J. Toxicol. 20, 113-120]. In the present study, we investigated the effects of perinatal Pb-exposure on the activity of heme oxygenase (HO), the enzyme that produces CO by the catabolism of heme. Male Sprague-Dawley rats were exposed to 0.2% Pb-acetate gestationally and lactationally up to postnatal day (PND) 21. The specific activity of HO was determined in the hippocampus and the cerebellum of rat brain at PND 7, 14, 21 and 35. HO activity was higher in the hippocampus as compared to the cerebellum at any postnatal age. HO activity increased progressively up to PND 21 followed by a marginal decrease at PND 35. Perinatal Pb-exposure did not induce any significant change in HO activity as compared to control rats. However, in vitro treatment of microsomal supernatant fractions prepared from the cerebellum and the hippocampus to different (5-50 micro M) concentrations of Pb increased HO activity whereas the activity decreased at 100 micro M. The selective NOS inhibitors, nitroarginine (NA) and nitroindazole (NI) had no significant effect on HO activity; however, nNOS activity was inhibited in a concentration-dependent manner. In contrast, the selective HO-inhibitor, zinc protoporphyrin-IX (ZnPP) inhibited both HO and nNOS activity levels. The NO-donor, sodium nitroprusside (SNP) did not alter HO activity except at a higher (100 micro M) concentration. These data suggest that Pb-exposure selectively modulates nNOS but not HO in developing rat brain.

Cytoprotective role of heme oxygenase (HO)-1 in human kidney with various renal diseases

BACKGROUND

We previously reported that glomerular changes in the renal specimen of a human case with heme oxygenase-1 (HO-1) deficiency were mild, but tubulointerstitial injury advanced progressively. This study examined the patterns of HO-1 production in the kidney in various renal diseases. Furthermore, the critical cytoprotective roles of HO-1 were evaluated in the kidney by comparing HO-1 production and expressions of carboxymethyllysine (CML) and pentosidine, both of which are markers of oxidative stress.

METHODS

Renal biopsy or autopsy materials were obtained from a total of 74 patients. Degrees of hematuria and proteinuria and the levels of urinary N-acetyl-beta-D-glucosaminidase (NAG), beta2-microglobulin (beta2m), and creatinine were evaluated. Immunohistochemical studies for HO-1, CML, and pentosidine expressions were performed with their specific antiserum.

RESULTS

HO-1 staining was observed within tubular epithelial cells in all of the renal diseases, but was not detected within intrinsic glomerular cells. HO-1 staining tended to be more intense within distal tubuli than in proximal tubuli. Within distal tubuli, there was no significant correlation between intensity of HO-1 staining and degree of hematuria or presence of proteinuria. Within proximal tubuli, HO-1 staining tended to be more intense with greater degrees of hematuria, presence of proteinuria, and moderate tubulointerstitial damage. Intense staining of CML and pentosidine was observed within renal tubular epithelial cells only in HO-1-deficient patients.

CONCLUSIONS

HO-1 plays important roles in protecting renal tubuli from oxidative injuries, as these cells are constantly exposed to various oxidative stresses. It is suggested that renal tubular epithelia are more susceptible to oxidative stress due to the lack of this critical enzyme in HO-1 deficiency.

Methemoglobin is a supplement for in vitro culture of human nasopharyngeal epithelial cells transformed by human papillomavirus type 16 DNA

NPC-N cells were normal human nasopharvngeal epithelial cells transformed by transfection with human papillomavirus type 16 deoxyribonucleic acid. Bovine pituitary extract (BPE) was one of the indispensable ingredients for in vitro culture of NPC-N cells in a serum-free medium. Chromatographic fractionation of BPE and subsequent immunoblotting analyses identified the hemoglobin growth-stimulating factor. Methemoglobin (metHb) was then synthesized, and also found to be growth stimulating. The growth-stimulating effect of metHb was abolished when NPC-N cells were cultured in a medium that also contained haptoglobin, a molecule that binds to hemoglobin. A defined medium consisting of insulin and metHb was then developed for optimal growth of NPC-N cells. MetHb kept under the conditions identical to those of cell culture released hemin which also enhanced the cell growth. Though all the degradation products of hemin are currently known to be physiologically significant. only ferric iron derived from metHb or hemin could stimulate the growth of NPC-N cells. Abnormal vasculature showing leaky walls and hemorrhage is a common feature of malignant tumors. Hemoglobin originating from extravasated red blood cells and subsequently oxidized to metHb because of the presence of activated inflammatory cells might contribute to the increased proliferation of cancerous cells.

Methaemoglobin enhances the proliferation of transformed human epithelial cells: a possible outcome of neovascularisation and haemorrhage in tumours?

The effect of human methaemoglobin (metHb), possibly derived from extravasated red blood cells in tumours showing neovascularisation and haemorrhage, on the growth of transformed human epithelial cells was investigated. MetHb stimulated the growth of immortalised epithelial cells or transformed cells at precrisis stage (cells have bypassed M1, but not M2, the two mortality checkpoints). The stimulatory effect was due to the release of haemin from metHb that was isolated by a Sephadex column and identified by its characteristic light absorption spectrum. Although all the degradation products of haemin are currently known to be physiologically significant, only ferric iron derived from metHb or haemin could stimulate cell growth. High concentrations of metHb or haemin inhibited cell growth possibly due to the generation of high concentrations of bilirubin. However, bilirubin formed in the cells of human body is known to be transported to the liver for further processing and excretion. Haemoglobin oxidised to where tumours show neovascularisation and haemorrhage likely contributes significantly to the increased proliferation of cancerous cells.

Neural roles for heme oxygenase: contrasts to nitric oxide synthase

The heme oxygenase (HO) and nitric oxide (NO) synthase (NOS) systems display notable similarities as well as differences. HO and NOS are both oxidative enzymes using NADPH as an electron donor. The constitutive forms of the enzyme are differentially activated, with calcium entry stimulating NOS by binding to calmodulin, whereas calcium entry activates protein kinase C to phosphorylate and activate HO2. Although both NO and carbon monoxide (CO) stimulate soluble guanylyl cyclase to form cGMP, NO also S-nitrosylates selected protein targets. Both involve constitutive and inducible biosynthetic enzymes. However, functions of the inducible forms are virtual opposites. Macrophage-inducible NOS generates NO to kill other cells, whereas HO1 generates bilirubin to exert antioxidant cytoprotective effects and also provides cytoprotection by facilitating iron extrusion from cells. The neuronal form of HO, HO2, is also cytoprotective. Normally, neural NO in the brain seems to exert some sort of behavioral inhibition. However, excess release of NO in response to glutamate's N-methyl-d-aspartate receptor activation leads to stroke damage. On the other hand, massive neuronal firing during a stroke presumably activates HO2, leading to neuroprotective actions of bilirubin. Loss of this neuroprotection after HO inhibition by mutant forms of amyloid precursor protein may mediate neurotoxicity in Familial Alzheimer's Disease. NO and CO both appear to be neurotransmitters in the brain and peripheral autonomic nervous system. They also are physiologic endothelial-derived relaxing factors for blood vessels. In the gastrointestinal pathway, NO and CO appear to function as coneurotransmitters, both stimulating soluble guanylyl cyclase to cause smooth muscle relaxation.

Stereologic description of the changing expression of constitutive nitric oxide synthase and heme oxygenase in the enteric plexuses of the pig small intestine during development

The similarities between heme oxygenase-2 (HO-2) and nitric oxide synthase (nNOS) and the transient expression of nNOS during development led us to investigate whether both systems are similarly affected by changes that occur during development and by regional differences along the small intestine. By combining NADPH diaphorase histochemistry and HO-2 immunohistochemistry on whole-mount preparations and by using stereologic methods, a qualitative and quantitative description of HO-2 and nNOS expression was obtained. Examinations were carried out on the small intestine of fetal, 1-2-day and 5-6-week-old pigs. In all age groups, three enteric plexuses were distinguished. The presence of HO-2-immunoreactive (HO-2-IR) and NADPH diaphorase-positive neurons corresponded to earlier morphological and physiological reports. Nevertheless, the total number of nitrergic neurons remained constant or decreased in the enteric plexuses, whereas the total number of HO-2-IR neurons displayed an overall increase. Changing concentrations of glucocorticoids, target-derived signals, presynaptic input, and an effect of HO-2 activity on nNOS synthesis are likely to play roles in the observed developmental changes. The numerical density of HO-2-IR neurons remained relatively constant along the intestinal tract; in contrast, the nitrergic neurons were most numerous in the inner submucous and myenteric plexus in the duodenum and ileum, respectively. It is believed that the duodenal nitrergic neurons in the inner submucous plexus could be involved in the regulation of duodenal secretion processes, whereas the region-dependent density in the myenteric plexus possibly forms the morphological basis for a regionally different participation of NO in the relaxation of the small intestine.

Developmental changes in heme-oxygenase-2 and bNOS expression in enteric neurons in the pig duodenum

There exists much parallelism between carbon monoxide- and nitric oxide-generating systems. Therefore, we wondered whether developmental and functional differences along the duodenum similarly affect, part of them, namely, heme oxygenase-2-(HO-2) and neural isoform of nitric oxide synthase- (nNOS) expressing neurons. By applying NADPH diaphorase histochemistry and HO-2 immunohistochemistry on whole-mount preparations and by using stereologic methods, a qualitative and quantitative description of HO-2 and nNOS expression was obtained. Examinations were carried out on the duodenum of fetal, neonatal and weaned pigs. At all ages, three enteric plexuses were readily distinguished. The presence of both enzymes fits in with other morphological and physiological reports. However, the expression of both enzymes significantly changed during development. The number of HO-2-IR neurons increased approximately 20-fold in the inner submucous and almost doubled in the myenteric plexus. In addition, the number of nNOS-expressing neurons displayed a significant decrease in the outer submucous plexus after weaning. High levels of glucocorticoids may cause the perinatally increased HO-2 expression, whereas an influence on nNOS expression is doubtful. Therefore, it seems that notwithstanding the high similarity between both systems, their expression is regulated differently in the pig duodenum.

Human heme oxygenase-1 gene transfer lowers blood pressure and promotes growth in spontaneously hypertensive rats

Heme oxygenase (HO) catalyzes the conversion of heme to biliverdin, with release of free iron and carbon monoxide. Both heme and carbon monoxide have been implicated in the regulation of vascular tone. A retroviral vector containing human HO-1 cDNA (LSN-HHO-1) was constructed and subjected to purification and concentration of the viral particles to achieve 5x10(9) to 1x10(10) colony-forming units per milliliter. The ability of concentrated infectious viral particles to express human HO-1 (HHO-1) in vivo was tested. A single intracardiac injection of the concentrated infectious viral particles (expressing HHO-1) to 5-day-old spontaneously hypertensive rats resulted in functional expression of the HHO-1 gene and attenuation of the development of hypertension. Rats expressing HHO-1 showed a significant decrease in urinary excretion of a vasoconstrictor arachidonic acid metabolite and a reduction in myogenic responses to increased intraluminal pressure in isolated arterioles. Unexpectedly, HHO-1 chimeric rats showed a simultaneous significant proportionate increase in somatic growth. Thus, delivery of HHO-1 gene by retroviral vector attenuates the development of hypertension and promotes body growth in spontaneously hypertensive rats.

Mechanism of carbon monoxide-induced relaxation in the guinea pig ileal smooth muscle

The mechanism of carbon monoxide (CO)-induced relaxation were investigated in the guinea-pig ileum. CO (10%) inhibited the 40 mM KCl-induced contraction. This effect was antagonized by ODQ (1 microM), a soluble guanylate cyclase inhibitor. In contrast, CO did not inhibit the 40 mM KCl-induced increase in cytosolic Ca2+ level ([Ca2+]i). Cumulative addition of KCl induced a graded increase in both [Ca2+]i and muscle tension. In the presence of CO, the increase in muscle tension was attenuated whereas the increase in [Ca2+]i was only slightly decreased. Thus, the [Ca2+]i-tension relationship constructed by cumulative addition of KCl shifted downwards in the presence of CO. Using the patch clamp, CO was found to have little effect on the peak Ba currents (I(Ba)) when voltage was stepped from -60 mV to 0 mV. From these results, we conclude that CO inhibits contraction of guinea-pig ileum mainly by the decrease in the sensitivity of contractile elements to Ca2+ via a cyclic GMP-dependent pathway but not by the inhibition of L-type Ca2+ channel.

Role of the haeme oxygenase/carbon monoxide pathway in mechanical nociceptor hypersensitivity

The cleavage of haeme by haeme oxygenase (HO) yields carbon monoxide (CO), a biologically active molecule which exerts most of its effects via activation of soluble guanylate cyclase (sGC). In the present study, we tested the hypothesis that endogenous CO could modulate inflammatory hyperalgesia. The intensity of hyperalgesia was investigated in a model of mechanical nociceptor hypersensitivity in rats. The intra-plantar (i.pl.) administration of the HO inhibitor, ZnDPBG (Zinc deuteroporphyrin 2,4-bis glycol), potentiated in a dose-dependent manner the mechanical nociceptor hypersensitivity evoked by i.pl. administration of carrageenan. The mechanical hypersensitivity evoked by i.pl. injection of interleukin-1beta (IL-1beta), tumour necrosis factor-alpha (TNF-alpha), but not interleukin-8 (IL-8), prostaglandin E(2) (PGE(2)) or dopamine, was also enhanced by ZNDPBG: Moreover, the haeme (HO substrate) injection in the paws reduced the hypersensitivity evoked by IL-1beta, but not PGE(2). Furthermore, i.pl. administration of the gas CO reduced the hypersensitivity elicited by PGE(2). The inhibitory effect of haeme and CO upon mechanical nociceptor hypersensitivity were counteracted by a soluble guanylate cyclase (sGC) inhibitor, ODQ (1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one), suggesting that this effect of CO is mediated via cyclic GMP. Finally, the inhibitory effect of CO upon mechanical nociceptor hypersensitivity was prevented by the NO synthase blocker, L-NMMA (N(G)-monomethyl L-arginine), suggesting that the impairment of mechanical hypersensitivity elicited by CO depends on the integrity of the NO pathway. In conclusion, the results presented in this paper imply that endogenously CO produced by HO plays an anti-hyperalgesic role in inflamed paws, probably by increasing the intracellular levels of cyclic GMP in the primary afferent neurone.

Haem oxygenase in enteric nervous system of human stomach and jejunum and co-localization with nitric oxide synthase

Recent evidence suggests that carbon monoxide (CO) may be a neurotransmitter, similar to nitric oxide (NO) in the enteric nervous system. The distribution of haem oxygenase (HO), the biosynthetic enzyme for CO, has been determined in the enteric nervous system of animals, but little is known about the distribution of HO in human gastrointestinal tract. The present study investigated the expression of HO and its colocalization with NO synthase (NOS), the biosynthetic enzyme for NO, in human antrum and jejunum. HO isoforms were identified using immunohistochemistry and NOS was identified by immunohistochemistry or NADPH-d histochemistry. HO-2 immunoreactive (IR) cell bodies in enteric ganglia and nerve fibres in longitudinal and circular muscle were found in both antrum and jejunum. Co-localization of HO-2 and NOS was about 40% in HO-2 containing cell bodies of myenteric ganglia and only 10% or less in cell bodies of submucous ganglia. HO-1 immunoreactivity was not detected in antrum or jejunum. The results suggest that CO is produced in human enteric ganglion neurones and indicate a possible role of CO as a neurotransmitter and possible interaction between HO and NOS pathways in inhibitory neurotransmission in the human gastrointestinal tract.

Atypical neural messengers

Notions of what constitutes a neurotransmitter have changed markedly with the advent in the past decade of synaptic molecules, which satisfy key neurotransmitter criteria but differ radically from classical transmitters. Thus, NO and carbon monoxide are neither stored in synaptic vesicles nor released by exocytosis. These gases do not act via traditional receptors on postsynaptic membranes. In addition, zinc, stored together with glutamate in synaptic vesicles, appears to act as an 'antagonist' co-transmitter at the NMDA receptor, and although localized exclusively to glia, D-serine fulfills most neurotransmitter criteria as an endogenous ligand for the 'glycine' site of NMDA receptors.

Suppression of the induction of long-term depression by carbon monoxide in rat cerebellar slices

Carbon monoxide (CO) suppresses brain functions at doses lower than that suppressing oxygen (O(2)) supply to the brain, and the cerebellum is one of the sites most susceptible to the neurotoxic effects of CO. We investigated the effects of CO on the induction of cerebellar long-term depression (LTD) in the synapses between parallel fibres (PFs) and Purkinje cells. CO, at concentrations between 8 nM and 5 microM, exhibited almost no effect on synaptic responses in Purkinje cells, O(2) consumption and NO release from PFs in rat cerebellar slices. However, the induction of LTD was significantly suppressed by CO at concentrations between 40 and 200 nM. The suppressive effect of 40 nM CO was antagonized by 10 microM NOR3, an NO donor. In contrast, CO exhibited no clear effect on the induction of LTD at concentrations between 1 and 5 microM. The induction of LTD, suppressed by 10 microM N(G)-nitro-L-arginine, an inhibitor of NO synthase, was not restored by 5 microM CO. CO is not only a neurotoxic substance but also a candidate for an intercellular messenger. delta-Aminolevulinate (30 microM), a substance facilitating endogenous CO production, suppressed the induction of LTD, and the effect of delta-aminolevulinate was antagonized by 10 microM NOR3. These findings suggest that CO may have a suppressive effect on the induction of cerebellar LTD at nanomolar concentrations, probably via its effects on NO/cGMP signalling.

Real-time measurements of endogenous CO production from vascular cells using an ultrasensitive laser sensor

Carbon monoxide (CO) has been implicated as a biological messenger molecule analogous to nitric oxide. A compact gas sensor based on a midinfrared laser absorption spectroscopy was developed for direct and real-time measurement of trace levels (in approximate pmol) of CO release by vascular cells. The midinfrared light is generated by difference frequency mixing of two nearinfrared lasers in a nonlinear optical crystal. A strong infrared absorption line of CO (4.61 microm) is chosen for convenient CO detection without interference from other gas species. The generation of CO from cultured vascular smooth muscle cells was detected every 20 s without any chemical modification to the CO. The sensitivity of the sensor reached 6.9 pmol CO. CO synthesis was measured from untreated control cells (0.25 nmol per 10(7) cells/h), sodium nitroprusside-treated cells (0.29 nmol per 10(7) cells/h), and hemin-treated cells (0.49 nmol per 10(7) cells/h). The sensor also detected decreases in CO production after the addition of the heme oxygenase (HO) inhibitor tin protoporphyrin-IX (from 0.49 to 0.02 nmol per 10(7) cells/h) and increases after the administration of the HO substrate hemin (from 0.27 to 0.64 nmol per 10(7) cells/h). These results demonstrate that midinfrared laser absorption spectroscopy is a useful technique for the noninvasive and real-time detection of trace levels of CO from biological tissues.